Bioinspired Insights into Silicic Acid Stabilization Mechanisms: The Dominant Role of Polyethylene Glycol-Induced Hydrogen Bonding

Mono- and disilicic acids were stabilized by uncharged polyethylene glycols (PEGs) in silica-supersaturated solutions (the starting solution contained 500 ppm/8.3 mM sodium orthosilicate, Na₂SiO₃·5H₂O, expressed as SiO₂) at pH = 7, most likely by hydrogen bonding between the silanol groups and −CH₂–CH₂–O–ether moieties. The stabilization was monitored by measuring molybdate-reactive silica and also by a combination of liquid- and solid-state ²⁹Si NMR spectroscopy. It depends on PEG concentration (20–100 ppm) and molecular weight (1550–20 000 Da). Two narrow ²⁹Si NMR signals characteristic for monosilicic acid (Q⁰) and disilicic acid (Q¹) can be observed in ²⁹Si NMR spectra of solutions containing PEG 10000 with intensities distinctly higher than the control, that is, in the absence of PEG. Silica-containing precipitates are observed in the presence of PEG, in contrast to the gel formed in the absence of PEG. These precipitates exhibit similar degrees of silica polycondensation as found in the gel as can be seen from the ²⁹Si MAS NMR spectra. However, the ²D HETCOR spectra show different ¹H NMR signal shifts: The signal due to H-bonded SiOH/H₂O, which is found at 6 ppm in the control, is shifted to ∼7 ppm in the PEG-containing precipitate. This indicates the formation of slightly stronger H-bonds than in the control sample, most likely between PEG and the silica species. The presence of PEG in these precipitates is unequivocally proven by ¹³C CP MAS NMR spectroscopy. The ¹³C signal of PEG significantly shifts and is much narrower in the precipitates as compared to the pristine PEG, indicating that PEG is embedded into the silica or at least bound to its surface (or both), and not phase separated. FT-IR spectra corroborate the above arguments. The H-bonding between silanol and ethereal O perturbs the band positions attributed to vibrations involving the O atom. This work may invoke an alternative way to envision silica species stabilization (prior to biosilica formation) in diatoms by investigating possible scenarios of uncharged biomacromolecules playing a role in biosilica synthesis

Influence of Polyamines and Related Macromolecules on Silicic Acid Polycondensation: Relevance to “Soluble Silicon Pools”?

The influence of a number of N-containing macromolecules on the polycondensation of silicic acid to form amorphous silica is studied by the combined use of ²⁹Si NMR spectroscopy and the silicomolybdate test. Polymeric additives include poly(allylamine hydrochloride) (PAH), the poly(aminoamide) dendrimer of generation 1 (PAMAM-1), poly(ethyleneimine) (PEI), and poly(vinylpyrrolidone) (PVP). These studies were performed under biologically relevant conditions (pH 5.4 and 7.0) using aqueous solutions of isotope-labeled sodium [²⁹Si]metasilicate as the precursor compound. It was found at pH 5.4 that all additives accelerate silicic acid polycondensation, except for PVP, which exerts a minor silicic acid stabilizing effect. At pH 7.0, polycondensation is much faster in the presence of PAMAM-1, PEI, and PAH. However, PVP significantly stabilizes mono- and disilicic acid. Silica precipitates were also studied by ²⁹Si NMR spectroscopy. The effect observed for PVP is striking and indicates that the silicic acid polycondensation is slowed, although the oligomers are immobilized by the PVP polymer. In contrast, the charged PAH attracts the oligomeric species and enhances the silicic acid polycondensation

Structural Systematics and Topological Analysis of Coordination Polymers with Divalent Metals and a Glycine-Derived Tripodal Phosphonocarboxylate

A novel family of four hybrid metal phosphonate coordination polymers is reported that are constructed from divalent metal ions (Ca, Sr, Ba, and Pb) and <b>BPMGLY</b> (bis-phosphonomethylglycine, a phosphonated derivative of glycine). These compounds (and their compositions) are <b>Ca-BPMGLY</b> (CaBPMGLY·H₂O), <b>Sr-BPMGLY</b> (SrBPMGLY·H₂O), <b>Ba-BPMGLY</b> (Ba_3.5(BPMGLY)₂·6H₂O), and <b>Pb-BPMGLY</b> (PbBPMGLY·H₂O). They were obtained by hydrothermal reactions in acidic aqueous solutions (pH range 2.3–5.7) and fully characterized by physicochemical methods and structural analysis. <b>Ca-BPMGLY</b>, <b>Sr-BPMGLY</b>, and <b>Pb-BPMGLY</b> have very similar 3D coordination polymer structures, and the latter two are isostructural. In contrast to the Ca, Sr, and Pb analogs, <b>Ba-BPMGLY</b> possesses a different 2D layered network. These four new compounds, together with our previously reported 2D coordination polymer <b>Mg-BPMGLY</b> (MgBPMGLY·2H₂O, Demadis et al.<i> Inorg. Chem.</i> <b>2012</b>, 51, 7889–7896), were topologically classified revealing (i) the uninodal 3-connected net with the <b>hcb</b> topology in <b>Mg-BPMGLY</b>, (ii) the uninodal 5-connected nets with the <b>bnn</b> and <b>vbj</b> topology in <b>Ca-BPMGLY</b> and <b>Sr-BPMGLY</b>, respectively, and (iii) the very complex topologically unique hexanodal 4,4,6,6,7,8-connected net in <b>Ba-BPMGLY</b>. The <b>vbj</b> topology was also identified in the related <b>Pb-BPMGLY</b> 3D framework. These topological features show that the complexity of BPMGLY-driven 2D and 3D metal–organic networks increases periodically following the Mg < Ca ≤ Sr ≪ Ba trend

Disruption of “Coordination Polymer” Architecture in Cu²⁺ Bis-Phosphonates and Carboxyphosphonates by Use of 2,2′-Bipyridine as Auxiliary Ligand: Structural Variability and Topological Analysis

The outcome of a synthesis involving a metal ion and a (poly)­phosphonic acid depends on a plethora of variables such as solution pH, reactant molar ratios, nature of the metal ion, number of phosphonate groups, and other “functional” moieties present on the ligand backbone. Products are usually coordination polymers of diverse dimensionality. Here we report that the use of a chelating auxiliary ligand (2,2′-bpy) can “disrupt” the polymeric architecture of the copper phosphonate, causing the isolation of a series of molecular complexes (mononuclear or binuclear) that incorporate both the phosphonate and the 2,2′-bpy ligands. Synthetic details, crystal structures, and intermolecular interactions (π–π stacking and hydrogen bonding) are discussed. The structures of the obtained Cu complexes are extended into 2D or 3D networks via multiple hydrogen bonds involving the molecular units and crystallization water molecules. These H-bonded networks have been classified from the topological viewpoint, revealing diverse topologies that also include their undocumented types

2D Corrugated Magnesium Carboxyphosphonate Materials: Topotactic Transformations and Interlayer “Decoration” with Ammonia

In this paper we report the synthesis and structural characterization of the 2D layered coordination polymer Mg­(BPMGLY)­(H₂O)₂ (BPMGLY = bis-phosphonomethylglycine, (HO₃PCH₂)₂N­(H)­COO^2–). The Mg ion is found in a slightly distorted octahedral environment formed by four phosphonate oxygens and two water molecules. The carboxylate group is deprotonated but noncoordinated. This compound is a useful starting material for a number of topotactic transformations. Upon heating at 140 °C one (of the two) Mg-coordinated water molecule is lost, with the archetype 2D structure maintaining itself. However, the octahedral Mg in Mg­(BPMGLY)­(H₂O)₂ is now converted to trigonal bipyramidal in Mg­(BPMGLY)­(H₂O). Upon exposure of the monohydrate Mg­(BPMGLY)­(H₂O) compound to ammonia, one molecule of ammonia is inserted into the interlayer space and stabilized by hydrogen bonding. The 2D layered structure of the product Mg­(BPMGLY)­(H₂O)­(NH₃) is still maintained, with Mg now acquiring a pseudo-octahedral environment. All of these topotactic transformations are also accompanied by changes in hydrogen bonding between the layers

Linking ³¹P Magnetic Shielding Tensors to Crystal Structures: Experimental and Theoretical Studies on Metal(II) Aminotris(methylenephosphonates)

The ³¹P chemical shift tensor of the phosphonate group [RC-PO₂(OH)]⁻ is investigated with respect to its principal axis values and its orientation in a local coordinate system (LCS) defined from the P atom and the directly coordinated atoms. For this purpose, six crystalline metal aminotris­(methylenephosphonates), <i>M</i>AMP·<i>x</i>H₂O with <i>M</i> = Zn, Mg, Ca, Sr, Ba, and (2Na) and <i>x</i> = 3, 3, 4.5, 0, 0, and 1.5, respectively, were synthesized and identified by diffraction methods. The crystal structure of water-free BaAMP is described here for the first time. The principal components of the ³¹P shift tensor were determined from powders by magic-angle-spinning NMR. Peak assignments and orientations of the chemical shift tensors were established by quantum-chemical calculations from first principles using the extended embedded ion method. Structure optimizations of the H-atom positions were necessary to obtain the chemical shift tensors reliably. We show that the ³¹P tensor orientation can be predicted within certain error limits from a well-chosen LCS, which reflects the pseudosymmetry of the phosphonate environment

An Unusual Michael-Induced Skeletal Rearrangement of a Bicyclo[3.3.1]nonane Framework of Phloroglucinols to a Novel Bioactive Bicyclo[3.3.0]octane

A novel skeletal rearrangement of bicyclo[3.3.1]nonane-2,4,9-trione (<b>16</b>) to an unprecedented highly functionalized bicyclo[3.3.0]octane system (<b>17</b>), induced by an intramolecular Michael addition, is presented. This novel framework was found to be similarly active to hyperforin (<b>1</b>), against PC-3 cell lines. A mechanistic study was examined in detail, proposing a number of cascade transformations. Also, reactivity of the Δ^7,10-double bond was examined under several conditions to explain the above results

An Unusual Michael-Induced Skeletal Rearrangement of a Bicyclo[3.3.1]nonane Framework of Phloroglucinols to a Novel Bioactive Bicyclo[3.3.0]octane

A novel skeletal rearrangement of bicyclo[3.3.1]nonane-2,4,9-trione (<b>16</b>) to an unprecedented highly functionalized bicyclo[3.3.0]octane system (<b>17</b>), induced by an intramolecular Michael addition, is presented. This novel framework was found to be similarly active to hyperforin (<b>1</b>), against PC-3 cell lines. A mechanistic study was examined in detail, proposing a number of cascade transformations. Also, reactivity of the Δ^7,10-double bond was examined under several conditions to explain the above results

Tuning Proton Conductivity in Alkali Metal Phosphonocarboxylates by Cation Size-Induced and Water-Facilitated Proton Transfer Pathways

The structural and functional chemistry of a family of alkali-metal ions with racemic <i>R</i>,<i>S</i>-hydroxyphosphonoacetate (<b>M-HPAA</b>; M = Li, Na, K, Cs) are reported. Crystal structures were determined by X-ray data (Li⁺, powder diffraction following an ab initio methodology; Na⁺, K⁺, Cs⁺, single crystal). A gradual increase in dimensionality directly proportional to the alkali ionic radius was observed. [Li₃(OOCCH­(OH)­PO₃)­(H₂O)₄]·H₂O (<b>Li-HPAA</b>) shows a 1D framework built up by Li-ligand “slabs” with Li⁺ in three different coordination environments (4-, 5-, and 6-coordinated). <b>Na-HPAA</b>, Na₂(OOCCH­(OH)­PO₃H)­(H₂O)₄, exhibits a pillared layered “house of cards” structure, while <b>K-HPAA</b>, K₂(OOCCH­(OH)­PO₃H)­(H₂O)₂, and <b>Cs-HPAA</b>, Cs­(HOOCCH­(OH)­PO₃H), typically present intricate 3D frameworks. Strong hydrogen-bonded networks are created even if no water is present, as is the case in <b>Cs-HPAA</b>. As a result, all compounds show proton conductivity in the range 3.5 × 10^–5 S cm^–1 (<b>Cs-HPAA</b>) to 5.6 × 10^–3 S cm^–1 (<b>Na-HPAA</b>) at 98% RH and <i>T</i> = 24 °C. Differences in proton conduction mechanisms, Grothuss (Na⁺ and Cs⁺) or vehicular (Li⁺ and K⁺), are attributed to the different roles played by water molecules and/or proton transfer pathways between phosphonate and carboxylate groups of the ligand HPAA. Upon slow crystallization, partial enrichment in the <i>S</i> enantiomer of the ligand is observed for <b>Na-HPAA</b>, while the <b>Cs-HPAA</b> is a chiral compound containing only the <i>S</i> enantiomer

Structural Variability in Multifunctional Metal Xylenediaminetetraphosphonate Hybrids

Two new families of divalent metal hybrid derivatives from the aromatic tetraphosphonic acids 1,4- and 1,3-<i>bis</i>(aminomethyl)­benzene-<i>N</i>,<i>N</i>′-<i>bis</i>(methylenephosphonic acid), (H₂O₃PCH₂)₂–N–CH₂C₆H₄CH₂–N­(CH₂PO₃H₂)₂ (designated herein as <b><i>p</i>-H₈L</b> and <b><i>m</i>-H₈L</b>) have been synthesized by crystallization at room temperature and hydrothermal conditions. The crystal structures of M­[(HO₃PCH₂)₂N­(H)­CH₂C₆H₄CH₂N­(H)­(CH₂PO₃H)₂(H₂O)₂]·2H₂O (M = Mg, Co, and Zn), <b>M–(<i>p</i>-H₆L)</b>, and M­[(HO₃PCH₂)₂N­(H)­CH₂C₆H₄CH₂N­(H)­(CH₂PO₃H)₂]·<i>n</i>H₂O (M = Ca, Mg, Co, and Zn and <i>n</i> = 1–1.5), <b><b>M–(<i>m</i>-H₆L)</b></b>, were solved ab initio by synchrotron powder diffraction data using the direct methods and subsequently refined using the Rietveld method. The crystal structure of the isostructural <b><b>M–(<i>p</i>-H₆L)</b></b> is constituted by organic–inorganic monodimensional chains where the phosphonate moiety acts as a bidentate chelating ligand bridging two metal octahedra. <b><b>M–(<i>m</i>-H₆L)</b></b> compounds exhibit a 3D pillared open-framework with small 1D channels filled with water molecules. These channels are formed by the pillaring action of the organic ligand connecting adjacent layers through the phosphonate oxygens. Thermogravimetric and X-ray thermodiffraction analyses of <b><b>M–(<i>p</i>-H₆L)</b></b> showed that the integrity of their crystalline structures is maintained up to 470 K, without significant reduction of water content, while thermal decomposition takes place above 580 K. The utility of <b><b>M–(<i>p</i>-H₆L)</b></b> (M = Mg and Zn) hybrid materials in corrosion protection was investigated in acidic aqueous solutions. In addition, the impedance data indicate both families of compounds display similar proton conductivities (σ ∼ 9.4 × 10^–5 S·cm^–1, at 98% RH and 297 K), although different proton transfer mechanisms are involved