Structural Fusion-Induced Activity Suppression in Copper Nanoclusters for Electrocatalytic Nitrate Reduction

Understanding how structural evolution influences catalytic behavior is a central challenge in chemistry. We establish an atomically precise platform to directly probe the catalytic consequences of structural fusion in copper nanoclusters and uncover a counterintuitive anti-emergent phenomenon, wherein increased structural complexity leads to suppressed activity. By integrating thiacalix[4]arene with an ortho-hydroxyl-substituted alkynyl ligand, we enable the in situ generation and directional templating of C₂ ^{2 -} dianions, achieving controlled fusion of two Cu₁₇ units into a well-defined supercluster, {(C₂)₆@Na₂Cu₄₀(TC4A)₆(3-HOhexC≡C)₆} (Cu₄₀). Precise regulation of the hydroxyl position allows selective isolation of the monomeric counterpart {NaCu₁₇(TC4A)₃(6-HOhexC≡C)₆} (Cu₁₇), providing a closely matched model pair to disentangle fusion effects. The generality of this C₂ ^{2 -}-templated fusion pathway is further supported by the isolation of Cu₂₂ and Cu₄₃ clusters. Comparative electrocatalytic analysis shows that, despite similar topological architectures, Cu₄₀ exhibits markedly inferior nitrate-reduction activity relative to the Cu₁₇ and Cu₂₂ monomers. Notably, Cu₁₇ delivers an optimal NH₃ Faradaic efficiency of 98.45% with a production rate of 2.91 mol·h^-1·g^-1 at -1.0 V. In situ spectroscopic experiments combined with DFT calculations reveal that fusion preserves the intrinsic nature of Cu active sites but reduces surface accessibility and perturbs local electronic environments, thereby suppressing interfacial *H formation and hindering hydrogenation of *NO intermediates.