Sanaz Khorasani

A single-crystal-to-single-crystal Diels–Alder reaction with mixed topochemical and topotactic behaviour

Electron donor/acceptor (EDA) interactions have been found to be very useful in engineering reactive heteromolecular crystals, but few examples have been reported in the literature. By utilising EDA interactions, crystals of charge-transfer (CT) complexes were formed with bis(N-allylimino)-1,4-dithiin as the electron acceptor and 9-bromoanthracene as the electron donor. The CT complex crystallised in the monoclinic P21/n space group with the crystal structure consisting of stacks of alternating electron donor and acceptor molecules in a 1 : 1 ratio. These crystals are able to undergo a solid-state Diels–Alder reaction with bis(N-allylimino)-1,4-dithiin as the dienophile and 9-bromoanthracene as the diene. Examination of close contacts indicates that the diene can theoretically react with the dienophile above or below it within a stack as the reaction distances are less than 3.5 A in both directions. A single crystal was selected and allowed to react at 30 °C, was analysed at various states of conversion by single-crystal X-ray diffraction, and was found to react by approximately 10% every 6 days, with the reaction occurring in a single direction along the CT stack axis. The solid-state reaction creates a void space which leads to a molecular conformational change within the crystal. Consequently, the single crystal started to show significant signs of deterioration after approximately 28% conversion but remained intact upon further reaction and was found to anneal as 100% conversion was approached, leading to the formation of new intermolecular interactions not present in the starting crystal. The solid-state reaction occurs topochemically when fewer than 28% or more than 80% of the molecules have reacted, with minimal motion during the reaction. In the conversion range of 28–80%, the reaction occurs in an almost topotactic manner with significant molecular motion and associated crystal deterioration.

1-(Anthracen-1-yl)pyrrolidine-2,5-dione

In the molecular structure of title compound, C18H13NO2, the succinimide ring is orientated away from the plane of the anthracene moiety by 71.94 (4)°. The crystal structure features three different types of intermolecular interactions, viz. C—H⋯O, C—H⋯π and π–π bonds. Molecules along the b axis stack on each other as a result of π–π interactions which have a centroid–centroid distance of 3.6780 (15) Å, while C—H⋯π interactions are present between neigbouring stacks. Also, acting between the stacks are the C—H⋯O interactions between the aromatic H atoms of the anthracene and the O atoms of the succinimide.

(Z)-Ethyl 2-hy-droxy-4-oxo-4-(1,4,5,6,8-penta-meth-oxy-naphthalen-2-yl)but-2-enoate.

The title compound, C21H24O9, crystallizes with two independent molecules in the asymmetric unit which are almost centrosymmetrically related to each other. The ethanoate group in one of the two molecules is disordered over two positions with a site-occupation factor of 0.880 (7) for the major occupied site. In the crystal, the 1,3-diketone group exists in the keto–enol isomeric form due to the stabilizing effect of the intramolecular O—H⋯O hydrogen bond present in this form. The compound packs as a layered structure in which C—H⋯π and C—H⋯O interactions are present within and between the layers.

Cyclo-octanaminium hydrogen succinate monohydrate.

In the title hydrated salt, C8H18N+·C4H5O4 −·H2O, the cyclooctyl ring of the cation is disordered over two positions in a 0.833 (3):0.167 (3) ratio. The structure contains various O—H.·O and N—H⋯O interactions, forming a hydrogen-bonded layer of molecules perpendicular to the c axis. In each layer, the ammonium cation hydrogen bonds to two hydrogen succinate anions and one water molecule. Each hydrogen succinate anion hydrogen bonds to neighbouring anions, forming a chain of molecules along the b axis. In addition, each hydrogen succinate anion hydrogen bonds to two water molecules and the ammonium cation.

Feedback mechanisms in single-crystal-to-single-crystal solid-state reactions

Reactions in single crystals offer an opportunity to study the movement of atoms and molecules during the reaction process at the atomic level through X-ray diffraction techniques. However, examples of single-crystal-to-single-crystal (SCSC) reactions are relatively uncommon as the reaction process often leads to complete crystal disintegration. Even more unusual are SCSC reactions involving two different molecules. The main reason for this is the lack of co-crystals with suitably orientated reactant molecules. A useful way around this problem is to use charge transfer (CT) interactions to pre-align the molecules of interest in the solid-state so as to allow a solid-state reaction to occur. In this case, the CT interactions usually lead to a structure composed of stacks in which the acceptor and donor molecules alternate. In the case of charge transfer complexes made of 1,4dithiintetracarboxylic type compounds and anthracene derivatives, it is usually possible to carry out [2+4] Diels-Alder cycloaddition reactions in the solid-state, in which the former act as dienophiles and the later as dienes. Modification of substituents on either the acceptor or donor molecule also has an effect on the course of the reaction in a single crystal. Several concepts have been proposed to explain how molecular reactions occur in the solid state with those of relevance to SCSC reactions having been reviewed in detail.[1] Of relevance to this work are the topochemical principle, the concept of a reaction cavity, and the identification of possible reaction affecting feedback mechanisms. Solid-state reactions where the structure of the product is determined by minimal motion from the coordinates of the starting materials are said to have occurred topochemically. However, very few reactions occur in this way resulting in interesting solid-state chemistry. Here we present examples of solid-state reactions in which interactions between reactant molecules, or between product molecules and reactant molecules influence the course of the reaction. One of these is the reaction of 9-methylanthracene with bis(N-cyclobutylimino)1,4-dithiin where the results show that steric effects between product molecules and reactant molecules during the SCSC reaction influence the formation of products along the b axis of the structure, resulting in a more ordered structure than initially suggested by X-ray crystal structure analysis (see Figure). [2] Other examples showing significant crystal rebuilding after reaction will also be presented. [3]

Cooperativity in the Reaction of 9-Methylanthracene With a 1,4-Dithiin Molecule

Solid-state chemistry involves the manipulation of molecules and materials through photochemical, thermal, or mechanical reaction methods. Single-crystal-to-single-crystal (SCSC) reactions are rare, but offer the opportunity to study reaction mechanisms and molecular motions in the solid state at the atomic level using single crystal X-ray diffraction. This allows the effect of the surrounding molecules, and hence the reaction cavity, on the reacting molecules to be examined which may ultimately lead to postcrystallization methods for creating new materials or reaction products that cannot easily be obtained via solution. SCSC reactions involving two different molecules are relatively uncommon. A convenient system that allows the study of such reactions is the [4+2] Diels-Alder reaction of 1,4-dithiintetracarboxylic type compounds with anthracene derivatives. In the work reported here, electron donor to acceptor interactions between 9-Methylanthracene and bis(N-cyclobutylimino)-1,4-dithiin lead to the formation of chiral charge transfer (CT) crystals [1]. These undergo a topochemical thermal SCSC [4 + 2] Diels-Alder reaction in the solid state. CT crystals were reacted at 40 °C, their structures determined by X-ray diffraction at various degrees of conversion, and examined using Hirshfeld surfaces and lattice energy calculations to find evidence of reaction cooperativity and feedback mechanisms. In this case, a maximum reaction conversion of around 96% was obtained indicating that the reaction is non-random within the charge transfer stacks, with close contacts between product molecules in the reacted crystal also providing some evidence for reaction cooperativity along the b axis perpendicular to the CT stacking axis.

(±)-3-Benz­yloxy-1-(4-meth­oxy­benz­yl)piperidine-2-thione

The title molecule, C20H23NO2S, adopts a twisted conformation in which the two aromatic rings connected to the central piperidine ring are orientated trans to each other. An intramolecular C—H⋯S contact occurs. In the crystal, C—H⋯π and C—H⋯O interactions act to stabilize the structure in three dimensions.

3-Methyl-1-tosyl-1H-indole-2-carbaldehyde

The title indole derivative, C17H15NO3S, crystallizes with two independent molecules in the asymmetric unit. The benzene ring of the tosyl group is almost perpedicular to the indole ring in both molecules, with interplanar angles of 82.60 (5)° and 81.82 (6)°. The two molecules are, as a consequence, able to form an almost centrosymmetric non-bonded dimer, in which the molecules are linked by pairs of C—H⋯π interactions. The crystal structure displays a three-dimensional network of C—H⋯O interactions. A π–π interaction occurs between inversion-related indole rings with a centroid–centroid distance of 3.6774 (16) Å and an interplanar angle of 1.53 (15)°. This interaction leads to a stacking of molecules along the a axis.

3-Hy-droxy-1-(4-meth-oxy-benz-yl)piperidin-2-one.

The title compound, C13H17NO3, adopts a conformation in which the aromatic ring and the mean plane of the piperidine ring are almost perpendicular to each other [dihedral angle = 79.25 (6)°]. The presence of the carbonyl group alters the conformation of the piperidine ring from a chair to a twisted half-chair conformation. In the crystal, pairs of strong O—H⋯O hydrogen bonds link the molecules into inversion dimers. Weak C—H⋯O interactions extend the hydrogen-bonding network into three dimensions.

1-(2-Hy-droxy-4,5-dimeth-oxy-phen-yl)ethanone.

The molecular structure of the title compound, C10H12O4, contains an intramolecular hydrogen bond between the phenol and acetyl substituents. In the crystal, C—H⋯π interactions act between the molecules in a cyclic manner to stabilize stacks of molecules along the b axis. Several C—H⋯O interactions are present between the stacks.