Achiral Ligands Research Articles

Separation of Enantiomers and CD Spectra of Au40(SCH2CH2Ph)24: Spectroscopic Evidence for Intrinsic Chirality

Chirality unveiled: Thiolate-protected Au40(SR)24 clusters were enantioenriched using an HPLC approach. CD spectra show strong mirror-image responses, indicating the intrinsic chirality of a cluster of unknown structure protected with achiral ligands.

ChemInform Abstract: A Versatile Ru Catalyst for the Asymmetric Transfer Hydrogenation of Both Aromatic and Aliphatic Sulfinylimines.

AbstractA ruthenium complex bearing an achiral amino alcohol ligand is prepared in situ.

Ion-paired chiral ligands for asymmetric palladium catalysis

Conventional chiral ligands rely on the use of a covalently constructed, single chiral molecule embedded with coordinative functional groups. Here, we report a new strategy for the design of a chiral ligand for asymmetric transition-metal catalysis; our approach is based on the development of an achiral cationic ammonium-phosphine hybrid ligand paired with a chiral binaphtholate anion. This ion-paired chiral ligand imparts a remarkable stereocontrolling ability to its palladium complex, which catalyses a highly enantioselective allylic alkylation of α-nitrocarboxylates. By exploiting the possible combinations of the achiral onium entities with suitable coordinative functionalities and readily available chiral acids, this approach should contribute to the development of a broad range of metal-catalysed, stereoselective chemical transformations.

Iron Nanoparticles Catalyzing the Asymmetric Transfer Hydrogenation of Ketones

Investigation into the mechanism of transfer hydrogenation using trans-[Fe(NCMe)CO(PPh(2)C(6)H(4)CH═NCHR-)(2)][BF(4)](2), where R = H (1) or R = Ph (2) (from R,R-dpen), has led to strong evidence that the active species in catalysis are iron(0) nanoparticles (Fe NPs) functionalized with achiral (with 1) and chiral (with 2) PNNP-type tetradentate ligands. Support for this proposition is given in terms of in operando techniques such as a kinetic investigation of the induction period during catalysis as well as poisoning experiments using substoichiometric amounts of various poisoning agents. Further support for the presence of Fe(0) NPs includes STEM microscopy imaging with EDX analysis, XPS analysis, and SQUID magnetometry analysis of catalytic solutions. Further evidence of Fe NPs acting as the active catalyst is given in terms of a polymer-supported substrate experiment whereby the NPs are too large to permeate the pores of a functionalized polymer. Final support is given in terms of a combined poisoning/STEM/EDX experiment whereby the poisoning agent is shown to be bound to the Fe NPs. This paper provides evidence of a rare example of asymmetric catalysis with nonprecious metal, zerovalent nanoparticles.

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.

Highly stable 3D homochiral coordination polymer with interweaving of double-stranded helices and extended metal-SO4-metal chains

Abstract Solvothermal synthesis of an achiral ligand N,N-bis(3-pyridyl) isophthalamide (bpip) with ZnSO4 under 120 °C yield a novel compound, [Zn(bpip)SO4]n, which displays chiral self-assembly into an infinite double-stranded helical coordination polymer consisting of two single-stranded helices with the same P handedness. These chiral infinite double-stranded helical chains are further interwoven by the 1D Zn–SO4–Zn chains to form a novel 3D framework structure with a rare 65.10 topology. On the basis of the results of TG-DSC analyses, the structure is thermally stable up to ~ 467.4 °C.

A Versatile Ru Catalyst for the Asymmetric Transfer Hydrogenation of Both Aromatic and Aliphatic Sulfinylimines

A highly efficient Ru catalyst based on an achiral, very simple, and inexpensive amino alcohol ligand (2-amino-2-methylpropan-1-ol) has been developed for the asymmetric transfer hydrogenation (ATH) of chiral N-(tert-butylsulfinyl)imines. This complex is able to catalyze the ATH of both aromatic and the most challenging aliphatic sulfinylimines by using isopropyl alcohol as the hydrogen source. The diastereoselective reduction of aromatic, heteroaromatic, and aliphatic sulfinylketimines, including sterically congested cases, over short reaction times (1-4 h), followed by desulfinylation of the nitrogen atom, affords the corresponding highly enantiomerically enriched (ee up to >99 %) α-branched primary amines in excellent yields. The same ligand was equally effective for the synthesis of both (R)- and (S)-amines by using the appropriate absolute configuration in the iminic substrate. DFT mechanistic studies show that the hydrogen-transfer process is stepwise. Moreover, the origin of the diastereoselectivity has been rationalized.

New chiral coordination polymers constructed from well elaborated achiral and chiral ligands

The elaboration of structurally related tetrazolate-based ligands from achiral to chiral led to the formation of two new chiral coordination polymers (CCPs). Compound 1 contains an infinite 1D zigzag Hg chain and a 1D helical water chain, its chirality is induced by the symmetry breaking of an achiral ligand while compound 2 is a 3D framework with a new 3-connected structural topology constructed from a homochiral ligand. The chiral natures of the two CCPs were both confirmed by circular dichroism (CD) spectra. The luminescent properties of compounds 1 and 2 were also investigated. Their strong luminescence suggest that both compounds may be excellent candidates as potential photoactive materials.

Metal–organic frameworks built from achiral cyclohex-1-ene-1,2-dicarboxylate: syntheses, structures and photoluminescence properties

Using achiral ligand cyclohex-1-ene-1,2-dicarboxylic acid (H2L), in the presence or absence of N-donor coligands, five metal(II) complexes formulated as Zn(L)(4,4′-bpy)·H2O (1), Cu(L)(4,4′-bpy)0.5 (2), Cd2(L)2(phen)2(H2O)2·3H2O (3), Zn5(L)4(μ3-OH)2(H2O)2 (4) and Cd(L)(H2O) (5) have been synthesized and structurally characterized by single-crystal X-ray diffractions. Complex 1 shows a homochiral network based on novel twisted hexanuclear cyclic units, which are fused by sharing metal ions into a 3D framework. Complex 2 exhibits a 2D framework constructed by 1D chain linked by 4,4′-bpy. 4,4′-bpy exhibits twisted and planar conformations in complexes 1 and 2. In complex 3, alternating 1D left-handed and right-handed helical chains are observed. Complexes 4 and 5 display different 2D frameworks based on a Zn5 unit and Cd–O–Cd infinite linkage, respectively. Complex 1 shows a strong CD signal, and complexes 4–5 are achiral. The twisted 4,4-bpy plays an important role in the formation of the chiral complex 1. The five complexes show different absorption and photoluminescence properties as well as different thermal stabilities.

First enantioseparation and circular dichroism spectra of Au38 clusters protected by achiral ligands

Bestowing chirality to metals is central in fields such as heterogeneous catalysis and modern optics. Although the bulk phase of metals is symmetric, their surfaces can become chiral through adsorption of molecules. Interestingly, even achiral molecules can lead to locally chiral, though globally racemic, surfaces. A similar situation can be obtained for metal particles or clusters. Here we report the first separation of the enantiomers of a gold cluster protected by achiral thiolates, Au38(SCH2CH2Ph)24, achieved by chiral high-performance liquid chromatography. The chirality of the nanocluster arises from the chiral arrangement of the thiolates on its surface, forming 'staple motifs'. The enantiomers show mirror-image circular dichroism responses and large anisotropy factors of up to 4×10−3. Comparison with reported circular dichroism spectra of other Au38 clusters reveals that the influence of the ligand on the chiroptical properties is minor.

Spontaneous resolution of a coordination polymer containing stereogenic five-coordinate Zn(ii) centers and achiral ligands with axially chiral conformation

A pair of chiral coordination polymers, [Zn(HPyIDC)]n (1a and 1b, H3PyIDC = 2-(pyridin-3-yl)-1H-imidazole-4,5-dicarboxylate), were assembled from an achiral precursor, H3PyIDC and Zn(II) ions via spontaneous resolution under solvothermal conditions. Crystal structure analysis showed that they are chiral three-dimensional (3D) coordination polymers based on the interconnection of six types of one-dimensional (1D) helical chains. They exhibit an intrinsic chiral (10,3)-a network. The chirality comes from both the stereogenic five-coordinate Zn(II) ions and the axially chiral ligands. Solid-state circular dichroism (CD) spectra measurements confirmed the occurrence of spontaneous resolution of [Zn(HPyIDC)]n and demonstrated that the resulting crystal of [Zn(HPyIDC)]n is a racemic mixture. Compound 1 showed strong fluorescence emission at room temperature.

A pair of 3D homochiral metal–organic frameworks: spontaneous resolution, single-crystal-to-single-crystal transformation and selective adsorption properties

A novel achiral hexacarboxylic ligand L was synthesized. Spontaneous resolution occurred during the reaction of L with Eu(NO(3))(3) to produce a pair of enantiomers of 1a and 1b with a formula of [EuL(NO(3))(3)(H(2)O)]·13H(2)O, which possesses 1D chiral open channels formed by triple-stranded helical chains. When heated to 60 °C, 1a releases coordinated and partial lattice H(2)O molecules to give a complex of [EuL(NO(3))(3)]·4H(2)O (1a'), accompanied by a single-crystal-to-single-crystal transformation from 1a to 1a', with the space group changing from P2(1) to P3(1)2. The dehydrated 1 can selectively adsorb water over organic solvents and exhibits reversible water adsorption upon dehydration and hydration.

Axially chiral metal–organic frameworks produced from spontaneous resolution with an achiral pyridyl dicarboxylate ligand

Three-dimensional chiral (10,3)-c MOFs are generated via symmetry breaking from achiral precursors, 5-(pyridine-3-yl)isophthalic acid and M2+ (M = Cd, Zn, Mn), and chiral transmission from molecular axially chiral conformations to framework chirality is established.

Homochiral assembly of polycatenated bilayers with mixing achiral ligands

Presented here is a new homochiral Cu(II) compound with a polycatenated bilayer structure constructed from flexible 4,4′-oxybisbenzoic acid and 1,3-di(4-pyridyl)propane, which shows an unusual topology with intrinsic chirality and provides a new route toward the construction of chiral metal–organic frameworks via the homochiral assembly of polycatenated bilayers.

Synthesis of Bis‐indole Carboxamides as G‐Quadruplex Stabilizing and Inducing Ligands

The design and synthesis of a series of bis-indole carboxamides with varying amine containing side chains as G-quadruplex DNA stabilising small molecules are reported. Their interactions with quadruplexes have been evaluated by means of Förster resonance energy transfer (FRET) melting analysis, UV/Vis spectroscopy, circular dichroism spectroscopy and molecular modelling studies. FRET analysis indicates that these ligands exhibit significant selectivity for quadruplex over duplex DNA, and the position of the carboxamide side chains is of paramount importance in G-quadruplex stabilisation. UV/Vis titration studies reveal that bis-indole ligands bind tightly to quadruplexes and show a three- to fivefold preference for c-kit2 over h-telo quadruplex DNA. CD studies revealed that bis-indole carboxamide with a central pyridine ring induces the formation of a single, antiparallel, conformation of the h-telo quadruplex in the presence and absence of added salt. The chirality of h-telo quadruplex was transferred to the achiral ligand (induced CD) and the formation of a preferred atropisomer was observed.

Chiral Nanoporous Metal‐Organic Frameworks with High Porosity as Materials for Drug Delivery

A chiral nanoporous metal-organic framework (MOF) with high porosity is obtained based on nontoxic zinc and achiral hexadentate ligand. It shows high drug loading and slow release of the proportion of the loaded drug with a complete delivery time of about one week when used as a material for adsorption and delivery of anticancer 5-fluorouracil.

Enantioselective Supramolecular Catalysis Induced by Remote Chiral Diols

A new method of creating libraries of chiral diphosphines is presented. Supramolecular coordination compounds based on Ti, Rh, achiral ditopic ligands, and chiral diols were synthesized by in situ mixing and used as catalysts in the asymmetric hydrogenation of (Z)-methyl 2-acetamido-3-phenylacrylate, giving ee's of up to 92%. The ditopic ligands contain a Schiff base that coordinates to the assembly metal Ti and a phosphine as a ligand for Rh. Chirality is introduced by coordination of the chiral diols to Ti. The controlling chiral center and the substrate are separated by as much as 13 Å.

Homo- and Heterochiral Aggregations of Samarium(III) Complexes with Acetate and Tripod Ligand Containing Three Imidazole Groups

Abstract Three SmIII complexes with tripod heptadentate N7 ligands containing three imidazole groups, [Sm(H3L2-Me)(ac)](ClO4)2·2MeOH·H2O (1), [Sm(H3L2-Me)(ac)](ClO4)2·2EtOH (2), and [Sm(H3L4-Me)(ac)](ClO4)2·MeOH (3), were synthesized and the crystal structures were determined, where H3L2-Me and H3L4-Me are the 1:3 condensation products of tris(2-aminoethyl)amine and either 2-methyl-4-formylimidazole and 4-methyl-5-formylimidazole, respectively, and ac denotes acetate ion. Each SmIII ion is coordinated by a tripod heptadentate N7 ligand and two oxygen atoms of the acetate ion. Due to the screw coordination arrangement of the achiral tripod ligand to the SmIII ion, the complex-cation [SmIII(H3L2- or 4-Me)(ac)]2+ gives Δ- and Λ-enantiomorphs. In compounds 1 and 2, two adjacent [Sm(H3L2-Me)(ac)]2+ species with the same chirality are linked by an intermolecular imidazole···acetate hydrogen bond to form a homochiral 1D chain structure. Adjacent chains with the same chirality are stacked to give a conglomerate crystal in 1, while adjacent chains with opposite chiralities are stacked to give a racemic crystal in 2. In compound 3, two adjacent [Sm(H3L4-Me)(ac)]2+ species with the opposite chiralities are doubly bridged by two intermolecular imidazole···acetate hydrogen bonds to form a heterochiral dimer and the adjacent dimers are connected by ClO4− ion through the hydrogen bonds to form a racemic crystal.

Helical Complexes of Chiral Quaterpyridines – Mononuclear CuII and Dinuclear CuI Complexes

AbstractTwo series of chiral helical copper–quaterpyridine complexes were prepared by reacting chiral ligands L1–4 with CuI and CuII precursors. The CuII complexes, [Cu(L)(H2O)(ClO4)2] or [Cu(L)(ClO4)2], were studied by ESI‐MS, elemental analysis, and CD spectroscopy, and the CuI complexes, [Cu2(L)2](PF6)2, were examined by NMR, ESI‐MS, elemental analysis, and CD spectroscopy. The crystal structures of [Cu(L1)(H2O)(ClO4)](ClO4) and [Cu(L3)(ClO4)2] were determined by X‐ray diffraction. They are monomeric, distorted octahedral complexes with the quaterpyridine ligand coordinated at the four equatorial positions and one or two perchlorates at the apical position. The crystal structure of [Cu(L1m)(H2O)(ClO4)](ClO4), which has an achiral quaterpyridine ligand L1m, the meso form of L1, was also determined. Comparison between [Cu(L1)(H2O)(ClO4)](ClO4) and [Cu(L1m)(H2O)(ClO4)](ClO4) shows small differences between the two structures. CD analyses of mononuclear CuII complexes of sterically demanding L2–4 indicate that these structures require a helical twist in the ligand. The crystal structure of [Cu2(L3)2](PF6)2 shows that it is a two‐metal two‐ligand double‐stranded helical structure. Solution NMR studies of the dinuclear CuI complexes show that each sample consists of two diastereomers with moderate to high diastereoselectivity (52–99 %). With L1m, head‐to‐head and head‐to‐tail configurational isomers were formed in an almost 1:1 ratio. Heterostranded helicates could also be formed with two different chiral ligands but selectivity favors the homostranded one. DFT calculations on the homostranded helicates yield evidence that interactions between the chiral groups of the ligand strands in the helicate are not solely responsible for determining the helical chirality of the CuI complexes.

Chiral information harvesting in dendritic metallopeptides

Long-range communication of stereochemical information is common in biological systems, particularly in enzyme-catalysed reactions. Here, we report the remote control of the dynamic chirality of metal centres, coordinated by 2,2'-bipyridine ligands bearing dynamic helical oligopeptides. The helical chirality of the oligopeptides is controlled by a stereocentre remote from the metal. We show that when a mixture of chiral and achiral peptide ligands is used, both the chirality of the metal centre and that of the achiral oligopeptide helices are significantly amplified. The amplification mechanism relies on several steps of chirality induction, first from a single chiral peptide to the helicity of an oligopeptide through the induction of propeller chirality at the metal centre, then on to induction of helical chirality in an achiral oligopeptide.