Laura J Sewell

University of Oxford, Oxford, England, United Kingdom

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Publications (8)57.82 Total impact

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    ABSTRACT: The cationic rhodium complex [Rh(PcPr3)2(η6-PhF)]+[B{3,5-(CF3)2C6H3}4]− (PcPr3 = triscyclopropylphosphine, PhF = fluorobenzene) was used as a catalyst for the hydrogenation of the charge-tagged alkyne [Ph3P(CH2)4C2H]+[PF6]−. Pressurized sample infusion electrospray ionization mass spectrometry (PSI-ESI-MS) was used to monitor reaction progress. Experiments revealed that the reaction is first order in catalyst and first order in hydrogen, so under conditions of excess hydrogen the reaction is pseudo-zero order. Alkyne hydrogenation was 40 times faster than alkene hydrogenation. The turnover-limiting step is proposed to be oxidative addition of hydrogen to the alkyne (or alkene)-bound complex. Addition of triethylamine caused a dramatic reduction in rate, suggesting a deprotonation pathway was not operative.
    Organometallics 06/2015; 34(12):3021-3028. DOI:10.1021/acs.organomet.5b00322 · 4.13 Impact Factor
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    ABSTRACT: A combined experimental and computational study on the fluxional processes involving the M-H and B-H positions in the sigma amine-borane complexes [M(PR3)2(H)2(η(2)-H3B·NMe3)][BAr(F)4] (M = Rh, Ir; R = Cy for experiment; R = Me, Cy for computation; Ar(F) = 3,5-(CF3)2C6H3) is reported. The processes studied are: B-H bridging/terminal exchange; reaction with exogenous D2 leading to exchange at M-H; and intramolecular M-H/B-H exchange. Experimentally it was found that B-H bridging/terminal exchange is most accessible and slightly favoured for Rh; D2/M-H exchange occurs at qualitatively similar rates for both M = Rh and Ir, while M-H/B-H exchange is the slowest overall, with the Ir congener having a lower barrier than Rh. Evidence for the isotopic perturbation of equilibrium is also reported for the BH/BD isotopologues of [Ir(PCy3)2(H)2(η(2)-H3B·NMe3)][BAr(F)4]. DFT calculations using model complexes (R = Me) qualitatively reproduce the relative rates of the various exchange processes for both M = Rh and Ir, i.e. barriers for B-H bridging/terminal exchange are less than those for M-H/H2 exchange, which in turn are less than those for M-H/B-H exchange. Which metal promotes these processes more effectively depends upon the nature of the rate-limiting transition state, which can change between Rh and Ir. Computational analysis of the full experimental system (R = Cy) reveals similar overall trends in terms of the relative ease of the various exchange processes. However, there are differences in the details, and these are discussed.
    Dalton Transactions 12/2013; 43(29). DOI:10.1039/c3dt52771a · 4.20 Impact Factor
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    ABSTRACT: The Rh(III) species Rh(PCy3)2H2Cl is an effective catalyst (2 mol %, 298 K) for the dehydrogenation of H3B·NMe2H (0.072 M in 1,2-F2C6H4 solvent) to ultimately afford the dimeric aminoborane [H2BNMe2]2. Mechanistic studies on the early stages in the consumption of H3B·NMe2H, using initial rate and H/D exchange experiments, indicate possible dehydrogenation mechanisms that invoke turnover-limiting N-H activation, which either precedes or follows B-H activation, to form H2B═NMe2, which then dimerizes to give [H2BNMe2]2. An additional detail is that the active catalyst Rh(PCy3)2H2Cl is in rapid equilibrium with an inactive dimeric species, [Rh(PCy3)H2Cl]2. The reaction of Rh(PCy3)2H2Cl with [Rh(PCy3)H2(H2)2][BAr(F)4] forms the halide-bridged adduct [Rh(PCy3)2H2(μ-Cl)H2(PCy3)2Rh][BAr(F)4] (Ar(F) = 3,5-(CF3)2C6H3), which has been crystallographically characterized. This dinuclear cation dissociates on addition of H3B·NMe2H to re-form Rh(PCy3)2H2Cl and generate [Rh(PCy3)2H2(η(2)-H3B·NMe2H)][BAr(F)4]. The fate of the catalyst at low catalyst loadings (0.5 mol %) is also addressed, with the formation of an inactive borohydride species, Rh(PCy3)2H2(η(2)-H2BH2), observed. On addition of H3B·NMe2H to Ir(PCy3)2H2Cl, the Ir congener Ir(PCy3)2H2(η(2)-H2BH2) is formed, with concomitant generation of the salt [H2B(NMe2H)2]Cl.
    Inorganic Chemistry 04/2013; 52(8). DOI:10.1021/ic302804d · 4.76 Impact Factor
  • Journal of the American Chemical Society 02/2012; 134(8):3932. DOI:10.1021/ja211731p · 12.11 Impact Factor
  • Laura J Sewell · Guy C Lloyd-Jones · Andrew S Weller
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    ABSTRACT: The multistage Rh-catalyzed dehydrocoupling of the secondary amine-borane H(3)B·NMe(2)H, to give the cyclic amino-borane [H(2)BNMe(2)](2), has been explored using catalysts based upon cationic [Rh(PCy(3))(2)](+) (Cy = cyclo-C(6)H(11)). These were systematically investigated (NMR/MS), under both stoichiometric and catalytic regimes, with the resulting mechanistic proposals for parallel catalysis and autocatalysis evaluated by kinetic simulation. These studies demonstrate a rich and complex mechanistic landscape that involves dehydrogenation of H(3)B·NMe(2)H to give the amino-borane H(2)B═NMe(2), dimerization of this to give the final product, formation of the linear diborazane H(3)B·NMe(2)BH(2)·NMe(2)H as an intermediate, and its consumption by both B-N bond cleavage and dehydrocyclization. Subtleties of the system include the following: the product [H(2)BNMe(2)](2) is a modifier in catalysis and acts in an autocatalytic role; there is a parallel, neutral catalyst present in low but constant concentration, suggested to be Rh(PCy(3))(2)H(2)Cl; the dimerization of H(2)B═NMe(2) can be accelerated by MeCN; and complementary nonclassical BH···HN interactions are likely to play a role in lowering barriers to many of the processes occurring at the metal center. These observations lead to a generic mechanistic scheme that can be readily tailored for application to many of the transition-metal and main-group systems that catalyze the dehydrocoupling of H(3)B·NMe(2)H.
    Journal of the American Chemical Society 02/2012; 134(7):3598-610. DOI:10.1021/ja2112965 · 12.11 Impact Factor
  • Laura J Sewell · Adrian B Chaplin · Andrew S Weller
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    ABSTRACT: The catalytic hydroboration of tert-butylethene using H(3)B·NMe(3) gives RH(2)B·NMe(3). With H(3)B·NMe(2)H tandem hydroboration under mild conditions/dehydrocoupling occurs that produces R(2)B=NMe(2) (R = H, CH(2)CH(2)(t)Bu).
    Dalton Transactions 06/2011; 40(29):7499-501. DOI:10.1039/c1dt10819k · 4.20 Impact Factor
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    ABSTRACT: We report the first insertion step at a metal center for the catalytic dehydropolymerization of H3B center dot NMeH2 to form the simplest oligomeric species, H3B center dot NMeHBH2 center dot NMeH2, by the addition of 1 equiv of H3B center dot NMeH2 to [Ir(PCy3)(2)(H)(2)(eta(2)-H3B center dot NMeH2)] [BAr4F] to give [Ir(PCy3)(2)(H)(2)(eta(2)-H3B center dot NMeHBH2 center dot NMeH2)] [BAr4F]. This reaction is also catalytic for the formation of the free linear diborazane, but this is best obtained by an alternative stoichiometric synthesis.
    Journal of the American Chemical Society 06/2011; 133(29):11076-9. DOI:10.1021/ja2040738 · 12.11 Impact Factor
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    ABSTRACT: [Rh(P(t)Bu(i)Bu(2))(2)][BAr(F)(4)], formed by removal of H(2) from [RhH(2)(P(t)Bu(i)Bu(2))(2)][BAr(F)(4)], is in rapid equilibrium between C-H activated Rh(III) isomers, but reacts as a masked 12-electron [Rh(P(t)Bu(i)Bu(2))(2)](+) Rh(I) cation.
    Dalton Transactions 08/2010; 39(32):7437-9. DOI:10.1039/c0dt00449a · 4.20 Impact Factor