Dihydrogen to dihydride isomerization mechanism in [(C5Me5)FeH2( Ph2PCH2CH2PPh2)]+ through the experimental and theoretical analysis of kinetic isotope effects

The isomerization of complex [Cp*Fe(dppe)(eta(2)-H-2)](+), generated in situ by low-temperature protonation of Cp*Fe(dppe)H with either HBF4 or CF3COOH, to the dihydride tautomer trans-[Cp*Fe(dppe)(H)(2)](+) is irreversible and follows first-order kinetics in the -10 to +15 degrees C range with Delta H-double dagger = 21.6 +/- 0.8 kcal mol(-1) and Delta S-double dagger = 5 +/- 3 eu. The isomerization rate constant is essentially independent of the nature and quantity of a strong acid. Density functional theory (DFT) calculations on various models, including the complete system at both the quantum mechanics/ molecular mechanics (QM/MM) and full QM levels, probe the relative importance of steric and electronic effects for the relative stability of the nonclassical and classical isomers and identify two likely isomerization mechanisms: a "direct" pathway involving simultaneous H-H bond breaking and cis-trans isomerization and a "via Cp" pathway involving agostic C5Me5H intermediates. Both pathways are characterized by activation energies in close correspondence with the experimental value (21.3 and 22.2 kcal mol(-1), respectively). Further kinetic studies were carried out for the Cp*Fe(dppe)H + CF3COOD and Cp*Fe(dppe)D + CF3COOD systems at 273 K. The [Cp*Fe(dppe)(eta(2)-HD)](+) complex establishes a very rapid isotope redistribution equilibrium with the eta(2)-H-2 and eta(2)-D-2 analogues. The equilibrium constant value (K = 3.3 +/- 0.3) indicates a significant equilibrium isotope effect. Simulation of the rate data provides access to the individual isomerization rate constants k(HH), k(HD), and k(DD) for the three isotopomers, yielding kinetic isotope effects: k(HH)/k(HD) = 1.24 +/- 0.01 and k(HD)/k(DD) = 1.58 +/- 0.01 (and, consequently, k(HH)/k(DD) = 1.96 +/- 0.02). The analysis of the DFT-calculated frequencies, using the [Cp*Fe(dhpe)H-2](+) model system, for the [Cp*Fe(dhpe)(eta(2)-XY)](+) isotopomers as well as transition states for the "direct" (TSdir) and "via Cp" (TSrot) pathways (X = H, D) allowed the computation of the expected isotope effects. A comparison with the experiment strongly suggests that the mechanism occurs via the "direct" pathway for the present system, although the small difference in the calculated energy barriers suggests that the "via Cp" pathway may be preferred in other cases.