H 2 Activation DOI: 10.1002/anie.201202218 Hydrogen Activation by an Intramolecular Boron Lewis Acid/ Zirconocene Pair** Santhosh Kumar Podiyanachari, Roland Frçhlich, Constantin G. Daniliuc, Jeffrey L. Petersen, Christian Mück-Lichtenfeld, Gerald Kehr, and Gerhard Erker* Small-molecule activation is a very important feature in stoichiometric synthetic chemistry as well as in catalysis. The cleavage and activation of dihydrogen is a typical example. [1] In many molecular systems hydrogen activation is carried out by oxidative addition of the H 2 molecule to a single transition- metal center, the Wilkinson catalyst being a typical example. [2] Alternatively transition-metal centers can split dihydrogen heterolytically with the assistance of a proton accepting main- group-element Lewis base, variants of the Noyori catalysts being typical examples. [3] We have now prepared a system featuring an inverse pair of functionalities, namely a bifunc- tional Group 4 metallocene/borane system where the main- group-element component may serve as a hydride acceptor. Herein we describe the synthesis of this new zirconocene/ RB(C 6 F 5 ) 2 pair and its reaction with dihydrogen under mild conditions. We first treated the aldimido zirconocene complex 1 [4] with trimethylsilylethynyl lithium. Product 2 (see Scheme 1) was isolated in over 90 % yield as a yellow solid. It was characterized by X-ray diffraction (see the Supporting Information) and shown to contain a close to linear “met- alla-2-aza allene” type unit. Subsequent treatment of compound 2 with one molar equivalent of “Piers borane” [HB(C 6 F 5 ) 2 ] [5] gave complex 4 (isolated in over 80 % yield; see Scheme 1 and Figure 1). Apparently, compound 4 is the product of a 1,1-hydrobora- tion reaction [6, 7] of HB(C 6 F 5 ) 2 with the pendant trimethylsilyl acetylide s-ligand. This reaction is typically initiated by s- alkynyl ligand transfer from zirconium to the boron Lewis acid. [7] Hydride shift from boron to carbon followed by coordination of the imino nitrogen atom to the boron Lewis acid then straightforwardly leads to 4. We then investigated the formally related, but in some essential aspects decidedly different reaction of the s-alkenyl/ s-alkynyl zirconocene complex 5 with HB(C 6 F 5 ) 2 . [8] Complex 5 was prepared by hydrozirconation of tert-butylacetylene [9] followed by treatment with the Li-C C-SiMe 3 reagent (see Scheme 2). It was then treated with one molar equivalent of HB(C 6 F 5 ) 2 in benzene at room temperature (12 h). Workup gave the product 7 as a dark red solid in 86 % yield. The X-ray crystal structure analysis revealed that one equivalent of tert- butylethene had apparently been eliminated during the reaction leaving the substituted 2-boryl-1-silylacetylene zir- conocene complex 7 behind (see Figure 2). Both the acety- lenic carbon atoms are strongly bonded to zirconium (Zr1– C1: 2.275(2), Zr1–C2: 2.390(2), C1–C2: 1.281(3) ). The C1– Scheme 1. Preparation of complexes 2 and 4 ; Cp = C 5 H 5 . Figure 1. Molecular structure of compound 4. Selected bond lengths [] and angles [8]: Zr1–C1 2.165(4), Zr1–C2 2.513(4), Zr1–N1 2.277(3), C1–C2 1.338(5), B1–C2 1.693(5), Si1–C1 1.872(4), N1–C3 1.269(4), B1–N1 1.578(5); C1-Zr1-N1 101.8(1), B1-N1-Zr1 93.2(2), C1- Zr1-C2 32.1(1), N1-Zr1-C2 69.7(1), C2-C1-Si1 121.5(3), C2-C1-Zr1 88.4(2), B1-C2-Zr1 82.5(2). [19] [*] Dr. S.K. Podiyanachari, Dr. R. Frçhlich, [+] Dr. C. G. Daniliuc, [+] Dr. C. Mück-Lichtenfeld, [++] Dr. G. Kehr, Prof.Dr. G. Erker Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Correnstrasse 40, 48149 Münster (Germany) E-mail: erker@uniso-muenster.de Prof. Dr. J. L. Petersen [+] Bennett Department of Chemistry, West Virginia University P.O. Box 6045, Morgantown, WV 26506 (USA) [ + ] X-ray crystal structure analyses [ ++ ] Computational chemistry [**] Financial support from the Deutsche Forschungsgemeinschaft is gratefully acknowledged. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201202218. A ngewandte Chemi e 1 Angew. Chem. Int. Ed. 2012, 51,1–6  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! Ü Ü