Surface organometallic chemistry on metals influence of the presence of functional groups grafted at the surface of Rh particles on the competitive hydrogenation of terminal and internal double bonds of unsaturated primary alcohols

Careful hydrogenolysis of organometallic compounds with group VIII metallic surfaces lead to surface organometallic fragments directly grafted on the metallic particles with well-defined structures. The presence of such surface organometallic fragments was demonstrated by various physical and chemical techniques (EXAFS, magnetic measurements, IR spectroscopy, Mössbauer, thermoreduction, etc.). Surface organometallic complexes with an average formula M_s[M′R_x]_y (M: Rh, Pt, Ni; M′: Sn, Ge, Si, Se; R: Me, Et, Bu) were proposed. It was recently demonstrated that organometallic fragments with functional groups can also be grafted on a platinum or rhodium surface. In this work, we demonstrate that organo-tin fragments with functional groups, grafted on a silica-supported rhodium surface can strongly modify the regio-selectivity of the hydrogenation of unsaturated primary alcohols. The rate of hydrogenation of trans-2-hexenol and 5-hexenol was determined at room temperature under 20 bar of hydrogen in heptane solution for Rh/SiO₂ catalysts which were either used as they were or were modified with Bu₃Sn(CH₂)₃OH or Bu₂Sn[(CH₂)₂ (O(CH₂)₂)₂OMe]₂. The rate of hydrogenation of the terminal double bond is slightly modified by the presence of functional groups, while that of the internal unsaturation decreases by a factor of 4. These results are explained by the formation of a hydrophilic layer around the metallic particle which inhibits the adsorption of the unsaturated alcohol by its internal double bond.