“…Polymorphism is generally believed to be an intractable phenomenon that is very often observed upon crystallization from the melt or solution and is not easy to control. − However, if the strategy for the control of polymorphism could be established, this phenomenon would be utilized as a fruitful methodology for producing several solid materials of different electronic or physicochemical properties from a given organic substance. , In this context, various methods such as changing the crystallization conditions, pseudo-seeding with a crystal of desired structure, or adding impurities or additives to inhibit the nucleation or the crystal growth of an undesired polymorphic form have been devised to obtain a desired polymorphic form by selective crystallization from the supersaturated solution.…”
A new mode of the solvent-assisted solid-to-solid transformation of the first-formed and least stable
γ-polymorphic form into the most stable ε-polymorphic one occurring during crystallization from the supersaturated
EtOH solution of (±)-[2-[4-(3-ethoxy-2-hydroxypropoxy)phenylcarbamoyl]ethyl]trimethylammonium benzenesulfonate
[(±)-2], which shows an unusual enantiomeric resolution phenomenon called Preferential Enrichment, has been
revealed. The crystal structure of the new ε-form, which was obtained as the monophasic powder sample by exhaustive
polymorphic transitions via three other metastable polymorphic forms in contact with the solvent, has been solved
from the powder X-ray diffraction data by the direct-space approach employing the Monte Carlo method with the
subsequent Rietveld refinement. By comparison of the crystal structure of the least stable γ-form with that of the
most stable ε-one, the mechanism of this polymorphic transition has been interpreted in terms of a new type of
rearrangement of weak intermolecular interactions caused by a slight molecular movement inside the crystal lattice,
in which intermolecular C(sp2)H···O interactions prominently control the crystal structure. This new finding
complements our recent report on the mechamisn of Preferential Enrichment.
“…Polymorphism is generally believed to be an intractable phenomenon that is very often observed upon crystallization from the melt or solution and is not easy to control. − However, if the strategy for the control of polymorphism could be established, this phenomenon would be utilized as a fruitful methodology for producing several solid materials of different electronic or physicochemical properties from a given organic substance. , In this context, various methods such as changing the crystallization conditions, pseudo-seeding with a crystal of desired structure, or adding impurities or additives to inhibit the nucleation or the crystal growth of an undesired polymorphic form have been devised to obtain a desired polymorphic form by selective crystallization from the supersaturated solution.…”
A new mode of the solvent-assisted solid-to-solid transformation of the first-formed and least stable
γ-polymorphic form into the most stable ε-polymorphic one occurring during crystallization from the supersaturated
EtOH solution of (±)-[2-[4-(3-ethoxy-2-hydroxypropoxy)phenylcarbamoyl]ethyl]trimethylammonium benzenesulfonate
[(±)-2], which shows an unusual enantiomeric resolution phenomenon called Preferential Enrichment, has been
revealed. The crystal structure of the new ε-form, which was obtained as the monophasic powder sample by exhaustive
polymorphic transitions via three other metastable polymorphic forms in contact with the solvent, has been solved
from the powder X-ray diffraction data by the direct-space approach employing the Monte Carlo method with the
subsequent Rietveld refinement. By comparison of the crystal structure of the least stable γ-form with that of the
most stable ε-one, the mechanism of this polymorphic transition has been interpreted in terms of a new type of
rearrangement of weak intermolecular interactions caused by a slight molecular movement inside the crystal lattice,
in which intermolecular C(sp2)H···O interactions prominently control the crystal structure. This new finding
complements our recent report on the mechamisn of Preferential Enrichment.
“…As the control experiment, crystallization began within 24 h in the same supersaturated 2-PrOH solution in the absence of the δ-form seed crystals, resulting in a substantial enantiomeric enrichment of 1a up to 70% ee in the mother liquor together with the exclusive formation of the α-form crystals . This difference in the crystallization rate can well be interpreted by assuming that the added δ-form seed crystals of (±)- 1b serve both as the additive to inhibit the nucleation or crystal growth process of the α-form 12 and as the substrate for heterogeneous nucleation of the δ-form of 1a . , The reached low ee value of 1a in the mother liquor in the crystallization with the δ-form seed crystals is supposed to be correlated with the slow crystallization, because even slow crystallization of the α-form crystals of (±)- 1a conducted at the lower concentrations does not give a high ee value of the solute in the mother liquor. 7 (a) Crystal structure of the δ-form of (±)- 1a .…”
Section: Resultsmentioning
confidence: 99%
“…18 This difference in the crystallization rate can well be interpreted by assuming that the added δ-form seed crystals of (()-1b serve both as the additive to inhibit the nucleation or crystal growth process of the R-form 12 and as the substrate for heterogeneous nucleation of the δ-form of 1a. 10,11 The reached low ee value of 1a in the mother liquor in the crystallization with the δ-form seed crystals is supposed to be correlated with the slow crystallization, because even slow crystallization of the R-form crystals of (()-1a conducted at the lower concentrations does not give a high ee value of the solute in the mother liquor. By comparison of the powder X-ray diffraction (XRD) patterns as shown in Figure 3, the crystal structure of the new polymorphic form of (()-1a has turned out to be isomorphous with that of the δ-form crystal of (()-1b.…”
Section: Resultsmentioning
confidence: 99%
“…Of these methods, the induced nucleation with the adequate seed crystals possessing an analogous target crystal structure is the most straightforward and promising for designed crystallization if the seed crystals are available, because the activation energy for heterogeneous nucleation on the surfaces of seed crystals is small enough to induce the assembly of molecules into supramolecular motifs affording the desired polymorph during the formation of prenucleation aggregates which give crystal nuclei after phase transition. 10 However, very often it was not an easy task to determine the crystal structure of a metastable crystalline phase due to the usual inaccessibility of single crystals suitable for X-ray crystallographic analysis. Recently, such difficulties have been overcome by the contemporary advances in the procedures for structure solution by using powder X-ray diffraction data measured with laboratory X-ray sources, albeit there are still some limitations.…”
Section: Introductionmentioning
confidence: 99%
“…In this context, it has been reported that changing the crystallization conditions such as solvents and temperatures, pseudoseeding the supersaturated solution with a crystal of desired structure on the basis of an epitaxy protocol, , or adding impurities or additives to inhibit the nucleation or the crystal growth of an undesired polymorph can afford a desired polymorph. Of these methods, the induced nucleation with the adequate seed crystals possessing an analogous target crystal structure is the most straightforward and promising for designed crystallization if the seed crystals are available, because the activation energy for heterogeneous nucleation on the surfaces of seed crystals is small enough to induce the assembly of molecules into supramolecular motifs affording the desired polymorph during the formation of prenucleation aggregates which give crystal nuclei after phase transition …”
Described are the successful, selective crystallization and characterization of one (δ-form) of the
metastable polymorphs of (±)-[2-[4-(3-ethoxy-2-hydroxypropoxy)phenylcarbamoyl]ethyl]dimethylsulfonium p-toluenesulfonate (1a), which is assumed to exist as a transient key-intermediate crystalline phase relevant to the
mechanism of Preferential Enrichment, an unusual enantiomeric resolution phenomenon observed upon crystallization of a certain kind of racemates from solvents. By pseudoseeding the supersaturated solution with the δ-form
seed crystals of an anlogous compound, the desired δ-form crystals of (±)-1a could be obtained as a monophasic
powder sample. This selective crystallization can be interpreted in terms of the interplay between an inhibition of
the nucleation or crystal growth process of the undesired stable α-polymorphic form and a heterogeneous nucleation
of the desired metastable δ-form through epitaxy. The physicochemical properties of the δ-form crystal such as
melting point and solubility have been compared with those of the stable α-form one to evaluate the relative stability
of the metastable δ-form. It has also been confirmed that polymorphic transition of the metastable δ-form crystal to
the stable α-form one occurs in contact with a solvent. On the basis of the δ-form crystal structure solved from its
powder X-ray diffraction data measured with a laboratory X-ray source by means of the direct-space approach
employing the Monte Carlo method and the subsequent Rietveld refinement, the origin of the metastability of the
δ-form crystal has been discussed.
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