Surface Organometallic Chemistry: Understanding the Multistep Process of Silica-Mediated Synthesis of Various Osmium Carbonyl Clusters from Supported α-[Os(CO)3Cl2]2

Reaction of silica-supported α-[Os(CO) 3 Cl 2 ] 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 K 2 CO 3 behaving as a stronger base than Na 2 CO 3 when supported on silica. Infrared evidence suggests that with a low basicity (for instance, molar ratio Na 2 CO 3: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 (Na 2 CO 3 or K 2 CO 3 ):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 H 2 of silica-supported [Os(CO) 3 (OH) 2 ] n in the presence of alkali-metal carbonates, which leads selectively to either neutral ([Os 3 (CO) 12 ], [H 4 Os 4 (CO) 12 ]) or anionic ([H 3 Os 4 (CO) 12 ] -, [Os 10 C(CO) 24 ] 2 - ) clusters, in accord with results obtained with supported α-[Os(CO) 3 Cl 2 ] 2 . There is direct and indirect evidence that the aggregation process occurs via silica-anchored [HOs 3 (CO) 10 (OSi⪪)] or silica-supported [HOs 3 (CO) 10 (OH)] species, followed by further condensation to [H 4 Os 4 (CO) 12 ] or [H 3 Os 4 (CO) 12 ] - according to the basicity of the surface. The nature and the quantity of added alkali carbonate (Na 2 CO 3 or K 2 CO 3 ), together with the temperature, influence the formation of either [H 3 Os 4 (CO) 12 ] - or [H 2 Os 4 (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 H 2 in the gas phase is also crucial in defining the relative stability and the reactivity of the surface species [H 3 Os 4 (CO) 12 ] - and [H 2 Os 4 (CO) 12 ] 2 - and their further condensation to specific carbonyl cluster anions.