Color Polymorphism: Understanding the Diverse Solid‐State Packing and Color in Dimethyl‐3,6‐dichloro‐2,5‐dihydroxyterephthalate

Dimethyl‐3,6‐dichloro‐2,5‐dihydroxyterephthalate (MCHT) is known to exist in three differently packed crystals having three different colors, namely yellow (Y), light yellow (LY), and white (W). Apart from the difference in their color, the molecules in the crystals also differ in their intramolecular O?H???O and O?H???Cl hydrogen bonds. Time‐dependent DFT calculations reveal the role of the various types of hydrogen bonds in controlling the color of the polymorphs. Mechanistic pathways that lead to such transformations in the crystal are elucidated by solid‐state dispersion‐corrected DFT studies. Relative stabilities of the various polymorphs rationalize the experimentally observed transformations between them. Calculations reveal that the minimum‐energy pathway for the conversion of the Y form to a W form is through stepwise disrotatory motion of the two ?OH groups through a hybrid intermediate having one intramolecular O?H???O and one O?H???Cl bond. The LY form is shown to exist on the higher‐energy pathway involving a concerted Y→W transformation.     

Synthesis and Molecular Structure of β‐Phosphinoyl Carboxamides: An Unexpected Case of Chiral Discrimination of Hydrogen‐Bonded Dimers in the Solid State

A series of N,P,P‐trisubstituted aminophosphanes react with diphenylcyclopropenone to afford an easily separable mixture of two diastereomeric α,β‐diphenyl‐β‐phosphinoyl carboxamides in good yields. X‐ray crystal structures reveal that these compounds associate into dimers, formed from two enantiomeric units linked by two bifurcated hydrogen bonds, whereby the oxygen atom of the phosphoryl group acts as a dual acceptor for the vicinal NH and CH of a carbonyl group of a neighbouring molecule. Studies on the interconversion between diastereomeric phosphinoyl carboxamides in basic solution show that the thermodynamically most stable isomer depends on the nature of the substituent at the nitrogen atom. Simple computational calculations explain this phenomenon.     

Systematic Investigation of Molecular Arrangements and Solid‐State Fluorescence Properties on Salts of Anthracene‐2,6‐disulfonic Acid with Aliphatic Primary Amines

Organic salts of anthracene‐2,6‐disulfonic acid (ADS) with a wide variety of primary amines have been fabricated, and their arrangements of anthracene molecules and solid‐state fluorescence properties investigated. Single‐crystal X‐ray studies reveal that the salts show seven types of crystal forms and corresponding molecular arrangements of anthracene moieties depending on the amine, while anthracene shows only one form and arrangement in the solid state. Depending on the molecular arrangements, the ADS salts exhibit various solid‐state fluorescence properties: spectral shift (30 nm) and suppression and enhancement of the fluorescence intensity. Especially the ADS salt with n‐heptylamine (nHepA), which shows discrete anthracene moieties in the crystal, exhibits the highest quantum yield (Φ_F=46.1±0.2 %) in the series of ADS salts, which exceeds that of anthracene crystal (Φ_F=42.9±0.2 %). From these systematic investigations on the arrangements and the solid‐state properties, the following factors are essential for high fluorescence quantum yield in the solid state: prevention of contact between π planes of anthracene moieties and immobilization of anthracene rings. In addition, such organic salts have potential as a system for modulating the molecular arrangements of fluorophores and the concomitant solid‐state properties. Thus, systematic investigation of this system constructs a library of arrangements and properties, and the library leads to remarkable strategies for the development of organic solid materials.     

Three polymorphic forms of the co-crystal 4,4'-bipyridine/pimelic acid and their structural, thermal, and spectroscopic characterization

Three crystal forms of the co-crystal 4,4'-bipy/pimelic acid (bipy: bipyridine), [NH(4)C(5)-C(5)H(4)N][HOOC(CH(2))(5)COOH], have been prepared and their relationship investigated by single-crystal X-ray diffraction, variable-temperature X-ray powder diffraction, differential scanning calorimetry and solid-state NMR spectroscopy. Both X-ray and NMR spectroscopic results indicate that no proton transfer takes place, that is, the three crystal forms are true co-crystals of neutral molecules. Forms I and II both convert into Form III at high temperature, Forms II and III being the thermodynamically stable forms at room and high temperature, respectively.     

NMR Studies of Active‐Site Properties of Human Carbonic Anhydrase II by Using 15N‐Labeled 4‐Methylimidazole as a Local Probe and Histidine Hydrogen‐Bond Correlations

By using a combination of liquid and solid‐state NMR spectroscopy, ¹⁵N‐labeled 4‐methylimidazole (4‐MI) as a local probe of the environment has been studied: 1) in the polar, wet Freon CDF₃/CDF₂Cl down to 130 K, 2) in water at pH 12, and 3) in solid samples of the mutant H64A of human carbonic anhydrase II (HCA II). In the latter, the active‐site His64 residue is replaced by alanine; the catalytic activity is, however, rescued by the presence of 4‐MI. For the Freon solution, it is demonstrated that addition of water molecules not only catalyzes proton tautomerism but also lifts its quasidegeneracy. The possible hydrogen‐bond clusters formed and the mechanism of the tautomerism are discussed. Information about the imidazole hydrogen‐bond geometries is obtained by establishing a correlation between published ¹H and ¹⁵N chemical shifts of the imidazole rings of histidines in proteins. This correlation is useful to distinguish histidines embedded in the interior of proteins and those at the surface, embedded in water. Moreover, evidence is obtained that the hydrogen‐bond geometries of His64 in the active site of HCA II and of 4‐MI in H64A HCA II are similar. Finally, the degeneracy of the rapid tautomerism of the neutral imidazole ring His64 reported by Shimahara et al. (J. Biol. Chem.­ 2007 , 282, 9646) can be explained with a wet, polar, nonaqueous active‐site conformation in the inward conformation, similar to the properties of 4‐MI in the Freon solution. The biological implications for the enzyme mechanism are discussed.     

Insights into the Tautomerism in meso‐Substituted Corroles: A Variable‐Temperature 1H, 13C, 15N,and 19F NMR Spectroscopy Study

Tris(pentafluorophenyl)corrole and its ¹⁵N‐enriched isotopomer were studied in [D₈]toluene solution by 1D and 2D variable‐temperature NMR techniques to establish the mechanisms of tautomerization of the NH protons inside the interior of the corrole macrocycle. Three such rate processes could be identified of which two modulate the spectral line shapes at temperatures above 205 K and the third is NMR‐inaccessible as it is very fast. The latter involves the proton engaged in an unsymmetrical proton sponge unit formed by two pyrrole nitrogen atoms. Temperature and concentration dependences of the two remaining processes were determined. One of them is purely intramolecular and the other is intermolecular at low temperatures, with growing contribution of an intramolecular mechanism at elevated temperatures. The proposed microscopic mechanisms of all these processes are semi‐quantitatively confirmed by quantum chemical calculations using density functional theory.     

A Designed Three‐Stranded β‐Sheet in an α/β Hybrid Peptide

The incorporation of β‐amino acid residues into the antiparallel β‐strand segments of a multi‐stranded β‐sheet peptide is demonstrated for a 19‐residue peptide, Boc‐LV^βFV^DPGL^βFVVL^DPGLVL^βFVV‐OMe (BBH19). Two centrally positioned ^DPro–Gly segments facilitate formation of a stable three‐stranded β‐sheet, in which β‐phenylalanine (^βPhe) residues occur at facing positions 3, 8 and 17. Structure determination in methanol solution is accomplished by using NMR‐derived restraints obtained from NOEs, temperature dependence of amide NH chemical shifts, rates of H/D exchange of amide protons and vicinal coupling constants. The data are consistent with a conformationally well‐defined three‐stranded β‐sheet structure in solution. Cross‐strand interactions between ^βPhe3/^βPhe17 and ^βPhe3/Val15 residues define orientations of these side‐chains. The observation of close contact distances between the side‐chains on the N‐ and C‐terminal strands of the three‐stranded β‐sheet provides strong support for the designed structure. Evidence is presented for multiple side‐chain conformations from an analysis of NOE data. An unusual observation of the disappearance of the Gly NH resonances upon prolonged storage in methanol is rationalised on the basis of a slow aggregation step, resulting in stacking of three‐stranded β‐sheet structures, which in turn influences the conformational interconversion between type I′ and type II′ β‐turns at the two ^DPro–Gly segments. Experimental evidence for these processes is presented. The decapeptide fragment Boc‐LV^βFV^DPGL^βFVV‐OMe (BBH10), which has been previously characterized as a type I′ β‐turn nucleated hairpin, is shown to favour a type II′ β‐turn conformation in solution, supporting the occurrence of conformational interconversion at the turn segments in these hairpin and sheet structures.     

Synthesis and Self‐Association of Double‐Helical AADD Arrays

The design and syntheses of four self‐complementary oligomers that contain an underlying AADD hydrogen bond sequence are presented, and their self‐association was examined in the solution and solid state. The molecular recognition between the two strands is highly sensitive to substitutions of their component heterocycles. Substitution with electron‐donating and ‐withdrawing groups and the influence of preorganization has a large effect on the overall stabilities of the complexes studied. In particular, a wide range (>10⁵ M ^?1) of stabilities with respect to substitutions at various positions in the AADD oligomers was demonstrated. In the most extreme case, the dimerization constant measured (K_dimer≥4.5×10⁷ M ^?1) is comparable to the most stable homodimers of neutral AADD arrays reported to date.     

Formation of a Hydrogen‐Bonded Heptazine Framework by Self‐Assembly of Melem into a Hexagonal Channel Structure

Self‐assembly of melem C₆N₇(NH₂)₃ in hot aqueous solution leads to the formation of hydrogen‐bonded, hexagonal rosettes of melem units surrounding infinite channels with a diameter of 8.9 Å. The channels are filled with strongly disordered water molecules, which are bound to the melem network through hydrogen bonds. Single‐crystals of melem hydrate C₆N₇(NH₂)₃ ? xH₂O (x≈2.3) were obtained by hydrothermal treatment of melem at 200 °C and the crystal structure (R $\bar 3$ c, a=2879.0(4), c=664.01(13) pm, V=4766.4(13)×10⁶ pm³, Z=18) was elucidated by single‐crystal X‐ray diffraction. With respect to the structural similarity to the well‐known adduct between melamine and cyanuric acid, the composition of the obtained product was further analyzed by solid‐state NMR spectroscopy. Hydrolysis of melem to cyameluric acid during syntheses at elevated temperatures could thus be ruled out. DTA/TG studies revealed that, during heating of melem hydrate, water molecules can be removed from the channels of the structure to a large extent. The solvent‐free framework is stable up to 430 °C without transforming into the denser structure of anhydrous melem. Dehydrated melem hydrate was further characterized by solid‐state NMR spectroscopy, powder X‐ray diffraction, and sorption measurements to investigate structural changes induced by the removal of water from the channels. During dehydration, the hexagonal, layered arrangement of melem units is maintained whereas the formation of additional hydrogen bonds between melem entities requires the stacking mode of hexagonal layers to be altered. It is assumed that layers are shifted perpendicular to the direction of the channels, thereby making them inaccessible for guest molecules.     

Ferrocenophanes with Interannular Nitrogen–Gallium Bridges. 1,3,2‐Diazagalla‐[3]Ferrocenophanes and an 1‐Aza‐3‐Azonia‐2‐Gallata‐[3]Ferrocenophane. Synthesis and Structure

1,1′‐Bis(trimethylsilylamino)ferrocene reacts with trimethyl‐ and triethylgallium to give the μ‐[ferrocene‐1,1′‐diyl‐bis(trimethylsilylamido)]tetraalkyldigallanes. These were converted into the 1,3‐bis(trimethylsilyl)‐2‐alkyl‐2‐pyridine‐1,3,2‐diazagalla‐[3]ferrocenophanes, of which the ethyl derivative was characterized by X‐ray structural analysis. Treatment of gallium trichloride with N,N′‐dilithio‐1,1′‐bis(trimethylsilylamino)ferrocene affords μ‐[ferrocene‐1,1′‐diyl‐bis(trimethylsilylamido)]tetrachlorodigallane along with bis(trimethylsilyl)‐2,2‐dichloro‐1‐aza‐3‐azonia‐2‐gallata‐[3]ferrocenophane as a side product, and both were structurally characterized by X‐ray analysis. The solution‐state structures of the new gallium compounds and aspects of their molecular dynamics in solution were studied by NMR spectroscopy (¹H, ¹³C, ²⁹Si NMR).     

Stacking Structure of Confined 1‐Butanol in SBA‐15 Investigated by Solid‐State NMR Spectroscopy

Understanding the complex thermodynamic behavior of confined amphiphilic molecules in biological or mesoporous hosts requires detailed knowledge of the stacking structures. Here, we present detailed solid‐state NMR spectroscopic investigations on 1‐butanol molecules confined in the hydrophilic mesoporous SBA‐15 host. A range of NMR spectroscopic measurements comprising of ¹H spin–lattice (T₁), spin–spin (T₂) relaxation, ¹³C cross‐polarization (CP), and ¹H,¹H two‐dimensional nuclear Overhauser enhancement spectroscopy (¹H,¹H 2D NOESY) with the magic angle spinning (MAS) technique as well as static wide‐line ²H NMR spectra have been used to investigate the dynamics and to observe the stacking structure of confined 1‐butanol in SBA‐15. The results suggest that not only the molecular reorientation but also the exchange motions of confined molecules of 1‐butanol are extremely restricted in the confined space of the SBA‐15 pores. The dynamics of the confined molecules of 1‐butanol imply that the ¹H,¹H 2D NOESY should be an appropriate technique to observe the stacking structure of confined amphiphilc molecules. This study is the first to observe that a significant part of confined 1‐butanol molecules are orientated as tilted bilayered structures on the surface of the host SBA‐15 pores in a time‐average state by solid‐state NMR spectroscopy with the ¹H,¹H 2D NOESY technique.     

Ultra‐wideline 27Al NMR Investigation of Three‐ and Five‐Coordinate Aluminum Environments

Ultra‐wideline ²⁷Al NMR experiments are conducted on coordination compounds with ²⁷Al nuclei possessing immense quadrupolar interactions that result from exceptionally nonspherical coordination environments. NMR spectra are acquired using a methodology involving frequency‐stepped, piecewise acquisition of NMR spectra with Hahn‐echo or quadrupolar Carr–Purcell Meiboom–Gill (QCPMG) pulse sequences, which is applicable to any half‐integer quadrupolar nucleus with extremely broad NMR powder patterns. Despite the large breadth of these central transition powder patterns, ranging from 250 to 700 kHz, the total experimental times are an order of magnitude less than previously reported experiments on analogous complexes with smaller quadrupolar interactions. The complexes examined feature three‐ or five‐coordinate aluminum sites: trismesitylaluminum (AlMes₃), tris(bis(trimethylsilyl)amino)aluminum (Al(NTMS₂)₃), bis[dimethyl tetrahydrofurfuryloxide aluminum] ([Me₂‐Al(μ‐OTHF)]₂), and bis[diethyl tetrahydrofurfuryloxide aluminum] ([Et₂‐Al(μ‐OTHF)]₂). We report some of the largest ²⁷Al quadrupolar coupling constants measured to date, with values of C_Q(²⁷Al) of 48.2(1), 36.3(1), 19.9(1), and 19.6(2) MHz for AlMes₃ , Al(NTMS₂)₃ , [Me₂‐Al(μ‐OTHF)]₂ , and [Et₂‐Al(μ‐OTHF)]₂ , respectively. X‐ray crystallographic data and theoretical (Hartree–Fock and DFT) calculations of ²⁷Al electric field gradient (EFG) tensors are utilized to examine the relationships between the quadrupolar interactions and molecular structure; in particular, the origin of the immense quadrupolar interaction in the three‐coordinate species is studied via analyses of molecular orbitals.     

Zn8[P12N24]O2 – ein Nitridophosphat‐oxid mit Sodalith‐Struktur

Zn₈P₁₂N₂₄O₂ – a Nitridophosphate Oxide with Sodalite Structure The reaction between zinc metal and phosphorus nitride imide PN(NH) was investigated. Surprisingly, no Zn₆P₁₂N₂₄ was formed as assumed in former investigations but phase pure Zn₈[P₁₂N₂₄]O₂ ( (Nr. 217), a = 8.2422(2) Å; Z = 1) was obtained due to contamination by a small amount of oxygen. The existence of Zn₈[P₁₂N₂₄]O₂ was formerly supposed, but neither its crystal structure nor its exact composition have been unequivocally reported so far. The stoichiometric formula was deducted from elemental analyses, XANES spectroscopy at the phosphorus K‐threshold and IR‐spectroscopy using the crystallographic results of electron diffraction, X‐ray powder diffraction and solid‐state NMR spectroscopy. Zn₈[P₁₂N₂₄]O₂ adopts the sodalite structure type and is thus isotypic with Zn₈[P₁₂N₂₄]X₂ with X = S, Se, Te and Zn₈[B₁₂O₂₄]O₂.     

Nanoparticles of [Fe(NH2‐trz)3]Br2⋅3 H2O (NH2‐trz=2‐Amino‐1,2,4‐triazole) Prepared by the Reverse Micelle Technique: Influence of Particle and Coherent Domain Sizes on Spin‐Crossover Properties

By changing the surfactant/water ratio , nanoparticles of the iron(II) spin crossover material, [Fe(NH₂‐trz)₃]Br₂ ? 3 H₂O (with NH₂‐trz=4‐amino‐1,2,4‐triazole), have been synthesised from 1 μm down to 30 nm (see figure). Magnetic and reflectivity experiments indicate that the critical size for observing a thermal hysteresis in this 1D polymer family is around 50 nm, and powder X‐ray diffraction shows that particles of about 30 nm are constituted by about one coherent domain.