Electronic Properties of Bis(η⁵-cyclopentadienyl)titanium 2,2'-Bipyridyl. A Singlet Molecule with a Low-Lying Triplet Excited State¹

Magnetic susceptibility and EPR studies show that (η⁵-C₅H₅)₂Ti(bpy) has a triplet excited state which is thermally accessible from the ground-state singlet. The temperature dependence of the magnetic susceptibility and the EPR spectra was measured and confirms that the singlet and triplet states are separated by approximately 600 cm^-1 in toluene solution and 750 cm^-1 in the solid state. In both solution and solid state (η⁵-C₅H₅)₂Ti(bpy) is monomeric so that the presence of extended Ti-Ti interactions is unlikely. The low-lying triplet most likely corresponds to a state in which one unpaired electron formally occupies a molecular orbital which is localized on (η⁵-C₅H₅)₂Ti while a second unpaired electron resides in the lowest energy π* orbital of the bipyridyl group. This conclusion is supported by Fenske-Hall molecular orbital calculations. The calculations and single crystal X-ray data also suggest that the preferred molecular geometries of the singlet and triplet states are not the same. The cationic complex formed by the one-electron oxidation of (η⁵-C₅H₅)₂Ti(bpy) has been isolated as a hexafluorophosphate salt. The magnetic susceptibility and EPR spectrum of this material are characteristic of a d¹ complex.