Inorganic-Salt-Free Reduction in Main-Group Chemistry: Synthesis of

Mar 20, 2017 - Importantly, inorganic salts were not generated in this reduction: i.e., this is a unique inorganic-salt-free method for the synthesis ...
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Inorganic-Salt-Free Reduction in Main-Group Chemistry: Synthesis of a Dibismuthene and a Distibene Paresh Kumar Majhi,† Hideaki Ikeda,‡ Takahiro Sasamori,*,† Hayato Tsurugi,‡ Kazushi Mashima,‡ and Norihiro Tokitoh† †

Institute for Chemical Research, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan

‡

S Supporting Information *

ABSTRACT: A dibromobismuthine and a dibromostibine that bear 4-tBu-2,6-[CH(SiMe3)2]2-C6H2 (Tbb) groups were reduced with 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene to afford the corresponding stable dibismuthene (TbbBi BiTbb) and the distibene (TbbSbSbTbb), respectively. The only byproducts obtained were easily removable tetramethylpyrazine and bromotrimethylsilane. Importantly, inorganic salts were not generated in this reduction: i.e., this is a unique inorganic-salt-free method for the synthesis of compounds with multiple bonds between heavier main-group elements.

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have been synthesized by the reductive coupling of dihalobismuthine derivatives RBiX2 (X = Cl, Br). For the reductive coupling of dihalobismuthines, the choice of reducing agent is crucial, as the resulting RBiBiR compounds undergo facile decomposition under harsh reduction conditions:9 i.e., the use of mild reducing agents under mild reaction conditions is required to access dibismuthenes. Commonly used synthetic methods include the use of alkali or alkaline-earth metals such as Li, Na, K, and Mg.7,8 Subsequently, the resulting dibismuthenes have to be separated from inorganic salts that are inevitably generated as byproducts. Considering that dibismuthenes cannot be subjected to conventional separation techniques such as column chromatography and/or GPC on account of their extremely high reactivity toward air and moisture, the extraction into hydrocarbon solvents such as hexane or benzene, followed by filtration, seems to be the logical choice for the isolation of dibismuthenes. However, if the targeted dibismuthene is only sparingly soluble in hydrocarbon solvents, it is quite challenging to remove inorganic salts completely from the reaction mixture. For example, the reduction of TbtBiX2 (X = Cl, Br) with Mg affords a purple solid, which is insoluble in nonpolar organic solvents.7a,8 The poor solubility of this purple solid prevents unambiguous identification and structural characterization. Nevertheless, after the stable precursor (TbtBiSe)3 was developed, which was subsequently treated with P(NMe2)3, we were able to isolate the first stable

ompounds that contain multiple bonds between heavier main-group elements are of great interest due to their unique chemical and physical properties, which include most importantly their energetically low π* and high π orbitals.1,2 For a long time, the isolation of such compounds with a “genuine” multiple bond, i.e., in the absence of stabilization from e.g. intramolecular coordination, has been considered highly challenging due to the inherently high reactivity of the multiple bonds. However, nowadays, several kinds of compounds that contain multiple bonds between heavier main-group elements are easily accessible, especially when the multiple bond is kinetically stabilized by sterically demanding groups.1,3 Since the isolation of the first stable diphosphene, Mes*PPMes* (Mes* = 2,4,6-tBu3-C6H2),4 a variety of compounds with multiple bonds between heavier group 15 elements have been isolated as stable compounds, and their characteristic properties have been extracted.1−5 Especially a dibismuthene, i.e., a compound with a BiBi π bond, has attracted much attention, as bismuth is the heaviest element among the stable elements in the periodic table.6 Despite the progress accomplished in the last decades on the chemistry of compounds with multiple bonds between group 15 elements, dibismuthene chemistry is still in its infancy and remains a challenging research area. Progress in this area of research has been slowed particularly by a lack of appropriate synthetic methods and the generally low stability of Bi(I) compounds. With that said, it is hardly surprising that only few stable dibismuthenes have been isolated (Scheme 1),7 since the isolation of the first stable dibismuthene TbtBiBiTbt (Tbt = 2,4,6-[CH(SiMe3)2]3-C6H2).8 In most cases, dibismuthenes © XXXX American Chemical Society

Received: February 24, 2017

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DOI: 10.1021/acs.organomet.7b00147 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Examples of Isolated Stable Dibismuthenes I−X

Scheme 2. Synthesis of Dibismuthene 4 and Distibene 5

induce a reaction. However, vigorous stirring of the reaction mixture for 1 h resulted in a turbid solution with a discernible color change from light yellow to deep greenish yellow. After it was stirred for another 13 h, the crude mixture was filtered and the residue was washed with n-hexane to furnish pure distibene 5 in 87% yield. Thus, the salt-free reduction using 1 is able to induce the reductive coupling of dibromobismuthines and dibromostibines, although the former proceeds more quickly than the latter. To investigate the difference in the reactivity between dibromobismuthine 2 and dibromostibine 3, we carried out DFT calculations. The theoretically optimized structural parameters of 2 and 3, calculated at the (U)B3PW91/TD(2d)(Sb,Bi)/6-311G(d)(C,H,Br,Si) level of theory, are consistent with those experimentally determined by single-crystal X-ray diffraction analyses (Figure 1).12 It

dibismuthene, TbtBiBiTbt, owing to the high solubility of the byproduct SeP(NMe2)3 in nonpolar organic solvents. Later, we discovered that the aforementioned purple solid obtained from the reduction of TbtBiX 2 with Mg is indeed dibismuthene.7a,8 However, in the absence of an alternative synthetic methodology, it was very difficult to identify the first ever isolated dibismuthene. Such serious problems in the synthesis and purification are not confined to dibismuthenes and affect most compounds with multiple bonds of heavier main-group elements. Therefore, the development of alternative reagents for the reductive coupling of dihalides that do not generate any inorganic salts as byproducts is highly desirable. In this context, 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (1)10 is a suitable prospect for the reductive coupling of main-group-element dihalides, given previous reports which demonstrate that 1 is able to reduce SbCl5 and BiCl3 to Sb(0) and Bi(0), respectively.10b Herein, we report the inorganic-salt-free reductive coupling of aryldibromobismuthine 2 and aryldibromostibine 3 to furnish the corresponding dibismuthene 4 and distibene 5. When dibromobismuthine 2, which was obtained from the reaction of TbbLi11 with BiBr3, was treated with 1 equiv of 1 in C6H6 at ambient temperature, the reaction mixture immediately turned deep violet with concomitant precipitation, indicating the facile formation of dibismuthene 4. The crude mixture included the expected dibismuthene 4, bromotrimethylsilane, and 2,3,5,6-tetramethylpyrazine. After filtration of the crude mixture, followed by washing with n-hexane in order to remove BrSiMe3 and tetramethylpyrazine, pure dibismuthene 4 was obtained in 81% yield (Scheme 2). In contrast, treatment of dibromostibine 3 with 1 under identical conditions did not

Figure 1. Molecular structures of (a) dibromobismuthine 2, (b) dibismuthene 4, (c) dibromostibine 3, and (d) distibene 5 (atomic displacement parameters set at 50% probability; hydrogen atoms omitted for clarity).

should be noted that 2 and 3 exist as monomers in the crystalline state, while previously reported aryldibromobismuthines and -stibines form halogen-bridged oligomers due to their high Lewis acidity.13 However, the differences in their reactivity toward 1 cannot be rationalized in terms of the energy levels of the LUMOs of 2 (−2.218 eV) and 3 (−2.230 eV). A more feasible explanation seems to be the larger electron B

DOI: 10.1021/acs.organomet.7b00147 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

Natsume at Nagoya University for the expert manufacturing of custom-tailored glassware. Computation time was provided by the Supercomputer Laboratory at the Institute for Chemical Research (Kyoto University).

affinity (EA) of 2 (2.360 eV) relative to that of 3 (2.208 eV). The reactivity difference should thus not be attributed to steric reasons, as calculations for less-hindered Ph models at the same level of theory showed similar EA trends (PhSbBr2, 2.029 eV; PhBiBr2, 2.236 eV). Considering previous reports on the reactivity of 1,10 the reduction of 2 should hence start with an electron transfer from 1 to 2. The molecular structures of 4 and 5 (Figure 1) exhibit a center of symmetry in the middle of the EE bond (E = Bi, Sb), and the BiBi (2.8537(5) Å) and SbSb (2.6677(3) Å) bond lengths reflect considerable double-bond character.5,7,8 The observed E−E−C angles in 4 (98.86(16)°) and 5 (100.72(7)°) are somewhat counterintuitive, given the intrinsic nature of Bi: i.e., high p character should be expected for its chemical bonds. In addition, 4 and 5 showed two characteristic absorption bands in their UV/vis spectra in n-hexane, due to their π−π* and n−π* transitions. The absorption maxima of 4 at λmax 518 nm (ε 5870) and 631 nm (ε 160) are bathochromically shifted relative to those of 5 at λmax 456 nm (ε 4060) and 580 nm (ε 80), suggesting that the BiBi π bond in 4 is weaker than the SbSb π bond in 5. In conclusion, we have demonstrated that 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (1) works as a reducing agent for the reductive coupling of dibromobismuthine 2 or dibromostibine 3. This new reducing agent broadens the available choice of reducing agents, especially for cases when the solubility of the reduced species in hydrocarbon solvents is low. We hope that this new “inorganic-salt-free reduction” should stimulate not only dipnictene chemistry but also other areas of chemistry that focus on compounds with multiple bonds between main-group elements.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00147. Synthetic details; analytical data, and NMR spectra (PDF) Crystallographic data for 2−5 (CIF) Cartesian coordinates of the theoretically optimized structures (XYZ)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail for T.S.: [email protected]. ORCID

Takahiro Sasamori: 0000-0001-5410-8488 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI grants (15H03777, 15K13640, and 24109013), grants-in-aid of Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks” [#2401] (15H00738, 15H00743). Preliminary X-ray diffraction data of 2 were collected at the BL40XU beamline of Spring-8 (JASRI, 2015B1074). P.K.M. thanks the JSPS for a postdoctoral fellowship. We thank Mr. Toshiaki Noda and Ms. Hideko C

DOI: 10.1021/acs.organomet.7b00147 Organometallics XXXX, XXX, XXX−XXX