Proton NMR study of the molecular and electronic structure of ferric chlorin complexes: evidence for .pi. 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 πspin 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.