New mixed-valence (Mn2Mn2III)-Mn-II clusters exhibiting an unprecedented Mn-II/III oxidation state distribution in their magnetically coupled cores

摘要

The synthesis and magnetic properties of the high-spin tetranuclear cluster [Mn-III Mn-2(II) (2)(O2CC(CH3)(3))(2)(teaH(2))(2)(teaH)(2)](O2CC(CH3)(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 Mn-2(II) O-2(2)} '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-III Mn-2(II) (2)(O2CPh)(4)(teaH)(2)(acac)(2)] center dot MeCN (2), [Mn-III Mn-2(II) (2)( teaH)(2)( acac)(4)(MeOH)(2)](ClO4)(2) ( 3) and [Mn-III Mn-2(II) (2)(bheapH)(2)(acac)(4)(MeOH)(2)](ClO4)(2) (4) (where bheapH(3) = 1-[N, N-bis(2-hydroxyethyl) amino]-2-propanol). Unlike any previously reported tetranuclear clusters containing the Mn-II Mn-2(III) (2) core, 2, 3 and 4 exhibit a reversal in their Mn-II Mn-2(III) (2) oxidation state distribution. In these clusters, the 'wing-tip' Mn atoms exhibit Mn-III ( S = 2) oxidation states while the MnII 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 Mn-2(III) O-2(3)} core while clusters 3 and 4 exhibit two mu(2)-O linkers within the {Mn-II Mn-2(III) O-2(2)} core. All display trigonal prismatic coordination around the MnII 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.