Tricarbonyl Rhenium(I) Complexes Containing a Bridging 2,5-Diphenyl-1,3,4-oxadiazole Ligand : Structural, Spectroscopic, Electrochemical, and Computational Characterization

The three complexes [Re2(ì-X1)(ì-X2)(CO)6(ì-ppd-êN3:êN4)] (X1, X2 ) H, 1; X1 ) H, X2 ) Cl, 2; X1, X2 ) Cl, 3; ppd ) 2,5-diphenyl-1,3,4-oxadiazole) have been synthesized by different routes, involving the reaction of [Re4(ì3-H)4(CO)12] with ppd for 1, the reaction of 1 with HCl for 2, and the reaction of [ReCl(CO)5] with ppd for 3. The three complexes possess a different number of valence electrons, so the formal Re-Re bond order varies from 2 to 1 to 0 in complexes 1, 2, and 3, respectively. This is reflected in the Re-Re bond distance (277.9, 297.9, and 358.5 pm in the same series) and in the stability of the complexes in the coordinating solvent acetonitrile (t1/2 for ppd displacement 13.6, 4.5, and 3.7 h, for 1, 2, and 3, respectively). Both experimental and calculated structures indicates that coordination induces a distortion from planarity of the diphenyloxadiazole moiety due to the interaction of the equatorial carbonyls with the bridging ppd, which increases on going from 1 to 2 to 3 (dihedral angle between the oxadiazole and the phenyl rings 18.4°, 23.3°, and 45.0°, respectively). The UV spectra show ð-ð* transitions of the oxadiazole ligand (which shift to higher energy on increasing the distortion from the planarity, from 252 to 267 nm) and metal-to-ligand charge transfer absorptions (from 300 to 362 nm). Upon irradiation between 340 and 380 nm, complex 2 only features a weak broad emission at 527 nm (Ö)0.02%), whereas upon excitation at 300 nm, the emission typical of free ppd is observed, suggesting photodissociation. Cyclic voltammetry investigations in acetonitrile showed that the three complexes exhibit ligand-centered irreversible reduction peaks (from -1.83 to -1.93 V vs Fc+|Fc), shifted to more positive values with respect to free ppd (-2.50 V). The shift however is smaller than in the analogous derivatives containing 1,2-diazines, suggesting a smaller electron depletion of the heterocycle ligand upon coordination. The complexes also show a metal-centered, bielectronic, irreversible oxidation peak (from 1.05 to 1.37 V vs Fc+|Fc). A combined density functional and time-dependent density functional (TD DFT) study allowed us to understand the factors affecting the stability of the three complexes and to rationalize their electrochemical and photophysical properties in terms of their electronic structure.