Correlations of axial ligand field strength and zero-field splittings in the carbon-13 NMR spectra of five- and six-coordinate high-spin iron(III) porphyrin complexes

Abstract

Carbon-13 NMR spectra have been recorded for complexes of high-spin iron(III) porphyrins in which the axial anionic ligand has been varied. The (Me2SO)2 complexes of high-spin iron(III) tetraphenylporphyrins ((TPP)Fe111) and natural-derivative porphyrins have also been examined. Replacement of a single anionic ligand with two solvent ligands induces striking changes in carbon-13 NMR spectra, with changes in the phenyl ortho-carbon and meso-carbon signals of (TPP)Fe111 on the order of several hundred ppm. This explains the seemingly anomalous upfield and downfield meso-proton shift values for fiveand six-coordinate species. Results are interpreted in terms of unpaired spin delocalization through bothand -type molecular orbitals, with a predominant mechanism of Fe--porphyrin "back-bonding" in six-coordinate complexes. A contribution from porphyrinFe charge transfer places large unpaired spin density at the meso-carbon atom of five-coordinate species. Modulation of the meso-position spin density by differing axial anionic ligands is apparent in variable NMR chemical shift values for the phenyl and meso-carbon atoms of (TPP)FeX complexes. Phenyl shifts show a positive correlation with the zero-field splitting parameter, D, as measured by far-infrared and magnetic methods. A correspondence is also apparent between meso-carbon NMR shift values and Fe(III)/Fe(II) redox potentials. These observations suggest the ligand field strength of the axial ligand dictates the magnitude of unpaired spin density at the meso position, with larger downfield shifts associated with weaker field ligands. On this basis the following ordering of ligand field strength is offered for high-spin iron(III) porphyrins: F" > OPh' > OPh(p-N02)~> N3" = OAc" > NCS' > Cl' > Br" > I".

NMR Hyperfine Shifts in Blue Copper Proteins: A Quantum Chemical Investigation

“…Every chemical bond has a bond critical point at which the first derivative of the charge density, ρ (r), is zero. 13,14 The ρ(r) topology is described by a real, symmetric, second-rank Hessian-of-ρ(r) tensor, and the tensor trace is related to the bond interaction energy by a local expression of the virial theorem: (5) where ∇ 2 ρ(r) is the Laplacian of ρ(r) and G(r) and V(r) are electronic kinetic and electronic potential energy densities, respectively. Negative and positive ∇ 2 ρ(r) values are associated with shared-electron (covalent) interactions and closed-shell (electrostatic) interactions, respectively.…”

“…3,4 For example, the iron centers in paramagnetic proteins (and model systems) have a >5000 ppm range of 13 C NMR shifts 5,6 and are well-correlated with electronic structure. 7 While 1 H NMR shifts are typically smaller, recent studies on several blue copper proteins (BCPs), amicyanin (Am), 8 azurin (Az), 9 plastocyanin (Pc), 10 pseudoazurin (Pa), 11 stellacyanin (St), 9 and rusticyanin (Rc), 11 revealed that the Cys-C β H 2 shifts are in the range of ~240-850 ppm, 9-11 since they are only three bonds removed from the paramagnetic (Cu II ) center.…”

Magnetic Transitions in Iron Porphyrin Halides by Inelastic Neutron Scattering and Ab Initio Studies of Zero-Field Splittings

“…12 Goff and co-workers have reported correlations of axial ligand field strength and zero-field splittings in the C-13 NMR spectra of 5-and 6-coordinate high-spin Fe(III) porphyrin complexes. 13 Ohya and Sato conducted a comparative study of Mossbauer spectra of -fold symmetry, their crystals are in the monoclinic system. Thus, the rhombic parameter E in eq 1 is required to explain ZFS properties of their crystalline samples.…”

Paramagnetic NMR Shifts for Saddle‐Shaped Five‐Coordinate Iron(III) Porphyrin Complexes with Intermediate‐Spin Structure

“…[7][8][9][10] For planar porphyrin ligands, such as high-spin FeTPPCl (or FeOEPCl), the meso-C signal appears at a downfield position of 500 ppm (or 380 ppm), and 368 ppm (or 246 ppm) for the admixed (5/2, 3/2) FeTPPClO 4 (or FeOEPClO 4 ), respectively (see Table 1; TPP = dianion of 5,10,15,20-tetraphenylporphyrin and OEP = dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin). [11,12] These downfield shifts at the meso-C atoms have been attributed by Cheng et al to interactions between the iron(III) d z 2 and the porphyrin a 2u orbitals. [13] However, for saddled porphyrin ligands, considering the main skeleton of the macrocycle, the five-coordinate saddleshaped Fe(OETPP)Cl has a local C 2v symmetry, which is lower than the C 4v symmetry of five-coordinate planar iron(III) porphyrin complexes.…”

“…Fe(TPP)X [7,11,12,14] porphyrin complexes in similar spin states so far, but this change has been postulated because of the difference in the geometric structures. [7] In this study, we employed density functional theory (DFT) calculations to correlate the Fe-porphyrin bonding interactions with calculated spin populations, including the total spin, the localized p spin, and the Fermi contact spin densities on the porphyrin macrocycle to elucidate these unexpected NMR data.…”