Versatile redox reactivity of triaryl-meso-substituted Ni(ii) porphyrin

Abstract

The electrochemical oxidation of nickel(II) 5,15-p-ditolyl-10-phenylporphyrin (1-Ni) leads to the formation of different coupling products, with the distribution depending on the nature of the solvent (CH₂Cl₂–CH₃CN, CH₂Cl₂, DMF), the cell configuration (2 or 3 compartments) and the number of electrons abstracted. In a two compartment configuration (anode and cathode in the same compartment) in a CH₂Cl₂–CH₃CN mixture, nickel(II) 5-chloro-10,20-p-ditolyl-15-phenylporphyrin (1-Ni-Cl) was isolated in good yield and its mechanism of formation is proposed. Switching to the three compartment configuration, the meso-β/meso-β doubly fused dimer (3-Ni) is detected as the major product whereas in pure CH₂Cl₂, the singly bonded meso-β dimer (2′-Ni) is the major product; in both cases a significant amount of 1-Ni-Cl is also produced. These results are in accordance with the cyclic voltammetry analysis of 1-Ni measured in CH₂Cl₂–CH₃CN and CH₂Cl₂ from which the voltammetric trace of 3-Ni is, respectively, appearing or not. In DMF the hydroxyporphyrin 1-Ni-OH was detected as the major product. Furthermore, meso-functionalization of 1-Ni was performed by controlled potential electrolysis with triphenylphosphine as the nucleophile leading to the phosphonium substituted derivative (1-Ni-P⁺) in good yield. Finally, unprecedented X-ray crystallographic structures of 1-Ni, 1-Ni-Cl, 1-Ni-P⁺ and 3-Ni are presented and their respective structural parameters compared.