Electrochemical, Computational, and Photophysical Characterization of New Luminescent Dirhenium–Pyridazine Complexes Containing Bridging OR or SR Anions

A series of [Re2(μ-ER)2(CO)6(μ-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)5O3SCF3] (for 2 and 4), [Re2(μ-OR)3(CO)6]− (for 3 and 5), or [Re4(μ3-OH)4(CO)12] (for 6). Single-crystal diffractometric
analysis showed that the two μ-phenolato derivatives (3 and 4) possess an idealized C2 symmetry, while the μ-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 π* orbitals of pyridazine,
whereas the highest occupied molecular orbitals (HOMOs) are mainly
constituted by the “t2g”
set of the Re atoms, with a strong Re–(μ-E) π*
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+|Fc). 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 ≈ 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).