Selective and mild hydrogen production using water and formaldehyde

With the increased efforts in finding new energy storage systems for mobile and stationary applications, an intensively studied fuel molecule is dihydrogen owing to its energy content, and the possibility to store it in the form of hydridic and protic hydrogen, for example, in liquid organic hydrogen carriers. Here we show that water in the presence of paraformaldehyde or formaldehyde is suitable for molecular hydrogen storage, as these molecules form stable methanediol, which can be easily and selectively dehydrogenated forming hydrogen and carbon dioxide. In this system, both molecules are hydrogen sources, yielding a theoretical weight efficiency of 8.4% assuming one equivalent of water and one equivalent of formaldehyde. Thus it is potentially higher than formic acid (4.4 wt%), as even when technical aqueous formaldehyde (37 wt%) is used, the diluted methanediol solution has an efficiency of 5.0 wt%. The hydrogen can be efficiently generated in the presence of air using a ruthenium catalyst at low temperature

Prediction of chemical shift in NMR: a review

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.Calculation of solution-state NMR parameters, including chemical shift values and scalar coupling constants, is often a crucial step for unambiguous structure assignment. Data-driven (sometimes called \textit{empirical}) methods leverage databases of known parameter values to estimate parameters for unknown or novel molecules. This is in contrast to popular \textit{ab initio} techniques which use detailed quantum computational chemistry calculations to arrive at parameter estimates. Data-driven methods have the potential to be considerably faster than ab inito techniques and have been the subject of renewed interest over the past decade with the rise of high-quality databases of NMR parameters and novel machine learning methods. Here we review these methods, their strengths and pitfalls, and the databases they are built on

Carbene catalyzed umpolung of alpha,beta-enals: a reactivity study of diamino dienols vs. azolium enolates, and the characterization of advanced reaction intermediates

Since their discovery by Bode and Glorius in 2004, N-heterocyclic carbene catalyzed conjugate umpolung reactions of alpha,beta-enals have been postulated to involve the formation of diamino dienols (homoenolates) and/or azolium enolates (enolates), typically followed by addition to electrophiles, e.g. Michael-acceptors. In this article, we provide evidence, for the first time, for the postulated individual and specific reactivity patterns of diamino dienols (gamma-C-C-bond formation) vs. azolium enolates (beta-C-C-bond formation). Our study is based on the pre-formation of well defined diamino dienols and azolium enolates, and the in situ NMR monitoring of their reactivities towards enone electrophiles. Additionally, reaction intermediates were isolated and characterized, inter alia by X-ray crystallography

TEFDDOLs (alpha,alpha,alpha ',alpha '-Tetrakis(perfluoroaryl/alkyl)-2,2 '-dimethyl-1,3-dioxolane-4,5-dimethanols): Highly Fluorinated Chiral H-Bond Donors and Bronsted Acids with Distinct H-Bonding Patterns and Supramolecular Architectures

The synthesis of six enantiopure alpha,alpha,alpha',alpha'-tetrakis(perfluoroalkyl/aryl)-2,2' -dimethyl-1,3-dioxolane-4,5-dimethanols (TEFDDOLs), by addition of perfluorinated organolithium reagents or Ruppert's reagent (TMS-CF3) to isopropylidene tartaric dichloride, is reported. X-ray crystal structures of the TEFDDOLs alone or in complexes with H-bond acceptors such as water and DABCO revealed that this new class of highly fluorinated chiral 1,4-diols forms distinct intra- and intermolecular H-bond patterns. Intramolecular OH-OH bonding accounts for the relatively high acidity of the perfluoroalkyl TEFDDOLs (pK(a) in DMSO: tetrakis-CF3, 5.7; tetrakis-C2F5, 2.4). For the tetrakis(perfluorophenyl) TEFDDOL, a quite unusual pseudo-anti conformation of the diol, with no intramolecular (and no intermolecular) OH-OH bonds, was found both in the crystal and in solution (DOSY and NOESY NMR). The latter conformation results from a total of four intramolecular OH-F-aryl, hydrogen bonds overriding OH-OH bonding. Due to their H-bonding properties, the TEFDDOLs are promising new building blocks for supramolecular and potentially catalytic applications

Anion Recognition with Hydrogen-Bonding Cyclodiphosphazanes

Modular cyclodiphosph(V)azanes are synthesised and their affinity for chloride and actetate anions were compared to those of a bisaryl urea derivative (1). The diamidocyclodiphosph(V)azanes cis-[{ArNHP(O)(mu-tBu)}(2)] [Ar=Ph (2) and Ar=m-(CF3)(2)Ph (3)] were synthesised by reaction of [{ClP(mu-NtBu)}(2)] (4) with the respective anilines and subsequent oxidation with H2O2. Phosphazanes 2 and 3 were obtained as the cis isomers and were characterised by multinuclear NMR spectroscopy, FTIR spectroscopy, HRMS and single-crystal X-ray diffraction. The cyclodiphosphazanes 2 and 3 readily co-crystallise with donor solvents such as MeOH, EtOH and DMSO through bidentate hydrogen bonding, as shown in the X-ray analyses. Cyclodiphosphazane 3 showed a remarkably high affinity (log[K]=5.42) for chloride compared with the bisaryl urea derivative 1 (log[K]=4.25). The affinities for acetate (AcO-) are in the same range (3: log[K]=6.72, 1: log[K]=6.91). Cyclodiphosphazane 2, which does not contain CF3 groups, exhibits weaker binding to chloride (log[K]=3.95) and acetate (log[K]=4.49). DFT computations and X-ray analyses indicate that a squaramide-like hydrogen-bond directionality and C-alpha-H interactions account for the efficiency of 3 as an anion receptor. The C-alpha-H groups stabilise the Z,Z-3 conformation, which is necessary for bidentate hydrogen bonding, as well as coordinating with the anion

Breslow Intermediates from Aromatic N-Heterocyclic Carbenes (Benzimidazolin-2-ylidenes, Thiazolin-2-ylidenes)

We report the first generation and characterization of elusive Breslow intermediates derived from aromatic N-heterocyclic carbenes (NHCs), namely benzimidazolin-2-ylidenes (NMR, X-ray analysis) and thiazolin-2-ylidenes (NMR). In the former case, the diamino enols were generated by reaction of the free N,N-bis(2,6-diisopropylphenyl)- and N,N-bis(mesityl)-substituted benzimidazolin-2-ylidenes with aldehydes while the dimer of 3,4,5-trimethylthiazolin-2-ylidene served as the starting material in the latter case. The unambiguous NMR identification of the first thiazolin-2-ylidene-based Breslow intermediate rests on double C-13 labeling of both the NHC and the aldehyde component. The acyl anion reactivity was confirmed by benzoin formation with excess aldehyde

Intramolecular rearrangement of the imine-amide ligand within the nickel coordination sphere affected by carbon monoxide

The interactions of the nickel imine-amide allyl complex [D-2-C=N (N) over cap Ni)CNi(eta(3)-allyl)] 1 with carbon monoxide and unsaturated hydrocarbons have been studied. It is shown that this complex reacts readily with carbon monoxide to form the nickel(0) diimine carbonyl complex [(2-(1-propenyl)-[1,10] phenanthroline) Ni(CO)(2)](2). During the process the ligand undergoes a deep transformation within the nickel coordination sphere. Specifically, the nickel-nitrogen sigma-bond turns to an N-donor bond with aromatization of a ring in the nitrogen-containing ligand. This novel heteroaromatic ligand 2-(1-propenyl)-[1,10] phenanthroline has been isolated; the nickel(0) diimine carbonyl complex 2 has been studied with X-ray diffraction method. The comparative spectral studies of complexes 1, 2, and 2-(1-propenyl)-[1,10] phenanthroline have been carried out with UV/vis, IR-FT, and 2D NMR spectroscopy. It has been shown that the planar 16-electron nickel(II) imine-amide allyl complex 1 is indifferent to olefins and acetylenes. Based on the NMR data, this fact can be explained by the inability of the pi-delta rearrangement into 1. (C) 2011 Elsevier B. V. All rights reserved

1,4-Bis-Dipp/Mes-1,2,4-Triazolylidenes: Carbene Catalysts That Efficiently Overcome Steric Hindrance in the Redox Esterification of alpha- and beta-Substituted alpha,beta-Enals

As reported by Scheidt and Bode in 2005, sterically nonencumbered alpha,beta-enals are readily converted to saturated esters in the presence of alcohols and N-heterocyclic carbene catalysts, e.g., benzimidazolylidenes or triazolylidenes. However, substituents at the alpha- or beta-position of the alpha,beta-enal substrate are typically not tolerated, thus severely limiting the substrate spectrum. On the basis of our earlier mechanistic studies, a set of N-Mes- or N-Dipp-substituted 1,2,4-triazolium salts were synthesized and evaluated as (pre)catalysts in the redox esterification of various alpha- or beta-substituted enals. In particular the 1,4-bis-Mes/Dipp-1,2,4-triazolylidenes overcome the above limitations and efficiently catalyze the redox esterification of a whole series of alpha/beta-substituted enals hitherto not amenable to NHC-catalyzed transformations. The synthetic value of 1,4-bis-Mes/Dipp-1,2,4-triazolylidenes is further demonstrated by the one-step bicyclization of 10-oxocitral to (racemic) nepetalactone in diastereomerically pure form