Proton NMR study of the molecular and electronic structure of ferric chlorin complexes: Evidence for π bonding by the orbital derived from the porphyrin a1u orbital

The proton NMR spectra of a variety of low-spin and high-spin ferric complexes of the naturally occurring chlorin derivative pyropheophorbide a methyl ester have been recorded and assigned. The complete assignments for the low-spin, dicyano complex were effected by a combination of partial isotope labeling, paramagnetic relaxation, multiplet structure, and nuclear Overhauser measurements in viscous solvent. The resulting contact shift pattern reflects π spin delocalization into both the highest filled MO equivalent to the porphyrin 3eπ(xz,yz), as well as into the former a1u orbital of the porphyrin. The saturation of a pyrrole ring to produce a chlorin is concluded to uniquely stabilize the orbital ground state of the iron, which allows the lone spin to π bond to the two unmodified trans-pyrroles. The pattern of contact shift asymmetry of the pyrroles adjacent to the saturated ring provides a potential model for interpreting NMR spectra of iron chlorin enzymes. Ferric pyropheophorbide a methyl ester yields a six-coordinate high-spin complex in dimethyl sulfoxide in both the presence and absence of chloride, indicating that high-spin ferric chlorins have a greater affinity for weak-field ligands than do analogous porphyrins. The ∼103 acceleration of the rate of macrocycle "inversion" of pyropheophorbide-iron chloride, as induced by associative halide exchange, supports a more flexible core for chlorins relative to porphyrins. © 1989 American Chemical Society