Surface Organometallic Chemistry: Easy Reductive Carbonylation of Silica-Supported [Re(CO)3(OH)]4to [Re2(CO)10] via Silica-Anchored [Re(CO)5(OSi⋮)] and the Thermal Behavior of Silica-Supported [Re2(CO)10]

Silica-supported [Re(CO)3(OH)]4 is easily converted to [Re2(CO)10] by reductive carbonylation under very mild conditions (1 atm CO). This reaction does not occur in solution, suggesting that the silica surface plays a unique role via the surface-anchored species [Re- (CO)5(OSit)]. This latter intermediate, of particular interest since organometallic species of the type [Re(CO)5(OR)] have so far eluded isolation, reacts with HCl, HReO4, and water to give [Re(CO)5Cl], [Re(CO)5OReO3], and [Re(CO)3(OH)]4, respectively. No evidence was reached for the previously proposed formation of [HRe3(CO)14] in the reductive carbonylation of silica-physisorbed [Re(CO)3(OH)]4. In addition, silica-supported [Re2(CO)10] can be easily reoxidized to [Re(CO)3(OH)]4 on the silica surface by thermal treatment at 150-250 °C under nitrogen. Some highly oxidized rhenium species such as ROReO3 (R ) H, Sit) are formed in parallel, as suggested by an XPS study and confirmed by further treatment under 1 atm of CO at 200 °C, which affords mixtures containing also [Re(CO)5OReO3]. The latter was isolated and fully characterized by X-ray diffraction. Obviously these oxidation reactions, in which the silica surface plays an important positive role, are faster in air than under nitrogen.