Electronic effects in transition metal porphyrins. 8. The effect of porphyrin substituents, axial ligands, steric crowding , solvent, and temperature on the 57Fe chemical shifts of a series of model heme complexes

Article abstract of DOI:10.1021/ja00141a027

The ⁵⁷Fe chemical shifts of a series of 94.5% ⁵⁷Fe enriched model heme complexes (those of tetraphenylporphyrin, TPP, and two of its p-phenyl substituted derivatives, tetramesitylporphyrin, TMP, and octaethylporphyrin, OEP), for which trimethylphosphine serves as at least one of the axial ligands, have been measured by recording the ³¹P NMR spectrum using double resonance at the appropriate ⁵⁷Fe frequency. The discovery of an approximate correlation between ³¹P and ⁵⁷Fe chemical shifts makes it easy to predict the ⁵⁷Fe chemical shift of new complexes, thus simplifying the search for the proper decoupling frequency. The ⁵⁷Fe chemical shifts of substituted phenyl TPP complexes increase as the electron-donating nature of the substituents increases, and most OEP complexes have chemical shifts that are 200-250 ppm larger than their TPP counterparts; both of these trends are attributed to increased π donor characteristics that increase the magnitude of the paramagnetic screening constant, σ^para. Mixed axial ligand complexes where trimethylphosphine is one of the ligands have larger ⁵⁷Fe chemical shifts when the other ligand (L) is a strong σ donor/weak π acceptor, but within a group of closely related ligands such as 4-substituted pyridines, the weakest σ donor/strongest π acceptor ligand produces the largest ⁵⁷Fe chemical shift (4-CNPy > Py > 4-NMe₂Py), in line with the predictions of the Ramsey formula for σ^para. Solvent effects on the ⁵⁷Fe chemical shift of [TMPFe(PMe₃)₂] are small and, in distinction to 6-coordinate low-spin cobalt(III) porphyrinates, do not follow any common measures of solvent polarity or donor strength (dielectric constant, Gutmann's donor number, Reichardt's E_T). ³¹P and ⁵⁷Fe chemical shifts both show linear temperature dependence, but with opposite slopes. The structures of two (PMe₃)₂ complexes of iron(II) porphyrinates, [(p-OCH₃)₄TPPFe(PMe₃) ₂]·2C₆D₆ and [OEPFe-(PMe₃)₂]ave also been determined. In both cases the porphyrinate core is essentially planar and the axial trimethylphosphine ligands have their methyl groups in staggered conformation.