ID and 2D 1H NMR studies of iron(III) complexes on geoporphyrins of the deoxophylloerythroetio structural type derived from oil shale

Abstract

AbstractOne‐ and two‐dimensional 1H NMR studies of low‐spin iron(III) geoporphyrins derived from the deoxophylloery‐throetioporphyrin series (the oil shale source) resulted in an unambiguous assignment of all resonances. Deoxophylloerythroetioporphyrin (DPEP) and its 17‐desethyl homologue were isolated from dictyonema shale as vanadyl complexes. They were demetallated and subsequently converted into iron(III) porphyrins under mild conditions. The 17‐propionic methyl ester analogue, i.e. deoxophylloerythrin methyl ester, was synthesized for comparative studies. A spectroscopic probe to identify iron(III) isocyclic porphyrins was established. The five‐membered isocyclic ring induced considerable asymmetry in the spin density distribution at the 3eπ bonding orbitals, placing the unpaired electron on the dxz orbital located along the N‐21—N–23 axis. The temperature dependence of the hyperfine shift is consistent with the thermal equilibrium between two non‐degenerated electronic states (dxz)2(dyz)1↔(dxz)1(dyz)2. The new structural factor generating the asymmetric spin density distribution in low‐spin iron(III) porphyrin complex was determined. The hyperfine shift pattern of high‐spin iron(III) isocyclic geoporphyrins provided useful probes for their detection in iron geoporphyrin mixtures by 1H NMR prior to their separation. Valuable information is derived from the number of observed signals of meso‐protons and characteristic, extremely downfield resonances of isocyclic ring hydrogens. Both features may be useful for the spectral identification of iron(III) porphyrins of the DPEP series from geological materials of relatively low purity.