Kinetics and thermodynamics of proton and hydrogen atom transfer from transition metal hydrides: Mechanistic implications and catalytic applications

We have compared the rate at which a proton is transferred to the hydride ligand of CPW(CO)₂(PMe₃)H with the rate at which one is transferred to the metal. Rate constants were obtained by 1H NMR for H/D exchange between CPW(CO)₂(PMe₃)H and 4- t-butyl-N,N-dimethylanilinium-N-d ₁; an unusual equilibrium isotope effect (EIE) of 0.19 was observed and explained by location of the vibrational normal modes of CPW(CO) ₂(PMe₃)H, 4-t-butyl-N,N-dimethylanilinium, and their deuterated analogs, via IR and Raman spectroscopy. A pK_a of 5.6(1) in CH₃CN was determined for CPW(CO)₂(PMe ₃)H₂+ by IR; the importance of homoconjugate pair interactions in protonation equilibria is illustrated and discussed. The exchange rate data and the rate constant for deprotonation of CPW(CO) ₂(PMe₃)H₂+ by 4-t-butyl- N,N-dimethylaniline, combined with the pKa data, provide quantitative evidence that the kinetic site of protonation of CPW(CO)₂(PMe ₃)H is the hydride ligand.; (η5-C₅Ph₅)Cr(CO)₃• can serve as an effective chain transfer catalyst for the free radical polymerization of methyl methacrylate (MMA). The mechanism of chain transfer involves H• 5Ph₅)Cr(CO)₃• (k_tr ), followed by H• transfer from (η5-C₅ Ph₅)Cr(CO)₃H to MMA to start a new chain ( k_H). The rates of H• transfer were obtained by 1H NMR for isotopic exchange between (η5-C₅R ₅)Cr(CO)₃H (R = H, Me, or Ph) and MMA-d ₅ or styrene-d₈. The second order k_H rate constants at 50°C were about an order of magnitude faster for styrene versus MMA. For MMA, k_H is primarily determined by sterics. For styrene, the rates of H• transfer were more uncertain due to hydrogenation and organic radical termination competing with isotopic exchange, and no steric effect was apparent. The rates of chain transfer (k_tr) to MMA and styrene were obtained by observing the growth of (η5-C₅Ph₅)Cr(CO) ₃• by UV-Vis. The values of k_H and k_tr give an indirect measure of the C-H bond strengths in the methyl-isobutyrl and methylbenzyl radicals.; We are also exploring the use of chromium hydrides for radical cyclization of dienes and enynes. H• transfer from chromium hydrides to internal double bonds is too slow to lead to cyclization. In contrast, hydrogen atom transfer was observed to external double bonds, but the cyclization rates were too slow.