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

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 pi-pi 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.