The synthesis of phosphonium indenylides and an investigation of their coordination to group 4 metals
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A class of phosphonium cyclopentadienylide ligands was first reported in 1956 by Ramirez and Levy, who synthesized triphenylphosphonium cyclopentadienylide. They discovered that this ylide was unusually inert, for instance being unreactive with ketones; they attributed this unusual stability to its ability to undergo charge delocalization. It was subsequently found that the ylide coordinates to metals in η5 fashion, and thus one might anticipate a rich chemistry involved with these ligands, although surprisingly little research on this fascinating ligand has been reported over the past 50 years. More recently, the coordination chemistry of its analogous compound, methyldiphenylphosphonium cyclopentadienylide, has been investigated, specifically using group six transition metals. As an extension of the latter work, we have now successfully developed a general synthetic route to similar phosphonium 1-indenylide compounds, IndPL3 (L3 = Ph3, PMePh2, PMe2Ph). The coordination of these compounds is of particular interest given that the compounds that form exhibit planar chirality and, as a result, enantioselective catalytic properties are anticipated. Furthermore, reactions involving two molar equivalents of phosphonium 1-indenylide, where two ligands coordinate, would produce diastereomers.It was our attempt to extend this work to group 4 metal halides, specifically to form coordination compounds of the type [TiCl3(IndPL3)]Cl and [TiCl2(IndPL3)2]Cl2. These compounds could be of great importance as enantioselective polymerization catalysts, or alternatively anti-cancer agents.