Rigid PN cages as 3-dimensional building blocks for crystalline or amorphous networked materials.

Three-dimensional covalent connectors are valuable synthons for accessing crystalline or amorphous networks. Currently, fused polycyclic alkanes are employed as connectors in this context. We debut phosphorus-nitrogen (PN) cages as new 3-dimensional (3-D) inorganic connectors that yield crystalline and amorphous networks, including examples with gas porosity. We show that the high tunability of PN cages accelerates network diversification and the presence of a responsive 31P NMR spectroscopic handle provides structural insight. Collectively, this work unlocks a new and convenient 3-D synthon for reticular chemistry.

A Robust, Divalent, Phosphaza-bicyclo[2.2.2]octane Connector Provides Access to Cage-Dense Inorganic Polymers and Networks.

While polymers containing chain or ring motifs in their backbone are ubiquitous, those containing well-defined molecular cages are very rare and essentially unknown for the inorganic elements. We report that a rigid and dinucleophilic cage (PNSiMe3)2(NMe)6, which is chemically robust and accessible on a multi-gram scale from commercial precursors, serves as a linear and divalent connector that forms cage-dense inorganic materials. Reaction of the cage with various ditopic P(III) dihalide comonomers proceeded via Me3SiCl elimination to give high molecular weight (30 000-70 000 g mol-1), solution-processable polymers that form free-standing films. The end groups of the polymers could be tuned to engender orthogonal reactivity and form block copolymers. Networked cage-dense materials could be accessed by using PCl3 as a tritopic P(III) linker. Detailed mechanistic studies implicate a stepwise polycondensation that proceeds via phosphino-phosphonium ion intermediates, prior to Me3SiCl loss. Thus, metathesis between the dinucleophilic cage and polyhalides represents a general strategy to making cage-dense polymers, setting the stage for systematically understanding the consequences of the three-dimensional microstructure on macroscopic material properties.

(PNSiMe3 )4 (NMe)6 : A Robust Tetravalent Phosphaza-adamantane Scaffold for Molecular and Macromolecular Construction.

Tetraarylmethanes and adamantanes are important rigid covalent connectors that play a four-way scaffolding role in molecular and materials chemistry. We report the synthesis of a new tetravalent phosphaza-adamantane cage, (PNSiMe3 )4 (NMe)6 (2), that shows high thermal, air, and redox stability due to its geometry. It nevertheless participates in covalent four-fold functionalization reactions along its periphery. The combination of a robust core and reactive corona makes 2 a convenient inorganic scaffold upon which tetrahedral molecular and macromolecular chemistry can be constructed. This potential is demonstrated by the synthesis of a tetrakis(bis(phosphine)iminium) ion (in compound 3) and the first all P/N poly(phosphazene) network (5).

A New Solid-State Proton Conductor: The Salt Hydrate Based on Imidazolium and 12-Tungstophosphate.

We report the structure and charge transport properties of a novel solid-state proton conductor obtained by acid-base chemistry via proton transfer from 12-tungstophosphoric acid to imidazole. The resulting material (henceforth named Imid3WP) is a solid salt hydrate that, at room temperature, includes four water molecules per structural unit. To our knowledge, this is the first attempt to tune the properties of a heteropolyacid-based solid-state proton conductor by means of a mixture of water and imidazole, interpolating between water-based and ionic liquid-based proton conductors of high thermal and electrochemical stability. The proton conductivity of Imid3WP·4H2O measured at truly anhydrous conditions reads 0.8 × 10-6 S cm-1 at 322 K, which is higher than the conductivity reported for any other related salt hydrate, despite the lower hydration. In the pseudoanhydrous state, that is, for Imid3WP·2H2O, the proton conductivity is still remarkable and, judging from the low activation energy (Ea = 0.26 eV), attributed to structural diffusion of protons. From complementary X-ray diffraction data, vibrational spectroscopy, and solid-state NMR experiments, the local structure of this salt hydrate was resolved, with imidazolium cations preferably orienting flat on the surface of the tungstophosphate anions, thus achieving a densely packed solid material, and water molecules of hydration that establish extremely strong hydrogen bonds. Computational results confirm these structural details and also evidence that the path of lowest energy for the proton transfer involves primarily imidazole and water molecules, while the proximate Keggin anion contributes with reducing the energy barrier for this particular pathway.

Heavy Metals Make a Chain: A Catenated Bismuth Compound.

Catenation is common for the light main-group elements whereas it is rare for the heavy elements. Herein, we report the first example of a neutral molecule containing a Bi4 chain. It is prepared in a one-step reaction between bismuth trichloride and bis(diisopropylphosphino)amine in methanol suspension. The same reaction carried out in dichloromethane gives quite different products. All products have been characterized spectroscopically and using single-crystal X-ray analysis.

The influence of hydrofluoric acid etching processes on the photocatalytic hydrogen evolution reaction using mesoporous silicon nanoparticles.

H2 has been identified as one of the potential energy vectors that can provide a sustainable energy supply when produced through solar-driven water-splitting reaction. Si is the second most abundant element in the Earth's crust and can absorb a significant fraction of the solar spectrum while presenting little toxicity risk, making it an attractive material for photocatalytic H2 production. Hydrogen-terminated mesoporous Si (mp-Si) nanoparticles can be utilized to effectively drive the hydrogen evolution reaction using UV-to-visible light. In this work, the response of the photocatalytic activity of mp-Si nanoparticles to a series of HF acid treatments was investigated. A two-step magnesiothermic reduction method was used to prepare crystalline mp-Si nanoparticles with a specific surface area of 573 m2 g-1. The HF etching process was optimized as a function of the amount of acid added and the reaction time. The reaction time did not influence the H2 evolution rate substantially. However, the amount of HF used did have a significant effect on the photocatalytic activity. In the presence of ≥1.0 mL HF acid per 0.010 g of Si, morphological damage was observed using electron microscopy. N2 adsorption measurements indicated that the pore size and surface area were also altered. Solution-phase 19F{1H} NMR studies indicated the formation of SiF5- and SiF62- when larger volumes of HF were used. Both factors, morphological damage and the presence of byproducts in the pores, likely result in a lowering of the photocatalytic H2 evolution rate. Under the optimized HF treatment conditions (0.5 mL of HF per 0.010 g of Si), a H2 evolution rate of 1398 ± 30 µmol g-1 h-1 was observed.

Solid-state nuclear magnetic resonance investigation of synthetic phlogopite and lepidolite samples.

In the present work, our aim is to decipher the cationic ordering in the octahedral and tetrahedral sheets of two Al-rich synthetic materials, namely, phlogopites of nominal composition K(Mg3-x Alx )[Al1+x Si3-x O10 ](OH)y F2-y and lepidolites in the system trilithionite-polylithionite with composition K (Lix Al3-x )[Al4-2x Si2x O10 ](OH)y F2-y , by directly probing the aluminium distribution through 27 Al and 17 O magic-angle spinning, multiple-quantum magic-angle spinning, and 27 Al-27 Al double-quantum single-quantum nuclear magnetic resonance (NMR) experiments. Notably, 27 Al-27 Al double-quantum single-quantum magic-angle spinning NMR spectra, recorded at 9.34 and/or 20.00 T, show the spatial proximity or avoidance of the Al species inside or between the sheets. In both studied minerals, the ensemble of NMR data suggests a preference for [4] Al in the tetrahedral sheet to occupy position close to the [6] Al of the octahedral sheets.

Periodicity in Structure, Bonding, and Reactivity for p-Block Complexes of a Geometry Constraining Triamide Ligand.

The use of pincer ligands to access non-VSEPR geometries at main-group centers is an emerging strategy for eliciting new stoichiometric and catalytic reactivity. As part of this effort, several different tridentate trianionic substituents have to date been employed at a range of different central elements, providing a patchwork dataset that precludes rigorous structure-function correlation. An analysis of periodic trends in structure (solid, solution, and computation), bonding, and reactivity based on systematic variation of the central element (P, As, Sb, or Bi) with retention of a single tridentate triamide substituent is reported herein. In this homologous series, the central element can adopt either a bent or planar geometry. The tendency to adopt planar geometries increases descending the group with the phosphorus triamide (1) and its arsenic congener (2) exhibiting bent conformations, and the antimony (3) and bismuth (4) analogues exhibiting a predominantly planar structure in solution. This trend has been rationalized using an energy decomposition analysis. A rare phase-dependent dynamic covalent dimerization was observed for 3 and the associated thermodynamic parameters were established quantitatively. Planar geometries were found to engender lower LUMO energies and smaller band gaps than bent ones, resulting in different reactivity patterns. These results provide a benchmark dataset to guide further research in this rapidly emerging area.

A Redox-Confused Bismuth(I/III) Triamide with a T-Shaped Planar Ground State.

Reaction of a tethered triamine ligand with Bi(NMe2 )3 gives a Bi triamide, for which a BiI electronic structure is shown to be most appropriate. The T-shaped geometry at bismuth provides the first structural model for edge inversion in bismuthines and the only example of a planar geometry for pnictogen triamides. Analogous phosphorus compounds exhibit a distorted pyramidal geometry because of different Bi-N and P-N bond polarities. Although considerable BiI character is indicated for the title Bi triamide, it exhibits reactivity similar to BiIII electrophiles, and expresses either a vacant or a filled p orbital at Bi, as evidenced by coordination of either pyridine N-oxide or W(CO)5 . The product of the former shows evidence of coordination-induced oxidation state change at bismuth.

Amorphous Quantum Nanomaterials.

In quantum materials, macroscopic behavior is governed in nontrivial ways by quantum phenomena. This is usually achieved by exquisite control over atomic positions in crystalline solids. Here, it is demonstrated that the use of disordered glassy materials provides unique opportunities to tailor quantum material properties. By borrowing ideas from single-molecule spectroscopy, single delocalized π-electron dye systems are isolated in relatively rigid ultrasmall (<10 nm diameter) amorphous silica nanoparticles. It is demonstrated that chemically tuning the local amorphous silica environment around the dye over a range of compositions enables exquisite control over dye quantum behavior, leading to efficient probes for photodynamic therapy (PDT) and stochastic optical reconstruction microscopy (STORM). The results suggest that efficient fine-tuning of light-induced quantum behavior mediated via effects like spin-orbit coupling can be effectively achieved by systematically varying averaged local environments in glassy amorphous materials as opposed to tailoring well-defined neighboring atomic lattice positions in crystalline solids. The resulting nanoprobes exhibit features proven to enable clinical translation.

A long-chain protic ionic liquid inside silica nanopores: enhanced proton mobility due to efficient self-assembly and decoupled proton transport.

We report enhanced protonic and ionic dynamics in an imidazole/protic ionic liquid mixture confined within the nanopores of silica particles. The ionic liquid is 1-octylimidazolium bis(trifluoromethanesulfonyl)imide ([HC8Im][TFSI]), while the silica particles are microsized and characterized by internal well connected nanopores. We demonstrate that the addition of imidazole is crucial to promote a proton motion decoupled from molecular diffusion, which occurs due to the establishment of new N-HN hydrogen bonds and fast proton exchange events in the ionic domains, as evidenced by both infrared and 1H NMR spectroscopy. An additional reason for the decoupled motion of protons is the nanosegregated structure adopted by the liquid imidazole/[HC8Im][TFSI] mixture, with segregated polar and non-polar nano-domains, as clearly shown by WAXS data. This arrangement, promoted by the length of the octyl group and thus by significant chain-chain interactions, reduces the mobility of molecules (Dmol) more than that of protons (DH), which is manifested by DH/Dmol ratios greater than three. Once included into the nanopores of hydrophobic silica microparticles, the nanostructure of the liquid mixture is preserved with slightly larger ionic domains, but effects on the non-polar ones are unclear. This results in a further enhancement of proton motion with localised paths of conduction. These findings demonstrate significant progress in the design of proton conducting materials via tailor-made molecular structures as well as by smart exploitation of confinement effects. Compared to other imidazole-based proton conducting materials that are crystalline up to 90 °C or above, the gel materials that we propose are useful for applications at room temperature, and can thus find applications in e.g. intermediate temperature proton exchange fuel cells.

125Te NMR Probes of Tellurium Oxide Crystals: Shielding-Structure Correlations.

The local environments around tellurium atoms in a series of tellurium oxide crystals were probed by 125Te solid-state NMR spectroscopy. Crystals with distinct TeOn units (n from 3 to 6), including Na2TeO3, α-TeO2 and γ-TeO2, Te2O(PO4)2, K3LaTe2O9, BaZnTe2O7, and CsYTe3O8 were studied. The latter four were synthesized through a solid-state process. X-ray diffraction was used to confirm the successful syntheses. The 125Te chemical shift was found to exhibit a strong linear correlation with the Te coordination number. The 125Te chemical-shift components (δ11, δ22, and δ33) of the TeO4 units were further correlated to the O-Te-O-bond angles. With the aid of 125Te NMR, it is likely that these relations can be used to estimate the coordination states of Te atoms in unknown Te crystals and glasses.

Mono- and Bis(imidazolidinium ethynyl) Cations and Reduction of the Latter To Give an Extended Bis-1,4-([3]Cumulene)-p-carboquinoid System.

An extended π-system containing two [3]cumulene fragments separated by a p-carboquinoid and stabilized by two capping N-heterocyclic carbenes (NHCs) has been prepared. Mono- and bis(imidazolidinium ethynyl) cations have also been synthesized from the reaction of an NHC with phenylethynyl bromide or 1,4-bis(bromoethynyl)benzene. Cyclic voltammetry coupled with synthetic and structural studies showed that the dication is readily reduced to a neutral, singlet bis-1,4-([3]cumulene)-p-carboquinoid as a result of the π-accepting properties of the capping NHCs.

Effect of boron oxide addition on the viscosity-temperature behaviour and structure of phosphate-based glasses.

In this study, nine phosphate-based glass formulations from the system P2 O5 -CaO-Na2 O-MgO-B2 O3 were prepared with P2 O5 content fixed as 40, 45 and 50 mol%, where Na2 O was replaced by 5 and 10 mol% B2 O3 and MgO and CaO were fixed to 24 and 16 mol%, respectively. The effect of B2 O3 addition on the viscosity-temperature behaviour, fragility index and structure of the glasses was investigated. The composition of the glasses was confirmed by ICP-AES. The viscosity-temperature behaviour of the glasses were measured using beam-bending and parallel -plate viscometers. The viscosity of the glasses investigated was found to shift to higher temperature with increasing B2 O3 content. The kinetic fragility parameter, m and F1/2 , estimated from the viscosity curve were found to decease with increasing B2 O3 content. The structural analysis was achieved by a combination of Fourier transform infrared spectroscopy and solid state nuclear magnetic resonance. 31 P solid-state magic-angle-spinning nuclear magnetic resonance (MAS-NMR) showed that the local structure of the glasses changes with increasing B2 O3 content. As B2 O3 was added to the glass systems, the phosphate connectivity increases as the as the Q1 units transforms into Q2 units. The 11 B NMR results confirmed the presence of tetrahedral boron (BO4 ) units for all the compositions investigated. Structural analysis indicates an increasing level of cross-linking with increasing B2 O3 content. Evidence of the presence of P-O-B bonds was also observed from the FTIR and 31 P NMR analysis. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 764-777, 2017.

Phase transformations during processing and in vitro degradation of porous calcium polyphosphates.

A 2-Step sinter/anneal treatment has been reported previously for forming porous CPP as biodegradable bone substitutes [9]. During the 2-Step annealing treatment, the heat treatment used strongly affected the rate of CPP degradation in vitro. In the present study, x-ray diffraction and (31)P solid state nuclear magnetic resonance were used to determine the phases that formed using different heat treating processes. The effect of in vitro degradation (in PBS at 37 °C, pH 7.1 or 4.5) was also studied. During CPP preparation, ß-CPP and γ-CPP were identified in powders formed from a calcium monobasic monohydrate precursor after an initial calcining treatment (10 h at 500 °C). Melting of this CPP powder (at 1100 °C), quenching and grinding formed amorphous CPP powders. Annealing powders at 585 °C (Step-1) resulted in rapid sintering to form amorphous porous CPP. Continued annealing to 650 °C resulted in crystallization to form a multi-phase structure of ß-CPP primarily plus lesser amounts of α-CPP, calcium ultra-phosphates and retained amorphous CPP. Annealing above 720 °C and up to 950 °C transformed this to ß-CPP phase. In vitro degradation of the 585 °C (Step-1 only) and 650 °C Step-2 annealed multi-phase samples occurred significantly faster than the ß-CPP samples formed by Step-2 annealing at or above 720 °C. This faster degradation was attributable to preferential degradation of thermodynamically less stable phases that formed in samples annealed at 650 °C (i.e. α-phase, ultra-phosphate and amorphous CPP). Degradation in lower pH solutions significantly increased degradation rates of the 585 and 650 °C annealed samples but had no significant effect on the ß-CPP samples.

Stimuli-Responsive Shapeshifting Mesoporous Silica Nanoparticles.

Stimuli-responsive materials have attracted great interest in catalysis, sensing, and drug delivery applications and are typically constituted by soft components. We present a one-pot synthetic method for a type of inorganic silica-based shape change material that is responsive to water vapor exposure. After the wetting treatment, the cross-sectional shape of aminated mesoporous silica nanoparticles (MSNs) with hexagonal pore lattice changed from hexagonal to six-angle-star, accompanied by the loss of periodic mesostructural order. Nitrogen sorption measurements suggested that the wetting treatment induced a shrinkage of mesopores resulting in a broad size distribution and decreased mesopore volume. Solid-state (29)Si nuclear magnetic resonance (NMR) spectroscopy of samples after wetting treatment displayed a higher degree of silica condensation, indicating that the shape change was associated with the formation of more siloxane bonds within the silica matrix. On the basis of material characterization results, a mechanism for the observed anisotropic shrinkage is suggested based on a buckling deformation induced by capillary forces in the presence of a threshold amount of water vapor available beyond a humidity of about 50%. The work presented here may open a path toward novel stimuli-responsive materials based on inorganic components.

Stimulation of apoptotic pathways in liver cancer cells: An alternative perspective on the biocompatibility and the utility of biomedical glasses.

A host of research opportunities with innumerable clinical applications are open to biomedical glasses if one considers their potential as therapeutic inorganic ion delivery systems. Generally, applications have been limited to repair and regeneration of hard tissues while compositions are largely constrained to the original bioactive glass developed in the 1960s. However, in oncology applications the therapeutic paradigm shifts from repair to targeted destruction. With this in mind, the composition-structure-property-function relationships of vanadium-containing zinc-silicate glasses (0.51SiO2-0.29Na2O-(0.20-X)ZnO-XV2O5, 0 ≤ X ≤ 0.09) were characterized in order to determine their potential as therapeutic inorganic ion delivery systems. Increased V2O5mole fraction resulted in a linear decrease in density and glass transition temperature (Tg).(29)Si MAS NMR peak maxima shifted upfield while(51)V MAS NMR peak maxima were independent of V2O5content and overlapped well with the spectra NaVO3 Increased V2O5mole fraction caused ion release to increase. When human liver cancer cells, HepG2, were exposed to these ions they demonstrated a concentration-dependent cytotoxic response, mediated by apoptosis. This work demonstrates that the zinc-silicate system studied herein is capable of delivering therapeutic inorganic ions at concentrations that induce apoptotic cell death and provide a simple means to control therapeutic inorganic ion delivery.

Composition-structure-properties relationship of strontium borate glasses for medical applications.

We have synthesized TiO2 doped strontium borate glasses, 70B2O3-(30-x)SrO-xTiO2 and 70B2 O3 -20SrO(10-x)Na2 O-xTiO2 . The composition dependence of glass structure, density, thermal properties, durability, and cytotoxicity of degradation products was studied. Digesting the glass in mineral acid and detecting the concentrations of various ions using an ICP provided the actual compositions that were 5-8% deviated from the theoretical values. The structure was investigated by means of (11)B magic angle spinning (MAS) NMR spectroscopy. DSC analyses provided the thermal properties and the degradation rates were measured by measuring the weight loss of glass disc-samples in phosphate buffered saline at 37°C in vitro. Finally, the MTT assay was used to analyze the cytotoxicity of the degradation products. The structural analysis revealed that replacing TiO2 for SrO or Na2 O increased the BO3/BO4 ratio suggesting the network-forming role of TiO2 . Thermal properties, density, and degradation rates also followed the structural changes. Varying SrO content predominantly controlled the degradation rates, which in turn controlled the ion release kinetics. A reasonable control (2-25% mass loss in 21 days) over mass loss was achieved in current study. Even though, very high concentrations (up to 5500 ppm B, and 1200 ppm Sr) of ions were released from the ternary glass compositions that saturated the degradation media in 7 days, the degradation products from ternary glass system was found noncytotoxic. However, quaternary glasses demonstrated negative affect on cell viability due to very high (7000 ppm) Na ion concentration. All the glasses investigated in current study are deemed fast degrading with further control over degradation rates, release kinetics desirable.

Composition-structure-property relationships for non-classical ionomer cements formulated with zinc-boron germanium-based glasses.

Non-classical ionomer glasses like those based on zinc-boron-germanium glasses are of special interest in a variety of medical applications owning to their unique combination of properties and potential therapeutic efficacy. These features may be of particular benefit with respect to the utilization of glass ionomer cements for minimally invasive dental applications such as the atruamatic restorative treatment, but also for expanded clinical applications in orthopedics and oral-maxillofacial surgery. A unique system of zinc-boron-germanium-based glasses (10 compositions in total) has been designed using a Design of Mixtures methodology. In the first instance, ionomer glasses were examined via differential thermal analysis, X-ray diffraction, and (11)B MAS NMR spectroscopy to establish fundamental composition - structure-property relationships for the unique system. Secondly, cements were synthesized based on each glass and handling characteristics (working time, Wt, and setting time, St) and compression strength were quantified to facilitate the development of both experimental and mathematical composition-structure-property relationships for the new ionomer cements. The novel glass ionomer cements were found to provide Wt, St, and compression strength in the range of 48-132 s, 206-602 s, and 16-36 MPa, respectively, depending on the ZnO/GeO2 mol fraction of the glass phase. A lower ZnO mol fraction in the glass phase provides higher glass transition temperature, higher N4 rate, and in combination with careful modulation of GeO2 mol fraction in the glass phase provides a unique approach to extending the Wt and St of glass ionomer cement without compromising (in fact enhancing) compression strength. The data presented in this work provide valuable information for the formulation of alternative glass ionomer cements for applications within and beyond the dental clinic, especially where conventional approaches to modulating working time and strength exhibit co-dependencies (i.e. the enhancement of one property comes at the expense of the other) and therefore limit development strategies.

A simple complex on the verge of breakdown: isolation of the elusive cyanoformate ion.

Why does cyanide not react destructively with the proximal iron center at the active site of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase, an enzyme central to the biosynthesis of ethylene in plants? It has long been postulated that the cyanoformate anion, [NCCO2](-), forms and then decomposes to carbon dioxide and cyanide during this process. We have now isolated and crystallographically characterized this elusive anion as its tetraphenylphosphonium salt. Theoretical calculations show that cyanoformate has a very weak C-C bond and that it is thermodynamically stable only in low dielectric media. Solution stability studies have substantiated the latter result. We propose that cyanoformate shuttles the potentially toxic cyanide away from the low dielectric active site of ACC oxidase before breaking down in the higher dielectric medium of the cell.