Enantiomeric self-recognition in homo- and heterodinuclear macrocyclic lanthanide(III) complexes

Distributive Nd-to-Yb Energy Transfer within Pure [YbNdYb] Heterometallic Molecules

Facile access to site-selective hetero-lanthanide molecules will open new avenues in the search of novel photophysical phenomena based on Ln-to-Ln' energy transfer (ET). This challenge demands strategies to segregate efficiently different Ln metal ions among different positions in a molecule. We report here the one-step synthesis and structure of a pure [YbNdYb] (1) coordination complex featuring short Yb···Nd distances, ideal to investigate a potential distributive (i.e., from one donor to two acceptors) intramolecular ET from one Nd³⁺ ion to two Yb³⁺ centers within a well-characterized molecule. The difference in ionic radius is the mechanism allowing to allocate selectively both types of metal ion within the molecular structure, exploited with the simultaneous use of two β-diketone-type ligands. To assist the photophysical investigation of this heterometallic species, the analogues [YbLaYb] (2) and [LuNdLu] (3) have also been prepared. Sensitization of Yb³⁺ and Nd³⁺ in the last two complexes, respectively, was observed, with remarkably long decay times, facilitating the determination of the Nd-to-Yb ET within the [YbNdYb] composite. This ET was demonstrated by comparing the emission of iso-absorbant solutions of 1, 2, and 3 and through lifetime determinations in solution and solid state. The comparatively high efficiency of this process corroborates the facilitating effect of having two acceptors for the nonradiative decay of Nd³⁺ created within the [YbNdYb] molecule.

Imine- and Amine-Type Macrocycles Derived from Chiral Diamines and Aromatic Dialdehydes

The condensation of aromatic dialdehydes with chiral diamines, such as 1,2-trans-diaminocyclohexane, leads to various enantiopure or meso-type macrocyclic Schiff bases, including [2 + 2], [3 + 3], [4 + 4], [6 + 6] and [8 + 8] condensation products. Unlike most cases of macrocycle synthesis, the [3 + 3] macrocycles of this type are sometimes obtained in high yields by direct condensation without a metal template. Macrocycles of other sizes from this family can often be selectively obtained in high yields by a suitable choice of metal template, solvent, or chirality of the building blocks. In particular, the application of a cadmium(II) template results in the expansion of the [2 + 2] macrocycles into giant [6 + 6] and [8 + 8] macrocycles. These imine macrocycles can be reduced to the corresponding macrocyclic amines which can act as hosts for the binding of multiple cations or multiple anions.

Sorting Phenomena and Chirality Transfer in Fluoride-Bridged Macrocyclic Rare Earth Complexes

The reaction of fluoride anions with mononuclear lanthanide(III) and yttrium(III) hexaaza-macrocyclic complexes results in the formation of dinuclear fluoride-bridged complexes. As indicated by X-ray crystal structures, in these complexes two metal ions bound by the macrocycles are linked by two or three bridging fluoride anions, depending on the type of the macrocycle. In the case of the chiral hexaaza-macrocycle L1 derived from trans-1,2-diaminocyclohexane, the formation of these μ2-fluorido dinuclear complexes is accompanied by enantiomeric self-recognition of macrocyclic units. In contrast, this kind of recognition is not observed in the case of complexes of the chiral macrocycle L2 derived from 1,2-diphenylethylenediamine. The reaction of fluoride with a mixture of mononuclear complexes of L1 and L2, containing two different Ln(III) ions, results in narcissistic sorting of macrocyclic units. Conversely, a similar reaction involving mononuclear complexes of L1 and complexes of achiral macrocycle L3 based on ethylenediamine results in sociable sorting of macrocyclic units and preferable formation of heterodinuclear complexes. In addition, formation of these heterodinuclear complexes is accompanied by chirality transfer from the chiral macrocycle L1 to the achiral macrocycle L3 as indicated by CPL and CD spectra.

Structure Determination of Europium Complexes in Solution Using Crystal-Field Splitting of the Narrow f-f Emission Lines

Nine nona-coordinated Eu(III) complexes (1-9) studied here have three unsymmetric β-diketonate ligands and one chiral Ph-Pybox ligand, which can produce eight possible coordination isomers, depending on the position of the three unsymmetric β-diketonate ligands. Substituents on the β-diketonate ligands cause a rational structural rearrangement upon crystallization. Substituents with higher polarity, including -CN, -F, -Cl, -Br, -OMe, and -OEt, employ intercomplex hydrogen bonding to generate an association complex through structural rearrangement upon crystallization. Substituents with lower polarity, including -CF3, -SMe, and -Me, cause the most energetically favorable isomer to crystallize directly from solution. These two crystal structures exhibit well-resolved f-f emission lines with characteristic Stark splitting structures. This work revealed that the configuration of the Eu(III) complexes in solution can be determined by systematic comparison of their Stark splitting structures to those obtained from the solid phase using density functional theory (DFT)-based predictions combined with circular dichroism data.

Helix-mediated over 1 nm-range chirality recognition by ligand-to-ligand interactions of dinuclear helicates

Long-range chirality recognition between the two chiral guest ligands can be tuned based on the helix distances (d_Ln–Ln = 11.5 and 14.0 Å) of bis-diketonate bridged dinuclear lanthanide complexes (2Th and 3Th, respectively) used as mediators. Both 2Th and 3Th form one-dimensional (1D) helical structures upon terminal binding of two chiral guest co-ligands (L^R or L^S). Long-range chiral self-recognition is achieved in self-assembly of 2Th with L^R and L^S to preferentially form homochiral assemblies, 2Th-L^R·L^R and 2Th-L^S·L^S, whereas there is no direct molecular interaction between the two guest ligands at the terminal edges. X-ray crystal structure analysis and density functional theory studies reveal that long-range chiral recognition is achieved by terminal ligand-to-ligand interactions between the bis-diketonate ligands and chiral guest co-ligands. Conversely, in self-assembly of 3Th with a longer helix length, statistical binding of L^R and L^S occurs, forming heterochiral (3Th-L^R·L^S) and homochiral (3Th-L^R·L^R and 3Th-L^S·L^S) assemblies in an almost 1 : 1 ratio. When phenyl side arms of the chiral guest co-ligands are replaced by isopropyl groups (L′^R and L′^S), chiral self-recognition is also achieved in the self-assembly process of 3Th with the longer helix length to generate homochiral (3Th-L′^R·L′^R and 3Th-L′^S·L′^S) assemblies as the favored products. Thus, subtle modification of the chiral guests is capable of achieving over 1.4 nm-range chirality recognition.

Long-range chirality recognition between the two chiral guest ligands can be tuned based on the helix distances (d_Ln–Ln = 11.5 and 14.0 Å) of bis-diketonate bridged dinuclear lanthanide complexes (2Th and 3Th, respectively).

Chiral molecularly imprinted polymeric stir bar sorptive extraction for naproxen enantiomer detection in PPCPs

Chiral micropollutant analysis in pharmaceuticals and personal care products (PPCPs) is interesting but challenging. We firstly developed a series of chiral molecularly imprinted polymeric (CMIP) stir bar sorptive extraction coatings by combining a chiral template with chiral functional monomers via a click reaction for naproxen enantiomer analysis in PPCPs. Heterochiral selectivity was observed in the molecule recognition of the CMIP coatings, which demonstrated good adsorption capability for the chiral template and its structurally similar chiral compounds. The coatings also exhibited excellent enrichment capability for chiral analytes in an aqueous matrix. The surface morphology and pore structure of the CMIP coatings were characterized. The molecular interactions between the chiral template and chiral functional monomer were investigated through UV-vis spectroscopy and theoretical calculations to prove the effective interactions existing in the heterochiral MIPs. The CMIP coatings were used to enrich naproxen enantiomers in chiral drug and environmental water samples, and satisfactory recoveries (83.98 %-118.88 %) with a relative standard deviation of 3.49 %-13.08 % were achieved. The heterochiral imprinted coating-based method provided a sensitive, selective, and effective enrichment strategy for chiral micropollutant analysis in PPCPs. This technique is critical for chiral molecule recognition and enantiomer analysis in complex samples.

Chirality transfer between hexaazamacrocycles in heterodinuclear rare earth complexes

Both the chiral hexaazamacrocyle L1 based on trans-1,2-diaminocyclohexane and the achiral hexaazamacrocyle L2 based on ethylenediamine form lanthanide(iii) dinuclear μ-hydroxo bridged complexes which have been characterized by NMR and CD spectroscopy. The homodinuclear complexes of the type [Ln₂(L1)₂(μ-OH)₂](NO₃)₄ (Ln = Nd^III, Eu^III, Tb^III and Yb^III) have been synthesized in the enantiopure form and the X-ray crystal structures of Nd^III, Eu^III and Yb^III derivatives have been determined. The heterodinuclear cationic complexes [Ln(L1)Ln'(L2)(μ-OH)₂X₂]ⁿ⁺ have been generated and characterized in solution by using the mononuclear complexes of L1 and L2 as substrates. While the formation of [LnLn'(L1)₂(μ-OH)₂X₂]ⁿ⁺ dinuclear complexes is accompanied by chiral narcissistic self-sorting, the formation of [Ln(L1)Ln'(L2)(μ-OH)₂X₂]ⁿ⁺ dinuclear complexes is accompanied by the sizable sociable self-sorting of macrocyclic units. The homodinuclear complexes [Y₂(L1)₂(μ-OH)₂X₂]ⁿ⁺ and [Ln₂(L2)₂(μ-OH)₂X₂]ⁿ⁺ (Ln = Dy^III, Pr^III and Nd^III) are CD silent in the visible region due to the lack of f-f transitions and the presence of an achiral ligand, respectively. In contrast, the heterodinuclear [Y(L1^S)Ln(L2)(μ-OH)₂X₂]ⁿ⁺ complexes give rise to CD signals arising from the f-f transitions because of the chirality transfer from the L1 macrocyclic unit to the L2 macrocyclic unit.

Ligand-to-Ligand Interactions That Direct Formation of D2-Symmetrical Alternating Circular Helicate

This work demonstrates that ligand-to-ligand interactions between achiral bis-β-diketonate (BTP) and chiral bis(4-phenyl-2-oxazolinyl)pyridine [( R)- or ( S)-Ph-Pybox] are successfully directed to the fabrication of a D₂-symmetrical alternating circular helicate with the general formula [( R)- or ( S)-Ph-Pybox]₄(Ln^III)₄(BTP)₆. The lanthanide(III) Ln^III assemblies (Ln^III₄- RRRR and Ln^III₄- SSSS) have a nanometer-size squarelike grid (interatomic distances > 10 Å). X-ray structure analysis revealed that the circular helicate contains two double helicate Ln^III₂L₂ units, where both show ( M)-helicity for Ln^III₄- RRRR and ( P)-helicity for Ln^III₄- SSSS, where π-π stacking interaction is present between the side arm of ( R)-Ph-Pybox (Ph₁) and the adjacent BTP ligand around the Eu^III metal center ( d_ππ = 3.636 Å: the diketonate plane···Ph₁ distance). The newly obtained circular lanthanide(III) helicate exists as single and homochiral diastereomers in solution (Ln^III₄- RRRR and Ln^III₄- SSSS), exhibiting circularly dichroism (CD) and circularly polarized luminescence (CPL). Conversely, the circular helicate favors the heterochiral arrangement (i.e., Ln^III₄- RRRR/Ln^III₄- SSSS).

Making a Right or Left Choice: Chiral Self-Sorting as a Tool for the Formation of Discrete Complex Structures

This review discusses chiral self-sorting-the process of choosing an interaction partner with a given chirality from a complex mixture of many possible racemic partners. Chiral self-sorting (also known as chiral self-recognition or chiral self-discrimination) is fundamental for creating functional structures in nature and in the world of chemistry because interactions between molecules of the same or the opposite chirality are characterized by different interaction energies and intrinsically different resulting structures. However, due to the similarity between recognition sites of enantiomers and common conformational lability, high fidelity homochiral or heterochiral self-sorting poses a substantial challenge. Chiral self-sorting occurs among natural and synthetic molecules that leads to the amplification of discrete species. The review covers a variety of complex self-assembled structures ranging from aggregates made of natural and racemic peptides and DNA, through artificial functional receptors, macrocyles, and cages to catalytically active metal complexes and helix mimics. The examples involve a plethora of reversible interactions: electrostatic interactions, π-π stacking, hydrogen bonds, coordination bonds, and dynamic covalent bonds. A generalized view of the examples collected from different fields allows us to suggest suitable geometric models that enable a rationalization of the observed experimental preferences and establishment of the rules that can facilitate further design.

Chirality- and sequence-selective successive self-sorting via specific homo- and complementary-duplex formations

Self-recognition and self-discrimination within complex mixtures are of fundamental importance in biological systems, which entirely rely on the preprogrammed monomer sequences and homochirality of biological macromolecules. Here we report artificial chirality- and sequence-selective successive self-sorting of chiral dimeric strands bearing carboxylic acid or amidine groups joined by chiral amide linkers with different sequences through homo- and complementary-duplex formations. A mixture of carboxylic acid dimers linked by racemic-1,2-cyclohexane bis-amides with different amide sequences (NHCO or CONH) self-associate to form homoduplexes in a completely sequence-selective way, the structures of which are different from each other depending on the linker amide sequences. The further addition of an enantiopure amide-linked amidine dimer to a mixture of the racemic carboxylic acid dimers resulted in the formation of a single optically pure complementary duplex with a 100% diastereoselectivity and complete sequence specificity stabilized by the amidinium–carboxylate salt bridges, leading to the perfect chirality- and sequence-selective duplex formation.

The recognition and self-sorting of chiral molecules is a vital feature of many biomolecules. Here, the authors report chirality- and sequence-specific self-sorting of organic strands containing carboxylic acid or amidine groups, leading to selective duplex formation.

Chiral mononuclear lanthanide complexes and the field-induced single-ion magnet behaviour of a Dy analogue

A rare series of enantiopure mononuclear lanthanide complex pairs were obtained by using a chiral macrocyclic ligand and thiocyanate. The Dy complex was revealed to be a rare seven-coordinated lanthanide-based SIM encapsulated in a macrocyclic ligand.

Olefin epoxidation by a (salen)Mn(iii) catalyst covalently grafted on glass beads

A newly synthesized robust (salen)Mn(iii) monolayer on glass bead substrates provides an active catalyst for asymmetric epoxidation of 6-cyano-2,2-dimethylchromene, cis-β-ethylstyrene, 1,2-dihydronaphthalene and indene.

Anion and solvent induced chirality inversion in macrocyclic lanthanide complexes

A series of the lanthanide(III) or yttrium(III) complexes of the type [LnL(NO3)(H2O)2](NO3)2, [LnL(NO3)(H2O)](NO3)2, [LnL(H2O)2](NO3)3, and [LnLCl(H2O)2]Cl2 where L is an all-R or all-S enantiomer (L(R) or L(S)) of the chiral hexaaza macrocycle, 2(R),7(R),18(R),23(R)- or 2(S),7(S),18(S),23(S)-1,8,15,17,24,31-hexaazatricyclo[25.3.1.1.0.0]-dotriaconta-10,12,14,26,28,30-hexaene, and Ln(III) = Sm(III), Tb(III), Ho(III), Er(III), Tm(III), Yb(III), Lu(III), or Y(III), have been synthesized and structurally characterized. The crystal structure of the free macrocycle shows a highly twisted molecule, preorganized for the formation of helical complexes. The crystal structures of the lanthanide(III) complexes show two different diastereomeric forms of the macrocycle with different configurations at the stereogenic amine nitrogen atoms: (RRRR) or (RSRS) (denoted as L(RI) and L(RII), respectively). The L(RI) diastereomeric form of the nitrate derivatives [LnL(NO3)(H2O)](NO3)2 (Ln = Ho, Er) and [LnL(H2O)2](NO3)3 (Ln = Tm, Yb, Lu) convert slowly to the L(RII) form in methanol or acetonitrile solutions, while this process is not observed for the L(RI) diastereomers of analogous chloride derivatives [LnL(H2O)2]Cl3 (Ln = Tm, Yb, Lu). On the other hand, the L(RI) → L(RII) conversion for these Tm(III), Yb(III), and Lu(III) chloride derivatives can be triggered by the addition of external nitrate anions. The circular dichroism (CD) and (1)H NMR data indicate initial fast exchange of axial chloride for axial nitrate ligand, followed by slow chirality inversion of the equatorial macrocyclic ligand.

Chiroptical spectra of a series of tetrakis((+)-3-heptafluorobutylyrylcamphorato)lanthanide(III) with an encapsulated alkali metal ion: circularly polarized luminescence and absolute chiral structures for the Eu(III) and Sm(III) complexes

The luminescence and circularly polarized luminescence (CPL) spectra of M(I)[Eu((+)-hfbc)(4)] show a similar behavior to the exciton CD in the intraligand π-π* transitions when the alkali metal ions and solvents are manipulated. There is a difference in susceptibility in solvation toward the alkali metal ions but not toward the Eu(III) ion, as in the case of axially symmetric DOTA-type compounds. The remarkable CPL in the 4f-4f transitions provide much more information on the stereospecific formation of chiral Eu(III) complexes, since CPL spectroscopy is limited to luminescent species and reflects selectively toward helicity of the local structural environment around the lanthanide(III). While in comparison, exciton CD reveals the chiral structural information from the helical arrangement of the four bladed chelates. Of special importance, the observation of the highest CPL activities measured to date for lanthanide(III)-containing compounds (i.e., Eu and Sm) in solution supports the theory that the chirality of lanthanide(III) in the excited state corresponds to that in the ground state, which was derived from the exciton CD.