COBALT CARBONYL-COMPLEXES WITH BRIDGING DIPHOSPHINE LIGANDS

Reduction of cobalt(II) chloride by Na[BH4] in the presence of CO and dmpm (=Me2PCH2-PMe2) gave either [Co2(CO)2(mu-CO)2(mu-dmpm)2] (1a) or [Co4(CO)5(mu-CO)3(mu-dmpm)2] (6), depending on the reaction stoichiometry. Complex 1a, in solution is in equilibrium with its isomer [Co2(CO)4(mu-dmpm)2] (2a), and the thermodynamic parameters for the reaction were determined by measuring the equilibrium constant as a function of temperature using FTIR. The activation energy for the isomerization was determined by variable-temperature NMR studies, which also indicated that 2a is weakly paramagnetic. Reaction of 1a with iodine or [Cu(NCMe)4]+ gave [Co2(mu-I)(mu-CO)(CO)2(mu-dmpm)2]+ (7) and [Co4Cu3(CO)8(mu-dmpm)2]+ (8), respectively. The molecular structures of 6 and 8 were characterized by X-ray diffraction (6, orthorhombic, P2(1)2(1)2(1), a = 14.953(2) angstrom, b = 11.386(1) angstrom, c = 16.836(1) angstrom, Z = 4, R = 0.038; 8, monoclinic, P2(1)/c, a = 10.767(2) angstrom, b = 20.092(2) angstrom, c = 22.370(3) angstrom, beta = 92.13-degrees, Z = 4, R = 0.076). Complex 6 contains an irregular tetrahedral Co4 cluster with two edges bridged by dmpm ligands and one face edge-bridged by carbonyl groups. The Co-Co bond lengths are 2.426(1)-2.541(2) angstrom. The cluster 8 contains a central copper atom with approximately square-planar stereo-chemistry, which is apparently unique in copper clusters.