Lewis acidic titanium species: The synthesis, structure, bonding and molecular modelling considerations of the complexes Ti(NR₂)₃CI (R = Me, Et)

Reaction of simple amides with TiCl₄ affords mixed amido-chloride species Ti(NR₂)_4-nCl_n The trisamide-chloride species Ti(NR₂)₃Cl can be prepared directly employing three equivalents of amide or by reaction Ti(NR₂)₄ with TiCl₄. The compound Ti(NMe₂)₃Cl, 1, crystallizes in the trigonal space group Ric, with a = 11.525(5), c = 14.939(3) A, Z = 6, and V = 1718(1) Å^;3. The compound Ti(NEt₂)₃Cl, 2, crystallizes in the monoclinic space group P2₁/c, with a = 8.385(2) Å, b = 15.958(2) Å, c = 14.230(4) Å, β= 107.79(1)°, Z = 4, and V = 1813(1) Å^;3. The geometry of the Ti coordination sphere in these complexes is best described as pseudo-tetrahedral. The structural data are consistent with Ti-N multiple bonding. Preliminary results of EHMO calculations are consistent with dπ-pπ Ti-N bonding. Attempts to replace thehalides with phosphides (LiPR2, R = Me, Et, Ph) led not to the Ti(IV) phosphido species, but rather to redox chemistry yielding Ti(III) amides and P₂R₄. The barrier to rotation about the Ti-N bonds has been considered. Variable temperature 'H NMR studies reveal that the barrier is small. Extended Hiickel total energy minimization calculations have been performed. In addition, MMX calculations of the barrier to Ti-N rotation are reported. The results of these calculations imply that the rotational barrier is dominated by steric effects.