Di- and poly-nuclear transition metal complexes as catalysts for the metal carbonyl substitution reaction. The role of supported metals and metal oxides as catalyst promoters

Abstract

Various di- and poly-nuclear transition metal complexes have been investigated as catalysts for the metal carbonyl substitution reaction. The complexes [{(η5-C5H4R)Fe(CO)2} 2] (R = H, Me, CO2Me, OMe, O(CH2)4OH) and [{(η5-C5H5)-Ru(CO)2} 2] are active catalysts for a range of substitution reactions including the probe reaction [Fe(CO)4(CNBut)] + ButNC → [Fe(CO)3(CNBut)2] + CO. [{(η5-C5Me5)Fe(CO)2}2] is catalytically active only on irradiation with visible light. For [{η5-C5H5)Fe(CO)2}2] and a range ofisocyanides RNC ( R = But, C6H5CH2, 2,6-Me2C6H3), catalyst modification by substitution with isocyanide is a major factor influencing the degree of the catalytic effects observed, e.g. [{(η5-C5H5)Fe(CO)(CNBut)}2] is approximately 35 times as active as [(η5-C5H5)2FE2(CO)3(CNBut)] for the [Fe(CO)4(CNBut)] → [Fe(CO)3(CNBut)2] conversion. Mechanistic studies on this system suggest that the catalytic substitution step probably involves a rapid intermolecular attack of isonitrile, possibly on a labile catalyst-substrate radical intermediate such as {[Fe(CO)4(CNR)][(η5-C5H5)Fe(CO)2]}; or on a reactive radical cation such as [Fe(CO)4(CNR)]+ generated via electron transfer between the substrate and the catalyst. Other transition metal complexes which also catalyze the substitution of CO by isocyanide in [Fe(CO)4(CNR)] (and [M(CO)6] (M = Cr, Mo, W), [Mn2(CO)10], [Re2(CO)10]) include [Ru3(CO)12], [H4Ru4(CO)12], [M4(CO)12] (M = Co, Ir) and [Co2(CO)8]. These reactions conform to the general mechanistic patterns established for [{(η5-C5H5)Fe(CO)2}2], suggesting a similar mechanism. A range of materials, notably PtO2, PdO and Pd/C, act as promoters for the homogeneous di- and poly-nuclear transition metal catalysts, and can even be used to induce activity in normally inactive dimer and cluster complexes e.g. [Os3(CO)12]. This promotion is attributed to at least three possible effects: the removal of catalyst inhibitors, a catalyzed substitution of the homogeneous catalyst partner, and a possible homogeneous-heterogeneous interaction which promotes the formation of catalytic intermediates.