A nuclear magnetic resonance and X-ray diffraction study on the metal salt complexes of dicarbonyl-stabilized ammonium and sulphonium ylides

Abstract

The metal salt complexes of some carbonyl-stabilized ammonium and sulphonium ylides were studied by n.m.r. spectroscopy and X-ray diffraction. The crystal structure of the trimethylammonium diformylmethylide (I)–NaClO₄ complex forms a continuous network between the octahedrally co-ordinated sodium cations, perchlorate anions, and the ylide molecules. The behaviour of ³J(CHO, CHO) of (I) in solution reveals that the trans,trans structure, in which a metal cation is bound simultaneously to both formyl oxygens, is formed only to a very small extent, if at all. The ¹J(CHO) and chemical shift data for (I) indicate that the cations prefer to bond to oxygen, not to the ylide carbon, which is the preferred site in sulphonium ylides. The complex formation free energies (at 25°C in acetone) range between –5.0 and 10.0 kJ mol^–1. The formation entropy for the AgBF₄ complex of 1-dimipthylsulphonio-4,4-dimethyl-2,6-dioxocyclohexanide is ca. –85 J K^–1 mol^–1.