Crystal structure and Hirshfeld surface analysis of bis(benzoato-κ2 O,O′)[bis(pyridin-2-yl-κN)amine]nickel(II)

The crystal structure of a new mononuclear NiII complex with bis(pyridin-2-yl)amine (dpyam) and benzoate (benz) is reported.


Chemical context
Nickel(II) complexes have been of wide interest in many fields such as coordination chemistry (Devereux et al., 2007;Lee et al., 2012) and bioinorganic chemistry (Morgant et al., 2006;Luo et al., 2007;Zianna et al., 2016), to name just a few. Generally, an Ni II ion is stable in its [Ar]3d 8 electronic configuration. Among the various types of Ni II complexes, mononuclear Ni II complexes containing mixed carboxylate and N-donor ligands have received considerable attention because of their interesting properties such as their behaviour catalysis in transesterification (Lee et al., 2012) and their occasional bioactivity (Zianna et al., 2016). One of the aims of our research group is to explore and study the coordination chemistry and bioactivities of new mononuclear complexes containing first row transition metal(II) ions and mixed ligands such as benzoate and N-donor bipyridine derivatives. Moreover we are interested in understanding the crystal structures and stability of the self-assembly between mononuclear units through non-covalent interactions, and the resulting properties of the material. Generally, a carboxylate group of e.g. a benzoate can give rise to various types of coordination modes, leading to a variety of coordination geometries and coordination frameworks, while a phenyl ring is able to providestacking interactions that can support crystal stability. For N-donor ligands, bipyridine derivatives can act as chelating agents to form mononuclear units as building blocks for constructing 1D, 2D and 3D supramolecular frameworks through weak interactions such as hydrogen bonding,stacking among others, depending on the exact nature of the ligand. As part of our ongoing research into the coordination chemistry and bioactivities of new discrete Ni II complexes containing benzoate and chelating N- ISSN 2056-9890 donor ligands, we have synthesized a new mononuclear Ni II complex containing benzoate (benz) and bis(pyridin-2-yl)amine (dpyam) mixed ligands, [Ni(dpyam)(benz) 2 ]. Herein, the crystal structure determination and Hirshfeld surface analysis of the title complex is reported.

Structural commentary
The title complex crystallizes in the monoclinic crystal system in the P2 1 /c space group. The asymmetric unit consists of one Ni II ion, one dpyam, and two benzoate ligands. The Ni II ion is six-coordinated by two nitrogen atoms from the dpyam chelating ligand and four oxygen atoms from two benzoate chelating ligands, adopting a cis-distorted octahedral geometry as shown in Fig. 1. The Ni-N and Ni-O bond lengths range from 2.032 (2) to 2.045 (2) Å and 2.041 (2) to 2.221 (2) Å , respectively, whereas the bond angles around the central Ni atom are 61.53 (7)-159.84 (8) (see Table 1). These values in the title complex are comparable to those of related Ni II complexes such as [Ni(bpy)(benz) 2 ] (bpy = 2,2 0 -bipyridine; Baruah et al., 2007), and are shorter than those of other isostructural metal(II) complexes with the same ligand set, such as [M(dpyam)(benz) 2 ], where M = Zn (Lee et al., 2007), Cd (Park et al., 2010) and Hg (Lee et al., 2012), because of the different sizes of the central metal ions.

Hirshfeld surface analysis
The interactions stabilizing the supramolecular framework of the title complex have been further studied by the analysis of the Hirshfeld surfaces and their two-dimensional fingerprint plots. These results were visualized using the program Crys-talExplorer ( View of the C-HÁ Á Á intermolecular interactions of the title complex.

Figure 4
View of the three-dimensional supramolecular network of the title complex. Hirshfeld surface of the title complex is shown in Fig. 5a.
Interactions are represented using different colours, red indicating distances closer than the sum of the van der Waals radii, white indicating distances near the van der Waals radii separation, and blue indicating distances longer than the van der Waals radii (McKinnon et al., 2007;Venkatesan et al., 2016). The strong intermolecular N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonding and C-HÁ Á Á interactions in the crystal of the title complex are represented as red spots on d norm . Selected two-dimensional fingerprint plots are shown in Fig. 5b for all contacts as well as individual HÁ Á ÁH, CÁ Á ÁH/HÁ Á ÁC, OÁ Á ÁH/HÁ Á ÁO and CÁ Á ÁC contacts, whose percentage contribution is also given. HÁ Á ÁH intermolecular contacts make the highest percentage contribution (44.0%), a result of the prevalence of hydrogen from the organic ligand. The CÁ Á ÁH/ HÁ Á ÁC and OÁ Á ÁH/HÁ Á ÁO intermolecular contacts are due to the attractive C-HÁ Á Á and hydrogen-bonding interactions with percentage contributions of 30.7 and 15.7%, respectively, indicating these to be the dominant stabilizing interactions in this crystal. The CÁ Á ÁC contacts, with a percentage contribution of only 4.8%, indicate that theinteractions in the crystal of the title complex are weak compared to the other types of interactions, despite their prominent apparent role when visually inspecting the crystal structure.

Characterization
The IR spectrum (see Fig. S1 in the supporting information) of the title complex presents characteristic peaks at 3323, 3219 and 3148 cm À1 for N-H stretching and 1642 cm À1 for N-H bending, 1595 cm À1 for C N aromatic stretching and 1421 cm À1 for C-N stretching in the coordinated dpyam ligand. Asymmetric and symmetric COO À peaks of the chelating benzoate ligand are present at 1528 and 1489 cm À1 , respectively. The peaks at 865, 772 and 687 cm À1 are assigned to C-H bending of aromatic rings. The peaks at 526 and 443 cm À1 have been assigned to Ni-O and Ni-N stretching, respectively (Zianna et al., 2016). The solid-state diffuse reflectance spectrum (Fig. S2) of the title complex presents three peaks at 391, 669 and 1044 nm that can be attributed to the allowed transitions 3 A 2g ! 3 T 1g (P), 3 A 2g ! 3 T 1g (F) and 3 A 2g ! 3 T 2g , respectively. In addition, the spectrum also shows a shoulder peak at 793 nm which can be attributed to a forbidden transition, 3 A 2g ! 1 E g . This spectroscopic feature agrees with the typical d-d transitions of the Ni II ion in a distorted octahedral geometry (Al-Riyahee et al., 2018).
A PXRD pattern of the title complex was collected at room temperature (Fig. S3). The result shows that the pattern of the as-synthesized bulk material matches its simulated pattern, confirming the phase purity of the title complex. Views of (a) the Hirshfeld surface mapped over d norm in the range À0.526 to +1.5208 (arbitrary units) and (b) Hirshfeld surface fingerprint plots for the HÁ Á ÁH, CÁ Á ÁH/HÁ Á ÁC, OÁ Á ÁH/HÁ Á ÁO and CÁ Á ÁC contacts of the title complex. order Ni II < Zn II < Cd II < Hg II in the corresponding complexes, leading to a different degree of distortion in their coordination spheres.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All hydrogen atoms were generated geometrically and refined isotropically using a riding model, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C). The H atom bonded to the N atom of dpyam was located in a difference-Fourier map and was freely refined.

Funding information
NW acknowledges Thammasat University Research Fund (Contract No. 34/2560) for financial support. The authors thank the Central Scientific Instrument Center (CSIC), Faculty of Science and Technology, Thammasat University, for funds to purchase the X-ray diffractometer and acknowledge the Center of Scientific Equipment for Advanced Research (CSEAR), Thammasat University, for facilities to conduct this research.

Bis(benzoato-κ 2 O,O′)[bis(pyridin-2-yl-κN)amine]nickel(II)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.