Metal Cluster Catalysis:  A Kinetic and Mechanistic Study of the Carbonylation of Methanol to Give Methyl Formate as Catalyzed by [Et₄N]₂[Fe₃(CO)₉E] (E = S, Se, Te)

The cluster compounds [Et 4 N] 2 [Fe 3 (CO) 9 E] (E = S, Se, Te) catalyze the homogeneous carbonylation of methanol to form methyl formate under moderate CO pressures (600−1200 psi) between 50 and 90 °C. A detailed kinetic analysis of this system shows that the reaction is first order with respect to cluster concentration and quasi-second order with respect to the pressure of CO. All three cluster compounds show similar behavior. The reaction appears to be almost independent of the [CH 3 O - ] total at values of [CH 3 O - ] total ≤ 3 × 10 - 3 M, but at higher concentrations it appears to be first-order dependent on [CH 3 O - ] total . This system affords the opportunity to compare the effects of different main group heteroatoms in a homologous series. The rates of the reaction were found to lie in the following order: Te (6 × 10 - 4 M·min - 1 ) > S (4 × 10 - 4 M·min - 1 ) > Se (2 × 10 - 4 M·min - 1 ) at 1200 psi and 90 °C. The activation energies were found to be 43 ± 11 kJ·mol - 1 (E = S), 76 ± 8 kJ·mol - 1 (E = Se), and 72 ± 7 kJ·mol - 1 (E = Te). The rate dependence upon CO pressure suggests a mechanism in which metal−metal bond opening is important.