Structure of Plastic Crystalline Succinonitrile: High‐Resolution in situ Powder Diffraction

The temperature dependent (150–290 K) crystal structure of the low‐temperature α‐phase, and high temperature β‐phase, of succinonitrile has been determined by high resolution in situ powder diffraction. The α‐phase has a monoclinic unit cell that contains four gauche molecules and belongs to the P2₁/a space group. The crystal undergoes a reversible first‐order phase transition at 233 K into the high temperature β‐phase. The lattice parameters increase with temperature and the phase transition leads to an abrupt 6.7 % increase in volume. The β‐phase crystallizes into a bcc‐structure that belongs to the space group. The high temperature phase; however, is a highly disordered plastic crystal at room temperature that contains both gauche and trans molecules. The non‐linearity in the overall isotropic temperature‐factor indicates other possible phase transitions in the temperature range of 233–250 K.     

Monoclinic‐to‐orthorhombic phase transition of the hexamethylenetetramine–2‐methylbenzoic acid (1/2) cocrystal with temperature‐dependent dynamic molecular disorder

As a function of temperature, the hexamethylenetetramine–2‐methylbenzoic acid (1/2) cocrystal, C₆H₁₂N₄·2C₈H₈O₂, undergoes a reversible structural phase transition. The orthorhombic high‐temperature phase in the space group Pccn has been studied in the temperature range between 165 and 300 K. At 164 K, a t₂ phase transition to the monoclinic subgroup P2₁/c space group occurs; the resulting twinned low‐temperature phase was investigated in the temperature range between 164 and 100 K. The domains in the pseudomerohedral twin are related by a twofold rotation corresponding to the matrix (100/00/00). Systematic absence violations represent a sensitive criterium for the decision about the correct space‐group assignment at each temperature. The fractional volume contributions of the minor twin domain in the low‐temperature phase increases in the order 0.259 (2) → 0.318 (2) → 0.336 (2) → 0.341 (3) as the temperature increases in the order 150 → 160 → 163 → 164 K. The transformation occurs between the nonpolar point group mmm and the nonpolar point group 2/m, and corresponds to a ferroelastic transition or to a t₂ structural phase transition. The asymmetric unit of the low‐temperature phase consists of two hexamethylenetetramine molecules and four molecules of 2‐methylbenzoic acid; it is smaller by a factor of 2 in the high‐temperature phase and contains two half molecules of hexamethylenetetramine, which sit across twofold axes, and two molecules of the organic acid. In both phases, the hexamethylenetetramine residue and two benzoic acid molecules form a three‐molecule aggregate; the low‐temperature phase contains two of these aggregates in general positions, whereas they are situated on a crystallographic twofold axis in the high‐temperature phase. In both phases, one of these three‐molecule aggregates is disordered. For this disordered unit, the ratio between the major and minor conformer increases upon cooling from 0.567 (7):0.433 (7) at 170 K via 0.674 (6):0.326 (6) and 0.808 (5):0.192 (5) at 160 K to 0.803 (6):0.197 (6) and 0.900 (4):0.100 (4) at 150 K, indicating temperature‐dependent dynamic molecular disorder. Even upon further cooling to 100 K, the disorder is retained in principle, albeit with very low site occupancies for the minor conformer.     

Collapse of Cylindrical Brushes with 2‐Isopropyloxazoline Side Chains Close to the Phase Boundary

A high‐molar‐mass cylindrical brush polymer with a main chain degree of polymerization of P_w = 1047 is synthesized by free‐radical polymerization of a poly‐2‐isopropyloxazoline macromonomer with P_n = 28. The polymerization is conducted above the lower phase transition temperature of the macromonomer, i.e., in the phase‐separated regime, which provides a sufficiently concentrated macromonomer phase mandatory to obtain high‐molar‐mass cylindrical brushes. Upon heating to the phase transition temperature, the hydrodynamic radius is observed to shrink from 34 to 27 nm. Further increase in temperature resulted in aggregated chains which were observed to coexist with single chains until eventually only aggregates of μm size were detectable.

     

The low‐temperature phase transition of 9‐methylfluoren‐9‐ol: comparison of the crystal structures at 100 and 200 K

Crystals of 9‐methyl­fluoren‐9‐ol, C₁₄H₁₂O, undergo a reversible phase transition at 176 (2) K. The structure of the high‐temperature α form at 200 K is compared with that of the low‐temperature β form at 100 K. Both polymorphs crystallize in space group P with Z = 4 and contain discrete hydrogen‐bonded R(8) ring tetramers arranged around crystallographic inversion centres. The most obvious changes observed on cooling the crystals to below 176 K are an abrupt increase of ca 0.5 Å in the shortest lattice translation, and a thermal transition with ΔH = 1 kJ mol^?1.     

Two reversible enantiotropic phase transitions in a pentacoordinate silicon complex with an O,N,O′‐tridentate valinate ligand

Dimethyl[N‐(4‐oxidopent‐3‐en‐2‐ylidene)valinato‐κ³O,N,O′]silicon(IV), C₁₂H₂₁NO₃Si, (II), crystallizes in the orthorhombic space group P2₁2₁2₁. The chiral compound undergoes two sharp enantiotropic phase transitions upon cooling. The first transformation occurs at 163 K to yield a unit cell with one axis having double length. This intermediate‐temperature form has the monoclinic space group P2₁. The second transition takes place at 142 K and converts the single crystal into the low‐temperature form in the orthorhombic space group P2₁2₁2₁. This transition proceeds under tripling of the a axis of the high‐temperature form. Both phase transitions are fully reversible and correspond to order–disorder transitions of the isopropyl group of the valine unit in the ligand backbone. The phase transitions presented here raise questions, since they do not fit into the rules of group–subgroup relationships.     

Reversible phase transition of 2‐carboxypyridinium perchlorate–pyridinium‐2‐carboxylate (1/1)

The title salt, C₆H₆NO₂⁺·ClO₄⁻·C₆H₅NO₂, was crystallized from an aqueous solution of equimolar quantities of perchloric acid and pyridine‐2‐carboxylic acid. Differential scanning calorimetry (DSC) measurements show that the compound undergoes a reversible phase transition at about 261.7 K, with a wide heat hysteresis of 21.9 K. The lower‐temperature polymorph (denoted LT; T = 223 K) crystallizes in the space group C2/c, while the higher‐temperature polymorph (denoted RT; T = 296 K) crystallizes in the space group P2/c. The relationship between these two phases can be described as: 2a_RT = a_LT; 2b_RT = b_LT; c_RT = c_LT. The crystal structure contains an infinite zigzag hydrogen‐bonded chain network of 2‐carboxypyridinium cations. The most distinct difference between the higher (RT) and lower (LT) temperature phases is the change in dihedral angle between the planes of the carboxylic acid group and the pyridinium ring, which leads to the formation of different ten‐membered hydrogen‐bonded rings. In the RT phase, both the perchlorate anions and the hydrogen‐bonded H atom within the carboxylic acid group are disordered. The disordered H atom is located on a twofold rotation axis. In the LT phase, the asymmetric unit is composed of two 2‐carboxypyridinium cations, half an ordered perchlorate anion with ideal tetrahedral geometry and a disordered perchlorate anion. The phase transition is attributable to the order–disorder transition of half of the perchlorate anions.     

Influence of Isotope Substitution on Lattice and Spin‐Peierls‐Type Transition Features in One‐Dimensional Nickel Bis‐dithiolene Spin Systems

Four new 1D spin‐Peierls‐type compounds, [D₅]1‐(4′‐R‐benzyl)pyridinium bis(maleonitriledithiolato)nickelate ([D₅]R‐Py; R=F, I, CH₃, and NO₂), were synthesized and characterized structurally and magnetically. These 1D compounds are isostructural with the corresponding non‐deuterated compounds, 1‐(4′‐R‐benzyl)pyridinium bis(maleonitriledithiolato)nickelate (R‐Py; R=F, I, CH₃, and NO₂). Compounds [D₅]R‐Py and R‐Py (R=F, I, CH₃, and NO₂) crystallize in the monoclinic space group P2₁/c with uniform stacks of anions and cations in the high‐temperature phase and triclinic space group P$\bar 1$ with dimerized stacks of anions and cations in the low‐temperature phase. Similar to the non‐deuterated R‐Py compounds, a spin‐Peierls‐type transition occurs at a critical temperature for each [D₅]R‐Py compound; the magnetic character of the 1D S=1/2 ferromagnetic chain for [D₅]F‐Py and the 1D S=1/2 Heisenberg antiferromagnetic chain for others appear above the transition temperature. Spin‐gap magnetic behavior was observed for all of these compounds below the transition temperature. In comparison to the corresponding R‐Py compound, the cell volume is almost unchanged for [D₅]F‐Py and shows slight expansion for [D₅]R‐Py (R=I, CH₃, and NO₂) as well as an increase in the spin‐Peierls‐type transition temperature for all of these 1D compounds in the order of F>I≈CH₃≈NO₂. The large isotopic effect of nonmagnetic countercations on the spin‐Peierls‐type transition critical temperature, T_C, can be attributed to the change in ω₀ with isotope substitution.     

Racemic 1,2‐diphenylbut‐3‐yn‐2‐ol

Molecules of the title compound, C₁₆H₁₄O, are chiral and crystallize in space group P with Z′ = 2, and with one R and one S mol­ecule in the asymmetric unit. The conformations of the phenyl rings in the two independent mol­ecules differ slightly. Supramolecular organization in the crystal is via tetrameric O—H?H(O) hydrogen‐bonded synthons formed separately by each conformer. These tetrameric synthons stack along the c axis via C[triple‐bond]C—H?O(H) hydrogen bonds. The only link between the conformer stacks is provided by weaker C_methylene—H and C_phenyl—H interactions with π_arene density.     

Monoclinic AlPO4 tridymite at 473 and 463 K from X‐ray powder data

Upon cooling from its hexagonal high‐temperature modification, AlPO₄ (aluminium phosphate) tridymite successively transforms to several displacively distorted forms, including a normal structure–incommensurate–lock‐in phase transition sequence. The space‐group symmetries in this series are P112₁, P112₁(αβ0) and P2₁2₁2₁, respectively. The distortion pattern of the intermediate P112₁ phase can be described as alternate shifts of adjacent layers of tetrahedra coupled with tilting of the tetrahedra. The symmetry and direction of the shifts are different from the analogous SiO₂ tridymite modification. The atomic displacement parameters of the O atoms are strongly anisotropic due to thermal motions of the rigid tetrahedra. Condensation of a lattice vibration mode results in the formation of an incommensurate structural modulation below 473 K. The 3+1 superspace‐group symmetry of the modulated phase is P112₁(αβ0).     

High‐ and low‐temperature La2RuO5 by powder neutron diffraction

The structure of dilanthanum ruthenium pentoxide was solved by powder neutron diffraction at room temperature and 1.5 K. High‐temperature La₂RuO₅ crystallizes in the monoclinic space group P2₁/c. Upon cooling, the sample undergoes a phase transition to the triclinic low‐temperature form (space group P). This transition leads to pronounced changes in the Ru—O—Ru bond distances, resulting in a dimerization of the ruthenium ions.     

Synchrotron‐radiation study of the two‐leg spin‐ladder (VO)2P2O7 at 120 K

The crystal structure of the ambient‐pressure phase of vanadyl pyrophosphate, (VO)₂P₂O₇, has been precisely determined at 120 K from synchrotron X‐ray diffraction data measured on a high‐quality single crystal. The structure refinement unambiguously establishes the orthorhombic space group Pca2₁ as the true crystallographic symmetry. Moreover, it improves the accuracy of previously published atomic coordinates by one order of magnitude, and provides reliable anisotropic displacement parameters for all atoms. Along the a axis, the structure consists of infinite two‐leg ladders of vanadyl cations, (VO)²⁺, which are separated by pyrophosphate anions, (P₂O₇)^4?. Parallel to the c axis, the unit cell comprises two alternating crystallographically inequivalent chains of edge‐sharing VO₅ square pyramids bridged by PO₄ double tetrahedra. No structural phase transition has been observed in the temperature range between 300 and 120 K.     

Syntheses,Structures, and Polymorphism of β‐Diketonato Complexes – Co(thd)3

Oxidation of Co(thd)₂ dissolved in different solvents has been investigated in air and oxygen atmosphere. In oxygen atmosphere and at the boiling point of the solvents this treatment leads to oxidation of Co^II to Co^III, but also to degradation of some of the thd ligands and formation of a new mixed‐ligand complex. Three pure‐cultivated crystalline Co(thd)₃ phases are reported: 1 (room‐temperature phase), 2 (low‐temperature phase), and 3 (metastable phase) and in addition there exists an amorphous Co(thd)₃ phase ( 4 ) with approximate composition Co(thd)₃·xH(thd); x = 0.06. Reaction of metal(II) oxides (MO, M = Mn, Fe, and Co) with H(thd) under air or O₂ atmosphere is an easy direct route to M(thd)₃ complexes. Structure determinations are reported for Co(thd)₃ ( 1 – 3 ) based on single‐crystal X‐ray diffraction data. Modification 1 crystallizes in space group with a = b = 18.8100(10), c = 18.815(2) Å at 295 K; R(wR2) = 0.180, modification 2 in space group C2/c with a = 28.007(12), b = 18.482(8), c = 21.356(9) Å, β = 97.999(5)° at 100 K; R(wR2) =0.211, and modification 3 in space group Pnma with a = 19.2394(15), b = 18.8795(15), c = 10.7808(8) Å at 100 K; R(wR2) = 0.193. The molecular structures of 1 – 3 all comprise a central Co atom octahedrally co‐ordinated by the ketonato O atoms of three thd ligands. The transformation between modifications 1 and 2 is of a fully reversible second‐order character. Modifications 1 and 3 are, on the other hand, related by a quasi‐reversible cycle. Heat treatment (specifically sublimation) of 1 leads to 3 whereas re‐crystallization or prolonged storage at room temperature is required to regenerate 1 . Co(thd)₃ has sufficient thermal stability to permit sublimation without degradation. The various forms of Co(thd)₃ are all diamagnetic, viz. a confirmation of the Co^III valence state.     

Phenakite‐Type BeP2N4—A Possible Precursor for a New Hard Spinel‐Type Material

BeP₂N₄ was synthesized in a multi‐anvil apparatus starting from Be₃N₂ and P₃N₅ at 5 GPa and 1500 °C. The compound crystallizes in the phenakite structure type (space group R$\bar 3$ , no. 148) with a=1269.45(2) pm, c=834.86(2) pm, V=1165.13(4)×10⁶ pm³ and Z=18. As isostructural and isovalence‐electronic α‐Si₃N₄ transforms into β‐Si₃N₄ at high pressure and temperature, we studied the phase transition of BeP₂N₄ into the spinel structure type by using density functional theory calculations. The predicted transition pressure of 24 GPa is within the reach of today’s state of the art high‐pressure experimental setups. Calculations of inverse spinel‐type BeP₂N₄ revealed this polymorph to be always higher in enthalpy than either phenakite‐type or spinel‐type BeP₂N₄. The predicted bulk modulus of spinel‐type BeP₂N₄ is in the range of corundum and γ‐Si₃N₄ and about 40 GPa higher than that of phenakite‐type BeP₂N₄. This finding implies an increase in hardness in analogy to that occurring for the β‐ to γ‐Si₃N₄ transition. In hypothetical spinel‐type BeP₂N₄ the coordination number of phosphorus is increased from 4 to 6. So far only coordination numbers up to 5 have been experimentally realized (γ‐P₃N₅), though a sixfold coordination for P has been predicted for hypothetic δ‐P₃N₅. We believe, our findings provide a strong incentive for further high‐pressure experiments in the quest for novel hard materials with yet unprecedented structural motives.     

Crystal structures of two polymorphic forms of DOTAM‐mono‐acid dihydrate

The structural chemistry of 2‐[4,7,10‐tris(carbamoylmethyl)‐1,4,7,10‐tetraazacyclododecan‐1‐yl]acetic acid dihydrate, C₁₆H₃₁N₇O₅·2H₂O, is described. The macrocyclic compound, also known by the abbreviation DOTAM‐mono‐acid, crystallized at room temperature and was isolated concomitantly as two polymorphic forms. The structures of both polymorphs were determined at 90 K. The first polymorph crystallized as a zwitterionic dihydrate [systematic name: 4,7,10‐tris(carbamoylmethyl)‐1‐(carboxylatomethyl)‐1,4,7,10‐tetraazacyclododecan‐1‐ium dihydrate] in the space group P2₁/n, with Z′ = 1. The second polymorph crystallized as a zwitterionic dihydrate in the space group P2₁ at 90 K, with Z′ = 2. The two independent molecules are related by a local center. In each polymorph, the zwitterion is formed between the negatively‐charged carboxylate group and the ring N atom that bears the acetate pendant arm. Extensive inter‐ and intramolecular hydrogen bonding exists in both polymorphic structures. In polymorph 1, an intermolecular hydrogen‐bonding network propagating parallel to the a direction creates an infinite chain. A second hydrogen‐bonding network is observed through a water molecule of hydration in the b direction. Polymorph 2 also has two intermolecular hydrogen‐bonding networks. One propagates parallel to the a direction, while the other propagates in the [10] direction. Increasing the temperature of polymorph 2 yields the same structure at T = 180 K, but the pseudocenter becomes exact at 299 K. The higher‐temperature structure has Z′ = 1 in the space group P2₁/c.     

Tunable Keplerate Type‐Cluster “Mo132” Cavity with Dicarboxylate Anions

The internal functionalization of the Keplerate‐type capsule Mo₁₃₂ has been carried out by ligand exchange leading to the formation of glutarate and succinate containing species isolated as ammonium or dimethylammonium salts. Solution NMR analysis is consistent with asymmetric inner dicarboxylate ions containing one carboxylato group grafted onto the inner side of the spheroidal inorganic shell while the second hangs toward the center of the cavity. Such a disposition has been confirmed by the single‐crystal X‐ray diffraction analysis of the glutarate containing {Mo₁₃₂} species. A detailed NMR solution study of the ligand‐exchange process allowed determining the binding constant K_L of acetate (AcO^?), succinate (HSucc^?) or glutarate (HGlu^?) ligands at the 30 inner coordinating sites, which vary such as K<K<K‐supported by the associated thermodynamic parameters Δ_rS* and Δ_rH*. Such a variation is mainly explained by a positive entropic gain attenuated by unfavorable steric effect. Furthermore, these results are completed by ¹H DOSY and ¹H EXSY NMR experiments which are in agreement with bulky guests firmly trapped within the cavity. At last, variable temperature ¹H NMR study below 290 K revealed a striking line broadening occurring abruptly within a 5 K range. Such an effect appears closely related to the presence of the ammonium cations suspected to be present within the cavity and then has been interpreted as an inner‐phase transition leading to a frozen state.     

The Crystal Structures of the High‐temperature Phases of Ag4Mn3O8

Two endothermic, reversible structural phase transitions of first order have been observed in Ag₄Mn₃O₈ by means of in‐situ powder diffraction and by differential scanning calorimetry. At a temperature of T = 477 K, Ag₄Mn₃O₈ undergoes a structural phase transition from the room temperature phase in space group P3₁21 to a phase in space group R32, and at T = 689 K a second phase transition to a structure in space group P4₃32 occurs. Whilst the Mn₃O₈ framework does not change significantly upon heating, rearrangements of the silver atoms, located in the cavities of the framework, were found to be the driving force behind the transitions. The structural changes with increasing temperature proceed along a path of minimal group‐supergroup relations between the respective space groups.     

Redetermination of hexa­carbonyl­(η5‐cyclo­penta­dienyl)­bis(μ3‐selenido)­diiron(II)­cobalt(II)

The structure of the title compound, hexa­carbonyl‐1κ³C,2κ³C‐[3(η⁵)‐cyclo­penta­dienyl]­bis(μ₃‐selenido)­diiron(II)­cobalt(II),[CoFe₂(μ₃‐Se)₂(C₅H₅)(CO)₆], was redetermined at room temperature and the correct C2/c space group was assumed instead of the previously reported P space group [Mathur et al. (1995). Organometallics, 14 , 2115–2118]. Analysis of the literature data showed that the previously reported triclinic parameters correspond to a primitive subcell of the actual monoclinic C‐centred cell with cell dimensions close to those found by us. The title compound appeared to be isostructural with the sulfur–selenium analogue.     

Novel C60‐Anchored Two‐Armed Poly(tert‐butyl acrylate): Synthesis and Characterization

Summary: A multistep synthetic procedure for preparing novel C₆₀‐anchored two‐armed poly(tert‐butyl acrylate) was developed. First, two‐armed poly(tert‐butyl acrylate) bearing a malonate ester core with well‐controlled molecular weight was synthesized through atom transfer radical polymerization. The effective Bingel reaction between C₆₀ and the well‐defined polymer was then carried out to yield C₆₀‐anchored polymer. GPC, ¹H NMR, and UV‐vis spectroscopy indicated that the C₆₀‐anchored polymer was a monosubstituted and ‘closed’ 6,6‐ring‐bridged methanofullerene derivative.

Schematic of a novel C₆₀‐anchored two‐armed polymer.     

Enantiotropic phase transition and twinning in 2,2,3,3,4,4‐hexafluoropentane‐1,5‐diol

Four crystal structure determinations of 2,2,3,3,4,4‐hexafluoropentane‐1,5‐diol (HFPD), C₅H₆F₆O₂, were conducted on a single specimen by varying the temperature. Two polymorphs of HFPD were found to be enantiotropically related as phases (I) and (II), both in the space group P1. These structures contain closely related R₄⁴(20) sheets. A structure determination was completed on form (Ia) at 283 K. Form (Ia) was then supercooled below the phase transition temperature at 279 to 173 K to give form (Ib) for a second structure determination. Metastable form (Ib) was transformed by momentary warming and recooling to give form (II) for a third structure determination at 173 K. Form (II) transformed to form (Ic) upon warming to 283 K. Enantiotropic phase transitions between phases (I) and (II) were confirmed with X‐ray powder diffraction and differential scanning calorimetry. Form (Ia) was found as a twin by nonmerohedry by a reflection in (011). This twinning persists in all phases described. Additional twinning was found after the phase (I) to phase (II) transformation. These two additional twin components are related to the first pair by a 180° rotation about the (012) plane. This latter pair of twins persisted as the specimen was warmed back to form (Ic) at 283 K.     

Concealed Cyclotrimeric Polymorph of Lithium 2,2,6,6‐Tetramethylpiperidide Unconcealed: X‐Ray Crystallographic and NMR Spectroscopic Studies

Lithium 2,2,6,6‐tetramethylpiperidide (LiTMP), one of the most important polar organometallic reagents both in its own right and as a key component of ate compositions, has long been known for its classic cyclotetrameric (LiTMP)₄ solid‐state structure. Made by a new approach through transmetalation of Zn(TMP)₂ with tBuLi in n‐hexane solution, a crystalline polymorph of LiTMP has been uncovered. X‐ray crystallographic studies at 123(2) K revealed this polymorph crystallises in the hexagonal space group P6₃/m and exhibited a discrete cyclotrimeric (C_3h) structure with a strictly planar (LiN)₃ ring containing three symmetrically equivalent TMP chair‐shaped ligands. The molecular structure of (LiTMP)₄ was redetermined at 123(2) K, because its original crystallographic characterisation was done at ambient temperature. This improved redetermination confirmed a monoclinic C2/c space group with the planar (LiN)₄ ring possessing pseudo (non‐crystallographic) C_4h symmetry. Investigation of both metalation and transmetalation routes to LiTMP under different conditions established that polymorph formation did not depend on the route employed but rather the temperature of crystallisation. Low‐temperature (freezer at ?35 °C) cooling of the reaction solution favoured (LiTMP)₃; whereas high‐temperature (bench) storage favoured (LiTMP)₄. Routine ¹H and ¹³C NMR spectroscopic studies in a variety of solvents showed that (LiTMP)₃ and (LiTMP)₄ exist in equilibrium, whereas ¹H DOSY NMR studies gave diffusion coefficient results consistent with their relative sizes.