DOI: 10.1002/chem.201201317 Synthesis, Structure, and Reactivity of Borole-Functionalized Ferrocenes Holger Braunschweig,* [a] Ching-Wen Chiu, [a] Daniela Gamon, [a] Martin Kaupp, [b] Ivo Krummenacher, [a] Thomas Kupfer, [a] Robert Müller, [b] and Krzysztof Radacki [a] Introduction The remarkable linear, nonlinear, and electrooptical proper- ties of boron-containing polymeric materials have stimulated significant interest in this area of research. [1–3] Particularly, systems containing tri-coordinated boron centers have proven their potential for diverse applications such as OLEDs, solar cells, and anion sensing. [1] The vacant p z orbi- tal at boron is readily accessible, which endows these mate- rials with strong Lewis acidity and promotes efficient elec- tron transport through conjugated systems. [1b, f, 4] The electro- philicity also enables the generation and isolation of numer- ous stable Lewis acid/base adducts with interesting nonlin- ear optical properties and uncommon reactivity patterns such as photoinduced rearrangement reactions. [5, 6] With ster- ically demanding Lewis bases, frustrated Lewis pairs (FLP) are formed, which have successfully been used in the activa- tion of small molecules such as H 2 . [7] In addition to the empty p z orbital at boron, boroles also feature an antiaro- matic 4p electron system, which further enhances the Lewis acidic nature of these species. In fact, boroles rank amongst the most Lewis acidic species known. Even weak donors such as ethers readily coordinate to boroles. [8] This reactivity usually involves dramatic color changes, which makes bor- oles attractive candidates for sensing applications. [9] Coordi- nation of sterically encumbered bases also offer the oppor- tunity for FLP formation. [9] In general, boroles represent highly colored species, a property that is closely related to their rather small HOMO–LUMO gap. It was nicely demon- strated that careful adjustment of the electronic properties of the exo-boron substituent provides easy access to a broad range of colors. [9] Numerous recently published articles have highlighted the unique chemistry of antiaromatic borole sys- tems. [9, 10] Their strong Lewis acidity enabled the metal-free activation of H 2 without requiring additives such as weakly coordinating Lewis bases (cf. FLPs). Thus, Piers perfluori- nated pentaarylborole was shown to be capable of readily breaking the H À H bond, even in the solid state. [10d, 11] The in- trinsic electron deficiency of boroles also becomes evident in the crystal structure of 1-ferrocenyl-2,3,4,5-tetraphenyl- Abstract: Herein, we report on the syn- thesis of ferrocenylborole [Fc- ACHTUNGTRENNUNG(BC 4 Ph 4 ) 2 ] featuring two borole moiet- ies in the 1,1’-positions. The results of NMR and UV/Vis spectroscopy and X- ray diffraction studies provided conclu- sive evidence for the enhanced Lewis acidity of the boron centers resulting from the conjugation of two borole fragments. This finding was further validated by the reaction of [Fc- ACHTUNGTRENNUNG(BC 4 Ph 4 ) 2 ] and the 4-Me-NC 5 H 4 adduct of monoborole [FcACHTUNGTRENNUNG(BC 4 Ph 4 )], which led to quantitative transfer of the Lewis base. The coordination chemistry of ferrocenylboroles was further studied by examining their reactivity towards several pyridine bases. Accordingly, the strong Lewis acidity of boroles in gen- eral was nicely demonstrated by the re- action of [FcACHTUNGTRENNUNG(BC 4 Ph 4 )] with 4,4’-bipyri- dine. Unlike common borane deriva- tives such as [FcBMe 2 ], which only forms a 2:1 adduct, we also succeeded in the isolation of a 1:1 Lewis acid/base adduct, with one nitrogen donor of 4,4’-bipyridine remaining uncoordinat- ed. In addition, the reduction chemistry of ferrocenylboroles [FcACHTUNGTRENNUNG(BC 4 Ph 4 )] and [FcACHTUNGTRENNUNG(BC 4 Ph 4 ) 2 ] has been studied in more detail. Thus, depending on the reducing agent and the reaction stoichiometry, chemical reduction of [FcACHTUNGTRENNUNG(BC 4 Ph 4 )] might lead to the migration of the borolediide fragment towards the iron center, affording dianions with either h 5 -coordinated C 5 H 4 or h 5 -coordinated BC 4 Ph 4 moieties. In contrast, no evi- dence for borole migration was ob- served during reduction of bisborole [FcACHTUNGTRENNUNG(BC 4 Ph 4 ) 2 ], which readily resulted in the formation of the corresponding tetraanion. Finally, our efforts to fur- ther enhance the borole ratio in ferro- cenylboroles aiming at the synthesis of [FcACHTUNGTRENNUNG(BC 4 Ph 4 ) 4 ] failed and, instead, gen- erated an uncommon ansa-ferrocene containing two borole fragments in the 1,1’-positions and a B 2 C 4 ansa-bridge. Keywords: boron · iron · ligand effects · reduction [a] Prof. H. Braunschweig, Dr. C.-W. Chiu, Dr. D. Gamon, Dr. I. Krummenacher, Dr. T. Kupfer, Dr. K. Radacki Institut für Anorganische Chemie Julius-Maximilians Universität Würzburg Am Hubland, 97074 Würzburg (Germany) Fax: (+ 49) 931-31-84623 E-mail : h.braunschweig@uni-wuerzburg.de [b] Prof. M. Kaupp, Dipl. Chem. R. Müller Institut für Chemie Technische Universität Berlin Straße des 17. Juni 135, 10623 Berlin (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201201317. Chem. Eur. J. 2012, 00,0–0  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! ÞÞ &1& FULL PAPER