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 = −2 J S 1 S 2 ) have been determined based on the magnetic susceptibility measurement. (I): Tetrakis(\mu-benzoylformato- O , O ′)bis(pyridine)dicopper(II), [Cu(C 8 H 5 O 3 ) 2 C 5 H 5 N] 2 , −2 J = 648 cm^{-1}. (II): Tetrakis(\mu-benzoylformato- O , O ′)bis(2-methylpyridine)dicopper(II), [Cu(C 8 H 5 O 3 ) 2 C 6 H 7 N] 2 , −2 J = 656 cm^{-1}. (III): Tetrakis(\mu-benzoylformato- O , O ′)bis(3-methylpyridine)dicopper(II) monobenzene solvate, [Cu(C 8 H 5 O 3 ) 2 C 6 H 7 N] 2 .C 6 H 6 , −2 J = 649 cm^{-1}. (IV): Tetrakis(\mu-benzoylformato- O , O ′)bis(4-methylpyridine)dicopper(II), [Cu(C 8 H 5 O 3 ) 2 C 6 H 7 N] 2 , −2 J = 625 cm^{-1}. (V): Bis(2,3-dimethylpyridinium) tetrakis(\mu-benzoylformato- O , O ′)bis(chloride)dicopper(II) dibenzene solvate, 2C 7 H 10 N.[Cu(C 8 H 5 O 3 ) 2 Cl] 2 .2C 6 H 6 , −2 J = 618 cm^{-1}. (VI): Tetrakis(benzoylformato- O , O ′)bis(caffeine)dicopper(II) dibenzene solvate, [Cu(C 8 H 5 O 3 ) 2 C 8 H 10 N 4 O 2 ] 2 .2C 6 H 6 , −2 J = 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 Cu...Cu 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 \alpha-keto group in the bridging carboxylate moiety. This is in accordance with a positive linear relationship between −2 J of the copper(II) carboxylates and the diagonal part of the 2 p 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 R COO^{-} ion).