Synthesis and Electrochemical Properties of Half-Sandwich Rhodium and Iridium Methyl Complexes

A series of complexes of the form [Cp­(*)­M­(bpy)­(CH₃)]I was accessed by treatment of CpM­(bpy) or Cp*M­(bpy) with methyl iodide (M = Rh, Ir; Cp = cyclopenta­dienyl; Cp* = pentamethyl­cyclopenta­dienyl; bpy = 2,2′-bipyridyl). Solid state structures (X-ray diffraction) reveal the expected distorted octahedral geometry, with Cp or Cp* bound in the η⁵ mode and bpy bound in the typical κ² mode. Electrochemical studies demonstrate that the Cp* complexes undergo a single, quasi-reversible one-electron reduction, whereas the Cp complexes undergo both a quasi-reversible one-electron reduction and a second, more negative, irreversible reduction. Electron paramagnetic resonance studies and comparisons between complexes of different metals suggest that the formulation of the singly reduced species is formally M­(III) complexes with a bound bpy anion radical. The second reduction observed in the Cp complexes, on the other hand, results in cleavage of the M–C bond. Taken together, the results suggest that the compounds have strong metal–methyl interactions, but these can be labilized upon reduction

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

Remodeling and Enhancing Schmidt Reaction Pathways in Hexafluoroisopropanol

The effect of carrying out two variations of the Schmidt reaction with ketone electrophiles in hexafluoroisopropanol (HFIP) solvent has been studied. When TMSN₃ is reacted with ketones in the presence of triflic acid (TfOH) promoter, tetrazoles are obtained as the major products. This observation is in contrast to established methods, which usually lead to amides or lactams arising from formal NH insertion as the major products. The full product profiles of several examples of this reaction are also reported and found to include mechanistically interesting products (e.g., double ring expansion). Application of TfOH promoter in HFIP was also found to promote the reaction of a hydroxyalkyl azide with a ketone, which affords lactams following nucleophilic opening of initially formed iminium ether more efficiently than previously reported methods

Saturation Kinetics in Phenolic O–H Bond Oxidation by a Mononuclear Mn(III)–OH Complex Derived from Dioxygen

The mononuclear hydroxomanganese­(III) complex, [Mn^III(OH)­(dpaq)]⁺, which is supported by the amide-containing N₅ ligand dpaq (dpaq = 2-[bis­(pyridin-2-ylmethyl)]­amino-<i>N</i>-quinolin-8-yl-acetamidate) was generated by treatment of the manganese­(II) species, [Mn^II(dpaq)]­(OTf), with dioxygen in acetonitrile solution at 25 °C. This oxygenation reaction proceeds with essentially quantitative yield (greater than 98% isolated yield) and represents a rare example of an O₂-mediated oxidation of a manganese­(II) complex to generate a single product. The X-ray diffraction structure of [Mn^III(OH)­(dpaq)]⁺ reveals a short Mn–OH distance of 1.806(13) Å, with the hydroxo moiety <i>trans</i> to the amide function of the dpaq ligand. No shielding of the hydroxo group is observed in the solid-state structure. Nonetheless, [Mn^III(OH)­(dpaq)]⁺ is remarkably stable, decreasing in concentration by only 10% when stored in MeCN at 25 °C for 1 week. The [Mn^III(OH)­(dpaq)]⁺ complex participates in proton-coupled electron transfer reactions with substrates with relatively weak O–H and C–H bonds. For example, [Mn^III(OH)­(dpaq)]⁺ oxidizes TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-ol), which has a bond dissociation free energy (BDFE) of 66.5 kcal/mol, in MeCN at 25 °C. The hydrogen/deuterium kinetic isotope effect of 1.8 observed for this reaction implies a concerted proton–electron transfer pathway. The [Mn^III(OH)­(dpaq)]⁺ complex also oxidizes xanthene (C–H BDFE of 73.3 kcal/mol in dimethylsulfoxide) and phenols, such as 2,4,6-tri-<i>t</i>-butylphenol, with BDFEs of less than 79 kcal/mol. Saturation kinetics were observed for phenol oxidation, implying an initial equilibrium prior to the rate-determining step. On the basis of a collective body of evidence, the equilibrium step is attributed to the formation of a hydrogen-bonding complex between [Mn^III(OH)­(dpaq)]⁺ and the phenol substrates

Remodeling and Enhancing Schmidt Reaction Pathways in Hexafluoroisopropanol

The effect of carrying out two variations of the Schmidt reaction with ketone electrophiles in hexafluoroisopropanol (HFIP) solvent has been studied. When TMSN₃ is reacted with ketones in the presence of triflic acid (TfOH) promoter, tetrazoles are obtained as the major products. This observation is in contrast to established methods, which usually lead to amides or lactams arising from formal NH insertion as the major products. The full product profiles of several examples of this reaction are also reported and found to include mechanistically interesting products (e.g., double ring expansion). Application of TfOH promoter in HFIP was also found to promote the reaction of a hydroxyalkyl azide with a ketone, which affords lactams following nucleophilic opening of initially formed iminium ether more efficiently than previously reported methods

Remodeling and Enhancing Schmidt Reaction Pathways in Hexafluoroisopropanol

The effect of carrying out two variations of the Schmidt reaction with ketone electrophiles in hexafluoroisopropanol (HFIP) solvent has been studied. When TMSN₃ is reacted with ketones in the presence of triflic acid (TfOH) promoter, tetrazoles are obtained as the major products. This observation is in contrast to established methods, which usually lead to amides or lactams arising from formal NH insertion as the major products. The full product profiles of several examples of this reaction are also reported and found to include mechanistically interesting products (e.g., double ring expansion). Application of TfOH promoter in HFIP was also found to promote the reaction of a hydroxyalkyl azide with a ketone, which affords lactams following nucleophilic opening of initially formed iminium ether more efficiently than previously reported methods

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

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