Synthesis And Characterization Of Coordination Polymers Based On 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic Acid)

As a kind of solid electrolyte,proton conductors play an important role in applications such as fuel cells,ion sensors and electrochromic devices.Proton conductors can be classified into anhydrous and hydrous types as well as low temperature,moderate temperature and high temperature types depending on the actual working temperature and humidity.The corresponding candidate materials are diverse,such as organic polymers,inorganic solid acids,solid oxides,coordination polymers,and some composite materials.Among them,coordination polymers(CPs)is a kind of inorganic-organic hybrid material formed by metal ions or clusters as nodes and organic ligands as bridges.Usually,CPs are highly crystalline,well-defined and adjustable in composition,which are benefit for construction of the high-performance proton conductors in terms of the application requirements.To obtain a proton conductor with practical application,two primary conditions must be met:high proton conductivity and high stability.Therefore,the challenge is how to achieve a proton conductive material with both high proton conductivity and stability by choosing the right metal nodes and bridging ligands and reasonable structural design.Compared with organic carboxylate compounds,metal phosphonates can provide more acidic protons and more free oxygen atoms to participate in the formation of a continuous H-bonding network,which improves proton migration in the materials.In addition,the phosphonic acid group is easy to coordinate with metal ions and tend to form a multi-dimensional structure,which is beneficial to enhance the thermal and chemical stability of the material.In this paper,a metallo-ligand M(notpH3)[M=Fe3+ orCo3+;notpH6=(?)1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid)]is used to react with Zn2+,UO22+ and Ca2+ ions,respectively.As a result,nine metal phosphonates with new structures are obtained.For these compounds,the relationship between the synthetic conditions and structural regulation,water stability and proton conduction were investigated in details.The details are shown as follow:1.Water stable 3d-3d heterometallic phosphonates based on notp ligand.Two new compounds FeZn2Cl(notp)(H2O)(1)and[CoZn(notpH)(H2O)]·1.5H2O(2)have been prepared by hydrothermal reactions of M(notpH3)[M=Fe or Co]and Zn(OAc)2 with a molar ratio of 1:2 under 120?.Both compounds show 2-D layered structures.In 1,all three phosphonate groups of the Fe(notp)3-unit are deprotonated,and each phosphonate group bridges one {ZnO4} and one {ZnO3Cl}coordination tetrahedron to form a 2-D layered structure.In 2,one phosphonate oxygen atom of the Co(notpH)2-unit is protonated,and the other two phosphonate groups respectively bridge one {ZnO4} and two {ZnO4} coordination tetrahedrons to form a 2-D wave-like layered structure.Both 1 and 2 are stable for 5 days in pure water at room temperature or 2 days at 100?,or 5 days in HCl solution with pH=1.0,respectively.Compound 1 remains original structure after being immersed in NaOH solution with pH=12 for 1 day,while 2 partially changes after immersion under the same conditions.The proton conductivities of 1 and 2 are relative humidity(RH)dependent.The proton conductivities of 1 and 2 are 1.10×10-10 and 1.85 ×10-9 S cm-1 at 50%RH and 25?,respectively.And the proton conductivities are increased to 4.31×10-7 and 1.01×10-6 S cm-1 at 95%RH and 25?,respectively.The Arrhenius activation energies Ea of 1 and 2 are estimated to be 0.45 eV and 0.39 eV at 95%RH and in the range of 15 to 55?,respectively.2.Water stable 3d-5f heterometallic phosphonates based on notp ligand.Four new compounds[Fe(UO2)(notpH)]·0.5H2O(3),[Fe2(UO2)(notpH2)2]·0.75H2O(4),[Co(UO2)(notpH)(H2O)]·5H2O(5)and[Co2(UO2)2(notpH)2(H2O)2]·6H2O(6)have been prepared by hydrothermal reactions of M(notpH3)[M=Fe or Co]and UO2(OAc)2 with various molar ratio under 120?.All compounds show 2-D layered structures.In 3,one phosphonate oxygen atom of the Fe(notpH)2-unit are protonated,and three phosphonate groups bridge four {UO6} coordination octahedrons to form a 2-D layered structure.In 4,two phosphonate oxygen atoms from two phosphonate groups of the Co(notpH)2-unit are protonated,and one protonated phosphonate group and another deprotonated phosphonate group bridge one different {UO6} coordination octahedron to form a 2-D layered structure.Like 3,one phosphonate oxygen atom of the Co(notpH)2-unit are protonated in 5,but three phosphonate groups bridge four{UO7} units with one water molecule coordinating to the UO22+ion,forming a 2-D layered structure.The layer topology of 6 is almost the same as that of 5,however,the packing modes among layers are different in both compounds due to the variance of the amount of lattice water molecules.3 is not only stable for 5 days in pure water at room temperature or 100?,or in HCl solution with pH=1.0,respectively,but also stable for 1 day in NaOH solution with pH=12.The proton conductivities of 3 and 5 are also RH dependent.The proton conductivities of 3 and 5 are 1.09×10-9 and 4.85×10-8 S cm-1 at 50%RH and 25 ?,respectively.And the proton conductivities are increased to 3.98×10-7 and 2.95×10-6 S cm-1 at 95%RH and 25?,respectively.The Arrhenius activation energies Ea of 3 and 5 are estimated to be 0.29 eV and 0.42 eV at 95%RH and in the range of 15 to 55?,respectively.3.Moisture stable 3d-4s-5d heterometallic phosphonates based on notp ligand.Three new compounds[Co?2Ca?2(notpH2)2(H2O)8](Ir?Cl6)·2H2O(7),[Co?2Ca?(notpH3)2(H2O)2](Ir?Cl6)·4H2O(8)and[Co?2(notpH4)2](Ir?Cl6)·6H2O(9)have been isolated from the solution of Co(notpH3),Ca2+and IrCl62-at pH=3.5,2.5 and 1.7,respectively.Controlling the protonation of the acid affects the Co:Ca ratio but does not affect its coordination or geometry to the cobalt center.The compounds represent different networks involving H-bonds between cations-anions-solvents where the proton conductivities of 1.27×10-6,6.10×10-6 and 14.3×10-6 S cm-1,respectively,under 95%RH and 25 ?,increase with the protonation of the phosphonate groups.Their high activation energies(0.72,0.78 and 0.60 eV,respectively)are consistent with a vehicle mechanism.All three compounds are stable under the 95%RH and 55? for 2 days.Only 8 exhibits slow magnetic relaxation associated with Ir?Cl62-5d5,low spin,S=? and L=1),which correlates with the absence of halogen-bond between neighboring Ir?Cl62-(Cl…C1>4.941?).