Halogenated triphenylgallium and -indium in frustrated Lewis pair activations and hydrogenation catalysis

Hexacoordinated tin complexes catalyse imine hydrogenation with H2

Frustrated Lewis pair (FLP) hydrogenation catalysts predominantly use alkyl- and aryl-substituted Lewis acids (LA) that offer a limited number of combinations of substituents, limiting our ability to tune their properties and, ultimately, their reactivity. Nevertheless, main-group complexes have numerous ligands available for such purposes, which could enable us to broaden the range of FLP catalysis. Supporting this hypothesis, we demonstrate here that hexacoordinated tin complexes with Schiff base ligands catalyse imine hydrogenation via activation of H2(g). As shown by hydrogen-deuterium scrambling, [Sn(tBu2Salen)(OTf)2] activated H2(g) at 25 °C and 10 bar of H2. After tuning the ligands, we found that [Sn(Salen)Cl2] was the most efficient imine hydrogenation catalyst despite having the lowest activity in H2(g) activation. Moreover, various imines were hydrogenated in yields up to 98% thereby opening up opportunities for developing novel FLP hydrogenation catalysts based on hexacoordinated LA of main-group elements.

What Distinguishes the Strength and the Effect of a Lewis Acid: Analysis of the Gutmann-Beckett Method

IUPAC defines Lewis acidity as the thermodynamic tendency for Lewis pair formation. This strength property was recently specified as global Lewis acidity (gLA), and is gauged for example by the fluoride ion affinity. Experimentally, Lewis acidity is usually evaluated by the effect on a bound molecule, such as the induced ³¹ P NMR shift of triethylphosphine oxide in the Gutmann-Beckett (GB) method. This type of scaling was called effective Lewis acidity (eLA). Unfortunately, gLA and eLA often correlate poorly, but a reason for this is unknown. Hence, the strength and the effect of a Lewis acid are two distinct properties, but they are often granted interchangeably. The present work analyzes thermodynamic, NMR specific, and London dispersion effects on GB numbers for 130 Lewis acids by theory and experiment. The deformation energy of a Lewis acid is identified as the prime cause for the critical deviation between gLA and eLA but its correction allows a unification for the first time.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Influence of the solvent on the Lewis acidity of antimony pentahalides and group 13 Lewis acids toward acetonitrile and pyridine

Energetic effects of solvation of SbF₅ , SbCl₅ , and 21 group 13 Lewis acids (LA) and their molecular complexes with acetonitrile and pyridine are evaluated using SMD approach. Compared to the gas phase, solvation increases the stability of boron- and aluminum-containing complexes but decreases the stability of gallium and indium-containing homologs due to larger solation energies of free LA. New Lewis acidity scales, based on the Gibbs energy of dissociation of the molecular complexes LA·pyridine and LA·acetonitrile in the gas phase, in benzene and acetonitrile solutions, are proposed.

Progressing the Frustrated Lewis Pair Abilities of N-Heterocyclic Carbene/GaR3 Combinations for Catalytic Hydroboration of Aldehydes and Ketones

Exploiting the steric incompatibility of the tris(alkyl)gallium GaR₃ (R = CH₂SiMe₃) and the bulky N-heterocyclic carbene (NHC) 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu), here we report the B-H bond activation of pinacolborane (HBPin), which has led to the isolation and structural authentication of a novel ion pair, [{ItBu-BPin}⁺{GaR₃(μ-H)GaR₃}^-] (2). Contrastingly, neither ItBu or GaR₃ was able to react with HBPin under the conditions of this study. Combining an NHC-stabilized borenium cation, [{ItBu-BPin}⁺], with an anionic dinuclear gallate, [{GaR₃(μ-H)GaR₃}^-], 2 proved to be unstable in solution at room temperature, evolving to the abnormal NHC-Ga complex [BPinC{{N(tBu)]₂CHCGa(R)₃}] (3). Interestingly, the structural isomer of 2, with the borenium cation residing at the C4 position of the carbene, [{aItBu-BPin}⁺{GaR₃(μ-H)GaR₃}^-] (4), was obtained when the abnormal NHC complex [aItBu·GaR₃] (1) was heated to 70 °C with HBPin, demonstrating that, under these forced conditions, it is possible to induce thermal frustration of the Lewis base/Lewis acid components of 1, enabling the activation of HBPin. Building on these stoichiometric studies, the frustrated Lewis pair (FLP) reactivity observed for the GaR₃/ItBu combination with HBPin could then be upgraded to catalytic regimes, allowing the efficient hydroboration of a range of aldehydes and ketones under mild reaction conditions. Mechanistic insights into the possible reaction pathway involved in this process have been gained by combining kinetic investigations with a comparative study of the catalytic capabilities of several gallium and borenium species related to 2. Disclosing a new cooperative partnership, reactions are proposed to occur via the formation of a highly reactive monomeric hydride gallate, [{ItBu-BPin}⁺{GaR₃(H)}^-] (I). Each anionic and cationic component of I plays a key role for success of the hydroboration, with the nucleophilic monomeric gallate anion favoring the transfer of its hydride to the C═O bond of the organic substate, which in turn is activated by coordination to the borenium cation.

A Phosphanyl-Phosphagallene that Functions as a Frustrated Lewis Pair

Phosphagallenes (1 a/1 b) featuring double bonds between phosphorus and gallium were synthesized by reaction of (phosphanyl)phosphaketenes with the gallium carbenoid Ga(Nacnac) (Nacnac=HC[C(Me)N(2,6-i-Pr2 C6 H3 )]2 ). The stability of these species is dependent on the saturation of the phosphanyl moiety. 1 a, which bears an unsaturated phosphanyl ring, rearranges in solution to yield a spirocyclic compound (2) which contains a P=P bond. The saturated variant 1 b is stable even at elevated temperatures. 1 b behaves as a frustrated Lewis pair capable of activation of H2 and forms a 1:1 adduct with CO2 .

Al/P- and Ga/P-Based Frustrated Lewis Pairs and Electronically Unsaturated Substrates: Ring Cleavage and Ring Closure, C-C and C-N Bond Formation

Al/P- and Ga/P-based frustrated Lewis pairs (FLPs) reacted with an azirine under mild conditions under cleavage of the heterocycle on two different positions. Opening of the C-C bond yielded an unusual nitrile-ylide adduct in which a C-N moiety coordinated to the FLP backbone. Cleavage of a C-N bond afforded the thermodynamically favored enamine adduct with the N atom bound to P and Al or Ga atoms. Ring closure was observed upon treatment of an Al/P FLP with electronically unsaturated substrates (4-(1-cyclohexenyl)-1-aza-but-1-en-3-ynes) and yielded by C-N bond formation hexahydroquinoline derivatives, which coordinated to the FLP through P-C and Al-C bonds. Diphenylcyclopropenone showed a diverse reactivity, which depending on steric shielding and the polarizing effect of Al or Ga atoms afforded different products. An AltBu₂ /P FLP yielded an adduct with the C=O group coordinated to P and Al. The dineopentyl derivative gave an equilibrium mixture consisting of a similar product and a simple adduct with O bound to Al and a three-coordinate P atom. Both compounds co-crystallize. The Ga/P FLP only formed the simple adduct with the same substrate. Rearrangement resulted in all cases in C₃ -ring cleavage and migration of a mesityl group from P to a former ring C atom by C-C bond formation. Diphenylthiocyclopropenone (evidence for the presence of P=C bonds) and an imine derivative afforded similar products.

FLP catalysis: main group hydrogenations of organic unsaturated substrates

This article is focused on recent developments in main group mediated hydrogenation chemistry and catalysis using "frustrated Lewis pairs" (FLPs). The broading range of substrates and catalyst systems is reviewed and the advances in catalytic reductions and the development of stereoselective, asymmetric reductions made since 2012 is considered.

Surprisingly Flexible Oxonium/Borohydride Ion Pair Configurations

We investigate the geometry of oxonium/borohydride ion pairs [ether-H⁽⁺⁾-ether][LA-H^(-)] with dioxane, THF, and Et₂O as ethers and B(C₆F₅)₃ as the Lewis acid (LA). The question is about possible location of the disolvated proton, [ether-H⁽⁺⁾-ether], with respect to the hydride of the structurally complex [LA-H^(-)] anion. Using Born-Oppenheimer molecular dynamics and a comparison of the potential and free energies of the optimized configurations, we show that herein considered ion pairs are much more flexible geometrically than previously thought. Conformers with different locations of cations with respect to anions are governed by a flat energy-landscape. We found a novel configuration in which oxonium is below [LA-H^(-)], with respect to the direction of borane → hydride vector, and the proton-hydride distance is ca. 6 Å. With calculations of the vibrational spectra of [ether-H⁽⁺⁾-ether][(C₆F₅)₃B-H^(-)] for dioxane, THF, and Et₂O as ethers, we investigate the manifestation of SSLB-type (short, strong, low-barrier) hydrogen bonding in the OHO motif of an oxonium cation.

Probing steric influences on electrophilic phosphonium cations: a comparison of [(3,5-(CF3)2C6H3)3PF]+ and [(C6F5)3PF]+

The electrophilic phosphonium cation (EPC) salt [(3,5-(CF₃)₂C₆H₃)₃PF][B(C₆F₅)₄] (2) can display catalytic activity greater than its thermodynamic acidity would suggest. The role of steric factors is explored.

Reactions of Al/P, Ga/P and P–H functionalized frustrated Lewis pairs with azides and a diazomethane – formation of adducts and capture of nitrenes

The Al- and Ga-based frustrated Lewis pairs (FLPs) Mes2P–C(MR2)=CH-R′ (1, M=Al, R= t Bu; 2, M=Al, R=CH2 t Bu; 3, M=Ga, R= t Bu) and the unique P–H functionalized FLP Mes(H)P–CH(AlR2)=C(H)- t Bu [4, R=CH(SiMe3)2] were treated with a variety of azides R′-N=N=N [R′= t Bu, SiMe3, Ph, CH2Ph, C6H4(4-Cl), C6H4(4-CF3), C6H4(4-Me), CH2C6H4(4-Cl), CH2C6H4(4- t Bu), C6H4(2-CH=CHPh)] in order to study systematically the influence of the substituents at nitrogen, phosphorus and the metal atoms on the reaction courses and the thermal stability of the products. Azide adducts (5–8) were isolated in which the terminal nitrogen atoms of the azides (Nγ γ) were bound to the phosphorus and the respective metal atoms resulting in four-membered PCMN heterocycles as the sole structural motif despite the wide range of substituents and the variation in the metal atoms of the FLPs. Thermal activation of selected azide adducts led to the elimination of N2 and the formation of the nitrene adducts 9–11 in which formally a transient, highly reactive nitrene N–R with an electron sextet nitrogen atom is trapped by the FLPs. For the first time FLPs were treated with a diazomethane, Me3Si–C(H)=N=N. Reactions with 1 and 2 afforded the adducts [Mes2P–C(AlR2)=CH-Ph](μ-N2CH–SiMe3) 12 (R= t Bu, CH2 t Bu) which had structures and spectroscopic properties similar to those of the corresponding azides. These compounds are thermally stable and do not eliminate dinitrogen upon warming or irradiation. Protonation of 12a with HCl in Et2O resulted in cleavage of the Al–N bond and formation of the zwitterionic phosphonium salt Mes2P[NH–N=C(H)–SiMe3]–C(AlCl t Bu2)=C(H)-Ph 13 with an intramolecular N–H···Cl hydrogen bond.