Solid‐state NMR study of dopant effects on the chemical properties of Mg‐, In‐, and Al‐doped SnP2O7

The effects of doped low‐valence cations on the properties of the SnP₂O₇ proton conductor at ambient temperature are investigated from changes in solid‐state NMR spectra and nuclear magnetic relaxation times. Although the T₁H values increased with decreasing acidity as a result of cation exchange, the ¹H chemical shifts moved to lower field in Al‐ and In‐doped materials compared with undoped ones. Furthermore, the shifts changed to higher field in Mg‐doped materials, suggesting the existence of different protonic species in those materials. The bulk phosphate chemical shifts in the ³¹P dipolar‐decoupling MAS NMR spectra were very similar, regardless of the nature and amount of the doping species. On the other hand, by ¹H/³¹P cross‐polarization MAS NMR, P₂O₇ signals interacting with an interstitial proton [Q¹(proton)] were observed in all the undoped and doped SnP₂O₇, while acidic P–OH‐type phosphate signals [Q¹(acid)] were additionally observed in the Mg‐doped conductor. The different affinity of the proton with the dopants and phosphates caused lower conductivity and larger activation energy in the Mg‐doped materials, compared with those in the In‐ and Al‐doped materials. Copyright © 2014 John Wiley & Sons, Ltd.     

Encapsulation of Metalloporphyrins in a Self‐Assembled Cubic M8L6 Cage: A New Molecular Flask for Cobalt–Porphyrin‐Catalysed Radical‐Type Reactions

The synthesis of a new, cubic M₈L₆ cage is described. This new assembly was characterised by using NMR spectroscopy, DOSY, TGA, MS, and molecular modelling techniques. Interestingly, the enlarged cavity size of this new supramolecular assembly allows the selective encapsulation of tetra(4‐pyridyl)metalloporphyrins (M^II(TPyP), M=Zn, Co). The obtained encapsulated cobalt–porphyrin embedded in the cubic zinc–porphyrin assembly is the first example of a catalytically active encapsulated transition‐metal complex in a cubic M₈L₆ cage. The substrate accessibility of this system was demonstrated through radical‐trapping experiments, and its catalytic activity was demonstrated in two different radical‐type transformations. The reactivity of the encapsulated Co^II(TPyP) complex is significantly increased compared to free Co^II(TPyP) and other cobalt–porphyrin complexes. The reactions catalysed by this system are the first examples of cobalt–porphyrin‐catalysed radical‐type transformations involving diazo compounds which occur inside a supramolecular cage.     

NMR spectral properties of the tetramantanes – nanometer‐sized diamondoids

Tetramantanes, and all diamondoid hydrocarbons, possess carbon frameworks that are superimposable upon the cubic diamond lattice. This characteristic is invaluable in assigning their ¹H and ¹³C NMR spectra because it translates into repeating structural features, such as diamond‐cage isobutyl moieties with distinctively complex methine to methylene signatures in COSY and HMBC data, connected to variable, but systematic linkages of methine and quaternary carbons. In all tetramantane C₂₂H₂₈ isomers, diamond‐lattice structures result in long‐range ⁴J_HH, W‐coupling in COSY data, except where negated by symmetry; there are two highly symmetrical and one chiral tetramantane (showing seven ⁴J_HH). Isobutyl‐cage methines of lower diamondoids and tetramantanes are the most shielded resonances in their ¹³C spectra (<29.5 ppm). The isobutyl methylenes are bonded to additional methines and at least one quaternary carbon in the tetramantanes. W‐couplings between these methines and methylenes clarify spin‐network interconnections and detailed surface hydrogen stereochemistry. Vicinal couplings of the isobutyl methylenes reveal positions of the quaternary carbons: HMBC data then tie the more remote spin systems together. Diamondoid ¹³C NMR chemical shifts are largely determined by α and β effects, however γ‐shielding effects are important in [123]tetramantane. ¹H NMR chemical shifts generally correlate with numbers of 1,3‐diaxial H–H interactions. Tight van der Waals contacts within [123]tetramantane's molecular groove, however, form improper hydrogen bonds, deshielding hydrogen nuclei inside the groove, while shielding those outside, indicated by Δδ of 1.47 ppm for geminal hydrogens bonded to C‐3,21 . These findings should be valuable in future NMR studies of diamondoids/nanodiamonds of increasing size. Copyright © 2015 John Wiley & Sons, Ltd.     

Identification of radical structures on 1‐pentamethylbenzyl‐3‐ethylimidazoliumsilver(I)bromide and 1,3‐bis(pentamethylbenzyl)‐4,5‐dimethylbenzimidazoliumsilver(I)bromide exposed to gamma rays: an EPR study

1‐Pentamethylbenzyl‐3‐ethylimidazoliumsilver(I)bromide and 1,3‐bis(pentamethylbenzyl)‐4,5dimethylbenzimidazoliumsilver(I)bromide and their Ag⁺ complexes were synthesized and their polycrystal forms were produced by recrystallization in dichloromethane/Et₂O solvent system. Structural determinations were carried out by ¹H NMR and ¹³C NMR with a Varian 400 NMR system using tetramethylsilane as internal standard and CDCl₃ as solvent. The disappearance of acidic N‐heterocyclic carbene proton showed the formation of Ag(I) complexes. Also, elemental analyses were carried out. Electron paramagnetic resonance (EPR) measurements were performed to determine the formed radical structure on the samples irradiated at the room temperature for 72 h by using ⁶⁰Co‐source with dose rate of 0.680 kGy. The EPR measurements were carried out in the temperature range of 200 K–450 K. Identical radicals were determined on the irradiated compounds. It was observed that the shapes of the spectra of the samples were independent of the temperature but, the resonance line intensities changed linearly with the temperature. Also, it was detected that the free radical on the 1‐pentamethylbenzyl‐3‐ethylimidazoliumsilver(I)bromide is not stable compared to that on the 1,3‐bis(pentamethylbenzyl)‐4,5dimethylbenzimidazoliumsilver(I)bromide. Copyright © 2016 John Wiley & Sons, Ltd.     

Encapsulation of Formaldehyde and Hydrogen Cyanide in an Open‐Cage Fullerene

Reaction of C₆₃NO₂(Ph)₂(Py) ( 1 ) with o‐phenylenediamine and pyridine produces a mixture of C₆₃H₄NO₂(Ph)₂(Py)(N₂C₆H₄) ( 2 ) and H₂O@ 2 . Compound 2 is a new open‐cage fullerene containing a 20‐membered heterocyclic orifice, which has been fully characterized by NMR spectroscopy, high‐resolution mass spectrometry, and X‐ray crystallography. The elliptical orifice of 2 spans 7.45 Å along the major axis and 5.62 Å along the minor axis, which is large enough to trap water and small organic molecules. Thus, heating a mixture of 2 and H₂O@ 2 with hydrogen cyanide and formaldehyde in chlorobenzene affords HCN@ 2 and H₂CO@ 2 , respectively. The ¹H NMR spectroscopy reveals substantial upfield shifts for the endohedral species (δ=?1.30 to ?11.30 ppm), owing to the strong shielding effect of the fullerene cage.     

Exceptionally Inert Lanthanide(III) PARACEST MRI Contrast Agents Based on an 18‐Membered Macrocyclic Platform

We report a macrocyclic ligand based on a 3,6,10,13‐tetraaza‐1,8(2,6)‐dipyridinacyclotetradecaphane platform containing four hydroxyethyl pendant arms (L¹) that forms extraordinary inert complexes with Ln³⁺ ions. The [EuL¹]³⁺ complex does not undergo dissociation in 1 M HCl over a period of months at room temperature. Furthermore, high concentrations of phosphate and Zn²⁺ ions at room temperature do not provoke metal‐complex dissociation. The X‐ray crystal structures of six Ln³⁺ complexes reveal ten coordination of the ligand to the metal ions through the six nitrogen atoms of the macrocycle and the four oxygen atoms of the hydroxyethyl pendant arms. The analysis of the Yb³⁺‐ and Pr³⁺‐induced paramagnetic ¹H NMR shifts show that the solid‐state structures are retained in aqueous solution. The intensity of the ¹H NMR signal of bulk water can be modulated by saturation of the signals of the hydroxy protons of Pr³⁺, Eu³⁺, and Yb³⁺ complexes following chemical‐exchange saturation transfer (CEST). The ability of these complexes to provide large CEST effects at 25 and 37 °C and pH 7.4 was confirmed by using CEST magnetic resonance imaging experiments.     

Synthesis and NMR elucidation of novel pentacycloundecane‐based peptides

The synthesis and NMR elucidation of two novel pentacycloundecane (PCU)‐based peptides are reported. The PCU cage amino acids were synthesised as racemates and the incorporation of the cage amino acid with (S)‐natural amino acids produced diastereomeric peptides. The diastereomeric ‘cage’ peptides were separated using preparative HPLC and the NMR elucidation of these PCU containing peptides are reported for the first time. The ¹H and ¹³C NMR spectra showed series of overlapping signals of the cage skeleton and that of the peptide, making it extremely difficult to resolve the structure using one‐dimensional NMR techniques only. The use of two‐dimensional NMR techniques proved to be a highly effective tool in overcoming this problem. Copyright © 2010 John Wiley & Sons, Ltd.     

Tight and Selective Caging of Chloride Ions by a Pseudopeptidic Host

The selective molecular recognition of chloride versus similar anions is a continuous challenge in supramolecular chemistry. We have designed and prepared a simple pseudopeptidic cage ( 1 a ) that defines a cavity suitable for the tight encapsulation of chloride. The interaction of the protonated form of 1 a with different inorganic anions was studied in solution by ¹H NMR spectroscopy and ESI‐MS, and in the solid state by X‐ray diffraction. The solution binding data showed that the association constants of 1 a to chloride are more than two orders of magnitude higher than to any other tested inorganic anion. Remarkably, 1 a displayed a high selectivity for chloride over other closely related halides such as bromide (selectivity=111), iodide (selectivity=719), and fluoride (selectivity >1000). Binding experiments (¹H NMR spectroscopy and ESI‐MS) suggested that 1 a has a high‐affinity (inner) binding site and an additional low‐affinity (external) binding site. The supramolecular complexes with F^?, Cl^?, and Br^? have been also characterized by the X‐ray diffraction of the corresponding [ 1 a? nHX] crystalline salts. The structural data show that the chloride anion is tightly encapsulated within the host, in a binding site defined by a very symmetric array of electrostatic H‐bonds. For the fluoride salt, the size of the cage cavity is too large and is occupied by a water molecule, which fits inside the cage efficiently competing with F^?. In the case of the bigger bromide, the mismatch of the anion inside the cage caused a geometrical distortion of the host and thus a large energetic penalty for the interaction. This minimalistic pseudopeptidic host represents a unique example of the construction of a simple well‐defined binding pocket that allows the highly selective molecular recognition of a challenging substrate.     

Complete assignment of the 1H and 13C NMR spectra of antimicrobial 4‐arylamino‐ 3‐nitrocoumarin derivatives

Herein, we describe the synthesis and complete assignment of the ¹H and ¹³C NMR chemical shifts of a series of antimicrobial 4‐arylamino‐3‐nitrocoumarin derivatives based on a combination of ¹H and ¹³C NMR, ¹H‐¹H‐COSY, NOESY, HSQC and HMBC experiments. Conformational effects upon the chemical shifts of the coumarin moiety arising from the anisotropy of the aryl side group are briefly discussed. This study provides the first complete and fully assigned NMR data for this important group of antimicrobial compounds and bridges the gap existing in the literature with regard to NMR structural data for 4‐arylamino‐3‐nitrocoumarins. Copyright © 2010 John Wiley & Sons, Ltd.     

Solid‐State NMR Study of the Tautomerism of Acetylacetone Included in a Host Matrix

The tautomerism of the enol form of acetylacetone (=pentane‐2,4‐dione; 1 ) inside a host cavity has been studied by means of solid‐state ¹³C‐NMR spectroscopy (SSNMR) using the variable‐temperature CPMAS technique. It appears that the enol form, 4‐hydroxypent‐3‐en‐2‐one ( 1a ), exists in an equilibrium with an identical tautomer ( 1c ) trough O H ⋅⋅⋅O proton transfer. The experimental results (energy barrier and chemical shifts) were rationalized by means of MP2 and GIAO calculations.     

NMR analysis of 6‐(2′,3′‐dihydro‐1′H‐inden‐1′‐yl)‐1H‐indene

¹H, ¹³C and two‐dimensional NMR analyses were applied to determine the NMR parameters of 6‐(2′,3′‐dihydro‐1′H‐inden‐1′‐yl)‐1H‐indene. The measurements were accomplished with 0.5 mg of the substance, this quantity being sufficient to determine the chemical shifts of all the H and C atoms, and also the appropriate coupling constants and to give the complete NMR resonance assignments of the molecule. The predicted patterns of the four different H atoms of the methylene groups of the indane structural element coincided completely with the complex patterns in the NMR spectra. Copyright © 2002 John Wiley & Sons, Ltd.     

Two Histidines in an α‐Helix: A Rigid Co2+‐Binding Motif for PCS Measurements by NMR Spectroscopy

Pseudocontact shifts (PCS) generated by paramagnetic metal ions present valuable long‐range information in the study of protein structural biology by nuclear magnetic resonance (NMR) spectroscopy. Faithful interpretation of PCSs, however, requires complete immobilization of the metal ion relative to the protein, which is difficult to achieve with synthetic metal tags. We show that two histidine residues in sequential turns of an α‐helix provide a binding site for a Co²⁺ ion, which positions the metal ion in a uniquely well‐defined and predictable location. Exchange between the bound and free cobalt is slow on the timescale defined by chemical shifts, but the NMR resonance assignments are nonetheless readily transferred from the diamagnetic to the paramagnetic NMR spectrum by an I_zS_z‐exchange experiment. The double‐histidine‐Co²⁺ motif offers a straightforward, inexpensive, and convenient way of generating precision PCSs in proteins.     

Release of the Water Molecule Encapsulated Inside an Open‐Cage Fullerene through Hydrogen Bonding Mediated by Hydrogen Fluoride

A reversible wetting/dewetting procedure is reported for an open‐cage fullerene with an 18‐membered orifice. In a homogeneous mixture of H₂O/EtOH/CHCl₃, water was encapsulated into the cavity of the open‐cage compound quantitatively at 80 °C. Addition of aqueous hydrogen fluoride into the water‐encapsulated complex removed the encapsulated water completely at room temperature. H‐bonding between the trapped water and fluoride is shown to play a key role for the water release process.     

Host–Guest Chemistry of a Water‐Soluble Pillar[5]arene: Evidence for an Ionic‐Exchange Recognition Process and Different Complexation Modes

The complexation of an anionic guest by a cationic water‐soluble pillararene is reported. Isothermal titration calorimetry (ITC), ¹H NMR, ¹H and ¹⁹F DOSY, and STD NMR experiments were performed to characterize the complex formed under aqueous neutral conditions. The results of ITC and ¹H NMR analyses showed the inclusion of the guest inside the cavity of the pillar[5]arene, with the binding constant and thermodynamic parameters influenced by the counter ion of the macrocycle. NMR diffusion experiments showed that although a fraction of the counter ions are expelled from the host cavity by exchange with the guest, a complex with both counter ions and the guest inside the pillararene is formed. The results also showed that at higher concentrations of guest in solution, in addition to the inclusion of one guest molecule in the cavity, the pillararene can also form an external complex with a second guest molecule.     

Nickel(II) meso‐Hydroxyporphyrin Complexes Revisited: Palladium‐Catalysed Synthesis,Electronic Structures of Derived Oxy Radicals,and Oxidative Coupling to a Dioxoporphodimethene Dyad

We report the synthesis and characterisation of new examples of meso‐hydroxynickel(II) porphyrins with 5,15‐diphenyl and 10‐phenyl‐5,15‐diphenyl/diaryl substitution. The OH group was introduced by using carbonate or hydroxide as nucleophile by using palladium/phosphine catalysis. The NiPor?OHs exist in solution in equilibrium with the corresponding oxy radicals NiPor?O^.. The 15‐phenyl group stabilises the radicals, so that the ¹H NMR spectra of {NiPor?OH} are extremely broad due to chemical exchange with the paramagnetic species. The radical concentration for the diphenylporphyrin analogue is only 1 %, and its NMR line‐broadening was able to be studied by variable‐temperature NMR spectroscopy. The EPR signals of NiPor?O^. are consistent with somewhat delocalised porphyrinyloxy radicals, and the spin distributions calculated by using density functional theory match the EPR and NMR spectroscopic observations. Nickel(II) meso‐hydroxy‐10,20‐diphenylporphyrin was oxidatively coupled to a dioxo‐terminated porphodimethene dyad, the strongly red‐shifted electronic spectrum of which was successfully modelled by using time‐dependent DFT calculations.     

1H NMR analysis of O‐methyl‐inositol isomers: a joint experimental and theoretical study

Density functional theory (DFT) calculations of ¹H NMR chemical shifts for l ‐quebrachitol isomers were performed using the B3LYP functional employing the 6‐31G(d,p) and 6‐311 + G(2d,p) basis sets. The effect of the solvent on the B3LYP‐calculated NMR spectrum was accounted for using the polarizable continuum model. Comparison is made with experimental ¹H NMR spectroscopic data, which shed light on the average uncertainty present in DFT calculations of chemical shifts and showed that the best match between experimental and theoretical B3LYP ¹H NMR profiles is a good strategy to assign the molecular structure present in the sample handled in the experimental measurements. Among four plausible O‐methyl‐inositol isomers, the l ‐quebrachitol 2a structure was unambiguously assigned based only on the comparative analysis of experimental and theoretical ¹H NMR chemical shift data. The B3LYP infrared (IR) spectrum was also calculated for the four isomers and compared with the experimental data, with analysis of the theoretical IR profiles corroborating assignment of the 2a structure. Therefore, it is confirmed in this study that a combined experimental/DFT spectroscopic investigation is a powerful tool in structural/conformational analysis studies. Copyright © 2012 John Wiley & Sons, Ltd.     

Solid‐state 13C, 15N and 29Si NMR characterization of block copolymers with CO2 capture properties

Natural abundance solid‐state multinuclear (¹³C, ¹⁵N and ²⁹Si) cross‐polarization magic‐angle‐spinning NMR was used to study structures of three block copolymers based on polyamide and dimethylsiloxane and two polyamides, one of which including ferrocene in its structure. Assignment of most of the resonance lines in ¹³C, ¹⁵N and ²⁹Si cross‐polarization magic‐angle‐spinning NMR spectra were suggested. A comparative analysis of ¹³C isotropic chemical shifts of polyamides with and without ferrocene has revealed a systematic shift towards higher δ ‐values (de‐shielding) explained as the incorporation of paramagnetic ferrocene into the polyamide backbone. In addition, the ¹³C NMR resonance lines for ferrocene‐based polyamide were significantly broadened, because of paramagnetic effects from ferrocene incorporated in the structure of this polyamide polymer. Single resonance lines with chemical shifts ranging from 88.1 to 91.5 ppm were observed for ¹⁵N sites in all of studied polyamide samples. ²⁹Si chemical shifts were found to be around ?22.4 ppm in polydimethylsiloxane samples that falls in the range of chemical shifts for alkylsiloxane compounds. The CO₂ capture performance of polyamide‐dimethylsiloxane‐based block copolymers was measured as a function of temperature and pressure. The data revealed that these polymeric materials have potential to uptake CO₂ (up to 9.6 cm³ g^?1) at ambient pressures and in the temperature interval 30–40 °C. Copyright © 2016 John Wiley & Sons, Ltd.     

The Effect of C4H and C5H on the Microstructure of Aqueous Solutions of 1‐Alkyl‐3‐methylimidazolium Tetrafluoroborate Ionic Liquids

As is well‐known, the C2?H proton of 1‐ethyl‐3‐methylimidazolium tetrafluoroborate ([Emim]BF₄) and 1‐butyl‐3‐methylimidazolium tetrafluoroborate ([Bmim]BF₄) has a strong ability to form hydrogen bonds. The purpose of this work is to evaluate the effect of the interactions of the C4?H and C5?H protons on the microstructure of [Emim]BF₄ and [Bmim]BF₄ with water by using ¹H NMR spectroscopy. The differences between the relative ¹H NMR chemical shifts of C2?H, C4?H, and C5?H and between the interaction‐energy parameters obtained from these chemical shifts are minor, thus suggesting that the interactions of C4?H and C5?H may have a considerable effect on the microstructure. To confirm this, the viscosities of the systems are estimated by using the interaction‐energy parameters obtained from the ¹H NMR chemical shifts of the three studied aromatic protons and water, showing that the interactions of C4?H and C5?H also play an important role in the microstructure.     

Z and E rotamers of N‐formyl‐1‐bromo‐4‐hydroxy‐3‐methoxymorphinan‐6‐one and their interconversion as studied by 1H/13C NMR spectroscopy and quantum chemical calculations

N‐Formyl‐1‐bromo‐4‐hydroxy‐3‐methoxymorphinan‐6‐one (compound 2 ), an important intermediate in the NIH Opiate Total Synthesis, presumably exists as a mixture of two rotamers (Z and E) in both CHCl₃ and DMSO at room temperature due to the hindered rotation of its N‐C18 bond in the amide moiety. By comparing the experimental ¹H and ¹³C chemical shifts of a single rotamer and the mixture of compound 2 in CDCl₃ with the calculated chemical shifts of the geometry optimized Z and E rotamers utilizing density functional theory, the crystalline rotamer of compound 2 was characterized as having the E configuration. The energy barrier between the two rotamers was also determined with the temperature dependence of ¹H and ¹³C NMR coalescence experiments, and then compared with that from the reaction path for the interconversion of the two rotamers calculated at the level of B3LYP/6‐31G*. Detailed geometry of the ground state and the transition states of both rotamers are given and discussed. Copyright © 2012 This article is a US Government work and is in the public domain in the USA.     

Bicyclo[6.3.0]undecapentaenyl Anion: The Next Higher Homolog of the Indenyl Anion with Exceptionally Large Ion‐Pairing Effects on its Tropicity

The title anion 1 was generated as a fairly thermally stable species in tetrahydrofuran (THF) and dimethylsulfoxide (DMSO) by the action of several bases (sodium hydride, potassium hydride, lithium diisopropylamide, and lithium hexamethyldisilazide) with appropriate bicyclo[6.3.0]undecapentaenes. Variable‐temperature ¹H NMR spectra of 1? Li⁺ in [D₈]THF reveal that the anion exhibits exceptionally large ion‐pairing effects; proton chemical shifts vary by more than 1 ppm as a function of ion‐pairing conditions. Thus, anion 1 , in a contact ion pair (Li⁺ at ambient temperature in THF), behaves as an aromatic cyclopentadienyl anion that is perturbed only slightly by the electronic effects of a paramagnetic cyclooctatetraene (COT), whereas 1 in a separated ion pair (Li⁺ at low temperatures in THF or at ambient temperature in DMSO) behaves as an overall paratropic species with a 12 π‐electron periphery. ¹³C NMR spectroscopy indicates no major skeletal rearrangement and only small variations of the electron density. The variable tropicity of 1 can be ascribed to small conformational changes of the molecule. In addition to its unusual, tunable tropicity, anion 1 can also serve as a versatile building block for the synthesis of cyclopentanoid conjugated systems fused to a fully unsaturated eight‐membered ring. A theoretical calculation predicts that the 10‐position of 1 should have the highest electron density. In agreement with this prediction, the reactions of 1 with electrophiles occur predominantly at the 10‐position. The corresponding ferrocene, two fulvenes, two diazo derivatives, and a COT‐fused azulene were obtained by the reactions of 1 with appropriate electrophiles.