Nuclear magnetic resonance spectra of porphyrins. Part 33. Ring currents in nickel(II) hydroporphyrins derived from anhydromesorhodoporphyrin XV

Abstract

The double dipole network model of the porphyrin macrocyclic ring current is used to investigate the ring currents in reduced (hydro) porphyrins derived from nickel(II) anhydromesorhodoporphyrin xv methyl ester (2). The reduction of ring D to the corresponding chlorin (3) results in a decrease of about 10% of the pyrrole ring current, and about 40% in the inner loop current. The marked effect of the nickel(II) atom on the inner loop ring current in the chlorin series is clearly identified; the inner loop ring current is about two-thirds of that previously found for the magnesium(II) and zinc(II) chlorins. Further reduction of rings A and C gives the corresponding isobacteriochlorin isomers [(4) and (5)], which show an additional decrease of the ring current, this result paralleling that found previously for the nickel(II) isobacteriochlorin from phylloerythrin. The proton chemical shifts in nickel(II) hexahydro-(6) and (7) and octahydro-porphyrins (8), in which one or two of the meso methine carbons have been reduced (and therefore interrupt the inner loop pathway), are best accounted for by the inclusion of a small inner loop ring current in one quadrant of the macrocycle (i.e. via the nickel atom), together with ring currents in the unreduced pyrrole rings.