Coordination engineering in single-atom covalent organic framework photocatalysts for solar-driven nitrogen fixation

Single-atom covalent organic frameworks (COFs) provide a modular platform for the photocatalytic nitrogen reduction reaction (NRR), yet the mechanistic interplay between local coordination environments and catalytic performance remains insufficiently understood. Herein, we employ spin-polarized density functional theory to unravel structure-function relationships in a series of metal-phthalocyanine-like single-atom catalysts (TM@pdiCOF, TM = Fe, Co, Ni) and their N-doped derivatives. Our calculations reveal that side-on N₂ adsorption yields superior activation through enhanced charge transfer, driven by a characteristic donation-back-donation interaction between N₂ and metal d-orbitals as elucidated by -pCOHP/ICOHP analyses. Thermodynamic profiles constructed within the computational hydrogen electrode framework identify the initial protonation (*NN → *NNH) as the potential-determining step. Coordination engineering via ligand N-doping systematically tailors the local metal environment, significantly lowering the limiting potentials (U_L), achieving competitive values of -0.59 V, -0.42 V, and -0.76 V for Fe@NpdiCOF, Co@3NpdiCOF, and Ni@2NpdiCOF, respectively, while thermodynamically suppressing the competing hydrogen evolution reaction. Furthermore, this heteroatom doping strategy induces a synergistic optimization of photophysical properties, manifesting as a pronounced spectral blue shift and moderate band-gap narrowing to enhance visible-light harvesting. These insights establish ligand modulation as a robust design principle to concurrently optimize reaction energetics and optical properties for efficient solar-driven nitrogen fixation.