Effect of phosphorus axial π-acceptors in iron(III) porphyrinates:: Characterization of low-spin complexes [(PPh(OMe)2)2Fe(T(p-Me)PP)]CF3SO3 and [(PPh(OEt)2)2Fe(T(p-Me)PP)]CF3SO3

The synthesis and characterization of the trifluoromethanesulfonato derivatives of bis(dimethyl phenylphosphonite)(tetrakis(p-methylphenyl)porphyrinato)iron(III) 1, and bis(diethyl phenylphosphonite)(tetrakis(p-methylphenyl)- porphyrinato)iron(III), 2, are reported: [(PPh(OMe)(2))(2)Fe(T(p-Me)PP)]CF3SO3, 1, and [(PPh(OEt)(2))(2)Fe(T(p-Me)PP)]CF3SO3, 2, (T(p-Me)PP = tetrakis(p-methylphenyl)porphyrinato). The crystal structure of complexes 1 and 2 have been determined. Both X-ray structures show that the porphyrinate rings are strongly ruffled. The equatorial Fe-N bond distances average to 1.971 (6) Angstrom for 1 and to 1.969 (7) Iq for 2, which is a quite short distance for low-spin iron(III) porphyrinate derivatives. The H-1 NMR isotropic shifts at 20 degrees C of the pyrrole protons of the two complexes, varied from -2 ppm for 2 to +3 ppm for 1 rather than the expected -20 to -30 ppm, based on previously studied bis-ligated complexes of low-spin ferric porphyrins. The electron paramagnetic resonance spectra of [(PPh(OMe)(2))(2)Fe(T(p-Me)PP)]CF3SO3, 1, and [(PPh(OEt)(2))(2)Fe(T(p-Me)PP)]CF3SO3, 2, in solution are axial, with g(perpendicular to) = 2.36 and g(parallel to) = 1.87-1.91 at 4 K, Sigma g(2) = 14.7. The Mossbauer spectra of 1 and 2 at 70 K have isomer shifts of 0.35 and 0.37 mm/s and quadrupole splittings of 1.23 and 1.66 mm/s, respectively. All physical properties are consistent with a low-spin iron(III) with an unusual ground-state configuration (d(xy),d(yz))(4)(d(xy))(1), as recently reported for [(4-CN-Py)(2)Fe(TPP)]ClO4 Safe, (M. K.; Walker, F. A.; Raitsimring, A. M.; Waiters, W. P.; Dolata, D. P.; Debrunner P. G.; Scheidt, W. R. J. Am. Chem. Sec. 1994, 116, 7760-7770). Crystal data for [(PPh(OMe)(2))(2)Fe(T(p-Me)PP)]CF3SO3, 1: a = 16.611(3) Angstrom, c = 20.722(4) Angstrom, trigonal, space group P3(2), V = 4951(2) Angstrom(3), Z = 3. Crystal data for [(PPh(OEt)(2))(2)Fe(T(p-Me)PP)]CF3SO3, 2: a = 24.017(2) Angstrom, b = 14.380(9) Angstrom c = 18.702(4) Angstrom, monoclinic, space group P2(1/a), V = 6457(6) Angstrom(3), Z = 4.