Tetraaquabis(pyridine-3-carbonitrile-κN 1)nickel(II) benzene-1,4-dicarboxylate tetrahydrate

The structure of a nickel(II) terephthalate complex, viz. tetraaquabis(pyridine-3-carbonitrile)nickel(II) benzene-1,4-dicarboxylate tetrahydrate is described.


Chemical context
Multi-carboxylate ligands with suitable spacers, especially benzene-multicarboxylate ligands, are frequent choices for coordination chemistry as they feature a broad range of coordination modes and can result in the formation of systems with variable complexity ranging from molecular complexes to metal-organic frameworks of different dimensionality (Janiak & Vieth, 2010;Kim et al., 2001). Benzene-1,4-dicarboxylate (terephthalate) ligands have received increased attention in the field of coordination chemistry, especially as building blocks for coordination polymers, mainly with porous networks with varied metal ions (Kim et al., 2003). As a result of the presence of conjugation, the terephthalate anion can provide an electronic pathway for delocalization of electrons belonging to the d-orbitals of the metal ion, thus changing its magnetic properties. The most important factor that affects magnetic exchange pathways between two metal centres is the proper choice of bridging ligands since they influence the magnetic strength and behaviour of the molecule (Massoud et al., 2006;Mukherjee et al., 2003;Rogan et al., 2000). Coordinated ligand systems containing electron-donor as well as acceptor sites also give rise to metallosupramolecular assemblies. Hence, pyridine-3-carbonitrile (3-NCpy) with the electron-withdrawing nitrile group as the acceptor along with the pyridyl nitrogen atom as the donor stands as a suitable ligand in this regard. Despite the availability of two potentially coordinating sites, not many compounds having pyridine-3carbonitrile as a bidentate bridging ligand are known (Heine et al., 2018). The nitrile group may also be expected to take part in hydrogen bonding andinteractions. In this work, we describe our results on the synthesis and crystal structure of a pyridine-3-carbonitrile-based Ni II Fig. 1. The compound consists of a complex dication, which is in association with four free water molecules and an uncoordinated terephthalate dianion, where the asymmetric unit contains half of these quantities. The Ni 2+ centre is situated on an inversion centre and coordinates to two axial pyridine-3-carbonitrile ligands and four equatorial water molecules forming the cationic complex [Ni(H 2 O) 4 (3-CNpy) 2 ] 2+ . The bond angles in the cationic part suggest that the complex contains an Ni 2+ ion in an approximately octahedral coordination environment [cis angles in the range of 88.66 (4)-91.33 (4) ]. The free terephthalate anion is also located on an inversion centre and has an angle of 14.54 (7) between the planes of the aromatic ring and of the carboxylate group. Furthermore, it does not coordinate to the Ni 2+ ion and remains fully deprotonated for charge balance. It also acts as a secondary acceptor to the cationic complex unit. The Ni-O bond lengths are 2.0381 (11) and 2.0519 (9) Å and are in agreement with similar complexes reported (Xiao et al., 2003;Ma & Xu, 2010;Ju et al., 2016). The Ni-N bond length of 2.1481 (11) Å is slightly longer than those in the similar complexes reported by Zukerman-Schpector et al. (2000) and Heine et al. (2018).

Figure 2
Packing structure of the complex showing the hydrogen bonds and C-HÁ Á ÁO andinteractions. showing the atom-labelling scheme (ellipsoids drawn at the 50% probability level; unlabelled atoms generated by the symmetry operations 2 À x, 2 À y, Àz for the cation and 1 À x, 1 À y, 1 À z for the anion).
network with an R 4 6 (12) motif, forming infinite chains in a zigzag fashion along the a-axis direction (Fig. 3). Finally, the three-dimensional network is further accomplished among others by the D 2 2 (7) O2-H2BÁ Á ÁO6 hydrogen bonds and C2-H2Á Á ÁO3 and C5-H5Á Á ÁO3 interactions (Table 1, Fig. 2). A comprehensive list of first and second level graph sets can be found in Table 2.

Database survey
A survey of the Cambridge Structural Database (CSD version 2020.2; Groom et al., 2016) for Ni II complexes involving an uncoordinated terephthalate dianion led us to a few results, some of which are as follows. In the complex [Ni(2,2 0 -bipy)-(H 2 O) 4 ](C 8 H 4 O 4 ) (2,2 0 -bipy = 2,2 0 -bipyridyl) (CSD refcode: WUWZET) reported by Xiao et al. (2003), the terephthalate anion acts as a synthon to generate a supramolecular network. The hydrogen bonds between the terephthalate anions and the [Ni(2,2 0 -bipy)(H 2 O) 4 ] 2+ cations produce a two-dimensional hydrogen-bonded architecture with double sheets. A similar compound, tetraaquabis(dimethylformamide)nickel(II) tetrachloroterephthalate, (QAMDUF; Ma & Xu, 2010) has a nearly ideal octahedral structure with the metal ion lying on an inversion center along with an uncomplexed and fully deprotonated terephthalate dianion. Another Ni II -terephthalate complex (AJUPEC; Ju et al., 2016) with 4,7-di(4-pyridyl)-2,1,3-benzothiadiazole as auxiliary ligand crystallizes in the monoclinic P2 1 /c space group. The terephthalate dianion remains uncoordinated and the Ni II ion sits in the centre of an octahedron constituted by two pyridyl N atoms in the apical positions and four water oxygen atoms constructing the equatorial plane. The independent cationic units are held together bystacking interactions and O-HÁ Á ÁO hydrogen bonding, generating a compact packing structure. A pyrazine-based Ni II -terephthalate complex (AGIWOC; Groeneman & Atwood, 2000) is a one-dimensional zigzag coordination polymer, where each nickel centre has two cispyrazine ligands along with four coordinated water molecules, giving rise to a distorted octahedral coordination environment. A survey of Ni II complexes involving pyridine-3carbonitrile as ligand led us to some other related structures. Heine et al. (2018) investigated the ability of pyridine-3carbonitrile to act as a mono-or bidentate ligand in complexes of the type [M II Br 2 (3-CNpy) x ] n with M II = Mn, Fe, Co, Ni and x = 1, 2 and 4, (CSD refcodes XOSNUR, XOSPAZ, XOSPAZ02) and found that the pyridine-3-carbonitrile ligand acted as bridging ligand in complexes with a metal:ligand ratio of 1:1 and as a terminal ligand with ratios of 1:2 and 1:4. In an adduct of Ni II acetylacetonate chelating with pyridine-3carbonitrile (MASTUV; Zukerman-Schpector et al., 2000), the Ni II atom is situated on a centre of symmetry and is octahedrally bonded to two equatorial AcAc groups and two pyridine-3-carbonitrile groups, which are axially coordinated in a trans configuration.

Figure 4
Hydrogen-bonded pattern associated with the R 4 4 (20) graph set andinteractions between two 3-NCpy rings. 0.832 g (8 mmol) of pyridine-3-carbonitrile was added and the resulting reaction mixture was stirred mechanically for 2 h. A light-green precipitate was formed. It was filtered, washed with water under suction and dried in a vacuum desiccator over fused CaCl 2 . Green prism-shaped single crystals of the title compound suitable for X-ray diffraction studies were obtained from the undisturbed aqueous reaction solutions after 24 h, yield 73% (1.675 g). The compound is air stable and insoluble in common organic solvents. The crystals remained indefinitely stable against dehydration under ambient conditions. IR spectroscopic data (KBr disc, cm À1 ): asym (OCO À ) 1568, sym (OCO À ) 1365, (C N) 1602, (CN py ) 2243, asym (OCO À ) 810, sym (OCO À ) 748. Decomposition point 270 C.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Ni1