Bismuth Compounds in Radical Catalysis: Transition Metal Bismuthanes Facilitate Thermally Induced Cycloisomerizations

Boryl Radical as a Catalyst in Enabling Intra- and Intermolecular Cascade Radical Cyclization Reactions: Construction of Polycyclic Molecules

Cascade radical cyclization constitutes an atom- and step-economic route for rapid assembly of polycyclic molecular skeletons. Although an array of redox-active metal catalysts has recently shown robust applications in enabling various catalytic cascade radical processes, the use of free organic radical as the catalyst, which is capable of triggering strategically distinct cascades, has rarely been developed. Here, we disclosed that the benzimidazolium-based N-heterocyclic carbene (NHC)-boryl radical is capable of catalyzing cascade cyclization reactions in both intra- and intermolecular pathways, assembling [5,5] fused bicyclic and [6,6,6] fused tricyclic molecules, respectively. The catalytic reactions start with the chemo- and regioselective addition of the boryl radical catalyst to a tethered alkene or alkyne moiety, followed by either an intramolecular formal [3+2] or an intermolecular [2+2+2] cycloaddition process to construct bicyclo[3.3.0]octane or tetrahydrophenanthridine skeletons, respectively. Eventually, a β-elimination occurs to release the boryl radical catalyst, completing a catalytic cycle. High to excellent diastereoselectivity is achieved in both catalytic reactions under substrate control.

Cyclic Hydrocarbon Frameworks Containing Two Bismuth Atoms: Towards 9,10-Dibismaanthracene

When bismuth atoms are incorporated into cyclic organic systems, this commonly goes along with strained or distorted molecular geometries, which can be exploited to modulate the physical and chemical properties of these compounds. In six-membered heterocycles, bismuth atoms are often accompanied by oxygen, sulfur or nitrogen as a second hetero-element. In this work, we present the first examples of six-membered rings, in which two CH units are replaced by BiX moieties (X=Cl, Br, I), resulting in dihydro-anthracene analogs. Their behavior in chemically reversible reduction reactions is explored, aiming at the generation of dibisma-anthracene (bismanthrene). Heterometallic compounds (Bi/Fe, Bi/Mn) are introduced as potential bismanthrene surrogates, as supported by bismanthrene-transfer to selenium. Analytical techniques used to investigate the reported compounds include NMR spectroscopy, high-resolution mass spectrometry, single-crystal X-ray diffraction analyses, and DFT calculations.

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.

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.

Bismuth in Dynamic Covalent Chemistry: Access to a Bowl-Type Macrocycle and a Barrel-Type Heptanuclear Complex Cation

Dynamic covalent chemistry (DCvC) is a powerful and widely applied tool in modern synthetic chemistry, which is based on the reversible cleavage and formation of covalent bonds. One of the inherent strengths of this approach is the perspective to reversibly generate in an operationally simple approach novel structural motifs that are difficult or impossible to access with more traditional methods and require multiple bond cleaving and bond forming steps. To date, these fundamentally important synthetic and conceptual challenges in the context of DCvC have predominantly been tackled by exploiting compounds of lighter p-block elements, even though heavier p-block elements show low bond dissociation energies and appear to be ideally suited for this approach. Here we show that a dinuclear organometallic bismuth compound, containing BiMe2 groups that are connected by a thioxanthene linker, readily undergoes selective and reversible cleavage of its Bi-C bonds upon exposure to external stimuli. The exploitation of DCvC in the field of organometallic heavy p-block chemistry grants access to unprecedented macrocyclic and barrel-type oligonuclear compounds.

Bismuth radical catalysis in the activation and coupling of redox-active electrophiles

Radical cross-coupling reactions represent a revolutionary tool to make C(sp³)-C and C(sp³)-heteroatom bonds by means of transition metals and photoredox or electrochemical approaches. However, the use of main-group elements to harness this type of reactivity has been little explored. Here we show how a low-valency bismuth complex is able to undergo one-electron oxidative addition with redox-active alkyl-radical precursors, mimicking the behaviour of first-row transition metals. This reactivity paradigm for bismuth gives rise to well-defined oxidative addition complexes, which could be fully characterized in solution and in the solid state. The resulting Bi(III)-C(sp³) intermediates display divergent reactivity patterns depending on the α-substituents of the alkyl fragment. Mechanistic investigations of this reactivity led to the development of a bismuth-catalysed C(sp³)-N cross-coupling reaction that operates under mild conditions and accommodates synthetically relevant NH-heterocycles as coupling partners.

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.

Synthesis, structure and properties of trivalent and pentavalent tricarbabismatranes

The first trivalent and pentavalent tricarbabismatranes were synthesized by the reaction of N(CH₂{2-LiC₆H₄})₃ with BiCl₃ and subsequent reaction with XeF₂, respectively. The trivalent bismatrane was easily oxidized by air, while the pentavalent bismatrane difluoride was relatively stable to air. A similar pentavalent bismatrance dichloride was prone to C-Cl bond reductive elimination even at room temperature.

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.

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.

Two Faces of the Bi−O Bond: Photochemically and Thermally Induced Dehydrocoupling for Si−O Bond Formation

The diorgano(bismuth)alcoholate [Bi((C₆H₄CH₂)₂S)OPh] (1‐OPh) has been synthesized and fully characterized. Stoichiometric reactions, UV/Vis spectroscopy, and (TD‐)DFT calculations suggest its susceptibility to homolytic and heterolytic Bi−O bond cleavage under given reaction conditions. Using the dehydrocoupling of silanes with either TEMPO or phenol as model reactions, the catalytic competency of 1‐OPh has been investigated (TEMPO=(tetramethyl‐piperidin‐1‐yl)‐oxyl). Different reaction pathways can deliberately be addressed by applying photochemical or thermal reaction conditions and by choosing radical or closed‐shell substrates (TEMPO vs. phenol). Applied analytical techniques include NMR, UV/Vis, and EPR spectroscopy, mass spectrometry, single‐crystal X‐ray diffraction analysis, and (TD)‐DFT calculations.

A new C-anionic tripodal ligand 2-{bis(benzothiazolyl)(methoxy)methyl}phenyl and its bismuth complexes

A new tripodal C-anionic ligand, 2-{bis(benzothiazolyl)(methoxy)methyl}phenyl (L), was stably generated by the reaction of the ligand precursor (L'), the corresponding bromide (2-BrC6H4)(MeO)C(C7H4NS)2 (C7H4NS = 2-benzothiazolyl), with nBuLi at -104 °C in the presence of TMEDA (N,N,N',N'-tetramethylethylenediamine). The ligand lithium salt reacted with BiCl3 to give a 2 : 1 complex L2BiCl. A 1 : 1 complex LBiCl2 was obtained in good yield by the redistribution reaction between L2BiCl and BiCl3. X-ray diffraction analysis revealed that the ligand L coordinated in an expected κ3-C,N,N' coordination mode in LBiCl2, while it coordinated in κ3-C,N,O and κ2-C,O coordination modes in L2BiCl. The ligand precursor reacted with BiX3 (X = Cl, Br) to give 1 : 1 complexes L'BiX3 and was found to act as a neutral tripodal C(π),N,N-ligand.

Bismuth species in the coordination sphere of transition metals: synthesis, bonding, coordination chemistry, and reactivity of molecular complexes

This contribution is focused on bismuth species in the coordination sphere of transition metals. In molecular transition metal complexes, three types of Bi-M bonding are considered, namely dative Bi→M interactions (with Bi acting as a donor), dative Bi←M interactions (with Bi acting as an acceptor) and covalent Bi-M interactions (M = transition metal). Synthetic routes to all three classes of compounds are outlined, the Bi-M bonding situation is discussed, trends in the geometric parameters and in the coordination chemistry of the compounds are addressed, and common spectroscopic properties are summarized. As an important part of this contribution, the reactivity of bismuth species in the coordination sphere of transition metal complexes in stoichiometric and catalytic reactions is highlighted.

Cationic Bismuth Aminotroponiminates: Charge Controls Redox Properties

The behavior of the redox‐active aminotroponiminate (ATI) ligand in the coordination sphere of bismuth has been investigated in neutral and cationic compounds, [Bi(ATI)₃] and [Bi(ATI)₂L_n][A] (L=neutral ligand; n=0, 1; A=counteranion). Their coordination chemistry in solution and in the solid state has been analyzed through (variable‐temperature) NMR spectroscopy, line‐shape analysis, and single‐crystal X‐ray diffraction analyses, and their Lewis acidity has been evaluated by using the Gutmann–Beckett method (and modifications thereof). Cyclic voltammetry, in combination with DFT calculations, indicates that switching between ligand‐ and metal‐centered redox events is possible by altering the charge of the compounds from 0 in neutral species to +1 in cationic compounds. This adds important facets to the rich redox chemistry of ATIs and to the redox chemistry of bismuth compounds, which is, so far, largely unexplored.

Bismuth Amides Mediate Facile and Highly Selective Pn-Pn Radical-Coupling Reactions (Pn=N, P, As)

The controlled release of well-defined radical species under mild conditions for subsequent use in selective reactions is an important and challenging task in synthetic chemistry. We show here that simple bismuth amide species [Bi(NAr2 )3 ] readily release aminyl radicals [NAr2 ]. at ambient temperature in solution. These reactions yield the corresponding hydrazines, Ar2 N-NAr2 , as a result of highly selective N-N coupling. The exploitation of facile homolytic Bi-Pn bond cleavage for Pn-Pn bond formation was extended to higher homologues of the pnictogens (Pn=N-As): homoleptic bismuth amides mediate the highly selective dehydrocoupling of HPnR2 to give R2 Pn-PnR2 . Analyses by NMR and EPR spectroscopy, single-crystal X-ray diffraction, and DFT calculations reveal low Bi-N homolytic bond-dissociation energies, suggest radical coupling in the coordination sphere of bismuth, and reveal electronic and steric parameters as effective tools to control these reactions.

Molecular bismuth(iii) monocations: structure, bonding, reactivity, and catalysis

Structurally defined, molecular bismuth(iii) cations show remarkable properties in coordination chemistry, Lewis acidity, and redox chemistry, allowing for unique applications in synthetic chemistry.

Well-Defined, Molecular Bismuth Compounds: Catalysts in Photochemically Induced Radical Dehydrocoupling Reactions

A series of diorgano(bismuth)chalcogenides, [Bi(di-aryl)EPh], has been synthesised and fully characterised (E=S, Se, Te). These molecular bismuth complexes have been exploited in homogeneous photochemically-induced radical catalysis, using the coupling of silanes with TEMPO as a model reaction (TEMPO=(tetramethyl-piperidin-1-yl)-oxyl). Their catalytic properties are complementary or superior to those of known catalysts for these coupling reactions. Catalytically competent intermediates of the reaction have been identified. Applied analytical techniques include NMR, UV/Vis, and EPR spectroscopy, mass spectrometry, single-crystal X-ray diffraction analysis, and (TD)-DFT calculations.

Methylbismuth: an organometallic bismuthinidene biradical

We report the generation, spectroscopic characterization, and computational analysis of the first free (non-stabilized) organometallic bismuthinidene, BiMe. The title compound was generated in situ from BiMe3 by controlled homolytic Bi-C bond cleavage in the gas phase. Its electronic structure was characterized by a combination of photoion mass-selected threshold photoelectron spectroscopy and DFT as well as multi-reference computations. A triplet ground state was identified and an ionization energy (IE) of 7.88 eV was experimentally determined. Methyl abstraction from BiMe3 to give [BiMe2]• is a key step in the generation of BiMe. We reaveal a bond dissociation energy of 210 ± 7 kJ mol-1, which is substantially higher than the previously accepted value. Nevertheless, the homolytic cleavage of Me-BiMe2 bonds could be achieved at moderate temperatures (60-120 °C) in the condensed phase, suggesting that [BiMe2]• and BiMe are accessible as reactive intermediates under these conditions.

Main-Group Metal Complexes in Selective Bond Formations Through Radical Pathways

Recent years have witnessed remarkable advances in radical reactions involving main-group metal complexes. This includes the isolation and detailed characterization of main-group metal radical compounds, but also the generation of highly reactive persistent or transient radical species. A rich arsenal of methods has been established that allows control over and exploitation of their unusual reactivity patterns. Thus, main-group metal compounds have entered the field of selective bond formations in controlled radical reactions. Transformations that used to be the domain of late transition-metal compounds have been realized, and unusual selectivities, high activities, as well as remarkable functional-group tolerances have been reported. Recent findings demonstrate the potential of main-group metal compounds to become standard tools of synthetic chemistry, catalysis, and materials science, when operating through radical pathways.

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.