Surface chiroselective assembly of enantiopure crystalline porous films containing bichiral building blocks

The development of chiral crystalline porous materials (CPMs) containing multiple chiral building blocks plays an important role in chiral chemistry and applications but is a challenging task. Herein, we report the first example of bichiral building block based enantiopure CPM films containing metal–organic cages (MOCs) and metal complexes. The functionalized substrate was immersed subsequently into homochiral metal complex (R)- or (S)-Mn(DCH)₃ (DCH = 1,2-diaminocyclohexane) and racemic Ti₄L₆ cage (L = embonate) solutions by a layer-by-layer growth method. During the assembly process, the substrate surface coordinated with (R)- or (S)-Mn(DCH)₃ can, respectively, layer-by-layer chiroselectively connect Δ- or Λ-Ti₄L₆ cages to form homochiral (R, Δ)- or (S, Λ)-CPM films with a preferred [111] growth orientation, tunable thickness and homogeneous surface. The resulting enantiopure CPM films show strong chirality, photoluminescence, and circularly polarized luminescence (CPL) properties as well as good enantioselective adsorption toward enantiomers of 2-butanol and methyl-lactate. The present in situ surface chiroselective strategy opens a new route to assemble homochiral CPM films containing multiple chiral building blocks for chiral applications.

Bichiral building block based enantiopure CPM films containing metal–organic cages (MOCs) and metal complexes are chiroselectively assembled on the substrate surface by a layer-by-layer method.     

Chiral polymer hosts for circularly polarized electroluminescence devices

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications. The fabrication of a CP PLED requires a polymer host that provides the appropriate chiral environment around the emitting dopant. However, chemical strategies for the design of chiral polymer hosts remain underdeveloped. We have developed new polymer hosts for CP PLED applications. These polymers were prepared through a free-radical polymerization of 3-vinylcarbazole with a chiral N-alkyl unit. This chiral unit forces the carbazole repeat units to form mutually helical half-sandwich conformers with preferred (P)-helical sense along the polymer main chain. Electronic circular dichroism measurements demonstrate the occurrence of chirality transfer from chiral monomers to achiral monomers during chain growth. The (P)-helical-sense-enriched polymer interacts diastereoselectively with an enantiomeric pair of new phosphorescent (R)- and (S)-dopants. The magnitude of the Kuhn dissymmetry factor (g_abs) for the (P)-helically-enriched polymer film doped with the (R)-dopant was found to be one order of magnitude higher than that of the film doped with the (S)-dopant. Photoluminescence dissymmetry factors (g_PL) of the order of 10⁻³ were recorded for the doped films, but the magnitude of diastereomeric enhancement decreased to that of g_abs. The chiral polymer host permits faster energy transfer to the phosphorescent dopants than the achiral polymer host. Our photophysical and morphological investigations indicate that the acceleration in the chiral polymer host is due to its longer Förster radius and improved compatibility with the dopants. Finally, multilayer CP PLEDs were fabricated and evaluated. Devices based on the chiral polymer host with the (R)- and (S)-dopants exhibit electroluminescence dissymmetry factors (g_EL) of 1.09 × 10⁻⁴ and −1.02 × 10⁻⁴ at a wavelength of 540 nm, respectively. Although challenges remain in the development of polymer hosts for CP PLEDs, our research demonstrates that chiroptical performances can be amplified by using chiral polymer hosts.

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications.     

Chiral information harvesting in helical poly(acetylene) derivatives using oligo(p-phenyleneethynylene)s as spacers

A chiral harvesting transmission mechanism is described in poly(acetylene)s bearing oligo(p-phenyleneethynylene)s (OPEs) used as rigid achiral spacers and derivatized with chiral pendant groups. The chiral moieties induce a positive or negative tilting degree in the stacking of OPE units along the polymer structure, which is further harvested by the polyene backbone adopting either a P or M helix.

A chiral harvesting transmission mechanism is described in poly(acetylene)s bearing oligo(p-phenyleneethynylene)s (OPEs) used as rigid achiral spacers and derivatized with chiral pendant groups.

During the last years, dynamic helical polymers have attracted the attention of the scientific community due to the possibility of tuning the helical sense and/or the elongation of the helical structure by using external stimuli.^1–14In the case of a chiral dynamic helical polymer, modifications in its structure—helical sense enhancement or helix inversion—arise from conformational changes induced at its chiral pendants—usually, with just one stereocenter—, by stimuli such as variations in solvent polarity or temperature, the addition of certain ions, and so on (Fig. 1a).¹⁵ On the other hand, if a helical polymer is achiral (i.e., bearing achiral pendants), the chiral amplification phenomena can emerge from interactions between the polymer and external chiral molecules.¹⁶ In both the above cases, the changes produced in the helical structures are related to the spatial dispositions adopted by the substituents or associated species at the pendant groups.^17–19Open in a separate windowFig. 1Several scenarios depicting conceptual representations of the transmission of chiral information. (a) Helical switch via chiral tele-induction. (b) Effect of distance on chiral tele-induction from multichiral pendants. (c) Helicity controlled by the conformational composition of achiral spacers.A step forward in the helical sense control of poly(phenylacetylene)s (PPA)s is to study different mechanisms of transmission of chiral information from the pendant to the polyene backbone by introducing achiral spacers. The goal is to demonstrate how far it is possible to place the chiral center and still have an effective chiral induction on the polyene backbone. Therefore, transmission of the chiral information from a remote position can occur through space, thus overpassing the distance generated by the spacer—tele-induction—(Fig. 1b),^20–28 or through the achiral spacer itself, producing in it a preferred structure, such as a helical structure and where the orientation of the achiral helix is further transmitted to the polyene backbone—conformational switch—(Fig. 1c).^29–31For the first mechanism—chiral tele-induction—, both flexible and rigid spacers have been designed.^20–28 In all cases, supramolecular interactions, such as H bonding or π–π stacking, generate organized structures. As a result, the chiral center is located into a specific orientation, producing an effective helical induction. Additionally, those studies allow evaluating how distances and sizes have an effect on this phenomenon.In the second strategy, the helix induction is transmitted through conformational changes along an achiral spacer which is harvested by the polyene. For instance, an achiral peptide or an achiral polymeric helix derivatized at one end with a chiral residue and linked to the polymer main chain at the other end. In such cases, changes in the absolute configuration or even just a conformational change at the chiral center can induce an opposite helical structure into the achiral spacer, which in turn will be harvested by the polymer main chain (Fig. 1c).^29–31Herein we will demonstrate another remote chiral induction mechanism based on a different chiral harvesting process. In this case, the chiral center does not produce a conformational change at the achiral spacer, but affects its array within the helical scaffold. Thus, to perform these studies we decided to introduce the use of oligo(p-phenyleneethynylene)s (m = 1, 2, 3) (OPEs) as rigid spacers to separate the distant chiral center from the polyene backbone. These OPE units have been used in the formation of benzene-1,3,5-tricarboxamide (BTA) based supramolecular helical polymers, demonstrating their ability to stack with a certain tilting degree commanded by the chiral center.^32–34Hence, in our design, the chiral moiety will determine the supramolecular chiral orientation of the OPE groups used as spacers, which is further harvested by the polyene backbone. The overall process yields a helix with a preferred screw sense (Fig. 2).Open in a separate windowFig. 2Conceptual side view and top view of the chiral information transmission mechanism from stereocenters at the far end of oligo(p-phenyleneethynylene) spacers to the polyene backbone via chiral harvesting.To perform these studies, we used as model compounds two PPAs—poly-(R)-1 and poly-(S)-1—derived from the 4-ethynylanilide of (S)- and (R)-α-methoxy-α-phenylacetic acid (MPA, m-(S/R)-1), whose helical structures and dynamic behaviors have been deeply studied by our group—poly-(R)-1 and poly-(S)-1—(Fig. 3).^35–46 By using these polymers as reference materials, four novel PPAs were designed introducing two OPE spacers—4-[(p-phenyleneethynylene)_n]ethynylanilide (n = 1, 2)—between the phenyl acetylene group and the (S)- or (R)-α-methoxy-α-phenylacetic acid (MPA) chiral group. Thus, monomers m-(S)- and m-(R)-2 and m-(S)- and m-(R)-3 (Fig. 3a) were prepared and submitted to polymerization by using a Rh(i) catalyst poly-(S)- and poly-(R)-2 and poly-(S)- and poly-(R)-3 (Fig. 3b) were obtained in high yield and showed Raman spectra characteristic of cis polyene backbones (see Fig. S11 and S12^†).Open in a separate windowFig. 3(a) Monomers and (b) polymers synthetized in this study.X-ray structures of the monomers show a preferred antiperiplanar (ap) orientation between the carbonyl and methoxy groups (O C–C–OMe) for m-(R)-2 and m-(S)-3, whereas in the case of m-(S)-1 a synperiplanar (sp) geometry is favoured (Fig. 4a).³⁵ In complementary studies, CD spectra of monomers m-(S)-[1–3] in CHCl₃ show negative Cotton effects, indicative of major ap conformations in solution (Fig. 4b),³⁵ further corroborated by theoretical calculations (see Fig. S10^†). Interestingly, the maximums of the Cotton effects in CD undergo a bathochromic shift—from 266 nm in m-1 to 327 nm in m-3—due to a larger conjugation of the π electrons (from the anilide to the alkyne group) when the length of the spacer increases (Fig. 4b).Open in a separate windowFig. 4(a) X-ray structures of m-(S)-1, m-(R)-2 and m-(S)-3. (b) CD traces of m-(S)- and m-(R)-1; m-(S)- and m-(R)-2; m-(S)- and m-(R)-3 in CHCl₃ (0.1 mg mL⁻¹). (c) CD spectra for poly-(S)- and poly-(R)-1 in CHCl₃ (0.1 mg mL⁻¹); poly-(S)- and poly-(R)-2 in DMSO (0.1 mg mL⁻¹); poly-(S)- and poly-(R)-3 in DMSO (0.1 mg mL⁻¹).CD studies of the polymer series bearing OPE spacers—poly-(R)- and poly-(S)-[2–3]—in different solvents show the formation of a PPA helical structure with a preferred helical sense, while the parent polymer, poly-1, devoid of the OPE unit, has a poor CD. This is a very interesting phenomena that indicates that the OPE spacers work as transmitters of the chiral information from remote chiral centers to the polyene backbone—placed at 1.7 nm for poly-2 and at 2.4 nm for poly-3—(Fig. 4a). These large distances between the chiral center and the polymer main chain mean that other mechanisms of chiral induction, such as chiral tele-induction effect, should be almost null in these cases.In these two polymers (poly-2 and poly-3), the chiral information transmission mechanism must occur in different sequential steps. First, the chiral centers possessing a major (ap) conformation induce a certain tilting degree (θ) in the achiral spacer array. This step resembles the helical induction mechanism found in supramolecular helical polymers bearing OPE units.^32–34 Next, the chiral array induced in the OPE units is harvested by the polyene backbone, resulting in an effective P or M helix induction (Fig. 2).^34,47Additional structural studies were carried out in poly-(S)-2 and poly-(S)-3 to obtain an approximated secondary structure of these polymers and determine their dynamic behaviour.From literature it is known that the conformational equilibrium of poly-1 can be altered in solution by the presence of metal ions. The addition of monovalent ions (e.g., Li⁺) stabilizes the ap conformer at the pendant group by cation–π interactions, while divalent ions (e.g., Ca²⁺) stabilize the sp conformations by chelation with the methoxy and carbonyl groups.^36,38,39,43 As a result, both the P or M helical senses can be selectively induced in poly-1 by the action of metal ions.Therefore, we decided to add different perchlorates of monovalent and divalent metal ions to solutions of poly-(S)-2 and poly-(S)-3 with the aim of determining the conformational composition at the pendant groups. Thus, when monovalent metal ions (Li⁺, Ag⁺ and Na⁺) are added to a chloroform solution of poly-(S)-2, a chiral enhancement is observed (Fig. 5d for Li⁺ and Fig. S16^† for Na⁺ and Ag⁺). IR and ⁷Li-NMR studies show that those ions stabilize the ap conformer at the pendant group in a similar fashion to poly-1, this is by coordination to the carbonyl group of the MPA (Fig. 5g) and the presence of a cation–π interaction with the aryl ring of the chiral (|Δδ| ⁷Li ca., 3.75 ppm) (Fig. 5f and ESI^†). Therefore, addition of Li⁺ produces a larger number of pendant groups with ap conformation among poly-2, which triggers a chiral enhancement effect through a cooperative process.Open in a separate windowFig. 5(a) Conceptual representation of the chiral information harvesting and top view of the 3D model for poly-(S)-2. (b) CD spectra of poly-(S)-2 (0.2 mg mL⁻¹) in DMSO vs. calculated ECD spectra. Full width at half-maximum (FWHM) equals 20 nm. (c) Low-resolution AFM image from a poly-(S)-2 monolayer and profile depicting the chain separation of the yellow highlighted area in the AFM image. (d) CD spectra showing the chiral enhancement after the addition of Li⁺ (50 mg mL⁻¹, THF) to a poly-(S)-2 solution (0.1 mg mL⁻¹, THF). (e) CD trace of poly-(S)-2 before and after the addition of a Ca²⁺ solution (50 mg mL⁻¹, THF). (f) ⁷Li-NMR spectra substantiating the cation–π interaction. (g) IR shifts observed for carbonyl and methoxy groups after the addition of LiClO₄ and Ca(ClO₄)₂ (50 mg mL⁻¹, THF) to a poly-(S)-2 solution (3 mg mL⁻¹, CHCl₃). The coordination modes of the MPA moiety with Li⁺ and Ca²⁺ are shown vertically in the middle of the figure.On the contrary, the addition of perchlorates of divalent metal ions, such as Ca²⁺and Zn²⁺, produced an inversion of the third Cotton band—310 nm—associated to the MPA moiety and the disappearance of both first and second Cotton effects (Fig. 5e for Ca²⁺ and Fig. S17^† for Zn²⁺). This is a very interesting outcome because, although the conformational equilibrium at the MPA group changes from ap to sp after the addition of Ca²⁺, the number of pendant groups with sp conformation do not reach the number needed to trigger the helix inversion process and in fact, a mixture of P and M helices at the polyene backbone is obtained.The helical structures adopted by both polymer systems, PPAs (poly-1) and poly[oligo(p-phenyleneethynylene)phenylacetylene]s (POPEPAs) (poly-2 and poly-3), are defined by two coaxial helices, one formed by the polyene backbone (internal helix, CD active) and the other constituted by the pendants (external helix, observed by AFM).These two helices can rotate in either the same or the opposite sense, depending on the dihedral angle between conjugated double bonds. Thus, internal and external helices rotate in the same direction in cis-cisoidal polymers, while they rotate in opposite directions in cis-transoidal ones.^14,42,48,49In order to find out an approximated helical structure for poly-(S)-2, DSC studies were performed. The thermogram shows a compressed cis-cisoidal polyene skeleton (see Fig. S13a^†), similar to the one obtained for poly-1.⁴² Moreover, AFM studies on a 2D crystal of poly-(S)-2 did not produce high-resolution AFM images, although some parameters such as helical pitch (c.a., 2.8 nm) and packing distance between helices of (c.a., 6 nm) could be extracted from the well-ordered monolayer analyzed (Fig. 5c).Previous structural studies in PPAs found that it is possible to correlate the internal helical sense with the Cotton band associated to the polyene backbone—CD (+), P_int; CD (−), M_int—.^50,51 Herein, the positive Cotton effect observed for the polyene backbone [CD_{365 nm} = (+)] in poly-(S)-2 is indicative of a P orientation of the internal helix, which correlates with a P orientation of the external helix in a cis-cisoidal polyene scaffold. To summarize, DSC, AFM and CD studies agree that poly-(S)-2 is made up of a cis-cisoidal framework with P_int and P_ext helicities (Fig. 5a).Computational studies [TD-DFT(CAM-B3LYP)/3-21G] were carried out on a P helix of an n = 9 oligomer of poly-(S)-2, possessing a cis-cisoidal polyene skeleton (ω₁ = +50°, ω₃ = −40°) and an antiperiplanar orientation of the carbonyl and methoxy groups at the pendants. The theoretical ECD spectrum obtained from these studies (Fig. 5b and see ESI^† for additional information) is in good agreement with the experimental one, indicating that our model structure is a good approximation of the helical structure adopted by poly-(S)-2.Next, a similar set of DSC and AFM studies were carried out for poly-(S)-3, that bears an OPE spacer with n = 2. The data showed that this polymer presents a compressed cis-cisoidal polyene skeleton, similar to those obtained for poly-1 and poly-2 (see Fig. S13b^†), with a helical pitch of 3.8 nm and a P_ext helical sense (Fig. 6a and c).Open in a separate windowFig. 6(a) Conceptual representation of the chiral information harvesting and top view of the 3D model for poly-(S)-3. (b) CD spectrum of poly-(S)-3 in THF (0.2 mg mL⁻¹) and comparison to the calculated ECD spectra. Full width at half-maximum (FWHM) equals 20 nm. (c) AFM image obtained from a poly-(S)-3 monolayer. (d) CD traces for poly-(S)-3 in THF polymerized at different temperatures.UV studies indicate that, in poly-(S)-3, the polyene backbone absorbs at ca. 380 nm, coincident with the first Cotton effect, that is positive (see Fig. S15b^†). Therefore, it reveals that poly-(S)-3 adopts a P_int helicity (Fig. 6b). Thus, as expected for cis-cisoidal scaffolds, the orientations of the two coaxial helices are coincident.Computational studies [TD-DFT(CAM-B3LYP)/3-21G] were carried out on a P helix of an n = 9 oligomer of poly-(S)-3, possessing a cis-cisoidal polyene skeleton (ω₁ = +63°, ω₃ = −40°) and an antiperiplanar orientation of the carbonyl and methoxy groups at the pendants. The theoretical results (Fig. 6b and see ESI^† for additional information) match with the experimental data, indicating that our model structure is a good approximation to the helical structure adopted by poly-(S)-3.Finally, the stimuli response properties of poly-(S)-3 were explored by CD. These experiments revealed that the addition of monovalent or divalent metal ions to a chloroform solution of poly-(S)-3 does not produce any significant effect in the structural equilibrium of this polymer (see Fig. S18^†). This fact, in addition to the previous results obtained from the interaction of poly-(S)-2 with divalent metal ions, corroborates the decrease of the dynamic character of helical PPAs when large OPEs are used as spacers.The poor dynamic behaviour was further demonstrated by polymerizing m-(S)-3 at a lower temperature (0 °C) (Fig. 6d). In this case, the region around 240–350 nm remains unaffected, indicating that the pendant is ordered in a similar manner in both batches of polymers, regardless of the temperature at which they were synthesized (20 °C and 0 °C). Interestingly, the magnitude of the first Cotton band is duplicated when the polymer is obtained at low temperature due to a stronger helical sense induction at the polyene backbone. This result indicates that a preorganization process may occur during polymerization, affecting the screw sense excess of the PPA.In conclusion, a novel chiral harvesting transmission mechanism has been described in poly(acetylene)s bearing oligo(p-phenylenethynylene)s as rigid spacers that place the chiral pendant group away from the polyene backbone, at a distance around ca. 1.7 nm for poly-2, and 2.4 nm for poly-3. Hence, the disposition of the chiral moiety affects the stacking of the OPE units within the helical structure, inducing a specific positive or negative tilting degree, which is further harvested by the polyene backbone inducing either a P or M internal helix.We believe that these results open new horizons in the development of novel helical structures by combining information from the helical polymers and supramolecular helical polymers fields, which leads to the formation of novel materials with applications in important fields such as asymmetric synthesis, chiral recognition or chiral stationary phases among others.     

Metal–metal bonded alkaline-earth distannyls

The first families of alkaline-earth stannylides [Ae(SnPh₃)₂·(thf)_x] (Ae = Ca, x = 3, 1; Sr, x = 3, 2; Ba, x = 4, 3) and [Ae{Sn(SiMe₃)₃}₂·(thf)_x] (Ae = Ca, x = 4, 4; Sr, x = 4, 5; Ba, x = 4, 6), where Ae is a large alkaline earth with direct Ae–Sn bonds, are presented. All complexes have been characterised by high-resolution solution NMR spectroscopy, including ¹¹⁹Sn NMR, and by X-ray diffraction crystallography. The molecular structures of [Ca(SnPh₃)₂·(thf)₄] (1′), [Sr(SnPh₃)₂·(thf)₄] (2′), [Ba(SnPh₃)₂·(thf)₅] (3′), 4, 5 and [Ba{Sn(SiMe₃)₃}₂·(thf)₅] (6′), most of which crystallised as higher thf solvates than their parents 1–6, were established by XRD analysis; the experimentally determined Sn–Ae–Sn′ angles lie in the range 158.10(3)–179.33(4)°. In a given series, the ¹¹⁹Sn NMR chemical shifts are slightly deshielded upon descending group 2 from Ca to Ba, while the silyl-substituted stannyls are much more shielded than the phenyl ones (δ¹¹⁹Sn/ppm: 1′, −133.4; 2′, −123.6; 3′, −95.5; 4, −856.8; 5, −848.2; 6′, −792.7). The bonding and electronic properties of these complexes were also analysed by DFT calculations. The combined spectroscopic, crystallographic and computational analysis of these complexes provide some insight into the main features of these unique families of homoleptic complexes. A comprehensive DFT study (Wiberg bond index, QTAIM and energy decomposition analysis) points at a primarily ionic Ae–Sn bonding, with a small covalent contribution, in these series of complexes; the Sn–Ae–Sn′ angle is associated with a flat energy potential surface around its minimum, consistent with the broad range of values determined by experimental and computational methods.

The complete series of heterobimetallic alkaline-earth distannyls [Ae{SnR₃}₂·(thf)_x] (Ae = Ca, Sr, Ba) have been prepared for R = Ph and SiMe₃, and their bonding and electronic properties have been comprehensively investigated.     

Characterization of Diastereomeric Equilibria of Pseudotetrahedral Bis[(R or S)‐N‐1‐(Ar)Ethylsalicylaldiminato‐κ2N,O]zinc(II) with Λ/Δ‐Chirality‐At‐Metal Induction

A family of bis[(R or S)‐N‐1‐(Ar)ethylsalicylaldiminato‐κ²N,O]‐Δ/Λ‐zinc(II) {Ar=C₆H₅ (ZnRL¹ or ZnSL¹), p‐CH₃OC₆H₄ (ZnRL² or ZnSL²) and p‐ClC₆H₄ (ZnRL³ or ZnSL³)} compounds was synthesized and investigated by multiple methods. They feature Λ/Δ‐chirality‐at‐metal induction along the pseudo‐C ₂ axis of the molecules. The chirality induction is quantitative in the solid state, explored by X‐ray crystallography and powder X‐ray diffraction (PXRD), where R or S‐ligated complexes diastereoselectively yield Λ or Δ‐configuration at the metal. On the other hand, Λ and Δ‐diastereomers co‐exist in solution. The Λ⇆Δ equilibrium is solvent‐ and temperature‐dependent. Electronic circular dichroism (ECD) spectra confirm the existence of a diastereomeric excess of Λ‐ZnRL¹⁻³ or Δ‐ZnSL¹⁻³ in solution. DSC analysis reveals thermally induced irreversible phase transformation from a crystalline solid to an isotropic liquid phase. ECD spectra were reproduced by DFT geometry optimizations and time‐dependent DFT (TD‐DFT) calculations, providing ultimate proof of the dominant chirality atmetal in solution.     

Lanthanide MOFs for inducing molecular chirality of achiral stilbazolium with strong circularly polarized luminescence and efficient energy transfer for color tuning

We present herein an innovative host–guest method to achieve induced molecular chirality from an achiral stilbazolium dye (DSM). The host–guest system is exquisitely designed by encapsulating the dye molecule in the molecule-sized chiral channel of homochiral lanthanide metal–organic frameworks (P-(+)/M-(−)-TbBTC), in which the P- or M-configuration of the dye is unidirectionally generated via a spatial confinement effect of the MOF and solidified by the dangling water molecules in the channel. Induced chirality of DSM is characterized by solid-state circularly polarized luminescence (CPL) and micro-area polarized emission of DSM@TbTBC, both excited with 514 nm light. A luminescence dissymmetry factor of 10⁻³ is obtained and the photoluminescence quantum yield (PLQY) of the encapsulated DSM in DSM@TbTBC is ∼10%, which is close to the PLQY value of DSM in dilute dichloromethane. Color-tuning from green to red is achieved, owing to efficient energy transfer (up to 56%) from Ln³⁺ to the dye. Therefore, this study for the first time exhibits an elegant host–guest system that shows induced strong CPL emission and enables efficient energy transfer from the host chiral Ln-MOF to the achiral guest DSM with the emission color tuned from green to red.

Homochiral Ln-MOFs are synthesized to encapsulate achiral dyes to induce strong circularly polarized luminescence with a luminescence dissymmetry factor of 10⁻³.     

Synthesis and Structure Elucidation of Novel Spirooxindole Linked to Ferrocene and Triazole Systems via [3 + 2] Cycloaddition Reaction

In the present work, a novel heterocyclic hybrid of a spirooxindole system was synthesized via the attachment of ferrocene and triazole motifs into an azomethine ylide by [3 + 2] cycloaddition reaction protocol. The X-ray structure of the heterocyclic hybrid (1″R,2″S,3R)-2″-(1-(3-chloro-4-fluorophenyl)-5-methyl-1H-1,2,3-triazole-4-carbonyl)-5-methyl-1″-(ferrocin-2-yl)-1″,2″,5″,6″,7″,7a″-hexahydrospiro[indoline-3,3″-pyrrolizin]-2-one revealed very well the expected structure, by using different analytical tools (FTIR and NMR spectroscopy). It crystallized in the triclinic-crystal system and the P-1-space group. The unit cell parameters are a = 9.1442(2) Å, b = 12.0872(3) Å, c = 14.1223(4) Å, α = 102.1700(10)°, β = 97.4190(10)°, γ = 99.1600(10)°, and V = 1484.81(7) ų. There are two molecules per unit cell and one formula unit per asymmetric unit. Hirshfeld analysis was used to study the molecular packing of the heterocyclic hybrid. H···H (50.8%), H···C (14.2%), Cl···H (8.9%), O···H (7.3%), and N···H (5.1%) are the most dominant intermolecular contacts in the crystal structure. O···H, N···H, H···C, F···H, F···C, and O···O are the only contacts that have the characteristic features of short and significant interactions. AIM study indicated predominant covalent characters for the Fe–C interactions. Also, the electron density (ρ(r)) at the bond critical point correlated inversely with the Fe–C distances.     

Two helices from one chiral centre – self organization of disc shaped chiral nanoparticles

Gold nanoparticles (AuNPs) have been prepared and surface-functionalized with a mixture of 1-hexanethiol co-ligands and chiral discogen ligands separated from a disulfide function via a flexible spacer. Polarized optical microscopy together with differential scanning calorimetry showed that the organic corona of the nanocomposite forms a stable chiral discotic nematic phase with a wide thermal range. Synchrotron X-ray diffraction showed that gold NPs form a superlattice with p2 plane symmetry. Analysis indicated that the organic corona takes up the shape of a flexible macrodisk. Synchrotron radiation-based circular dichroism signals of thin films are significantly enhanced on the isotropic-LC transition, in line with the formation of a chiral nematic phase of the organic corona. At lower temperatures the appearance of CD signals at longer wavelengths is associated with the chiral organisation of the NPs and is indicative of the formation of a second helical structure. The decreased volume required and the chiral environment of the disc ligands drives the nanoparticles into columns that arrange helically, parallel to the shortest axis of the two dimensional lattice.

Gold nanoparticles (NPs) were surface functionalized with hexanethiol groups and chiral nematic discogen ligands. A superlattice, liquid crystal behaviour and helix formation of the discs and a second helical organisation of the NPs were detected.

The exploration of chiral nanomaterials is a rapidly evolving fascinating field with uses ranging from catalysis, chiral molecular sensing, stereoselective separations to super-resolution imaging.^1–3 Bottom-up approach design strategies for nanoparticles (NPs) or nanorods (NRs) as composites in liquid crystal (LC) matrices have been developed, making use of progress in synthesis strategies and characterization techniques.^4–6 For optical applications symmetry breaking by introducing chirality into the systems at the nanoscale is critical.⁷ So far, efforts to obtain chiroptical NP-LC systems have relied mainly on NPs doped in LC hosts: either chiral nematic templates disperse NPs into chiral assemblies⁸ or the NPs functionalized with chiral ligands are included in achiral nematic LCs;⁹ both strategies aim to transfer and amplify chiral effects.^10,11 Beyond the scientific challenge of generating nanocomposites where the interplay of bottom-up structuring enables plasmonic interactions of the NPs, leading to novel, so called metamaterials properties, there are new uses ranging from novel display devices,¹² to more advanced optical communications using spin information between LEDs¹³ and in quantum teleportation for optical computing.¹⁴Precise size control of NPs in LC matrices in combined systems has been found critical to avoid phase separation or precipitation of NPs.¹⁵ For intrinsic liquid crystalline NP-LC systems, targeting of the LC phase range to be close to ambient is of utmost importance, as delamination of ligands occurs at elevated temperatures, even for very stable thiol functions. Degradation tends to occur above 100 °C.¹⁶ Inclusion of chiral groups in the organic corona can lead to chiral LC phase behavior, but there is little evidence that the NPs experience a chiral distortion in their packing; experimental evidence indicates that the nanocomposites minimize energy by packing in non-chiral superlattices and chirality is confined to the corona.¹⁷Here we show that a combination of small NPs with a carefully designed chiral organic coating results in chiral assembly behaviour of both the NPs and the organic corona. Our approach is strictly modular. The surface of the gold NPs is covered and passivated by hexylthiol groups and chiral lipoic acid derivates bearing via a spacer the mesogenic groups. We note that lipoic acid, a naturally occurring chiral antioxidant explored elsewhere for the treatment of several diseases, is bound with a dithiolate bridge via two atoms to the Au surface (see Chart 1).¹⁸ This places the chiral moiety rigidly in position onto the NPs, making thus a chiral surface.¹⁹ A pentaalkynylbenzene (PA) derivative was selected as mesogenic group, as this structural motif is known to promote reliably discotic nematic phase (N_D) behaviour.²⁰ The much larger volume of the PA group, when compared to typical rod shaped mesogens, whilst also delivering LC behaviour close to ambient, is anticipated to impact more strongly on the spatial organization of neighbouring NPs when compared to calamitic functionalized NP systems.¹⁷ The formation of helically ordered superstructures is studied by a combination of dedicated thin film optical polarizing microscopy, X-ray diffraction and synchrotron radiation-based circular dichroism (SRCD). A structural model is proposed based on a chiral nematic phase made up of AuNPs coated with chiral shells forming flexible macrodisks which in turn assemble in a superlattice.^21–23Open in a separate windowChart 1Sketch of the functionalized gold nanoparticle system AuDLC*.The design, synthesis and chemical characterization of the chiral discogen is described in the ESI.^† The precursor disk-like mesogen which forms exclusively a N_D phase above 100 °C consists of six aromatic rings flanked by flexible pentyloxy chains and an undecyloxy chain bearing a terminal hydroxy group (Fig. S1^†). The attachment of the flexible (R)-(+)-1,2-dithiolane-3-pentanoic tail was predicted to promote the desired phase. The new target discogen was obtained in a high yield (93.1%) reaction. For the synthesis and purification of LC-functionalized AuNPs, denoted as AuDLC*, a synthetic pathway was developed involving first the preparing and purifying of the alkylthiol coated NPs to which LC groups are linked, using an exchange reaction (for details see the Scheme 1 in ESI^†).^24,25The thermal behaviour of the chiral discogen was examined by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The target discogen shows clearly a chiral nematic phase with the characteristic bundles of oily streaks (Fig. S2^†). On heating (Fig. S3^†), phase formation was observed at 49.0 °C from the crystalline state, with the material turning into an isotropic liquid at 95.7 °C (ΔH = 0.20 J g⁻¹ or 0.28 kJ mol⁻¹). Formation of the mesophase from the isotropic state on cooling started from 94.5 °C with an enthalpy change of 0.12 J g⁻¹ (0.17 kJ mol⁻¹), followed by a slow crystallisation occurring at 5.1 °C. In brief, the chiral phase behaviour of the target ligand is exclusively chiral nematic, enantiotropic and with a wide thermal range of up to 89.4 °C.The ¹H-NMR spectra of the investigated AuDLC* nanocomposite and the comparison with the spectra of the monomer ligand (Fig. S4^†) indicate that the chiral discogen with the disulfide group are chemically attached onto the surface of AuNPs with high efficiency. A typical feature for such NPs are the broadened ¹H-NMR spectra caused by the reduced mobility of alkyl chains of the ligands when anchored to NPs and additionally the sharp peaks present in the free ligand have disappeared, typical for such systems.²⁶ The absence of mobile free discogen using thin layer chromatography (silica, with dichloromethane as mobile phase) confirms this result. The amount of chiral discogen bounded onto the gold surface relative to the number of 1-hexanethiol groups was determined from the ¹H-NMR spectra of AuDLC*. The ratio of hydrocarbons to discogens was found to be 2 : 3. Transmission Electron Microscopy (TEM) investigations confirm that this sequential synthesis affords nanoparticles of low polydispersity (Fig. 1a); the average particle size is ∼2.5 ± 0.5 nm. Thermo-gravimetric analysis (TGA) was performed (Fig. S6^†) and gives the weight fraction of gold in AuDLC* to be 47.8% (wt/wt). The mesogen density per Au NP was calculated (ESI^† Part 7) by combining the ¹H-NMR results (Fig. S4^†) with the model developed by Gelbart et al.²⁷ On the basis of these data, we obtain that there are on average 211 Au atoms per particle, and a total number of 53 ligands per particle, of which 32 are chiral discogens and 21 are hexylthiol groups (Table S1^†). This is in accord with a ligand density of up to ∼6.3 ligands per nm².²⁸ Additionally, the characteristic peaks in the UV-vis spectra of AuDLC* arise from the combination of gold (∼500 nm) and LC ligands (absorption maxima at 238, 261 and 351 nm with two shoulder peaks at 378 and 421 nm) (Fig. S7^†).Open in a separate windowFig. 1(a) TEM image of 2.5 nm AuDLC* nanocomposite (inset: the size distribution of AuDLC*). POM micrographs of AuDLC* (b) 86.5 °C (90° crossed polarizer) (×100 μm). (c) 37.8 °C (90° crossed polarizer) (×100 μm). (d) DSC of AuDLC* at heating and cooling rate of 10.0 °C min⁻¹ (e) SAXS diffractograms of AuDLC* on heating from 30 °C to 140 °C. (f) SRCD spectra of sheared AuDLC*. Recorded in 5 °C steps on cooling from 100 °C to 30 °C. The enlarged plots in dotted regions A–C are shown in Fig. S12b–d.^† The CD intensities at a selected wavelengths in each region (A, B and C) are plotted as a function of temperature in Fig. S13^† and these plots clearly show the difference in the formation process between the two helical structures.The mesophase behaviour of the AuDLC* nanocomposite was first characterized by POM observation. As shown in Fig. 1b and c, upon cooling from the isotropic liquid, the colour in the POM texture changes from light green to yellow and we associate this with the helical pitch in AuDLC* increasing with decreasing temperature. In some regions of the POM slides, a chiral nematic phase with the typical Grandjean texture (Fig. S8a and b^†) is found. The LC textures in AuDLC* do recover after gently pressing the microscope coverslip indicating that the LC-decorated NPs form a stable phase and birefringent textures remain stable at ambient (Fig. S9^†). DSC investigations (Fig. 1d) confirm that the AuDLC* nanocomposite shows a thermodynamically stable (enantiotropic) mesophase. In DSC experiments on cooling a small peak associated with the isotropic to phase transition (92.2 °C) can be detected, and at a low temperature (3.8 °C), the glass transition occurs. The transition peak for the transition is wider than in the pure chiral discogen, typical for such systems.⁴ This is associated with the reduced mobility of the LC groups attached to the NPs. The thermal transitions determined by DSC for the pure chiral discogen and AuDLC* are collected in Table 1, showing the low values typical for transitions.²⁰ In contrast to the free chiral ligand, the NPs coated with the chiral ligand and hexylthiol groups show a small decrease in transition temperatures. This could be attributed to both the plastifying effects of the hexylthiol co-ligands and packing constraints of the LC groups due to the attachment to the NPs. This view is supported by the DSC data for the transition at 0.043 J g⁻¹ (0.12 kJ mol⁻¹, considering the evaluated 32 chiral discogen ligands per particle).Transition temperatures (°C) of free chiral discogen and AuDLC*, as determined by DSC (second cooling at rate of 10.0 °C min⁻¹)^a

New Alk(en)ylhydroxycyclohexanes with Tyrosinase Inhibition Potential from Harpephyllum caffrum Bernh. Gum Exudate

This work presents the first report on the phytochemical investigation of Harpephyllum caffrum Bernh. gum exudate. A known cardanol, 3-heptadec-12′-Z-enyl phenol (1) and three new alk(en)ylhydroxycyclohexanes, namely, (1R,3R)-1,3-dihydroxy-3-[heptadec-12′(Z)-enyl]cyclohexane (2) (1S,2S,3S,4S,5R)-1,2,3,4,5-pentahydroxy-5-[octadec-13′(Z)-enyl]cyclohexane (3) and (1R,2S,4R)-1,2,4-trihydroxy-4-[heptadec-12′(Z)-enyl]cyclohexane (4) were isolated from the gum. The structures of the compounds were determined by extensive 1D and 2D NMR spectroscopy and HR-ESI-MS data. The ethanolic extract of the gum was found to be the most potent tyrosinase inhibitor with IC₅₀ of 11.32 µg/mL while compounds 2 and 3, with IC₅₀ values of 24.90 and 26.99 µg/mL, respectively, were found to be potential anti-tyrosinase candidates from the gum. Gum exudate may be a potential source for non-destructive harvesting of selective pharmacologically active compounds from plants. The results also provide evidence that H. caffrum gum may find application in cosmetics as a potential anti-tyrosinase agent.     

Versatile Reactivity of MnII Complexes in Reactions with N-Donor Heterocycles: Metamorphosis of Labile Homometallic Pivalates vs. Assembling of Endurable Heterometallic Acetates

Reaction of 2,2′-bipyridine (2,2′-bipy) or 1,10-phenantroline (phen) with [Mn(Piv)₂(EtOH)]_n led to the formation of binuclear complexes [Mn₂(Piv)₄L₂] (L = 2,2′-bipy (1), phen (2); Piv⁻ is the anion of pivalic acid). Oxidation of 1 or 2 by air oxygen resulted in the formation of tetranuclear Mn^II/III complexes [Mn₄O₂(Piv)₆L₂] (L = 2,2′-bipy (3), phen (4)). The hexanuclear complex [Mn₆(OH)₂(Piv)₁₀(pym)₄] (5) was formed in the reaction of [Mn(Piv)₂(EtOH)]_n with pyrimidine (pym), while oxidation of 5 produced the coordination polymer [Mn₆O₂(Piv)₁₀(pym)₂]_n (6). Use of pyrazine (pz) instead of pyrimidine led to the 2D-coordination polymer [Mn₄(OH)(Piv)₇(µ₂-pz)₂]_n (7). Interaction of [Mn(Piv)₂(EtOH)]_n with FeCl₃ resulted in the formation of the hexanuclear complex [Mn^II₄Fe^III₂O₂(Piv)₁₀(MeCN)₂(HPiv)₂] (8). The reactions of [MnFe₂O(OAc)₆(H₂O)₃] with 4,4′-bipyridine (4,4′-bipy) or trans-1,2-(4-pyridyl)ethylene (bpe) led to the formation of 1D-polymers [MnFe₂O(OAc)₆L₂]_n·2nDMF, where L = 4,4′-bipy (9·2DMF), bpe (10·2DMF) and [MnFe₂O(OAc)₆(bpe)(DMF)]_n·3.5nDMF (11·3.5DMF). All complexes were characterized by single-crystal X-ray diffraction. Desolvation of 11·3.5DMF led to a collapse of the porous crystal lattice that was confirmed by PXRD and N₂ sorption measurements, while alcohol adsorption led to porous structure restoration. Weak antiferromagnetic exchange was found in the case of binuclear Mn^II complexes (J_Mn-Mn = −1.03 cm⁻¹ for 1 and 2). According to magnetic data analysis (J_Mn-Mn = −(2.69 ÷ 0.42) cm⁻¹) and DFT calculations (J_Mn-Mn = −(6.9 ÷ 0.9) cm⁻¹) weak antiferromagnetic coupling between Mn^II ions also occurred in the tetranuclear {Mn₄(OH)(Piv)₇} unit of the 2D polymer 7. In contrast, strong antiferromagnetic coupling was found in oxo-bridged trinuclear fragment {MnFe₂O(OAc)₆} in 11·3.5DMF (J_Fe-Fe = −57.8 cm⁻¹, J_Fe-Mn = −20.12 cm⁻¹).     

New Bifunctional Bis(azairidacycle) with Axial Chirality via Double Cyclometalation of 2,2′-Bis(aminomethyl)-1,1′-binaphthyl

As a candidate for bifunctional asymmetric catalysts containing a half-sandwich C–N chelating Ir(III) framework (azairidacycle), a dinuclear Ir complex with an axially chiral linkage is newly designed. An expedient synthesis of chiral 2,2′-bis(aminomethyl)-1,1′-binaphthyl (1) from 1,1-bi-2-naphthol (BINOL) was accomplished by a three-step process involving nickel-catalyzed cyanation and subsequent reduction with Raney-Ni and KBH₄. The reaction of (S)-1 with an equimolar amount of [IrCl₂Cp*]₂ (Cp* = η⁵–C₅(CH₃)₅) in the presence of sodium acetate in acetonitrile at 80 °C gave a diastereomeric mixture of new dinuclear dichloridodiiridium complexes (5) through the double C–H bond cleavage, as confirmed by ¹H NMR spectroscopy. A loss of the central chirality on the Ir centers of 5 was demonstrated by treatment with KOC(CH₃)₃ to generate the corresponding 16e amidoiridium complex 6. The following hydrogen transfer from 2-propanol to 6 provided diastereomers of hydrido(amine)iridium retaining the bis(azairidacycle) architecture. The dinuclear chlorido(amine)iridium 5 can serve as a catalyst precursor for the asymmetric transfer hydrogenation of acetophenone with a substrate to a catalyst ratio of 200 in the presence of KOC(CH₃)₃ in 2-propanol, leading to (S)-1-phenylethanol with up to an enantiomeric excess (ee) of 67%.     

Asymmetric synthesis of dihydro-1,3-dioxepines by Rh(ii)/Sm(iii) relay catalytic three-component tandem [4 + 3]-cycloaddition

Heterocycles have been widely used in organic synthesis, agrochemical, pharmaceutical and materials science industries. Catalytic three-component ylide formation/cycloaddition enables the assembly of complex heterocycles from simple starting materials in a highly efficient manner. However, asymmetric versions remain a yet-unsolved task. Here, we present a new bimetallic catalytic system for tackling this challenge. A combined system of Rh(ii) salt and chiral N,N′-dioxide–Sm(iii) complex was established for promoting the unprecedented tandem carbonyl ylide formation/asymmetric [4 + 3]-cycloaddition of aldehydes and α-diazoacetates with β,γ-unsaturated α-ketoesters smoothly, affording various chiral 4,5-dihydro-1,3-dioxepines in up to 97% yield, with 99% ee. The utility of the current method was demonstrated by conversion of products to optically active multi-substituted tetrahydrofuran derivatives. A possible reaction mechanism was provided to elucidate the origin of chiral induction based on experimental studies and X-ray structures of catalysts and products.

Catalytic asymmetric tandem carbonyl ylide formation/[4 + 3]-cycloaddition of β,γ-unsaturated α-ketoesters, aldehydes and α-diazoacetates was achieved by using a bimetallic rhodium(ii)/chiral N,N′-dioxide–Sm(iii) complex catalyst.     

Use of a cyclo-P4 building block – a way to networks of host–guest assemblies

Despite the proven ability to form supramolecular assemblies via coordination to copper halides, organometallic building blocks based on four-membered cyclo-P₄ ligands find only very rare application in supramolecular chemistry. To date, only three types of supramolecular aggregates were obtained based on the polyphosphorus end-deck complexes Cp^RTa(CO)₂(η⁴-P₄) (1a: Cp^R = Cp′′; 1b: Cp^R = Cp′′′), with none of them, however, possessing a guest-accessible void. To achieve this target, the use of silver salts of the weakly coordinating anion SbF₆⁻ was investigated as to their self-assembly in the absence and in the presence of the template molecule P₃Se₄. The two-component self-assembly of the building block 1a and the coinage-metal salt AgSbF₆ leads to the formation of 1D or 3D coordination polymers. However, when the template-driven self-assembly was attempted in the presence of an aliphatic dinitrile, the unprecedented barrel-like supramolecular host–guest assembly P₃Se₄@[{(Cp′′Ta(CO)₂(η⁴-P₄))Ag}₈]⁸⁺ of 2.49 nm in size was formed. Moreover, cyclo-P₄-based supramolecules are connected in a 2D coordination network by dinitrile linkers. The obtained compounds were characterised by mass-spectrometry, ¹H and ³¹P NMR spectroscopy and X-ray structure analysis.

A one-pot self-assembly template-controlled reaction is reported to result in a 2D coordination network of first host-guest assemblies P₃Se₄@[{(Cp′′Ta(CO)₂(η⁴-P₄))Ag}₈]⁸⁺ of 2.49 nm in size based on an organometallic complex with a cyclo-P₄ end-deck.     

Highest-Tc single-component homochiral organic ferroelectrics

Organic single-component ferroelectrics with low molecular mass have drawn great attention for application in organic electronics. However, the discovery of high-T_c single-component organic ferroelectrics has been very scarce. Herein, we report a pair of homochiral single-component organic ferroelectrics (R)-10-camphorsulfonylimine and (S)-10-camphorsulfonylimine under the guidance of ferroelectric chiral chemistry. They crystallize in the chiral–polar space group P2₁, and their mirror image relations have been identified using vibrational circular dichroism spectra. They both exhibit 422F2 multiaxial ferroelectricity with T_c as high as 429 K. Besides, they possess superior acoustic impedance characteristics with a value of 2.45 × 10⁶ kg s⁻¹ m⁻², lower than that of PVDF. To our knowledge, enantiomeric (R and S)-10-camphorsulfonylimine show the highest T_c among the known organic single-component ferroelectrics and low acoustic impedance well matching with that of bodily tissues. This work promotes the development of high-performance organic single-component ferroelectrics and is of great inspiration to explore their application in next-generation flexible smart devices.

A pair of enantiomeric organic ferroelectrics (R and S)-10-camphorsulfonylimine show the highest T_c among the known single-component organic ferroelectrics.     

Ammoides pusilla Essential Oil: A Potent Inhibitor of the Growth of Fusarium avenaceum and Its Enniatin Production

Enniatins are mycotoxins produced by Fusarium species contaminating cereals and various agricultural commodities. The co-occurrence of these mycotoxins in large quantities with other mycotoxins such as trichothecenes and the possible synergies in toxicity could lead to serious food safety problems. Using the agar dilution method, Ammoides pusilla was selected among eight Tunisian plants for the antifungal potential of its essential oil (EO) on Fusarium avenaceum mycelial growth and its production of enniatins. Two EO batches were produced and analyzed by GC/MS-MS. Their activities were measured using both contact assays and fumigant tests (estimated IC50 were 0.1 µL·mL⁻¹ and 7.6 µL·L⁻¹, respectively). The A. pusilla EOs and their volatiles inhibited the germination of spores and the mycelial growth, showing a fungistatic but not fungicidal activity. The accumulation of enniatins was also significantly reduced (estimated IC50 were 0.05 µL·mL⁻¹ for the contact assays and 4.2 µL·L⁻¹ for the fumigation assays). The most active batch of EO was richer in thymol, the main volatile compound found. Thymol used as fumigant showed a potent fungistatic activity but not a significant antimycotoxigenic activity. Overall, our data demonstrated the bioactivity of A. pusilla EO and its high potential to control F. avenaceum and its enniatins production in agricultural commodities.     

Structure,Luminescent Sensing and Proton Conduction of a Boiling-Water-Stable Zn(II) Metal-Organic Framework

A novel Zn(II) metal-organic framework [Zn₄O(C₃₀H₁₂F₄O₄S₈)₃]_n, namely ZnBPD-4F4TS, has been constructed from a fluoro- and thiophenethio-functionalized ligand 2,2′,5,5′-tetrafluoro-3,3′,6,6′-tetrakis(2-thiophenethio)-4,4′-biphenyl dicarboxylic acid (H₂BPD-4F4TS). ZnBPD-4F4TS shows a broad green emission around 520 nm in solid state luminescence, with a Commission International De L’Eclairage (CIE) coordinate at x = 0.264, y = 0.403. Since d¹⁰-configured Zn(II) is electrochemically inert, its photoluminescence is likely ascribed to ligand-based luminescence which originates from the well-conjugated system of phenyl and thiophenethio moieties. Its luminescent intensities diminish to different extents when exposed to various metal ions, indicating its potential as an optical sensor for detecting metal ion species. Furthermore, ZnBPD-4F4TS and its NH₄Br-loaded composite, NH₄Br@ZnBPD-4F4TS, were used for proton conduction measurements in different relative humidity (RH) levels and temperatures. Original ZnBPD-4F4TS shows a low proton conductivity of 9.47 × 10⁻¹⁰ S cm⁻¹ while NH₄Br@ZnBPD-4F4TS shows a more than 25,000-fold enhanced value of 2.38 × 10⁻⁵ S cm⁻¹ at 40 °C and 90% RH. Both of the proton transport processes in ZnBPD-4F4TS and NH₄Br@ZnBPD-4F4TS belong to the Grotthuss mechanism with E_a = 0.40 and 0.32 eV, respectively.     

Ferro- and Antiferromagnetic Interactions in Oxalato-Centered Inverse Hexanuclear and Chain Copper(II) Complexes with Pyrazole Derivatives

Two novel copper(II) complexes of formulas {[Cu(4-Hmpz)₄][Cu(4-Hmpz)₂(µ₃-ox-κ²O¹,O²:κO^2′:κO^1′)(ClO₄)₂]}_n (1) and {[Cu(3,4,5-Htmpz)₄]₂[Cu(3,4,5-Htmpz)₂(µ₃-ox-κ²O¹,O²:κO^2′:κO^1′)(H₂O)(ClO₄)]₂[Cu₂(3,4,5-Htmpz)₄(µ-ox-κ²O¹,O²:κ²O^2′,O^1′)]}(ClO₄)₄·6H₂O (2) have been obtained by using 4-methyl-1H-pyrazole (4-Hmpz) and 3,4,5-trimethyl-1H-pyrazole (3,4,5-Htmpz) as terminal ligands and oxalate (ox) as the polyatomic inverse coordination center. The crystal structure of 1 consists of perchlorate counteranions and cationic copper(II) chains with alternating bis(pyrazole)(µ₃-κ²O¹,O²:κO^2′:κO^1′-oxalato)copper(II) and tetrakis(pyrazole)copper(II) fragments. The crystal structure of 2 is made up of perchlorate counteranions and cationic centrosymmetric hexanuclear complexes where an inner tetrakis(pyrazole)(µ-κ²O¹,O²:κ²O^2′,O^1′-oxalato)dicopper(II) entity and two outer mononuclear tetrakis(pyrazole)copper(II) units are linked through two mononuclear aquabis(pyrazole)(µ₃-κ²O¹,O²:κO^2′:κO^1′-oxalato)copper(II) units. The magnetic properties of 1 and 2 were investigated in the temperature range 2.0–300 K. Very weak intrachain antiferromagnetic interactions between the copper(II) ions through the µ₃-ox-κ²O¹,O²:κO^2′:κO^1′ center occur in 1 [J = −0.42(1) cm⁻¹, the spin Hamiltonian being defined as H = −J∑S_1,i · S_2,i+1], whereas very weak intramolecular ferromagnetic [J = +0.28(2) cm⁻¹] and strong antiferromagnetic [J’ = −348(2) cm⁻¹] couplings coexist in 2 which are mediated by the µ₃-ox-κ²O¹,O²:κO^2′:κO^1′ and µ-ox-κ²O¹,O²:κ²O^2′,O^1′ centers, respectively. The variation in the nature and magnitude of the magnetic coupling for this pair of oxalato-centered inverse copper(II) complexes is discussed in the light of their different structural features, and a comparison with related oxalato-centered inverse copper(II)-pyrazole systems from the literature is carried out.     

Resolution of P-Sterogenic 1-Phenylphosphin-2-en-4-one 1-Oxide into Two Enantiomers by (R,R)-TADDOL and Conformational Diversity of the Phosphinenone Ring and TADDOL in the Crystal State

The resolution of racemic 1-phenylphosphin-2-en-4-one 1-oxide (2), was achieved through the fractional crystallization of its diastereomeric complexes with (4R,5R)-(−)-2,2-dimethyl -α,α,α′,α′-tetraphenyl-dioxolan-4,5-dimethanol (R,R-TADDOL) followed by the liberation of the individual enantiomers of 2 by flash chromatography on silica gel columns. The resolution process furnished the two enantiomers of 2 of 99.1 and 99.9% e.e. at isolated yields of 62 and 59% (counted for the single enantiomer), respectively. The absolute configurations of the two enantiomers were established by means of X-ray crystallography of their diastereomerically pure complexes, i.e., (R)-2•R,R)-TADDOL and (S)-2•(R,R)-TADDOL. The structural analysis revealed that in the (R)-2•(R,R)-TADDOL complex, the P-phenyl substituent occupied a pseudoequatorial position, whereas in (S)-2•(R,R)-TADDOL, it appeared in both the pseudoequatorial and the pseudoaxial positions in four symmetrically independent molecules. Concurrent conformational changes of the TADDOL molecules were best described by the observed changes of a pseudo-torsional CO...OC angle that could be considered as a possible measure of TADDOL conformation in its receptor–ligand complexes. The structural analysis of the (R,R)-TADDOL molecule revealed that efficiency of this compound for use as an effective resolving factor comes from its ability to flexibly fit its structure to both enantiomers of a ligand molecule, producing a rare case of resolution for both pure enantiomers with one chiral separating agent. The resolved (R)-2 was used to assign the absolute configuration of a recently described (−)-1-phenylphosphin-2-en-4-one 1-sulfide by chemical correlation. In addition, an attempted stereoretentive reduction of (R)-2 by PhSiH₃ at 60 °C revealed an unexpectedly low barrier for P-inversion in 1-phenylphosphin-2-en-4-one.     

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.     

Ion polarisation-assisted hydrogen-bonded ferroelectrics in liquid crystalline domains

An alkylamide-substituted (−NHCOC₁₀H₂₁) hydrogen-bonded dibenzo[18]crown-6 derivative (1) was prepared to stabilise the ionic channel structure in a discotic hexagonal columnar (Col_h) liquid crystal. The introduction of simple M⁺X⁻ salts such as Na⁺PF₆⁻ and K⁺I⁻ into the ionic channel of 1 enhanced the ionic conductivity of the Col_h phase of the M⁺·(1)·X⁻ salts, with the highest ionic conductivity reaching ∼10⁻⁶ S cm⁻¹ for K⁺·(1)·I⁻ and Na⁺·(1)·PF₆⁻ at 460 K, which was approximately 5 orders of magnitude higher than that of 1. The introduction of non-ferroelectric 1 into the ferroelectric N,N′,N′′-tri(tetradecyl)-1,3,5-benzenetricarboxamide (3BC) elicited a ferroelectric response from the mixed Col_h phase of (3BC)_x(1)_1−x with x = 0.9 and 0.8. The further doping of M⁺X⁻ into the ferroelectric Col_h phase of (3BC)_0.9(1)_0.1 enhanced the ferroelectric polarisation assisted by ion displacement in the half-filled ionic channel for the vacant dibenzo[18]crown-6 of (3BC)_0.9[(M⁺)_0.5·(1)·(X⁻)_0.5]_0.1.

An alkylamide-substituted (−NHCOC₁₀H₂₁) hydrogen-bonded dibenzo[18]crown-6 derivative (1) was prepared to stabilise the ionic channel structure in a discotic hexagonal columnar (Col_h) liquid crystal.