STRUCTURES AND MAGNETIC-PROPERTIES OF DIMERIC COPPER(II) BENZOYLFORMATES

Structures of six dimeric copper(II) benzoylformates (I)-(VI) have bee n determined by single crystal X-ray diffraction and the magnitudes of the spin-exchange interaction (H=-2JS(1)S(2)) have been determined ba sed on the magnetic susceptibility measurement. (I): Tetrakis(mu-benzo ylformato-O, O')bis(pyridine)dicopper(II), [Cu(C8H5O3)(2)C5H5N](2), -2 J = 648 cm(-1). (II): Tetrakis (mu-benzoylformato-O,O')bis(2-methylpyr idine) dicopper(II), [Cu(C8H5O3)(2)C6H7N](2), -2J = 656 cm(-1). (III): zoylformato-O,O')bis(3-methylpyridine)dicopper(II) monobenzene solvat e, [Cu(C8H5O3)(2)- C6H7N](2).C6H6 -2J = 649 cm(-1). (IV): Tetrakis(mu- benzoylformato-O, O')bis(4-methylpyridine)dicopper(II), [Cu(C8H5O3)(2) C6H7N](2), -2J = 625 cm(-1). (V): Bis(2,3-dimethylpyridinium) (mu-benz oylformato-O,O')-bis(chloride)dicopper(II) dibenzene solvate, 2C(7)H(1 0)N.- [Cu(C8H5O3)(2)Cl](2).2C(6)H(6), -2J = 618 cm(-1). (VI): Tetrakis (benzoylformato-O, O')bis(caffeine)dicopper(II) dibenzene solvate, [C u(C8H5O3)(2)C8H10N4O2](2).2C(6)H(6), -2J = 651 cm(-1) (caffeine = 3,7- dihydro-1,3,7-trimethyl-1H-purine-2,6-dione). In the binuclear cage st ructure the coordination geometry around the Cu-II atoms is typical sq uare pyramidal with Cu ... Cu distances 2.725 (1)-2.843 (1) Angstrom. An ab initio molecular orbital calculation for the benzoylformate ion indicates that the unusually strong antiferromagnetic interaction in d imeric copper(II) benzoylformates is attributed to the electronic effe ct of the alpha-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 pop ulation of the carboxylate C atom in the symmetrical HOMO, highest occ upied molecular orbital (x is parallel to the C-R bond axis in the RCO O- ion).