The effect of axial ligand plane orientation on the isotropic shifts of the dichelated protohemin cyanide complexes of Traylor and Berzinis

In 1980 Traylor and Berzinis published the NMR spectrum of the mixture of t wo geometrical isomers of the dichelated protohemin cyanide prepared from t he 6,7-bis-(3-N-imidazolylpropyl)propionamide of protohemin (J. APn. Chem. Sac, 1980, 102, 2844). We have prepared these same isomers and investigated their H-1 NMR spectra by modern 1- and 2D NMR techniques. The results show that the eight pyrrole-methyl resonances having a spread of 26.5 ppm at -5 7 degrees C divide into two sets of four widely spaced pyrrole-methyl reson ances that have shifts 3,8 much greater than 5,1 for the isomer having the 6-substituted imidazole bound (isomer I), and 5,1 much greater than 3,8 for the isomer having the 7-substituted imidazole bound (isomer II). NMR techn iques not available in 1980 (NOESY/EXSY, COSY, ROESY, DQF-COSY) have allowe d nearly complete assignment of the NMR spectra, except for several meso-H resonances and the resonances of some of the protons of the alkyl side chai ns of each isomer, which all fall in a narrow chemical shift range in the, diamagnetic region. The imidazole planes are found not to be lying directly over the porphyrin nitrogens, but are shifted by about 10 degrees counterc lockwise with respect to that line for isomer I, which yields the order 3 > 8 >> 5 > 1, and 10 degrees clockwise of that line for isomer II, which yie lds the order 5 > 1 much greater than 3 > 8.