Surface Catalytic Sites Prepared from [HRe(CO)5] and [H3Re3(CO)12]: Mononuclear, Trinuclear, and Metallic Rhenium Catalysts Supported on Magnesia.

Abstract

MgO-supported catalysts were prepared from [HRe(CO)5] and [H,Re3(CO),,] and characterized by extraction of surface organometallics, infrared and ultraviolet/visible spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS analysis and other data show that [H,Re,(CO),,] was initially deprotonated on the MgO surface, giving a surface-bound anion with a structure comparable to that of the salt [Ph,As] [H2Re,(C0)12] and having a Re-Mg distance of 2.39 8,. Heating of the supported cluster anion in helium to 225 OC led to oxidation and breakup of the cluster framework, giving a mononuclear complex formulated as [Re(C0)3(0MgJ3(]w here the braces refer to groups terminating the bulk oxide). The distances characterizing the bonding of the Re to the support are Re-O = 2.15 8, and Re-Mg = 2.80 8,. A Re-Re distance of 3.94 8, was observed, consistent with the decomposition of the cluster on the support to form ensembles consisting of three of the Re subcarbonyls, for which a structural model is presented. Treatment of this sample in hydrogen at 350 'C gave a Re species with oxygen neighbors at average distances of 1.94 and 2.45 8,. Heating of this sample to 500 OC in hydrogen led to reduction and conversion of most of the Re into metal crystallites. The several samples were tested as catalysts, as described in a companion paper.