CHARACTERIZATION OF 5-COORDINATE AND 6-COORDINATE IRON(III) COMPLEXES OF N-METHYLPORPHYRINS

The coordination chemistry of iron(III) in the environment of N-methylporphyrins has been expanded to include a crystallographic characterization of a high-spin, five-coordinate complex, [N-methyltetra-p-tolylporphyrinFeIIICI]-[SbCl6]·1.5(toluene), which crystallizes in the triclinic space group Pi with a = 13.170 (6) Å, b = 14.591 (1) Å, c = 15.702 (14) Å, α = 83.78 (7)°, 0 = 79.82 (6)°, γ = 74.34° at 130 K, refined to R = 0.069 for 4842 data and 375 parameters, and spectroscopic (1H and 2H NMR) characterization of low-spin (S = 1/2) six-coordinate forms with two strong-field axial ligands (cyanide, imidazole, or substituted imidazole). Thus the steric effects of N-methyl substitution do not preclude the addition of axial ligands on the same side of the porphyrin plane, but planar ligands like imidazole lie with their planes perpendicular to the Cs plane of the complex. Rotation about the Fe-N imidazole bond on the same side of the plane as the porphyrin N-methyl group is restricted, and as a consequene all pyrrole protons of the N-methylated porphyrin are nonequivalent. Titration of [(N-MeTTP)FeIIICl][SbCI6] with cyanide proceeds through the formation of high-spin [(N-MeTTP)FeIIICN][SbCl6], with the cyanide bound on the opposite face from the N-methyl substituent, and a second, low-spin species which must have the cyanide ligand bound on the same side as the N-methyl substituent and either a chloride or no ligand in the other axial site. N-Methylporphyrins are readily prepared in 90% yield through the use of methyldiphenylsulfonium tetrafluoroborate, and this has allowed the preparation of the sterically encumbered N-methyltetramesitylporphyrin for the first time. The thianthrene cation radical proves to be an excellent oxidant for the preparation of iron(III) complexes of N-methylporphyrins. © 1990, American Chemical Society. All rights reserved.