Solid State Coordination Chemistry and Supramolecular Assemblies of M(II)/Benzilato/2,2’-Dipyridylamine Systems

Two new mixed-ligand complexes of benzilato (bz) and 2,2’-dipyridylamine (dipyam), 1 [Ni(bz)(dipyam)2](bz)·2MeOH and 2 [Cu(bz)2(dipyam)]·EtOH, have been synthesized by refluxing benzilic acid and 2,2’-dipyridylamine with nickel(II) acetate for 1 and copper(II)carbonate hydroxide for 2. The new compounds have been characterized by elemental analysis, IR and electronic absorption spectroscopy, magnetic measurements, thermogravimetric analysis and singlecrystal X-ray diffraction. The role of non-covalent interactions, such as hydrogen bonds, π···π and/or C-H···π, in the creation of supramolecular assemblies is analyzed.


Introduction
In recent decades, research in metallosupramolecular chemistry has gone together with an important attention to studies of non-covalent intermolecular interactions and to analyses of self-assembled structures. These extended frameworks of coordination compounds present great interest because of their fascinating topologies and their potential applications [1][2][3]. Benzilato ligand is a α-hydroxycarboxylato which may exhibit various coordination modes and provide potential intermolecular interactions such as hydrogen bonds involving the hydroxyl and carboxylate functionalities, and π···π and C-H···π interactions through the phenyl substituents [4]. The probability of formation of supramolecular arrays can be increased by the use of the auxiliary ligand 2,2'-dipyridylamine with N-H group potentially donor in hydrogen bonds and with aromatic rings suitable to establish π···π and/or C-H···π interactions. The work described here contains the preparation, properties and a detailed supramolecular structural analysis of two new mixedligand complexes of nickel(II)and copper(II)with benzilato and 2,2'-dipyridylamine.
The infrared spectra of both compounds contain bands in the 3000-3250 cm -1 region assigned to the N-H (dipyam) stretching frequency and in the 3300-3450 cm -1 region corresponding to ν(OH) of benzilate and solvation molecules. The intense bands that appear around 1650 cm -1 , which in most cases overlap with bands associated with the 2,2'-dipyridylamine ligand, are assigned to the asymmetric COOvibration. The bands observed between 1340 and 1370 cm -1 correspond to the symmetric COOvibration in the benzilate ions [5]. These bands are consistent The electronic spectra of the complexes were recorded at the solid state by diffuse reflectance. The electronic spectrum of nickel(II)compound 1 shows two bands at 935 and 570 nm which are typical for octahedral symmetry. Therefore, assignments of the observed absorption bands can be made to 3 T 1g (P)  3 A 2g (F) (ν 1 ) and 3 T 1g (F)  3 A 2g (F) (ν 2 ) transitions, respectively. The band corresponding to 3 T 2g (F)  3 A 2g (F) (ν 3 ) transition is overlapped with the charge-transfer band [6]. The magnetic moment value at room temperature for 1 is 3.11 B.M. being consistent with an octahedral stereochemistry for the nickel(II)complexes [7]. The spectrum of copper(II)complex 2 exhibit a broad band at around 810 nm, which is characteristic of a copper(II)d-d transition in a tetragonal field with the copper(II)ion in a distorted square-based pyramidal coordination environment [6]. The magnetic moment value at room temperature of 1.65 B.M. in 2 lies in the range expected for pentacoordinated complexes of copper(II) [7].
Thermogravimetric analysis (TGA) was performed to investigate the thermal stability of compounds 1 and 2. The degradation of species was investigated by monitoring the evolved gases by infrared spectroscopy. The thermal behavior is very similar in both compounds showing three mass loss steps. The first step corresponds to the loss of the solvation molecules, the second mass loss corresponds to pyrolysis of the benzilato anions followed, in a last step, by the loss of 2,2'-dipyridylamine. Finally, the complexes give residues of NiO at 410°C (1) and CuO at 475°C (2).

Structural analysis
Selected interatomic distances are listed in Table 1 and the main hydrogen parameters in Table 2. Figures 1 and 2 show representations of complex structures of 1 and 2, respectively, together with the atom-numbering schemes used.   [23][24][25].
In both compounds, the nature of benzilato, 2,2'dipyridylamine and the solvation molecules (methanol and ethanol) allows the formation of supramolecular assemblies based on hydrogen bonding (Table 2) and π···π and/or C-H···π interactions.
In 1, the cationic complex, the benzilate counterion and the methanol molecules of asymmetric unit are linked by hydrogen bonds generating a discrete supramolecule with a 14-membered ring (Table 2; Figure 3a). These supramolecules are joined by additional hydrogen bonds involving a N-H group of one 2,2'-dipyridylamine as donor and non-coordinated carboxylate oxygen atom as acceptor, resulting a dimeric assembly (Table 2; Figure 3b). In addition, the dimers establish a weak non-classical hydrogen bonds (C19-H19···O23 ii , ii: -x+2,-y+1,-z) forming  In 2, the intramolecular hydrogen bonds involving the hydroxyl group and the non-coordinated carboxylate oxygen atoms of the monodentade benzilato ligand are the strongest interaction. In this case, the supramolecular organization is a consequence of the existence of intermolecular hydrogen bonds and π···π interactions. Two different types of hydrogen bonds between the bidentade-chelating benzilato ligands and the ethanol molecules form a polymeric chain along the crystallographic b axis. One of them involve the hydroxyl group of bidentade benzilato ligand as donor and the oxygen of ethanol molecule as acceptor and, in the second, the hydroxyl group of ethanol acts as donor and towards the non-coordinated carboxylate oxygen of bidentade benzilato ligand is the acceptor (Table 2; Figure 4a). The chains are linked by other hydrogen bonds, involving a N-H group of the 2,2'-dipyridylamine as donor and non-coordinated carboxylate

Experimental section
General information: All reagents and solvents were obtained commercially and were used as supplied. Elemental analyses (C, H, N) were carried out on a Fisons EA-1108 microanalyser. Melting points (m.p.) were measured with a Gallenkamp MBF-595 apparatus. IR spectra were recorded from KBr discs (4000-400 cm -1 ) or polyethylene-sandwiched Nujol mulls (500-100 cm -1 ) on a Bruker Vector 22 and IFS66v spectrophotometers, respectively. A Shimadzu UV-3101PC spectrophotometer was used to obtain electronic spectra in the solid state. Magnetic susceptibility measurements were performed at room temperature using a Johnson Matthey Alfa MSB-MK1 Gouy balance. TG/DTG analysis profiles were recorded under a 100 mL.min -1 air flow using a TA Instruments Hi-Res TGA2950 Thermobalance coupled to a Bruker Tensor 27 550 FT-IR spectrophotometer for identification of evolved gases.

Synthesis of complexes: [Ni(bz)(dipyam) 2 ](bz)·2MeOH (1)
A solution of benzilic acid (2.0 mmol) in 10 mL of EtOH and a solution of 2,2'-dipyridylamine (2.0 mmol) in 10 mL of i PrOH were added to a solution of Ni(AcO) 2 ·4H 2 O acetate (1.0 mmol) in 10 mL of EtOH. The mixture was refluxed for 2 h, left to cool to room temperature and stirred for a week yielding a purple precipitate that was filtered out and dried in vacuum. Purple single crystals of 1 were obtained by recrystallization of the  of data, indexing reflections, and the determination of lattice parameters, SAINT [27] for integration of intensity of reflections, and SADABS [28] for scaling and empirical absorption correction. The structure was solved by dual-space algorithm using the program SHELXT [29]. All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares calculations on F 2 using the program SHELXL [30] with OLEX2 [31]. Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters. Drawings were produced with PLATON [32] and Mercury [33]. Crystal data and   structure refinement parameters are reported in Table 3. CCDC 1811239 and 1811240 contain the supplementary data for 1 and 2, these data can be obtained free of charge via https://www. ccdc.cam.ac.uk/structures/.

Conclusion
In summary, two novel mixed-ligand of nickel(II)and copper(II) formed with benzilato and 2,2'-dipyridylamine have been isolated and characterized. The supramolecular organizations due to noncovalent intermolecular interactions have been analyzed.  Table 3 Crystal data and structure refinement for 1 and 2.