(1)H NMR spectra and electronic structure of reduced iron porphyrins: Fe(II), Fe(I) and Fe(0) porphyrins

Article abstract of DOI:10.1016/0022-2860(93)85001-B

Fe(II), Fe(I) and Fe(0) porphyrins have been generated by stepwise reduction with a sodium mirror in vacuum and their (1)H NMR spectra have been recorded and analyzed.Fe(II) porphyrins (Fe(II)P) have been examined in three spin states, an S = 0 state in pyridine-d8, an S = 1 state in benzene-d6 and an S = 2 state in tetrahydrofuran-d8 (THF-d8).The analysis of isotropic shifts for low-spin Fe(II)P (S = 0) has indicated that no charge transfer has been observed.The ground state configuration is (d_xy)²(d_xz,d_yx)⁴.The contact shifts for intermediate-spin Fe(II)P (S = 1) reflect P -> Fe ? charge transfer.The proposed electron configuration is (d_xy)² (d_z²)² (d_xz,d_yz)², which agrees with Moessbauer data.The pattern of contact shifts for high-spin Fe(II)P (S = 2) is consistent with ? spin transfer, which suggests that the d_(x²-y²) orbital possesses an unpaired spin.The electron configuration is (d_xy)² (d_xz,d_yz)² (d_z²)¹ (d_(x²-y²))¹.Our results for Fe(II)P (S = 1,2) agree with the literature (1)H NMR data.In the case of Fe(I)P in THF, the separation of isotropic shifts into the dipolar and contact contributions has shown the dominance of the latter.The observed shifts indicate negative ? spin density on pyrrole and meso carbon atoms of the ligand, which seems to be due to a strong ?-? spin polarization effect.When this fact is taken into account the pattern of contact shifts is consistent with ? spin transmission involving both P -> Fe ? charge transfer out of the ligand-filled molecular (3e(?)) orbital and Fe -> P ?^* charge transfer into the ligand highest unoccupied (4e(?^*)) molecular orbital.The occurrence of the unpaired spin in this molecular orbital is consistent with ?-radical anion formulation which was found by X-ray crystallography.An S = 1/2 spin state determined by magnetic moment measurements agrees with the most probable electron configuration (d_xy)² (d_xz,d_yz)³ (d_z²)².In the case of Fe(0)P, the isotropic shifts were found to be small, providing evidence of some spin transfer.The ground state configuration is (d_xy)² (d_xz,d_yz)⁴ (d_z²)².