Abstract:
Modifications on the ligand environment of Milstein
ruthenium(II) pincer hydride catalysts have been proposed to fine-tune
the activation free energy, ΔG⧧ for the key step of H2 elimination in the
water splitting reaction. This study conducted at the B3LYP level of
density functional theory including the solvation effect reveals that
changing the bulky t-butyl group at the P-arm of the pincer ligand by
methyl or ethyl group can reduce the ΔG⧧ by a substantial margin, ∼10
kcal/mol. The reduction in the steric effect of the pincer ligand causes
exothermic association of the water molecule to the metal center and leads
to significant stabilization of all the subsequent reaction intermediates and
the transition state compared to those of the original Milstein catalyst that
promotes endothermic association of the water molecule. Though
electron donating groups on the pyridyl unit of the pincer ligand are
advantageous for reducing the activation barrier in the gas phase, the effect is only 1−1.4 kcal/mol compared to that of an
electron withdrawing group. The absolute minimum of the electrostatic potential at the hydride ligand and carbonyl stretching
frequency of the catalyst are useful parameters to gauge the effect of ligand environment on the H2 elimination step of the water
splitting reaction.
