9 research outputs found
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><sub><b>12</b></sub>) was
synthesized and characterized. The <i><b>H</b></i><sub><b>12</b></sub> 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><sub>3</sub> symmetric “compressed tetrahedron” structure,
or one in which there are two sets of three stacked pyridine units
related by a pseudo-S<sub>4</sub> 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., H<sub>2</sub>O·4H<sub>2</sub>O or F<sup>–</sup>·4H<sub>2</sub>O, as observed crystallographically. In solution,
however, <sup>19</sup>F NMR spectroscopy indicates that <i><b>H</b></i><sub><b>12</b></sub> encapsulates fluoride
ion through direct amide hydrogen bonding. By collectively combining
one-dimensional <sup>1</sup>H, <sup>13</sup>C, and <sup>19</sup>F
with two-dimensional <sup>1</sup>H–<sup>1</sup>H COSY, <sup>1</sup>H–<sup>13</sup>C HSQC, and <sup>1</sup>H–<sup>19</sup>F 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><sub>3</sub> 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><sub><b>12</b></sub>) was
synthesized and characterized. The <i><b>H</b></i><sub><b>12</b></sub> 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><sub>3</sub> symmetric “compressed tetrahedron” structure,
or one in which there are two sets of three stacked pyridine units
related by a pseudo-S<sub>4</sub> 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., H<sub>2</sub>O·4H<sub>2</sub>O or F<sup>–</sup>·4H<sub>2</sub>O, as observed crystallographically. In solution,
however, <sup>19</sup>F NMR spectroscopy indicates that <i><b>H</b></i><sub><b>12</b></sub> encapsulates fluoride
ion through direct amide hydrogen bonding. By collectively combining
one-dimensional <sup>1</sup>H, <sup>13</sup>C, and <sup>19</sup>F
with two-dimensional <sup>1</sup>H–<sup>1</sup>H COSY, <sup>1</sup>H–<sup>13</sup>C HSQC, and <sup>1</sup>H–<sup>19</sup>F 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><sub>3</sub> symmetry. A complex deuterium
exchange process for the fluoride complex can also be unraveled by
multiple NMR techniques
Macrocyclic Influences in CO<sub>2</sub> Uptake and Stabilization
Two 24-member diamine-tetraamido
macrocycles (R = H and CH<sub>3</sub>), readily synthesized in one
or two steps, were found to
react with CO<sub>2</sub> rapidly and efficiently (100% conversion
within 1 min at rt). The resulting carbamate formation was demonstrated
by <sup>1</sup>H, <sup>13</sup>C NMR, ESI-MS, and X-ray crystallography.
The crystal structure clearly showed the carbamate group (N-CO<sub>2</sub><sup>–</sup>) 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<sup>*</sup>
<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>)(CH<sub>3</sub>CN) 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 <sup>1</sup>H NMR spectroscopy in CDCl<sub>3</sub>. 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. <sup>1</sup>H 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 H<sub>2</sub>PO<sub>4</sub><sup>–</sup> in DMSO-<i>d</i><sub><i></i>6</sub> with association constants, <i>K</i><sub>a</sub> = 1.09 × 10<sup>4</sup> and >10<sup>5</sup> M<sup>–1</sup>, respectively, and moderate selectivity for Cl<sup>–</sup> with <i>K</i><sub>a</sub> = 1.70 × 10<sup>3</sup> and 5.62 × 10<sup>4</sup> M<sup>–1</sup>, respectively.
Crystallographic studies for the Cl<sup>–</sup> and HSO<sub>4</sub><sup>–</sup> 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 HSO<sub>4</sub><sup>–</sup> complex. The pyridine-containing
macrocycle folds so that the two pyridine units are face-to-face.
The two I<sup>–</sup> ions are chelated to the two amides adjacent
to a given pyridine. In the structure of the thiophene containing
macrocycle with two BPh<sub>4</sub><sup>–</sup> 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