Solid State and Solution Characterization of Chiral, Conformationally Mobile Tripodal Ligands

Enantioselective and Chemoselective Optical Detection of Chiral Organic Compounds without Resorting to Chromatography

Enantiorecognition and resolution are of essential importance in many diverse areas of science. Whenever there arises a need to analyze/investigate enantiomers in different situations chromatography stands up in our minds immediately. Nevertheless, chemoselective and enantioselective recognition/discrimination (without going for separation) constitutes a different perception and requirement. The techniques using chiroptical sensing cause detection based on molecular interactions induced in different manners. Enantioselective sensing of monosaccharides in γ-cyclodextrin assembly and by diboronic acid based fluorescent sensors, application of bi-naphthol and H8 BINOL based sensors and dendrimers, metal-to-ligand charge transfer transitions in CD, exciton-coupled circular dichroism, surface enhanced Raman spectroscopy, and enantioselective indicator displacement sensor arrays for enantioselective recognition/detection of chiral organic compounds, such as amines, amino acids/alcohols, and hydroxycarboxylic acids have been discussed in progressive manner with mechanistic explanations, wherever available. Besides, the chiroptical vs LC approach has been discussed. The present paper is focused on certain different non-chromatographic optical techniques and aims to extend an understanding and a view to consider such techniques which have been successful in selective detection, and determination of absolute configuration and enantiomeric excess, (without resorting to separation vis-à-vis LC) and that have potential use in high-throughput chiral assay and combinatorial search for asymmetric catalysts and reagents.

A Quinoline-Appended Cyclodextrin Derivative as a Highly Selective Receptor and Colorimetric Probe for Nucleotides

The design and development of specific recognition and sensing systems for biologically important anionic species has received growing attention in recent years, as they play significant roles in biology, pharmacy, and environmental sciences. Herein, a new supramolecular sensing probe L1 was developed for highly selective differentiation of nucleotides. L1 displayed extremely marked absorption and emission differentiation upon binding with nucleotide homologs of AMP, ADP, and ATP, due to the divergent spatial orientations of guests upon binding, which allowed for a naked-eye colorimetric differentiation for nucleotides. A differentiating mechanism was unambiguously rationalized by using various spectroscopic studies and theoretical calculations. Furthermore, we successfully demonstrated that L1 can be applied to the real-time monitoring of the enzyme-catalyzed phosphorylation/dephosphorylation processes and thus demonstrated an unprecedented visualizable strategy for selectively differentiating the structurally similar nucleotides and real-time monitoring of biological processes via fluorescent and colorimetric changes.

Taking advantage of Co(ii) induced enhanced VCD for the fast and sensitive determination of enantiomeric excess

Large Co(ii)-induced VCD signals provide a method for determining the enantiomeric excess of α-amino acids. This strategy can be followed for building new VCD protocols.

Synthesis and structural analyses of phenylethynyl-substituted tris(2-pyridylmethyl)amines and their copper(ii) complexes

Three new tris(2-pyridylmethyl)amine-based ligands possessing phenylethynyl units have been prepared using Sonogashira couplings and substitution reactions. Copper(ii) complexes of those tetradentate ligands have also been synthesized. Solid-state structures of the six new compounds have been determined by single-crystal X-ray diffraction analyses. Examination of the molecular structures of the ligands revealed the expected triangular geometries with virtually undeformed carbon-carbon triple bonds. While the tertiary nitrogen of the free ligands seem to be prevented from participation in supramolecular non-covalent interactions by the pyridyl hydrogen at the 3-position, the pyridyl nitrogens play a crucial role in the packing mode of the crystal structure. The nitrogens form weak hydrogen bonds, varied in length between 2.32 and 2.66 Å, with the pyridyl hydrogen of its neighbouring molecule. The [NH-C] contacts enforce one-dimensional columnar assemblies on ligands that organize into wall-like structures, which in turn assemble into three-dimensional structures through CH-π interactions. Structural analyses of Cu(ii) complexes of the ligands revealed propeller-like structures caused by steric crowding of three pyridine ligands. The copper complexes of the ligands having three phenylethynyl substituents showed a remarkably deformed carbon-carbon triple bond enforced by a steric effect of the three phenyl groups. Most significantly, a total of seventy non-covalent interactions, classified into twelve types of hydrogen-involving short contacts, were identified in this study. The phenylethynyl substituent participated in forty-two interactions as a hydrogen bond acceptor, and its role was more distinctive in the crystal structures of the Cu(ii) complexes.

Redox-configurable ambidextrous catalysis: structural and mechanistic insight

A helically chiral copper complex is used as a switchable asymmetric catalyst capable of delivering either enantiomer of a Michael addition reaction.

A ligand capable of adopting two pseudo-enantiomeric helically chiral states when bound to copper has been applied as an asymmetric catalyst in the Michael addition of malonate substrates to nitrostyrenes. The absolute configuration of the helically chiral ligand is inverted upon oxidation/reduction of the copper center. In this way, the handedness of the Michael addition product (R/S) can be selected based on the handedness of the catalyst (Λ/Δ). Exciton coupled circular dichroism (ECCD) was used to identify which of the two pseudo-enantiomeric forms the catalyst adopted after reduction/oxidation, with additional support from X-ray crystallographic data. The synthesis of the ligand was achieved in five steps with an overall 61% yield. Enantiomeric excesses of the Michael addition products of up to 72% (S) and 70% (R) were obtained in acetonitrile. The ability to choose the handedness of the product based on the chiral state of the catalyst has been demonstrated with several different solvents, bases, nitrostyrene/malonate substrates, and prochiral malonate substrates. A combination of molecular modelling, crystal structure and kinetic data suggest that one urea moiety of the catalyst ligand likely binds the nitrostyrene substrate while blocking the Re face of the nitrostyrene in the transition state.

Multicomponent One-pot Reactions Towards the Synthesis of Stereoisomers of Dipicolylamine Complexes

Reported are multi-component one-pot syntheses of chiral complexes [M(L(R) OR')Cl2 ] or [M(L(R) SR')Cl2 ] from the mixture of an N-substituted ethylenediamine, pyridine-2-carboxaldehyde, a primary alcohol or thiol and MCl2 utilizing in-situ formed cyclized Schiff bases where a C-O bond, two stereocenters, and three C-N bonds are formed (M=Zn, Cu, Ni, Cd; R=Et, Ph; R'=Me, Et, nPr, nBu). Tridentate ligands L(R) OR' and L(R) SR' comprise two chiral centers and a hemiaminal ether or hemiaminal thioether moiety on the dipicolylamine skeleton. Syn-[Zn(L(Ph) OMe)Cl2 ] precipitates out readily from the reaction mixture as a major product whereas anti-[Zn(L(Ph) OMe)Cl2 ] stays in solution as minor product. Both syn-[Zn(L(Ph) OMe)Cl2 ] and anti-[Zn(L(Ph) OMe)Cl2 ] were characterized using NMR spectroscopy and mass spectrometry. Solid-state structures revealed that syn-[Zn(L(Ph) OMe)Cl2 ] adopted a square pyramidal geometry while anti-[Zn(L(Ph) OMe)Cl2 ] possesses a trigonal bipyramidal geometry around the Zn centers. The scope of this method was shown to be wide by varying the components of the dynamic coordination assembly, and the structures of the complexes isolated were confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. Syn complexes were isolated as major products with Zn(II) and Cu(II) , and anti complexes were found to be major products with Ni(II) and Cd(II) . Hemiaminals and hemiaminal ethers are known to be unstable and are seldom observed as part of cyclic organic compounds or as coordinated ligands assembled around metals. It is now shown, with the support of experimental results, that linear hemiaminal ethers or thioethers can be assembled without the assistance of Lewis acidic metals in the multi-component assembly, and a possible pathway of the formation of hemiaminal ethers has been proposed.

Pyridylalkylamine ligands and their palladium complexes: structure and reactivity revisited by NMR

Pyridylmethylamines or pma are versatile platforms for different catalytic transformations. Five pma-ligands and their respective Pd complexes have been studied by liquid state NMR. By comparing (1)H, (13)C and (15)N chemical shifts for each pma/pma-Pd couple, a general trend for the metallacycle atoms concerns variations of the electronic distribution at the pendant arm, especially at the nitrogen atom of the ligand. Moreover, the increase of the chemical shift of the pendant arm nitrogen atom from primary to tertiary amine is also related to the increase of crowding within the complex. This statement is in good agreement with X-ray data collected for several complexes. Catalytic results for the Suzuki-Miyaura reaction involving the pma-Pd complexes showed within this series that a sterically crowded and electron-rich ligand in the metallacycle was essential to reach the coupling product with a good selectivity. In this context, NMR study of chemical shifts of all active nuclei especially in the metallacycle could give a trend of reactivity in the studied family of pma-Pd complexes.

Cooperative anion recognition in copper(II) and zinc(II) complexes with a ditopic tripodal ligand containing a urea group

The ability of Cu(II) and Zn(II) complexes of the ditopic receptor H2L [1-(2-((bis(pyridin-2-ylmethyl)amino)methyl)phenyl)-3-(3-nitrophenyl)urea] for anion recognition is reported. In the presence of weakly coordinating anions such as ClO4(-), the urea group binds to the metal ion (Cu(II) or Zn(II)) through one of its nitrogen atoms. The study of the interaction of the metal complexes with a variety of anions in DMSO shows that SO4(2-) and Cl(-) bind to the complexes through a cooperative binding involving simultaneous coordination to the metal ion and different hydrogen-bonding interactions with the urea moiety, depending on the shape and size of the anion. On the contrary, single crystal X-ray diffraction studies show that anions such as NO3(-) and PhCO2(-) form 1:2 complexes (metal/anion) where one of the anions coordinates to the metal center and the second one is involved in hydrogen-bonding interaction with the urea group, which is projected away from the metal ion. Spectrophotometric titrations performed for the Cu(II) complex indicate that this system is able to bind a wide range of anions with an affinity sequence: MeCO2(-) ∼ Cl(-) (log K11 > 7) > NO2(-) > H2PO4(-) ∼ Br(-) > HSO4(-) > NO3(-) (log K11 < 2). In contrast to this, the free ligand gives much weaker interactions with these anions. In the presence of basic anions such as MeCO2(-) or F(-), competitive processes associated with the deprotonation of the coordinated N-H group of the urea moiety take place. Thus, N-coordination of the urea unit to the metal ion increases the acidity of one of its N-H groups. DFT calculations performed in DMSO solution are in agreement with both an anion-hydrogen bonding interaction and an anion-metal ion coordination collaborating in the stabilization of the metal salt complexes with tetrahedral anions.

Determination of amino acid enantiopurity and absolute configuration: synergism between configurationally labile metal-based receptors and dynamic covalent interactions

Reliable determination of the enantiomeric excess of free amino acids can be obtained by measuring the induced circular dichroism of a multicomponent assembly formed by a modified tris(2-pyridylmethyl)amine ligand, a zinc salt, and the amino acid of interest. The systems furnish reliable information for all natural amino acids.

Dynamic multicomponent hemiaminal assembly

A simple approach to generating in situ metal-templated tris-(2-picolyl)amine-like multicomponent assemblies with potential applications in molecular recognition and sensing is reported. The assembly is based on the reversible covalent association between di-(2-picolyl)amine and aldehydes. Zinc ion is best for inducing assembly among the metal salts investigated, whereas 2-picolinaldehyde is the best among the heterocyclic aldehydes studied. Although an equilibrium constant of 6.6×10(3) M(-1) was measured for the assembly formed by 2-picolinaldehdye, di-(2-picolyl)amine, and zinc triflate, the equilibrium constants for other systems are in the 10(2) M(-1) range. X-ray structural analysis revealed that zinc adopts trigonal-bipyramidal geometry within the assembled ligand. The diversity and equilibrium of the assemblies are readily altered by simply changing concentrations, varying components, or adding counteranions.

A stereodynamic tripodal ligand with three different coordinating arms: synthesis and zinc(II), copper(I) complexation study

A tetradentate tripodal ligand containing a chiral center and three different coordinating arms was designed and synthesized. Its complexation properties with Zn(II) and Cu(I) were studied by NMR and optical spectroscopy. NMR experiments demonstrated the formation of two diastereomers, indicating the stabilization of the central tertiary amine configuration by metal coordination. The inversion of pyramidalization of the central tertiary amine of the ligand was found to be highly dependent upon metal ion, solvent, and temperature. Dynamic NMR measurements were used to estimate the energy of activation required for nitrogen atom inversion. Finally, absorption and circular dichroism measurements confirmed the expectation that metal complexes of the ligand gave rise to circular dichroism but that such spectra were not characterized by exciton-coupling, in contrast to previously described ligands containing two identical arms with strong chromophores.

Structures, metal ion affinities, and fluorescence properties of soluble derivatives of tris((6-phenyl-2-pyridyl)methyl)amine

Metal complexes of tris((6-phenyl-2-pyridyl)methyl)amine (2) have hydrophobic cavities that potentially accommodate small molecules. However, the utility of this attractive motif has been hampered by the poor solubility of such complexes in many common solvents. In this study, two tripodal ligands (3, tris-[6-(3,4,5-trimethoxy-phenyl)-pyridin-2-ylmethyl]-amine, and 4, tris((6-(3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)pyridin-2-yl)methyl)amine) derived from 2 were prepared with enhanced solubility in organic and aqueous solvents. The X-ray crystallographic analyses of selected ligands and complexes revealed that the hydrophobic cavities inside the zinc complexes were retained after derivatization. Fluorescence, nuclear magnetic resonance (NMR), and potentiometric titration studies, which were enabled by the improved solubility, were performed to investigate the binding properties of the soluble ligands (3 and 4) with metal ions such as Zn(2+) and Cu(2+). When saturating quantities of Zn(2+) ions are added to ligand 3 in acetonitrile, the fluorescence emission maximum exhibits a pronounced red shift of approximately 80 nm (from 376 to 457 nm) and is enhanced by a factor of >100 when measured at 520 nm. The fluorescence properties of the Zn(2+) ion-coordinated ligands in the Zn(3) complex are consistent with a charge-transfer character in the excited state, with possible contributions from a planarization of the pyridyl-trimethoxyphenyl groups in the excited state, and from excitonic interactions.

Dichlorido{2-hydroxy-imino-N'-[1-(2-pyrid-yl)ethyl-idene]propanohydrazide-κN,N',O}zinc(II) hemihydrate

The title compound, [ZnCl(2)(C(10)H(12)N(4)O(2))]·0.5H(2)O, was readily prepared by the reaction between ZnCl(2) and 2-hydroxy-imino-N'-[1-(2-pyrid-yl)ethyl-idene]propanohydrazide. The Zn atom has a distorted trigonal-bipyramidal geometry with two Cl atoms and one azomethine N atom in the equatorial plane and one pyridine N atom and one amide O atom in the axial positions. In the crystal structure, complex mol-ecules are connected in pairs by N-H⋯Cl hydrogen bonds, formed between the amide NH of one mol-ecule and the Cl atom of a neighboring one. Mol-ecular pairs are connected by hydrogen bonds involving the uncoordinated water mol-ecule, which lies on a twofold axis.

Targeted Guanine Oxidation by a Dinuclear Copper(II) Complex at Single Stranded/Double Stranded DNA Junctions

A dinuclear copper(II) complex [Cu(II)2(PD'O-)(H2O)2](ClO4)3 (5) with terminal Cu(II)-H(2)O moieties and a Cu...Cu distance of 4.13 A (X-ray structure) has been synthesized and characterized by EPR spectroscopy (ferromagnetic coupling observed) and cyclic voltammetry. Dizinc(II) and mononuclear copper(II) analogues [Zn(II)2(PD'O-)(H2O)2]3+ (7) and [Cu(II)(mPD'OH)(H2O)]2+ (6), respectively, have also been synthesized and structurally characterized. Reacting 5/MPA/O(2) (MPA = 3-mercaptopropionic acid) with DNA leads to a highly specific oxidation of guanine (G) at a junction between single- and double-stranded DNA. Mass spectrometric analysis of the major products indicates a gain of +18 and +34 amu relative to initial DNA strands. The most efficient reaction requires G at the first and second unpaired positions of each strand extending from the junction. Less reaction is observed for analogous targets in which the G cluster is farther from the junction or contains less than four Gs. Consistent with our previous systems, the multinuclear copper center is required for selective reaction; mononuclear complex 6 is not effective. Hydrogen peroxide as a substitute for MPA/O2 also does not lead to activity. Structural analysis of a [Cu(II)2(PD'O-)(G)]3+ complex (8) and dizinc analogue [Zn(II)(2)(PD'O-)(G)](ClO4)3 (9) (G = guanosine) reveals coordination of the G O6 and N7 atoms with the two copper (or zinc) centers and suggests that copper-G coordination likely plays a role in recognition of the DNA target. The Cu2-O2 intermediate responsible for guanine oxidation appears to be different from that responsible for direct-strand scission induced by other multinuclear copper complexes; the likely course of reaction is discussed.

Detection of zinc ions by differential circularly polarized fluorescence excitation

A new chiroptical spectroscopic approach, differential circularly polarized fluorescence excitation (CPE), can be used to provide a selective method for detecting the presence of zinc ions. The approach utilizes the same instrumentation as fluorescence-detected circular dichroism and provides strong contrast in metal detection due to response to both chelation-enhanced fluorescence and circular dichroism upon metal ion binding. The observed contrast is therefore better than either of the parent spectroscopic detection methods. CPE also provides a strategy to reduce interference from background such as protein-based tryptophan fluorescence.

Luminescent lanthanide complexes with stereocontrolled tris(2-pyridylmethyl)amine ligands: chirality effects on lanthanide complexation and luminescence properties

A series of tris(2-pyridylmethyl)amines including one and two asymmetric centers were synthesized in a stereo-controlled fashion as potential ligands of lanthanide cations. The reaction of chiral pyridylethyl methanesulfonates and bis(pyridylmethyl)amines occurred via an S(N)2 mechanism with complete inversion of asymmetric centers and gave the stereocontrolled tris(2-pyridylmethyl)amines, the stereochemical purity of which was ascertained by GPC, NMR, X-ray, and polarimetry experiments. They formed stable Tb(3+) and Eu(3+) complexes having 1:1, 1:2, and 1:3 stoichiometry (metal:ligand) in CH(3)CN solutions. NMR and UV titration experiments revealed that their complexation behaviors were rarely influenced by ligand chirality but significantly affected by the nature of the counteranion and the concentration ratio of metal to ligand. The Tb(3+) and Eu(3+) complexes with these tripodal ligands exhibited characteristic luminescence spectra upon excitation for pyridine chromophores (260 nm), the intensities of which were largely dependent on the ligand chirality. The meso isomer of the disubstituted tripods particularly exhibited the enhanced terbium luminescence ca. three times more than its diastereomer and un- and monosubstituted tripods. Direct excitation at the lanthanide center had similar chirality effects on the luminescence profiles, indicating that the stereochemistry of the employed ligand largely influenced the lanthanide emitting processes. Since the ligand chirality finely modified the local coordination environments around the lanthanide center, the use of stereocontrolled ligands is applicable in design of the luminescent lanthanide complexes.

LZnX complexes of tripodal ligands with intramolecular RN–H hydrogen bonding groups: structural implications of a hydrogen bonding cavity, and of X/R in the hydrogen bonding geometry/strength

Tripodal ligands N(CH2Py)3-n(CH2Py-6-NHR)n(R=H, n=1-3 L1-3, n=0 tpa; R=CH2tBu, n=1-3 L'1-3) are used to investigate the effect of different hydrogen bonding microenvironments on structural features of their LZnX complexes (X=Cl-, NO3-, OH-). The X-ray structures of [(L2)Zn(Cl)](BPh4)2.0.5(H2O.CH3CN), [(L3)Zn(Cl)](BPh4)3.CH3CN, [(L'1)Zn(Cl)](BPh4) 1', [(L'2)Zn(Cl)](BPh4)2'.CH3OH, and [(L'3)Zn(Cl)](BPh4)3' have been determined and exhibit trigonal bipyramidal geometries with intramolecular (internal) N-HCl-Zn hydrogen bonds. The structure of [(L'2)Zn(ONO2)]NO3 4'.H2O with two internal N-HO-Zn hydrogen bonds has also been determined. The axial Zn-Cl distance lengthens from 2.275 A in [(tpa)Zn(Cl)](BPh4) to 2.280-2.347 A in 1-3, 1'-3'. Notably, the average Zn-N(py) distance is also progressively lengthened from 2.069 A in [(tpa)Zn(Cl)](BPh4) to 2.159 and 2.182 A in the triply hydrogen bonding cavity of 3 and 3', respectively. Lengthening of the Zn-Cl and Zn-N(py) bonds is accompanied by a progressive shortening of the trans Zn-N bond from 2.271 A in [(tpa)Zn(Cl)](BPh4) to 2.115 A in 3 (2.113 A in 3'). As a result of the triply hydrogen bonding microenvironment the Zn-Cl and Zn-N(py) distances of 3 are at the upper end of the range observed for axial Zn-Cl bonds, whereas the axial Zn-N distance is one of shortest among N4 ligands that induce a trigonal bipyramidal geometry. Despite the rigidity of these tripodal ligands, the geometry of the intramolecular RN-HX-Zn hydrogen bonds (X=Cl-, OH-, NO3-) is strongly dependent on the nature of X, however, on average, similar for R=H, CH2tBu.

First Row Divalent Transition Metal Complexes of Aryl-Appended Tris((pyridyl)methyl)amine Ligands: Syntheses, Structures, Electrochemistry, and Hydroxamate Binding Properties

Divalent manganese, cobalt, nickel, and zinc complexes of 6-Ph(2)TPA (N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine; [(6-Ph(2)TPA)Mn(CH(3)OH)(3)](ClO(4))(2) (1), [(6-Ph(2)TPA)Co(CH(3)CN)](ClO(4))(2) (2), [(6-Ph(2)TPA)Ni(CH(3)CN)(CH(3)OH)](ClO(4))(2) (3), [(6-Ph(2)TPA)Zn(CH(3)CN)](ClO(4))(2) (4)) and 6-(Me(2)Ph)(2)TPA (N,N-bis((6-(3,5-dimethyl)phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine; [(6-(Me(2)Ph)(2)TPA)Ni(CH(3)CN)(2)](ClO(4))(2) (5) and [(6-(Me(2)Ph)(2)TPA)Zn(CH(3)CN)](ClO(4))(2) (6)) have been prepared and characterized. X-ray crystallographic characterization of 1A.CH(3)()OH and 1B.2CH(3)()OH (differing solvates of 1), 2.2CH(3)()CN, 3.CH(3)()OH, 4.2CH(3)()CN, and 6.2.5CH(3)()CN revealed mononuclear cations with one to three coordinated solvent molecules. In 1A.CH(3)()OH and 1B.2CH(3)()OH, one phenyl-substituted pyridyl arm is not coordinated and forms a secondary hydrogen-bonding interaction with a manganese bound methanol molecule. In 2.2CH(3)()CN, 3.CH(3)()OH, 4.2CH(3)()CN, and 6.2.5CH(3)()CN, all pyridyl donors of the 6-Ph(2)TPA and 6-(Me(2)Ph)(2)TPA ligands are coordinated to the divalent metal center. In the cobalt, nickel, and zinc derivatives, CH/pi interactions are found between a bound acetonitrile molecule and the aryl appendages of the 6-Ph(2)TPA and 6-(Me(2)Ph)(2)TPA ligands. (1)H NMR spectra of 4 and 6 in CD(3)NO(2) solution indicate the presence of CH/pi interactions, as an upfield-shifted methyl resonance for a bound acetonitrile molecule is present. Examination of the cyclic voltammetry of 1-3 and 5 revealed no oxidative (M(II)/M(III)) couples. Admixture of equimolar amounts of 6-Ph(2)TPA, M(ClO(4))(2).6H(2)O, and Me(4)NOH.5H(2)O, followed by the addition of an equimolar amount of acetohydroxamic acid, yielded the acetohydroxamate complexes [((6-Ph(2)TPA)Mn)(2)(micro-ONHC(O)CH(3))(2)](ClO(4))(2) (8), [(6-Ph(2)TPA)Co(ONHC(O)CH(3))](ClO(4))(2) (9), [(6-Ph(2)TPA)Ni(ONHC(O)CH(3))](ClO(4))(2) (10), and [(6-Ph(2)TPA)Zn(ONHC(O)CH(3))](ClO(4))(2) (11), all of which were characterized by X-ray crystallography. The Mn(II) complex 8.0.75CH(3)()CN.0.75Et(2)()O exhibits a dinuclear structure with bridging hydroxamate ligands, whereas the Co(II), Ni(II), and Zn(II) derivatives all exhibit mononuclear six-coordinate structures with a chelating hydroxamate ligand.

Redox inversion of helicity in propeller-shaped molecules derived from s-methyl cysteine and methioninol

One-electron reduction inverts the helicity of copper complexes formed from derivatives of S-methylcysteine and methioninol. The change in conformation of the organic ligand is followed in the exciton-coupled circular dichroism (ECCD) spectra of the complexes.

Derivatization, complexation, and absolute configurational assignment of chiral primary amines: application of exciton-coupled circular dichroism

We report here a sensitive method for the determination of the absolute configurations of primary amines using exciton-coupled circular dichroism (ECCD). The method works on a microgram scale by derivatization of chiral amines with quinoline chromophores. Complexation of the chiral ligands with metal ion fixes the geometry of the chromophores, resulting in a twist that is governed by the asymmetric carbon configuration and steric environment of the amine. The absolute configurations of the primary amines can be interpreted from the couplets of the ECCD spectra of the derivatized complexes. Crystal structures, 2D NMR studies, and semiempirical calculations provide structural evidence for our model.

Crystal-driven distortion of ligands in copper coordination complexes: conformational pseudo-enantiomers

The ligand in [Cu(6)CH3CN]-(ClO4)2 adopts two conformations that may be described as "pseudo-enantiomers" in that they possess approximately mirror-image molecular helicity, although they differ in the orientation of a single methyl substituent. The two conformations differ in energy as judged by analogy to other compounds studied previously, computation of the relative free energies of formation of the isomers, and measurement of solution circular dichroism spectra. The solid-state structures of both single enantiomer and racemic forms of the complex were determined by X-ray crystallography. In the chiral complex, a quasi-racemate was observed with both pseudo-enantiomeric conformers present in the asymmetric unit. Packing forces induce a higher energy conformation in order to achieve higher apparent symmetry in the solid state. In contrast, the racemic complex only displayed a single conformation corresponding to the lower energy one of the two observed in the single enantiomer structure.

Cu(I/II) redox control of molecular conformation and shape in chiral tripodal ligands: binary exciton-coupled circular dichroic states

This manuscript describes the design, synthesis, and characterization of a coordination complex that demonstrates reversible interconversion between two redox states that show very different chiroptical properties. Reported here are three chiral N(4)-tetradentate ligands, prepared in enantiopure form by asymmetric synthesis, and their circular dichroic and other properties, as well as those of numerous Cu(I) and Cu(II) complexes. While the chiroptical spectra of the free ligands are unremarkable, the CD spectra of the complexes are very intense, and spectral features indicate that they derive from exciton coupling. Comparison of complexes with one, two, or three chromophores provides strong supportive evidence for this assignment, and semiquantitative correlation with intensity as rationalized by additivity. A Cu(I)(L)/Cu(II)(L) redox couple was identified that showed dramatically different circular dichroic properties that was further shown to be easily oxidized and reduced chemically between two states with remarkable retention of signal. A structural hypothesis for the interconversion is given and supported by a variety of spectroscopic evidence.

Supramolecular detection of metal ion binding: ligand conformational control of cholesteric induction in nematic liquid crystalline phases

Tripodal tetradentate ligands may act as chemosensor molecules. Their ability to torque a nematic into a cholesteric phase increases upon complexation with copper ion. Moreover, changes in overall shape of the complexes induced by different metals and counter ions were transferred sensitively to the supramolecular level, observed by proportionate changes in the degree of twisting. Modification of the oxidation state of the metal center also gave large changes in twisting power; this suggests potential application in electrochemical molecular switches. The handedness of the induced cholesteric phase is related to the stereochemistry of the ligand: The small amount of chiral dopant needed for the LC technique (less than 2 nmol) suggests the possible determination of the absolute configuration of the parent primary amines of the ligands.

Electron-induced inversion of helical chirality in copper complexes of N,N-dialkylmethionines

Stereodynamic complexes of copper were found to undergo inversion of a helical chiral element upon oxidation or reduction. The amino acid methionine was derivatized by the attachment of two chromophores to the nitrogen atom. The resultant ligands formed stable complexes with Cu(I) and Cu(II) salts. For a derivative of a given absolute chirality, the complexes afford nearly mirror image circular dichroism spectra. The spectral changes originate from reorientation of the nitrogen-attached chromophores due to a conformation interconversion driven by the exchange of a carboxylate for a sulfide ligand. The electrically induced chirality inversion coupled with strong interactions with polarized light is unique and may lead to novel chiral molecular devices.

Absolute configurations of N,N-dialkyl alpha-amino acids and beta-amino alcohols from exciton-coupled circular dichroism spectra of Cu(II) complexes

[formula: see text] A circular dichroism technique has been applied to the absolute configurational assignment of acyclic alpha-amino acids and beta-amino alcohols with single stereogenic centers via a one-step derivitization procedure requiring microgram quantities of material. Metal ions fix the geometrical relationship of two chromophores, affording CD spectra that agree with theory on the basis of the expected conformation of the ligands in the metal complexes.