Chemistry and Structure of a Host–Guest Relationship: The Power of NMR and X‑ray Diffraction in Tandem

An amine/amide mixed covalent organic tetrahedral cage <b>1</b> (<i><b>H</b></i>_<b>12</b>) was synthesized and characterized. The <i><b>H</b></i>_<b>12</b> cage contains 12 amide NH groups plus four tertiary amine N groups, the latter of which are positioned in a pseudo-tetrahedral array. Crystallographic findings indicate that the tetrahedral host can adopt either a pseudo-<i>C</i>₃ symmetric “compressed tetrahedron” structure, or one in which there are two sets of three stacked pyridine units related by a pseudo-S₄ axis. The latter conformation is ideal for encapsulating small pentameric clusters, either a water molecule or a fluoride ion surrounded by a tetrahedral array of water molecules, i.e., H₂O·4H₂O or F^–·4H₂O, as observed crystallographically. In solution, however, ¹⁹F NMR spectroscopy indicates that <i><b>H</b></i>_<b>12</b> encapsulates fluoride ion through direct amide hydrogen bonding. By collectively combining one-dimensional ¹H, ¹³C, and ¹⁹F with two-dimensional ¹H–¹H COSY, ¹H–¹³C HSQC, and ¹H–¹⁹F HETCOR NMR techniques, the solution binding mode of fluoride can be ascertained as consisting of four sets of independent structural subunits with <i>C</i>₃ symmetry. A complex deuterium exchange process for the fluoride complex can also be unraveled by multiple NMR techniques

Chemistry and Structure of a Host–Guest Relationship: The Power of NMR and X‑ray Diffraction in Tandem

An amine/amide mixed covalent organic tetrahedral cage <b>1</b> (<i><b>H</b></i>_<b>12</b>) was synthesized and characterized. The <i><b>H</b></i>_<b>12</b> cage contains 12 amide NH groups plus four tertiary amine N groups, the latter of which are positioned in a pseudo-tetrahedral array. Crystallographic findings indicate that the tetrahedral host can adopt either a pseudo-<i>C</i>₃ symmetric “compressed tetrahedron” structure, or one in which there are two sets of three stacked pyridine units related by a pseudo-S₄ axis. The latter conformation is ideal for encapsulating small pentameric clusters, either a water molecule or a fluoride ion surrounded by a tetrahedral array of water molecules, i.e., H₂O·4H₂O or F^–·4H₂O, as observed crystallographically. In solution, however, ¹⁹F NMR spectroscopy indicates that <i><b>H</b></i>_<b>12</b> encapsulates fluoride ion through direct amide hydrogen bonding. By collectively combining one-dimensional ¹H, ¹³C, and ¹⁹F with two-dimensional ¹H–¹H COSY, ¹H–¹³C HSQC, and ¹H–¹⁹F HETCOR NMR techniques, the solution binding mode of fluoride can be ascertained as consisting of four sets of independent structural subunits with <i>C</i>₃ symmetry. A complex deuterium exchange process for the fluoride complex can also be unraveled by multiple NMR techniques

Macrocyclic Influences in CO₂ Uptake and Stabilization

Two 24-member diamine-tetraamido macrocycles (R = H and CH₃), readily synthesized in one or two steps, were found to react with CO₂ rapidly and efficiently (100% conversion within 1 min at rt). The resulting carbamate formation was demonstrated by ¹H, ¹³C NMR, ESI-MS, and X-ray crystallography. The crystal structure clearly showed the carbamate group (N-CO₂^–) formed was tightly bound within the macrocyclic cavity, held by five internal hydrogen bonds, and stabilized by intramolecular carbamate-ammonium salt-bridge formation

Pyridine-2,6-dicarboxamide pincer-based macrocycle: a versatile ligand for oxoanions, oxometallates, and transition metals^*

<p>A tetracarboxamide-based macrocycle has shown a general aptitude for binding both anions and metal cations. Crystallographic studies indicate that in several instances quite similar structures are obtained. The macrocycle can be readily synthesised by a one-step condensation. Smaller oxoanions (sulfate, oxalate, and nitrite) bind within a folded macrocyclic cleft. Larger oxoanions and oxometallates (dihydrogen phosphate, dichromate, and perrhenate) dangle below the folded macrocycle. Smaller divalent metal ions (nickel(II) and copper(II)) form binuclear complexes with the metal ions bound within the folded macrocycle. The larger palladium(II) also forms a ditopic complex with the two pincers units, but the macrocycle lies in a more planar, non-folded conformation.</p

Chemical Mustard Containment Using Simple Palladium Pincer Complexes: The Influence of Molecular Walls

Six amide-based NNN palladium­(II) pincer complexes Pd­(<b>L</b>)­(CH₃CN) were synthesized, characterized, and examined for binding the sulfur mustard surrogate, 2-chloroethyl ethyl sulfide (CEES). The complexes all bind readily with CEES as shown by ¹H NMR spectroscopy in CDCl₃. The influence of para-substituents on the two amide phenyl appendages was explored as well as the effect of replacing the phenyl groups with larger aromatic rings, 1-naphthalene and 9-anthracene. While variations of the para-substituents had only a slight influence on the binding affinities, incorporation of larger aromatic rings resulted in a significant size-related increase in binding, possibly due to increasing steric and electronic interactions. In crystal structures of three CEES-bound complexes, the mustard binds through the sulfur atom and lies along the aromatic walls of the side appendages approximately perpendicular to the pincer plane, with increasingly better alignment progressing from phenyl to 1-naphthalene to 9-anthracene

Molecular Thioamide ↔ Iminothiolate Switches for Sulfur Mustards

SNS platinum­(II) pincer complexes reversibly bind and release the surrogate half sulfur mustard, 2-chloroethyl ethyl sulfide (CEES). The switch-like behavior of the pincers is attributed to a reversible transformation between the thioamide and iminothiolate forms of the pincer skeleton under slightly acidic and basic conditions, respectively. An amide-based palladium­(II) pincer complex also binds CEES, as confirmed crystallographically and by NMR

Influence of Charge on Anion Receptivity in Amide-Based Macrocycles

Binding and structural aspects of anions with tetraamido/diquaternized diamino macrocyclic receptors containing <i>m</i>-xylyl, pyridine, and thiophene spacers are reported. ¹H NMR studies indicate that the quaternized receptors display higher affinities for anions compared to corresponding neutral macrocycles. The macrocycles containing pyridine spacers consistently display higher affinity for a given anion compared to those with either <i>m</i>-xylyl or thiophene spacers. The <i>m</i>-xylyl- and pyridine-containing receptors exhibit high selectivity for H₂PO₄^– in DMSO-<i>d</i>_<i></i>6 with association constants, <i>K</i>_a = 1.09 × 10⁴ and >10⁵ M^–1, respectively, and moderate selectivity for Cl^– with <i>K</i>_a = 1.70 × 10³ and 5.62 × 10⁴ M^–1, respectively. Crystallographic studies for the Cl^– and HSO₄^– complexes indicate that the <i>m</i>-xylyl-containing ligand is relatively elliptical in shape, with the two charges at ends of the major axis of the ellipse. The anions are hydrogen bonded with the macrocycle but are outside the ligand cavity. In the solid state, an unusual low-barrier hydrogen bond (LBHB) was discovered between two of the macrocycle’s carbonyl oxygen atoms in the HSO₄^– complex. The pyridine-containing macrocycle folds so that the two pyridine units are face-to-face. The two I^– ions are chelated to the two amides adjacent to a given pyridine. In the structure of the thiophene containing macrocycle with two BPh₄^– counterions, virtually no interaction was observed crystallographically between the macrocycle and the bulky anions

Pyrazinetetracarboxamide: A Duplex Ligand for Palladium(II)

Tetraethylpyrazine-2,3,5,6-tetracarboxamide forms a dipalladium­(II) complex with acetates occupying the fourth coordination sites of the two bound metal ions. Crystallographic results indicate that the “duplex” dipincer has captured two protons that serve as the counterions. The protons lie between adjacent amide carbonyl groups with very short O···O distances of 2.435(5) Å. In the free base, the adjacent carbonyl groups are farther apart, averaging 3.196(3) Å. While the dipalladium­(II) complexes stack in an ordered stepwise fashion along the <i>a</i> axis, the free base molecules stack on top of each other, with each pincer rotated by about 60° from the one below

Characterizing Hydrogen-Bond Interactions in Pyrazinetetracarboxamide Complexes: Insights from Experimental and Quantum Topological Analyses

Experimental and topological analyses of dipalladium­(II) complexes with pyrazinetetracarboxamide ligands containing tetraethyl (<b>1</b>), tetrahexyl (<b>2</b>), and tetrakis­(2-hydroxyethyl) ethyl ether (<b>3</b>) are described. The presence of two very short O---O distances between adjacent amide carbonyl groups in the pincer complexes revealed two protons, which necessitated two additional anions to satisfy charge requirements. The results of the crystal structures indicate carbonyl O---O separations approaching that of low barrier hydrogen bonds, ranging from 2.413(5) to 2.430(3) Å. Solution studies and quantum topological analyses, the latter including electron localization function, noncovalent interaction, and Bader’s quantum theory of atoms in molecules, were carried out to probe the nature of the short hydrogen bonds and the influence of the ligand environment on their strength. Findings indicated that the ligand field, and, in particular, the counterion at the fourth coordination site, may play a subtle role in determining the degree of covalent association of the bridging protons with one or the other carbonyl groups