Synthesis, Aromaticity, and Optical and Redox Properties of Nickel(II), Palladium(II), and Platinum(II) Complexes of <i>meso</i> ‐Substituted 5‐Monoazaporphyrin Derivatives

There has been great interest in the development of azaporphyrin-based photosensitizers. Herein, we report a series of group 10 metal (Ni, Pd, Pt) complexes of 10,15,20-triaryl-5-monoazaporphyrin (Ar₃MAP; Ar = 4-MeC₆H₄) and their meso-N-substituted derivatives (RAr₃MAP; R = Ph, ethynyl, BF₃). The freebase of Ar₃MAP, prepared by annulative condensation between 5,10,15-tris(4-methylphenyl)bilane and ammonium carbamate in the presence of an iron(III) salt, was converted into metal complexes by treatment with the corresponding metal salts. The copper-catalyzed meso-N-phenylation of Ar₃MAP with a diphenyliodonium salt afforded PhAr₃MAP salts. The substantial basicity of the meso-N atom of Ar₃MAP was further demonstrated by meso-N-ethynylation with an ethynyl(phenyl)iodonium salt and Lewis acid-base complexation with BF₃•OEt₂. The aromatic character and the optical and redox properties of the new MAP derivatives were assessed using spectroscopic and voltammetric measurements and density functional theory calculations. It was revealed that adding a positive charge to the MAP ring weakened the diatropic ring-current effect, red-shifted the Q bands, and anodically shifted the π-centered redox processes. Transient absorption measurements of the Pd and Pt complexes also provide valuable information regarding the effects of meso-N substituents and central metals on the excited-state dynamics of MAP dyes.