Synthetic Endeavors toward Titanium Based Frustrated Lewis Pairs with Controlled Electronic and Steric Properties

A new family of cationic Ti complexes 4' with a pendant phosphine of general formula [(CpCpTiOAr)-Ti-P][BPh4] (Cp = eta(5)-C5H5; Cp-P = eta(5)-C5H4(CMe2)PR2) has been prepared in four steps from 6,6-dimethylfulvene. These complexes were designed to behave as Ti based frustrated Lewis pairs (FLPs). The key synthetic step is a reduction oxidation sequence from [(CpCpTiClOAr)-Ti-P] complexes 3 using lithium phosphide salts as the reductants and ferricinium tetraphenylborate as the oxidant. Four complexes have been structurally characterized by X-ray diffraction and show elongated Ti-P bonds, above 2.60 angstrom. One complex (4b': OAr = 2,6-Me2C6H3; PR2 = PCy2) reacted with benzaldehyde to form a typical FLP activation product. Complex 4b' also reacted with 2 equiv of trans-chalcone to form a 10-membered Ti phosphonium macrocycle (6b') by extrusion of 6,6-dimethylfulvene.