Variable-temperature 1H NMR Research Articles

Halogen effect on mixed-valence state of biosmocenium(II, IV) salts

The mixed-valence biosmocenium(II, IV)–Cl adduct [(C5H5)OsII(μ2-η5:η5-C10H8) (C5H5)OsIVCl]Y(1a:YPF6, 1b:YBF4) and biosmocenium-Br adduct [(C5H5)OsII(μ2-η5:η5-C10H8) (C5H5)OsIVBr]PF6(2) were prepared. The structures of 1b and 2 were determined by single crystal X-ray analysis, and it was found that Os-X bond was fixed on cis position to the other half of osmocenyl moiety. Although 1b and 2 had the trapped-valence state in solid state, valence-detrapping depending on temperature was observed in solution state by using variable-temperature 1H NMR spectroscopy. It was found that intramolecular electron exchange reaction OsII–OIV ⇄ OsIV–OsII occurred accompanied by halogen migration. The activation energy of the reaction was determined. The energy decreased in the order A > 2 > 1a depending on a kind of halogen ligand bonded to Os center, and the tendency can be explained by HSAB theory.

Star Diblock Copolymer Concentration Dictates the Degree of Dispersion of Carbon Black Particles in Nonpolar Media: Bridging Flocculation versus Steric Stabilization

The solution behavior of a polystyrene–hydrogenated polyisoprene star diblock copolymer (Mn ∼ 384 K; 6 mol % polystyrene) is examined in nonpolar media. Variable temperature 1H NMR studies using deuterated n-dodecane confirm that the outer polystyrene blocks are only partially solvated in n-dodecane at 25 °C: the apparent polystyrene content of 3.2 ± 0.2 mol % remains essentially constant on heating up to 100 °C. Physical adsorption of this star diblock copolymer onto carbon black particles is examined, with particular attention being paid to the effect of copolymer concentration on colloidal stability. An isotherm is constructed for copolymer adsorption onto carbon black from n-dodecane at 20 °C using a supernatant depletion assay based on UV spectroscopy analysis of the aromatic chromophore in the polystyrene block. Langmuir-type adsorption is observed with a maximum adsorbed amount, Γ, of ∼2.2 ± 0.1 mg m–2. In addition, thermogravimetric analysis is used to directly determine the amount of adsorbed cop...

G-quadruplex induced chirality of methylazacalix[6]pyridine via unprecedented binding stoichiometry: en route to multiplex controlled molecular switch.

Nucleic acid based molecular device is a developing research field which attracts great interests in material for building machinelike nanodevices. G-quadruplex, as a new type of DNA secondary structures, can be harnessed to construct molecular device owing to its rich structural polymorphism. Herein, we developed a switching system based on G-quadruplexes and methylazacalix[6]pyridine (MACP6). The induced circular dichroism (CD) signal of MACP6 was used to monitor the switch controlled by temperature or pH value. Furthermore, the CD titration, Job-plot, variable temperature CD and 1H-NMR experiments not only confirmed the binding mode between MACP6 and G-quadruplex, but also explained the difference switching effect of MACP6 and various G-quadruplexes. The established strategy has the potential to be used as the chiral probe for specific G-quadruplex recognition.

Synthesis and structural characterisation of amides from picolinic acid and pyridine-2,6-dicarboxylic acid.

Coupling picolinic acid (pyridine-2-carboxylic acid) and pyridine-2,6-dicarboxylic acid with N-alkylanilines affords a range of mono- and bis-amides in good to moderate yields. These amides are of interest for potential applications in catalysis, coordination chemistry and molecular devices. The reaction of picolinic acid with thionyl chloride to generate the acid chloride in situ leads not only to the N-alkyl-N-phenylpicolinamides as expected but also the corresponding 4-chloro-N-alkyl-N-phenylpicolinamides in the one pot. The two products are readily separated by column chromatography. Chlorinated products are not observed from the corresponding reactions of pyridine-2,6-dicarboxylic acid. X-Ray crystal structures for six of these compounds are described. These structures reveal a general preference for cis amide geometry in which the aromatic groups (N-phenyl and pyridyl) are cis to each other and the pyridine nitrogen anti to the carbonyl oxygen. Variable temperature 1H NMR experiments provide a window on amide bond isomerisation in solution.

Synthesis and characterization of a potential bifunctional C60-Ih fullerene-based catechol amide ligand

The new C60-Ih fullerene-based catechol amide derivatives C60{C[COO(CH2)nCAM]2} [n = 2, 3 or 4; CAM = 2,3-Ph(OH)2CONH] were prepared using a modified Bingel-type reaction of C60-Ih and characterized by variable-temperature 1H NMR spectroscopy.

Four-Coordinate Iron(II) Diaryl Compounds with Monodentate N-Heterocyclic Carbene Ligation: Synthesis, Characterization, and Their Tetrahedral-Square Planar Isomerization in Solution.

The salt elimination reactions of (IPr2Me2)2FeCl2 (IPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with the corresponding aryl Grignard reagents afford [(IPr2Me2)2FeAr2] (Ar = Ph, 3; C6H4-p-Me, 4; C6H4-p-(t)Bu, 5; C6H3-3,5-(CF3)2, 6) in good yields. X-ray crystallographic studies revealed the presence of both tetrahedral and trans square planar isomers for 3 and 6 and the tetrahedral structures for 4 and 5. Magnetic susceptibility and (57)Fe Mössbauer spectrum measurements on the solid samples indicated the high-spin (S = 2) and intermediate-spin (S = 1) nature of the tetrahedral and square planar structures, respectively. Solution property studies, including solution magnetic susceptibility measurement, variable-temperature (1)H and (19)F NMR, and absorption spectroscopy, on 3-6, as well as an (57)Fe Mössbauer spectrum study on a frozen tetrahydrofuran solution of tetrahedral [(IPr2Me2)2(57)FePh2] suggest the coexistence of tetrahedral and trans square planar structures in solution phase. Density functional theory calculations on (IPr2Me2)2FePh2 disclosed that the tetrahedral and trans square planar isomers are close in energy and that the geometry isomerization can occur by spin-change-coupled geometric transformation on four-coordinate iron(II) center.

Preparation and characterization of CS-g-PNIPAAm microgels and application in a water vapour-permeable fabric

Chitosan-graft-poly(N-isopropylacrylamide) (CS-g-PNIPAAm) was synthesised using sonication with and without the crosslinker, N,N′-methylenebisacrylamide (MBA). FTIR, variable-temperature 1H NMR spectroscopy, atomic force microscopy, UV–vis spectrophotometry, differential scanning calorimetry, and dynamic light scattering were used to characterize the microgels’ chemical constituents, structures, morphologies, lower critical solution temperatures (LCSTs), and thermo- and pH-responsiveness. The chemical structures of the two CS-g-PNIPAAm materials were found to be similar and both exhibited dual responsiveness towards temperature and pH. The microgel containing MBA had a higher LCST, smaller diameter, and more compact structure, but exhibited opposite pH- and similar thermo-responsiveness. Although the structure of the microgel particles prepared without crosslinking was unstable, the stability of the crosslinked microgel particles enabled them to be finished onto fabric. Because the microgel prepared with MBA retains thermosensitivity, it can be used to impart controllable water vapour permeability properties. The incorporation of the MBA-crosslinked CS-g-PNIPAAm microgel particles in cotton fabric was accomplished by a simple pad-dry-cure procedure from an aqueous microparticle dispersion. The water vapour permeation of the finished fabric was measured at 25 and 40°C and 50 and 90% relative humidities. The finished fabric displayed an obviously high water vapour permeability at 40°C.

Effects of the PEG molecular weight of a PEG-lipid and cholesterol on PEG chain flexibility on liposome surfaces

Abstract A variable-temperature 1H NMR study was performed to investigate the effects of the molecular weight of poly(ethylene glycol) (PEG) in PEG-lipids and cholesterol addition to the lipid bilayer on PEG chain flexibility at the liposomal surface. PEG-lipids, i.e., l -α-distearoylphosphatidylethanolamine (DSPE)-PEG, with PEG molecular weights of 750, 2000, and 5000 were modified to liposomes of ca. 100 nm. The 1H NMR peak of PEG in DSPE-PEG was overlapped by broad and sharp peaks, corresponding to rigid and flexible PEG components, respectively. When the PEG molecular weight was increased, the PEG peak became sharp, indicating that long-chain PEGs were more flexible on the liposome surface. The proportion of flexible components projecting into the water phase increased as the PEG chain length increased. Peak sharpening also occurred when the cholesterol content was increased from 0 to 30 mol%, demonstrating that cholesterol incorporation into the lipid bilayer enhanced the PEG chain flexibility. In addition, the PEG chain flexibility significantly increased when cholesterol was delocalized in the lipid bilayer at concentrations above 20 mol%. Lateral diffusion of the lipid with the presence of cholesterol in the lipid bilayer significantly affected the PEG chain flexibility.

X-ray structure and conformational study of tris(2-(dimethylamino)phenyl)methanol: Roles of hydrogen bond in conformational conversions of molecules

Tris(2-(dimethylamino)phenyl)methanol, (2-(Me)2N-C6H4)3COH, was synthesized and characterized by using NMR, EA and ESI-MS. Single crystal of the 2-syn/1-anti conformer of the tertiary alcohol was obtained and analyzed by X-ray diffraction. The variable-temperature 1H NMR experiments reveal that, in solution, the tertiary alcohol is an equilibrium mixture of four conformers, all-syn, 2-syn/1-anti, 1-syn/2-anti and all-anti. The results show that hydrogen bond plays an important role in both intra- and inter-conformational conversions of molecules. The intra-conformational conversions within specific conformers are just helical changes. The inter-conformational conversions between conformers occurred via one-ring flip process.

ChemInform Abstract: Synthesis and Biological Evaluation of the Osteoblast Proliferating Cyclic Peptides Dianthins G and H.

Abstract The first synthesis and osteoblast proliferative activity of the naturally occurring cyclic peptides dianthins G and H is described. The greater potency of naturally occurring dianthin G over dianthin H at physiological concentrations mirrored the osteoblast proliferative activity observed for synthetic dianthins G and H. Six alanine-scan analogues of the more potent dianthin G were also synthesised and osteoblast assays revealed that four of the six residues can be further modified for improved activity. We also confirmed by variable temperature 1H NMR spectroscopic analysis that the sets of major and minor signals observed for dianthins G and H in DMSO-d6 are in fact due to cis–trans rotational isomers of the proline ring.

Aluminabenzene–Zirconium Complexes: Intramolecular Coordination of Chloride to Aluminum

The reaction of the previously reported anionic aluminabenzene with zirconium chlorides (CpZrCl3, Cp*ZrCl3, and ZrCl4) gave novel zirconium complexes bearing aluminabenzene ligand(s). The X-ray crystallographic analysis of the resulting complexes revealed that the aluminabenzene ligand(s) coordinated to the zirconium center in an η5-fashion as a six-electron donor. Chloride ligand(s) on the zirconium center intramolecularly coordinated to the Lewis acidic aluminum atom in the aluminabenzene ring. The variable-temperature 1H NMR experiment showed a dynamic process for dissociation and recoordination of chloride to the aluminum center.

Chiral Dopants Derived from Ephedrine/Pseudoephedrine: Structure and Medium Effects on the Helical Twisting Power

Chiral dopants were obtained by acylation of enantiomerically pure ephedrine and pseudoephedrine with promesogenic carbonyl reagents. The products have been investigated with respect to their chiral transfer ability on nematic host matrices characterized by extreme differences of the dielectric anisotropy. It has been found that the medium dependence of the helicity induction nearly disappears at reduced temperatures. Based on variable temperature 1H NMR studies on monoacylated homologues, the estimated coalescence temperatures and free activation enthalpies for the hindered rotation around C‒N bonds could be correlated with the helical twisting power. Measurements by dielectric spectroscopy reveal the correlation between the molar mass of substituents linked to the chiral building block and the dynamic glass transition of corresponding chiral dopants. Furthermore, the effect of intramolecular and intermolecular hydrogen bonds has been studied by ATR-FTIR spectroscopy.

Structures of tetrasilylmethane derivatives (XMe2Si)2C(SiMe3)2 (X = H, Cl, Br) in the gas phase, and their dynamic structures in solution.

The structures of the molecules (XMe2Si)2C(SiMe3)2, where X = H, Cl, Br, have been determined by gas electron diffraction (GED) using the SARACEN method of restraints, with all analogues existing in the gas phase as mixtures of C1- and C2-symmetric conformers. Variable temperature (1)H and (29)Si solution-phase NMR studies, as well as (13)C NMR and (1)H/(29)Si NMR shift correlation and (1)H NMR saturation transfer experiments for the chlorine and bromine analogues, are reported. At low temperatures in solution there appear to be two C1 conformers and two C2 conformers, agreeing with the isolated-molecule calculations used to guide the electron diffraction refinements. For (HMe2Si)2C(SiMe3)2 the calculations indicated six conformers close in energy, and these were modeled in the GED refinement.

1H Solid-State NMR Study of Nanothin Nafion Films

The unique behaviors of Nafion nanothin films with thicknesses of 10 nm (ultrathin) and 160 nm (thin) were evaluated using variable-temperature and variable-humidity solid-state 1H NMR spectroscopy. These unprecedented measurements of nanothin films stacked within an NMR rotor represent a remarkable experimental achievement and demonstrate that 1H NMR spectroscopy of such minute amounts of ionomer might be possible within active catalyst layers in polymer electrolyte fuel-cell electrodes. This study was motivated by the observation, in a separate work, of thickness-dependent and highly suppressed conductivity in nanothin films of Nafion (4–300 nm) compared to counterpart free-standing Nafion membranes. Trends in the line width and, more precisely, the T2 relaxation, as probed using a Hahn echo, showed that the local mobility within the hydrogen-bonded domain is equivalent for 10 and 160 nm films and is governed by the fast exchange limit in terms of NMR time scales. Subtle differences in the chemical shif...

Dynamic stereoisomerization in inherently chiral bimetallic [2]catenanes

Stereoisomerization and the unprecedented phenomenon of metal translocation in the absence of redox processes were probed in two inherently chiral bimetallic [2]catenanes by using a combination of variable-temperature (1)H NMR and CD spectroscopies, X-ray crystallography, and DFT calculations.

Structures of Tetrasilylmethane Derivatives C(SiXMe2)4 (X=H, F, Cl, Br) in the Gas Phase and their Dynamic Structures in Solution

Abstract The structures of the molecules C(SiXMe2)4 (X=H, F, Cl, Br) have been determined by gas electron diffraction (GED). Ab initio calculations revealed nine potential minima for each species, with significant ranges of energies. For the H, F, Cl, and Br derivatives nine, seven, two, and two conformers were modelled, respectively, as they were quantum-chemically predicted to be present in measurable quantities. Variable-temperature 1H and 29Si solution-phase NMR studies and, where applicable, 13C NMR, 1H/29Si NMR shift-correlation, and 1H NMR saturation-transfer experiments are reported for C(SiXMe2)4 (X=H, Cl, Br, and also I). At low temperature in solution two conformers (one C1-symmetric and one C2-symmetric) are observed for each of C(SiXMe2)4 (X=Cl, Br, I), in agreement with the isolated molecule ab initio calculations carried out as part of this work for X=Cl, Br. C(SiHMe2)4 is present as a single C1-symmetric conformer in solution at the temperatures at which the NMR experiments were performed.

Highly Electron-Rich Pincer-Type Iron Complexes Bearing Innocent Bis(metallylene)pyridine Ligands: Syntheses, Structures, and Catalytic Activity

The first neutral bis(metallylene)pyridine pincer-type [ENE] ligands (E = SiII, GeII) were synthesized, and their coordination chemistry and reactivity toward iron was studied. First, the unprecedented four-coordinate complexes κ2E,E′-[ENE]FeCl2 were isolated. Unexpectedly and in contrast to other related pyridine-based pincer-type Fe(II) complexes, the N atom of pyridine is reluctant to coordinate to the Fe(II) site due to the enhanced σ-donor strength of the E atoms, which disfavors this coordination mode. Subsequent reduction of κ2Si,Si′-[SiNSi]FeCl2 with KC8 in the presence of PMe3 or direct reaction of the [ENE] ligands using Fe(PMe3)4 produced the highly electron-rich iron(0) complexes [ENE]Fe(PMe3)2. The reduction of the iron center substantially changes its coordination features, as shown by the results of a single-crystal X-ray diffraction analysis of [SiNSi]Fe(PMe3)2. The iron center, in the latter, exhibits a pseudosquare pyramidal (PSQP) coordination environment, with a coordinative (pyridine)N→Fe bond, and a trimethylphosphine ligand occupying the apical position. This geometry is very unusual for Fe(0) low-spin complexes, and variable-temperature 1H and 31P NMR spectra of the [ENE]Fe(PMe3)2 complexes revealed that they represent the first examples of configurationally stable PSQP-coordinated Fe(0) complexes: even after heating at 70 °C for >7 days, no changes are observed. The substitution reaction of [ENE]Fe(PMe3)2 with CO resulted in the isolation of [ENE]Fe(CO)2 and the hitherto unknown κ2E,E′-[ENE]Fe(CO)2L (L = CO, PMe3) complexes. All complexes were fully characterized (NMR, MS, XRD, IR, and 57Fe Mössbauer spectroscopy), showing the highest electron density on the iron center for pincer-type complexes reported to date. DFT calculations and 57Fe Mössbauer spectroscopy confirmed the innocent behavior of these ligands. Moreover, preliminary results showed that these complexes can serve as active precatalysts for the hydrosilylation of ketones.

A variable temperature study of the transport properties of aqueous solutions of VOSO4 and NH4VO3 in 2 M H2SO4

Variable temperature ionic conductivity, viscosity and 1H NMR spin-lattice relaxation times, linewidths and chemical shifts measurements were determined for various concentrations of aqueous ammonium metavanadate (NH4VO3) and vanadyl sulfate (VOSO4) over the temperature range of 20–100 °C. Concentration ranges of 0.05–2 M and 0.05–0.2 M were investigated for VOSO4 and NH4VO3 respectively in 2 M H2SO4. Results show ion and proton transport processes that are mediated strongly by increasing solution viscosity, especially in the case of VOSO4. Activation energies calculated from ionic conductivity and solution viscosity show greater energy being needed for translational ion dynamics with increasing concentration for both solutions. Additionally, data suggest that with greater metal ion concentration ion association is more prevalent for the vanadyl ion. The differences between the transport capabilities of the two solution are discussed in terms of the 1H NMR, ionic conductivity and viscosity data.

A CuN2S2 core with a high potential copper(II/I) couple. Use of a surface modified electrode and generation of five coordinate copper(I)

Using the 1:2 condensate of benzil dihydrazone and 2-(methylthio)benzaldehyde as a ligand (L), a cationic copper(I) complex [CuL]PF6·0.5THF (THF≡tetrahydrofuran; 1) is synthesized. The X-ray crystal structure of the desolvated species [CuL]PF6 has been determined and is found to contain a CuN2S2 core. The corresponding copper(II) complex could not be obtained. The cyclic voltammograms of 1 at a Pt or a glassy carbon electrode show features of adsorption, but it shows a quasireversible Cu(II/I) couple at a gold electrode modified by thiocyanate. The redox potential is 0.79V versus NHE. Complex 1 reacts with PPh3 in CH2Cl2 in 1:1M proportion and subsequent addition of n-hexane yields [CuL(PPh3)]PF6·0.25n-hexane·4H2O (2). Our bp86/6-311g(2d,p) calculations indicate two plausible structures A and B for the cation in 2. In A, copper(I) is 5-coordinate with a trigonal bipyramidal N2S2P coordination sphere (the P atom and an azine N occupy the apices) and in B, the metal has a tetrahedral N2SP coordination sphere with one of the two S atoms of L remaining uncoordinated. Binding of PPh3 to the metal in [CuL]+ is theoretically exothermic; structure B is energetically more stable than A by 4.5kcalmol−1. In solution there exists an equilibrium between A and B. From variable temperature 1H NMR of 2, an average ΔH0 value of −2.8kcalmol−1 is obtained for the equilibrium thus showing that enthalpy wise B is more stable than A which matches well with the DFT results. The equilibrium is not favoured entropically. Existence of the equilibrium is also apparent in the 31P NMR of 2.

2]Rotaxane with multiple functional groups.

High-yield syntheses of Cu(II)- and Ni(II)-templated [2]pseudorotaxane precursors (CuPRT and NiPRT, respectively) were achieved by threading bis(azide)bis(amide)-2,2'-bipyridine axle into a bis(amide)tris(amine) macrocycle. Single-crystal X-ray structural analysis of CuPRT revealed complete threading of the axle fragment into the wheel cavity, where strong aromatic π-π stacking interactions between two parallel arene moieties of the wheel and the pyridyl unit of axle are operative in addition to metal ion templation. Attachment of a newly developed bulky stopper molecule with a terminal alkyne to CuPRT via a Cu(I)-catalyzed azide-alkyne cycloaddition reaction failed as a result of dethreading of the azide-terminated axle under the reaction conditions. However, the synthesis of a metal-free [2]rotaxane containing triazole with other functionalities in the axle was achieved in ∼45% yield upon coupling between azide-terminated NiPRT and the alkyne-terminated stopper. The [2]rotaxane was characterized by mass spectrometry, 1D and 2D NMR (COSY, DOSY, and ROESY) experiments. Comparative solution-state NMR studies of the [2]rotaxane in its unprotonated and protonated states were carried out to locate the position of the wheel on the axle of the metal-free [2]rotaxane. Furthermore, a variable-temperature (1)H NMR study in DMSO-d6 of [2]rotaxane supported the kinetic inertness of the interlocked structure, where the newly developed stopper prevents dethreading of the 30-membered wheel from the axle.