Distorted Porphyrins with High Stability: Synthesis and Characteristic Electronic Properties of Mono- and Di-Nuclear Tricarbonyl Rhenium Tetraazaporphyrin Complexes

Mono- and di-nuclear tricarbonyl Re(I) tetraazaporphyrin complexes (Re(1)TAP and Re(2)TAP) are investigated and compared with Re(I) phthalocyanine complexes (Re1Pc and Re2Pc). Although Re2Pc is unstable in polar solvents, and easily undergoes demetallation reaction, the coordination of the TAP ligand significantly improves the tolerance toward polar solvents, affording more stability to Re(2)TAP. Additionally, the incorporation of [Re(CO)(3)](+) unit(s) and the TAP ligand results in remarkable positive shifts in both oxidation and reduction potentials. Consequently, the more positive oxidation potentials of the ReTAP complexes significantly increase the tolerance toward oxidation, while the reduction potential indicates that Re(2)TAP is suitable for a soluble electron acceptor. In contrast to Re1Pc and Re2Pc, Re(1)TAP and Re(2)TAP show unique broad Q bands, which can be attributed to the admixture of the pi-pi* and metal-to-ligand charge transfer characters, owing to the lowered pi orbital energy in the TAP complexes. This study is useful for controlling electronic properties and realizing high stability in Pc analogues.

Automated summary

The study reveals that the TAP ligand enhances the stability and tolerance of Re(2)TAP towards polar solvents, while also resulting in positive shifts in oxidation and reduction potentials for the ReTAP complexes. The unique broad Q bands in Re(1)TAP and Re(2)TAP are attributed to the lowered pi orbital energy in the TAP complexes, which contribute to the admixture of pi-pi* and metal-to-ligand charge transfer characters.