Time-resolved optical and infrared spectral studies of intermediates generated by photolysis of trans-RhCl(CO)PR3)(2). Roles played in the photocatalytic activation of hydrocarbons

Described are picosecond and nanosecond time-resolved optical (TRO) spectra l and nanosecond time-resolved infrared (TRIR) spectral studies of intermed iates generated when the rhodium(I) complexes trans-RhCl(CO)L-2 (L = PPh3 ( I), P(p-tolyl)(3) (II), or PMe3 (III) are subjected to photoexcitation. Eac h of these species, which are precursors in the photocatalytic activation o f hydrocarbons, undergoes CO labilization to form an intermediate concluded to be the solvated complex RhCl(SoL)L-2 (A(i)). The picosecond studies dem onstrate that an initial transient is formed promptly (<30 ps), which decay s to Al with lifetimes ranging from 40 to 560 ps depending upon L and the m edium. This is proposed on the basis of ab initio calculations to be a meta l-to-ligand charge transfer (MLCT) excited state. Second-order rate constan ts (k(CO)) for reaction of the A(i) with CO were determined, and these depe nd on the nature of L and the solvent, the slowest rate being for Ar in tet rahydrofuran (k(CO) = 7.1 x 10(6) M-1 s(-1)), the fastest being for Am in d ichloromethane (1.3 x 10(9) M-1 s(-1)). Each Ar also undergoes competitive unimolecular reaction with solvent to form long-lived transients with TRIR properties suggesting these to be Rh(III) products of oxidative addition. A lthough this was mostly suppressed by the presence of higher concentrations of CO (which trapped A(i) to re-form the starting complexes in each case), both TRO and TRIR experiments indicate that a fraction of the oxidative ad dition could not be quenched. Thus, the short-lived MLCT state or a vibrati onally hot species formed during the decay of this excited state appears to participate directly in C-H activation.