Structural characterization of photoreduced complete and incomplete cubane-type molybdenum oxide clusters in CO and their catalytic behavior in propene metathesis reaction

SiO2-impregnated complete and incomplete cubane-type molybdenum oxide clusters such as (RhCp*)₄Mo₄O₁₆] (I) and (RhCp*)₂Mo₃O₉(OMe)₄] (II) were photoreduced with a UHP-Hg lamp (> 365 nm) in CO, resulting in the formation of catalytically active species for propene metathesis at 300 K. The CO-photoreduced Mo oxide clusters were characterized by EXAFS, IR, XPS and TPD technique. The results suggest that the Mo-O_4c (four-centered bridging oxygens) in I were specifically reduced with CO under the illumination to produce two sets of subcarbonyl species, e.g., those characteristic of the IR bands at 2061 and 2021 cm^–1, which were thermally inactive for the¹³CO exchange reaction but very active under illumination at 300 K, possibly assignable to Mo(CO)x (x = 2, 3), whereas those at 2092 and 2035 cm^–1 due to Rh(CO)₂, which were readily exchangeable with¹³CO at 300 K in dark. Removal of both carbonyls attached on Rh and Mo in photoreduced I and II by evacuation at 375–440 K led to the formation of oxygen-deficient Mo⁴+/Mo⁵⁺ sites, which exhibited high catalytic activities in propene metathesis at 300 K to produce an equimolar mixture of ethene and 2-butenes. The CO-photoreduced incomplete cubane Mo oxide cluster (II) exhibited higher activities and higher trans/cis ratios of 2-butenes in the reaction, compared with those on the photoreduced I.