School of Chemistry - Research Publications

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    Selectivity Effects in Bimetallic Catalysis: Role of the Metal Sites in the Decomposition of Formic Acid into H-2 and CO2 by the Coinage Metal Binuclear Complexes [dppmMM(H)](+)
    Zavras, A ; Krstic, M ; Dugourd, P ; Bonacic-Koutecky, V ; O'Hair, RAJ (Wiley, 2017-04-07)
    Design of new bimetallic catalysts requires an understanding of how cooperative effects of the metal sites influences reactivity. Here we show how switching one or both of the silver atoms in binuclear silver hydride cations, [dppmAg2(H)]+ (dppm=1,1‐Bis(diphenylphosphino)‐methane), with all combinations of copper and/or gold maintains selective dehydrogenation of formic acid, enhancing reactivity by up to 2 orders of magnitude. This is a key step in the selective, catalyzed extrusion of carbon dioxide from formic acid, HO2CH, with important applications in hydrogen storage and in situ generation of H2. Decarboxylation of [dppmMM′(O2CH)]+ through collision induced dissociation regenerates [dppmMM′(H)]+. DFT calculations provide insights into these cooperative effects. The copper homobinuclear catalyst performs best overall.
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    Role of Ligand in the Selective Production of Hydrogen from Formic Acid Catalysed by the Mononuclear Cationic Zinc Complexes [(L)Zn(H)]+ (L=tpy, phen, and bpy)
    Piacentino, EL ; Parker, K ; Gilbert, TM ; O'Hair, RAJ ; Ryzhov, V (WILEY-V C H VERLAG GMBH, 2019-07-25)
    A series of zinc-based catalysts was evaluated for their efficiency in decomposing formic acid into molecular hydrogen and carbon dioxide in the gas phase using quadrupole ion trap mass spectrometry experiments. The effectiveness of the catalysts in the series [(L)Zn(H)]+ , where L=2,2':6',2''-terpyridine (tpy), 1,10-phenanthroline (phen) or 2,2'-bipyrydine (bpy), was found to depend on the ligand used, which turned out to be fundamental in tuning the catalytic properties of the zinc complex. Specifically, [(tpy)Zn(H)]+ displayed the fastest reaction with formic acid proceeding by dehydrogenation to produce the zinc formate complex [(tpy)Zn(O2 CH)]+ and H2 . The catalysts [(L)Zn(H)]+ are reformed by decarboxylating the zinc formate complexes [(L)Zn(O2 CH)]+ by collision-induced dissociation, which is the only reaction channel for each of the ligands used. The decarboxylation reaction was found to be reversible, since the zinc hydride complexes [(L)Zn(H)]+ react with carbon dioxide yielding the zinc formate complex. This reaction was again substantially faster for L=tpy than L=phen or bpy. The energetics and mechanisms of these processes were modelled using several levels of density functional theory (DFT) calculations. Experimental results are fully supported by the computational predictions.
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    Gas-phase functionalized carbon-carbon coupling reactions catalyzed by Ni (II) complexes
    Piacentino, EL ; Rodriguez, E ; Parker, K ; Gilbert, TM ; O'Hair, RAJ ; Ryzhov, V (WILEY, 2019-06)
    Gas-phase C-C coupling reactions mediated by Ni (II) complexes were studied using a linear quadrupole ion trap mass spectrometer. Ternary nickel cationic carboxylate complexes, [(phen)Ni (OOCR1 )]+ (where phen = 1,10-phenanthroline), were formed by electrospray ionization. Upon collision-induced dissociation (CID), they extrude CO2 forming the organometallic cation [(phen)Ni(R1 )]+ , which undergoes gas-phase ion-molecule reactions (IMR) with acetate esters CH3 COOR2 to yield the acetate complex [(phen)Ni (OOCCH3 )]+ and a C-C coupling product R1 -R2 . These Ni(II)/phenanthroline-mediated coupling reactions can be performed with a variety of carbon substituents R1 and R2 (sp3 , sp2 , or aromatic), some of them functionalized. Reaction rates do not seem to be strongly dependent on the nature of the substituents, as sp3 -sp3 or sp2 -sp2 coupling reactions proceed rapidly. Experimental results are supported by density functional theory calculations, which provide insights into the energetics associated with the C-C bond coupling step.
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    Models Facilitating the Design of a New Metal-Organic Framework Catalyst for the Selective Decomposition of Formic Acid into Hydrogen and Carbon Dioxide
    O'Hair, RAJ ; Mravak, A ; Krstic, M ; Bonacic-Koutecky, V (WILEY-V C H VERLAG GMBH, 2019-05-20)
    Abstract Here we describe a new conceptual approach for the design of a heterogeneous metal‐organic framework (MOF) catalyst based on UiO‐67 for the selective decarboxylation of formic acid, a reaction with important applications in hydrogen storage and in situ generation of H2. Models for the {CuH} reactive catalytic site at the organic linker are assessed. In the first model system, gas‐phase mass spectrometry experiments and DFT calculations on a fixed charge bathophen ligated copper hydride complex, [(phen*)Cu(H)]2−, were used to demonstrate that it acts as a catalyst for the selective decomposition of formic acid into H2 and CO2 via a two‐step catalytic cycle. In the first step liberation of H2 to form the carboxylate complex, [(phen*)Cu(O2CH)]2− occurs, which in the second step selectively decomposes via CO2 extrusion to regenerate the hydride complex. DFT calculations on four other model systems showed that changing the catalyst to neutral [(LCu(H)] complexes or embedding it within a MOF results in mechanisms which are essentially identical. Thus catalytic active sites located on the organic linker of a MOF appear to be close to a gas‐phase environment, thereby benefitting from the favorable characteristics of gas‐phase reactions and validating the use of gas‐phase models to design new MOF based catalysts.
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    Unimolecular reactivity of organotrifluoroborate anions, RBF3-, and their alkali metal cluster ions, M(RBF3)2- (M = Na, K; R = CH3, CH3CH2, CH3(CH2)3, CH3(CH2)5, c-C3H5, C6H5, C6H5CH2, CH2CHCH2, CH2CH, C6H5CO)
    Bathie, FLB ; Bowen, CJ ; Hutton, CA ; O'Hair, RAJ (WILEY, 2018-07-15)
    RATIONALE: Potassium organotrifluoroborates (RBF3 K) are important reagents used in organic synthesis. Although mass spectrometry is commonly used to confirm their molecular formulae, the gas-phase fragmentation reactions of organotrifluoroborates and their alkali metal cluster ions have not been previously reported. METHODS: Negative-ion mode electrospray ionization (ESI) together with collision-induced dissociation (CID) using a triple quadrupole mass spectrometer were used to examine the fragmentation pathways for RBF3- (where R = CH3 , CH3 CH2 , CH3 (CH2 )3 , CH3 (CH2 )5 , c-C3 H5 , C6 H5 , C6 H5 CH2 , CH2 CHCH2 , CH2 CH, C6 H5 CO) and M(RBF3 )2- (M = Na, K), while density functional theory (DFT) calculations at the M06/def2-TZVP level were used to examine the structures and energies associated with fragmentation reactions for R = Me and Ph. RESULTS: Upon CID, preferentially elimination of HF occurs for RBF3- ions for systems where R = an alkyl anion, whereas R- formation is favoured when R = a stabilized anion. At higher collision energies loss of F- and additional HF losses are sometimes observed. Upon CID of M(RBF3 )2- , formation of RBF3- is the preferred pathway with some fluoride transfer observed only when M = Na. The DFT-calculated relative thermochemistry for competing fragmentation pathways is consistent with the experiments. CONCLUSIONS: The main fragmentation pathways of RBF3- are HF elimination and/or R- loss. This contrasts with the fragmentation reactions of other organometallate anions, where reductive elimination, beta hydride transfer and bond homolysis are often observed. The presence of fluoride transfer upon CID of Na(RBF3 )2- but not K(RBF3 )2- is in agreement with the known fluoride affinities of Na+ and K+ and can be rationalized by Pearson's HSAB theory.
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    How to Translate the [LCu2(H)]+-Catalysed Selective Decomposition of Formic Acid into H2 and CO2 from the Gas Phase into a Zeolite.
    Krstic, M ; Jin, Q ; Khairallah, GN ; O'Hair, RAJ ; Bonacic-Koutecky, V (WILEY-V C H VERLAG GMBH, 2018-03-07)
    Abstract Translating a homogenous catalyst into a heterogeneous catalyst requires a fundamental understanding of how the catalyst “fits” into the zeolite and how the reaction is influenced. Previous studies of bimetallic catalyst design identified a potent copper homobinuclear catalyst, [(L)Cu2(H)]+ for the selective decomposition of formic acid. Here, a close interplay between theory and experiment shows how to preserve this selective reactivity within zeolites. Gas‐phase experiments and DFT calculations showed that switching from 1,1‐bis(diphenylphosphino)‐methane ligand to the 1,8‐naphthyridine ligand produced an equally potent catalyst. DFT calculations show that this new catalyst neatly fits into a zeolite which does not perturb reactivity, thus providing a unique example on how “heterogenization” of a homogenous catalyst for the selective catalysed extrusion of carbon dioxide from formic acid can be achieved, with important application in hydrogen storage and in situ generation of H2.
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    Synthesis and X-Ray Crystallographic Characterisation of Frustum-Shaped Ligated [Cu18H16(DPPE)6]2+ and [Cu16H14(DPPA)6]2+ Nanoclusters and Studies on Their H2 Evolution Reactions
    Li, J ; Ma, HZ ; Reid, GE ; Edwards, AJ ; Hong, Y ; White, JM ; Mulder, RJ ; O'Hair, RAJ (WILEY-V C H VERLAG GMBH, 2018-02-09)
    We report new structural motifs for Cu nanoclusters that conceptually represent seed crystals for large face-centred cubic (FCC) crystal growth. Kinetically controlled syntheses, high resolution mass spectrometry experiments for determination of the dication formulae and crystallographic characterisation were carried out for [Cu18 H16 (DPPE)6 ][BF4 ][Cl] (DPPE=bis(diphenylphosphino)ethane) and [Cu16 H14 (DPPA)6 ][(BF4 )2 ] (DPPA=bis(diphenylphosphino)amine) polyhydrido nanoclusters, which feature the unprecedented bifrustum and frustum metal-core architecture in metal nanoclusters. The Cu18 nanocluster contains two Cu9 frustum cupolae and the Cu16 nanocluster has one Cu9 frustum cupola and a Cu7 distorted hexagonal-shape base. Gas-phase experiments revealed that both Cu18 H16 and Cu16 H14 cores can spontaneously release H2 upon removal of one bisphosphine capping ligand.
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    Chemical Ionization Mass Spectrometry: 50 Years on
    O'Hair, RAJ (SPRINGER, 2016-11)
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    Gas-Phase Intercluster Thiyl-Radical Induced C-H Bond Homolysis Selectively Forms Sugar C2-Radical Cations of Methyl D-Glucopyranoside: Isotopic Labeling Studies and Cleavage Reactions
    Osburn, S ; Speciale, G ; Williams, SJ ; O'Hair, RAJ (SPRINGER, 2017-07)
    A suite of isotopologues of methyl D-glucopyranosides is used in conjunction with multistage mass spectrometry experiments to determine the radical site and cleavage reactions of sugar radical cations formed via a recently developed 'bio-inspired' method. In the first stage of CID (MS2), collision-induced dissociation (CID) of a protonated noncovalent complex between the sugar and S-nitrosocysteamine, [H3NCH2CH2SNO + M]+, unleashes a thiyl radical via bond homolysis to give the noncovalent radical cation, [H3NCH2CH2S• + M]+. CID (MS3) of this radical cation complex results in dissociation of the noncovalent complex to generate the sugar radical cation. Replacement of all exchangeable OH and NH protons with deuterons reveals that the sugar radical cation is formed in a process involving abstraction of a hydrogen atom from a C-H bond of the sugar coupled with proton transfer to the sugar, to form [M - H• + D+]. Investigation of this process using individual C-D labeled sugars reveals that the main site of H/D abstraction is the C2 position, since only the C2-deuterium labeled sugar yields a dominant [M - D• + H+] product ion. The fragmentation reactions of the distonic sugar radical cation, [M - H•+ H+], were studied by another stage of CID (MS4). 13C-labeling studies revealed that a series of three related fragment ions each contain the C1-C3 atoms; these arise from cross-ring cleavage reactions of the sugar. Graphical Abstract ᅟ.