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.