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(CO)3(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.
The reactivity (e.g., toward hydrolysis, alcoholysis, reduction by CO or H2) of various [Os3(CO)10(μ-H)(μ-OSiPh2R‘)] (R‘ = Ph, OH, OSiPh2OH) clusters and the thermal behavior of [Os3(CO)10(μ-H)(μ-OSiEt3)] have been studied with the aim of clarifying by a molecular approach some aspects of the surface chemistry of silica-anchored [Os3(CO)10(μ-H)(μ-OSi⋮)]. Their easy and selective reduction to [Os3(CO)12] (under CO) and to [H4Os4(CO)12] (under H2) suggests that [Os3(CO)10(μ-H)(μ-OSi⋮)] does not require, as a reactive intermediate, a previous hydrolysis to the more reactive molecular species [Os3(CO)10(μ-H)(μ-OH)] in order to generate different osmium carbonyl clusters in their silica-mediated synthesis starting from OsCl3 or [Os(CO)3Cl2]2. The thermal behavior of [Os3(CO)10(μ-H)(μ-OSiEt3)] dissolved in triethylsilanol (to mimic a silica surface with many available surface silanols) or triglyme (to mimic a highly dehydroxylated silica surface) gives an answer to the controversy on the nature of the products formed by thermal degradation on the silica surface of [Os3(CO)10(μ-H)(μ-OSi⋮)]. In triethylsilanol, oxidation occurs to give a Os(II) hydrido carbonyl species which, on the basis of chemical and spectroscopic evidence, we suggest to be [Os(CO)3(μ-OSiEt3)2(OSiEt3)(H)Os(CO)2] n (n = probably 2), whereas in triglyme an aggregation to high-nuclearity clusters such as [H4Os10(CO)24]2- and [H5Os10(CO)24]- occurs. Therefore, it is shown for the first time that molecular models not only are a tool to define structural aspects but also may be a springboard to understand and clarify by a molecular approach aspects of the reactivity of organometallic species on the silica surface.
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