Supramolecular Chirality in Coordination Chemistry

Subtle Stereochemical Effects Influence Binding and Purification Abilities of an FeII4L4 Cage

A tetrahedral Fe^II₄L₄ cage assembled from the coordination of triangular chiral, face-capping ligands to iron(II). This cage exists as two diastereomers in solution, which differ in the stereochemistry of their metal vertices, but share the same point chirality of the ligand. The equilibrium between these cage diastereomers was subtly perturbed by guest binding. This perturbation from equilibrium correlated with the size and shape fit of the guest within the host; insight as to the interplay between stereochemistry and fit was provided by atomistic well-tempered metadynamics simulations. The understanding thus gained as to the stereochemical impact on guest binding enabled the design of a straightforward process for the resolution of the enantiomers of a racemic guest.

Steroid-Aromatics Clathrates as Chiroptical Materials with Circularly Polarized Luminescence and Phosphorescence

Solid-state host-guest complexation shows great potential in the fabrication of chiroptical and phosphorescent materials. Developing chiral hosts toward achiral guests with a wide guest scope would expand the chiroptical application, which however remains a major challenge. Here, we report the steroid-aromatic compound complexation in the solid state that could realize effective chirality transfer, circularly polarized luminescence, and room temperature phosphorescence (RTP). Progesterone shows cocrystallization behavior toward a wide scope of guests through CH-π interaction, which also offers a rigid yet chiral microenvironment to entrap aromatic luminophores within the cavities or channels. Depending on the geometry of the guests, the handedness of the Cotton effects and circularly polarized luminescence could be tuned. Host-guest complexation not only gave rise to Cotton effects and circularly polarized luminescence but also stabilized the triplet state of bromo-compounds to achieve RTP and circularly polarized phosphorescence. This work first illustrates the application of steroid complexation in the chiroptical and phosphorescent materials, which shows potential in the display and information aspects.

Bicarbonate insertion triggered self-assembly of chiral octa-gold nanoclusters into helical superstructures in the crystalline state

Constructing atomically precise helical superstructures of high order is an extensively pursued subject for unique aesthetic features and underlying applications. However, the construction of cluster-based helixes of well-defined architectures comes with a huge challenge owing to their intrinsic complexity in geometric structures and synthetic processes. Herein, we report a pair of unique P- and M-single stranded helical superstructures spontaneously assembled from R- and S-Au8c individual nanoclusters, respectively, upon selecting chiral BINAP (2,2'-bis(diphenylphosphino)-1,1'-binaphthalene) and hydrophilic o-H2MBA (o-mercaptobenzoic acid) as protective ligands to induce chirality and facilitate the formation of helixes. Structural analysis reveals that the chirality of the Au8c individual nanoclusters is derived from the homochiral ligands and the inherently chiral Au8 metallic kernel, which was further corroborated by experimental and computational investigations. More importantly, driven by the O-H⋯O interactions between (HCO3 -)2 dimers and achiral o-HMBA- ligands, R/S-Au8c individual nanoclusters can assemble into helical superstructures in a highly ordered crystal packing. Electrospray ionization (ESI) and collision-induced dissociation (CID) mass spectrometry of Au8c confirm the hydrogen-bonded dimer of Au8c individual nanoclusters in solution, illustrating that the insertion of (HCO3 -)2 dimers plays a crucial role in the assembly of helical superstructures in the crystalline state. This work not only demonstrates an effective strategy to construct cluster-based helical superstructures at the atomic level, but also provides visual and reliable experimental evidence for understanding the formation mechanism of helical superstructures.

Chiral Water-Soluble Molecular Capsules With Amphiphilic Interiors

We present the synthesis of new chiral water-soluble dimeric capsules by the multicomponent Mannich reaction between charged amino acids (glutamic acid or arginine), resorcinarene, and formaldehyde and by subsequent self-assembly. The zwitterionic character of the backbones enables electrostatic interactions between arms and induces self-assembly of dimeric capsules, namely, (L-ArgR)₂ and (L-GluR)₂, in water with a wide range of pH, as demonstrated by NMR, diffusion coefficient measurement, and circular dichroism. The assembly/disassembly processes are fast on the NMR timescale. This mode of dimerization leaves side chains available for additional interactions and creates chiral cavities of mixed hydrophobic/hydrophilic character. According to this characteristic, capsules do not bind fully nonpolar or fully polar guests but effectively encapsulate a variety of chiral molecules with mixed polar/apolar characters (aliphatic and aromatic aldehydes, epoxides, alcohols, carboxylic acids, amines, and amino acids) with moderate strength. We also demonstrate the formation of heterocapsules (GluR) (ArgR) (homo- and heterochiral) that utilize additional interactions between charged acidic and basic side chains and have better encapsulation properties than those of the homodimers.

Solvent Drives Switching between Λ and Δ Metal Center Stereochemistry of M8L6 Cubic Cages

An enantiopure ligand with four bidentate metal-binding sites and four (S)-carbon stereocenters self-assembles with octahedral Zn^II or Co^II to produce O-symmetric M₈L₆ coordination cages. The Λ- or Δ-handedness of the metal centers forming the corners of these cages is determined by the solvent environment: the same (S)-ligand produces one diastereomer, (S)₂₄-Λ₈-M₈L₆, in acetonitrile but another with opposite metal-center handedness, (S)₂₄-Δ₈-M₈L₆, in nitromethane. Van ’t Hoff analysis revealed the Δ stereochemical configuration to be entropically favored but enthalpically disfavored, consistent with a loosening of the coordination sphere and an increase in conformational freedom following Λ-to-Δ transition. The binding of 4,4′-dipyridyl naphthalenediimide and tetrapyridyl Zn-porphyrin guests did not interfere with the solvent-driven stereoselectivity of self-assembly, suggesting applications where either a Λ- or Δ-handed framework may enable chiral separations or catalysis.

Supramolecular Helical Assemblies of Dirhodium(II) Paddlewheels with 1,4-Diazabicyclo[2.2.2]octane: A Remarkable Substituent Effect on the Helical Sense Preference and Amplification of the Helical Handedness Excess of Metallo-Supramolecular Helical Polymers

We report unique coordination-driven supramolecular helical assemblies of a series of dirhodium(II) tetracarboxylate paddlewheels bearing chiral phenyl- or methyl-substituted amide-bound m-terphenyl residues with triethylene glycol monomethyl ether (TEG) or n-dodecyl tails through a 1:1 complexation with 1,4-diazabicyclo[2.2.2]octane (DABCO). The chiral dirhodium complexes with DABCO in CHCl₃/n-hexane (1:1) form one-handed helical coordination polymers with a controlled propeller chirality at the m-terphenyl groups, which are stabilized by intermolecular hydrogen-bonding networks between the adjacent amide groups at the periphery mainly via a cooperative nucleation-elongation mechanism as supported by circular dichroism (CD), vibrational CD, and variable-temperature (VT) absorption and CD analyses. The VT visible-absorption titrations revealed the temperature-dependent changes in the degree of polymerization. The columnar supramolecular helical structures were elucidated by X-ray diffraction and atomic force microscopy. The helix sense of the homopolymer carrying the bulky phenyl and n-dodecyl substituents is opposite those of other chiral homopolymers despite having the same absolute configuration at the pendants. A remarkably strong "sergeants and soldiers" (S&S) effect was observed in most of the chiral/achiral copolymers, while the copolymers of the bulky chiral phenyl-substituted dirhodium complexes with n-dodecyl chains displayed an "abnormal" S&S effect accompanied by an inversion of the helix sense, which could be switched to a "normal" S&S effect by changing the solvent composition. A nonracemic dirhodium complex of 20% enantiomeric excess bearing the less bulky chiral methyl substituents with n-dodecyl chains assembled with DABCO to form an almost one-handed helix (the "majority rule" (MR) effect), whereas the three other nonracemic copolymers showed a weak MR effect.

Structural Chemistry of Giant Metal Based Supramolecules

The review presents a bird-eye view on the state of research in the field of giant nonbiological discrete metal complexes and ions of nanometer size, which are structurally characterized by means of single-crystal X-ray diffraction, using the crystal structure as a common key feature. The discussion is focused on the main structural features of the metal clusters, the clusters containing compact metal oxide/hydroxide/chalcogenide core, ligand-based metal-organic cages, and supramolecules as well as on the aspects related to the packing of the molecules or ions in the crystal and the methodological aspects of the single-crystal neutron and X-ray diffraction of these compounds.

Sterics and Hydrogen Bonding Control Stereochemistry and Self-Sorting in BINOL-Based Assemblies

Here we demonstrate how the hydrogen-bonding ability of a BINOL-based dialdehyde subcomponent dictated the stereochemical outcome of its subsequent self-assembly into one diastereomeric helicate form when bearing free hydroxy groups, and another in the case of its methylated congener. The presence of methyl groups also altered the self-sorting behavior when mixed with another, short linear dialdehyde subcomponent, switching the outcome of the system from narcissistic to integrative self-sorting. In all cases, the axial chirality of the BINOL building block also dictated helicate metal center handedness during stereospecific self-assembly. A new family of stereochemically pure heteroleptic helicates were thus prepared using the new knowledge gained. We also found that switching from Fe^II to Zn^II, or the incorporation of a longer linear ligand, favored heteroleptic structure formation.

Assembly-Induced Strong Circularly Polarized Luminescence of Spirocyclic Chiral Silver(I) Clusters

Spirocyclic Ag₉ clusters, as a new form of intrinsically chiral metal clusters, were constructed through vertex-sharing of two in-situ-generated heteroaryl diide-centered metal rings. Such core-peripheral type clusters exhibit versatile photoluminescent and chiroptical behavior under different aggregation conditions. In contrast to a ligand-based fluorescence emission in a diluted solution of the clusters, a solvent polarity-caused assembly gives rise to new cluster-based phosphorous luminescence owing to radiative mode switching and aggregation-induced emission. Assembly of cluster enantiomers leads to micrometer-long helical nanofibers, whose handedness is determined by absolute configuration of individual spirocyclic clusters. Benefiting from exciton couplings of helical arrangements of chelating ligands at molecular and microscopic levels, the assembled film of cluster enantiomers exhibits circularly polarized luminescence with a high anisotropy factor (0.16).

Tunable Supramolecular Chirogenesis in the Self-Assembling of Amphiphilic Porphyrin Triggered by Chiral Amines

Supramolecular chirality is one of the most important issues in different branches of science and technology, as stereoselective molecular recognition, catalysis, and sensors. In this paper, we report on the self-assembly of amphiphilic porphyrin derivatives possessing a chiral information on the periphery of the macrocycle (i.e., D- or L-proline moieties), in the presence of chiral amines as co-solute, such as chiral benzylamine derivatives. The aggregation process, steered by hydrophobic effect, has been studied in aqueous solvent mixtures by combined spectroscopic and topographic techniques. The results obtained pointed out a dramatic effect of these ligands on the morphology and on the supramolecular chirality of the final self-assembled structures. Scanning electron microscopy topography, as well as fluorescence microscopy studies revealed the formation of rod-like structures of micrometric size, different from the fractal structures formerly observed when the self-assembly process is carried out in the absence of chiral amine co-solutes. On the other hand, comparative experiments with an achiral porphyrin analogue strongly suggested that the presence of the prolinate moiety is mandatory for the achievement of the observed highly organized suprastructures. The results obtained would be of importance for unraveling the intimate mechanisms operating in the selection of the homochirality, and for the preparation of sensitive materials for the detection of chiral analytes, with tunable stereoselectivity and morphology.

Beyond Chiral Organic (p-Block) Chromophores for Circularly Polarized Luminescence: The Success of d-Block and f-Block Chiral Complexes

Chiral molecules are essential for the development of advanced technological applications in spintronic and photonic. The best systems should produce large circularly polarized luminescence (CPL) as estimated by their dissymmetry factor (g _lum), which can reach the maximum values of -2 ≤ g _lum ≤ 2 when either pure right- or left-handed polarized light is emitted after standard excitation. For matching this requirement, theoretical considerations indicate that optical transitions with large magnetic and weak electric transition dipole moments represent the holy grail of CPL. Because of their detrimental strong and allowed electric dipole transitions, popular chiral emissive organic molecules display generally moderate dissymmetry factors (10^-5 ≤ g _lum ≤ 10^-3). However, recent efforts in this field show that g _lum can be significantly enhanced when the chiral organic activators are part of chiral supramolecular assemblies or of liquid crystalline materials. At the other extreme, chiral Eu^III- and Sm^III-based complexes, which possess intra-shell parity-forbidden electric but allowed magnetic dipole transitions, have yielded the largest dissymmetry factor reported so far with g _lum ~ 1.38. Consequently, 4f-based metal complexes with strong CPL are currently the best candidates for potential technological applications. They however suffer from the need for highly pure samples and from considerable production costs. In this context, chiral earth-abundant and cheap d-block metal complexes benefit from a renewed interest according that their CPL signal can be optimized despite the larger covalency displayed by d-block cations compared with 4f-block analogs. This essay thus aims at providing a minimum overview of the theoretical aspects rationalizing circularly polarized luminescence and their exploitation for the design of chiral emissive metal complexes with strong CPL. Beyond the corroboration that f-f transitions are ideal candidates for generating large dissymmetry factors, a special attention is focused on the recent attempts to use chiral Cr^III-based complexes that reach values of g _lum up to 0.2. This could pave the way for replacing high-cost rare earths with cheap transition metals for CPL applications.

Cavitand-Based Pd-Pyridyl Coordination Capsules: Guest-Induced Homo- or Heterocapsule Selection and Applications of Homocapsules to the Protection of a Photosensitive Guest and Chiral Capsule Formation

A 2 : 4 mixture of tetrakis[4-(4-pyridyl)phenyl]cavitand (1) or tetrakis[4-(4-pyridyl)phenylethynyl]cavitand (2) and Pd(dppp)(OTf)₂ self-assembles into a homocapsule {1₂  ⋅ [Pd(dppp)]₄ }⁸⁺  ⋅ (TfO^- )₈ (C1) or {2₂  ⋅ [Pd(dppp)]₄ }⁸⁺  ⋅ (TfO^- )₈ (C2), respectively, through Pd-Npy coordination bonds. A 1 : 1 : 4 mixture of 1, 2, and Pd(dppp)(OTf)₂ produced a mixture of homocapsules C1, C2, and a heterocapsule {1 ⋅ 2 ⋅ [Pd(dppp)]₄ }⁸⁺  ⋅ (TfO^- )₈ (C3) in a 1 : 1 : 0.98 mole ratio. Selective formation (self-sorting) of homocapsules C1 and C2 or heterocapsule C3 was controlled by guest-induced encapsulation under thermodynamic control. Applications of Pd-Npy coordination capsules with the use of 1 were demonstrated. Capsule C1 serves as a guard nanocontainer for trans-4,4'-diacetoxyazobenzene to protect against the trans-to-cis photoisomerization by encapsulation. A chiral capsule {1₂  ⋅ [Pd((R)-BINAP)]₄ }⁸⁺  ⋅ (TfO^- )₈ (C5) was also constructed. Capsule C5 induces supramolecular chirality with respect to prochiral 2,2'-bis(alkoxycarbonyl)-4,4'-bis(1-propynyl)biphenyls by diastereomeric encapsulation through the asymmetric suppression of rotation around the axis of the prochiral biphenyl moiety.

Mechanistic Interplay between Light Switching and Guest Binding in Photochromic [Pd2Dithienylethene4] Coordination Cages

Photochromic [Pd₂L₄] coordination cages based on dithienylethene (DTE) ligands L allow triggering guest uptake and release by irradiation with light of different wavelengths. The process involves four consecutive electrocyclic reactions to convert all chromophores between their open and closed photoisomeric forms. So far, guest affinity of the fully switched species was elucidated, but mechanistic details concerning the intermediate steps remained elusive. Now, a new member of the DTE cage family allows unprecedented insight into the interplay between photoisomerization steps and guest location inside/outside the cavity. Therefore, the intrinsic chirality of the DTE backbones was used as reporter for monitoring the fate of a chiral guest. In its "open" photoisomeric form ( o-L, [Pd₂( o-L)₄] = o-C), the C₂-symmetric DTE chromophore quickly converts between energetically degenerate P and M helical conformations. After binding homochiral 1 R-( -) or 1 S-( +) camphor sulfonate ( R-CSA or S-CSA), guest-to-host chirality transfer was observed via a circular dichroism (CD) signal for the cage-centered absorption. Irradiating the R/S-CSA@ o-C host-guest complexes at 313 nm produced configurationally stable "closed" photoisomers, thus locking the induced chirality with an enantiomeric excess close to 25%. This value (corresponding to chiral induction for one out of four ligands), together with DOSY NMR, ion mobility mass spectrometry, and X-ray structure results, shows that closure of the first photoswitch is sufficient to expel the guest from the cavity.

Chiral hydrogen-bonded supramolecular capsules: synthesis, characterization and complexation of C70

Hydrogen-bonded nanocapsules were generated by multi-component self-assembly of phosphoric acids and amidines and could be used as hosts for C₇₀.

Face and edge directed self-assembly of Pd12 tetrahedral nano-cages and their self-sorting

Reactions of a cis-blocked Pd(ii) 90° acceptor [cis-(tmeda)Pd(NO3)2] (M) with 1,4-di(1H-tetrazol-5-yl)benzene (H2L1 ) and [1,3,5-tri(1H-tetrazol-5-yl)benzene] (H3L2 ) in 1 : 1 and 3 : 2 molar ratios respectively, yielded soft metallogels G1 and G2 [tmeda = N,N,N',N'-tetramethylethane-1,2-diamine]. Post-metalation of the gels G1 and G2 with M yielded highly water-soluble edge and face directed self-assembled Pd12 tetrahedral nano-cages T1 and T2, respectively. Such facile conversion of Pd(ii) gels to discrete tetrahedral metallocages is unprecedented. Moreover, distinct self-sorting of these two tetrahedral cages of similar sizes was observed in the self-assembly of M with a mixture of H2L1 and H3L2 in aqueous medium. The edge directed tetrahedral cage (T1) was successfully used to perform Michael reactions of a series of water insoluble nitro-olefins assisted by encapsulation into the cage in aqueous medium.

Efficient optical resolution of water-soluble self-assembled tetrahedral M4L6 cages with 1,1′-bi-2-naphthol

Self-assembled tetrahedral M₄L₆ cages were successfully resolved into their ΔΔΔΔ and ΛΛΛΛ enantiomers by (R)-/(S)-1,1′-bi-2-naphthol.

Meso-helicates with rigid angular tetradentate ligand: design, molecular structures, and progress towards self-assembly of metal-organic nanotubes

The self-assembly of two novel metallosupramolecular complexes of the general formulas [L2M2(CH3CN)4][BF4]4 (M = Co, 1a; M = Ni, 1b), where L stands for the tetradentate ligand 3,5-bis[4-(2,2'-dipyridylamino)phenylacetylenyl]toluene, is reported together with their molecular structures ascertained by single-crystal X-ray diffraction studies. Complexes 1a and 1b are isostructural and show the formation of dinuclear meso-helicates with the two octahedral metal centers displaying respectively Δ and Λ configurations. These meso-helicates display large nanocavities with metal---metal separation distance of >2 nm; furthermore, π-π-stacking occurs among individual units to form one-dimensional (1D) polymers which further autoassemble in another direction through π-π contacts among neighboring chains to generate a two-dimensional (2D) network with regular nanocavities. Our approach might be of interest to prepare metal-organic nanotubes via a bottom-up strategy depending on the assembling functional ligand and the geometry of molecular building block.

Planar-chiral metal complexes comprised of square-planar metal and achiral tetradentate ligands: design, optical resolution, and thermodynamics

Planar-chiral palladium complexes {[[N,N'-[1,4-butanediylbis(oxy-7,1-naphthalenediyl)]bis(2-pyridinecarboxamidato)](2-)-κN(1),κN(1)',κN(2),κN(2)']palladium (PdL(4)) and [[2,2'-[1,4-butanediylbis[[(oxy-7,1-naphthalenediyl)imino]methyl]]dipyrrolato](2-)-κN(1),κN(1)',κN(2),κN(2)']palladium (PdL(5))} were synthesized from achiral tetradentate ligands N,N'-[1,4-butanediylbis(oxy-7,1-naphthalenediyl)]bis(2-pyridinecarboxamide) (H(2)L(4)) and N,N'-bis[(1H-pyrrol-2-yl)methylidene]-7,7'-(1,4-butanediyldioxy)bis(1-naphthalenamine) (H(2)L(5)) bearing two dissymmetric bidentate units at both ends and a Pd(II) ion, respectively. The palladium complexes were crystallized in the monoclinic space group P2(1)/n with the unit cell parameters a = 16.5464(6) Å, b = 11.3534(4) Å, c = 17.6697(7) Å, β = 115.5300(10)°, and Z = 4 for PdL(4) and a = 17.2271(8) Å, b = 10.1016(5) Å, c = 17.9361(9) Å, β = 105.6310(10)°, and Z = 4 for PdL(5). The planar-chiral structures of PdL(4) and PdL(5) were confirmed by single-crystal X-ray analyses, resulting in the fact that the crystals were racemic mixtures. The racemic mixtures were successfully resolved by using chiral high-performance liquid-chromatography techniques. Racemizations of the complexes were found to be drastically dependent on the arrangement of the charged or uncharged metal-binding N atoms of the ligands.

Molecular recognition of hydrocarbon guests by a supramolecular capsule formed by the 4:4 self-assembly of tris(Zn(2+)-cyclen) and trithiocyanurate in aqueous solution

We have previously reported that the trimeric Zn(2+)-cyclen complex (tris(Zn(2+)-cyclen), [Zn(3)L(1)](6+)) and the trianion of trithiocyanuric acid (TCA(3-)) assembled in a 4:4 ratio to form a cuboctahedral supramolecular cage, [(Zn(3)L(1))(4)(TCA(3-))(4)](12+) (hereafter referred to as a Zn-cage), in neutral aqueous solution (cyclen=1,4,7,10-tetraazacyclododecane). Herein, we examined the molecular recognition of C(1)-C(12) hydrocarbons (C(n)H((2n+2)) (n≈1-12)), cyclopentane, cyclododecane, cis-decalin, and trans-decalin by the Zn-cage under normal atmospheric pressure. This cage complex was also able to encapsulate guest molecules that had larger volumes than that of the inner cavity of the Zn-cage, thereby suggesting that the inner shape of the Zn-cage was flexible. Computational simulations of Zn-cage-guest complexes provided support for this conclusion. Moreover, the solvent-accessible surface areas (SASA) of the Zn-cage host, guest molecules, and the Zn-cage-guest complexes were calculated and the data were used to explain the order of stability determined by the guest-replacement experiments. The storage of volatile molecules in aqueous solution by the Zn-cage is also discussed.

Interconversion between the enantiomers of chiral five-coordinate Me3Pt(IV) complexes

The dinuclear Me(2)Pt(II) complexes of 3,4-bis(quinolin-8-yl)thiophene (1a), 3,4-bis(6 trifluoromethoxyquinolin-8-yl)thiophene (1b), and 3,4-bis(2-methylquinolin-8-yl)thiophene (1c) react with MeOTf (OTf = trifluoromethanesulfonate) to afford the corresponding chiral mononuclear five-coordinate Me(3)Pt(IV) complexes [PtMe(3)(1a)]OTf (3a), [PtMe(3)(1b)]OTf (3b), and [PtMe(3)(1c)]OTf (3c), respectively. [PtMe(3)(1c)]BAr(F)(4) (3d) (where BAr(F)(4) = [B{C(6)H(3)-3,5-(CF(3))(2)}(4)]) has also been synthesized for structural study. While 3a appears to be symmetric in solution and asymmetric in solid state, 3c and 3d are asymmetric in both solution and solid state. The chirality originates from interligand repulsion, rather than any unsymmetrical ligand. Variable-temperature NMR and computational studies suggest a ligand-twisting isomerization pathway for the interconversion of the enantiomers, rather than the rotational exchange of three CH(3) ligands on the metal center.