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 [CpCp^PTiOAr]­[BPh₄] (Cp = η⁵-C₅H₅; Cp^P = η⁵-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 [CpCp^PTiClOAr] 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 Å. 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.