Anion-induced switching of the helical orientation of a chiral indolocarbazole dimer

An indolocarbazole dimer, containing chiral urea appendages, that adopts a helically folded conformation by intramolecular hydrogen bonds as proven by ¹H NMR and circular dichroism (CD) spectroscopy has been prepared. Owing to the preferential formation of one helical conformer, strong CD signals appear in relatively non-polar solvents such as chloroform (CHCl₃) and dichloromethane (CH₂Cl₂) but the signal is negligible in dimethyl sulfoxide (DMSO). In addition, the optical rotation of the dimer is highly sensitive to the polarity of solvents. For example, the magnitude of the specific rotation ([α]_D) is ? 934° in CH₂Cl₂ and ? 657° in CHCl₃ but it is only ? 75° in DMSO. These observations suggest that the dimer folds to a helical structure by intramolecular hydrogen bonds in relatively non-polar solvents but exists in an unfolded extended conformation in polar solvents such as DMSO. The dimer strongly binds anions such as chloride, acetate and sulfate by multiple hydrogen bonds. In addition, anion binding leads to considerable CD spectral changes with the different pattern and degree of Cotton effects depending on the kind of anions. The dimer may be therefore utilised for the construction of an anion-responsive chiroptical sensor or switch.     

Conformational Panorama and Chirality Controlled Structure–Energy Relationship in a Chiral Carboxylic Acid Dimer

Chirality recognition in dimers of tetrahydro‐2‐furoic acid (THFA) was studied in a conformer‐specific manner using rotational spectroscopy and theoretical approaches. THFA shows a strong preference for the trans‐ over the cis‐COOH configuration. Two drastically different scenarios are possible for the detectable (THFA)₂: a kinetically preferred dimer bound by feeble interactions between two trans THFAs or a thermodynamically favored dimer with a double hydrogen‐bonded ring structure between two cis subunits. To identify the conformers responsible for the extremely dense rotational spectra observed, it was essential not only to locate several hundred homo/heterochiral (THFA)₂ minima in ab initio calculations but also to evaluate the energetic connectivities among the minima. The study further reveals an interesting chirality dependent structure–energy ordering relationship. A method for enantiomeric excess (ee) determination of THFA is presented using a recently proposed chiral self‐tag approach.     

Di‐μ‐chlorido‐bis[chlorido(4′‐p‐tolyl‐2,2′:6′,2′′‐terpyridine‐κ3N,N′,N′′)nickel(II)]: a supramolecular system constructed by C—H...Cl interactions

The title complex, [Ni₂Cl₄(C₂₂H₁₇N₃)₂], was synthesized solvothermally. The molecule is a centrosymmetric dimer with the unique Ni^II centre in a distorted octahedral N₃Cl₃ coordination environment. The chloride bridges are highly asymmetric. In the 4′‐p‐tolyl‐2,2′:6′,2′′‐terpyridine ligand, the p‐tolyl group is perfectly coplanar with the attached pyridine ring, and this differs from the situation found in previously reported compounds; however, there are no π–π interactions between the ligands. The terminal Cl atom forms four intermolecular C—H...Cl hydrogen bonds with one methyl and three methine groups. The methyl group also forms intermolecular C—H...π interactions with a pyridine ring. These nonclassical hydrogen bonds extend the molecule into a three‐dimensional network.     

2′‐Deoxy‐5‐propynyluridine: a nucleoside with two conformations in the asymmetric unit

The title compound, 1‐(2‐deoxy‐β‐d ‐erythro‐pentofuranosyl)‐5‐(prop‐1‐ynyl)pyrimidin‐2,4(1H,3H)‐dione, C₁₂H₁₄N₂O₅, shows two conformations in the crystalline state: conformer 1 adopts a C2′‐endo (close to ²E; S‐type) sugar pucker and an anti nucleobase orientation [χ = −134.04 (19)°], while conformer 2 shows an S sugar pucker (twisted C2′‐endo–C3′‐exo), which is accompanied by a different anti base orientation [χ = −162.79 (17)°]. Both molecules show a +sc (gauche, gauche) conformation at the exocyclic C4′—C5′ bond and a coplanar orientation of the propynyl group with respect to the pyrimidine ring. The extended structure is a three‐dimensional hydrogen‐bond network involving intermolecular N—H...O and O—H...O hydrogen bonds. Only O atoms function as H‐atom acceptor sites.     

Polymorphs and Polymorphic Cocrystals of Temozolomide

Crystal polymorphism in the antitumor drug temozolomide (TMZ), cocrystals of TMZ with 4,4′‐bipyridine‐N,N′‐dioxide (BPNO), and solid‐state stability were studied. Apart from a known X‐ray crystal structure of TMZ (form 1), two new crystalline modifications, forms 2 and 3, were obtained during attempted cocrystallization with carbamazepine and 3‐hydroxypyridine‐N‐oxide. Conformers A and B of the drug molecule are stabilized by intramolecular amide N? H???N_imidazole and N? H???N_tetrazine interactions. The stable conformer A is present in forms 1 and 2, whereas both conformers crystallized in form 3. Preparation of polymorphic cocrystals I and II (TMZ?BPNO 1:0.5 and 2:1) were optimized by using solution crystallization and grinding methods. The metastable nature of polymorph 2 and cocrystal II is ascribed to unused hydrogen‐bond donors/acceptors in the crystal structure. The intramolecularly bonded amide N–H donor in the less stable structure makes additional intermolecular bonds with the tetrazine C?O group and the imidazole N atom in stable polymorph 1 and cocrystal I, respectively. All available hydrogen‐bond donors and acceptors are used to make intermolecular hydrogen bonds in the stable crystalline form. Synthon polymorphism and crystal stability are discussed in terms of hydrogen‐bond reorganization.     

1,N6‐Etheno‐2′‐deoxytubercidin hemihydrate

The title compound [systematic name: 7‐(2‐deoxy‐β‐d ‐erythro‐pentofuranosyl)‐7H‐imidazo[1,2‐c]pyrrolo[2,3‐d]pyrimidine hemihydrate], 2C₁₃H₁₄N₄O₃·H₂O or (I)·0.5H₂O, shows two similar conformations in the asymmetric unit. These two conformers are connected through one water molecule by hydrogen bonds. The N‐glycosylic bonds of both conformers show an almost identical anti conformation with χ = −107.7 (2)° for conformer (I‐1) and −107.0 (2)° for conformer (I‐2). The sugar moiety adopts an unusual N‐type (C3′‐endo) sugar pucker for 2′‐deoxyribonucleosides, with P = 36.8 (2)° and τ_m = 40.6 (1)° for conformer (I‐1), and P = 34.5 (2)° and τ_m = 41.4 (1)° for conformer (I‐2). Both conformers and the solvent molecule participate in the formation of a three‐dimensional pattern with a `chain'‐like arrangement of the conformers. The structure is stabilized by intermolecular O—H...O and O—H...N hydrogen bonds, together with weak C—H...O contacts.     

Synthesis and properties of <Emphasis Type="Italic">O,O</Emphasis>-dialkyl [1-hydroxy-3-(dialkylamino)-2,2-dimethylpropyl]phosphonates

O,O-Dialkyl [1-hydroxy-3-(dialkylamino)-2,2-dimethylpropyl]phosphonates were prepared for the first time. By means of NMR ¹H, IR spectroscopy and quantum-chemical calculations the presence in them of various H-bonds was established. In the crystalline state P=O…HO intermolecular hydrogen bonds favor the formation of cyclic dimer associates D _P=O. In the liquid state and concentrated solutions P=O…HO and N…HO intermolecular hydrogen bonds cause the formation of cyclic dimer associates D _P=O and D_N, and intramolecular hydrogen bonds provide the existence of different conformations of the monomer form MN, the most stable among them with the non-strained six-membered …NCCCOH… ring.     

1,6‐Anhydro‐2,3‐di‐O‐benzyl‐5C‐[(R)‐ethoxycarbonyl(hydroxy)methyl]‐β‐l‐altrofuranose

The crystal structure of the title compound, C₂₄H₂₈O₈, has been determined. The conformation of the furan­ose ring can be described as 58% ideal envelope ^OE conformer and 42% ideal twisted ^OT₁ conformer. The 1,3‐dioxane ring adopts a chair conformation with the anhydro‐O atom pointing upwards. Both phenyl rings are quasi‐perpendicular to the mean plane of the furan­ose ring. The hydrogen bonding is intermolecular and consists of infinite chains parallel to the a axis.     

Hexa­methyl­enetetra­mine–4‐nitro­catechol–water (1/2/1)

In the title adduct, 1,3,5,7‐tetra­aza­tri­cyclo[3.3.1.1^3,7]dec­ane–4‐nitro­benzene‐1,2‐diol–water (1/2/1), C₆H₁₂N₄·2C₆H₅NO₄·H₂O, the hexa­methyl­ene­tetra­mine mol­ecule acts as an acceptor of intermolecular O—H?N hydrogen‐bonding interactions from the water mol­ecule and the hydroxy groups of one of the two symmetry‐independent 4‐nitro­catechol mol­ecules. The structure is built from molecular layers which are stabilized by three intermolecular O—H?O, two intermolecular O—H?N and four intermolecular C—H?O hydrogen bonds. The layers are further interconnected by one additional intermolecular O—H?N and two intermolecular C—H?O hydrogen bonds.     

Probing the relationships between molecular conformation and intermolecular contacts in N,N‐dibenzyl‐N′‐(furan‐2‐carbonyl)thiourea

In the crystal structure of the title compound, C₂₀H₁₈N₂O₂S, molecules are linked by bifurcated C—H...O hydrogen‐bond interactions, giving rise to chains whose links are composed of alternating centrosymmetrically disposed pairs of molecules and characterized by R₂²(10) and R₂²(20) hydrogen‐bonding motifs. Also, N—H...S hydrogen bonds form infinite zigzag chains along the [010] direction, which exhibit the C(4) motif. Hirshfeld surface and fingerprint plots were used to explore the intermolecular interactions in the crystal structure. This analysis confirms the important role of C—H...O hydrogen bonds in the molecular conformation and in the crystal structure, providing a potentially useful tool for a full understanding of the intermolecular interactions in acylthiourea derivatives.     

Geometry of an Isolated Dimer of Imidazole Characterised by Rotational Spectroscopy and Ab Initio Calculations

An isolated, gas‐phase dimer of imidazole is generated through laser vaporisation of a solid rod containing a 1:1 mixture of imidazole and copper in the presence of an argon buffer gas undergoing supersonic expansion. The complex is characterised through broadband rotational spectroscopy and is shown to have a twisted, hydrogen‐bonded geometry. Calculations at the CCSD(T)(F12*)/cc‐pVDZ‐F12 level of theory confirm this to be the lowest‐energy conformer of the imidazole dimer. The distance between the respective centres of mass of the imidazole monomer subunits is determined to be 5.2751(1) Å, and the twist angle γ describing rotation of one monomer with respect to the other about a line connecting the centres of mass of the monomers is determined to be 87.9(4)°. Four out of six intermolecular parameters in the model geometry are precisely determined from the experimental rotational constants and are consistent with results calculated ab initio.     

A pseudo‐quadruple hydrogen‐bonding motif consisting of six N—H⋯O hydrogen bonds in trimethoprim formate

The title compound, trimethoprim (TMP) formate [systematic name: 2,4‐di­amino‐5‐(3,4,5‐tri­methoxy­benzyl)­pyrimidin‐1‐ium formate], C₁₄H₁₉N₄O₃⁺·CHO₂^?, reveals a pseudo‐quadruple hydrogen‐bonding motif consisting of six N—H?O hydrogen bonds involving two unpaired TMP cations and two formate anions which are symmetrically disposed. The hydrogen‐bonding motif is strikingly comparable with that observed in other TMP salts where the amino­pyrimidine moieties of the TMP cations are centrosymmetrically paired. These conserved hydrogen‐bonding motifs may serve as robust synthons in crystal engineering and design. The characteristic pseudo‐quadruple hydrogen‐bonding motif and other intermolecular hydrogen bonds operating in the crystal form a two‐dimensional supramolecular sheet structure.     

cis‐Dichloridobis(1,10‐phenanthroline‐5,6‐diol‐κ2N,N′)manganese(II): a supramolecular chain formed via O—H...Cl bonds and aromatic stacking

The title compound, [MnCl₂(C₁₂H₈N₂O₂)₂], displays a novel supramolecular chain formed by intermolecular O—H...Cl hydrogen bonds and aromatic stacking. The molecule has crystallographically imposed twofold symmetry with the Mn^II atom on the twofold axis. In the 1,10‐phenanthroline‐5,6‐diol ligand, each H atom of the two hydroxy groups is oriented towards the other hydroxy O atom. Both hydroxy groups form intermolecular O—H...Cl hydrogen bonds with a single Cl atom of an adjacent molecule. These hydrogen bonds connect the molecules via operation of the molecular twofold axis and the centre of inversion of the crystal lattice, forming a doubly‐bridged one‐dimensional structure with Mn atoms as the nodes. Strong aromatic π‐stacking between two antiparallel neighbouring 1,10‐phenanthroline‐5,6‐diol ligands also helps to stabilize the chain.     

2,2,2‐Trifluoro‐N‐(1a,2,7,7a‐tetrahydronaphtho[2,3‐b]oxiren‐3‐yl)acetamide by X‐ray powder diffraction

The title compound, C₁₂H₁₀F₃NO₂, an important precursor in the preparation of benzovesamicol analogues for the diagnosis of Alzheimer's disease, was prepared by the epoxidation of 5,8‐dihydronaphthalen‐1‐amine using 3‐chloroperoxybenzoic acid. The structure was determined by X‐ray powder diffraction, multinuclear NMR spectroscopy and FT–IR spectroscopy. A pair of molecules form intermolecular N—H...O hydrogen bonds, involving the amino and oxirene groups, to produce a dimer.     

Similarities and differences in the structures of 5‐bromo‐6‐hydroxy‐7,8‐dimethylchroman‐2‐one and 6‐hydroxy‐7,8‐dimethyl‐5‐nitrochroman‐2‐one

The title compounds, C₁₁H₁₁BrO₃, (I), and C₁₁H₁₁NO₅, (II), respectively, are derivatives of 6‐hydroxy‐5,7,8‐trimethylchroman‐2‐one substituted at the 5‐position by a Br atom in (I) and by a nitro group in (II). The pyranone rings in both molecules adopt half‐chair conformations, and intramolecular O—H...Br [in (I)] and O—H...O_nitro [in (II)] hydrogen bonds affect the dispositions of the hydroxy groups. Classical intermolecular O—H...O hydrogen bonds are found in both molecules but play quite dissimilar roles in the crystal structures. In (I), O—H...O hydrogen bonds form zigzag C(9) chains of molecules along the a axis. Because of the tetragonal symmetry, similar chains also form along b. In (II), however, similar contacts involving an O atom of the nitro group form inversion dimers and generate R₂²(12) rings. These also result in a close intermolecular O...O contact of 2.686 (4) Å. For (I), four additional C—H...O hydrogen bonds combine with π–π stacking interactions between the benzene rings to build an extensive three‐dimensional network with molecules stacked along the c axis. The packing in (II) is much simpler and centres on the inversion dimers formed through O—H...O contacts. These dimers are stacked through additional C—H...O hydrogen bonds, and further weak C—H...O interactions generate a three‐dimensional network of dimer stacks.     

Di(phenylpropylamino)gossypol: a derivative of the dimeric natural product gossypol

Di(phenylpropylamino)gossypol [systematic name: 2,2′‐bis{1,6‐dihydroxy‐5‐isopropyl‐8‐[(3‐phenylpropylamino)methylidene]naphthalen‐7‐one}, C₄₈H₅₂N₂O₆, was formed by reaction of the dimeric natural product gossypol with 3‐phenylpropylamine. The structure of this compound has its two naphthalene ring systems oriented approximately perpendicular to each other, and the two pendant phenylpropyl groups have different conformations. One of these side groups is considerably disordered at room temperature but less so at 120 K. The enantiomeric molecules form centrosymmetric dimers that are supported by intermolecular hydrogen bonds and by hydrophobic interactions between a pair of naphthalene rings. Two additional hydrogen bonds tie the dimer pairs into layers. Unlike gossypol and many gossypol Schiff base derivatives, the title compound crystallizes without the inclusion of solvent, which appears to occur because of the size and flexibility of its phenylpropyl pendent groups.     

Chloro(2‐{[5‐(2‐hydroxy­benzyl‐O)‐1,5‐di­aza‐1‐cyclo­octyl‐N,N′]­methyl}phenolato‐O)­cadmium(II)

The crystal structure of the title complex, [Cd(C₂₀H₂₅­N₂­O₂)­Cl], reveals a hydrogen‐bonded dimer composed of neutral molecules. The Cd^II center is five‐coordinated by two O atoms of the pendant arms, two nitro­gen donors of the 1,5‐di­aza­cyclo­octane ring and a chloride anion. The coordination geometry of the complex could be described as a distorted square pyramid. The 1,5‐di­aza­cyclo­octane backbone adopts a boat/chair configuration and the two phenol/phenolato groups have a dihedral angle of 101.3 (2)° between them. The coordinated phenolate and phenolic groups of inversion‐related mol­ecules form strong intermolecular O—H?O hydrogen bonds.     

2,2,2‐Trinitroethanol

2,2,2‐Trinitroethanol, C₂H₃N₃O₇, at 100 (2) K has Z′ = 2 in the space group P2₁/c. The structure displays intramolecular O—H...O hydrogen bonds, as well as intermolecular O—H...O and C—H...O hydrogen bonding; the O—H...O hydrogen bonds, forming R₄⁴(8) rings, and dipolar nitro–nitro interactions account for the high density of 1.839 Mg m⁻³.     

2‐Ethyl‐6‐methylpyridin‐3‐ol and its salt bis(2‐ethyl‐3‐hydroxy‐6‐methylpyridinium) succinate

The title compounds, C₈H₁₁NO, (I), and 2C₈H₁₂NO⁺·C₄H₄O₄²⁻, (II), both crystallize in the monoclinic space group P2₁/c. In the crystal structure of (I), intermolecular O—H...N hydrogen bonds combine the molecules into polymeric chains extending along the c axis. The chains are linked by C—H...π interactions between the methylene H atoms and the pyridine rings into polymeric layers parallel to the ac plane. In the crystal structure of (II), the succinate anion lies on an inversion centre. Its carboxylate groups interact with the 2‐ethyl‐3‐hydroxy‐6‐methylpyridinium cations via intermolecular N—H...O hydrogen bonds with the pyridine ring H atoms and O—H...O hydrogen bonds with the hydroxy H atoms to form polymeric chains, which extend along the [01] direction and comprise R₄⁴(18) hydrogen‐bonded ring motifs. These chains are linked to form a three‐dimensional network through nonclassical C—H...O hydrogen bonds between the pyridine ring H atoms and the hydroxy‐group O atoms of neighbouring cations. π–π interactions between the pyridine rings and C—H...π interactions between the methylene H atoms of the succinate anion and the pyridine rings are also present in this network.