A Class of Heptaphyrins with NIR Absorption Modulated by Metal Coordination and Nucleophilic Substitution

The intensive interest in expanded porphyrins can be attributed to their appealing photoelectric and coordination behavior. In this work, an N-confused heptaphyrin 1 was synthesized by an acid-catalyzed [3+4] condensation reaction. The introduction of an N-confused pyrrolic unit into the heptaphyrin macrocycle led to the formation of a figure-eight-like conformation with nonsymmetrical "NNNN" and "NNNC" coordination cavities employable for bimetallic coordination. As a result, chelation of 1 with Zn(II) and Cu(II) afforded mono-Zn(II) complex 2 and bis-Cu(II) complex 3, respectively, with the metal atoms exhibiting distorted square-planar geometries. In complex 3, an oxygen atom is attached to the α-C atom of N-confused pyrrole D, and thus the N and C atoms of ring D participate in coordination within the two cavities. Interestingly, treatment of 1 with Cs₂CO₃ in MeOH resulted in regioselective substitution of all the seven para-F atoms in the meso-C₆F₅ groups as well as the α-H of ring D by eight methoxy moieties. Complex 3 displays a red-shifted absorption band edge of ca. 2200 nm, compared to that of ca. 1600 nm observed for 1. This work provides an example of incorporating an N-confused pyrrole to construct expanded porphyrins with distinctive coordination behavior and tunable NIR absorption.