Supramolecular Assemblies in Pb(II) Complexes with Hydrazido-Based Ligands

: Herein, we describe the synthesis and single crystal X-ray di ﬀ raction characterization of several Pb(II) complexes using Schi ﬀ base hydrazido-based ligands and di ﬀ erent counterions (NO 3 − , I − and ClO 4 ). In the three complexes reported in this work, the lead(II) metal exhibits a high coordination number (n > 8) and thus it is apparently not involved in tetrel bonding interactions. Moreover, the aromatic ligands participate in noncovalent interactions that play an important role in the formation of several supramolecular assemblies in the solid state of the three Pb(II) complexes. These assemblies have been analyzed by means of Hirshfeld surface analysis and DFT calculations. radii of the two atoms to the surface respectively. The 2D ﬁngerprint plot displays the summary of available contacts in the crystal [42–46] that are generated by binning ( d e , d i ) pairs in the interval of 0.01 Å. The 2D histogram was produced as a function of the fraction of surface points in that bin (essentially a pixel) that varies from few points to many points i.e., from blue through green to red. The Hirshfeld surfaces are plotted with d norm using red-white-blue color scheme, where red color indicates shorter contacts, white shows the contacts around the vdW separation, and blue designate longer contacts. Additionally, another colored property of the surface named “shape-index” has been discussed in this manuscript. Shape-index is a measure of “which shape”, and extremely sensitive in low curvature areas. The Hirshfeld surface calculation was performed using the program Crystal Explorer 3.1 . [47].


Introduction
The large radius of lead(II) is responsible of its versatile coordination chemistry and its coordination number usually ranges from four to nine. Consequently, it has been used for the preparation of new hybrid inorganic-organic compounds, polymers and complexes [1][2][3][4]. It is well-known that the utilization of lead(II) has negative implications related to environmental issues and health; however, it used for the synthesis of materials with interesting properties like semiconductors, NLO (non-linear optical) and ferroelectric materials [5][6][7][8]. In fact, nowadays the technical, economic and social relevance of Pb is beyond all doubt [9].

Materials
Ligands were synthesized following the protocol previously published by us [24].

Synthesis of the Crystals 1-3
In order to grow single-crystals, we used a special glassware that consists in a branched tube designed by us [31] for doing the reaction and crystallization in a single step (see reference [31] for a detailed explanation). The detailed synthesis of 1 is explained here. Complexes 2 and 3 were synthesized using the same synthetic protocol, utilizing Pb(NO 3 ) 2 and L 1 for 2 and PbI 2 and HL 2 for 3.
Synthesis of complex 1: Pb(NO 3 ) 2 , L 1 and NaClO 4 (0.164 g, 0.500 mmol; 0.113 g, 0.500 mmol; and 0.061 g, 0.500 mmol; respectively) were introduced in the special glassware (main arm of the tube). Both arms of the branched tube were filled with 15 ml of MeOH and the main arm introduced in an oil bath at 60 • C and the branched arm was preserved at room temperature. In the branched arm, crystals of 1 were isolated after 6 days. The crystals were filtered off and, subsequently, washed with CH 3 COCH 3 and Et 2 O, and finally dried in air.

Characterization
X-ray Diffraction analyses. Single crystal X-ray diffraction of compounds (1-3) were collected on Bruker APEX-II CCD diffractometer at 293(2) K with MoKα radiation (λ = 0.71073 Å). The Bruker SAINT [32] program was used for data reduction and by using multi-scan method [33], the absorption correction was applied. The structure of (1-3) were solved by using the program SHELXS-14 [34] and refined by SHELXL-18 [35]. All hydrogen atoms were located at their geometrically perfect positions and an isotropic refinement was employed. All calculations were performed using the programs WinGX system V2014.1 [36] and PLATON [37]. The details of the crystal data and structure refinement factors are included in Table 1. (CCDC 1919248-1919250) contain the supplementary crystallographic data of compounds (1-3) respectively.

Hirshfeld Surface Analysis
Hirshfeld surface (H-S) [38][39][40] is generated based on electron distribution of investigating molecule. H-S is distinctive [41] for every crystal structure. The normalized contact distance (d norm ) calculated by Equation (1) is used to locate both inner and outer intermolecular interactions simultaneously on a single Hirshfeld surface [38]. Where, d e and d i are the distances from the point to the nearest nucleus external and internal to the surface respectively; r i vdw and r e vdw are the internal and external van der Walls (vdW) radii of the two atoms to the surface respectively. The 2D fingerprint plot displays the summary of available contacts in the crystal [42][43][44][45][46] that are generated by binning (d e , d i ) pairs in the interval of 0.01 Å. The 2D histogram was produced as a function of the fraction of surface points in that bin (essentially a pixel) that varies from few points to many points i.e., from blue through green to red. The Hirshfeld surfaces are plotted with d norm using red-white-blue color scheme, where red color indicates shorter contacts, white shows the contacts around the vdW separation, and blue designate longer contacts. Additionally, another colored property of the surface named "shape-index" has been discussed in this manuscript. Shape-index is a measure of "which shape", and extremely sensitive in low curvature areas. The Hirshfeld surface calculation was performed using the program Crystal Explorer 3.1. [47].

Computational Details
For the calculations we have used the X-ray coordinates where only the position of the H-atoms were optimized using DFT calculations at the PBE0 [48]-D [49]/def2-SVP [50] level of theory. Gaussian-09 [51] program package was used for the calculations. The def2-svp basis set that uses ECPs (effective core potentials) [52] for Pb and I atoms. Table 1. Crystallographic data and structure refinement factors of (1-3).

Structural Analysis
The molecular view of compound (1) is included in Figure 2 with partial atom-numbering. Four nitrogen and two oxygen atoms from hydrazide and 2-pyridinyl groups of two ligands form a distorted PbN 4 O 2 chromophore. The oxygen atom from the nitrate anion occupies the seventh coordination site with a distance of 2.844(2)Å ( Table 2). The N(4A) atom of the ligand occupies eighth coordination site. The long Pb-N(4A) and Pb-O3 coordination bonds (>2.8 Å) can be also described as strong (noncovalent) tetrel bonds instead of coordination bonds. It fact, these distances are approximately in the middle of ΣR cov (sum of covalent radii: 2.12 for Pb + O and 2.17 Pb + N) and ΣR vdw (sum of van der Waals radii: 3.54 for Pb + O and 3.57 Pb + N). Based on this simple geometric criterion, both interactions can be either described as tetrel bonds or weak coordination bonds. Therefore, in case of compound 1, where the Pb-N(4A) and Pb-O3 distances are significantly longer than ΣR cov , the structure can be also defined as a mononuclear [Pb(L1) 2 ][NO 3 ]·[ClO 4 ] hemidirected (n = 6) complex self-assembled by two strong Pb· · · N4 tetrel bonds.  (1) with partial atom numbering. The unlabeled counterpart is produced through the symmetry (1 − x, −y, 1−z). Perchlorate anion is not included for clarity. Ellipsoids are drawn at 30% probability. Table 2. Selected bond lengths (Å) and bond angles ( • ).
The ORTEP view of compound (2) is shown in Figure 5 with partial atom numbering. The compound (2) crystallized in space group P-1 in which the asymmetric unit consists of half molecular moiety. The full moiety is generated by the symmetry operation of an inversion center. The Pb atom in the asymmetric unit is located on the inversion center at (0, 0, 1 2 ) and possesses an octahedral coordination whose equatorial planes are generated by one oxygen and one nitrogen atoms from the ligand and their symmetry related counterparts of another ligand. Two iodine atoms (I1 and I1*. * = −x, 2 − y, 1 − z) occupy the trans axial positions, consequently forming a PbO 2 N 2 I 2 chromophore. The apical Pb-I bond length is much longer in comparison to the equatorial bond lengths. A second nitrogen atom N(3) from the ligand lies at a longer distance (2.917Å) than the equatorial Pb(1)-N(1) bond-length (Table 2). In this complex both Pb(1)-N(1) and Pb(1)-N(3) are significantly longer than ΣR cov . values and closer to ΣR vdw values, and thus they can be also considered as tetrel bonds.  (2) with partial atom numbering. The unlabeled counterpart is produced through the symmetry (−x, 2 − y, 1 − z). Thermal ellipsoids have been generated at 30% probability.

Hirshfeld Surface Analysis
Following the pattern of the solid-state structure of all compounds, we were interested to quantify the noncovalent interactions. In this investigation, the contacts responsible in building supramolecular assemblies are evaluated. The Hirshfeld surface analysis of the investigating compounds are performed and mapped over d norm (Figure 9, left column) in the range (−0.474 to 1.891Å), (−0.471 to 1.154Å), and (−1.295 to 1.522Å) for (1), (2) and (3) respectively. The shape-index plot of all three compounds are shown in Figure 9 (middle column). The scattered points of the fingerprint plots (Figure 9, right column) evidence all contacts that are exhibited by the structures. The symmetry-generated counterpart of the structures were not considered for this calculation. Therefore, the large depression on the metal centers and the bridging N/O atoms are due to the symmetry generated counterpart of the structure. For instance, the Pb· · · N/N· · · Pb interaction is evidenced by dual discrete spikes in the breakdown fingerprint plot ( Figure S3) of compound (1) since one nitrogen atom binds with another Pb atom through the inversion center whereas for compounds (2) and (3), only one spike is evident (Figures S4 and S5). That means there exists both Pb· · · N (0.7%) and N· · · Pb (0.7%) interaction in compound (1) but for compounds (2) and (3), only Pb· · · N interaction exists. There is no signature of N/O· · · Pb interactions in compounds (2) and (3). Due to the variety of the bridging mode, the contribution are different and the Pb· · · N/N· · · Pb interaction comprises 1.4%, 2.7%, and 1.4% of the total Hirshfeld surface area of compound (1), (2), and (3) respectively. The I· · · H/H· · · I contacts in compound (2) are evidenced by two distinct spikes on the fingerprint plot, where I· · · H interaction contributed more (12.1%) compare to the H· · · I counterpart (4.7%). It is clearly evident that the spikes in the breakdown fingerprint plots correspond to the O· · · H/H· · · O interactions that are very piercing and distinct for compound (1) ( Figure S3) whereas no such distinct spikes are evidenced ( Figure S4) for compound (2) and compound (3) exhibits one sharp spike ( Figure S5). The spikes in different regions of the fingerprint plot comprises 43.5% in (1), only 5.6% in (2), and 25.7% in (3). The N· · · H/H· · · N contribution is also higher in compound (1) compare to compounds (2) and (3) (see Figures S3-S5). In all the structures, the N· · · H contact contributed more than it's H· · · N counterpart. Similarly, we have analyzed and quantified all contacts that are exhibited by the structures are included in Figures S3-S5. Following the structural description, we have analyzed π-π contacts by inspecting shape-index surfaces (Figure 9). The adjacent red and blue triangles on the shape-index surface that are marked by the red circle are characteristic of π-π stacking contacts [56,57]. The π-π interaction contributed 4.6% in (1), 5.9% in (2), and 2.6% in (3) that are evidenced by breakdown fingerprint plots. The H· · · H contacts comprised of 16.0% in (1), 40.1% in (2), and 34.4% in (3) that are shown in Figures S3-S5).

Theoretical Study
This study is intended to analyze the π-π and anion-π interactions observed in the crystal packing of the Pb(II) compounds.
For compound 1, we have first analyzed the anion-π interaction described above (see Figure 4). A careful inspection of the structure shows that the interaction of perchlorate anion with the ligand is ditopic. That is, one O-atom is over the coordinated pyridine ring of the nicotine moiety and another one with shortest distance (3.08 Å, see Figure 10) is located over the C atom of the hydrazido group (π-hole interaction) [58,59]. This C atom is a good π-hole donor [60] due to the adjacent Pb-O coordination bond that enhances the π-acidity of C. We have computed the interaction energy of the model shown in Figure 10a, which is very large (∆E 1 = −173.9 kcal/mol) because of the significant electrostatic attraction between the dicationic [Pb 2 (L 1 ) 4 (NO 3 ) 2 ] 2+ moiety and the anions. To evaluate the importance of the O· · · π-hole contact we have utilized a model where chlorate anion instead of perchlorate was used (see Figure 10b). By doing so, the interaction energy weakens to ∆E 2 = −156.1 kcal/mol and, consequently, the contribution of each O· · · π-hole interaction can be estimated by difference, which is −8.9 kcal/mol [ 1 2 *(∆E 1 − ∆E 2 )]. This interaction energy is consistent with other π-hole interactions [60]. In complex (2) the π-π interactions previously described in Figure 6 have been studied, which are crucial in the crystal packing of this complex. In Figure 11, we show the self-assembled dimer used to evaluate the interaction energetically. The resulting binding energy is strong (∆E 3 = −26.5 kcal/mol) due to the antiparallel arrangement of the pyridine rings and the coordination of the ligand to the Pb(II) that enhances the dipole-dipole interaction. Finally, in compound (3) we have studied the intricate combination of interactions that are established between the perchlorate anion and the complex (see Figure 12). The four O-atoms of the anion are involved in several contacts, which are OH· · · O H-bonding interactions with the coordinated H 2 O ligand, CH· · · O H-bonds with the aromatic H-atoms and finally, an interesting O· · · chelate ring (CR) interaction. The latter interaction has been previously described in the literature, where the chelate rings are able to interact favorably with anions or lone pair donor atoms [61]. We have computed the binding energy for the interaction of one perclorate (we have considered the system as binary, [Pb(L 3 ) 2 (H 2 O)][(ClO 4 )]· · · [(ClO 4 )]), which is very large due to the pure electrostatic attraction between the positive Pb(II) complex and the anion, ∆E 4 = −79.5 kcal/mol. To evaluate the strength of the OH· · · O H-bond, another model has been computed where the coordinated water molecule has been eliminated. Therefore, the H-bond is not formed and the interaction energy weakens to ∆E 5 = −72.1 kcal/mol and, consequently the H-bond contributes in −7.4 kcal/mol.

Conclusions
The synthetic protocol to obtain three new Pb(II) coordination compounds involving nicotinhydrazido and picolinhydrazido-ligands and I − and ClO 4 − counter anions is reported in this manuscript. The complexes were characterized in the solid state by X-ray diffraction. In compound (1), the perchlorate interacts with the organic ligands via ditopic anion-π and O· · · π-hole interactions. These interactions have been analyzed theoretically using the DFT-D method. Moreover, in compound (3), the anions interact with the Pb-complex through a network of OH· · · O and CH· · · O H-bonding interactions and also unconventional anion· · · CR (chelate ring interactions). Finally, in compound (2), we have studied the strong and antiparallel pyridine-pyridine stacking assembly that is reinforced by their coordination to Pb(II).

Funding:
The publication was prepared with the support of the RUDN University Program 5-100. This research was funded by MINECO/AEI from Spain, project number CTQ2017-85821-R FEDER funds.