Symmetry and the Role of the Anion Sublattice in Aurivillius Oxyfluoride Bi2TiO4F2.

The photocatalytic and dielectric behaviors of Aurivillius oxyfluorides such as Bi2TiO4F2 depend sensitively on their crystal structure and symmetry but these are not fully understood. Our experimental work combined with symmetry analysis demonstrates the factors that influence anion order and how this might be tuned to break inversion symmetry. We explore an experimental approach to explore anion order, which combines Rietveld analysis with strain analysis.

Characterization of and Structural Insight into Struvite-K, MgKPO4·6H2O, an Analogue of Struvite.

Struvite-K (MgKPO4·6H2O) is a magnesium potassium phosphate mineral with naturally cementitious properties, which is finding increasing usage as an inorganic cement for niche applications including nuclear waste management and rapid road repair. Struvite-K is also of interest in sustainable phosphate recovery from wastewater and, as such, a detailed knowledge of the crystal chemistry and high-temperature behavior is required to support further laboratory investigations and industrial applications. In this study, the local chemical environments of synthetic struvite-K were investigated using high-field solid-state 25Mg and 39K MAS NMR techniques, alongside 31P MAS NMR and thermal analysis. A single resonance was present in each of the 25Mg and 39K MAS NMR spectra, reported here for the first time alongside the experimental and calculated isotropic chemical shifts, which were comparable to the available data for isostructural struvite (MgNH4PO4·6H2O). An in situ high-temperature XRD analysis of struvite-K revealed the presence of a crystalline-amorphous-crystalline transition that occurred between 30 and 350 °C, following the single dehydration step of struvite-K. Between 50 and 300 °C, struvite-K dehydration yielded a transient disordered (amorphous) phase identified here for the first time, denoted δ-MgKPO4. At 350 °C, recrystallization was observed, yielding ß-MgKPO4, commensurate with an endothermic DTA event. A subsequent phase transition to γ-MgKPO4 was observed on further heating, which reversed on cooling, resulting in the α-MgKPO4 structure stabilized at room temperature. This behavior was dissimilar from that of struvite exposed to high temperature, where NH4 liberation occurs at temperatures >50 °C, indicating that struvite-K could potentially withstand high temperatures via a transition to MgKPO4.

Framework Effects on Activation and Functionalisation of Methane in Zinc-Exchanged Zeolites.

The first selective oxidation of methane to methanol is reported herein for zinc-exchanged MOR (Zn/MOR). Under identical conditions, Zn/FER and Zn/ZSM-5 both form zinc formate and methanol. Selective methane activation to form [Zn-CH3 ]+ species was confirmed by 13 C MAS NMR spectroscopy for all three frameworks. The percentage of active zinc sites, measured through quantitative NMR spectroscopy studies, varied with the zeolite framework and was found to be ZSM-5 (5.7 %), MOR (1.2 %) and FER (0.5 %). For Zn/MOR, two signals were observed in the 13 C MAS NMR spectrum, resulting from two distinct [Zn-CH3 ]+ species present in the 12 MR and 8 MR side pockets, as supported by additional NMR experiments. The observed products of oxidation of the [Zn-CH3 ]+ species are shown to depend on the zeolite framework type and the oxidative conditions used. These results lay the foundation for developing structure-function correlations for methane conversion over zinc-exchanged zeolites.

Tweaking the Charge Transfer: Bonding Analysis of Bismuth(III) Complexes with a Flexidentate Phosphane Ligand.

To account for the charge transfer and covalent character in bonding between P and Bi centers, the electronic structures of [P(C6H4-o-CH2SCH3)3BiCln](3-n)+ (n = 0-3) model species have been investigated computationally. On the basis of this survey a synthetic target compound with a dative P→Bi bond has been selected. Consecutively, the highly reactive bismuth cage [P(C6H4-o-CH2SCH3)3Bi]3+ has been accessed experimentally and characterized. Importantly, our experiments (single-crystal X-ray diffraction and solid-state NMR spectroscopy) and computations (NBO and AIM analysis) reveal that the P···Bi bonding in this trication can be described as a dative bond. Here we have shown that our accordion-like molecular framework allows for tuning of the interaction between P and Bi centers.

A One-Pot Divergent Sequence to Pyrazole and Quinoline Derivatives.

The hydroxy-pyrazole and 3-hydroxy-oxindole motifs have been utilised in several pharma and agrochemical leads but are distinctly underrepresented in the scientific literature due to the limited routes of preparation. We have developed a one-pot procedure for their synthesis starting from simple isatins. The method employs cheap and easy-to-handle building blocks and allows easy isolation.

Weak Pnictogen Bond with Bismuth: Experimental Evidence Based on Bi-P Through-Space Coupling.

To study pnictogen bonding involving bismuth, flexible accordion-like molecular complexes of the composition [P(C6 H4 -o-CH2 SCH3 )3 BiX3 ], (X=Cl, Br, I) have been synthesised and characterised. The strength of the weak and mainly electrostatic interaction between the Bi and P centres strongly depends on the character of the halogen substituent on bismuth, which is confirmed by single-crystal X-ray diffraction analyses, DFT and ab initio computations. Significantly, 209 Bi-31 P through-space coupling (J=2560 Hz) is observed in solid-state 31 P NMR spectra, which is so far unprecedented in the literature, delivering direct information on the magnitude of this pnictogen interaction.

Tin and Lead Phosphanido Complexes: Reactivity with Chalcogens.

The reactivity of tin and lead phosphanido complexes with chalogens is reported. The addition of sulfur to [(BDI)MPCy2] (M = Sn, Pb; BDI = CH{(CH3)CN-2,6-iPr2C6H3}2) results in the formation of phosphinodithioates [(BDI)MSP(S)Cy2] regardless of the conditions; however, when selenium is added to [(BDI)MPCy2], a selenium insertion product, phosphinoselenoite [(BDI)MSePCy2], can be isolated. This compound readily reacts with additional selenium to form the phosphinodiselenoate complex [(BDI)MSeP(Se)Cy2]. In contrast, the addition of selenium to [(BDI)SnP(SiMe3)2] results in the formation of the heavy ether [(BDI)SnSeSiMe3]. Differences in the solution and solid-state molecular species of tin phosphinoselenoite and phosphinodiselenoate complexes were probed using multinuclear solution and solid-state NMR spectroscopy.

Characterization of Two Distinct Amorphous Forms of Valsartan by Solid-State NMR.

Valsartan (VAL) is an antihypertensive drug marketed in an amorphous form. Amorphous materials can have different physicochemical properties depending on preparation method, thermal history, etc., but the nature of such materials is difficult to study by diffraction techniques. This study characterizes two different amorphous forms of valsartan (AR and AM) using solid-state NMR (SSNMR) as a primary investigation tool, supported by solution-state NMR, FT-IR, TMDSC, and dissolution tests. The two forms are found to be clearly distinct, with a significantly higher level of structural arrangement in the AR form, as observed in (13)C, (15)N, and (1)H SSNMR. (13)C and (15)N NMR indicates that the fully amorphous material (AM) contains an approximately equal ratio of cis-trans conformers about the amide bond, whereas the AR form exists mainly as one conformer, with minor conformational "defects". (1)H ultrafast MAS NMR shows significant differences in the hydrogen bonding involving the tetrazole and acid hydrogens between the two materials, while (15)N NMR shows that both forms exist as a 1,2,3,4-tetrazole tautomer. NMR relaxation times show subtle differences in local and bulk molecular mobility, which can be connected with the glass transition, the stability of the glassy material, and its response to aging. Counterintuitively the fully amorphous material is found to have a significantly lower dissolution rate than the apparently more ordered AR material.

Synthesis, Structure, and Reactivity of Anionic sp(2) -sp(3) Diboron Compounds: Readily Accessible Boryl Nucleophiles.

Lewis base adducts of tetra-alkoxy diboron compounds, in particular bis(pinacolato)diboron (B2 pin2 ), have been proposed as the active source of nucleophilic boryl species in metal-free borylation reactions. We report the isolation and detailed structural characterization (by solid-state and solution NMR spectroscopy and X-ray crystallography) of a series of anionic adducts of B2 pin2 with hard Lewis bases, such as alkoxides and fluoride. The study was extended to alternative Lewis bases, such as acetate, and other diboron reagents. The B(sp(2) )-B(sp(3) ) adducts exhibit two distinct boron environments in the solid-state and solution NMR spectra, except for [(4-tBuC6 H4 O)B2 pin2 ](-) , which shows rapid site exchange in solution. DFT calculations were performed to analyze the stability of the adducts with respect to dissociation. Stoichiometric reaction of the isolated adducts with two representative series of organic electrophiles-namely, aryl halides and diazonium salts-demonstrate the relative reactivities of the anionic diboron compounds as nucleophilic boryl anion sources.

Structure of cellulose microfibrils in primary cell walls from collenchyma.

In the primary walls of growing plant cells, the glucose polymer cellulose is assembled into long microfibrils a few nanometers in diameter. The rigidity and orientation of these microfibrils control cell expansion; therefore, cellulose synthesis is a key factor in the growth and morphogenesis of plants. Celery (Apium graveolens) collenchyma is a useful model system for the study of primary wall microfibril structure because its microfibrils are oriented with unusual uniformity, facilitating spectroscopic and diffraction experiments. Using a combination of x-ray and neutron scattering methods with vibrational and nuclear magnetic resonance spectroscopy, we show that celery collenchyma microfibrils were 2.9 to 3.0 nm in mean diameter, with a most probable structure containing 24 chains in cross section, arranged in eight hydrogen-bonded sheets of three chains, with extensive disorder in lateral packing, conformation, and hydrogen bonding. A similar 18-chain structure, and 24-chain structures of different shape, fitted the data less well. Conformational disorder was largely restricted to the surface chains, but disorder in chain packing was not. That is, in position and orientation, the surface chains conformed to the disordered lattice constituting the core of each microfibril. There was evidence that adjacent microfibrils were noncovalently aggregated together over part of their length, suggesting that the need to disrupt these aggregates might be a constraining factor in growth and in the hydrolysis of cellulose for biofuel production.

Crystal polymorphs of barbital: news about a classic polymorphic system.

Barbital is a hypnotic agent that has been intensely studied for many decades. The aim of this work was to establish a clear and comprehensible picture of its polymorphic system. Four of the six known solid forms of barbital (denoted I(0), III, IV, and V) were characterized by various analytical techniques, and the thermodynamic relationships between the polymorph phases were established. The obtained data permitted the construction of the first semischematic energy/temperature diagram for the barbital system. The modifications I(0), III, and V are enantiotropically related to one another. Polymorph IV is enantiotropically related to V and monotropically related to the other two forms. The transition points for the pairs I(0)/III, I(0)/V, and III/IV lie below 20 °C, and the transition point for IV/V is above 20 °C. At room temperature, the order of thermodynamic stability is I(0) > III > V > IV. The metastable modification III is present in commercial samples and has a high kinetic stability. The solid-state NMR spectra provide information on aspects of crystallography (viz., the asymmetric units and the nature of hydrogen bonding). The known correlation between specific N-H···O═C hydrogen bonding motifs of barbiturates and certain IR characteristics was used to predict the H-bonded pattern of polymorph IV.

Nanostructure of cellulose microfibrils in spruce wood.

The structure of cellulose microfibrils in wood is not known in detail, despite the abundance of cellulose in woody biomass and its importance for biology, energy, and engineering. The structure of the microfibrils of spruce wood cellulose was investigated using a range of spectroscopic methods coupled to small-angle neutron and wide-angle X-ray scattering. The scattering data were consistent with 24-chain microfibrils and favored a "rectangular" model with both hydrophobic and hydrophilic surfaces exposed. Disorder in chain packing and hydrogen bonding was shown to increase outwards from the microfibril center. The extent of disorder blurred the distinction between the I alpha and I beta allomorphs. Chains at the surface were distinct in conformation, with high levels of conformational disorder at C-6, less intramolecular hydrogen bonding and more outward-directed hydrogen bonding. Axial disorder could be explained in terms of twisting of the microfibrils, with implications for their biosynthesis.

NMR characterisation of dynamics in solvates and desolvates of formoterol fumarate.

Solid-state NMR is used to characterise dynamics in the ethanol solvate of the pharmaceutical material formoterol fumarate and its associated desolvate. Jump rates and activation barriers for dynamic processes such as phenyl ring rotation and methyl group rotational diffusion are derived from 1D-EXSY and (13)C spin-lattice relaxation times respectively. (2)H and (13)C spin-lattice relaxation times measured under magic-angle spinning conditions are used to show that the fumarate ion in the desolvate is undergoing small-amplitude motion on a frequency scale of 100s of MHz at ambient temperature with an activation parameter of about 32 kJ mol(-1). Exact calculations of relaxation times under MAS provide a simple and robust means to test motional models in cases where relaxation rate maxima are observed, including for systems where the crystal structure of the material is unknown.

Spectroscopic and structural characterization of the CyNHC adduct of B2pin2 in solution and in the solid state.

The Lewis base adduct of B(2)pin(2) and the NHC (1,3-bis(cyclohexyl)imidazol-2-ylidene), which was proposed to act as a source of nucleophilic boryl groups in the ß-borylation of α,ß-unsaturated ketones, has been isolated, and its solid state structure and solution behavior was studied. In solution, the binding is weak, and NMR spectroscopy reveals a rapid exchange of the NHC between the two boron centers. DFT calculations reveal that the exchange involves dissociation and reassociation of NHC rather than an intramolecular process.

Group 14 metal terminal phosphides: correlating structure with |J(MP)|.

A series of heavier group 14 element, terminal phosphide complexes, M(BDI)(PR(2)) (M = Ge, Sn, Pb; BDI = CH{(CH(3))CN-2,6-iPr(2)C(6)H(3)}(2); R = Ph, Cy, SiMe(3)) have been synthesized. Two different conformations (endo and exo) are observed in the solid-state; the complexes with an endo conformation have a planar coordination geometry at phosphorus (M = Ge, Sn; R = SiMe(3)) whereas the complexes possessing an exo conformation have a pyramidal geometry at phosphorus. Solution-state NMR studies reveal through-space scalar coupling between the tin and the isopropyl groups on the N-aryl moiety of the BDI ligand, with endo and exo exhibiting different J(SnC) values. The magnitudes of the tin-phosphorus and lead-phosphorus coupling constants, |J(SnP)| and |J(PbP)|, differ significantly depending upon the hybridization of the phosphorus atom. For Sn(BDI)(P{SiMe(3)}(2)), |J(SnP)| is the largest reported in the literature, surpassing values attributed to compounds with tin-phosphorus multiple-bonds. Low temperature NMR studies of Pb(BDI)(P{SiMe(3)}(2)) show two species with vastly different |J(PbP)| values, interpreted as belonging to the endo and exo conformations, with sp(2)- and sp(3)-hybridized phosphorus, respectively.

NMR characterisation of structure in solvates and polymorphs of formoterol fumarate.

The solid-state structures of two polymorphs and two alcoholates (ethanol and isopropanol) of formoterol fumarate have been investigated by a combination of NMR techniques. First-principles shielding computations are combined with NMR data to successfully relate peaks to their crystallographic positions for the solvates, including atoms that are in equivalent molecular positions. The uncharacterised structure of the asolvate form C is found to contain a single formoterol ion and half a fumarate ion in its asymmetric unit. HETCOR experiments for the ethanolate and form C allow proton chemical shifts to be determined and give improved (13)C resolution for the former compound. Desolvation of the solvates to form C has been monitored under the conditions of the NMR experiment. Differential rates of phenylene ring flipping are observed in the different forms. Carbon-13 relaxation times and (2)H NMR are used to probe dynamics of the fumarate ion.

Alkali Metal Salts of 10,12-Pentacosadiynoic Acid and Their Dosimetry Applications.

Wide-dose-range 2D radiochromic films for radiotherapy, such as GAFchromic EBT, are based on the lithium salt of 10,12-pentacosadiynoic acid (Li-PCDA) as the photosensitive component. We show that there are two solid forms of Li-PCDA-a monohydrated form A and an anhydrous form B. The form used in commercial GAFchromic films is form A due to its short needle-shaped crystals, which provide favorable coating properties. Form B provides an enhanced photoresponse compared to that of form A, but adopts a long needle crystal morphology, which is difficult to process. The two forms were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, CP-MAS 13C solid-state NMR spectroscopy, and thermogravimetric analysis. In sum, these data suggest a chelating bridging bidentate coordination mode for the lithium ions. The sodium salt of PCDA (Na-PCDA) is also reported, which is an ionic cocrystal with a formula of Na+PCDA-·3PCDA. The PCDA and PCDA- ligands display monodentate and bridging bidentate coordination to the sodium ion in contrast to the coordination sphere of the Li-PCDA forms. In contrast to its lithium analogues, Na-PCDA is photostable.

Zinc(II) silsesquioxane complexes and their application for the ring-opening polymerization of rac-lactide.

In this Communication, we report the unprecedented solid-state structures for a series of zinc(II) silsesquioxane complexes. Initial catalytic data for the ring-opening polymerization of rac-lactide are also presented together with analogous heterogeneous species.

The crystal engineering of radiation-sensitive diacetylene cocrystals and salts.

In this work we develop photoreactive cocrystals/salts of a commercially-important diacetylene, 10,12-pentacosadiynoic acid (PCDA, 1) and report the first X-ray crystal structures of PCDA based systems. The topochemical reactivity of the system is modified depending on the coformer used and correlates with the structural parameters. Crystallisation of 1 with 4,4'-azopyridine (2), 4,4'-bipyridyl (3), and trans-1,2-bis(4-pyridyl)ethylene (4) results in unreactive 2 : 1 cocrystals or a salt in the case of 4,4'-bipiperidine (5). However, salt formation with morpholine (6), diethylamine (7), and n-butylamine (8), results in highly photoreactive salts 12·7 and 1·8 whose reactivity can be explained using topochemical criteria. The salt 1·6 is also highly photoreactive and is compared to a model morpholinium butanoate salt. Resonance Raman spectroscopy reveals structural details of the photopolymer including its conformational disorder in comparison to less photoactive alkali metal salts and the extent of solid state conversion can be monitored by CP-MAS NMR spectroscopy. We also report an unusual catalysis in which amine evaporation from photopolymerised PCDA ammonium salts effectively acts as a catalyst for polymerisation of PCDA itself. The new photoreactive salts exhibit more reactivity but decreased conjugation compared to the commercial lithium salt and are of considerable practical potential in terms of tunable colours and greater range in UV, X-ray, and γ-ray dosimetry applications.

Superbasicity of a bis-guanidino compound with a flexible linker: a theoretical and experimental study.

The bis-guanidino compound H(2)C{hpp}(2) (I; hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) has been converted to the monocation [I-H](+) and isolated as the chloride and tetraphenylborate salts. Solution-state spectroscopic data do not differentiate the protonated guanidinium from the neutral guanidino group but suggest intramolecular "-N-H...N=" hydrogen bonding to form an eight-membered C(3)N(4)H heterocycle. Solid-state CPMAS (15)N NMR spectroscopy confirms protonation at one of the imine nitrogens, although line broadening is consistent with solid-state proton transfer between guanidine functionalities. X-ray diffraction data have been recorded over the temperature range 50-273 K. Examination of the carbon-nitrogen bond lengths suggests a degree of "partial protonation" of the neutral guanidino group at higher temperatures, with greater localization of the proton at one nitrogen position as the temperature is lowered. Difference electron density maps generated from high-resolution X-ray diffraction studies at 110 K give the first direct experimental evidence for proton transfer in a poly(guanidino) system. Computational analysis of I and its conjugate acid [I-H](+) indicate strong cationic resonance stabilization of the guanidinium group, with the nonprotonated group also stabilized, albeit to a lesser extent. The maximum barrier to proton transfer calculated using the Boese-Martin for kinetics method was 2.8 kcal mol(-1), with hydrogen-bond compression evident in the transition state; addition of zero-point vibrational energy values leads to the conclusion that the proton transfer is barrierless, implying that the proton shuttles freely between the two nitrogen atoms. Calculations determining the gas-phase proton affinity and the pK(a) in acetonitrile both indicate that compound I should behave as a superbase. This has been confirmed by spectrophotometric titrations in MeCN using polyphosphazene references, which give an average pK(a) of 28.98 +/- 0.05. Triadic analysis indicates that the dominant term causing the high basicity is the relaxation energy.