INFRARED SPECTROSCOPIC CHARACTERIZATION OF MOLYBDENUM CARBONYL SPECIES FORMED BY ULTRAVIOLET PHOTOREDUCTION OF SILICA-SUPPORTED MO(VI) IN CARBON-MONOXIDE

The molybdenum carbonyl species formed by ultraviolet photoreduction of Mo6+/SiO2 in CO were characterized with Fourier transform infrared spectroscopy and temperature-programmed decomposition (TPDE). Mo6+/SiO2 samples containing 0.07-6.4% Mo were prepared from MoCl5,Mo2(eta3-C3H5)4, and (NH4)6Mo7O24.4H2O. Four molybdenum carbonyl species have been identified: mer-Mo4+(CO)3, cis-Mo4+ (CO)2, linear Mo4+(CO), and MO(CO)6. The stoichiometry of photoreduction, decomposition, and reoxidation supports the +4 oxidation state for the mono-, di-, and tricarbonyls. The C2, symmetry of mer-Mo(CO)3 resulted in an IR spectrum consisting of a weak (A1), symmetric trans C-O stretch at 2181 cm-1, a strong B2 antisymmetric trans C-O stretch at 2141 cm-1, and a strong (A1)2 cis C-O stretch at 2108 cm-1. The mer-MO(CO)3 structure successfully predicted the observed frequencies and intensities of partially substituted (Mo(CO)x(CO)3-x)-C-12-O-16-C-13-O-16 and (Mo(CO)x(CO)3-x)-C-12-O-16-C-13-O-18 (x = 1, 2). Mo(CO)3 was stable at 300 K with CO partial pressures above 60 Torr. At 193 K, this species was stable under vacuum. Evacuation of mer-Mo(CO)3 at 300 K led to CO ligand loss, resulting in the sequential formation of cis-Mo(CO)2, linear Mo(CO), and finally, CO-free Mo4+. The tricarbonyl assignment is supported by the TPDE pattern, in which two CO ligands were released near 350 K, followed by desorption of the final CO group near 440 K. The stable species are photoformed in the sequence mer-Mo4+(CO)3, cis-Mo4+(CO)2, and Mo(CO)6. Photoformed Mo(CO)6 was found to both physisorb on silica and chemisorb on reduced molybdenum cations.