Electrochemical, computational, and photophysical characterization of new luminescent dirhenium-pyridazine complexes containing bridging OR or SR anions

A series of [Re-2(mu-ER)(2)(CO)(6)(mu-pydz)] complexes have been synthesized (E = S, R = C6H5, 2; E = O, R = C6F5, 3; C6H5, 4; CH3, and 5; H, 6), starting either from [Re(CO)(5)O3SCF3] (for 2 and 4), [Re-2(mu-OR)(3)(CO)(6)](-) (for 3 and 5), or [Re-4(mu(3)-OH)(4)(CO)(12)] (for 6). Single-crystal diffractometric analysis showed that the two mu-phenolato derivatives (3 and 4) possess an idealized C-2 symmetry, while the mu-benzenethiolato derivative (2) is asymmetrical, because of the different conformation adopted by the phenyl groups. A combined density functional and time-dependent density functional study of the geometry and electronic structure of the complexes showed that the lowest unoccupied molecular orbital (LUMO) and LUMO +1 are the two lowest-lying pi* orbitals of pyridazine, whereas the highest occupied molecular orbitals (HOMOs) are mainly constituted by the "t(2g)" set of the Re atoms, with a strong Re-(mu-E) pi* character. The absorption spectra have been satisfactorily simulated, by computing the lowest singlet excitation energies. All the complexes exhibit one reversible monoelectronic reduction centered on the pyridazine ligand (ranging from -1.35 V to -1.53 V vs Fc(+)lFc). The benzenethiolato derivative 2 exhibits one reversible two-electron oxidation (at 0.47 V), whereas the OR derivatives show two close monoelectronic oxidation peaks (ranging from 0.85 V to 1.35 V for the first peak). The thioderivative 2 exhibits a very small electrochemical energy gap (1.9 eV, vs 2.38-2.70 eV for the OR derivatives), and it does not show any photoluminescence. The complexes containing OR ligands show from moderate to poor photoluminescence, in the range of 608-708 nm, with quantum yields decreasing (ranging from 5.5% to 0.07%) and lifetimes decreasing (ranging from 550 ns to 9 ns) (3 > 4 > 6 approximate to 5) with increasing emission wavelength. The best emitting properties, which are closely comparable to those of the dichloro complex (1), are exhibited by the pentafluorophenolato derivative (3).