Electronic communication in homobimetallic anthracene-bridged ·5-cyclopentadienyl derivatives of rhodium(I): generation and characterization of the cation-radicals species [L2Rh{C5H4CH2(9,10-anthrylene)CH2C5H4}RhL2]+.

9,10-Bis(cyclopentadienylmethyl)anthracene (1) is obtained by reacting 9,10-bis(bromomethyl) anthracene with cyclopentadienylsodium and transformed into its dithallium(I) derivative 3 on reaction with thallium ethoxide. The reaction of 3 with the chloro derivatives of rhodium(I) of formula [RhClL2]2 (L ) è2-C2H4 or CO; L2 ) è4-C7H8) leads to the corresponding bimetallic complexes [L2Rh{C5H4CH2(9,10-anthrylene)CH2C5H4}RhL2], 4 (L ) è2-C2H4), 5 (L ) CO), and 6 (L2 ) è4-C7H8), in 13, 22, and 55% yields, respectively. All complexes have been characterized by elemental analysis, particle beam mass spectrometry, 1H NMR, and FT-IR. The UV-vis spectra (280-800 nm) of 4-6 are indicative of the existence of strong electronic interactions among the anthrylic chromophore and the two cyclopentadienylRhL2 moieties. When excited at ca. 370 nm, 1 becomes an efficient light-emitting molecule, while 4-6 are poorly luminescent compounds. The fluorescence spectra of all the complexes present the vibrational structure typical of the anthrylic fluorophore but have low intensity: 6, 3, and 15% of the one observed for 9-methylanthracene, taken as the reference compound, respectively for 4, 5, and 6. The study of the electrochemical behavior of 4-6 in strictly aprotic conditions allows a satisfactory interpretation of the observed electrode processes and furnishes information about the location of the redox sites along with the thermodynamic characterization of the corresponding redox processes. These data show that the occurrence of an intramolecular charge-transfer process between the photoexcited 9,10-anthrylenic moiety and the cyclopentadienylRhL2 unit is a possible route for the observed quenching of emission in the compounds 4-6. The one-electron oxidation of compounds 4-6 by thallium(III) trifluoroacetate in a 1:1 dichloromethane/1,1,1,3,3,3- hexafluoropropan-2-ol mixture leads to the formation of the corresponding radical cations [L2Rh{C5H4CH2(9,10-anthrylene)CH2C5H4}RhL2]+. Two of them, i.e., 4+ (L ) è2-C2H4) and 5+ (L ) CO), give rise to highly resolved EPR spectra which allow one to describe such species as average-valence [Rh+1/2, Rh+1/2] complexes. DFT calculations of spin density distribution confirm the EPR results and allow a further insight into the structure of 4+ and 5+ complexes.