QUANTITATIVE MEASUREMENTS OF CPRH(CO)(2) (CP = ETA(5)-C5H5) PHOTOCHEMISTRY IN VARIOUS HYDROCARBON SOLUTIONS - MECHANISMS FOR LIGAND PHOTOSUBSTITUTION AND INTERMOLECULAR C-H AND SI-H BOND ACTIVATION REACTIONS

The quantitative solution photochemistry of CpRh(CO)(2) (Cp = eta(5)-C 5H5) involving ligand substitution and intermolecular C-H and Si-H bon d activation processes has been investigated in several hydrocarbon so lvents at room temperature following excitation in the region 313-458 nm. These photoreactions have been monitored by UV-vis and FTIR spectr oscopy, and the absolute quantum efficiencies (phi(cr)), determined to be in the 0.0007-0.31 range, are dependent on the entering ligand con centration, excitation wavelength, and solvent; The observed wavelengt h dependence is consistent with distinct reaction pathways occurring f rom two rapidly dissociating ligand-field (LF) excited states. Analysi s of the quantitative photochemical results has led to a comprehensive mechanistic description for all of the various competing reaction pat hways in the photochemistry of CpRh(CO)(2). In the absence of an enter ing ligand, a carbonyl-bridged trans-Cp2Rh2(CO)(3) complex is identifi ed as the major photochemical reaction product; this species is formed with a low quantum efficiency. When excess triethylsilane (Et3SiH) is present in the solution, the CpRh(CO)(2) complex is converted cleanly on irradiation to the silyl hydride CpRh(CO)(SiEt3)H photo-product. Q uantum efficiencies recorded for the Si-H activation process are depen dent on the Et3SiH concentration in the range of 0.001-0.3 M, exhibiti ng saturation-type kinetics. Kinetic analysis of the phi(cr) data impl icates a solvated CpRh(CO) primary photoproduct which is scavenged com petitively by Et3SiH and CpRh(CO) under these solution conditions. Whe n excess triphenylarsine (AsPh3) and triphenylphosphine (PPh3) ligands are present in the hydrocarbon solution, the monosubstituted CpRh(CO) AsPh3 and CpRh(CO)PPh3 photoproducts are formed cleanly and completely . Quantum efficiencies obtained for these ligand substitution reaction s exhibit-an increasing linear dependence with [L] in the range 0.05-0 .3 M; kinetic analysis implicates a solvated (eta(3)-Cp)Rh(CO)(2) prim ary photoproduct which is competitively scavenged by AsPh3 and PPh3. I n contrast, pyridine is determined to be too poor a nucleophile to eff ectively scavenge this intermediate. Variations in the quantum efficie ncies over a range of alkane, aromatic, and chlorinated hydrocarbon so lvents are shown to be dependent on nonradiative deactivation pathways from CpRh(CO)(2) and are not affected by the subsequent oxidative-add ition step.