Structure and Catalytic Properties of Homo- and Heterometallic Iron(II)-Based Di(2-pyridyl)ketone Oxoclusters

Electrochemical water splitting is essential for reducing our dependence on fossil fuels through green hydrogen production and requires the design of new, low-cost, 3d transition-metal-based catalysts for the sluggish oxygen evolution reaction (OER). We report on the synthesis, characterization, and OER performance of new types of homo- and heterometallic iron(II)-based di(2-pyridyl)ketone cubanes, 1-[Fe₄(dpy-C{OH}O)₄(OAc)₃(H₂O)]ClO₄ and 2-[Fe₂Ni₂(dpy-C{OH}O)₄(OAc)₃]ClO₄ (dpy = di(2-pyridyl)diol), referred to as 1-{Fe₄O₄} and 2-{Fe₂Ni₂O₄}. The heterometallic oxocluster is the first sought-after molecular cutout of the key active {H₂O-Fe₂Ni₂(OR)₂-OH₂} motif, bridging molecular and heterogeneous OER catalysts as a model to understand the key catalytic synergisms in powerful NiFe oxide-based materials. The precise positions of Fe(II) and Ni(II) cations within the 2-{Fe₂Ni₂O₄} oxocluster, along with the detailed structural and electronic features, were elucidated with a variety of methods, including extended advanced X-ray absorption spectroscopy and total neutron scattering techniques, accompanied by time-dependent density functional theory (TDDFT) calculations. (Spectro)electrochemical analyses showed that 2-{Fe₂Ni₂O₄} exhibits synergisms between Fe(II) and Ni(II) centers, tuning both metals' redox potentials and the molecular stability of the mixed-valent {(Fe^III)₂Ni₂O₄} precatalyst species. 2-{Fe₂Ni₂O₄} displays a maximum j_cat of 1.75 mA/cm² for the OER and a Faradaic efficiency of 84.8% under turnover conditions. Although the oxocluster experienced notable degradation during the OER, significant metal (oxy)hydroxide formation could be excluded. 2-{Fe₂Ni₂O₄} is introduced as a new molecular platform for in-depth studies of NiFe synergisms, along with ligand engineering or polymer matrix strategies.