Intermediates in the Time-Resolved and Matrix Photochemistry of (η5-Cyclopentadienyl)rhodium Complexes: Roles of Alkane Activation and Rhodium-Rhodium Bond Formation

Article abstract of DOI:10.1021/ja00204a008

Pulsed laser photolysis studies of solutions of Cp2Rh2(CO)3, CpRh(CO)2, and CpRh(C2H4)CO (Cp = η⁵-C5H5) are reported using UV/vis and IR detection.The identification of the transients is supported by earlier studies of Cp2Rh2(CO)3 in hydrocarbon glasses, by prior matrix studies of CpRh(CO)2 and CpRh(CO)(C2H4), and by matrix isolation experiments on Cp2Rh2(CO)3 reported here.Matrix photolysis (20 K, Ar, N2, CH4, and CO matrices) of Cp2Rh2(CO)3 generates 2 reversibly.Isotopic labeling and polarized photolysis demonstrates that the Rh2(CO)2 unit is planar with CO bridges perpendicular to the RhRh axis.Metal-metal bond fission of Cp2Rh2(CO)3 is observed only in CO-doped matrices.The major photoproduct of Cp2Rh2(CO)3 in solution is 2, which reacts with CO to re-form starting material with k₂ = (1.2+/-0.1)E3 dm³mol^-1s^-1 at 292 K (ΔH(excit) = 22.3+/-2.5 kJ mol^-1, ΔS(excit) = -110+/-10 J mol^-1K^-1).The first observable product of photolysis of CpRh(CO)2 in cyclohexane at room temperature is CpRh(CO)(C6H11)H, formed by insertion of CpRhCO into the solvent C-H bonds within 400ns.In the absence of other ligands, CpRh(CO)(C6H11)H decays by complex kinetics (probably by reaction with its reductive elimination product CpRhCO) to form 2 and then slowly to Cp2Rh2(CO)3.The formation of 2 is quenched by high concentrations of other ligands (Et3SiH, P(OPh)3).The reactions with ligands exhibit saturation kinetics, from which the rate of reductive elimination of CpRh(CO)(C6H11)H is measured as k_-1 = (2.7+/-0.3)E3 s^-1.The IR experiments show that photolysis of CpRh(C2H4)CO gives a higher yield of CpRh(CO)(C6H1)H than that from CpRh(CO)2.These experiments show that matrix isolation and room-temperature photolysis experiments are fully consistent, that the kinetics of formation of CpRh(CO)(alkyl)H are favorable, but that this C-H activation product is exceptionally unstable with respect to formation of other products via reductive elimination of alkane.