1H NMR and EPR studies of the electronic structure of low-spin iron(III) isocyanide mesotetraphenylchlorin complexes: a (dxz,dyz)4(dxy)1 configuration from 293 to 4 K

Abstract

Reaction of (tetraphenylchlorinato)iron(III) chloride with eight equivalents of tert-butyl isocyanide and 2,6-xylyl isocyanide in the presence of zinc amalgam afforded Fe(TPC)(t-BuNC)₂1 and Fe(TPC)(2,6-xylylNC)₂2, respectively. The synthesis and characterization of the trifluoromethanesulfonato derivatives of (tetraphenylchlorinato)iron(III) 3, bis(tert-butyl isocyanide)(tetraphenylchlorinato)iron(III) 4, and bis(2,6-xylyl isocyanide)(tetraphenylchlorinato)iron(III) 5 are reported: [Fe(TPC)]CF₃SO₃3, [Fe(TPC)(t-BuNC)₂] CF₃SO₃4 and [Fe(TPC)(2,6-xylylNC)₂]CF₃SO₃5. The ¹H NMR isotropic shifts at 20 °C of the pyrrole protons of the two complexes 4 and 5, varied from 5 ppm for 4 to 8 ppm for 5 rather than the expected −10 to −30 ppm, based on previously studied bis-ligated complexes of low-spin iron(III) chlorins. EPR spectra of [Fe(TPC)(t-BuNC)₂]CF₃SO₃4 in solution are axial, with g_⊥ = 2.15 and g_∥ = 1.97 at 4 K, Σg² = 14.7. All physical properties are consistent with a low-spin iron(III) with an unusual ground-state configuration (d_xz,d_yz)⁴(d_xy)¹.