Molecular, Supramolecular Structures Combined with Hirshfeld and DFT Studies of Centrosymmetric M(II)-azido {M=Ni(II), Fe(II) or Zn(II)} Complexes of 4-Benzoylpyridine

: The supramolecular structures of the three metal (II) azido complexes [Fe(4bzpy) 4 (N 3 ) 2 ]; 1 , [Ni(4bzpy) 4 (N 3 ) 2 ]; 2 and [Zn(4bzpy) 2 (N 3 ) 2 ] n ; 3 with 4-benzoylpyridine ( 4bzpy ) were presented. All complexes contain hexa-coordinated divalent metal ions with a slightly distorted octahedral MN 6 coordination sphere. Complexes 1 and 2 are monomeric with terminal azido groups while 3 is one-dimensional coordination polymer containing azido groups with µ (1,1) and µ (1,3) bridging modes of bonding. Hirshfeld analysis was used to quantitatively determine the different contacts affecting the molecular packing in the studied complexes. The most common interactions are the polar O . . . H and N . . . H interactions and the hydrophobic C . . . H contacts. The charges at the M(II) sites are calculated to be 1.004, 0.847, and 1.147 e for complexes 1–3 , respectively. The degree of asymmetry is the highest in the case of the terminal azide in complexes 1 and 2 while was found the lowest in the µ (1,1) and µ (1,3) azide bonding modes in the Zn(II) complex 3 . These facts were further explained in terms of atoms in molecules (AIM) topological parameters. in metal complexes.


Introduction
In supramolecular solid-state chemistry of metal-organic complexes, the framework constructions are based on the arrangement of the molecular building blocks by strong hydrogen bonds or dative coordination bonds. An important class of supramolecular chemistry known as metallosupramolecular chemistry which is based upon the self-assembly of metal ions and organic ligands [1,2]. Metallosupramolecular chemistry has an important contribution to the spectacular development of crystal engineering [3][4][5][6][7][8][9][10]. Self-assembly is one of the most appropriate synthetic routes to build metallosupramolecular structures with interesting multidimensional and topological structures [11][12][13][14][15][16][17][18][19][20]. These can be achieved by proper selection of the suitable metal ion, ligand, and co-ligand [21,22]. The coordination number and geometry, charge, HSAB behavior of the metal ion as well as the denticity, shape, size, HSAB behavior of the ligands are vital factors in controlling the desired network topology [21,22]. Azide ion N 3 is linear and symmetric with equal N-N distances; the covalent azides are linear but asymmetric with unequal N-N distances [23]. The importance of the coordinated azides is due to the capability of the azide group to have different modes of bonding as it acts as a terminal ligand (mono-dentate) or bridging ligand with several types of bonding modes (Scheme 1). Furthermore, the azide complexes may contain more than one of these modes of bonding in the same compound. Dori and Ziolo [24] have divided the azide complexes into three main groups (terminal, end-on bridging, end-to-end bridging). Hence, the azide anion is a versatile ligand that can bridge metal centers [25]. The coordinated azide was found to be linear and asymmetric in the first investigated complex [Co(NH 3 ) 5 N 3 ](N 3 ) 2 of the terminal mode of bonding [26]. Now, many azido complexes have been investigated showing different types of bonding modes. In light of the interesting coordination behavior of azide-containing complexes, the aim of the present work is to shed light on the structural diversity of three metal-azido complexes (Ni(II), Fe(II), and Zn(II)) with 4-benzoylpyridine (4bzpy) as N-donor organic ligand. The molecular and supramolecular structure features of these complexes were also investigated and discussed.
Symmetry 2021, 13, x FOR PEER REVIEW 2 of 14 coordinated azides is due to the capability of the azide group to have different modes of bonding as it acts as a terminal ligand (mono-dentate) or bridging ligand with several types of bonding modes (Scheme 1). Furthermore, the azide complexes may contain more than one of these modes of bonding in the same compound. Dori and Ziolo [24] have divided the azide complexes into three main groups (terminal, end-on bridging, end-to-end bridging). Hence, the azide anion is a versatile ligand that can bridge metal centers [25]. The coordinated azide was found to be linear and asymmetric in the first investigated complex [Co(NH3)5N3](N3)2 of the terminal mode of bonding [26]. Now, many azido complexes have been investigated showing different types of bonding modes. In light of the interesting coordination behavior of azide-containing complexes, the aim of the present work is to shed light on the structural diversity of three metal-azido complexes (Ni(II), Fe(II), and Zn(II)) with 4-benzoylpyridine (4bzpy) as N-donor organic ligand. The molecular and supramolecular structure features of these complexes were also investigated and discussed. Scheme 1. Coordination modes of azide ligands in metal complexes.

Materials and Instrumentation
The CHN analyses were carried out using a Perkin-Elmer analyzer. The Ni, Fe, and Zn were analyzed by a Perkin-Elmer Analyst 300, AAS atomic absorption spectrophotometer. The 4-benzoylpyridine ligand was purchased from Aldrich Company and other chemicals were of analytical grade quality and used without further purification.

Materials and Instrumentation
The CHN analyses were carried out using a Perkin-Elmer analyzer. The Ni, Fe, and Zn were analyzed by a Perkin-Elmer Analyst 300, AAS atomic absorption spectrophotometer. The 4-benzoylpyridine ligand was purchased from Aldrich Company and other chemicals were of analytical grade quality and used without further purification.

Computational Details
With the aid of the Gaussian 09 software package [33], natural charge calculations [34] were performed using WB97XD and MPW1PW91 methods [35,36] at the X-ray structure coordinates and employing the TZVP basis sets. Atoms in molecules (AIM) [37] topology analyses were performed using the Multiwfn program [38]. The structure of 1 comprised two intramolecular C-H...N interactions with donor-acceptor distances of 3.254(2) and 3.123(2) Å for the C22-H22…N2 and C26-H26…N1 interactions, respectively (Table 3). In addition, the neutral complex units are connected with each other via weak C-H…O interactions with a donor-acceptor distance of 3.419(2) Å. Presentation of the inter-and intra-molecular contacts, as well as the molecular packing, is shown in Figure 1B,C, respectively. Similar to 1, the [Ni(4bzpy)4(N3)2]; 2 is a neutral complex comprising the central Ni(II) ion coordinated to four 4bzpy molecules and two terminally coordinated azide ions in trans positions ( Figure 2). Moreover, complex 2 possesses an inversion center located at the Ni atom, hence the asymmetric unit comprised half molecule. The

Hirshfeld Analysis
The different Hirshfeld surfaces [39][40][41][42] are shown in Figure S1 (Data) while the intermolecular contacts and their percentages are shown in Figure 4. In the case of complex 3, the Hirshfeld calculations were performed for one of the disordered parts of this complex as the results of the two parts are almost the same. The polar O…H and N…H hydrogen bonding interactions as well as the hydrophobic C…H contacts are the most common interactions in the crystal structure of the studied systems. The O…H contact percentages are 12.8, 13.1 and 10.5% for 1, 2, and 3   Symm. Codes: 1 -x, -y + 2, -z + 1; 2 x + 1, y, z; 3 -x + 1, -y + 2, -z + 1.

Hirshfeld Analysis
The different Hirshfeld surfaces [39][40][41][42] are shown in Figure S1 (Data) while the intermolecular contacts and their percentages are shown in  5% for 1, 2, and 3 H contacts contributed by 16.3, 16.2, and 26.5%, for 1, 2, and 3  It is clear that these interactions appeared as red regions with shorter distances than the vdWs radii sum of the two atoms sharing the contact ( Figure 5). The O…H contacts are the shortest in complexes 1 and 2 compared to 3. On other hand, the N…H and C…H interactions appeared the shortest in the case of complex 2. The percentages of the C…C/C…N contacts are 4.3, 3.9, and 10.0% from the whole fingerprint area of complexes 1, 2, and 3, respectively, suggesting the presence of some π-π stacking interactions which are considered of less significance as these interactions appeared as blue regions in the dnorm maps. The polymeric nature of complex 3 was revealed by the presence of a large red region corresponding to the Zn-N(azido) coordination interactions (lower right part of Figure 5). It is clear that these interactions appeared as red regions with shorter distances than the vdWs radii sum of the two atoms sharing the contact (  1, 2, and 3, respectively, suggesting the presence of some π-π stacking interactions which are considered of less significance as these interactions appeared as blue regions in the d norm maps. The polymeric nature of complex 3 was revealed by the presence of a large red region corresponding to the Zn-N(azido) coordination interactions (lower right part of Figure 5).

Natural Charges
Natural charges calculated using the NBO method employing two DFT functional (MPW1PW91 and Wb97XD) are listed in Table 5. Since the results of the two DFT functionals are almost the same, the discussion will be therefore limited to one of these two methods for simplicity. The charge at the Fe, Ni, and Zn centers are calculated using the MPW1PW91 method to be 1.004, 0.847, and 1.147 e, respectively. For the two monomeric complexes, the organic ligand compensated the charge of the divalent metal ion (M(II)) to a higher extent in the case of complex 2 than that in complex 1. The four 4bzpy ligands compensated the Ni(II) by about 0.596 e while the two azide groups donated 0.556 e to the Ni(II) ion. The corresponding values in the Fe(II) complexes are 0.414 and 0.562 e, respectively. In contrast, complex 3 has four azido groups coordinating the Zn(II) in µ (1,1) and µ (1,3) bonding fashion. Each one of the µ (1,1) and µ(1,3) azido groups has a natural charge less than −1 by 0.304 and 0.319 e, respectively, indicating that the four azido groups compensated the Zn(II) charge to a higher extent (~0.622 e) compared to those in complexes 1 and 2. These results are further revealed by the lower negative charge density transferred from the two 4bzpy ligand units (0.19 e) in this complex.

Natural Charges
Natural charges calculated using the NBO method employing two DFT function (MPW1PW91 and Wb97XD) are listed in Table 5. Since the results of the two DF functionals are almost the same, the discussion will be therefore limited to one of the two methods for simplicity. The charge at the Fe, Ni, and Zn centers are calculated usi the MPW1PW91 method to be 1.004, 0.847, and 1.147 e, respectively. For the tw monomeric complexes, the organic ligand compensated the charge of the divalent me ion (M(II)) to a higher extent in the case of complex 2 than that in complex 1. The fo 4bzpy ligands compensated the Ni(II) by about 0.596 e while the two azide grou donated 0.556 e to the Ni(II) ion. The corresponding values in the Fe(II) complexes a

AIM Studies
It is well known covalent azides are linear and asymmetric while the azide ion (N 3 ) is symmetric with equal N-N distances [23]. A rationale for the inequivalence within the bound azide was given by considering the ground state electronic structure of the azide in terms of contribution [43] from two resonance structures (A) and (B) shown in Figure 6. Topological parameters of the atoms in molecules (AIM) [37] theory have great importance in describing the nature and strength of atom-atom interactions [44][45][46][47][48][49]. In this study, we employed these parameters to describe the degree of asymmetry of the azido group in the three complexes presented in this work. The electron density ( (r)) at the (3, −1) bond critical point (BCP; Figure 7) is a good measure for the bond strength. The N-N distances (d N-N ) and the corresponding topological parameters at the N-N BCPs in the studied complexes are summarized in Table 6. It is clear that in all complexes, the two N-N bonds of an azido group are not identical. The degree of asymmetry is the highest in the case of the terminal azide in the Fe(II) and Ni(II) complexes. This could be simply confirmed by calculating the difference (∆d) between the two N-N bond distances in these azido groups which are also listed in the same table. The most symmetric situation occurred in the azide groups of the Zn(II) complex where the two azido groups coordinating the Zn(II) ion either in a µ (1,3) or µ(1,1) mode of bonding. Interestingly, the electron density ( (r)) topological parameter correlated well with the N-N distances of the azido groups ( Figure 8). There is a clear dramatic decrease in the (r) values with increasing N-N distances which could be used as a measure for the degree of asymmetry of the coordinated azido group. In addition, the high (r) and negative ∇ 2 (r) at the N-N BCPs indicate clear covalent interactions.      On other hand, the AIM parameters for the M-N bonds are listed in Table 7. Based on the low electron density (ρ(r) < 0.10 au) values, positive H(r) and positive ∇ 2 ρ(r) as well as V(r)/G(r) < 1, one could conclude that all M-N bonds belong to closed-shell On other hand, the AIM parameters for the M-N bonds are listed in Table 7. Based on the low electron density ( (r) < 0.10 au) values, positive H(r) and positive ∇ 2 (r) as well as V(r)/G(r) < 1, one could conclude that all M-N bonds belong to closed-shell interactions. It is clear that the M-N interactions of the terminal azido groups in the Fe and Ni complexes have some higher covalency as indicated from the slightly negative H(r) and V(r)/G(r) ratios are slightly higher than 1 [50][51][52][53].