Bismuthinidene

General structure of a bismuthinidene.

Bismuthinidenes are a class of organobismuth compounds, analogous to carbenes. These compounds have the general form R-Bi, with two lone pairs of electrons on the central bismuth(I) atom. Due to the unusually low valency and oxidation state of +1, most bismuthinidenes are reactive and unstable, though in recent decades, both transition metals and polydentate chelating Lewis base ligands have been employed to stabilize the low-valent bismuth(I) center through steric protection and π donation either in solution or in crystal structures. Lewis base-stabilized bismuthinidenes adopt a singlet ground state with an inert lone pair of electrons in the 6s orbital. A second lone pair in a 6p orbital and a single empty 6p orbital make Lewis base-stabilized bismuthinidenes ambiphilic.

Synthesis

Transition metal-stabilized bismuthinidene

The earliest examples of bismuthinidene complexes used transition metal chemistry to stabilize the Bi(I) center. These methods generally leveraged the ability of simple bismuth(I) halides or methylbismuth to ligate to tungsten, manganese, and chromium carbonyl complexes. These complexes were occasionally found to oligomerize, forming Bi-Bi single or double bonds to form bismuthane or bismuthene moieties. One of the first examples of a monomeric bismuthinidene was discovered by Balasz et al., who used R = 2-(dimethylaminomethyl)phenyl to chelate a Bi(I) center through a combination of strong C-Bi and weak N-Bi interactions. Although the molecule quickly formed a cyclic oligomer, upon reaction with two equivalents of tungsten pentacarbonyl, monomeric crystalline RBi[W(CO)₅]₂ was isolated.

Synthesis of an early example of monomeric bismuthinidene.

N,C,N-chelated bismuthinidene

Monomeric bismuthinidenes were not stabilized without the use of transition metal complexes until 2010, when Libor Dostál's research group reported the isolation of a bismuth(I) center stabilized only by the N,C,N-pincer ligand L = 2,6-bis[N-(2’,6’-dimethylphenyl)ketimino]phenyl. This complex was first synthesized by reacting the precursor molecule LBi^IIICl₂ with two equivalents of the reducing agent K[B(iBu)₃H] to yield isolable crystals of stable [C₆H₃-2,6-(C(Me)=N-2′,6′-Me₂C₆H₃)₂]Bi. A slightly simpler N,C,N-chelating ligand was soon used to create the bismuthinidene [C₆H₃-2,6-(CH=NtBu)₂], which became widely used in later bismuthinidene studies and is occasionally referred to as "Dostál's bismuthinidene". In fact, many analogs of this compound have been synthesized, often replacing the imine tert-butyl groups with other bulky organic groups or replacing the two imine arms with disubstituted amine arms.

Synthesis of Dostál's bismuthinidene, the most common example of an N,C,N-chelated bismuthinidene.

N,C-chelated bismuthinidene

Dostál's group later synthesized a monomeric bismuthinidene coordinated only by a bidentate N,C-chelating ligand. When the bismuth dichloride [C₆H₂-2-(CH=N-2’,6’-iPr₂C₆H₃)-4,6-(tBu)₂]BiCl₂ is reduced by two equivalents of K[B(iBu)₃H], isolable dark violet crystals of [C₆H₂-2-(CH=N-2’,6’-iPr₂C₆H₃)-4,6-(tBu)₂]Bi appear. In contrast to the earlier transition metal-stabilized [2-(Me₂NCH2)C₆H₄]Bi[W(CO)₅]₂, the tert-butyl group ortho to the bismuth atom in this N,C-chelated bismuthinidene sterically block the partially empty p-type orbital on the bismuth atom, kinetically stabilizing it without the use of transition metals. In addition, calculated nucleus-independent chemical shift indices (NICS) and anisotropy of current-induced density (ACID) analysis show that the BiC₃N ring of the molecule was stabilized by a certain degree of aromatic character due and may be classified as a benzazabismole to the delocalization of six π electrons, despite the nominally dative Bi-N bond. Unlike N,C,N-coordinated bismuthinidenes, this N,C-coordinated species requires the pendant nitrogen atom to be in an imine group, as replacement of the Dipp-substituted imine arm with a diethyl-substituted amine arm resulted in rapid dimerization to a dibismuthene species.

Synthesis of N,C-chelated bismuthinidene, forming a single five-membered aromatic BiC₃N ring.

Carbene-stabilized bismuthinidene

In 2019, Wang et al. isolated a novel carbene-stabilized bismuthinidene with an exocyclic bismuth(I) center. Phenylbismuth dichloride, stabilized by a diethyl/diisopropylphenyl-substituted cyclic alkyl amino carbene (^Et2CAAC), reacts with one equivalent of the beryllium(0) complex Be(^Et2CAAC)₂ in toluene to give stable, isolable red crystals of the carbene-stabilized bismuthinidene (^Et2CAAC)Bi-Ph. Despite the exposed, exocyclic bismuth(I) center, the compound can exist without dimerization for up to two weeks in the solid state. Density functional theory (DFT) calculations showed that this is a result of partial double bond character between the carbene carbon and the bismuth(I) center, wherein the p-type lone pair of electrons on the bismuth atom interact with the partially-filled p orbitals on the carbene carbon. However, the charge on the bismuth atom as determined by natural population analysis (NPA) was much lower than in bismuth(III)-carbon bonds, supporting the compound's classification as a bismuthinidene.

Synthesis of a CAAC carbene-stabilized bismuthinidene complex.

Structural Properties

As with other heavier carbene analogs, the structural and electronic properties of bismuthinidenes are in large part driven by the inert-pair effect, wherein the large energy gap between the bismuth atom’s 6s and 6p orbitals disfavors the formation of sp hybrid orbitals. In stark contrast to their lighter congeners phosphinidenes, whose smaller phosphorus 3s-3p energy gap favors a triplet ground state, bismuthinidenes generally have a singlet ground state on account of the larger bismuth 6s-6p energy gap.

The structural and electronic properties of bismuthinidenes in general are clearly exemplified by Dostal’s N,C,N-stabilized bismuthinidene, which is the most commonly used bismuthinidene in the literature to date. Optimization of the tert-butyl imino version of this compound at the M06/cc-pVTZ level of theory reveals that, as in other nontrigonal pnictogen compounds, the central bismuth atom is coplanar with the N,C,N-chelating ligand, adopting a T-shaped C_2v coordination mode. The Wiberg bond index (WBI) between the bismuth and carbon atoms is 1.09, while the Bi-C bond distance is 2.2156 Å, slightly shorter than the sum of these atoms’ single-bonded covalent radii (Σ_cov(Bi,C) = 2.26 Å). On the other hand, the WBI of the Bi-N bonds is only 0.34, and the Bi-N bond distance is 2.500 Å, significantly longer than the sum of these atoms’ covalent radii (Σ_cov(Bi,N) = 2.22 Å). This agrees with calculations based on the quantum theory of atoms in molecules (QTAIM), which show that the electron density at the bond critical point between Bi and N is only 0.049, significantly lower than the electron density of 0.114 at the Bi-C bond critical point. Natural bond orbital (NBO) calculations show that these weaker dative bonds arise from weak σ donation of the nitrogen atoms’ lone pairs into an empty 6p orbital on the central bismuth atom. These two n_N → p*_Bi interactions stabilize the bismuthinidene by as much as 382 kJ/mol. Additionally, the amount of sigma donation from the pendant nitrogen atoms may be increased or decreased by replacing the tert-butyl groups on the pendant nitrogen atoms with aryl groups containing electron-donating groups or electron-withdrawing groups, respectively. One lone pair resides in the bismuth atom’s 6s orbital and generally remains inert, while the other resides in the 6p orbital oriented perpendicular to the plane of the central ring, which also comprises the highest occupied molecular orbital (HOMO).

Reactivity

Theoretically, bismuthinidenes are both Lewis acidic and Lewis basic due to their empty and filled p-type orbitals, respectively. In practice, both N,C,N- and N,C-chelated bismuthinidenes lose much of their Lewis acidic character due to n_N → p*_Bi donor-acceptor interactions. However, the Lewis basicity of bismuthinidenes, particularly Dostál's N,C,N-chelated bismuthinidene, allows them to cycle predictably between stable Bi(I) and Bi(III) oxidation states depending on the reaction conditions, allowing them to act as catalysts for a variety of different reactions, including transfer hydrogenations, deoxygenations, hydrodefluorinations, and dihydrogen reduction. In addition, bismuthinidenes react intrinsically with certain alkyl halides, dichalcogenides, and alkynes to form Bi(III) species.

Summary of the reactivity of Dostál's bismuthinidene.

Intrinsic Reactivity

Transfer hydrogenation of azoarenes

In 2019, Wang et al., who leveraged the catalytic activity of Dostál's bismuthinidene to catalyze a transfer hydrogenation reaction between ammonia-borane and azoarenes to form the corresponding arylhydrazines with good functional group tolerance. The reaction's catalytic cycle proceeds through the oxidative addition of two hydrogen atoms from ammonia-borane to the bismuth(I) center, forming a highly unstable bismuthine intermediate. Subsequent reductive elimination transfers the two hydrogen atoms across the pi bond of an azoarene molecule, restoring the bismuthinidene and forming arylhydrazine. A similar bismuthinidene-catalyzed transfer hydrogenation reaction reduces nitroarenes to the corresponding aryl hydroxyl amines.

Deoxygenation of nitrous oxide

The Bi(I)/Bi(III) redox couple has also been applied to catalyze the deoxygenation of nitrous oxide.

The catalytic cycle for the hydrodefluorination of polyfluoroarenes via Phebox-Bi(I) involves C-F oxidative addition (a), Bi(III)-F to Bi(III)-H ligand metathesis (b), and C-H reductive elimination (c).

When Dostál's bismuthinidene is exposed to gaseous N₂O, the reaction mixture changes color from green to yellow and evolves dinitrogen gas. The color change is due to the formation of an arylbismuth oxide dimer with two μ-oxo bridging moieties forming a Bi₂O₂ center, consistent with the propensity of bismuth(III) oxides to spontaneously dimerize or polymerize. However, a modified version of Dostál's bismuthinidene with ketimine arms and m-terphenyl substituents on the ketimine nitrogen atoms disfavors dimerization, instead forming a rare monomeric organobismuth(III) hydroxide upon reaction with N₂O. In either case, reduction of the product with pinacolborane (HBpin) returns the bismuth(III) centers to the bismuth(I) state and yields a mixture of HOBpin and (pinB)₂O, completing the catalytic cycle.

Hydrodefluorination of polyfluoroarenes

The electronic properties of the N,C,N-pincer ligand may be tuned with electron withdrawing groups to promote the reactivity of bismuthinidenes toward aryl C-F bonds. One example is Phebox-Bi(I), an N,C,N-chelated bismuthinidene stabilized by a 2,6-bis(oxazolinyl)phenyl (Phebox) pincer ligand. Unlike Dostál's bismuthinidene, which has only shown reactivity towards pentafluoropyridine, Phebox-Bi(I) has demonstrated a propensity to add to C-F bonds in a variety of perfluorinated arenes, including pentafluoropyridine, substituted pentafluorobenzenes, highly fluorinated phosphine compounds, octafluoronaphthalene, and decafluorobiphenyl. After oxidative addition to the aryl C-F bond, the resulting Phebox-Bi(III)(fluoroaryl) fluoride intermediate may undergo ligand metathesis with diethylsilane, replacing the Bi(III)-F bond with a Bi(III)-H bond. The unstable Bi(III) hydride then undergoes aryl C-H reductive elimination, regenerating Phebox-Bi(I) and the hydrodefluorinated product. The catalyst usually targets C-F bonds para to any electron-withdrawing substituents or heteroatoms on the fluorinated substrate. Reaction rates decrease significantly when the fluorinated substrate contains an electron-donating group.

Reduction of dihydrogen from acetic acid

Electrochemical studies indicate that Dostál's bismuthinidene may serve as an electrocatalyst for the formation of hydrogen gas. Cyclic voltammetry and DFT calculations indicate that, under reducing conditions, acetic acid binds to the central bismuth atom, transferring a hydrogen atom to bismuth and generating an N,C,N-chelated bismuth(III) acetate hydride intermediate. Rearrangement of the acetate ligand from the equatorial to the axial position allows a second equivalent of acetic acid to bind to the bismuth center, eliminating H₂ in the process. Two-electron reduction releases acetate ligands, regenerating the bismuthinidene catalyst.

Oxidative addition toward alkyl halides and diphenyldichalcogenides

The low valency of bismuthinidenes renders them reactive toward carbon-polar group bonds. Oxidative addition reactions between Dostál’s bismuthinidene and primary C(sp³)-X bonds are particularly favorable for X = I or OTf, converting the bismuth(I) center to a bismuth(III) alkyl halide or alkyl triflate. This is true even for longer fluorinated alkyl halides up to six carbon atoms in length. Steric hindrance prevents the activation of tert-butyl iodide by Dostal’s bismuthinidene, although a metastable analog with amine pincer arms rather than imine pincer arms does participate in oxidative addition even with bulky tertiary C-X bonds, likely because the increased rotational mobility of the amine arms allows them to rotate away from the incoming bulky alkyl group in the transition state.

This metastable analog of Dostál’s bismuthinidene, as well as N,C-stabilized bismuthinidene, are also reactive toward diphenyldichalcogenides. While the former yields stable crystals of [2,6-C₆H₃(CH₂NMe₂)₂]Bi(III)(EPh)₂ (E = S, Se, Te) upon reaction with PhEEPh, the latter yields [2-C₆H₄(CH=NC₆H₃(i-Pr)₂-2,6)]₂Bi(III)(EPh) with two N,C ligands and only one phenyl chalcogenolate. Metastable N,C,N- and doubly N,C-chelated bismuth(III) phenyl tellurolates are particularly unstable and decompose to form a mixture of products. Oxidative additions of N,C,N-chelated bismuthinidene to diaryldisulfides are tolerant to a variety of aryl functional groups, including pyridyl, thiazolyl, thienyl, and aminophenyl groups.

Mechanism for the Diels-Alder reaction between Dostál's bismuthinidene and DMAD, with the diene region of the bismuthinidene highlighted in green. Note that the trivalent Bi-centered resonance structure contributes little to the overall structure and simply illustrates the hidden diene character of the compound.

Hetero Diels-Alder reaction with alkynes

In 2019, Kořenková et al. discovered that Dostál's bismuthinidene behaves as a masked heterocyclic diene in the presence of the electron-deficient alkyne dimethyl acetylenedicarboxylate (DMAD), performing a hetero Diels-Alder [4+2] cycloaddition reaction to yield CO₂Me-disubstituted 1-bisma-1,4-dihydro-iminonaphthalene, effectively converting one of the pendant imine arms of the bismuthinidene into a nitrogen-bridged bismacyclohexadiene, with the bismuth(III) atom serving as a bridgehead and the second imine arm largely losing coordination with the bismuth(III) center. Energy profiles generaed by theoretical studies show that the free energy barrier between the 1-bisma-1,4-dihydro-iminonaphthalene species and its corresponding bismanaphthalene is only 6.7 kJ/mol. A similar cycloaddition reaction between Dostál's bismuthinidene and methyl propiolate yields an iminonaphthalene only as an intermediate, as the bridgehead bismuth atom is quickly attacked by a deprotonated second equivalent of methyl propiolate, breaking the Bi-N bond and yielding a boat-shaped bismacyclohexadiene moiety. Though technically unbridged, the axial amine group on the bismacyclohexadiene ring remains datively coordinated to the bismuth heteroatom.

References

attribution: contains text from CC-BY published article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6728098/