Two Novel Schiff Base Manganese Complexes as Bifunctional Electrocatalysts for CO₂ Reduction and Water Oxidation

One mononuclear Mn(III) complex [Mn^IIIL(H₂O)(MeCN)](ClO₄) (1) and one hetero-binuclear complex [(Cu^IILMn^II(H₂O)₃)(Cu^IIL)₂](ClO₄)₂·CH₃OH (2) have been synthesized with the Schiff base ligand (H₂L = N,N'-bis(3-methoxysalicylidene)-1,2-phenylenediamine). Single crystal X-ray structural analysis manifests that the Mn(III) ion in 1 has an octahedral coordination structure, whereas the Mn(II) ion in 2 possesses a trigonal bipyramidal configuration and the Cu(II) ion in 2 is four-coordinated with a square-planar geometry. Electrochimerical catalytic investigation demonstrates that the two complexes can electrochemically catalyze water oxidation and CO₂ reduction simultaneously. The coordination environments of the Mn(III), Mn(II), and Cu(II) ions in 1 and 2 were provided by the Schiff base ligand (L) and labile solvent molecules. The coordinately unsaturated environment of the Cu(II) center in 2 can perfectly facilitate the catalytic performance of 2. Complexes 1 and 2 display that the over potentials for water oxidation are 728 mV and 216 mV, faradaic efficiencies (FEs) are 88% and 92%, respectively, as well as the turnover frequency (TOF) values for the catalytic reduction of CO₂ to CO are 0.38 s^-1 at -1.65 V and 15.97 s^-1 at -1.60 V, respectively. Complex 2 shows much better catalytic performance for both water oxidation and CO₂ reduction than that of complex 1, which could be owing to a structural reason which is attributed to the synergistic catalytic action of the neighboring Mn(III) and Cu(II) active sites in 2. Complexes 1 and 2 are the first two compounds coordinated with Schiff base ligand for both water oxidation and CO₂ reduction. The finding in this work can offer significant inspiration for the future development of electrocatalysis in this area.