Liquid coordination complexes of Lewis acidic metal chlorides: Lewis acidity and insights into speciation

Abstract

Coordination complexes of Lewis acidic metal chlorides AlCl₃, GaCl₃, InCl₃, SbCl₃, SnCl₄, SnCl₂, ZnCl₂ and TiCl₄ with trioctylphosphine (P₈₈₈) and trioctylphosphine oxide (P₈₈₈O) were synthesised. All compounds formed liquid coordination complexes (LCCs) at ambient temperature, although decomposition via a redox mechanism was detected in some cases. The Lewis acidity of the metal chlorides (measured in 1,2-dichloroethane solutions) and the LCCs (measured neat) was quantified by using the Gutmann acceptor number (AN) approach. In general, LCCs were equally or more Lewis acidic than the corresponding metal chlorides. The AN values were compared with the catalytic activity of selected LCCs in a model Diels–Alder reaction. Insight into speciation of LCCs was gained using multinuclear NMR spectroscopy, revealing that most LCCs comprised charge-neutral complexes rather than ionic ones. The relationship between the speciation, Lewis acidity (AN scale) and catalytic activity is discussed in detail. This approach reveals several new, promising catalytic systems, such as P₈₈₈O–InCl₃, with Lewis acidity enhanced compared to chloroindate ionic liquids, and P₈₈₈O–TiCl₄, with hydrolytic stability enhanced with respect to neat TiCl₄.