Structures and Magnetic Properties of Dimeric Copper(II) Benzoylformates

Structures of six dimeric copper(II) benzoylformates (I)-(VI) have been determined by single-crystal X-ray diffraction and the magnitudes of the spin-exchange interaction (H = -2JS₁S₂) have been determined based on the magnetic susceptibility measurement. (I): Tetrakis(-benzoylformato-O,O')bis(pyridine)dicopper(II), [Cu(C₈H₅O₃)₂C₅H₅N]₂, -2J = 648 cm^-1. (II): Tetrakis(-benzoylformato-O,O')bis(2-methylpyridine)dicopper(II), [Cu(C₈H₅O₃)₂C₆H₇N]₂, -2J = 656 cm^-1. (III): Tetrakis(-benzoylformato-O,O')bis(3-methylpyridine)dicopper(II) monobenzene solvate, [Cu(C₈H₅O₃)₂C₆H₇N]₂.C₆H₆, -2J = 649 cm^-1. (IV): Tetrakis(-benzoylformato-O,O')bis(4-methylpyridine)dicopper(II), [Cu(C₈H₅O₃)₂C₆H₇N]₂, -2J = 625 cm^-1. (V): Bis(2,3-dimethylpyridinium) tetrakis(-benzoylformato-O,O')bis(chloride)dicopper(II) dibenzene solvate, 2C₇H₁₀N.[Cu(C₈H₅O₃)₂Cl]₂.2C₆H₆, -2J = 618 cm^-1. (VI): Tetrakis(benzoylformato-O,O')bis(caffeine)dicopper(II) dibenzene solvate, [Cu(C₈H₅O₃)₂C₈H₁₀N₄O₂]₂.2C₆H₆, -2J = 651 cm^-1 (caffeine = 3,7-dihydro-l,3,7-trimethyl-1 H-purine-2,6-dione). In the binuclear cage structure the coordination geometry around the Cu^II atoms is typical square pyramidal with CuCu distances 2.725 (1)-2.843  (1) Å. An ab initio molecular orbital calculation for the benzoylformate ion indicates that the unusually strong antiferromagnetic interaction in dimeric copper(II) benzoylformates is attributed to the electronic effect of the -keto group in the bridging carboxylate moiety. This is in accordance with a positive linear relationship between -2J of the copper(II) carboxylates and the diagonal part of the 2p_x orbital population of the carboxylate C atom in the symmetrical HOMO, highest occupied molecular orbital (x is parallel to the C-R bond axis in the RCOO^- ion).