New mixed-valence MnII2MnIII2 clusters exhibiting an unprecedented MnII/IIIoxidation state distribution in their magnetically coupled cores

Abstract

The synthesis and magnetic properties of the high-spin tetranuclear cluster [Mn^III₂Mn^II₂(O₂CC(CH₃)₃)₂(teaH₂)₂(teaH)₂](O₂CC(CH₃)₃)₂ (1) (where teaH₃ = triethanolamine) is described. Complex 1 is the pivalate analogue of our previously reported family of tetranuclear mixed-valence carboxylate clusters. The teaH₂⁻ and teaH²⁻ anions in complex 1 act as oxygen donors in the {Mn^III₂Mn^II₂O₂} ‘butterfly’ core. Detailed dc and ac magnetic susceptibility measurements and magnetisation isotherms have been made and show that intra-cluster ferromagnetic coupling is occuring between the S = 2 Mn^III and S = 5/2 Mn^II ions to yield a S = 9 ground state and the g, J_bb and J_wb parameters have been deduced (b = body, w = wingtip). Incorporation of the acetylacetonate (acac⁻) ligand has led to three new clusters: [Mn^III₂Mn^II₂(O₂CPh)₄(teaH)₂(acac)₂]·MeCN (2), [Mn^III₂Mn^II₂(teaH)₂(acac)₄(MeOH)₂](ClO₄)₂ (3) and [Mn^III₂Mn^II₂(bheapH)₂(acac)₄(MeOH)₂](ClO₄)₂ (4) (where bheapH₃ = 1-[N,N-bis(2-hydroxyethyl)amino]-2-propanol). Unlike any previously reported tetranuclear clusters containing the Mn^II₂Mn^III₂ core, 2, 3 and 4 exhibit a reversal in their Mn^II₂Mn^III₂ oxidation state distribution. In these clusters, the ‘wing-tip’ Mn atoms exhibit Mn^III (S = 2) oxidation states while the Mn^II ions occupy the central ‘body’ positions. Furthermore, the cores in 2, 3 and 4 contain at least one µ₂-oxygen based bridging ion as opposed to the standard two µ₃-oxygen bridges previously reported. More precisely, cluster 2 exhibits one µ₃-O bridge and two µ₂-bridges in a {Mn^II₂Mn^III₂O₃} core while clusters 3 and 4 exhibit two µ₂-O linkers within the {Mn^II₂Mn^III₂O₂} core. All display trigonal prismatic coordination around the Mn^II centres. These structural and oxidation state differences lead to very different magnetic coupling interactions between the four Mn^II/III centres compared to 1. Direct current magnetic susceptibility measurements and magnetisation isotherms show that clusters 3 and 4 have ground states of S = 1. The g, J_bb and J_wb parameters have been deduced.