Synthesis, Structure, and Reactivity of Titanacyclopentadiene Complexes Bearing Ancillary Pyridine Diamide Ligands

Titanium complexes bearing the pyridine diamide ligands [2,6-(RNCH2)2NC5H3]2- (R = 2,6-diisopropylphenyl (BDPP); R = 2,6-dimethylphenyl (BDMP)) have been synthesized. Reduction of the dichloride precursors (BDPP)TiCl2 (1a) and (BDMP)TiCl2 (1b) with excess 1% Na/Hg amalgam in the presence of >2 equiv of internal (PhC⋮CPh, EtC⋮CEt, PrC⋮CPr) or terminal (HC⋮CSiMe3, PhC⋮CH) alkynes yields metallacyclopentadiene derivatives in good yield. The α,β‘-substituted titanacycle (BDPP)Ti[C4H2(SiMe3)2] (5a) was characterized by X-ray crystallography and is best described as a distorted square pyramid with the metallacycle carbon C(4) occupying the apical position. The α,α‘-substituted titanacycle (BDMP)Ti[C4H2(SiMe3)2] (5b) reacts with excess 3-hexyne and 4-octyne to give the asymmetric metallacycles (BDMP)Ti[C4Et2H(SiMe3)] (7b) and (BDMP)Ti[C4Pr2H(SiMe3)] (8b), respectively. No cyclotrimerization of alkyne is observed. Ligand activation is observed in certain cases for complexes bearing the BDMP ligand.