Substituent Effects at the β‑Positions of the Nonfused Pyrroles in a Quadruply Fused Porphyrin on the Structure and Optical and Electrochemical Properties

We have synthesized 2, a derivative of zinc­(II) quadruply fused porphyrinato (Zn^IIQFP) that is tetrabrominated at the β-positions of the two nonfused pyrroles, by treatment of Zn^IIQFP with N-bromosuccinimide. X-ray diffraction analysis of a single crystal obtained from a THF solution of 2 by vapor diffusion of ethanol (EtOH) revealed that 2 formed an unprecedented dimeric structure, (2)₂-L (L = EtOH), in which one of the brominated QFP ligands acts as a bridging ligand in an unprecedented μ-η³:η¹ coordination mode. In the dimeric structure, the two QFP ligands showed a unique η³ coordination mode for both Zn^II centers. In (2)₂-EtOH, one of the pyrrolic nitrogen atoms of the two nonfused pyrroles dissociates from the Zn^II center, and the dissociated pyrrolic nitrogen atom coordinates to the Zn^II center of the other molecule in the dimer. The Zn^II center having the μ-η³:η¹-QFP ligand is coordinated by an EtOH molecule, and the other Zn^II center is coordinated by the η³-QFP ligand and one nitrogen atom of the bridging QFP ligand. The dimeric structure is stable and maintained even in a solution of noncoordinating solvents such as dichloromethane. The bromo groups of 2 can be substituted with phenyl groups under Suzuki coupling conditions to afford the tetraphenyl derivative, 3. Furthermore, the effects of the substituents at the β-positions on the optical and electrochemical properties and Lewis acidity of the Zn^II centers have been investigated. The redox potentials were positively shifted by introduction of electron-withdrawing groups at the β-positions, and the shift widths exhibited a linear correlation to the Hammett parameters of the substituents.