Anion and solvent induced chirality inversion in macrocyclic lanthanide complexes

Exploring the Spatial Arrangement of Simple 18-Membered Hexaazatetraamine Macrocyclic Ligands in Their Metal Complexes

Hexaazamacrocyclic Schiff bases have been extensively combined with lanthanoid (Ln) ions to obtain complexes with a highly axial geometry. However, the use of flexible hexaazatetraamine macrocycles containing two pyridines and acyclic spacers is rather uncommon. Accordingly, we obtained [DyL(OAc)2]OAc·7H2O·EtOH and [DyLMe2(Cl)2]Cl·2H2O, where L and LMe2 are the 18-membered macrocycles 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane and 3,10-dimethyl-3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane, respectively, which contain ethylene and methylethylene spacers between their N3 moieties. [DyL(OAc)2]OAc·7H2O·EtOH represents the first crystallographically characterized lanthanoid complex of L, while [DyLMe2(Cl)2]Cl·2H2O contributes to increasing the scarce number of LnIII compounds containing LMe2. Furthermore, the crystal structure of L·12H2O was solved, and it was compared with those of other related macrocycles previously published. Likewise, the crystal structures of the DyIII complexes were compared with those of the lanthanoid and d-metal complexes of other 18-membered N6 donor macrocycles. This comparison showed some effect of the spacers employed, as well as the influence of the size of the ancillary ligands and the metal ion. Additionally, the distinct folding behaviors of these macrocycles influenced their coordination geometries. Moreover, the luminescent properties of [DyL(OAc)2]OAc·7H2O·EtOH and [DyLMe2(Cl)2]Cl·2H2O were also investigated, showing that both complexes are fluorescent, with the emission being sensitized by the ligands.

Acceleration and deceleration of chirality inversion speeds in a dynamic helical metallocryptand by alkali metal ion binding

We report that the chirality inversion kinetics of a trinickel(II) cryptand can be controlled by guest recognition in the cryptand cavity. When the guest was absent, the nickel(II) cryptand underwent a dynamic interconversion between the P and M forms in solution, preferring the M form, with a half-life of t1/2 = 4.99 min. The P/M equilibrium is reversed to P-favored by binding with an alkali metal ion in the cryptand cavity. The timescale of this M→P inversion kinetics was both notably accelerated and decelerated by the guest binding (t1/2 = 0.182 min for K+ complex; 186 min for Cs+ complex); thus, the equilibration rate constants differed by up to 1000-fold depending on the guest metal ions. This acceleration/deceleration can be explained in terms of the virtual binding constants at the transition state of the P/M chirality inversion; K+ binding more stabilizes the transition state rather than the P and M forms to result in the acceleration.

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.

Tripyrrin-armed isosmaragdyrins: synthesis, heterodinuclear coordination, and protonation-triggered helical inversion

Oxidative ring closure of linear oligopyrroles is one of the synthetic approaches to novel porphyrinoids with dinuclear coordination sites and helical chirality. The spatial arrangement of the pyrrolic groups of octapyrrole (P8) affected the position of the intramolecular oxidative coupling of the pyrrolic units; tripyrrin-armed isosmaragdyrin analogue (1) containing a β,β-linked bipyrrole moiety was synthesized regioselectively in a high yield by using FeCl₃. Ni^II-coordination at the armed tripyrrin site of 1 allowed the formation of diastereomeric helical twisted complexes (2A and 2B) and succeeding Cu^II-coordination at the macrocyclic core afforded heterodinuclear Ni^II/Cu^II-complexes (3A and 3B). Each of them comprised a pair of separable enantiomers, exhibiting P- and M-helices, respectively. Notably, diastereomeric interconversion from 2A to 2B was quantitatively achieved as a consequence of helical transformation under acidic conditions.

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.

Supramolecular chirality transformation driven by monodentate ligand binding to a coordinatively unsaturated self-assembly based on C 3-symmetric ligands

Monodentate ligand binding is facilitated by supramolecular chirality transformations from propeller-shaped chirality into single-twist chirality by altering the self-assembly of C 3-symmetric chiral ligands. The C 3-symmetric chiral ligands (Im R 3Bz and Im S 3Bz) contain three chiral imidazole side arms (Im R  and Im S ) at the 1,3,5-positions of a central benzene ring. Upon coordination to zinc ions (Zn2+), which have a tetrahedral coordination preference, the C 3-symmetric chiral ligands assemble, in a stepwise manner, into a propeller-shaped assembly with a general formula (Im( R or S ) 3Bz)4(Zn2+)3. In this structure each Zn2+ ion coordinates to the three individual imidazole side arms. The resulting assembly is formally coordinatively unsaturated (coordination number, n = 3) and capable of accepting monodentate co-ligands (imidazole: ImH2) to afford a coordinatively saturated assembly [(ImH2)3(Im R 3Bz)4(Zn2+)3]. The preformed propeller-shaped chirality is preserved during this transformation. However, an excess of the monodentate co-ligand (ImH2/Zn2+ molar ratio of ∼1.7) alters the propeller-shaped assembly into a stacked dimer assembly [(ImH2) m (Im R 3Bz)2(Zn2+)3] (m = 4-6) with single-twist chirality. This switch alters the degree of enhancement and the circular dichroism (CD) pattern, suggesting a structural transition into a chiral object with a different shape. This architectural chirality transformation presents a new approach to forming dynamic coordination-assemblies, which have transformable geometric chiral structures.

Synthesis, structure, photoluminescent, optical and magnetic properties of a novel thulium p-hydroxybenzenesulfonate complex

A novel thulium p-hydroxybenzenesulfonate complex [Tm(C6H5O4S)2(H2O)6](C6H5O4S)·3H2O (1) was solvothermally synthesized and characterized by elemental analyses, photoluminescence, solid-state UV/vis diffuse reflectance, magnetic measurements and single-crystal X-ray diffraction. Complex 1 features an ionic structure with the thulium ion possessing a square antiprismatic geometry. Complex 1 crystallized in the monoclinic system with space group P21. Photoluminescent measurements with solid-state sample demonstrate that the anti-Stokes emission bands in the red/NIR spectral region 710 and 812 nm are observed from the Tm3+ 4f intrashell transitions from the 3F2,3 excited states to the 3H6 and 1G4 excited state to the 3H5 state, respectively. Solid-state UV/vis diffuse reflectance spectra of complex 1 show the existence of a wide optical band gap of 3.56 eV. Variable-temperature magnetic susceptibility and field dependence magnetization measurements are also studied and the magnetic susceptibility obeys the Curie-Weiss law (χ m =c/(T−θ)) with the value C being of 8.6 K and a negative Weiss constant θ being of −0.2 K.

Group 13 element containing conformationally rigid "N-E-N" heteroatomic bridged [3.3](2,6)pyridinophanes (E = B, Al)

The first examples of group 13 element containing pyridinophanes have been assembled using heteroatom N-E-N bridges (E = B, Al). The presence of B and Al as acceptor atoms in the bridges and their coordination with pyridine nitrogen has a very strong influence on the conformational rigidity of the pyridinophanes.

Chiral Cryptates Derived from a Hexaazamacrocycle

The reactions of hexaazamacrocycle 1 with 2,6-bis(bromomethyl)pyridine or 2,6-bis[(tosyloxy)methyl)]pyridine in the presence of appropriate carbonates result in the formation of derivatives of cryptand 6: enantiopure azacryptates of sodium and potassium. Crystal structures of these compounds indicate interaction of a metal ion with four pyridine nitrogen atoms and four tertiary amine atoms. The competition reactions monitored by NMR spectroscopy indicate preferential binding of Na⁺ over K⁺ as well as higher affinity of 6 for Na⁺ in comparison with the [2.2.1] cryptand.

Transmetalation for Flexible Pendant-Armed Schiff-Base Macrocyclic Complexes Influenced by Halide Effects

Three 46-membered [2 + 2] pendant-armed Schiff-base macrocyclic dinuclear Cd^II and Cu^II complexes (2a, 2b, and 3b) and one 23-membered [1 + 1] Cu^II macrocycle 4a were prepared from the template-directed condensation reactions between 1,2-bis(2-aminoethoxy)-ethane and extended Cl-dialdehyde in the presence of CdX₂ and CuX₂ (X = Cl and Br), in which halide effects play important roles in the organization of final macrocyclic complexes in addition to the dominant influence of metal cations. Transmetalation was intensively studied among these Cd^II and Cu^II complexes with large and flexible macrocyclic ligands, including two previously synthesized dinuclear Zn^II macrocycles (1a and 1b). Our results indicate that Zn^II → Cu^II and Cd^II → Cu^II transmetalation proceeds more quickly than that from Cd^II to Zn^II, and all the processes are found to be irreversible. It is noted that a [2 + 2] heterodinuclear Cd^IIZn^II macrocyclic intermediate could be detected by ESI-MS together with [2 + 2] homodinuclear Cd^II and Zn^II macrocyclic complexes. Furthermore, distinct halide behavior was observed in the in situ Cd^II → Cu^II and Zn^II → Cu^II metal-ion exchange. That is to say, [2 + 2] macrocycles (1a and 2a) could be converted to [1 + 1] macrocycles 4a and 4b under the reflux condition in the case of CuCl₂, accompanied by the configurational transformation from half-folded dinuclear Zn^II and Cd^II to unfolded Cu^II macrocyclic skeleton. In contrast, CuBr₂ leads to a highly efficient transmetalation to corresponding [2 + 2] dinuclear Cu^II complex 3b from both 1b and 2b no matter the experimental condition used.

Benzhydryl Ethers of Tartaric Acid Derivatives: Stereochemical Response of a Dynamically Chiral Propeller

The benzhydryl (diphenylmethyl) group is a molecular propeller that can act as a chirality reporter if it is introduced nearby a stereogenic center by making an ether bond. The hydrophobic character of the benzhydryl group allows transformation of insoluble natural tartaric acid derivatives into soluble entities in a nonpolar environment. Electronic circular dichroism spectra, recorded within the short-wavelength region of the phenyl ¹ B transitions (190-200 nm) shows strong bisignate Cotton effects. The signs and magnitudes of these Cotton effects are a function of absolute configuration and conformation of the molecule and do not primarily arise from exciton coupling of chiral benzhydryl chromophores. In crystals, the main-chain conformation is stabilized by intramolecular hydrogen bonds and CH-CO dipolar interactions. The number of the donor NH groups has a pronounced effect on the preferred conformations and inclusion properties of benzhydryl-(R,R)-tartaric acid diamides. Evidence is shown for the solvent dependency of the conformations of NH amides of tartaric acid diphenylmethyl ethers.

Chiral metal-macrocycle frameworks: supramolecular chirality induction and helicity inversion of the helical macrocyclic structures

Porous molecular solids composed of discrete macrocycles/cages have great potential for catalysis, separation and sensing techniques. Dynamic structural transformation of the host building blocks, especially a helicity inversion responsive to chemical triggers, is central to upgrading the spatial functions. Here we have achieved the syntheses of homochiral porous molecular solids composed of helical metal macrocycles through supramolecular chirality induction to both enantiomorphic forms with the aid of two different enantiopure sugar-derived lactones in the crystallization process. Moreover, we found that the helicity of the macrocyclic skeletons can be inverted in the crystalline state only by changing the type of solvent. This finding would lead to dynamic control of space chirality in connection with optical resolution, chiral amplification and asymmetric reactions.

Protonated Hexaazamacrocycles as Selective K(+) Receptors

Protonated hexaazamacrocycle [M•H](+) is able to detect K(+) ions present at ppb level in methanolic solutions containing 10(-5) M of Na(+) ions. The high sensitivity and selectivity of [M•H](+) for K(+) is ascribed to the favorable energy balance between the K(+) ion desolvation and its coordination to the [M•H](+) macrocycle, which allows the formation of the corresponding adduct before the Coulombic explosion of the ESI-MS nanodroplets.

Homochiral self-assembly of biocoordination polymers: anion-triggered helicity and absolute configuration inversion

The different natures of the weakly coordinating anions - triflate or perchlorate - in the Cu2+-mediated self-assembly of cytidine monophosphate nucleotide play a fundamental role in the homochiral resolution process, yielding one-dimensional copper(ii) coordination polymers of opposite helicity that can be easily inverted, in a reversible way, by changing the nature of the anion as revealed by circular dichroism experiments both in solution and in the solid state.

Lanthanide(iii) and lead(ii) complexes of a chiral nonaaza macrocyclic amine based on 1,2-diaminocyclopentane

We report the synthesis, spectroscopic characterization and X-ray crystal structures of a series of helical complexes of a new chiral macrocycle based on diaminocyclopentane fragments. In the case of a Pb(ii) derivative the NMR data indicate a dynamic process corresponding to partial wrapping and unwrapping of the macrocycle resulting in the switching of the helix axis.

Temperature-induced one-dimensional chiral Ag(i) linear chains and left-handed 21 helices: DFT studies, luminescence and SHG response

Two pairs of chiral coordination polymers have been synthesized with the same materials at 25 and 90 °C, respectively.