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 π–π*
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 × 103 M–1 cm–1) at lower energy (364 nm) than the meta position
(331 nm, 16.0 × 103 M–1 cm–1)