α‐Tris(2,4‐pentanedionato‐κ2O,O′)aluminium(III) at 240, 210, 180, 150 and 110 K: a new δ phase at 110 K

The crystal structure of the title compound, [Al(C₅H₇O₂)₃], has been investigated by a multi‐temperature measurement to provide information on thermal vibrations and disorder in the structure. At 110 K, the structure of a new δ polymorph could be determined. A disorder–order phase transition takes place between 150 and 110 K and is klassengleich. The unit‐cell volume increases by a factor of three and the diffraction pattern shows weak supercell reflections.     

Three polymorphs (α, β, and δ) of d‐mannitol at 100 K

In the monoclinic δ polymorph of d ‐mannitol, C₆H₁₄O₆, both the mol­ecule and the packing have approximate twofold rotational symmetry. The P2₁ structure thus approximates space group C222₁, and the α′ polymorph, previously reported in that space group, is almost certainly identical to the δ polymorph. However, torsion angles along the main backbone of the mol­ecule deviate from twofold symmetry by as much as 7.4 (3)° and the hydrogen‐bonding pattern does not conform to the higher symmetry. The α polymorph reported here is identical to the previously reported κ polymorph, and the low‐temperature structure of the β polymorph agrees well with previously reported room‐temperature determinations. The range of C—O bond lengths over the three polymorphs is 1.428 (2)–1.437 (4) Å, and the range of C—C distances is 1.515 (4)–1.5406 (19) Å. The δ polymorph has the highest density of the three, both at room temperature and at 100 K.     

α‐Tris(2,4‐pentanedionato‐κ2O,O′)cobalt(III) at 240, 210, 180, 150 and 110 K

The crystal structure of the title compound, [Co(C₅H₇O₂)₃], has been investigated by a multi‐temperature measurement. In contrast to the isomorphous Al compound, the title compound exists in the studied temperature range as its monoclinic α polymorph (space group P2₁/c) and does not undergo a phase transition. Rigid‐body TLS analyses have been performed and the anisotropic thermal expansion tensor α_ij has been determined. The cell axes show a linear expansion behavior with respect to the temperature, but the slope is significantly different. A possible explanation are the different strengths of different intermolecular C—H...O contacts, which run in different crystallographic directions.     

Partial ordering of tripivaloylmethane at 110 K

Tripivaloylmethane [systematic name: 4‐(2,2‐dimethylpropanoyl)‐2,2,6,6‐tetramethylheptane‐3,5‐dione], C₁₆H₂₈O₃, is known to crystallize at room temperature in the space group R3m with three molecules in the unit cell. The molecules are conformationally chiral and pack so that each molecular site is occupied with equal probability by the two enantiomers. Upon cooling to 110 K, the structure partially orders; two molecules in the unit cell order into two different conformations of opposite chirality, while the third remains disordered. The symmetry of the resulting crystal is P3, with each of the molecules lying about a different threefold rotation axis. This paper describes an unusual case of order–disorder phase transition in which the structure partially orders by changes of molecular conformation in the single crystals. Such behaviour is of interest in the study of phase transitions and molecular motion in the solid state.     

Hexakis(urea‐κO)zinc(II) dinitrate at 110 and 250 K: uniaxial negative thermal expansion

The crystal structure of the title compound, [Zn{CO(NH₂)₂}₆](NO₃)₂, has been determined at 110 and 250 K. The structure is stabilized by 12 individual hydrogen bonds, both intra‐ and intermolecular. Analysis of the thermal expansion tensor, based on unit cells determined over a temperature range of 180 K, shows uniaxial compression in the direction of the b axis during warming. The hydrogen bonds form layers perpendicular to this axis and these layers are connected by coordinative bonds parallel to the axis. As expected, the intermolecular hydrogen bonds expand during warming. Surprisingly, the coordinative bonds contract, accompanied by changes in the O—Zn—O angles. Overall, this behaviour can be described as an accordion‐like effect.     

Copper(II) hypophosphite: the α‐ and β‐forms at 270 and 100 K,and the γ‐form at 270 K

Copper(II) hypophosphite has been shown to exist as several polymorphs. The crystal structures of monoclinic α‐, ortho­rhombic β‐ and ortho­rhombic γ‐Cu(H₂PO₂)₂ have been determined at different temperatures. The geometry of the hypophosphite anion in all three polymorphs is very close to the idealized one, with point symmetry mm2. Despite having different space groups, the structures of the α‐ and β‐polymorphs are very similar. The polymeric layers formed by the Cu atoms and the hypophosphite ions, which are identical in the α‐ and β‐polymorphs, stack in the third dimension in different ways. Each hypophosphite anion is coordinated to three Cu atoms. On cooling, a minimum amount of contraction was observed in the direction normal to the layers. The structure of the polymeric layers in the γ‐­polymorph is quite different. There are two symmetry‐independent hypophosphite anions; the first is coordinated to two Cu atoms, while the second is coordinated to four Cu atoms. In all three polymorphs, the Cu atoms are coordinated by six O atoms of six hypophosphite anions, forming tetragonal bipyramids; in the α‐ and β‐polymorphs, there are four short and two long Cu—O distances, while in the γ‐polymorph, there are four long and two short Cu—O distances.     

A centrosymmetric polymorph of chloridotris(triphenylphosphine‐κP)copper(I) at 150 K

This study characterizes a new polymorph of the title compound, [CuCl(C₁₈H₁₅P)₃], and analyses the influence of the extensive network of weak hydrogen‐bonding interactions in the generation of this different crystal structure. The compound crystallizes in the centrosymmetric space group C2/c with two crystallographically independent molecules per asymmetric unit, in contrast with the previously determined polymorph which crystallizes in the noncentrosymmetric space group P3 with three crystallographically independent molecules in the asymmetric unit, each with crystallographically imposed C3 symmetry [Gill, Mayerle, Welcker, Lewis, Ucko, Barton, Stowens & Lippard (1976). Inorg. Chem. 15 , 1155–1168]. The geometries of the two molecules of the title compound are very similar, with each having an approximately tetrahedral coordination around the central Cu atom and approximate C_s molecular symmetry. The molecules pack in chains parallel to the crystallographic b axis, connected by C—H...Cl and C—H...π(phenyl) hydrogen bonds.     

Two polymorphs of anhydrous 4‐pyridone at 100 K

Two polymorphs of the title compound, C₅H₅NO, (I), have been obtained from ethanol. One polymorph crystallizes in the monoclinic space group C2/c [henceforth (I)‐M], while the other crystallizes in the orthorhombic space group Pbca [henceforth (I)‐O]. In the two forms, the lattice parameters, cell volume and packing motifs are very similar. There are also two independent molecules of 4‐pyridone in each asymmetric unit. The molecules are linked by N—H...O hydrogen bonds into one‐dimensional zigzag chains extending along the b axis in the (I)‐M polymorph and along the a axis in the (I)‐O polymorph, with the graph set C₂²(12). The structures are stabilized by weak C—H...O hydrogen bonds linking adjacent chains, thus forming a ring with the graph set R₆⁵(28). The significance of this study lies in the analysis of the hydrogen‐bond interactions occurring in these structures. Analyses of the crystal structures of the two polymorphs of 4‐pyridone are helpful in elucidating the mechanism of the generation of spectroscopic effects observed in the IR spectra of these polymorphs in the frequency range of the N—H stretching vibration band.     

1,3,4‐Triphenyl‐7‐trifluoromethyl‐1H‐pyrazolo[3,4‐b]quinoline at 293 and 100 K

In the structure of the title compound, C₂₉H₁₈F₃N₃, belonging to the space group P6₅ (or P6₁), three symmetry‐independent molecules are arranged in two chains, with two molecules alternating along the 3₂ axes, whereas the remaining molecule forms a chain along [0001] due to the 6₅ screw axis. The conformation of each of the molecules is stabilized by an intramolecular C—H...N hydrogen bond, with C...N distances in the range 2.964 (6)–3.069 (5) Å at room temperature (293 K) and 2.943 (4)–3.084 (4) Å at low temperature (100 K). One molecule has its –CF₃ group ordered even at 293 K, which can be explained only by considering its involvement in two weak intermolecular C—H...F interactions, with C...F distances in the range 3.084 (6)–3.302 (5) Å at 293 K and 3.070 (3)–3.196 (3) Å at 100 K, and also a C—F...N interaction, with a C...N distance of 3.823 (5) Å at 293 K and 3.722 (4) Å at 100 K. The trifluoromethyl groups in the two remaining molecules are disordered at 293 K, whereas at 100 K the continuous (dynamic) positional disorder of one of the –CF₃ groups (of the molecule forming the chain along [0001]) is totally eliminated while the –CF₃ group disorder remains for the third molecule.     

trans‐4‐Bromo‐ONN‐azoxy­benzene at 100 K

The crystal structure of the α isomer of trans‐4‐bromo­azoxy­benzene [systematic name: trans‐1‐(bromophenyl)‐2‐phenyl­diazene 2‐oxide], C₁₂H₉BrN₂O, has been determined by X‐ray dif­frac­tion. The geometries of the two mol­ecules in the asymmetric unit are slightly different and are within ∼0.02 Å for bond lengths, ∼2° for angles and ∼3° for torsion angles. The azoxy bridges in both mol­ecules have the typical geometry observed for trans‐azoxy­benzenes. The crystal network contains two types of planar mol­ecules arranged in columns. The torsion angles along the Ar—N bonds are only 7 (2)°, on either side of the azoxy group.     

Low‐temperature study of 3‐acetylindole at 110 K

The title compound, C₁₀H₉NO, contains an acetyl group that is nearly coplanar with the indole ring system, with an angle between the planes of the heterocyclic ring and the acetyl group of 1.75 (17)°. The planes of the benzene and pyrrole rings in the indole system make a dihedral angle of 2.05 (11)°. Each molecule in the unit cell is linked through N—H...O hydrogen bonds to two other molecules, forming hydrogen‐bonded chains in the [101] direction with graph set C(6). The significance of this study lies in the analysis of the interactions occurring via hydrogen bonds in this structure, as well as in the comparison drawn between the molecular structure of the title compound and those of several other indole derivatives possessing a 3‐carbonyl group. The correlation between the IR spectrum of this compound and the structural data is also discussed.     

β‐Carboline (norharman)

The structure of β‐carboline, also called norharman (systematic name: 9H‐pyrido[3,4‐b]indole), C₁₁H₈N₂, has been determined at 110 K. Norharman is prevalent in the environment and the human body and is of wide biological interest. The structure exhibits intermolecular N—H...N hydrogen bonding, which results in a one‐dimensional herringbone motif. The three rings of the norharman molecule collectively result in a C‐shaped curvature of 3.19 (13)° parallel to the long axis. The diffraction data show shorter pyridyl C—C bonds than those reported at the STO‐3G level of theory.     

A second monoclinic polymorph of S‐­(4‐tolyl) 4‐toluenethiosulfonate at 150 and 293 K

The title compound, C₁₄H₁₄O₂S₂, contains discrete mol­ecules and is a polymorph of a structure reported previously by Caputo, Palumbo, Nardelli & Pelizzi [Gazz. Chim. Ital. (1984), 114 , 421–430]. The present structure contains no intermolecular C—H?O hydrogen bonds, whereas in the previous polymorph, the mol­ecules are linked into continuous chains by such interactions.     

Polymorphism of dinitro[tris(2‐aminoethyl)amine]cobalt(III) chloride

Three polymorphs of bis(nitrito‐κN)[tris(2‐aminoethyl)amine‐κ⁴N,N′,N′′,N′′′]cobalt(III) chloride, [Co(NO₂)₂(C₆H₁₈N₄)]Cl, have been structurally characterized in the 100–300 K temperature range. Two orthorhombic polymorphs are related by a solid‐state enantiotropic order–disorder k2 phase transition at ca 152 K. The third, monoclinic, polymorph crystallizes as a nonmerohedral twin. In the structure of the high‐temperature (300 K) orthorhombic polymorph, the Co^III complex cation resides on a crystallographic mirror plane, whereas the Cl⁻ anion occupies a crystallographic twofold axis. In the unit cell of the monoclinic polymorph, the cationic Co^III complex is in a general position, whose charge is balanced by two halves of two Cl⁻ anions, each residing on a crystallographic twofold axis.     

Apparent shortening of the Csp3—Csp3 bond analysed via a variable‐temperature X‐ray diffraction study in racemic 1,1′‐binaphthalene‐2,2′‐diyl diethyl bis(carbonate)

Crystal structure determination at room temperature [292 (2) K] of racemic 1,1′‐binaphthalene‐2,2′‐diyl diethyl bis(carbonate), C₂₆H₂₂O₆, showed that one of the terminal carbon–carbon bond lengths is very short [Csp³—Csp³ = 1.327 (6) Å]. The reason for such a short bond length has been analysed by collecting data sets on the same crystal at 393, 150 and 90 K. The values of the corrected bond lengths clearly suggest that the shortening is mainly due to positional disorder at two sites, with minor perturbations arising as a result of thermal vibrations. The positional disorder has been resolved in the analysis of the 90 K data following the changes in the unit‐cell parameters for the data sets at 150 and 90 K, which appear to be an artifact of a near centre of symmetry relationship between the two independent molecules in the space group P at these temperatures. Indeed, the unit cell at low temperature (150 and 90 K) is a supercell of the room‐temperature unit cell.     

catena‐Poly[[triphenyltin(IV)]‐μ‐hydroxo‐κ2O:O] at 120 K

The structure of the title compound, [Sn(C₆H₅)₃(OH)]_n, has been re‐investigated at 120 (2) K. The hydroxyl H atom was readily located and the threefold coordination about the O atom is planar. There are no hydrogen bonds involving the hydroxyl group, either as donor or as acceptor.     

A third polymorph of 4‐(2,6‐difluorophenyl)‐1,2,3,5‐dithiadiazolyl

The crystal structure of a third polymorphic form of the known 4‐(2,6‐difluorophenyl)‐1,2,3,5‐dithiadiazolyl radical, C₇H₃F₂N₂S₂, is reported. This new polymorph represents a unique crystal‐packing motif never before observed for 1,2,3,5‐dithiadiazolyl (DTDA) radicals. In the two known polymorphic forms of the title compound, all of the molecules form cis‐cofacial dimers, such that two molecules are π‐stacked with like atoms one on top of the other, a common arrangement for DTDA species. By contrast, the third polymorph, reported herein, contains two crystallographically unique molecules organized such that only 50% are dimerized, while the other 50% remain monomeric radicals. The dimerized molecules are arranged in the trans‐antarafacial mode. This less common dimer motif for DTDA species is characterized by π–π interactions between the S atoms [S...S = 3.208 (1) Å at 110 K], such that the two molecules of the dimer are related by a centre of inversion. The most remarkable aspect of this third polymorph is that the DTDA dimers are co‐packed with monomers. The monomeric radicals are arranged in one‐dimensional chains directed by close lateral intermolecular contacts between the two S atoms of one DTDA heterocycle and an N atom of a neighbouring coplanar DTDA heterocycle [S...N = 2.857 (2) and 3.147 (2) Å at 110 K].     

Two polymorphs of chlorpropamide: the δ‐form and the high‐temperature ɛ‐form

The ɛ‐form of chlorpropamide [systematic name: 4‐chloro‐N‐(propylaminocarbonyl)benzenesulfonamide], C₁₀H₁₃ClN₂O₃S, has been obtained as single crystals from solution (and not as a polycrystalline sample by heating the α‐, γ‐ or δ‐forms). The results of anisotropic structure refinements for the ɛ‐ and δ‐forms are reported. The density of the δ‐polymorph is the highest, and that of the ɛ‐polymorph the lowest, among the five known chlorpropamide polymorphs. The main intermolecular hydrogen‐bonding pattern in polymorphs δ and ɛ is the same as in polymorphs α, β and γ, but the conformations differ. The densities of the polymorphs were found to depend on the molecular conformations.     

Two polymorphs of 20‐desmethyl‐β‐carotene

Two polymorphs of 20‐desmethyl‐β‐carotene (systematic name: 20‐nor‐β,β‐carotene), C₃₉H₅₄, in monoclinic and triclinic space groups, were formed in the same vial by recrystallization from pyridine and water. Each polymorph crystallizes with the complete molecule as the asymmetric unit, and the two polymorphs show differing patterns of disorder. The β end rings of both polymorphs have the 6‐s‐cis conformation, and are twisted out of the plane of the polyene chain by angles of −53.2 (8) and 47.3 (8)° for the monoclinic polymorph, and −43.6 (3) and 56.1 (3)° for the triclinic polymorph. The cyclohexene end groups are in the half‐chair conformation, but the triclinic polymorph shows disorder of one ring. Overlay of the molecules shows that they differ in the degree of nonplanarity of the polyene chains and the angles of twist of the end rings. The packing arrangements of the two polymorphs are quite different, with the monoclinic polymorph showing short intermolecular contacts of the disordered methyl groups with adjacent polyene chain atoms, and the triclinic polymorph showing π–π stacking interactions of the almost parallel polyene chains. The determination of the crystal structures of the two title polymorphs of 20‐desmethyl‐β‐carotene allows information to be gained regarding the structural effects on the polyene chain, as well as on the end groups, versus that of the parent compound β‐carotene. The absence of the methyl group is known to have an impact on various functions of the title compound.     

4‐Dimethyl­amino‐β‐nitro­styrene and 4‐dimethyl­amino‐β‐ethyl‐β‐nitro­styrene at 100 K

The structures of 4‐dimethyl­amino‐β‐nitro­styrene (DANS), C₁₀H₁₂N₂O₂, and 4‐dimethyl­amino‐β‐ethyl‐β‐nitro­styrene (DAENS), C₁₂H₁₆N₂O₂, have been solved at T = 100 K. The structure solution for DANS was complicated by the presence of a static disorder, characterized by a misorientation of 17% of the mol­ecules. The mol­ecule of DANS is almost planar, indicating significant conjugation, with a push–pull effect through the styrene skeleton extending up to the terminal substituents and enhancing the dipole moment. As a consequence of this conjugation, the hexa­gonal ring displays a quinoidal character; the lengths of the C—N [1.3595 (15) Å] and C—C [1.448 (2) Å] bonds adjacent to the benzene ring are shorter than single bonds. The mol­ecules are stacked in dimers with anti­parallel dipoles. In contrast, the mol­ecule of DAENS is not planar. The ethyl substituent pushes the nitro­propene group out of the benzene plane, with a torsion angle of −21.9 (3). Nevertheless, the mol­ecule remains conjugated, with a shortening of the same bonds as in DANS.