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

A thermal reaction using a series of [14]tribenzotriphyrins(2.1.1) (TriPs, 1a-d) with Rh-2(C8H12)Cl-2 provides Rh-III-TriP complexes (2a-d) in 40-52% yields. The complexes were characterized by mass spectrometry, UV-visible absorption and H-1 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 pi -> pi* transition of the [14]tribenzotriphyrin(2.1.1) pi-system.