Ligand Design DOI: 10.1002/anie.201404753 B À H, C À H, and B À C Bond Activation: The Role of Two Adjacent Agostic Interactions** Audrey Cassen, Yann Gloaguen, Laure Vendier, Carine Duhayon, Amalia Poblador-Baha- monde, Christophe Raynaud, Eric Clot, Gilles Alcaraz,* and Sylviane Sabo-Etienne* Abstract: Tuning the nature of the linker in a L ~ BHR phosphinoborane compound led to the isolation of a ruthenium complex stabilized by two adjacent, d-C ÀH and e-B sp2 ÀH, agostic interactions. Such a unique coordination mode stabil- izes a 14-electron “RuH 2 P 2 ” fragment through connected s- bonds of different polarity, and affords selective B À H, C À H, and B À C bond activation as illustrated by reactivity studies with H 2 and boranes. The design of functional ligands is an important strategy to achieve specific properties for a large variety of applications, and most notably in catalysis. Polyfunctional ligands offer many advantages: 1) several anchoring points to stabilize the metal center, 2) preferred geometrical positions for improve- ment of selectivity, 3) potential hemilabile character and access to a vacant site for further functionalization, 4) access to a large variety of functional groups with different steric and electronic properties. For example, the use of pincer-type ligands displaying a so-called non-innocent behavior led to impressive achievements in the field of small-molecule bond activation. [1] As part of our ongoing research program on the design and coordination of bifunctional L ~ BHR ligands aimed at examining the B À H activation mode at a metal center, we recently disclosed a new family of ruthenium complexes incorporating a phosphinoborane ligand, Ph 2 P ~ BH(NiPr 2 ). [2] Herein, and in the absence of any pincer-type strategy which is likely to force the coordination, [3] we report the first complex exhibiting two adjacent agostic interactions of a different nature, h 2 -C À H and h 2 -B À H. This unique coordi- nation mode affords selective B ÀH, C ÀH, and B ÀC bond activation as demonstrated by reactivity studies. The new phosphino(benzyl)(amino)borane 1 was pre- pared and added to the bis(dihydrogen) ruthenium complex 2 in toluene at room temperature (Scheme 1). After work-up and precipitation in cold pentane at À55 8C, a yellow powder, analyzed as [RuH 2 {h 2 -HBDh 2 -HC-HB(NiPr 2 )CH 2 C 6 H 4 PPh 2 }- (PCy 3 )] (3), was isolated in 70 % yield. The complex 3 was fully characterized by NMR spectros- copy and X-ray crystallography. At room temperature, the 31 P{ 1 H} NMR spectrum of 3 displays an AB pattern at d = 72.10 and 68.66 ppm, thus corresponding to the PCy 3 and the PPh 2 groups, respectively. The large 2 J PP coupling constant of 241 Hz is indicative of a trans disposition of the two different phosphines. The 11 B{ 1 H} NMR spectrum exhibits a broad signal at d = 35.5 ppm, which is slightly more upfield than that of 1 (d = 41 ppm). The 1 H NMR spectrum at room temper- ature is featureless in the hydride region. At 183 K, four broad signals of equal intensity are observed at d = À13.22, À8.58, À6.71, and À1.00 ppm, in accordance with a ruthenium surrounded by four hydrogen atoms. The signal at d = À6.71 ppm was the only one to sharpen upon boron decou- pling. The four hydride signals remained broad upon phos- phorus decoupling. At 213 K, the signal at d = À1.00 ppm appeared as a well-resolved doublet with a coupling constant value of 15.0 Hz, which is consistent with a 2 J HH constant for geminal CH 2 hydrogen atoms, whereas the two hydrides at higher fields coalesced into a broad signal at approximately d = À11 ppm. (see Figures S1–S5 in the Supporting Informa- tion). [4] The broad 13 C{ 1 H} NMR resonance at d = À11.24 ppm was assigned to the carbon atom bound to boron, thus at a much higher field than that in 1 (br, d = 27.1 ppm). A 2D-HSQC 13 C/ 1 H{ 31 P} experiment clearly showed this strongly shielded carbon atom correlating with the two magnetically non-equivalent benzylic hydrogen atoms at d = À1.00 and 1.30 ppm, also identified by 2D- COSY 1 H{ 31 P} NMR spectroscopy (see Figures S6 and S7). To Scheme 1. Synthesis of the bis(agostic) phosphinobenzyl- (amino)borane ruthenium complex 3. [*] A. Cassen, Y. Gloaguen, L. Vendier, C. Duhayon,G. Alcaraz, S. Sabo-Etienne L.C.C. (Laboratoire de Chimie de Coordination) CNRS, UniversitØ de Toulouse, UPS, INPT 205 route de Narbonne 31077 Toulouse (France) E-mail: gilles.alcaraz@lcc-toulouse.fr sylviane.sabo@lcc-toulouse.fr Homepage: http://www.lcc-toulouse.fr/lcc/spip.php?article433 A. Poblador-Bahamonde, C. Raynaud, E. Clot Institut Charles Gerhardt, CNRS, UniversitØ de Montpellier 2 CNRS 5253, cc 1501 Place Eugne Bataillon, 34095 Montpellier (France) C. Raynaud Sorbonne UniversitØs, UPMC Univ. Paris 06, CNRS UMR 7616 Laboratoire de Chimie ThØorique 75005 Paris (France) [**] We thank the CNRS and the ANR for support through the ANR program ReBAB (ANR-11-BS07-0015). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201404753. A ngewandte Chemi e 7569 Angew. Chem. Int. Ed. 2014, 53, 7569 –7573  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim