Synthesis, characterization and electrochemistry of iron porphyrin complexes of the form (por)Fe(O(CH ₂ ) _n SiMe ₃ ) (por = OEP, TPP, TpivPP; n = 0 or 1) and their reaction with NO

The five-coordinate iron(III) porphyrin complexes, (por)Fe(OSiMe 3 ) and (por)Fe(OCH 2 SiMe 3 ) (por = OEP, TPP, TpivPP) have been synthesized and characterized by spectroscopy. The magnetic susceptibility data obtained by the Evans method for the complexes indicate that they are high-spin or intermediate-spin species, depending on the axial ligand. The X-ray crystal structures of (OEP)Fe(OSiMe 3 ), (TPP)Fe(OSiMe 3 ), (TpivPP)Fe(OSiMe 3 ), and (TPP)Fe(OCH 2 SiMe 3 ) revealed five-coordinate square pyramidal complexes with O-bound ligands in the axial position. The Fe-O bond lengths are in the 1.819 - 1.827 Å range, and the average Fe-N bond lengths are 2.066 - 2.093 Å. In general, the Fe-O((CH 2 ) n SiMe 3 ) (n = 0, 1) bonds of these complexes are longer than the related Fe-O(aryl), and a distortion of the porphyrin macrocycles due to steric interaction with the axial ligands could influence the spin states. The compounds react with NO both in powdered form or in CH 2 Cl 2 to form (por)Fe(NO)(O(CH 2 ) n SiMe 3 ) and (por)Fe(NO), generating (por)Fe(NO)(NO 2 ) upon exposure to air. A comparison of the IR data of the five-coordinate (por)Fe(OR) and six-coordinate (por)Fe(NO)(OR) shows the possibility of trans influence of NO on the axial ligand, and this was supported by DFT calculations. The redox behaviors of (TpivPP)Fe(OSiMe 3 ) and (TpivPP)Fe(OCH 2 SiMe 3 ) studied by CV and IR spectroelectrochemistry reveal irreversible porphyrin-centered oxidations and reversible reductions. IR spectroelectrochemistry data also suggest that electroreduction of (TpivPP)Fe(OSiMe 3 ) is accompanied by conformational change and spin state changes resulting in shifts of the ν CO band to lower wavenumbers. Chemical reduction of (TpivPP)Fe(OSiMe 3 ) supports the formation of the reduced [(TpivPP)Fe(OSiMe 3 )]- species.