Low-temperature synthesis and magnetism of novel frameworks

Abstract

Solvothermal and ionothermal syntheses have been used in this work for the generation of novel frameworks containing oxy-anions in the presence of organic amines and ionic liquids. The new phases have been characterised using X-ray diffraction techniques, elemental analysis, thermogravimetric analysis, spectroscopic methods and SQUID magnetometry. The role of the 1-ethyl-3-methylimidazolium bromide ([EMIM] Br) ionic liquid has been investigated, resulting in the formation of the (NH4)2M2(SO4)3 (M=Mn, Fe) langbeinite-type phases through an unusual redox process in which sulphur is oxidised to sulphate under vacuum conditions. Both materials exhibit paramagnetic behaviour. The use of the [EMIM] Br ionic liquid has also afforded the determination of the crystal structure of the mixed-anion barium carbonate chloride Ba3Cl4CO3, previously only obtainable in polycrystalline form. It consists of a complex three-dimensional network, in which the barium and carbonate ions define chains which are cross-linked via chloride ions. Solvothermal synthesis has been employed for the investigation of the system MSO4:amine:H2SO4:H2O (amine= ethylenediamine (en) and triethylenetetramine (trien)) yielding the formation of novel transition metal sulphates. A common building block M4O20 has been identified and effectively incorporated in the structure of [Mn4(SO4)8(OH)2(H2O)2](enH2)5, [Fe3(SO4)3(OH)2(H2O)2](NH4)2 and the hybrid [Mn3(SO4)3(OH)2(trienH2)], in which trienH2 2+ cations serve to link [Mn3(SO4)3(OH)2]2- layers into a three-dimensional structure. Both the manganese hybrid and the iron sulphate constitute examples of frustrated systems. The iron phase exhibits a complex magnetic behaviour and is magnetically ordered at room temperature. En and trien have been successfully linked to the metal centre in [M(SO4)(trien)] and [Mn(SO4)(en)], generating two- and three-dimensional hybrid structures, respectively. The layered sulphates [Co3(SO4)3(OH)2](enH2) and [Mn(SO4)2](enH2), in which layers are charge balanced by enH2 2+ cations, have also been prepared. The former constitutes a rare example of Kagome layer structure with a unique connectivity, whose magnetic properties showed the presence of geometrical magnetic frustration. The successful addition of rare-earths into the reactions has resulted in the formation of new rare-earth sulphates with different dimensionalities. Using ii ethylenediamine the formation of layered structures was favoured and [Ln2(SO4)6(H2O)2](enH2)3, [Dy(SO4)2](enH2)0.5 and the hybrid [Ln2(SO4)6(enH)2](enH2)2, in which the metal centre is directly connected to the organic amine through Ln-N bond, were obtained together with the one-dimensional [La(SO4)3](enH2)1.5. The ribbon-like [Dy2(SO4)6(H2O)](trienH4)1.5 and [Sm4(SO4)10(H2O)4](pipH2)4, built up from threedimensional motifs in which channels of eight- and twelve-membered rings are generated, have also been prepared employing triethylenetetramine and piperazine, respectively. The rare-earth ions exhibit paramagnetic behaviour with the exception of the samarium sulphate, which is magnetically ordered at room temperature.