Kinetics and Thermodynamics of H• Transfer from (η5-C5R5)Cr(CO)3H (R = Ph, Me, H) to Methyl Methacrylate and Styrene

The rates of H/D exchange have been measured between (a) the activated olefins methyl methacrylate-d5 and styrene-d8, and (b) the Cr hydrides (η5-C5Ph5)Cr(CO)3H (2a), (η5-C5Me5)Cr(CO)3H (2b), and (η5-C5H5)Cr(CO)3H (2c). With a large excess of the deuterated olefin the first exchange goes to completion before subsequent exchanges begin, at a rate first order in olefin and in hydride. (Hydrogenation is insignificant except with styrene and CpCr(CO)3H; in most cases, the radicals arising from the first H• transfer are too hindered to abstract another H•.) Statistical corrections give the rate constants kreinit for H• transfer to the olefin from the hydride. With MMA, kreinit decreases substantially as the steric bulk of the hydride increases; with styrene, the steric bulk of the hydride has little effect. At longer times, the reaction of MMA or styrene with 2a gives the corresponding metalloradical 1a as termination depletes the concentration of the methyl isobutyryl radical 3 or the α-methylbenzyl radical 4; computer simulation of [1a] as f(t) gives an estimate of ktr, the rate constant for H• transfer from 3 or 4 back to Cr. These rate constants imply a ΔG (50 °C) of +11 kcal/mol for H• transfer from 2a to MMA, and a ΔG (50 °C) of +10 kcal/mol for H• transfer from 2a to styrene. The CH3CN pKa of 2a, 11.7, implies a BDE for its Cr−H bond of 59.6 kcal/mol, and DFT calculations give 58.2 kcal/mol for the Cr−H bond in 2c. In combination the kinetic ΔG values, the experimental BDE for 2a, and the calculated ΔS values for H• transfer imply a C−H BDE of 45.6 kcal/mol for the methyl isobutyryl radical 3 (close to the DFT-calculated 49.5 kcal/mol), and a C−H BDE of 47.9 kcal/mol for the α-methylbenzyl radical 4 (close to the DFT-calculated 49.9 kcal/mol). A solvent cage model suggests 46.1 kcal/mol as the C−H BDE for the chain-carrying radical in MMA polymerization.