Research On Molecular Based Magnetic Materials With Pentagonal Bipyramid Coordination Geometry

Molecular magnetic materials have been of intense interest all over the world in the last few years.Among them,those with high Tc,large coercive force or magnetic bistability(spin transition,single molecule magnets(SMMs),single ion magnets(SIMs),single chain magnets(SCMs)etc.),multifunctionality(coexistence or synergism of magnetism and conductivity,porosity,chirality,etc.)have attracted considerable attention.An important factor to consider is magnetic anisotropy when designing molecular magnetic materials.High magnetic anisotropy is essential for hard magnets and has a direct effect on the magnetic susceptibility,coercivity of magnetic materials.More interestingly,magnetic anisotropy has a decisive effect on SMMs,SIMs and SCMs,which directly determine the intrinsic properties of bistable magnetic materials,such as magnetic relaxation energy barriers,blocking temperatures,and so on.Theoretical calculations predict that the pentagonal bipyramid geometries have a strong axial symmetry,which is conducive to the construction of materials with large anisotropy.Based on the above consideration,pentagonal bipyramid[MoⅢ(CN)7]4-building block was selected to construct compounds.Besides,DyⅢ,YbⅢ and a chelate acylhydrazone ligand were used to design compounds with pentagonal bipyramid coordination geometries.The research results are listed in the following:1.Three-dimensional(3D)compounds with magnetic ordering based on[MoⅢ(CN)7]4-Five 3D compounds were synthesized based on[MoⅢ(CN)7]4-.By introducing different organic cations such as dimethylamine hydrochloride,ethylamine hydrochloride,ethylamidine hydrochloride into Mn2+/[MoⅢ(CN)7]4-system,we obtained compounds {A2Mn5(H20)10[Mo(CN)7]3·8H2O}n(A =[NH2(CH3)2]+(1),[NH3CH2CH3]+(2),[(NH3NH)CCH3]+(3)).Magnetic studies revealed that compound 1 and 2 both behaved as a ferrimagnets and ordered below 80 K and 50 K,respectively.The self-assembly of[MoⅢ(CN)7]4-and Fe2+ resulted in a 3D compound{Fe2(H20)5[Mo(CN)7]·5H2O}n(4).Adding[NH2(CH3)2]+ during the synthesis of 4,a new compound {[NH2(CH3)2]2Fe5(H2O)10[Mo(CN)7]3·8H2O}n(5)with distinct three-dimensional topology were gained.Magnetic studies of 4 and 5 revealed that both are magnets ordering below 65 K.2.Synthesis and magnetic properties of zero-dimensional(0 D)compounds based on[MoⅢ(CN)7]4-In this part,we designed and synthesized five five-dentate chelating ligands and fourteen mononuclear precursors based on them.Assembly of these precursors with[MoⅢ(CN)7]4-or mixing 3d transition metal ions M2+(M = V,Cr,Mn,Fe,Co,Ni)with chelating ligands in solvents,then diffuse them into[MoⅢ(CN)7]4-solution slowly,we expected to gained SMMs.However,owing to the sensitivity of[MoⅢ(CN)7]4-to oxygen and light,adverse hydrophilic-lipophilic property compared to chelating ligands,it is very hard to grow single crystal.Although we tried many times,only three compounds[Fe(Btzp)(H2O)]2[Mo(CN)7]·18H2O(6),[Co(Btzp)(H20)]2[Mo(CN)7]·18H2O(7),[Co(Py5Me2)2[Mo(CN)8]·10H2O(8)were obtained.Compounds 6 and 7 are trinuclear molecule based on[MoⅢ(CN)7]4-.During the synthesis of compound 8,[MoⅢ(CN)7]4-was oxidized into[MoⅣ(CN)8]4-even in rigorous anaerobic atmosphere.Magnetic studies revealed no slow relaxation due to the fact that magnetic anisotropy decreases significantly when the local geometry deviates from the ideal pentagonal bipyramid.3.Reversible On-Off Switching of Anisotropic Magnetic Coupling and Single-Molecule Magnetism triggered by a Solid-State Crystal-to-CrystalReactionA four-dentate chelating ligands L1(N,N’-bis[(1H-imidazol-4-yl)methylene]-2,2-dimethylpropane-1,3-diamine)were synthesized.The diffusion of solution of LI and Mn2+ into aqueous solution of[MoⅢ(CN)7]4-lead to a trinuclear compound[Mn(L1)(H2O)]2[Mo(CN)7]·2H2O(9).MoⅢ centers exhibit pentagonal bipyramid geometry and linked two Mn2+ ions through two axial cyanide-groups.Magnetic studies showed that compound 9 behaved as a SMM with effective energy barrier of 44.9 cm-1,which is higher than that of 40.5 cm-1(58.5 K)for the first reported Mn2Mo SMM.More interestingly,the trinuclear Mn2Mo molecule could undergone reversible single-crystal-to-single-crystal(SCSC)transformation to a hexanuclear Mn4Mo2 compound[Mn(L)(H2O)]2[Mn(L)]2[Mo(CN)7]2(10)when heated in the vacuum drying oven.This drastic structural transformation involving the breaking and reforming of coordination bonds leads to obvious changes in the color.The distortion of the pentagonal bipyramidal geometry of[Mo(CN)7]4-in 10 disrupts the anisotropic exchange interactions that lead to SMM behavior in 9.Hence compound 10 behaves as a simple paramagnet despite its higher ground state spin value.In a word,this work realized the reversible on-off switching of anisotropic magnetic coupling and single-molecule magnetism triggered by a solid-state crystal-to-crystal reaction.4.Synthesis and magnetic properties of 4f SIMs with pentagonal bipyramid coordination geometries LigandH4L2(H4L2=2,6-bis[(3-methoxysalicylidene)hydrazinecarbonyl]-pyridine)were synthesized and used to react with DyCl3·6H2O or YbCl3·6H2O.When the molar ratio of H4L2:LnCl3·6H20 equals to 1:1,we obtained compound[Ln(H3L2)Cl2(DMF)2]·DMF·2H20(Ln=Dy,llDy;Ln=Yb,12Yb),while the molar ratio changed to 2:1,[Ln(H3L2)Cl2(DMF)2]·H4L2((Ln=Dy,13Dy;Ln=Yb,14yb))were gained.Structural characterization indicates that changes of the mole ratio of reactants result in different guset molecules in the lattice.Compound[Ln(H3L2)Cl2(DMF)2]DMF·2H2O and[Ln(H3L2)Cl2(DMF)2]·H4L2 have the same magnetic building block[Ln(H3L)(DMF)2Cl2]except some subtle differences in the local coordination geometry symmetry of LnⅢ centers.This maintains the ligand field around the LnⅢ ion well,and leaves the symmetry as the only variate for the comparison of the magnetic properties.Magnetic data analysis reveals that the subtle structure variations trigger remarkable distinct slow magnetic relaxation processes of compounds 12Yb and 14Yb,while 11Dy and 13Dy show similar slow magnetic relaxation.