Two
independent synthetic routes to η2-imine titanocene
complexes were developed. On one hand side, ligand exchange reactions of bis(trimethylsilyl)acetylene
by (p-Tolyl)HCNPh (3) employing
the Rosenthal reagent Cp2Ti{η2-C2(SiMe3)2} (1) lead to Cp2Ti{η2-(p-Tolyl)CHNPh} (5), exhibiting a titanaaziridine
structure. On the other hand, the direct reductive complexation of 3 by using Cp2TiCl2 (2)
and Mg as reducing agent leads also to 5, one of the
rare known titanoceneaziridines without additional ligands. By using
the ketimine (p-Tolyl)2CNPh (4) instead of the aldimine 3, an unexpected coordination
mode was found by X-ray diffraction, exhibiting an azatitanacyclopent-4-ene
structure involving one tolyl fragment. In such a way, via the reductive
complexation of 4, employing 2 or Cp*TiCl3 (12), the 1-aza-2-titanacyclopent-4-ene complexes 6 and 13 are formed. Density functional calculations
at the M06-2X level identify these new complexes 6 and 13 as 1-aza-2-titanacyclopent-4-enes, in agreement with an
analysis based on the experimental structural parameters. A theoretical
study of the bonding between the titanocene fragment and the imine
ligand reveals that steric factors are more pronounced for titanaaziridines
and disfavor their formation compared to azatitanacyclopentenes. This
provides a rationalization for the preferred formation of titanoceneaziridines
in the case of aldimine ligands and azatitanacyclopentenes when ketimines
are applied. Whereas titanoceneaziridine 5 undergoes
insertion reactions into the Ti–C carbon σ-bond with
aldehydes, ketones, or carbodiimides to the five-membered titanacycles 20 and 21, complex 6 appears to
be inert in comparable reactions.