Titanium(IV) alkoxy-N-heterocyclic carbenes: Structural preferences of alkoxide and bromide adducts

A series of titanium(IV) adducts of an amido-tethered N-heterocyclic carbene (NHC) of the form Ti(L)((OPr)-Pr-i)(n)(Br)(3-n) (n = 1-3, L = (BuNHCH2CH2)-Bu-t[C{(NBu)-Bu-t(CHCH)N}]) have been synthesized and characterized. Structural characterization of the n = 2 complex shows marked bending of the two ligands cis (and perpendicular) to the plane of the NHC group in the direction of the carbene, suggestive of a pseudo-back-bonding interaction between adjacent ligands and the carbene p orbital, consistent with a bonding model recently proposed for d(0) metal-NHC complexes. However, inspection of a space-filling model and the calculation of bond order using DFT methods suggest that the bending is due to repulsions between lone pairs on the adjacent pi-donor ligands in the complexes and not any specific interaction between the NHC and cis pi-donor ligands.