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

Abstract

We have synthesized 2, a derivative of zinc(II) quadruply fused porphyrinato (ZnIIQFP) that is tetrabrominated at the β-positions of the two nonfused pyrroles, by treatment of ZnIIQFP 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 ZnII centers. In (2)2-EtOH, one of the pyrrolic nitrogen atoms of the two nonfused pyrroles dissociates from the ZnII center, and the dissociated pyrrolic nitrogen atom coordinates to the ZnII center of the other molecule in the dimer. The ZnII center having the μ-η3:η1-QFP ligand is coordinated by an EtOH molecule, and the other ZnII 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 ZnII 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.