Customizing stable Ni(ii)-based cubanes with bidirectional chelation-bridging ligands enables atomic manipulation of...

The well-known [M₄O₄] cubane represents one of the most important catalytic models; however, analogous coordination clusters for effective electrocatalysis were greatly hindered by the limited stability of the conventional [M₄L₄] motif with dual-site ligands. Aiming to obtain a highly stable cubane for exposure and atomic manipulation of all four active metal sites, a pair of clusters were customized as [Ni₄(L1)₄(C₂H₅OH)₄] (Ni₄(L1)₄) and [Ni₄(L2)₃(C₂H₅O)(NO₃)(C₂H₅OH)₃] (Ni₄(L2)₃). A series of ligand advantages including bidirectional chelation-bridging coordination, semi-enclosed tridentate O, N, O sites and Schiff-base-linked rigid biphenyl backbone, prevent concurrent dissociation of Ni(ii) and µ ₃-O node in the cubane. Compared to Ni₄(L1)₄, a key innovation lies in generation of the Ni₄(L2)₃ core and incorporation of terminally coordinated nitrate, thereby creating dynamic weakly coordinating metal sites. Through optimized exposure and precisely engineered Ni(ii) active sites, Ni₄(L2)₃ demonstrates exceptional electrocatalytic activity for the urea oxidation reaction (UOR), requiring a record-low potential of 1.29 and 1.38 V to deliver a current density of 10 and 100 mA cm^-2, respectively, outperforming all known coordination clusters, along with stable operation exceeding 120 hours. Mechanistic studies utilizing systematic in situ characterization reveal that the Ni(ii) centers formed via nitrate coordination are essential for stabilizing the key NCO^- intermediate and steering the reaction pathway towards N₂ generation. DFT calculations further demonstrate that Ni₄(L2)₃ displays a more pronounced electron delocalization effect upon urea adsorption compared to Ni₄(L1)₄, resulting in a marked improvement in N₂ product selectivity. This work showcases the powerful capacity of molecular customization to achieve tailored stability, regulate active metal sites, and attain exceptional electrocatalytic performance via a new reaction mechanism.