Spontaneous resolution of a novel chiral coordination polymer through supramolecular interactions and solvent symmetry breaking

The spontaneous resolution reaction of racemic trans-2,3-dihydro-2,3-dipyridyl-benzo[e]indole 1 with Cd(ClO₄)₂·6H₂O in the presence of 2-butanol under solvothermal reaction conditions favors the formation of crystal 2 [P-Cd(R,R,-1)₂(ClO₄)₂], while a similar reaction in the presence of ethanol only favors the formation of crystal 3 [M-Cd(S,S,-1)₂(ClO₄)₂]. The crystal structural determination shows that both 2 and 3 crystallize in chiral enantiomorphous space groups (P6₁22 and P6₅22) and their structures are 1D infinite chain, and are just enantiomorphous pairs most like. The spontaneous resolution process displays estimated ee values of ca. +0.6 for 2-butanol and ca. −0.4 for ethanol. Enantiomerically pure (S,S)-trans-2,3-dihydro-2,3-dipyridyl-benzo[e]indole (S,S,-1) can be obtained through the decomposition of mechanically separated 3. Additionally (S,S,-1) also crystallizes in a chiral space group (P2₁). The CD (circular dichroism) spectra of both 2 and 3 in the solid state are also approximately enantiomorphous pairs. However, their fluorescent spectra in the solid state display a moderate difference in maximum emission peaks (Δλ = 19 nm). Crystal data for 2: C₄₄H₃₄Cl₂N₆O₈Cd, M = 958.07, hexagonal, P6₁22, a = 10.5488(5), c = 68.256(4) Å, α = γ = 90°, β = 120°, V = 6577.8(6) Å³, Z = 6, D_c = 1.451 mg m⁻³, R₁ = 0.0498, wR₂ = 0.1124, μ = 0.679 mm⁻¹, S = 0.623, Flack χ = −0.02(6). For space group P6₅22, R₁ = 0.0670, wR₂ = 0.1602, S = 0.725 with a Flack value of 1.03(7); Crystal data for 3, C₄₄H₃₄Cl₂N₆O₈Cd, M = 958.07, hexagonal, P6₅22, a = 10.5446(3), c = 68.265(3) Å, V = 6573.3(4) Å³, Z = 6, D_c = 1.452 mg m⁻³, R₁ = 0.0444,wR₂ = 0.1002, μ = 0.679 mm⁻¹, S = 0.558, Flack χ = 0.01(5). For space group P6₁22, R₁ = 0.0501, wR₂ = 0.1178, S = 0.599 with a Flack value of 1.00(5). The low Flack parameter indicates that the absolute configurations of 2 and 3 are stated; Crystal data for (S,S)-1, C₂₂H₁₇N₂, M = 323.39, orthorhombic, P2₁2₁2₁, a = 9.2598(7), b = 9.4617(8), c = 19.1452(16) Å, V = 1677.4(2) Å³, Z = 4, D_c = 1.281 mg m⁻³, R₁ = 0.0417, wR₂ = 0.1191, T = 293 K, μ = 0.077 mm⁻¹, S = 0.862.     

Modulated nematic structures and chiral symmetry breaking in 2D

ABSTRACT

We have studied the properties of biaxial particles interacting via an anisotropic pair potential, involving second-rank quadrupolar and third-rank octupolar coupling terms, using Monte Carlo simulation. The particles occupy the sites of a 2D square lattice and the interactions are restricted to nearest neighbours. The system exhibits spontaneous chiral symmetry breaking from an isotropic phase to a chiral modulated nematic phase, composed of ambidextrous chiral domains. When twofold axes of quadrupolar and octupolar tensors coincide this modulated phase appears to be the ambidextrous cholesteric phase with pitch comparable to a few lattice spacings. The associated phase transition is first order.     

Degree of supersaturation-regulated chiral symmetry breaking in one crystal

This paper aimed at studying chiral symmetry-breaking phenomena in one crystal. Preferential crystallization of racemic asparagines was carried out in nonseeded stagnant solutions through slow cooling. By varying the supersaturation, only one transparent crystal could be obtained at enough low supersaturation of dl-asparagine, and the crystal was not pure enantiomer with crystal enantiomeric excess increasing inversely with the degree of supersaturation. Crystal enantiomeric excess can amount up to 85% in one transparent crystal. Because no secondary nucleation occurred except for stochastic primary nucleation, we suggest that primary nucleation and competition between l- and d-nuclei were considered to be a mechanism for asymmetry amplification. High-performance capillary electrophoresis coupled with laser-induced fluorescence was used to separate and quantify l- and d-asparagine and the enantiomeric excess value can be calculated according to their concentration.     

Chiral memory via chiral amplification and selective depolymerization of porphyrin aggregates

Chiral memory at the supramolecular level is obtained via a new approach using chiral Zn porphrins and achiral Cu porphyrins. In a "sergeant-and-soldiers" experiment, the Zn "sergeant" transfers its own chirality to Cu "soldiers" and, after chiral amplification, the "sergeant" is removed from the coaggregates by axial ligation with a Lewis base. After this extraction, the preferred helicity observed for the aggregates containing achiral Cu porphyrins reveals a chiral memory effect that is stable and can be erased and partially restored upon subsequent heating and cooling.     

Helicity-driven chiral self-sorting supramolecular polymerization with Ag+: right- and left-helical aggregates

The study of chiral self-sorting is extremely important for understanding biological systems and for developing applications for the biomedical field. In this study, we attempted unprecedented chiral self-sorting supramolecular polymerization accompanying helical inversion with Ag⁺ in one enantiomeric component. Bola-type terpyridine-based ligands (R-L¹ and S-L¹) comprising R- or S-alanine analogs were synthesized. First, R-L¹ dissolved in DMSO/H₂O (1 : 1, v/v) forms right-handed helical fibers (aggregate I) via supramolecular polymerization. However, after the addition of AgNO₃ (0.2–1.1 equiv.) to the R-L¹ ligand, in particular, it was found that aggregate II with left-handed helicity is generated from the [R-L¹(AgNO₃)₂] complex through the [R-L¹Ag]⁺ complex via the dissociation of aggregate I by a multistep with an off pathway, thus demonstrating interesting self-sorting properties driven by helicity and shape discrimination. In addition, the [R-L¹(AgNO₃)₂] complex, which acted as a building block to generate aggregate III with a spherical structure, existed as a metastable product during the formation of aggregate II in the presence of 1.2–1.5 equiv. of AgNO₃. Furthermore, the AFM and CD results of two samples prepared using aggregates I and III with different volume ratios were similar to those obtained upon the addition of AgNO₃ to free R-L¹. These findings suggest that homochiral self-sorting in a mixture system occurred by the generation of aggregate II composed of the [R-L¹Ag]⁺ complex via the rearrangement of both, aggregates I and III. This is a unique example of helicity- and shape-driven chiral self-sorting supramolecular polymerization induced by Ag⁺ starting from one enantiomeric component. This research will improve understanding of homochirality in complex biological models and contribute to the development of new chiral materials and catalysts for asymmetric synthesis.

Chiral self-sorting supramolecular polymerization of bola-type terpyridine-based ligands (R-L¹ and S-L¹) comprising R- or S-alanine analogs occurred upon addition of Ag⁺ in one enantiomeric component.     

Kinetically controlled Ag+-coordinated chiral supramolecular polymerization accompanying a helical inversion

We report kinetically controlled chiral supramolecular polymerization based on ligand–metal complex with a 3 : 2 (L : Ag⁺) stoichiometry accompanying a helical inversion in water. A new family of bipyridine-based ligands (d-L1, l-L1, d-L2, and d-L3) possessing hydrazine and d- or l-alanine moieties at the alkyl chain groups has been designed and synthesized. Interestingly, upon addition of AgNO₃ (0.5–1.3 equiv.) to the d-L1 solution, it generated the aggregate I composed of the d-L1AgNO₃ complex (d-L1 : Ag⁺ = 1 : 1) as the kinetic product with a spherical structure. Then, aggregate I (nanoparticle) was transformed into the aggregate II (supramolecular polymer) based on the (d-L1)₃Ag₂(NO₃)₂ complex as the thermodynamic product with a fiber structure, which led to the helical inversion from the left-handed (M-type) to the right-handed (P-type) helicity accompanying CD amplification. In contrast, the spherical aggregate I (nanoparticle) composed of the d-L1AgNO₃ complex with the left-handed (M-type) helicity formed in the presence of 2.0 equiv. of AgNO₃ and was not additionally changed, which indicated that it was the thermodynamic product. The chiral supramolecular polymer based on (d-L1)₃Ag₂(NO₃)₂ was produced via a nucleation–elongation mechanism with a cooperative pathway. In thermodynamic study, the standard ΔG° and ΔH_e values for the aggregates I and II were calculated using the van''t Hoff plot. The enhanced ΔG° value of the aggregate II compared to that of the formation of aggregate I confirms that aggregate II was thermodynamically more stable. In the kinetic study, the influence of concentration of AgNO₃ confirmed the initial formation of the aggregate I (nanoparticle), which then evolved to the aggregate II (supramolecular polymer). Thus, the concentration of the (d-L1)₃Ag₂(NO₃)₂ complex in the initial state plays a critical role in generating aggregate II (supramolecular polymer). In particular, NO₃⁻ acts as a critical linker and accelerator in the transformation from the aggregate I to the aggregate II. This is the first example of a system for a kinetically controlled chiral supramolecular polymer that is formed via multiple steps with coordination structural change.

The nanoparticles were transformed into the supramolecular polymer as the thermodynamic product, involving a helical inversion from left-handed to right-handed helicity.     

Mirror symmetry breaking of silicon polymers—from weak bosons to artificial helix

From elemental particles to human beings, matter and living worlds in our universe are dissymmetric with respect to mirror symmetry. Since the early 19th century, the origin of biomolecular handedness has been puzzling scientists. Nature's elegant bottom‐up preference, however, sheds light on new concepts of generating, amplifying, and switching artificial polymers, supramolecules, liquid crystals, and organic crystals that can exhibit ambidextrous circular dichroism in the UV/Visible region with efficiency in production under milder ambient conditions. In the 1920s, Kipping, who first synthesized polysilanes with phenyl groups, had much interest in the handedness of inorganic and organic substances from 1898 to 1909 in his early research life. Polysilanes—which are soluble Si‐Si bonded chain‐like near‐UV chromophores that carry a rich variety of organic groups—may become a bridge between animate and inanimate polymer systems. The present account focuses on several mirror symmetry breaking phenomena exemplified in polysilanes carrying chiral and/or achiral side groups, which are in isotropic dilute solution, as polymer particles dispersed in solution, and in a double layer film immobilized at the solid surface, and subtle differences in the helix, by dictating ultimately ultraweak chiral forces at subatomic, atomic, and molecular levels. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 000–000; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200900018     

A chiral interdigitated supramolecular network assembled from single‐stranded helical tubes

The amino‐functionalized helical chiral one‐dimensional coordination polymer catena‐poly[[bis(pyridine‐κN)zinc(II)]‐μ‐2‐aminobenzene‐1,4‐dicarboxylato‐κ⁴O¹,O^1′:O⁴,O^4′], [Zn(C₈H₅NO₄)(C₅H₅N)₂]_n, has an extended structure that is assembled from 2‐aminobenzene‐1,4‐dicarboxylate anions and Zn²⁺ cations and which presents a left‐handed 4₃ helix with a pitch of 25.6975 (9) Å. All the pyridine rings and all the amino groups point away from the helix to generate a hollow tube with a cross‐section of approximately 8 × 8 Å running parallel to the crystallographic c direction. Each single‐stranded helix is interdigitated with four neighbouring helices via N—H...O hydrogen bonds, which gives rise to a dense homochiral three‐dimensional supramolecular network.     

Self-assembly of helical supramolecular channels from chiral aminopyrimidine hydrogen bonding motifs in the solid state

The H-bond mediated self-assembly of the chiral C₂-symmetric bis-(2-amino-4-chloro-pyrimidines) 3 and 4 allows for the molecular recognition directed generation of helical superstructures. In the former case, unoccupied channel structures defined by the cylindrical interior of the derived supramolecular helix result, as revealed by X-ray crystallographic analysis using a synchrotron source. Upon crystallization, racemic 3 spontaneously resolves to form homochiral crystals exhibiting a helical packing motif identical to that determined for optically pure 3. The data provide insight into the interplay of the different structural and interactional features of the molecular components to the generation of the channel structure and suggest design strategies toward porous organic molecular solids of variable size.     

Synthesis of complementary double-stranded helical oligomers through chiral and achiral amidinium-carboxylate salt bridges and chiral amplification in their double-helix formation

A series of complementary molecular strands from 2-mer to 5-mer that are composed of m-terphenyl units bearing chiral/achiral amidine or achiral carboxyl groups linked via Pt(II) acetylide complexes were synthesized by sequential stepwise reactions, and their chiroptical properties on the double-helix formation were investigated by circular dichroism (CD) and (1)H NMR spectroscopies. In CHCl(3), the "all-chiral" amidine strands consisting of (R)- or (S)-amidine units formed preferred-handed double helices with the complementary achiral carboxylic acid strands through the amidinium-carboxylate salt bridges, resulting in characteristic induced CDs in the Pt(II) acetylide complex regions, indicating that the chiral substituents on the amidine units biased a helical sense preference. The Cotton effect patterns and intensities were highly dependent on the molecular lengths. The complementary double-helix formation was also explored using the chiral/achiral amidine strands with different sequences in which a chiral amidine unit was introduced at the center (center-chiral) or a terminus (edge-chiral) of the amidine strands. The effect of the sequences of the chiral and achiral amidine units on the amplification of chirality (the "sergeants and soldiers" effect) in the double-helix formation was investigated by comparing the CD intensities with those of the corresponding all-chiral amidine double helices with the same molecular lengths. Variable-temperature CD experiments of the all-chiral and chiral/achiral amidine duplexes demonstrated that the Pt(II)-linked complementary duplexes are dynamic and their chiroptical properties including the chirality transfer from the chiral amidine unit to the achiral amidine ones are significantly affected by the molecular lengths, sequences, and temperatures. On the basis of the above results together with molecular dynamics simulation results, key structural features of the Pt(II)-linked oligomer duplexes and the effect of the chiral/achiral amidine sequences on the amplification of chirality are discussed.     

Hydrogen bond induced chiral symmetry breaking in the crystallization of nickel(II) complexes

A pair of enantiomers formulated as {Δ-cis-[Ni(en)₂OAc][ClO₄]} _n (Δ-1) and {Λ-cis-[Ni(en)₂OAc][ClO₄]} _n (Λ-1) (en = diaminoethane, OAc^? = acetate anion) were obtained when nickel acetate was reacted with diaminoethane and sodium perchlorate in the absence of any chiral source, whereas the reactions of nickel acetate with 1,3-propanediamine and sodium perchlorate only gave a centrosymmetric complex [Ni(1,3-pn)₂OAc][ClO₄] (2) (1,3-pn = 1,3-propanediamine). Single-crystal X-ray diffraction analyses of all three complexes indicated that the central Ni(II) atoms all have a distorted octahedral coordination geometry, being coordinated by four nitrogen atoms of the diamine ligands, plus two oxygen atoms of OAc^?. In complexes Δ-1 and Λ-1, the monomers of {Δ-cis-[Ni(en)₂OAc]}⁺ and {Λ-cis-[Ni(en)₂OAc]}⁺ are connected through intermolecular hydrogen bonds to generate one-dimensional right- and left-handed homochiral helical chains, respectively, while the monomers of [Ni(1,3-pn)₂OAc]⁺ are linked by similar intermolecular hydrogen bonds to form one-dimensional zigzag chains instead of helical chains. The chiral natures of complexes Δ-1 and Λ-1 have been confirmed by circular dichroism spectroscopy.     

Spontaneous and induced chiral symmetry breaking of stereolabile pillar[5]arene derivatives upon crystallisation

Stereolabile pillar[5]arene (P[5]) derivatives, which are dynamic racemic mixtures in solution on account of their low inversion barriers, were employed as platforms to study chiral symmetry breaking during crystallisation. In the solid state, we showed that crystal enantiomeric excess of a conglomerate-forming P[5] derivative can be obtained by handpicking and Viedma ripening without the intervention of external chiral entities. On the other hand, in the presence of ethyl d/l-lactate as both optically-active solvents and chiral guests, the handedness of P[5] derivative crystals, either forming racemic compounds or conglomerates upon condensation, can be directed and subsequently inverted in a highly controllable manner.

Stereolabile pillar[5]arene derivatives, which are dynamic racemic mixtures in solution on account of their low inversion barriers, were employed as platforms to study chiral symmetry breaking during crystallisation.