Tetrameric and Polymeric Zn(II) Coordination Complexes of 4-Diallylaminobenzoic Acid and Their Applications in the Electroreduction of CO₂ and Schottky Diode Behavior

Electrochemical reduction of CO2 into C-fuels using metal and metal oxides nanoparticles, molecular complexes, etc. is a challenging task in view of a sustainable environment, renewable energy sources, and transformation of waste-to-wealth. Zn(II), d10 redox-innocent coordination complexes have recently been used as efficient and selective catalysts in the electroreduction of CO2. Toward this goal, we have synthesized two complexes, tetranuclear (0D) [Zn4(μ4-O)(DABA)6] (1) and polynuclear (1D) [Zn2(DABA)4(4,4′-BPY)]n (2), in one pot (HDABA = 4-diallylamino-benzoic acid and 4,4′-BPY = 4,4′-bipyridine) and characterized them by single-crystal X-ray diffraction measurements and other spectroscopic data. The electroreduction of CO2 (CO2RR) using complexes 1 and 2 as catalysts produced HCOOH, HCHO, and CH3OH. The reduced products are identified by 13C NMR spectral data. It is observed that compound 1 is catalytically more efficient than compound 2; when compound 1 demonstrates the current −5.63 mA mg–1, compound 2 shows a much lower value (−22.27 mA mg–1). Impedance spectroscopy shows that in complex 2, Z′ decreases with a diminishing frequency compared to that of complex 1; this implies that the dc resistivity of 2 is lower than that of 1. The theoretically calculated energy gap values (ΔE(HOMO–LUMO)) are 4.4 eV (1) and 2.81 eV (2), and the conductivity is 2.29 × 10–7 S/m (1) and 2.54 × 10–3 S/m (2). About 104 times enhancement of conductivity of 2 is due to not only coordination of 4,4′-BPY to Zn(II) but also to subsidization by the coordination polymer and noncovalent superstructure.