Solvent Study of the Kinetics of Molybdenum Radical Self-Termination

The kinetics of (n-butylCp)Mo(CO)3 (n-butylCp is n-butyl-η5-cyclopentadienyl) radical self-termination to form a nonequilibrium mixture of trans- and gauche-[(n-butylCp)Mo(CO)3]2 and the kinetics of the gauche-to-trans isomerization have been determined in the liquid solvents n-heptane, tetrahydrofuran, xenon (350 bar), and CO2 (350 bar) at 283 K by step-scan FTIR spectroscopy. The overall rate constant for the disappearance, 2k R, of the (n-butylCp)Mo(CO)3 radical increases with decreasing solvent viscosity as expected, except in CO2, which is anomalously slower. The slower overall termination rate in liquid CO2 is consistent with the formation of a transient molybdenum radical−CO2 complex. The observed overall rate constants for (n-butylCp)Mo(CO)3 self-termination, 2k R, are (7.9 ± 0.5) × 109 M-1 s-1 in xenon; (3.2 ± 0.5) × 109 M-1 s-1 in heptane; (2.2 ± 0.8) × 109 M-1 s-1 in THF; and (1.7 ± 0.5) × 109 M-1 s-1 in CO2. The first determinations of the radical self-termination-to-gauche rate constants, k G, are presented. The values of k G are much slower than the corresponding recombination to trans, k T, reflecting a steric contribution to the rate. The rate of isomerization (rotation about the molydenum−molybdenum bond) from gauche to trans is unaffected by the solvent and is 3 times faster than the reported isomerization rate for the nonsubstituted [CpMo(CO)3]2 molecule.