New Insights Into the Electronic-Structures of Solid-State Dioxygen Adducts of N,N′-Ethylenebis(Salicylaldiminato)Cobalt(II), Co(Salen), and of Active Precursors

Oxygen-active forms of [Co( salen )], derived from solvated forms such as [Co( salen )].C6H6, [Co( salen )]CHCl3 and Co( salen )(C5H5N), have long been known to bind dioxygen reversibly in the solid state. It has generally been accepted that the product of oxygenation is the diamagnetic peroxo -bridged species [Co( salen )]2O2. However, the present detailed study of temperature-dependent magnetic susceptibilities and of electron spin resonance spectra clearly shows that this is an oversimplification. The data show that the oxygenated species also contain the paramagnetic superoxo -like species [O2Co( salen )] as well as dimeric 'inactive' [Co( salen )]2 centres . Prolonged oxygenation produces also a second monomeric Co(salen) component although the e.s.r . spectra of both CoIIspecies are diminished in intensity relative to the [O2Co( salen )] signal. The magnetic properties of the oxygen-active Co( salen ) samples are compatible with ferromagnetically coupled pairs of Co( salen ) molecules, with further weak antiferromagnetic coupling between such sets of pairs. These magnetostructural features appear to be related to the solid-state oxygen reactivity. The solvated precursors also show weak magnetic interactions but different in detail to those in the desolvated materials.