Bonding and 13 C-NMR properties of coinage metal tris(ethylene) and tris(norbornene) complexes: Evaluation of the role of relativistic effects from DFT calculations

The π-complexes of cationic coinage metal ions (Cu(I), Ag(I), Au(I)) provide useful experimental support for understanding fundamental characteristics of bonding and ¹³ C-NMR patterns of the group 11 triad. Here, we account for the role of relativistic effects on olefin-coinage metal ion interaction for cationic, homoleptic tris-ethylene, and tris-norbornene complexes, [M(η² -C₂ H₄ )₃ ]⁺ and [M(η² -C₇ H₁₀ )₃ ]⁺ (M = Cu, Ag, Au), as representative case of studies. The M-(CC) bond strength in the cationic, tris-ethylene complexes is affected sizably for Au and to a lesser extent for Ag and Cu (48.6%, 16.7%, and 4.3%, respectively), owing to the influence on the different stabilizing terms accounting for the interaction energy in the formation of coinage metal cation-π complexes. The bonding elements provided by olefin → M σ-donation and olefin ← M π-backbonding are consequently affected, leading to a lesser covalent interaction going down in the triad if the relativistic effects are ignored. Analysis of the ¹³ C-NMR tensors provides further understanding of the observed experimental values, where the degree of backbonding charge donation to π₂ *-olefin orbital is the main influence on the observed high-field shifts in comparison to the free olefin. This donation is larger for ethylene complexes and lower for norbornene counterparts. However, the bonding energy in the later complexes is slightly stabilized given by the enhancement in the electrostatic character of the interaction. Thus, the theoretical evaluation of metal-alkene bonds, and other metal-bonding situations, benefits from the incorporation of relativistic effects even in lighter counterparts, which have an increasing role going down in the group.

Carbene chemistry of arsenic, antimony, and bismuth: origin, evolution and future prospects

The discovery of the first isolable N-heterocyclic carbene in 1991 ushered in a new era in coordination chemistry. The remarkable bonding properties of carbenes have led to their rapid proliferation as auxiliary ligands for a wide range of transition metals and main group elements. In the case of group 15, while carbene-stabilized nitrogen and phosphorus compounds are extensively studied, the scope of research has shrunk significantly from arsenic to bismuth. This is essentially attributed to the decrease in stability of the C-E bond upon descending the group. Even so, modulating the carbene backbone or introducing alternative synthetic strategies not only alleviates the stability issues but also offers promising results in terms of the bonding and reactivities of these compounds. The purpose of the present perspective is to provide a comprehensive overview of the origins and development of carbene chemistry of arsenic, antimony, and bismuth, as well as to highlight the future prospects of this field.

N-Heterocyclic carbene derivatives to modify gold superatom characteristics. Tailorable electronic and optical properties of [Au11(PPh3)7LCl2]+ as a cluster from relativistic DFT

Atomically precise gold superatoms are useful building blocks whose properties can be tuned by the proper choice of ligands in the protecting ligand layer. Herein, different N-heterocyclic carbene (NHC) derivatives of the prototypical [Au₁₁(PPh₃)₈Cl₂]⁺ cluster were evaluated by the replacement of a single ligand, which led to isoelectronic [Au₁₁(PPh₃)₇(NHC)Cl₂]⁺ species, enabling further understanding of the possible changes in the resulting cluster properties. Our results reveal the great variation in the HOMO-LUMO gap and optical features when going from strong to weak σ-donor NHC ligands. The Au₁₁ core retains similar features throughout the series, and the lowest unoccupied orbital (LUMO) is further stabilized, indicating greater π*-NHC character for the weaker σ-donor ligands, which favors directional core-ligand optical charge transfer to a single ligand. The ligand-tailored behavior of the [Au₁₁(PPh₃)₇LCl₂]⁺ cluster underlies its tunable characteristics, indicating its potential use in novel devices as building blocks of nanostructured materials, which favors further versatility and applications of superatomic clusters.

Base-stabilized formally zero-valent mono and diatomic molecular main-group compounds

Various compounds are known for transition metals in their formal zero-oxidation state, while similar compounds of main-group elements are recently realized and limited to only a few examples. Lewis-base-stabilized mono and diatomic molecular species (B₂, C, C₂, Si, Si₂, Ge, Ge₂, Sn, P₂, As₂, Sb₂) represent groundbreaking examples of main-group compounds with formally zero-oxidation state. In recent years, the isolation of low-valent main-group compounds has attracted increasing attention of both experimental and theoretical chemists. This is not only due to their fascinating electronic structures and exceptional reactivities, but also their use as valuable precursors for the synthesis of exotic yet important chemical species. This has led to a better understanding of the intricate balance of the donor-acceptor properties of the ligand(s) used to stabilize elements in a formally zero-oxidation state. Owing to the unusual oxidation state of the central element, many compounds containing formally zero-valent elements can efficiently activate otherwise inert small molecules. This review describes the synthesis, characterization, and reactivity of reported mono and diatomic formal zero-oxidation state main-group compounds. This review also emphasizes the comparative description of systems where different ligands are used to stabilize an element in its formal zero-oxidation state.

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.

Stabilization of a bismuth–bismuth double bond by anionic N-heterocyclic carbenes

Anionic N-heterocyclic carbenes have been employed for the isolation of the first dicarbene–dibismuth complex; the resulting dibismuthene features a trans-bent geometry with a Bi–Bi double bond and short intramolecular Bi–C_ipso contacts.

Dibismuthates as Linking Units for Bis-Zwitterions and Coordination Polymers

Adducts of bismuth trihalides BiX₃ (X = Cl, Br, I) and the PS₃ ligand (PS₃ = P(C₆H₄-o-CH₂SCH₃)₃) react with HCl to form inorganic/organic hybrids with the general formula [HPS₃BiX₄]₂. On the basis of their solid-state structures determined by single-crystal X-ray diffraction, these compounds exhibit discrete bis-zwitterionic assemblies consisting of two phosphonium units [HPS₃]⁺ linked to a central dibismuthate core [Bi₂X₈]^2– via S→Bi dative interactions. Remarkably, the phosphorus center of the PS₃ ligand undergoes protonation with hydrochloric acid. This is in stark contrast to the protonation of phosphines commonly observed with hydrogen halides resulting in equilibrium. To understand the important factors in this protonation reaction, ³¹P NMR experiments and DFT computations have been performed. Furthermore, the dibismuthate linker was utilized to obtain the coordination polymer {[AgPS₃BiCl₃(OTf)]₂(CH₃CN)₂}_∞, in which dicationic [Ag(PS₃)]₂²⁺ macrocycles containing five-coordinate silver centers connect the dianionic [Bi₂Cl₆(OTf)₂]^2– dibismuthate fragments. The bonding situation in these dibismuthates has been investigated by single-crystal X-ray diffraction and DFT calculations (NBO analysis, AIM analysis, charge distribution).

The potential of dibismuthates [Bi₂X₈]²⁻ as building blocks for the synthesis of bis-zwitterions and coordination polymers has been shown. The structures of these compounds and the bonding in the dibismuthate linkers have been studied by single-crystal X-ray diffraction and DFT calculations.

Hexaphenyl carbodiphosphorane adducts of heavier group 15 element trichlorides: syntheses, properties and reactivities

Herein, we present a series of hexaphenyl carbodiphosphorane (CDP^Ph) adducts of heavier group 15 trichlorides ECl₃ (E = P-Bi). The reaction with PCl₃ yields the known salt [CDP^Ph-PCl₂][Cl] ([1][Cl]), the heavier element trichlorides ECl₃ (E = Sb (4), Bi (5)) give the neutral adducts CDP^Ph-ECl₃ which were characterised crystallographically and spectroscopically. The reaction of CDP^Ph with AsCl₃ does not yield CDP^Ph-AsCl₃ (2), but in the presence of GaCl₃ the corresponding salt [CDP^Ph-AsCl₂][GaCl₄] ([3][GaCl₄]) is formed. DFT (density functional theory) calculations were carried out to examine the molecular frontier orbitals in 1⁺-5. Additional reactivity studies revealed an intramolecular electrophilic aromatic substitution (S_EAr) in 1⁺, which represents an excellent starting point for further selective C-P bond formation reactions.

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.

Highly Reactive Cyclic(alkyl)(amino) Carbene- and N-Heterocyclic Carbene-Bismuth(III) Complexes: Synthesis, Structure, and Computations

Cyclic(alkyl)(amino) carbene (CAAC)-stabilized complexes of phosphorus, one of the lightest group 15 elements, are well-established and can often be obtained in high yields. In contrast, analogous CAAC compounds of bismuth, the heaviest nonradioactive member of group 15, are unknown. Indeed, reactivity increases as you descend the group, and as a result there are only a few examples of N-heterocyclic carbene (NHC)-bismuth complexes. Moreover, activated bismuth compounds often readily extrude bismuth metal, making isolation of stable complexes highly challenging. We report that CAACs react with phenylbismuth dichloride (PhBiCl₂) to afford ^Et2CAAC-Bi(Ph)Cl₂ and ^CyCAAC-Bi(Ph)Cl₂. Significantly, these complexes represent the first structurally characterized examples of CAAC-coordination to bismuth. The CAAC-stabilized bismuth compounds can also be obtained from air-stable salts, [^Et2CAAC-H]₂²⁺ [Cl₂(Ph)Bi(μ-Cl₂)Bi(Ph)Cl₂]^2- and [^CyCAAC-H]₂²⁺ [Cl₂(Ph)Bi(μ-Cl₂)Bi(Ph)Cl₂]^2-, by deprotonation with potassium bis(trimethylsilyl)amide, K[N(SiMe₃)₂]. The electronic effects of the ligand on the bismuth center were investigated by comparing the CAAC-Bi(Ph)Cl₂ complexes to the NHC analogues, SIPr-Bi(Ph)Cl₂(THF) and IPr-Bi(Ph)Cl₂ (SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazole-2-ylidene; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene). Interestingly, the "normal" IPr-Bi(Ph)Cl₂ slowly isomerizes to the "abnormal" carbene complex, Cl₂(Ph)Bi-IPr-H, at -37 °C. In the solid-state, the CAAC-, NHC-, and abnormal NHC-bismuth compounds exhibit Bi atomic centers in unique coordination environments. The complexes were fully characterized by NMR, elemental analysis, and single crystal X-ray diffraction studies. In addition, the bonding was probed by natural bond orbital (NBO) calculations.

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 and crystal structure of three new bismuth(III) arylsulfonatocarboxylates

Three new bismuth arylsulfonatocarboxylates [Bi(OH)(SB)] (1), [Bi4(ST)2(HST)O2(H2O)2]·H2O (2) and [Bi4(ST)2O3(H2O)2] (3) were synthesized under solvothermal reaction conditions at 180°C using the potassium or sodium salt of 4-sulfobenzoic acid (H2SB) and 2-sulfoterephthalic acid (H3ST), respectively. The compounds were characterized in detail and the crystal structures were determined from single crystal X-ray diffraction data. Phase purity was confirmed by powder X-ray diffraction and elemental analysis. Structural comparisons to the only three other known bismuth sulfonatocarboxylates are presented. Due to the higher reaction temperatures employed for the synthesis of the title compounds a higher degree of condensation of the BiOx polyhedra (X=7 or 8) to tetrameric units, 1D chains or a 2D layer is observed. Connection through the organic linker molecules leads to the formation of 3D coordination polymers in all three title compounds.

N-Heterocyclic carbene adducts of the heavier group 15 tribromides. Normal to abnormal isomerism and bromide ion abstraction

The reactivity of the heavier group 15 tribromides, SbBr₃ and BiBr₃, towards 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) is described.

Bismuth(iii)dichalcogenones as highly active catalysts in multiple C–C bond formation reactions

Thirteen new bismuth(iii) dichalcogenone derivatives with diversified structural motifs were successfully isolated and used as potential catalysts for the synthesis of triaryl- or triheteroarylmethanes.

Phosphine complexes of lone pair bearing Lewis acceptors

An overview of the synthesis, structures and reaction chemistry of coordination complexes featuring an acceptor with at least one lone pair and at least one phosphine donor is presented. One or more examples of complexes have been structurally-characterized for the majority of p-block elements but few are known for most elements. The unusual condition of a p-block element centre accommodating a lone pair of electrons and offering a low energy LUMO gives the element centre the potential to behave as both a Lewis acid and a Lewis Base. The structural diversity and reactivity of the phosphine complexes highlights new directions in main group chemistry and by comparison with transition metal coordination chemistry, the featured complexes demonstrate significant configurational and stereochemical flexibility. Ligand exchange, oxidation and reduction chemistry at the lone pair bearing acceptor centre reveals unusual reactivity and an interesting class of ligands and inorganic reagents, with new possibilities for catalysis or small molecule activation.