Synthon In Crystal Engineering Research Articles

Trityl-Based Lanthanide-Supramolecular Assemblies Exhibiting Slow Magnetic Relaxation.

The triphenylmethane (trityl) group has been recognized as a supramolecular synthon in crystal engineering, molecular machine rotors and stereochemical chirality inductors in materials science. Herein we demonstrate for the first time how it can be utilized in the domain of molecular magnetic materials through shaping of single molecule magnet (SMM) properties within the lanthanide complexes in tandem with other non-covalent interactions. Trityl-appended mono- (HL1 ) and bis-compartmental (HL2 ) hydrazone ligands were synthesized and complexated with Dy(III) and Er(III) triflate and nitrate salts to generate four monometallic (1-4) and two bimetallic (5, 6) complexes. The static and dynamic magnetic properties of 1-6 were investigated, revealing that only ligand HL1 induces assemblies (1-4) capable of showing SMM behaviour, with Dy(III) congeners (1, 2) able to exhibit the phenomenon also under zero field conditions. Theoretical ab initio studies helped in determination of Dy(III) energetic levels, magnetic anisotropic axes and corroborated magnetic relaxation mechanisms to be a combination of Raman and quantum tunnelling in zero dc field, the latter being cancelled in the optimum non-zero dc field. Our work represents the first study of magneto-structural correlations within the trityl Ln-SMMs, leading to generation of slowly relaxing zero-field dysprosium complexes within the hydrogen-bonded assemblies.

Two Dimensional Triptycene End‐Capping and Its Influence on the Self‐Assembly of Quinoxalinophenanthrophenazines†

Comprehensive SummaryIn this report we investigated two‐dimensionally triptycene end‐capped QPPs in terms of their solution and solid‐state behavior. For this purpose, a triphenylene based ortho‐diamine decorated with two triptycenyl units as well as a phenylene diamine with two non‐annulated triptycene units have been synthesized. Sequences of condensation reactions with a pyrene‐based tetraketone and ortho‐diamines yielded a series of QPPs and UV/Vis investigations of the corresponding compounds led to the conclusion, that the QPPs form dimers in solution, which was further supported by MALDI‐TIMS‐TOF‐MS. Single‐crystal X‐ray analysis of the triply and quadruply triptycene end‐capped QPPs furthermore showed short π‐π‐distances of 3.3—3.4 Å and a perfect shape match during the dimerization of the triply triptycenyl end‐capped QPP making it possible synthon for crystal engineering.

Pillarplexes – porous organometallic synthons in crystal engineering

Pillarplexes – porous organometallic synthons in crystal engineering

Twinning-Based Organosuperelasticity and Chirality in a Single Crystal of an Achiral Donor–Acceptor Type Schiff Base Induced by Charge-Transfer Interactions

A charge-transfer (CT) interaction stabilizes molecular assemblies and can form a useful supramolecular synthon in crystal engineering. Herein we demonstrate preparation of a chiral single crystal showing twinning-based organosuperelasticity by forming a CT complex of an achiral donor–acceptor type Schiff base: N-(2,3,4,5,6-pentafluorophenyl)-1-phenylmethanimine. Pseudo-180° symmetry of a single-component CT complex originating from an antiparallel molecular assembly is the key to mechanical twinning. The chiral crystallization induced by CT interactions is also noteworthy because of the potential development of materials showing CT-based functions coupled with chirality.

Application of Halogen Bonding to Organocatalysis: A Theoretical Perspective.

The strong, specific, and directional halogen bond (XB) is an ideal supramolecular synthon in crystal engineering, as well as rational catalyst and drug design. These attributes attracted strong growing interest in halogen bonding in the past decade and led to a wide range of applications in materials, biological, and catalysis applications. Recently, various research groups exploited the XB mode of activation in designing halogen-based Lewis acids in effecting organic transformation, and there is continual growth in this promising area. In addition to the rapid advancements in methodology development, computational investigations are well suited for mechanistic understanding, rational XB catalyst design, and the study of intermediates that are unstable when observed experimentally. In this review, we highlight recent computational studies of XB organocatalytic reactions, which provide valuable insights into the XB mode of activation, competing reaction pathways, effects of solvent and counterions, and design of novel XB catalysts.

Crystal Engineering with Multipoint Halogen Bonding: Double Two-Point Donors and Acceptors at Work.

The combination of singly or doubly bidentate halogen‐bond donors with double bidentate acceptors was investigated as a supramolecular synthon in crystal engineering. The crystal topologies obtained feature novel halogen‐bonding motifs like double two‐point recognition and infinite chains or networks based on two‐point interactions. Induced conformational changes in the double bidentate halogen‐bond donors could be exploited to obtain different 1D and 2D networks. All solid‐state studies were accompanied by DFT calculations to predict and rationalize the outcome.

Face-to-face stacking of caffeinium and [PMVI12O40]3− ions: A synthon for crystal engineering with purine bases

Ionic polyoxometales (Hcaf)3[PM12O40]·6H2O [M = Mo, W; caf = caffeine] exhibit unusual supramolecular structure dominated by multiple anion-π interactions, which occur at both axial sides of the caffeinium cations. These cations, when stacked to every of six square faces of POM cuboctahedra, afford supramolecular boxes and the resulting non-covalent cubic framework (α-Po topology) is generated by face-sharing of the boxes.

Cocrystallization of Nutraceuticals

Cocrystallization has emerged over the past decade as an attractive technique for modification of the physicochemical properties of compounds used as active pharmaceutical ingredients (APIs), complementing more traditional methods such as salt formation. Nutraceuticals, with associated health benefits and/or medicinal properties, are attractive as coformers due to their ready availability, known pharmacological profile, and natural origin, in addition to offering a dual therapy approach. Successful studies of favorably altering the physicochemical properties of APIs through cocrystallization with nutraceuticals are highlighted in this review. Many of the key functional groups commonly seen in nutraceuticals (e.g., acids, phenols) underpin robust supramolecular synthons in crystal engineering. This review assesses the structural data available to date across a diverse range of nutraceuticals, both in pure form and in multicomponent materials, and identifies the persistent supramolecular features present. T...

The multiple phenyl embrace: a useful synthon in crystal engineering

The multiple phenyl embrace: a useful synthon in crystal engineering

Does intermolecular SO⋯H–C–SO hydrogen bonding in sulfoxides and sulfones provide a robust supramolecular synthon in the solid state?

Hydrogen bonding between the sulfur oxygens and the acidic α-hydrogens in sulfoxides and sulfones is proposed as a supramolecular synthon in crystal engineering. A systematic analysis of supramolecular interactions in the solid state of a series of structurally related aryl benzyl sulfides, sulfoxides and sulfones was undertaken to establish the extent to which such hydrogen bonds persist as a structure determining feature in the solid state. The impact of the level of oxidation at sulfur, steric and electronic effects of substituents on the aryl rings and methyl substitution α to the sulfur functional group on the solid state structure of the compounds have been explored. The impact of stereochemical features, including relative and absolute stereochemistry, is also discussed.

2,4,6-Tris(4-cyanophenoxy)-1,3,5-triazine (CNPOT): Trigonal synthons in crystal engineering

The substituted triazine 2,4,6-tris(4-cyanophenoxy)-1,3,5-triazine (CNPOT) was synthesised for the first time by a ball-mill activated solid-state reaction. A single crystal X-ray structure refinement revealed a channel structure and a novel trigonal cyano synthon. The (CN)3 motif is discussed in comparison to the currently known trigonal substituted triazines described in crystal engineering.

Designing neutral coordination networks using inorganic supramolecular synthons: Combination of coordination chemistry and C–H⋯Cl hydrogen bonding

Crystal structure determination of two compounds, [(L3)MnIICl2(EtOH)] (1) and [(L6)FeIIICl3] (2) [L3 = 2-[3-(2′-pyridyl)pyrazol-1-ylmethyl]pyridine; L6 = methyl[2-(2-pyridyl)ethyl](2-pyridylmethyl)amine], the former one reported for the first time, demonstrates that these coordination units can be used effectively as supramolecular synthons in crystal engineering to construct neutral networks. In continuation of our recent contribution on inorganic crystal engineering from the standpoint of metal–ligand coordination chemistry and C–H⋯Cl2MII hydrogen-bonding, in this work we have investigated the potential of coordination modules present in 1 and 2 in bringing about generality and diversity of C–H⋯Cl hydrogen bonding interactions. Special attention has been directed to the influence of additional (i) coordinated solvent molecule, which can act as hydrogen bond donor/acceptor and (ii) chloride ion in the assembly of supramolecular architectures via noncovalent interactions. Inorganic supramolecular synthons 1 and 2 give rise to molecular staircase and zig-zag network, respectively. Re-examination of previously reported compound [(L6)ZnCl2] with extended limit of hydrogen bonding interaction reveals additional helical network.

Molecular bricklaying: the protonated benzimidazole moiety as a synthon for crystal engineering

The crystal structures of three salts of diprotonated 1,2-bis(1H-benzimidazol-2-yl)ethane, 1, with tetrahalogenometallates are reported. The crystal lattices are constructed from two types of interactions, the stacking of the protonated benzimidazole groups and N–H⋯Cl hydrogen bonding between the cations and the complex anions. Depending on the conformation of the cation, two- or three-dimensionally stacked structures are formed. A similar packing motif is found with the related cation formed by 1,3-bis(1H-benzimidazol-2-yl)propane, 2, and this suggests that the protonated benzimidazole group may be a useful synthon for crystal engineering.

Cycloalkanone-Based Threo and Erythro Aldols as Supramolecular Synthons in Crystal Engineering

Aldols are bifunctional molecules possessing OH and CO groups, which are able to act as both hydrogen donors and/or acceptors. Despite their high potential as supramolecular synthons, in comparison to their two carbon less homologues, carboxylic acids, the structures of aldols, and their crystals have not been systematically studied. We have designed synthons on the basis of the ab initio calculated syn-CO/OH and OC−C(α)−C(β) eclipsed conformation of the simplest aldol and have synthesized 16 cycloalkanone-based threo/erythro aldols, which are systematically installed with structural determinants. The molecular conformations, the hydrogen-bonding patterns, the location of OH protons, the crystal structures, and the packing efficiency have been analyzed by X-ray diffraction to relate synthons with crystals. Generally, the stable syn/eclipsed conformations are maintained in the C(3)-unsubstituted cyclohexanone-based aldols, and the racemic synthons crystallize via an S(6) monomer or a heterochiral dimer by way of van der Waals forces. The enantiomerically pure aldols, however, tend to form intermolecular hydrogen bonds generating a helix chain. Introduction of a C(3) substituent completely destroys the stable conformation yielding an unusual C(2)−C(3) trans-diaxial conformation. This enables the formation of an intermolecularly hydrogen-bonded long chain column with a screw axis or glide plane, which can crystallize through CH···O or van der Waals interaction.

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], C(14)H(19)N(4)O(3)(+).CHO(2)(-), 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.

On the Reliability of C−H···O Interactions in Crystal Engineering: Synthesis and Structure of Two Hydrogen Bonded Phosphonium Bis(aryloxide) Salts

Two related phosphonium aryloxides, [Ph3PCH3]+2[CH2(C6H2-3,5-tert-butyl-4-O)2]2-, 1a, and [Ph3PC2H5]+2[CH2(C6H2-3,5-tert-butyl-4-O)2]2-, 1b have been synthesized by protonation of nonstabilized phosphorus ylides with 4,4‘-methylenebis(2,6-di-tert-butylphenol) LH2. The crystal structures of 1a and 1b have been determined by single-crystal X-ray diffraction, and that of 1b has also been determined by low-temperature (20 K) neutron diffraction. Both crystallize as CH3CN solvates, and both exhibit polymeric supramolecular structures via extensive C−H···O hydrogen bonding. The predominant structural motif is chelation of the aryloxide oxygen atom of the anion by phosphonium alkyl and aryl C−H groups, such as has been previously observed for phosphonium monoaryloxide salts. The use of this motif as a supramolecular synthon in crystal engineering is explored. In the case of 1a, in addition to the expected intermolecular C−H···O hydrogen bonding pattern, C−H···π interactions between the anion and this solvent are observed. In the structure of 1b, C−H···π interactions between the cation and anion lead to an unpredicted supramolecular structure. The possible significance of this feature is discussed the context of the general use of weak hydrogen bonds in crystal engineering.

Halogen trimer synthons in crystal engineering: low-temperature X-ray and neutron diffraction study of the 1:1 complex of 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine with tribromobenzene

The title complex has been studied using low-temperature X-ray (150 K) and neutron (100 K) diffraction. Molecules of the triazine host form a two-dimensional hexagonal network mediated by trigonally symmetric Cl(3) synthons having Cl.Cl interactions of 3.441 (3) A, a C-Cl.Cl angle of 165 degrees and a Cl.Cl-C angle of 105 degrees, close to the ideal values of 180 and 90 degrees, respectively. The guest molecules are of an appropriate size to fit the hexagonal networks and interact with the host via C-H.pi (phenyl) and C-Br.pi (phenyl) interactions which stabilize the overall structure. Both C-donor bond vectors are directed more closely towards the mid-point (X) of an individual aromatic bond, rather than the ring centroid, with H.X 2.817 (9) A and C-H.X 174.0 (9) degrees, and Br.X 3.353 (4) A and C-Br.X 158.1 (2) degrees.

Stereochemistry and Stereoelectronics of Azines. 13. Conformational Effects on the Quadrupolarity of Azines. An Ab Initio Quantum-Mechanical Study of a Lateral Synthon

The quadrupole moment of formaldazine, H2C=N-N=CH2, has been studied for the trans structure (Ð(C-N-N-C) = τ = 180≡) and a series of gauche structures (τ > 120≡). Restricted Hartree-Fock theory, second-order Moller-Plesset theory, and quadratic CI theory have been used in conjunction with the basis sets 6-31G*, 6-31G**, 6-311G** and 6-311++G**. Formaldazine is a quadrupolar molecule with primitive quadrupole moment tensor components of Qxx = -22.4, Qyy = -20.4 and Qzz = -25.6 DA at the theoretical level QCISD/6-311++G**. The examination of the theoretical level dependency shows that the reliable computation of a quadrupole moment requires the use of a flexible basis set. A large part of the component Qzz = -25.6 DA is due to the π-system and compares, on a per electron basis, with the Qzz value of benzene. Conformational changes of the azines in the range 120° < τ < 180≡ have but a minute effect on the energy and are associated with only minor electronic relaxation. These conformational changes alter the quadrupole moment tensor components less than ΔQxx = +0.4, ΔQyy = +1.6 and ΔQzz = -1.0 DA at QCISD/6-311++G**//QCISD/6-31G*. The direction of these changes is explained by consideration of the rotation of the CN-π-systems and a small reduction of the CN bond polarity in the gauche structures. The Qzz component of formaldazine is representative of the quadrupole moment tensor component along the direction of the C2 axis of the azine bridge as such. Hence, the results of this study suggest that azines can engage in strong quadrupole-quadrupole interactions and can be employed as lateral synthons in crystal engineering.

Tribenzylidenemethane complexes of ruthenium. Synthesis and effects of the diastereoisomerism on the crystal packing

Reaction of the tribenzylidenemethane (TBM) dianion with [RuCl2(C6H6)]2 yields a diastereoisomeric mixture of the metal complexes with formula [Ru(TBM)(C6H6)], which can be separated by column chromatography and fractional crystallisation. The crystal structures of both compounds were determined by X-ray diffraction on single crystals. The arrangement of the phenyl rings has a major influence on the build-up of the crystal lattice. The molecule containing the propeller-shaped ligand of C3 symmetry crystallises in a layer structure with a packing arrangement that is caused by the fragment TBM. The close similarity with the crystal structure of [Ta(TBM)(η5-C5H5)(CH3)2] shows that the fragment TBM can serve as a synthon for crystal engineering.

Novel supramolecular synthon in crystal engineering: ionic complexes of 4,4′-bipyridine and 1,2-bis(2-pyridyl)ethylene with 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone

4,4′-Bipyridine (BPY) and 1,2-bis(2-pyridyl)ethylene (2-PDE) form bifurcated hydrogen bonds with 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (chloranilic acid, 2H-CLA) to afford [BPY-2H]2+[CLA]2– 1 and [2-PDE-2H]2+[CLA]2– 2; their crystal structures exhibit a new type supramolecular synthon and links as molecular chains and zigzag tapes, respectively.