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The intermolecular C−H bond activation photochemistry of (HBPz‘3)Rh(CO)2 (Pz‘ = 3,5-dimethylpyrazolyl) has been investigated in various alkane solutions at 293 K. Excitations have been performed at 313−458 nm into the lowest energy absorption band of this complex, and the reactions have been monitored throughout photolysis by in situ UV−visible and FTIR spectroscopy. The spectral results reveal that the photochemistry is exceptionally clean and that the reagent complex can be completely converted to the corresponding (HBPz‘3)Rh(CO)(R)H photoproduct at each of the irradiation wavelengths. Absolute photochemical quantum efficiencies (φCH) for these reactions have been determined and illustrate that the intermolecular C−H bond activation process is strongly wavelength dependent. Very effective conversion (φCH = 0.31−0.34) is attained upon near-UV excitation at 313 or 366 nm. In contrast, inefficient conversion (φCH = 0.010−0.011) is observed on visible excitation at 458 nm. The differences in reactivities are interpreted on a photophysical scheme in which the irradiations produce two ligand field (LF) electronically excited states with different primary photoproducts. The thermal chemistry of (HBPz‘3)Rh(CO)2 in room-temperature solution is characterized as involving facile η3 ↔ η2 ligand interconversions. Accordingly, the complex is readily protonated to form [{η2-HBPz‘2(Pz‘H)}Rh(CO)2]BF4, and when PPh3 is added to a solution of (HBPz‘3)Rh(CO)2, the ligand substitution product, (HBPz‘3)Rh(CO)PPh3, is formed immediately. In contrast, there is no evidence of thermal ligand scavenging of (HBPz‘3)Rh(CO)2 by pyridine and upon irradiation in pyridine solutions the photochemistry is dominated by the C−H activation reaction of the hydrocarbon solvent. In triethylsilane solutions the observed spectral changes reveal that intermolecular Si−H bond activation takes place readily on excitation of (HBPz‘3)Rh(CO)2 at 366 nm. The thermal chemistry of the analogous square planar complex (H2BPz2)Rh(CO)2 has been investigated, and the complex has been found to be reactive toward PPh3 and not R−H in room-temperature solution under dark conditions. These experimental observations imply that the long-wavelength (458 nm) photochemistry and thermal chemistry of (HBPz‘3)Rh(CO)2 are associated with a (η2-HBPz‘3)Rh(CO)2 intermediate that is unable to facilitate hydrocarbon C−H bond activation. Importantly, the observed results also suggest that the short-wavelength (313, 366 nm) photochemistry proceeds via an extremely short-lived monocarbonyl (HBPz‘3)Rh(CO) complex that undergoes efficient C−H activation.

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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.

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