The Reductive Chemistry of Nickel Hydroporphyrins. Evidence for a Biologically Significant Difference between Porphyrins, Hydroporphyrins, and Other Tetrapyrroles

Article abstract of DOI:10.1021/ja00227a020

The chemical and electrochemical reductions of nickel porphyrin and hydroporphyrin complexes in the octaethyl, tetraphenyl, and methylated octaethyl series were investigated in nonaqueous media.The potentials for reduction of the complexes were determined by cyclic voltammetry in three solvents.A single, reversible, one-electron reduction was observed near -1.5 V versus SCE for most complexes in acetonitrile and dimethylformamide.Ni(OEiBC) and Ni(DMOEiBC) had voltammograms with shapes characteristic of electrocatalytic processes when reduced in methylene chloride.Other complexes were generally reduced irreversibly (i_p,ap,c) in this solvent.UV-vis, EPR, and 1H NMR spectroscopy were used to characterize the reduced species obtained by electrochemical and chemical means.The site of reduction depends upon the method of reduction (and the time scale of the method), the saturation level of the macrocycle, and the identity of the substituents on the macrocycle.Ni(OEP) is reduced to the anion radical Ni(OEP)^.-, which undergoes further reduction to afford the stable, diamagnetic phlorin anion complex Ni(OEPH)^-.Ni(OEC) and Ni(DMOEiBC) are reduced to transient nickel(I) complexes.Ni^I(OEC)^- reacts further to give the chlorin-phlorin anion complex Ni(OECH)^-.Ni(OEiBC) is reduced to Ni^I(OEiBC)^-.Aside from reduced F₄₃₀, Ni^I(OEiBC)^- is the first stable nickel(I) tetrapyrrole and is the only known nickel(I) complex that has a ?-system extending over the entire macrocycle.Chemical reductions of Ni(TPP), Ni(TPC), and Ni(TPiBC) produce mixtures of anion radical, phlorin anion, and phlorin dianion species.The macrocycles that appear best able to accomodate the large, approximately 2.1 Angstroem Ni-N distance required by nickel(I) are those that were shown to ruffle in neutral, low-spin nickel(II) complexes.One consequence of ruffling is a reduction in the macrocycle core size to give smaller Ni-N distances (1.92 Angstroem) than typically observed in a porphyrin environment.Apparently, the hole size/ligand field strength of hydroporphyrins can be varied over a wide range at little cost in energy.Further consideration of the conformational energy of these complexes suggests that a fundamental difference between porphyrins, hydroporphyrins, corrins, oxoporphyrins, and other tetrapyrrole macrocycles is their optimal hole size and the range of hole sizes that are readily accessible in their complexes.The effects of the stereochemistry of the macrocycle substituents are discussed, and an explanation is developed for the widely varying affinities of nickel tetrapyrroles for axial ligands.