Studies Of Metal Phosphonates Based On Chiral And Racemic Phosphonate Ligands

Metal phosphonates are a class of important inorganic-organic hybrid materials with potential applications in exchange and sorption, optics, catalysis, biotechnology and magnetism. Chiral coordination polymers have attracted intensive current interest mainly due to their potential applications in the areas of enantioselective catalysis, chiral separation and chiral materials. As a result, the incorporation of chiral information into the metal phosphonate compounds becomes an important subject. However, examples of chiral or enantioenriched metal phosphonates are rather few thus far. In this thesis, we shall focus on the assembly of new metal phosphonates based on chiral or racemic phosphonate ligands. The achievements are obtained as follows:1. Racemic metal phosphonates based on1-phosphonomethyl-2-benzimidazol-piperidine phosphonateThe first examples of metal phosphonates based on a racemic1-phosphonomethyl-2-benzimidazol-piperidine (pbpH2) are obtained under hydrothermal conditions, namely,[M(pbp)(H2O)2]·3H2O [M=Ni (1), Co (2)] and Cu(pbp)·2H2O (3). Compounds1and2are isostructural. The metal ions are six-coordinated with a distorted octahedral geometry. The pbp2-behaves as a tetra-dentate ligand. The amino nitrogen (N1) and one benzimidazol nitrogen (N2) atoms chelate the metal atom. Two of the three phosphonate oxygen atoms (O1, O2) bridge the equivalent metal atoms to form a chiral single chain running along the a-axis with the benzimidazol groups arranged on the same side of the chain. The adjacent single chains containing R-pbp2-and S-pbp2-ligands, respectively, are fused together through hydrogen bond interactions, leading to a racemic double chain. The double chains are packed in the lattice thorugh hydrogen bond interactions to form a three-dimensional supramolecular network structure. For compound3, the Cu(II) ion is four-coordinated with a planar geometry. It is chelated by the pbp2-ligand through N1, N2and01atoms, and bridged by the phosphonate group from another equivalent pbp2-ligand. An infinite helical chain is thus constructed with the benzimidazol groups arranged on the two sides of the chain. The chains with opposite helicity are alternatively arranged in the lattice to furnish a final racemic structure. Dominant antiferromagnetic interactions are mediated between the magnetic centers in compounds1-3.2. Racemic metal phosphonates based on2-phenyl-2-(phosphonomethylamino)acetic acidThe first examples of metal phosphonates based on a racemic2-phenyl-2-(phosphonomethylamino)acetic acid (ppaH3) are obtained under hydrothermal conditions, namely, Co3(ppa)2(4,4’-bpy)2(H2O)4-2H2O (4) and Ni4(ppaH)4(4,4’-bpy)2(H2O)2·2H2O (5). Complex4crystallizes in P21/c space group. It contains two crystallographically independent cobalt atoms. The ppa3-behaves as a penta-dentate ligand. The equivalent Co2atoms are bridged by two O-P-0units from R-and S-ppa3-ligands, resulting in a racemic dimer. The dimers are connected by Co1through O-P-0units to form an infinite racemic chain. The chains are cross-linked by4,4’-bpy, leading to a three-dimensional framework structure. In compound5, four crystallographically independent nickel atoms are found. The ppaH2-ligands are either tetra-dentate or penta-dentate. The equivalent Ni atoms are linked by O-P-0into dimers. Neighboring dimers of Nil2and Ni22(or Ni32and Ni42) are connected by O-C-O units, forming infinite chain I (or chain Ⅱ) running along the c-axis. Each chain contains equal amount of R-and S-ppa3-ligands, thus is racemic. Chains Ⅰ and Ⅱ are each linked by4,4’-bpy, resulting in layers Ⅰ and Ⅱ, respectively, in the ac plane. These layers are packed alternatively along the b-axis with weak inter-layer hydrogen bond and van del Waals interactions, thus forming a3D supramolecular structure. The magnetic properties of4and5are studied, which show dominant antiferromagnetic interactions.3. Homochiral lanthanide phosphonates based on (1-phenylethylamino)methylphosphonic acidHomochiral lanthanide phosphonates (R)-or (S)-[Ln(pempH)3]·xH2O [Ln=Ho (6,7), Er (8,9)],(R)-[Tb3(pempH)8(pempH2)Cl]-xH2O [Ln=Ho (10), Er (11)] are obtained through reactions of (R)/(S)-(1-phenylethylamino)methylphosphonic acid (pempH2) and Ln(NO3)3under hydrothermal conditions. Compounds6and7,8and9are enatiomers, respectively. Although single crystals suitable for structural determination cannot be obtained for compounds6-9, they are isostructural to compounds (R)-and (S)-[Tb(pempH)3]·xH2O based on their powder XRD patterns. Accordingly, the crystal structures of6-9can be described as below:(R)/(S)-[Ln(pempH)3]·xH2O (Ln=Ho, Er) crystallize in hexagonal space group P65and P61, respectively. Each shows a ID helical chain structure, where the equivalent Ln(III) ions are triply bridged by two μ3-O(P) and one O-P-0units. Each Ln(III) ion is eight-coordinated with the eight coordination sites provided by the phosphonate oxygen atoms from three pairs of pempH-ligands. Each helical chain is constructed by right-or left-handed triple-strands formed from three phosphonate groups of three pempH-species which bridge two adjacent Ln(III) ions along the crystallographic61screw axis. The helical chains are packed through supramolecular interactions, generating channels running along the c-axis where the lattice water molecules reside.Compounds10and11are also enantiomers. Single crystal structural determination reveals that both crystallize in orthorhombic chiral space group P212121and show a chain structure. In this case, there are three crystallographically distinguished Er(Ⅲ) ions in the asymmetric unit, two of which are eight-coordinated and the third one is seven-coordinated. They are triply bridged by two μ3-O(P) and one O-P-O units, forming a Er3trimer. The neighboring trimers are further connected by one μ3-O(P) and two O-P-O units, thus leading to an infinite helical chain running along the b-axis. The Cl-anions serve as counterions and reside in the interchain spaces together with the lattice water molecules. Magnetic properties of compounds6,8,10and11are investigated. Slow magnetization relaxation is observed in all cases under a dc field at low temperature, indicating single molecular magnet behaviors.