Surface-functionalized zirconium phosphate (ZrP) nanoparticles were synthesized using a combination of ion exchange and self-assembly techniques. The surface of ZrP was used as a platform to deposit tetravalent metal ions by direct ion exchange with the protons of the surface phosphate groups. Subsequently, phosphonic acids were attached to the metal ion layer, effectively functionalizing the ZrP nanoparticles. Use of axially oriented bisphosphonic acids led to the ability to build layer-by-layer assemblies from the nanoparticle surface. Varying the metal ion and ligand used allowed designable architectures to be synthesized on the nanoparticle surface. X-ray powder diffraction, XPS, electron microprobe, solid-state NMR, FTIR, and TGA were used to characterize the synthesized materials.

Three heterometallic aggregates, [(CoII)2(GdIII)2(tBuPO3)2(O2CtBu)2(HO2CtBu)2(NO3)4]·NEt3 (1), [(CoII)2(CoIII)2(GdIII)3(μ3-OH)2(tBuPO3)2(O2CtBu)9(deaH)2(H2O)2] (2), and (CoIII)2(GdIII)5(μ2-OH)(μ3-OH)2(tBuPO3)2(O2CtBu)10(HO2CtBu)(deaH)2]·MeOH (3), were successfully isolated in reactions of [Co2(μ-OH2)(O2CtBu)4]·(HO2CtBu)4, Gd(NO3)3·6H2O, tBu-PO3H2, and diethanolamine (deaH3) by varying the stoichiometry of the reactants and/or changing the solvent. The structures of the final products were profoundly affected by these minor changes in stoichiometry or a change in solvent. The metal–oxo core of these complexes displays a hemicubane or a defective dicubane-like view. Bond valence sum calculations and bond lengths indicate the presence of CoII centers in compound 1, mixed valent Co centers (CoII/CoIII) in compound 2, and only CoIII centers in compound 3 as required for the charge balances and supported by the magnetic measurements. Magnetic studies reveal significant magnetic entropy changes for complexes 1–3 (−ΔSm values of 28.14, 25.06, and 29.19 J kg–1 K–1 for 3 K and 7 T, respectively). This study shows how magnetic refrigeration can be affected by anisotropy, magnetic interactions (ferro- or antiferromagnetic), the metal/ligand ratio, and the content of GdIII in the molecule.

ConspectusTransition metal based high nuclearity molecular magnetic cages are a very important class of compounds owing to their potential applications in fabricating new generation molecular magnets such as single molecular magnets, magnetic refrigerants, etc. Most of the reported polynuclear cages contain carboxylates or alkoxides as ligands. However, the binding ability of phosphonates with transition metal ions is stronger than the carboxylates or alkoxides. The presence of three oxygen donor sites enables phosphonates to bridge up to nine metal centers simultaneously. But very few phosphonate based transition metal cages were reported in the literature until recently, mainly because of synthetic difficulties, propensity to result in layered compounds, and also their poor crystalline properties. Accordingly, various synthetic strategies have been followed by several groups in order to overcome such synthetic difficulties. These strategies mainly include use of small preformed metal precursors, proper choice of coligands along with the phosphonate ligands, and use of sterically hindered bulky phosphonate ligands. Currently, the phosphonate system offers a library of high nuclearity transition metal and mixed metal (3d–4f) cages with aesthetically pleasing structures and interesting magnetic properties.This Account is in the form of a research landscape on our efforts to synthesize and characterize new types of phosphonate based high nuclearity paramagnetic transition metal cages. We quite often experienced synthetic difficulties with such versatile systems in assembling high nuclearity metal cages. Few methods have been emphasized for the self-assembly of phosphonate systems with suitable transition metal ions in achieving high nuclearity. We highlighted our journey from 2005 until today for phosphonate based high nuclearity transition metal cages with VIV/V, MnII/III, FeIII, CoII, NiII, and CuII metal ions and their magnetic properties. We observed that slight changes in stoichiometry, reaction conditions, and presence or absence of coligand played crucial roles in determining the final structure of these complexes. Most of the complexes included are regular in geometry with a dense arrangement of the above-mentioned metal centers in a confined space, and a few of them also resemble regular polygonal solids (Archimedean and Platonic). Since there needs to be a historical approach for a comparative study, significant research output reported by other groups is also compared in brief to ensure the potential of phosphonate ligands in synthesizing high nuclearity magnetic cages.

Zirconium and tin phosphonates were synthesized to determine ion-exchange preference and radiolytic stability. The Sn and Zr phosphonate materials were found to have preference for highly charged ions (3+) over lower charged ions (1+, 2+). The materials were exposed to 3.18 × 106 Gray from a Co-60 gamma source and were shown to retain their structure and performance. Ion-exchange of radioactive Am-241 with varying pH was also explored. In this paper, we describe these unusual ion-exchangers that are simple to prepare, reproducible, and do not require phosphate addition.

Layered metal phosphonate–phosphate hybrid materials are known to be ion-exchange materials. Hybrids with zirconium metal centers were synthesized at varying phosphonate–phosphate ratios in order to explore the function and charge preference. The zirconium hybrid materials were found to have a range of applicable uses with preference for highly charged ions (3+) over lower charged ions (1+ and 2+). The addition of a large excess of phosphate altered the selectivity, and these materials were able to remove all ions from solution regardless of charge. In this paper, we describe newly synthesized compounds that are simple to prepare, reproducible, stable, and offer a variety of separation schemes.

Two heterometallic CoIII–GdIII nanomagnets (Co2Gd6 and Co2Gd9) with defective dicubane-like cores were isolated from the same set of reactants by varying the reaction conditions. These are the first examples of cobalt(III)–gadolinium(III) phosphonate compounds and a rare class of compounds with large 4f ratio among the reported 3d–4f complexes. Magnetic studies reveal large magnetic entropy changes for both complexes (−ΔSm = 27.81 and 33.07 J kg–1 K–1, respectively at 3 K and 7 T).

We report a rare example of the preparation of HKUST-1 metal–organic framework nanoplatelets through a step-by-step seeding procedure. Sodium ion exchanged zirconium phosphate, NaZrP, nanoplatelets were judiciously selected as support for layer-by-layer (LBL) assembly of Cu(II) and benzene-1,3,5-tricarboxylic acid (H3BTC) linkers. The first layer of Cu(II) is attached to the surface of zirconium phosphate through covalent interaction. The successive LBL growth of HKUST-1 film is then realized by soaking the NaZrP nanoplatelets in ethanolic solutions of cupric acetate and H3BTC, respectively. The amount of assembled HKUST-1 can be readily controlled by varying the number of growth cycles, which was characterized by powder X-ray diffraction and gas adsorption analyses. The successful construction of HKUST-1 on NaZrP was also supported by its catalytic performance for the oxidation of cyclohexene.

Inorganic–organic hybrid materials were synthesized by covalent attachment of epoxides to the surface of zirconium phosphate (ZrP) nanoplatelets. X-ray powder diffraction, FTIR, and TGA were utilized to confirm the presence of the modifiers and exclusive functionalization of the ZrP surface. NMR experiments were conducted to confirm the formation of P–O–C bonds between surface phosphate groups and epoxide rings. The applicability of the organically modified products was demonstrated by their use as fillers in a polymer matrix. Subsequently, a two step intercalation and surface modification procedure was utilized to prepare polymer nanocomposites that were imparted with functionality through the encapsulation of molecules within the interlayer of surface modified ZrP.Keywords: inorganic layered materials; polymer nanocomposites; self-assembled monolayers; surface modification; tetravalent metal phosphates;

The intercalation of inorganic layered materials has resulted in a wide range of applicability. In such cases the applicability of the material is largely dependent upon the species intercalated within the layer, and the layered material acts largely as a host. Recently, the surface modification of inorganic layered materials has been investigated and it has been shown that the exterior layers can be exclusively functionalized. The advent of surface chemistry allows for the synthesis of particles with both a controlled interlayer and surface. This approach can be used to tailor nanoparticles for specific applications. Herein we review the surface chemistry of α-zirconium bis(monohydrogen orthophosphate) monohydrate (Zr(HPO4)2·H2O, α-ZrP) along with some applications of recent interest. Not only can these reactions be applied to α-ZrP, but similar chemistry can also be expanded to other layered materials and systems.

Incorporation of the same ligand into three different aluminum phenylenediphosphonates (Al(H2O)(O3PC6H4PO3H) (1), Al4(H2O)2(O3PC6H4PO3)3 (2), and Al4(H2O)4(O3PC6H4PO3)2.84(OH)0.64 (3)) was accomplished by varying the synthetic conditions. The compounds have different sorption properties; however, all exhibit reversible dehydration behavior. The structures of the hydrated and dehydrated phases were determined from powder X-ray diffraction data. Compounds 2 and 3 were found to be microporous, while compound 1 was found to be nonporous. The stability of the dehydrated phase and the resulting porosity was found to be influenced by the change in the structure upon loss of water.

Surface-functionalized zirconium phosphate (ZrP) nanoparticles were synthesized using a combination of ion exchange and self-assembly techniques. The surface of ZrP was used as a platform to deposit tetravalent metal ions by direct ion exchange with the protons of the surface phosphate groups. Subsequently, phosphonic acids were attached to the metal ion layer, effectively functionalizing the ZrP nanoparticles. Use of axially oriented bisphosphonic acids led to the ability to build layer-by-layer assemblies from the nanoparticle surface. Varying the metal ion and ligand used allowed designable architectures to be synthesized on the nanoparticle surface. X-ray powder diffraction, XPS, electron microprobe, solid-state NMR, FTIR, and TGA were used to characterize the synthesized materials.

Organically surface-modified α-zirconium phosphate was obtained by reacting the surface P–O–H groups of α-zirconium phosphate nanoparticles (α-ZrP) with octadecyltrichlorosilane (OTS). Surface functionalization of α-ZrP with OTS was accomplished using a one-step synthesis producing highly hydrophobic nanoparticles. The formation of P–O–Si bonds arising from nucleophilic attack of POH to the silane was confirmed by solid-state NMR experiments. The surface coverage of the organic modifier was characterized by TGA, AFM, and FTIR. In addition, we show the applicability of this system with a photoinduced electron-transfer reaction in a nonpolar solvent. Using an organically surface-modified α-ZrP previously loaded with tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)32+), the quenching of the luminescence of Ru(bpy)32+ in the presence of p-benzoquinone was monitored; a static quenching constant (Ks) value of 8.82 × 102 M–1 and a dynamic quenching constant (KD) value of 6.99 × 102 M–1 were obtained.Keywords: layered compounds; photoinduced electron transfer; silanes; surface chemistry; surface functionalization; tetravalent phosphonates;

A one-step hydrothermal synthesis with small amines and 1,3,5-benzenetriphosphonic acid was used to prepare single crystals of isostructural anionic metal–organic frameworks (MOF): Zn2.5(H)0.4–0.5(C6H3O9P3)(H2O)1.9–2(NH4)0.5–0.6 and Zn2.5(H)0.75(C6H3O9P3)(H2O)2(CH3NH3)0.25. The ammonium ions are exchangeable with lithium ions. The MOF exhibits reversible dehydration, and the process was studied by two complementary methods: solid state NMR and in situ X-ray diffraction. These experiments revealed three different phases. The crystal structures of all phases have been determined, showing loss in volume of the structure due to a phase change. The ammonium ions remain in the structure and are forced to occupy the larger pores due to a reduction in free volume. The change in positions of the guest molecules in the framework has an effect on the potential conductivity properties of the materials. Changes in framework and guest molecules due to negative expansion have an effect on other physical and chemical properties and need to be explored.

An aluminium carboxymethylphosphonate of composition (NH4)2Al(H1/2O3PCH2CO2)2 has been prepared hydrothermally. The aluminium cation is chelated by six membered rings formed from bonding by both the carboxylate and phosphonate oxygens. These chelate rings in turn form larger eight membered rings by connecting to similar chelate groups to form chains running along the a-axis.

A series of novel organic–inorganic hybrids based on trivalent lanthanide (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho) and 1,4-phenylbis(phosphonate) formulated as Ln[O3P(C6H4)PO3H] has been obtained as single phases under hydrothermal conditions. In the praseodymium compound (Za1), single crystals have been obtained and the crystal structure has been determined. Za1 crystallizes in the monoclinic space group, C2/c, with a = 5.6060(4) Å, b = 20.251(7) Å, c = 8.2740(6) Å, β = 108.52(1)°. All other compounds are isostructural to Za1 as confirmed by Rietveld refinement using X-ray powder diffraction data. Compounds are characterized by thermal analyses (TG-MS and SDTA), elemental analysis, IR spectra, and X-ray thermodiffraction analysis. Their visible photoluminescence properties are also discussed.

The intercalation of inorganic layered materials has resulted in a wide range of applicability. In such cases the applicability of the material is largely dependent upon the species intercalated within the layer, and the layered material acts largely as a host. Recently, the surface modification of inorganic layered materials has been investigated and it has been shown that the exterior layers can be exclusively functionalized. The advent of surface chemistry allows for the synthesis of particles with both a controlled interlayer and surface. This approach can be used to tailor nanoparticles for specific applications. Herein we review the surface chemistry of α-zirconium bis(monohydrogen orthophosphate) monohydrate (Zr(HPO4)2·H2O, α-ZrP) along with some applications of recent interest. Not only can these reactions be applied to α-ZrP, but similar chemistry can also be expanded to other layered materials and systems.

An aluminium carboxymethylphosphonate of composition (NH4)2Al(H1/2O3PCH2CO2)2 has been prepared hydrothermally. The aluminium cation is chelated by six membered rings formed from bonding by both the carboxylate and phosphonate oxygens. These chelate rings in turn form larger eight membered rings by connecting to similar chelate groups to form chains running along the a-axis.