Highly Selective Ferric Ion Sorption and Exchange by Crystalline Metal Phosphonates Constructed from Tetraphosphonic Acids

Advances and Challenges in the Creation of Porous Metal Phosphonates

In the expansive world of porous hybrid materials, a category of materials that has been rather less explored than others and is gaining attention in development is the porous metal phosphonates. They offer promising features towards applications which demand control over the inorganic–organic network and interface, which is critical for adsorption, catalysis and functional devices and technology. The need to establish a rationale for new synthesis approaches to make these materials in a controlled manner is by itself an important motivation for material chemists. In this review, we highlight the various synthetic strategies exploited, discussing various metal phosphonate systems and how they influence the properties of porous metal phosphonates. We discuss porous metal phosphonate systems based on transition metals with an emphasis on addressing challenges with tetravalent metals. Finally, this review provides a brief description of some key areas of application that are ideally suited for porous metal phosphonates.

High-Throughput Synthesis of Pillared-Layered Magnesium Tetraphosphonate Coordination Polymers: Framework Interconversions and Proton Conductivity Studies

Novel pillared-layered framework materials were synthesized by high-throughput or microwave-assisted methodology that contain Mg2+ and the zwitterionic linker HDTMP (hexamethylenediamine-N,N,N′,N′-tetrakis(methylenephosphonic acid)). Three compounds were structurally characterized by X-ray powder diffraction. In the compound {Mg[(HO3PCH2)2N(CH2)6N(CH2PO3H2)2]·(H2O)}n(1), obtained at 140 °C by hydrothermal or microwave-assisted reaction, the layers are built by isolated Mg2+ octahedra coordinated by oxygen atoms from six different zwitterionic HDTMP ligands. Each amino-bis(methylenephosphonate) moiety links three Mg2+ ions, bridging two of them through one phosphonate group and connecting the third polyhedron in a monodentate fashion. In Compound 2, {Mg[(HO3PCH2)2N(CH2)6N(CH2PO3H2)2]}n, hydrothermally synthesized at 180 °C, the layers are composed of bidentate amino-bis(methylenephosphonate) moieties connected to three Mg2+ ions, with one of the phosphonate groups acting as a bridging ligand. Various subtle structural changes are noted for the other two compounds. Thermodiffraction of 1 reveals that a crystalline-to-crystalline phase transformation occurs concomitantly with its dehydration, leading to a new anhydrous phase, namely, {Mg[(HO3PCH2)2N(CH2)6N(CH2PO3H2)2]}n(1deh). This process is fully reversible upon equilibrating the solid at room temperature. The reported compounds can adsorb ammonia and CO2. Compound 1 exhibits a moderate proton conductivity, ~1.5 × 10−5 S·cm−1 at 80 °C and 95% RH, that increases a half order of magnitude after experiencing a complete dehydration/rehydration process, 1→1deh→1.

The Effect of Hydrogen Bonding in Enhancing the Ionic Affinities of Immobilized Monoprotic Phosphate Ligands

Environmental remediation requires ion-selective polymers that operate under a wide range of solution conditions. In one example, removal of trivalent and divalent metal ions from waste streams resulting from mining operations before they enter the environment requires treatment at acidic pH. The monoethyl ester phosphate ligands developed in this report operate from acidic solutions. They have been prepared on polystyrene-bound ethylene glycol, glycerol, and pentaerythritol, and it is found that intra-ligand hydrogen bonding affects their metal ion affinities. The affinity for a set of trivalent (Fe(III), Al(III), La(III), and Lu(III)) and divalent (Pb(II), Cd(II), Cu(II), and Zn(II)) ions is greater than that of corresponding neutral diethyl esters and phosphonic acid. In an earlier study, hydrogen bonding was found important in determining the metal ion affinities of immobilized phosphorylated polyol diethyl ester coordinating ligands; their Fourier transform infrared (FTIR) band shifts indicated that the basicity of the phosphoryl oxygen increased by hydrogen bonding to auxiliary –OH groups on the neighboring polyol. The same mechanism is operative with the monoprotic resins along with hydrogen bonding to the P–OH acid site. This is reflected in the FTIR spectra: the neutral phosphate diethyl ester resins have the P=O band at 1265 cm⁻¹ while the monoethyl ester resins have the band shifted to 1230 cm⁻¹; hydrogen bonding is further indicated by the broadness of this region down to 900 cm⁻¹. The monoprotic pentaerythritol has the highest metal ion affinities of the polymers studied.

Novel amine templated three-dimensional zinc-organophosphonates with variable pore-openings

Amine templated three-dimensional zinc-organophosphates with different ring sizes have been obtained by varying the organic amine molecules. The figure shows the various net structures obtained during the study.

A comparative structural study of alkane- α,ω-diphosphonic acids

In a comparative structural study, the solid state structures within the homologous series of alkane-α,ω-diphosphonic acids, H2O3P–(CH2) n –PO3H2 with n = 6–12, have been characterised by powder and single crystal X-ray diffraction. The crystal structures of the odd-numbered diphosphonic acids were found to be homotypic. For the even-numbered diphosphonic acids – including the two already known polymorphs of butane-1,4-diphosphonic acid – two different types are found. Basically, all alkane-α,ω-diphosphonic acids exhibit pillared-layered structures with their terminal groups forming two-dimensional hydrogen bonded networks which are covalently bridged by alkylene chains. Structural differences occur within the hydrogen bonding systems as well as in the arrangement of the alkylene chains. Based on the existence of different structure types of the alkane-α,ω-diphosphonic acids, the progression of their melting points can be explained.

Mixed-solvothermal synthesis, structures, surface photovoltage, luminescence and molecular recognition properties of three new transition metal phosphonates with 3D framework and supramolecular structures

Three new transition metal(ii) phosphonates have been synthesized under mixed-solvothermal conditions. Surface photovoltage and field-induced surface photovoltage spectroscopy of compounds 1 and 2 have been studied. Compound 3 could be used for the sensitive sensing of DMF.

A dual-functional Cd(ii)–organic-framework demonstrating selective sensing of Zn2+ and Fe3+ ions exclusively and size-selective catalysis towards cyanosilylation

A dual-functional metal–organic-framework can selectively sense Zn²⁺ and Fe³⁺ ions over mixed metal ions and displays size-selective catalysis towards cyanosilylation of aldehydes.

Syntheses, structures, luminescence and molecular recognition properties of four new cadmium carboxyphosphonates with 2D layered and 3D supramolecular structures

Four new cadmium carboxyphosphonates with 2D layered and 3D supramolecular structure have been hydrothermally synthesized. Their luminescence and molecular recognition properties have been studied.