ELECTRONIC EFFECTS IN TRANSITION-METAL PORPHYRINS .7. SYNTHESIS AND SPECTROSCOPIC INVESTIGATIONS OF A SERIES OF OCTASUBSTITUTED PORPHYRIN ISOMERS AND THEIR IRON(III) COMPLEXES

The synthesis, from a mixture of 3,4-diethylpyrrole, 3,4-bis(N,N-dieth ylcarbamoyl)pyrrole, and formaldehyde, of six pyrrole-substituted porp hyrin isomers and their characterization by UV-visible and H-1 NMR spe ctroscopy is reported. Spectral band shifts of the six isomers are irr egular, but generally in the order OEPH2 < E6A2PH2 < t-E4A4PH2 < c-E4A 4PH2 > E2A6PH2 > OAPH2 for the four visible bands, while the Soret ban d shifts to longer wavelengths as diethyl substituents are replaced by carbamoyls. The relative intensities of the visible bands I-IV vary i n accord with the predictions of Falk (Porphyrins and Metalloporphyrin s, Elsevier: New York, 1964), based on the symmetry of the molecules. The H-1 NMR spectra of these six free-base porphyrins show the general effects of the ethyl groups in strengthening the ring current relativ e to the amide groups, but in addition demonstrate the much stronger r ing current in the trans relative to the cis isomer of E4A4PH2. Both t he high-spin and low-spin iron(III) complexes of the six pyrrole-subst ituted porphyrins were also synthesized and characterized by H-1 NMR s pectroscopy at several temperatures. In all of the low-spin Fe(III) po rphyrin isomers, the pattern of spin delocalization within the porphyr in ring is sensitive to both the nature of the substituents and their pattern of substitution. In both high- and low-spin Fe(III) complexes of the mixed substituent porphyrins, the methylene protons of the ethy l substituents exhibit diastereotopism due to the lack of coplanarity of the amide substituents with the porphyrin ring. A combination of CO SY and NOESY investigations of [E6A2PFe(N-MeIm)2]+ performed at -45 an d -60-degrees-C in CD2Cl2 allowed the assignment of all methylene prot ons of the three unique types of ethyl groups to their respective posi tions. The EPR spectra of the bis(N-methylimidazole) complexes of the same series of compounds show a marked difference between the cis and trans isomers, with much larger g anisotropy for the trans isomer than the cis (DELTAg(c) = 1.042; DELTAg(t) = 1.496, where DELTAg = g1 - g3 ). As a consequence of this difference, the calculated rhombicity (V/D ELTA) of the cis isomer is greater than 2/3, if it is assumed that g1 = g(zz), g2 = g(yy), and g3 = g(xx). The EPR parameters of cis-[E4A4PF e(N-MeIm)2]+ suggest that the principal magnetic axis may be in the-pl ane of the macrocyclic ring, at least at 8 K.