Tuning the structure, reactivity and magnetic communication of nitride-bridged uranium complexes with the ancillary ligands

The reactivity of the nitride ligand is increased in complexes of uranium(iv) when bound by the OSi(OtBu)3 ligand as opposed to N(SiMe3)2, but magnetic exchange coupling is decreased.


Introduction
Uranium nitride compounds are of high interest because of their recently discovered ability to promote the stoichiometric transformation of small molecules such as CO 2 , CO and N 2 , 1 which are widely available C1 and N feedstocks for the synthesis of high-value organic compounds. 2 Notably, examples of small molecule activation by nitride compounds remain scarce 3 both in d-block 4 and in f-block chemistry. Moreover, uranium nitrides are driving experimental and computational studies because they are well suited to probe the nature of bonding in actinide compounds 5,6 and to promote magnetic communication between uranium centers in various oxidation states. 1i,7 The number of isolated terminal and bridging uranium nitrides continues to increase, 1a,1e,1f,1i,5b-5d,7b,8 but their synthesis remains far from trivial in part because of their high reactivity that may result in solvent deprotonation, C-H activation 8i,9 or C-O cleavage 1d of the ancillary ligand to afford amido or imido complexes. Moreover, studies of the reactivity of uranium nitrides have so far been limited to three ligand systems, 1c,1d,1f-1i namely, complexes of the tris(tertbutoxy)siloxide ligand OSi(O t Bu) 3 , the U 5+ -U 5+ nitride [(U(N[t-Bu]Ar) 3 ) 2 (m-N)][B(Ar F ) 4 ] with amide ligands, and lastly terminal nitrides supported by the tripodal tris-amido ligand Tren TIPS (Tren TIPS ¼ N(CH 2 CH 2 -NSi i Pr 3 ) 3 ). The nitride in complexes of the siloxide and Tren TIPS ligands acts as a strong nucleophile and reacts with CO 2 , CO, and CS 2 to give products of N-C bond formation. 1c,1d,1f-1i The siloxide supported complexes also effect the cleavage of H 2 leading to N-H formation. 1c,1d,1f-1i In contrast, the aforementioned [(U(N[t-Bu]Ar) 3 ) 2 (m-N)][B(Ar F ) 4 ] was reported unreactive with CO but showed electrophilic reactivity towards cyanide resulting in the formation of a cyanoimide bridged U 4+ -U 4+ complex. 5c The geometric and electronic parameters leading to the observed differences in reactivity remain unclear. Moreover, the effects of the inner-sphere cation on the reactivity of the siloxide supported complex [Cs{(LO) 3 U} 2 (m-N)] (LO ¼ ( t BuO) 3 SiO), [Cs]-1 (Chart 1), 8g,1b was not elucidated. It occurred to us that the bulky silylamide ligand (LN ¼ N(SiMe 3 ) 2 ) is well suited for determining ligand effects on the reactivity and magnetic properties of bridging nitrides because it is more electron-donating than the siloxide ligand LO with only a slightly higher steric demand. 10 Earlier room temperature studies from the Hayton group 8f with the LN ligand led to the synthesis of the methylenebridged U 4+ -U 4+ nitride complex [Na(dme) 3 ][(LN) 2 U(m-N)(m-k 2 -C,N-CH 2 SiMe 2 NSiMe 3 )-U(LN) 2 ] (dme ¼ 1,2-dimethoxyethane), 3 (Chart 1).
Here we report the synthesis of three new diuranium U 4+ -U 4+ nitride complexes, namely the all siloxide [NBu 4  Comparative reactivity studies with small molecules, CO, CO 2 , and H 2 showed dramatic differences between the nitride complexes depending on the supporting ligands. No reactivity was shown by the all-amide complex 2, but high nucleophilic reactivity of the nitride was observed with CO, CO 2 , and H 2 for the all-siloxide complex [NBu 4 ]-1. Moreover, the presence of only one siloxide ligand out of six supporting ligands is sufficient to enable CO, CO 2 activation by complex 5. Computational studies inferred that the different behavior can be interpreted in terms of differences in the U]N]U bonding between the allsiloxide and the all-silylamide ligand. The increased orbital overlap between the U]N]U centers found in the allsilylamide complex results in stronger magnetic coupling between the two uranium centers and reduced nucleophilic character of the bridging nitride compared to the all-siloxide one.

Results and discussion
Preparation of the nitride complexes Compound [NBu 4 ]-1 is obtained in 70% yield using a method similar to that used to prepare the previously reported Cs analogue [Cs]-1 8g in which reduction of azide by [U(LO) 2 (LO)] 2 in thf is done at low temperature (À40 C). Single-crystals of [NBu 4 ]-1 were obtained aer storage of an 8 : 1 toluene/hexane solution at À40 C. Complex 2 can also be synthesized in thf, but in toluene precipitates as a brown powder in 58% yield. On several occasions while investigating the low temperature synthesis of 2 in toluene-d 8 , single crystals of 2 grew in the NMR tube, which allowed its structural characterization. The 1 H NMR spectrum of 2 at À60 C in thf-d 8 shows two narrow and four broad resonances attributed to the SiMe 3 groups, suggesting a rigid structure under these conditions. As the temperature is elevated to room temperature, the resonances become increasingly broadened until they are indistinguishable from the baseline; at room temperature, only those of the [NBu 4 ] + cation are observed.
Previously, it was shown that [Cs]-1 decomposes slowly at room temperature and more rapidly at 80 C to afford the tert- , 1d due to the high nucleophilic character of the bridging nitride in [Cs]-1. Monitoring the 1 H NMR spectrum of [NBu 4 ]-1 at room temperature or at 80 C demonstrates that its thermal stability is similar to that of [Cs]-1, but an imide product could not be identied in the decomposition mixture.
In order to investigate the effect of the uranium oxidation state on the spectroscopic features and reactivity of the N(SiMe 3 ) 2 nitride complexes, a U 4+ -U 5+ analogue of 2, namely [{(LN) 3 U} 2 (m-N)], 4, was also prepared. Complex 4 was obtained in 60% yield by oxidation of 2 with 1 equiv. of AgBPh 4 , Scheme 2. The 1 H NMR spectrum of 4 is similar to that of 2 in that it consists of two narrow and four broad resonances at À60 C and at room temperature, the resonances broaden and become indistinguishable from the baseline. Single crystals of 4 characterizable by X-ray crystallography were obtained by cooling a concentrated toluene solution to À40 C.
In order to probe further ligand effects on reactivity and bonding, the synthesis of OSi(O t Bu) 3 /N(SiMe 3 ) 2 mixed-ligand uranium nitride complexes was pursued. We rst explored protonolysis reactions and attempted to synthesize [NBu 4 ]-1 by reacting 2 with 6 equiv. of HOSi(O t Bu) 3 , but 1 H NMR spectroscopy indicated that no reaction occurred between these two reagents at room temperature. This suggests that the U-N bonds in complex 2 are signicantly more resistant to protonolysis compared to those in the U(III) complex [U(LN) 3 ] 11 that reacts rapidly with HOSi(O t Bu) 3 to afford the protonolysis product [U(LO) 2 (m-LO)] 2 . 12 However, the cyclometalate U 4+ nitride complex, [Na(dme) 3 ] [(LN) 2 U(m-N)(m-k 2 -C,N-CH 2 SiMe 2 NSiMe 3 )U(LN) 2 ], 3, 8f reacts immediately with HOSi(O t Bu) 3 , affording a mixed-ligand nitride of N(SiMe 3 ) 2 and OSi(O t Bu) 3 in 70% yield, [Na(dme) 3 ][(LN) 3 U(m-N)U(LN) 2 (LO)], 5, Scheme 3. The room temperature 1 H NMR spectrum of 5 in thf-d 8 displays three resonances; two resonances at 1.08 ppm and À0.02 ppm are attributed to the N(SiMe 3 ) 2 ligands and another broad resonance at À5.40 ppm has been assigned to OSi(O t Bu) 3 . Single crystals of 5 were grown by diffusion of hexane into an Et 2 O solution of 5 in the presence of dme.

Structural characterization of the nitride-bridged complexes
The solid-state structures of complexes [NBu 4 ]-1, 2, 4 and 5 were determined by X-ray diffraction studies. The metrical parameters are presented in Table 1, including those previously reported for complex [Cs]-1 8g and [Na(dme) 2 (tmeda)]-3 (ref. 8f) for comparison. The molecular structure of 4 ( Fig. S39 †) and a discussion of the metrical parameters of 4 are presented in the ESI. † Complex [NBu 4 ]-1 crystallizes in the I2 space group and its molecular structure (Fig. 1) shows the presence of a U 4+ -U 4+ complex bridged by a nitride ligand. In the solid-state structure of [NBu 4 ]-1, the U 4+ ions of [NBu 4 ]-1 are disordered between two positions that were rened with 50% occupancy. Analysis of the X-ray data of [NBu 4 ]-1 reveals signicant differences in the overall structure with respect to the previously reported [Cs]-1, but the disorder of the U-N-U core is such that its parameters are not reliable. In [NBu 4 ]-1, each U(IV) ion is four-coordinate and in a pseudo tetrahedral coordination environment bound by three k 1 -OSi(O t Bu) 3 ligands and one m-nitride ligand, which bridges the two [(LO) 3 U] + units. The OSi(O t Bu) 3 ligands are in a staggered orientation with respect to those of the adjacent [(LO) 3 U] + moiety as reected in the 6(2) twist angle. In the previously reported 8g structure of [Cs]-1, the binding situation is more complicated. There are one k 1 -OSi(O t Bu) 3 , one    (1) , and one at 174 (1) ).
The structure of the [{(LN) 3 U} 2 (m-N)] À anion ( Fig. 1b) in 2 shows the presence of a dinuclear U(IV) complex with two [(LN) 3 U] + units bridged by a nitride ligand. The coordination geometry of each four-coordinate uranium ion is pseudo tetrahedral and the arrangement of the N(SiMe 3 ) 2 ligands in the two [((LN) 3 U] + units is staggered as reected by the 1.7(4) twist angle. The 178.7(2) and 180.0 U-N-U angles found in 2 are larger than that found in the siloxide complex Cs-1 and are very close to linearity. For comparison, the angle in the previously reported [NBu 4 The other metrical parameters in [NBu 4 ][{((3,5-Me 2 C 6 H 3 )( t Bu) N) 3 U} 2 (m-N)] are similar to 2 (see Table S2 in the ESI †), but changes between the U 4+ -U 4+ and U 4+ -U 5+ complexes differ in the two classes of compounds. The U-N nitride distances in the U 4+ -U 5+ compound 4 are different suggesting the presence of localized valence. A detailed structural comparison of 2 and 4 The molecular structure of the mixed-ligand complex 5 shows a dinuclear nitride-bridged U(IV) complex with an outer sphere cation, [Na(dme) 3 ] + , (Fig. 2). The [(LN) 2 (LO)U(m-N) U(LN) 3 ] À anion shows two different ligand environments for the two U(IV) centers; one U(IV) is bound by two N(SiMe 3 ) 2 amide ligands and one k 1 -OSi(O t Bu) 3 siloxide ligand while the second U(IV) is ligated by three amides. Both U(IV) ions are fourcoordinate in a pseudotetrahedral coordination environment and the ligands are gauche with respect to those of the adjacent U(IV) ion. A comparison of the structure of 5 with those of [Cs]-1 and 2 shows they have similar U-N nitride and U-O siloxide and U-N amide bond distances. Interestingly, the presence of only one siloxide ligand results in a signicant deviation of the U-N-U angle from linearity. The value of the U-N-U angle in 5 (168.4 (3) ) is close to that found in Cs-1 (170.3(3) ) containing six siloxide ligands. These structural data indicate that the ligands signicantly affect the solid-state metrical parameters of the U]N]U core.
We assessed the small molecule reactivity of the complexes to explore the effect of ancillary ligands on the reactivity.

Small molecule reactivity
The reactivity of the newly prepared nitride-bridged complexes with CO, CO 2 , and H 2 was investigated and compared with that of [Cs]-1 1b-1d,1h with the objective of relating differences in reactivity to structure. The reactivity of 5-10 mM solutions of the complexes was monitored by 1 H and 13 C NMR spectroscopies and X-ray crystallography when single crystals were obtainable. The NMR spectra and additional X-ray characterization data are presented in the ESI. †  CO. The siloxide complexes [Cs]-1 and [NBu 4 ]-1 show immediate reactivity with 1-2 equiv. of CO. As previously reported, [Cs]-1 reacts cleanly with CO in toluene-d 8 to afford the m-oxo cyanide complex, [Cs{(LO) 3 U} 2 (m-CN)(m-O)]. 1c When [Cs]-1 is reacted in thfd 8 , the 1 H NMR spectrum of the reaction mixture is more complicated, but hydrolysis of the reaction residue with pD ¼ 12 D 2 O and analysis by 13 C NMR spectroscopy indicates that 13 CN À is also produced under these conditions.
In our previous work, computational studies had pointed out the importance of cooperative binding of CO by the multimetallic U-N-Cs nitride fragment. Therefore, we investigated the reaction of the [NBu 4 ]-1 that does not contain a bound Cs + cation. The reaction of [NBu 4 ]-1 with 13 CO in both toluene-d 8 and thf-d 8 also showed conversion of 13 CO to 13 CN À . In toluened 8 , the 1 H NMR spectrum of the reaction mixture is more complicated than for the . Notably, the inner sphere cesium cation binds four siloxide oxygen atoms and the bridging oxo, which could hold together the overall structure and prevent scrambling. In the thf-d 8 reaction of [NBu 4 ]-1 with 13 CO, only a single product was observed in the 1 H NMR spectrum that resonates at 2.66 ppm. In the 13 C spectrum of the reaction mixture in thf-d 8 , we identied a resonance at 634.7 ppm, which we have assigned to a uranium bound 13 CN À ligand. In both the toluene-d 8 and thf-d 8 reactions, when the reaction residue is hydrolysed with pD ¼ 12 D 2 O, 13 CN À is observed as the only product by 13 C NMR spectroscopy. The spectra are shown in the ESI (Fig. S15-S19 †).
A solution of the diuranium(IV) amide complex 2 was monitored by 1 H NMR spectroscopy aer exposure to 1 atm of CO. Aer 5 h, only minor resonances consistent with decomposition were observed. The U 4+ -U 5+ amide complex 4 also showed no reactivity when exposed to 1 atm of CO.
The 13 CO reactivity of the mixed-ligand 5 was explored in thfd 8 (Fig. S23-S26 †) but not in toluene-d 8 due to its insolubility. Complex 5 reacts with 1-2 equiv. of 13 CO, but more slowly than the [Cs]-1 and [NBu 4 ]-1 complexes. Full consumption of 5 was observed only aer ca. 24 h and the 1 H NMR spectrum appeared complicated. Analysis of the reaction mixture by 13 C NMR spectroscopy in thf-d 8 did not allow an assignment of a resonance for a bound 13 CN À ligand. However, aer removal of the volatiles and hydrolysis with pD ¼ 12 D 2 O, 13 CN À was identied by 13 C NMR spectroscopy. Attempts to grow single crystals from reactions with 1-2 equiv. of natural isotope abundance CO produced only amorphous powders.
To test whether a U(IV) nitride complex without a OSi(O t Bu) 3 ligand would react with CO, we examined 3 (Fig. S20-S22 †). We report only its reactivity in thf-d 8 due to its limited solubility in toluene-d 8 . The 1 H NMR spectra in thf-d 8 from reactions of 3 with either 1 atm of CO or 1-2 equiv. of 13 CO appear identical (see Fig. S24 and S25 in the ESI †) suggesting that the product distribution is not affected by the stoichiometry of CO. In the 1-2 equiv. of 13 CO reaction, complete consumption of 3 was observed aer ca. 9 h. Similar to the 5 result, we did not identify a resonance attributable to a bound 13 CN À ligand in the thf-d 8 13 C NMR spectrum, but did observe 13  The CO insertion chemistry has been observed previously in mononuclear uranium(IV) metallacycles, 14 but these results demonstrate its extension to uranium nitride complexes. The reactivity of the all-siloxide complexes [Cs]-1 and [NBu 4 ]-1, which effect the cleavage of CO under ambient conditions, differs signicantly from that of the all-amide complexes 2 and 4, which do not react with CO despite differences in their oxidation states. The CO cleavage reactivity is reinstated in the mixed ligand amide siloxide 5 and methanide 3 complexes, although in the case of 3, CO insertion into the U-C bond also occurs concomitantly with CO cleavage. These results suggest reduced nucleophilic character of the nitride in the all-amide complexes compared to those with siloxide or methanide ligands.
CO 2 . Previously, our group reported the reaction of [Cs]-1 with 2-3 equiv. of CO 2 in toluene, which produced the rst example of a metal dicarbamate species, [Cs{(LO) 3 U} 2 (m-NC 2 O 4 )], from CO 2 addition to a metal nitride. 1d We found that under analogous conditions, [NBu 4 ]-1 reacts similarly with CO 2 to afford the dicarbamate complex, [NBu 4 ][{(LO) 3 U} 2 (m-NC 2 O 4 )], 8, Scheme 5. Single crystals of 8 characterizable by X-ray diffraction were grown from a 9 : 1 toluene/hexane mixture at À40 C, Fig. 4. The 1 H NMR spectrum generated from crystals of 8 in toluene-d 8 shows a broad resonance ranging 2-0 ppm assigned to OSi(O t Bu) 3 of 8. The 13 C NMR spectrum in toluened 8 shows a broad resonance at 141.5 ppm assigned to the bound dicarbamate ligand. Hydrolysis of crystals of 8 with pD ¼ 12 D 2 O and analysis by 13 NMR spectroscopy showed only D 13 CO 3 À . Similar results were obtained when the reaction was performed in thf and hydrolysis of the reaction residue similarly showed only D 13  We explored the CO 2 reactivity of the amide complexes 2 and 4 with CO 2 but found that exposure of thf-d 8 solutions of these complexes to 1 atm of CO 2 does not produce any change in their respective 1 H NMR spectra indicating no reactivity.
The bridging nitrides in the heteroleptic amide methanide, 3, and amide siloxide, 5, both react with 2 equiv. or 1 atm CO 2 in thf-d 8 to produce a complicated mixture of products. The products could not be isolated, but hydrolysis with pD ¼ 12 D 2 O of the reaction residues aer reactions of 3 or 5 with 2-6 equiv. 13 CO 2 showed N 13 CO À as the only product originating of the nitride; evidence for a dicarbamate product was not obtained in either case. The formation of cyanate indicates that the reaction of 3 and 5 with excess CO 2 proceeds with deoxygenation and N-C bond formation. Similar reactivity leading to the formation of oxo cyanate complexes had been reported for [Cs]-1 at substoichiometric ratios of CO 2 (ref. 1d) and for terminal U(V) nitrides. 1g These results show that the replacement of one amide ligand (out of six ligands) by one siloxide is sufficient to promote nitride transfer to CO 2 . The different reactivity of 3 and 5 compared to [NBu 4 ]-1 indicates that bridging nitrides in complexes supported by amide ligands are less nucleophilic and react preferentially with one CO 2 molecule with the rate of (NCO 2 ) 3À cleavage to yield NCO À and O 2À being faster than the addition of a second CO 2 molecule. All of the spectra are shown in Fig. S27-S35 (Fig. S36-S38 †).
Complex 9  The bridging hydride was located in the Fourier difference map and the U-H distances (2.31(4)Å and 2.23(4)Å) are similar to those found in the Cs analogue (2.18(6) and 2.36(6)Å), 1h and those determined by neutron diffraction studies for the complex [U(C 5 Me 5 ) 2 (m-H) 2 ] 2 of 2.134(9)Å. 15 In the cesium complex, the cesium cation lies at the apical position of the hydride ligand with a Cs-H distance of 2.73(6)Å. The Cs-H interaction is probably rendering more labile the U-H binding and facilitating the H 2 elimination when the H 2 headspace is removed. In contrast, the absence of the coordinated cesium cation evidently reduces its reactivity towards H 2 elimination. H 2 cleavage was not observed for the heteroleptic complexes, 3 and 5, nor for the all-amide complexes 2 and 4 at ambient conditions. This can be explained in term of their reduced nucleophilic character.
Overall, the correlations between structural parameters and nucleophilic reactivity are not obvious except that the all-amide complexes with the linear U]N]U angle are the least reactive. The different reactivity with H 2 of the mixed-ligand complex 5 and the all-siloxide complexes 1 suggests a reduced nucleophilicity of the nitride in complex 5 in spite of the presence of a bent U]N]U angle. This suggests that the nucleophilicity of the nitride is affected by ligand electronics which do not always result in obvious structural differences.
In order to further investigate how differences in bonding could explain the observed difference in reactivity we turned to other methods.

Magnetic properties
Magnetic communication between uranium centers can provide important information on the nature of bonding 5b,7a,8f,16 and lead to attractive magnetic properties. 17 Examples of unambiguous magnetic coupling are rare in uranium chemistry and mostly limited to antiferromagnetic coupling between uranium(V) centers with values of c maximum in the c versus T plots ranging from 5 to 77 K. 7a,16b,18 Stronger antiferromagnetic coupling with the highest value of c maximum at 110 K was reported by Cummins and Diaconescu for an arene-bridged U(III) dimer. 19 In contrast, two examples of unambiguous magnetic coupling between two U(IV) ions were reported for diuranium(IV) chalcogenide complexes with c maxima at 3 and 20 K. 20 Strong antiferromagnetic coupling has been also suggested from the $60 K maximum in the c versus T plot of an amide supported nitride-bridged U 4+ -U 4+ also presenting a linear U]N]U core, complex. 7b The c versus T plot for complex 2 (Fig. 6) shows the magnetic behavior of an antiferromagnetically coupled dinuclear complex with a maximum in the c versus T plot of approximately 90 K. Below 20 K, an increase of the magnetic susceptibility is observed which we attribute to small amounts of paramagnetic impurities. Thus, the linear U(IV)]N]U(IV) unit in the all amide complex exhibits a strong coupling between the two f 2 ions higher than for any other U 4+ -U 4+ complex.
For the all-siloxide complexes [NBu 4 ]-1 and [Cs]-1 (Fig. 6), and for the all-amide U 4+ -U 5+ complex 4 (Fig. S43 †), the c versus  T plot shows a continuous increase and for [NBu 4 ]-1 and [Cs]-1 approaches a temperature-independent plateau at low temperature. This behavior is characteristic of magnetically isolated uranium ions in complexes. 8e,18f,20b,21 A similar behavior was also reported for the amide methanide complex 3, which was interpreted in terms of a localized U]N bonding interactions of the bridging nitrido ligand (as suggested by the inequivalent U-N nitride distances). 8f A shorter U-U distance is found in the solidstate structure of the all-siloxide complexes [NBu 4 ]-1 (4.107(2)Å) and Cs-1 (4.1214(4)Å) compared to the amide complex 2 (4.15Å), but coupling is observed only in the amide complex. The c versus T plot for the mixed siloxide-amide complex 5 (Fig. 6) shows the magnetic behavior of an antiferromagnetically coupled U 4+ -U 4+ complex with a maximum in the c versus T at 55 K, signicantly lower than for the all-amide complex 2. In this complex, the U-N-U angle (168 ) is similar to what found in all siloxide complex [Cs]-1 suggesting that there is no a relation between this structural parameter and the magnetic properties. The dramatic difference in the magnetic properties of complexes 2 and 5 compared to complexes [NBu 4 ]-1 and [Cs]-1 are likely to arise from the electronic properties of the supporting ligands, which evidently lead to increased orbital overlap between the nitride and the two uranium(IV) centers in the amide supported complex, enhancing the electronic communication between the two uranium centers. Such communication is reduced when one amide out of six is replaced by one siloxide. This is in line with the reactivity the nitride ligand in complex 5 which is increased compared to the all-amide complexes, but still lower than for allsiloxide complexes. In order to assess more thoroughly the U-N-U bonding scheme, we have computed the electronic structures of the three complexes.

Computational studies
Depending on the arrangement of the two 5f electrons on each U IV center, the synthesized complexes are characterized either by a quintet, triplet, or singlet ground-state. Computations on the all-siloxide 1, the all-amide 2 and the mixed-ligand 5 complexes for each of these spin-states were performed at the DFT level (B3LYP/Def2-SVP/Def-SDD and M06L(-D3)/Def2-SVP/ Def-SDD (see ESI †)). The two formally degenerate states (triplet and singlet) were treated within a broken-symmetry (BS) formalism, starting from a spin-ip ansatz from the nondegenerate high-spin quintet state. According to both density functionals, the all-amide is characterized by a singlet antiferromagnetic ground-state (exchange couplings: J B3LYP (UU) ¼ À15.02 cm À1 and J M06L (UU) ¼ À50.15 cm À1 ), while the quintet state is the most stable for the all-siloxide. Assigning the multiplicity of the mixed-ligand compound is more delicate, since this compound is more sensitive to the choice of the density functional, which has to be able to capture simultaneously the asymmetry in the ligand environment between the two uranium centers and mimic the effects of state degeneracies (see ESI †). This complex results in a quintet with M06L and in a singlet anti-ferromagnetic state (exchange coupling: J B3LYP (UU) ¼ À9.44 cm À1 ) with B3LYP. Although the results with B3LYP are in line with experimental magnetic properties for all the complexes, the relatively small energy differences (see ESI †) between spin-states has to be taken with care.
Beside the differences in magnetic properties, Mayer bond order (MBO) analysis 22 on the three complexes reveals a correlation between the increased reactivity of the complexes and a decreased bond-order between the uranium centers and the bridging nitrogen, despite the relatively small magnitude of the bond-order differences (Fig. 7 top panel). An opposite trend is found when considering the bond order between the uranium centers and the non-bridging ligands, suggesting that the modulation of the reactivity is linked to the relative strength of the O-U  bond, compared to the N-U (Fig. 7 bottom panel). The overall reduced bond order between the bridging nitrogen and the uranium centers in the presence of oxygen-based ligands indicate an increased localization of the electron density on the bridging nitrogen and a consequent increase of its nucleophilic character.

Conclusions
In summary, four new examples of diuranium nitride bridged complexes have been prepared and structurally characterized, the siloxide supported [NBu 4 ]-1, the amide supported 2 and 4, and the amide siloxide 5. The previously reported amide methanide complex 3 8f demonstrated to be a useful precursor for the preparation of the mixed-ligand amide siloxide complex 5. The small molecule reactivity of the nitride group in the [NBu 4 ]-1, 2, 3 and 5 has been investigated with CO, CO 2 , and H 2 . Although [Cs]-1 and [NBu 4 ]-1 show differences in structure, their reactivity is similar. Both complexes convert CO to CN À , react with CO 2 to give dicarbamate (N(CO 2 ) 2 ) 3À , and react with H 2 to yield imide hydride complexes. The only observed effect of the presence of an inner sphere cesium cation is that reversible H 2 cleavage is observed in the Cs + case versus NBu 4 + where it is irreversible.
The overall reactivity of the described nitride complexes can be summarized according to the ligand; the all-amide complexes 2 and 4 are unreactive with CO, CO 2 , and H 2 whereas the all-OSi(O t Bu) 3 complexes 1 react with all of these substrates. The heteroleptic complexes 3 and 5 react with CO and CO 2 but not H 2 . Complexes 1, 3, and 5 convert of CO to CN À but their reactivity with CO 2 is different; the all-OSi(O t Bu) 3 1 convert CO 2 to dicarbamate, whereas the mixed ligand complexes 3 and 5 favor NCO À formation. Finally, only the all-OSi(O t Bu) 3 complexes 1 react with H 2 and give imide/hydride complexes.
These results demonstrate that the all-OSi(O t Bu) 3 ligand environment in U 4+ -U 4+ nitride complexes allows reactivity that is not observed when amide ligands are used, (e.g., dicarbamate formation and reversible and irreversible H 2 heterolysis), and highlights the unique chemistry that can be obtained in this coordination environment. The all-OSi(O t Bu) 3 ligand environment provides nitrides that are highly nucleophilic and more reactive than those with amide ligands. In contrast, the allsilylamide ligand promotes magnetic exchange coupling but gives less reactive complexes. Computational analysis at the DFT level shows that increasing reactivity of the complexes correlates with a decrease in bond order of the bridging nitride, as well as with an increase in the bond order of the ancillary ligands.

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