Bis[μ-4-(1H-imidazol-3-ium-1-yl)benzoato-κ2 O:O′]bis[(methanol)tris(nitrato-κ2 O,O′)terbium(III)]

In the centrosymmetric dinuclear title complex, [Tb2(NO3)6(C10H8N2O2)2(CH3OH)2], the Tb atoms are bridged by the carboxylate groups of the two 4-(1H-imidazol-3-ium-1-yl)benzoate (iba) ligands. The iba ligand adopts a zwitterionic form with a protonated imidazole group. The Tb atom adopts a distorted tricapped trigonal–prismatic coordination geometry and is coordinated by six O atoms of three chelating nitrate ions, one O atom of the methanol molecule and two O atoms of two iba ligands. The intramolecular Tb⋯Tb separation is 5.1419 (3) Å. O—H⋯O and N—H⋯O hydrogen bonds connect complex molecules into a two-dimensional network.

In the centrosymmetric dinuclear title complex, [Tb 2 (NO 3 ) 6 -(C 10 H 8 N 2 O 2 ) 2 (CH 3 OH) 2 ], the Tb atoms are bridged by the carboxylate groups of the two 4-(1H-imidazol-3-ium-1-yl)benzoate (iba) ligands. The iba ligand adopts a zwitterionic form with a protonated imidazole group. The Tb atom adopts a distorted tricapped trigonal-prismatic coordination geometry and is coordinated by six O atoms of three chelating nitrate ions, one O atom of the methanol molecule and two O atoms of two iba ligands. The intramolecular TbÁ Á ÁTb separation is 5.1419 (3) Å . O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds connect complex molecules into a twodimensional network.

Related literature
For the preparation of 4-(1H-imidazol-1-yl)benzoic acid (iba), see: Zhang et al. (2007a). To the best of our knowledge, the title compound is the first f-block complex of the iba ligand.
Comment 4-(1H-Imidazol-1-yl)benzoic acid (iba) is a potential bridging ligand possessing two coordinating groups: a carboxylate group and an imidazole group. This ligand was originally prepared by Chen's group (Zhang et al., 2007a). The iba ligand was recently employed to prepare a couple of discrete complexes of d-block metals such as Mn (Gao et al., 2008) and Ni (Zhang et al., 2007b). It was also utilized for the preparation of coordination polymers of d-block metals such as Co (Zhang et al., 2007a), Cu (Niu, et al., 2009), Zn (Bai et al., 2009), and Cd (Aijaz et al., 2009;Bai et al., 2009;Gao et al., 2009). By contrast, the corresponding f-block complexes, discrete or polymeric, are not known at present. In this context, we attempted to prepare f-block coordination polymers by using the iba ligand. Inconsistent with our expectation, however, a discrete molecular species (the title complex) was produced. We herein report the preparation and structure of a terbium complex, the first f-block complex of the iba ligand.
The local coordination environment of the Tb III ion in the title complex is presented in Fig. 1, which shows two iba ligands linking two 9-coordinate Tb III ions. The asymmetric unit consists of only half the formula unit, and the other half is generated by crystallographic inversion center. All atoms occupy general positions, and the inversion point is located at the center of the title complex. The imidazole nitrogen (N2) in the iba ligand remains uncoordinated and is protonated.
As a result, the iba ligand has a net charge of zero and acts as a zwitterion in which a positive charge is on the imidazole N atom and a negative charge is on the carboxylate group. It would be meaningful to notice that the imidazole N atom is coordinated to the metals in all known d-block metal complexes and coordination polymers. Additional coordination of the iba carboxylate group to the metals led to the formation of 2-D or 3-D coordination polymers (Zhang et al., 2007a;Niu, et al., 2009;Bai et al., 2009;Aijaz et al., 2009;Bai et al., 2009;Gao et al., 2009). The fact that the N2 atom is not bonded to the Tb III ion in the title complex can probably be explained on the basis of the hard-soft acid-base concept. The hard Tb III ion is expected to have a preference to coordinating to the harder oxygen atom (in the carboxylate group) over the softer nitrogen atom (in the imidazole group).
Each Tb III ion is coordinated to six O atoms from three NO 3 -, one O atom from CH 3 OH, and two O atoms from two bis(monodentate) iba ligands. The [TbO 9 ] core forms a distorted tricapped trigonal prism. Two Tb III ions and two carboxylate groups form a central eight-membered ring. All nitrate ligands act as bidentate ligands. The Tb···Tb separation is 5.1419 (3) Å, which is much longer than the sum (4.0 Å) of van der Waals radii of the two Tb III ions and therefore rules out direct interaction between the two metal ions. The N-H of the imidazole group and O-H of the coordinated methanol molecule participate in the intermolecular hydrogen bonds, which connect the molecules of the title complexe into a two-dimensional network (Fig. 2).

Refinement
The H atom of the methanol OH group was generated in the idealized position [O-H = 0.81Å, U iso (H) = 1.5U eq (O)] and refined in a riding model approximation. The NH hydrogen atom was located in difference Fourier maps and freely refined.
The remaining H atoms were generated in idealized positions (C-H = 0.93-0.96 Å) and refined as riding with U iso (H) = 1.2U eq (C). Fig. 1. Molecular structure of the title complex showing 50% probability displacement ellipsoids. Symmetry code for the atoms with A in their labels: -x + 1, -y + 1, -z + 2.