Investigation of Oxidative Behaviour and Structural Insights of (3,4,5-Trimethoxyphenyl)Porphyrin Ni(II) Complex: Experimental and Theoretical Study

Abstract

Meso 5,10,15,20-tetrakis(3,4,5-trimethoxyphenyl)porphyrin [H₂T(3,4,5-OCH₃)PP] and its Ni complex [T(3,4,5-OCH₃)PP Ni] (complex 1) have been prepared. Its oxidation characteristics were examined using diverse analytical techniques, such as UV-visible spectrophotometry (UV-Vis), electron paramagnetic resonance (EPR), cyclic voltammetry (CV), and nuclear magnetic resonance (NMR) with antimony pentachloride (SbCl₅) or trifluoroacetic acid (TFA). The structure of the titled compound has been confirmed by single-crystal analysis. Cyclic voltammetry study of complex 1 provides that Ni^II is converted to Ni^III porphyrin through π-cation radical formation which is further EPR supported by IR spectrum peaks at 1276 cm^–1 with the addition of SbCl₅. The confirmation of formation of the π-cation radical has been established by the emergence of a novel peak at 649.5 nm in the UV-visible spectrum. This peak, indicative of oxidation, was observed at the central Ni ion, distinguishing it from the porphyrin macrocycle. Additionally, the EPR spectrum informed the formation of Ni(III) from Ni(II) porphyrin which is oxidized with SbCl₅ or CF₃COOH (TFA) as \(d_{z}^{2}\) orbital has a single electron with high spin, S = 3/2 ground state in complex 1. Crystallographic analysis and Hirshfeld surface analysis, show hydrogen bonding and molecular packing in the crystal lattice. DFT calculation and experimental HOMO–LUMO gap in solvent closely align with the theoretically determined HOMO–LUMO energy difference (0.92 eV) for complex 1.