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Agostic Interaction and Intramolecular Proton Transfer from the Protonation Agostic interaction and intramolecular proton SPECIAL FEATURE transfer from the protonation of dihydrogen ortho metalated ruthenium complexes Andrew Toner†, Jochen Matthes†‡, Stephan Gru¨ ndemann‡, Hans-Heinrich Limbach‡, Bruno Chaudret†, Eric Clot§, and Sylviane Sabo-Etienne†¶ †Laboratoire de Chimie de Coordination du Centre National de la Recherche Scientifique, Associe´a` l’Universite´Paul Sabatier, 205 route de Narbonne, 31077 Toulouse Cedex 04, France; ‡Institute of Chemistry, Freie Universita¨t Berlin, Takustrasse 3, D-14195 Berlin, Germany; and §Laboratoire de Structure et Dynamique des Syste`mes Mole´culaires et Solides (Centre National de la Recherche Scientifique, Unite´Mixte de Recherche 5253), Institut Charles Gerhardt, case courrier 14, Universite´Montpellier II, Place Euge`ne Bataillon, 34000 Montpellier, France Edited by Jay A. Labinger, California Institute of Technology, Pasadena, CA, and accepted by the Editorial Board January 17, 2007 (received for review October 13, 2006) Protonation of the ortho-metalated ruthenium complexes insertion of an olefin into the aromatic C–H bond ortho to an i phenylpyridine (ph-py) (1), benzoquinoline activating ketone group (Eq. 1) (30). In this system, the key-2 ؍ RuH(H2)(X)(P Pr3)2 [X i ؉ ؊ bq) (2)] and RuH(CO)(ph-py)(P Pr3)2 (3) with [H(OEt2)2] [BAr؅4] intermediate is an ortho-metalated complex resulting from ortho) CF3)2C6H3)4B]) under H2 atmosphere yields the corre- C–H bond activation, thanks to chelating assistance with the donor)-3,5)] ؍ BAr؅4) sponding cationic hydrido dihydrogen ruthenium complexes group. Coordination of the olefin, olefin insertion into Ru–H and i phenylpyridine (ph-py) (1-H); ben- C–C coupling are the subsequent steps needed to close the catalytic ؍RuH(H2)(H-X)(P Pr3)2][BAr؅4][X] zoquinoline (bq) (2-H)] and the carbonyl complex [RuH(CO)(H-ph- cycle as proposed by Kakiuchi and Murai (8). i py)(P Pr3)2][BAr؅4] (3-H). The complexes accommodate an agostic C–H interaction characterized by NMR and in the case of 1-H by x-ray diffraction. Fluxional processes involve the hydride and dihydrogen R R CHEMISTRY ligands in 1-H and 2-H and the rotation of the phenyl ring displaying O Ru(H)2(CO)(PPh3)3 O the agostic interaction in 1-H and 3-H. NMR studies (lineshape analysis + [1] R' of the temperature-dependent NMR spectra) and density functional toluene, 135 °C R' theory calculations are used to understand these processes. Under vacuum, one equivalent of dihydrogen can be removed from 1-H and 2-H leading to the formation of the corresponding cationic ortho- We have shown that, when using the bis-dihydrogen complex, ph-py (1-THF), bq ؍ metalated complexes [Ru(H )(THF)(X)(PiPr ) ]؉ [X 2 3 2 Ru(H)2(H2)2(PCy3)2, as catalyst precursor, the C–C coupling of (2-THF)]. The reaction is fully reversible. Density functional theory ethylene with acetophenone or benzophenone was catalytic at room calculations and NMR data give information about the reversible temperature (31, 32). The activity was recently improved by replac- mechanism of C–H activation in these ortho-metalated ruthenium ing the tricyclohexylphosphines by two tricyclopentylphosphines complexes. Our study highlights the subtle interplay between key (33). Moreover, we were able to isolate key intermediates of general ligands such as hydrides, ␴-dihydrogen, and agostic bonds, in C–H formula RuH(H2)(o-C6H5R)(PRЈ3)2,(Rϭ COCH3, COC6H5) that activation processes. proved to be ortho-metalated species (31). These compounds with a ketone chelating group present a very limited solubility in most of ͉ ͉ ͉ ͉ C–H activation density functional theory hydrogen transfer NMR the solvents. It was thus easier to perform an in-depth study on sigma bonds analogous complexes, with the R substituent replaced by an aro- matic N-heterocycle. In such a case, chelation is assisted by nitrogen atalytic transformation of alkanes and arenes via activation coordination to the metal center. Indeed, we recently reported the Cof an inert C–H bond is of considerable interest and remains properties of a series of ortho-metalated ruthenium hydrido com- a challenge to chemists (1–13). Since the 1980s, many informa- plexes Ru(H)(H2)(X)(PiPr3)2 [X ϭ 2-phenylpyridine (ph-py), ben- tion have been gathered on the stoichiometric transformations of zoquinoline (bq), phenylpyrazole (ph-pz)] resulting from C–H a C–H bond at a transition metal center (14–26). Different activation of the corresponding functionalized arene (34). These mechanisms are operative depending on the metal, the ligand set compounds display remarkable exchange couplings between the and the nature of the media in which the reaction is performed. hydride and the dihydrogen ligand. Despite the nonactivity of these They differ, inter alia, by the way the R–H moiety interacts with species toward the Murai reaction, we believed that a study focused the transition metal before activation. Oxidative addition pro- on protonation could bring some general information, especially on ␴ ceeds from a complex where the C–H bond interacts as a Lewis the interplay between key ligands such as ␴-dihydrogen or agostic ␴ base with the metal, whereas -bond metathesis does not require bonds, as well as on hydrogen transfer processes. precoordination of the substrate. An alternative mechanism based on the properties of ␴ complexes is now under consider- ation for late transition metals. Such a ␴-complex assisted Author contributions: H.-H.L., B.C., and S.S.-E. designed research; A.T., J.M., and S.G. metathesis mechanism (␴-CAM) allows substrate functionaliza- performed research; A.T., J.M., S.G., H.-H.L., B.C., E.C., and S.S.-E. analyzed data; E.C. and tion by ␴-ligand substitution. It involves the interconversion of at S.S.-E. wrote the paper; and E.C. performed DFT calculations. least two ␴ complexes and no change in oxidation state, thanks The authors declare no conflict of interest. to the intermediacy of secondary interactions (27). This article is a PNAS direct submission. J.A.L. is a guest editor invited by the Editorial Board. The inert character of the C–H bond is reflected by its poor Abbreviations: DFT, density functional theory; TS, transition state. properties as a ligand, even though alkane complexes have been ¶To whom correspondence should be addressed. E-mail: [email protected]. observed (28, 29). It is thus necessary to promote the interaction of This article contains supporting information online at www.pnas.org/cgi/content/full/ one particular C–H bond to observe selective activation. A major 0608979104/DC1. breakthrough was proposed by Murai in 1993 with the selective © 2007 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0608979104 PNAS ͉ April 24, 2007 ͉ vol. 104 ͉ no. 17 ͉ 6945–6950 Downloaded by guest on October 1, 2021 i + i + + i P Pr3 P Pr P Pr3 3 PiPr H H 3 H H υ H 2 H Ru Ru H Ru H H 9 N N H N THF Ru H i H i PiPr N P Pr3 P Pr3 3 υ H 1 H5 T/K 11-H1-THF i P Pr3 1-H 303 PiPr i + i + 3 P Pr3 P Pr3 H H H H H Ru Ru Ru H N H N H N THF H PiPr i i 3 P Pr3 P Pr3 263 2 2-H 2-THF i PiPr + P Pr3 3 H H CO Ru Ru 213 NCO N H i i P Pr3 P Pr3 3 3-H Chart 1. Structures of the ortho-metalated complexes 1–3, 1-THF, and 2-THF 173 and the corresponding agostic complexes 1-H, 2-H, and 3-H. H5 H9 The selectivity of the Murai’s reaction relies on the existence of an agostic precursor, which corresponds to the very first step of the 9.0 8.0 7.0 6.0 5.0 4.0 activation process of the arene substrate. We have already com- δ / ppm municated about the characterization of such a complex, namely i ϩ Ϫ [RuH(H2)(H-ph-py)(P Pr3)2] [BArЈ4] (1-H), showing two coor- Fig. 1. Aromatic region of the experimental (top line) and simulated (bottom 1 i ϩ Ј Ϫ dinated ␴-bonds: an agostic C–H bond and an H2 ligand (35). 1-H line) H-NMR spectra (500.13 MHz) of [RuH(H2)(H-ph-py)(P Pr3)2] [BAr 4] (1-H) was prepared by protonation of the ortho-metalated complex under 800 mbar H2 in THF-d8. Only the exchange between H5 and H9 was i simulated. Ru(H)(H2)(ph-py)(P Pr3)2. We now report the results of a com- bined experimental and theoretical study on a series of agostic complexes aimed at describing their structure, the influence of the 10(11.3Ϯ0.3)exp((Ϫ35.6 Ϯ 1.8) kJ⅐molϪ1/RT)sϪ1 (173 K Յ T Յ 303 various ligands, as well as the C–H activation processes occurring K; k ϭ 18,000 sϪ1 at 263 K). The value of the activation energy of within these systems. Part of this work has been communicated (35). the phenyl rotation process is the addition of the bond dissociation Results energy of the agostic interaction plus the intrinsic barrier of the Ϫ1 For further details, see supporting information (SI) Text, Figs. 7–12, phenyl ring rotation itself. The present value of Ea (35.6 kJ mol ) and Tables 2–6. in complex 1-H is low, in agreement with the weak coordination of the C–H bond. The chemical shift found for the frozen agostic Synthesis and Properties of Agostic Complexes.
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