On the clathrate forms of syndiotactic poly(p-methylstyrene)

A classification for clathrate structures of syndiotactic poly(p-methylstyrene) in two classes (α and β), on the basis of X-ray diffraction analyses as well as of the different observed behavior with respect to the thermal and acetone treatment, is presented. A structural correlation between α and β class clathrates and forms I and II, respectively, is evidenced. It is also suggested that form I and form II could be the “emptied forms” of the α and β class clathrates, respectively. Similar steric hindrance of the molecules inducing clathrate structures, within each class has been observed.     

Comparative analysis of the clathrate forms of syndiotactic polystyrene,poly (p-methylstyrene) and poly (m-methylstyrene)

The crystal structures of the clathrate forms of syndiotactic polystyrene (s-PS), poly (p-methylstyrene) (s-PPMS) and poly (m-methylstyrene) (s-PMMS) containing guest molecules having widely different steric hindrance are compared in detail. Common features and differences concerning the packing of the chains, the shape and the dimensions of the cavities and the stability of the forms deprived of the guest molecules are pointed out. A new clathrate form of s-PPMS containing CS₂ is also described.     

Influence of the guest molecules on the structural organization of the clathrates of syndiotactic poly(p‐methylstyrene)

The influence of the shape and size of guest molecules on the structural organization of the α and β class clathrates of syndiotactic poly(p‐methylstyrene) is described, through the analysis of the packing model proposed for the crystal structures of the clathrate forms containing o‐dichlorobenzene and tetrahydrofuran. Preliminary data of the crystal structure of the s‐PPMS clathrate containing benzene and a comparison with the crystal structure of the clathrate forms of syndiotactic polystyrene are also presented.     

Crystal structures of syndiotactic poly(p-methylstyrene) clathrates with two guest molecules having very similar shape and volume

In the present paper we report on the clathrate structures of syndiotactic poly(p-methylstyrene) (s-PPMS) containing chlorobenzene (cell constants: a = 23.5 Å, b = 12.0 Å, c = 7.9 Å and γ = 112.4°; space group: P2₁/a) and toluene (cell constants: a = 19.5 Å, b = 13.5 Å, c = 7.9 Å and γ = 90°; space group: P2₁). Despite the extreme similarity between the shape and volume of these two molecules, they give rise to completely different clathrate structures, the first belonging to α class, the second belonging to β class. Moreover the clathrate form containing chlorobenzene represent the first case in which a α class clathrate form of s-PPMS contains two guest molecules in each cavity while the crystal structure here proposed for the s-PPMS/toluene clathrate represents a new example of chiral crystalline phase in which the polymer helices assume all the same chirality in the lattice. These results underline the fact that the choice of a particular structural organization in the process of clathrates’ formation of s-PPMS is not easily referable only to steric effects but seems to be dependant even on the chemical structure of the guest molecules. A comparison with syndiotactic polystyrene is also done.     

Polymorphism of the co-crystalline forms of syndiotactic polystyrene with chloroform: Crystal structure of the δ clathrate

The crystal structure of the δ clathrate form of syndiotactic polystyrene (s-PS) containing CHCl₃, a molecule having a pivotal role in respect to the co-crystalline phases formation of this polymer, has been determined through X-ray diffraction data and molecular mechanics calculations. Analogously to all the other δ clathrate forms of s-PS, this structure presents a monoclinic unit cell (cell constants a = 1.77 nm, b = 1.32 nm, c = 0.78 nm and γ = 121.5°) in which the s(2/1)2 polymer helices and guest molecules are packed according the space group P2₁/a. At variance with all the other δ clathrate forms of s-PS whose crystal structure has been reported in the literature, probably due to the not planar shape of the chloroform guest molecule, in this structure guest molecules occupy each centrosymmetric cavity in a very low efficient way, giving rise to a disorder in the positioning of the guest molecules along the b + a/2 direction of the unit cell. A comparison with the ε type clathrate with the same guest, for which some preliminary results have been reported too, is also presented.     

Steric effects on excimer formation in isotactic and atactic polystyrene and poly(ar-methylstyrene)s in solution

Effects of tacticity and steric hindrance on excimer formation were investigated in isotactic and atactic polystyrene, poly(o-methylstyrene), poly(m-methylstyrene), and poly(p-methylstyrene) in the presence and absence of a quencher (CCl₄). The calculated rate constants for excimer formation in the isotactic polymers except for poly(o-methylstyrene) were almost the same and larger than those in the corresponding atactic polymers. These results indicate that excimer formation was due to not only rotational sampling but also energy migration to trapping sites. It was found that steric hindrance on excimer formation was intimately related to the excition diffusion length in the polymer chain.     

Clathrates with tetrahydrofuran of styrene‐p‐methyl styrene co‐syndiotactic copolymers

Clathrates with tetrahydrofuran of styrene‐p‐methyl styrene co‐syndiotactic copolymers have been characterized by wide‐angle X‐ray scattering and differential scanning calorimetry. Stable clathrate structures with THF molecules have been observed in whole range of composition. In particular, syndiotactic polystyrene like and syndiotactic poly‐p‐methylstyrene like clathrate phases are obtained for p‐methylstyrene contents lower and higher than 35% by mol, respectively.     

Powder X‐ray studies of meso‐hexamethyl propylene amine oxime (meso‐HMPAO) in two different phases

Two different forms of meso‐3,3′‐[2,2‐dimethylpropane‐1,3‐diylbis(azanediyl)]dibutan‐2‐one dioxime, commonly called meso‐hexamethyl propylene amine oxime (HMPAO), C₁₃H₂₈N₄O₂, designated α and β, were isolated by fractional crystallization and their crystal structures were determined by powder X‐ray diffraction using the direct‐space method with the parallel tempering algorithm. The α form was first crystallized from acetonitrile solution, while the β form was obtained by recrystallization of the α phase from diethyl ether. The α form crystallizes in the triclinic system (space group P), with one molecule in the asymmetric unit, while the crystal of the β form is monoclinic (space group P2₁/n), with one molecule in the asymmetric unit. In both phases, the molecules have similar conformations and RS/EE geometric isomerism. The crystal packing of the two phases is dominated by intermolecular hydrogen‐bonding interactions between the two O—H oxime groups of an individual molecule and the amine N atoms of two different adjacent molecules, which lead to segregation of extended poly(meso‐HMPAO) one‐dimensional chains along the c direction. The structures of the two phases are primarily different due to the different orientations of the molecules in the chains.     

Structural and Dynamical Properties of n-Alkane Molecules in Clathrate Phase of Syndiotactic Polystyrene

Syndiotactic polystyrene (sPS) forms a clathrate phase with a variety of compounds. Not only rigid molecules but also flexible molecules can be stored in the cavities of the clathrate phase. To clarify the adjustment mechanism of a flexible guest molecule to the sPS clathrate system, the host and guest structures were investigated by means of solid-state ¹³C NMR and Raman spectroscopy, and X-ray diffractometry for the sPS clathrates with a series of n-alkanes from n-hexane to n-decane. Although the 010 spacing of the host sPS lattice expanded slightly on going from n-hexane to n-heptane, it decreased markedly at n-octane and then increased gradually with the chain length of guest n-alkane. The conformational change of guest n-alkane molecules was involved in this anomalous change in the 010 spacing. Majority of the n-hexane and n-heptane molecules took extended chain structures in the clathrates, whereas all longer n-alkanes took bent chain structures. The mean-square displacement of hydrogen atoms in the clathrates was estimated by quasielastic neutron scattering experiments. It was confirmed that the host lattice contraction suppressed thermal motion of the clathrate system.     

Crystal structures of the clathrate inclusion compounds formed by the host bis(isothiocyanato) tetrakis (4-methylpyridine) iron (II) with benzene,m-xylene andp-xylene as guests

Single crystal X-ray structures of three 1 : 1 (guest: host) compounds in which the Fe(NCS)₂ (4-methylpyridine)₄ complex is the host component and benzene,m-xylene andp-xylene are the respective guest components, are reported. The crystals of the inclusion compounds are tetragonalI4₁/a, with:a = 17.08(1),c = 23.66(3) Å (benzene clathrate);a = 17.17(1),c = 24.02(2) Å (m-xylene clathrate) anda = 17.12(1),c = 23.93(3) Å (p-xylene clathrate);Z = 8. The host complex molecule has the octahedral type of coordination of the N₆ donor system. The isothiocyanate ligands are intrans positions and related by twofold axial symmetry. The symmetry axis runs diagonally between the neighbouring, equatorial 4-methylpyridine (MePy) ligands. The two symmetrically independent McPy ligands form angles ofca. 45 and 55° with the equatorial Fe-N₄ plane. The absorption sites for guest molecules lie on centres of symmetry. Non-centrosymmetricm-xylene molecules occupy these sites randomly distributed over two orientations related by symmetry. The structures belong to the class oforganic zeolites since the cavities occupied by the guest molecules are interconnected to form a three-dimensional network.     

Molecular‐Complex Formation of Syndiotactic Polystyrene with Stable Radical Molecules

Summary: The formation of a molecular‐complex crystalline phase of syndiotactic polystyrene (sPS) that contains a stable nitroxide radical compound, 2,2,6,6‐tetramethylpiperidinyl‐N‐oxyl (TEMPO), is confirmed by IR and electron spin resonance (ESR) spectroscopy, X‐ray diffractometry, and thermogravimetric analysis. Through a guest exchange procedure assisted by a plasticizing agent, the original guest (chloroform) contained in the starting clathrate phase is completely replaced by TEMPO. Although the conformational regularity of the sPS helices in the resultant crystalline phase that contains TEMPO is similar to that in the starting clathrate phase, the host lattice expands in the 010 direction. The guest TEMPO molecules exhibit a significantly broadened ESR signal because of their highly concentrated state in the complex crystalline phase.

Thermogravimetric measurement of a powder sample of the sPS/TEMPO complex.     

Effect of Guest–Host Hydrogen Bonding on the Structures and Properties of Clathrate Hydrates

To provide improved understanding of guest–host interactions in clathrate hydrates, we present some correlations between guest chemical structures and observations on the corresponding hydrate properties. From these correlations it is clear that directional interactions such as hydrogen bonding between guest and host are likely, although these have been ignored to greater or lesser degrees because there has been no direct structural evidence for such interactions. For the first time, single‐crystal X‐ray crystallography has been used to detect guest–host hydrogen bonding in structure II (sII) and structure H (sH) clathrate hydrates. The clathrates studied are the tert‐butylamine (tBA) sII clathrate with H₂S/Xe help gases and the pinacolone + H₂S binary sH clathrate. X‐ray structural analysis shows that the tBA nitrogen atom lies at a distance of 2.64 Å from the closest clathrate hydrate water oxygen atom, whereas the pinacolone oxygen atom is determined to lie at a distance of 2.96 Å from the closest water oxygen atom. These distances are compatible with guest–water hydrogen bonding. Results of molecular dynamics simulations on these systems are consistent with the X‐ray crystallographic observations. The tBA guest shows long‐lived guest–host hydrogen bonding with the nitrogen atom tethered to a water HO group that rotates towards the cage center to face the guest nitrogen atom. Pinacolone forms thermally activated guest–host hydrogen bonds with the lattice water molecules; these have been studied for temperatures in the range of 100–250 K. Guest–host hydrogen bonding leads to the formation of Bjerrum L‐defects in the clathrate water lattice between two adjacent water molecules, and these are implicated in the stabilities of the hydrate lattices, the water dynamics, and the dielectric properties. The reported stable hydrogen‐bonded guest–host structures also tend to blur the longstanding distinction between true clathrates and semiclathrates.     

The structure - properties relations in the clathrates of N1 (NCS)2 (4-methylpyridine)4

The Ni(NCS)₂(4-methylpyridine)₄ molecule shows significant confomational flexibility of the four piooline and the two isothiocyanate ligands. Both intramolecular non-bonded energy calculations and x-ray crystal structures support the idea that the host molecule may adopt different conformations (molecular shape) in order to bind, in clathrate formation, guest molecules of different shape and size. This versatility enables the Ni(NCS)₂(4-methylpyridine)₄ complex to form many different crystal structures able to absorb both small molecules, e.g. noble gases, and relatively large condensed aromatic hydrocarbons. The type of the crystalline structure formed defines “primary” properties of the clathrate. Secondary effects are observed when varying lattice parameters of the host structure (dilatation - contraction) by introducing guest component of suitable composition.     

Novel series of clathrate compounds of the three-dimensional metal complex hosts (N-methyl-1,3-diaminopropane)cadmium(II) tetracyanonickelate(II), (N,N-Dimethyl-1,3-diaminopropane)cadmium(II) Tetracyanonickelate(II), and (2-Hydroxyethylmethylamine) cadmium(II) Tetracyanonickelate(II)

The clathrate compounds of the title three-dimensional metal complex hosts have been prepared with various aliphatic, alicyclic, and aromatic guest molecules. The typical structure has been demonstrated for a cyclohexane clathrate (N-methyl-1,3-diaminopropane)cadmium(II) tetracyanonickelate(II)-cyclohexane(2/1) by X-ray single crystal analysis. The powder X-ray diffraction data were assigned for the other clathrates to the tetragonal systems isostructural to the cyclohexane clathrate in which the box-like cavity with the approximate dimensions of 7 × 7 × 5 ų accommodates the guest molecule.     

High-temperature phases of poly(p-xylylene)

The crystal structure of poly(p-xylylene), as polymerized, is the α form. This transforms irreversibly to the β from by annealing or drawing. To clarify the mechanism of this transition, structural changes of the α and β crystals were examined with a high-temperature stage in the electron microscope. Two high-temperature phases, β₁ and β₂, were found and their structures were analyzed. In these structures lattice distortions due to rotational and translational motions of chains are in troduced, especially in the β₂ form. The α → β transition is induced through such a disordered phase. The statistical arrangement of a molecule in the β-form unit cell results from freezing the disorder in the high-temperature phases.     

Temperature-dependent selectivity of inclusion by <Emphasis Type="Italic">trans</Emphasis>-9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid

From acetophenone solution with a small amount of water, a widely used clathrate forming compound, trans-9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid (DED), crystallizes either in the form of a clathrate with acetophenone (25°C) or in the form of hydrate (5°C). The clathrate of DED with acetophenone is triclinic, space group P-1 with the unit cell parameters: a = 8.5002(2) Å, b = 12.5247(8) Å, c = 12.8251(8) Å; α = 62.876(2)°, β = 80.454(2)°, γ = 89.789(2)°; V = 1194.4(1) ų, Z = 2; the molar ratio DED:acetophenone is 2:3. The clathrate is of channel type; the system of mutually intersecting channels propagates in [100] and [01–1] directions in the structure. The guest molecules of acetophenone are included in the channels and do not form any H-bonds with the host molecules of DED. The hydrate of DED crystallizes in monoclinic system, in space group C2/c, with the unit cell parameters: a = 31.770(6) Å, b = 8.503(2) Å, c = 12.888(3) Å; β = 104.26(3)°; V = 3374(1) ų, Z = 8; the molar ratio DED:water is 1:3. One of two carboxylic groups of the molecule of DED is deprotonated and the proton is incorporated into the hydroxonium ion H₃O⁺. The crystal structure of the hydrate of DED is of layer type with well distinguished hydrophobic and hydrophilic parts.     

Crystal structure of the Hofmann-dma type clathrate

Seven new Hofmann-dma type clathrates Cd(dma)₂Ni(CN)₄-xG (x = 1, G = aniline, 2,3-xylidine, 2,4-xylidine, 2,5-xylidine, 2,6-xylidine, 3,5-xylidine and x = 2, G = 2,4,6-trim ethylaniline) were prepared by replacing the amine in a Hofmann type clathrate Cd(NH₃)₂Ni(CN)₄-2G by dimethylamine (dma). The structure of the Hofmann-dma type clathrate is formed with stacked host two-dimensional metal complexes of Cd(dma)₂ Ni(CN)₄ and guest molecules accommodated in the space between the stacked host complexes. This basic structure scheme is the same as that of the Hofmann type clathrate. However, the guest species accommodated in the Hofmann-dma type clathrate are more various than those of the Hofmann type clathrate, and their crystal structures are classified into four types depending on the geometry of the guest species. In order to clarify the structure of the Hofmann-dma type clathrate, single crystal X-ray diffraction experiments were canied out on the seven new clathrates, and the crystal structures of the o-, m- and p- toluidine clathrates were refined. The X-ray structure analyses showed that the host two-dimensional metal complex of the Hofmann-dma tvpe clathrate has stmctural flexibility to form a puckered structure, which results from the angular distortion of the bond between Cd and N of the cyanide bridge in the host two-dimensional complex. This stmctural flexibility of the host complex leads to the diversity of crystal structures and guest species in Hofmann-dma type clathrates. Translated fromZhurnal Strukturnmoi Khimii, Vol. 40, No. 5, pp. 898–926, September–October, 1999.     

New crystal structures of β-[Ni(NCS)2(4-methylpyridine)4] clathrates with furan,tetrahydrofuran, methylene chloride,benzene + ethanol and methylcellosolve as guest molecules

Single crystal X-ray structures of clathrates of -[Ni(NCS)₂(4-methylpyridine)₄] with furan, tetrahydrofuran, benzene + ethanol, methylene chloride, and methylcellosolve as guests molecules are reported. The location of the guest molecule in the partially decomposed clathrate with methylene chloride was defined by X-ray diffraction and compared with the fully occupied one. The host lattices of all clathrates studied are tetragonal (I4₁/a) and do not differ significantly from typical -phase clathrates of [M(NCS)₂(4-methylpyridine)₄] (M = divalent metal cation). Arrangements of guest molecules represent different types of packing: one type of guest molecule occupies both possible types of positions, two different guest molecules occupy different positions, with only one type of positions occupied by one type of guest molecule. Possible stoichiometries of clathrates with -type lattices are discussed.Presented at the Sixth International Seminar on Inclusion Compounds, Istanbul, Turkey, 27–31 August 1995.     

Coordination steric effect of N,N-dimethylformamide, N,N-dimethylacetamide and N-methyl-2-pyrrolidone on the assembly of coordination polymers

Three coordination polymers 1, 2 and 3 have been synthesized in DMF (N,N-dimethylformamide), DMA (N,N-dimethylacetamide) and NMP (N-methyl-2-pyrrolidone), respectively. In 1, DMF solvent molecule coordinates to zinc ion as an ancillary ligand, and 1D chain structure is obtained. 2 and 3 are isostructural, in which solvent molecules, DMA and NMP, do not coordinate to zinc ions, and 1D double stranded chain structures are formed. The coordination steric hindrance of the solvents is suggested as the decisive factor of the assemblies. Crystallography and thermoanalysis reveal that 2 and 3 are more stable and also include more guest solvent molecules than 1.     

Pseudodimorphism of the Trans-9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic Acid Clathrates with Acetic and Propionic Acids

The versatile host compound trans-9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid (1) forms under ambient conditions isostructural complexes with acetic and propionic acids being true clathrates without host-guest type H-bonds. A new modification of the clathrate between 1 and acetic acid (1a) is obtained at sub-room temperature (5 °C) while for preparation of the new crystal form of the clathrate with propionic acid (1b) crystallization temperature should be increased up to 50 °C. Crystal structures of the pseudodimorphs show that homo carboxylic acid dimers existing in the conventional phases are also observed here, demonstrating the new compounds to be of same clathrate type. Crystal data: for 1a: triclinic P-1, a = 8.626(2) Å, b = 9.073(2) Å, c = 12.042(2) Å, α = 76.34(3)°, β = 77.41(3)°, γ = 84.13(3)°, V = 892.5(4) ų, Z = 2, R = 0.0446 for 3171 reflections; for 1b: monoclinic C2/c, a = 13.268(3) Å, b = 12.636(3) Å, c = 21.786(4) Å, β = 90.56(3)°, V = 3652.3(14) ų, Z = 8, R = 0.0618 for 2365 reflections.