New platinum and ruthenium Schiff base complexes for water splitting reactions

Abstract

New platinum(II) and ruthenium(II) mononuclear complexes with naphthalene-based Schiff base ligands L₁ (H₂–selnaph) and L₂ (H₂–selnaph–COOH) were synthesized: Pt–selnaph (1), Pt–selnaph–COOH (2), Ru–selnaph(4-picoline)₂ (3), and Ru–selnaph(isoquinoline)₂ (4). The complexes were characterized by NMR spectroscopy, matrix-assisted laser desorption/ionization time-of-flight spectrometry, and elemental analysis, and their electrochemical and photophysical properties were investigated. The luminescent complexes 1 and 2 were used as photosensitizers for visible-light driven hydrogen production reactions in the presence of sacrificial electron donor triethylamine and cocatalyst precursor K₂PtCl₄ aqueous solution. When complex 2 was attached to the surface of TiO₂ by a carboxyl group, enhanced hydrogen photogeneration was achieved compared with complex 2 alone, with turnover numbers of about 84 after 12 h irradiation. Calculations based on electrochemical and spectroscopic data also confirmed the feasibility of electron injection through the carboxyl group of complex 2 into the conduction band of TiO₂ for hydrogen production reactions. Complexes 3 and 4 were found to be efficient stable water oxidation (NH₄)₂Ce(NO₃)₆-driven catalysts with a first-order reaction behavior. A turnover frequency of 5.34 min⁻¹ was achieved for complex 3, while complex 4 exhibited an enhanced turnover frequency of 11.9 min⁻¹ in pH 1.0 aqueous solution. Turnover numbers up to 1400 and 2060 were obtained after 6.5 h of reaction for 3 and 4, respectively. Unique mechanistic information for water splitting is also presented through electrochemical, spectroscopic and ESI-MS high-valent ruthenium-oxo intermediate investigations.