Crystal structure of tetrakis(μ-2,4,6-trimethylbenzoato-κ2 O:O′)bis[(nicotinamide-κN 1)copper(II)]

Four carboxylate groups of 2,4,6-trimethylbenzoate anions bridge two CuII cations to form a binuclear complex. Distorted square-pyramidal coordinations are completed by the pyridine N atoms of nicotinamide molecules.

In the title binuclear Cu II complex, [Cu 2 (C 10 H 11 O 2 ) 4 (C 6 H 6 N 2 O) 2 ], the two Cu II cations [CuÁ Á ÁCu = 2.5990 (5) Å ] are bridged by four 2,4,6-trimethylbenzoate (TMB) anions. The four nearest O atoms around each Cu II cation form distorted square-planar arrangements and the distorted square-pyramidal coordinations are completed by the pyridine N atoms of nicotinamide molecules at distances of 2.164 (2) and 2.165 (2) Å , respectively. The Cu II cations are displaced by À0.2045 (3) and 0.2029 (3) Å from the corresponding planes formed by the nearest four O atoms. In the molecule, the dihedral angles between the planes of the benzene rings and the adjacent carboxylate groups are 80.6 (2), 51.4 (2), 24.4 (2) and 32.5 (2) , while the planes of the pyridine rings are oriented at a dihedral angle of 11.28 (10) . In the crystal, bifurcated N-HÁ Á ÁO and weak C-HÁ Á ÁO hydrogen bonds link the molecules, enclosing R 2 2 (8) and R 4 4 (8) ring motifs, into a three-dimensional network. The structure contains a solvent-accessible void of 72 Å 3 , but there is no solvent molecule located within this void. The crystal studied was an inversion twin refined with a minor component of 0.488 (8).

Chemical context
Nicotinamide (NA) is one form of niacin. A deficiency of this vitamin leads to loss of copper from the body, known as pellagra disease. Victims of pellagra show unusually high serum and urinary copper levels (Krishnamachari, 1974). It is thus of interest to determine the manner in which copper interacts with niacin and nicotinamide. In the structures of some complexes obtained from the reactions of Cu II ions with NA, e.g. [Cu(sal) 2 (NA) 2 ] (sal is salicylate) (Hoang et al., 1993) and [Cu(C 7 H 3 ClFO 2 ) 2 (NA) 2 ] (Hoang et al., 1995), NA is a monodentate ligand coordinating to Cu II via its pyridine N atom. In its rare earth complexes, NA coordinates to the rare earth ion via only the O atoms of the substituents, not by the pyridine N atom (Poray-Koshits et al., 1976). Coordination via the amide N atom may also occur. Hence, NA may form molecular or polymeric structures affecting such properties of the compounds as their solubility.
The structure-function-coordination relationships of the arylcarboxylate ion in Cu II complexes of benzoic acid derivatives may change depending on the nature and position of the substituted groups on the benzene ring, the nature of the additional ligand molecule or solvent, and the pH and temperature of synthesis as in Zn II complexes of benzoic acid derivatives Nadzhafov et al., 1981;Antsyshkina et al., 1980;Adiwidjaja et al., 1978). When pyri-dine and its derivatives are used instead of water molecules, the structure is completely different (Catterick et al., 1974). In this context, we synthesized a Cu II -containing compound with 2,4,6-trimethylbenzoate (TMB) and NA ligands, namely tetrakis(-2,4,6-trimethylbenzoato-2 O:O 0 )bis[(nicotinamide-N 1 )copper(II)], [Cu 2 (TMB) 4 (NA) 2 ], and report herein its crystal structure.

Structural commentary
The binuclear title complex, [Cu 2 (TMB) 4 (NA) 2 ], contains two Cu II atoms surrounded by four TMB and two NA ligands (Fig. 1) van Niekerk & Schoening, 1953). In metallic copper, the Cu-Cu bond length is 2.55 Å (Lee, 1986). The title complex has the smallest CuÁ Á ÁCu distance after metallic copper. Therefore, a weak orbital interaction may exist between the two Cu atoms.

Refinement
The experimental details including the crystal data, data collection and refinement are summarized in Table 3. N-and C-bound H atoms were positioned geometrically, with N-H = 0.86 Å (for NH 2 ) and C-H = 0.93 and 0.96 Å for aromatic and methyl H atoms, respectively, and constrained to ride on their parent atoms, with U iso (H) = kU eq (C,N), where k = 1.5 for methyl H atoms and k = 1.2 for NH 2 and aromatic H atoms.

Synthesis and crystallization
The title compound was prepared by the reaction of CuSO 4 (0.40 g, 2.5 mmol) in H 2 O (100 ml) and nicotinamide (0.61 g, 5 mmol) in H 2 O (25 ml) with sodium 2,4,6-trimethylbenzoate (0.93 g, 5 mmol) in H 2 O (150 ml). The mixture was set aside to crystallize at ambient temperature for three weeks, giving green single crystals.

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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.