INDIRECT COOPERATIVE EFFECTS LEADING TO SYNERGISM IN BIMETALLIC HOMOGENEOUS CATALYSTS CONTAINING AZOLATES AS BRIDGING LIGANDS

The binuclear compounds [H(CO)(PPh3)2Ru(mu-bim)Ir(COD)] (1) (bim = 2,2'-biimidazolate, COD = 1,5-cyclooctadiene) and [H(CO)(PPh3)2Ru(mu-pz)2Ir(TFB)] (2) (pz = pyrazolate, TFB = tetrafluorobenzobarrelene) are more active catalysts for the hydrogenation of cyclohexene than the mononuclear parent compounds [RuH(Hbim)(CO)(PPh3)2], [Ir(Hbim)(COD)], [RuH(pz)(CO)(Hpz)(PPh3)2], and [Ir(TFB)-(Hpz)2]BF4. In the presence of 1, the reaction rate is first order with respect to the concentration of 1 and cyclohexene, second order with respect to hydrogen pressure, and inversely proportional to the concentration of added phosphine. For the reaction catalyzed by 2, the experimental data are in accordance with a rate expression of the form -d[cyclohexene]/dt = k[2][cyclohexene]P(H2) [P(H2) = hydrogen pressure]. On the basis of the kinetic results and experimental evidence, the origin of the catalytic synergism is assigned to indirect cooperative effects between the metallic centers of the binuclear complexes. The kinetic investigation of the hydrogenation of cyclohexene catalyzed by [Ir(mu-pz)(TFB)]2 (3) is also reported, suggesting that the full catalytic cycle involves binuclear species.