Racemic Ligand Research Articles

Lantern-Shaped Structure Induced by Racemic Ligands in Red-Light-Emitting Metal Halide with Near 100 % Quantum Yield and Multiple-Stimulus Response.

Organic-inorganic metal halides (OIMHs) have become a research hotspot in recent years due to their excellent luminescent properties and tunable emission wavelengths. However, the development of efficient red-light-emitting OIMHs remains a significant challenge. This work reports three Mn-based OIMHs derived from 1-methyl-1,2,3,4-tetrahydroisoquinoline hydrobromide: racemic one (Rac-TBM) and chiral ones (R-TBM and S-TBM). As a result of the synergism of chiral organic ligands inducing a unique lantern-shaped hybrid structure containing both tetrahedra and octahedra, Rac-TBM exhibits red-light emission with near-unity luminescence quantum yield. In comparison, the chiral counterparts R/S-TBM display strong green emission and circularly polarized luminescence (CPL) with a glum value up to ±2.5×10-2. Interestingly, a mixture of R- and S-TBM can transform into Rac-TBM, successfully achieving a sensitive and reversible switch between red light of octahedra and green light of tetrahedra under external stimuli. The outstanding luminescent properties allow Rac-TBM to be utilized not only for X-ray radioluminescence with a detection limit down to 46.29 nGys-1, but also for advanced information encryption systems to achieve leak-proof decryption.

Lantern‐Shaped Structure Induced by Racemic Ligands in Red‐Light‐Emitting Metal Halide with Near 100 % Quantum Yield and Multiple‐Stimulus Response

AbstractOrganic‐inorganic metal halides (OIMHs) have become a research hotspot in recent years due to their excellent luminescent properties and tunable emission wavelengths. However, the development of efficient red‐light‐emitting OIMHs remains a significant challenge. This work reports three Mn‐based OIMHs derived from 1‐methyl‐1,2,3,4‐tetrahydroisoquinoline hydrobromide: racemic one (Rac‐TBM) and chiral ones (R‐TBM and S‐TBM). As a result of the synergism of chiral organic ligands inducing a unique lantern‐shaped hybrid structure containing both tetrahedra and octahedra, Rac‐TBM exhibits red‐light emission with near‐unity luminescence quantum yield. In comparison, the chiral counterparts R/S‐TBM display strong green emission and circularly polarized luminescence (CPL) with a glum value up to ±2.5×10−2. Interestingly, a mixture of R‐ and S‐TBM can transform into Rac‐TBM, successfully achieving a sensitive and reversible switch between red light of octahedra and green light of tetrahedra under external stimuli. The outstanding luminescent properties allow Rac‐TBM to be utilized not only for X‐ray radioluminescence with a detection limit down to 46.29 nGys−1, but also for advanced information encryption systems to achieve leak‐proof decryption.

Relay PHOS: Ligands with Fluxional Chirality in Asymmetric Palladium-Catalyzed Allylic Alkylation.

A modular route toward the synthesis of P,N ligands containing a fluxional group along the pyrazoline ring core is described. The racemic ligands were accessed in three steps from commercially available fluoroacetophenone in overall yields ranging from 18 to 76%. The enantiopure ligands were obtained using semi-preparative chiral high-performance liquid chromatography and chiral enantioselective phase-transfer catalysis. The effectiveness of the new ligands was assessed in palladium-catalyzed allylic alkylation with diphenylpropenyl acetate and dimethylpropenyl acetate. Under optimized conditions, diphenylpropenyl acetate underwent alkylation with dimethyl malonate in 98% yield and 94% ee. In general, the enantioselectivity for the product correlates with the size of the ligand fluxional group; the larger the fluxional group, the higher the selectivity.

Anion-induced chiral assembly: construction of Ag(i) coordination polymers for photocatalytic degradation of organic dyes

Assembly of a racemic ligand with different Ag(i) salts leads to homo/heterochiral polymers. The photocatalytic experiments indicate CPs 4–8 are active for the degradation of rhodamine B and methylene blue under UV irradiation.

Structure Identification of Dynamic Zinc(II)-ligand Assemblies in Solution Based on Ligand Chirality

Abstract This study demonstrates that pendant chirality on building ligands functions as a valuable indicator for identifying metal-ligand dynamic assemblies in solution: Racemic ligands (L(R) and L(S)) with zinc ions (Zn2+) give (Zn2+)2(L(R))n(L(S))(4−n) (n = 0–4) assemblies with statistical L(R) and L(S) positioning, which can be analyzed by 1H NMR spectroscopy with a statistical model.

Five Porous Complexes Constructed from a Racemic Ligand: Synthesis, Chiral Self-Assembly, Iodine Adsorption, and Desorption Properties.

Herein, a chiral bispyridyl ligand (L) was designed and synthesized using a Schiff base condensation reaction, followed by a 1,3-H shift. Five complexes, [Zn2L2(OAc)4] (1), {[CdLCl2(DMF)]·4H2O}n (2), [Cd2L2I4]·4H2O (3), {[CdL2(H2O)2](NO3)2·2CH3OH}n (4), and [Hg2L2Cl4]·2DMF (5), were synthesized and characterized upon its reaction with Zn(II), Cd(II), or Hg(II) ions, respectively. X-ray crystallography shows that the organic compound exists as a racemic ligand with equal amounts of its R- and S-isomers, and all of the synthesized complexes exhibit heterochiral self-assembly via a chiral self-discrimination process. Complexes 1, 3, and 5 exist as centrosymmetric binuclear metallamacrocycles, while complexes 2 and 4 exist as 1D looped-chain coordination polymers. Inspired by the assembled structures of the five complexes, I2 adsorption/desorption measurements for the complexes were carried out. The results show that complexes 1 and 5 exhibit good adsorption capacities toward I2 in n-hexane and in water, respectively.

Zinc Complexes Bearing Dinucleating Bis(imino‐ pyridine)binaphthol Ligands: Highly Active and Robust Catalysts for the Lactide Polymerization

AbstractWe report the synthesis of new hexadentate ligands bearing imine and pyridine nitrogen neutral donors with a binaphthol backbone. Such ligands have been tailored to have two coordinative pockets thus promoting the formation of bimetallic complexes. By direct reaction of the enantiomerically pure and the racemic ligand with two equivalents of zinc amide precursor, the bimetallic complexes S‐1 and rac‐1 were respectively obtained. Both complexes were highly active in the polymerization of lactides under several reaction conditions, preserving exceptional activities even when working in technical grade melted lactide, thus showing the robustness of the catalysts. Interestingly, complex S‐1 allows the production of either linear or purely macrocyclic PLAs by back‐biting reactions, modifying the reaction conditions. Mechanistic studies have been carried out in NMR‐scale solution to get information on the active species of the polymerization process.

Optically-pure triptycene-based metallomacrocycles and homochiral self-sorting assisted by ladder formation.

Optically-pure triptycene-based metallomacrocycles are, for the first time, synthesized by the coordination-driven self-assembly of enantiopure triptycene-derived ladder-type bis(benzo[f]isoquinoline) ligands with a cis-platinum(II) complex. A pair of enantiomeric homochiral metallomacrocycles is also produced by the coordination-driven homochiral self-sorting of the corresponding racemic ligands, which relies on the shape-persistent nature of the ladder-structured ligands.

Coordination-Driven Self-Assembly of Complexes Constructed from Two Helical Ligands: Synthesis, Structures, and Selective Gas Adsorption Properties.

Two helical ligands (L1 and L2) were designed and synthesized by a Schiff base condensation reaction. Eight complexes, {[Zn(L1)I2]·H2O}n (1), [Cd2(L1)2I4(CH3OH)2] (2), [Hg2(L1)2I4] (3), [Ag(L1)NO3]n (4), [Ag2(L1)2(NO3)2DMSO]·H2O (5), {[Zn2(L2)2Cl4]·2CHCl3}n (6), {[Ag(L2)]·NO3}n (7), and {[Ag(L2)NO3]·CH3OH}n (8), were synthesized and characterized based on these two ligands. The crystal structures show that both Schiff base compounds exist as racemic ligands with equal amounts of P- and M-helicity, and the assembly of these racemic ligands with metal ions can lead to homochiral or heterochiral complexes via a chiral self-recognition or self-discrimination process. Complexes 2, 3, and 5 exist as heterochiral metallomacrocycles with a figure-eight conformation. Complexes 1, 6, and 8 exist as one-dimensional (1D) homochiral helical chain coordination polymers, while complexes 4 and 7 exist as 1D heterochiral helical chain coordination polymers. Furthermore, gas and vapor adsorption measurements show that all of the synthesized complexes exhibit good selective adsorption capacities toward methanol and ethanol vapor over N2, H2, and O2.

Synthesis, crystal structures, electrochemical and gas adsorption properties of two 1D silver(I) coordination polymers constructed from racemic helical ligands

Two helical ligands (L1 and L2) were designed and synthesized by Schiff base condensation reaction. Two Ag(I) coordination polymers (CPs), namely, {[Ag2(L1)2(NO3)2]·3H2O}n (1) and [AgL2NO3]n (2) were synthesized and characterized via elemental analysis, FT-IR spectra and single-crystal X-ray diffraction techniques. The crystal structures show that both Schiff base compounds exist as racemic ligands with equal amounts of P- and M-helicity. CP 1 exists as a 1D homochiral helical chain polymer via self-recognition process, and CP 2 exists as a 1D looped chain polymer. The cyclic voltammetric properties of the two CPs were investigated. Furthermore, gas and vapor adsorption measurements show that both CPs exhibit good adsorption capacity toward CH3OH over CO2 and N2.

Toward Coordination Cages with Hybrid Chirality: Amino Acid-Induced Chirality on Metal Centers.

Tripodal chiral ligands containing amino acid residues and salicyl-acylhydrazone units were synthesized and used to obtain coordination cages through deprotonation and coordination to gallium. These coordination cages have Ga3L2 stoichiometry and pinwheel geometry with two types of chiral centers built into their walls: stereogenic centers at the amino acid backbones and stereoselectively induced centers at metal ions. The pinwheel geometry is unique among analogous cages and originates from the partial flexibility of the ligands. Despite the flexibility, the ligands induce the chirality of metal centers in a highly stereoselective way, leading to the formation of cages that are single diastereoisomers. It has also been demonstrated that stereoselectivity is a unique feature of cage geometry and leads to effective chiral self-sorting: homochiral cages can be obtained selectively from the mixtures of racemic ligands. The configuration of metal centers was determined by circular dichroism, TD DFT calculation, and X-ray crystallography.

Synthesis, crystal structures and antibacterial properties of four complexes derived from mono- or diquinoline-substituted acylhydrazone ligands

Two quinoline-substituted acylhydrazone ligands (L1 and L2) were synthesized from the reaction of 2-aminobenzoylhydrazide and quinoline-8-carboxaldehyde in a molar ratio of 1:1 or 1:2 respectively. Four complexes, namely, [Cd(L1)Cl2] (1), [Pd(L1-)Cl] (2), [Ag2(L2)2(NO3)2]‧2CH3OH (3) and [Zn(L2)Cl2]·H2O (4) were obtained by the solvent evaporation method. The four complexes were determined structurally by single-crystal X-ray diffraction and characterized by infrared spectroscopy, element analysis and powder X-ray diffraction. The crystal structures reveal that both of the two acylhydrazone compounds act as tridentate ligands in the complexes. Besides, chiral self-assembly process occurs in the reaction of the racemic ligand L2 with Ag(I) or Zn (II) ions. Furthermore, thermal stability and antibacterial properties of the complexes have been investigated as well.

SYNTHESIS, STRUCTURE, AND PHOTOLUMINESCENT PROPERTIES OF LANTHANIDE(III) COMPLEXES WITH A LIGAND BASED ON 1,10-PHENANTHROLINE AND BORNEOL

Chiral complexes [Ln((–)-L)2(NO3)3] (Ln = Sm (1), Eu (2), Tb (3)) with an optically active ligand (–)-L containing 1,10-phenanthroline and (–)-borneol moieties and also [Ln((±)-L)2(NO3)3]·1.5MeCN (Ln = Sm (4), Eu (5), Tb (6)) compounds with a racemic ligand (±)-L based on 1,10-phenanthroline and (+)-, (–)-borneol are synthesized. According to single crystal X-ray diffraction data, mononuclear complexes 1-3 are isostructural and crystallize in the non-centrosymmetric space group P43212. The crystal structures of isostructural complexes 4-6 crystallizing in the centrosymmetric space group C2/c contains (+)- and (–)-enantiomeric molecules of the mononuclear complex and MeCN molecules. In complexes 1-6, the (–)-L, (±)-L, and $$\text{NO}_{3}^{-}$$ ligands perform the bidentate chelate function (N4O6 coordination polyhedron of the Ln atom). Complexes 2 and 3 have high quantum yields (φ = 52% and 41%) and long-life luminescence (τ = 1.4 ms and 1.2 ms) in the solid state at 300 K. The luminescence quantum yields of chiral complexes 1-3 are found to several times exceed the quantum yields of racemic complexes 4-6.

Possible Physical Basis of Mirror Symmetry Effect in Racemic Mixtures of Enantiomers: From Wallach’s Rule, Nonlinear Effects, B–Z DNA Transition, and Similar Phenomena to Mirror Symmetry Effects of Chiral Objects

Effects associated with mirror symmetry may be underlying for a number of phenomena in chemistry and physics. Increase in the density and melting point of the 50%L/50%D collection of enantiomers of a different sign (Wallach’s rule) is probably based on a physical effect of the mirror image. The catalytic activity of metal complexes with racemic ligands differs from the corresponding complexes with enantiomers as well (nonlinear effect). A similar difference in the physical properties of enantiomers and racemate underlies L/D inversion points of linear helical macromolecules, helical nanocrystals of magnetite and boron nitride etc., B–Z DNA transition and phenomenon of mirror neurons may have a similar nature. Here we propose an explanation of the Wallach effect along with some similar chemical, physical, and biological phenomena related to mirror image.

Impact of Enantiomeric Ligand Composition on the Photophysical Properties of Chiral Ag29 Nanoclusters

Abstract Chiral surface ligands have often been employed to impart optical activity to metal nanoclusters, metal nanoparticles and semiconductor nanocrystals. They are considered to form a chiral structure to a certain degree whereas the effect of such chiral structure on the global physicochemical properties apart from chiroptical ones has been unexplored. We herein demonstrate the impact of optical purity of chiral surface ligand on the emission property of silver nanocluster (NCs). Chiral bidentate α-dihydrolipoic acid (DHLA) with varied enantiomeric excess (ee) values was employed as a surface capping ligand to prepare a series of silver NCs, displaying identical absorption and emission profiles typical for Ag29 NCs. Interestingly, the emission quantum yields exhibited a clear dependence on the enantiopurity of DHLA. The more enantiopure DHLA afforded more emissive NCs. This |ee|-dependent emission efficiency was discussed in association with the orientation of ligands on the Ag29 NCs. The surface structures of Ag29(dithiolate)12 models composed of enantiopure and racemic ligand systems were compared with the aid of DFT calculations, suggesting that the enantiopure surface is more stable with one-handed ligand orientation. Two-dimensional NMR technique also supported the observation that well-defined ligand orientations depend on the enantiomeric composition of chiral ligand.

Ni-Catalyzed Enantioconvergent Coupling of Epoxides with Alkenylboronic Acids: Construction of Oxindoles Bearing Quaternary Carbons

We have developed a nickel- nickel/bisphosphine-catalyzed stereoconvergent cross-coupling reaction of epoxides with alkenylboronic acids. Racemic spiroepoxyoxindoles were converted to chiral homoal...

Heterochiral or Homochiral Self‐Assembly: Mercury(II), Cadmium(II), and Spontaneous Resolution of Silver(I) Complexes Derived from a Racemic Bis(pyridyl) Ligand

A novel racemic bis(pyridyl) ligand (rac‐L) was designed and synthesized by Schiff condensation reaction. Five complexes, [Hg2(LR)(LS)Cl4]·2CH3CH2OH (1), [Cd(LR)(LS)Cl2]n (2), {[Cd(LR)(LS)SO4(H2O)]·11H2O}n (3), {[Ag(LR)2]NO3·CH3CN}n (4a) and {[Ag(LS)2]NO3·CH3CN}n (4b) have been synthesized and characterized. The different geometric features of complexes 1–4 were determined by single‐crystal X‐ray diffraction. The crystal structures show that the self‐assembly with racemic ligands can lead to homochiral or heterochiral complexes, through self‐recognition or self‐discrimination of the ligand units. Complex 1 exists as a 26‐membered binuclear heterochiral macrocycle. Complexes 2 and 3 form 1D mesomeric looped chain coordination polymers with different heterochiral self‐assembly types. More interestingly, two novel 2D homochiral coordination polymers 4a and 4b were obtained as racemic conglomerates via spontaneous resolution. Furthermore, the thermal stabilities and luminescence properties of complexes 1–4 were investigated.

Monomeric and dimeric nitrate lanthanide(III) and yttrium(III) coordination compounds of (2 + 2) imine macrocycle derived from 2,6-diformylpyridine and trans-1,2-diaminocyclopentane

Monomeric nitrate coordination compounds [Ln(L1)](NO3)3·nH2O (Ln = Y(III), Tm(III), Yb(III) and Lu(III)); n = 0, 0.75 or 3)11For simplicity of description we included the Y(III) in the abbreviation of Ln(III), commonly reserved for lanthanide(III) ions. It is well known that although yttrium does not belong to the lanthanides, the ionic radius of the Y(III) cation is similar to ionic radii of lanthanide(III) cations. Y(III) coordination compounds resemble very much in their chemical properties those of the respective lanthanide(III) coordination compounds. of the chiral macrocyclic (2 + 2) imine L1 (L1RRRR, L1SSSS and/or L1rac), derived from enantiomerically pure or racemic trans-1,2-diaminocyclopentane (DACP) and 2,6-diformylpyridine (DFP), have been synthesized in the templated condensation of precursors in the presence of appropriate metal salts. From these monomers their homochiral μ-hydroxodimers [Ln2(L1)2(μ-OH)2](NO3)4·nH2O (Ln = Y(III), Tm(III), Yb(III) and Lu(III)); n = 4, 5 or 7) have been obtained and isolated by addition of NaOH to the respective monomeric form of coordination compound solution. All synthesized coordination compounds have been characterized by NMR spectroscopy, mass spectrometry, elemental analyses and/or circular dichroism (CD). 1H NMR signals of CDCl3/CD3OD solutions of the diamagnetic Y(III) and Lu(III) monomers have been assigned on the basis of their COSY and HMQC spectra, and for the remaining paramagnetic lanthanide coordination compounds (Tm(III) and Yb(III)) the signals were tentatively assigned on the basis of linewidths analyses. 1H NMR signals of D2O solutions of diamagnetic Y(III) and Lu(III) representatives of dimeric species have been established taking into account their COSY, HMQC, HMBC and NOESY spectra. The NOESY spectra confirm that the structure of the dimmers observed in solids is also maintained in a solution.The axial ligand exchange in all monomeric coordination compounds was investigated by 1H NMR titration experiments with acetate and chloride anions in organic solvents as well as with hydroxide anion in D2O. The formation of homochiral heterobimetallic μ-hydroxodimers [Ln’Ln”(L1)2(μ-OH)2](NO3)4 was investigated in a solution by mixing the solutions of two different [Ln’2(L1)2(μ-OH)2](NO3)4·nH2O and [Ln”2(L1)2(μ-OH)2](NO3)4·nH2O homochiral homobimetallic precursors. This experiment proves the chiral recognition between two macrocyclic units of the same chirality in a solution. The μ-hydroxodimers may convert with time to the corresponding peroxo-dimers of the general formula [Ln2(L1)2(μ-η2: η2-O2)](NO3)4.The X-ray crystal structures of the representative monomeric and dimeric coordination compounds [Yb(L1RRRR)(NO3)2]NO3, [Yb2(L1SSSS)2(OH)2(H2O)2](NO3)4·2MeOH·2H2O, [Yb2(L1RRRR)2(OH)2(H2O)2][Yb2(L1SSSS)2(OH)2(H2O)2](NO3)8·1.5MeOH·5H2O and [Y2(L1SSSS)2(O2)(NO3)2](NO3)2·2MeOH with enantiomerically pure or racemic ligand L1 have been determined. The structure of [Yb2(L1RRRR)2(OH)2(H2O)2][Yb2(L1SSSS)2(OH)2(H2O)2] (NO3)8·1.5MeOH·5H2O crystal confirms chiral recognition of macrocyclic units of the same chirality in solids.

Chirality Control in Crystalline Ni(II) Complexes of Thiophosphorylated Thioureas

Chirality control over the formation of Ni(II) complexes with chiral thiophosphorylated thioureas was achieved via breaking the symmetry of nickel coordination geometry by the introduction of the pyridine ligand, while centrosymmetric meso-complexes are formed from racemic ligands in case of square-planar nickel coordination. Centrosymmetric heterochiral arrangement is observed in crystals of ligands themselves through N–H⋅⋅⋅S hydrogen bonds in intermolecular dimers. Molecular homochirality in tetragonal pyramidal complexes is further transferred to supramolecular homochiral arrangement via key–lock steric interactions.

Heterometallic Copper\u2013Vanadium Compounds: Crystal Structures of Polymers [Cu(im)4(V2O4(mand)2)]n and [Cu(im)4(V2O4((S)-mand)2)]n\xb72nH2O (im\u2009=\u2009imidazole, mand\u2009=\u2009mandelato2\u2212)

Two new 1D polymeric heterometallic copper–vanadium compounds were prepared. The polymers are constructed from [Cu(im)4]2+ cations that are coordinated to two terminal oxido ligands of [V2O4(mand)2]2− anions. The stronger coordination in [Cu(im)4V2O4(mand)2]n (1) that contains the racemic mandelato ligand is manifested by a shorter Cu‒O bond distance 2.4095(12) Å, while the weaker interaction in [Cu(im)4(V2O4((S)-mand)2)]n·2nH2O (2) is exhibited by Cu‒O bond distances 2.4547(16) Å and 2.5413(16) Å. The vanadate anion in compound 2 carries only the (S)-enantiomer of the initial mandelic acid and differs from the anion in 1 in parallel cis orientation of the phenyl groups of the mandelato ligand. FT-IR spectroscopy was used for the confirmation of the coordination mode of mandelato ligand. Strong bands corresponding to the vibrations of carboxyl groups can be observed around 1650 and at 1344 cm−1. The stretching vibration of deprotonated hydroxyl group in the mandelato ligand occurs at 1045 and 1065 cm−1 for 1 and 2, respectively. In addition, the very strong, characteristic band corresponding to ν(V=O) vibration can be observed at 931 cm−1 for 1 and 925 cm−1 for 2, as well as in Raman spectrum.Graphic The polymeric structures of two new vanadium-copper heterometallic complexes are constructed from [Cu(imidazole)4]2+ cations that are coordinated to two terminal oxido ligands of [V2O4(mandelato)2]2− anions with different orientation of the phenyl groups depending on the chirality of the mandelato ligand.