Photochemical C−H Bond Activation Reactivity of (HBPz‘3)Rh(CO)2 (Pz‘ = 3,5-Dimethylpyrazolyl) in Alkane Solutions

Abstract

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.