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-Ph4(η5-C4COH)Ru(CO)2H] (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 (η2-cyclopentadienone)ruthenium amine intermediate. The presence of Ph groups on the Cp ring facilitates the ring slippage
that occurs on imine coordination. This η2-intermediate finally rearranges to the corresponding
(η4-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.