Tetraaquabis[3-(pyridin-4-yl)benzoato-κN]cobalt(II)

In the title compound, [Co(C12H8NO2)2(H2O)4], the Co atom lies on a twofold rotation axis and has an N2O4 octahedral coordination environment formed by four O atoms of water molecules in the equatorial plane and two apical N atoms of pyridine groups. An intricate three-dimensional supramolecular network is formed by multiple O—H⋯O hydrogen bonds between the coordinated water molecules and the uncoordinated carboxylate groups.

In the title compound, [Co(C 12 H 8 NO 2 ) 2 (H 2 O) 4 ], the Co atom lies on a twofold rotation axis and has an N 2 O 4 octahedral coordination environment formed by four O atoms of water molecules in the equatorial plane and two apical N atoms of pyridine groups. An intricate three-dimensional supramolecular network is formed by multiple O-HÁ Á ÁO hydrogen bonds between the coordinated water molecules and the uncoordinated carboxylate groups.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: YK2023).

Comment
The synthesis and exploration of multifunctional metal-organic complexes is a current topic of chemical research (M. Ruben, et al., 2003). Pyridyl-containing multi-carboxylic acids, a kind of gracious multifunctional spacers, have been widely used to construct various extended metal-organic frameworks with attracting properties (Y. Huang, et al., 2007). Pyridylbenzoate ligands are typical unsymmetrical spacers, but up to now their coordination chemistry has been discussed uncommonly. From our best knowledge, only one coordination compound of 3-(pyridin-4-yl)benzoic acid (PBC) of pyridylbenzoate family was synthesized and characterized up to now (J. Luo, et al., 2007). Herein we report a new Co(II) complex with PBC, namely, [Co(PBC) 2 (H 2 O) 4 ] (1).
As showed in Fig. 1, (1) is a mononuclear complex with a twofold axis passing through the Co(II) center along b axis and equally splitting the whole complex molecule. In (1), the Co(II) center is ligated by four O atoms of coordinated water molecules in the equatorial plane, whereas two PBC act as monodentate N-donating ligands with their pyridyl nitrogen atoms occupying the axial positions. Thus the cobalt(II) ion is six-coordinated and has octahedral coordination geometry.
The bond distances Co-O and Co-N range from 2.098 (3) to 2.148 (3) Å, while the in-plane and axis-transition angles are 175.22 (10) and 175.97 (15) °, respectively, indicating a slight distortion of the octahedral coordination sphere around the Co(II) center.
Owing to the self-effacement of the versatile carboxyl groups in coordination chemistry (W. G. Lu, et al., 2008), the potential multifunctional PBC ligands in (1) act as terminal ligand only, rather than bridging one, very dissimilar to that in literature structure (J. Luo, et al., 2007). Further aggregation of monomers (1) is performed by the multiple hydrogen bonds between the coordinated water molecules (as donors) and the uncoordinated carboxylate groups (as acceptors) (Table   1). Hydrogen-bonding system among monomers (1) is rather complicated: each coordination water molecule forms two O-H···O hydrogen bonds with carboxylate groups of neighbouring complex molecules, while every carboxylate group of PBC forms three hydrogen bonds. Consequently, every monomer acts as a novel six-connected supramolecular synthon to connect with six adjacent monomers. For example, as shown in Fig. 2, the O4 of the carboxylate group of PBC ligates to two water molecules from two neighboring monomers, and as a result, monomers (1) are regularly arrayed in the ab<ι> plane and linked into 2D layers by strong hydrogen bonding (O1···O4 iii , 2.743 (4) Å; O2···O4, 2.714 (4) Å). The layer structure is stabilized by forceful face-to-face π···π stacking interactions between adjacent benzoate and pyridyl groups of PBC with a centroid to centroid distance of 3.60 (1) Å. Intriguingly, the dihedral angle between benzoate and pyridyl groups in PBC is equal to 26.9 (0) ° to meet the formation of hydrogen bonds. The layers are further bound together to create the 3D supramolecular architecture by hydrogen bonds between the O3 atom of the carboxylate group of PBC and two water molecules in the adjacent complex molecule.
supplementary materials sup-2 Experimental The title compound, (1), was prepared according to the following process. A mixture of Co(NO 3 ) 2 .6H 2 O (0.029 g, 0.1 mmol), PBC (0.040 g, 0.2 mmol) and deionized water (10 ml) was adjusted to the pH value about 7 by adding 0.1 M sodium hydroxide solution and then sealed into a 25 ml Teflon-lined stainless autoclave. The autoclave was heated at 160 °C for 3 days. After cooling to room temperature gradually, dark-red block crystals being characterized as the previously reported complex (J. Luo, et al., 2007) were obtained in 30% yield (based on Co). Allowing the red filtrate to evaporate slowly at ambient temperature for two months, pink crystals of (1) suitable for X-ray analysis were obtained in 58% yield (based on Co).