Six-Coordinate Titanium Complexes of a Tripodal Aminetris(phenoxide) Ligand:  Synthesis, Structure, and Dynamics

The five-coordinate titanium(IV) alkoxide LTi(O^tBu) (LH₃ = tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine) is protonolyzed readily by the conjugate acids of monoanionic bidentate ligands, both symmetrical (tropolone, acetylacetone, di-p-toluoylmethane) and unsymmetrical (8-hydroxyquinoline, salicylaldehyde, 2,6-diformyl-p-cresol, anthrarufin). The geometry of these complexes, which is pseudo-octahedral with the tripodal ligand adopting a chiral, propeller-like conformation, has been confirmed in four cases by X-ray crystallography. Variable-temperature NMR spectroscopy indicates that the six-coordinate complexes undergo two dynamic processes. First, the ligands undergo a twisting motion that results in racemization, a process which is over 10⁴ times faster than in five-coordinate complexes. The rate acceleration upon binding of an equatorial ligand is ascribed to steric repulsions with one of the cis phenoxides; the dynamics of a binuclear dibenzyl phosphate-bridged compound, which has a unique conformation of the tripodal ligand, indicates that flexing the cis phenoxide is the rate-limiting step in racemization. Second, the complexes undergo a process that interchanges the inequivalent arms of the tripodal ligand. This process involves a trigonal twist that shifts the bidentate ligand between clefts in the tripod. The intermediate geometry in the reaction appears to be a transition state and not a long-lived intermediate, as judged from the relative rates of interconversion of tripod arms and chelate ends in the ditoluoylmethane complex. Tripod arm interchange takes place without partial dissociation of the bidentate chelate, a reaction that has been observed on a slower time scale in one case.