Metal-Free Stabilization of Monomeric Antimony(I): A Carbene-Supported Stibinidene

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.

Triplet bismuthinidenes featuring unprecedented giant and positive zero field splittings

Isolation of triplet pnictinidenes, which bear two unpaired electrons at the pnictogen centers, has long been a great challenge due to their intrinsic high reactivity. Herein, we report the syntheses and characterizations of two bismuthinidenes MsFluindtBu-Bi (3) and MsFluind*-Bi (4) stabilized by sterically encumbered hydrindacene ligands. They were facilely prepared through reductions of the corresponding dichloride precursors with 2 molar equivalents of potassium graphite. The structural analyses revealed that 3 and 4 contain a one-coordinate bismuth atom supported by a Bi-C single σ bond. As a consequence, the remaining two Bi 6p orbitals are nearly degenerate, and 3 and 4 possess triplet ground states. Experimental characterizations with multinuclear magnetic resonance, magnetometry and near infrared spectroscopy coupled to wavefunction based ab initio calculations concurred to evidence that there exist giant and positive zero field splittings (>4300 cm-1) in their S = 1 ground states. Hence even at room temperature the systems almost exclusively populate the lowest-energy nonmagnetic Ms = 0 level, which renders them seemingly diamagnetic.

1,3-Imidazole-Based Mesoionic Carbenes and Anionic Dicarbenes: Pushing the Limit of Classical N-Heterocyclic Carbenes

Classical N-heterocyclic carbenes (NHCs) featuring the carbene center at the C2-position of 1,3-imidazole framework (i.e. C2-carbenes) are well acknowledged as very versatile neutral ligands in molecular as well as in materials sciences. The efficiency and success of NHCs in diverse areas is essentially attributed to their persuasive stereoelectronics, in particular the potent σ-donor property. The NHCs with the carbene center at the unusual C4 (or C5) position, the so-called abnormal NHCs (aNHCs) or mesoionic carbenes (iMICs), are however superior σ-donors than C2-carbenes. Hence, iMICs have substantial potential in sustainable synthesis and catalysis. The main obstacle in this direction is rather demanding synthetic accessibility of iMICs. The aim of this review article is to highlight recent advances, particularly by the author's research group, in accessing stable iMICs, quantifying their properties, and exploring their applications in synthesis and catalysis. In addition, the synthetic viability and use of vicinal C4,C5-anionic dicarbenes (ADCs), also based on an 1,3-imidazole framework, are presented. As will be apparent on following pages, iMICs and ADCs hold potentials in pushing the limit of classical NHCs by enabling access to conceptually new main-group heterocycles, radicals, molecular catalysts, ligands sets, and more.

Compounds with Alternating Single and Double Bonds of Antimony and Silicon; Isoelectronic to Ethane-1,2-diimine

We present an approach for preparing chain-type unsaturated molecules with low oxidation state Si(I) and Sb(I) supported by amidinato ligands that exploit to generate heavy analogues of ethane 1,2-diimine. The reduction of antimony dihalide (R-SbCl₂) with KC₈ in the presence of silylene chloride afforded L─(Cl)Si═Sb─Tip (1) and L(Cl)Si═Sb─^TerPh (2), respectively. Compounds 1 and 2 further undergo reduction with KC₈ to produce Tip─Sb═LSi─LSi═Sb─Tip (3) and ^TerPh─Sb═LSi─LSi═Sb─^TerPh (4). The solid-state structures and DFT studies show that all compounds have σ-type lone pairs at each Sb atom. It forms a strong pseudo-π-bond with Si. The pseudo-π-bond is formed by the hyperconjugative donation of the π-type lone pair at Sb to the Si-N σ* MO. The quantum mechanical studies indicate that compounds 3 and 4 has delocalized pseudo-π-MOs arising from hyperconjugative interactions. Hence, 1 and 2 can be considered as isoelectronic to imine, while 3 and 4 are isoelectronic to ethane-1,2-diimine. The proton affinity studies indicate that the pseudo-π-bond resulting from the hyperconjugative interaction is more reactive than the σ-type lone pair.

Switching from Heteronuclear Allyl Cations to Vinyl Cations by Using a Chemical Charge Trap

Halide abstraction of the carbene-coordinated pnictinidenes (^MecAAC)EGa(Cl)L (E = As 1, Sb 2, Bi 3, ^MecAAC = [H₂C(CMe₂)₂NDipp]C; L = HC[C(Me)NDipp]₂; Dipp = 2,6-i-Pr₂C₆H₃) yielded the series of cationic group 15 compounds [(^MecAAC)EGaL][Al(OR^F)₄] (E = As 4, Sb 5; Al(OR^F)₄ = Al(OC(CF₃)₃)₄) and [(^MecAAC)EGaL][B(Ar^F)₄] (E = Sb 6, Bi 7; B(Ar^F)₄ = B[C₆H₃(CF₃)₂]₄), which were characterized by heteronuclear NMR spectroscopy and sc-XRD. The electronic nature of the cations [(^MecAAC)EGaL]⁺ is controlled by the central pnictogen atom, according to quantum chemical calculations. The calculations furthermore demonstrated that compounds containing the lighter pnictogens (E = N, P) are best described as heteronuclear allyl cations, whereas heavier pnictogen atoms (E = As, Sb, Bi) serve as a trap for the positive charge, resulting in carbene-stabilized heterovinyl-type structures.

Group 15 and 16 Nitrene-Like Pnictinidenes

Pnictinidenes are an increasingly relevant species in main group chemistry and generally exhibit proclivity for the triplet electronic ground state. However, the elusive singlet electronic states are often desired for chemical applications. We predict the singlet-triplet energy differences (ΔE_ST =E_Singlet -E_Triplet ) of simple group 15 and 16 substituted pnictinidenes (Pn-R; Pn=P, As, Sb, or Bi) with highly reliable focal-point analyses targeting the CCSDTQ/CBS level of theory. The only cases we predict to have favorable singlet states are P-PH₂ (-3.2 kcal mol^-1 ) and P-NH₂ (-0.2 kcal mol^-1 ). ΔE_ST trends are discussed in light of the geometric predictions as well as qualitative natural bond order analysis to elucidate some of the important electronic structure features. Our work provides a rigorous benchmark for the ΔE_ST of fundamental Pn-R moieties and provides a firm foundation for the continued study of heavier pnictinidenes.

Phosphine-Stabilized Pnictinidenes

The reaction of the intramolecular germylene‐phosphine Lewis pair (o‐PPh₂)C₆H₄GeAr* (1) with Group 15 element trichlorides ECl₃ (E=P, As, Sb) was investigated. After oxidative addition, the resulting compounds (o‐PPh₂)C₆H₄(Ar*)Ge(Cl)ECl₂ (2: E=P, 3: E=As, 4: E=Sb) were reduced by using sodium metal or LiHBEt₃. The molecular structures of the phosphine‐stabilized phosphinidene (o‐PPh₂)C₆H₄(Ar*)Ge(Cl)P (5), arsinidene (o‐PPh₂)C₆H₄(Ar*)Ge(Cl)As (6) and stibinidene (o‐PPh₂)C₆H₄(Ar*)Ge(Cl)Sb (7) are presented; they feature a two‐coordinate low‐valent Group 15 element. After chloride abstraction, a cyclic germaphosphene [(o‐PPh₂)C₆H₄(Ar*)GeP] [B(C₆H₃(CF₃)₂)₄] (8) was isolated. The ³¹P NMR data of the germaphosphene were compared with literature examples and analyzed by quantum chemical calculations. The phosphinidene was treated with [iBu₂AlH]₂, and the product of an Al−H addition to the low‐valent phosphorus atom (o‐PPh₂)C₆H₄(Ar*)Ge(H)P(H)Al(C₄H₉)₂ (9) was characterized.

Stabilization of the Elusive Antimony(I) Cation and Its Coordination Complexes with Transition Metals

Upon stabilization by 5,6-bis(diisopropylphosphino)acenaphthene to form compound 1, the fugitive antimony (I) cation exhibited nucleophilic behavior towards coinage metals. Compound 1 was strategically synthesized at room temperature from SbCl₃ , the bis(phosphine), and trimethylsilyl trifluoromethanesulfonate taken in a 1:2:3 ratio, whereby the bis(phosphine) plays the dual role of a reductant and a supporting ligand. The generation of 1 involves two-electron oxidation of the ligand to form a P-P bonded diphosphonium dication. Compound 1 was separated from this dication to give both products in pure form in moderate yields. Despite the overall positive charge, the Sb^I site in 1 was found to bind to metal centers, forming complexes with Au^I , Ag^I and Cu^I . Compound 1 reduced Cu^II to Cu^I and formed a coordination complex with the resulting Cu^I species. The effects of the electron-rich bis(phosphine) and the constrained peri geometry in stabilizing and enhancing the nucleophilicity of 1 have been rationalized through computational studies.

Indirect Access to Carbene Adducts of Bismuth- and Antimony-Substituted Phosphaketene and Their Unusual Thermal Transformation to Dipnictines and [(NHC)2OCP][OCP]

The synthesis and thermal redox chemistry of the first antimony (Sb)– and bismuth (Bi)–phosphaketene adducts are described. When diphenylpnictogen chloride [Ph₂PnCl (Pn = Sb or Bi)] is reacted with sodium 2-phosphaethynolate [Na[OCP]·(dioxane)_x], tetraphenyldipnictogen (Ph₂Pn–PnPh₂) compounds are produced, and an insoluble precipitate forms from solution. In contrast, when the N-heterocyclic carbene adduct (NHC)–PnPh₂Cl is combined with [Na[OCP]·(dioxane)_x], Sb– and Bi–phosphaketene complexes are isolated. Thus, NHC serves as an essential mediator for the reaction. Immediately after the formation of an intermediary pnictogen–phosphaketene NHC adduct [NHC–PnPh₂(PCO)], the NHC ligand transfers from the Pn center to the phosphaketene carbon atom, forming NHC–C(O)P-PnPh₂ [Pn = Sb (3) or Bi (4)]. In the solid state, 3 and 4 are dimeric with short intermolecular Pn–Pn interactions. When compounds 3 and 4 are heated in THF at 90 and 70 °C, respectively, the pnictogen center Pn^III is thermally reduced to Pn^II to form tetraphenyldipnictines (Ph₂Pn–PnPh₂) and an unusual bis-carbene-supported OCP salt, [(NHC)₂OCP][OCP] (5). The formation of compound 5 and Ph₂Pn–PnPh₂ from 3 or 4 is unique in comparison to the known thermal reactivity for group 14 carbene–phosphaketene complexes, further highlighting the diverse reactivity of [OCP]⁻ with main-group elements. All new compounds have been fully characterized by single-crystal X-ray diffraction, multinuclear NMR spectroscopy (¹H, ¹³C, and ³¹P), infrared spectroscopy, and elemental analysis (1, 2, and 5). The electronic structure of 5 and the mechanism of formation were investigated using density functional theory (DFT).

An N-heterocyclic carbene (NHC) was used to support the otherwise unstable Ph₂Sb—P=C=O and Ph₂Bi—P=C=O moieties. Exploration of the thermal chemistry of these NHC−phosphaketene adducts reveals the formation of the salt [NHC₂OCP][OCP]. This present work demonstrates the thermal chemistry of the 2-phospaethynolate anion with heavier pnictogens (Sb and Bi).

Carbodicarbene Bismaalkene Cations: Unravelling the Complexities of Carbene versus Carbone in Heavy Pnictogen Chemistry

We report a combined experimental and theoretical study on the first examples of carbodicarbene (CDC)-stabilized bismuth complexes, which feature low-coordinate cationic bismuth centers with C=Bi multiple-bond character. Monocations [(CDC)Bi(Ph)Cl][SbF6 ] (8) and [(CDC)BiBr2 (THF)2 ][SbF6 ] (11), dications [(CDC)Bi(Ph)][SbF6 ]2 (9) and [(CDC)BiBr(THF)3 ][NTf2 ]2 (12), and trication [(CDC)2 Bi][NTf2 ]3 (13) have been synthesized via sequential halide abstractions from (CDC)Bi(Ph)Cl2 (7) and (CDC)BiBr3 (10). Notably, the dications and trication exhibit C ⇉ Bi double dative bonds and thus represent unprecedented bismaalkene cations. The synthesis of these species highlights a unique non-reductive route to C-Bi π-bonding character. The CDC-[Bi] complexes (7-13) were compared with related NHC-[Bi] complexes (1, 3-6) and show substantially different structural properties. Indeed, the CDC ligand has a remarkable influence on the overall stability of the resulting low-coordinate Bi complexes, suggesting that CDC is a superior ligand to NHC in heavy pnictogen chemistry.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Isolation of Cyclic(Alkyl)(Amino) Carbene-Bismuthinidene Mediated by a Beryllium(0) Complex

The long-sought carbene-bismuthinidene, (CAAC)Bi(Ph), has been synthesized. Notably, this represents both the first example of a carbene-stabilized subvalent bismuth complex and the extension of the carbene-pnictinidene concept to a non-toxic metallic element (Bi). The bonding has been investigated by single-crystal X-ray diffraction studies and DFT calculations. This report also highlights the hitherto unknown reducing and ligand transfer capability of a beryllium(0) complex.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Synthesis, Spectroscopic Characterization, and Redox Reactivity of a Cyclic (Alkyl) Amino Carbene-Derived Arsamethine Cyanine Dye

In our efforts to prepare a diarsenic allotrope supported by two cyclic alkyl amino carbene (CAAC) ligands we stumbled upon the synthesis of the first carbene-supported chloroarsinidene 3, which has been fully characterized by a combination of NMR spectroscopic and XRD methods. Although further reduction of 3 was not possible, we found that addition of a second equivalent of CAAC in refluxing toluene afforded the first example of a crystallographically characterized arsamethine cyanine dye (4). The arsenic(I) dye is structurally similar to phosphorus analogues, and contains an arsenide anion with two stereochemically active lone pairs supported by two iminium ligands. The UV/Visible spectrum and redox chemistry of 4 were also explored. Upon reduction with one equivalent of KC₈ , 3 is reduced to the originally targeted CAAC₂ As₂ allotrope 6, whereas oxidation provides access to the first example of an arsenic(II) radical dication (5).

Transition-Metal-like Behavior of Main Group Elements: Ligand Exchange at a Phosphinidene

(Phosphino)phosphaketenes (>P-P═C═O) behave as (phosphino)phosphinidene-carbonyl adducts (>P-P←:C═O). CO scrambling was observed using ¹³C labeled CO, and exchange reactions with phosphines afford the corresponding (phosphino)phosphinidene-phosphine adducts (>P-P←:PR₃). The latter react with isonitriles and singlet carbenes giving (phosphino)phosphinidene-isonitrile (>P-P←:CNR) and -carbene adducts (>P-P←:C<). Based on experimental results and DFT calculations, it is shown that these "ligand" exchange reactions occur via an associative mechanism as classically observed with transition metal complexes.