Reduction of Titanium Supported by a <i>σ</i>-/<i>π</i>-Bonded Tripyrrole Ligand: Ligand C−N Bond Cleavage and Coordination of Olefin and Arene with an Inverse Sandwich Structure

The formally Ti(I)/Ti(II) mixed-valence toluene complex {2,5-[(C4H3N)CPh2]2[C4H2N(Me)]}Ti (μ,η6-C7H8)Ti[{2,5-[(C4H3N)CPh2]2[C4H2N(Me)]}][K(DME)2]·toluene (1) with an inverse sandwich type of structure has been obtained from the reduction of {2,5-[(C4H3N)CPh2]2[C4H2N(Me)]}TiCl with potassium in toluene. The bridging molecule of toluene in the paramagnetic 1 shows a visible distortion due to a substantial amount of metal-to-ring back-bonding. Complex 1 was always the only detectable product, even in cases when lower than stoichiometric amounts of reductant were employed. DFT calculations have been carried out to elucidate the electronic structure of the mixed-valence 1 in order to clarify the reason for its apparent thermodynamic stability. The most energetically favorable model comprises two divalent Ti centers connected to an organic radical anion. The olefin adducts {2,5-[(C4H3N)CPh2]2[C4H2N]}Ti(η2-trans-PhHCCHPh)[K(DME)] (2) and {2,5-[(C4H3N)CPh2]2[C4H2N (Me)]}Ti(η2-trans-PhHCCHPh) (3) were obtained from identical reactions carried out in the presence of trans-stilbene. Complexes 2 and 3 may be regarded as the result of the oxidative addition of a transient divalent species to the trans-stilbene molecule. In the case of 2 however, the reaction is accompanied by the cleavage of the C−N bond of the π-bound alkylated pyrrole ring.