Synergistic Roles of Hydroxyl Groups and Coordinating Ligands in Governing Metal-Specific Electrocatalytic Nitrate Reduction in Covalent Organic Framework

Abstract Electrocatalytic nitrate reduction is often framed in terms of metal identity, yet the decisive factor governing reactivity remains the local coordination environment in which that metal is embedded. Here, we demonstrate that a covalent organic framework (COF) can be engineered to encode a hydroxyl–ligand cooperative microenvironment that actively dictates metal-specific catalytic behavior. Using a bipyridine-functionalized, hydroxyl-bearing COF (TU-42) as a platform, we show that postsynthetic incorporation of Fe and Ni produces two fundamentally distinct single-site architectures: a relatively open, electronically adaptive Fe–N/O manifold and a coordination-saturated Ni–N/O/Cl environment. Through a combined electrochemical–spectroscopic–theoretical analysis, we reveal that framework hydroxyl groups do not merely tune the electronic structure but establish directional hydrogen-bonding interactions with coordinated acetate ligands in TU-42-Fe, dynamically modulating the ligand field and redox accessibility of the active site. This cooperative effect lowers the barrier for nitrate activation, biases reaction topology toward energetically favorable pathways, and enhances the coupling between electron flux and ammonia formation. In contrast, the more rigid and saturated Ni coordination sphere imposes an energetic penalty for substrate access and restricts mechanistic flexibility. This work establishes a design principle in which secondary-sphere interactions embedded within a reticular scaffold govern reaction pathways, offering a route to control electrocatalysis beyond conventional metal-centered strategies.