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

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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 {(FeIII)2Ni2O4} precatalyst species. 2-{Fe2Ni2O4} displays a maximum jcat 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.