Homobimetallic anthracene-bridged eta-5-cyclopentadienyl derivatives of rhodium(I) and iridium(I): large molecules or supramolecular species?

The reaction of 9,10-bis[(cyclopentadienylmethyl)thallium(I)]anthracene (2), obtained from 9,10-bis(cyclopentadienylmethyl)anthracene (1), with the chloro derivatives of rhodium(I) of formula [RhClL2]2 (L = η2-C8H14 or L2 = η4-C8H12) leads to the corresponding bimetallic complexes [L2Rh{C5H4CH2(9,10-anthrylene)CH2C5H4}RhL2] 3 (L = η2-C8H14) and 4 (L2 = η4-C8H12), in 22.8% and 15.0% yields, respectively. Analogously, by reacting 2 with [IrClL2]2 (L = η2-C8H14 or L2 = η4-C8H12), the corresponding bimetallic iridium(I) complexes [L2Ir{C5H4CH2(9,10-anthrylene)CH2C5H4}IrL2] 5 (L = η2-C8H14) and 6 (L2 = η4-C8H12) were obtained, in 24.5% and 43.0% yields, respectively. All complexes have been characterised by elemental analysis, mass spectrometry, and 1H NMR. The structure of 4 was elucidated also by single crystal X-ray diffraction: it crystallises in the P21/c space group with a = 19:932(11), b = 6:4417(4), c = 12:377(2) Å; alpha = 90°, beta = 100:90(4)°, γ = 90°. V = 1560:5(9) Å3. Z = 2, Dcalc = 1:606 g•cm-1, R1 = 0:0449 [I > σ(I)], wR2 = 0:1121. The UV–Vis spectra (280–530 nm) of 3–6 are indicative of the existence of strong electronic interactions among the 9,10-anthrylene chromophore and the two cyclopentadienylML2 moieties. When excited at ca. 370 nm, 1 results to be an efficient light-emitting molecule, while the fluorescence emission of the 9,10-anthrylene chromophore is almost completely quenched in complexes 3–6. The study of the electrochemical behaviour of 3–6 in strictly aprotic conditions allows a satisfactory interpretation of the observed electrode processes and gives information about the location of the redox sites along with the thermodynamic characterisation of the corresponding redox processes. These data show that the occurrence of an intramolecular charge-transfer process between the photo-excited 9,10-anthrylene group and the cyclopentadienylML2 moiety is a possible route for the observed quenching of emission in the compounds 3–6. The one-electron oxidation of compounds 3–6 by thallium(III) trifluoroacetate leads to the formation of the corresponding cation radicals. Three of them, i.e., 3+, 5+ and 6+, give rise to good X-band EPR spectra that were fully interpreted by computer simulation as well as by semi-empirical calculations (PM3 level) of the spin density distribution.