New mixed-valence Mn_2~(II)Mn_2~(III) 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_2~(III)Mn_2~(II)(O_2CC(CH_3)_3)_2(teaH_2)_2(teaH)_2](O_2CC(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_2~(III)Mn_2~(II)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 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_2~(III)Mn_2~(II)(O_2CPh)_4(teaH)_2(acac)_2] centre dot MeCN (2),[Mn_2~(III)Mn_2~(II)(teaH)_2(acac)_4(MeOH)_2](ClO_4)_2 (3) and [Mn_2~(III)Mn_2~(II)(bheapH)_2(acac)_4(MeOH)_2](ClO_4)_2 (4) (where bheapH_3=l-[N,N-bis(2-hydroxyethyl)amino]-2-propanol).Unlike any previously reported tetranuclear clusters containing the Mn_2~(II)Mn_2~(III) core,2,3 and 4 exhibit a reversal in their Mn_2~(II)Mn_2~(III) oxidation state distribution.In these clusters,the 'wing-tip' Mn atoms exhibit Mn~(III) (S=2) oxidation states while the Mn" 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_2~(II)Mn_2~(III)O_3} core while clusters 3 and 4 exhibit two mu_2-O linkers within the {Mn_2~(II)Mn_2~(III)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.