Bromination of a Stable 1,2‐Bis(ferrocenyl)dibismuthene: Synthesis of a 1,2‐Dibromodibismuthane Derivative

ABSTRACT

The bromination reaction of the stable bis(ferrocenyl)dibismuthene, Fc*Bi=BiFc* (1, Fc* = 2,5‐bis(3,5‐di‐t‐butylphenyl)ferrocenyl), with 0.5 equiv of BiBr₃ resulted in the formation of the first stable 1,2‐dibromodibismuthane derivative, Fc*(Br)Bi–Bi(Br)Fc* (3). The structure of 3 was revealed by X‐ray crystallographic analysis, showing its unique structure with the bromine atom intramolecularly coordinating to the bismuth atom.

INTRODUCTION

Tetraorganodibismuthanes (R₂Bi–BiR₂) are a unique class of compounds due to their physical and chemical properties of the Bi–Bi single bond 1. However, there are only a few examples of stable dibismuthanes (see Scheme 1), because it has been known that dibismuthanes with a Bi–Bi single bond are thermally labile and severely air‐sensitive species due to their weak Bi–Bi bond. They readily react with aerobic oxygen, electrophiles, nucleophiles, and free radicals to undergo the Bi–Bi bond cleavage 1, 2. Not only such high reactivity but also their thermal instability would also be the reason for the difficulty in isolation of stable dibismuthanes. For example, tetramethyldibismuthane, Me₂Bi–BiMe₂, undergoes disproportionation, giving trimethylbismuthane (Me₃Bi) and bismuth metal with the half‐life time of ca. 6 h in a benzene solution 3. With such background, a dibismuthane has been attractive species as one of reactive species. The most convenient synthetic methods should be (i) oxidative coupling of methallated bismuthane R₂BiM (M = Li, Na) and (ii) reductive coupling of monohalobismuthane R₂BiX (X = Cl, Br, I) 1. For example, Me₂BiNa prepared by the reaction of Me₃Bi with Na was oxidized by 1,2‐dichloroethane to give Me₂Bi–BiMe₂ [method (i)] 3. A reductive coupling reaction of the couple of Ph₂BiCl with Na 4 or Ph₂BiI with Cp₂Co 5 afforded Ph₂Bi–BiPh₂, which is one of the stable dibismuthanes [method (ii)]. On the other hand, the reduction of dihalobismuthane, DisBiCl₂ (Dis = CH(SiMe₃)₂), with Mg was reported to give neither 1,2‐dihalodibismuthane, Dis(Cl)Bi–Bi(Cl)Dis, nor dibismuthene, DisBi=BiDis, but cyclic oligobismuthanes (DisBi)_n 6. Thus, there is no example of isolation of a 1,2‐dihalodibismuthane derivative due to the difficulty in finding an appropriate synthetic method.

SCHEME 1

Examples of the synthesis of dibismuthanes.

It has been demonstrated that steric protection afforded by sterically demanding substituents should effectively stabilize the reactive chemical bonds of heavier main group elements even in the case of the 6th row elements 7. Thus, we have succeeded in the synthesis of the stable dibismuthenes, TbtBi=BiTbt 8, BbtBi=BiBbt 9, and Fc*Bi=BiFc* 10 (Tbt = 2,4,6‐tris[bis(trimethylsilyl)methyl]phenyl, Bbt = 2,6‐bis[bis(trimethylsilyl)methyl]‐4‐tris[bis(trimethylsilyl)methyl]phenyl, Fc* = 2,5‐bis(3,5‐di‐t‐butylphenyl)ferrocenyl) by the reductive coupling reaction of the corresponding dihalobismuthane, RBiX₂, with using Mg metal. Especially 1,2‐bis(ferrocenyl)dibismuthene, Fc*Bi=BiFc* (1), is a unique class of compounds as a d–π electron system bearing heavier main group elements, exhibiting interesting redox behavior 10. During the course of our investigation on the reactivity of stable dibismuthenes, it was found that the reaction of Fc*Bi=BiFc* (1) with BiBr₃ afforded the first stable 1,2‐dibromodibismuthane derivative, meso‐Fc*(Br)Bi–Bi(Br)Fc* (3) (see Scheme 2), suggesting the brominating ability of BiBr₃ toward a dibismuthene. The isolated 1,2‐dibromodibismuthane 3 was structurally characterized, showing its unique bromine‐bridging structure.

SCHEME 2

Synthesis of the first stable 1,2‐dibromodibismuthane.

RESULTS AND DISCUSSION

1,2‐Bis(ferrocenyl)dibismuthene 1 was prepared according to the reported procedure, where dibromobismuthane Fc*BiBr₂ (2) was reduced by Mg metal (2.0 equiv) in THF at room temperature (r.t.) for 15 min 10. When dibismuthene 1 was treated with BiBr₃ (0.5 equiv) in THF at r.t. for 48 h, the color of the solution was changed from violet to red‐purple. After a purification procedure, 1,2‐dibromo‐1,2‐bis(ferrocenyl)dibismuthane 3 was obtained as purple crystals in 34% yield. The treatment of dibismuthene 1 with an excess amount of BiBr₃ (3.0 equiv) in THF at r.t. for 1 h directly afforded dibromobismuthane 2 quantitatively as judged by ¹H NMR spectrum. Thus, BiBr₃ was found to be an appropriate brominating reagent for the controlled bromination of dibismuthene 1.

The results of X‐ray crystallographic analysis of dibromobismuthane 2 and 1,2‐dibromodibismuthane 3 are shown in Figs. 1 and 2. As shown in Fig. 1a, dibromobismuthane 2 was found to exist as a monomeric form, though the previously reported dibromobismuthane bearing a bulky aryl substituent, Ar_MesBiBr₂ (Ar_Mes = 2,6‐dimesitylphenyl), was reported to exhibit its dimeric structure in the crystalline state 11. Thus, compound 2 is the first example of a tricoordinated organodihalobismuthane existing as a monomeric form in the crystalline state. In the structure of dibromobismuthane 2, all the bond angles around the central Bi atom (Br1–Bi–Br2, Br1–Bi–C, and Br2–Bi–C) are close to 90°, suggesting high p‐character of the Bi–Br and Bi–C bonds. It should be noted that one of the Bi–Br bonds (Bi–Br1) is almost perpendicular to the Cp ring connected to the Bi atom, and the Bi atom is deviated and tilted from the Cp plane by 16.6° to the inside of the ferrocenyl moiety. Thus, the Bi–Fe distance (3.4655(10) Å) is relatively short as compared with the sum of the corresponding van der Waals radii (3.66 Å). In addition, the Bi–Br1 bond length (2.6688(8) Å) is slightly longer than the other (Bi–Br2, 2.6443(8) Å). These structural features would indicate the weak donation from Fe d‐electrons toward the Bi–Br1 σ* orbital. Theoretically optimized structural parameters for 2 in the gas phase were found to be in good agreement with those experimentally observed (Fig. 1) (B3PW91/6–31G(d) [6‐31G(2d) for Br, Lanl2dz for Fe, TZ(2d) for Bi]). In particular, the unique structural features of 2 such as the short contact between the Fe and Bi atoms, the slightly longer Bi–Br1 bond than Bi–Br2, and so on, were reproduced by the theoretical calculations, suggesting these unique structural features would not be due to the packing force in the crystalline state but arise from the intrinsic nature of a ferrocenyldibromobismuthane.

Figure 1

Molecular structure of dibromobismuthane 2. Displacement ellipsoids were drawn at the 50% probability level. (a) Total view other than hydrogen atoms. (b) Selected view with omission of hydrogen atoms and Dtp groups. Selected structural parameters of 2 (Å and deg): C1–Bi, 2.188(7); Bi–Br1, 2.6688(8); Bi–Br2, 2.6443(8); Bi···Fe, 3.4655(10); Br1–Bi–Br2, 95.86(2), C1–Bi–Br1, 94.74(18), C1–Bi–Br2, 101.48(16). (c) Theoretically optimized structure at B3PW91/[6‐31G(d) for C,H, lanl2dz for Fe, 6‐31G(2d) for Br, TZ(2d) for Bi] level. Selected structural parameters of 2 (Å and deg): C1–Bi, 2.200; Bi–Br1, 2.688; Bi–Br2, 2.655; Bi···Fe, 3.467; Br1–Bi–Br2, 97.62, C1–Bi–Br1, 98.96, C1–Bi–Br2, 102.68.

Figure 2

Molecular structure of 1,2‐dibromodibismuthane 3. Displacement ellipsoids were drawn at the 50% probability level. Hexane molecules are omitted for clarity. (a) Total view other than hydrogen atoms. (b) Selected view with omission of hydrogen atoms and Dtp groups. Selected structural parameters of 3 (Å and deg): C1–Bi1, 2.213(11); Bi1–Bi2, 3.0129(6); Bi1–Br1, 2.8535(10); Bi1···Fe1, 3.2997(15); C2–Bi2, 2.229(10); Bi2–Br2, 2.6889(11); Bi2···Br1, 3.4586(11). (c) Theoretically optimized structure at the B3PW91/[6‐31G(d) for C,H, lanl2dz for Fe, 6‐31G(2d) for Br, TZ(2d) for Bi] level. Selected structural parameters of 3 (Å and deg): C1–Bi1, 2.212; Bi1–Bi2, 3.016; Bi1–Br1, 2.755; Bi1···Fe1, 3.358; C2–Bi2, 2.224; Bi2–Br2, 2.727; Bi2···Br1, 3.359.

1,2‐Dibromodibismuthane 3 exhibits unique unsymmetrical structure with one bridging bromine atom (Fig. 2). In the crystalline state of 3, the Bi1–Bi2 bond length is 3.0129(6) Å, which is slightly longer than but almost comparable to that of Ph₂Bi–BiPh₂ (2.990 Å) 12, suggesting the existence of a considerable Bi–Bi σ‐bond. The structural features around the Bi1 moiety are similar to those of dibromobismuthane 2. That is, the Bi–Br1 bond is close to be vertical to the tethered Cp plane, and the Bi1 seems to get closer to the Fe atom with the relatively short Fe···Bi distance of 3.2997(15) Å, which is shorter than that of 3, indicating the more effective interaction. On the other hand, the Bi2 is far from the Fe atom without any effective contact (Bi2···Fe2, ca. 4.07 Å), and it is slightly deviated from the Cp plane by 16.8° toward the opposite site from the Fe2 moiety, probably due to the repulsion between the Br2 and Fe2 moiety (Br2···Fe2, ca. 4.6 Å). The most striking structural feature in 3 should be the intramolecularly bridging Br2 atom between Bi1 and Bi2 atoms (Br1···Br2, 3.4586(11) Å) probably due to the weak coordination of the lone pair of Br1 the σ* orbital of the Bi2–Br2 bond. Even in a solution, the ¹H NMR spectrum of C₆D₆ solution of 3 suggested its unsymmetrical structure. That is, it should be noted that signals for the free Cp (C₅H₅) moieties of its two Fc* groups were observed independently [δ = 3.78 (s, 5H), 4.25 (s, 5H)], and signals for the β‐protons of the Cp rings tethered with Bi–Br moieties appeared as two AB quartet signals [δ = 4.45 (d, 1H, ³J_HH = 2.5 Hz), 4.56 (d, 1H, ³J_HH = 2.5 Hz), 4.61 (d, 1H, ³J_HH = 2.0 Hz), 4.70 (d, 1H, ³J_HH = 2.0 Hz)]. Such spectral features should be explained by the unsymmetrical structure of 3 in the solution similar to that in the crystalline state.

Since we wonder whether the bromo‐bridging structure would somewhat stabilize the 1,2‐dibromodibismuthane, the following calculations have been performed. The unique Br‐bridged structure of 3 was reproduced in the theoretically optimized structure of the less hindered model of 1,2‐dibromo‐1,2‐diferrocenyldibismuthane, meso‐Fc(Br)Bi–Br(Br)Fc (4‐C1) (Fig. 3) (B3PW91/6–31G(d) [6‐31G(2d) for Br, Lanl2dz for Fe, TZ(2d) for Bi]). The optimized structure of 4 with Ci symmetry (4‐Ci) exhibits less stable SCF energies than 4‐C1 by 2.2 kcal/mol. In addition, the calculated Bi–Bi bond dissociation energy of 4‐C1 (34 kcal/mol) is slightly higher than that of 4‐Ci (32.5 kcal/mol) (the bond dissociation energies have been calculated with considering basis set superposition error corrections), though both of the Bi–Bi bond lengths are almost the same (2.985 Å). Thus, the intramolecular Br‐coordination would contribute to slight extent to fasten the Bi–Bi bond. Interestingly, the corresponding phenyl derivative, meso‐Ph(Br)Bi–Bi(Br)Ph (5), was computed to give the minimum structure with Ci symmetry (5‐Ci), whereas the Br‐bridged structure of 5‐C1 was not found at the energy minimum. Thus, the intramolecular‐Br bridging in 4‐C1 would be characteristic of a ferrocenyl‐substituted dibromodibismuthane. It should be noted that not the ferrocenyl‐substitution itself but the Br‐bridging structure caused by the ferrocenyl‐substitution would be helpful for the slightly fastened Bi–Bi bond in 4‐C1, because the calculated Bi–Bi bond dissociation energy of 5‐Ci (32.1 kcal/mol) is similar to that of 4‐Ci. The computed frontier orbitals (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) of real molecule 3 are shown in Fig. 4. The optimized structural parameters of 3 were in good agreement with those experimentally observed (Fig. 3), and the HOMO and LUMO were found to be dominantly composed of the d orbital of the Fe moiety and the conjugated Bi–Br σ* orbitals, respectively. While the UV–vis spectra of 3 in hexane showed three characteristic absorptions at λ_max = 293 (ε 12,000), 373 (ε 6,200), and 503 (ε 2,500) nm, the longest absorption (λ_max = 503 nm) should contain the contribution of the HOMO–LUMO d–σ* electron transitions to some extent on the basis of time‐dependent density functional theory calculations (TD‐B3PW91/[6‐31G(d) for C,H, lanl2dz for Fe, 6‐31G(2d) for Br, TZ(2d) for Bi]//opt level). Several numbers of excited states due to electron transitions from d electrons (Fe) to the σ* and π*(aryl and Cp moieties) were calculated around 518–549 nm.

Figure 3

Theoretically optimized structures of 1,2‐dibromodibismuthanes (a) 4‐C1, (b) 4‐Ci, and (c) 5‐C1. Selected bond lengths (Å): (a) 4‐C1, Bi1–Bi2, 2.985; Bi1–Br1, 2.708; Bi2–Br2, 2.749; Bi2···Br1, 4.502. (b) 4‐Ci, Bi1–Bi1*, 2.984; Bi1–Br1, 2.675 (c) 5‐C1, Bi1–Bi1*, 2.980; Bi1–Br1, 2.667.

Figure 4

Depictions of calculated LUMO and HOMO of 1,2‐dibromodibismuthane 3 (B3PW91/[6‐31G(d) for C,H, lanl2dz for Fe, 6‐31G(2d) for Br, TZ(2d) for Bi] level).

CONCLUSIONS

We demonstrated that the bromination of the extremely hindered 1,2‐bis(ferrocenyl)dibismuthene 1, Fc*Bi=BiFc*, with BiBr₃ gave the corresponding 1,2‐dibromodibismuthane. The newly obtained dibromodibismuthane 3 was structurally characterized, showing its unique Br‐bridging structure. Further investigation is currently in progress to elucidate the physical properties of dibromodibismuthane 3.

EXPERIMENTAL

General Procedures

All reactions were carried out under an argon atmosphere or in a degassed and sealed tube, unless otherwise noted. All solvents were purified by standard methods and then dried by using an Ultimate Solvent System (Glass Contour) 13. Benzene‐d₆ for the NMR spectroscopy was dried by using a potassium mirror prior to use. ¹H NMR (300 MHz) and ¹³C NMR (75 Hz) spectra were measured in C₆D₆ with a JEOL AL‐300 spectrometer using C₆HD₅ (δ = 7.15 ppm) for ¹H NMR spectra, and C₆D₆ (δ = 128.0 ppm) for ¹³C NMR spectra as internal standards, respectively. High‐resolution mass spectrometry data were obtained from a JEOL JMS‐700 spectrometer (FAB) or a Bruker microTOF (APPI‐TOF). Electronic spectra were recorded on a Shimadzu‐1700 UV–vis spectrophotometer. All melting points were determined on a Büchi melting point M‐565. All elemental analyses were performed in the Microanalytical Laboratory of the Institute for Chemical Research, Kyoto University, Kyoto, Japan. 1,2‐Bis(ferrocenyl)dibismuthene 1 was prepared according to the reported procedure 10.

Synthesis of 1,2‐Dibromo‐1,2‐bis(ferrocenyl)dibismuthane (3)

To a THF (1.0 mL) solution of 1,2‐bis(ferrocenyl)dibismuthene 1 (20.6 mg, 13.0 μmol), BiBr₃ (3.0 mg, 6.69 μmol) at room temperature was added. After the reaction mixture was stirred for 48 h, the color of the solution changed to purple. The solvent was evaporated under reduced pressure, and hexane (20 mL) was added to the residue. The hexane solution was filtrated through Celite, and after the removal of the solvent the residue was recrystallized from Et₂O/hexane to afford 1,2‐dibromo‐1,2‐bis(ferrocenyl)dibismuthane 3 (7.4 mg, 4.4 μmol, 34%), 1: purple crystals; 220°C (decomp.); ¹H NMR (300 MHz, C₆D₆) δ = 1.22 (s, 36H), 1.42 (s, 36H), 3.78 (s, 5H), 4.25 (s, 5H), 4.45 (d, 1H, ³J_HH = 2.5 Hz), 4.56 (d, 1H, ³J_HH = 2.5 Hz), 4.61 (d, 1H, ³J_HH = 2.0 Hz), 4.70 (d, 1H, ³J_HH = 2.0 Hz), 6.88 (brs, 2H), 7.19 (brs, 2H), 7.30 (brs, 1H), 7.39 (brs, 2H), 7.41 (brs, 2H), 7.54 (brs, 1H), 7.61 (brs, 1H), 8.33 (brs, 1H); UV–vis, λ_max = 293 (ε 12,000), 373 (6,200), 503 (2,500) nm (hexane); HRMS (FAB), m/z: 1700.4331 ([M + H]⁺), calcd. for C₇₆H₉₉⁸¹Br₂⁵⁴Fe⁵⁶FeBi₂: 1700.4340.

X‐Ray Crystallographic Analysis of 2 and [3·1.5(hexane)]

The intensity data were collected on a Rigaku Saturn70 CCD(system) with VariMax Mo Optic using MoKα radiation (λ = 0.71070 Å). Single crystals suitable for X‐ray analysis were obtained by slow recrystallization from hexane. The single crystals were mounted on a glass fiber. The structures were solved by a direct method (SHELXS‐97) 14 and refined by full‐matrix least‐squares procedures on F² for all reflections (SHELXL‐97) 15. All hydrogen atoms were placed using AFIX instructions, whereas all other atoms were refined anisotropically. Crystal data for 2: C₃₈H₄₉BiBr₂Fe, M = 930.42, T = 103(2) K, monoclinic, P2₁/n (no. 14), a = 10.9157(3) Å, b = 32.1469(9) Å, c = 10.9652(4) Å, β = 104.373(3)°, V = 3727.3(2) Å³, Z = 4, D_calc = 1.658 gcm⁻³, μ = 7.271 mm⁻¹, 2θ_max = 51.0, 30,209 measured reflections, 6851 independent reflections (R_int = 0.0566), 6/4 refined parameters, goodness of fit (GOF) = 1.075, R₁ = 0.0418 and wR₂ = 0.0997 [I > 2σ(I)], R₁ = 0. 0553 and wR₂ = 0.1104 [for all data], largest difference peak and hole 3.957 and –1.424 e.Å⁻³. Crystal data for [3·1.5(hexane)]: C₈₅H₁₁₉Bi₂Br₂Fe₂, M = 1830.28, T = 103(2) K, monoclinic, P2₁/n (no. 14), a = 19.7568(6) Å, b = 20.0197(8) Å, c = 21.4201(9) Å, β = 107.987(3)°, V = 8058.1(5) Å³, Z = 4, D_calc = 1.509 gcm⁻³, μ = 5.740 mm⁻¹, 2θ_max = 51.0, 66,467 measured reflections, 14,941 independent reflections (R_int = 0.1088), 847 refined parameters, GOF = 1.017, R₁ = 0.0624 and wR₂ = 0.1447 [I > 2σ(I)], R₁ = 0.1079 and wR₂ = 0.1704 [for all data], largest difference peak and hole 3.260 and –1.959 e.Å⁻³. CCDC 978280 and 978281 contains the supplementary crystallographic data for 2 and [3·1.5(hexane)], respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223‐336‐033; or e‐mail: [e-mail].