Synthesis of Silicon and Germanium-Containing Heterosumanenes

Aug 23, 2017 - The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02254. Experimenta...
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Synthesis of Silicon and Germanium-Containing Heterosumanenes via Rhodium-Catalyzed Cyclodehydrogenation of Silicon/ Germanium−Hydrogen and Carbon−Hydrogen Bonds Dandan Zhou,† Ya Gao,† Bingxin Liu,† Qitao Tan,*,† and Bin Xu*,†,‡,§ †

Department of Chemistry, Innovative Drug Research Center, Shanghai University, Shanghai 200444, China State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China § Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China ‡

S Supporting Information *

ABSTRACT: A three-step synthesis of C3-symmetric trisilasumanene and trigermasumanene, heteroanalogues of the πbowl sumanene, was achieved using a threefold rhodiumcatalyzed cyclodehydrogenation of Si/Ge−H and C−H bonds as the key step. Trigermasumanene was proven to adopt a planar geometry by single crystal X-ray diffraction for the first time. The optical properties were also investigated by UV−vis and fluorescence spectroscopy. iloles (silacyclopentadienes) are silicon-based π-conjugated molecules that possess intriguing photophysical and electronic properties owing to their low-lying LUMO derived from σ*−π* conjugation between the σ* orbital of the two exocyclic Si−C bonds with the π* orbital.1 9-Silafluorene, a silole embedded in a biphenyl framework, has recently received much attention due to its great potentials as organic materials.2 Germanium locates below silicon in the periodic table, but due to the d-block contraction,3 it has similar covalent radii with silicon (1.22 Å versus 1.17 Å) which has subtle effects on the molecular packing and morphology compared to Si.4 As a result of the same d-block contraction effect, the electronegativity of Ge is much closer to C than that of Si which makes arylgermanes much more stable to bases and nucleophiles than the corresponding arylsilanes.5 Therefore, it is interesting to dope Si or Ge atoms to the framework of other π-extended aromatic compounds. Buckybowls are bowl-shaped aromatic hydrocarbons resembling the fragments of fullerenes or the cap of single-walled carbon nanotubes, which possess both concave and convex πsurfaces.6 Besides their unique geometry and properties, buckybowls are intriguing precursors for bottom-up synthesis of fullerene,7 uniform carbon nanotubes (CNTs),8 and warped nanographenes.9 Recent advances indicate buckybowls are promising carbon-rich materials.10 Sumanene 1, named after the Hindi word “suman” for a type of flower by Mehta et al.,11 is a C3v-symmetric buckybowl (Figure 1a), which represents a basic unit of C60. It posed a great challenge to synthetic chemists due to its deep bowl depth. In 2003, the synthesis of sumanene was achieved by Sakurai et al.12 Since then, many interesting properties have been studied, such as unique crystal packing,13 bowl-to-bowl inversion,14 metal complexes,15 elec-

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© 2017 American Chemical Society

tron mobility,16 and bowl chirality,17 and it is also a precursor or template for the synthesis of large buckybowls.18 Doping of heteroatoms to the aromatic frameworks of sumanene can drastically modulate their physical and chemical properties.19 In 2012, we reported triazasumanene by replacement of three sp2 carbons on the rim benzene rings with nitrogen atoms (Figure 1a), which shows very stable chirality ascribable to the higher bowl inversion energy (42.2 kcal mol−1) than that of sumanene (20.3 kcal mol−1).20 Sumanene has three benzylic carbons which have been replaced by various atoms such as S,21 Se,21b Te22 and P,23 leading to various heterosumanenes. Group 14 elements including Si, Ge, and Sn have also been incorporated into sumanene (Figure 1b).24 In 2010, Kawashima et al. reported the first synthesis of silasumanene from 2,3,6,7,10,11-hexabutoxytriphenylene through bromination followed by repeated lithiation/silylation/sila-Friedel−Crafts reaction (Figure 1b).24a This pioneering synthesis provided the first silasumanene but suffered from a tedious route and low total yield (∼0.7%). Furthermore, only trisilasumanene containing alkoxyl groups could be prepared by this method. Later, Saito et al. reported the synthesis of pristine silasumanene and germasumanene by repeated lithiation and silylation of the bay regions of triphenylene.24d,e However, only n-Bu-containing trisilasumanene could be prepared due to the methyl/butyl exchange under the harsh conditions (n-BuLi/ TMEDA, hexane, 60 °C). This method also suffered from low yield and tedious procedures. Therefore, the exploration of efficient and general synthesis of Si or Ge-containing sumanenes is the prerequisite for potential applications as Received: July 22, 2017 Published: August 23, 2017 4628

DOI: 10.1021/acs.orglett.7b02254 Org. Lett. 2017, 19, 4628−4631

Letter

Organic Letters Scheme 1. Synthesis of Trisilasumanene 5 and Trigermasumanene 6

(100 °C) afforded germasumanene 6 in 80% yield. Moreover, treatment of 5a with excess n-BuLi afforded all-butyl substituted silasumanene 5c in 72% yield through Me/n-Bu exchange. The single crystal of 6 suitable for X-ray crystallographic analysis was obtained by slow evaporation of its solution in hexane. It was the first germanium-containing sumanene that has been resolved by X-ray diffraction. Similar to previously reported trisilasumanene, trigermasumanene 6 has a planar framework (Figure 2a and 2b), as a result of the replacement of the benzylic carbons by larger-sized Ge atoms. The C−C bonds in the hub benzene ring alter from 1.395 to 1.460 Å, while there is no remarkable alternation of the C−C bonds in the three external benzenes, indicating that the higher aromaticity of the external benzenes than the central benzene ring. The bond

Figure 1. Sumanene and heterosumanenes.

materials. We herein report the first synthesis of pristine allmethyl substituted sila- and germasumanenes from 1,5,9triiodotriphenylene using a threefold rhodium-catalyzed cyclodehydrogenation of silicon/germanium−hydrogen and carbon−hydrogen bonds as the key step (Figure 1c). The geometry of trigermasumanene was investigated by single crystal X-ray crystallographic analysis. The synthesis of trisilasumanene and trigermasumanene is demonstrated in Scheme 1. The synthesis started from 1,5,9triaminotriphenylene 1, which was readily prepared on decagram scale from inexpensive starting materials through two operationally simple steps.25 Compound 1 was converted to 1,5,9-triiodotriphenylene 2 under classic Sandmeyer reaction conditions. Then, lithiation of 2 followed by addition of chlorodimethylsilane or chlorodiisobutylsilane afforded 3a and 3b in moderate yields. Similarly, treatment of 2 with n-BuLi and dichlorodimethylgermane followed by reduction with LiAlH4 provided compound 4 in 38% yield. Recently, several elegant examples on the synthesis of 9silafluorenes from arylhydrosilanes through C−H and Si−H activation have been reported.24a,26 In 2010, Takai et al. reported an elegant rhodium-catalyzed synthesis of silafluorene derivatives via cleavage of silicon−hydrogen and carbon− hydrogen bonds,26a which has been utilized in the synthesis various silicon-containing compounds.26b−d The treatment of 3a with a catalytic amount of Willkinson’s catalyst, RhCl(PPh3)3, and excess 3,3-dimethyl-1-butene as a hydrogen acceptor afforded the desired all-methyl substituted trisilasumanene 5a in 45% yield via a threefold cyclodehydrogenation of Si−H and C−H bonds. Similarly, trisilasumanene 5b was obtained from 3b in 52% yield. In a similar manner, the cyclodehydrogenation of 4 under relatively mild conditions

Figure 2. X-ray crystallographic analysis of germasumanene 6 (pink is the Ge atoms). (a) Top view; (b) side view; (c) packing model (spacefilled); (d) packing with C−H···π distances. Selected bond lengths [Å]: C(1)−C(2) 1.399, C(2)−C(3) 1.398, C(3)−C(4) 1.383, C(4)− C(5) 1.409, C(5)−C(6) 1.402, C(1)−C(6) 1.403, C(6)−C(7) 1.460, C(7)−C(8) 1.395, C(8)−C(9) 1.448, C(9)−C(10) 1.404, C(5)− C(10) 1.447, C(1)−Ge(1) 1.951, C(11)−Ge(1) 1.980, C(12)−Ge(1) 1.939, C(13)−Ge(1) 1.932. 4629

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Organic Letters

observation that the incorporation of silicon atoms leads to lower LUMO energy levels.1 The calculated HOMO levels match the experimental results through cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements (Figure S1 and S2, see Supporting Information). This observation also indicates that Ge-doped materials are electronically more similar to the corresponding carbon analogues than Si-doped ones, probably as a result of the dblock contraction effect. In conclusion, we have succeeded in the synthesis of trisilasumanene and trigermasumanene using a threefold Rhcatalyzed cyclodehydrogenation of Si−H/Ge−H and C−H bonds as the key step. This three-step procedure from readily available starting materials provides a shortcut to homogeneous methyl or isobutyl substituted Si or Ge containing sumanenes without substituents on their peripheral carbons, which are difficult targets through known methods. X-ray crystallographic analysis demonstrates that the germasumanene core adopts a planar geometry, and it stacks in a herringbone model mainly due to the strong intermolecular C−H···π interactions. The doping of silicon and germanium atoms significantly stabilizes the LUMOs of sumanene, while having little effect on the HOMO energy levels. This work demonstrates the power of the transition-metal-catalyzed cyclodehydrogenative reactions for the synthesis of polycyclic aromatic hydrocarbons and their heteroanalogues. Further studies of the trisilasumanene and trigermasumanene as organic materials or for the synthesis of nanographenes are in progress.29

lengths of internal C−Ge bonds range from 1.95 to 1.98 Å, while the external C−Ge bonds are significantly shorter (1.932 and 1.939 Å). The C−Ge−C bond angle is almost rectangular (89.8°). Germasumanene 6 adopts a herringbone packing mainly due to the C−H···π interactions (Figure 2c and 2d).27 The optical properties of 5a and 6 were investigated as shown in Figure 3. Compounds 5a and 6 have very similar

Figure 3. Absorption (solid line) and emission spectra (dashed line) of 5a (blue) and 6 (red) in CH2Cl2.

absorption profiles with the highest absorption peak at 270 nm (ε = 1.4 × 105 M−1 cm−1 for 5a, 9.9 × 104 M−1 cm−1 for 6) and two shoulder peaks at 260 and 293 nm. The intense absorption bands of 5a and 6 at 270 nm are slightly blue-shifted from that of sumanene (278 nm).28 Both 5a and 6 in CH2Cl2 solution emit in the violet/blue regions. To further understand the effect of heteroatom doping on the electronic properties, we performed density functional theory (DFT) calculations on 5a, 6, and sumanene. Calculation demonstrates that both 5a and 6 are planar, as in the X-ray crystal structure of 6, while sumanene is bowl-shaped expectedly. Figure 4 shows their frontier orbitals and energy

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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.orglett.7b02254. Experimental details, characterization data, NMR, MS, FT-IR spectra for all new compounds (PDF) X-ray crystallographic data for compound 6 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Qitao Tan: 0000-0002-6220-651X Bin Xu: 0000-0002-9251-6930 Figure 4. DFT calculated frontier molecular orbitals and energy levels of silasumanene 5a, germasumanene 6, and sumanene at the B3LYP/ 6-311+G(d,p) level of theory.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Nos. 21672140, 21672136, and 21302123) for financial support. The authors thank Prof. Hongmei Deng (Laboratory for Microstructures, SHU) for NMR spectroscopic measurements and Prof. Xiang He (Department of Chemistry, SHU) for X-ray crystallographic analysis. The calculations were supported by High Performance Computing Center, Shanghai University (zq4000).

levels. The HOMO and LUMO orbitals of sumanene are mainly delocalized over the triphenylene core. In comparison, the HOMOs of silasumanene 5a and germasumanene 6 are delocalized over the triphenylene core, while their LUMOs are delocalized over the whole sumanene skeleton indicating the σ*−π* conjugations of the siloles. As a result, the HOMO energy levels of 5a, 6, and sumanene are quite similar, while the LUMO energy levels decrease significantly in the order sumanene > 6 > 5a. This result is consistent with the 4630

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DOI: 10.1021/acs.orglett.7b02254 Org. Lett. 2017, 19, 4628−4631