Mechanistic study of the [(dpp-bian)Re(CO)3Br] electrochemical reduction using in situ EPR spectroscopy and computational chemistry

The [(α-diimine)Re(CO)3(Hal)] complexes are able to act as efficient catalysts for electrochemical reduction of CO2 into energy-rich compounds. Among the α-diimine ligands, the 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) attracted recently increased attention. Reaction of [Re(CO)5Br] with dpp-bian leads to formation of [(dpp-bian)Re(CO)3Br], which was isolated and characterized by XRD as solvent-free [(dpp-bian)Re(CO)3Br] (1), and two solventomorphs [(dpp-bian)Re(CO)3Br]·C6H5CH3 (1-C7H8) and [(dpp-bian)Re(CO)3Br]·0.5CH3CN (1-0.5CH3CN). Electrochemical reduction of 1 in DMF and CH3CN has been studied using CV and in situ EPR spectroelectrochemistry. According to the experimental results, complex 1 undergoes in DMF multistep reduction via a number of intermediates, two of which were detected using in situ EPR spectroscopy. A careful theoretical analysis of the multistep reduction mechanism including calculations of the thermodynamics of elementary reactions and electronic structures of proposed intermediates has been performed. General scheme of the electrochemical reduction, which is essentially ligand-centered, has been proposed and supported by full-electron DFT calculations with scalar relativistic DKH2 and ZORA Hamiltonians.