s2fieldsofstudy:
Chemistry
Materials Science

text:
Understanding the active sites of Fe–N–C materials and their properties in the ORR catalysis system

Metal–N–C-based catalysts prepared by pyrolysis are frequently used in the oxygen reduction reaction (ORR). Zeolitic imidazolate frameworks (ZIFs), a type of metal organic framework (MOF), are selected as precursors due to their special structure and proper pore sizes. A series of Fe–N–C catalysts with different concentrations of 2-methylimidazole were prepared with a simple solvothermal-pyrolysis method, and the transformation productivity, morphology and ORR activity were investigated. It was found that the Fe–N–C catalyst with a 2-methylimidazole concentration of 0.53 mol L−1 had the best performance. In 0.1 M KOH solution, the half-wave potential was 0.852 V (vs. RHE), with the highest electrochemically active surface area (ECSA) of 94.1 cm2, and the ORR reaction was dominated by a 4-electron process. The current only decreased by 10.5% after 50 000 s of chronoamperometry (CA), while the half-wave potential only decreased 20 mV in 3 M methanol. Additionally, this catalyst cannot be poisoned by Cl− and SO32− ions in the ORR process. Finally, some typical ions including SCN−, Fe(CN)63− and Fe(CN)64− were used to inhibit the active sites, and it was determined that Fe(ii) is the real active species. The series of synthesis and testing experiments has significance in guiding optimization of the synthesis conditions and analysis of the mechanism of active sites in Fe–N–C materials.

Introduction
With society's increasing demand for energy, the energy crisis has become a worldwide problem. Development of a highly efficient energy conversion device is a promising way to solve this problem. Fuel cells (FCs) have attracted extensive attention worldwide due to their advantages of clean products, high energy density, and highly efficient energy conversion. Alkaline fuel cells (AFCs) and proton exchange membrane fuel cells (PEMFCs) 1,2 are of particular interest due to their low operating temperature, which has more application prospects than other fuel cell designs. However, the sluggish ORR kinetics in the cathode have so far restricted their application and development. Therefore, the most effective catalysts of platinum group metals (PGMs) are typically selected to improve the cathode kinetic process. The main problem with PGM catalysts is the high price of Pt and its poor reserves in the Earth's crust, 3 which demand that researchers investigate new catalysts. Among the PGM-free catalysts, transition metals coordinate with nitrogen and form a metal-N x (M-N x ) site in the carbon skeleton, which has been investigated as a highly active site in the alkaline system of the ORR process [4][5][6][7] and has the potential to replace PGMs as a catalyst in AFCs. Fe-N-C is regarded as one of the most active sites, [8][9][10] while the abundant reserves and low price of Fe further indicate that Fe-N-C is worth investigating.
MOF derivatives with high conductivity and a high number of M-N-C sites are obtained by pyrolysis and carbonization of MOFs. 11,12 ZIFs, as typical MOF materials, are usually selected to synthesize M-N-C catalysts because of their high specic surface area, proper pore sizes and composition, structure and morphology, which are all easy to design. [13][14][15] ZIF-8 has been widely investigated in ORR electrocatalysis. The current research focuses on the control of synthesis, 10,16,17 design of pore patterns and morphology, 18-21 multicomponent doping [21][22][23][24] and composites with other materials 25-28 to achieve catalysts with high performance, in addition to discussing the catalytic mechanism. For the optimization of synthesis conditions, research is focused on the type and dosage of metal and the process of heat treatment. However, there are a few reports on tuning the concentration of the precursor to change the morphology of the ZIF. Zhang and coworkers changed the volume of methanol and synthesized a series of Fe-N-C catalysts with different particle sizes. 29 Their research showed that ORR activity is related to the particle sizes, which suggested that the inuence of precursor concentrations on ORR activity can be used to optimize the synthesis conditions. The change in the volume of methanol can signicantly affect the concentration of various precursors. Hence, variation of a single precursor is still worthy of investigation. On the other hand, some typical ions from the atmosphere and tap water, such as Cl À and SO 3 2À , may have an attenuation effect on ORR activity in real FC conditions, but there are few studies on their effect on activity. Wang and coworkers selected poly-m phenylenediamine (PmPDA) to synthesize Fe-N-C catalysts. 30 Their research showed that the Fe-N-C catalyst had a decrease in ORR half-wave potential with the addition of Cl À and SO 3
Whether this phenomenon appears in other types of Fe-N-C catalysts remains unknown. In addition, in the study of the active site of Fe-N-C catalysts, further research is needed to understand whether the different valence states of the Fe site have the same catalytic properties. Potassium thiocyanide, KSCN, is used as an inhibiting reagent for Fe sites, but the effect of this reagent on other valence states of Fe sites and the effect of other inhibiting reagents on Fe sites is still unclear. Despite the progress made in this eld, it remains unclear which valence state of Fe provides the best ORR activity.
In this work, a simple solvothermal-pyrolysis combination method is used to synthesize FeNC catalysts with different concentrations of 2-methylimidazole, and the relationships between 2-methylimidazole concentration and productivity, phase, morphology and ORR performance are analysed to optimize the synthesis conditions. The change in ORR activity can be explained by the trend in productivity and surface state. The optimized product has a high half-wave potential, high durability, strong ability to resist methanol poisoning and good ORR catalytic kinetic performance. The addition of Cl À and SO 3 2À in solution do not decrease the half-wave potential, but instead increase it, although the mechanism for this effect still needs further investigation. Instead of the typical SCN À ion used to inhibit Fe(III), Fe(CN) 6 4À and Fe(CN) 6 3À are used to block Fe(III) and Fe(II) sites, and the degree of half-wave potential attenuation conrms that Fe(II) is the real active site.  3 were dissolved in methanol. The solution was transferred into a Teon-lined stainless-steel autoclave for the solvothermal process. The product was washed and dried aer separation by centrifugation. The obtained powder was carbonized at 900 C for 3 h in argon. The specic synthesis processes are shown in ESI † and the dosage of synthesis are shown in Table S1. † The molar quantities of 2-methylimidazole were varied to produce different samples at 12 mmol, 16 mmol, 24 mmol, 32 mmol and 36 mmol. The as-prepared ZIFs are named FeZIF-X (where X is the number of mmols of 2-methylimidazole). The carbonized samples are named Fe-NC-X (where X is the number of mmols of 2-methylimidazole).
2.2.2 Synthesis of NC-24. The synthesis of NC-24 is the same as that of FeNC-24, with the absence of Fe(acac) 3 .

Characterization methods
The phase identication of the powder samples was performed with X-ray diffraction (XRD, Smartlab 3) at a scan rate of 10 degrees ( ) per min from 5 to 80 and confocal laser microscopy Raman spectroscopy (Raman, Renishaw) from 900 cm À1 to 2000 cm À1 . The morphology was investigated by scanning electron microscopy (SEM, JSM-7900) and transmission electron microscopy (TEM, JEM-2100F). Valence states were analysed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) with an Al Ka radiation source (note: the XPS results aer the CA test of FeNC-24 were obtained without the addition of binder. The elemental composition was analysed by an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima8300)).

Electrochemical measurements
The electrochemical measurements were obtained in a threeelectrode system, with the working electrode, a cylindrical glassy carbon electrode with a diameter of 5 mm, functioning as a rotating disk electrode (RDE, Pine AFMSRCE); the counter electrode, a carbon rod; the reference electrode, a Hg/HgO electrode in 0.1 M KOH and an electrolyte of 0.1 M KOH. The following experiments included cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA) and Electrochemical Impedance Spectroscopy (EIS).
Generally, a 6 mg Fe-NC-X powder sample (2 mg for commercial Pt/C) was prepared in a centrifuge tube. Then, 1 mL ethanol and 0.5 mL 0.2% Naon ethanol solution were added to the powder with ultrasonic dispersion for 10 min to disperse them into a slurry. Then, 20 mL of slurry was added dropwise onto the glassy carbon electrode with a microsyringe and dried. The theoretical mass of FeNC catalysts on the electrode is 8.00 Â 10 À4 mg (2.67 Â 10 À4 mg for commercial Pt/C).
In the chronoamperometry (CA) test, a glassy carbon piece of 10 Â 10 mm was used as the working electrode. The formulation of the electrode material slurry was dropwise added onto the electrode at 200 mL for each side.  Fig. 1(b). Except mixed phase of Fe and Fe 2 O 3 for FeNC-12, the other samples show no crystal phase structure. It demonstrates that the crystallinity of these samples are limited during the heat treatment at 900 C for 3 h. While for FeNC-12, the difference might be attribute to the extremely low productivity (Fig. S1 †) of its FeZIF, resulting in the higher molar ratio of Fe(acac) 3 in the composite, which is favorable for the sintering of Fe species during the high-temperature process. Fig. 1(c) shows the Raman spectra of the typical three named as FeNC-16, 24, 32. The broad peaks around 1340 cm À1 and 1580 cm À1 attribute to the unordered carbon and ordered graphitic carbon. FeNC-24 has the highest peak ratio of I D /I G ¼ 1.12, demonstrating the higher defects in carbon structure with the doping of N, Zn and Fe atoms, which is benecial to its promotion in activity. 32,33 The SEM images of FeNC-24 are shown in Fig. 2(a). It is observed that the achieved materials are in particles, uniformly distributed in this area. Fig. S2 † shows SEM images and histogram of particle size distribution of typical FeNCs. The result of particle size distribution statistics show that the average sizes of such samples are less than 1 mm. Fig. S3 † are SEM images of typical FeZIFs. The diameter of FeZIF particles are also uniformed, which demonstrates that FeNCs remain the dispersity of FeZIFs even aer pyrolysis. As shown in Fig. S4(a), † the dispersity of FeZIF-12 is worse than typical FeZIFs. Pyrolyzed FeNC-12 shows a different morphology from other samples with the appearance of silk-liked structures around the morphology of ZIF-liked carbon (Fig. S4(b-d) †). The silk-liked structure corresponding to the carbon nano tubes (CNTs) which was formed with the catalyzing of sintered iron particles during the heat treatment. 34,35 This result is consistent with XRD results.

Phase and morphology analysis
The summary of average sizes of FeZIFs and FeNCs are shown in Fig. 2(b). The average size of obtained FeZIF distribution from 1.18 mm to 0.67 mm while that counterpart of FeNC is from 0.63 mm to 0.39 mm. It can be observed that the average sizes of FeZIFs and FeNCs decrease with the increase of 2methylimidazole dosage. It also can be discovered that the sizes of FeZIFs are larger than that counterpart of FeNCs, which  might be caused by the consumption in volume and mass during the pyrolysis of ZIFs. Fig. 2(c) is the TEM image of FeNC-24. The shape of these particles clearly indicate that carbonized samples remain the dodecahedral structure of ZIF-8. The result in Fig. 2(d and e) show the amorphous structure of FeNC-24 from the bulk to the surface. And the rough structure on the surface might appears during the evaporation of zinc in the ZIFs. Fig. S5 † shows the image of FeNC-24 in high-angle annular dark-eld scanning transmission electron microscopy (HAADF-STEM). No lightspots are observed, indicating metal particles does not existing in this sample, which manifests metal is not seriously sintered and formed in amorphous structure. Fig. 2(f)  The XPS results are demonstrated in Fig. 3. For Fe species, the binding energy of Fe 2+ 2p 3/2 , Fe 3+ 2p 3/2 , Fe 2+ 2p 1/2 , Fe 3+ 2p 1/2 respectively located at $711.5 eV, 714.5 eV, 724.5 eV, 727.5 eV, respectively ( Fig. 3(a-c)). It can be observed that the valence state of Fe are in +2 and +3 in such samples, and the proportion of +2 is higher than +3, indicating Fe prefer +2 form aer the process of heat treatment. For N species, the form of N in samples are in pyridine N, M (Fe or Zn)-N, pyrrole N, graphitic N, oxide N, the binding energy respectively assigned to $398.5 eV, 399.4 eV, 400.2 eV, 401.1 eV, 402.1 eV, respectively ( Fig. 3(d-f)). The appearance of M-N indicates the coordination of metal and N. And in C 1 s pattern (Fig. S6 †), C-N bond is observed, indicating the structure of M(Fe or Zn)-N-C is formed. The graphitic N have the ability in control the geometric and electronic structure of carbon framework 36 and improve the limiting current density of ORR catalysts. 33 Table  S2 † provide the atomic ratio of Fe measured by XPS and ICP-OES. FeNC-24 shows higher surface Fe atomic ratio than that of bulk. On the contrary, the trend of FeNC-16 and 32 are distinct. Generally, XPS can measure the metal distribution on surface while ICP-OES measure that counterpart of bulk. The data in this table cannot give consistent result among the three samples, which might be caused by the inhomogeneity of MOF samples aer pyrolysis at high temperature.

Electrochemical properties and analysis
The electrochemical properties of FeNC-X, NC-24, Pt/C samples are tested in 0.1 mol L À1 KOH solution. For CV and LSV tests, argon is saturated in electrolyte solution to ensure sample activated in the non-oxygen solution. Then, ORR activity is measured in oxygen saturated electrolyte. For stability tests, oxygen is saturated in electrolyte solution and kept plugging in electrolyte.
The LSV curves of FeNC-16, 24, 32, NC-24 and Pt/C are shown in Fig. 4(a). The half-wave potential of FeNC-24 is 0.852 V, which is 9 mV lower than that of commercial Pt/C(0.861 V) and is the best among the three typical FeNCs. NC-24 has much lower halfwave-potentials than FeNCs, demonstrating Fe might play the key roles in ORR electrocatalysis.
As shown in Fig. S7, † the FeNC-24 has the best ORR activity among all FeNCs. The half-wave potential of FeNCs rst increased and then decreased with the increase of 2-methylimidazole dosage. Combined with production and morphology, it might be explained that: when the dosage of 2-methylimidazole is to a low level, the larger particle size of Fe will be obtained during the heat treatment, which is a negative factor for ORR. On the other hand, the much higher dosage of 2methylimidazole will induce the extremely high production of ZIF, leading to the decrease of the density of Fe active sites, which is also unfavorable for ORR. The medium dosage of 2methylimidazole will lead to the reasonable particle sizes and production, which is benecial to the ORR catalyst. The ECSA tting of such samples is shown in Fig. 4(b). The original curves are exhibited in Fig. S8. † The value of ECSA is calculated in Table S3. † It is demonstrated that FeNC-24 has the highest ECSA among these materials, which might provide more redox sites for ORR catalysis.
The electron transfer number and yield of H 2 O 2 are measured in RRDE test (Fig. 4(c)). The electron transfer number is close to 4, demonstrating ORR is basically a four-electron process. The productivity of H 2 O 2 is blow to 10% even in the apex of the curve for both FeNC-24 and Pt/C. The result of K-L equation tting also support above conclusion (Fig. 4(d) and S9 †). The calculated average electron transfer number is 3.91, which is in accordance with RRDE curve results.
The CA data is shown in Fig. 4(e). The current density decrease with increasing of time. At 10000 s, 30000 s and 50000 s, the retention rates of current are respectively 96.3%, 94.0% and 89.5% for FeNC-24. And that counterpart of Pt/C are 88.0%, 85.1% and 82.0% (The original curves are shown in Fig. S10. †) For each times, FeNC-24 has higher retention rate of current than that of Pt/C, demonstrating FeNC-24 is much better durability than that of commercial Pt/C. The TEM and EDS-mapping image of FeNC-24 aer CA test is shown in Fig. S11. † It is clear that catalysts still remain the similar morphology and components as original aer stability test. These results indicate the excellent durability of FeNC-24, and the change of carbon framework is negligible during the CA test. Fig. 4(f) exhibits the LSV curve of FeNC-24 with the addition of different concentration of methanol. The half-wave potential and limiting diffusion current density decrease with increasing of methanol concentration. In the electrolyte with 3 M methanol, the half-wave potential le shied$20 mV. In contrast with commercial Pt/C (Fig. S12 †), the decrease of FeNC-24 is slight. Therefore it is suggested that FeNC-24 has satisfactory tolerance toward methanol. A previous research 37 demonstrate that FeNC sites has no activity in catalyzing the decomposition of small molecule alcohols, that is distinct with some noble metals. The poisoning of FeNC material is caused by the lower conductivity of solution with the addition of methanol and the blockage of pathway in ORR. The pores of FeNC-24 are not blocked by the methanol, presenting the less degree of activity.
The Tafel plot of such samples are presented in Fig. S13. † Except NC-24, FeNC samples in this graph have lower slope in the selected scope than commercial Pt/C, proving that FeNCs electrode possess better kinetic of ORR. And the EIS plots and equivalent circuit of FeNCs are shown in Fig. S14. † The solution resistances(R s ) are same while the polarization resistances (R p ) of FeNC-16, 24, 32 are respectively 294 U, 201 U, 160 U. The R p decrease with the increase of the dosage of 2-methylimidazole. It is attribute to the increase of productivity, improving the amount of carbon in FeNC, which promoting the conductivity of FeNC.
Some typical ions are added into the electrolyte to simulate the electrochemical environment in PEMFC. As shown in Fig. S15(a), † with the added of 1 mM SO 3 2À , a very-slight positive shi (4 mV) is observed in half-wave potential of FeNC-24, indicating that SO 3 2À has no obvious toxic effect to this material. This very-slight improvement in half-wave potential is caused by the increase in the concentration of conductive ions.
There is a larger increase in activity with the addition of Cl À (Fig. S15(b) †). It possible relate to the pitting mechanism in a previous research in steel corrosion. 38 The effect in ORR activity of SO 3 2À and Cl À is widely divergent with Wang and coworkers' experiments, 30 demonstrating that the real impact and mechanism of poisoning toxic ions in different systems of FeNC material are highly important. From the electrochemical measurements, Fe components are usually considered as important active sites for ORR activity. For XPS results, Fe could be divided into two valence of +2 and +3. Hence, the addition of complex ions are used to investigated which valence is the real active site for ORR. The LSV curves are shown in Fig. S16. † SCN À is added to block Fe(III), which shows that it has no affect in half-wave potential. It is proposed that Fe(II) might be the real source of activity rather than Fe(III oxides into Fe(CN) 6 3À , which blocks Fe(II), caused the small negative shi in half-wave potential. Some researches 11,[39][40][41] demonstrate that KSCN has powerful ability in reducing the activity of Fe active sites. To sum up, for FeNC materials in different electron structure, skeleton structure and in even micro area environment, the real active site should to be analysed in specic circumstance. The change in half-wave potential is summarized in Fig. 5(a). The XPS result of FeNC-24 aer CA test is shown in Fig. 5(b). The valence states of Fe are still in +2 and +3 state, and +2 state is also the main valence state in the sample, which indicating the change in surface valence state of Fe is little aer the CA test. Thus, the result of ion experiments is still applicable to sample aer durability test.

Conclusions
A series of FeNC materials were synthesized by solvothermalpyrolysis method with different dosage of 2-methylimidazole in this work.
No obvious crystalline structure was observed in these materials. And the N and metal atoms doped in carbon skeleton to form lots of defects and FeNC active sites. In addition, the particle sizes of FeNCs were uniformed and could be controlled by the dosage of 2-methylimidazole to some extent. Electrochemical measurements demonstrated that the ORR activity of samples increased rst and then decreased with the increasing of 2-methylimidazole.
FeNC-24 had the highest half-wave potential of 0.852 V and ECSA among FeNCs. The half wave potential of FeNC-24 was only 9 mV lower than that of Pt/C. On the other hand, it also had favourable stability of ORR. The CA tests in the 50 000 s, retention rate of current density is 89.5%, and the morphology had no distinct difference aer CA test. The tolerance of methanol test indicated the decrease in half-wave potential of FeNC-24 was only 20 mV in 3 M methanol, which was much lower than that of Pt/C. And the Tafel slope was 122.6 mV dec À1 , lower than Pt/C. Meanwhile, the ORR activity cannot be decreased by Cl À , SO 3 2À pollution ions.
The block test of activity sites indicated that Fe(II) was an important active site, while the valence state had no change aer the ORR test.
In this research, the synthesis conditions was optimized, and the real iron active species were revealed by the experiment of complex ions, which provided a new thinking and methods in the research of FeNC materials.

Conflicts of interest
There are no conicts to declare.


Ag2O versus Cu2O in the Catalytic Isomerization of Coordinated Diaminocarbenes to Formamidines: A Theoretical Study

DFT theoretical calculations for the Ag2O-induced isomerization process of diaminocarbenes to formamidines, coordinated to Mn(I), have been carried out. The reaction mechanism found involves metalation of an N-H residue of the carbene ligand by the catalyst Ag2O and the formation of a key transition state showing a μ-η2:η2 coordination of the formamidinyl ligand between manganese and silver, which allows a translocation process of Mn(I) and silver(I) ions between the carbene carbon atom and the nitrogen atom, before the formation of the formamidine ligand is completed. Calculations carried out using Cu2O as a catalyst instead of Ag2O show a similar reaction mechanism that is thermodynamically possible, but highly unfavorable kinetically and very unlikely to be observed, which fully agrees with experimental results.

Introduction
For many years silver(I) oxide has been widely used as a catalyst in different organic transformations, leading to the synthesis of a variety of organic molecules such as pyrroles, imidazolines, β-keto esthers, or benzothiophenes [1][2][3][4][5], as well as in many stoichiometric reactions, such as the synthesis of N-heterocyclic carbenes (NHC) based on Ag 2 O [6][7][8]. In spite of that, mechanistic studies of reactions involving Ag 2 O are really scarce, with some notable exceptions being those related to the ethylene epoxidation reaction [9][10][11] and the generation of NHC-Ag(I) complexes used in further transmetalation processes [12]. We have recently described an efficient experimental procedure to transform isocyanides into formamidines by reaction with primary amines [13,14], involving a key step tautomerization of diaminocarbenes coordinated to Mn(I), mediated by Ag 2 O, into formamidines. This behavior is apparently exclusive to Ag 2 O, as isostructural Cu 2 O does not show any activity in the same reaction, nor do other metallic oxides of variable nature.
Free protic diaminocarbenes are unknown, as their formamidine tautomers are much more stable [15]. On the contrary, protic diaminocarbene complexes are very stable [16,17] and, as shown in the present case, can be transformed into the corresponding formamidine complexes by addition of a catalyst (Scheme 1). Interestingly, we have similarly demonstrated that coordinated NHCs isomerize to imidazoles under the same conditions [13,14]. In order to shed light on the mechanism of this intriguing reaction and to make a contribution to the rather scarcely developed field of mechanistic studies of organic and organometallic transformations assisted by Ag 2 O, we have now carried out DFT theoretical calculations supporting the finding that the exclusive behavior of Ag 2 O in the catalytic isomerization of diaminocarbenes to formamidines is essentially based on an DFT theoretical calculations supporting the finding that the excl in the catalytic isomerization of diaminocarbenes to formamidine an ideal combination of the basic character of the oxide ion and of the Ag + ion. Both features promote, respectively, proton tra carbon and carbene carbon atom transfer from Mn(I) to Ag(I) formation of formamidines.

Experimental Section
The detailed experimental procedure for the synthesis and characterization (FTIR and NMR) of 1, 2, and 3 complexes has be [12,13]. Selected crystallographic data and structure refinement are given here for the first time in Table S1 of the Supplementary shows the experimental molecular structure together with the ato and selected bond lengths and angles. Additional details of the determination of 3⋅ − 4 ClO are given in the CIF file (CCDC depo The rather disordered floppy ligands could be finally modeled constrained/restrained algorithms. Further details about the ref may be found elsewhere [17,18]. Scheme 1. Transformation of free and coordinated diaminocarbenes into formamidines.

Experimental Section
The detailed experimental procedure for the synthesis and spectroscopic structural characterization (FTIR and NMR) of 1, 2, and 3 complexes has been described previously [13,14]. Selected crystallographic data and structure refinement parameters for 3·ClO − 4 are given here for the first time in Table S1 of the Supplementary Material, while Figure 1 shows the experimental molecular structure together with the atomic numbering scheme and selected bond lengths and angles. Additional details of the X-ray crystal structure determination of 3·ClO − 4 are given in the CIF file (CCDC deposition number 2044950). The rather disordered floppy ligands could be finally modeled by means of convenient constrained/restrained algorithms. Further details about the refinement protocols used may be found elsewhere [18,19]. DFT theoretical calculations supporting the finding that the exclusive behavior of Ag2O in the catalytic isomerization of diaminocarbenes to formamidines is essentially based on an ideal combination of the basic character of the oxide ion and the Lewis acid character of the Ag + ion. Both features promote, respectively, proton transfer from nitrogen to carbon and carbene carbon atom transfer from Mn(I) to Ag(I) preceding the ultimate formation of formamidines.

Scheme 1.
Transformation of free and coordinated diaminocarbenes into formamidines.

Experimental Section
The detailed experimental procedure for the synthesis and spectroscopic structural characterization (FTIR and NMR) of 1, 2, and 3 complexes has been described previously [12,13]. Selected crystallographic data and structure refinement parameters for 3⋅ − 4 ClO are given here for the first time in Table S1 of the Supplementary Material, while Figure 1 shows the experimental molecular structure together with the atomic numbering scheme and selected bond lengths and angles. Additional details of the X-ray crystal structure determination of 3⋅ − 4 ClO are given in the CIF file (CCDC deposition number 2044950).
The rather disordered floppy ligands could be finally modeled by means of convenient constrained/restrained algorithms. Further details about the refinement protocols used may be found elsewhere [17,18].

Computational Details
Density functional theory (DFT) computations have been performed with the GAUS-SIAN09 and ADF2012 program packages [20,21], starting from our experimental geometry for cation 3, here included, and from the one of a derivative of cation 1, obtained from X-ray single-crystal diffraction data and previously published [22], using the B3P86-D3(BJ) Becke's three-parameter exchange functional with the non-local Perdew correlation functional, as well as the M06-D3 hybrid functional of Truhlar and Zhao [23][24][25][26], which are both well known for obtaining reliable results with transition-metal organometallic compounds [27][28][29][30]. The standard all-electron 6-31+G(d) and 6-311++G(2df,2pd) basis sets were employed for C, H, N, O, Cu, and Mn atoms at different steps of the procedure. As it is usual practice when large compounds involving transition-metal atoms is concerned, for geometry optimizations, the smaller basis set was used, while, for the final single-point electronic structure calculations, the larger basis set was utilized, since it would be impossible for the optimization process to either converge or finish at a reasonable time if the larger basis set were used for the geometry optimization processes [31][32][33][34]. The LanL2DZ effective core potential and the large all-electron AQZP basis set were used for the Ag atom [35,36] (again, the former in the geometry optimization and the latter in the single-point electronic structure calculations).
The usual Berny algorithm with the quasi-Newton rational function optimization (RFO) method and the Pulay s Direct Inversion procedure for SCF cycles were used for most of the optimizations made, although in some cases (the saddle points of reaction paths), the slower but more reliable quadratically convergent SCF procedure and the Newton-Raphson optimization algorithm were utilized instead [37,38]. Vibrational analysis was performed at every stationary point found, within the B3P86-D3(BJ)/6-31+G(d)/LanL2DZ(Ag) level of theory, characterizing intermediate compounds I1, I2, I3, I4, and I5 as minima (no imaginary frequencies) in mechanism M1, intermediate compounds I6, I7, I8, I9, and I10 as minima in mechanism M2, and intermediate compounds I11 and I12 as minima in (partial) mechanism M3. The same level of theory was used to simulate modeling reactants 1 + Ag 2 O in mechanisms M1 and M3, reactants 1 + Cu 2 O in mechanism M2, products 2 + Ag 2 O, 2 + Ag 2 O and 2" + Ag 2 O in mechanism M1, products 2 + Cu 2 O, 2 + Cu 2 O and 2" + Cu 2 O in mechanism M2, and products 3 + Ag 2 O in mechanism M3. Subsequently, using these minima as starting points, transition states (each one of them having one imaginary frequency) were searched between minima in the three mechanisms within the B3P86/LanL2DZ level of theory. Additionally, application of the intrinsic reaction coordinate (IRC) method showed that each transition state found in reaction paths effectively connects with their corresponding minima [39,40]. All reactants, products, intermediates, and transition states have an electric charge of +1 e, except for Ag 2 O and Cu 2 O, and also have a singlet state as the ground state.
Every ground state wavefunction obtained for each compound was analyzed and found to be stable. Solvent effects were taken into account using the polarizable continuum model (PCM) with ε = 8.93 (dichloromethane) [41]. The anion (ClO − 4 ) contribution to the energy has not been taken into account, since the classical electrostatic interaction between anion and cations is assumed to be approximately constant along the three reactions in solution (this contribution may be estimated to be, using pure Coulomb classical approximation, −12.5 kcal mol −1 from the Cl − · · · · · · H + experimental distance in compound 3·ClO − 4 : 2.973 Å). The simulation of catalysts Ag 2 O and Cu 2 O started by theoretically optimizing the experimental geometry from their crystalline structures, following a procedure similar to the one made on previous theoretical studies [9][10][11][12]. The additional stabilizing effect of the lattice for the catalytic action has been estimated by the simulation of Ag 14 O 7 and Cu 14 O 7 (001) solid surfaces (Figure 2), using the model M06-D3/6-311++G(3df,3pd)/QZPDKH(Ag) [42,43], to be −124.5 kcal mol −1 and -156.8 kcal mol −1 , respectively.  Scaled zero-point energies (ZPE) and thermal components (translational, rota and vibrational) were added to the final electronic energies in order to obtain re enthalpies and Gibbs free energies at room temperature and normal pressure (scale used for ZPE: 0.9804) [43]. Gibbs energies were determined, as it is usual practi adding the translational, rotational, and vibrational components of the total energy ZPE and the entropy term), calculated at every stationary point found using the s basis set, to the electronic part of the total energy, which has been calculated usi larger basis set, as explained above. In order to check the reliability of the result different DFT functionals have been used, the B3P86-D3(BJ) functional and the M functional, giving results really close to each other, showing differences not highe 0.1 kcal mol −1 . The B3P86-D3(BJ)/6-311++G(2df,2pd)/AQZP(Ag) is the final adopted for further discussion in the main text of this paper, while in the Supplem Material, results using the M06-D3 functional are included (see Tables S9-S11).

Ag2O mechanism M1
The overall transformation of diaminocarbene complex 1 into formamidine d tive 2 mediated by Ag2O is summarized in Scheme 2. The graphic representation M1 mechanism may be seen in Figure 3, while theoretically optimized geometries molecules involved are depicted in Figure 4, and selected bond distances and angl collected in Tables 1 and 2, respectively (See Figure 1 for the atomic numbering sc used in all tables and the forthcoming discussion). Bearing in mind that the reactio follows the sequence (1+Ag2O) - Figure 4 and Table 1 are the following. The tautom tion process from cation 1 to cation 2 may be divided into two main sequences. Th sequence goes from cation complex 1, where the Mn atom of the [Mn] ⊕ group ([M [Mn(CO)3(bipy)] ⊕ ) is bonded to the C1 atom of the acyclic diaminocarbene (ADC) l onto intermediate I4, where the Mn atom is bonded to the N1 atom of the same lig its deprotonated form and the Ag atom is bonded to the carbene carbon atom C1 the reaction path passing through another three intermediates and three transition (see Table 1). In this sequence there is first a relatively easy and fast proton transfe the reactant (1) to the catalyst (Ag2O) (see Figure   Scaled zero-point energies (ZPE) and thermal components (translational, rotational, and vibrational) were added to the final electronic energies in order to obtain reliable enthalpies and Gibbs free energies at room temperature and normal pressure (scale factor used for ZPE: 0.9804) [44]. Gibbs energies were determined, as it is usual practice, by adding the translational, rotational, and vibrational components of the total energy (plus ZPE and the entropy term), calculated at every stationary point found using the smaller basis set, to the electronic part of the total energy, which has been calculated using the larger basis set, as explained above. In order to check the reliability of the results, two different DFT functionals have been used, the B3P86-D3(BJ) functional and the M06-D3 functional, giving results really close to each other, showing differences not higher than 0.1 kcal mol −1 . The B3P86-D3(BJ)/6-311++G(2df,2pd)/AQZP(Ag) is the final model adopted for further discussion in the main text of this paper, while in the Supplementary Material, results using the M06-D3 functional are included (see Tables S9-S11).

Ag 2 O Mechanism M1
The overall transformation of diaminocarbene complex 1 into formamidine derivative 2 mediated by Ag 2 O is summarized in Scheme 2. The graphic representation of this M1 mechanism may be seen in Figure 3, while theoretically optimized geometries for all molecules involved are depicted in Figure 4, and selected bond distances and angles are collected in Tables 1 and 2 Table 1 are the following. The tautomerization process from cation 1 to cation 2 may be divided into two main sequences. The first sequence goes from cation complex 1, where the Mn atom of the where the Mn atom is bonded to the N1 atom of the same ligand in its deprotonated form and the Ag atom is bonded to the carbene carbon atom C1, with the reaction path passing through another three intermediates and three transition states (see Table 1). In this sequence there is first a relatively easy and fast proton transfer from the reactant (1) to the catalyst (Ag 2 O) (see Figure 3, where low energetic barriers are shown to reach intermediate I3) through a significant increase of the N1-H1 bond distance, from the typical covalent bond value of 1.015 Å in 1 to the hydrogen-bond value of 2.192 Å in I2. At the same time, H1-O1 distance decreases from 1.582 Å in I1 to 0.983 Å in I2. The proton transfer is totally completed in intermediate I3 (H1-O1 0.967 Å) with parallel coordination of N1 to the silver atom (N1-Ag1 2.134 Å). However, the critical step in the first sequence of the mechanism is given in its second part by the formation of the transition state TS(3-4), for which both Mn1 and Ag1 atoms are attached to both N1 and C1 atoms, a structure where Mn1-C1 and Ag1-N1 bonds are simultaneously breaking, at the same time as Mn1-N1 and Ag1-C1 bonds are forming. On the other hand, it is noteworthy that both C1-N1 and C1-N2 bond distances, as well as N1-C1-N2 bond angle, do not change significantly along this whole first sequence of the mechanism, not even in the transition states (see Tables S3 and S4 of the Supplementary Material). ls 2022, 15, x FOR PEER REVIEW worthy that both C1-N1 and C1-N2 bond distances, as well not change significantly along this whole first sequence of the transition states (see Tables S3 and S4 of  The second sequence of the M1 mechanism starts from rotation of the protonated catalyst in its own plane until it r to create the hydrogen bond O1-H1…C1 in the transition st gradual transformation of this interaction into a C1-H1 p forthcoming separation of product and catalyst. Along th Materials 2022, 15, x FOR PEER REVIEW 6 of 18   Figure 1 for the atomic numbering scheme. The second sequence of the M1 mechanism starts from the intermediate I4 with a rotation of the protonated catalyst in its own plane until it reaches an orientation useful to create the hydrogen bond O1-H1 . . . C1 in the transition state TS(4-5), followed by the gradual transformation of this interaction into a C1-H1 pure covalent bond, and the forthcoming separation of product and catalyst. Along this process, the C1-H1 bond distance decreases from 1.856 Å in TS(4-5) to 1.087 Å in product 2 (see Table 1), while H1-O1 bond distance consequently increases from 0.968 Å in I4 to 1.807 Å in TS(5-Products).
Rather interestingly, two alternative products (2 and 2") are in principle possible (see Figure 4), although they are less stable than product 2 ( Figure 3). As may be seen in Table 1, bond distances and angles are quite similar for the three possible products. Nevertheless, the main structural difference between these products is the relative orientation of the NHMe group with respect to the metallic fragment, which is located anti in 2 and syn in 2 and 2", thus minimizing the steric hindrance, and so the repulsion energy, in the more stable product 2. For their part, compounds 2 and 2" differ in the relative orientation of C1-H1 and N2-H2 groups in the formamidine ligand, which are trans in 2 and cis in 2" (H1-C1-N2-H2 torsion angle is −171.1 • in 2 and 2.6 • in 2"; see Figure 4), which in turn leads to methyl and phenyl groups in the same ligand to also have different relative orientations in both products (syn in 2 and anti in 2"). Therefore, the methyl group is farther from the bipy ligand of the [Mn] ⊕ group in 2 than in 2", thus minimizing the repulsion energy between methyl and bipy ligands in the former.  Rather interestingly, two alternative products (2' and 2'') are in principle possible (see Figure 4), although they are less stable than product 2 ( Figure 3). As may be seen in Table 1, bond distances and angles are quite similar for the three possible products Nevertheless, the main structural difference between these products is the relative ori entation of the NHMe group with respect to the metallic fragment, which is located ant in 2 and syn in 2' and 2'', thus minimizing the steric hindrance, and so the repulsion en ergy, in the more stable product 2. For their part, compounds 2' and 2'' differ in the rela   From the foregoing discussion, it is not surprising that the thermodynamically preferred path in mechanism M1 leads to product 2, with the most negative value of ∆G 0 298 (1 → 2) = −12.4 kcal mol −1 found for this exergonic process, instead to product 2 , with a value of ∆G 0 298 (1 → 2 ) = −7.5 kcal mol −1 , or to product 2" with a value of ∆G 0 298 (1 → 2 ) = +1.8 kcal mol −1 found for this unfavorable path (see Figure 3), a result in complete agreement with experimental data, where only a product is observed compatible with the current mechanism; i.e., product 2 (but see below, mechanism M3, for completely different products, starting from the same reactants) [13,14]. Calculated standard Gibbs free energies at room temperature for all compounds along the favorable path of the M1 mechanism are collected in Figure 3 and in Table S9, showing ∆G 0 298 (Intermediate → TS) values of the same order of magnitude than those found in similar reactions involving the bulky [Mn(CO) 3 (bipy)] ⊕ group, although slightly higher [45,46]. Since the estimated lattice stabilization energy for the catalyst is of the order of one hundred kcal mol −1 , as already mentioned in the Computational Details section, it is hardly surprising that transition barriers are of the order of several tenths of kcal mol −1 . As already mentioned in the Introduction and Computational Details sections, the experimentally observed process studied in previous papers is in fact heterogeneously catalyzed by a solid Ag 2 O surface [13,14], while the simulation performed currently is made in a simplified homogeneous media instead; therefore, the energy barriers are consequently much higher here than in the real physical process, which is computationally unaffordable using ab initio methods. Very likely, dispersion and other non-covalent long-range interactions between the solid surfaces and the reactants (and possibly with the solvent and the perchlorate anion as well), which could not be properly modeled by the state-of-the-art ab initio methods, reduce the high barriers in the experimentally observed process, but they scarcely affect the proposed mechanism and, more importantly, the energy differences calculated between reactants and products. Therefore, this is clearly a kinetically-controlled process that could not be experimentally observed in a reasonable time at room temperature in the absence of the surface provided by the solid catalyst. Two rate-determining steps with similar energy barriers are found in this mechanism: the fourth step (I3 → I4), which proceeds through transition state TS(3-4) with the simultaneous breaking of Mn1-C1 and Ag1-N1 bonds, and the fifth step (I4 → I5), which proceeds through transition state TS(4-5) with the additional breaking of the Ag1-C1 bond. On the other hand, the product-determining step is the sixth one (I5 → 2+Ag 2 O) since all previous steps are common to the three possible outcomes 2, 2 , and 2". In addition, due to the steric restrictions involving the rotation of Ag 2 O between intermediates I2 and I3, the reaction is very likely to proceed at the edges and defects of the catalyst surface.

Cu 2 O Mechanism M2
The overall transformation of diaminocarbene complex 1 into formamidine derivative 2 mediated by Cu 2 O is summarized in Scheme 3. The graphic representation of the M2 mechanism may be seen in Figure 5, while theoretically optimized geometries for all molecules involved are depicted in Figure 6, and selected bond distances and angles are collected in Tables 3 and 4 Table 3 are the following. The tautomerization process from cation 1 to cation 2 using Cu 2 O as catalyst instead of Ag 2 O follows qualitatively, at least from a geometrical point of view, a mechanism closely related to the previously detailed M1 mechanism, taking place also here through five intermediate complexes, I6 to I10, which may be divided again into two main sequences. The first sequence goes from cation complex 1, where the Mn atom of the [Mn] ⊕ group is bonded to the C1 atom of the ADC ligand, onto intermediate I9, where the Mn atom is bonded to the N1 atom of the same ligand, with the reaction path passing through another three intermediates and three transition states (see Table 3). Within this sequence there is first a proton transfer from the reactant (1) to the catalyst (Cu 2 O) through a significant increase of the N1-H1 bond distance, from the typical covalent bond value of 1.015 Å in 1 to the hydrogen-bond value of 2.067 Å in I7. At the same time, H1-O1 distance decreases from 1.678 Å in I6 to 0.966 Å in I9. In the first part of this sequence, ending in intermediate I7, H1 atom is completely transferred from the N1 atom of the ADC ligand to the O1 atom of the Cu 2 O catalyst (see Figure 6). However, here again the critical step in the first sequence of the mechanism is given in its second part by the formation of the transition state TS(8-9), for which both Mn1 and Cu1 atoms are attached to both N1 and C1 atoms, a structure where Mn1-C1 and Cu1-N1 bonds are simultaneously breaking, at the same time as Mn1-N1 and Cu1-C1 bonds are forming (see Figure 5 for a view of the energy barrier involved in this step). On the other hand, it is noteworthy that both C1-N1 and C1-N2 bond distances, as well as N1-C1-N2 bond angle, not only do not change significantly along this first sequence of mechanism M2, but also are nearly equal to the ones of mechanism M1 (compare their values in Tables S5 and S6 with those in Tables S3  and S4    Calculated standard Gibbs free energies at room temperature for all compounds along the thermodynamically favorable path of the M2 mechanism are collected in Figure   5 and in Table S10 Figures 3 and 5). Considering the two rate-determining steps (fourth and fifth steps), the quotient between rate constants in both mechanisms is [46]: The second sequence of the M2 mechanism starts, similarly to mechanism M1, from the intermediate I9 with a rotation of the protonated catalyst in its own plane until it reaches an orientation useful to create the hydrogen bond O1-H1· · · C1 in the transition state TS(9-10), followed by the gradual transformation of this interaction into a C1-H1 pure covalent bond, and the forthcoming separation of product and catalyst. Along this process, the C1-H1 distance decreases from 2.297 Å in TS(9-10) to 1.092 Å in product 2 (see Table 3), while H1-O1 distance consequently increases from 0.966 Å in I9 to 2.096 Å in TS(10-Products). Note that exactly the same main products (2, 2 and 2") are obtained through both mechanisms M2 and M1. Similarly to mechanism M1, due to the steric restrictions, involving in the current case the rotation of Cu 2 O between intermediates I7 and I8, the reaction is very likely to proceed at the edges and defects of the catalyst surface.
Calculated standard Gibbs free energies at room temperature for all compounds along the thermodynamically favorable path of the M2 mechanism are collected in Figure 5 and in Table S10, showing ∆G 0 298 (Intermediate → TS) values clearly higher than the ones obtained in mechanism M1 (compare Figures 3 and 5). Considering the two rate-determining steps (fourth and fifth steps), the quotient between rate constants in both mechanisms is [47]: Therefore, the M2 mechanism, although thermodynamically possible, is highly unfavorable kinetically and very unlikely to be observed experimentally at room temperature, even in the presence of a catalyst's solid surface [13,14]. By comparing equivalent intermediates in both mechanisms for the fourth step, I3 and I8 (see Figures 4 and 6, respectively), it is clearly seen an attractive interaction that may be present in I8 between the Cu2 atom and the phenyl group of the ADC ligand (Cu2-C* distance is 2.503 Å, with C* the centroid of the phenyl group), which will reasonably be absent in I3 between the Ag2 atom and the phenyl group, much stabilizing then the Cu intermediate with respect to its transition state TS(8-9), and thus explaining the larger difference in rate constants for this step between both mechanisms. In fact, as may be clearly seen in Figure 7a,b, Cu2 s atomic orbitals (AOs) contribute appreciably to the HOMO molecular orbital in I8, while Ag2 s AOs do not contribute at all in I3. Moreover, the Boys-Foster localized MO depicted in Figure 7c shows rather neatly this interaction, which is present in the Cu intermediate but absent in the Ag one. An additional contribution for the stabilization of intermediate I8 respect to I3 may be the dissociation energy of the Cu(I)-N bond, which is higher than that of the Ag(I)-N bond for a variety of nitrogen ligands [48][49][50]. Therefore, the M2 mechanism, although thermodynamically possible, is highly unfavorable kinetically and very unlikely to be observed experimentally at room temperature, even in the presence of a catalyst's solid surface [12,13]. By comparing equivalent intermediates in both mechanisms for the fourth step, I3 and I8 (see Figures 4 and 6, re-

Ag2O mechanism M3
As mentioned above, another product is observed experimentally for the tautomerization of cation 1 catalyzed by Ag2O; namely, compound 3 (Scheme 1, Figure 1), which must be obtained through a different mechanism. Since the main difference between products 2 and 3 is just the proton which is transferred between nitrogen and carbon atoms (H1 and H2, respectively), as well as the nitrogen atom which is finally bonded to Mn1 (N1 and N2, respectively), it was assumed that the reaction path in mechanism M3 leading to product 3 should sensibly be very similar to the one leading to product 2 in mechanism M1. Therefore, it seemed unnecessary to develop again the whole mechanism, and then only the most significant features for mechanism M3 will be detailed here. Theoretically optimized geometries for the molecules involved in the partial mechanism M3 are depicted in Figure 8, and selected bond distances and angles are collected in Tables 5 and 6, respectively. Bearing in mind that the reaction path involved in the current mechanism is (1+Ag2O) -I11 -TS(11-12) -I12 -... -(3+Ag2O) -(3+Ag14O7), the most relevant features from Figure 8 and Table 5 are the following. A comparison between intermediate I11 (Figure 8) and intermediate I1 (Figure 4) shows that the first step in both mechanisms is basically the same, with the formation of the N2-H2…O1 hydrogen bond in mechanism M3 instead of the almost equivalent N1-H1…O1 hydrogen bond in mechanism M1. The stabilizing effect through a Ag1-phenyl attractive interaction in I1 (Ag1-C* distance is 3.108 Å, with C* the centroid of the phenyl group) is likely to be substituted in I11 by a similar Ag1-bipy stabilizing interaction (Ag1-C* distance is 3.068 Å, with C* the centroid of the closest pyridine group). Moreover, not only N1-H1 and H1-O1 bond distances in I1 (1.080 Å and 1.582 Å, respectively; see Table 1) are not far from their equivalents N2-H2 and H2-

Ag 2 O Mechanism M3
As mentioned above, another product is observed experimentally for the tautomerization of cation 1 catalyzed by Ag 2 O; namely, compound 3 (Scheme 1, Figure 1), which must be obtained through a different mechanism. Since the main difference between products 2 and 3 is just the proton which is transferred between nitrogen and carbon atoms (H1 and H2, respectively), as well as the nitrogen atom which is finally bonded to Mn1 (N1 and N2, respectively), it was assumed that the reaction path in mechanism M3 leading to product 3 should sensibly be very similar to the one leading to product 2 in mechanism M1. Therefore, it seemed unnecessary to develop again the whole mechanism, and then only the most significant features for mechanism M3 will be detailed here. Theoretically optimized geometries for the molecules involved in the partial mechanism M3 are depicted in Figure 8, and selected bond distances and angles are collected in Tables 5 and 6, respectively. Bearing in mind that the reaction path involved in the current mechanism is  Table 5 are the following. A comparison between intermediate I11 ( Figure 8) and intermediate I1 (Figure 4) shows that the first step in both mechanisms is basically the same, with the formation of the N2-H2 . . . O1 hydrogen bond in mechanism M3 instead of the almost equivalent N1-H1 . . . O1 hydrogen bond in mechanism M1. The stabilizing effect through a Ag1-phenyl attractive interaction in I1 (Ag1-C* distance is 3.108 Å, with C* the centroid of the phenyl group) is likely to be substituted in I11 by a similar Ag1-bipy stabilizing interaction (Ag1-C* distance is 3.068 Å, with C* the centroid of the closest pyridine group). Moreover, not only N1-H1 and H1-O1 bond distances in I1 (1.080 Å and 1.582 Å, respectively; see Table 1) are not far from their equivalents N2-H2 and H2-O1 bond distances in I11 (1.039 Å and 1.753 Å, respectively; see Table 5) but also N1-H1-O1 and N2-H2-O1 bond angles are quite similar (163.6 • and 156.2 • , respectively; see Tables S4 and S8 in Supplementary Material), not to mention the Ag1-O1-Ag2 bond angle in both intermediates (105.2 • in I1 and 104.9 • in I11), among many other similarities. Thus, intermediates I11 and I12, as well as the transition state TS (11)(12), are almost equivalent to I2, I3, and TS(2-3), respectively, in the sense that the bipyridine group in the latter plays the same role as the phenyl group in the former. From I11 to I12 there is just a rotation of approximately 90 • of the [Mn] ⊕ group about the C1-Mn1-(CO) ax axis, passing through the transition state TS (11-12). Consequently, through this step there is only a slight increase in N2-H2 bond distances (from 1.039 Å in I11 to 1.043 Å in I12) and a slight decrease in H2-O1 distances (from 1.753 Å in I11 to 1.702 Å in I12), coming with an increase in N2-H1-O1 angles (from 156.2 • in I11 to 163.1 • in I12) and a decrease in Ag1-O1-Ag2 bond angles (from 104.9 • in I11 to 100.2 • in I12).
156.2°, respectively; see Tables S4 and S8 in Supplementary Material), not to mention the Ag1-O1-Ag2 bond angle in both intermediates (105.2° in I1 and 104.9° in I11), among many other similarities. Thus, intermediates I11 and I12, as well as the transition state TS (11)(12), are almost equivalent to I2, I3, and TS(2-3), respectively, in the sense that the bipyridine group in the latter plays the same role as the phenyl group in the former. From I11 to I12 there is just a rotation of approximately 90° of the [Mn] ⊕ group about the C1-Mn1-(CO)ax axis, passing through the transition state TS (11)(12). Consequently, through this step there is only a slight increase in N2-H2 bond distances (from 1.039 Å in I11 to 1.043 Å in I12) and a slight decrease in H2-O1 distances (from 1.753 Å in I11 to 1.702 Å in I12), coming with an increase in N2-H1-O1 angles (from 156.2° in I11 to 163.1° in I12) and a decrease in Ag1-O1-Ag2 bond angles (from 104.9° in I11 to 100.2° in I12).
Calculated standard Gibbs free energies at room temperature for the above compounds along the path of the M3 mechanism are collected in Table S11, from which the value ( ) is obtained. Consequently, M3 is a thermodynamically possible mechanism, which may be also kinetically favorable and very likely to be observed experimentally. Considering all the results and data explained above, the experimentally observed fact that both products 2 and 3 are obtained within very similar amounts [12,13] is fully justified by our current calculations.

(exp)
----123.0 (2) 119.6 (2) --Calculated standard Gibbs free energies at room temperature for the above compounds along the path of the M3 mechanism are collected in Table S11, from which the value ∆G 0 298 (1 → 3) = −11.9 kcal mol −1 is obtained. Consequently, M3 is a thermodynamically possible mechanism, which may be also kinetically favorable and very likely to be observed experimentally. Considering all the results and data explained above, the experimentally observed fact that both products 2 and 3 are obtained within very similar amounts [13,14] is fully justified by our current calculations.

Conclusions
We have carried out DFT theoretical calculations that support a plausible mechanism for the experimentally observed Ag 2 O-catalyzed isomerization of diaminocarbenes to formamidines, coordinated to Mn(I). This mechanism involves metalation of an N-H residue of the carbene ligand by the catalyst Ag 2 O followed by a translocation process of the Mn(I) and Ag(I) ions between the carbene carbon atom and the nitrogen atom, which proceeds through a key transition state showing a µ-η 2 :η 2 coordination of the formamidinyl bridging ligand between manganese and silver, and leading to two different products 2 and 3, as it has been experimentally observed. Additional products 2 and 2" are also theoretically possible, although less stable thermodynamically. Calculations using Cu 2 O as a catalyst instead of Ag 2 O show a similar reaction mechanism thermodynamically possible, leading to the same 2 and 2 products, but highly unfavorable kinetically to be observed, which fully agree with our experimental results.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/ma15020491/s1, Figure S1: Graphic representation of compounds in mechanism M1; Figure S2: Graphic representation of compounds in mechanism M2; Figure S3: Graphic representation of compounds in mechanism M3; Table S1: Crystallographic data of complex 3; Table S2: Cartesian coordinates of all reactants, products, intermediates, and transition states for mechanisms M1, M2, and M3; Table S3: Bond lengths of theoretically optimized structures in mechanism M1; Table S4: Bond angles of theoretically optimized structures in mechanism M1; Table  S5: Bond lengths of theoretically optimized structures in mechanism M2; Table S6: Bond angles of theoretically optimized structures in mechanism M2; Table S7: Bond lengths of theoretically optimized structures in mechanism M3; Table S8: Bond angles of theoretically optimized structures in mechanism M3; Table S9: Standard Gibbs energies at room temperature for mechanism M1; Table S10: Standard Gibbs energies at room temperature for mechanism M2; Table S11: Standard Gibbs energies at room temperature for mechanism M3. Additionally, checkpoint files for each compound (those of geometry optimization procedures, frequency and thermodynamic properties calculations, and electronic energy calculations, both in gas phase and with solvent) are available from the authors upon request.


Synthesis and antitumor activity evaluation in vitro of 4-aminoquinazoline derivatives containing 1,3,4-thiadiazole

In order to search for new antitumor drugs with high efficiency and low toxicity, a series of novel 4-aminoquinazoline derivatives containing 1,3,4-thiadiazole group were designed, synthesized and evaluated for antiproliferative activity against four human cancer cell lines (H1975, PC-3, MCF-7 and HGC-27) using MTT assay in vitro. Among them, compound N-(5-((3,5-dichlorobenzyl)thio)-1,3,4-thiadiazol-2-yl)-2-((4-(phenylamino)quinazolin-2-yl)thio)acetamide (15o) showed good anti-tumor proliferation activity against four tested cancer cell lines, with IC50 values of 1.96 ± 0.15 μM to PC-3 cell. The antitumor activity was significantly better than that of gefitinib. Further mechanism studies showed that compound 15o inhibited the migration ability of PC-3 tumor cells in a concentration dependent and time-dependent manner, and blocked the cell cycle in S phase. At the same time, cell cloning experiment further proved that compound 15o significantly inhibited the colony formation of PC-3 cells, The inhibition rate was as high as 92% at 2.0 μM. Graphical abstract Graphical abstract

Introduction
Malignant tumor is a tissue with abnormal physiological function formed by excessive proliferation or abnormal differentiation of tumor cells. At present, the clinical treatment methods of malignant tumors include surgical treatment, radiotherapy, chemotherapy, immunotherapy and targeted therapy, among which chemotherapy is still an effective method for the treatment of malignant tumors [1][2][3][4]. However, the adverse side effect and the development of resistance to traditional anticancer drugs call for an urgent exploration of new anticancer agents [5,6].

Chemistry
The synthetic strategy to prepare the target compounds was depicted in Fig. 2. First dissolved KOH in water, and then dissolved 5-amino-1,3,4-thiadiazole-2-thiol in the aqueous solution of potassium hydroxide. After the system became clear, substituted benzyl chloride compounds are added to the system and reacted at room temperature for 8 h to obtain compounds (9a-o). Then, compounds (10a-o) were acquired from the reaction of compound (9a-o) with chloroacetyl chloride at room temperature for 3.5 h. Next, Aniline, triethylamine and carbon disulfide were added to acetone solution and reacted at room temperature for two hours to obtain a yellow solid. The solid obtained by suction filtration was compound (12). Then, compound (13) was acquired from the reaction of compound (12) with triphosgene in chloroform at room temperature for 0.5 h, that was further reacted with o-anthranilonitrile in pyridine at 100°C for 8 h to get compound (14). Finally, dissolved compound (14) in the aqueous solution of potassium hydroxide. After the system became clear, dissolved compounds (10a-o) with 1,4-dioxane and added it to the above system reacted at 70°C for two hours to obtain white solid, and then filter to obtain compounds (15ao). The structure of target compounds were confirmed by 1 H NMR, 13 C NMR, IR and HRMS.

MTT assay
The in vitro antiproliferative activities of all the prepared compounds (15a-o) were evaluated against H1975 (Human lung cancer cell line), PC-3 (Human prostate cancer cell line), MCF-7 (Human breast cancer cell line) and HGC-27 (Human gastric carcinoma cell line) by MTT assay. Gefitinib was selected as positive reference drugs. The IC 50 values (concentration required to achieve 50% inhibition of  Table 1. The synthetic strategy to prepare the target compounds was depicted in Fig. 2. First dissolved KOH in water, and then dissolved 5-amino-1,3,4-thiadiazole-2thiol in the aqueous solution of potassium hydroxide. After the system became clear, substituted benzyl chloride compounds are added to the system and reacted at room temperature for 8 h to obtain compounds (9a-o). Then, compounds (10a-o) were acquired from the reaction of compound (9a-o) with chloroacetyl chloride at room temperature for 3.5 h. Next, Aniline, triethylamine and carbon disulfide were added to acetone solution and reacted at room temperature for two hours to obtain a yellow solid. The solid obtained by suction filtration was compound (12). Then, compound (13) was acquired from the reaction of compound (12) with triphosgene in chloroform at room temperature for 0.5 h, that was further reacted with o-anthranilonitrile in pyridine at 100°C for 8 h to get compound (14). Finally, dissolved compound (14) in the aqueous solution of potassium hydroxide. After the system became clear, dissolved compounds (10a-o) with 1,4-dioxane and added it to the above system reacted at 70°C for two hours to obtain white solid, and then filter to obtain compounds (15a-o). The structure of target compounds were confirmed by 1 H NMR, 13 C NMR, IR and HRMS.

Effect of compound 15o against normal human cell lines
To further investigate the toxicity of these compounds, compound 15o which had best antiproliferative activities against tumour cell lines in Table 1 was picked out to further determine its effects on normal human gastric epithelial cell (GES-1) and human normal esophageal epithelial cells (HEEC). As shown in Table 2, it showed weak or no toxicity against GES-1 and HEEC.

Cell cycle arrest by compound 15o
Compound 15o was chosen to further evaluate its possible anticancer mechanism of action against PC-3 cell based on the above results. After 24 h treatment with compound 15o, PC-3 cells were arrested in S phase (Fig. 3). With the increase of compound 15o concentration, the percentage of

Cell migration inhibition arrest by compound 15o
As shown in Fig. 4, with the extension of time and the increase of compound 15o concentration, the cell migration area gradually decreases, indicating that the migration ability of PC-3 cells decreases, and compound 15o inhibits the migration of PC-3 cells in a time-dependent and concentration-dependent manner.

Cell clone arrest by compound 15o
As shown in Fig. 5, compared with the control group, with the increase of compound 15o concentration, the number of cell clone formation decreased and the cell community decreased significantly, indicating that compound 15o significantly inhibited the colony formation of PC-3 cells, The inhibition rate was as high as 92% at 2.0 μM.

Conclusion
In this work, a series of novel 4-aminoquinazoline derivatives bearing 1,3,4-thiadiazole group were designed, synthesized and evaluated for antiproliferative activity against four human cancer cell lines (H1975, PC-3, MCF-7, and HGC-27). The results suggested that most compounds had potent anti-proliferative activity. Especially, the compound (15o) exhibited the best antiproliferative activity against H1975, PC-3, MCF-7, and HGC-27 cell lines. Further studies showed that compound 15o could inhibit the cloning and migration of PC-3 cell and block the cell cycle in S phase. The above facts illustrated that this work would have remarkable implications for further design and development of a possible antitumor agent.

Materials
Reagents and solvents were purchased from commercial sources and were used without further purification. High resolution mass spectra (HRMS) were recorded on a Waters Micromass Q-T of Micromass spectrometer by electrospray ionizaton (ESI). 1

Synthesis of compounds
Compounds (9-10) [23,24] and compounds (12)(13)(14) [25] were synthesized according to the published literature. Potassium hydroxide (4.96 mmol) was dissolved in water (15 ml) then compound (8) (4.50 mmol) was added. Stirred at room temperature until the solid disappeared. Different benzyl chloride compounds were added to the system and reacted at room temperature for 8 h to obtain large amount of white precipitation. Then compounds (9a-o) were obtained by filtration and drying. The compounds 9a-o (2.33 mmol) were added to 25 ml round bottom flask, then 8 ml of 1,4-dioxane was added to dissolve them, then chloroacetyl chloride (2.79 mmol) was slowly added to the system and reacted at room temperature for 3.5 h. After the monitored reaction, poured the reaction system into the ice water mixture to precipitate a large amount of solid, filtered and dried to obtain the compounds (10a-o). Aniline (5.37 mmol) was added to a 25 ml round bottomed flask, dissolved in 5 ml acetone, and then triethylamine (26.84 mmol) and carbon disulfide (26.84 mmol) were added in turn. After reaction at room temperature for 2 h, a large amount of precipitation is generated, filtered and dried to obtain compound (12). Triphosgene (0.98 mmol) was added to a 25 ml round bottom flask and dissolved by adding 6 ml of chloroform. Then compound (12) (1.97 mmol) was added to the above system and reacted at room temperature for 0.5 h. Compound (13) was separated by column chromatography after the reaction was detected by thin layer chromatography. O-aminobenzonitrile (6.77 mmol) was added to a 50 ml round bottom flask, followed by 10 ml of pyridine and compound 13 (8.13 mmol) at 100°C for 8 h. After the reaction was detected by TLC, 10 ml of mixed solution of petroleum ether and ethanol was added to the reaction system to produce a large amount of precipitation. Compound (14) was obtained by filtration and drying. General procedure for synthesis of derivatives (15a-o). Potassium hydroxide (1.18 mmol) was dissolved in water (20 ml) then compound (14) (0.79 mmol) was added. When the system became transparent, compounds (10a-o) (0.75 mmol) were dissolved in dioxane (3 ml) and added to the reaction system, the mixture was heated to 70°C for 2 h, a large amount of white precipitate was precipitated out. After the reaction was completed, filtered (the filter cake was washed with water and dioxane for three times), dried and then crude products were purified by column chromatography to obtain compounds (15a-o).

Evaluation of antiproliferative activity
Cells in the logarithmic growth phase cells at 3000-5000 cells/well were seeded in 96-well plates for 24 h. Then the cells were treated in triplicate with various concentrations of each compound at 37°C in the humidified 5% CO 2 atmosphere for 72 h. And added MTT to each well at a final concentration of 5 mg/ml. After incubated for 4 h in 37°C, the medium was aspirated. Add 150 μL DMSO to each well to dissolve formazan and the plate was shaken on a shaker for 10 min. The OD vaules was measured by microplate reader at a wavelength of 490 nm and The IC 50 values were obtained using SPSS 20.0 software. Results were Mean ± SD of three independent experiments.

Cell cycle distribution assay
The PC-3 cells in logarithmic growth phase were divided into 2 × 10 5 / well were inoculated in 6-well plates and treated overnight at 37°C and 5% CO 2 . The concentrations were 0 (blank control group), 0.5, 1.0 and 2.0 μM. PC-3 cells were treated with compound 15o and treated for 24 h at 37°C and 5% CO 2 .Then cells were harvested and fixed with 70% ethanol for 8 h at 4°C. The fixed cells were washed and resuspended using phosphate buffer saline (PBS) containing 50 mg/mL propidium iodide (PI) and 10 mg/mL RNaseA. Then cell suspension was incubated for 40 min in a dark place at room temperature. After that, samples were analyzed for DNA content with flow cytometry. (Becton, Dickinson and Company, NJ).

Cell migration experiment
The PC-3 cells in logarithmic growth phase were divided into 4 × 10 5 /well were inoculated in 6-well plates and treated overnight at 37°C and 5% CO 2 . When the cell density reached 85%, The 200 μL gun head evenly marks the "cross" at the bottom of the 6-well plates. The cells were treated with concentrations were 0 (blank control group), 0.5, 1.0 and 2.0 μM, and photographed at the same position of each well with a microscope at 0, 12 and 24 h to record the trend of cell migration.
Cell clone arrest PC-3 cells in logarithmic growth stage were inoculated in 6-well plates at 1000/ well, and cultured overnight at 37°C and 5% CO 2 . PC-3 cells were treated with compound 15o at concentrations of 0 (blank control group), 0.5, 1.0 and 2.0 μM under the condition of 37°C and 5% CO 2 for 8 days. When the cell community visible to the naked eye appeared at the bottom of the 6-well plate, the culture was terminated. The cells were fixed at 4°C with 4% paraformaldehyde for 0.5 h, and stained with 0.1% crystal violet for 20 min.


Reactions of a Dioxidomolybdenum(VI) Complex with Thionation Reagents—Formation of Mo(IV) Species with Sulfur Donors

Molecular molybdenum complexes with sulfur donor ligands are generally studied as soluble model compounds for molybdenum enzymes essential for life. The dioxidomolybdenum(VI) complex with tetradentate aminobisphenolate ligand undergoes a reaction with thionation reagent P2S5 or its organic derivative, Lawesson’s reagent, to yield stable Mo(IV) aminobisphenolate complexes, where pristine oxido ligands have been replaced by bidentate sulfur donors tetrasulfide, S42− or (4-methoxyphenyl)phosphonotrithioate residue derived from Lawesson’s reagent. This is in contrast to the behaviour of analogous dioxidotungsten(VI) complex, which, under similar conditions, yields W(VI) S2 systems. The overall cis,trans,cis geometry of the parent dioxidomolybdenum(VI) aminobisphenolate is retained, namely, the neutral nitrogen donors are in cis positions, phenolate oxygens are trans to each other and sulfur donors are cis. Although formally Mo(IV), thus d2 system, the studied complexes have diamagnetic singlet electron configurations as a result of the axially compressed octahedral structures.

Introduction
Molybdenum sulfides are useful catalysts in a number of important reactions used to activate small molecules, e.g., hydrogen evolution reaction, CO 2 reduction and N 2 fixation [1]. Moreover, molybdenum-sulfur bonds are found in all molybdenum enzymes; therefore, molecular molybdenum sulfide complexes are investigated as soluble model compounds for the active sites of such catalysts as well as for metal enzymes [2][3][4][5]. In principle, diverse metal-organic sulfide complexes can be synthesized by the reaction of simple metal sulfides and ligand precursors, but in practice, they are mostly made of easily available oxido complexes by simple thionation reactions, i.e., oxido-to-sulfido substitutions. Typically, the thionations are run using H 2 S [6-10], B 2 S 3 [11][12][13][14], (R 3 Si) 2 S/R 3 SiSH [15][16][17][18], P 2 S 5 [12], or Lawesson's reagent (2,4-bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane-2,4-disulfide) [9,19]. We have earlier used the reaction of dioxidotungsten(VI) complexes with P 2 S 5 and its soluble derivative, Lawesson's reagent, to prepare stable disulfidotungsten(VI) complexes supported with tetradentate aminobisphenolates [20]. As equivalent Mo and W dioxido compounds have, in many cases, very similar molecular and crystals structures and rather similar chemical properties, we might expect parallel reactivity in the thionation reactions as well. However, under identical reaction conditions, the dioxidomolybdenum(VI) aminobisphenolates did not yield the anticipated disulfido derivatives; instead, the reactions led to the reduction of the metals' centres as the isolated products were identified as Mo(IV) complexes. In this article, we report the reactivity of dioxidomolybdenum(VI) aminobisphenolate with thionation reagents and structural characterization of two Mo(IV) species with sulfur donor chelate ligands.

Results and Discussion
The oxido-to-sulfido substitution of dioxidomolybdenum(VI) aminobisphenolates was studied, allowing complex 1 to react with either P 2 S 5 or Lawesson's reagent in 1,2dichloroethane at reflux temperature following the substitution procedure used previously to synthesize corresponding disulfidotungsten(VI) compounds. As a result, the reactions yielded dark mixtures of several strongly coloured, poorly stable compounds (seen by thin-layer chromatography), which could not be isolated or further characterized. However, 1 reacted with P 2 S 5 at room temperature in a toluene solution to form a dark solution, which allowed the isolation of green 2 in ca. 15% yield (Scheme 1). The reaction was repeated using Lawesson's reagent under identical conditions, which again yielded 2 and brownish-red 3 along with several unstable compounds. The isolated yields of major products in the reaction with Lawesson's reagent depended on the reaction time-after a 20 h reaction, 3 was isolated in ca. 40% yield together with 5% of 2, whereas 2 was found in ca. 35% yield with only a minor amount of 3 after one-week reaction.
Molecules 2022, 27, x FOR PEER REVIEW 2 of 8 isolated products were identified as Mo(IV) complexes. In this article, we report the reactivity of dioxidomolybdenum(VI) aminobisphenolate with thionation reagents and structural characterization of two Mo(IV) species with sulfur donor chelate ligands.

Results and Discussions
The oxido-to-sulfido substitution of dioxidomolybdenum(VI) aminobisphenolates was studied, allowing complex 1 to react with either P2S5 or Lawesson's reagent in 1,2dichloroethane at reflux temperature following the substitution procedure used previously to synthesize corresponding disulfidotungsten(VI) compounds. As a result, the reactions yielded dark mixtures of several strongly coloured, poorly stable compounds (seen by thin-layer chromatography), which could not be isolated or further characterized. However, 1 reacted with P2S5 at room temperature in a toluene solution to form a dark solution, which allowed the isolation of green 2 in ca. 15% yield (Scheme 1). The reaction was repeated using Lawesson's reagent under identical conditions, which again yielded 2 and brownish-red 3 along with several unstable compounds. The isolated yields of major products in the reaction with Lawesson's reagent depended on the reaction time-after a 20 h reaction, 3 was isolated in ca. 40% yield together with 5% of 2, whereas 2 was found in ca. 35% yield with only a minor amount of 3 after one-week reaction. The molecular structures of 2 and 3 were studied by NMR spectroscopy and X-ray diffraction studies. The NMR spectra of 2 show the anticipated chemical shifts for the tetradentate aminobisphenolate ligand. For example, in the 1 H NMR spectrum, the benzylic methylene protons are visible as two two-proton doublets at 5.66 and 4.30 ppm, respectively, accompanied by a two-proton singlet at 4.56 ppm for the methylene protons in the picoline arm. The XRD data shows that the product is virtually a Cs symmetric neutral molecule, where two oxides have been substituted by one S4 2− dianion to form a new Mo (IV) complex ( Figure 1). The overall cis,trans,cis geometry of the parent complex 1 has been retained, i.e., the neutral nitrogen donors are in cis positions, phenolate oxygens are trans to each other and sulfur donors are cis (see Figure 1). Compared to 1, the metaldonor distances to the aminobisphenolate ligand are somewhat shorter in 2, whereas the O1-Mo-O2 and N8-Mo-N37 bite angles are slightly larger (see Table 1). The Mo-S distances and S-Mo-S angle are closely similar to the corresponding bonding parameters found previously for the dianionic tetrasulfido ligand in known Mo(IV) complexes [3,4,9,21,22]. In the same way, the 1 H and 13 C NMR spectra of 3 show chemical shifts for the coordinated aminobisphenolate ligand, while the number of the chemical shifts indicates a C1 symmetric structure. The spectra also show the presence of 4-methoxyphenyl group originated from Lawesson's reagent. Correspondingly, the XRD studies show that the complex 3 is a neutral species, wherein one tetradentate aminobisphenolate and one bidentate, dianionic (4-methoxyphenyl) phosphonotrithioate surround the Mo(IV) ion The molecular structures of 2 and 3 were studied by NMR spectroscopy and X-ray diffraction studies. The NMR spectra of 2 show the anticipated chemical shifts for the tetradentate aminobisphenolate ligand. For example, in the 1 H NMR spectrum, the benzylic methylene protons are visible as two two-proton doublets at 5.66 and 4.30 ppm, respectively, accompanied by a two-proton singlet at 4.56 ppm for the methylene protons in the picoline arm. The XRD data shows that the product is virtually a C s symmetric neutral molecule, where two oxides have been substituted by one S 4 2− dianion to form a new Mo (IV) complex ( Figure 1). The overall cis,trans,cis geometry of the parent complex 1 has been retained, i.e., the neutral nitrogen donors are in cis positions, phenolate oxygens are trans to each other and sulfur donors are cis (see Figure 1). Compared to 1, the metal-donor distances to the aminobisphenolate ligand are somewhat shorter in 2, whereas the O1-Mo-O2 and N8-Mo-N37 bite angles are slightly larger (see Table 1). The Mo-S distances and S-Mo-S angle are closely similar to the corresponding bonding parameters found previously for the dianionic tetrasulfido ligand in known Mo(IV) complexes [3,4,9,21,22]. In the same way, the 1 H and 13 C NMR spectra of 3 show chemical shifts for the coordinated aminobisphenolate ligand, while the number of the chemical shifts indicates a C 1 symmetric structure. The spectra also show the presence of 4-methoxyphenyl group originated from Lawesson's reagent. Correspondingly, the XRD studies show that the complex 3 is a neutral species, wherein one tetradentate aminobisphenolate and one bidentate, dianionic (4-methoxyphenyl) phosphonotrithioate surround the Mo(IV) ion (see Figure 2). The overall geometry around the metal centre is cis,trans,cis and the phenolate groups are in trans positions, i.e., the aminobisphenolate ligand is symmetrically coordinated. Therefore, the asymmetry seen in the NMR spectra rises from the phosphonotrithioate ligand. The metal-donor distances to the aminobisphenolate ligand are considerably shorter than in 2 ( Table 1). The bite angles O1-Mo-O2 and N8-Mo-N37 are slightly larger and the S-Mo-S angle is clearly smaller than in 2, obviously due to the rigidity of the phosphonotrithioate ligand. Complexes 2 and 3 are formally Mo(IV) complexes with d 2 electron configurations, and are therefore expected to show paramagnetic properties caused by the two odd electrons. However, both compounds are diamagnetic, as evidenced by the well-resolved NMR spectra. Principally, the d 2 metal centres may have either paramagnetic triplet or diamagnetic singlet electron configurations, depending on the coordination geometry. According to the visual inspection of the single crystal structures and further calculation of the continuous shape measures (CShM's, see Table S2 Supplementary Materials) [23], 2 and 3 are octahedral complexes with axially compressed structures with four long bonds in xy-planes (Mo-N8, Mo-N37, Mo-S1 and Mo-S2/S4) and two shorter ones (Mo-O1 and Mo-O2) along z axes. This compression causes the splitting of the d xy , d xz and d yz orbital set, the first lower in energy compared to the other ones, which results singlet ground state system with both electrons on the d xy orbital. The diamagnetic ground states of 2 and 3 were also confirmed by DFT calculations using the respective crystal structures of the complexes as the basis of coordinates for computational analyses. The singlet-triplet gap of 2 is 103 kJ/mol, whereas in 3 the triplet lies 35 kJ/mol above the singlet ground state, which affirms the singlet ground states of the complexes evident from the experimental data. For 3, the lowest triplet state at the PBE0/def2-TZVP level shows Mo(IV) with d 2 electron configuration, whereas for the lowest triplet state of 2, the Mulliken spin densities (see ESI) in fact suggest a d 1 Mo centre with the other unpaired electron localized at the S 4 -fragment while the d 2 triplet state could not be established even with an attempt of using fragment-based initial assignment of partial charges and spins separately for ligands and metal.
(see Figure 2). The overall geometry around the metal centre is cis,trans,cis and the phenolate groups are in trans positions, i.e., the aminobisphenolate ligand is symmetrically coordinated. Therefore, the asymmetry seen in the NMR spectra rises from the phosphonotrithioate ligand. The metal-donor distances to the aminobisphenolate ligand are considerably shorter than in 2 ( Table 1). The bite angles O1-Mo-O2 and N8-Mo-N37 are slightly larger and the S-Mo-S angle is clearly smaller than in 2, obviously due to the rigidity of the phosphonotrithioate ligand. Complexes 2 and 3 are formally Mo(IV) complexes with d 2 electron configurations, and are therefore expected to show paramagnetic properties caused by the two odd electrons. However, both compounds are diamagnetic, as evidenced by the well-resolved NMR spectra. Principally, the d 2 metal centres may have either paramagnetic triplet or diamagnetic singlet electron configurations, depending on the coordination geometry. According to the visual inspection of the single crystal structures and further calculation of the continuous shape measures (CShM's, see Table S2 Supplementary Material) [23], 2 and 3 are octahedral complexes with axially compressed structures with four long bonds in xy-planes (Mo-N8, Mo-N37, Mo-S1 and Mo-S2/S4) and two shorter ones (Mo-O1 and Mo-O2) along z axes. This compression causes the splitting of the dxy, dxz and dyz orbital set, the first lower in energy compared to the other ones, which results singlet ground state system with both electrons on the dxy orbital. The diamagnetic ground states of 2 and 3 were also confirmed by DFT calculations using the respective crystal structures of the complexes as the basis of coordinates for computational analyses. The singlet-triplet gap of 2 is 103 kJ/mol, whereas in 3 the triplet lies 35 kJ/mol above the singlet ground state, which affirms the singlet ground states of the complexes evident from the experimental data. For 3, the lowest triplet state at the PBE0/def2-TZVP level shows Mo(IV) with d 2 electron configuration, whereas for the lowest triplet state of 2, the Mulliken spin densities (see ESI) in fact suggest a d 1 Mo centre with the other unpaired electron localized at the S4-fragment while the d 2 triplet state could not be established even with an attempt of using fragment-based initial assignment of partial charges and spins separately for ligands and metal.      3 tach = 1,3,5-tri-tert-butyl-1,3,5-triazacyclohexane) [3,9]. Although the formation mechanism of the reduced products 2 and 3 is not clear, we may suppose that the initial step is the oxido-to-sulfido substitution to form MoS 2 (L), similarly to the reactions of analogous W(VI) complexes. The unstable disulfido reduces then to eliminate elemental sulfur yielding a reactive Mo(IV) intermediate MoS(L). This intermediate reacts further with Lawesson's reagent or other sulfur-containing species in the reaction mixture to form distinct isolatable products 2 and 3. Both 2 and 3 are stable in the solid state as well as in inert solvents under open atmosphere. Although 3 decomposes in a prolonged reaction while 2 is formed, isolated and purified 3 is stable and can be stored in a toluene solution for a week without any noticeable formation of 2. In general, the formation of these Mo(IV) complexes demonstrates the relative inclination of Mo(VI) toward reduction which is in stark contrast to the behaviour of W(VI), although these to M(VI) species often reflect each other's reactivity.
In conclusion, the oxido-to-sulfido substitution of a dioxidomolybdenum(VI) aminobisphenolate 1 was studied using different thionation reagents. The reaction with P 2 S 5 leads to the formation of Mo(IV) aminobisphenolate complex 2 with a bidentate sulfur chelate, i.e., tetrasulfide, S 4 2− . The parallel reaction with Lawesson's reagent, the organic derivative of P 2 S 5 , yields also complex 2 together with complex 3 as the initial major product, of which the latter has a (4-methoxyphenyl)phosphonotrithioate residue derived from Lawesson's reagent coordinated to the Mo(IV) centre. Both compounds 2 and 3 have an overall geometry of cis,trans,cis, that is, the neutral nitrogen donors are in cis positions, phenolate oxygen atoms are trans to each other and sulfur donors are cis in the axially compressed octahedral structures. Both 2 and 3 are stable in the solid state as well as in inert solvents under ambient atmosphere. The studied complexes are formally Mo(IV) with d 2 metal centres. However, they are diamagnetic due to the singlet electron configurations as a result of the axially compressed octahedral structures. Similar thionation reactions of comparable dioxidotungsten(VI) aminobisphenolates are known to yield disulfidotungsten(VI) complexes, so these results demonstrate the higher inclination.

Materials and Methods
Complex 1 was prepared as reported previously [25]. Toluene was dried over 3A molecular sieves. Other chemicals were from commercial sources and were used as purchased. The synthetic reactions were run under a nitrogen atmosphere, whereas all isolations and analyses were conducted under open atmosphere. The IR spectra were measured using a Bruker VERTEX 70 FTIR instrument in transmittance mode, and peaks are reported in wavenumbers (cm -1 ) and intensities (b = broad, w = weak, m = medium, s = strong, vs = very strong). All NMR spectra were recorded on a Bruker Avance III 500 MHz instrument ( 1 H: 500.08 MHz, 13 C: 125.75 MHz) equipped with a broad-band smart probe and were referenced to residual CHCl 3 solvent signals ( 1 H: δ 7.26, 13 C: δ 77.16). The NMR samples were kept under vacuum prior to the measurements to remove possible solvate molecules. The samples were dissolved in acetonitrile for mass spectrometric analysis. The mass spectra were obtained by quadrupole-Orbitrap mass spectrometer (QExactive TM , Thermo Fisher Scientific GmbH, Bremen, Germany) using direct infusion, negative electrospray ionization and full scan at m/z 150-2000 with the resolution of 140,000. The calibration was performed by Pierce ESI Negative Ion Calibration Solution (Thermo Fisher Scientific Inc., Waltham, MA, USA). The data was processed with Thermo Xcalibur Qual Browser software (Version 3.0.63, Thermo Fisher Scientific Inc., Waltham, MA, USA). Single crystal X-ray diffraction data were collected with Rigaku Oxford Diffraction custom system consisting of microfocus MicroMax™-007 HF rotating anode generator producing monochromatized Cu K α1 radiation and HyPix-6000HE detector. Data collection and reduction were done using the CrysAlis Pro software [26], whereas crystal structures were solved and refined using SHELXS and SHELXL programs within the Olex 2 interface. DFT calculations were conducted using Gaussian 16 software [27]. The geometries were taken from single crystal X-ray structures and the C-H bond lengths were normalized. The relative energies of singlet and triplet spin states were determined as single point calculations at the PBE0/def2-TZVP (effective core potential for Mo) level using these geometries [28,29].
2: Method a: First, 0.35 g (0.50 mmol) of 1 and 0.15 g (0.68 mmol) of P 2 S 5 were mixed in 20 mL of dry toluene. The dark mixture was stirred for 24 h, the solvent was evaporated and the green product (60 mg, 15%) was isolated by a silica column chromatography using CH 2 Cl 2 as an eluent. Method b: First, 0.50 g (0.75 mmol) of 1 and 0.61 g (1.50 mmol) of Lawesson's reagent were mixed in 20 mL of dry toluene and the solution was kept for week at room temperature. Afterwards, 132 mg (35%) of 2 as well as 33 mg (5%) of 3 were isolated by column chromatography using CH 2 Cl 2 as an eluent. The crystals of 2 for XRD analyses were grown from acetonitrile at room temperature. 3: First, 3.9 g (5.8 mmol) of 1 and 2.4 g (5.9 mmol) of Lawesson's reagent were mixed in 35 mL of dry toluene and the reaction mixture was stirred under a nitrogen atmosphere for 20 h at room temperature. Next, 3.55 g of brown precipitate was isolated and recrystallized from hot acetonitrile to obtain 2.08 g (39%) of brownish-red crystals. The filtrate was evaporated and 0.