Surface Organometallic Chemistry:  Synthesis and X-ray Characterization of Novel Silanolate Surface Models [Re2(CO)8(μ-H)(μ-OSiR2R‘)] and of the First Models with Two Homo and Hetero Metal Carbonyl Fragments Linked to Vicinal or Geminal Silanols

Reaction of [Re2(CO)8(THF)2] with R2R‘SiOH (R = Et, Ph; R‘ = Et, Ph, OH, OSiPh2OH) gives the new molecular models [Re2(CO)8(μ-H)(μ-OSiR2R‘)], which mimic a surface [Re2(CO)8(μ-H)(μ-OSi⋮)] species anchored to isolated, geminal, or vicinal silanols. The bridging structure has been confirmed by the X-ray structure of [Re2(CO)8(μ-H)(μ-OSiPh2OH)]. Thermal treatment (70 °C) of [Re2(CO)8(μ-H)(μ-OH)] with excess Et3SiOH is another way to form [Re2(CO)8(μ-H)(μ-OSiEt3)], which can be reconverted into the hydroxo complex by addition of excess water at the same temperature. Both [Re2(CO)8(μ-H)(μ-OSiEt3)] and [Re2(CO)8(μ-H)(μ-OH)] are readily transformed into [Re2(CO)10] under 1 atm of CO at 150 °C. These molecular models, which are the only neutral carbonyl rhenium complexes bearing silanolate ligands reported up to now, constitute a new tool to clarify the first step of the surface chemistry of the photochemical interaction of [Re2(CO)10] with a rather inert support such as silica. Although the free silanol OH of both [Re2(CO)8(μ-H)(μ-OSiPh2OSiPh2OH)] and [Re2(CO)8(μ-H)(μ-OSiPh2OH)] is much less reactive than the OH of the corresponding noncoordinated silanol, it reacts at 25 °C with excess [Re2(CO)8(THF)2] to give the first bimetallic models [Re2(CO)8(μ-H)(μ-OSiPh2OSiPh2O-μ)(μ-H)Re2(CO)8] and [Re2(CO)8(μ-H)(μ-OSiPh2O-μ)(μ-H)Re2(CO)8], respectively. The latter complex, less stable to hydrolysis, is of particular interest because it constitutes the first molecular model of two metal carbonyl fragments linked to geminal surface silanols, thus suggesting that this kind of bimetallic system may occur also on a surface. Remarkably, the reaction of the rather unreactive [Os3(CO)10(μ-H)(μ-OSiPh2OSiPh2OH)] cluster with the very reactive [Re2(CO)8(THF)2] affords [Re2(CO)8(μ-H)(μ-OSiPh2OSiPh2O-μ)(μ-H)Os3(CO)10], the first heterobimetallic molecular model whose structure has been determined by X-ray diffraction.