The [FeFe]-hydrogenase enzymes catalyze
hydrogen oxidation and production efficiently with binuclear Fe metal
centers. Recently the bioinspired H<sub>2</sub>-producing model system
Fe<sub>2</sub>(adt)(CO)<sub>2</sub>(dppv)<sub>2</sub> (adt=azadithiolate
and dppv=diphosphine) was synthesized and studied experimentally.
In this system, the azadithiolate bridge facilitates the formation
of a doubly protonated ammonium-hydride species through a proton relay.
Herein computational methods are utilized to examine this system in
the various oxidation states and protonation states along proposed
mechanistic pathways for H<sub>2</sub> production. The calculated
results agree well with the experimental data for the geometries,
CO vibrational stretching frequencies, and reduction potentials. The
calculations illustrate that the NH···HFe dihydrogen
bonding distance in the doubly protonated species is highly sensitive
to the effects of ion-pairing between the ammonium and BF<sub>4</sub><sup>–</sup> counterions, which are present in the crystal
structure, in that the inclusion of BF<sub>4</sub><sup>–</sup> counterions leads to a significantly longer dihydrogen bond. The
non-hydride Fe center was found to be the site of reduction for terminal
hydride species and unsymmetric bridging hydride species, whereas
the reduced symmetric bridging hydride species exhibited spin delocalization
between the Fe centers. According to both experimental measurements
and theoretical calculations of the relative p<i>K</i><sub>a</sub> values, the Fe<sub>d</sub> center of the neutral species
is more basic than the amine, and the bridging hydride species is
more thermodynamically stable than the terminal hydride species. The
calculations implicate a possible pathway for H<sub>2</sub> evolution
that involves an intermediate with H<sub>2</sub> weakly bonded to
one Fe, a short H<sub>2</sub> distance similar to the molecular bond
length, the spin density delocalized over the two Fe centers, and
a nearly symmetrically bridged CO ligand. Overall, this study illustrates
the mechanistic roles of the ammonium-hydride interaction, flexibility
of the bridging CO ligand, and intramolecular electron transfer between
the Fe centers in the catalytic cycle. Such insights will assist in
the design of more effective bioinspired catalysts for H<sub>2</sub> production
