Magnetothermal Studies of a Series of Coordination Clusters Built from Ferromagnetically Coupled {(Mn4Mn6III)-Mn-II} Supertetrahedral Units

Three high-nuclearity mixed valence manganese^II/III coordination clusters, have been synthesised, that is, [Mn^III₆Mn^II₄(μ₃-O)₄(HL¹)₆(μ₃-N₃)₃(μ₃-Br)(Br)](N₃)_0.7/(Br)_0.3⋅3 MeCN⋅2 MeOH (1) (H₃L¹=3-methylpentan-1,3,5-triol), [Mn^III₁₁Mn^II₆(μ₄-O)₈(μ₃-Cl)₄(μ,μ₃-O₂CMe)₂(μ,μ-L²)₁₀Cl_2.34(O₂CMe)_0.66(py)₃(MeCN)₂]⋅7 MeCN (2) (H₂L²=2,2-dimethyl-1,3-propanediol and py is pyridine), and [Mn^III₁₂Mn^II₇(μ₄-O)₈(μ₃-η¹N₃)₈(HL³)₁₂(MeCN)₆]Cl₂⋅10 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 Mn^II ions with six Mn^III 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.