STRUCTURE AND PROPERTIES OF AN UNSYMMETRICALLY SUBSTITUTED 6-COORDINATE IRON(III) PORPHYRIN

The six-coordinate azido(pyridine)(tetraphenylporphinato)iron(III) complex, Fe(TPP)N3py, has been prepared and its structure has been determined by diffraction techniques. The monoclinic unit cell contains four Fe(TPP)N3py·1/2py formulations and has a = 14.519 (3) Å, b = 23.651 (5) Å, c = 13.030 (3) Å, and β = 103.94 (1)°. The space group is P21/n. The calculated and experimental densities are 1.28 and 1.27 g/cm3 at 22 ± 1 °C. Diffracted intensities have been recorded for 4200 reflections classified as observed. The final discrepancy factors are R = 8.6% and = 11.2%. The nearly tetragonal geometry and bond distances of the coordination group are consistent with a low-spin ferric ion. This spin state has been confirmed by a temperature-dependent magnetic analysis by using the Faraday method over the temperature range from 77 to 297 K. The effective magnetic moments range from 2.09 (1) µB at 77 K to 2.33 (1) µB at 297 K and are characteristic of a low spin state with spin-orbit coupling. Equilibrium constants for the addition of pyridine, N-methylimidazole, and imidazole to Fe(TPP)N3 in CH2C12 at 23 ± 1 °C have been determined from the visible spectra. The equilibrium observed is Fe(TPP)N3 + B (K1) ⇄ Fe(TPP)N3B where the product is six-coordinate. K1 for pyridine is small, confirming the instability observed for Fe(TPP)N3py in the magnetic study. For imidazole, Fe(TPP)N3 + 2B (β2) ⇄ Fe(TPP)B2+N3 - is observed in addition to the other equilibrium. © 1979, American Chemical Society. All rights reserved.