Reduction of CO₂ by pyridine monoimine molybdenum carbonyl complexes: Cooperative metal-ligand binding of CO₂

[(^ArPMI)Mo(CO)₄] complexes (PMI=pyridine monoimine; Ar=Ph, 2,6-di-iso-propylphenyl) were synthesized and their electrochemical properties were probed with cyclic voltammetry and infrared spectroelectrochemistry (IR-SEC). The complexes undergo a reduction at more positive potentials than the related [(bipyridine)Mo(CO)₄] complex, which is ligand based according to IR-SEC and DFT data. To probe the reaction product in more detail, stoichiometric chemical reduction and subsequent treatment with CO₂ resulted in the formation of a new product that is assigned as a ligand-bound carboxylate, [( iPr 2PhPMI)Mo(CO)₃(CO₂)]^2-, by NMR spectroscopic methods. The CO₂ adduct [( iPr 2PhPMI)Mo(CO)₃(CO₂)]^2- could not be isolated and fully characterized. However, the C-C coupling between the CO₂ molecule and the PDI ligand was confirmed by X-ray crystallographic characterization of one of the decomposition products of [( iPr 2PhPMI)Mo(CO)₃(CO₂)]^2-.