New pentanuclear mixed valence Co(ii)–Co(iii) complexes of “short” salen homologues

Abstract

We describe herein a series of novel pentanuclear Co(II)–Co(III) mixed-valence cationic complexes of formula [Co₅L₄(μ₃-OH)₂]⁺ {L²⁻ = 2,2′-[1,1-methanediylbis(nitrilomethylidene)diphenate] or its arylmethane analogues}, obtained by controlled oxidation of [Co₂L₂] precursors. The [Co₅(μ₃-OH)₂(salNO₂ben)₄]I·2(CH₃)₂CO and [Co₅(μ₃-OH)₂(salClben)₄]I·2DMF·CH₂Cl₂ compounds have been studied by X-ray methods and turn out to be isostructural. The structural data show that the cobalt atoms in the pentanuclear units are bridged by μ-O(phenolato) atoms and μ₃-OH groups and approximate three types of co-ordination geometries: tetrahedral, octahedral and trigonal-bipyramidal. The X-ray structure of one proligand, namely 2,2′-[1,1-(4-nitrophenylmethanediyl)bis(nitrilomethylidene)diphenol] is also reported. The χT(T) behaviour of the complex with this ligand has been studied in the 4–300 K range. The data clearly show that ferromagnetic as well as (much weaker) antiferromagnetic exchange interactions occur within the cluster. A simple exchange model has been developed which nicely reproduces the experimental data.