HOMO based two electrons and one-electron oxidation in planar and nonplanar methoxy-substituted nickel tetraphenylporphyrins

Abstract

A series of methoxy-substituted nickel tetra phenylporphyrins were synthesized by a direct method to correlate their electrochemical properties with nonplanar distortion of the porphyrin rings. From the newly synthesized nickel tetraphenylporphyrin complexes with o,p-dimethoxy [T(2,4-OMe)PPNi], o,m-dimethoxy [T(2,5-OMe)PPNi], m,p-dimethoxy [T(3,4-OMe)PPNi], and o,o^′, p-trimethoxy [T(2,4,6-OMe)PPNi] substituted phenyl groups, the planar T(2,4,6-OMe)PPNi structure elucidated by X-ray crystallography shows two well separated one-electron oxidations at +0.85 V and +1.22 V. Among previously reported nickel tetraphenylporphyrins with p-methoxy [T(4-OMe)PPNi] and m,m',p-trimethoxy [T(3,4,5-OMe)PPNi] substituted phenyl groups, the structurally characterized highly distorted T(3,4,5-OMe)PPNi shows a two-electron oxidation occurring at the potential of +1.05 V. Density functional theory (DFT) calculations performed on the nickel porphine systems show lowering of HOMO-LUMO gap in distorted ruffled or saddled systems with change in the energy level of HOMO and HOMO-1. While the decrease in HOMO-LUMO gap can explain the lowering in the difference between first oxidation potential and first reduction potential, Δ |Ox1-Red1|, the degeneracy or near-degeneracy of HOMO and HOMO-1 in distorted systems explains the two-electron oxidation process due to pseudo-Jahn-Teller distortion in highly distorted nickel porphyrins.