Constructing Co₄(SO₄)₄ Clusters within Metal-Organic Frameworks for Efficient Oxygen Electrocatalysis

Multinuclear metal clusters are ideal candidates to catalyze small molecule activation reactions involving the transfer of multiple electrons. However, synthesizing active metal clusters is a big challenge. Herein, on constructing an unparalleled Co₄(SO₄)₄ cluster within porphyrin-based metal-organic frameworks (MOFs) and the electrocatalytic features of such Co₄(SO₄)₄ clusters for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is reported. The reaction of Co^II sulfate and metal complexes of tetrakis(4-pyridyl)porphyrin under solvothermal conditions afforded Co₄-M-MOFs (M═Co, Cu, and Zn). Crystallographic studies revealed that these Co₄-M-MOFs have the same framework structure, having the Co₄(SO₄)₄ clusters connected by metalloporphyrin units through Co─N_pyridyl bonds. In the Co₄(SO₄)₄ cluster, the four Co^II ions are chemically and symmetrically equivalent and are each coordinated with four sulfate O atoms to give a distorted cube-like structure. Electrocatalytic studies showed that these Co₄-M-MOFs are all active for electrocatalytic OER and ORR. Importantly, by regulating the activity of the metalloporphyrin units, it is confirmed that the Co₄(SO₄)₄ cluster is active for oxygen electrocatalysis. With the use of Co porphyrins as connecting units, Co₄-Co-MOF displays the highest electrocatalytic activity in this series of MOFs by showing a 10 mA cm^-2 OER current density at 357 mV overpotential and an ORR half-wave potential at 0.83 V versus reversible hydrogen electrode (RHE). Theoretical studies revealed the synergistic effect of two proximal Co atoms in the Co₄(SO₄)₄ cluster in OER by facilitating the formation of O─O bonds. This work is of fundamental significance to present the construction of Co₄(SO₄)₄ clusters in framework structures for oxygen electrocatalysis and to demonstrate the cooperation between two proximal Co atoms in such clusters during the O─O bond formation process.