| Metal phosphonates have received increasing attentions because of their potentials to be used in gas adsorption and separation,catalysis,magnetism,and etc.Particular efforts in this field have been devoted to the assembly of new compounds with versatile architectures and interesting properties.A number of new metal phosphonate compounds have been synthesized by using several approaches:1)employing phosphonate ligands functionalized with different organic groups,ii)introducing rigid or flexible co-ligands,iii)using different organic and/or inorganic templates,iv)varying the reaction conditions,and v)using post-synthetic methods etc.Some of them show very interesting chemical and physical properties.In this thesis work,we focus on the construction of new cobalt phosphonates coupled with magnetic and absorption properties.For this purpose,we selected two phosphonic acids which have not been described in the literature,namely,5-phosphonatophenyl-1,2,4-tricarboxylic acid(pbtcHs)and 4-(ethoxycarbonyl)-naphthalen-1-yl)phosphonic acid(4-cnppH2).Based on these two ligands,we obtained a series of new cobalt phosphonates with the incorporation of the rigid or flexible organic co-ligands.The contents are given below:1.Cobalt phosphonates containing pbtcHs and terminal co-ligands(1)Two mixed ligated cobalt phosphonates Co2(pbtcH)(2,2'-bpy)2(H2O)(1)and Co2(pbtcH)(phen)2(H2O)(2)with layer structures have been synthesized,where 2,2'-bpy is 2,2'-bipyridine and phen is 1,10-phenanthroline.Compound 1 gives a rare example of coordination polymers that experiences a successive reversible two-step single-crystal to single-crystal to single-crystal(SC-SC-SC)structural transformation upon heating and rehydration.The dehydrated compound la has a porous structure,arising from the re-orientation of the 2,2'-bpy molecules induced by the release of coordination water.For compound 2,the SC-SC transformation involves the cleavage and formation of six Co-0 bonds,forming a carboxylate/phosphonate bridged Co(?)linear chain within the layer.The structural transformations of two compounds are also monitored by in-situ IR measurements.This work provides a new route to produce porous hybrid materials through post-synthetic thermal treatment of a nonporous material.(2)Two new mixed-ligated metal phosphonates,M2(pbtcH)(phen)2(H2O)2[M(?)=Co(3),Ni(4)]have been synthesized and characterized.Both show one-dimensional double chain structures,where the M(phen)(H2O)moieties are chelated and bridged by pbtcH4-through carboxylate and phosphonate oxygen atoms.The chains are connected by hydrogen bond interactions and ?-? stacking,forming a three-dimensional supramolecular structure.The spectroscopic properties are investigated by using infrared spectroscopy(IR)and ultraviolet-visible spectroscopy(UV-vis).Their magnetic properties are also studied.2.Cobalt phosphonates containing pbtcHs and bridging co-ligands(1)Three novel mixed ligated cobalt phosphonates Co5(1,2-bix)2(pbtc)2(H2O)4·10H2O(5)?Co5(1,3-bix)2(pbtc)2(H20)4·10H2O(6)?Co5(1,4-bix)2(pbtc)2(H2O)4·6H2O(7)have been obtained by introducing the flexible 1,2-bis(imidazol-1-ylmethyl)-benzene(1,2-bix),1,3-bis(imidazol-1-ylmethyl)-benzene(1,3-bix)and 1,4-bis(imidazol-l-ylmethyl)-benzene(1,4-bix)co-ligands into the Co-pbtc system.In all three compounds,inorganic chains made up of alternatively arranged Co3O2 trimers and Co2 dimers are found,which are further linked by the pbtc5-ligands into a layer.The most significant structural difference is found for the three compounds is the stacking of the layers.In compound 5,the Co3O2 trimers and CO2 dimers in the same chain(or layer)are connected by 1,2-bix,thus forming a layer structure.While in compound 6,the equivalent Co3O2 trimers or Co2 dimers from neighboring layers are connected by 1,3-bix,leading to a three-dimensional open-framework structure.In the case of compound 7,an open-framework structure is also found but the 1,4-bix links both Co3O2 trimer and Co2 dimer from the adjacent layers.Such connection modes cause the differences in the Co…Co distances between the layers,which decreases in the order of 7(10.290 A)>5(9.868 A)>6(9.764 A).Magnetic studies reveal that all three compounds show long-range magnetic ordering with a spin-canted structure,coupled with spin-glass behavior below their Curie temperatures.The Curie temperature decreases in the order of 7(25 K)>5(23 K)>6(22 K).While the canting angles,estimated from the magnetization curves at 1.8 K,also decreases in the order of 7>5>6.Apparently,the magnetic behaviors of the cobalt-pbtc compounds may be modulated by using isomeric co-ligands.In addition,compound 5 can undergo a reversible SC-SC transformation by releasing/re-absoprtion of the lattice water molecules in-situ,resulting in compound Co5(1,2-bix)2(pbtc)2(H2O)4(5a).The structure of 5a is similar to that of 5,except that the packing of the layers is changed and the inter-layer Co...Co distance is reduced.As a result,paramagentism instead of weak ferromagnetism is observed in 5a.This result demonstrates that the magnetic behaviors of the Co-ptbc systems can also be tuned by dehydration and re-hydration process.(2)By introducing rigid secondary ligands such as 4,4'-bpy and 2,2'-bipyrimidine,compounds Co5(pbtc)2(4,4'-bpy)2(H2O)4·2H20(8)and Co5(pbtc)2(bpm)(H2O)6·4H2O(9)have been synthesized.Compound 8 shows a similar open-framework structure to compound 7,but the Co...Co distance between layers is much smaller.Consequently,no long-range magnetic ordering is observed above 1.8 K.Compound 9 shows a novel 3D open-framework structure,where the chains made up of pentamers of edge-sharing CoO6 and CoN2O4 octahedra are connected by bpm and pbtc5-ligands into a 3D structure.The Co...Co distance over the bpm bridge is much shorter than those over bix ligands in 5-7.Thus no long-range magnetic ordering is observed above 1.8 K.3.Cobalt phosphonate based on 4-cnppH2Under hydrothermal/solventhermal conditions,compound Co(4-cnpp)(10)has been synthesized which shows a layer structure.The layer topology of this compound is related to the other cobalt monophosphonates.However,the coordination environment of cobalt in 10 is distorted tetrahedral instead of octahedral.By introducing rigid bridging ligands such as 4,4'-bpy and 3,6-bis(pyridin-3-yl)-1,2,4,5-tetrazine(3-pytz),Co2(4-cnpp)2(4,4'-bpy)(H2O)(11)and Co2(4-cnpp)2(3-pytz)(12)have been obtained.Both contain similar double chains of corner-sharing cobalt polyhedra and PO3C tetrahedra.But in the case of 11,the cobalt ions within the chain are further connected by a bridging water molecule,thus forming a dimer.In compound 11,the inorganic chains are cross-linked by 4,4'-bpy to form a 3D open-framework structure.While in compound 12,the chains are linked by 3-pytz forming a layer structure.The inter-layer spaces are filled with the organic groups of the 4-cnpp2-ligands.Dominant antiferromagnetic interactions are found in 10-12. |
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