SOLUTION PHOTOCHEMISTRY OF (ETA-5-C5R5)RH(CO)2 (R = H, ME) COMPLEXES - PATHWAYS FOR PHOTOSUBSTITUTION AND C-H/SI-H BOND ACTIVATION REACTIONS

The detailed ligand photosubstitution chemistry of CpRh(CO)2 and Cp*Rh(CO)2 (CP = eta-5-C5H5; Cp* = eta-5-C5R5) has been investigated in deoxygenated decalin solutions containing a series of phosphine and phosphite (PR3) ligands. The solution photoreactions have been monitored by diode-array UV-visible and FTIR spectroscopy following excitation at 313 and 458 nm. Spectral sequences recorded over the time of photolysis reveal that these photoreactions involve clean and complete conversions to the corresponding CpRh(CO)PR3 and Cp*Rh(CO)PR3 products. The reactions are attributed to occur from ligand field (LF) excited states. Photochemical quantum efficiencies have been determined and in each system they exhibit a linear relationship with the entering PR3 ligand concentration and a dependence on the nature of the scavenging nucleophile; these observations illustrate that the ligand photosubstitution reactions proceed via an associative mechanism. A comparison of the quantum efficiency results obtained for the Cp and Cp* systems indicates that the nature of the cyclopentadienyl ring substantially affects the reaction efficiency, and it is suggested that the primary photoproduct involved is formed as a consequence of a eta-5 --> eta-3 ring slip process. The photochemistry of CpRh(CO)2 and Cp*Rh(CO)2 has also been studied in triethylsilane (Et3SiH) solution to determine the nature of the Si-H bond activation reaction. In these cases the observed quantum efficiency values are independent of Et3SiH concentration, illustrating that these reactions proceed via a CO dissociative process. Ligand photosubstitution quantum efficiencies obtained following 313-nm excitation provide further evidence for the participation of the dissociative mechanism arising from a higher energy LF excited state. The experimental results lead to a postulated mechanistic scheme that represents both the ligand photosubstitution and photochemical C-H/Si-H bond activation pathways and to a description of the reaction intermediates involved.