SURFACE CATALYTIC SITES PREPARED FROM [HRE(CO)5] AND [H3RE3(CO)12] - MONONUCLEAR, TRINUCLEAR, AND METALLIC RHENIUM CATALYSTS SUPPORTED ON MGO

MgO-supported catalysts were prepared from [HRe(CO)5] and [H3Re3(CO)12] 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 [H3Re3(CO)12] was initially deprotonated on the MgO surface, giving a surface-bound anion with a structure comparable to that of the salt [Ph4As][H2Re3(CO)12] and having a Re-Mg distance of 2.39 Å. Heating of the supported cluster anion in helium to 225°C led to oxidation and breakup of the cluster framework, giving a mononuclear complex formulated as [Re(CO)3{OMg}3] (where 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 Å and Re-Mg = 2.80 Å. A Re-Re distance of 3.94 Å 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 Å. Heating of this sample to 500 °C 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. © 1990 American Chemical Society.