The Hydrogen Catalyst Cobaloxime: A Multifrequency EPR and DFT Study of Cobaloxime’s Electronic Structure

Solar fuels research aims to mimic photosynthesis and
devise integrated
systems that can capture, convert, and store solar energy in the form
of high-energy molecular bonds. Molecular hydrogen is generally considered
an ideal solar fuel because its combustion is essentially pollution-free.
Cobaloximes rank among the most promising earth-abundant catalysts
for the reduction of protons to molecular hydrogen. We have used multifrequency
EPR spectroscopy at X-band, Q-band, and D-band combined with DFT calculations
to reveal electronic structure and establish correlations among the
structure, surroundings, and catalytic activity of these complexes.
To assess the strength and nature of ligand cobalt interactions, the
BF2-capped cobaloxime, Co­(dmgBF2)2, was studied in a variety of different solvents with a range of
polarities and stoichiometric amounts of potential ligands to the
cobalt ion. This allows the differentiation of labile and strongly
coordinating axial ligands for the Co­(II) complex. Labile, or weakly
coordinating, ligands such as methanol result in larger g-tensor anisotropy than strongly coordinating ligands such as pyridine.
In addition, a coordination number effect is seen for the strongly
coordinating ligands with both singly ligated LCo­(dmgBF2)2 and doubly ligated L2Co­(dmgBF2)2 . The presence of two strongly coordinating axial ligands
leads to the smallest g-tensor anisotropy. The relevance
of the strength of the axial ligand(s) to the catalytic efficiency
of Co­(dmgBF2)2 is discussed. Finally, the influence
of molecular oxygen and formation of Co­(III) superoxide radicals LCo­(dmgBF2)2O2• is studied.
The experimental results are compared with a comprehensive set of
DFT calculations on Co­(dmgBF2)2 model systems
with various axial ligands. Comparison with experimental values for
the “key” magnetic parameters such as g-tensor and 59Co hyperfine coupling tensor allows the
determination of the conformation of the axially ligated Co­(dmgBF2)2 complexes. The data presented here are vital
for understanding the influence of solvent and ligand coordination
on the catalytic efficiency of cobaloximes.