Synthesis, characterization and electrochemistry of rhodium(iii) complexes of meso-substituted [14]tribenzotriphyrin(2.1.1)

Abstract

A thermal reaction using a series of [14]tribenzotriphyrins(2.1.1) (TriPs, 1a–d) with Rh₂(C₈H₁₂)Cl₂ provides Rh^III–TriP complexes (2a–d) in 40−52% yields. The complexes were characterized by mass spectrometry, UV-visible absorption and ¹H NMR spectroscopy. Single crystal X-ray analysis reveals that 2b adopts a dome-shaped conformation. The rhodium(III) ion is coordinated by the three pyrrole nitrogen atoms, two chloride ions and the nitrogen atom of an acetonitrile (CH₃CN) solvent molecule. The optical spectra can be assigned using Michl's perimeter model. The L and B bands of the 2a–d complexes lie at ca. 600 and 500 nm, respectively, and are markedly red shifted relative to those of 1a–d. A reversible one-electron oxidation and two reversible one-electron reductions are observed in the cyclic voltammograms of 2a–d in CH₂Cl₂. The redox potentials are consistent with the optical data and the relatively narrow HOMO–LUMO gaps that are predicted in DFT calculations. TD-DFT calculations have been used to assign a third intense spectral band at 375 nm to a higher energy π → π* transition of the [14]tribenzotriphyrin(2.1.1) π-system.