Probing the mechanism of carbon-hydrogen bond activation by photochemically generated hydridotris(pyrazolyl)borato carbonyl rhodium complexes: New experimental and theoretical investigations

Fast time-resolved infrared (TRIR) experiments and density functional (DFT) calculations have been used to elucidate the complete reaction mechanism between alkanes and photolytically activated hydridotris(pyrazoly-1-yl) boratodicarbonylrhodium. TRIR spectra were obtained after photolysis of Rh(Tp^4-tBu-3,5-Me)(CO)₂ in n-heptane, n-decane, and cyclohexane and of Rh(Tp^3,5-Me)(CO)₂ in n-heptane and cyclohexane. Initial photolysis produces a coordinatively unsaturated, 16-electron monocarbonyl species that vibrationally relaxes to an intermediate with v_co of 1971 cm^-1 in n-heptane solution (species A). DFT calculations on Rh(Tp^3,5Me)(CO)-RH (RH = C₂H ₆, C₆H₁₂) suggest that A is the triplet state of a five-coordinate, square-pyramidal Rh(κ³-Tp ^3,5-Me)(CO)-RH, in which the alkane is weakly bound. Within the first 2 ns, a new transient grew in at 1993 cm^-1 (species B). The calculations show that the observed species B is the singlet state of a four-coordinate Rh(κ²-Tp^3,5-Me)(CO)(RH), in which the alkane is strongly bound and one pyrazolyl ring is rotated, decoordinating one N. The transient due to B grew at the same rate as A partially decayed. However, A did not decay completely, but persisted in equilibrium with B throughout the time up to 2500 ps. The v(CO) bands due to A and B decayed at the same rate as a band at 2026 cm^-1 grew in (τ ca. 29 ns, n-heptane). The latter band can be readily assigned to the final alkyl hydride products, Rh(κ³-Tp^4-tBu-3,5-Me)(CO)R(H) and Rh(κ³-Tp^3,5-Me)(CO)R(H) (species D). The experimental data do not allow the elucidation of which of the two alkane complexes, A or B, is C-H activating, or whether both of the complexes react to form the final product. The calculations suggest that a third intermediate (species C) is the C-H activating species, that is, the final product D is formed from C and not directly from either A or B. Species C is nominally a five-coordinate, square-pyramidal Rh(κ^21/2-Tp ^3,5-Me)(CO)(RH) complex with a strongly bound alkane and one pyrazolyl partially decoordinated, but occupying the apical position of the square pyramid. Intermediate C is unobserved, as the calculations predict it possesses the same CO stretching frequency as the parent dicarbonyl. The unobserved species is predicted to lie on the reaction path between A and B and to be in rapid equilibrium with the four-coordinate species B.