Acid-Base Adducts of Catalytically Active Titanium(IV) Lewis Acids

A series of monomeric Lewis acid-base adducts of the Diels-Alder catalyst Ti(O-2,6-Me(2)C(6)H(3))(2)Cl(2) have been synthesized from bidentate diphosphines and diamines, Ti(O-2,6-Me(2)C(6)H(3))(2)Cl(2)L(2) (L(2) = dmpe, depe, dpeda, and dmeda). X-ray crystal structures of Ti(O-2,6-Me(2)C(6)H(3))(2)Cl(2)(dmpe) and Ti(O-2,6-Me(2)C(6)H(3))(2)Cl(2)(dpeda) establish a distorted octahedral coordination environment with trans-chloride ligands. Bidentate ligands that were also studied but did not form isolable complexes with the Ti(IV) Lewis acid include dppe, tmeda, and binam. Through pairwise exchange reactions a qualitative ranking of relative bidentate ligand binding strengths to the Lewis acid were obtained (dmeda >/= dpeda > dmpe >/= depe > tmeda > binam > dppe). The ranking is readily rationalized using hard-soft electronic arguments except for tmeda, which requires that unfavorable steric interactions be invoked.