The peptide NCbz‐Val‐Tyr‐OMe and aromatic π–π interactions

The peptide N‐benzyloxycarbonyl‐L‐valyl‐L‐tyrosine methyl ester or NCbz‐Val‐Tyr‐OMe (where NCbz is N‐benzyloxycarbonyl and OMe indicates the methyl ester), C₂₃H₂₈N₂O₆, has an extended backbone conformation. The aromatic rings of the Tyr residue and the NCbz group are involved in various attractive intra‐ and intermolecular aromatic π–π interactions which stabilize the conformation and packing in the crystal structure, in addition to N—H...O and O—H...O hydrogen bonds. The aromatic π–π interactions include parallel‐displaced, perpendicular T‐shaped, perpendicular L‐shaped and inclined orientations.     

3‐Oxa‐6,8‐diaza‐1,2:4,5‐dibenzocycloocta‐1,4‐dien‐7‐one: a three‐dimensional network assembled by hydrogen‐bonding, π–π and edge‐to‐face interactions

The title compound, C₁₃H₁₀N₂O₂, is the first structure in which the urea moiety is incorporated into an eight‐membered ring. Two mol­ecules are found in the asymmetric unit, which are almost identical in their conformation and their hydrogen‐bond pattern. The carbonyl O atom acts as a double acceptor for the NH groups of two adjacent mol­ecules. In this way, infinite tapes are formed, which are connected viaπ–π and edge‐to‐face interactions in the second and third dimension. This hierarchical order of interactions is confirmed by molecular mechanics calculations. Force‐field and semi‐empirical calculations for a single mol­ecule did not find the envelope conformation present in the crystal, indicating instead a C_s conformation. Only with a model consisting of a hydrogen‐bonded dimer or a larger hydrogen‐bonded section was a conformation found that was similar to the one present in the crystal.     

Synthesis, π‐Face‐Selective Aggregation,and π‐Face Chiral Recognition of Configurationally Stable C3‐Symmetric Propeller‐Chiral Molecules with a π‐Core

The C₃‐symmetric propeller‐chiral compounds (P,P,P)‐ 1 and (M,M,M)‐ 1 with planar π‐cores perpendicular to the C₃‐axis were synthesized in optically pure states. (P,P,P)‐ 1 possesses two distinguishable propeller‐chiral π‐faces with rims of different heights named the (P/L)‐face and (P/H)‐face. Each face is configurationally stable because of the rigid structure of the helicenes contained in the π‐core. (P,P,P)‐ 1 formed dimeric aggregates in organic solutions as indicated by the results of ¹H NMR, CD, and UV/Vis spectroscopy and vapor pressure osmometry analyses. The (P/L)/(P/L) interactions were observed in the solid state by single‐crystal X‐ray analysis, and they were also predominant over the (P/H)/(P/H) and (P/L)/(P/H) interactions in solution, as indicated by the results of ¹H and 2D NMR spectroscopy analyses. The dimerization constant was obtained for a racemic mixture, which showed that the heterochiral (P,P,P)‐ 1 /(M,M,M)‐ 1 interactions were much weaker than the homochiral (P,P,P)‐ 1 /(P,P,P)‐ 1 interactions. The results indicated that the propeller‐chiral (P/L)‐face interacts with the (P/L)‐face more strongly than with the (P/H)‐face, (M/L)‐face, and (M/H)‐face. The study showed the π‐face‐selective aggregation and π‐face chiral recognition of the configurationally stable propeller‐chiral molecules.     

The tropolone–isobutylamine complex: a hydrogen‐bonded troponoid without dominant π–π interactions

Tropolone long has served as a model system for unraveling the ubiquitous phenomena of proton transfer and hydrogen bonding. This molecule, which juxtaposes ketonic, hydroxylic, and aromatic functionalities in a framework of minimal complexity, also has provided a versatile platform for investigating the synergism among competing intermolecular forces, including those generated by hydrogen bonding and aryl coupling. Small members of the troponoid family typically produce crystals that are stabilized strongly by pervasive π–π, C—H…π, or ion–π interactions. The organic salt (TrOH·iBA) formed by a facile proton‐transfer reaction between tropolone (TrOH) and isobutylamine (iBA), namely isobutylammonium 7‐oxocyclohepta‐1,3,5‐trien‐1‐olate, C₄H₁₂N⁺·C₇H₅O₂⁻, has been investigated by X‐ray crystallography, with complementary quantum‐chemical and statistical‐database analyses serving to elucidate the nature of attendant intermolecular interactions and their synergistic effects upon lattice‐packing phenomena. The crystal structure deduced from low‐temperature diffraction measurements displays extensive hydrogen‐bonding networks, yet shows little evidence of the aryl forces (viz. π–π, C—H…π, and ion–π interactions) that typically dominate this class of compounds. Density functional calculations performed with and without the imposition of periodic boundary conditions (the latter entailing isolated subunits) documented the specificity and directionality of noncovalent interactions occurring between the proton‐donating and proton‐accepting sites of TrOH and iBA, as well as the absence of aromatic coupling mediated by the seven‐membered ring of TrOH. A statistical comparison of the structural parameters extracted for key hydrogen‐bond linkages to those reported for 44 previously known crystals that support similar binding motifs revealed TrOH·iBA to possess the shortest donor–acceptor distances of any troponoid‐based complex, combined with unambiguous signatures of enhanced proton‐delocalization processes that putatively stabilize the corresponding crystalline lattice and facilitate its surprisingly rapid formation under ambient conditions.     

5‐Chloro‐2‐hydroxybenzophenone,featuring O—H...O,C—H...O,C—H...π and π–π interactions

Molecules of the title compound, C₁₃H₉ClO₂, contain an intramolecular O—H...O hydrogen bond, and the two aromatic rings are inclined at 57.02 (3)° with respect to one another. The crystal structure is supported by C—H...O, C—H...π and π–π interactions.     

Inter‐ and intramolecular C—H...π interactions in morphine bis­(1‐naph­tho­ate)

The crystal structure of morphine bis­(1‐naph­tho­ate) [or 7,8‐di­de­hydro‐4,5‐epoxy‐17‐methyl­morphinan‐2,6‐diyl bis­(naph­thal­ene‐1‐carboxyl­ate)], C₃₉H₃₁NO₅, determined at 123 K, shows extensive C—H...π interactions in the crystal lattice. Of particular interest is an intramolecular C—H...π interaction within the unit cell between the two naphthoyl groups. Comparison of the opiate scaffolds of morphine bis­(1‐naph­tho­ate) and morphine shows only a small increase in strain due to the steric bulk of the naphthoyl groups. The crystal packing shows distinct areas of packing for the naphthalene/aromatic groups and the opiate backbone. Extensive inter‐ and intramolecular C—H...π interactions lead to a densely packed aromatic region in the crystal lattice.     

Supramolecular networks supported by the anion…π linkage of Keggin‐type heteropolyoxotungstates

Anion…π interactions are newly recognized weak supramolecular forces which are relevant to many types of electron‐deficient aromatic substrates. Being less competitive with respect to conventional hydrogen bonding, anion…π interactions are only rarely considered as a crystal‐structure‐defining factor. Their significance dramatically increases for polyoxometalate (POM) species, which offer extended oxide surfaces for maintaining dense aromatic/inorganic stacks. The structures of tetrakis(caffeinium) μ₁₂‐silicato‐tetracosa‐μ₂‐oxido‐dodecaoxidododecatungsten trihydrate, (C₈H₁₁N₄O₂)₄[SiW₁₂O₄₀]·3H₂O, (1), and tris(theobrominium) μ₁₂‐phosphato‐tetracosa‐μ₂‐oxido‐dodecaoxidododecatungsten ethanol sesquisolvate, (C₇H₉N₄O₂)₃[PW₁₂O₄₀]·1.5C₂H₅OH, (2), support the utility of anion…π interactions as a special kind of supramolecular synthon controlling the structures of ionic lattices. Both caffeinium [(HCaf)⁺ in (1)] and theobrominium cations [(HTbr)⁺ in (2)] reveal double stacking patterns at both axial sides of the aromatic frameworks, leading to the generation of anion…π…anion bridges. The latter provide the rare face‐to‐face linkage of the anions. In (1), every square face of the metal–oxide cuboctahedra accepts the interaction and the above bridges yield flat square nets, i.e. {(HCaf⁺)₂[SiW₁₂O₄₀]^4?}_n. Two additional cations afford single stacks only and they terminate the connectivity. Salt (2) retains a two‐dimensional (2D) motif of square nets, with anion…π…anion bridges involving two of the three (HTbr)⁺ cations. The remaining cations complete a fivefold anion…π environment of [PW₁₂O₄₀]^3?, acting as terminal groups. This single anion…π interaction is influenced by the specific pairing of (HTbr)⁺ cations by double amide‐to‐amide hydrogen bonding. Nevertheless, invariable 2D patterns in (1) and (2) suggest the dominant role of anion…π interactions as the structure‐governing factor, which is applicable to the construction of noncovalent linkages involving Keggin‐type oxometalates.     

Hydrogen‐bonding versusπ–π stacking interactions in dipyrido[f,h]quinoxaline‐6,7‐dicarbonitrile and 6,7‐dicyanodipyrido[f,h]quinoxalin‐1‐ium chloride dihydrate

The solvent‐free title compound, C₁₆H₆N₆, is an aromatic derivative of phenanthroline with an extended π system. It exhibits a remarkable π–π columnar stacking in the crystal structure, with interplanar distances of 3.229 (3) and 3.380 (3) Å, the shorter spacing being between the two molecules within the asymmetric unit. Adjacent units along the stacked arrays are rotated in‐plane with respect to one another by approximately 120°. The hydrochloride derivative, C₁₆H₇N₆⁺·Cl⁻·2H₂O, in which one of the phenanthroline N atoms has been protonated, crystallized as a dihydrate. The supramolecular organization in this compound is characterized by continuous hydrogen bonding between the component species, yielding two‐dimensional hydrogen‐bonded networks. This study demonstrates the high significance of the π–π stacking interactions in the solvent‐free aromatic system and how they can be undermined by introducing hydrogen‐bonding capacity into the ligand.     

C—H...π interactions in five ethynyl‐substituted [2.2]paracyclophanes: further examples of the `7,11' packing pattern

The monosubstituted derivative 4‐ethynyl[2.2]paracyclophane, C₁₈H₁₆, (I), and the four disubstituted isomers, 4,12‐, (II), 4,13‐, (III), 4,15‐, (IV), and 4,16‐diethynyl[2.2]paracyclophane, (V), all C₂₀H₁₆, show the usual distortions of the [2.2]paracyclophane framework. The crystal packing is analyzed in terms of C—H...π interactions, some with H...π as short as 2.47 Å, in which the cyclophane rings and/or the triple‐bond systems may act as acceptors. For compounds (I) and (IV), the known `7,11'‐type cyclophane packing is observed, with a herring‐bone pattern of molecules in a layer structure.     

Structural insights into methyl‐ or methoxy‐substituted 1‐(α‐aminobenzyl)‐2‐naphthol structures: the role of C—H…π interactions

Aminobenzylnaphthols are a class of compounds containing a large aromatic molecular surface which makes them suitable candidates to study the role of C—H…π interactions. We have investigated the effect of methyl or methoxy substituents on the assembling of aromatic units by preparing and determining the crystal structures of (S,S)‐1‐{(4‐methylphenyl)[(1‐phenylethyl)amino]methyl}naphthalen‐2‐ol, C₂₆H₂₅NO, and (S,S)‐1‐{(4‐methoxyphenyl)[(1‐phenylethyl)amino]methyl}naphthalen‐2‐ol, C₂₆H₂₅NO₂. The methyl group influenced the overall crystal packing even if the H atoms of the methyl group did not participate directly either in hydrogen bonding or C—H…π interactions. The introduction of the methoxy moiety caused the formation of new hydrogen bonds, in which the O atom of the methoxy group was directly involved. Moreover, the methoxy group promoted the formation of an interesting C—H…π interaction which altered the orientation of an aromatic unit.     

4‐Methoxy‐1‐naphthol: chains formed by O—H...O hydrogen bonds and π–π stacking interactions

The structure of 4‐methoxy‐1‐naphthol, C₁₁H₁₀O₂, (I), contains an intermolecular O—H...O hydrogen bond which links the molecules into a simple C(2) chain running parallel to the shortest crystallographic b axis. This chain is reinforced by intermolecular π–π stacking interactions. Comparisons are drawn between the crystal structure of (I) and those of several of its simple analogues, including 1‐naphthol and some monosubstituted derivatives, and that of its isomer 7‐methoxy‐2‐naphthol. This comparison shows a close similarity in the packing of the molecules of its simple analogues that form π‐stacks along the shortest crystallographic axes. A substantial spatial overlap is observed between adjacent molecules in such stacks. In this group of monosubstituted naphthols, the overlap depends mainly on the position of the substituents carried by the naphthalene moiety, and the extent of the overlap depends on the substituent type. By contrast with (I), in the crystal structure of the isomeric 7‐methoxy‐2‐naphthol there are no O—H...O hydrogen bonds or π–π stacking interactions, and sheets are formed by O—H...π and C—H...π interactions.     

Two azo pigments based on β‐naphthol

There has been much discussion in the literature of the azo–hydrazone tautomerism of pigments. All commercial azo pigments with β‐naphthol as the coupling compound adopt the hydrazone tautomeric form (Ph—NH—N=C) in the solid state. In contrast, the red pigments 1‐[4‐(dimethylamino)phenyldiazenyl]‐2‐naphthol, C₁₈H₁₇N₃O, (1a), and 1‐[4‐(diethylamino)phenyldiazenyl]‐2‐naphthol, C₂₀H₂₁N₃O, (1b), have been reported to be azo tautomers or a mixture of azo and hydrazone tautomers in the solid state. To prove these observations, both compounds were synthesized, recrystallized and their crystal structures redetermined by single‐crystal structure analysis. Difference electron‐density maps show that the H atoms of the hydroxyl groups are indeed bonded to the O atoms. Nevertheless, a small amount of the hydrazone form seems to be present. Hence, the compounds are close to being `real' azo compounds. Compound (1a) crystallizes with a herring‐bone structure and compound (1b) forms a rare double herring‐bone structure.     

Remarkable π–π stacking of dipyrido[f,h]quinoxaline‐6,7‐dicarbonitrile in its ethanol solvate

The title compound, C₁₆H₆N₆·C₂H₆O, is an ethanol solvate of an aromatic phenanthroline‐based flat ligand. The latter exhibits a remarkable π–π stacking in the crystal structure, with interplanar distances of 3.27 and 3.40 Å, which directs the columnar organization of the ligands. The ethanol solvent molecule is located in channels between these columns, being hydrogen bonded to one of the N‐atom sites of the phenanthroline fragment.     

Triclinic and orthorhombic polymorphs of 2‐iodo‐4‐nitro­aniline: interplay of hydrogen bonds,nitro⃛I interactions and aromatic π–π‐stacking interactions

In the triclinic polymorph of 2‐iodo‐4‐nitro­aniline, C₆H₅IN₂O₂, space group P, the mol­ecules are linked by paired N—­H?O hydrogen bonds into C(8)[R(6)] chains of rings. These chains are linked into sheets by nitro?I interactions, and the sheets are pairwise linked by aromatic π–π‐stacking interactions. In the orthorhombic polymorph, space group Pbca, the mol­ecules are linked by single N—H?O hydrogen bonds into spiral C(8) chains; the chains are linked by nitro?O interactions into sheets, each of which is linked to its two immediate neighbours by aromatic π–π‐stacking inter­actions, so producing a continuous three‐dimensional ­structure.     

Synthesis and Crystal Structure of a [Mo8O26]4– Cluster Derivative with 4‐MePyH+. First β‐Octamolybdate Derivative with π–π Stacking

Dioxobis(pyridine‐2‐thiolate‐N, S)molybdenum(VI) (MoO₂(Py‐S)₂), reacts with of 4‐methylpyridine (4‐MePy) in acetonitrile, by slow diffusion, to afford the title compound. This has been characterized by elemental analysis, IR and ¹H NMR spectroscopy. The X‐ray single crystal structure of the complex is described. Structural studies reveal that the molecular structure consists of a β‐Mo₈O₂₆ polyanion with eight MoO₆ distorted edge‐shared octahedra with short terminal Mo–O bonds (1.692–1.714 Å), bonds of intermediate length (1.887–1.999 Å) and long bonds (2.150–2.473 Å). Two different types of hydrogen bonds have been found: N–H···O (2.800–3.075 Å) and C–H···O (3.095–3.316 Å). The presence of π–π stacking interactions and strong hydrogen bonds are presumably responsible for the special disposition of the pyridinic rings around the polyanion cluster.     

C—H⋯π, π–π and C—H⋯Cl interactions in chloro‐substituted Schiff bases and 4‐chloro‐N‐[4‐(di­methyl­amino)­benzyl­idene]­aniline

Molecular packing analyses were carried out on 15 crystal data sets of chloro‐substituted Schiff bases, including that of the title compound, C₁₅H₁₅ClN₂. C—H⋯π and π–π interactions play a major role in the molecular self‐assembly in the crystal. The former interactions favor mol­ecules assembling into a screw, with a non‐centrosymmetric crystal structure. When the molecular dipole is small, π–π interactions favor a parallel, but not usually antiparallel, mode of packing. Weak C—H⋯X hydrogen bonds (X = Cl or Br) and X⋯X interactions seem to be a secondary driving force in packing. The title mol­ecule takes the trans form and the two benzene rings are twisted around the central linkage in opposite directions. In the crystal structure, mol­ecules interact through C—H⋯π and π–π interactions, forming a `dimer' and further forming double chains along [001]. The double chains are extended along [10] through C—H⋯Cl hydrogen bonds, forming double layers in (010). In the third direction, there are only ordinary, weaker, van der Waals interactions, which explains the crystal habit (i.e. thin plate).     

(Z)‐3‐Methyl‐N‐(7‐nitroacridin‐3‐yl)‐2,3‐dihydro‐1,3‐benzothiazol‐2‐imine from laboratory powder diffraction data

The title compound, C₂₁H₁₄N₄O₂S, belongs to a family of molecules possessing nonlinear optical properties in solution. Its structure has been solved from laboratory X‐ray powder diffraction data using a new direct‐space structure solution method, where the atomic coordinates are directly used as parameters and the molecular geometry is described by restraints. The molecular packing is controlled by two systems of π–π interactions and one weak edge‐to‐face interaction.     

Herringbone Pattern and CH–π Bonding in the Crystal Architecture of Linear Polycyclic Aromatic Hydrocarbons

The herringbone pattern is a pervasive structural motive found in most molecular crystals involving aromatic compounds. A plot of the experimental sublimation enthalpies of members of increasing size of the acene, phenacene and p‐phenyl families versus the number of carbons uncovers a linear relationship between the two magnitudes, suggesting a major role of CH–π bonding. In this work we undertake the task of evaluating the relevance of the edge‐to‐face interaction (or CH–π bond) in the overall reticular energy of the crystal, to quantitatively assess the importance of this structural element. Following a heuristic approach, we considered the series of acenes, phenacenes and p‐phenyls and analyzed the edge‐to‐face interaction between the molecules as they occur in the experimental crystal network. Isolation of the relevant molecular dimers allows to incorporate some of the most sophisticated tools of quantum chemistry and get a reliable picture of the isolated bond. When compared to the experimental sublimation energy, our results are conclusive: this sole interaction is the largest contribution to the lattice energy, and definitively dictates the crystal architecture in all the studied cases. Elusive enough, the edge‐to‐face interaction is mainly dominated by correlation interactions, specifically in the form of dispersion and, to a less extent, of charge‐transfer terms. A suggestive picture of the bond has been obtained by displaying the differences in local electron densities calculated by either correlated or non‐correlated methods.     

The β‐polymorph of phenazine

In β‐phenazine, C₁₂H₉N₂, the mol­ecules show a sandwich herring‐bone type of packing. The experimental crystal structure shows very good agreement with that predicted earlier from systematic searches of potential packing arrangements for the known unit cell [Hammond, Roberts, Smith & Docherty (1999). J. Phys. Chem. B, 103 , 7762–7770].     

Construction of π‐Surface‐Metalated Pillar[5]arenes which Bind Anions via Anion–π Interactions

By simple ligand exchange of the cationic transition‐metal complexes [(Cp*)M(acetone)₃](OTf)₂ (Cp*=pentamethylcyclopentadienyl and M=Ir or Rh) with pillar[5]arene, mono‐ and polynuclear pillar[5]arenes, a new class of metalated host molecules, is prepared. Single‐crystal X‐ray analysis shows that the charged transition‐metal cations are directly bound to the outer π‐surface of aromatic rings of pillar[5]arene. One of the triflate anions is deeply embedded within the cavity of the trinuclear pillar[5]arenes, which is different to the host–guest behavior of most pillar[5]arenes. DFT calculation of the electrostatic potential revealed that the metalated pillar[5]arenes featured an electron‐deficient cavity due to the presence of the electron‐withdrawing transition metals, thus allowing encapsulation of electron‐rich guests mainly driven by anion–π interactions.