Spectroelectrochemistry and excited-state absorption spectroscopy of rhenium(I)α,α′-diimine complexes

Abstract

Thin-layer electrochemistry and spectroelectrochemistry in 0.1 mol dm^–3 NBu₄PF₆–dmf (dmf = dimethylformamide) have been used to characterize spectroscopically the sequentially reduced forms of [Re(bipy)(CO)₃Cl], [Re(dpp)(CO)₃Cl] and [{Re(CO)₃Cl}₂(dpp)][dpp = 2,3-bis(2-pyridyl)pyrazine, bipy = 2,2′-bipyridine]. Following the first reduction of each complex, the spectra exhibit similar absorption features in the near-UV region from 350 to 400 nm with a tailing absorption to ca. 500–600 nm. The excited-state absorption spectra of [Re(dpp)(CO)₃Cl] and [{Re(CO)₃Cl}₂(dpp)] have also been obtained and exhibit similar features as in the spectroelectrochemical measurements. The spectra are consistent with reduction of the α,α′-dimine ligand and are dominated by π→π* transition of the ligand radical anion. Addition of a second electron to the monometallic complexes yields spectra that are characterized by a broad absorption band in the range 530–580 nm (ε= 10 900–12 100 dm³ mol^–1 cm^–1). In comparison, addition of a second electron to [{Re(CO)₃Cl}₂(dpp)] also yields a broad absorption at 550 nm (ε= 8800 dm³ mol^–1 cm^–1) and loss of the absorption at 357 nm, which is analogous to the spectrum for the doubly reduced [Re(dpp)(CO)₃Cl] complex. Further reduction of the bimetallic complex results in identical spectral features as the doubly reduced complex with the main difference being an increase in absorbance over the entire UV/VIS region and a red shift of the lowest energy absorption to 576 nm. The spectroelectrochemical measurements for [{Re-(CO)₃Cl}₂(dpp)] suggest the second and third reductions result in reduction of the metal centres contrary to previous reports for electron localization in the bimetallic complex, which describe the second reduction as ligand based.