Publications
54 results found
Westhead O, Barrio J, Bagger A, et al., 2023, Near ambient N2 fixation on solid electrodes versus enzymes and homogeneous catalysts, Nature Reviews Chemistry, Vol: 7, Pages: 184-201, ISSN: 2397-3358
The Mo/Fe nitrogenase enzyme is unique in its ability to efficiently reduce dinitrogen to ammonia at atmospheric pressures and room temperature. Should an artificial electrolytic device achieve the same feat, it would revolutionise fertilizers and even provide an energy dense, truly carbon-free fuel. This Review provides a coherent comparison of recent progress made in dinitrogen fixation on (i) solid electrodes, (ii) homogeneous catalysts and (iii) nitrogenases. Specific emphasis is placed on systems for which there is unequivocal evidence that dinitrogen reduction has taken place. By establishing the cross-cutting themes and synergies between these systems, we identify viable avenues for future research.
Westhead O, Barrio J, Bagger A, et al., 2023, Near ambient N<sub>2</sub> fixation on solid electrodes versus enzymes and homogeneous catalysts (vol 7, pg 184, 2023), NATURE REVIEWS CHEMISTRY, Vol: 7, Pages: 225-225
Paparakis A, Turnell-Ritson RCC, Sapsford JSS, et al., 2023, Tin-catalyzed reductive coupling of amines with CO<sub>2</sub> and H<sub>2</sub>, CATALYSIS SCIENCE & TECHNOLOGY, Vol: 13, Pages: 637-644, ISSN: 2044-4753
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Sapsford J, Csokas D, Turnell-Ritson R, et al., 2021, Transition metal-free direct hydrogenation of esters via a frustrated Lewis pair, ACS Catalysis, Vol: 11, Pages: 9143-9150, ISSN: 2155-5435
‘Frustrated Lewis pairs’ (FLPs) continue to exhibit unique reactivity for the reduction of organic substrates, yet to date the catalytic hydrogenation of an ester functionality has not been demonstrated. Here we report that iPr3SnNTf2 (1-NTf2; Tf = SO2CF3) is a more potent Lewis acid than the previously studied iPr3SnOTf; in an FLP with 2,4,6-collidine/2,6-lutidine (col/lut) this translates to faster H2 activation and the catalytic hydrogenolysis of an ester bond by a by a main-group compound, furnishing alcohol and ether (minor) products. The reaction outcome is sensitive to the steric and electronic properties of the substrate; CF3CO2Et and simple formates (HCO2Me, HCO2Et) are catalytically reduced, whereas related esters CF3CO2nBu and CH3CO2Et show only stoichiometric reactivity. A computational case study on the hydrogenation of CF3CO2Et and CH3CO2Et reveals that both share a common mechanistic pathway, however, key differences in the energies of a Sn-acetal intermediate and transition states emerge, favoring CF3CO2Et reduction. The alcohol products reversibly inhibit 1-NTf2/lut via formation of resting-state species 1-OR/[1·(1-OR)]+[NTf2]–, however the extra energy required to regenerate 1-NTf2/lut exacerbates the unfavorable reduction energy profile for CH3CO2Et, ultimately preventing turnover. These findings will assist the design of future main-group catalysts for ester hydrogenation, with improved performance.
Sapsford JS, Csókás D, Scott DJ, et al., 2020, Establishing the role of triflate anions in H2 activation by a cationic triorganotin(IV) lewis acid, ACS Catalysis, Vol: 10, Pages: 7573-7583, ISSN: 2155-5435
Cationic Lewis acids (LAs) are gaining interest as targets for frustrated Lewis pair (FLP)-mediated catalysis. Unlike neutral boranes, which are the most prevalent LAs for FLP hydrogenations, the Lewis acidity of cations can be tuned through modulation of the counteranion; however, detailed studies on such anion effects are currently lacking in the literature. Herein, we present experimental and computational studies which probe the mechanism of H2 activation using iPr3SnOTf (1-OTf) in conjunction with a coordinating (quinuclidine; qui) and noncoordinating (2,4,6-collidine; col) base and compare its reactivity with {iPr3Sn·base}{Al[OC(CF3)3]4} (base = qui/col) systems which lack a coordinating anion to investigate the active species responsible for H2 activation and hence resolve any mechanistic roles for OTf– in the iPr3SnOTf-mediated pathway.
Bennett EL, Lawrence EJ, Blagg RJ, et al., 2019, A new mode of chemical reactivity for metal-free hydrogen activation by Lewis acidic boranes, Angewandte Chemie - International Edition, Vol: 58, Pages: 8362-8366, ISSN: 0570-0833
We herein explore whether tris(aryl)borane Lewis acids are capable of cleaving H2 outside of the usual Lewis acid/base chemistry described by the concept of frustrated Lewis pairs (FLPs). Instead of a Lewis base we use a chemical reductant to generate stable radical anions of two highly hindered boranes: tris(3,5-dinitromesityl)borane and tris(mesityl)borane. NMR spectroscopic characterization reveals that the corresponding borane radical anions activate (cleave) dihydrogen, whilst EPR spectroscopic characterization, supported by computational analysis, reveals the intermediates along the hydrogen activation pathway. This radical-based, redox pathway involves the homolytic cleavage of H2 , in contrast to conventional models of FLP chemistry, which invoke a heterolytic cleavage pathway. This represents a new mode of chemical reactivity for hydrogen activation by borane Lewis acids.
Mercea DM, Howlett MG, Piascik AD, et al., 2019, Enantioselective reduction of N-alkyl ketimines with frustrated Lewis pair catalysis using chiral borenium ions, Chemical Communications, Vol: 55, Pages: 7077-7080, ISSN: 1359-7345
Enantioselective reduction of ketimines was demonstrated using chiral N-heterocyclic carbene (NHC)-stabilised borenium ions in frustrated Lewis pair catalysis. High levels of enantioselectivity were achieved for substrates featuring secondary N-alkyl substituents. Comparative reactivity and mechanistic studies identify key determinants required to achieve useful enantioselectivity and represent a step forward in the further development of enantioselective FLP methodologies.
Sapsford JS, Gates SJ, Doyle LR, et al., 2019, Cp*Fe(Me2PCH2CH2PMe2)(CHO): Hydride shuttle reactivity of a thermally stable formyl complex, Inorganica Chimica Acta, Vol: 488, Pages: 201-207, ISSN: 0020-1693
[Cp*Fe(Me2PCH2CH2PMe2)(CO)]+ [BArF24]− has been synthesised and characterised using single crystal X-ray diffraction, NMR and IR spectroscopies. Reduction of the CO ligand using Na[Et3BH] produces the corresponding neutral formyl complex Cp*Fe(Me2PCH2CH2PMe2)(CHO), that is very thermally stable, and which is attributed to the electron-releasing properties of the spectator ligands. This compound is a potent hydride donor which exists in equilibrium with [Et3BH]−, Et3B, and the structural isomer (η4-C5Me5H)Cp*Fe(Me2PCH2CH2PMe2)(CO), resulting from reversible hydride migration to the Cp* ligand.
Turnell-Ritson RC, Sapsford JS, Cooper RT, et al., 2018, Base-induced reversible H-2 addition to a single Sn(II) centre, Chemical Science, Vol: 9, Pages: 8716-8722, ISSN: 2041-6520
A range of amines catalyse the oxidative addition (OA) of H2 to [(Me3Si)2CH]2Sn (1), forming [(Me3Si)2CH]2SnH2 (2). Experimental and computational studies point to ‘frustrated Lewis pair’ mechanisms in which 1 acts as a Lewis acid and involve unusual late transition states; this is supported by the observation of a kinetic isotope effect Image ID:c8sc03110j-t1.gif for Et3N. When DBU is used the energetics of H2 activation are altered, allowing an equilibrium between 1, 2 and adduct [1·DBU] to be established, thus demonstrating reversible oxidative addition/reductive elimination (RE) of H2 at a single main group centre.
Piascik AD, Li R, Wilkinson HJ, et al., 2018, Fe-catalyzed conversion of N2 to N(SiMe3)3 via an fe-hydrazido resting state, Journal of the American Chemical Society, Vol: 140, Pages: 10691-10694, ISSN: 1520-5126
The catalytic conversion of N2 to N(SiMe3)3 by homogeneous transition metal compounds is a rapidly developing field, yet few mechanistic details have been experimentally elucidated for 3 d element catalysts. Herein we show that Fe(PP)2(N2) (PP = R2PCH2CH2PR2; R = Me, 1Me; R = Et, 1Et) are highly effective for the catalytic production of N(SiMe3)3 from N2 (using KC8/Me3SiCl), with the yields being the highest reported to date for Fe-based catalysts. We propose that N2 fixation proceeds via electrophilic Nβ silylation and 1e- reduction to form unstable FeI(NN-SiMe3) intermediates, which disproportionate to 1Me/Et and hydrazido FeII[N-N(SiMe3)2] species (3Me/Et); the latter act as resting states on the catalytic cycle. Subsequent 2e- reduction of 3Me/Et leads to N-N scission and formation of [N(SiMe3)2]- and putative anionic Fe imido products. These mechanistic results are supported by both experiment and DFT calculations.
Doyle LR, Scott DJ, Hill PJ, et al., 2018, Reversible coordination of N<inf>2</inf>and H<inf>2</inf>to a homoleptic: S = 1/2 Fe(i) diphosphine complex in solution and the solid state, Chemical Science, Vol: 9, Pages: 7362-7369, ISSN: 2041-6520
The synthesis and characterisation of the S = 1/2 Fe(i) complex [Fe(depe)2]+[BArF4]-([1]+[BArF4]-), and the facile reversible binding of N2and H2in both solution and the solid state to form the adducts [1·N2]+and [1·H2]+, are reported. Coordination of N2in THF is thermodynamically favourable under ambient conditions (1 atm; ΔG298= -4.9(1) kcal mol-1), while heterogenous binding is more favourable for H2than N2by a factor of ∼300. [1·H2]+[BArF4]-represents a rare example of a well-defined, open-shell, non-classical dihydrogen complex, as corroborated by ESR spectroscopy. The rapid exchange between N2and H2coordination under ambient conditions is unique for a paramagnetic Fe complex.
Ashley AE, Fuchter MJ, Tighe C, et al., 2018, Direct reductive amination of carbonyl compounds catalyzed by a moisture tolerant Tin (IV) Lewis acid, Advanced Synthesis and Catalysis, Vol: 360, Pages: 1066-1071, ISSN: 1615-4150
Despite the ever-broadening applications of main-group ‘frustrated Lewis pair’ (FLP) chemistry to both new and established reactions, their typical intolerance of water, especially at elevated temperatures (>100 °C), represents a key barrier to their mainstream adoption. Herein we report that FLPs based on the Lewis acid iPr3SnOTf are moisture tolerant in the presence of moderately strong nitrogenous bases, even under high temperature regimes, allowing them to operate as simple and effective catalysts for the reductive amination of organic carbonyls, including for challenging bulky amine and carbonyl substrate partners.
Cooper RT, Sapsford JS, Turnell-Ritson RC, et al., 2017, Hydrogen activation using a novel tribenzyltin Lewis acid, Royal Society of London. Philosophical Transactions A. Mathematical, Physical and Engineering Sciences, Vol: 375, ISSN: 1364-503X
Over the last decade there has been an explosion in the reactivity and applications of frustrated Lewis pair (FLP) chemistry. Despite this, the Lewis acids (LAs) in these transformations are often boranes, with heavier p-block elements receiving surprisingly little attention. The novel LA Bn3SnOTf (1) has been synthesised from simple, inexpensive starting materials and has been spectroscopically and structurally characterised. Subtle modulation of the electronics at the tin centre has led to an increase in its Lewis acidity in comparison with previously reported R3SnOTf LAs, and has facilitated low temperature hydrogen activation and imine hydrogenation. Deactivation pathways of the R3Sn+ Lewis acid core have also been investigated.
Scott DJ, Fuchter MJ, Ashley AE, 2017, Designing effective ‘frustrated Lewis pair’ hydrogenation catalysts, Chemical Society Reviews, Vol: 46, Pages: 5689-5700, ISSN: 1460-4744
The past decade has seen the subject of transition metal-free catalytic hydrogenation develop incredibly rapidly, transforming from a largely hypothetical possibility to a well-established field that can be applied to the reduction of a diverse variety of functional groups under mild conditions. This remarkable change is principally attributable to the development of so-called ‘frustrated Lewis pairs’: unquenched combinations of bulky Lewis acids and bases whose dual reactivity can be exploited for the facile activation of otherwise inert chemical bonds. While a number of comprehensive reviews into frustrated Lewis pair chemistry have been published in recent years, this tutorial review aims to provide a focused guide to the development of efficient FLP hydrogenation catalysts, through identification and consideration of the key factors that govern their effectiveness. Following discussion of these factors, their importance will be illustrated using a case study from our own research, namely the development of FLP protocols for successful hydrogenation of aldehydes and ketones, and for related moisture-tolerant hydrogenation.
Piascik AD, Crawford AD, Hill PJ, et al., 2017, Cationic silyldiazenido complexes of the Fe(diphosphine)2(N2)platform: structural and electronic models for an elusive firstintermediate in N2 fixation, Chemical Communications (London), Vol: 53, Pages: 7657-7660, ISSN: 0009-241X
The first cationic Fe silyldiazenido complexes, [Fe(PP)2(NN–SiMe3)]+[BArF4]− (PP = dmpe/depe), have been synthesised and thoroughly characterised. Computational studies show the compounds to be useful structural and electronic surrogates for the more elusive [Fe(PP)2(NN–H)]+, which are postulated intermediates in the H+/e− mediated fixation of N2 by Fe(PP)2(N2) species.
Scott DJ, Phillips NA, Sapsford JS, et al., 2016, Versatile catalytic hydrogenation using a simple tin(IV) Lewis acid, Angewandte Chemie, Vol: 128, Pages: 14958-14962, ISSN: 0044-8249
Despite the rapid development of frustrated Lewis pair (FLP) chemistry over the last ten years, its application in catalytic hydrogenations remains dependent on a narrow family of structurally similar early main‐group Lewis acids (LAs), inevitably placing limitations on reactivity, sensitivity and substrate scope. Herein we describe the FLP‐mediated H2 activation and catalytic hydrogenation activity of the alternative LA iPr3SnOTf, which acts as a surrogate for the trialkylstannylium ion iPr3Sn+, and is rapidly and easily prepared from simple, inexpensive starting materials. This highly thermally robust LA is found to be competent in the hydrogenation of a number of different unsaturated functional groups (which is unique to date for main‐group FLP LAs not based on boron), and also displays a remarkable tolerance to moisture.
Scott DJ, Phillips NA, Sapsford JS, et al., 2016, Versatile Catalytic Hydrogenation Using A Simple SnIV Lewis Acid, Angewandte Chemie International Edition, Vol: 55, Pages: 14738-14742, ISSN: 1433-7851
Despite the rapid development of frustrated Lewis pair (FLP) chemistry over the last ten years, its application in catalytic hydrogenations remains dependent on a narrow family of structurally similar early main-group Lewis acids (LAs), inevitably placing limitations on reactivity, sensitivity and substrate scope. Herein we describe the FLP-mediated H2 activation and catalytic hydrogenation activity of the alternative LA iPr3SnOTf, which acts as a surrogate for the trialkylstannylium ion iPr3Sn+, and is rapidly and easily prepared from simple, inexpensive starting materials. This highly thermally robust LA is found to be competent in the hydrogenation of a number of different unsaturated functional groups (which is unique to date for main-group FLP LAs not based on boron), and also displays a remarkable tolerance to moisture.
Hill PJ, Doyle LR, Crawford AD, et al., 2016, Selective catalytic reduction of N2 to N2H4 by a simple Fe complex, Journal of the American Chemical Society, Vol: 138, Pages: 13521-13524, ISSN: 1520-5126
The catalytic fixation of N2 by molecular Fe compounds is a rapidly developing field, yet thus far few complexes can effect this transformation, and none are selective for N2H4 production. Herein we report that the simple Fe(0) complex Fe(Et2PCH2CH2PEt2)2(N2) (1) is an efficient catalyst for the selective conversion of N2 (> 25 molecules N2 fixed) into N2H4, attendant with the production of ca. one molecule of NH3. Notably, the reductant (CoCp*2) and acid (Ph2NH2OTf) used are considerably weaker than conventional chemical H+ and e– sources used in previous demonstrations of N2 turnover by synthetic Fe compounds. These results show that the direct catalytic conversion of N2 to the hydrazine oxidation state on molecular Fe complexes is viable, and indicates that the mechanism of NH3 formation by such systems may proceed via Fe-N2H4 intermediates.
Ashley AE, Doyle LR, Hill P, et al., 2016, Teaching old compounds new tricks: efficient N2 fixation by simple Fe(N2)(diphosphine)2 complexes, Dalton Transactions, Vol: 45, Pages: 7550-7554, ISSN: 1477-9226
The Fe(0) species Fe(N2)(dmpe)2 exists in equilibrium with the previously unreported dimer, [Fe(dmpe2)2(μ-N2)]. For the first time these complexes, alongside Fe(N2)(depe)2, are shown unambiguously to produce N2H4 and/or NH3 upon addition of triflic acid; for Fe(N2)(depe)2 this represents one of the highest electron conversion efficiencies for Fe complexes to date.
Scott DJ, Simmons TR, Lawrence EJ, et al., 2015, Facile Protocol for Water-Tolerant "Frustrated Lewis Pair"-Catalyzed Hydrogenation, ACS Catalysis, Vol: 5, Pages: 5540-5544, ISSN: 2155-5435
Despite rapid advances in the field of metal-free, “frustrated Lewis pair” (FLP)-catalyzed hydrogenation, the need for strictly anhydrous reaction conditions has hampered wide-scale uptake of this methodology. Herein, we report that, despite the generally perceived moisture sensitivity of FLPs, 1,4-dioxane solutions of B(C6F5)3 actually show appreciable moisture tolerance and can catalyze hydrogenation of a range of weakly basic substrates without the need for rigorously inert conditions. In particular, reactions can be performed directly in commercially available nonanhydrous solvents without subsequent drying or use of internal desiccants.
Blagg RJ, Lawrence EJ, Resner K, et al., 2015, Exploring structural and electronic effects in three isomers of tris{bis(trifluoromethyl)phenyl}borane: towards the combined electrochemical-frustrated Lewis pair activation of H2, Dalton Transactions, Vol: 45, Pages: 6023-6031, ISSN: 1477-9226
Three structural isomers of tris{bis(trifluoromethyl)phenyl}borane have been studied as the acidic component of frustrated Lewis pairs. While the 3,5-substituted isomer is already known to heterolytically cleave H2 to generate a bridging-hydride; ortho-substituents in the 2,4- and 2,5-isomers quench such reactivity through electron donation into the vacant boron pz orbital and steric blocking of the boron centre; as shown by electrochemical, structural and computational studies. Electrochemical studies of the corresponding borohydrides identify that the two-electron oxidation of terminal-hydrides occurs at more positive potentials than observed for [HB(C6F5)3]−, while the bridging-hydride oxidizes at a higher potential still, comparable to that of free H2.
Lawrence EJ, Blagg RJ, Hughes DL, et al., 2015, A Combined "Electrochemical-Frustrated Lewis Pair" Approach to Hydrogen Activation: Surface Catalytic Effects at Platinum Electrodes, CHEMISTRY-A EUROPEAN JOURNAL, Vol: 21, Pages: 900-906, ISSN: 0947-6539
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Cooper RT, Chadwick FM, Ashley AE, et al., 2015, Double CO<sub>2</sub> activation by 14-electron η<SUP>8</SUP>-permethylpentalene titanium dialkyl complexes, CHEMICAL COMMUNICATIONS, Vol: 51, Pages: 11856-11859, ISSN: 1359-7345
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Chadwick FM, Ashley AE, Cooper RT, et al., 2015, Group 9 bimetallic carbonyl permethylpentalene complexes, DALTON TRANSACTIONS, Vol: 44, Pages: 20147-20153, ISSN: 1477-9226
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Hill PJ, Herrington TJ, Rees NH, et al., 2015, H<sub>2</sub> activation by a highly electron-deficient aralkylated organoborane, DALTON TRANSACTIONS, Vol: 44, Pages: 8984-8992, ISSN: 1477-9226
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Scott DJ, Fuchter MJ, Ashley AE, 2014, Nonmetal catalyzed hydrogenation of carbonyl compounds, Journal of the American Chemical Society, Vol: 136, Pages: 15813-15816, ISSN: 0002-7863
Solutions of the Lewis acid B(C6F5)3 in 1,4-dioxane are found to effectively catalyze the hydrogenation of a variety of ketones and aldehydes. These reactions, the first to allow entirely metal-free catalytic hydrogenation of carbonyl groups under relatively mild reaction conditions, are found to proceed via a “frustrated Lewis pair” mechanism in which the solvent, a weak Brønsted base yet moderately strong donor, plays a pivotal role.
Herrington TJ, Ward BJ, Doyle LR, et al., 2014, Bypassing a highly unstable frustrated Lewis pair: dihydrogen cleavage by a thermally robust silylium-phosphine adduct, CHEMICAL COMMUNICATIONS, Vol: 50, Pages: 12753-12756, ISSN: 1359-7345
Scott DJ, Fuchter MJ, Ashley AE, 2014, Metal-free hydrogenation catalyzed by an air-stable borane: use of solvent as a frustrated Lewis base, Angewandte Chemie International Edition, Vol: 53, Pages: 10218-10222, ISSN: 1433-7851
In recent years ‘frustrated Lewis pairs’ (FLPs) have been shown to be effective metal‐free catalysts for the hydrogenation of many unsaturated substrates. Even so, limited functional‐group tolerance restricts the range of solvents in which FLP‐mediated reactions can be performed, with all FLP‐mediated hydrogenations reported to date carried out in non‐donor hydrocarbon or chlorinated solvents. Herein we report that the bulky Lewis acids B(C6Cl5)x(C6F5)3−x (x=0–3) are capable of heterolytic H2 activation in the strong‐donor solvent THF, in the absence of any additional Lewis base. This allows metal‐free catalytic hydrogenations to be performed in donor solvent media under mild conditions; these systems are particularly effective for the hydrogenation of weakly basic substrates, including the first examples of metal‐free catalytic hydrogenation of furan heterocycles. The air‐stability of the most effective borane, B(C6Cl5)(C6F5)2, makes this a practically simple reaction method.
Lawrence EJ, Herrington TJ, Ashley AE, et al., 2014, Metal-free dihydrogen oxidation by a borenium cation: A Combined electrochemical/frustrated Lewis pair approach, Angewandte Chemie - International Edition, Vol: 53, Pages: 9922-9925, ISSN: 1433-7851
In order to use H2 as a clean source of electricity, prohibitively rare and expensive precious metal electrocatalysts, such as Pt, are often used to overcome the large oxidative voltage required to convert H2 into 2 H+ and 2 e-. Herein, we report a metal-free approach to catalyze the oxidation of H2 by combining the ability of frustrated Lewis pairs (FLPs) to heterolytically cleave H2 with the in situ electrochemical oxidation of the resulting borohydride. The use of the NHC-stabilized borenium cation [(IiPr2)(BC 8H14)]+ (IiPr2=C3H 2(NiPr)2, NHC=N-heterocyclic carbene) as the Lewis acidic component of the FLP is shown to decrease the voltage required for H2 oxidation by 910 mV at inexpensive carbon electrodes, a significant energy saving equivalent to 175.6 kJ mol-1. The NHC-borenium Lewis acid also offers improved catalyst recyclability and chemical stability compared to B(C6F5)3, the paradigm Lewis acid originally used to pioneer our combined electrochemical/frustrated Lewis pair approach. © 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
Gates SJ, Ashley AE, 2014, Homogeneous conversion of CO to ethylene using Fe-cyclopentadienyl complexes, 248th National Meeting of the American-Chemical-Society (ACS), Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
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