Search

A mixed-ligand metal-organic framework (MOF) material composed of both amine- and hydroxyl-bearing linkers, KSU-1, was reacted with a variety of isocyanates. The hydroxyl groups reacted to a greater extent than the amines, in conflict with the previously observed relative nucleophilicities of these functionalities in the same MOF. When immobilized individually in monofunctional MOFs, the amine-functionalized linker was more reactive than the hydroxyl linker, indicating that the reactivity reversal observed in KSU-1 is due to the groups' mutual confinement within the MOF., Competing Interests: Conflicts of interest There are no conflicts to declare.

The amino group of 2-amino-5-(4-halophenyl)-1,3,4-chalcogenadiazole has been replaced with bromo/iodo substituents to obtain a library of four compositionally related compounds. These are 2-iodo-5-(4-iodophenyl)-1,3,4-thiadiazole, C 8 H 4 I 2 N 2 S, 2-bromo-5-(4-bromophenyl)-1,3,4-selenadiazole, C 8 H 4 Br 2 N 2 Se, 2-bromo-5-(4-iodophenyl)-1,3,4-selenadiazole, C 8 H 4 BrIN 2 Se, and 2-bromo-5-(4-iodophenyl)-1,3,4-thiadiazole, C 8 H 4 BrIN 2 S. All were isostructural and contained bifurcated Ch...N (Ch is chalcogen) and X...X (X is halogen) interactions forming a zigzag packing motif. The noncovalent Ch...N interaction between the chalcogen-bond donor and the best-acceptor N atom appeared preferentially instead of a possible halogen bond to the same N atom. Hirshfeld surface analysis and energy framework calculations showed that, collectively, a bifurcated chalcogen bond was stronger than halogen bonding and this is more structurally influential in this system.

A structure-property analysis of ten compositionally and chemically similar co-crystals of N -(pyridin-2-yl)alkylamides and carboxylic acids shows that three co-crystals of targets bearing a methyl chain were brittle, while the remaining co-crystals of targets bearing ethyl or propyl chains were flexible. Five of these displayed elastic deformation while two displayed plastic deformation. Compounds with different mechanical behaviour (brittle, plastic, and elastic deformation) in response to external mechanical stimuli could be organized into structurally similar groups based on the presence of specific intermolecular interactions and packing features in each crystal structure.

In this study, we investigated how the presence of multiple intermolecular interaction sites influences the heteromeric supramolecular assembly of N-[(3-pyridinylamino) thioxomethyl] carbamates with fluoroiodobenzenes. Three targets—R-N-[(3-pyridinylamino) thioxomethyl] carbamate (R = methyl, ethyl, and isobutyl)—were selected and crystallized, resulting in three parent structures, five co-crystals, and one co-crystal solvate. Three hydrogen-bonded parent crystal structures were stabilized by N-H···N hydrogen bonding and assembled into layers that stacked on top of one another. Molecular electrostatic potential surfaces were employed to rank binding sites (Npyr > C=S > C=O) in order to predict the dominant interactions. The N-H⋯H hydrogen bond was replaced by I⋯Npyr in 3/6 cases, I⋯C=S in 4/6 cases, and I⋯O=C in 1 case. Interestingly, the I⋯C=S halogen bond coexisted twice with I⋯Npyr and I⋯O=C. Overall, the MEPs were fairly reliable for predicting co-crystallization outcomes; however, it is crucial to also consider factors such as molecular flexibility. Finally, halogen-bond donors are capable of competing for acceptor sites, even in the presence of strong hydrogen-bond donors.

In order to explore how σ-hole potentials, as evaluated by molecular electrostatic potential (MEP) calculations, affect the ability of halogen atoms to engage in structure-directing intermolecular interactions, we synthesized four series of ethynyl halogen-substituted amide containing pyridines (activated targets); (N-(pyridin-2-yl)benzamides (Bz-act-X), N-(pyridin-2-yl)picolinamides (2act-X), N-(pyridin-2-yl)nicotinamides (3act-X) and N-(pyridin-2-yl) isonicotinamides (4act-X), where X = Cl/Br/I. The molecules are deliberately equipped with three distinctly different halogen-bond acceptor sites, π, N(pyr), and O[double bond, length as m-dash]C, to determine binding site preferences of different halogen-bond donors. Crystallographic data for ten (out of a possible twelve) new compounds were thus analyzed and compared with data for the corresponding unactivated species. The calculated MEPs of all the halogen atoms were higher in the activated targets in comparison to the unactivated targets and were in the order of iodine ≈ chloroethynyl < bromoethynyl < iodoethynyl. This increased positive σ-hole potential led to a subsequent increase in propensity for halogen-bond formation. Two of the four chloroethynyl structures showed halogen bonding, and all three of the structurally characterized bromoethynyl species engaged in halogen bonding. The analogous unactived species showed no halogen bonds. Each chloroethynyl donor selected a π-cloud as acceptor and the bromoethynyl halogen-bond donors opted for either π or N(pyr) sites, whereas all halogen bonds involving an iodoethynyl halogen-bond donor (including both polymorphs of Bz-act-I) engaged exclusively with a N(pyr) acceptor site.

In order to explore how specific atom-to-atom replacements change the electrostatic potentials on 1,3,4-chalcogenadiazole derivatives, and to deliberately alter the balance between intermolecular interactions, four target molecules were synthesized and characterized. DFT calculations indicated that the atom-to-atom substitution of Br with I, and S with Se enhanced the σ-hole potentials, thus increasing the structure directing ability of halogen bonds and chalcogen bonds as compared to intermolecular hydrogen bonding. The delicate balance between these intermolecular forces was further underlined by the formation of two polymorphs of 5-(4-iodophenyl)-1,3,4-thiadiazol-2-amine; Form I displayed all three interactions while Form II only showed hydrogen and chalcogen bonding. The results emphasize that the deliberate alterations of the electrostatic potential on polarizable atoms can cause specific and deliberate changes to the main synthons and subsequent assemblies in the structures of this family of compounds.

The interplay between hydrogen bonds (HBs) and halogen bonds (XBs), has been addressed by co-crystallizing two halogen bond donors, 1,4-diiodotetrafluorbenzene(DITFB) and 1,3,5-trifluoro-2,4,6-triiodobenzene(TITFB) with four series of targets; N-(pyridin-2-yl)benzamide (Bz-X), N-(pyridin-2-yl)picolinamides (2Pyr-X), N-(pyridin-2-yl)nicotinamides (3Pyr-X), N-(pyridin-2-yl)isonicotinamides (4Pyr-X); X=H/Cl/Br/I. The structural outcomes were compared with interactions in the targets themselves. 13 co-crystals were analysed by single-crystal X-ray diffraction (SCXRD). In all three co-crystals from the 2Pyr series, the intramolecular HB remained intact while the XB donors engaged with the N(pyr) or O=C sites. In the ten co-crystals from the other three series, the intermolecular HBs present in the individual targets were disrupted in 9/10 cases. Overall, the acceptor sites selected by the halogen-bond donors in these targets were distributed as follows; N(pyr)=81 %, O=C (15 %) or π (4 %)., (© 2021 Wiley-VCH GmbH.)

Nucleophilic substitution of F atoms in 5,6-difluorobenzo[c][1,2,5]thiadiazole (DFBT) for carbazole could be potentially interesting as a novel way of synthesizing building blocks for new conjugated materials for applications in organic chemistry. The crystal structures of 5,6-bis(9H-carbazol-9-yl)benzo[c][1,2,5]thiadiazole (DCBT), C 30 H 18 N 4 S, and its hydrate, C 30 H 18 N 4 S·0.125H 2 O, were investigated using single-crystal X-ray analysis. The hydrate contains two symmetry-independent DCBT molecules. The dihedral angles between the plane of the central benzothiadiazole fragment and that of the carbazole units vary between 50.8 and 69.9°, indicating conformational flexibility of the DCBT molecule in the crystals, which is consistent with quantum chemical calculations. The analysis of the crystal packing of DCBT revealed that the experimental triclinic structure could be described as a distortion from a hypothetical higher-symmetry monoclinic structure. The quantum chemical calculations of two possible monoclinic structures, which are related to the experimental structure by a shifting of molecular layers, showed that the proposed structures are higher in energy by 5.4 and 10.1 kcal mol -1 . This energy increase is caused by less dense crystal packings of the symmetric structures, which results in a decrease of the number of intermolecular interactions.

In the title mol-ecular salt, C 6 H 16 NO + ·Cl - , two of the C-C-N-O groups in the cation adopt a gauche conformation [torsion angles = 62.86 (11) and -54.95 (13)°] and one an anti conformation [-177.82 (10)°. The cation and anion are linked by an O-H⋯Cl hydrogen bond. The extended structure displays C-H⋯Cl and C-H⋯O hydrogen bonds, resulting in layers lying parallel to the (100) plane: further C-H⋯Cl contacts connect the sheets into a three-dimensional network.

The vibrational and structural behavior of a novel, energetic crystal, dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50), was examined over a broad pressure range to elucidate its structural and chemical stability at high pressures. Raman measurements were performed on single crystals compressed to 50 GPa in a diamond anvil cell, and data were obtained over the entire frequency range of TKX-50 Raman activity. The Raman spectroscopy results were complemented by density functional theory (DFT) calculations to provide vibrational mode assignments and to gain insight into pressure effects on the vibrational and crystal response of TKX-50. Several features were observed in Raman spectra measured in the ranges 4-10, 10-13, and 32-36 GPa. We suggest that the changes between 32 and 36 GPa may be associated with a phase transformation. In addition, a number of vibrational modes showed intensity exchange and avoided crossing of vibrational frequency at various pressures, characteristic of the coupling of modes. Despite all these pressure effects, the compression of TKX-50 to 50 GPa and the subsequent release of pressure did not result in any irreversible spectral changes, demonstrating its remarkable chemical stability. DFT calculations, using the PBE functional with an empirical dispersion correction by the Grimme, PBE-D method, were used to calculate pressure effects on Raman frequencies and unit cell parameters. The calculated Raman shifts to 20 GPa are in good overall agreement with the measured shifts over a broad range of frequencies. The calculations also show that TKX-50 exhibits anisotropic compressibility, with a highly incompressible response along the a axis. The calculated bulk modulus, a measure of average stiffness, of TKX-50 is significantly higher than the calculated or measured bulk moduli of other energetic crystals. We suggest that the strong intermolecular interactions and the coupling of vibrational modes may potentially contribute to the shock insensitivity of TKX-50. This work demonstrates the robust high-pressure response of TKX-50, making this crystal attractive for practical applications.

Theoretical investigations to evaluate the viability of extended nonmetal atom chains on the basis of molecular models with the general formula Mn F4n+2 (M=S and Se) and corresponding solid-state systems exhibiting direct SS or SeSe bonding were performed. The proposed high-symmetry molecules were found to be minima on the potential energy surface for all Sn F4n+2 systems studied (n=2-9) and for selenium analogues up to n=6. Phonon calculations of periodic structures confirmed the dynamic stability of the -(SF4 -SF4 )∞ - chain, whereas the analogous -(SeF4 -SeF4 )∞ - chain was found to have a number of imaginary phonon frequencies. Chemical bonding analysis of the dynamically stable -(SF4 -SF4 )∞ - structure revealed a multicenter character of the SS and SF bonds. A novel definition and abbreviation (ENAC) are proposed by analogy with extended metal atom chain (EMAC) complexes., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Pressure effects on the Raman vibrations of an energetic crystal FOX-7 (1, 1-diamino-2, 2-dinitroethene) were examined using density functional theory (DFT) calculations. High accuracy calculations were performed with a periodic plane-wave DFT method using norm-conserving pseudopotentials. Different exchange-correlation functionals were examined for their applicability in describing the structural and vibrational experimental data. It is shown that the PBE functional with an empirical dispersion correction by Grimme, PBE-D method, reproduces best the molecular geometry, unit cell parameters, and vibrational frequencies. Assignments of intramolecular Raman active vibrations are provided. The calculated pressure dependence of Raman shifts for the intramolecular and lattice modes were found to be in good agreement with the experimental data; in particular, the calculations predicted correctly a decrease of frequencies for the NH2 stretching modes with pressure. Also, in accord with experiments, the calculations indicated some instances of modes mixing/coupling with increasing pressure. This work demonstrates that the dispersion-corrected PBE functional can account for the structural and vibrational properties of FOX-7 crystal at ambient and high pressures.

We have investigated the structural and electronic properties of the B(17)(-) and B(18)(-) clusters using photoelectron spectroscopy (PES) and ab initio calculations. The adiabatic electron detachment energies of B(17)(-) and B(18)(-) are measured to be 4.23 ± 0.02 and 3.53 ± 0.05 eV, respectively. Calculated electron detachment energies are compared with experimental data, confirming the presence of one planar C(2v) ((1)A(1)) isomer for B(17)(-) and two nearly isoenergetic quasi-planar C(3v) ((2)A(1)) and C(s) ((2)A') isomers for B(18)(-). The stability and planarity/quasi-planarity of B(17)(-) and B(18)(-) are ascribed to σ- and π-aromaticity. Chemical bonding analyses reveal that the nature of π-bonding in B(17)(-) and B(18)(-) is similar to that in the recently elucidated B(16)(2-) and B(19)(-) clusters, respectively. The planar B(17)(-) cluster can be considered as an all-boron analogue of naphthalene, whereas the π-bonding in the quasi-planar B(18)(-) is reminiscent of that in coronene.

Anion photoelectron spectra of Ga(x)N(y)(-) cluster anions, in which x = 4-12, y = 1 and x = 7-12, y = 2, were measured. Ab initio studies were conducted on Ga(x)N(y)(-) cluster anions in which x = 4-7, y = 1 and Ga(7)N(2)(-), providing their structures and electronic properties. The photoelectron spectra were interpreted in terms of the computational results. This allowed for identification of the isomers present in the beam experiments for specific Ga(x)N(-) cluster anions (x = 4-7). The unexpected presence of Ga(x)N(2)(-) species is also reported.

Bulk carbon and boron form very different materials, which are also reflected in their clusters. Small carbon clusters form linear structures, whereas boron clusters are planar. For example, it is known that the B(5)(-) cluster possesses a C(2v) planar structure and C(5)(-) is a linear chain. Here we study B/C mixed clusters containing five atoms, C(x)B(5-x)(-) (x = 1-5), which are expected to exhibit a planar to linear structural transition as a function of the C content. The C(x)B(5-x)(-) (x = 1-5) clusters were produced and studied by photoelectron spectroscopy; their geometric and electronic structures were investigated using a variety of theoretical methods. We found that the planar-to-linear transition occurs between x = 2 and 3: the global minimum structures of the B-rich clusters, CB(4)(-) and C(2)B(3)(-), are planar, similar to B(5)(-), and those of the C-rich clusters, C(3)B(2)(-) and C(4)B(-), are linear, similar to C(5)(-).

Atomic clusters often show unique, size-dependent properties and have become a fertile ground for the discovery of novel molecular structures and chemical bonding. Here we report an investigation of the B₁₉⁻ cluster, which shows chemical bonding reminiscent of that in [10]annulene (C₁₀H₁₀) and [6]circulene (C₂₄H₁₂). Photoelectron spectroscopy reveals a relatively simple spectrum for B₁₉⁻, with a high electron-binding energy. Theoretical calculations show that the global minimum of B₁₉⁻ is a nearly circular planar structure with a central B₆ pentagonal unit bonded to an outer B₁₃ ring. Chemical bonding analyses reveal that the B₁₉⁻ cluster possesses a unique double π-aromaticity in two concentric π-systems, with two π-electrons delocalized over the central pentagonal B₆ unit and another ten π-electrons responsible for the π-bonding between the central pentagonal unit and the outer ring. Such peculiar chemical bonding does not exist in organic compounds; it can only be found in atomic clusters.

We demonstrated in our joint photoelectron spectroscopic and ab initio study that wheel-type structures with a boron ring are not appropriate for designing planar molecules with a hypercoordinate central carbon based on the example of CB(8), and CB(8)(-) clusters. We presented a chemical bonding model, derived from the adaptive natural density partitioning analysis, capable of rationalizing and predicting planar structures either with a boron ring or with a carbon atom occupying the central hypercoordinate position. According to our chemical bonding model, in the wheel-type structures the central atom is involved in delocalized bonding, while peripheral atoms are involved in both delocalized bonding and two-center two-electron (2c-2e) sigma-bonding. Since carbon is more electronegative than boron it favors peripheral positions where it can participate in 2c-2e sigma-bonding. To design a chemical species with a central hypercoordinate carbon atom, one should consider electropositive ligands, which would have lone pairs instead of 2c-2e peripheral bonds. Using our extensive chemical bonding model that considers both sigma- and pi-bonding we also discuss why the AlB(9) and FeB(9)(-) species with octacoordinate Al and Fe are the global minima or low-lying isomers, as well as why carbon atom fits well into the central cavity of CAl(4)(2-) and CAl(5)(+). This represents the first step toward rational design of nano- and subnano-structures with tailored properties.

The structure and electronic properties of the Al(8)N(-) and Al(8)N clusters were investigated by combined photoelectron spectroscopy and ab initio studies. Congested photoelectron spectra were observed and experimental evidence was obtained for the presence of multiple isomers for Al(8)N(-). Global minimum searches revealed several structures for Al(8)N(-) with close energies. The calculated vertical detachment energies of the two lowest-lying isomers, which are of C(2v) and C(s) symmetry, respectively, were shown to agree well with the experimental data. Unlike the three-dimensional structures of Al(6)N(-) and Al(7)N(-), in which the dopant N atom has a high coordination number of 6, the dopant N atom in the two low-lying isomers of Al(8)N(-) has a lower coordination number of 4 and 5, respectively. The competition between the Al-Al and Al-N interactions are shown to determine the global minimum structures of the doped aluminum clusters and results in the structural diversity for both Al(8)N(-) and Al(8)N.

The structures and bonding of CB6-, C2B5-, and CB62- are investigated by photoelectron spectroscopy and ab initio calculations. It is shown that the global minimum structures for these systems are distorted heptacyclic structures. The previously reported hexacyclic structures with a hypercoordinate central carbon atom are found to be significantly higher in energy and were not populated under current experimental conditions. The reasons why carbon avoids hypercoordination in these planar carbon-boron clusters are explained through detailed chemical-bonding analyses.

The electronic and geometrical structures of Al7N- are investigated using photoelectron spectroscopy and ab initio calculations. Photoelectron spectra of Al7N- have been obtained at three photon energies with six resolved spectral features at 193 nm. The spectral features of Al7N- are relatively broad, in particular for the ground state transition, indicating a large geometrical change from the ground state of Al7N- to that of Al7N. The ground state vertical detachment energy is measured to be 2.71 eV, whereas only an upper limit of approximately 1.9 eV can be estimated for the ground state adiabatic detachment energy due to the broad detachment band. Global minimum searches for A7N- and Al7N are performed using several theoretical methods. Vertical electron detachment energies are calculated using three different methods for the lowest energy structure and compared with the experimental data. Calculated results are in excellent agreement with the experimental data. The global minimum structure of Al7N- is found to possess C3v symmetry, which can be viewed as an Al atom capping a face of a N-centered Al6N octahedron. In the ground state of Al7N, however, the capping Al atom is pushed inward with the three adjacent Al-Al distances being stretched outward. Thus, even though Al7N still possesses C3v symmetry, it is better viewed as a N-coordinated by seven Al atoms in a cage-like structure. The chemical bonding in Al7N- is discussed on the basis of molecular orbital and natural bond analysis.

Aromaticity is an important concept in chemistry primarily for organic compounds, but it has been extended to compounds containing transition-metal atoms. Recent findings of aromaticity and antiaromaticity in all-metal clusters have stimulated further research in describing the chemical bonding, structures and stability in transition-metal clusters and compounds on the basis of aromaticity and antiaromaticity, which are reviewed here. The presence of d-orbitals endows much more diverse chemistry, structure and chemical bonding to transition-metal clusters and compounds. One interesting feature is the existence of a new type of aromaticity-delta-aromaticity, in addition to sigma- and pi-aromaticity which are the only possible types for main-group compounds. Another striking characteristic in the chemical bonding of transition-metal systems is the multi-fold nature of aromaticity, antiaromaticity or even conflicting aromaticity. Separate sets of counting rules have been proposed for cyclic transition-metal systems to account for the three types of sigma-, pi- and delta-aromaticity/antiaromaticity. The diverse transition-metal clusters and compounds reviewed here indicate that multiple aromaticity and antiaromaticity may be much more common in chemistry than one would anticipate. It is hoped that the current review will stimulate interest in further understanding the structure and bonding, on the basis of aromaticity and antiaromaticity, of other known or unknown transition-metal systems, such as the active sites of enzymes or other biomolecules which contain transition-metal atoms and clusters.

The extensive search for the global minimum structure of Hf3 at the B3LYP/LANL2DZ level of theory revealed that D3h 3A2' (1a1'(2)1a2''(2)1e'(4)2a1'(2)1e''2) and D3h 1A1' (1a1'(2)2a1'(2)1e'(4)1a2''(2)3a1'2) are the lowest triplet and singlet states, respectively, with the triplet state being the lowest one. However, at the CASSCF(10,14)/Stuttgart+2f1g level of theory these two states are degenerate, indicating that at the higher level of theory the singlet state could be in fact the global minimum structure. The triplet D3h 3A2' (1a1'21a2''(2)1e'(4)2a1'(2)1e''2) structure is doubly (sigma- and pi-) aromatic and the singlet D3h 1A1' (1a1'(2)2a1'(2)1e'(4)1a2''(2)3a1'2) structure is the first reported triply (sigma-, pi-, and delta-) aromatic system.

The electronic and geometrical structure of a nitrogen-doped Al6- cluster (Al6N-) is investigated using photoelectron spectroscopy and ab initio calculations. Photoelectron spectra of Al6N- have been obtained at three photon energies with seven resolved spectral features. The electron affinity of Al6N has been determined to be 2.58 +/- 0.04 eV. Global minimum structure searches for A6N- and its corresponding neutral form are performed using several theoretical methods. Vertical electron detachment energies, calculated using three different methods for the lowest energy structure and a low-lying isomer, are compared with the experimental data. The ground-state structure of Al6N- is established from the joint experimental and theoretical study to consist of an Al2 dimer bonded to the top of a quasi-planar tetracoordinated N unit, Al4N-, or it can be viewed as a distorted trigonal prism structure with the N atom bonded in one of the prism faces. For neutral Al6N, three low-lying isomers are found to compete for the global minimum, two of which are built from the tetracoordinated Al4N unit. The chemical bonding in Al6N- is discussed on the basis of molecular orbital and natural bond analyses.

The electronic and geometrical structures of three nitrogen-doped aluminum clusters, Al(x)N(-) (x=3-5), are investigated using photoelectron spectroscopy and ab initio calculations. Well-resolved photoelectron spectra have been obtained for the nitrogen-doped aluminum clusters at four photon energies (532, 355, 266, and 193 nm). Global minimum structure searches for Al(x)N(-) (x=3-5) and their corresponding neutrals are performed using several theoretical methods. Vertical electron detachment energies are calculated using three different methods for the lowest energy structures and low-lying isomers are compared with the experimental observations. Planar structures have been established for all the three Al(x)N(-) (x=3-5) anions from the joint experimental and theoretical studies. For Al(5)N(-), a low-lying nonplanar isomer is also found to contribute to the experimental spectra, signifying the onset of two-dimensional to three-dimensional transition in nitrogen-doped aluminum clusters. The chemical bonding in all the planar clusters has been elucidated on the basis of molecular orbital and natural bond analyses.

The novel title furazan-containing macrocycle (systematic name: 6,9,14,17-tetraoxa-2,3,5,7,16,18-hexaazatricyclo[13.3.0.0(4,8)]octadeca-4,7,15,18-tetraene), C8H10N6O4, (I), is the first macrocycle where the furazan rings are connected via a hydrazine group. In spite of the strain in the 12-membered macrocycle of (I), the geometry of the furazan fragment is the same in (I) and in its acyclic analogue 1,8-bis(5-aminofurazan-4-yloxy)-3,6-dioxaoctane, C10H16N6O6, (II). In both compounds, the participation of the furazan rings in intermolecular hydrogen bonding equalizes the N-O bonds within the furazan rings, in contrast with rings which do not participate in such interactions.

In earlier papers, we described the synthesis and structures of bis(3-nitrofurazan-4-yl) ether, C(4)N(6)O(7), (I), bis[3-(nitro-N,N,O-azoxy)furazan-4-yl] ether, C(4)N(10)O(9), (II), and bis[3-(5H-[1,2,3]triazolo[4,5-c]furazan-5-yl)furazan-4-yl] ether, C(8)N(14)O(5), (III). Here we compare the structures of (I)-(III) with those of four 3-cyanodifurazanyl ethers, namely bis(3-cyanofurazan-4-yl) ether, C(6)N(6)O(3), (IV), 3-cyanofurazanyl 3-nitrofurazanyl ether, C(5)N(6)O(5), (V), 3,4-bis(3-cyanofurazan-4-yloxy)furazan, C(8)N(8)O(5), (VI), and bis[3-(3-cyanofurazan-4-yloxy)furazan-4-yl]diazene, C(10)N(12)O(6), (VII). It was found that the geometric parameters of the difurazanyl ether fragments are similar in these structures and therefore not influenced by substituent effects; however, the conformation of this fragment is different, viz. structures (I), (III), (V) and (VI) have approximate C(2) symmetry, and structures (II), (IV) and (VII) have C(s) symmetry. Dense crystal packing (1.626-1.898 Mg m(-3)) is characteristic for all these hydrogen-free compounds. A linear correlation is also determined between crystal density and 'molecular density' (M/V), where M is the mass of a molecule and V is the molecular volume.

Syntheses and X-ray structural investigations have been carried out for (E)-(4-hydroxyphenyl)(4-nitrophenyl)diazene, C(12)H(9)N(3)O(3), (Ia), (E)-(4-methoxyphenyl)(4-nitrophenyl)diazene, C(13)H(11)N(3)O(3), (IIIa), and (E)-[4-(6-bromohexyloxy)phenyl](4-cyanophenyl)diazene, C(19)H(20)BrN(3)O, (IIIc). In all of these compounds, the molecules are almost planar and the azobenzene core has a trans geometry. Compound (Ia) contains four and compound (IIIc) contains two independent molecules in the asymmetric unit, both in space group P1 (No. 2). In compound (Ia), the independent molecules are almost identical, whereas in crystal (IIIc), the two independent molecules differ significantly due to different conformations of the alkyl tails. In the crystals of (Ia) and (IIIa), the molecules are arranged in almost planar sheets. In the crystal of (IIIc), the molecules are packed with a marked separation of the azobenzene cores and alkyl tails, which is common for the solid crystalline precursors of mesogens.