Laura J Sewell

University of Oxford, Oxford, England, United Kingdom

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Publications (7)51.42 Total impact

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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 · 4.10 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 · 4.79 Impact Factor
  • Journal of the American Chemical Society 02/2012; 134(8):3932. DOI:10.1021/ja211731p · 11.44 Impact Factor
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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 · 11.44 Impact Factor
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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 · 4.10 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 · 11.44 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 · 4.10 Impact Factor