Synthesis,Structure, and Reactivity of Diazene Adducts: Isolation of iso‐Diazene Stabilized as a Borane Adduct

This work describes the synthesis and full characterization of a series of GaCl₃ and B(C₆F₅)₃ adducts of diazenes R¹?N?N?R² (R¹=R²=Me₃Si, Ph; R¹=Me₃Si, R²=Ph). Trans‐Ph?N?N?Ph forms a stable adduct with GaCl₃, whereas no adduct, but instead a frustrated Lewis acid–base pair is formed with B(C₆F₅)₃. The cis‐Ph?N?N?Ph ? B(C₆F₅)₃ adduct could only be isolated when UV light was used, which triggers the isomerization from trans‐ to cis‐Ph?N?N?Ph, which provides more space for the bulky borane. Treatment of trans‐Ph?N?N?SiMe₃ with GaCl₃ led to the expected trans‐Ph?N?N?SiMe₃ ? GaCl₃ adduct but the reaction with B(C₆F₅)₃ triggered a 1,2‐Me₃Si shift, which resulted in the formation of a highly labile iso‐diazene, Me₃Si(Ph)N?N; stabilized as a B(C₆F₅)₃ adduct. Trans‐Me₃Si?N?N?SiMe₃ forms a labile cis‐Me₃Si?N?N?SiMe₃ ? B(C₆F₅)₃ adduct, which isomerizes to give the transient iso‐diazene species (Me₃Si)₂N?N ? B(C₆F₅)₃ upon heating. Both iso‐diazene species insert easily into one B?C bond of B(C₆F₅)₃ to afford hydrazinoboranes. All new compounds were fully characterized by means of X‐ray crystallography, vibrational spectroscopy, CHN analysis, and NMR spectroscopy. All compounds were further investigated by DFT and the bonding situation was assessed by natural bond orbital (NBO) analysis.     

Borohydrides from Organic Hydrides: Reactions of Hantzsch’s Esters with B(C6F5)3

We report herein that the reaction between a series of Hantzsch’s ester analogues 1 a – d with the Lewis acidic species B(C₆F₅)₃ results in facile transfer of hydride to boron. The main products of this reaction are pyridinium borohydride salts 2 a – d , which are obtained in high to moderate yields. The N‐substituted substrates (N‐Me, N‐Ph) reacted in high yield 90–98 % and the connectivity of the products were confirmed by an X‐ray crystallographic analysis of the N‐Me borohydride salt 2 a . Unsubstituted Hanztsch’s ester 1 a reacted less effectively generating only 60 % of the corresponding borohydride salt, with the balance of the material sequestered as the ester‐bound Lewis acid–base adduct 3 a . Formation of the Lewis acid–base adduct could be minimized by increasing the steric bulk about the ester groups as in 1 d . The connectivity of the carbonyl‐bound adduct was confirmed by an X‐ray crystallographic analysis of 3 e the product of the reaction of methyl ketone 1 e with B(C₆F₅)₃. We also explored the generation of these pyridinium salts by employing frustrated Lewis pair methodology. However, the reaction of mixtures of the corresponding pyridine and B(C₆F₅)₃ with hydrogen gas only resulted in formation of trace amounts of the pyridinium borohydride, along with the Lewis acid–base adduct of the starting material and B(C₆F₅)₃. The 1,2‐dihydropyridine adduct was the final product of this reaction. This was ascribed to the low basicity of the pyridine nitrogen and the complicating formation of an ester bound Lewis acid–base adduct.     

Aggregation Behavior of a Six‐Membered Cyclic Frustrated Phosphane/Borane Lewis Pair: Formation of a Supramolecular Cyclooctameric Macrocyclic Ring System

A new six‐membered cyclic frustrated phosphane/borane Lewis pair was liberated from its HB(C₆F₅)₂ adduct by treatment with vinylcyclohexane. The system is an active frustrated Lewis pair that undergoes cycloaddition reactions with suitable π reagents and it splits dihydrogen. At room temperature in solution the new compound is a monomer, however, in the crystal and in solution at low temperature it aggregates to a thermodynamically favoured supramolecular macrocyclic cyclooctamer.     

Borane Adducts of Hydrazoic Acid and Organic Azides: Intermediates for the Formation of Aminoboranes

The reaction of HN₃ with the strong Lewis acid B(C₆F₅)₃ led to the formation of a very labile HN₃?B(C₆F₅)₃ adduct, which decomposed to an aminoborane, H(C₆F₅)NB(C₆F₅)₂, above ?20 °C with release of molecular nitrogen and simultaneous migration of a C₆F₅ group from boron to the nitrogen atom. The intermediary formation of azide–borane adducts with B(C₆F₅)₃ was also demonstrated for a series of organic azides, RN₃ (R=Me₃Si, Ph, 3,5‐(CF₃)₂C₆H₃), which also underwent Staudinger‐like decomposition along with C₆F₅ group migration. In accord with experiment, computations revealed rather small barriers towards nitrogen release for these highly labile azide adducts for all organic substituents except R=Me₃Si (m.p. 120 °C, T_dec=189 °C). Hydrolysis of the aminoboranes provided C₆F₅‐substituted amines, HN(R)(C₆F₅), in good yields.     

1,1‐Hydroboration and a Borane Adduct of Diphenyldiazomethane: A Potential Prelude to FLP‐N2 Chemistry

Diphenyldiazomethane reacts with HB(C₆F₅)₂ and B(C₆F₅)₃, resulting in 1,1‐hydroboration and adduct formation, respectively. The hydroboration proceeds via a concerted reaction involving initial formation of the Lewis adduct Ph₂CN₂BH(C₆F₅)₂. The highly sensitive adduct Ph₂CN₂(B(C₆F₅)₃) liberates N₂ and generates Ph₂CB(C₆F₅)₃. DFT computations reveal that formation of Ph₂CN₂B(C₆F₅)₃ from carbene, N₂, and borane is thermodynamically favourable, suggesting steric frustration could preclude carbene–borane adduct formation and affect FLP‐N₂ capture.     

3‐Bromo‐2‐Pyrone: An Alternative and Convenient Route to Functionalized Phosphinines

The facile access to 3‐bromo‐2‐pyrone allows the preparation of 6‐bromo‐2‐trimethylsilyl‐phosphinine by a [4+2] cycloaddition with Me₃Si‐C≡P for the first time. The regioselectivity of this reaction could be verified by means of single crystal X‐ray diffraction of the corresponding W⁰ complex. In the presence of ZnBr₂ and dppp (1,3‐bis(diphenylphosphino)propane) as a bidentate ligand, the bromo‐phosphinine quantitatively undergoes a Negishi cross‐coupling reaction with PhLi that selectively leads to 6‐phenyl‐2‐trimethylsilyl‐phosphinine. This heterocycle could again be characterized by means of X‐ray diffraction as a W⁰ complex. These results describe a new and convenient route to 2,6‐disubstituted phosphinines that makes use of readily available starting materials.     

Incorporation of Boron into Uranium Metallacycles: Synthesis,Structure, and Reactivity of Boron-Containing Uranacycles Derived from Bis(alkynyl)boranes

The reaction of uranacyclopropene complex (C₅Me₅)₂U[η²-1,2-C₂(SiMe₃)₂] with B-aryl bis(alkynyl)borane PhB(C≡CPh)₂ led to the first six-membered uranium metallaboracycle, while the reaction with B-amino bis(alkynyl)borane (Me₃Si)₂NB(C≡CPh)₂ afforded an unexpected uranaborabicyclo[2.2.0] complex via [2+2] cycloaddition. The reaction with CuCl revealed the non-innocent property of the rearranged bis(alkynyl)boron species towards oxidant. The reactions with isocyanide DippNC: (Dipp=2,6-iPr₂-C₆H₃) and isocyanate tBuNCO afforded the novel uranaborabicyclo[3.2.0] complexes. All new complexes have been structurally characterized. DFT calculations were performed to provide more insights into the electronic structures and the reaction mechanism.     

Boron Lewis Acid‐Catalyzed Hydroboration of Alkenes with Pinacolborane: BArF3 Does What B(C6F5)3 Cannot Do!

The transition‐metal‐free hydroboration of various alkenes with pinacolborane (HBpin) initiated by tris[3,5‐bis(trifluoromethyl)phenyl]borane (BAr^F₃) is reported. The choice of the boron Lewis acid is crucial as the more prominent boron Lewis acid tris(pentafluorophenyl)borane (B(C₆F₅)₃) is reluctant to react. Unlike B(C₆F₅)₃, BAr^F₃ is found to engage in substituent redistribution with HBpin, resulting in the formation of Ar^FBpin and the electron‐deficient diboranes [H₂BAr^F]₂ and [(Ar^F)(H)B(μ‐H)₂BAr^F₂]. These in situ‐generated hydroboranes undergo regioselective hydroboration of styrene derivatives as well as aliphatic alkenes with cis diastereoselectivity. Another ligand metathesis of these adducts with HBpin subsequently affords the corresponding HBpin‐derived anti‐Markovnikov adducts. The reactive hydroboranes are regenerated in this step, thereby closing the catalytic cycle.     

The Non-Ancillary Nature of Trimethylsilylamide Substituents in Boranes and Borinium Cations

The known boranes (R(Me₃Si)N)₂BF (R=Me₃Si 1 , tBu 2 , C₆F₅ 3 , o-tol 4 , Mes 5 , Dipp 6 ) and borinium salts (R(Me₃Si)N)₂B][B(C₆F₅)₄] (R=Me₃Si 7 , tBu 8 ) are prepared and fully characterized. Compound 7 is shown to react with phosphines to generate [R₃PSiMe₃]⁺ and [R₃PH]⁺ (R=Me, tBu). Efforts to generate related borinium cations via fluoride abstraction from (R(Me₃Si)N)₂BF (R=C₆F₅ 3 , o-tol 4 , Mes 5 ) gave complex mixtures suggesting multiple reaction pathways. However for R=Dipp 6 , the species [(μ-F)(SiMe₂N(Dipp))₂BMe][B(C₆F₅)₄] was isolated as the major product, indicating methyl abstraction from silicon and F/Me exchange on boron. These observations together with state-of-the-art DFT mechanistic studies reveal that the trimethylsilyl-substituents do not behave as ancillary subsitutents but rather act as sources of proton, SiMe₃ and methyl groups.     

Borane‐Catalyzed Cross‐Metathesis Strategy for Facile Transformation of Cyclic (Alkyl)(Amino)Germylenes

A borane B(C₆F₅)₃‐catalyzed metathesis reaction between the Si?C bond in the cyclic (alkyl)(amino)germylene (CAAGe) 1 and the Si?H bond in a silane (R₃SiH; 2 ) is reported. Mechanistic studies propose that the initial step of the reaction involves Si?H bond activation to furnish an ionic species [ 1 ‐SiR₃]⁺[HB(C₆F₅)₃]^?, from which [Me₃Si]⁺[HB(C₆F₅)₃]^? and an azagermole intermediate are generated. The former yields Me₃SiH concomitant with the regeneration of B(C₆F₅)₃ whereas the latter undergoes isomerization to afford CAAGes bearing various silyl groups on the carbon atom next to the germylene center. This strategy allows the straightforward synthesis of eight new CAAGes starting from 1 .     

Front Cover: Reactivity of a Free Aluminylene towards Boron Lewis Acids: Accessing Aluminum-Boron-Bonded Species (Eur. J. Inorg. Chem. 25/2023)

The reactivity of the free aluminylene [N]-Al ( 1 ) ([N]=1,8-bis(3,5-di-tert-butylphenyl)-3,6-di-tert-butylcarbazolyl) towards boron Lewis acids is investigated. A facile oxidative addition reaction of 1 with Ph₂BOBPh₂ furnishes an exceedingly scarce example of the free alumaborane [N]-Al(BPh₂)(OBPh₂) ( 2 ) with an Al−B electron-sharing bond. By contrast, complexation of 1 with B(C₆F₅)₃ and HB(C₆F₅)₂ gives rise to the corresponding Lewis adducts [N]-Al→B(C₆F₅)₃ ( 3 ) and [N]-Al→BH(C₆F₅)₂ ( 4 ), respectively, with an Al→B dative bond. Crystallization of 4 in Et₂O produces the adduct [N]-Al(Et₂O)→BH(C₆F₅)₂ ( 5 ). Quantum chemical calculations are carried out to understand the formation of 2 as well as the bonding situation of 3 and 5 .     

How Frustrated Lewis Acid/Base Systems Pass through Transition‐State Regions: H2 Cleavage by [tBu3P/B(C6F5)3]

We investigate the transition‐state (TS) region of the potential energy surface (PES) of the reaction tBu₃P+H₂+B(C₆F₅)₃→tBu₃P‐H⁽⁺⁾+^(?)H?B(C₆F₅)₃ and the dynamics of the TS passage at room temperature. Owing to the conformational inertia of the phosphane???borane pocket involving heavy tBu₃P and B(C₆F₅)₃ species and features of the PES E(P???H, B???H | B???P) as a function of P???H, B???H, and B???P distances, a typical reactive scenario for this reaction is a trajectory that is trapped in the TS region for a period of time (about 350 fs on average across all calculated trajectories) in a quasi‐bound state (scattering resonance). The relationship between the timescale of the TS passage and the effective conformational inertia of the phosphane???borane pocket leads to a prediction that isotopically heavier Lewis base/Lewis acid pairs and normal counterparts could give measurably different reaction rates. Herein, the predicted quasi‐bound state could be verified in molecular collision experiments involving femtosecond spectroscopy.     

Formylborane Formation with Frustrated Lewis Pair Templates

Boranes R₂BH react with carbon monoxide by forming the respective borane carbonyl compounds R₂BH(CO). The formation of (C₆F₅)₂BH(CO) derived from the Piers borane, HB(C₆F₅)₂, is a typical example. Subsequent CO‐hydroboration does not take place, since the formation of the formylborane is usually endothermic. However, an “η²‐formylborane” was formed by CO‐hydroboration with the Piers borane at vicinal phosphane/borane frustrated Lewis pair (FLP) templates. Subsequent treatment with pyridine liberated the intact formylborane from the FLP framework, and (pyridine)(C₆F₅)₂B? CHO was then isolated as a stable compound. This product underwent typical reactions of carbonyl compounds, such as Wittig olefination.     

A Silylene–Borane Lewis Pair as a Tool for Trapping a Water Molecule: Silanol Formation and Dehydrogenation

The Lewis acid B(C₆F₅)₃ and the cyclic silane (Ar^N₂Si)₃ ( 1 ) (Ar^N=o-(CH₃)₂NCH₂C₆H₄) are useful precursors to access the silylene(II)–borane adduct Ar^N₂Si-B(C₆F₅)₃ ( 2 ). Treatment of 2 with water led to coordination and gave the Lewis pair (Ar^N₂H₂O)Si-B(C₆F₅)₃ ( 3 ) that exhibits a hydrogen-bond-stabilized silanol unit. It can be converted into the siloxane [(HAr^N)₂SiOB(C₆F₅)₃]₂O ( 6 ) by dehydrogenation in the presence of a base. Heteronuclear NMR spectroscopic data to characterize the compounds were supported by quantum-chemical calculations.     

Probing the Charge Transfer in a Frustrated Lewis Pair by Resonance Raman Spectroscopy and DFT Calculations

A classical Lewis adduct derives from a covalent bond between a Lewis acid and a base. When the adduct formation is precluded by means of steric hindrance the association of the respective acid-base molecular system is defined as a frustrated Lewis pair (FLP). In this work, the archetypal FLP Mes₃P/B(C₆F₅)₃ was characterized for the first time by resonance Raman spectroscopy, and the results were supported by density functional theory (DFT) calculations. The charge transfer nature of the lowest energy electronic transition, from phosphine to borane, was confirmed by the selective enhancement of the Raman bands associated to the FLP chromophore at resonance condition. Herein, we demonstrate the use of resonance Raman spectroscopy as a distinguished technique to probe the weak interaction involved in FLP chemistry.     

Metal‐Free Hydrogenation Catalyzed by an Air‐Stable Borane: Use of Solvent as a Frustrated Lewis Base

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.     

Planar N‐Heterocyclic Carbene Diarylborenium Ions: Synthesis by Cationic Borylation and Reactivity with Lewis Bases

The NHC–borane adduct (IBn)BH₃ ( 1 ) (NHC= N‐heterocyclic carbene; IBn=1,3‐dibenzylimidazol‐2ylidene) reacts with [Ph₃C][B(C₆F₅)₄] through sequential hydride abstraction and dehydrogenative cationic borylation(s) to give singly or doubly ring closed NHC–borenium salts 2 and 3 . The planar doubly ring closed product [C₃H₂(NCH₂C₆H₄)₂B][B(C₆F₅)₄] is resistant to quaternization at boron by Et₂O coordination, but forms classical Lewis acid–base adducts with the stronger donors Ph₃P, Et₃PO, or 1,4‐diazabicyclo[2.2.2]octane (DABCO). Treatment of 3 with tBu₃P selectively yields the unusual oligomeric borenium salt trans‐[(C₃H₂(NCH₂C₆H₄)₂B)₂(C₃H₂(NCHC₆H₄)₂B)][B(C₆F₅)₄] ( 7 ).     

B(C6F5)3‐Catalyzed Transfer Hydrogenation of Imines and Related Heteroarenes Using Cyclohexa‐1,4‐dienes as a Dihydrogen Source

The strong boron Lewis acid tris(pentafluorophenyl)borane, B(C₆F₅)₃, is shown to abstract a hydride from suitably donor‐substituted cyclohexa‐1,4‐dienes, eventually releasing dihydrogen. This process is coupled with the FLP‐type (FLP=frustrated Lewis pair) hydrogenation of imines and nitrogen‐containing heteroarenes that are catalyzed by the same Lewis acid. The net reaction is a B(C₆F₅)₃‐catalyzed, i.e., transition‐metal‐free, transfer hydrogenation using easy‐to‐access cyclohexa‐1,4‐dienes as reducing agents. Competing reaction pathways with or without the involvement of free dihydrogen are discussed.     

A New Method of Synthesis of Monomeric Aryloxide Complexes of Manganese(II): Structures of Manganese Phenoxides

Monomeric manganese(II) complexes of bulky alkyl/ aryl‐substituted phenoxides, [Mn{C₆H_nR_mO}₂(DME)] [ 1 : R = C₆H₅, n = 3, m = 2 (2,6); 2 : R = Me₃C, n = 3, m = 2 (2,6); 3 : R = Me₃C, n = 2, m = 3 (2,4,6)] were prepared in yields of 37, 44 and 72 %, respectively, from the reaction of manganese powder, Hg(C₆F₅)₂ and the corresponding phenol in the presence of a little mercury in dimethoxyethane (DME). The compounds were characterized spectroscopically and by magnetic measurements. The single crystal structures of 1 and 3 and also of the mixed phenoxide complex [Mn{(C₆H₃(C₆H₅)₂‐2,6‐O}{C₆H₃(Me₃C)₂‐2,6‐O}(DME)] 4 are reported.     

Photoinduced and Thermal Single-Electron Transfer to Generate Radicals from Frustrated Lewis Pairs

Archetypal phosphine/borane frustrated Lewis pairs (FLPs) are famed for their ability to activate small molecules. The mechanism is generally believed to involve two-electron processes. However, the detection of radical intermediates indicates that single-electron transfer (SET) generating frustrated radical pairs could also play an important role. These highly reactive radical species typically have significantly higher energy than the FLP, which prompted this investigation into their formation. Herein, we provide evidence that the classical phosphine/borane combinations PMes₃/B(C₆F₅)₃ and PtBu₃/B(C₆F₅)₃ both form an electron donor–acceptor (charge-transfer) complex that undergoes visible-light-induced SET to form the corresponding highly reactive radical-ion pairs. Subsequently, we show that by tuning the properties of the Lewis acid/base pair, the energy required for SET can be reduced to become thermally accessible.