59Co NMR Studies of Diamagnetic Porphyrin Complexes in the Solid Phase

Solid state 59Co nuclear magnetic resonance (NMR) spectra were recorded on diamagnetic complexes with general structure Co(Por)L2, where Por ) tetraphenylporphyrin, tetramethoxyphenylporphyrin, and octaethylporphyrin; and L ) imidazole, methylimidazole, pyridine, and isoquinoline. Measurements were carried out at different magnetic field strengths (4.7, 7.1, and 11.7 T) on both static and spinning samples. Iterative fitting of these data yielded the shielding and quadrupolar parameters that characterize the various cobalt(III) sites in the complexes. These results are only in partial agreement with data inferred from previous solution NMR measurements. The values measured for the anisotropic components of the 59Co NMR tensors also deviate from traditional correlations observed for simpler nonaromatic solid cobalt complexes. Possible sources for the discrepancies observed between the solution and the solid phase parameters as well as for the anomalous behavior observed with the anisotropic 59Co coupling constants are discussed.