Electronic properties of new homobimetallic anthracene-bridged η5-cyclopentadienyl derivatives of iridium(I) and of the corresponding cation radicals [L2Ir{C5H4CH2(9,10-anthrylene)CH2C5H4}IrL2]+

The bimetallic complexes [L2Ir{C5H4CH2(9,10-anthrylene)CH2C5H4}IrL2] (3) (L = η2-C2H4) and (4) (L = CO) were obtained by reacting the thallium(I) derivative of 9,10-bis(cyclopentadienylmethyl)-anthracene (1), i.e. [Tl{C5H4CH2(9,10-anthrylene)CH2C5H4}Tl] (2), with [IrCl(η2-C2H4)4] and [IrCl(C5H5N)(CO)2], respectively, and characterized by elemental analysis, MS, 1H NMR, UV–Vis (290–490 nm) spectroscopy, and FT-IR. When excited at wavelengths ranging from 333 to 383 nm, 1 results to be fluorescent, while 3 and 4 show the almost complete quenching of the anthrylene fluorescence. The electrochemical behaviour of 3 and 4 has been studied and compared with that of the monometallic complexes, i.e. (η5-9-anthrylmethylcyclopentadienyl)-bis(η2-ethylene)iridium(I) (5), whose preparation and X-ray structure are reported here, and the already described (η5-9-anthrylmethylcyclopentadienyl)dicarbonyliridium(I) (6). The study allows the interpretation of the electrode processes and gives information about the location of the redox sites along with the thermodynamic characterization of the redox processes. On this basis, the intramolecular charge-transfer process between the photo-excited anthrylenic moiety and one cyclopentadienylIrL2 unit is suggested to be a possible route for the quenching of the anthrylene fluorescence. The oxidation of 3 and 4 by [bis(trifluoroacetoxy)iodo]benzene (PIFA) and thallium(III) trifluoroacetate (TTFA), respectively, produces the radical cations 3+ and 4+, which, on the base of their EPR spectra, are described as average-valence [Ir+1.5, Ir+1.5] species. DFT calculations of spin density distribution confirm the EPR results and allow a further insight into the structure of such radicals. Differences and analogies lying between the electronic and conformational structure of the bimetallic, 3+ and 4+, and the monometallic, 5+ and 6+, cation radicals are discussed by comparing the EPR spectra and the spin density distribution maps.

Graphical abstractA study is reported aimed to investigate how the active sites of new homobimetallic anthracene-bridged cyclopentadienyl derivatives of iridium(I) interact in those conditions (chemical, photophysical, and electrochemical) which can induce the intramolecular transmission of electronic effects.Figure optionsDownload full-size imageDownload as PowerPoint slide