Synthesis, molecular structure, and spectroelectrochemistry of a nitrosyl iron porphyrin containing an unsymmetrical xanthene-linked porphyrin core

Synthetic nitrosyl porphyrins with meso-aryl substituents are potential models for the biologically-important NO-bound P460 heme cofactor. A five-coordinate iron nitrosyl tetraaryl-porphyrin (HTPPX-CO₂H)Fe(NO) containing a xanthene-based meso substituent has been prepared. The crystal structure of this formally {FeNO}⁷ complex reveals an ordered axial and bent NO ligand (∠FeNO = 142.5(6) Å) displaying an off-axis tilt of the nitrosyl N atom from the heme normal by 9.2. Surprisingly, the porphyrin core does not display the expected asymmetry in FeN(por) distances frequently observed in iron nitrosyl porphyrins. The redox behavior as determined by cyclic voltammetry reveals, in contrast to most (por)Fe(NO) compounds, a fast NO dissociation after electrooxidation in CH₂Cl₂ to result in a net chemically-irreversible oxidation at E_pa = +0.77 V vs Ag/AgCl. IR spectroelectrochemistry reveals a recombination, on the spectroelectrochemistry time-scale, of the dissociated NO on oxidation with electrogenerated [(HTPPX-CO₂H)Fe]⁺.