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 Nicholas Dunwoody and Alistair J. Lees* Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902-6016 Received October 2, 1997 X 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 photo- product. 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 photoprod- ucts 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. Introduction The photochemistry of CpML 2 and Cp*ML 2 (Cp ) η 5 - C 5 H 5 , Cp* ) η 5 -C 5 Me 5 ;M ) Rh, Ir; L ) CO, olefin, PR 3 ) complexes has been vigorously investigated ever since the discovery of their intermolecular C-H bond activa- tion reactivity upon light excitation. 1 Initial synthetic studies provided competitive rates of C-H bond activa- tion for several of these systems. 1,2 Subsequently, a number of photochemical methods have been used to investigate these important reaction mechanisms. These experiments have included low-temperature matrix isolation, 3 laser-flash photolysis (including ultrafast spectroscopy), 4 solvation in liquefied noble gases, 5 and quantum efficiency determination. 6 Measurements on X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) (a) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1982, 104, 352. (b) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1983, 105, 3929. (c) Hoyano, J. K.; Graham, W. G. J. Am. Chem. Soc. 1982, 104, 3723. (d) Hoyano J. K.; McMaster, A. D.; Graham, W. A. G. J. Am. Chem. Soc. 1983, 105, 7190. (e) Periana, R. A.; Bergman, R. G. Organometallics 1984, 3, 508. (f) Wax, M. J.; Stryker, J. M.; Buchanan, J. M.; Kovac, C. A.; Bergman, R. G. J. Am. Chem. Soc. 1984, 106, 1121. (g) Janowicz, A. H.; Periana, R. A.; Buchanan, J. M.; Kovac, C. A.; Stryker, J. M.; Wax, M. J.; Bergman, R. G. Pure Appl. Chem. 1984, 56, 13. (h) Periana, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1984, 106, 7272. (i) Arndtsen, B. A.; Bergman, R. G.; Mobley, A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154. (2) (a) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1982, 104, 4240. (b) Jones, W. D.; Feher, F. J. Organometallics 1983, 2, 562. (c) Jones, W. D.; Feher, F. J. Organometallics 1983, 2, 686. (d) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1984, 106, 1650. (e) Jones, W. D.; Feher, F. J. Inorg. Chem. 1984, 23, 2376. (f) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1985, 107, 620. (3) (a) Rest, A. J.; Whitwell, I.; Graham, W. A. G.; Hoyano, J. K.; McMaster, A. D. J. Chem. Soc., Chem. Commun. 1984, 624. (b) Rest, A. J.; Whitwell, I.; Graham, W. A. G.; Hoyano, J. K.; McMaster, A. D. J. Chem. Soc., Dalton Trans. 1987, 1181. (c) Haddleton, D. M.; Perutz, R. N. J. Chem. Soc., Chem. Commun. 1985, 1372. (d) Haddleton, D. M. J. Organomet. Chem. 1986, 311, C21. (e) Haddleton, D. M.; McCramley, A.; Perutz, R. N. J. Am. Chem. Soc. 1988, 110, 1810. (f) Bloyce, P. E.; Rest, A. J.; Whitwell, I.; Graham, W. A. G.; Holmes- Smith, R. J. Chem. Soc., Chem. Commun. 1988, 846. 5770 Organometallics 1997, 16, 5770-5778 S0276-7333(97)00855-8 CCC: $14.00 © 1997 American Chemical Society