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

Abstract

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.

Chemical and Thermal Behavior of the Molecular Models [Os3(CO)10(μ-H)(μ-OSiR2R‘)] (R = Et, Ph; R‘ = Et, Ph, OH, OSiPh2OH): Molecular Approach to the Clarification of the Surface Chemistry of the Silica-Anchored Cluster [Os3(CO)10(μ-H)(μ-OSi⋮)]

“…We investigated first the reactivity of the model compound [Os 3 (CO) 10 (μ-H)(μ-OSiPh 2 OSiPh 2 OH)], because at the temperature (ca. 150−200 °C) of the synthesis of various osmium carbonyl clusters starting from OsCl 3 or [Os(CO) 3 Cl 2 ] 2 supported on silica in the presence of alkali-metal carbonates, , ca. 62% of the total surface silanols are vicinal, whereas ca.…”

“…Reduction of [Os 3 (CO) 10 ( μ -H)( μ -OSiPh 2 R ‘ )] (R ‘ = Ph, OH, OSiPh 2 OH) and [Os 3 (CO) 10 ( μ -H)( μ -OH)] . Recently some of us reported that both silica-anchored [Os 3 (CO) 10 (μ-H)(μ-OSi⋮)] and silica-supported [Os 3 (CO) 10 (μ-H)(μ-OH)] can be selectively reduced to various carbonyl clusters ([Os 3 (CO) 12 ], [H 4 Os 4 (CO) 12 ], [H 3 Os 4 (CO) 12 ] - , [H 2 Os 4 (CO) 12 ] 2- , [Os 5 C(CO) 14 ] 2- , and [Os 10 C(CO) 24 ] 2- ) by reductive carbonylation or hydrogenation, under appropriate conditions, in the presence of alkali-metal carbonates. , For a given set of reaction parameters (nature and quantity of alkali-metal carbonate, temperature, gas-phase composition), the selectivity starting from silica-anchored [Os 3 (CO) 10 (μ-H)(μ-OSi⋮)] and silica-supported [Os 3 (CO) 10 (μ-H)(μ-OH)] is similar to that observed starting from silica-supported [Os(CO) 3 Cl 2 ] 2 . , This evidence does not allow us to discriminate between silica-anchored [Os 3 (CO) 10 (μ-H)(μ-OSi⋮)] and silica-supported [Os 3 (CO) 10 (μ-H)(μ-OH)] as intermediates in the silica-mediated synthesis of different osmium carbonyl clusters starting from [Os(CO) 3 Cl 2 ] 2 . , In fact, these two potential intermediates may interconvert under reaction conditions; therefore, due to the presence of surface water, the surface species [Os 3 (CO) 10 (μ-H)(μ-OSi⋮)] could first hydrolyze to the more reactive molecular species [Os 3 (CO) 10 (μ-H)(μ-OH)], which could be the key intermediate also in the surface-mediated synthesis of [H 4 Os 4 (CO) 12 ] by easy hydrogenation of silica physisorbed [Os 3 (CO) 12 ]. , …”

“…In the easy reductive carbonylation (1 atm of CO, 150 °C) of silica-physisorbed [Os(CO) 3 Cl 2 ] 2 to K[H 3 Os 4 (CO) 12 ] in the presence of K 2 CO 3 (molar ratio K 2 CO 3 :Os = 10:1), silica-anchored [Os 3 (CO) 10 (μ-H)(μ-OSi⋮)] and silica-supported [Os 3 (CO) 10 (μ-H)(μ-OH)] were suggested as possible intermediate species. , To confirm the involvement of these species as reactive intermediates, we tried a model homogeneous reaction. When an anhydrous ethylene glycol solution of [Os 3 (CO) 10 (μ-H)(μ-OSiPh 2 OSiPh 2 OH)] is heated under a flux of CO (1 atm) at 160 °C for 14 h in the presence of K 2 CO 3 (molar ratio K 2 CO 3 :Os = 10:1), K[H 3 Os 4 (CO) 12 ] is formed selectively, as shown by infrared spectroscopy of the resulting solution.…”

“…Because of the usefulness of molecular models for getting a better insight into surface organometallic species, − the synthesis and structural characterization of molecular organometallic compounds bearing silanolate ligands as models of the silica-anchored surface species [Os 3 (CO) 10 (μ-H)(μ-OSi⋮)], the first well-described organometallic carbonyl cluster species covalently bound to silica, has attracted increasing attention. 1a,,,6b, However, this approach was limited to the identification of the structural features. Very little is known about the surface chemistry and thermal decomposition of [Os 3 (CO) 10 (μ-H)(μ-OSi⋮)], which not only is a catalyst for olefin isomerization and hydrogenation but which also seems to be an intermediate in the synthesis of various neutral and anionic osmium carbonyl clusters starting from OsCl 3 or [Os(CO) 3 Cl 2 ] 2 supported on silica in the presence of alkali-metal carbonates. , In particular, its thermal decomposition has been first reported by one of us to give rise to Os(II) carbonyl 1 or even to Os(II) hydrido carbonyl species anchored to the surface of silica. In contrast, another group suggested the thermal formation on the surface of high-nuclearity osmium carbonyl clusters …”

On the Origins and Development of “Surface Organometallic Chemistry”

“…In addition, aided by profound knowledge of the nature and reactivity of some surface organometallic species, it was possible to identify the various steps and the nature of intermediates involved in the nucleation processes occurring on the surface in the selective growth of very large clusters such as for instance in the case of [Os 5 C(CO) 14 ] 2 − and [Os 10 C(CO) 24 ] 2 − [52] . As this subject is treated in detail elsewhere in this book it is not covered here.…”