HETERONUCLEAR NMR-STUDIES OF COBALAMINS .10. N-15 NMR-STUDIES OF [N-15]CYANOCOBALT CORRINS

15N NMR chemical shifts and13N-13C coupling constants have been determined for cyanocobalamin (CNCbl) and several other monocyano and dicyanocobalt corrins enriched in15N or in15N and13C in the axial cyanide ligand(s). The15N chemical shift is found to be nearly as sensitive (ca. 75%) to changes in trans axial ligation as the13C chemical shift of13CN cyanocobalt corrins. All alterations in the cobalt coordination sphere have opposite effects on the13C and15N chemical shifts; i.e. there is a strictly inverse relationship between δ15Nand δ13c. This suggests that the resonance structure Co+=C=N-indicative of dπ-pπ metal-to-ligand back-bonding is important in these cobalt-cyanide complexes and that its contribution to the overall bonding is highly sensitive to trans axial ligation effects.l5N chemical shifts andl5N-l3C coupling constants of enriched, base-off CNCbl have also been studied as a function of acidity in sulfuric acid/water mixtures and have permitted an assignment of the previously demonstrated protonation of the axial cyanide ligand in such media. The results are only consistent with a conjugate acid species that is N-bound (rather than the normal C-bound) and protonated at the cyanide carbon. © 1990, American Chemical Society. All rights reserved.