Models for partially hydroxylated silica and alumina and the modeling of metal-support interaction of triosmium clusters on silica and alumina

Surface complexes formed from the reactions of Os₃(CO)₁₂ with silica and alumina are generally represented as (μ-H)(μ-OSi) Os₃(CO)₁₀ and (μ-H)(μ-OAl)Os₃(CO)₁₀, with a surface oxygen atom bridging two osmium atoms, and as (μ-OSi)₂Os₃(CO)₁₀ and (μ-OAl)Os₃(CO)₁₀, in which two surface oxygens bridge the same two osmium atoms. These surface complexes have been modeled employing computer graphics techniques. Models for partially hydroxylated silica and gamma-alumina have been developed and metal-support interaction between (μ-H)Os₃(CO)₁₀, Os₃(CO)₁₀, and local regions of support surfaces have been studied. Van der Waals interaction energies, nonbonded contacts as a function of orientation of the cluster with respect to the surface, and possible relaxation of the cluster have been considered. The results indicate that the (μ-H)Os₃(CO)₁₀ unit appears to bind readily to selected oxygen sites of the silica and alumina with little or no steric restraint. On the other hand, the cluster unit Os₃(CO)₁₀ is not readily accommodated on either the silica or alumina surfaces by means of a double-oxygen bridge to two osmium atoms without extreme disruption of the surface or the cluster.