Synthesis, Reactivity, and Insights into the Lewis Acidity of Mononuclear Titanocene Imido Complexes Bearing Sterically Demanding Terphenyl Moieties
Fischer M., Wolff MC., Del Horno E., Schmidtmann M., Beckhaus R.
The titanocene imido complex 2 with a m-terphenyl moiety is synthesized by reacting Cp2Ti(η2-Me3SiC2SiMe3) (1) with m-terphenyl azide. Subsequent reactivity studies with tetrahydrofuran, pyridine, and nitriles verify that these substrates act as two-electron donors, thus binding end-on to the titanium center to give the donor-stabilized titanocene imides 3a,b and 4a-c. Given the potential of the titanium center to act as an electron acceptor, 2 was reacted with phosphine oxides (OPEt3/OPPh3) according to the Gutmann-Beckett method to get information about the Lewis acidity of 2 and to yield 5a,b. Complex 5a is one of the rare examples of structurally characterized titanium complexes with an η1-bound cyclopentadienyl ligand. Further insights into the Lewis acidity of 2 were obtained from the nitrile adducts 4a-c through IR spectroscopy and NMR spectroscopy (δ(19F) and 1JC,F employing 4-fluorobenzontrile). In reactions of 2 with terminal alkynes, the respective titanium amido acetylides 6a,b were obtained, whereas reactions with internal alkynes demonstrate that 2 is not very prone to undergoing [2 + 2] cycloaddition reactions.