Water-soluble platinum and palladium porphyrins with peripheral ethyl phosphonic acid substituents: synthesis, aggregation in solution, and photocatalytic properties

Abstract

Water-soluble porphyrins have garnered significant attention due to their broad range of applications in biomedicine, catalysis, and material chemistry. In this work, water-soluble platinum(II) and palladium(II) complexes with porphyrins bearing ethyl phosphonate substituents, namely, Pt/Pd 10-(ethoxyhydroxyphosphoryl)-5,15-di(p-carboxyphenyl)porphyrins (M3m, M = Pt(II), Pd(II)) and Pt/Pd 5,10-bis(ethoxyhydroxyphosphoryl)-10,20-diarylporphyrins (M1d–M3d; aryl = p-tolyl (1), mesityl (2), p-carboxyphenyl (3)), were synthesized by alkaline hydrolysis of the corresponding diethyl phosphonates M6m and M4d–M6d. NMR, UV–vis, and fluorescence spectroscopy revealed that the mono-phosphonates M3m tend to form aggregates in aqueous media, while the bis-phosphonates M3d exist predominantly as monomeric species across a wide range of concentrations (10⁻⁶–10⁻³ M), ionic strengths (0–0.81 M), and pH values (4–12). Single-crystal X-ray diffraction studies of the diethyl phosphonates Pt6d and Pd6d revealed that π–π stacking of the aromatic macrocycles is sterically hindered in the crystals, providing a rationale for the low degree of solution aggregation observed for ethyl phosphonate M3d. Photophysical studies of M3m and M1d–M3d demonstrated that these compounds are phosphorescent and generate singlet oxygen in aqueous solutions. Pd(II) complex Pd3d is an excellent photocatalyst for the oxidation of sulfides using di-oxygen in a solvent mixture (MeCN/H₂O, 4 : 1 v/v). Under these conditions, various alkyl and aryl sulfides were quantitatively converted into the desired sulfoxides. For the oxygenation of mixed alkyl–aryl sulfides, Pd3d outperforms Pd(II) meso-tetrakis(p-carboxyphenyl)porphyrin (PdTCPP). This photocatalyst can be recycled and reused to afford sulfoxides with no loss of product yield.