2-Aminopyrrolines:  New Chiral Amidinate Ligands with a Rigid Well-Defined Molecular Structure and Their Coordination to Ti^IV

The use of an amino-oxazolinate (NN_ox = κ²-2,6-dimethylphenylamido-4(S)-isopropyloxazoline) as a chiral analogue to amidinate ligands in the chemistry of titanium was found to lead to undesired side reactions. The reaction of 2,6-dimethylphenylamido-4(S)-isopropyloxazoline with [Ti(NMe₂)₄] afforded the bis(amidinato) complex [Ti(NN_ox)₂(NMe₂)₂] (2) which was thermally converted to the ring-opened decomposition products [Ti(NN_ox){κ³-N(2,6-C₆H₃Me₂)C(NMe₂)NC(ⁱPr)CH₂O}(NMe₂)] (3) and [Ti{κ³-N(2,6-C₆H₃Me₂)C(NMe₂)-NC(ⁱPr)CH₂O}₂] (4). The NMR spectra of 4 recorded at low temperature displayed two sets of resonances corresponding to two symmetric isomers in a 2:5 ratio, the probable geometries of which were established by ONIOM (QM/MM) simulations. To suppress ring opening of the oxazolines, their oxygen atom was formally replaced by a CH₂ group in the synthesis of a series of amino-pyrroline protioligands 2-RN(H)(5-C₄H₅NR‘) (HN^RN^R‘). Their reaction with [Ti(NMe₂)₄] gave the thermally stable complexes [Ti(N^RN^R‘)₂(NMe₂)₂], of which three derivatives were characterized by X-ray diffraction. They are stereochemically dynamic and undergo reversible ligand rearrangements in solution, for which the activation parameters were determined by variable-temperature ¹H NMR spectroscopy.