An umpolung of Lewis acidity/basicity at nitrogen by deprotonation of a cyclic (amino)(aryl)nitrenium cation

Recent advances in the chemistry of isolable carbene analogues with group 13-15 elements

Carbenes (R2C:), compounds with a divalent carbon atom containing only six valence shell electrons, have evolved into a broader class with the replacement of the carbene carbon or the RC moiety with main group elements, leading to the creation of main group carbene analogues. These analogues, mirroring the electronic structure of carbenes (a lone pair of electrons and an empty orbital), demonstrate unique reactivity. Over the last three decades, this area has seen substantial advancements, paralleling the innovations in carbene chemistry. Recent studies have revealed a spectrum of unique carbene analogues, such as monocoordinate aluminylenes, nitrenes, and bismuthinidenes, notable for their extraordinary properties and diverse reactivity, offering promising applications in small molecule activation. This review delves into the isolable main group carbene analogues that are in the forefront from 2010 and beyond, spanning elements from group 13 (B, Al, Ga, In, and Tl), group 14 (Si, Ge, Sn, and Pb) and group 15 (N, P, As, Sb, and Bi). Specifically, this review focuses on the potential amphiphilic species that possess both lone pairs of electrons and vacant orbitals. We detail their comprehensive synthesis and stabilization strategies, outlining the reactivity arising from their distinct structural characteristics.

Unveiling N-Fused Nitreniums as Potent Catalytic Photooxidants

The first example of a hitherto-unknown facet of catalytic photooxidant capability of nitrenium cations is reported herein. The fundamental limitation of inability of the traditional and reported nitreniums to achieve the excited-state redox potential beyond +2.0 V (vs Ag/AgCl), the primary requirement for a powerful photooxidant, is addressed in this work by developing a structurally unique class of N-fused nitrenium cations, with the required structural engineering involving extensive π-conjugation through ring fusion at the nitrenium site, which enabled significant lowering of the LUMO energy and easy reduction at the excited state (excited-state redox potential up to +2.5 V vs Ag/AgCl), facilitated by effective delocalization/stabilization of the generated radical. This finding opens a new way to discover novel and useful (photo)catalytic properties of nitrenium cations beyond just Lewis acidity.

Advances in CO2 activation by frustrated Lewis pairs: from stoichiometric to catalytic reactions

The rise of CO2 concentrations in the environment due to anthropogenic activities results in global warming and threatens the future of humanity and biodiversity. To address excessive CO2 emissions and its effects on climate change, efforts towards CO2 capture and conversion into value adduct products such as methane, methanol, acetic acid, and carbonates have grown. Frustrated Lewis pairs (FLPs) can activate small molecules, including CO2 and convert it into value added products. This review covers recent progress and mechanistic insights into intra- and inter-molecular FLPs comprised of varying Lewis acids and bases (from groups 13, 14, 15 of the periodic table as well as transition metals) that activate CO2 in stoichiometric and catalytic fashion towards reduced products.

Nitrenium ions as new versatile reagents for electrophilic amination

Herein we report the utilization of N-heterocyclic nitrenium ions - easily prepared, bench-stable and non-oxidating nitrogen sources for the efficient electrophilic amination of aliphatic and aromatic organometallic nucleophiles, towards the facile and general preparation of primary amines. To this end, a plethora of abundant organolithium and organomagnesium reagents were combined with nitrenium salts to generate a variety of previously unexplored N-alkyl and N-aryl triazanes. Through the simple hydrogenolysis of these relatively stable triazanes, we have prepared a diverse scope of primary amines, including linear and branched aliphatic as well as (hetero)aromatic amines possessing various stereo-electronic substituents. Furthermore, we present the facile synthesis of valuable 15N-labelled primary amines from easily prepared 15N-labelled nitrenium salts, as well as a one-pot approach to biologically relevant primary amines. Finally, a recyclable variant of the nitrenium precursor was prepared and a simple recovery protocol was developed to improve the atom-economy of this procedure.

Computational Design of Frustrated Lewis Pairs as a Strategy for Catalytic Hydrogen Activation and Hydrogenation Catalyst

Catalytic hydrogenation is one of the most important reaction types commonly used in chemistry and chemical industry. Recently, there has been significant interest in developing a metal-free hydrogenation catalyst to avoid the problems caused by using heavy transition metal catalysts. On the basis of the advances of metal-free hydrogen activation with frustrated Lewis pairs (FLPs, e.g. tBu₃P/B(C₆F₅)₃) which often uses boron as a Lewis acid center, we computationally explored the prospect for phosphorus(V) and sulfur(VI) as Lewis acid centers to construct FLPs for hydrogen activation and hydrogenation. We found out that the proposed FLPs with P(V)- or S(VI)-centered Lewis acid can also activate H₂ with a mechanism similar to that used by the conventional FLPs. A heterolytic cleavage of H-H is achieved when electrons are donated simultaneously from the σ orbital of H₂ to the empty orbital of the Lewis acid center and from the lone-pair orbital of the Lewis base center to the σ* orbital of H₂. The multiple C-H···F hydrogen bonds further aid the association of the pairs for H₂ activation. Some of our designed FLPs possess kinetics and thermodynamics for developing hydrogenation catalysts. This computational exploration could inspire experimental development of a new type of FLPs with P(V) or S(VI) or a Lewis acid partner for FLPs for reversible H₂ activation.

Multimodal Reactivity of N-H Bonds in Triazanes and Isolation of a Triazinyl Radical

The employment of nitrogen Lewis acids based on nitrenium cations has been increasingly featured in the fields of main group chemistry and catalysis. A formally reduced form of nitrenium D─cyclic triazanes E─are intriguing chemical compounds, the chemistry of which is completely unexplored. In this work, we reveal that N-H-triazanes exhibit unusual N-H bond properties; namely, they can serve as protons, hydrides, or hydrogen atom donors. This unique multimodal reactivity provides an N-cation, N-anion, or N-radical from the same species. It allowed us to isolate, for the first time, a stable naphto[1,2,3]triazinyl radical, which was fully characterized both computationally and experimentally, including its monomeric X-ray structure. Moreover, this radical can be prepared directly from the nitrenium cation by a single electron reduction (E = -0.46 V), and this process is reversible. We envision versatile uses of this radical in synthetic and materials chemistry.

Benzotriazolium Salts: Emergent Readily Accessible Bench-Stable Lewis Acid Catalysts

In this work, benzotriazolium salts have been introduced as efficient, readily accessible, bench-stable Lewis acid catalysts. Though these sorts of N-heterocyclic compounds have found wide applications as ionic liquids or electrolytes, their Lewis acid catalytic activity remained unexplored. Herein, their potential as Lewis acid catalysts was demonstrated in two prototypical allylic and Nazarov cyclization reactions, showing a matching reactivity and allowing low catalytic loadings (down to 0.5 mol %).

Recent Advances in Group 14 and 15 Lewis Acids for Frustrated Lewis Pair Chemistry

Frustrated Lewis pairs (FLP) which rely on the cooperative action of Lewis acids and Lewis bases, played a prominent role in the advancement of main-group catalysis. While the early days of FLP chemistry witnessed the dominance of boranes, there is a growing body of reports on alternative Lewis acids derived from groups 14 and 15. This short review focuses on the discovery of such non-boron candidates reported since 2015.

Investigating Tetrel-Based Neutral Frustrated Lewis Pairs for Hydrogen Activation

Tetrel Lewis acids are a prospective alternative to commonly employed neutral boranes in frustrated Lewis pair (FLP) chemistry. While cationic tetrylium Lewis acids, being isolobal and iso(valence)electronic, are a natural replacement to boranes, neutral tetrel Lewis acids allude as less trivial options due to the absence of a formally empty p orbital on the acceptor atom. Recently, a series of intramolecular geminal FLPs (C₂F₅)₃E-CH₂-P(tBu)₂ (E = Si, Ge, Sn) featuring neutral tetrel atoms as acceptor sites has been reported for activation of small molecules including H₂. In this work, through density functional theory computations, we elucidate the general mechanistic picture of H₂ activation by this family of FLPs. Our findings reveal that the acceptor atom derives the required Lewis acidity utilizing the antibonding orbitals of its adjacent bonds with the individual contributions depending on the identity of the acceptor and the donor atoms. By varying the identity of the Lewis acid and Lewis base sites and attached substituents, we unravel their interplay on the energetics of the H₂ activation. We find that switching the donor site from P to N significantly affects the synchronous nature of the bond breaking/formations along the reaction pathway, and as a result, N-bearing FLPs have a more favorable H₂ activation profile than those with P. Our results are quantitatively discussed in detail within the framework of the activation-strain model of reactivity along with the energy-decomposition analysis method. Finally, the reductive elimination decomposition route pertinent to the plausible extension of the H₂ activation to catalytic hydrogenation by these FLPs is also examined.

The Reactivity of Isomeric Nitrenium Lewis Acids with Phosphines, Carbenes, and Phosphide

Alkylation of spiro[fluorene-9,3'-indazole] at N(1) and N(2) with tBuCl affords the nitrenium cations [C6 H4 N2 (tBu)C(C12 H8 )][BF4 ], 1 and 2, respectively. Compound 1 converts to 2 over the temperature range 303-323 K with a free energy barrier of 28±5 kcal mol-1 . Reaction of 1 with PMe3 afforded the N-bound phosphine adduct [C6 H4 N(tBu)N(PMe3 )C(C12 H8 )]BF4 ] 3. However, phosphines attack 2 at the para-carbon atom of the aryl group with concurrent cleavage of N(2)-C(1) bond and proton migration to C(1) affording [(R3 P)C6 H3 NN(tBu)CH(C12 H8 )][BF4 ] (R=Me 4, nBu 5). Analogous reactions of 1 and 2 with the carbene SIMes prompt attack at the para-carbon with concurrent loss of H. affording the radical cation salts [(SIMes)C6 H3 N(tBu)NC(C12 H8 ). ][BF4 ] 6 and [(SIMes)C6 H3 NN(tBu)C(C12 H8 ). ][BF4 ] 7, whereas reaction of 2 with BAC gives the Lewis acid-base adduct, [C6 H4 N(BAC)N(tBu)C(C12 H8 )][BF4 ] 8. Finally, reactions of 1 and 2 with KPPh2 result in electron transfer affording (PPh2 )2 and the persistent radicals C6 H4 N(tBu)NC(C12 H8 ). and C6 H4 NN(tBu)C(C12 H8 ). . The detailed reaction mechanisms are also explored by extensive DFT calculations.

Reaction of Phosphines with 1-Azido-(2-halogenomethyl)benzene Giving Aminophosphonium-Substituted Indazoles

The reaction between a 1-azido-(2-halogenomethyl)benzene and a phosphine gives different products depending on the nature of the halogen, the phosphine itself, and the solvent employed. While PPh₃ (2 equiv) reacts with the chloro reagent in toluene to give the expected iminophosphorane-phosphonium adduct, trialkylphosphines (PCy₃ and PEt₃) surprisingly furnish an aminophosphonium substituted by a zwitterionic indazole. The bicyclic product can also form from PPh₃ using the bromo reagent in acetonitrile. A mechanism is proposed for this cyclization based on DFT calculations.

Reactivity of Diarylnitrenium Ions

Hydride abstraction from diarylamines with the trityl ion is explored in an attempt to generate a stable diarylnitrenium ion, Ar₂ N⁺ . Sequential H-atom abstraction reactions ensue. The first H-atom abstraction leads to intensely colored aminium radical cations, Ar₂ NH^.+ , some of which are quite stable. However, most undergo a second H-atom abstraction leading to ammonium ions, Ar₂ NH₂ ⁺ . In the absence of a ready source of H-atoms, a unique self-abstraction reaction occurs when Ar=Me₅ C₆ , leading to a novel iminium radical cation, Ar=N^.+ Ar, which decays via a second self H-atom abstraction reaction to give a stable iminium ion, Ar=N⁺ HAr. These products differ substantially from those derived via photochemically produced diarylnitrenium ions.

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.

An arene-stabilized η5-pentamethylcyclopentadienyl antimony dication acts as a source of Sb+ or Sb3+ cations

The dicationic compound [(η⁵-Cp*)Sb(tol)][B(C₆F₅)₄]₂ (1) (tol = toluene), which exhibits strong Lewis acidity, reacts with Lewis bases to provide Sb⁺ or Sb³⁺ cations.

Aryldiazonium Salts as Nitrogen-Based Lewis Acids: Facile Synthesis of Tuneable Azophosphonium Salts

Inspired by the commercially available azoimidazolium dyes (e.g., Basic Red 51) that can be obtained from aryldiazonium salts and N-heterocyclic carbenes, we developed the synthesis of a unique set of arylazophosphonium salts. A range of colours were obtained by applying readily tuneable phosphine donor ligands and para-substituted aryldiazonium salts as nitrogen-based Lewis acids. With cyclic voltammetry, a general procedure was designed to establish whether the reaction between a Lewis acid and a Lewis base occurs by single-electron transfer or electron-pair transfer.

Stable Diporphyrinylaminyl Radical and Nitrenium Ion

Nitrenium ions, isoelectronic nitrogen counterparts of carbenes, are important intermediates in various biological and chemical processes. Herein we describe the first synthesis and characterization of a stable nitrenium ion without resonance stabilization by adjoining amino groups. Namely, a stable salt of a diporphyrinylnitrenium ion was synthesized by stepwise oxidation of the corresponding diporphyrinylamine through a stable aminyl radical. The nitrenium ion exhibits characteristic features such as a singlet ground state, enhanced double-bond character of the central C-N bonds, no reactivity toward water and methanol, and negative solvatochromic behavior.

Nitrogen-Based Lewis Acids Derived from Phosphonium Diazo Cations

Reaction of PPh₃ and [(p-ClC₆ H₄ )N₂ ][BF₄ ] affords [(p-ClC₆ H₄ )N(PPh₃ )N(PPh₃ )][BF₄ ] 1, while reaction with (Ph₂ PCH₂ )₂ gave [(p-ClC₆ H₄ )(NPh₂ PCH₂ )₂ )][BF₄ ] 2. These species confirm the Lewis acidity of [(p-ClC₆ H₄ )N₂ (PR₃ )][BF₄ ] cations at N. In contrast, use of bulky phosphines afford the species [ArN₂ (PR₃ )][BF₄ ] (R=tBu 3, Mes 4). Compound 3 undergoes one electron reduction to give the stable radical [(p-ClC₆ H₄ )N₂ (PtBu₃ )]^. 5. Combination of 3 and PtBu₃ acts as an FLP to effect (SPh)₂ cleavage, generating [PhSPtBu₃ ]⁺ and the radical [ArN₂ (PR₃ )]^. . Collectively, these data affirm the ability of the cations [ArN₂ (PR₃ )]⁺ to behave as one or two electron acceptors.

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.