Lewis Acids DOI: 10.1002/anie.201403700 Highly Electron-Deficient and Air-Stable Conjugated Thienylboranes** Xiaodong Yin, Jiawei Chen, Roger A. Lalancette, Todd B. Marder,* and Frieder Jäkle* Abstract: Introduced herein is a series of conjugated thienyl- boranes, which are inert to air and moisture, and even resist acids and strong bases. X-ray analyses reveal a coplanar arrangement of the thiophene rings, an arrangement which facilitates p–p conjugation through the boron atoms despite the presence of highly bulky 2,4,6-tri-tert-butylphenyl (Mes*) or 2,4,6-tris(trifluoromethyl)phenyl ( F Mes) groups. Short B···F contacts, which lead to a pseudotrigonal bipyramidal geometry in the F Mes species, have been further studied by DFT and AIM analysis. In contrast to the Mes* groups, the highly electron- withdrawing F Mes groups do not diminish the Lewis acidity of boron toward F À anions. These compounds can be lithiated or iodinated under electrophilic conditions without decomposi- tion, thus offering a promising route to larger conjugated structures with electron-acceptor character. In recent years, main-group elements have been successfully incorporated into the backbone of conjugated organic oligomers and polymers, commonly resulting in unusual properties and improved performance. [1] Among these con- jugated hybrids, boron-containing materials have attracted considerable attention. [2] As a result of interactions between the empty p orbital of boron and p-conjugated systems, desirable optical and electronic properties are achieved, which in turn enable applications in optoelectronics and sensory materials. [3] Two mesityl groups are generally suffi- cient to stabilize three-coordinate aryl or vinyl boron com- pounds with respect to attack by air, water, and most nucleophiles. [3f–n] However, a significant drawback remains the susceptibility to degradation upon attack by nucleophiles in other systems, especially in the case of conjugated organo- boranes with high Lewis acidity. [4] The apparent dichotomy that higher electron deficiency is desirable for application purposes, but also tends to induce degradation, has become an obstacle to further progress. Several strategies have been explored to stabilize organo- boranes while still promoting effective p–p conjugation. Bulky groups, such as 2,4,6-tri-isopropylphenyl (Tip) [5] and 2,4,6-tri-tert-butylphenyl (Mes*) [6] have been successfully utilized to sterically protect the boron atom from attack by nucleophiles. In an alternative approach, tricoordinate bor- anes have been embedded into cyclic and extended planar conjugated structures. [6, 7] For applications in organic elec- tronics and sensing schemes, however, it is desirable not only to increase the stability, but also to maximize the electron- acceptor properties. A promising strategy in this respect is the use of F Mes groups, [8] which combine steric hindrance with strongly electron-withdrawing character. Theoretical studies by Marder and Weber et al. showed that replacement of Mes with F Mes groups in 5,5’-bis(dimesitylboryl)-2,2’-bithiophene significantly lowers the LUMO energy. [9] We introduce herein a new series of conjugated organoboranes with Mes* and F Mes groups. The influence of the F Mes moiety is clearly demonstrated, and the organoboranes serve as promising building blocks for conjugated oligomers and polymers. To facilitate p–p conjugation with boron we utilize thiophene moieties, because their small ring size should favor adoption of a coplanar structure. Another advantage is the high reactivity of the thiophene a position which lends itself to further derivatization. The compound 1 was readily prepared starting from 2- (trimethylstannyl)thiophene by Sn–B exchange and then treatment with a bulky aryl lithium reagent, Mes*Li or F MesLi, to give the borane monomers BDT and FBDT in an overall yield of 44 % and 43 %, respectively (Scheme 1). Scheme 1. Synthesis of sterically shielded (oligo)thienylboranes. [*] Dr. X. Yin, J. Chen, Prof. R. A. Lalancette, Prof. Dr. F. Jäkle Department of Chemistry, Rutgers University - Newark 73 Warren Street, Newark, NJ 07102 (USA) E-mail: fjaekle@rutgers.edu Homepage: http://chemistry.rutgers.edu/fjaekle Prof. Dr. T. B. Marder Institut für Anorganische Chemie Julius-Maximilians-Universität Würzburg Am Hubland, 97074 Würzburg (Germany) E-mail: todd.marder@uni-wuerzburg.de [**] This material is based upon work supported by the National Science Foundation under Grant No. CHE-1112195. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201403700. A ngewandte Chemi e 9761 Angew. Chem. Int. Ed. 2014, 53, 9761 –9765  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim