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

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 octaethylpoxphyrin, OEP), for which trimethylphosphine serves as at least one of the axial ligands, have been measured by recording the 3iP 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 JC donor characteristics that increase the magnitude of the paramagnetic screening constant, dpara. Mixed axial ligand complexes where trimethylphosphine is one of the ligands have larger 57Fe chemical shifts when the other ligand (L) is a strong u donortweak JC acceptor, but within a group of closely related ligands such as 4-substituted pyridines, the weakest u donorhtrongest n acceptor ligand produces the largest 57Fe chemical shift (4-CNPy > Py > 4-NMezPy), in line with the predictions of the Ramsey formula for dpxa. Solvent effects on the 57Fe chemical shift of [TMPFe(PMe&] are small and, in distinction to 6-coordinate low-spin cobalt(II1) porphyrinates, do not follow any common measures of solvent polarity or donor strength (dielectric constant, Gutmann’s donor number, Reichardt’s ET). 31P and 57Fe chemical shifts both show linear temperature dependence, but with opposite slopes. The structures of two (PMe3)z complexes of iron(I1) porphyrinates, [@OCH3)4TPPFe(PMe3)2]-2CsDs and [OEPFe(PMe3)2], have also been determined. In both cases the porphyrinate core is essentially planar and the axial trimethylphosphine ligands have their methyl groups in staggered conformation.