Role of Ligand Protonation in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst

Recent
work has shown that Cp*Rh­(bpy)
[Cp* = pentamethylcyclopentadienyl, bpy = 2,2′- bipyridine]
undergoes endo protonation at the [Cp*] ligand in
the presence of weak acid (Et3NH+; pKa = 18.8 in MeCN). Upon exposure to stronger
acid (e.g., DMFH+; pKa = 6.1),
hydrogen is evolved with unity yield. Here, we study the mechanisms
by which this catalyst evolves dihydrogen using density functional
theory (M06) with polarizable continuum solvation. The calculations
show that the complex can be protonated by weak acid first at the
metal center with a barrier of 3.2 kcal/mol; this proton then migrates
to the ring to form the detected intermediate, a rhodium­(I) compound
bearing endo η4-Cp*H. Stronger acid
is required to evolve hydrogen, which calculations show happens via
a concerted mechanism. The acid approaches and protonates the metal,
while the second proton simultaneously migrates from the ring with
a barrier of ∼12 kcal/mol. Under strongly acidic conditions,
we find that hydrogen evolution can proceed through a traditional
metal–hydride species; protonation of the initial hydride to
form an H–H bond occurs before migration of the hydride (in
the form of a proton) to the [Cp*] ring (i.e., H–H bond formation
is faster than hydride–proton tautomerization). This work demonstrates
the role of acid strength in accessing different mechanisms of hydrogen
evolution. Calculations also predict that modification of the bpy
ligand by a variety of functional groups does not affect the preference
for [Cp*] protonation, although the driving force for protonation
changes. However, we predict that exchange of bpy for a bidentate
phosphine ligand will stabilize a rhodium­(III) hydride, reversing
the preference for bound [Cp*H] found in all computed bpy derivatives
and offering an appealing alternative ligand platform for future experimental
and computational mechanistic studies of H2 evolution.