All posts by Pau

Solid State NMR supports a new study on hydride composite for hydrogen storage

A hydride composite featuring mutual destabilisation and reversible boron exchange: Ca(BH4)2–Mg2NiH4

N. Bergemann, C. Pistidda, C. Milanese, M. Aramini, S. Huotari, P. Nolis, A. Santoru, M. R. Chierotti, A.-L. Chaudhary, M. D. Baro, T. Klassen and M. Dornheim.

http://dx.doi.org/10.1039/C8TA04748K

ABSTRACT:The system Ca(BH4)2–Mg2NiH4 is used as a model to prove the unique possibility to fully reverse the borohydride decomposition process even in cases where the decomposition reaction leads to undesired stable boron containing species (boron sinks). The formation of MgNi2.5B2 directly from Ca(BH4)2 or from CaB12H12 and amorphous boron allows an unexpectedly easy transfer of the boron atoms to reversibly form Ca(BH4)2 during rehydrogenation. In addition, to the best of our knowledge, the mutual destabilisation of the starting reactants is observed for the first time in Ca(BH4)2 based Reactive Hydride Composite (RHC) systems. A detailed account of dehydrogenation and rehydrogenation reaction mechanisms as the function of applied experimental conditions is given.

  

Solid-State-NMR a useful tool for the characterization of Hydrogen Storage Composite System

In Situ Formation of TiB2 Nanoparticles for Enhanced Dehydrogenation/Hydrogenation Reaction Kinetics of LiBH4–MgH2 as a Reversible Solid-State Hydrogen Storage Composite System

Fahim Karimi* , María V. C. Riglos, Antonio Santoru , Armin Hoell, Vikram S. Raghuwanshi, Chiara Milanese, Nils Bergemann, Claudio Pistidda, Pau Nolis , Maria D. Baro, Gökhan Gizer, Thi-Thu Le, P. Klaus Pranzas, Martin Dornheim, Thomas Klassen, Andreas Schreyer, and Julián Puszkiel

J. Phys. Chem. C, Article ASAP
DOI: 10.1021/acs.jpcc.8b02258
Publication Date (Web): May 9, 2018
ABSTRACT:To enhance the dehydrogenation/rehydrogenation kinetic behavior of the LiBH4–MgH2 composite system, TiF4 is used as an additive. The effect of this additive on the hydride composite system has been studied by means of laboratory and advanced synchrotron techniques. Investigations on the synthesis and mechanism upon hydrogen interaction show that the addition of TiF4 to the LiBH4–MgH2 composite system during the milling procedure leads to the in situ formation of well-distributed nanosized TiB2 particles. These TiB2 nanoparticles act as nucleation agents for the formation of MgB2 upon dehydrogenation process of the hydride composite system. The effect of TiB2 nanoparticles is maintained upon cycling.

Multiplicity-edited 1H-1H TOCSY experiment

Pau Nolis and Teodor Parella

Magnetic Resonance in Chemistry 2017 (DOI: 10.1002/mrc.4695)

Abstract

A 1H-1H TOCSY experiment incorporating 13C multiplicity information is proposed. In addition, broadband 1H homodecoupling in the indirect dimension can be implemented using a perfect BIRD module that affords exclusive 1H chemical shift evolution with full decoupling of all heteronuclear and homonuclear (including 2JHH) coupling constants. As a complement to the normal TOCSY and the recent PSYCHE-TOCSY experiments, this novel multiplicity-edited TOCSY experiment distinguishes between CH/CH3 (phased up) and CH2 (phased down) cross-peaks which facilitates resonance analysis and assignment.

Tripeptides studied by NMR

Stereoselectivity of Proline / Cyclobutane Amino Acid-Containing Peptide Organocatalysts for Asymmetric Aldol Additions: a Rationale

Ona Illa, Oriol Porcar-Tost, Carme Robledillo, Carlos Elvira, Pau Nolis, Oliver Reiser, Vicenç Branchadell, and Rosa M. Ortuño

J. Org. Chem., Just Accepted Manuscript
DOI: 10.1021/acs.joc.7b02745
Publication Date (Web): November 29, 2017

Abstract
Several α,β,α- or α,γ,α-tripeptides, consisting of a central cyclobutane β- or γ-amino acid being flanked by two (D)- or (L)-proline residues, have been synthesized and tested as organocatalysts in asymmetric aldol additions. High yields and enantioselectivities have been achieved with α,γ,α-tripeptides, being superior to the peptides containing a cyclobutane β-amino acid residue. This can probably be due to their high rigidity, which hinders the peptide catalysts to adopt the proper active conformation. This reasoning correlates with the major conformation of the peptides in the ground state, as suggested by 1H NMR and computational calculations. The configuration of the aldol products is controlled by the proline chirality, and consequently, the R/S configuration of aldol products can be tuned by the use of either commercially available (D)- or (L)-proline enantiomers. The enantioselectivity in the aldol reactions is reversed if the reactions are carried out in the presence of water or other protic solvents such as methanol. Spectroscopic and theoretical investigations revealed that this effect is not the consequence of conformational changes in the catalyst but rather caused by the participation of a water molecule in the rate determining transition state, in such a way that the preferential nucleophilic attack is oriented to the opposite enantiotopic aldehyde face.

Folding peptides studied by NMR

The relevance of the relative configuration in the folding of hybrid peptides containing β-cyclobutane amino acids and γ-amino-L-proline residues

O. Illa, J.A. Olivares, P. Nolis, R.M. Ortuño

DOI: 10.1016/j.tet.2017.09.011

Four new series of diastereomeric β,γ-di- and β,γ-tetrapeptides derived from conveniently protected (1R,2S)- and (1S,2S)-2-aminocyclobutane-1-carboxylic acid and cis- and trans-γ-amino-l-proline joined in alternation have been synthesized. High resolution NMR experiments show that peptides containing trans-cyclobutane amino acid residues adopt a more folded structure in solution than those containing a cis-cyclobutane residue, which adopt a strand-like structure. The cis/trans relative configuration of the cyclobutane residue is the origin of the folding pattern of each peptide due to either intra- or inter-residue hydrogen-bonded ring formation, whereas the cis/trans isomerism of the γ-amino-l-proline residue does not have a significantly relevant role on the folding ability of these peptides.

Solid-state NMR supports a study on Rh nanoparticles

NHC-stabilised Rh nanoparticles: Surface study and application in the catalytic hydrogenation of aromatic substrates

F. Martinez-Espinar, P. Blondeau, P. Nolis, B. Chaudret, C. Claver, S. Castillón and C. Godard

DOI: 10.1016/j.jcat.2017.08.010

Highlights

  • Synthesis and characterisation of small RhNPs stabilised by N-heterocyclic carbenes.
  • Evidence of the location of the ligands on the faces, edges and apexes of the NPs.
  • Hydrogenation of aromatic ketones, phenols and N-heteroaromatic substrates.
  • Tuning of the selectivity as a function of the reaction conditions.
  • Full reduction of quinoline under mild conditions with total selectivity.

13C{1H} CP-MAS spectra of (a) L, (b) Rh0.4, (c) Rh0.4 + CO and (d) 13C-Rh0.4.

Pd-complex characterization by NMR

Dissimilar catalytic behavior of molecular or colloidal palladium systems with a new NHC ligand

Fernando Gómez-Villarraga,  Jonathan De Tovar,  Miguel Guerrero,  Pau Nolis,  Teodor Parella,  Pierre Lecante,  Nuria Romero,  Lluís Escriche,  Roger Bofill,  Josep Ros,  Xavier Sala,  Karine Philippot  and  Jordi García-Antón 

DOI: 10.1039/C7DT02729J

Abstract

In this work, we describe the synthesis of a new N-heterocyclic carbene (NHC) ligand, derived from a hybrid pyrazole-imidazolium scaffold, namely 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-3H-imidazol-2-ylidene (L). This ligand has been used as a stabilizer for the organometallic synthesis of palladium(0) nanoparticles (Pd NPs). L presents a better stabilizing effect than its pre-carbenic HLCl counterpart, allowing the formation of isolated Pd NPs while HLCl yields aggregated ones. Additionally, molecular Pd(II) coordination compounds of L and HLCl were synthesized and characterized to better understand the coordination modes of these ligands. Both molecular and colloidal Pd systems have been further tested in catalytic C–C coupling processes. Three different types of reactions have been observed depending on the catalytic system: (i) the Suzuki–Miyaura reaction takes place with Pd molecular complexes; (ii) a secondary reaction, the dehalogenation of the substrate, is always detected and (iii) the C–C homocoupling between two molecules of bromoarenes is observed with colloidal catalysts.

Ruthenium complexes studied by NMR

Mononuclear ruthenium compounds bearing N-donor and N-heterocyclic carbene ligands: structure and oxidative catalysis
Hai Jie Liu,   M. Gil-Sepulcre,   L. Francás,   P. Nolis,   T. Parella,   J. Benet-Buchholz,   X. Fontrodona,   J. García-Antón,   N. Romero,   A. Llobet,   Ll. Escriche,   R. Bofill and   X. Sala
Dalton Trans., 2017, Accepted Manuscript

DOI: 10.1039/C6DT04729G

ABSTRACT A new CNNC carbene-phthalazine tetradentate ligand has been synthesised, which under reaction with [Ru(T)Cl3] (T = trpy, tpm, bpea; trpy = 2,2′;6′,2″-terpyridine; tpm = tris(pyrazol-1-yl)methane; bpea = N,N-bis(pyridin-2-ylmethyl)ethanamine) in MeOH or iPrOH undergoes a C-N bond scission due to the nucleophilic attack of a solvent molecule, with the subsequent formation of the mononuclear complexes cis-[Ru(PhthaPz-OR)(trpy)X]n+, [Ru(PhthaPz-OMe)(tpm)X]n+ and trans,fac-[Ru(PhthaPz-OMe)(bpea)X]n+ (X = Cl, n = 1; X = H2O, n = 2; PhthaPz-OR = 1-(4-alkoxyphthalazin-1-yl)-3-methyl-1H-imidazol-3-ium), named 1a+/2a2+ (R = Me), 1b+/2b2+ (R = iPr), 3+/42+ and 5+/62+, respectively. Interestingly, regulation of the stability regions of the different Ru oxidation states is obtained by the different ligand combinations, going from 62+, where Ru(III) is clearly stable and mono-electronic transfers are favoured, to 2a2+/2b2+, where Ru(III) is almost unstable with regards to its disproportion. The catalytic performance of the Ru-OH2 complexes in chemical water oxidation at pH 1.0 points to poor stability (ligand oxidation), with subsequent evolution of CO2 together with O2, especially for 42+ and 62+. In electrochemically driven water oxidation, the highest TOF values are obtained for 2a2+ at pH 1.0. In alkene epoxidation, complexes favouring bi-electronic transfer processes show better performances and selectivities than those favouring mono-electronic transfers, while alkenes containing electron-donor groups promote better performances than those bearing electron-withdrawers. Finally, when cis-β-methylstyrene is employed as substrate, no cis/trans isomerization takes place, thus indicating the existence of a stereospecific process.

Solving JHH measurement in overcrowded regions

Accurate measurement of JHH in overlapped signals by a TOCSY-edited SERF Experiment

André Fredi, Pau Nolis* and Teodor Parella*

Magnetic Resonance in ChemistryDOI: 10.1002/mrc.4572

Selective Refocusing (GSERF or the recent PSYCHEDELIC) experiments were originally designed to determine all proton-proton coupling constants (JHH) for a selected proton resonance. They work for isolated signals on which selective excitation can be successfully applied but, as happens in other selective experiments, fail for overlapped signals. To circumvent this limitation, a doubly-selective TOCSY-GSERF scheme is presented for the measurement of JHH in protons resonating in crowded regions. This new experiment takes advantage of the editing features of an initial TOCSY transfer to uncover hidden resonances that become accessible to perform the subsequent frequency-selective refocusing.

Organogels studied by NMR

Studies on cycloalkane‐based bisamide organogelators: A new example of stochastic chiral symmetry breaking induced by sonication

Ortuno, R. M., Pi-Boleda, B., Sans, M., Campos, M., Nolis, P., Illa, O., Estévez, J. C. and Branchadell, V. (2016), Chem. Eur. J.. Accepted Author Manuscript. doi:10.1002/chem.201604818

Enantiomerically pure C16-alkyl amides derived from cis and trans cycloalkane-1,2-dicarboxylic acids, respectively, have been synthesized and their behaviour as organogelators has been investigated. These compounds include cis/trans diastereomeric cyclobutane and cyclohexane derivatives with the aim to explore the influence of the ring size as well as the relative configuration in their hierarchical self-assembly to form gels. High resolution 1H NMR spectroscopy studies allowed the determination of the dynamics of the gelation process in [D8]-toluene and the sol-gel transition temperature. The morphology and size of the aggregates have been investigated and results have shown that, in the case of cyclobutane derivatives, the cis/trans stereochemistry is not relevant for the gelation behaviour and the properties of the soft-materials obtained, but it is remarkable for cyclohexane diamides, which are better organogelators. The four compounds produce chiral aggregates despite that two of them are meso achiral molecules. We show in this work that this fact is an example of stochastic symmetry breaking induced by sonication. The self-assembly of these molecules has been modelled providing information and support about the structure and the chirality of the aggregates.