Pfeiffer Effect Research Articles

Pfeiffer effect on configurationally labile dyes within ternary complexes with metal ions and enantiopure macrocycles.

A configurationally-labile helical dye, 2,4,5,7-tetranitrofluorenone oximate, is used to probe complexes made of enantiopure macrocycles and mono/divalent metal ions. Induced electronic circular dichroism (ECD) and 1H NMR responses are amplified at room temperature only in the presence of K+ and Na+ ions despite larger binding efficiency with alkaline earth metal ions.

2,4,5,7-Tetranitrofluorenone Oximate for the Naked-Eye Detection of H-Bond Donors and the Chiroptical Sensing of Enantiopure Reagents.

Hydrogen bonding greatly influences rates and equilibrium positions of chemical reactions, conformations, and sometimes even stereochemistry. This study reports on tetranitrofluorenone oximate, a novel dye capable of naked-eye detection of hydrogen-bond donating species (HBDs) and of rapid determination of H-bond donation strength by hypsochromic shift monitoring. In addition, the molecule possesses atropisomeric conformations, of M and P configuration, as evidenced in solid and solution state studies by X-ray diffraction and electronic circular dichroism (ECD), respectively. In the latter case, enantiopure bis-thioureas were the most effective HBDs to promote a chiral induction (diastereoselective recognition, Pfeiffer effect); the ECD results being rationalized by time-dependent density functional theory (TDDFT) calculations. Based on these experiments, bis-thioureas were used as chiral reagents in asymmetric 1,3-dipolar cycloadditions of structurally-related nitrones; the ECD sensing of the stereoinduction between bis-thioureas and the oximate serving as an indirect method of selection of the most effective HBD for asymmetric synthesis.

Chirogenesis and Pfeiffer Effect in Optically Inactive EuIII and TbIII Tris(β-diketonate) Upon Intermolecular Chirality Transfer From Poly- and Monosaccharide Alkyl Esters and α-Pinene: Emerging Circularly Polarized Luminescence (CPL) and Circular Dichroism (CD).

We report emerging circularly polarized luminescence (CPL) at 4f-4f transitions when lanthanide (EuIII and TbIII) tris(β-diketonate) embedded to cellulose triacetate (CTA), cellulose acetate butyrate (CABu), D-/L-glucose pentamethyl esters (D-/L-Glu), and D-/L-arabinose tetramethyl esters (D-/L-Ara) are in film states. Herein, 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate (fod) and 2,2,6,6-tetramethyl-3,5-heptanedione (dpm) were chosen as the β-diketonates. The glum value of Eu(fod)3 in CABu are +0.0671 at 593 nm (5F1) and −0.0059 at 613 nm (5F2), respectively, while those in CTA are +0.0463 and −0.0040 at these transitions, respectively. The glum value of Tb(fod)3 in CABu are −0.0029 at 490 nm (5F6), +0.0078 at 540 nm (5F5), and −0.0018 at 552 nm (5F5), respectively, while those in CTA are −0.0053, +0.0037, and −0.0059 at these transitions, respectively. D-/L-Glu and D-/L-Ara induced weaker glum values at 4f-4f transitions of Eu(fod)3, Tb(fod)3, and Tb(dpm)3. For comparison, Tb(dpm)3 in α-pinene showed clear CPL characteristics, though Eu(dpm)3 did not. A surplus charge neutralization hypothesis was applied to the origin of attractive intermolecular interactions between the ligands and saccharides. This idea was supported from the concomitant opposite tendency in upfield 19F-NMR and downfield 1H-NMR chemical shifts of Eu(fod)3 and the opposite Mulliken charges between F-C bonds (fod) and H-C bonds (CTA and D-/L-Glu). An analysis of CPL excitation (CPLE) and CPL spectra suggests that (+)- and (–)-sign CPL signals of EuIII and TbIII at different 4f-4f transitions in the visible region are the same with the (+)-and (–)-sign exhibited by CPLE bands at high energy levels of EuIII and TbIII in the near-UV region.

Chirality Induction in Crystalline Solids Containing Sandwich-type [Ln(α2-P2W17O61)2]17- Polyoxotungstates and Proline.

Chiral crystalline compounds of the series of [Ln(α2-P2W17O61)2]17- polyanions embracing nearly all the lanthanide (Ln) ions were successfully isolated using Pro (proline) as a chiral director. In each compound, the polyanion and Pro were associated through a stereoselective interaction involving N(H)···O hydrogen bonding. 31P NMR and circular dichroism studies revealed the induction of chirality by Pro in the racemic aqueous systems of the [Ln(α2-P2W17O61)2]17- polyanions, i.e., the Pfeiffer effect. Mechanistically, this racemization was regulated by competitive enantiomeric interactions between the l-/d-Pro and l-/d-polyanions, with only one racemization route (the on-site rotation of the [α2-P2W17O61)]10- unit) observed, independent of the nature of Ln.

The Importance of Solvent Effects on the Mechanism of the Pfeiffer Effect.

The Pfeiffer effect is observed when an optically active compound such as an amino acid is introduced to a solution containing a labile racemic metal complex, and an equilibrium shift is obtained. The “perturbation” results in an excess of one enantiomer over the other. The shift is a result of a preferential outer sphere interaction between the introduced chiral species and one enantiomeric form (Λ or Δ) of a labile metal complex. Speculations regarding the mechanism of the Pfeiffer effect have attributed observations to a singular factor such as pH, solvent polarity, or numerous other intermolecular interactions. Through the use of the lanthanide(III) complexes [Tb(DPA)3]3− and [Eu(DPA)3]3− (where DPA = 2,6-pyridinedicarboxylate) and the amino acids l-serine and l-proline; it is becoming clear that the mechanism is not so simply described as per the preliminary findings that are discussed in this study. It appears that the true mechanism is far more complicated than the attribute just a singular factor. This work attempts to shine light on the fact that understanding the behavior of the solvent environment may hypothetically be the key to offering a more detailed description of the mechanism.

Antenna Biphenols: Development of Extended Wavelength Chiroptical Reporters.

Molecular hosts capable of chiroptical sensing of complexed guest molecules offer an attractive alternative to conventional methods for the analysis of the absolute configuration and enantiopurity. Sensors based on the Pfeiffer effect rely on complexation-driven asymmetric transformation of the first kind and can produce a chiroptical signal against an otherwise null background. To be most effective, the wavelength of the induced chiroptical sensor readout should be free and clear of interfering signals coming from the sample under investigation. In this study, we report the introduction of stereodynamic zinc complexes of antenna biphenols, a new class of sensors bearing antenna-like appendages that can extend the wavelength of the chiroptical signal while also improving enantioselective guest recognition.

On chirality induction in the crystalline solid-containing sandwich-type [Ln(α-PW11O39)2]11−polyoxotungstate and proline

For the racemisation, the [Ln(α-PW11O39)2]11−polyanion with light Ln experienced the ‘on-site rotation’ mechanism, while the [Ln(α-PW11O39)2]11−polyanion with heavy Ln followed the “dissociation–association” mechanism.

Formation of Chiral Heteronuclear LnIII Assemblies by Ion Pair Formation

AbstractCationic LnIII complexes with the chiral macrocycle L, (P)‐[LnLRRRRRR]3+, interact with anionic LnIII tris(dipicolinate) complexes, [Ln′(dpa)3]3–, forming tight hydrogen‐bonded ion pairs in solution. Formation of these pairs has been documented by the DFT calculations, and a strong paramagnetic shift of the signals of the diamagnetic cation interacting with the paramagnetic anion was observed. This ion pair interaction leads to spectral enantiodifferentiation of the Λ‐[Ln′(dpa)3]3– and Δ‐[Ln′(dpa)3]3– enantiomers of the anionic complex.Although no Pfeiffer effect was observed for the (P)‐[LnLRRRRRR]3+ diastereomers of the cationic complex, the (M)‐[LnLRRRRRR]3+ diastereomers induce CD signals in the anionic [Ln′(dpa)3]3– partner. These spectra indicate a Pfeiffer effect, i.e. a shift of equilibrium between the Λ‐[Ln′(dpa)3]3– and Δ‐[Ln′(dpa)3]3– enantiomers. This effect arises from the ion pair interaction between the anionic complex [Ln′(dpa)3]3– and the intact cationic complex as well as between [Ln′(dpa)3]3– and L. The protonated macrocycle (H3LRRRRRR)3+ forms highly insoluble salts with [Ln′(dpa)3]3–. These supramolecular compounds are formed by the reaction of L, acids and [Ln′(dpa)3]3– anions. The same product can also be formed by the reaction of (M)‐[LnLRRRRRR]3+ with three equivalents of dipicolinic acid. The model structure of (H3LRRRRRR)[Ln′(dpa)3] corresponds with a linear polymer with alternate cationic macrocycles and anionic [Ln′(dpa)3]3– complexes interacting through charge interactions and multiple hydrogen bonds.

Multispectroscopic DNA-binding studies of a tris-chelate nickel(II) complex containing 4,7-diphenyl 1,10-phenanthroline ligands

A tris-chelate nickel(II) complex, [Ni(DIP)3](NO3)2 · 3H2O in which DIP = 4,7-diphenyl 1,10 phenanthroline, was synthesized and characterized by spectroscopic (1H NMR and UV–vis) and elemental analysis techniques. Binding interaction of this complex with calf thymus-DNA (CT-DNA) was investigated using emission, absorption, circular dichroism (CD), viscosity and DNA thermal denaturation studies. In absorption spectrum of the complex, as the concentration of DNA increased, appearance of an isobestic point proved the new [Ni(DIP)3]2+–DNA complex formation. The intrinsic binding constant (Kb = 4.34 × 104 M−1) is comparable to groove binding complexes. In fluorimetric studies, the dynamic enhancement constants (kD) and bimolecular enhancement constant (kB) were calculated and showed that the fluorescence enhancement was initiated by a static process in the ground state. Furthermore, the thermodynamic studies showed that the reaction is exothermic and enthalpy favored (ΔH = −58.41 kJ/mol). A strong CD spectrum in the UV and visible region develops upon addition of CT-DNA into the racemate solution of Ni(II) complex (Pfeiffer effect). This has revealed that a shift in diastereomeric inversion equilibrium takes place in the solution to yield an excess of one of the DNA complex diastereomers. Finally, all of the experimental results prove that the minor groove binding must be predominant.

Chiral Synthesis of a Mononuclear Nickel(II) Complex Formed from an Achiral Tripodal Amine Ligand: Spontaneous Resolution

A neutral chiral mononuclear Ni(II) complex cis-[Ni(NCS)(2)(tren)] (1) [tren = tris(2-aminoethyl)amine] has been isolated in a reaction of nickel(II) salt with an achiral ligand tren and NH(4)SCN in a MeOH-H(2)O (1:1) mixture. The crystal structure of 1 reveals the encapsulation of Ni(II) ions in a facial {NiN(6)} coordination environment that includes four nitrogen donors from the tetradentate chelate tren and two nitrogen donors from two cis-NCS (monodentate) ligands. The chirality of 1 arises from symmetry breaking of the cis-(NCS)(2)-Ni(II) octahedral complex by the tetradentate chelate tren. Compound 1 exhibits both its enantiomers in a DMF solution through a Pfeiffer effect when l- and d-arabinose are used as environment substances. The particular synthesis, described herein, offers total spontaneous resolution, as is evidenced from crystal structure and circular dichroism spectral studies.

An Unsymmetrical Mixed‐Valent Divanadium(IV/V) Complex

AbstractIn acetonitrile medium, reaction of bisacetylacetonatooxovanadium(IV) {[VO(acac)2]} and acetylhydrazine (CH3CONHNH2) in a 2:1 molar ratio provides a novel, unsymmetrical, chiral, mixed‐valent, triply bridged, dinuclear vanadium(IV/V) complex having the formula [(Hdmpz)OV(μ‐acac)(μ‐O)(μ‐O2CCH3)VO(acac)] (Hdmpz = 3,5‐dimethylpyrazole). The molecular structure of the complex was confirmed by X‐ray crystallography. It crystallizes in the chiral space group P212121 indicating a spontaneous resolution in the crystal. The complex is one‐electron paramagnetic and redox active. It displays VVVIV to VVVV oxidation and VVVIV to VIVVIV reduction responses. The crystal structure and EPR measurements revealed the trapped‐valent character of the complex in the solid state as well as in the solution phase. Circular dichroism spectroscopic measurements with a powdered sample revealed that the bulk material is not racemic. In solution, the existence of both enantiomers of the complex is demonstrated by the Pfeiffer effect with the use of L‐ and D‐arabinose as environment substances.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

NMR enantiodifferentiation of quaternary ammonium salts of Tröger base

Hexacoordinated phosphorus BINPHAT anion 1 is an efficient NMR chiral solvating agent for quaternary ammonium cations (quats) derived from Tröger base leading to large separations of the proton signals of the enantiomers and even in polar solvent media such as CD(3)CN (Delta Delta delta up to 0.12 ppm). Quite surprisingly, this efficacy in the NMR split efficiency is not translated into a supramolecular stereocontrol (Pfeiffer effect) of the cation configuration by the anion.

Conformationally-locked metallomacrocycles—prototypes for a novel type of axial chirality

A series of ditopic ligands incorporating two 2,2′:6′,2″-terpyridine (tpy) metal-binding domains linked to a central naphthalenediyl core by ethyleneoxy chains of various lengths have been prepared and their iron(II) complexes prepared. The major thermodynamic products of complexation are ferracycles of [1 + 1], [2 + 2] or [3 + 3] stoichiometry. In the case of the [1 + 1] complexes, the linker between the two tpy domains is spatially restricted and the cations exhibit chirality associated with the conformation about the central N–Fe–N bonds. The dynamic processes by which the enantiomeric forms interconvert are investigated in the presence of chiral anions by NMR and circular dichroism spectroscopy making use of the Pfeiffer effect. We have shown that ditopic bis(2,2′:6′,2″-terpyridine) ligands give rise to conformationally restricted complexes with iron(II) centres. The absolute configuration of the cations in the diastereomeric ion pairs obtained through the Pfeiffer effect has been calculated using TDDFT methods.

DNA-Binding Properties of Iron(II) Mixed-Ligand Complexes Containing 1,10-Phenanthroline and Dipyrido[3,2-a:2’,3’-c]phenazine

An iron(II) mixed-ligand complex with 1,10-phenanthroline (phen) and dipyrido[3,2-a:2’,3’- c]phenazine (dppz), [Fe(phen)2(dppz)]2+, has been synthesized. The DNA-binding properties of the mixed-ligand complex have been studied in terms of equilibrium binding constant, thermodynamic parameter, thermal denaturation as well as Pfeiffer effect upon binding to DNA. The spectrophotometric titration of [Fe(phen)2(dppz)]2+ with calf thymus DNA (ct-DNA) has shown that the iron(II) mixed-ligand complex binds effectively to ct-DNA in an intercalation mode as indicated by remarkable hypochromicity (ca. 36%) and moderate bathochromic shift (8 nm) of the absorption spectra. This intercalative mode is supported by a significant increase (Δ Tm = 21 °C) in the melting temperature (Tm) of ct-DNA at R([complex]/[ct-DNA]) = 1.5. The binding of [Fe(phen)2(dppz)]2+ to ct-DNA is entropically driven as characterized by a positive enthalpy change and a large negative TΔ S term. An intense CD signal in the UV and visible region develops upon addition of ct-DNA to the racemate solution of [Fe(phen)2(dppz)]2+. This has revealed that a shift in diastereomeric inversion equilibrium takes place in the solution to yield an excess of one enantiomer of the DNA-iron(II) complex (Pfeiffer effect). The striking resemblance of the CD spectral profiles to those of the corresponding Δ -enantiomer indicates that Δ -[Fe(phen)2(dppz)]2+ is preferentially bound to ct-DNA

Chirality of a Strandberg-type heteropolyanion [S2Mo5O23]4−

The heteropolyanion [S2Mo5O23]4− having C2 symmetry is dissymmetric and hence expected to be chiral. The heteropoly-compound [NMe4]4[S2Mo5O23] exhibits its both enantiomers in DMSO solution by a Pfeiffer effect when l- and d-arabinose are used as environment substances.

From chiral counterions to twisted membranes.

In membranes, the chirality of the amphiphile constituents is sometimes expressed at a supramolecular scale of nanometers or micrometers. We have recently reported that membranes of nonchiral dicationic n-2-n amphiphiles can also be chirally twisted upon interacting with chiral tartrate counterions. Here, we demonstrate that the mechanism of the chiral induction by counterions involves specific anion-cation recognition and the induction of conformationally labile chirality in the cations. Single-crystal X-ray diffraction shows that the amphiphilic cations exist as a mixture of chiral conformers. (1)H NMR data establish a specific recognition between tartrate and n-2-n cations and show that chiral conformers also exist in solution. Circular dichroism (CD) in the UV-vis shows a sharp conformational change of tartrate ions from anti to gauche when bound to the chiral cationic membranes. This is confirmed by CD in the infrared region which also shows concomitant induced CD bands in the vibrations of the n-2-n amphiphiles. These results represent the first example of the so-called Pfeiffer effect in a membrane. They provide a general framework for designing new tunable membrane systems. Our work also includes the first application of vibrational circular dichroism in the study of chiral conformations of amphiphiles in membranes and demonstrates the very high potential of this technique.

Nature of equilibrium shifts in racemic praseodymium(iii) tris(2,2′-oxydiacetate) induced by interaction with chiral probes

The effects of L-proline and its derivatives N-acetyl-L-proline, L-proline methyl ester, and L-proline benzyl ester on the equilibrium shift in the racemic Pr(ODA)33− complex, in which ODA is 2,2′-oxydiacetate, have been studied by means of circular dichroism (CD) and 1H NMR spectroscopy. While N-acetyl-L-proline did not induce any optical activity in Pr(ODA)33−, the addition of L-proline, L-proline methyl ester, and L-proline benzyl ester led to the appearance of the Pfeiffer effect in this lanthanide complex. The signs of the induced CD signals at 484 nm which correspond to the 3P0 ← 3H4 transition implied that L-proline interacts more favourably with the Λ-enantiomer of the Pr(ODA)33−, while L-proline methyl ester and L-proline benzyl ester preferably interact with the Δ-enantiomer of the same complex. The 1H NMR studies suggested that L-proline and its esters adopt different conformations upon interaction with Pr(ODA)33−. The dependence of the absorption dissymmetry factor, gabs, on the concentration of the L-proline indicated that the Pfeiffer effect in this lanthanide complex is best described by the associated model. Measurements in the solution of various pH, ionic strength, and dielectric constant suggested that the chiral discriminatory interaction is a combination of electrostatic and hydrophobic forces, while the hydrogen bonding effects are less important.

DNA binding of iron(II) mixed-ligand complexes containing 1,10-phenanthroline and 4,7-diphenyl-1,10-phenanthroline

Absorption spectroscopy and circular dichroism (CD) have been used to characterize the DNA binding of [Fe(phen) 3] 2+, [Fe(phen) 2(DIP)] 2+ and [Fe(phen)(DIP) 2] 2+ where phen and DIP stand for 1,10-phenanthroline and 4,7-diphenyl-1,10-phenanthroline, respectively. Both [Fe(phen) 3] 2+ and [Fe(phen) 2(DIP)] 2+ bind weakly to calf thymus DNA (CT-DNA) in an electrostatic mode, while [Fe(phen)(DIP) 2] 2+ binds more strongly to CT-DNA, possibly in an intercalation mode. The hypochromicity, red shift and K b increase in the order [Fe(phen) 3] 2+<[Fe(phen) 2(DIP)] 2+<[Fe(phen)(DIP) 2] 2+ in accordance with the increase in size and hydrophobicity of the iron(II) complexes. The thermodynamic parameters obtained suggest that the DNA binding of both [Fe(phen) 3] 2+ and [Fe(phen) 2(DIP)] 2+ is entropically driven, while that of [Fe(phen)(DIP) 2] 2+ is enthalpically driven. A strong CD spectrum in the UV and visible region develops upon addition of CT-DNA into the racemate solution of each iron(II) complex (Pfeiffer effect). This has revealed that a shift in diastereomeric inversion equilibrium takes place in the solution to yield an excess of one of the DNA-complex diastereomers. The striking resemblance of the CD spectral profiles to those of the pure Δ-enantiomer indicates that the Δ-enantiomer of the iron(II) complexes is preferentially bound to CT-DNA. The mechanism of the development of Pfeiffer CD is proposed on the basis of kinetic studies on the DNA binding of the racemic iron(II) complexes.

Enantioselective outer-sphere complexation of lanthanide(III) tris (pyridine-2,6-dicarboxylate) chelates with vitamin B 12 derivatives. An NMR and CD study

The binding of three corrinoids, cyanocobalarnin (vitamin B 12, CNCbl), aquacobalamin (vitamin B 12a, H 2OCbl +) and dicyanocobinamide ((CN) 2Cbi), collectively denoted by Q, to the Δ- and Λ-enantiomer of Ln(DPA) 3 3− complexes (Ln=lanthanide, DPA=2,6-pyridine dicarboxylate) in aqueous solution at room temperature is investigated by NMR and Circular Dichroism (CD) spectroscopy. Tb(DPA) 3 3− induces shifts of the 1H-NMR resonances of Q. A formula, describing the dipolar interaction between the proton spin and the paramagnetic moment of the lanthanide complex, is fitted to the lanthanide induced shifts, yielding the position of Tb(DPA) 3 3− and the orientation of its magnetic axis with respect to the corrinoid. The magnitude of the shifts provide the value of K avg, an average of the association constants of the two diastereomeric complexes {Δ-Tb(DPA) 3 3−Q) and {Λ-Tb(DPA) 3 3−Q}. We find K avg=4±0.5 (B 12) and 0.8±0.2 M −1 ((CN) 2Cbi). The value for H 2OCbl + depends strongly on the solution’s ionic strength I: K avg=(7±1)×10 1, 22±5 and 16±7 M −1 at I=0.007, 0.12 and 0.22 M, respectively. The addition of the corrinoids to a (racemic) solution of Nd(DPA) 3 3−, induces CD in the electronic Nd(III) transitions in the 700–800 nm region (ICD). This Pfeiffer effect is attributed to an excess of Λ-Nd(DPA) 3 3− in the enantioselective, outer-sphere association with the corrinoids. From the magnitude of the ICD as a function of [Q], in conjunction with the (indirectly) determined CD effect of Λ-Nd(DPA) 3 3−, values are determined for K Δ− K Λ. We find K Δ− K Λ=−24±2 M −1 for H 2OCbl + at pH 6.7 and I=0.12 M and −12±2 M −1 at I=0.22 M; −4.4±0.5 M −1 for CNCbl and −0.9±0.3 M −1 for (CN) 2Cbi. Adopting the view that in the lanthanide series Ln(DPA) 3 3− chelates are isostructural, the results of the NMR and CD experiments are combined to estimate the enantioselectivity in the binding E b, defined as ( K Δ− K Λ)/2 K avg. We find E b=−0.5±0.2 for H 2OCbl +, −0.7±0.2 for CNCbl and −0.7±0.3 for (CN) 2Cbi. On the basis of the NMR data, a molecular structure is proposed for the two diastereomeric {Ln(DPA) 3 3−Q} complexes. The bonding involves hydrogen bonds between the amide proton on the a and g side chain of the corrinoid and the carboxylate oxygen of two DPA ligands.

Inversion of stereoselectivity in the association between tris(oxalato)cobaltate(III) and bovine serum albumin by the addition of dioxane to aqueous solutions

Measuring the Pfeiffer effect of the addition of bovine serum albumin (BSA) as an optically active environment to a tris(oxalato)cobaltate(III) anion, [Co(ox) 3] 3−, we found that the Λ-enantiomer of the complex anion is enriched in 5 or 10% dioxane–water solutions but the Δ-enantiomer is concentrated in aqueous solution, where the enantiomeric excess is 72% (Λ rich) in 10% dioxane–water solution.