Nickel(II), cobalt(II), and iron(II) lacunar complexes having very small cavities: syntheses, properties, and reactions with dioxygen

Abstract

The syntheses and properties of novel lacunar macrobicyclic complexes of a ligand containing a very small cavity around one of the metal-ion axial co-ordination sites are described. The cobalt(II) and iron(II) complexes do not reversibly bind dioxygen under any explored conditions, unlike all other congenors previously reported. This behaviour is presumed to be a consequence of the restrictive volume accessible for dioxygen co-ordination within the cavity of the structure. Despite the lack of dioxygen binding, the iron(II) complex autoxidizes extremely fast, with a second-order rate constant of (7.5 ± 0.5)× 10^–5 Torr^–1 s^–1 at –31 °C; this behaviour constitutes evidence for an outer-sphere electron-transfer oxidation mechanism.