Magnetothermal studies of a series of coordination clusters built from ferromagnetically coupled {MnII4MnIII6} supertetrahedral units

8 páginas, 6 figuras, 1 tabla.
Three high-nuclearity mixed valence manganeseII/III coordination clusters, have been synthesised, that is, [MnIII6MnII4(μ3-O)4(HL1)6(μ3-N3)3(μ3-Br)(Br)](N3)0.7/(Br)0.33 MeCN2 MeOH (1) (H3L1=3-methylpentan-1,3,5-triol), [MnIII11MnII6(μ4-O)8(μ3-Cl)4(μ,μ3-O2CMe)2(μ,μ-L2)10Cl2.34(O2CMe)0.66(py)3(MeCN)2]7 MeCN (2) (H2L2=2,2-dimethyl-1,3-propanediol and py is pyridine), and [MnIII12MnII7(μ4-O)8(μ3-η1N3)8(HL3)12(MeCN)6]Cl210 MeOH MeCN (3) (H3L3=2,6-bis(hydroxymethyl)-4-methylphenol) with high ground-spin states, S=22, 28±1, and 83/2, respectively; their magnetothermal properties have been studied. The three compounds are based on a common supertetrahedral building block as seen in the Mn10 cluster. This fundamental magnetic unit is made up of a tetrahedron of MnII ions with six MnIII ions placed midway along each edge giving an inscribed octahedron. Thus, the fundamental building unit as represented by compound 1 can be described as a Mn10 supertetrahedron. Compounds 2 and 3 correspond to two such units joined by a common edge or vertex, respectively, resulting in Mn17 and Mn19 coordination clusters. Magnetothermal studies reveal that all three compounds show interesting long-range magnetic ordering at low temperature, originating from negligible magnetic anisotropy of the compounds; compound 2 shows the largest magnetocaloric effect among the three compounds. This is as expected and can be attributed to the presence of a small magnetic anisotropy, and low-lying excited states in compound 2.
S.N. gratefully acknowledges the mobility grant from MAGMANet (grant No. 515767-2) to carry out the research in Karlsruhe, Leiden and Zaragoza. M.E. thanks the Spanish Ministry for Science and Innovation for grants MAT2009-13977-C03 and CSD2007-00010.