Borylene Reduction DOI: 10.1002/anie.201303798 Reductive Borylene–CO Coupling with a Bulky Arylborylene Complex** Holger Braunschweig,* Rian D. Dewhurst, Christian Hçrl, Krzysztof Radacki, ChristopherW. Tate, Alfredo Vargas, and Qing Ye Dedicated to Professor Werner Uhl on the occasion of his 60th birthday Walter Hiebers seminal work on metal carbonyl compounds not only established these complexes as one of the most important classes of organometallics, but also disclosed important reactivity patterns, most notably the Hieber base reaction, which gives access to anionic carbonyl complexes. [1] Such carbonylates, for example, [Fe(CO) 4 ] 2 (Collmans reagent) [2] or [Cr(CO) 5 ] 2 , [3] have found widespread applica- tion in organic and organometallic synthesis, especially for the formation of transition-metal–element bonds, yielding for example, silylene (L x M = SiR 2 ), [4] gallylene (L x M  GaR), [5] or borylene complexes. [6] The latter have attracted consid- erable interest in the past decade owing to their isoelectronic relationship to carbonyls [7] and their importance for borylene- based functionalizations, such as transmetallation, [8] transfer to unsaturated organic substrates, [9] coupling reactions, [10] or metathesis. [11] Despite the increasing number of reactivity patterns emerging from borylene complexes, and the signifi- cance of the aforementioned carbonylates, it is surprising that no attempts have been made towards corresponding anionic borylene complexes. Herein we report the generation of a unique bulky terminal arylborylene [(OC) 5 Cr(BAr’)] (Ar’ = 2,6-(2,4,6-iPr 3 C 6 H 2 ) 2 C 6 H 3 ) and compare aspects of its bonding and unprecedented reduction chemistry with the better- known aminoborylene [(OC) 5 Cr{BN(SiMe 3 ) 2 }]. [6a] Reaction of Na 2 [Cr(CO) 5 ] with Cl 2 BAr’ in a facile salt elimination reaction led to the formation and subsequent isolation of [(OC) 5 Cr(BAr’)] as a yellow crystalline solid in 75 % yield (Scheme 1). The route was also successful with Br 2 BAr’ but led to lower yields of isolated product (23 %). The 11 B NMR shift of the compound was identified at d B = 150 ppm, falling between values acquired for chromium amino- (d B = 92 ppm) [6a] and hypersilylborylenes (d B = 204 ppm). [12] It is also very close to the shifts reported for trans-[(Me 3 P)(OC) 3 Fe(BDur)] (d B = 146 ppm) [13] and [h 5 - C 5 Me 5 Fe(CO) 2 (BMes)] + [BAr f 4 ]  (d B = 145 ppm) [6c] com- plexes (Dur = 2,3,4,5-Me 4 C 6 H, Mes = mesityl; Ar f = 3,5- (CF 3 ) 2 C 6 H 3 . To confirm the identity of 1, its molecular structure was determined by single-crystal X-ray diffraction (Figure 1). [23] Complex 1 adopts C 2v symmetry in the crystal. The Cr1  B1 distance is 190.4(5) pm, and the Cr1-B1-C1 angle of 175.9(4) o shows that there is a very slight deviation from a linear arrangement for these three atoms. The susceptibility of the boron center in borylene com- plexes to nucleophilic attack is well-documented and can result in the cleavage of the MB bond. [10a, 11a,b, 14] Similar reactivity has been observed in isoelectronic metal carbyne chemistry, with the other possibility being an initial attack at the metal center. [15] With this in mind, we decided to investigate the reactivity of the title compound toward the addition of electrons and compare its reduction chemistry with the aminoborylene [(OC) 5 Cr{BN(SiMe 3 ) 2 }] . Reaction of 1 with 2 equiv of KC 8 in THF gave a vibrant red compound upon work-up and isolation in quantitative yield with no identified side products (Scheme 1). This compound was found to be extremely air- and moisture-sensitive and only stable in the presence of THF. 11 B NMR spectroscopy gave a single signal at d B = 23 ppm, which is in the typical range for 3/4 coordinate boron and the 1 H NMR spectrum indicated Scheme 1. Top: synthesis and reduction of borylene complex 1; bottom: reduction of borylene complex 3. [*] Prof. Dr. H. Braunschweig, Dr. R. D. Dewhurst, Dipl.-Chem. C. Hçrl, Dr. K. Radacki, Dr. C.W. Tate, Dr. A. Vargas, Dr. Q. Ye Institut für Anorganische Chemie Julius-Maximilians-Universität Würzburg Am Hubland, 97074 Würzburg (Germany) E-mail: h.braunschweig@uni-wuerzburg.de Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/ Braunschweig/ [**] Financial support from the European Research Council (ERC Advanced Grant to H.B.) is gratefully acknowledged. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201303798. . Angewandte Communications 10120  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2013, 52, 10120 –10123