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(II)QFP) that is tetrabrominated at the beta-positions of the two nonfused pyrroles, by treatment of Zn(II)QFP 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 mu-eta(3):eta(1) coordination mode. In the dimeric structure, the two QFP ligands showed a unique eta(3) 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 mu-eta(3):eta(1)-QFP ligand is coordinated by an EtOH molecule, and the other Zn-II center is coordinated by the eta(3)-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 beta-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 beta-positions, and the shift widths exhibited a linear correlation to the Hammett parameters of the substituents.