Electronic structure of low-spin ferric chlorins: characterization of bis(1-methylimidazole)(meso-tetraphenylchlorinato)iron(III) triflate

Abstract

The synthesis and characterization of the trifluoromethanesulfonato derivative of bis(1-methylimidazole)tetrakis(phenyl)chlorinatoiron(III) [Fe(TPC)(1-MeIm)₂]CF₃SO₃1 are reported. The crystal structure of complex 1 has been determined. The X-ray structure shows that the porphyrinate rings are weakly distorted. The metal–nitrogen distances to the reduced pyrrole N(2), 2.016 (6) Å and to the pyrrole trans to it N(4), 2.032 (5) Å are longer than the remaining two nitrogens: N(1), 2.005 (6) Å and N(3), 1.972 (6) Å leading to a core-hole expansion of the macrocycle due to the reduced pyrrole. The two axial ligand planes are parallel. The ¹H NMR isotropic shifts at 20 °C of the different pyrrole protons of 1 varied from −8.6 ppm to −42.6 ppm according to bis-ligated complexes of low-spin ferric chlorins. The EPR spectrum of [Fe(TPC)(1-MeIm)₂]CF₃SO₃1 in solution is rhombic and shows the principal g-values g₁ = 2.47, g₂ = 2.42 and g₃ = 1.78 (Σg² = 15.1). These spectroscopic observations are indicative of a metal-based electron in the d_π orbital for the [Fe(TPC)(1-MeIm)₂]CF₃SO₃ (1) complex with a (d_xy)² (d_xzd_yz)³ ground state at any temperature.