| As one of the important branches of polyoxometalates chemistry,polyoxometalates-based transition metal clusters have found potential applications in the fields of catalysis,pharmacy,magnetism,electrochemistry and photochemistry.However,due to the flexible coordination modes of transition metals,subtle changes in reaction conditions will cause the entire system to self-assemble in different ways,often making it difficult to achieve directed and controlled synthesis.In view of the above problems,we selected a number of inorganic phosphates and organophosphonates as bridging ligands to achieve effective and directional control of the structures and physicochemical properties,particularly magnetism,of the transitional metal clusters.This dissertation mainly is focused on manganese clusters.We started from trivacant Keggin-type heteropolytungstates and dodecanuclear manganese clusters of mixed valence as precursors,using bridging oxygen,phosphate/diphosphate,?,?-alkyldiphosphonates(H4Ln)and phenyltriphosphate(H6BTP)for intercluster connections.The controllable assembly of manganese clusters was realized,and nine manganese clusters based on Keggin-type heteropolytungstate with novel structures were designed and synthesized.Single crystal X-ray diffraction,infrared spectroscopy,nuclear magnetic resonance spectroscopy,element analysis,thermogravimetric analysis,X-ray photoelectron spectroscopy analysis and magnetic measurement methods are used for structural characterization and analysis.Through the analysis and comparison of synthetic conditions,molecular structures and magnetic properties,the effect of bridging ligands on structure and magnetic properties is studied.Taking advantage of the fact that Keggin-type silicotungstates can undergo a certain degree of transformation in aqueous solution,three monomers and dimers from different reaction systems with different manganese clusters are assembled into a hexamer,(Me2NH2)25Na12[Mn2?Mn9?(OH)8(H2O)2(A-SiW9O34)4(A-SiW10O37)2]·68H2O(1),through bridging oxygen atoms.The four monomers in the polyanion are all Keggin-type silicotungstates with vacancies filled by Mn?/?ions,while the S-type dimer is held together with the help of monomers,counter cations and water molecules.It remains stable within a wide range of conditions,including solvent,temperature,p H,etc.At low temperatures,the antiferromagnetic intra-cluster(J?–?=-3.23 and-3.97 cm-1,J?–?=-6.73 cm-1)and inter-cluster(z J=-0.06 cm-1)in the monomers causes theXMT values continuously drop over the while temperature range.Extending the bridging ligands to monophosphate and oligophosphate ions,a trivacant Keggin-type phosphotungstate-based{Mn3?Mn?O3}(Mn4)monomer,(Me2NH2)5[Mn3?Mn?O3(OAc)3(B-PW9O34)]·8H2O(2),was first synthesized.The ground state spin value of{Mn4}at low temperature is still 9/2.Because the central heteroatom tetrahedron and{Mn4}are connected in different ways,that is,the Jahn–Teller axes of the three Mn?are almost parallel in the A-type,but intersect at a point in the B-type.This change affects the bond length and bond angle in{Mn4},resulting in a change in the strength of the interaction within the manganese cluster(J?–?=8.36 cm-1,J?–?=-25.82 cm-1).With the introduction of phosphate linkers,a tetramer,(Me2NH2)14Na10[(Mn3?Mn?O3)4(PO4)4(OAc)4(B-PW9O34)4]·24H2O(3),is obtained.The ground state spin value of each{Mn4}still maintains 9/2 at low temperature(J?–?=8.53 and 9.08 cm-1,J?–?=-25.84 and-23.07 cm-1).The introduction of diphosphate leads to the formation of a hexamer,Cs25Na9[(Mn3?Mn?O3)2Mn8?(OH)4(P2O7)2(PO4)2(B-PW9O34)2(B-PW10O37)4]·60H2O(4).In order to ensure that all the P–O bonds of the inorganic phosphate ligands are coordinated,{Mn4}monomer is converted into{Mn2?(OH)}(Mn2)monomer in aqueous solution.Phosphate and diphosphate thus can be used as templates to induce structural transformation between compounds 2-4.The bridging ligands were then extended from inorganic phosphates to organic phosphonates.The flexible phosphonates ligand H4Ln with different chain lengths were introduced similarly to inorganic phosphate ligands.In order to control the molecular structure,the alkyl chains have to be deformed to a different degree to ensure that the distance between the phosphonate groups at both ends remains constant to connect the same manganese clusters monomers.In the end,two tetramers with similar structures,(Me2NH2)14Na8[(B-PW9O34)2(A-PW10O37)2Mn?10Mn2?O6(OH)2(OAc)2(L4)2]·26H2O(5)and(Me2NH2)20Na2[(B-PW9O34)2(A-PW10O37)2Mn?10Mn2?O6(OH)2(OAc)2(L5)2]·30H2O(6),were synthesized.The introduction of H4L4 and H4L5 still keeps the intra-cluster interaction of{Mn4}(S=9/2)and{Mn2}(S=0)unchanged,but drastically changes the inter-cluster interactions,thereby altering the ground state spin value of the molecule from 9(compound 5)to 0(compound 6).With longer H4L6,due to the limited degree of deformation of the alkyl chain and the high energy required for large deformation,an octamer,Rb29Na15[(A-PW9O34)8Mn?20(OH)8(H2O)8(L6)6]·92H2O(7),was synthesized instead.Moving from flexible phosphonates to rigid phosphonates,we introduced phenyltriphosphate into the phosphonate–polyoxometalates hybrid system.The assembly process was apparently sensitive to temperature change;at different temperatures,two tetramers with different configurations were constructed:a tetrahedron,(Me2NH2)21Na7[Mn?12(OH)12(A-SiW9O34)4(BTP)2]·57H2O(8),and a planar tetramer,(Me2NH2)24Na8[Mn?10Cl O3(OH)3(A-SiW9O34)(A-SiW10O37)3(BTP)2]·27H2O(9).Unlike the alkyl chain in H4Ln,which can be deformed,the rigid benzene ring in H6BTP leads to a fixed orientation of the phosphonate groups in both.Although they have similar{Mn4}and{Mn2}monomers,the composition ratios of the two monomers are not the same when forming tetramers,they are 2:2(compound 5)and 1:3(compound 9),respectively.The structural differences caused by the different phosphonate ligands also result in very different magnetic behavior for the two compounds at low temperatures:notably,the?MT values at 2 K are 50.67 and 8.08 emu·K/mol,respectively. |
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