Rh 4(CO) 12-catalyzed hydroformylation of cyclopentene promoted with HMn(CO) 5. Another example of Rh 4(CO) 12/HMn(CO) 5 bimetallic catalytic binuclear elimination

Abstract

A detailed in situ high-pressure FTIR spectroscopic study was performed to investigate the bimetallic origins of catalytic synergism in homogeneous catalysis. The reaction chosen was the homogeneous catalyzed hydroformylation of cyclopentene to cyclopentanecarbox-aldehyde, starting with unmodified rhodium and manganese carbonyls as catalyst precursors in n-hexane as solvent. The spectra were analyzed by an advanced signal processing and statistical technique. Only four organometallic spectra could be found. These were RCORh-(CO) 4, Rh 4(CO) 12, HMn(CO) 5, and Mn 2(CO) 10. A very significant increase in aldehyde formation was observed in the experiments when both rhodium carbonyl and manganese carbonyl complexes were used simultaneously. The kinetics of product formation shows a distinct linear-bilinear form in observable organometallics; k 1[RCORh(CO) 4][H 2][CO] -1 + k2[RCORh(CO) 4] [HMn(CO) 5] [CO] -1.6. The determined activation parameters for the bilinear term were ΔH ‡ = 47 ± 8 kJ/mol and ΔS ‡ = -88 ± 28 J/(mol K). The results, particularly the bilinear term and the negative entropy of activation, suggest that the origin of synergism is the presence of bimetallic catalytic binuclear elimination: namely, the hydride attack on the acyl species. The present results are consistent with observations and conclusions reached for a previously studied Rh 4(CO) 12/HMn(CO) 5/3,3-dimethylbut-1-ene hydroformylation system.