Nuclear magnetic resonance study of metal complexes of the ‘capped’ porphyrins

Abstract

The cobalt(II) derivatives of the ‘capped’ porphyrins CP2, CP3, and CP2(N) were studied by ¹H n.m.r. spectroscopy. From an analysis of the paramagnetic shifts in the n.m.r. spectrum the cap–porphyrin separations were deduced and the overall conformation examined. It was found that the cavity size, as implied by the cap–porphyrin separation, increased in the order CoCP3, CoCP2, then CoCP2(N). The cap proton resonance linewidths were consistent with this ordering. This order correlates with the observed O₂ affinity of the iron(II) capped porphyrins. A comparison of the solvent effects on the n.m.r. spectrum of CoCP2 and CoCP3 indicates binding of solvent on the cavity side in CoCP3. We also report a comparison of CP2(N) and its zinc derivative with CP2 and ZnCP2 which shows close similarity in their behaviour.