New mixed-valence Mn^II₂Mn^III₂clusters exhibiting an unprecedented Mn^II/IIIoxidation state distribution in their magnetically coupled cores

The synthesis and magnetic properties of the high-spin tetranuclear cluster [Mn(III)(2)Mn(II)(2)(O(2)CC(CH(3))(3))(2)(teaH(2))(2)(teaH)(2)](O(2)CC(CH(3))(3))(2) (1) (where teaH(3) = triethanolamine) is described. Complex 1 is the pivalate analogue of our previously reported family of tetranuclear mixed-valence carboxylate clusters. The teaH(2)(-) and teaH(2-) anions in complex 1 act as oxygen donors in the {Mn(III)(2)Mn(II)(2)O(2)} "butterfly" core. Detailed dc and ac magnetic susceptibility measurements and magnetisation isotherms have been made and show that intra-cluster ferromagnetic coupling is occurring 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(III)(2)Mn(II)(2)(O(2)CPh)(4)(teaH)(2)(acac)(2)].MeCN (2), [Mn(III)(2)Mn(II)(2)(teaH)(2)(acac)(4)(MeOH)(2)](ClO(4))(2) (3) and [Mn(III)(2)Mn(II)(2)(bheapH)(2)(acac)(4)(MeOH)(2)](ClO(4))(2) (4) (where bheapH(3) = 1-[N,N-bis(2-hydroxyethyl)amino]-2-propanol). Unlike any previously reported tetranuclear clusters containing the Mn(II)(2)Mn(III)(2) core, 2, 3, and 4 exhibit a reversal in their Mn(II)(2)Mn(III)(2) 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 mu(2)-oxygen based bridging ion as opposed to the standard two mu(3)-oxygen bridges previously reported. More precisely, cluster 2 exhibits one mu(3)-O bridge and two mu(2)-bridges in a {Mn(II)(2)Mn(III)(2)O(3)} core while clusters 3 and 4 exhibit two mu(2)-O linkers within the {Mn(II)(2)Mn(III)(2)O(2)} 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.

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