Polar Cobalt Phosphonates:Synthesis And Properties

Due to their potential applications in the fields of dielectrics,ferroelectrics,nonlinear optics and so on,polar crystalline materials have received much interest recently.Different approaches have been developed to achieve polar materials by introducing Jahn-Teller distorted or lone-pair cations,chiral ligands,asymmetric ligands and polar guest molecules into the systems.On the other hand,as an important class of inorganic-organic hybrid materials,metal phosphonates have attracted great attention because of their significance in ion-exchange and absorption,catalysis,proton conductivity,magnetism and optics etc.Although a number of metal phosphonates have been reported,polar metal phosphonates are rather rare.In this thesis,chiral or asymmetric phosphonate ligands were used to construct polar materials and the obtained materials show interesting properties-The details are described in the following sections.1.Polar two-dimensional layered cobalt phosphonates based on chiral phosphonate ligandsTwo pairs of enantiomeric CoⅡ compounds with formulas Co2(μ3-OH)(cyamp)(CnH2n+1 COO)[cyampH2=(S)-or(R)-((1-cyclohexylethyl)amino)methylphosphonic acid;n=1(1);n=7(2)]are reported.The structures of S-1 and S-2 are determined by single crystal structural analyses.Both crystallize in a monoclinic polar space group P21,and exhibit layered structures in which the Δ-type chains of corner-sharing Co3(μ3-OH)triangles are connected by the phosphonate groups.The interlayer spaces are filled with the organic groups of the phosphonate and carboxylate ligands.Therefore,the distances between the layers can be manipulated by the length of the alkyl chain of the carboxylate ligands,from 14.6 (?) in 1 to 20.0 (?) in 2.Magnetic studies have been carried out for compounds S-1 and S-2.Both show metamagnetism at low temperature.The critical field decreases with increasing interlayer distance from 8.18 kOe for S-1 to 7.01 kOe for S-2 at 1.8 K.The CD spectra confirm the enantiopure nature of the two pair compounds,and compounds S-1(R-1)and S-2(R-2)exhibit inverse Cotton effects at the range of 480-600 nm due to the vicinal effect induced by asymmetric octanoic ligand in the compound.2.Polar or apolar one-dimensional cobalt phosphonates based on asymmetric phosphonate ligandsAn apolar one-dimensional cobalt phosphonate(NH4)3[Co2(bamdpH)2(HCOO)(H2O)2](3)(bamdpH4=((benzylazanediyl)bis(methylene))diphosphonic acid)is obtained by reacting Co(NO3)2 with bamdpH4 in a water-formamide mixture.Compound 3 crystallizes in a centrosymmetric space group C2/c,the distorted CoⅡ octahedra are bridged by O-P-O linkages forming chain structures.The two neighbouring chains are further linked by HCOO-resulting in ladder-like chains.In the structure,the HCOO-and NH4+ ions are disordered,which,however,do not show temperature-dependent disorder-order transition.Although compound 3 shows high thermal stability(ca.160℃)under dry condition,it can undergo a humidity-induced(RH=95%)single-crystal to single-crystal transformation above 35 ℃,leading to compound[Co(bamdpH2)(H2O)2]·2H2O(4).Compound 4 also crystallizes in a centrosymmetric space group P21/c,and it shows a single chain structure in which the cobalt atoms are cross-linked by the O-P-O bridges.Compound 4 can be futher transformed to a third phase[Co(bamdpH2)(H2O)2]·H2O(5)by kept at low humidity condition(RH<10%).Obviously,the HCOO-and NH4+ions in compound 3 are released after transformation at highly humid condition,indicating the hydrolysis of the ions.MS spectra and an indophenol assay procedure are used to detect the products after the hydrolysis.Further,IR spectra and proton conductivity measurements are applied to study the dynamics of the process.When the reaction solvent is changed to water,a polar cobalt phosphonate Co(bamdpH2)(H2O)(6)is obtained.It shows a linear chain structure with zigzag arranging CoⅡ ions bridged by μ-O(P)and O-P-O linkages.The neighboring chains are connected by moderately strong hydrogen bonds forming a supramolecular layer.The interlayer spaces are filled with the organic groups of the phosphonate ligands.Compound 6 displays the coexistence of single chain magnet behavior and canted antiferromagnetism below 2.8 K and the magnetic dynamics is strongly dependent on the synthetic methods.Three bulk samples are synthesized by using different cobalt salts and pH adjusters,i.e.,[Co(NO3)2/bamdpH4/NaOH]for 6-bulk-a,[Co(CH3COO)2/bamdpH4/HNO3]for 6-bulk-b and[Co(CH3COO)2/bamdpH4]for 6-bulk-c.Magnetic studies reveal that the relaxation process is accelerated from sample 6-bulk-a to 6-bulk-c,due to increasing amounts of crystalline defects.It is the first systematic study of the defect effect on magnetic relaxation for undoped single-chain magnets(SCMs),which is helpful to understand the relaxation mechanisms of SCMs.3.Polar two-dimensional layered cobalt phosphonates based on asymmetric phosphonate ligandsFour perovskite-type layered cobalt phosphonates,namely,Co(4-mopp)(H2O)(7),Co(4-mobp)(H2O)(8),Co(3-mopp)(H2O)(9),Co(3-mobp)(H2O)(10)(4-or 3-moppH2=(4-or 3-methoxyphenyl)phosphonic aicd,4-or 3-mobpH2=(4-or 3-methoxybenzyl)phosphonic acid),are obtained by assembling cobalt salts with the corresponding phosphonate ligand under hydrothermal conditions.Compounds 7,8 and 10 crystallize in the polar space groups Pmn21,Pmn21 and Pna21,respectively,while compound 9 crystallizes in a centrosymmetric space group P21/n.These four compounds have almost the same layered backbones,where the neighbouring CoⅡions are bridged by μ-O(P)and O-P-O linkages.The average nearest Co…Co distances within the layer are similar in the four compounds,however,the average Co-O-Co angles are different,where those of compounds 8 and 10 are 2o larger than those of 7 and 9.This difference contributes to different magnetic properties,where the larger ones result in stronger anti-ferromagnetic interactions.In compounds 7 and 8,rotational behaviors of the polar Me-O groups are observed,confirmed by dielectric measurements.Their rotational rates are strongly dependent on the steric hindrances from the neighbouring Me-O groups.Although no phase transitions are observed in the four compounds,the dehydrated phases of compounds 9 and 10,namely,9-de and 10-de,show phase transitions at temperatures above 200℃.These transitions are confirmed by DSC,variable-temperature PXRD patterns and dielectric constant measurements.