Surface organometallic chemistry: Understanding the multistep process of silica-mediated synthesis of various osmium carbonyl clusters from supported alpha-[Os(CO)(3)Cl-2](2)

Reaction of silica-supported α-[Os(CO)3Cl2]2 in the presence of alkali-metal carbonates affords reactive surface osmium(II) species. The nature of the latter depends on the basicity given to the silica surface, with K2CO3 behaving as a stronger base than Na2CO3 when supported on silica. Infrared evidence suggests that with a low basicity (for instance, molar ratio Na2CO3:Os = 2:1), surface species such as [Os(CO)3(OR)2]n (R = H, Si≤) are initially formed; an increase of the surface basicity (molar ratio (Na2CO3 or K2CO3):Os = (10-20):1) leads to the formation of probably anionic {[Os(CO)3(OR)2]m(OR)}- (R = H, Si≤; m > 1) entities up to the less reactive species [Os(CO3(OH)3]-. The high reactivity of these surface species is confirmed by the controlled reduction by CO or H2 of silica-supported [Os(CO)3-(OH)2]n in the presence of alkali-metal carbonates, which leads selectively to either neutral ([Os3(CO)12], [H4Os4(CO)12]) or anionic ([H3Os4(CO)12]-, [Os10C(CO)24]2-) clusters, in accord with results obtained with supported α-[Os(CO)3Cl2]2. There is direct and indirect evidence that the aggregation process occurs via silica-anchored [HOs3(CO)10(OSi)] or silica-supported [HOs3(CO)10(OH)] species, followed by further condensation to [H4Os4(CO)12] or [H3Os4(CO)12]-according to the basicity of the surface. The nature and the quantity of added alkali carbonate (Na2CO3 or K2CO3), together with the temperature, influence the formation of either [H3Os4(CO)12]- or [H2Os4(CO)12]2-, which can act as intermediates for further condensation to cluster anions of higher nuclearity. In addition to these reaction parameters, the amount of H2 in the gas phase is also crucial in defining the relative stability and the reactivity of the surface species [H3Os4(CO)12]- and [H2Os4(CO)12]2- and their further condensation to specific carbonyl cluster anions.