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

摘要

In 1980 Traylor and Berzinis published the NMR spectrum of the mixture of two geometrical isomers of the dichelated protohemin cyanide prepared from the 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 -57 degrees C divide into two sets of four widely spaced pyrrole-methyl resonances 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 techniques not available in 1980 (NOESY/EXSY, COSY, ROESY, DQF-COSY) have allowed 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 chains 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 counterclockwise 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 yields the order 5 > 1 much greater than 3 > 8. [References: 17]