Olefin Hydro-metathesis DOI: 10.1002/ange.201007254 “Hydro-metathesis” of Olefins: A Catalytic Reaction Using a Bifunctional Single-Site Tantalum Hydride Catalyst Supported on Fibrous Silica (KCC-1) Nanospheres** Vivek Polshettiwar,* Jean Thivolle-Cazat,* Mostafa Taoufik,* Francois Stoffelbach, Sebastien Norsic, and Jean-Marie Basset* Low-temperature skeletal cleavage and the formation of CC bonds are of prime importance in the petrochemical industry because the transformation of crude oil into hydrocarbons having different numbers of carbon atoms is often necessary. In this regard, the skeletal transformation of olefins into valuable products remains an important challenge in chemis- try. Any new reaction related to this challenge is important. In 1991, we discovered that the highly electrophilic early- transition-metal hydride [(SiO) 3 Zr-H] supported on silica [1–4] could activate the C  H and C  C bonds of alkanes or polyolefins and could also catalyze the hydrogenolysis of these hydrocarbons into a range of gasolines. [5] Later, in 1997, we found that the highly electrophilic silica-supported tantalum hydride [(SiO) 2 TaH] [6, 7] could transform any light alkane into its lower and higher homologues by both cleavage and formation of C  H and C  C bonds. We called this new catalytic reaction “alkane metathesis” by analogy to “olefin metathesis” [Eq. (1)]. [6–8] Herein we disclose that tantalum 2C n H 2nþ2 Ð C ni H 2ðniÞþ2 þ C nþ1 H 2ðnþiÞþ2 n  2 i ¼ 1,2, ... ðn1Þ ð1Þ hydride (TaH) supported on fibrous silica nanospheres (KCC- 1) can catalyze, in the presence of hydrogen, the direct conversion of olefins into alkanes having higher and lower numbers of carbon atoms; therefore we refer to the reaction as “hydro-metathesis” [Eq. (2)]. This novel reaction has 2 ðC n H 2n þ H 2 ÞÐ C ni H 2ðniÞþ2 þ C nþ1 H 2ðnþiÞþ2 þ C n H 2nþ2 n  2 i ¼ 1,2, ... ðn1Þ ð2Þ excellent catalytic performance and unexpectedly high turn- over numbers as compared to the now classical alkane metathesis. For the first time, this silica-supported tantalum hydride shows remarkable catalytic stability, with an excellent potential of regeneration. In the case of propane metathesis, kinetic studies carried out at very low contact time, in a continuous flow reactor, revealed that the primary products of this reaction were olefins and H 2 . [9] This observation, among many others, as well as elementary steps known in tantalum organometallic chemistry, led us to propose a mechanism based on the following key steps: 1) paraffin C  H bond activation leading to a metal/alkyl species with subsequent formation of an olefin and a metal hydride by b-hydride elimination; 2) a-hydrogen abstraction from the same metal/alkyl species leading to the formation of a metallocarbene; 3) olefin metathesis on this metallocarbene; and 4) hydrogenation of the new olefins on the metal hydride (see Scheme S1 in the Supporting Information). [9] Thus, the tantalum hydride in this metathesis reaction acts as a trifunctional single-site system (dehydrogenation/metathesis/hydrogenation). In this work, we observed that TaH/KCC-1 not only transforms any olefin in the presence of hydrogen at moderate temperatures into the expected corresponding alkane, but also transforms the same olefin into alkanes having a higher and lower number of carbon atoms. Impor- tantly, in our quest of nanocatalysts, [10] we used our recently discovered high-surface-area silica nanospheres having a unique fibrous morphology (KCC-1) as the catalyst sup- port. [11] The KCC-1-supported tantalum hydride (TaH/KCC-1) was prepared by grafting the [Ta( = CHtBu)(CH 2 tBu) 3 ] com- plex onto fibrous silica nanospheres, and then treatment under hydrogen atmosphere at 150 8C for 12 hours. Infrared (IR) spectroscopy was used to monitor the grafting by sublimation of [Ta(=CHtBu)(CH 2 tBu) 3 ] (80 8C) onto a KCC-1 silica disk, which was previously dehydroxylated under vacuum at 500 8C for 8 hours; the data indicated that the intensity of the peak corresponding to isolated silanol groups (n(Si-OH) = 3747 cm 1 ) was sharply reduced (more than 70%; Figure 1). At the same time, characteristic IR bands appeared at between 2700 and 3000 cm 1 and at 1366 and 1465 cm 1 which corresponded to the CH stretching and C  H bending vibrations, respectively, and neopentane (NpH, Np = neopentyl) was evolved in the gas phase. These obser- vations and the resistance of the IR C  H vibrations bands to a desorption process at 80 8C indicated that the Ta/hydrocarbyl complex was chemically grafted onto KCC-1. Additional treatment of the grafted Ta/hydrocarbyl complex under a [*] Dr. V. Polshettiwar, Prof.Dr. J.-M. Basset KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology, Thuwal (KSA) E-mail: vivek.pol@kaust.edu.sa jeanmarie.basset@kaust.edu.sa Dr. J. Thivolle-Cazat, Dr. M. Taoufik, Dr. F. Stoffelbach, S. Norsic UniversitØ de Lyon 1, Institut de Chimie de Lyon; CPE Lyon; CNRS, UMR 5265 C2P2, LCOMS; Bâtiment 308F 43 Blvd du 11 Novembre 1918 F-69616, Villeurbanne Cedex (France) E-mail: thivolle@cpe.fr [**] We thank ESCPE-Lyon, the CNRS, and the KAUST for financial and logistic support and Anne Baudouin for NMR spectra acquisition. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201007254. Angewandte Chemie 2799 Angew. Chem. 2011, 123, 2799 –2803 # 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim