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) spectral and nanosecond time-resolved infrared (TRIR) spectral studies of intermediates generated when the rhodium(I) complexes trans-RhCl(CO)L2 (L = PPh3 (I), P(p-tolyl)3 (II), or PMe3 (III)) are subjected to photoexcitation. Each of these species, which are precursors in the photocatalytic activation of hydrocarbons, undergoes CO labilization to form an intermediate concluded to be the solvated complex RhCl(Sol)L2 (A(i)). The picosecond studies demonstrate that an initial transient is formed promptly (<30 ps), which decays to A(i) with lifetimes ranging from 40 to 560 ps depending upon L and the medium. This is proposed on the basis of ab initio calculations to be a metal-to-ligand charge transfer (MLCT) excited state. Second-order rate constants (kCO) for reaction of the A(i) with CO were determined, and these depend on the nature of L and the solvent, the slowest rate being for A(I) in tetrahydrofuran (kCO = 7.1 x 10(6) M(-1) x s(-1)), the fastest being for A(III) in dichloromethane (1.3 x 10(9) M(-1) x s(-1)). Each A(i) 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. Although 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 addition could not be quenched. Thus, the short-lived MLCT state or a vibrationally hot species formed during the decay of this excited state appears to participate directly in C-H activation.