Methylene spacer-regulated structural variation in cobalt (II/III) complexes with bridging acetate and salen- or salpn-type Schiff-base ligands

Abstract

Two linear, trinuclear mixed-valence complexes, [Co^II{(μ-L¹)(μ-OAc)Co^III(OAc)}₂] (1) and [Co^II{(μ-L²)(μ-OAc)Co^III(OAc)}₂] (2) and two mononuclear Co^III complexes [Co^III{L³}(OAc)] (3) and [Co^III{L⁴}(OAc)] (4) were prepared and the molecular structures of 1, 2 and 4 elucidated on the basis of X-ray crystallography [OAc = Acetate ion, H₂L¹ = H₂ Salen = 1,6-bis(2-hydroxyphenyl)-2,5-diazahexa-1,5-diene, H₂L² = H₂Me₂Salen = 2,7-bis(2-hydroxyphenyl)-2,6-diazaocta-2,6-diene, H₂L³ = H₂Salpn = 1,7-bis(2-hydroxyphenyl)-2,6-diazahepta-1,6-diene, H₂L⁴ = H₂Me₂Salpn = 2,8-bis(2-hydroxyphenyl)-3,7-diazanona-2,7-diene]. In complexes 1 and 2, the acetate groups show both monodentate and bridging bidentate coordination modes, whereas chelating bidentate acetate is present in 4. The terminal Co^IIIN₂O₄ centres in 1 and 2 exhibit uniform facial arrangements of both non-bridged N₂O and bridging O₃ donor sets and the Co^II centre is coordinated to six (four phenoxo and two acetato) oxygen atoms of the bridging ligands. The effective magnetic moment at room temperature corresponds to the presence of high-spin Co^II in both 1 and 2. The complexes 1 and 2 are thus Co^III(S= 0)–Co^II(S = 3/2)–Co^III(S = 0) trimers. Complexes 3 and 4 are monomeric and diamagnetic containing low-spin Co^III(S = 0) with chelating tetradentate Schiff base and bidentate acetate. Calculations based on DFT rationalise the formation of trinuclear or mononuclear complexes.