Low-temperature NMR Research Articles

Dynamic Solid-State NMR Experiments Reveal Structural Changes for a Methyl Silicate Nanostructure on Deuterium Substitution

Structural characterizations of three different solid–gas reaction products, recently obtained from abraded solid-state silicate free radicals reacting with two isotopically enriched methane gases, 13CH4 and CD4 (a possible sink for methane on MARS) and with 13CO2, are derived from various dynamic solid-state NMR experiments. These include cross-polarization/depolarization zero-cross times (ZCTs), variable temperature (VT) NMR to study 3-site jump CH3/CD3 activation energies (Ea), and 13CO2/13CH3 molecular species as a spy to determine the approximate diameters for the channel structures for some of these structures. Literature Ea data indicate that l-alanine and 4-CH3-phenanthrene exhibit the highest known Ea values (= 20–22.6 kJ/mol) for CH3 3-site jump motions. The ZCTs for these two compounds are 120 and 162 μs, respectively, indicative of the high Ea values for CH3/CD3 groups. Determination of Ea for 4-CD3-phenanthrene by low-temperature 2H MAS NMR experiments supplemented the previously reported liquid-state Ea value (Ea = 21 kJ/mol) for 4-CH3-phenanthrene. Finally, such experiments also revealed the structural difference for the free-radical reaction products with 13CH4 and CD4, i.e, a change from helical to chain structure.

Nickel-Catalyzed Copolymerization of Ethylene and Vinyltrialkoxysilanes: Catalytic Production of Cross-Linkable Polyethylene and Elucidation of the Chain-Growth Mechanism.

Copolymerizations of ethylene with vinyltrialkoxysilanes using cationic (α-diimine)Ni(Me)(CH3CN)+ complexes 4a,b/B(C6F5)3 yield high molecular weight copolymers exhibiting highly branched to nearly linear backbones depending on reaction conditions and catalyst choice. Polymerizations are first-order in ethylene pressure and inverse-order in silane concentration. Microstructural analysis of the copolymers reveals both in-chain and chain-end incorporation of -Si(OR)3 groups whose ratios depend on temperature and ethylene pressure. Detailed low-temperature NMR spectroscopic investigations show that well-defined complex 3b (α-diimine)Ni(Me)(OEt2)+ reacts rapidly at -60 °C with vinyltrialkoxysilanes via both 2,1 and 1,2 insertion pathways to yield 4- and 5-membered chelates, respectively. Such chelates are the major catalyst resting states but are in rapid equilibrium with ethylene-opened chelates, (α-diimine)Ni(R)(C2H4)+ complexes, the species responsible for chain growth. Chelate rearrangement via β-silyl elimination accounts for formation of chain-end -Si(OR)3 groups and constitutes a chain-transfer mechanism. Chelate formation and coordination of the Ni center to the ether moiety, R-O-Si, of the vinylsilane somewhat decreases the turnover frequency (TOF) relative to ethylene homopolymerization, but still remarkably high TOFs of up to 4.5 × 105 h-1 and overall productivities can be achieved. Activation of readily available (α-diimine)NiBr2 complexes 2 with a combination of AlMe3/B(C6F5)3/[Ph3C][B(C6F5)4] yields a highly active and productive catalyst system for the convenient synthesis of the copolymer, a cross-linkable PE. For example, copolymers containing 0.23 mol % silane can be generated at 60 °C, 600 psig ethylene over 4 h with a productivity of 560 kg copolymer/g Ni. This method offers an alternative route to these materials, normally prepared via radical routes, which are precursors to the commercial cross-linked polyethylene, PEX-b.

Synthesis and reactivity of a cationic palladium complex as possible intermediate in a Suzuki-Miyaura cross-coupling reaction

A stoichiometric reaction of a palladium fluorido complex with an aromatic boronic ester yielded a fluorinated 4-aryl phenylalanine derivative (aryl=4-C6H4SF5) as Suzuki-Miyaura cross-coupling product. Due to the high reactivity of the metal complex, the reaction proceeded smoothly at ambient temperature. Low-temperature NMR investigations revealed a possible role of trans‐[Pd{BF(4-C6H4SF5)(pin)}(PheEt)(PiPr3)2] (5, PheEt=bound phenylalanine derivative=4-C6H4CH2C{NHC(O)CH3}(CO2Et)2, pin=pinacolato=O2C2Me4) as a potential intermediate of the transmetallation step. Compound 5 resulted from fluoride transfer from palladium to boron. The similar complex trans-[Pd(BF4)(PheEt)(PiPr3)2] (7) was generated on treatment of the fluorido complex with NaBF4. Complex 7 is not stable at room temperature. Degradation gave the phosphonium salt [PiPr3PheEt][BF4] (4). Interestingly, the same compound was also found in the initially mentioned cross-coupling reaction as minor product.

Influence of tannin on aqueous layers at a surface of hydrophilic and hydrophobic nanosilicas

Pharmaceutical tannin was stirred with hydrophilic nanosilica A-300, hydrophobic nanosilica AM1 or their mixture (1:1). Tannin microparticles were strongly grinded during mechanical treatment with nanosilica to form much smaller tannin micro- and nanoparticles. The composites were studied with addition of water or acidic solution of HCl or F3CCOOD using low-temperature 1H NMR spectroscopy, NMR cryoporometry, microscopy, infrared spectroscopy, and quantum chemistry. Treated composites are characterized by reduced affinity toward water in comparison to hydrophilic A-300, but greater than that for initial tannin or hydrophobic AM1. The composites are characterized by increased mobility of organic molecules interacting with adsorbed water, acidic solution or chloroform. Consequently, grinded tannin can be more active in composites with nanosilicas than in the initial powder with microparticles. Chloroform affects location of bound water or acidic solution in voids between silica nanoparticles or weakly porous tannin particles because water and chloroform are practically immiscible and tend to reduce the contact area of one to another. Therefore, smaller water molecule can locate in narrow nanopores inaccessible for CDCl3 or in broad pores to form large domains. Upon location of a mixture of tannin with A-300/AM1 with bound water or acidic solution in the chloroform medium, hydrophobic AM1 nanoparticles can form a shell around hydrophilic A-300/tannin particles with bound water. In this case, contacts between bound water and chloroform can be minimal. Therefore, the system can be stable. This stability is higher for the composites with tannin than that for A-300/AM1 alone.

Lewis Acid Assisted C–CN Cleavage of Benzonitrile Using [(dippe)NiH

Reactions of [(dippe)NiH]2 with benzonitrile and varying concentrations of Lewis acids (primarily BPh3 and BF3) have shown a dramatic variation of reaction rate compared to the same reaction without Lewis acids. When less than one equivalent of Lewis acid is used, the reaction rate is as much as 100 times greater than without Lewis acid. Boron exchange was observed with less than one equivalent of Lewis acid allowing the formation of a Ni(0)–η2-aryl complex (observed by low-temperature NMR and calculations) to which the Lewis acid is postulated to re-coordinate as the rate-limiting step allowing the formation of a stable Ni(II) product. When one equivalent or greater of Lewis acid is used, the reaction shows dramatic inhibition even compared to the reaction without Lewis acid. Lewis acid dissociation can be considered as the rate-limiting step under these conditions. The overall work detailed herein has shown the importance of Lewis acids in the activation of benzonitriles due to the strengthening of the C–N bond allowing for increased stabilization of the products.

First Detection of Unprotected 1,2-Anhydro Aldopyranoses

The unprotected 1,2-anhydro aldopyranoses have been detected for the first time using low-temperature NMR spectroscopy. The formation of these unstable chemical species was successfully achieved through an intramolecular dehydration of saccharides in aqueous media under extremely mild and essentially neutral conditions.

The features of absorption of aqueous–organic mixtures by polyurethane–poly(2-hydroxyethyl methacrylate) matrix by the data of NMR spectroscopy

The features of hydration of the interpenetrating polymer network consisting of 83% polyurethane and 17% poly(2-hydroxyethyl methacrylate) as well as the initial polymers are studied by the method of lowtemperature NMR spectroscopy. It is shown that the two-component polymer matrix is capable of absorption of significant amounts of water in the form of clusters, with the size strongly depending on the presence of organic compounds, such as chloroform and trifluoroacetic acid. Thus, the changes in the medium composition allow one to control the state of water absorbed by the polymer in a wide range. The found effect may be used to create materials with the controlled delivery of biologically active substances whose retention by the matrix is determined to a great extent by the energy of binding of water by the polymer.

Quantifying the Kinetics of the Gilch Polymerization toward Alkoxy-Substituted Poly(p-phenylene vinylene)

The Gilch polymerization is one of the most popular routes toward high molecular weight alkoxy-substituted poly(p-phenylenevinylenes) (PPV) applied in, for instance, organic electronics and bioimaging. As the interplay between optoelectronic performance and (synthesis-related) defects represents an active area of research, control over the polymerization is of utmost importance. In this work we quantify for the first time the rate constants of the reaction steps of the Gilch polymerization. We obtain these values by fitting concentration transients of various key intermediates, measured by in situ low-temperature (−68 °C) 1H NMR spectroscopy, to kinetic models based on sets of coupled rate equations. The modeling not only accounts for the usual processes of initiation, propagation, transfer, and radical recombination but also involves the side reaction cascade associated with the presence of residual water in the reaction mixture. The results demonstrate that chain growth initiation by active monomer dime...

Tricyanomethane and Its Ketenimine Tautomer: Generation from Different Precursors and Analysis in Solution, Argon Matrix, and as a Single Crystal.

Solutions of azidomethylidenemalononitrile were photolyzed at low temperatures to produce the corresponding 2H-azirine and tricyanomethane, which were analyzed by low-temperature NMR spectroscopy. The latter product was also observed after short thermolysis of the azide precursor in solution whereas irradiation of the azide isolated in an argon matrix did not lead to tricyanomethane, but to unequivocal detection of the tautomeric ketenimine by IR spectroscopy for the first time. When the long-known "aquoethereal" greenish phase generated from potassium tricyanomethanide, dilute sulfuric acid, and diethyl ether was rapidly evaporated and sublimed, a mixture of hydronium tricyanomethanide and tricyanomethane was formed instead of the previously claimed ketenimine tautomer. Under special conditions of sublimation, single crystals of tricyanomethane could be isolated, which enabled the analysis of the molecular structure by X-ray diffraction.

Synthesis of Heterocycle-Containing 9,9-Diarylfluorenes Using Superelectrophiles.

A superacid-promoted method for the synthesis of 9,9-diarylfluorenes is described. The chemistry involves cyclizations and arylations with biphenyl-substituted heterocyclic ketones and a mechanism is proposed involving superelectrophilic intermediates. The key reactive intermediates-dicationic and trication fluorenyl cations have been observed by low-temperature NMR and the mechanism has been further studied using DFT calculations.

Superacids Based on Pentafluoroorthotellurate Derivatives of Aluminum.

The new Lewis acid Al(OTeF5 )3 and its acetonitrile adduct CH3 CN→Al(OTeF5 )3 were obtained by a simple one-step synthesis in batches of up to 15 g. Al(OTeF5 )3 and the adduct were characterized by vibrational spectroscopy (IR, Raman) and quantum-chemical calculations. Furthermore, five different salts of the new weakly coordinating anion [Al(OTeF5 )4 ]- were prepared in a two-step procedure. [Ph4 P][Al(OTeF5 )4 ], Cs[Al(OTeF5 )4 ], [Ph3 C][Al(OTeF5 )4 ], as well as the protonated benzene derivatives [C9 H13 ][Al(OTeF5 )4 ] and [C6 H7 ][Al(OTeF5 )4 ] were characterized by low-temperature single-crystal X-ray diffraction and NMR spectroscopy. Arenium salts have rarely been characterized in the solid state and were synthesized in this work in a simplified fashion.

Syntheses and structures of square planar diplatinum butadiynediyl complexes with two different monophosphine ligands on each terminus; probing the feasibility of a new type of inorganic atropisomerism

Reactions of trans-(C6F5)(p-tol3P)2Pt(Cl) (PtCl) and R2PhP (1.0 equiv; R = a/Me, b/p-t-BuC6H4, c/p-MeOC6H4, d/n-Pr; CH2Cl2/rt (a) or toluene/reflux (b-d)) give mainly trans-(C6F5)(R2PhP)(p-tol3P)Pt(Cl) (Pt′Cl-a-d, 89-29%) and some disubstitution products trans-(C6F5)(R2PhP)2Pt(Cl) (Pt″Cl-a-d, 4–13%). No substitution occurs with t-Bu2PhP. However, (cod)(C6F5)Pt(Cl) and t-Bu2PhP (2.7 equiv; toluene/reflux) react to give trans-(C6F5)(t-Bu2PhP)2Pt(Cl) (Pt″Cl-e, 64%), which upon treatment with p-tol3P (1.0 equiv, toluene/reflux) yields trans-(C6F5)(t-Bu2PhP)(p-tol3P)Pt(Cl) (Pt′Cl-e, 91%). Additions of excess butadiyne to Pt′Cl-a-d (CH2Cl2, cat. CuI, HNEt2) afford the butadiynyl complexes trans-(C6F5)(R2PhP)(p-tol3P)Pt(C≡C)2H (Pt′C4H-a-d, 36–77%), but Pt′Cl-e does not similarly react. Cross couplings of Pt′Cl-a-c and Pt′C4H-a-c (cat. CuI, HNEt2) give mixtures of diplatinum butadiynyl complexes in which the two unlike phosphine ligands scramble over all four positions (PtC4Pt, PtC4Pt′-a-c, PtC4Pt″-a-c, Pt′C4Pt′-a-c, Pt′C4Pt″-a-c, Pt″C4Pt″-b; TLC separable, 27-2% each). A modified coupling recipe is tested with Pt′Cl-b,d and Pt′C4H-b,d (t-BuOK, KPF6, cat. CuCl), and gives Pt′C4Pt′-b,d (21–76%) with only traces of scrambling. Crystal structures of Pt″Cl-e, Pt′C4H-a, Pt′C4Pt′-a-d, and PtC4Pt″-b are determined, and the endgroup/endgroup interactions analyzed. Low temperature NMR spectra do not reveal any dynamic processes.

Suzuki-Miyaura Cross-Coupling Reactions of Highly Fluorinated Arylboronic Esters: Catalytic Studies and Stoichiometric Model Reactions on the Transmetallation Step.

Fluorinated 4-aryl phenylalanine amino acid derivatives (aryl=2-C5 NF4 , 4-C6 H4 SF5 , 2-C6 H4 SCF3 , C6 F5 ) were obtained in Suzuki-Miyaura cross-coupling reactions of phenylalanine precursors with highly fluorinated aryl boronic acid and esters in the presence of CsF. PdII complexes that possessed phenyl alanine derived ligands, such as trans-[PdBr{4-C6 H4 CH2 CH{NHC(O)CH3 }CO2 Et}(PiPr3 )2 ], were used as catalysts. Stoichiometric model reactions indicate the intermediate generation of the boronate Cs[BF(2-C5 NF4 )(pin)] (pin=pinacolato=O2 C2 Me4 ). The transmetallation step with Cs[BF(4-C6 H4 SF5 )(pin)], [NMe4 ][BF(2-C5 NF4 )(pin)] and/or the fluorido complex trans-[PdF{4-C6 H4 CH2 C{NHC(O)CH3 }(CO2 Et)2 }(PiPr3 )2 ] was investigated. A comparison of the reactions of trans-[PdX{4-C6 H4 CH2 C{NHC(O)CH3 }(CO2 Et)2 }(PiPr3 )2 ] (X=Br, F) with 2-Bpin-C5 NF4 or [NMe4 ][BF(2-C5 NF4 )(pin)] revealed the remarkable high reactivity of the fluorido complex towards 2-BpinC5 NF4 . Low-temperature NMR studies indicated the ionic species trans-[Pd{4-C6 H4 CH2 C{NHC(O)CH3 }(CO2 Et)2 }(PiPr3 )2 ][BF(2-C5 NF4 )(pin)] to be a highly reactive intermediate.

George A. Olah (1927-2017).

George A. Olah (1927-2017).

Secondary Interactions Arrest the Hemiaminal Intermediate To Invert the Modus Operandi of Schiff Base Reaction: A Route to Benzoxazinones.

Discovered by Hugo Schiff, condensation between amine and aldehyde represents one of the most ubiquitous reactions in chemistry. This classical reaction is widely used to manufacture pharmaceuticals and fine chemicals. However, the rapid and reversible formation of Schiff base prohibits formation of alternative products, of which benzoxazinones are an important class. Therefore, manipulating the reactivity of two partners to invert the course of this reaction is an elusive target. Presented here is a synthetic strategy that regulates the sequence of Schiff base reaction via weak secondary interactions. Guided by the computational models, reaction between 2,3,4,5,6-pentafluoro-benzaldehyde with 2-amino-6-methylbenzoic acid revealed quantitative (99%) formation of 5-methyl-2-(perfluorophenyl)-1,2-dihydro-4H-benzo[d][1,3]oxazin-4-one (15). Electron donating and electron withdrawing ortho-substituents on 2-aminobenzoic acid resulted in the production of benzoxazinones 9-36. The mode of action was tracked using low temperature NMR, UV-vis spectroscopy, and isotopic (18O) labeling experiments. These spectroscopic mechanistic investigations revealed that the hemiaminal intermediate is arrested by the hydrogen-bonding motif to yield benzoxazinone. Thus, the mechanistic investigations and DFT calculations categorically rule out the possibility of in situ imine formation followed by ring-closing, but support instead hydrogen-bond assisted ring-closing to prodrugs. This unprecedented reaction represents an interesting and competitive alternative to metal catalyzed and classical methods of preparing benzoxazinone.

Influence of Nanosilica on Water-phase Transitions in Hygroscopic Systems

By low-temperature 1H NMR spectroscopy and thermogravimetry methods, the effect of hydrophobic (AM1) and hydrophilic (A-300) nanosilica on the process of evaporation and freezing water, linked to hygroscopic grape seed extract, biopreparation Enoxil (Ex), has been studied. It is shown that nanosilica modifies the character of binding water in composite systems. The total amount of water retained in the composite becomes substantially smaller than in pure biopreparation. This effect can be used to stabilize lyophilized biological tissues and cell cultures.

Doubly Diastereoconvergent Preparation and Microsolvation-Controlled Properties of (Z)- and (E)-1'-Lithio-1'-(2,6-dimethylphenyl)propenes.

A doubly diastereoconvergent reaction can ad libitum generate either one or the other of two diastereomeric products with complete consumption of the diastereomeric precorsors or their mixtures. Thus, the preparation of configurationally pure (Z)-1'-lithio-1'-(2,6-dimethylphenyl)propene [(Z)-1] from any Z,E mixture of the corresponding bromoalkenes with n-butyllithium succeeded by means of a user-friendly (E)-1 → (Z)-1 configurational interconversion. The subsequent treatment of (Z)-1 with a minimum amount of THF afforded exclusively (E)-1 as the other diastereomeric product and was mediated by a beneficial (Z)-1 → (E)-1 interconversion. This behavior provided microsolvation-controlled choices of highly diastereoselective derivatizations of 1. Low-temperature 13C NMR spectra established that (Z)-1 was dissolved as a trisolvated monomer in THF but as a disolvated dimer in monodentate, ethereal, non-THF solvents, whereas (E)-1 was always monomeric. Backed by such knowledge, kinetic experiments indicated that the electrophiles 1-bromobutane or ClSiMe3 in Et2O reacted at 32 °C with the tiny (NMR-invisible) population of monomeric (Z)-1 that was formed in a mobile equilibrium from the inactive, predominantly dimeric (Z)-1. The equilibration of monomeric (Z)-1 and (E)-1 in THF as the solvent was fast (seconds on the 1H NMR time scale), whereas the corresponding stereoinversion of both solvated and unsolvated (E)-1 → (Z)-1 in non-THF solvents occurred on the laboratory time scale (minutes at ambient temperatures). Dicyclopropyl ketone added rapidly to the monomers (Z)-1&3THF and (E)-1&3THF with a rate ratio of at least 14:1 in THF at -78 °C. Di-tert-butyl ketone added less rapidly to the less shielded (Z)-1 [but never to (E)-1]; this singly diastereoconvergent process was much more slowly reversible in THF.

High performance of H3BO3 modified USY and equilibrium catalyst with tailored acid sites in catalytic cracking

Increasing the cracking ability of catalyst by adjusting its surface acid property is of great significance for oil refining industry. The effects of boric acid on the structure and surface acidity of ultra stabilized Y (USY) and FCC equilibrium catalyst during the modification were investigated. The structural and surface properties of the prepared samples were characterized by low-temperature N2 sorption, XRD, 27Al and 29Si MAS NMR, and FTIR using pyridine probe molecule. Modified USY and equilibrium catalyst possessed reduced Lewis acid sites. The Brönsted/Lewis acid ratios of USY increased from 0.64 to 1.56 after modification, at the same time, the microporous surface areas increased, due to the nonframework and framework dealumination during the modification. The catalytic cracking tests were performed on a bench-scale micro reactor at 500 °C and atmospheric pressure, using n-dodecane as a modeling reactant and vacuum gas oil as conventional feedstock. The results showed that modified samples with high Brönsted acid sites had better cracking abilities and increased hydrogen transfer activity than raw samples.

Influence of the “wetting–drying” compaction on the adsorptive characteristics of nanosilica A-300

The methods of low-temperature 1H NMR and IR spectroscopy, densitometry, and adsorption from solutions are applied to study the influence of “wetting–drying” compaction on the adsorptive characteristics of the nanosilica A-300 in relation to water and gelatin. It was shown that if the bulk density changes from 0.05 to 0.25 g/ml, the protein adsorptive capacity of the nanosilica decreases no more than by 30%, which, apparently, is connected with the decrease of the part of the surface, available for the protein molecules. Interfacial energy in relation to water increases from 37 to 52 mJ/m2 if the bulk density increases and this is connected with the increase of the contribution from minor clusters (nanodrops) adsorbed on the surface water.

Unexpected Skeletal Rearrangement in the Gold(I)/Silver(I)-Catalyzed Conversion of 7-Aryl-1,6-enynes to Bicyclo[3.2.0]hept-6-enes via Hidden Brønsted Acid Catalysis

Cycloaddition of an isotopically labeled 7-phenyl-1,6-enyne catalyzed by a mixture of LAuCl [L = P(t-Bu)2o-biphenyl)] and AgSbF6 forms the corresponding 6-phenylbicyclo[3.2.0]hept-6-ene with concomitant scrambling of the olefinic CPh (C6) and CH (C7) groups of the product. The extent of C6/C7 scrambling was sensitive to the nature of the silver salt and was likewise observed in the cycloaddition of the 7-phenyl-1,6-enyne catalyzed by mixtures of [LAuNCMe]+SbF6– and Bronsted acids such as HOTf. These observations and low-temperature NMR analysis of organic and organometallic intermediates in the cycloaddition process were in accord with a mechanism for C6/C7 scrambling initiated by protonation of free bicyclo[3.2.0]hept-7-ene to generate a bicyclo[3.2.0]heptyl cation that undergoes reversible alkyl migration, presumably involving bicyclo[4.1.0]- and bicyclo[3.1.1]heptyl cations prior to elimination of the C7 proton.