DFT study of an inner-sphere mechanism in the hydrogen transfer from a hydroxycyclopentadienyl ruthenium hydride to imines

A combination of the DFT method with the computational description of environmental effects by solvent was applied to a theoretical study of the hydrogen transfer to imines by [2,3,4,5-Ph-4(eta(5)-C4COH)Ru(CO)(2)H] (2) within a molecular model that closely mimics the authentic reaction conditions. A consistent polarizable continuum solvent model (PCM) was instrumental and necessary in achieving stability of the computational model. Environmental effects by solvent were also considered in an extended model with an addition of explicit solvent molecules within the PCM. The study elucidates an inner-sphere mechanism in detail. Intermediate complexes and transition states are characterized. Three distinct energy barriers along the reaction coordinate are predicted when solvent effects are taken into account. The imine coordinates to ruthenium via ring slippage with an energy barrier of about 15 kcal/mol. Close in energy (12 kcal/mol) is the transition state of the hydride transfer, which gives an (eta(2)-cyclopentadienone)ruthenium amine intermediate. The presence of Ph groups on the Cp ring facilitates the ring slippage that occurs on imine coordination. This eta(2)-intermediate finally rearranges to the corresponding (eta(4)-cyclopentadienone)ruthenium amine complex via a transition state at 9 kcal/mol. The stable ruthenium amine complex was verified against an X-ray structure of the corresponding complex. Inclusion of the solvent (by PCM or explicit molecules) was required to stabilize low-hapticity intermediates and transition state structures.