Dihydrogen and Ethylene Activation by a Sterically Distorted Distibene

Advancing metallomimetic catalysis through structural constraints of cationic PIII species

In recent years, the concept of structural constraints on the main-group (MG) centers has emerged as a powerful strategy to enhance their reactivity. Among these, structurally constrained (SC) phosphorus centers have garnered significant attention due to their ability to cycle between two stable oxidation states, P(III) and P(V), making them highly promising for small molecule activation and catalysis. Structural constraints grant phosphorus centers transition metal (TM)-like reactivity, enabling the activation of small molecules by these SC P(III) centers, a reactivity previously inaccessible with conventional phosphines or other phosphorus derivatives. This feature article reviews recent advances in the chemistry of cationic, structurally constrained P(III) (CSCP) compounds, emphasizing their ability to mimic TM behavior in small-molecule activation and catalysis, particularly through the key elementary steps of TM-based catalysis, such as oxidative addition (OA), migratory insertion (MI), ligand metathesis (LM), reductive elimination (RE), etc. The development of these SC cationic P(III) species highlights the interplay between structural constraints and cationic charge, facilitating analogous metallomimetic reactivity in other main-group elements.

Transient Triplet Metallopnictinidenes M-Pn (M = PdII, PtII; Pn = P, As, Sb): Characterization and Dimerization

Nitrenes (R-N) have been subject to a large body of experimental and theoretical studies. The fundamental reactivity of this important class of transient intermediates has been attributed to their electronic structures, particularly the accessibility of triplet vs singlet states. In contrast, electronic structure trends along the heavier pnictinidene analogues (R-Pn; Pn = P-Bi) are much less systematically explored. We here report the synthesis of a series of metallodipnictenes, {M-Pn═Pn-M} (M = PdII, PtII; Pn = P, As, Sb, Bi) and the characterization of the transient metallopnictinidene intermediates, {M-Pn} for Pn = P, As, Sb. Structural, spectroscopic, and computational analysis revealed spin triplet ground states for the metallopnictinidenes with characteristic electronic structure trends along the series. In comparison to the nitrene, the heavier pnictinidenes exhibit lower-lying ground state SOMOs and singlet excited states, thus suggesting increased electrophilic reactivity. Furthermore, the splitting of the triplet magnetic microstates is beyond the phosphinidenes {M-P} dominated by heavy pnictogen atom induced spin-orbit coupling.

Combining Distibene, Diazoolefins, and Visible Light: Synthesis and Reactivity of Inorganic Rings

The chemistry of heterocycles containing "diaza" units has been extensively studied due to their applications ranging from pharmaceuticals to advanced materials. In contrast, heterocycles incorporating heavier elements, such as Sb and Bi, remain exceedingly rare and lack straightforward synthetic methodologies. Herein, we present a comprehensive experimental and theoretical investigation of the first diazadistiboylidenes (1a, 1b), synthesized via a [3 + 2]-cycloaddition between a distibene and diazoolefins. These stiboylidenes are key intermediates to promote selective nucleophilic substitution, leading to a rare example of diantimonyl anion. Furthermore, upon visible-light irradiation, we could isolate the first example of methylenedistibiranes, heavier analogs of methylenediaziridine (C2H4N2). These findings offer a novel platform for heavy dipnictogen chemistry, showcasing that diazoolefins, in combination with visible light, can facilitate the formation of unprecedented heavy heterocycles and serve as a platform to promote CO2 activation.

Carbodiphosphorane-Activated Distibene and Dibismuthene Dications

Low-valent antimony and bismuth have emerged as novel platforms for achieving reversible small-molecule activation at main-group metals. Although various examples of oxidative addition reactions at monomeric Sb(I) and Bi(I) have been reported, the chemistry of the heavy group 15 Sb(I)═Sb(I)/Bi(I)═Bi(I) double bonds toward small molecules remains largely unexplored. In this study, we present a straightforward synthesis of distibene and dibismuthene dications coordinated with a neutral carbodiphosphorane (CDP) ligand. The nonbonding interactions between the occupied p-orbital at the CDP ligand and the π-bonding orbital of the Sb═Sb/Bi═Bi bonds yield compounds with exceptionally small HOMO-LUMO gaps. In addition, the reduction of steric hindrance compared to known neutral derivatives stabilized with bulky aryl groups allows for better accessibility of the double bonds. This high reactivity is demonstrated in the oxidative addition of distibene to diphenyldisulfide as well as in [2+2] cycloadditions to alkynes. Additionally, the Sb═Sb bond reversibly adds to 2,3-dimethylbutadiene in a [4+2] cycloaddition reaction.

C-H bond activation at antimony(III): synthesis and reactivity of Sb(III)-oxyaryl species

We report on the synthesis, structure and reactivity of [{NCNMe4}Sb(C6H2-tBu2-3,5-O-4)] (3), an organoantimony(III)-oxyaryl species obtained upon Csp2-H bond activation in a phenolate ligand and stabilised by the monoanionic pincer {NCNMe4}-. The mechanism leading to the formation of 3 is highly sensitive to steric considerations. It was probed experimentally and by DFT calculations, and a number of intermediates and related complexes were identified. All data agree with successive heterolytic bond cleaving and bond forming processes involving charged species, rather than a pathway involving free radicals as previously exemplified with congeneric bismuth species. The nucleophilic behaviour of the oxyaryl ligand in 3, a complex that features both zwitterionic and quinoidal attributes, was illustrated in derivatisation reactions. In particular, insertion of CS2 in the Sb-Coxyaryl bond generates [{NCNMe4}Sb(S2C-C6H2-tBu2-3,5-O-4)].

Synthesis of a stable crystalline nitrene

Nitrenes are a highly reactive, yet fundamental, compound class. They possess a monovalent nitrogen atom and usually a short life span, typically in the nanosecond range. Here, we report on the synthesis of a stable nitrene by photolysis of the arylazide MSFluindN3 (1), which gave rise to the quantitative formation of the arylnitrene MSFluindN (2) (MSFluind is dispiro[fluorene-9,3'-(1',1',7',7'-tetramethyl-s-hydrindacen-4'-yl)-5',9''-fluorene]) that remains unchanged for at least 3 days when stored under argon atmosphere at room temperature. The extraordinary life span permitted the full characterization of 2 by single-crystal x-ray crystallography, electron paramagnetic resonance spectroscopy, and superconducting quantum interference device magnetometry, which supported a triplet ground state. Theoretical simulations suggest that in addition to the kinetic stabilization conferred by the bulky MSFluind aryl substituent, electron delocalization across the central aromatic ring contributes to the electron stabilization of 2.

Light-Dependent Reactivity of Heavy Pnictogen Double Bonds

The chemistry of light dipnictenes has been widely investigated in the last century with remarkable achievements especially for azobenzene derivatives. In contrast, distibenes and dibismuthenes are relatively rare and show very limited reactivity. Herein, we have designed a protocol using visible light to enhance the reactivity of heavy dipnictenes. Exploiting the distinctive π-π* transition, we have been able to isolate unique examples of dipnictene-cobalt complexes. The reactivity of the distibene complex was further exploited using red light in the presence of a diazoolefin to access an unusual four-membered bicyclo[1.1.0]butane analog, containing only a single carbon atom. These findings set the bases to a conceptually new strategy in heavy element double bonds chemistry where visible light is at the front seat of bond activation.

Phosphaarsenes - Combining Phospha- and Arsa-Wittig-Reagents

Dipnictenes of the type RPn=PnR (Pn=P, As, Sb, Bi) can be viewed as dimers of the corresponding pnictinidenes R-Pn. Phosphanylidene- and arsanylidenephosphoranes (R-Pn(PMe3); Pn=P, As) have been shown to be versatile synthetic surrogates for the delivery of pnictinidene fragments. We now report that thermal treatment of 1 : 1 mixtures of R-P(PMe3) and R'-As(PMe3) gives access to arsaphosphenes of the type RP=AsR'. Three examples are presented and the properties and reactivity of Mes*P=AsDipTer (1) (Mes*=2,4,6-tBu3-C6H2; DipTer=2,6-(2,6-iPr2C6H3)2-C6H3) were studied in detail. Solid state 31P NMR spectroscopy revealed a large 31P NMR chemical shift anisotropy with a span of ca. 920 ppm for 1 while computational methods were employed to investigate this pronounced magnetic deshielding of the P atom in 1. In the presence of the carbene IMe4 (IMe4=:C(MeNCMe)2) 1 is shown to be split into the corresponding NHC adducts Mes*P(IMe4) and DipTerAs(IMe4), which is additionally shown for diarsenes.

Recent advances in the stabilization of monomeric stibinidene chalcogenides and stibine chalcogenides

The elucidation of novel bonding situations at heavy p-block elements has greatly advanced recent efforts to access useful reactivity at earth-abundant main-group elements. Molecules with unsaturated bonds between heavier, electropositive elements and lighter, electronegative elements are often highly polarized and competent in small-molecule activations, but the reactivity of these molecules may be quenched by self-association of monomers to form oligomeric species where the polar, unsaturated groups are assembled in a head-to-tail fashion. In this Frontier, we discuss the synthetic strategies employed to isolate monomeric σ2,λ3-stibinidene chalcogenides (RSbCh) and monomeric σ4,λ5-stibine chalcogenides (R3SbCh). These classes of molecules each feature polarized antimony-chalcogenide bonds (Sb = Ch/Sb+-Ch-). We highlight how the synthesis and isolation of these molecules has led to the discovery of novel reactivity and has shed light on fundamental aspects of inorganic structure and bonding. Despite these advances, there are critical aspects of this chemistry that remain underdeveloped and we provide our perspective on yet-unrealized synthetic targets that may be achieved with the continued development of the strategies described herein.

From Neutral Diarsenes to Diarsene Radical Ions and Diarsene Dications

Diarsene [L(MeO)GaAs]2 (L=HC[C(Me)N(Ar)]2, Ar=2,6-iPr2C6H3, 4) reacts with MeOTf and MeNHC (MeNHC=1,3,4,5-tetra-methylimidazol-2-ylidene) to the diarsene [L(TfO)GaAs]2 (5) and the carbene-coordinated diarsene [L(MeO)GaAsAs(MeNHC)Ga(OMe)L] (6). The NHC-coordination results in an inversion of the redox properties of the diarsene 4, which shows only a reversible reduction event at E1/2=-2.06 V vs Fc0/+1, whereas the carbene-coordinated diarsene 6 shows a reversible oxidation event at E1/2=-1.31 V vs Fc0/+1. Single electron transfer reactions of 4 and 6 yielded [K[2.2.2.]cryp][L(MeO)GaAs]2 (8) and [L(MeO)GaAsAs(MeNHC)-Ga(OMe)L][B(C6F5)4] (9) containing the radical anion [L(MeO)GaAs]2⋅- (8⋅-) and the NHC-coordinated radical cation [L(MeO)GaAsAs(MeNHC)Ga(OMe)L]⋅+ (9⋅+), respectively, while the salt-elimination reaction of the triflate-coordinated diarsene 5 with Na[B(C6F5)4] gave [LGaAs]2[B(C6F5)4]2 (11) containing the dication [LGaAs]2 2+ (112+). Compounds 1-11 were characterized by 1H and 13C NMR, EPR (8, 9), IR, and UV-Vis spectroscopy and by single crystal X-ray diffraction (sc-XRD). DFT calculations provided a detailed understanding of the electronic nature of the diarsenes (4, 6) and the radical ions (8⋅-, 9⋅+), respectively.

Theoretical Investigation on Dialumenes toward Dihydrogen Activation: Mechanism and Ligand Effect

Herein, we report a computation study based on the density functional theory calculations to understand the mechanism and ligand effect of the base-stabilized dialumenes toward dihydrogen activation. Among all of the examined modes of dihydrogen activation using the base-stabilized dialumene, we found that the concerted 1,2-hydrogenation of the Al═Al double bond is kinetically more preferable. The concerted 1,2-hydrogenation of the Al═Al double bond adopts an electron-transfer model with certain asynchrony. That is, the initial electron donation from the H-H σ bonding orbital to the empty 3p orbital of the Al1 center is followed by the backdonation from the lone pair electron of the Al2 center to the H-H σ antibonding orbital. Combined with the energy decomposition analysis on the transition states of the concerted 1,2-hydrogenation of the Al═Al double bond and the topographic steric mapping analysis on the free dialumenes, we ascribe the higher reactivity of the aryl-substituted dialumene over the silyl-substituted analogue in dihydrogen activation to the stronger electron-withdrawing effect of the aryl group, which not only increases the flexibility of the Al═Al double bond but also enhances the Lewis acidity of the Al═Al core. Consequently, the aryl-substituted dialumene fragment suffers less geometric deformation, and the orbital interactions between the dialumene and dihydrogen moieties are more attractive during the 1,2-hydrogenation process. Moreover, our calculations also predict that the Al═Al double bond has a good tolerance with the stronger electron-withdrawing group (-CF3) and the weaker σ-donating N-heterocyclic carbene (NHC) analogue (e.g., triazol carbene and NHSi). The reactivity of the dialumene in dihydrogen activation can be further improved by introducing these groups as the supporting ligand and the stabilizing base on the Al═Al core, respectively.

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.

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.

Hydrobismuthation: Insertion of Unsaturated Hydrocarbons into the Heaviest Main Group Element Bond to Hydrogen

The bismuth hydride (2,6-Mes2H3C6)2BiH (1, Mes = 2,4,6-trimethylphenyl), which has a Bi-H 1H NMR spectroscopic signal at δ = 19.64 ppm, was reacted with phenylacetylene at 60 °C in toluene to yield [(2,6-Mes2C6H3)2BiC(Ph)=CH2] (2) after 15 min. Compound 2 was characterized by 1H, 13C NMR, and UV-vis spectroscopy, single crystal X-ray crystallography, and calculations employing density functional theory. Compound 2 is the first example of a hydrobismuthation addition product and displays Markovnikov regioselectivity. Computational methods indicated that it forms via a radical mechanism with an associated Gibbs energy of activation of 91 kJ mol-1 and a reaction energy of -90 kJ mol-1.