Substrate and solvent influence on the photochemical C-H bond activation reactivity of (HBPz'(3))Rh(CO)(2) (Pz'=3,5-dimethylpyrazolyl)

The photochemically-induced intermolecular C-H bond activation reaction of (HBPz'(3))Rh(CO)(2) (Pz' = 3,5-dimethylpyrazolyl) has been investigated in various hydrocarbon solutions at 293 K following excitation at 366 and 458 nm. UV-visible and FTIR spectra recorded throughout photolysis illustrate that the dicarbonyl complex can be converted readily to the corresponding (HBPz'(3))Rh(CO)(R)H derivatives at each of the excitation wavelengths. The photochemistry proceeds without interference from secondary photoprocesses or thermal reactions and the reactivity has been measured quantitatively with the determination of absolute quantum efficiencies for intermolecular C-H bond activation (phi(CH)). These measurements indicate that the C-H activation reaction proceeds very efficiently (phi(CH) = 0.13-0.32) on excitation at 366 nm but is much less effective (phi(CH) = 0.0059-0.011) on photolysis at 458 nm for each of the hydrocarbon substrates. The observed dependence of phi(CH) on irradiation wavelength is consistent with different reactivities from two rapidly dissociating low-energy ligand field (LF) excited states and the generation of monocarbonyl (HBPz'(3))Rh(CO) and ligand-dechelated (eta(2)-HBPz'(3))Rh(CO)(2) intermediates upon UV and visible excitation, respectively. The former species is attributed to be responsible for the unusually efficient C-H bond activation, whereas it is suggested that the latter complex effectively lowers the quantum efficiency by undergoing a facile eta(2)-->eta(3) ligand rechelation process. Significantly, the photoefficiencies are found to be unaffected on increasing the dissolved CO concentration, illustrating that the monocarbonyl reaction intermediate is extremely short-lived and is solvated before CO is able to coordinate. Additionally, the lack of a [CO] dependence on phi(CH) indicates that this solvated intermediate is not subject to a competitive back-reaction with CO prior to the C-H activation step, illustrating that the quantum efficiencies in (HBPz'(3))Rh(CO)(2) appear to be solely determined by the branching ratio between the dissociative and nondissociative routes. At any particular excitation wavelength the photoefficiencies are observed to be similar across the series of alkanes but are significantly reduced for the aromatic solvents, even though the aryl hydrido photoproducts are found to be more thermodynamically stable. These phi(CH) differences are also rationalized in terms of photophysical effects on the upper LF level and are related to variations in the nonradiative relaxation rates for the excited (HBPz'(3))Rh(CO)(2) complex in the hydrocarbon solutions.