Quantitative Measurements of CpRh(CO)₂ (Cp = η⁵-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 = η 5 -C 5 H 5 ) involving ligand substitution and intermolecular C−H and Si−H bond activation processes has been investigated in several hydrocarbon solvents at room temperature following excitation in the region 313−458 nm. These photoreactions have been monitored by UV−vis and FTIR spectroscopy, and the absolute quantum efficiencies (φ cr ), determined to be in the 0.0007−0.31 range, are dependent on the entering ligand concentration, excitation wavelength, and solvent. The observed wavelength dependence is consistent with distinct reaction pathways occurring from two rapidly dissociating ligand-field (LF) excited states. Analysis of the quantitative photochemical results has led to a comprehensive mechanistic description for all of the various competing reaction pathways in the photochemistry of CpRh(CO) 2 . In the absence of an entering ligand, a carbonyl-bridged trans -Cp 2 Rh 2 (CO) 3 complex is identified as the major photochemical reaction product; this species is formed with a low quantum efficiency. When excess triethylsilane (Et 3 SiH) is present in the solution, the CpRh(CO) 2 complex is converted cleanly on irradiation to the silyl hydrido CpRh(CO)(SiEt 3 )H photoproduct. Quantum efficiencies recorded for the Si−H activation process are dependent on the Et 3 SiH concentration in the range of 0.001−0.3 M, exhibiting saturation-type kinetics. Kinetic analysis of the φ cr data implicates a solvated CpRh(CO) primary photoproduct which is scavenged competitively by Et 3 SiH and CpRh(CO) under these solution conditions. When excess triphenylarsine (AsPh 3 ) and triphenylphosphine (PPh 3 ) ligands are present in the hydrocarbon solution, the monosubstituted CpRh(CO)AsPh 3 and CpRh(CO)PPh 3 photoproducts are formed cleanly and completely. Quantum efficiencies obtained for these ligand substitution reactions exhibit an increasing linear dependence with [L] in the range 0.05−0.3 M; kinetic analysis implicates a solvated (η 3 -Cp)Rh(CO) 2 primary photoproduct which is competitively scavenged by AsPh 3 and PPh 3 . In contrast, pyridine is determined to be too poor a nucleophile to effectively scavenge this intermediate. Variations in the quantum efficiencies over a range of alkane, aromatic, and chlorinated hydrocarbon solvents are shown to be dependent on nonradiative deactivation pathways from CpRh(CO) 2 and are not affected by the subsequent oxidative-addition step.