Engineering Spin States of Isolated Copper Species in a Metal–Organic Framework Improves Urea Electrosynthesis

The catalytic activities are generally believed to be relevant to the electronic states of their active center, but understanding this relationship is usually difficult. Here, we design two types of catalysts for electrocatalytic urea via a coordination strategy in a metal-organic frameworks: Cu^III-HHTP and Cu^II-HHTP. Cu^III-HHTP exhibits an improved urea production rate of 7.78 mmol h^-1 g^-1 and an enhanced Faradaic efficiency of 23.09% at - 0.6 V vs. reversible hydrogen electrode, in sharp contrast to Cu^II-HHTP. Isolated Cu^III species with S = 0 spin ground state are demonstrated as the active center in Cu^III-HHTP, different from Cu^II with S = 1/2 in Cu^II-HHTP. We further demonstrate that isolated Cu^III with an empty [Formula: see text] orbital in Cu^III-HHTP experiences a single-electron migration path with a lower energy barrier in the C-N coupling process, while Cu^II with a single-spin state ([Formula: see text]) in Cu^II-HHTP undergoes a two-electron migration pathway.