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

Abstract

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.

Carbene–pnictinidenes represent a class of molecules, which feature a carbene bound to an electron‐rich Group 15 element.1 Interest in these types of molecules has grown substantially in recent years, and applications in chemical synthesis and catalysis continue to be discovered.2 These pnictinidene adducts may be characterized as a hybrid of three resonance structures (Figure 1).3 However, the specific structure is dependent on the type of carbene, the pnictogen (Pn) element, and the substituent (R) on Pn. Among this class of compounds, carbene–phosphinidenes are the most well‐established and the electron‐rich phosphorus center has been used to coordinate to main‐group elements and transition metals.1, 2, 3, 4 The resulting compounds have even been employed in homogenous catalysis.5 It is noteworty that all of the carbene–pnctinidenes that have been reported to date feature non‐metal (P)1, 2, 3, 4 or metalloids (As, Sb) as the Pn element.6, 7 We sought to extend the carbene–pnictinidene concept to a fully metallic element (Bi).

Figure 1.

Resonance structures of carbene–pncitinidene complexes, (E=NR or CR₂).

Bismuthinidenes are extremely rare, and dicoordinate bismuth compounds are typically highly reactive. As such, chemists have utilized sterically bulky groups to protect the bismuth center.8, 9, 10 A prominent example is the isolation of the first compound containing a Bi=Bi double bond by Tokitoh et al. in 1997.8 Since then, a number of compounds have been reported, which feature Bi=Bi double bonds stabilized by a variety of formally anionic ligands (Figure 2, A).9

Figure 2.

Dicoordinate bismuth complexes, A (previous work, L=monoanionic ligands), B (previous work, bismuth metal in five‐membered ring), and C (this work).

Additionally, a dicoordinate monomeric bismuth complex was synthesized by Dostal ultilizating an N–C chelating ligand, in which the Bi atom is stabilized by an N‐donor, which results in the incoorpation of the Bi atom in a ring (Figure 2, B).10 The resulting C₃NBi ring was reported to possess aromatic character. In contrast, the synthesis of a carbene‐bismuthinidene would result in an exocyclic subvalent bismuth center (Figure 2, C). These types of complexes have eluded chemists. Indeed, current knowledge of carbene–bismuth chemistry pales in comparison to the chemistry of carbene‐phosphinidene,4 ‐arsinidene,6 and ‐stibinidene.7 This is in part due to metallic nature of bismuth, which results in significant differences in stability and reactivity compared to light Pn elements. Dutton and co‐workers reported the first examples of N‐heterocyclic carbene–bismuth complexes; however, attempts to reduce the bismuth(III) chloride adducts to subvalent species were unsuccessful.11 Goicoechea and co‐workers synthesized NHC–bismuth(III) tribromides,12 but the reactivity with reducing agents was not reported. A recent comprehensive review of the field highlighted the difficulty in synthesizing NHC adducts of heavier Pn elements, which was partly attributed to the destabilization of the 2p_π–np_π interaction as you descend the group.13 Consequently, we sought to exploit the stronger σ‐donating and π‐accepting properties of cyclic(alkyl)(amino) carbene (CAAC), which was first prepared by Bertrand and co‐workers.14

Recently, we synthesized a series of bismuth(III) complexes [i.e., carbene–Bi(Ph)Cl₂)] by combining NHCs and CAACs with phenylbismuth dichloride, PhBiCl₂(THF).15 The CAAC Bi^III compounds were obtained in higher yield and were more resistant to isomerization or decomposition compared to the analogous NHC complexes. It is also noteworthy that the (CAAC)Bi(Ph)Cl₂ complexes represented the first examples of compounds with CAAC coordination to bismuth. In an effort to advance carbene–bismuth chemistry, we sought to synthesize a subvalent bismuth compound stabilized by CAAC. Herein, we report the synthesis of the first carbene‐stabilized bismuthinidene complex (^Et2CAAC)Bi–Ph (3), which was formed by hitherto unknown methods, which utilize a beryllium(0) complex as a reducing agent and ligand transfer reagent.

Reduction of (^Et2CAAC)Bi(Ph)Cl₂ with two equivalents of potassium graphite (KC₈) in toluene at −37 °C resulted in a color change from pale yellow to red over one hour (Scheme 1). The solvent was removed, and the residue was extracted with hexane to give a red solution. The UV/Vis spectrum recorded at room temperature of the red solution in hexane showed a new absorption band at λ _max 516 nm, which was attributed to the formation of a reduced bismuth species (Figure 3). However, the red solution rapidly decolorizes, and an insoluble black precipitate was observed. This precipitate was attributed to bismuth metal, obtained through decomposition of the target compound. Therefore, we reasoned that long reaction times associated with using KC₈, potential reactive intermediates, or the strongly reducing nature of KC₈ contributed to the decomposition of the Bi species. Though the solution was initially colored, it is likely that the true quantity of reduced Bi species in solution was actually very small. Therefore, only free CAAC ligand could be isolated. We then turned our attention to soluble reducing agents that would rapidly produce an insoluble by‐product to make isolation facile.

Scheme 1.

Attempt to reduce Bi^III starting material with KC₈.

Figure 3.

Normalized electronic absorption spectrum. Transition A, λ=324 nm (free CAAC). Transition B, λ=516 nm (new Bi species).

Recently, Braunschweig and co‐workers seminally reported the synthesis of a Be⁰ compound, Be(^Me2CAAC)₂.16 Though there were no reports of Be⁰ being used as a reducing agent, we speculated that a Be⁰ compound might facilitate the removal of the chloride atoms. Additionally, our previous calculations showed that coordination of the CAAC results in the weakening of the Bi−Cl bonds, indicating that a strong reducing agent may not be necessary.15 The Be⁰ complex was also selected due to its high solubility in most organic solvents including hexanes.16 Therefore, we prepared compounds 1 and 2 in 85 % and 79 % yield, respectively, according to a modified procedure (Scheme 2). Colorless rod‐shaped crystals of 1 suitable for a single crystal X‐ray diffraction study were grown from toluene at −37 °C (Figure S11 in the Supporting Information). Purple single crystals of compound 2 suitable for X‐ray diffraction analysis were obtained from a hexane solution at −37 °C. The molecular structure of 2 in Figure 4 shows two ^Et2CAAC coordinated to one Be atom and the bonding is similar to Be(^Me2CAAC)₂.

Scheme 2.

Synthesis of beryllium(0) reducing agent.

Figure 4.

Molecular structure of compound 2.17

A yellow suspension of (^Et2CAAC)Bi(Ph)Cl₂ was added to a purple solution of Be(^Et2CAAC)₂ in toluene at −37 °C (Scheme 3). The combined homogeneous reaction mixture was kept at −37 °C for 30 minutes to give a dark red solution containing some bismuth metal. After workup, the first carbene–bismuthinidene complex was isolated as red single crystals in 10 % yield. To the best of our knowledge, this is the first example of a Be⁰ complex being used as a reducing agent. Surprisingly, hexane‐ and toluene‐soluble 2 can also act as a ligand‐transfer reagent (Scheme 3). Indeed, red crystals of compound 3 were obtained from the direct reaction of PhBiCl₂(THF) and 2, which results in a deep red color after 30 minutes at −37 °C. However, there was no significant change in the reaction yield. The low isolated yield is due to the extreme reactivity of compound 3 in solution. The ¹H NMR spectrum of 3 in [D₈]toluene at 0 °C showed a characteristic doublet at 8.91 ppm for the Bi–Ph(ortho‐H) (Figure S6 in the Supporting Information). However, the [D₈]toluene solution shows decomposition within five minutes (Figure S6b in the Supporting Information), indicated by gradual decolorizing of the solution and concomitant formation of bismuth metal and the neutral ^Et2CAAC ligand. The ¹³C NMR spectrum of 3 in [D₈]toluene at 0 °C showed a characteristic singlet at 190 ppm for the carbene carbon, compared to 317 ppm for the free ^Et2CAAC (Figure S7 in the Supporting Information). Indeed, this ¹³C NMR resonance is in accord with (CAAC)P(Ph) complexes 191–208 ppm.4a Compound 3 is a thermally stable crystalline material which decomposes at 100 °C. Solid samples stored at room temperature under a dry argon atmosphere retain their color and crystallinity for approximately two weeks.

Scheme 3.

Synthesis of cyclic(alkyl)(amino) carbene‐bismuthinidene, 3. Reduction and ligand transfer mediated by a Be⁰ complex.

Red air‐ and moisture‐sensitive rod‐shaped crystals of 3 suitable for a single‐crystal X‐ray diffraction study were grown from hexane solution at −37 °C. The structure of 3 shows a dicoordinate bismuth center in a bent arrangement, with two Bi−C bonds (Figure 5). Unlike previously reported bismuthinidene complexes stabilized by monoanionic chelating NCN ligands,10 3 features an exocyclic monomeric bismuth center and is stabilized by the formation of a carbene–metal interaction with partial double bond character. Remarkably, there are no additional interactions with the bismuth atom and it remains open without dimerization (Figure 6). There is no example of a carbene‐bismuthinidene compound that can be used to compare to compound 3. The Bi−C_carbene bond in 3 [Bi1−C1: 2.199(2) Å] is the shortest bond in all reported C_carbene−Bi bonds (2.35–2.4566 Å) and significantly shorter than that in the Bi^III compound ^Et2CAAC‐Bi(Ph)Cl₂ [2.4566(15) Å].11, 12, 15 Notably, the Bi1−C1 bond [2.199(2) Å] is shorter than the covalent Bi−C_Ph bond [Bi1‐C23: 2.278(2) Å] in compound 3, which is indicative of partial double bond character of between CAAC and bismuth metal center. There is no significant difference in the Bi−C_Ph bond in compound 3 [2.278(2) Å] and ^Et2CAAC−Bi(Ph)Cl₂ [2.2732(16) Å].15

Figure 5.

Molecular structure of 3 (thermal ellipsoids at 50 % probability; H atoms omitted for clarity). Selected bond lengths [Å] and angles [°]: Bi1−C1: 2.199(2); Bi1−C23: 2.278(2); N1−C1: 1.334(3); N1−C11: 1.452(2); N1−C4: 1.516(3); C1−C2: 1.519(3); N1‐C1‐Bi1: 119.65(14); C1‐Bi1‐C23: 99.60(8); C1‐N1‐C11:120.45(17); C1‐N1‐C4: 115.43(16); C11‐N1‐C4: 124.08(16); N1‐C1‐C2: 109.53(17).17

Figure 6.

Space filling model: front view (left) and side view (right).

To gain further insight into the bonding of 3, we carried out DFT calculations at the ωB97XD/cc‐pVTZ(‐PP) level of theory (for details see Supporting Information). The gas‐phase optimized structure of 3 is in agreement with the X‐ray structure (e.g., Bi−C_carbene distances: 2.195 vs. 2.199 Å, respectively). We also performed computations on a simplified system (3M), in which the Dipp, Ph, and Et substituents were replaced by methyl groups (Figure 7). The geometrical parameters of 3M are comparable to compound 3. On the optimized geometries natural bond orbital (NBO) calculations, natural population analysis (NPA), and atoms‐in‐molecules (AIM) analysis have been carried out (see the Supporting Information for details).

Figure 7.

Resonance structures of CAAC–bismuthinidene compound 3M.

The Bi−C_carbene distance of 2.182 Å in 3M is significantly shorter than the Bi−C_methyl distance (2.286 Å). The Wiberg bond index (WBI) value of the Bi−C_carbene bond in 3M (1.28) shows partial double bond character [WBI: Bi−C (0.87), Bi=C bond (1.84)]. The electron density value of 0.107 a.u. at the bond critical point also indicates the strengthening of the Bi−C_carbene bond [H₂C=Bi−Me (0.136 a.u.); Me₃Bi (0.099 a.u.)]. Furthermore, the ellipticity of this bond (ϵ=0.282), which accounts for the π character, equals that of a prototype Bi=C bond (ϵ=0.282 in H₂C=Bi−Me). The relative weakness of the Bi−C_carbene bond compared to a true Bi=C bond necessitates the consideration of additional resonance structures.

Based on natural resonance theory (NRT) analysis of the electronic structure of 3M, the weight of the double‐bonded species A (42.4 %) exceeds that of zwitterionic B (20.6 %), with the remaining 37 % comprised of a series of minor contributors (less than 2 % each). Representation C (Figure 7) depicts 3M as a donor–acceptor complex between a carbene and a low‐coordinate Bi−Me moiety. However, the Bi−C_carbene bond (WBI: 1.28 in 3M) is much stronger than the dative interaction observed for an adduct of CAAC and a Bi(Ph)Cl₂ unit (WBI: 0.60).15 This representation does however reflect the fragmentation of 3 into free CAAC. Collectively, the data for 3M supports a hybrid structure with a Bi−C_carbene interaction that is between a single and a double bond.

Further information may be deduced from the frontier molecular orbitals of 3M and H₂C=Bi−Me (Figure 8). The HOMO of H₂C=Bi−Me represents a π(Bi−C) interaction, whereas the LUMO is a π*(Bi−C) antibonding orbital. In contrast, the HOMO of 3M can be interpreted as a partial π bond or as a p‐type lone pair of electrons on Bi showing interaction towards C_carbene, and the LUMO is a π*(C−N) antibonding orbital.

Figure 8.

LUMO and HOMO orbitals of 3M and H₂C=BiMe at the ωB97XD/cc‐pVDZ(‐PP) level (contour value at 0.07).

In summary, we have synthesized and structurally characterized the first carbene‐bismuthinidene complex, (CAAC)Bi(Ph), which contains the shortest Bi−C_carbene bond. This represents the first example of a carbene‐stabilized subvalent bismuth complex. We have also reported the first example of a beryllium(0) complex being used as a reducing agent and a ligand‐transfer reagent. This may open the door to new reduction chemistry and allow access to other highly reactive main‐group molecules. Because the chemistry of carbene‐pnictinidene has grown to be a thriving area of active research,4 we expect that the synthesis of the missing metallic analogue will spark new research into heavier Group 15 carbene chemistry. We are currently investigating the reactivity of the title compound and utilization of the electron‐rich Bi center as neutral ligand to form metal–metal bonds.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supplementary

G. Wang, L. A. Freeman, D. A. Dickie, R. Mokrai, Z. Benkő, R. J. Gilliard, Chem. Eur. J. 2019, 25, 4335.