Ligation of Carbon Monoxide at Iron and Cobalt Single-Metal-Atom Sites: Competition for the +1 Oxidation State in a Surface-Confined Organic Network

Abstract Cobalt coordination to a FeTPyP monolayer on graphene yields a two-dimensional FeTPyP–Co metal-organic network, stabilizing highly reactive Fe(I) sites within a biomimetic coordination architecture. The assembly process triggers an electronic reorganization that activates the pristine Fe(II) centers, enabling competitive CO ligation. A thorough synergistic investigation combining in situ photon-in/photon-out and photon-in/electron-out methods with density-functional theory simulations is presented. Distinct vibronic fingerprints reveal the complexity of the system, with anharmonic hot-band features associated with CO ligation at Co sites and a red-shifted mode uniquely assigned to CO ligation at Fe sites. Spectroscopic and theoretical results consistently indicate pronounced charge transfer and oxidation-state changes induced by adsorption. The bonding nature is site-specific, with linear CO at Co and tilted coordination at Fe. Quantitative pressure-dependent measurements uncover weak, fully reversible adsorption at room temperature, highlighting dynamic ligand-exchange processes and mild anti-cooperativity driven by indirect long-range, network-mediated interactions.