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 (CO ₂ ). While the properties of the metal center are crucial for the ability of the complexes to electrochemically activate CO ₂ , 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 CO ₂ in their reduced states. The Ni(0) d ¹⁰ species associates with CO ₂ to form a putative Aresta-type Ni-η ² -CO ₂ complex, where electron transfer to CO ₂ through back-bonding is insufficient to enable electrocatalytic activity. By contrast, the Co(0) d ⁹ intermediate binding CO ₂ 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 d ⁷ -to-d ⁹ configuration in the reduced low oxidation state (+I or 0) are characteristics that foster electrochemical CO ₂ activation for complexes based on redox-innocent ligands.