NMR exchange dynamics studies of metal-capped cyclodextrins reveal multiple populations of host-guest complexes in solution.

Metal-capped molecular hosts are unique in supramolecular chemistry, benefitting from the inner cavity's hydrophobic nature and the metal center's electrochemical properties. It is shown here that the paramagnetic properties of the metals in lanthanide-capped cyclodextrins (Ln-α-CDs and Ln-β-CDs) are a convenient NMR indicator for different populations of host-guest complexes in a given solution. The paramagnetic guest exchange saturation transfer (paraGEST) method was used to study the exchange dynamics in systems composed of Ln-α-CDs or Ln-β-CDs with fluorinated guests, revealing multiple co-existing populations of host-guest complexes exclusively in solutions containing Ln-β-CDs.

Measurement of Water Saturation in Soybean Oil.

In a previous study, it was observed that survivability was low when attempting to cryopreserve sperm cells in a nanoliter-sized droplet protected under soybean oil, in stark contrast to the high survival rates in milliliter-sized droplets. In this study, infrared spectroscopy was used to provide an estimate of the saturation concentration of water in soybean oil. By following the time evolution of the infrared absorption spectrum of water-oil mixtures, the saturation of water in soybean oil was found to reach equilibrium after 1 h.

Fluorophore spectroscopy in aqueous glycerol solution: the interactions of glycerol with the fluorophore.

A common perception exists that glycerol provides an inert-like environment modifying viscosity and index of refraction by its various concentrations in aqueous solution. Said perception is herein challenged by investigating the effects of the glycerol environment on the spectroscopic properties of fluorescein, as a representative fluorophore, using steady-state and time-resolved techniques and computational chemistry. Results strongly suggest that the fluorescence quantum yield, measured fluorescence lifetime (FLT), natural lifetime and calculated fluorescence lifetime are all highly sensitive to the presence of glycerol.

The formyloxyl radical: electrophilicity, C-H bond activation and anti-Markovnikov selectivity in the oxidation of aliphatic alkenes.

In the past the formyloxyl radical, HC(O)O˙, had only been rarely experimentally observed, and those studies were theoretical-spectroscopic in the context of electronic structure. The absence of a convenient method for the preparation of the formyloxyl radical has precluded investigations into its reactivity towards organic substrates. Very recently, we discovered that HC(O)O˙ is formed in the anodic electrochemical oxidation of formic acid/lithium formate.

Manganese Catalyzed Hydrogenation of Carbamates and Urea Derivatives.

We report the hydrogenation of carbamates and urea derivatives, two of the most challenging carbonyl compounds to be hydrogenated, catalyzed for the first time by a complex of an earth-abundant metal. The hydrogenation reaction of these CO-derived compounds, catalyzed by a manganese pincer complex, yields methanol in addition to amine and alcohol, which makes this methodology a sustainable alternative route for the conversion of CO to methanol, involving a base-metal catalyst. Moreover, the hydrogenation proceeds under mild pressure (20 bar).

Cost-effective density functional theory (DFT) calculations of equilibrium isotopic fractionation in large organic molecules.

The application of stable isotopes to address a wide range of biochemical, microbiological and environmental problems is hindered by the experimental difficulty and the computational cost of determining equilibrium isotopic fractionations (EIF) of large organic molecules. Here, we evaluate the factors that impact the accuracy of computed EIFs and develop a framework for cost-effective and accurate computation of EIFs by density functional theory (DFT). We generated two benchmark databases of experimentally determined EIFs, one for H isotopes and another for the isotopes of the heavy atoms C, N and O.

Evaluating Transition Metal Barrier Heights with the Latest Density Functional Theory Exchange-Correlation Functionals: The MOBH35 Benchmark Database.

A new database of transition metal reaction barrier heights (MOBH35) is presented. Benchmark energies (forward and reverse barriers and reaction energy) are calculated using DLPNO-CCSD(T) extrapolated to the complete basis set limit using a Weizmann-1-like scheme. Using these benchmark energies, the performance of a wide selection of density functional theory (DFT) exchange-correlation functionals, including the latest from the Martin, Truhlar, and Head-Gordon groups, is evaluated.

Dynamic Interactions in Synthetic Receptors: A Guest Exchange Saturation Transfer Study.

The accumulated knowledge regarding molecular architectures is based on established, reliable, and accessible analytical tools that provide robust structural and functional information on assemblies. However, both the dynamicity and low population of noncovalently interacting moieties within studied molecular systems limit the efficiency and accuracy of traditional methods. Herein, the use of a saturation transfer-based NMR approach to study the dynamic binding characteristics of an anion to a series of synthetic receptors derived from bambusuril macrocycles is demonstrated.

The cationic dye basic orange 21 (BO21) as a potential fluorescent sensor.

The present study investigates the fluorescence properties of BO21 and their dependence on various intracellular conditions. The results obtained with cell-free solutions indicate that the influences of pH and temperature on the fluorescence spectra are negligible, while viscosity, various proteins and heparin have significant influence. In the presence of heparin, a red shift of the emission spectrum (from 515 to 550 nm) is observed, suggesting that this shift cannot simply be attributed to electrostatic interaction between BO21 and the polyanionic heparin, but rather to aggregation of BO21 on the polyanion.

Self-Assembled Hybrid Materials Based on Organic Nanocrystals and Carbon Nanotubes.

Organic crystalline materials are used as dyes/pigments, pharmaceuticals, and active components of photonic and electronic devices. There is great interest in integrating organic crystals with inorganic and carbon nanomaterials to create nanocomposites with enhanced properties. Such efforts are hampered by the difficulties in interfacing organic crystals with dissimilar materials.

Evaluation of the Factors Impacting the Accuracy of C NMR Chemical Shift Predictions using Density Functional Theory-The Advantage of Long-Range Corrected Functionals.

The various factors influencing the accuracy of C NMR calculations using density functional theory (DFT), including the basis set, exchange-correlation (XC) functional, and isotropic shielding calculation method, are evaluated. A wide selection of XC functionals (over 70) were considered, and it was found that long-range corrected functionals offer a significant improvement over the other classes of functionals. Based on a thorough study, it is recommended that for calculating NMR chemical shifts (δ) one should use the CSGT method, the COSMO solvation model, and the LC-TPSSTPSS exchange-correlation functional in conjunction with the cc-pVTZ basis set.

Amplifying undetectable NMR signals to study host-guest interactions and exchange.

The characteristics of host-guest systems, such as molecular recognition, complexation, encapsulation, guest composition, and dynamic exchange, are manifested by changes in the chemical shifts (Δ) in the NMR spectrum. However, in cases where NMR signals cannot be detected, due to low concentrations, poor solubility, or relatively fast exchange, an alternative is needed. Here, we show that by using the magnetization transfer (MT) method, the undetectable NMR signals of host-guest assemblies can be amplified by two orders of magnitude.

Enolonium Species-Umpoled Enolates.

Enolonium species/iodo(III)enolates of carbonyl compounds have been suggested to be intermediates in a wide variety of hypervalent iodine induced chemical transformations of ketones, including α-C-O, α-C-N, α-C-C, and α-carbon-halide bond formation, but they have never been characterized. We report that these elusive umpoled enolates may be made as discrete species that are stable for several minutes at -78 °C, and report the first spectroscopic identification of such species. It is shown that enolonium species are direct intermediates in C-O, C-N, C-Cl, and C-C bond forming reactions.

Mechanism of the Copper/TEMPO-Catalyzed Aerobic Oxidation of Alcohols.

Identifying the mechanism of a catalytic reaction is paramount for designing new and improved catalysts. Several alternative catalytic cycles for the copper/2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)-catalyzed aerobic oxidation of alcohols to the corresponding aldehydes or ketones were examined using DFT at the SMD(CH CN)-RIJCOSX-DSD-PBEB95/def2-TZVP//DF-PBE /def2-SVP level of theory. A catalytic cycle in which TEMPO remains coordinated to copper throughout was identified as the most likely mechanism.

Mechanistic Aspects of Aryl-Halide Oxidative Addition, Coordination Chemistry, and Ring-Walking by Palladium.

This contribution describes the reactivity of a zero-valent palladium phosphine complex with substrates that contain both an aryl halide moiety and an unsaturated carbon-carbon bond. Although η(2) -coordination of the metal center to a C=C or C≡C unit is kinetically favored, aryl halide bond activation is favored thermodynamically. These quantitative transformations proceed under mild reaction conditions in solution or in the solid state.

Synthesis and stability of cyclic α-hydrogen nitroxides.

Nitroxides (nitroxyl radicals) hold a unique place in science due to their stable radical nature. We have recently reported the first design concept providing a general solution to the problem of designing and preparing monocyclic α-hydrogen nitroxides. The initial studies were limited to aryl derivatives.

A novel liquid organic hydrogen carrier system based on catalytic peptide formation and hydrogenation.

Hydrogen is an efficient green fuel, but its low energy density when stored under high pressure or cryogenically, and safety issues, presents significant disadvantages; hence finding efficient and safe hydrogen carriers is a major challenge. Of special interest are liquid organic hydrogen carriers (LOHCs), which can be readily loaded and unloaded with considerable amounts of hydrogen. However, disadvantages include high hydrogen pressure requirements, high reaction temperatures for both hydrogenation and dehydrogenation steps, which require different catalysts, and high LOHC cost.

Design concept for α-hydrogen-substituted nitroxides.

Stable nitroxides (nitroxyl radicals) have many essential and unique applications in chemistry, biology and medicine. However, the factors influencing their stability are still under investigation, and this hinders the design and development of new nitroxides. Nitroxides with tertiary alkyl groups are generally stable but obviously highly encumbered.

Iron dicarbonyl complexes featuring bipyridine-based PNN pincer ligands with short interpyridine C-C bond lengths: innocent or non-innocent ligand?

A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and characterized by multinuclear NMR spectroscopy ((1) H, (13) C, (15) N, (31) P), IR spectroscopy, cyclic voltammetry, (57) Fe Mössbauer spectroscopy, XPS spectroscopy, and single-crystal X-ray diffraction. The complexes with the general formula [(R-PNN)Fe(CO)2 ] (5: R-PNN=tBu-PNN=6-[(di-tert-butylphosphino)methyl]-2,2'-bipyridine, 6: R-PNN=iPr-PNN=6-[(diisopropylphosphino)methyl]-2,2'-bipyridine, and 7: R-PNN=Ph-PNN=6-[(diphenylphosphino)methyl]-2,2'-bipyridine) feature differently P-substituted PNN pincer ligands. Complexes 5 and 6 were obtained by reduction of the corresponding dihalide complexes [(R-PNN)Fe(X)2 ] (1: R=tBu, X=Cl; 2: R=tBu, X=Br; 3: R=iPr, X=Cl; 4: R=iPr, X=Br) in the presence of CO.

On the Innocence of Bipyridine Ligands: How Well Do DFT Functionals Fare for These Challenging Spin Systems?

The electronic structures of a number of iron, cobalt, vanadium, and titanium complexes with the 2,2'-bipyridine (bpy) ligand were considered using the multireference CASSCF and NEVPT2 methods. Many of these systems have been studied in the past using B3LYP and were then found to contain the bpy ligand as a radical anion. For many of the cases, this is contradicted by our multireference calculations.

Activation of nitriles by metal ligand cooperation. Reversible formation of ketimido- and enamido-rhenium PNP pincer complexes and relevance to catalytic design.

The dearomatized complex cis-[Re(PNP(tBu)*)(CO)2] (4) undergoes cooperative activation of C≡N triple bonds of nitriles via [1,3]-addition. Reversible C-C and Re-N bond formation in 4 was investigated in a combined experimental and computational study. The reversible formation of the ketimido complexes (5-7) was observed.

Authorizing multiple chemical passwords by a combinatorial molecular keypad lock.

A combinatorial fluorescent molecular sensor operates as a highly efficient molecular security system. The ability of a pattern-generating molecule to process diverse sets of chemical inputs, discriminate among their concentrations, and form multivalent and kinetically stable complexes is demonstrated as a powerful tool for processing a wide range of chemical "passwords" of different lengths. This system thus indicates the potential for obtaining unbreakable combination locks at the molecular scale.