Catalytic Activation of N2O at a Low-Valent Bismuth Redox Platform

Bisgermylene-Stabilized Stannylone: Catalytic Reduction of Nitrous Oxide and Nitro Compounds via Element-Ligand Cooperativity

This study describes the synthesis, structural characterization, and catalytic application of a bis(germylene)-stabilized stannylone (2). The reduction of digermylated stannylene (1) with 2.2 equiv of potassium graphite (KC8) leads to the formation of stannylone 2 as a green solid in 78% yield. Computational studies showed that stannylone 2 possesses a formal Sn(0) center and a delocalized 3-c-2-e π-bond in the Ge2Sn core, which arises from back-donation of the p-type lone pair electrons on the Sn atom to the vacant orbitals of the Ge atoms. Stannylone 2 can serve as an efficient precatalyst for the selective reduction of nitrous oxide (N2O) and nitroarenes (ArNO2) with the formation of dinitrogen (N2) and hydrazines (ArNH-NHAr), respectively. Exposure of 2 with N2O (1 atm) resulted in the insertion of two oxygen atoms into the Ge-Ge and Ge-Sn bonds, yielding the germyl(oxyl)stannylene (3). Moreover, the stoichiometric reaction of 2 with 1-chloro-4-nitrobenzene afforded an amido(oxyl)stannylene (4) through the complete scission of the N-O bonds of the nitroarene. Stannylenes 3 and 4 serve as catalytically active species for the catalytic reduction of nitrous oxide and nitroarenes, respectively. Mechanistic studies reveal that the cooperation of the low-valent Ge and Sn centers allows for multiple electron transfers to cleave the N-O bonds of N2O and ArNO2. This approach presents a new strategy for catalyzing the deoxygenation of N2O and ArNO2 using a zerovalent tin compound.

Calcium-Ligand Cooperation Promoted Activation of N2O, Amine, and H2 as well as Catalytic Hydrogenation of Imines, Quinoline, and Alkenes

Bond activation and catalysis using s-block metals are of great significance. Herein, a series of calcium pincer complexes with deprotonated side arms have been prepared using pyridine-based PNP and PNN ligands. The complexes were characterized by NMR and X-ray crystal diffraction. Utilizing the obtained calcium complexes, unprecedented N2O activation by metal-ligand cooperation (MLC) involving dearomatization-aromatization of the pyridine ligand was achieved, generating aromatized calcium diazotate complexes as products. Additionally, the dearomatized calcium complexes were able to activate the N-H bond as well as reversibly activate H2, offering an opportunity for the catalytic hydrogenation of various unsaturated molecules. DFT calculations were applied to analyze the electronic structures of the synthesized complexes and explore possible reaction mechanisms. This study is an important complement to the area of MLC and main-group metal chemistry.

Phosphorus-Ligand Redox Cooperative Catalysis: Unraveling Four-Electron Dioxygen Reduction Pathways and Reactive Intermediates

The reduction of dioxygen to water is crucial in biology and energy technologies, but it is challenging due to the inertness of triplet oxygen and complex mechanisms. Nature leverages high-spin transition metal complexes for this, whereas main-group compounds with their singlet state and limited redox capabilities exhibit subdued reactivity. We present a novel phosphorus complex capable of four-electron dioxygen reduction, facilitated by unique phosphorus-ligand redox cooperativity. Spectroscopic and computational investigations attribute this cooperative reactivity to the unique electronic structure arising from the geometry of the phosphorus complex bestowed by the ligand. Mechanistic study via spectroscopic and kinetic experiments revealed the involvement of elusive phosphorus intermediates resembling those in metalloenzymes. Our result highlights the multielectron reactivity of phosphorus compound emerging from a carefully designed ligand platform with redox cooperativity. We anticipate that the work described expands the strategies in developing main-group catalytic reactions, especially in small molecule fixations demanding multielectron redox processes.

Stabilizing Monoatomic Two-Coordinate Bismuth(I) and Bismuth(II) Using a Redox Noninnocent Bis(germylene) Ligand

The formation of isolable monatomic BiI complexes and BiII radical species is challenging due to the pronounced reducing nature of metallic bismuth. Here, we report a convenient strategy to tame BiI and BiII atoms by taking advantage of the redox noninnocent character of a new chelating bis(germylene) ligand. The remarkably stable novel BiI cation complex 4, supported by the new bis(iminophosphonamido-germylene)xanthene ligand [(P)GeII(Xant)GeII(P)] 1, [(P)GeII(Xant)GeII(P) = Ph2P(NtBu)2GeII(Xant)GeII(NtBu)2PPh2, Xant = 9,9-dimethyl-xanthene-4,5-diyl], was synthesized by a two-electron reduction of the cationic BiIIII2 precursor complex 3 with cobaltocene (Cp2Co) in a molar ratio of 1:2. Notably, owing to the redox noninnocent character of the germylene moieties, the positive charge of BiI cation 4 migrates to one of the Ge atoms in the bis(germylene) ligand, giving rise to a germylium(germylene) BiI complex as suggested by DFT calculations and X-ray photoelectron spectroscopy (XPS). Likewise, migration of the positive charge of the BiIIII2 cation of 3 results in a bis(germylium)BiIIII2 complex. The delocalization of the positive charge in the ligand engenders a much higher stability of the BiI cation 4 in comparison to an isoelectronic two-coordinate Pb0 analogue (plumbylone; decomposition below -30 °C). Interestingly, 4[BArF] undergoes a reversible single-electron transfer (SET) reaction (oxidation) to afford the isolable BiII radical complex 5 in 5[BArF]2. According to electron paramagnetic resonance (EPR) spectroscopy, the unpaired electron predominantly resides at the BiII atom. Extending the redox reactivity of 4[OTf] employing AgOTf and MeOTf affords BiIII(OTf)2 complex 7 and BiIIIMe complex 8, respectively, demonstrating the high nucleophilic character of BiI cation 4.

Bismuth in Radical Chemistry and Catalysis

Whereas indications of radical reactivity in bismuth compounds can be traced back to the 19th century, the preparation and characterization of both transient and persistent bismuth-radical species has only been established in recent decades. These advancements led to the emergence of the field of bismuth radical chemistry, mirroring the progress seen for other main-group elements. The seminal and fundamental studies in this area have ultimately paved the way for the development of catalytic methodologies involving bismuth-radical intermediates, a promising approach that remains largely untapped in the broad landscape of synthetic organic chemistry. In this review, we delve into the milestones that eventually led to the present state-of-the-art in the field of radical bismuth chemistry. Our focus aims at outlining the intrinsic discoveries in fundamental inorganic/organometallic chemistry and contextualizing their practical applications in organic synthesis and catalysis.

Nitrous oxide activation by picoline-derived Ni-CNP hydrides

Oxygen atom transfer (OAT) from N2O to the Ni-H bond of proton-responsive picoline-derived CNP nickel complexes has been investigated both experimentally and theoretically. These Ni-CNP complexes efficiently catalyse the reduction of N2O with pinacolborane (HBpin) under mild conditions.

Ligand-enforced geometric constraints and associated reactivity in p-block compounds

The geometry at an element centre can generally be predicted based on the number of electron pairs around it using valence shell electron pair repulsion (VSEPR) theory. Strategies to distort p-block compounds away from these predicted geometries have gained considerable interest due to the unique structural outcomes, spectroscopic properties or reactivity patterns engendered by such distortion. This review presents an up-to-date group-wise summary of this exciting and rapidly growing field with a focus on understanding how the ligand employed unlocks structural features, which in turn influences the associated reactivity. Relevant geometrically constrained compounds from groups 13-16 are discussed, along with selected stoichiometric and catalytic reactions. Several areas for advancement in this field are also discussed. Collectively, this review advances the notion of geometric tuning as an important lever, alongside electronic and steric tuning, in controlling bonding and reactivity at p-block centres.

Mechanistic Studies on the Bismuth-Catalyzed Transfer Hydrogenation of Azoarenes

Organobismuth-catalyzed transfer hydrogenation has recently been disclosed as an example of low-valent Bi redox catalysis. However, its mechanistic details have remained speculative. Herein, we report experimental and computational studies that provide mechanistic insights into a Bi-catalyzed transfer hydrogenation of azoarenes using p-trifluoromethylphenol (4) and pinacolborane (5) as hydrogen sources. A kinetic analysis elucidated the rate orders in all components in the catalytic reaction and determined that 1 a (2,6-bis[N-(tert-butyl)iminomethyl]phenylbismuth) is the resting state. In the transfer hydrogenation of azobenzene using 1 a and 4, an equilibrium between 1 a and 1 a ⋅ [OAr]2 (Ar=p-CF3 -C6 H4 ) is observed, and its thermodynamic parameters are established through variable-temperature NMR studies. Additionally, pKa -gated reactivity is observed, validating the proton-coupled nature of the transformation. The ensuing 1 a ⋅ [OAr]2 is crystallographically characterized, and shown to be rapidly reduced to 1 a in the presence of 5. DFT calculations indicate a rate-limiting transition state in which the initial N-H bond is formed via concerted proton transfer upon nucleophilic addition of 1 a to a hydrogen-bonded adduct of azobenzene and 4. These studies guided the discovery of a second-generation Bi catalyst, the rate-limiting transition state of which is lower in energy, leading to catalytic transfer hydrogenation at lower catalyst loadings and at cryogenic temperature.

Bi-Catalyzed Trifluoromethylation of C(sp2)-H Bonds under Light

We disclose a Bi-catalyzed C-H trifluoromethylation of (hetero)arenes using CF3SO2Cl under light irradiation. The catalytic method permits the direct functionalization of various heterocycles bearing distinct functional groups. The structural and computational studies suggest that the process occurs through an open-shell redox manifold at bismuth, comprising three unusual elementary steps for a main group element. The catalytic cycle starts with rapid oxidative addition of CF3SO2Cl to a low-valent Bi(I) catalyst, followed by a light-induced homolysis of Bi(III)-O bond to generate a trifluoromethyl radical upon extrusion of SO2, and is closed with a hydrogen-atom transfer to a Bi(II) radical intermediate.

Accelerating σ-Bond Metathesis at Sn(II) Centers

Molecular main-group hydride catalysts are attractive as cheap and Earth-abundant alternatives to transition-metal analogues. In the case of the latter, specific steric and electronic tuning of the metal center through ligand choice has enabled the iterative and rational development of superior catalysts. Analogously, a deeper understanding of electronic structure-activity relationships for molecular main-group hydrides should facilitate the development of superior main-group hydride catalysts. Herein, we report a modular Sn-Ni bimetallic system in which we systematically vary the ancillary ligand on Ni, which, in turn, tunes the Sn center. This tuning is probed using Sn L1 XAS as a measure of electron density at the Sn center. We demonstrate that increased electron density at Sn centers accelerates the rate of σ-bond metathesis, and we employ this understanding to develop a highly active Sn-based catalyst for the hydroboration of CO2 using pinacolborane. Additionally, we demonstrate that engineering London dispersion interactions within the secondary coordination sphere of Sn allows for further rate acceleration. These results show that the electronics of main-group catalysts can be controlled without the competing effects of geometry perturbations and that this manifests in substantial reactivity differences.

Variation in pnictogen-oxygen bonding unlocks greatly enhanced Brønsted basicity for the monomeric stibine oxide

Phosphine oxides and arsine oxides feature highly polarized pnictoryl groups (Pn+-O-/Pn = O; Pn = P, As) and react as Brønsted bases through O-centered lone pairs. We recently reported the first example of a monomeric stibine oxide, Dipp3SbO (Dipp = diisopropylphenyl), allowing periodic trends in pnictoryl bonding to be extended to antimony for the first time. Computational studies suggest that, as the pnictogen atom becomes heavier, delocalization of electron density from the O-centered lone pairs to the Pn-C σ* orbitals is attenuated, destabilizing the lone pairs and increasing the donor capacity of the pnictine oxide. Herein, we assess the Brønsted basicity of a series of monomeric pnictine oxides (Dipp3PnO; Pn = P, As, and Sb). Stoichiometric reactivity between Dipp3PnO and a series of acids demonstrates the greatly enhanced ability of Dipp3SbO to accept protons relative to the lighter congeners, consistent with theoretical isodesmic reaction enthalpies and proton affinities. 1H NMR spectrometric titrations allow for the pKaH,MeCN determination of Dipp3AsO and Dipp3SbO, revealing a 106-fold increase in Brønsted basicity from Dipp3AsO to Dipp3SbO. The increased basicity can be exploited in catalysis; Dipp3SbO exhibits dramatically increased catalytic efficiency in the Brønsted base-catalyzed transesterification between p-nitrophenyl acetate and 2,2,2-trifluoroethanol. Our results unambiguously confirm the drastic increase in Brønsted basicity from Dipp3PO < Dipp3AsO < Dipp3SbO, a direct consequence of the variation in the electronic structure of the pnictoryl bond as the pnictogen atom increases in atomic number.

Isolation and characterization of bis(silylene)-stabilized antimony(I) and bismuth(I) cations

Monovalent group 15 cations L₂Pn + (L = σ-donor ligands, Pn = N, P, As, Sb, Bi) have attracted significant experimental and theoretical interest because of their unusual electronic structures and growing synthetic potential. Herein, we describe the synthesis of a family of antimony(I) and bismuth(I) cations supported by a bis(silylene) ligand [(TBDSi₂)Pn][BAr^F₄] (TBD = 1, 8, 10, 9-triazaboradecalin; Ar^F = 3,5-CF₃-C₆H₃; Pn = Sb, (2); Bi, (3)). The structures of 2 and 3 have been unambiguously characterized spectroscopically and by X-ray diffraction analysis and DFT calculations. They feature bis-coordinated Sb and Bi atoms which exhibit two lone pairs of electrons. The reactions of 2 and 3 with methyl trifluoromethane sulfonate provide a approach for the preparation of dicationic antimony(III) and bismuth(III) methyl complexes. Compounds 2 and 3 serve as 2e donors to group 6 metals (Cr, Mo), giving rise to ionic antimony and bismuth metal carbonyl complexes 6-9.

Synthesis and isolation of a triplet bismuthinidene with a quenched magnetic response

Large Spin-Orbit Coupling (SOC) is an intrinsic property of the heavy-elements that directly affects the electronic structures of the compounds. Herein we report the synthesis and characterization of a mono-coordinate bismuthinidene featuring a rigid and bulky ligand. All magnetic measurements (SQUID, NMR) point to a diamagnetic compound. However, multiconfigurational quantum chemical calculations predict the ground state of the compound to be dominated (76%) by a spin-triplet. The apparent diamagnetism is explained by an extremely large SOC induced positive zero-field-splitting of more than 4500 cm^-1 that leaves the M_S = 0 magnetic sublevel thermally isolated in the electronic ground state.

Insertion of CO2 and CS2 into Bi-N bonds enables catalyzed CH-activation and light-induced bismuthinidene transfer

The uptake and release of small molecules continue to be challenging tasks of utmost importance in synthetic chemistry. The combination of such small molecule activation with subsequent transformations to generate unusual reactivity patterns opens up new prospects for this field of research. Here, we report the reaction of CO₂ and CS₂ with cationic bismuth(iii) amides. CO₂-uptake gives isolable, but metastable compounds, which upon release of CO₂ undergo CH activation. These transformations could be transferred to the catalytic regime, which formally corresponds to a CO₂-catalyzed CH activation. The CS₂-insertion products are thermally stable, but undergo a highly selective reductive elimination under photochemical conditions to give benzothiazolethiones. The low-valent inorganic product of this reaction, Bi(i)OTf, could be trapped, showcasing the first example of light-induced bismuthinidene transfer.

Synthesis, Isolation, and Characterization of Two Cationic Organobismuth(II) Pincer Complexes Relevant in Radical Redox Chemistry

Herein, we report the synthesis, isolation, and characterization of two cationic organobismuth(II) compounds bearing N,C,N pincer frameworks, which model crucial intermediates in bismuth radical processes. X-ray crystallography uncovered a monomeric Bi(II) structure, while SQUID magnetometry in combination with NMR and EPR spectroscopy provides evidence for a paramagnetic S = 1/2 state. High-resolution multifrequency EPR at the X-, Q-, and W-band enable the precise assignment of the full g- and ²⁰⁹Bi A-tensors. Experimental data and DFT calculations reveal both complexes are metal-centered radicals with little delocalization onto the ligands.

Understanding the Organometallic Step: SO2 Insertion into Bi(III)-C(Ph) Bond

Heavier main-group element-catalyzed reactions provide an increasingly attractive tool to perform transformations mimicking the behaviors of transition metal catalysts. Recently, Magre and Cornella reported a Bi-catalyzed synthesis of aryl sulfonyl fluorides, which involves a fundamental organometallic step of SO₂ insertion into the Bi-Ph bond. Our theoretical studies reveal that i) the ability of hypervalent coordination of the Bi(III) center allows facile coordination sphere expansion for the SO₂ coordination via one oxygen atom; and ii) the high polarity of the Bi-Ph bond makes the Ph migration from the Bi(III) center feasible. These features enable the heavier main group element to resemble the transition metal having flexibility for ligand association and dissociation. Furthermore, iii) the available π electron pair of the migrating Ph group stabilizes the SO₂ insertion transition state by maintaining interaction with the Bi(III) center during migration. The insight helps us better understand the heavier main-group catalysis.

Oxidations of N-coordinated Arsinidene and Stibinidene by Substituted Quinones: A Remarkable Follow-Up Reactivity

The reactivity of pnictinidenes [2-(DippN=CH)-6-(DippNHCH₂ )C₆ H₃ ]E (where E=As (1) or Sb (2)) toward substituted ortho- and para-quinones is reported. The central pnictogen atom is easily oxidized by ortho-quinones closing five-membered EO₂ C₂ ring. The oxidized antimony derivatives are stable species, while in the case of arsenic compounds the hydrogen of the pendant amino NHCH₂ group cleaves one newly formed As-O bonds leading to the closure of a new azaarsole ring. Furthermore, a heating of these arsenic heterocycles resulted in a C-H bond activation at the NCH₂ group involved in this heterocycle followed by a reductive elimination of corresponding catechols and arsinidene [2,6-(DippN=CH)C₆ H₃ ]As. Using of para-quinones, resulted in the oxidation of the central atom with a concomitant hydrogen migration from NHCH₂ group even in the case of the antimony derivatives. The reductive elimination of hydroquinones is in this case feasible for all compounds. Studied compounds were characterized by multi-nuclear NMR, IR and Raman spectroscopy and single-crystal X-ray diffraction analysis. The theoretical study focusing the key compounds and reactions is also included.

Widening the Landscape of Small Molecule Activation with Gold-Aluminyl Complexes: A Systematic Study of E-H (E=O, N) Bonds, SO2 and N2 O Activation

The electronic features of gold-aluminyl complexes have been thoroughly explored. Their similarity with Group 14 dimetallenes and other metal-aluminyl complexes suggests that their reactivity with small molecules beyond carbon dioxide could be accessed. In this work, the reactivity of the [^t Bu₃ PAuAl(NON)] (NON=4,5-bis(2,6 diisopropylanilido)-2,7-ditert-butyl-9,9-dimethylxanthene) complex towards water, ammonia, sulfur dioxide and nitrous oxide is computationally explored. The reaction mechanisms computed for each substrate strongly suggest that all activation processes are in principle experimentally feasible. Electronic structure analysis highlights that, in all cases, the reactivity is driven by the presence of the poorly polarized electron-sharing gold-aluminyl bond, which induces a radical-like reactivity of the complex towards all the substrates. A flat topology of the potential energy surface (PES) has been found for the reaction with N₂ O, where two almost isoenergetic transition states can be located along the same reaction coordinate with different geometries, suggesting that the N₂ O binding mode may not be a good indicator of the nature of N₂ O activation in a cooperative bimetallic reactivity. In addition, the catalytic potentialities of these complexes have been explored in the framework of nitrous oxide reduction. The study reveals that the [^t Bu₃ PAuAl(NON)] complex might be an efficient catalyst towards oxidation of phosphines (and boranes) via N₂ O reduction. These findings underline recurring trends in the novel chemistry of gold-aluminyl complexes and call for experimental feedbacks.

Catalytic Mechanisms of Transfer Hydrogenation of Azobenzene with Ammonia Borane by Pincer Bismuth Complex: Crucial Role of C=N Functional Group on the Pincer Ligand

Transfer hydrogenation of azobenzene with ammonia borane mediated by pincer bismuth complex 1 was systematically investigated through density functional theory calculations. An unusual metal-ligand cooperation mechanism was disclosed, in which the saturation/regeneration of the C=N functional group on the pincer ligand plays an essential role. The reaction is initiated by the hydrogenation of the C=N bond (saturation) with ammonia borane to afford 3^CN , which is the rate-determining step with Gibbs energy barrier (ΔG^≠ ) and Gibbs reaction energy (ΔG) of 25.6 and -7.3 kcal/mol, respectively. 3^CN is then converted to a Bi-H intermediate through a water-bridged pathway, which is followed up with the transfer hydrogenation of azobenzene to produce the final product N,N'-diphenylhydrazine and regenerate the catalyst. Finally, the catalyst could be improved by substituting the phenyl group for the tert-butyl group on the pincer ligand, where the ΔG^≠ value (rate-determining step) decreases to 24.0 kcal/mol.

Ni-Catalyzed Oxygen Transfer from N2O onto sp3-Hybridized Carbons

Herein we disclose a catalytic synthesis of cycloalkanols that harnesses the potential of N₂O as an oxygen transfer agent onto sp³-hybridized carbons. The protocol is distinguished by its mild conditions and wide substrate scope, thus offering an opportunity to access carbocyclic compounds from simple precursors even in an enantioselective manner. Preliminary mechanistic studies suggest that the oxygen insertion event occurs at an alkylnickel species and that N₂O is the O transfer reagent.

Radical Activation of N-H and O-H Bonds at Bismuth(II)

The development of unconventional strategies for the activation of ammonia (NH₃) and water (H₂O) is of capital importance for the advancement of sustainable chemical strategies. Herein we provide the synthesis and characterization of a radical equilibrium complex based on bismuth featuring an extremely weak Bi-O bond, which permits the in situ generation of reactive Bi(II) species. The ensuing organobismuth(II) engages with various amines and alcohols and exerts an unprecedented effect onto the X-H bond, leading to low BDFE_X-H. As a result, radical activation of various N-H and O-H bonds─including ammonia and water─occurs in seconds at room temperature, delivering well-defined Bi(III)-amido and -alkoxy complexes. Moreover, we demonstrate that the resulting Bi(III)-N complexes engage in a unique reactivity pattern with the triad of H⁺, H^-, and H^• sources, thus providing alternative pathways for main group chemistry.

Mechanism of the Aryl-F Bond-Forming Step from Bi(V) Fluorides

In this article, we describe a combined experimental and theoretical mechanistic investigation of the C(sp²)–F bond formation from neutral and cationic high-valent organobismuth(V) fluorides, featuring a dianionic bis-aryl sulfoximine ligand. An exhaustive assessment of the substitution pattern in the ligand, the sulfoximine, and the reactive aryl on neutral triarylbismuth(V) difluorides revealed that formation of dimeric structures in solution promotes facile Ar–F bond formation. Noteworthy, theoretical modeling of reductive elimination from neutral bismuth(V) difluorides agrees with the experimentally determined kinetic and thermodynamic parameters. Moreover, the addition of external fluoride sources leads to inactive octahedral anionic Bi(V) trifluoride salts, which decelerate reductive elimination. On the other hand, a parallel analysis for cationic bismuthonium fluorides revealed the crucial role of tetrafluoroborate anion as fluoride source. Both experimental and theoretical analyses conclude that C–F bond formation occurs through a low-energy five-membered transition-state pathway, where the F anion is delivered to a C(sp²) center, from a BF₄ anion, reminiscent of the Balz–Schiemann reaction. The knowledge gathered throughout the investigation permitted a rational assessment of the key parameters of several ligands, identifying the simple sulfone-based ligand family as an improved system for the stoichiometric and catalytic fluorination of arylboronic acid derivatives.

Organopnictogen(III) bis(arylthiolates) containing NCN-aryl pincer ligands: from synthesis and characterization to reactivity

Salt elimination reactions between organopnictogen(III) dichlorides, RPnCl₂ [R¹ = 2,6-(Me₂NCH₂)₂C₆H₃, Pn = Sb (1), Bi (2); R² = 2,6-{MeN(CH₂CH₂)₂NCH₂}₂C₆H₃, Pn = Sb (3), Bi (4); R³ = 2,6-{O(CH₂CH₂)₂NCH₂}₂C₆H₃, Pn = Sb (5), Bi (6)] and 2 equivalents of KSC₆H₃Me₂-2,6 afforded the isolation of a series of new NCN-chelated monoorganopnictogen(III) bis(arylthiolates), RPn(SC₆H₃Me₂-2,6)₂ [R¹, Pn = Sb (7), Bi (8); R², Pn = Sb (9), Bi (10); R³, Pn = Sb (11), Bi (12)]. Compounds 7 and 8 are unstable upon exposure to a dry O₂ atmosphere and their aerobic decomposition yields the monoorganopnictogen(III) oxides, cyclo-[2,6-(Me₂NCH₂)₂C₆H₃Pn(μ-O)]₂ [Pn = Sb (13), Bi (14)] with concomitant formation of the corresponding disulfide, ArS-SAr (Ar = C₆H₃Me₂-2,6). The oxidative addition of elemental sulfur or selenium to 7 undergoes a similar reaction path and gives stable heterocyclic species cyclo-[2,6-(Me₂NCH₂)₂C₆H₃Sb(μ-E)]₂ [E = S (15), Se (16)]. The reaction of 12 with I₂ (1 : 1 molar ratio) gives the diiodide [2,6-{O(CH₂CH₂)₂NCH₂}₂C₆H₃]BiI₂ (17), along with the S-S oxidative coupling by-product, ArS-SAr. The use of an excess of iodine affords the crystallization of a 2 : 1 iodine adduct of 17 (17·0.5I₂), built through halogen bonding. All new compounds were characterized by multinuclear NMR spectroscopy and ESI-MS as well as single crystal X-ray diffraction (except compounds 9 and 10).

Synthesis and Structure of Mono-, Di-, and Trinuclear Fluorotriarylbismuthonium Cations

A series of cationic fluorotriarylbismuthonium salts bearing differently substituted aryl groups (Ar = 9,9-Me₂-9H-xanthene, Ph, Mes, and 3,5-tBu-C₆H₃) have been synthesized and characterized. While the presence of simple phenyl substituents around the Bi center results in a polymeric structure with three Bi centers in the repeating monomer, substituents at the ortho- and meta-positions lead to cationic mono- and dinuclear fluorobismuthonium complexes, respectively. Preparation of all compounds is accomplished by fluoride abstraction from the parent triaryl Bi(V) difluorides using NaBAr^F (BAr^F– = B[C₆H₃-3,5-(CF₃)₂]₄^–). Structural parameters were obtained via single crystal X-ray diffraction (XRD), and their behavior in solution was studied by NMR spectroscopy. Trinuclear and binuclear complexes are held together through one bridging fluoride (μ-F) between two Bi(V) centers. In contrast, the presence of Me groups in both ortho-positions of the aryl ring provides the adequate steric encumbrance to isolate a unique mononuclear nonstabilized fluorotriarylbismuthonium cation. This compound features a distorted tetrahedral geometry and is remarkably stable at room temperature both in solution (toluene, benzene and THF) and in the solid state.

Bismuth Redox Catalysis: An Emerging Main-Group Platform for Organic Synthesis

Bismuth has recently been shown to be able to maneuver between different oxidation states, enabling access to unique redox cycles that can be harnessed in the context of organic synthesis. Indeed, various catalytic Bi redox platforms have been discovered and revealed emerging opportunities in the field of main group redox catalysis. The goal of this perspective is to provide an overview of the synthetic methodologies that have been developed to date, which capitalize on the Bi redox cycling. Recent catalytic methods via low-valent Bi(II)/Bi(III), Bi(I)/Bi(III), and high-valent Bi(III)/Bi(V) redox couples are covered as well as their underlying mechanisms and key intermediates. In addition, we illustrate different design strategies stabilizing low-valent and high-valent bismuth species, and highlight the characteristic reactivity of bismuth complexes, compared to the lighter p-block and d-block elements. Although it is not redox catalysis in nature, we also discuss a recent example of non-Lewis acid, redox-neutral Bi(III) catalysis proceeding through catalytic organometallic steps. We close by discussing opportunities and future directions in this emerging field of catalysis. We hope that this Perspective will provide synthetic chemists with guiding principles for the future development of catalytic transformations employing bismuth.

Reversible addition of tin(II) amides to nitriles

Lappert's stannylene (Sn[N(SiMe₃)₂]₂) has been reacted with various nitriles, dinitriles and trinitriles with the formation of heteroleptic amidotin(II) amidinates of the general formulae [RC(NSiMe₃)₂]SnN(SiMe₃)₂, R'{[C(NSiMe₃)₂]SnN(SiMe₃)₂}₂ and R''{[C(NSiMe₃)₂]SnN(SiMe₃)₂}₃, where R = Ph (1), 2-(CN)-C₆H₄ (2), 3-(CN)-C₆H₄ (3); R' = 1,3-C₆H₄ (4), 1,4-C₆H₄ (5) and R'' = 1,3,5-C₆H₃ (6). The reactions of amidotin(II) benzamidinate 1 with an excess of benzonitrile proceed to homoleptic tin(II) bis(benzamidinate) [PhC(NSiMe₃)₂]₂Sn, which reversibly eliminates benzonitrile and 1 when warmed. The premise of reversibility has been supported by a multinuclear time-dependent NMR study and a theoretical (DFT) description. On the other hand, magnesium(II) bis(benzamidinate) [PhC(NSiMe₃)₂]₂Mg (8) and lanthanum(II) tris(benzamidinate) [PhC(NSiMe₃)₂]₃La (7) have been synthesised from appropriate metal hexamethyldisilazides and benzonitrile.

Computational mechanism investigation of bismuth (BiIII/BiV) redox-catalyzed fluorination of arylboronic esters

DFT calculations reveal details of the redox catalytic mechanism of a heavier main group element, bismuth.

Computational mechanism investigation of Bi(i)/Bi(iii) redox-catalyzed hydrodefluorination (HDF) of polyfluoroarenes

DFT calculations reveal details of the redox catalytic mechanism of non-transition-metal bismuth.

Redox-Neutral Organometallic Elementary Steps at Bismuth: Catalytic Synthesis of Aryl Sulfonyl Fluorides

A Bi-catalyzed synthesis of sulfonyl fluorides from the corresponding (hetero)aryl boronic acids is presented. We demonstrate that the organobismuth(III) catalysts bearing a bis-aryl sulfone ligand backbone revolve through different canonical organometallic steps within the catalytic cycle without modifying the oxidation state. All steps have been validated, including the catalytic insertion of SO₂ into Bi-C bonds, leading to a structurally unique O-bound bismuth sulfinate complex. The catalytic protocol affords excellent yields for a wide range of aryl and heteroaryl boronic acids, displaying a wide functional group tolerance.

Unveiling the Electronic Structure of the Bi(+1)/Bi(+3) Redox Couple on NCN and NNN Pincer Complexes

Low-valent group 15 compounds stabilized by pincer ligands have gained particular interest, given their direct access to fine-tune their reactivity by the coordination pattern. Recently, bismuth has been employed in a variety of catalytic transformations by taking advantage of the (+1/+3) redox couple. In this work, we present a detailed quantum–chemical study on the electronic structure of bismuth pincer complexes from two different families, namely, bis(ketimine)phenyl (NCN) and triamide bismuthinidene (NNN). The use of the so-called effective oxidation state analysis allows the unambiguous assignation of the bismuth oxidation state. In contrast to previous studies, our calculations suggest a Bi(+1) assignation for NCN pincer ligands, while Bi(+3) character is found for NNN pincer complexes. Notably, regardless of its oxidation state, the central bismuth atom disposes of up to two lone pairs for coordinating Lewis acids, as indicated by very high first and second proton affinity values. Besides, the Bi–NNN systems can also accommodate two Lewis base ligands, indicating also ambiphilic behavior. The effective fragment orbital analysis of Bi and the ligand allows monitoring of the intricate electron flow of these processes, revealing the noninnocent nature of the NNN ligand, in contrast with the NCN one. By the dissection of the electron density into effective fragment orbitals, we are able to quantify and rationalize the Lewis base/acid character.

Effective oxidation state analysis sheds light on the electronic structure of chemical systems. The oxidation state of bismuthinidene pincer complexes can be assigned as Bi(+1) or Bi(+3) depending on the nature of the ligands. Despite this assignation, the reactivity pattern as Lewis base or acid is similar. The occupation of the effective fragment orbitals gives a straightforward method to quantify the reactivity.

Deoxygenation of Nitrous Oxide and Nitro Compounds Using Bis(N-Heterocyclic Silylene)Amido Iron Complexes as Catalysts

Herein, we report the efficient degradation of N₂ O with a well-defined bis(silylene)amido iron complex as catalyst. The deoxygenation of N₂ O using the iron silanone complex 4 as a catalyst and pinacolborane (HBpin) as a sacrificial reagent proceeds smoothly at 50 °C to form N₂ , H₂ , and (pinB)₂ O. Mechanistic studies suggest that the iron-silicon cooperativity is the key to this catalytic transformation, which involves N₂ O activation, H atom transfer, H₂ release and oxygenation of the boron sites. This approach has been further developed to enable catalytic reductions of nitro compounds, producing amino-boranes with good functional-group tolerance and excellent chemoselectivity.

Three-Component Coupling of Acyl Fluorides, Silyl Enol Ethers, and Alkynes by P(III)/P(V) Catalysis

We report herein on the phosphine-catalyzed hydrovinylation reaction by three-component coupling of acyl fluorides, silyl enol ethers, and alkynoates. The key to the success of the reaction is the formal transmetalation between pentacoordinate P(V) species (i.e., fluorophosphorane) and a silyl enol ether, which allows for C-C bond formation between the polarity-mismatched sites. The bond formation that cannot be attained even by transition metal catalysis is accomplished by a P(III)/P(V) manifold.

Nitrous oxide as a diazo transfer reagent: the synthesis of triazolopyridines

Nitrous oxide is a potential diazo transfer reagent, but its applications in organic chemistry are scarce. Here, we show that triazolopyridines and triazoloquinolines are formed in the reactions of metallated 2-alkylpyridines or 2-alkylquinolines with N₂O. The reactions can be performed under mild conditions and give synthetically interesting triazoles in moderate to good yields.

Synthesis and reactivity of NHC-coordinated phosphinidene oxide

Here we report the synthesis of an N-heterocyclic carbene (NHC)-stabilised phosphinidene oxide by the controlled oxygenation of a phosphinidene under ambient conditions. This compound can be further oxygenated to a phosphinidene dioxide. The stoichiometric reduction of a phosphinidene oxide with KC₈ resembles the pinacol coupling reaction-the reduction of a carbonyl compound. We also looked at the stoichiometric oxidation of NHC-coordinated phosphinidene, phosphinidene oxide and phosphinidene dioxide with [NO][SbF₆].