Tetrel Bonding and Other Non-Covalent Interactions Assisted Supramolecular Aggregation in a New Pb(II) Complex of an Isonicotinohydrazide

A new supramolecular Pb(II) complex [PbL(NO2)]n was synthesized from Pb(NO3)2, N’-(1-(pyridin-2-yl)ethylidene)isonicotinohydrazide (HL) and NaNO2. [PbL(NO2)]n is constructed from discrete [PbL(NO2)] units with an almost ideal N2O3 square pyramidal coordination environment around Pb(II). The ligand L− is coordinated through the 2-pyridyl N-atom, one aza N-atom, and the carbonyl O-atom. The nitrite ligand binds in a κ2-O,O coordination mode through both O-atoms. The Pb(II) center exhibits a hemidirected coordination geometry with a pronounced coordination gap, which allows a close approach of two additional N-atoms arising from the N=C(O) N-atom of an adjacent molecule and from the 4-pyridyl N-atom from the another adjacent molecule, yielding a N4O3 coordination, constructed from two Pb–N and three Pb–O covalent bonds, and two Pb⋯N tetrel bonds. Dimeric units in the structure of [PbL(NO2)]n are formed by the Pb⋯N=C(O) tetrel bonds and intermolecular electrostatically enforced π+⋯π− stacking interactions between the 2- and 4-pyridyl rings and further stabilized by C–H⋯π intermolecular interactions, formed by one of the methyl H-atoms and the 4-pyridyl ring. These dimers are embedded in a 2D network representing a simplified uninodal 3-connected fes (Shubnikov plane net) topology defined by the point symbol (4∙82). The Hirshfeld surface analysis of [PbL(NO2)] revealed that the intermolecular H⋯X (X = H, C, N, O) contacts occupy an overwhelming majority of the molecular surface of the [PbL(NO2)] coordination unit. Furthermore, the structure is characterized by intermolecular C⋯C and C⋯N interactions, corresponding to the intermolecular π⋯π stacking interactions. Notably, intermolecular Pb⋯N and, most interestingly, Pb⋯H interactions are remarkable contributors to the molecular surface of [PbL(NO2)]. While the former contacts are due to the Pb⋯N tetrel bonds, the latter contacts are mainly due to the interaction with the methyl H-atoms in the π⋯π stacked [PbL(NO2)] molecules. Molecular electrostatic potential (MEP) surface calculations showed marked electrostatic contributions to both the Pb⋯N tetrel bonds and the dimer forming π+⋯π− stacking interactions. Quantum theory of atoms in molecules (QTAIM) analyses underlined the tetrel bonding character of the Pb⋯N interactions. The manifold non-covalent interactions found in this supramolecular assembly are the result of the proper combination of the polyfunctional multidentate pyridine-hydrazide ligand and the small nitrito auxiliary ligand.


Results and Discussion
The reaction of a mixture of Pb(NO3)2 and HL with NaNO2 in MeOH led to a new supramolecular heteroleptic coordination compound [PbL(NO2)]n (Scheme 1). The compound was isolated as a crystalline air-and moisture-stable solid that was characterized through elemental analysis, FTIR spectroscopy, single-crystal X-ray diffraction, and Hirshfeld surface analysis.
The FTIR spectrum of [PbL(NO2)]n contains characteristic bands for the C=O and C=N bonds at 1632 and 1590 cm −1 , respectively. The methyl C-H fragments showed resonances at 2921 cm −1 .
Compound [PbL(NO2)]n crystallized in the monoclinic space group P21/n, with one coordination unit [PbL(NO2)] in the asymmetric unit. The deprotonated L − and the nitrito NO2 − ligands were covalently bound to the metal center, yielding an almost ideal N2O3 square pyramidal coordination environment around the Pb(II) atom (Figure 1), as evidenced from the so-called τ5-descriptor of about 0.08 (Table 1) [43]. The ligand L − is tridentately coordinated through the Scheme 1. Syntheses of [PbL(NO 2 )] n (this work) and [PbL'(NO 2 )] n [37] (tetrel bonds shown as dashed lines).
In view of the interesting structure of the previously described nitrito nicotinohydrazide compound [Pd(L'(NO 2 )] n [37], we reacted a mixture of Pb(NO 3 ) 2 and 1-(pyridin-2-yl)ethylideneisonicotinohydrazide (HL) with NaNO 2 and received the corresponding isonicotino isomer [PbL(NO 2 )] n , which turned out to have a completely different structure compared with [PbL'(NO 2 )] n (Scheme 1). The nature and energetic features of the two Pb···N tetrel bonds were studied by density functional theory (DFT) and molecular electrostatic potential (MEP) calculations, which demonstrate the presence of a σ-hole at the Pb(II) ion.

Results and Discussion
The reaction of a mixture of Pb(NO 3 ) 2 and HL with NaNO 2 in MeOH led to a new supramolecular heteroleptic coordination compound [PbL(NO 2 )] n (Scheme 1). The compound was isolated as a crystalline air-and moisture-stable solid that was characterized through elemental analysis, FTIR spectroscopy, single-crystal X-ray diffraction, and Hirshfeld surface analysis.
The FTIR spectrum of [PbL(NO 2 )] n contains characteristic bands for the C=O and C=N bonds at 1632 and 1590 cm −1 , respectively. The methyl C-H fragments showed resonances at 2921 cm −1 .
Compound [PbL(NO 2 )] n crystallized in the monoclinic space group P2 1 /n, with one coordination unit [PbL(NO 2 )] in the asymmetric unit. The deprotonated L − and the nitrito NO 2 − ligands were covalently bound to the metal center, yielding an almost ideal N 2 O 3 square pyramidal coordination environment around the Pb(II) atom (Figure 1), as evidenced from the so-called τ 5 -descriptor of about 0.08 (Table 1) [43].    This structure is completely different from that of the previously reported nicotinohydrazide isomer [PbL'(NO2)]n [37]. In this coordination polymer, Pb(II) shows a distorted, non-octahedral six-fold coordination in which the pending 3-pyridyl N-atom bridges between two metals (Scheme 1 and Table 1). The
The Pb-N bond lengths in [PbL(NO 2 )] n are 2.462(4) and 2.583(4) Å, with Pb-N(N) < Pb-N 2-Py . The Pb-O distance with the carbonyl oxygen atom is shorter and of 2.387(3) Å. These values are in line with data from related hydrazide Pb(II) complexes [32,35,[37][38][39]42]. The same bonds with the nitrite oxygen atoms differ significantly. Particularly, while one of the bonds has the same length as that formed with the carbonyl O atom, the second bond with 2.768(4) Å is the longest within the covalent bonds formed by Pb(II) in the structure of [PbL(NO 2 )] n .
The covalent bond distances around the Pb(II) atom in [PbL(NO 2 )] n are very similar to the nicotinohydrazide isomer [PbL'(NO 2 )] n [37], and, in both cases, the corresponding organic ligand is almost completely planar, as can be seen from the torsion angles of only about 3 • between the mean planes formed by the pyridyl rings ( Table 1).
The covalent bonds in the central coordination unit are markedly concentrated on one hemisphere of the coordination environment (Figure 1), representing a hemidirected coordination geometry with a pronounced coordination gap, due to the 6s 2 lone pair. This allows for two additional N-atoms from adjacent molecules, the aza and the 4-pyridyl N-atoms, to approach the Pb(II) and form two Pb· · · N tetrel bonds ( Figure 2). The Pb· · · N=C(O) tetrel bond (3.235(4) Å) is significantly longer than the sum of the covalent radii (2.17 Å) and shorter than the sum of van der Waals radii (3.57 Å), thus supporting its non-covalent nature. It leads to the formation of centrosymmetric dimers [PbL(NO 2 )] 2 ( Figure 2), which are further reinforced by bilateral π· · · π stacking interactions, formed between the 2and 4-pyridyl rings ( Figure 2 and Table 2). These dimers are interlinked through bilateral C-H· · · π intermolecular interactions, formed by one of the methyl H-atoms and the 4-pyridyl ring ( Figure 2 and Table 3). The slightly shorter Pb· · · N 4-Py tetrel bond (3.039(5) Å) leads to a supramolecular aggregation of the dimers into a 2D layer structure (Figure 3). From a topological perspective, this 2D metal-organic layer in [PbL(NO 2 )] n is assembled from the 3-connected Pb(II) nodes and 3-connected L linkers and can be classified as a uninodal 3-connected fes (Shubnikov plane net) [44] topology defined by the point symbol (4·8 2 ). Thus, the overall topology of [PbL(NO 2 )] n is a supramolecular 3D framework, formed by multiple non-covalent interactions, such as Pb· · · N tetrel bonds, π· · · π stacking, and C-H· · · π interactions.  [37], and, in both cases, the corresponding organic ligand is almost completely planar, as can be seen from the torsion angles of only about 3° between the mean planes formed by the pyridyl rings ( Table 1).
The covalent bonds in the central coordination unit are markedly concentrated on one hemisphere of the coordination environment (Figure 1), representing a hemidirected coordination geometry with a pronounced coordination gap, due to the 6s 2 lone pair. This allows for two additional N-atoms from adjacent molecules, the aza and the 4-pyridyl N-atoms, to approach the Pb(II) and form two Pb⋯N tetrel bonds ( Figure 2). The Pb⋯N=C(O) tetrel bond (3.235(4) Å) is significantly longer than the sum of the covalent radii (2.17 Å) and shorter than the sum of van der Waals radii (3.57 Å), thus supporting its non-covalent nature. It leads to the formation of centrosymmetric dimers [PbL(NO2)]2 ( Figure 2), which are further reinforced by bilateral π⋯π stacking interactions, formed between the 2-and 4-pyridyl rings ( Figure 2 and Table 2). These dimers are interlinked through bilateral C-H⋯π intermolecular interactions, formed by one of the methyl H-atoms and the 4-pyridyl ring ( Figure 2 and Table 3). The slightly shorter Pb⋯N4-Py tetrel bond (3.039(5) Å) leads to a supramolecular aggregation of the dimers into a 2D layer structure ( Figure 3). From a topological perspective, this 2D metal-organic layer in [PbL(NO2)]n is assembled from the 3-connected Pb(II) nodes and 3-connected L linkers and can be classified as a uninodal 3-connected fes (Shubnikov plane net) [44] topology defined by the point symbol (4•8 2 ). Thus, the overall topology of [PbL(NO2)]n is a supramolecular 3D framework, formed by multiple non-covalent interactions, such as Pb⋯N tetrel bonds, π⋯π stacking, and C-H⋯π interactions.  ] n , formed through Pb· · · N=C(O) tetrel bonds, π· · · π stacking, and C-H· · · π interactions (ellipsoids are drawn at 50% probability level). Color code: H = black, C = gold, N = blue, O = red, and Pb = magenta; cyan dashed line = Pb· · · N tetrel bond, yellow dashed line = π· · · π stacking interaction, and gray dashed line = C-H· · · π interaction.   Table 3. C-H· · · π ring interactions distances (Å) and angles ( • ) for [PbL(NO 2 )] n a .

C-H(I) d[C-H(I)]
Cg (   This stands in complete contrast to the previously reported nicotinohydrazide isomer [PbL'(NO2)]n, in which a similar Pb⋯N=C(O) tetrel bond and a very dissimilar Pb⋯ONO tetrel bond lead to dimers, which are interconnected through the Pb-N3-Py coordinative bond, thus forming a zigzag polymeric structure and only one remarkable π⋯π stacking interaction with a centroidcentroid distance of 3.615(3) Å [37].
To further examine the intermolecular interactions in the crystal of [PbL(NO2)]n, we used a Hirshfeld surface analysis [45]. This analysis provides an excellent overview over all intermolecular interactions originating from a species, regardless of its nature. Together with the analysis of the nature of these interactions (XRD data, molecular electrostatic potential (MEP), binding energies from DFT, and quantum theory of atoms in molecules (QTAIM) calculations), it gives a complete picture, whose forces contribute to the molecular and crystal structure. The 2D fingerprint plots [46]  This stands in complete contrast to the previously reported nicotinohydrazide isomer [PbL'(NO 2 )] n , in which a similar Pb· · · N=C(O) tetrel bond and a very dissimilar Pb· · · ONO tetrel bond lead to dimers, which are interconnected through the Pb-N 3-Py coordinative bond, thus forming a zigzag polymeric structure and only one remarkable π· · · π stacking interaction with a centroid-centroid distance of 3.615(3) Å [37].
To further examine the intermolecular interactions in the crystal of [PbL(NO 2 )] n , we used a Hirshfeld surface analysis [45]. This analysis provides an excellent overview over all intermolecular interactions originating from a species, regardless of its nature. Together with the analysis of the nature of these interactions (XRD data, molecular electrostatic potential (MEP), binding energies from DFT, and quantum theory of atoms in molecules (QTAIM) calculations), it gives a complete picture, whose forces contribute to the molecular and crystal structure. The 2D fingerprint plots [46] of the basic coordination unit [PbL(NO 2 )] were generated, using CrystalExplorer 3.1 [47], to visualize proportions of the intermolecular interactions. Furthermore, we calculated the enrichment ratios (E) [48] of the intermolecular contacts to estimate the probability of two chemical species to be in contact. The Hirshfeld surface analysis showed that the intermolecular H···X (X = H, C, N, O) contacts occupy an overwhelming majority of the molecular surface of the basic coordination unit [PbL(NO 2 )] ( Figure 4 and Table 4). The shortest H···H contacts are shown in the corresponding 2D fingerprint plot at d e + d i ≈ 2.3 Å, and a clear splitting of the H···H fingerprint was found (Figure 4), which is due to the shortest contact being between three atoms, rather than for a direct two-atom contact [45]. The H···C contacts in the corresponding 2D plot of [PbL(NO 2 )] were found in the form of «wings», with the shortest being d e + d i ≈ 2.8 Å (Figure 4). This is characteristic for C-H· · · π type of interactions [45]. The H···N and H···O contacts are shown in the corresponding 2D fingerprint plots as two «horns», with the shortest being d e + d i ≈ 2.4-2.5 Å (Figure 4).
Molecules 2020, 25, x FOR PEER REVIEW 7 of 15 of the basic coordination unit [PbL(NO2)] were generated, using CrystalExplorer 3.1 [47], to visualize proportions of the intermolecular interactions. Furthermore, we calculated the enrichment ratios (E) [48] of the intermolecular contacts to estimate the probability of two chemical species to be in contact.
The Hirshfeld surface analysis showed that the intermolecular H•••X (X = H, C, N, O) contacts occupy an overwhelming majority of the molecular surface of the basic coordination unit [PbL(NO2)] ( Figure 4 and Table 4). The shortest H•••H contacts are shown in the corresponding 2D fingerprint plot at de + di ≈ 2.3 Å, and a clear splitting of the H•••H fingerprint was found (Figure 4), which is due to the shortest contact being between three atoms, rather than for a direct two-atom contact [45]. The H•••C contacts in the corresponding 2D plot of [PbL(NO2)] were found in the form of «wings», with the shortest being de + di ≈ 2.8 Å (Figure 4). This is characteristic for C-H⋯π type of interactions [45]. The H•••N and H•••O contacts are shown in the corresponding 2D fingerprint plots as two «horns», with the shortest being de + di ≈ 2.4-2.5 Å (Figure 4).    [47]. b The "enrichment ratios" were not computed when the "random contacts" were lower than 0.9%, as they are not meaningful [48].
Furthermore, the structure of [PbL(NO 2 )] is also characterized through intermolecular C···C and C···N interactions (Figure 4) observed as the area at d e = d i ≈ 1.7-2.0 Å. They correspond to intermolecular π· · · π stacking interactions between the pyridyl rings. Importantly, intermolecular Pb···N and, most interestingly, Pb···H interactions are also remarkable contributors into the molecular surface of [PbL(NO 2 )]. The former contacts are shown in the corresponding 2D fingerprint plot as two sharp «spikes», with the shortest being d e + d i ≈ 3.0 Å (Figure 4), and correspond to the Pb· · · N tetrel bonds. The latter contacts are mainly due to the interaction with the methyl H-atoms arising from an adjacent π· · · π stacked [PbL(NO 2 )] unit.
The 2D plot of [PbL(NO 2 )] exhibits points at large d e and d i (Figure 4), which are similar to those observed in the 2D plots of benzene [45] and have been observed in similar compounds [49][50][51][52][53][54]. They correspond to regions without any close contacts to adjacent molecules.
The favorable H···X (X = H, N, O, Pb) contacts in the structure of [PbL(NO 2 )] n show enrichment ratios E HH/HN/HO/PbH close to or even higher than unity ( Table 4). The H···C, C···N, and N···N contacts are much less favored, and they are consistent with smaller enrichment ratios of E HC/CN/NN = 0.82-0.87. This is explained by the presence of the highly probable C···C (E CC = 2.45) and Pb···N (E PbN = 3.46) contacts in the structure of the compound. Remaining contacts are discouraged with enrichment ratios ranging from 0.00 to 0.48 (Table 4).
As the Pb· · · N=C(O) tetrel bonding and the π· · · π stacking interactions are the dominating forces for the formation of the dimers in the solid state of [PbL(NO 2 )] n (Figures 2 and 3), we further analyzed the donor-acceptor properties of the basic coordination unit [PbL(NO 2 )] through DFT calculations.
First, we computed the molecular electrostatic potential (MEP) surface and found the highest positive values at the Pb(II) atom (+26.7 kcal/mol) representing the σ-hole ( Figure 5). The most negative part is located at the 4-pyridyl N-atom (−40.8 kcal/mol) of the ligand L − , which is more electron-richer than the nitrito N-and O-atoms (−31.4, −33.3, and −37.6 kcal/mol). Consequently, the Pb· · · N 4-Py tetrel bonding interaction is the most favored contact from an electrostatic point of view in line with the observed structure, d(Pb· · · N 4-Py ) = 3.039(5) Å ( Table 1). The second Pb· · · N=C(O) tetrel bond is much weaker (3.235(4) Å) and consistent with a significantly less negative value at the N=C(O) N-atom (−21.3 kcal/mol). From these MEP calculations, a relatively high probability of a Pb· · · ONO tetrel bond can be concluded. However, no such bond was observed in the structure of [PbL(NO 2 )] n . In contrast to this, in the nicotinohydrazide isomer [PbL'(NO 2 )] n , such a Pb· · · ONO tetrel bond was observed, although not very dominant and rather long (3.299(4) Å, Table 1).
Molecules 2020, 25, x FOR PEER REVIEW 9 of 15 line with the observed structure, d(Pb⋯N4-Py) = 3.039(5) Å (Table 1). The second Pb⋯N=C(O) tetrel bond is much weaker (3.235(4) Å) and consistent with a significantly less negative value at the N=C(O) N-atom (−21.3 kcal/mol). From these MEP calculations, a relatively high probability of a Pb⋯ONO tetrel bond can be concluded. However, no such bond was observed in the structure of [PbL(NO2)]n. In contrast to this, in the nicotinohydrazide isomer [PbL'(NO2)]n, such a Pb⋯ONO tetrel bond was observed, although not very dominant and rather long (3.299(4) Å, Table 1). An interesting feature of the surface is that the MEP values over the coordinated and non-coordinated pyridine ring centers represent opposite signs ( Figure 5). As a consequence, they are well suited to form strong, electrostatically enforced π + ⋯π -interactions [6] between the coordinated and non-coordinated pyridine rings of the Lligand, in line with the dimer-promoting stacking interactions found in [PbL(NO2)]n ( Figure 2 and Table 2).
The DFT-calculated binding energy of these antiparallel stacked dimers composed of electrostatically enforced π + ⋯π -and Pb⋯N=C(O) tetrel interactions ( Figure 6) is quite large (ΔE1 = −24.3 kcal/mol), in agreement with the MEP surface analysis. In contrast to this, the calculated interaction energy of the Pb⋯N4-Py tetrel bond is moderately strong (ΔE2 = −10.2 kcal/mol) and comparable to recently reported values [32,35]. When looking at the bond (red spheres), ring (yellow spheres), and cage (blue spheres) critical points (CPs), we found that the π + ⋯π − interaction is characterized by two bond CPs that interconnect two atoms of each pyridine ring ( Figure 6). The π⋯π stacking is further characterized by additional CPs that interconnect the chelate rings and more ring and cage CPs. Moreover, the two Pb⋯N=C(O) tetrel bonds are characterized by bond CPs and bond paths ( Figure 6). The Pb⋯N4-Py tetrel bond contributes, together with a C-H⋯O interaction, to the overall binding energy of entire assembly ( Figure 6). The energy associated to this C-H⋯O interaction was estimated by using the kinetic energy density predictor (E = 0.5 × Vr) to 1.3 kcal/mol. The Pb⋯N4-Py tetrel bond interaction is thus significantly stronger in keeping with the quite short experimentally observed distance of 3.039(5) Å. Thus, this tetrel bond dominates this type of intermolecular interaction and drives the formation of the uninodal 3-connected fes (Shubnikov plane net) topology. An interesting feature of the surface is that the MEP values over the coordinated and non-coordinated pyridine ring centers represent opposite signs ( Figure 5). As a consequence, they are well suited to form strong, electrostatically enforced π + · · · πinteractions [6] between the coordinated and non-coordinated pyridine rings of the Lligand, in line with the dimer-promoting stacking interactions found in [PbL(NO 2 )] n ( Figure 2 and Table 2).
The DFT-calculated binding energy of these antiparallel stacked dimers composed of electrostatically enforced π + · · · πand Pb· · · N=C(O) tetrel interactions ( Figure 6) is quite large (∆E 1 = −24.3 kcal/mol), in agreement with the MEP surface analysis. In contrast to this, the calculated interaction energy of the Pb· · · N 4-Py tetrel bond is moderately strong (∆E 2 = −10.2 kcal/mol) and comparable to recently reported values [32,35]. When looking at the bond (red spheres), ring (yellow spheres), and cage (blue spheres) critical points (CPs), we found that the π + · · · π − interaction is characterized by two bond CPs that interconnect two atoms of each pyridine ring ( Figure 6). The π· · · π stacking is further characterized by additional CPs that interconnect the chelate rings and more ring and cage CPs. Moreover, the two Pb· · · N=C(O) tetrel bonds are characterized by bond CPs and bond paths ( Figure 6). The Pb· · · N 4-Py tetrel bond contributes, together with a C-H· · · O interaction, to the overall binding energy of entire assembly ( Figure 6). The energy associated to this C-H· · · O interaction was estimated by using the kinetic energy density predictor (E = 0.5 × Vr) to 1.3 kcal/mol. The Pb· · · N 4-Py tetrel bond interaction is thus significantly stronger in keeping with the quite short experimentally observed distance of 3.039 (5)

Reagents
All reactants were used as received, without further purification. The ligand HL was synthesized by following the available protocol [37].

Synthesis of [PbL(NO2)]n
A solution of HL (0.024 g, 0.1 mmol) in MeOH (10 mL) was added dropwise to a solution of Pb(NO3)2 (0.033 g, 0.1 mmol), in the same solvent (10 mL). To this mixture, a solution of NaNO2 (0.014 g, 0.2 mmol) in MeOH (5 mL) was added slowly under stirring. The resulting mixture was stirred at room temperature for 30 min and was left undisturbed for slow evaporation. After about 4 days, yellow X-ray suitable single crystals were formed. Crystals were isolated by filtration. Yield: 0.038 g (78% based on Pb(NO3)2

Single-Crystal X-Ray diffraction of [PbL(NO2)]n
The data were collected with a Bruker AXS SMART APEX2 CCD (Bruker, Ettlingen, Germany) diffractometer, operating at 179(2) K and using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). Cell refinement, indexing, and scaling of the datasets were performed, using the program Bruker Smart Apex and Saint packages [55]. The structure was solved by direct methods and refined by full-matrix least-squares on F 2 with anisotropic displacement parameters for all non-hydrogen atoms, using the program SHELXL [56]. The H atoms were included as riding contributions with fixed isotropic displacement parameters in idealized positions. All the calculations were carried out by using the WinGX System, V2013.3 [57]. Crystal structure

Reagents
All reactants were used as received, without further purification. The ligand HL was synthesized by following the available protocol [37].

Single-Crystal X-Ray Diffraction of [PbL(NO 2 )] n
The data were collected with a Bruker AXS SMART APEX2 CCD (Bruker, Ettlingen, Germany) diffractometer, operating at 179(2) K and using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). Cell refinement, indexing, and scaling of the datasets were performed, using the program Bruker Smart Apex and Saint packages [55]. The structure was solved by direct methods and refined by full-matrix least-squares on F 2 with anisotropic displacement parameters for all non-hydrogen atoms, using the program SHELXL [56]. The H atoms were included as riding contributions with fixed isotropic displacement parameters in idealized positions. All the calculations were carried out by using the WinGX System, V2013.3 [57]. Crystal structure determination and refinement data are given in Table 5. CCDC-2014301 contains the supplementary crystallographic data for this paper (Supplementary Materials). These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk).

DFT Calculations
Gaussian-16 package [58] was used to perform the density functional theory calculations reported herein. In particular, the PBE1PBE-D3 functional [59,60], in combination with the def2-TZVP basis set [61,62], was employed for computing the binding energies and the molecular electrostatic potential (MEP) surfaces. The crystallographic coordinates were used for the calculations. The 0.001 a.u. isosurface was used for mapping the MEP onto the van der Waals surface. The QTAIM analysis [63] was carried out at the same level of theory and using the program AIMAll [64].