A readily available chiral Ag-based N-heterocyclic carbene complex for use in efficient and highly enantioselective Ru-catalyzed olefin metathesis and Cu-catalyzed allylic alkylation reactions

A new chiral bidentate N-heterocyclic carbene (NHC) ligand has been designed and synthesized. The NHC ligand bears a chiral diamine backbone and an achiral biphenol group; upon metal complexation (derived from Ag(I), Ru(II), or Cu(II)), the diamine moiety induces >98% diastereoselectivity such that the biaryl unit coordinates to the metal center to afford the desired complex as a single atropisomer. Because the ligand does not require optically pure biaryl amino alcohols, its synthesis is significantly shorter and simpler than the related first generation ligands bearing a chiral binaphthyl-based amino alcohol. The chiral NHC ligand can be used in the preparation of highly effective Ru- and Cu-based complexes (prepared and used in situ from the Ag(I) carbene) that promote enantioselective olefin metathesis and allylic alkylations with scope that is improved from previously reported protocols. In many cases, transformations promoted by the chiral NHC-based complexes proceed with higher enantioselectivity and reactivity than was observed with previously reported complexes.     

Helical and network coordination polymers based on a novel C2-symmetric ligand : SHG enhancement through specific metal coordination

Cu(I) and Ag(I) coordination polymers of an axially chiral "push-pull" ligand possess respectively 2-D network and helical structures and the coordination mode strongly influences the solid state SHG reduction/enhancement with respect to the free ligand.     

Production of chiral matrices with hybrid silver–thiocholesterol nanosystems and study of their properties

A new type of chiral matrix based on silver–thiocholesterol hybrid nanosystems adsorbed on silica gel has been proposed. The molar ratio of stabilized thiocholesterol (L) ligand and silver (Ag) was found to have little effect on the size of the resulting silver nanoparticles (SNPs). The average diameter of SNPs was 2.7 ± 0.4, 2.2 ± 0.4, and 2.1 ± 0.6 nm upon the ratios Ag: L = 1: 5, Ag: L = 1: 2, and Ag: L = 1: 0.5, respectively. The resulting chiral matrices possess enantioselectivity relative to the 1,1’-binaphthyl-2,2’-diamine (BNDA) and trifluoroanthranyl ethanol (TFAE) optical isomers. The TFAE optical isomers were successfully separated using thin layer chromatography (α = 1.56).     

Coordination polymers from the self-assembly of silver(I) salts and two nonlinear aliphatic dinitrile ligands, cis-1,3-cyclopentanedicarbonitrile and cis-1,3-bis(cyanomethyl)cyclopentane: synthesis, structures, and photoluminescent properties

Six coordination polymers with aliphatic dinitrile ligands, {[Ag(cpdcn)2]ClO4}n (6a), {[Ag(cpdcn)2]PF6}n (6b), {[Ag(cpdcn)2]SbF6}n (6c, cpdcn = cis-1,3-cyclopentanedicarbonitrile), {[Ag(bcmcp)2] ClO4}n (7a), {[Ag(bcmcp)2]PF6}n (7b), {[Ag(bcmcp)2]SbF6}n, (7c, bcmcp = cis-1,3-bis(cyanomethyl)cyclopentane) have been synthesized and structurally characterized by IR spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and X-ray crystallography. Both ligands used in this study are meso-compounds; while the ligand cpdcn is structurally rigid, the ligand bcmcp has greater conformational flexibility. X-ray crystallography has revealed that structures 6a-c consist of chiral 1D-polymers. The structure of complexes 7a and 7b are best described as a 2D chiral (4,4) square mesh with 3-fold parallel interpenetration. Surprisingly, complex 7c was characterized to be an achiral 1D coordination polymer. The synthesis of the ligands, IR spectra of the free and coordinated CN groups, DSC and TGA, and the photoluminescent properties of complexes 6a-c and 7a-c are also discussed.     

Self-assembled polymers of silver(I) with a chiral diphosphine ligand

Silver(I) salts, AgX, form self-assembled polymers with the chiral diphosphine ligand R,R-trans-C(6)H(10)(NHCO-2-C(6)H(4)PPh(2))(2), 1, of formula {Ag(2)X(2)(μ-1)}(n) and ring-opening polymerization of the trans chelate complex [Ag(1)]X has given the helical polymer [{Ag(μ-1)}(n)]X(n), when X = CF(3)SO(3).     

Chiral molecular assemblies from a novel achiral amphiphilic 2-(heptadecyl) naphtha[2,3]imidazole through interfacial coordination

A novel amphiphilic compound 2-(heptadecyl) naphtha[2,3]imidazole (NpImC17) was synthesized, and its coordination with AgNO(3) in situ in the monolayer at the air/water interface and ex situ in the Langmuir-Blodgett (LB) film on solid substrate has been investigated. It has been found that interfacial coordination between NpImC17 and Ag(I) ion occurred both in the monolayer and in the LB film. It is interesting to note that the Ag(I)-coordinated ultrathin film became chiral although the ligand itself is achiral. It was suggested that the chirality of the Ag(I)-coordinated LB film was developed due to the formation of a helical coordination polymer through the interfacial coordination. To the best of our knowledge, this is the first report on the formation of chiral monolayer and LB films from an achiral molecule through interfacial coordination.     

Novel chiral N4S2- and N6S3-donor macrocyclic ligands: synthesis,protonation constants,metal-ion binding and asymmetric catalysis in the Henry reaction

New hydrophobic chiral macrocyclic ligands L1-L3 with chiral diamino and thiophene moieties have been synthesized by the Schiff base condensation approach. Protonation constants of L1 and L2 were determined by potentiometry titration. Metal-ion binding experiments exhibited that L1 and L3 are pronounced in selective recognition, Ag+, Cu2+ and Ca2+ ions among the surveyed metal ions (Cu2+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+, Ag+, Li+, Na+, K+, and Ca2+). L1 was found to spectroscopically detect the presence of Cu2+ and Ca2+ to function as a multiple readout sensor. The detection limit for Ca2+ ions was found to be 9.8 x 10(-5) M in CH2Cl2-MeOH solution. The trimeric chiral ligand L3 has been shown to be an efficient auxiliary in a Zn(II)-mediated enantioselective Henry reaction.     

Sulfoxides as stereochemical controllers in intermolecular Heck reactions

The study of a variety of substituted sulfoxides as chiral auxiliaries in intermolecular Heck reactions of sulfinyldihydrofurans and sulfinylcyclopentenes with different iodoarenes is reported. In the presence of [Pd(OAc)2]/Ag2CO3 and a bidentate phosphine ligand, synthetically useful yields and asymmetric inductions were obtained. By far the best diastereoselectivities were obtained by the use of the palladium-coordinating O-(N,N-dimethylamino)phenylsulfinyl group. By final removal of the chiral auxiliary, these sulfoxide-stereocontrolled asymmetric Heck processes were applied to the enantioselective synthesis of 1-aryl-substituted and 1,3-diaryl-substituted dihydrofurans and cyclopentenes.     

Coordination chemistry of tetradentate N-donor ligands containing two pyrazolyl-pyridine units separated by a 1,8-naphthyl spacer: dodecanuclear and tetranuclear coordination cages and cyclic helicates

The tetradentate ligand L(naph) contains two N-donor bidentate pyrazolyl-pyridine units connected to a 1,8-naphthyl core via methylene spacers; L45 and L56 are chiral ligands with a structure similar to that of L(naph) but bearing pinene groups fused to either C4 and C5 or C5 and C6 of the terminal pyridyl rings. The complexes [Cu(L(naph))](OTf) and [Ag(L(naph))](BF4) have unremarkable mononuclear structures, with Cu(I) being four-coordinate and Ag(I) being two-coordinate with two additional weak interactions (i.e., "2 + 2" coordinate). In contrast, [Cu4(L(naph))4][BF4]4 is a cyclic tetranuclear helicate with a tetrafluoroborate anion in the central cavity, formed by an anion-templating effect; electrospray mass spectrometry (ESMS) spectra show the presence of other cyclic oligomers in solution. The chiral ligands show comparable behavior, with [Cu(L45)](BF4) and [Ag(L45)](ClO4) having similar mononuclear crystal structures and with the ligands being tetradentate chelates. In contrast, [Ag4(L56)4](BF4)4 is a cyclic tetranuclear helicate in which both diastereomers of the complex are present in the crystal; the two diastereomers have similar gross geometries but are significantly different in detail. Despite their different crystal structures, [Ag(L45)](ClO4) and [Ag4(L56)4](BF4)4 behave similarly in solution according to ESMS studies, with a range of cyclic oligomers (up to Ag9L9) forming. With transition-metal dications Co(II), Cu(II), and Cd(II), L(naph) generates a series of unusual dodecanuclear coordination cages [M12(L(naph))18]X24 (X- = ClO4- or BF4-) in which the 12 metal ions occupy the vertices of a truncated tetrahedron and a bridging ligand spans each of the 18 edges. The central cavity of each cage can accommodate four counterions, and each cage molecule is chiral, with all 12 metal trischelates being homochiral; the crystals are racemic. Extensive aromatic stacking between ligands around the periphery of the cages appears to be a significant factor in their assembly. The chiral analogue L45 forms the simpler tetranuclear, tetrahedral coordination cage [Zn4(L45)6](ClO4)(8), with one anion in the central cavity; the steric bulk of the pinene chiral auxiliaries prevents the formation of a dodecanuclear cage, although trace amounts of [Zn12(L45)18](ClO4)24 can be detected in solution by ESMS. Formation of [Zn4(L45)6](ClO4)8 is diastereoselective, with the chirality of the pinene groups controlling the chirality of the tetranuclear cage.     

Circular dichroism and UV-Vis absorption spectroscopic monitoring of production of chiral silver nanoparticles templated by guanosine 5'-monophosphate

Herein we report the chemical reduction of silver ions incorporated into chiral supramolecular nanostructures by NaBH(4) in buffered (basic) and unbuffered conditions. In situ self-assembly of guanosine 5'-monophosphate (5'-GMP) templated by Ag(I) and generation of silver nanoparticles (NPs) were continuously monitored by CD and UV-Vis spectroscopy measurements. 5'-GMP has been identified as an efficient chiral organic ligand to complex silver ions into a hierarchical helical nanostructure and is a useful capping agent for stabilizing silver NPs with a size diameter lower than 20 nm. The observation of opposite signed bands in the CD spectra of Ag(I)/5'-GMP complexes at different pH has suggested the existence of opposite-handed supramolecular helical structures depending on pH. Both helical supramolecular structures induce chirality in the silver NPs during their growth of the same handedness as shown by the CD signals in the plasmon resonance band.     

Syntheses, structures and antimicrobial activities of water-soluble silver(I)-oxygen bonding complexes with chiral and racemic camphanic acid (Hca) ligands

Water-soluble, relatively light-stable, chiral and achiral silver(I) complexes [[Ag(2)(ca)(2)]](n) and [[Ag(2)(ca)(2)(Hca)(2)]](n)(R- and S-Hca =(1R,4S)- and (1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylic acid, respectively) prepared from the reaction of Ag(2)O with chiral and racemic Hca in 1:2 and 1:4 molar ratios were characterized by elemental analysis, TG/DTA, FTIR, and solution ((1)H, (13)C and (109)Ag) and solid-state ((13)C) NMR spectroscopy. Crystallography revealed that unique 2(1) helical polymer and zigzag structures were formed on self-assembly of the dimeric units in the crystals of [[Ag(2)(S-ca)(2)]](n) and three [[Ag(2)(ca)(2)(Hca)(2)]](n). In the crystal of [[Ag(2)(S-ca)(2)]](n) two 2(1) helices and a loop were observed in the stair-like polymer structure, whereas zigzag and a loop were seen in the crystals of three [[Ag(2)(ca)(2)(Hca)(2)]](n). Carbon NMR spectra in the solid state and in D(2)O indicated that these polymeric structures were loosely bound and fast ligand-exchange reactions took place in aqueous solution. The complexes, [[Ag(2)(ca)(2)]](n) and [[Ag(2)(ca)(2)(Hca)(2)]](n), showed a wide spectrum of effective antimicrobial activity as anticipated for weak silver(i)-O bonding complexes. Similar antimicrobial activity of [[Ag(2)(ca)(2)]](n) and [[Ag(2)(ca)(2)(Hca)(2)]](n) against selected microorganisms suggested that ligand exchangeability played an important role as well as the coordination geometry of the silver(i) ion.     

Enantioselective Ag(I)-catalyzed [3+2] cycloaddition of azomethine ylides using a chiral ferrocene-based phosphine–phosphoramidite ligand having a stereogenic P-center

A series of chiral ferrocene-based phosphine–phosphoramidite ligands (PPFAPhos) have been employed in the Ag(I)-catalyzed asymmetric [3+2] cycloaddition of azomethine ylides with dimethyl maleate. The results showed that the ligand with a stereogenic P-center in the phosphino moiety displayed the best diastereo- and enantioselectivities, in which up to 99% enantiomeric excesses and 99/1 endo/exo selectivities have been achieved.     

Chiral Phosphate in Rhodium‐Catalyzed Asymmetric [2+2+2] Cycloaddition: Ligand,Counterion, or Both?

Investigations based on NMR spectroscopy, mass spectrometry, and DFT calculations shed light on the metallic species generated in the rhodium‐catalyzed asymmetric [2+2+2] cycloaddition reaction between diynes and isocyanates with the chiral phosphate TRIP. The catalytic mixture comprising [{Rh(cod)Cl}₂], 1,4‐diphenylphosphinobutane (dppb), and Ag(S)‐TRIP actually gives rise to two species, both having an effect on the stereoselectivity. One is a rhodium(I) complex in which TRIP is a weakly coordinating counterion, whereas the other is a bimetallic Rh/Ag complex in which TRIP is a strongly coordinating X‐type ligand.     

Rapid Conversion of a Au9Ag12 into a AuxAg16-x Nanocluster via Bisphosphine Ligand Engineering

The [Au_xAg_16-x(SAdm)₈(Dppe)₂] nanocluster with aggregation-induced emission (AIE) was synthesized from a non-fluorescent [Au₉Ag₁₂(SAdm)₄(Dppm)₆Cl₆](SbF₆)₃ nanocluster via a ligand-exchange engineering (Dppe=1,2-Bis(diphenylphosphino)ethane, Dppm=Bis(diphenylphosphino)methane, HSAdm=1-Adamantanethiol). The nanocluster has a Au-doped icosahedral Au_xAg_13-x core, capped by two Ag(SR)₃, one Ag(SR)₂ and two Dppe ligands. By changing the achiral Dppe ligand into a chiral dbpb ligand ((2S,3S)-(-)-Bis(diphenylphosphino)butane or (2R,3R)-(+)-2,3-Bis(diphenylphosphino)butane), chiral nanoclusters are obtained. ESI-MS and UV-vis spectroscopy were performed to track the reaction. This work provides guidance for the construction of new clusters by etching clusters with multidentate phosphine ligands.     

Enantioselective Alkynylation of Benzaldehyde Catalyzed by New Chiral Lewis Acid Ligand: Zinc-Amide Complex

The enantioselective alkynylzinc addition to aldehydes is very useful for the synthesis of chiral propargyl alcohols which are important versatile building blocks of many biologically active compounds and natural products1. A series of chiral oxazolidines were conveniently synthesized from amino acids in three steps with good yields. The use of chiral Lewis acid ligand: zinc-amide complex in situ generated from this oxazolidine with alkylzinc as chiral catalysts for the enantioselective alkyny…     

Bio-inspired synthesis of chiral silver nanoparticles in mucin glycoprotein--the natural choice

A method of synthesizing stable chiral Ag nanoparticles inside a natural mucin glycoprotein is demonstrated. The reaction is carried out without the help of an external reducing agent, by utilizing the reducing properties of the host mucin. A chiral spectrum is detected in the visible range, indicating the formation of a new type of chiral Ag nanoparticles-containing biomaterial.     

Chiral anion-mediated asymmetric ring opening of meso-aziridinium and episulfonium ions

Reactions proceeding through cationic intermediates that lack a Lewis or Br?nsted basic site present a challenge for traditional asymmetric catalysis based on chiral metals or organocatalysts. We present an enantioselective ring opening of tetrasubstituted meso-aziridinium ions with alcohol nucleophiles proceeding through a chiral ion pair with a binaphthol-phosphate anion. The reaction is initiated by silver-induced ring closure of beta-chloroamines using the Ag salt of the chiral anion as in situ generated catalyst. Use of insoluble Ag2CO3 as silver source is essential to obtain high enantioselectivity; we believe the chiral phosphate acts as a "chiral anion phase transfer catalyst" to bring silver ion into the organic phase. The chiral anion concept can also be extended to the related asymmetric opening of meso-episulfonium ions generated by protonation of trichloroacetimidates vicinal to sulfides.     

Silver Films with Hierarchical Chirality

Physical fabrication of chiral metallic films usually results in singular or large‐sized chirality, restricting the optical asymmetric responses to long electromagnetic wavelengths. The chiral molecule‐induced formation of silver films prepared chemically on a copper substrate through a redox reaction is presented. Three levels of chirality were identified: primary twisted nanoflakes with atomic crystal lattices, secondary helical stacking of these nanoflakes to form nanoplates, and tertiary micrometer‐sized circinates consisting of chiral arranged nanoplates. The chiral Ag films exhibited multiple plasmonic absorption‐ and scattering‐based optical activities at UV/Vis wavelengths based on their hierarchical chirality. The Ag films showed chiral selectivity for amino acids in catalytic electrochemical reactions, which originated from their primary atomic crystal lattices.     

Stereoselective synthesis of chiral pinene[5,6]bipyridine ligands and their coordination chemistry

The C(2)-symmetric chiral pinene[5,6]bipyridine V (Chart 1) was synthesized according to a procedure published by our group recently (Kolp, B.; Abeln, D.; Stoeckli-Evans, H.; Zelewsky, A. v. Eur. J. Inorg. Chem. 2001, 1207). A series of stereoselectively alkylated derivatives (Va-Vo) (Table 1) was prepared. The solid-state structures of the compounds Vc and Vk were determined by single-crystal X-ray diffraction, where both compounds show a transoid conformation of the bipyridine unit and proved to be alkylated stereoselectively from the sterically less hindered side of the pinene moiety. The X-ray structure of the cobalt complex 4 shows the metal ion to be tetrahedrally coordinated by one chiral bipyridine V and two chloride ligands. If 2 equiv of ligand V was used, 2:1 complexes were obtained with Cu(I), Ag(I), and Co(II) ions.     

Toward quartz and cristobalite: spontaneous resolution, structures, and characterization of chiral silica-mimetic silver(I)-organic materials

An alpha-quartz-mimetic chiral coordination network of [Ag(L1)(CF3SO3)]n (L1=5,5'-bipyrimidine), after treatment with PF6- anions, undergoes a solution-state structural transformation toward [Ag(L1)(PF6)]n with a cristobalite-mimetic chiral structures. This structural transformation is accompanied by substantial enhancement in the fluorescent intensity and in the second-harmonic-generation response. The results also demonstrate an effective design strategy based on the spontaneous resolution route for the preparation of chiral architectures.