Systematic Variation of 3d Metal Centers in a Redox-Innocent Ligand Environment: Structures, Electrochemical Properties, and Carbon Dioxide Activation.

Coordination compounds of earth-abundant 3d transition metals are among the most effective catalysts for the electrochemical reduction of carbon dioxide (CO2). While the properties of the metal center are crucial for the ability of the complexes to electrochemically activate CO2, systematic variations of the metal within an identical, redox-innocent ligand backbone remain insufficiently investigated. Here, we report on the synthesis, structural and spectroscopic characterization, and electrochemical investigation of a series of 3d transition-metal complexes [M = Mn(I), Fe(II), Co(II), Ni(II), Cu(I), and Zn(II)] coordinated by a new redox-innocent PNP pincer ligand system. Only the Fe, Co, and Ni complexes reveal distinct metal-centered electrochemical reductions from M(II) down to M(0) and show indications for interaction with CO2 in their reduced states. The Ni(0) d10 species associates with CO2 to form a putative Aresta-type Ni-η2-CO2 complex, where electron transfer to CO2 through back-bonding is insufficient to enable electrocatalytic activity. By contrast, the Co(0) d9 intermediate binding CO2 can undergo additional electron uptake into a formal cobalt(I) metallacarboxylate complex able to promote turnover. Our data, together with the few literature precedents, single out that an unsaturated coordination sphere (coordination number = 4 or 5) and a d7-to-d9 configuration in the reduced low oxidation state (+I or 0) are characteristics that foster electrochemical CO2 activation for complexes based on redox-innocent ligands.