Near ambient N2 fixation on solid electrodes versus enzymes and homogeneous catalysts.

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 revolutionize fertilizer production and even provide an energy-dense, truly carbon-free fuel. This Review provides a coherent comparison of recent progress made in dinitrogen fixation on solid electrodes, homogeneous catalysts and 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.

Establishing the Role of Triflate Anions in H2 Activation by a Cationic Triorganotin(IV) Lewis Acid.

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.

Enantioselective reduction of N-alkyl ketimines with frustrated Lewis pair catalysis using chiral borenium ions.

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.

A New Mode of Chemical Reactivity for Metal-Free Hydrogen Activation by Lewis Acidic Boranes.

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.

Reversible coordination of N2 and H2 to a homoleptic S = 1/2 Fe(i) diphosphine complex in solution and the solid state.

The synthesis and characterisation of the S = 1/2 Fe(i) complex [Fe(depe)2]+[BArF4]- ([1]+[BArF4]-), and the facile reversible binding of N2 and H2 in both solution and the solid state to form the adducts [1·N2]+ and [1·H2]+, are reported. Coordination of N2 in THF is thermodynamically favourable under ambient conditions (1 atm; ΔG 298 = -4.9(1) kcal mol-1), while heterogenous binding is more favourable for H2 than N2 by 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 N2 and H2 coordination under ambient conditions is unique for a paramagnetic Fe complex.

Fe-Catalyzed Conversion of N2 to N(SiMe3)3 via an Fe-Hydrazido Resting State.

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 Nß 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.

Direct Reductive Amination of Carbonyl Compounds Catalyzed by a Moisture Tolerant Tin(IV) Lewis Acid.

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.

Base-induced reversible H2 addition to a single Sn(ii) centre.

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 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.

Hydrogen activation using a novel tribenzyltin Lewis acid.

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 synthesized from simple, inexpensive starting materials and has been spectroscopically and structurally characterized. 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+ LA core have also been investigated.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

Designing effective 'frustrated Lewis pair' hydrogenation catalysts.

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.

Cationic silyldiazenido complexes of the Fe(diphosphine)2(N2) platform: structural and electronic models for an elusive first intermediate in N2 fixation.

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.

Selective Catalytic Reduction of N2 to N2H4 by a Simple Fe Complex.

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 that the mechanism of NH3 formation by such systems may proceed via Fe-N2H4 intermediates.

Versatile Catalytic Hydrogenation Using A Simple Tin(IV) Lewis Acid.

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 iPr3 SnOTf, which acts as a surrogate for the trialkylstannylium ion iPr3 Sn+ , 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.

Teaching old compounds new tricks: efficient N2 fixation by simple Fe(N2)(diphosphine)2 complexes.

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.

Exploring structural and electronic effects in three isomers of tris{bis(trifluoromethyl)phenyl}borane: towards the combined electrochemical-frustrated Lewis pair activation of H2.

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.

Facile Protocol for Water-Tolerant "Frustrated Lewis Pair"-Catalyzed Hydrogenation.

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.

Group 9 bimetallic carbonyl permethylpentalene complexes.

We describe the synthesis, structure and bonding of the first iridium and rhodium permethylpentalene complexes, syn-[M(CO)2]2(µ:η(5):η(5)-Pn*) (M = Rh, Ir). In fact, [Ir(CO)2]2(µ:η(5):η(5)-Pn*) is the first iridium pentalene complex. An interesting preference for the isolation of the sterically more demanding syn-isomer is observed and substantiated by DFT analysis. Upon photolysis, the rhodium analogue yields an unusual tetrameric species Rh4(CO)6(µ:η(3):η(5)-Pn*)2 with bridging carbonyls and Rh-Rh bonds, which has been characterised by single crystal X-ray diffraction and by solution NMR spectroscopy.

Double CO2 activation by 14-electron η(8)-permethylpentalene titanium dialkyl complexes.

The novel 14 electron species η(8)-Pn*TiR2 (Pn* = C8Me6; R = Me, CH2Ph) have been synthesised and spectroscopically and structurally characterised. Subsequent reaction with CO2 leads to the activation and double insertion of CO2 into both Ti-alkyl bonds to form the electronically saturated η(8)-Pn*Ti(κ(2)-O2CR)2 (R = Me, CH2Ph) complexes.