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

A new family of cationic Ti complexes <b>4</b>′ with a pendant phosphine of general formula [CpCp<sup>P</sup>TiOAr]­[BPh<sub>4</sub>] (Cp = η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>; Cp<sup>P</sup> = η<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>(CMe<sub>2</sub>)­PR<sub>2</sub>) 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<sup>P</sup>TiClOAr] complexes <b>3</b> 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 (<b>4b</b>′: OAr = 2,6-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>; PR<sub>2</sub> = PCy<sub>2</sub>) reacted with benzaldehyde to form a typical FLP activation product. Complex <b>4b</b>′ also reacted with 2 equiv of <i>trans</i>-chalcone to form a 10-membered Ti phosphonium macrocycle (<b>6b</b>′) by extrusion of 6,6-dimethylfulvene.