A novel Cd4O4 cubane cluster-based complex and its bifunctional catalytic activity aiming for artificial photosynthesis

Abstract

Artificial photosynthesis represents a promising route for sustainable energy conversion, utilizing efficient catalysts for both water oxidation and CO₂ reduction. Cubane clusters as tunable molecular platforms bridging homogeneous and heterogeneous catalysis offer practical strategies for designing molecular catalysts for integrated artificial photosynthetic systems. We introduce a novel cubane cluster complex [Cd(dpk(H)(O))(Cl)]₄·2H₂O (1), achieved by the solvothermal method, as the first Cd₄O₄ molecular catalyst exhibiting bifunctional activity that is important in artificial photosynthesis. The complex is found to be formed by four di(pyridin-2-yl)methanol ligands, which were formed by degradation of di-2-pyridylketone azine under solvothermal conditions, and coordinated through deprotonated ions (dpk(H)(O⁻)). Four Cd(II) ions are bridged by four oxygen atoms of four dpk(H)(O⁻) to form a cubane structure and each of the Cd(II) centers is additionally coordinated by two N atoms of two different dpk(H)(O⁻) ligands and a chloride ion to obtain an octahedral geometry. The cubane structure of the complex is confirmed by single-crystal X-ray diffraction, which reveals extensive noncovalent interactions in the lattice which are quantified by Hirshfeld surface analysis. The solid-state band gap (E_g), based on the Kubelka–Munk model, is found to be 2.53 eV and signifies the utility. DFT analysis indicates the highest positive Mulliken atomic charges on cadmium metal centers, while the bridging oxygen atoms are with the highest negative charges. For the catalytic study, a glassy carbon electrode was modified with carbon black and the cubane cluster complex, which exhibited rapid and effective charge transfer across the interface. The water oxidation and CO₂ reduction properties of the heterogenous electrode were found to be promising as it exhibits a catalytic current at an onset potential of 1.031 V under Ar and −1.746 V under a CO₂ atmosphere.