Synthesis of a Preorganized Hybrid Macrobicycle with Distinct Amide and Amine Clefts: Tetrahedral versus Spherical Anions Binding Studies

A new <i>C</i>_3<i>v</i> symmetric amido-amine hybrid macrobicycle, <b>L</b> is synthesized toward anion recognition in its protonated states. <b>L</b> contains tri-amide and tetra-amine clefts separated by <i>p</i>-phenylene spacers. The solid-state structure of methanol-encapsulated <b>L</b> exhibits an overall cavity length of ∼12.0 Å where the amide and amine -NH protons are converged toward the center of the respective cavities. Conformational analysis of <b>L</b> in solution is established by NOESY NMR. Anion binding of [H₃<b>L</b>]³⁺ with spherical (Cl^–, Br^–, I^–) and tetrahedral (ClO₄^–, SO₄^2–) anions are carried out by isothermal titration calorimeter in dimethylsulfoxide. The association of halides with [H₃<b>L</b>]³⁺ is endothermic and entropy driven. However, association of tetrahedral anions is exothermic in nature and both entropy- and enthalpy-driven. The overall association constants show the following order: HSO₄^– > Br^–> Cl^– ≈ ClO₄^–. Single crystal X-ray structures of ClO₄^– and Br^– complexes of protonated <b>L</b> show encapsulation of ClO₄^– in the amide cleft of [H₂<b>L</b>]²⁺ (complex <b>1</b>) and encapsulation of Br^– in the ammonium cleft of [H₃<b>L</b>]³⁺ (complex <b>2</b>). Further, preorganization of <b>L</b> toward encapsulation of spherical and tetrahedral anions is established by comparing its amide, amine, and overall cavity dimensions with <b>1</b> and <b>2</b>

[2]Rotaxane with Multiple Functional Groups

High-yield syntheses of Cu­(II)- and Ni­(II)-templated [2]­pseudorotaxane precursors (CuPRT and NiPRT, respectively) were achieved by threading bis­(azide)­bis­(amide)-2,2′-bipyridine axle into a bis­(amide)­tris­(amine) macrocycle. Single-crystal X-ray structural analysis of CuPRT revealed complete threading of the axle fragment into the wheel cavity, where strong aromatic π–π stacking interactions between two parallel arene moieties of the wheel and the pyridyl unit of axle are operative in addition to metal ion templation. Attachment of a newly developed bulky stopper molecule with a terminal alkyne to CuPRT via a Cu­(I)-catalyzed azide–alkyne cycloaddition reaction failed as a result of dethreading of the azide-terminated axle under the reaction conditions. However, the synthesis of a metal-free [2]­rotaxane containing triazole with other functionalities in the axle was achieved in ∼45% yield upon coupling between azide-terminated NiPRT and the alkyne-terminated stopper. The [2]­rotaxane was characterized by mass spectrometry, 1D and 2D NMR (COSY, DOSY, and ROESY) experiments. Comparative solution-state NMR studies of the [2]­rotaxane in its unprotonated and protonated states were carried out to locate the position of the wheel on the axle of the metal-free [2]­rotaxane. Furthermore, a variable-temperature ¹H NMR study in DMSO-<i>d</i>₆ of [2]­rotaxane supported the kinetic inertness of the interlocked structure, where the newly developed stopper prevents dethreading of the 30-membered wheel from the axle

Synthesis of a Preorganized Hybrid Macrobicycle with Distinct Amide and Amine Clefts: Tetrahedral versus Spherical Anions Binding Studies

A new <i>C</i>_3<i>v</i> symmetric amido-amine hybrid macrobicycle, <b>L</b> is synthesized toward anion recognition in its protonated states. <b>L</b> contains tri-amide and tetra-amine clefts separated by <i>p</i>-phenylene spacers. The solid-state structure of methanol-encapsulated <b>L</b> exhibits an overall cavity length of ∼12.0 Å where the amide and amine -NH protons are converged toward the center of the respective cavities. Conformational analysis of <b>L</b> in solution is established by NOESY NMR. Anion binding of [H₃<b>L</b>]³⁺ with spherical (Cl^–, Br^–, I^–) and tetrahedral (ClO₄^–, SO₄^2–) anions are carried out by isothermal titration calorimeter in dimethylsulfoxide. The association of halides with [H₃<b>L</b>]³⁺ is endothermic and entropy driven. However, association of tetrahedral anions is exothermic in nature and both entropy- and enthalpy-driven. The overall association constants show the following order: HSO₄^– > Br^–> Cl^– ≈ ClO₄^–. Single crystal X-ray structures of ClO₄^– and Br^– complexes of protonated <b>L</b> show encapsulation of ClO₄^– in the amide cleft of [H₂<b>L</b>]²⁺ (complex <b>1</b>) and encapsulation of Br^– in the ammonium cleft of [H₃<b>L</b>]³⁺ (complex <b>2</b>). Further, preorganization of <b>L</b> toward encapsulation of spherical and tetrahedral anions is established by comparing its amide, amine, and overall cavity dimensions with <b>1</b> and <b>2</b>

Formation and Transmetalation Mechanisms of Homo- and Heterometallic (Fe/Zn) Trinuclear Triple-Stranded Side-by-Side Helicates

A novel linear hybrid tris-bidentate neutral ligand having 2,2′-bipyridine and two terminal triazolylpyridine coordination sites (<b>L</b>) was efficiently synthesized and explored in the synthesis of trinuclear triple-stranded homometallic side-by-side helicates <b>L</b>₃Fe₃(OTf)₆ (<b>1</b>) and <b>L</b>₃Zn₃(OTf)₆ (<b>2</b>), in which the three metal centers display alternating Λ and Δ configurations. Selective formation of the analogous heterometallic side-by-side helicate <b>L</b>₃Fe₂Zn­(OTf)₆ (<b>3</b>) was achieved from a mixture of <b>L</b>, Fe­(CH₃CN)₂(OTf)₂, and Zn­(OTf)₂ (1:1:1) in acetonitrile at room temperature. Various analytical techniques, i.e., single-crystal X-ray diffraction and NMR and UV/vis spectroscopy, were used to elucidate the sequence of the metal atoms within the heterometallic helicate, with the Zn²⁺ at the central position. The formation of <b>3</b> was also achieved starting from either <b>L</b>₃Zn₃(OTf)₆ or <b>L</b>₃Fe₃(OTf)₆ by adding Fe­(CH₃CN)₂(OTf)₂ or Zn­(OTf)₂, respectively. ESI-MS and ¹H NMR studies elucidated different transmetalation mechanisms for the two cases: While a Zn²⁺-to-Fe²⁺ transmetalation occurs by the stepwise exchange of single ions on the helicate <b>L</b>₃Zn₃(OTf)₆ at room temperature, this mechanism is almost inoperative for the Fe²⁺-to-Zn²⁺ transmetalation in <b>L</b>₃Fe₃(OTf)₆, which is kinetically trapped at room temperature. In contrast, dissociation of <b>L</b>₃Fe₃(OTf)₆ at higher temperature is required, followed by reassembly to give <b>L</b>₃Fe₂Zn­(OTf)₆. The reassembly follows an interesting mechanistic pathway when an excess of Zn­(OTf)₂ is present in solution: First, <b>L</b>₃Zn₃(OTf)₆ forms as the high-temperature thermodynamic product, which is then slowly converted into the thermodynamic heterometallic <b>L</b>₃Fe₂Zn­(OTf)₆ product at room temperature. The temperature-dependent equilibrium shift is traced back to significant entropy differences resulting from an enhancement of the thermal motion of the ligands at high temperature, which destabilize the octahedral iron terminal complex and select zinc in a more stable tetrahedral geometry

Polyamide–Polyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase

Polyamide–polyamine hybrid macrobicycle <b>L</b> is explored with respect to its ability to bind α,ω-dicarboxylate anions. Potentiometric studies of protonated <b>L</b> with the series of dianions from succinate (suc^2–) through glutarate (glu^2–), α-ketoglutarate (kglu^2–), adipate (adi^2–), pimelate (pim^2–), suberate (sub^2–), to azelate (aze^2–) have shown adipate preference with association constant value of <i>K</i> = 4900 M^–1 in a H₂O/DMSO (50:50 <i>v/v</i>) binary solvent mixture. The binding constant increases from glu^2– to adi^2– and then continuously decreases with the length of the anion chain. Further, potentiometric studies suggest that hydrogen bonding between the guest anions and the amide/ammonium protons of the receptor also contributes to the stability of the associations along with electrostatic interactions. Negative-mode electrospray ionization of aqueous solutions of host–guest complexes shows clear evidence for the selective formation of 1:1 complexes. Single-crystal X-ray structures of complexes of the receptor with glutaric acid, α-ketoglutaric acid, adipic acid, pimelic acid, suberic acid, and azelaic acid assist to understand the observed binding preferences. The solid-state structures reveal a size/shape complementarity between the host and the dicarboxylate anions, which is nicely reflected in the solution state binding studies