Site selectivity in carbonyl reduction of mixed Fe 2Co 2 clusters. Formyl intermediates in the formation of hydrido derivatives

Abstract

The mixed metal cluster Co 2Fe 2(CO) 11(μ 4-PPh) 2 (II) undergoes a site-selective reduction of one of the carbonyl ligands by triethylborohydride reagent to afford a single formyl derivative Co 2Fe 2(CO) 10(PPh) 2CHO − (III) in excellent yields at −78°C. The same terminal CO bound to cobalt also selectively undergoes addition of methyllithium to afford the corresponding acetyl derivative (V). Whereas V is thermally robust at room temperature, the formyl derivative III is a transient species which readily loses carbon monoxide at −30°C to afford the μ 2-hydride Co 2Fe 2(CO) 10(PPh) 2H − in quantitative yields. All of these site-selective reactions are compared to the corresponding transformations which were previously observed in the homonuclear analogue Co 4(CO) 10(μ 4-PPh) 2 (I). The structural similarities of homo- and heteronuclear clusters I and II, respectively, are considered in the context of the fluxional behavior of their bound carbonyl ligands. The comparisons are extended to the IR, 1H and 13C NMR properties of their formyl, hydrido, and acetyl derivatives.