Solid state N-15 and C-13 NMR study of several metal 5,10,15,20-tetraphenylporphyrin complexes

The principal values of both the C-13 and N-15 chemical shift tensors are reported for the Zn, Ni, and Mg 5,10,15,20-tetraphenylporphyrin (TPP) complexes. The principal values of the N-15 chemical shift tensors were obtained from static powder patterns of N-15-enriched samples. Due to overlap between the powder patterns of the different carbons, the C-13 values were obtained using the recently developed magic angle turning (MAT) 2D experiment on unenriched materials. The measured principal values are presented along with theoretical calculations of the chemical shift tensors and a discussion of the effects that the metal bonding has on the chemical shift tensors in these compounds. Both the isotropic chemical shift and the principal values of the N-15 chemical shift tensor are nearly identical for the Mg and Zn complexes. The N-15 isotropic chemical shift for the NiTPP, however, changes by nearly 80 ppm relative to the Mg and Zn values, with large changes observed in each of the three principal values. Calculations show that the differences between the N-15 chemical shifts are almost entirely determined by the metal-nitrogen separation. In addition, both the experimental data and the calculations show only very minor differences in the C-13 chemical shift tensor components as the metal is changed.