B Exchange: A Versatile Route to π

Communication pubs.acs.org/JACS

Catalytic B−C Coupling by Si/B Exchange: A Versatile Route to π‑Conjugated Organoborane Molecules, Oligomers, and Polymers Artur Lik, Lars Fritze, Lars Müller, and Holger Helten* Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany S Supporting Information *

this, the boron centers of the reactants must be particularly kinetically stabilized to survive the cross-coupling process. Recently, Liu, Jäkle, and co-workers applied an AB type procedure (type b; i.e., using a single monomer containing both complementary functional groups required for condensation) to synthesize an azaborine polymer.5d Routes c−e, on the other hand, make use of B−C bond forming processes.6−8 The currently most sophisticated method available, developed by Jäkle and co-workers, uses distannyl compounds (X = SnMe3) in combination with diboryl species (c) or boranes (d) with Y = Br.7 Polycondensations via such Sn/B exchange reactions generally proceed under mild conditions, which is advantageous for controlled polymerizations. A major drawback of the use of organotin compounds, however, is their pronounced toxicity. This prompted us to seek for a suitable alternative. We considered organosilicon compounds as the ideal candidates. Si/B exchange reactions are generally slower than analogous Sn/B processes. For instance, PhSiMe3 and BBr3 react smoothly at room temperature to yield PhBBr2. However, subsequent replacement of a second bromine by a phenyl group using PhSiMe3 to give Ph2BBr requires harsh reaction conditions, i.e., heating of the neat reactants at 180 °C in a sealed ampule for 24 h.12 Certainly, these conditions are not suitable for controlled polymerizations, but we speculated that it might be possible to initiate such processes under milder conditions by a strong reagent. Herein, we demonstrate the use of Si/B exchange as a facile route to conjugated organoborane molecules, oligomers, and polymers.13 Si/B exchange condensations are effectively initiated at ambient temperature with the electrophilic silyl reagents Me3SiOTf and Me3SiNTf2 (Tf = SO2CF3). This can be performed catalytically, and in some cases, AB type monomers can be generated and subsequently oligomerized through this approach in a controlled manner. We started our investigations with the reaction between 2(trimethylsilyl)thiophene (1a) and 2-(dibromoboryl)thiophene (2a) to give bromo-bis(thien-2-yl)borane (3a;5c Figure 2). In fact, some condensation was observed by 1H NMR spectroscopy at ambient temperature in CH2Cl2 already without an initiator added.14 However, the reaction did not go to completion within 12 d (Figure 3 and Supporting Information (SI), Figure S3). We anticipated that it could be accelerated through the presence of a halogen-abstracting reagent. For this, we chose Me3SiOTf and Me3SiNTf2, which we applied in

ABSTRACT: Conjugated organoboranes have emerged as attractive hybrid materials for optoelectronic applications. Herein, a highly efficient, environmentally benign catalytic B−C bond formation method is presented that uses organosilicon compounds, dibromoboranes, and the metal-free organocatalyst Me3SiNTf2. This Si/B exchange approach has been successfully applied to the synthesis of arylborane molecules 4a−c, oligomers 8a,b, and polymers 8a′,b′. Photophysical investigations, supported by TDDFT calculations, reveal highly effective π-conjugation in thienyl- and furylborane species; the latter are also highly emissive. π-Conjugated organoboranes have emerged as an important class of organic−inorganic hybrid compounds in materials science.1,2 Intriguing optical and electronic features result from the incorporation of the vacant p orbital of boron into extended π-systems, thus often leading to highly emissive and/or effectively electron-transporting materials.1,2 This has enabled applications in organic (opto)electronic devices (OFETs, OLEDs, photovoltaic cells)1 or as chemosensors for certain anions or amines.1,3 In recent years, donor−acceptor-type conjugated oligomers and polymers comprising trivalent boron centers alternating with electron-rich π-systems came into the focus of intense research activity.1a,d−f,4−11 Rational synthetic routes to cyclolinear extended species that feature B−Caryl linkages follow one of the general polycondensation strategies a−e (Figure 1).4 The types a and b utilize transition metal-catalyzed C−C coupling reactions of organoborane (co)monomers.5,11 For

Figure 1. Strategies for the synthesis of arylborane oligomers or polymers (Ar, Ar′ = arenes; R = organic substituent; X/Y = complementary leaving groups). © 2017 American Chemical Society

Received: February 21, 2017 Published: April 10, 2017 5692

DOI: 10.1021/jacs.7b01835 J. Am. Chem. Soc. 2017, 139, 5692−5695

Communication

Journal of the American Chemical Society

Figure 2. Synthesis of 4a−c via catalytic B−C coupling.

Figure 4. Molecular structures of 4b,c in the solid state (H atoms omitted).

oligofurans, which also show greater planarity than analogous oligothiophenes.15 The twist angle between the phenyl groups in 4c, on the other hand, amounts to 43.8°. The furan derivative 4b also shows the shortest B−C bond lengths [d(B− Cfuryl) = 1.527(6), 1.529(6), 1.542(5), 1.547(5) Å]. The B− Cphenyl bonds of 4c are significantly longer (1.579(3), 1.581(3) Å), i.e., in the same range with the B−Cthienyl bonds of Mes*B(2-thienyl)2 (1.589(6), 1.568(4) Å).5c The UV−vis spectra of 4a,b (Figure 5; Table 1) display a major band, which is, according to our TD-DFT calculations

Figure 3. Consumption of 1a (by 1H NMR) during the reaction with 2a, without catalyst (red), with 5 mol% of Me3SiOTf (blue), and with 5 mol% of Me3SiNTf2 (green).

substoichiometric amounts (5 mol%). Me3SiOTf yielded significant rate acceleration with 95% conversion after 5 d. Finally, the stronger electrophilic species Me3SiNTf2 proved to be a highly efficient catalyst: complete and selective conversion to 3a was observed within 72 min. The volatile condensation byproduct, Me3SiBr, was easily removed at reduced pressure. Subsequent derivatization of 3a with TipLi (Tip = 2,4,6triisopropylphenyl) afforded the air- and moisture-stable triarylborane 4a. As furan-containing organoborane oligomers and polymers were unknown, so far, we carried out the reaction of 1b with 2b in the presence of Me3SiNTf2 (5 mol%). This yielded full conversion to 3b within less than 2 h at ambient temperature. After reaction with Mes*Li (Mes* = 2,4,6-tri-tert-butylphenyl), 4b was obtained, which was purified via column chromatography. The bulky Mes* substituent was chosen in this case because it was also effective in stabilizing a furylborane polymer (8b′; see below). We further considered as reactants the less activated phenyl species 1c and 2c. Condensation thereof proceeded significantly slower and required increased substrate concentration (4 M) and higher catalyst loadings (25 mol%). Nevertheless, the reaction was fully selective with 95% conversion to 3c after 3 d at room temperature. In this case, the final Mes*-substituted product, 4c, turned out to be air-sensitive, but we succeeded in isolating it via crystallization. The constitution of 4a−c was unambiguously ascertained by NMR spectroscopy and mass spectrometry. The solid state structures of 4b,c were determined by singlecrystal X-ray diffraction (Figure 4). The asymmetric unit of 4b contains two independent molecules with similar metrical data. In both 4b,c the Mes* group is nearly perpendicular oriented to the respective BC3 plane [dihedral angles: 83.0°/87.0° (4b), 83.4° (4c)]. The furan rings of 4b are almost perfectly coplanar, adopting anti-conformation. The interplanar angle between these rings is only 4.6° and 7.7°, respectively, which is even smaller than that between the thiophene rings in Mes*B(2thienyl)2 (19.0°).5c This parallels observations made for

Figure 5. UV−vis absorption spectra of 4a/8a′ and 4b/8b′ in THF.

Table 1. Photophysical Dataa for 4a−c, 8a,b, and 8a′,b′, and GPC Datab for 8a,b and 8a′,b′

a

compd

λabs,max/nm

λem,max/nm

Φf/%

Mn

Mw

DPn

4a 4b 4c 8a 8a′ 8b 8b′

325 315 271 409 412 407 411

410 400 − 421 455 422 426

3.1 24.0 − 0.6 0.8 70.3 71.1

− − − 1550 3270 1880 3260

− − − 1760 4100 2890 5220

− − − 4 10 5 9

In THF, except for 4c (in CH2Cl2). standards.

b

In THF, vs polystyrene

(B3LYP-D3(BJ)/def2-SV(P)),16 assigned to a π−π* transition from the HOMO−2 to the LUMO (Figure 6; the HOMO → LUMO process is of low probability). The HOMO−2 is delocalized over the hetaryl rings, while the LUMO shows significant contribution from the pπ orbital on boron. Hence, the excitation process is associated with intramolecular charge transfer (ICT) from the hetaryl groups to the boron center. When going from the thiophene (4a) to the furan system (4bTip), the energy levels of the relevant orbitals are increased to approximately the same degree, resulting in similar excitation energies. This correlates with the more electron-rich nature of furan vs thiophene. The absorption band of compound 4c appears at significantly higher energy. While 4c is basically 5693

DOI: 10.1021/jacs.7b01835 J. Am. Chem. Soc. 2017, 139, 5692−5695

Communication

Journal of the American Chemical Society

point during the polycondensation process. The products 8a,b, obtained after incorporation of the aryl side groups, however, were readily soluble in organic solvents such as CH2Cl2, THF, and n-pentane. In view of this solubilizing effect of the Tip and Mes* substituents, we decided to explore polymerizations starting from 9a,b, in which the growing polymer chain, 10a,b, carries one of these groups on every second boron atom and should, therefore, show increased solubility (Route B). For the reaction of 9a we also changed the solvent to the more polar odichlorobenzene (o-DCB), from which we anticipated further enhancement of the solubility of the macromolecular intermediate 10a. Precipitation of 10a was observed after 2 d. It was subjected to centrifugation/decantation prior to derivatization in toluene with TipLi to 8a′. In an attempt to adopt analogous conditions for the synthesis of 8b′, the intermediate 10b underwent decomposition during the centrifugation process. Therefore, the polymerization was repeated in CH2Cl2, which gave 8b′ after derivatization without a prior centrifugation step. The GPC analyses revealed that 8a′,b′ were of significantly increased molecular weight (Table 1). In the UV−vis spectra of 8a,b/8a,b′, the π−π* absorption band appears significantly red-shifted from that of their respective small molecule congeners 4a,b (Figure 5; Table 1). The shift is in a similar range with that of a recently reported Mes*-substituted poly(thienylborane)5f with respect to Mes*B(2-thienyl)2,5c and evidences highly effective π-conjugation. The furylborane species 8b and 8b′ showed intense blue fluorescence. In conclusion, we have developed a highly efficient, environmentally benign B−C bond formation method and demonstrated its application for the synthesis of conjugated organoborane molecules, oligomers, and polymers. Thereby, we applied a catalytic B−C coupling17 process for polycondensation for the first time. Our new approach offers several advantages over previous routes. Silyl groups are chemically robust and can be introduced into organic substrates by facile methods. The starting materials are less toxic than those employed previously. Notably, the condensation byproduct, Me3SiBr, is less harmful and easily removed. As the formation of B−C bonds is fundamental for the construction of organoboron compounds in general, we believe that our new method will be of use in various fields of chemical research.

Figure 6. Energy levels and plots of the LUMO and the HOMO−2 (isovalue: 0.04 au) of 4a and 4bTip.

nonemissive, compound 4a shows weak and the difurylborane 4b shows intense fluorescence. We then attempted polycondensations using our catalytic approach (Figure 7, Route A). Careful addition of BBr3 (1

Figure 7. Synthesis of oligomers 8a,b and polymers 8a′,b′.

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equiv) to 5a,b in n-pentane at −78 °C and subsequent warming to 0 °C selectively yielded the AB monomers 6a,b, which were obtained in considerable purity after removal of the volatiles in vacuo (see SI, Figures S35−S40). In the case of 6a, minor byproducts were removed by filtration at −40 °C. Catalytic polycondensation of 6a,b was then performed in CH2Cl2 at ambient temperature. Subsequent derivatization of 7a with TipLi gave 8a, which was purified by precipitation with ethanol. As the product obtained upon treating 7b with TipLi was found to be unstable toward ethanol, we employed Mes*Li for derivatization in this case. After workup this yielded 8b, which proved to be perfectly air- and moisture-stable. The NMR spectra of 8a,b are consistent with the proposed structures. The SiMe3 end groups, detected by 1H and 13C NMR spectroscopy, were partially cleaved off, giving rise to a new signal in the aromatic region. Analysis of 8a,b by GPC revealed that these species were oligomeric and almost monodisperse, featuring on average 4 and 5 repeat units, respectively (Table 1). A plausible reason for this is the observed precipitation of 7a,b at some

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01835. Full experimental and computational details (PDF) X-ray crystallographic data for 4b,c (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Holger Helten: 0000-0003-1273-3685 Notes

The authors declare no competing financial interest. 5694

DOI: 10.1021/jacs.7b01835 J. Am. Chem. Soc. 2017, 139, 5692−5695

Communication

Journal of the American Chemical Society

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Holthausen, M. C.; Jäkle, F.; Wagner, M. Angew. Chem., Int. Ed. 2009, 48, 4584. (10) For side chain boron-functionalized oligomers and polymers, see, e.g.: (a) Zhao, C.-H.; Wakamiya, A.; Inukai, Y.; Yamaguchi, S. J. Am. Chem. Soc. 2006, 128, 15934. (b) Li, H.; Sundararaman, A.; Venkatasubbaiah, K.; Jäkle, F. J. Am. Chem. Soc. 2007, 129, 5792. (c) Welch, G. C.; Bazan, G. C. J. Am. Chem. Soc. 2011, 133, 4632. (d) Cheng, F.; Bonder, E. M.; Jäkle, F. J. Am. Chem. Soc. 2013, 135, 17286. (e) Wan, W.-M.; Cheng, F.; Jäkle, F. Angew. Chem., Int. Ed. 2014, 53, 8934. (11) For polymers with tetra-coordinate boron, see, e.g.: (a) Meng, G.; Velayudham, S.; Smith, A.; Luck, R.; Liu, H. Macromolecules 2009, 42, 1995. (b) Nagai, A.; Chujo, Y. Macromolecules 2010, 43, 193. (c) Kim, B.; Ma, B.; Donuru, V. R.; Liu, H.; Fréchet, J. M. J. Chem. Commun. 2010, 46, 4148. (d) Novoa, S.; Paquette, J. A.; Barbon, S. M.; Maar, R. R.; Gilroy, J. B. J. Mater. Chem. C 2016, 4, 3987. (12) Haubold, W.; Herdtle, J.; Gollinger, W.; Einholz, W. J. Organomet. Chem. 1986, 315, 1. (13) We recently developed Si/B exchange polycondensation as a route to polymers with B−N linkages: (a) Lorenz, T.; Lik, A.; Plamper, F. A.; Helten, H. Angew. Chem., Int. Ed. 2016, 55, 7236. (b) Ayhan, O.; Eckert, T.; Plamper, F. A.; Helten, H. Angew. Chem., Int. Ed. 2016, 55, 13321. (c) Lorenz, T.; Crumbach, M.; Eckert, T.; Lik, A.; Helten, H. Angew. Chem., Int. Ed. 2017, 56, 2780. (14) It is conceivable that this reaction might have been initiated by an unknown species that may be present in small amounts. However, no analytical evidence for such a species could be obtained. (15) (a) Gidron, O.; Diskin-Posner, Y.; Bendikov, M. J. Am. Chem. Soc. 2010, 132, 2148. (b) Bunz, U. H. F. Angew. Chem., Int. Ed. 2010, 49, 5037. (16) For better comparison with 4a, the furylborane congener was also calculated with Ar′ = Tip (i.e., 4bTip). (17) For catalytic B−C coupling reactions yielding molecular compounds, see, e.g.: (a) Mkhalid, I. A. I.; Barnard, J. H.; Marder, T. B.; Murphy, J. M.; Hartwig, J. F. Chem. Rev. 2010, 110, 890. (b) Del Grosso, A.; Pritchard, R. G.; Muryn, C. A.; Ingleson, M. J. Organometallics 2010, 29, 241. (c) Farràs, P.; Olid-Britos, D.; Viñas, C.; Teixidor, F. Eur. J. Inorg. Chem. 2011, 2011, 2525. (d) Chow, W. K.; Yuen, O. Y.; So, C. M.; Wong, W. T.; Kwong, F. Y. J. Org. Chem. 2012, 77, 3543. (e) Zhao, D.; Xie, Z. Angew. Chem., Int. Ed. 2016, 55, 3166.

ACKNOWLEDGMENTS Support by the DFG (Emmy Noether Programme) and the COST action CM1302 (SIPs) is gratefully acknowledged. We thank Q. Guo and K.-N. Truong for X-ray diffraction data collection, Dr. K. Beckerle for GPC measurements, and Prof. Dr. J. Okuda for generous support and helpful discussions.

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REFERENCES

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DOI: 10.1021/jacs.7b01835 J. Am. Chem. Soc. 2017, 139, 5692−5695