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Quantitative Measurements of CpRh(CO) 2 (Cp = η 5 -C 5 H 5 ) Photochemistry in Various Hydrocarbon Solutions: Mechanisms for Ligand Photosubstitution and Intermolecular C−H and Si−H Bond Activation Reactions

1997; American Chemical Society; Volume: 16; Issue: 26 Linguagem: Inglês

10.1021/om970855t

1520-6041

Nicholas Dunwoody, Alistair J. Lees,

Lanthanide and Transition Metal Complexes

The quantitative solution photochemistry of CpRh(CO)2 (Cp = η5-C5H5) 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 (φcr), 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-Cp2Rh2(CO)3 complex is identified 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 hydrido CpRh(CO)(SiEt3)H photoproduct. Quantum efficiencies recorded for the Si−H activation process are dependent on the Et3SiH concentration in the range of 0.001−0.3 M, exhibiting saturation-type kinetics. Kinetic analysis of the φcr data implicates a solvated CpRh(CO) primary photoproduct which is scavenged competitively by Et3SiH and CpRh(CO) under these solution conditions. When 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 reactions exhibit an increasing linear dependence with [L] in the range 0.05−0.3 M; kinetic analysis implicates a solvated (η3-Cp)Rh(CO)2 primary photoproduct which is competitively scavenged by AsPh3 and PPh3. 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.