Inorganic Chemistry, Vol.51, No.23, 12738-12747, 2012
Diimine Triscarbonyl Re(I) of Isomeric Pyridyl-fulvene Ligands: an Electrochemical, Spectroscopic, and Computational Investigation
The synthesis and characterization of a novel family of positively charged fac-[Re(bpy)(CO)(3)(L)]PF6 (bpy = 2,2'-bipyridine) complexes are reported, where L is a pyridine functionalized in para or meta position with a fulvene moiety, namely, 4-fluoren-9-ylidenemethyl-pyridine (pFpy) and 3-fluoren-9-ylidenemethyl-pyridine (mFpy). The complexes were prepared in high yield (86%) by direct addition at room temperature of the corresponding pyridine to the tetrahydrofuran (THF) adduct fac-[Re(bpy)(CO)(3)(THF)][PF6] precursor. Both ligand and complex structures were fully characterized by a variety of techniques including X-ray crystallography. The complexes did not exhibit the expected triplet mixed metal ligand-to-ligand charge transfer (MLLCT) emission, because of its deactivation by the non-emissive triplet excited state of fulvene. The absorption profile shows that the MLLCT is overshadowed by the fulvene centered pi-pi* transition of higher molar absorptivity as shown by time dependent density functional theory (TD-DFT) calculations. The position of the fulvene on the pyridyl ring has a large effect on this transition, the para position displaying a much higher absorption coefficient (21.3 x 10(3) M-1 cm(-1)) at lower energy (364 nm) than the meta position (331 nm, 16.0 x 10(3) M-1 cm(-1))