Organometallics 2010, 29, 3973–3978 DOI: 10.1021/om100607e
3973
The Bridging Acetylene to Bridging Vinylidene Rearrangement in a Triruthenium Carbonyl Cluster: A DFT Mechanistic Study Javier A. Cabeza*,† and Enrique Perez-Carre~ no‡ †
Departamento de Quı´mica Org anica e Inorg anica-IUQOEM, Universidad de Oviedo-CSIC, E-33071 Oviedo, Spain, and ‡Departamento de Quı´mica Fı´sica y Analı´tica, Universidad de Oviedo, E-33071 Oviedo, Spain Received June 23, 2010
A DFT computational study on the reaction that transforms the triruthenium cluster [Ru3( μ3-κ2HNNMe2)( μ3-κ2-HCCH2)( μ-κ2-HCCH)(CO)7] (1), which contains an edge-bridging acetylene ligand, into the derivative [Ru3( μ3-κ2-HNNMe2)( μ3-κ2-HCCH2)(μ-κ1-CCH2)(CO)7] (2), which contains an edgebridging vinylidene ligand, is reported. The reaction pathway with the lowest energy barrier (pathway 1, 37.4 kcal mol-1) is a four-step process that releases 10.7 kcal mol-1. It involves the participation of a hydrido-alkynyl intermediate (arising from the oxidative addition of an acetylene C-H bond). It has been shown that the polymetallic (at least bimetallic) nature of the system and the hemilabile character of the face-capping vinyl ligand play important roles in the operating mechanism. Seven alternative reaction pathways, one of them also involving an acetylene C-H oxidative addition mechanism and the remaining six involving a 1,2-hydrogen-shift of an H atom of the edge-bridging acetylene, were also found to connect compounds 1 and 2 on the potential energy surface of the system, but they all are kinetically disfavored with respect to pathway 1. Introduction The transformation of acetylene into vinylidene is very endothermic (ca. 45 kcal mol-1), and consequently, free vinylidene is an unstable species.1-4 However, when these species are attached to a transition metal, the acetylene to vinylidene rearrangement is a thermodynamically favored process (vinylidene complexes are generally more stable than their corresponding acetylene precursors) that has been experimentally observed on many occasions, mostly in mononuclear complexes but also in bi- and trinuclear derivatives (Figure 1).5 *Author for correspondence. E-mail:
[email protected]. (1) (a) Ervin, K. M.; Ho, J.; Lineberger, W. C. J. Chem. Phys. 1989, 91, 5974. (b) Ervin, K. M.; Groner, S.; Barlow, S. E.; Giles, M. K.; Harrison, A. G.; Bierbaum, V. M.; De Puy, C. H.; Lineberger, W. C.; Ellison, G. B. J. Am. Chem. Soc. 1990, 112, 5750. (2) Chen, Y.; Jonas, D. M.; Kinsey, J. L.; Field, R. W. J. Chem. Phys. 1989, 91, 3976–3987. (3) Gallo, M. M.; Hamilton, T. P.; Schaefer, H. F. J. Am. Chem. Soc. 1990, 112, 8714. (4) (a) Jensen, J. H.; Morokuma, K.; Gordon, M. S. J. Chem. Phys. 1994, 110, 1981. (b) Chen, W. C.; Yu, C. H. Chem. Phys. Lett. 1997, 277, 245. (5) For reviews on the synthesis and uses in organic synthesis of (mostly mononuclear) vinylidene complexes, see: (a) Bruce, M. I. Chem. Rev. 1991, 91, 197. (b) Bruneau, C.; Dixneuf, P. H. Acc. Chem. Res. 1999, 32, 311. (c) Puerta, M. C.; Valerga, P. Coord. Chem. Rev. 1999, 193-195, 977. (d) Werner, H. Angew. Chem., Int. Ed. 1990, 29, 1077. (e) Selegue, J. P. Coord. Chem. Rev. 2004, 248, 1543. (f) Katayama, H.; Ozawa, F. Coord. Chem. Rev. 2004, 248, 1703. (g) Valyaev, D. A.; Semeikin, O. V.; Ustynyuk, N. A. Coord. Chem. Rev. 2004, 248, 1679. (h) Werner, H. Coord. Chem. Rev. 2004, 248, 1693. (i) Bruneau, C.; Dixneuf, P. H. Angew. Chem., Int. Ed. 2006, 45, 2176. (j) Varela, J. A.; Saa, C. Chem.-Eur. J. 2006, 12, 6450. (k) Metal Vinylidenes and Allenylidenes in Catalysis; Bruneau, C.; Dixneuf, P. H., Eds.; Wiley-VCH: Weinhein, 2008. (6) (a) Wolf, J.; Werner, H.; Serhadli, O.; Ziegler, M. L. Angew. Chem., Int. Ed. Engl. 1983, 22, 414. (b) Werner, H.; H€ohn, A. J. Organomet. Chem. 1984, 272, 105. (c) García-Alonso, F. J.; H€ohn, A.; Wolf, J.; Otto, H.; Werner, H. Angew. Chem., Int. Ed. Engl. 1985, 24, 406. (d) Dziallas, M.; Werner, H. J. Chem. Soc., Chem. Commun. 1987, 852.
(7) (a) Bianchini, C.; Masi, D.; Meli, A.; Peruzzini, M.; Ramı´ rez, J. A.; Vacca, A.; Zanobini, F.; Zanello, P. Organometallics 1989, 8, 2179. (b) Bianchini, C.; Meli, A.; Peruzzini, M.; Zanobini, F.; Zanello, P. Organometallics 1990, 9, 241. (c) Bianchini, C.; Peruzzini, M.; Vacca, A.; Zanobini, F. Organometallics 1991, 10, 3697. (8) Bruce, M. I.; Wong, F. S.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 1982, 2203. (9) De los Rı´ os, I.; Jimenez-Tenorio, M.; Puerta, M. C.; Valerga, P. J. Am. Chem. Soc. 1997, 119, 6529. (10) Ikeda, Y.; Yamaguchi, T.; Kanao, K.; Kimura, K.; Kamimura, S.; Mutoh, Y.; Tanabe, Y.; Ishii, Y. J. Am. Chem. Soc. 2008, 130, 16856. (11) Bassetti, M.; Cadierno, V.; Gimeno, J.; Pasquini, C. Organometallics 2008, 27, 5009. (12) Bullock, R. M. J. Chem. Soc., Chem. Commun. 1989, 165. (13) Cadierno, V.; Gamasa, M. P.; Gimeno, J.; Perez-Carre~ no, E.; Garcı´ a-Granda, S. Organometallics 1999, 18, 2821. (14) Cadierno, V.; Gamasa, M. P.; Gimeno, J.; Gonzalez-Bernardo, C.; Perez-Carre~ no, E.; Garcı´ a-Granda, S. Organometallics 2001, 20, 5177.
r 2010 American Chemical Society
Published on Web 08/03/2010
Figure 1. Representative types of acetylene to vinylidene isomerizations in transition metal complexes.
Many efforts have been devoted, both from experimental6-16 and theoretical approaches,12-28 to determine the mechanisms operating in the transformation of terminal alkynes into
pubs.acs.org/Organometallics
3974
Organometallics, Vol. 29, No. 17, 2010
Cabeza and P erez-Carre~ no Scheme 1
vinylidene ligands on the coordination sphere of mononuclear transition metal complexes. These studies have shown that in electron-poor dn (n e 6) metal systems the alkyne-vinylidene rearrangement proceeds either via a direct 1,2-hydrogen-shift (pathway A of Scheme 1)13-15,17,20-22 or via an indirect 1,2hydrogen-shift that involves a previous slippage process to a κ2-C-H alkyne intermediate (pathway B).18 However, in electron-rich Ru(II) and in many d8 metal systems, mostly Rh(I), (15) Garcı´ a-Yebra, C.; L opez-Mardomingo, C.; Fajardo, M.; Anti~ nolo, A.; Otero, A.; Rodrı´ guez, A.; Vallat, A.; Luca, D.; Mugnier, Y.; Carb o, J. J.; Lled os, A.; Bo, C. Organometallics 2000, 19, 1749. (16) Bustelo, E.; Carb o, J. J.; Lled os, A.; Mereiter, K.; Puerta, M. C.; Valerga, P. J. Am. Chem. Soc. 2003, 125, 3311. (17) (a) Silvestre, J.; Hoffmann, R. Helv. Chim. Acta 1985, 68, 1461. (b) Computational Modeling of Homogeneous Catalysis; Maseras, F.; Lled os, A., Eds.; Kluwer: Boston, 2002; Chapter 6. (c) Reference , Chapter 4. (d) Lynam, J. M. Chem.-Eur. J. 2010, 16, 8238. (18) Wakatsuki, Y.; Koga, N.; Yamazaki, H.; Morokuma, K. J. Am. Chem. Soc. 1994, 116, 8105. (19) Wakatsuki, Y.; Koga, N.; Werner, H.; Morokuma, K. J. Am. Chem. Soc. 1997, 119, 360. (20) De Angelis, F.; Sgamellotti, A.; Re, N. Organometallics 2002, 21, 2715. (21) De Angelis, F.; Sgamellotti, A.; Re, N. Organometallics 2002, 21, 4291. (22) Stegmann, R.; Frenking, G. Organometallics 1998, 17, 2089. (23) (a) De Angelis, F.; Sgamellotti, A.; Re, N. Dalton Trans. 2004, 3225. (b) Perez-Carre~ no, E.; Paoli, P.; Ienco, A.; Mealli, C. Eur. J. Inorg. Chem. 1999, 1315. (c) Cowley, M. J.; Lynam, J. M.; Slattery, J. M. Dalton Trans. 2008, 4552. (24) De Angelis, F.; Sgamellotti, A.; Re, N. Organometallics 2007, 26, 5285. (25) (a) Grotjahn, D. B.; Zeng, X.; Cooksy, A. L. J. Am. Chem. Soc. 2006, 128, 2798. (b) Grotjahn, D. B.; Zeng, X.; Cooksy, A. L.; Kassel, W. S.; Di Pasquale, A. G.; Zakharov, L. N.; Rheingold, A. L. Organometallics 2008, 27, 3626. (26) Baya, M.; Crochet, P.; Esteruelas, M. A.; L opez, A. M.; Modrego, J.; O~ nate, E. Organometallics 2001, 20, 4291. (27) Tokunaga, M.; Suzuki, T.; Koga, N.; Fukushima, T.; Horiuchi, A.; Wakatsuki, Y. J. Am. Chem. Soc. 2001, 123, 11917. (28) (a) Oliv an, M.; Eisenstein, O.; Caulton, K. G. Organometallics 1997, 16, 2227. (b) Olivan, M.; Clot, E.; Eisenstein, O.; Caulton, K. G. Organometallics 1998, 17, 3091.
Scheme 2
this isomerization takes place via a hydrido-alkynyl intermediate (oxidative addition pathway C).16-19,23-25 Although some controversy has arisen as to whether the 1,3-hydrogen-shift that transforms the hydrido-alkynyl intermediate into the final vinylidene product follows a bimolecular (or intermolecular, not shown in Scheme 1) or an intramolecular pathway,19 conclusive crossover experiments and in-depth theoretical studies have demonstrated that the bimolecular process can be ruled out.24,25 One alternative to this step involves deprotonation of the M-H unit by an external base and reprotonation at the alkynyl C2 atom.7b A fourth possibility (pathway D), observed in osmium complexes, implies deprotonation of the alkyne complex and reprotonation at C2.26 An additional mechanism, studied in some ruthenium complexes, features initial protonation of the coordinated alkyne, followed by a 1,2-shift of the vinyl C-H toward the metal (pathway E).27 One variant of this process, observed in osmium complexes, starts with a preformed M-H species that transfers the hydride to the alkyne rather than requiring protonation.28 Rearrangements of edge-bridging κ2-alkynes to edgebridging κ1-vinylidene ligands29,30 (Figure 1, central scheme) are well represented in the alkyne chemistry of binuclear29 and higher-nuclearity30 complexes. Face-capping κ2-alkyne
Article
Organometallics, Vol. 29, No. 17, 2010
to face-capping κ2-vinylidene rearrangements (Figure 1, bottom scheme), although rarer, have also been observed in some transition metal carbonyl cluster complexes.31 However, no mechanistic studies (experimental or theoretical) on alkyne to vinylidene rearrangements in non-mononuclear systems have been hitherto reported. In this context, we have reported that, upon thermolysis in toluene at reflux temperature, the triruthenium cluster [Ru3( μ3-κ2-HNNMe2)( μ3-κ2-HCCH2)( μ-κ2-HCCH)(CO)7] (1), which contains an edge-bridging acetylene ligand (in addition to a face-capping 1,1-dimethylhydrazido ligand, a face-capping vinyl ligand, and 7 CO ligands), is cleanly converted to the edge-bridging vinylidene derivative [Ru3( μ3-κ2-HNNMe2)( μ3κ2-HCCH2)( μ-κ1-CCH2)(CO)7] (2) (Scheme 2).30 The lack of experimental and theoretical mechanistic studies on the rearrangement of bridging alkynes to bridging vinylidene ligands and our experience in using DFT methods to explore potential energy surfaces associated with processes that involve transition metal carbonyl clusters32 led us to undertake a mechanistic DFT study of the reaction that transforms compound 1 into 2 (Scheme 2). We now report the results of such a study. Although various reaction pathways have been found to connect compounds 1 and 2, that with the lowest energy barrier involves a hydrido-alkynyl intermediate that arises from the oxidative addition of an acetylene C-H bond.
Computational Details Density functional theory (DFT) calculations were carried out using Becke’s three-parameter hybrid exchange-correlation functional33 and the B3LYP nonlocal gradient correction.34 The LanL2DZ basis set, with relativistic effective core potentials, was used for the Ru atoms.35 The basis set used for the remaining atoms was 6-31G, with addition of (d,p)-polarization.36 When working with large molecules with heavy atoms, it has been previously shown that the B3LYP/LanL2DZ/6-31G(d,p) approximation (29) For examples of μ-κ2-alkyne to μ-κ1-vinylidene rearrangements in binuclear complexes, see: (a) Field, J. S.; Haines, R. J.; Sundermeyer, J.; Woollam, S. F. J. Chem. Soc., Dalton Trans. 1993, 3749. (b) Wang, L. S.; Cowie, M. Organometallics 1995, 14, 2374. (c) Wang, L. S.; Cowie, M. Organometallics 1995, 14, 3040. (d) Xiao, J.; Cowie, M. Organometallics 1993, 12, 463. (e) Mague, J. T.; De Vries, S. H. Inorg. Chem. 1982, 21, 1632. (f) Antwi-Nsiah, F. H.; Oke, O.; Cowie, M. Organometallics 1996, 15, 506. (g) Doherty, N. M.; Elschenbroich, C.; Kneuper, H. J.; Knox, S. A. R. J. Chem. Soc., Chem. Commun. 1985, 170. (30) Cabeza, J. A.; del Rı´ o, I.; Martı´ nez-Mendez, L.; Perez-Carre~ no, E. Chem.-Eur. J. 2006, 12, 7694. 2 2 (31) For examples of μ3-κ -alkyne to μ3-κ -vinylidene rearrangements in trinuclear cluster complexes, see: (a) Deabate, S.; Giordano, R.; Sappa, E. J. Cluster Sci. 1997, 8, 407. (b) Takao, T; Takemori, T; Moriya, M.; Suzuki, H. Organometallics 2002, 21, 5190. (c) Roland, E.; Bernhardt, W.; Vahrenkamp, H. Chem. Ber. 1985, 118, 2858. (d) Albiez, T.; Bernhardt, W.; von Schnering, C.; Roland, E.; Bantel, H.; Vahrenkamp, H. Chem. Ber. 1987, 120, 141. (e) Tasi, M.; Bernhardt, W.; Vahrenkamp, H. Z. Naturforsch. 1990, B45, 647. (f) Bernhardt, W.; von Schnering, C.; Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1986, 25, 279. (32) (a) Cabeza, J. A.; del Rı´ o, I.; Fernandez-Colinas, J. M.; PerezCarre~ no, E.; V azquez-Garcı´ a, D. Organometallics 2010, 29, in press (published on the Web: DOI: 10.1021/om100148z). (b) Cabeza, J. A.; del Rı´ o, I.; Goite, M. C.; Perez-Carre~ no, E.; Pruneda, V. Chem.-Eur. J. 2009, 15, 7339. (c) Cabeza, J. A.; Fernandez-Colinas, J. M.; Perez-Carre~no, E. Organometallics 2009, 28, 2420. (d) Cabeza, J. A.; del Río, I.; PerezCarre~ no, E.; Sanchez-Vega, M. G.; Vazquez-García, D. Angew. Chem., Int. Ed. 2009, 48, 555. (e) Cabeza, J. A.; Perez-Carre~no, E. Organometallics 2008, 27, 4697. (f) Cabeza, J. A.; del Río, I.; García-Granda, S.; Moreno, M.; Perez-Carre~ no, E.; Suarez, M. Organometallics 2004, 23, 5849. (33) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (34) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev., B 1988, 37, 785. (35) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (36) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
3975
used in this paper provides an acceptable balance between accuracy and computing time.32,37 No simplified model compounds were used for the calculations. Crystallographically determined structures provided initial geometries for the optimization of compounds 1 and 2.30 Calculation of analytical frequencies for all stationary points provided one imaginary eigenvalue for transition states and positive eigenvalues for reactants, products, and intermediates. IRC calculations were used to verify that the transition states found were connected to the proposed intermediates, reactant, or product. All energies given in this article are potential energies calculated in the gas phase and are relative to that of compound 1 (E = 0.0 kcal mol-1). It was verified that the potential energy profiles are similar to the free energy profiles, the Gibbs energies being 2-5 kcal mol-1 lower than the electronic energies (Supporting Information). Among all possible reaction pathways for the transformation of 1 into 2, only alkyne C-H oxidative addition and alkyne 1,2-hydrogen shift pathways were studied because, in the absence of other reagents, they have been shown to be the only mechanisms that operate for the alkyne-vinylidene rearrangement in mononuclear systems. All calculations were carried out without symmetry constraints utilizing the Gaussian-03 package.38
Results and Discussion Oxidative Addition Pathways. Two alternative reaction pathways, both involving hydrido-alkynyl intermediates arising from the oxidative addition of an acetylene C-H bond, were found on the potential energy surface of the studied system. Figure 2 displays the relative energy profile of the transformation of the alkyne triruthenium cluster 1 into the vinylidene derivative 2 via pathway 1, along with the optimized structures of the stationary points involved in it. Table 1 shows the evolution of a selection of interatomic distances on going from 1 to 2 through the corresponding intermediates (i) and transition states (ts). Pathway 1 starts converting compound 1, which contains an acetylene ligand bridging the Ru1-Ru2 edge, into an intermediate, i1, which has a terminal hydride ligand attached to Ru1 (H1-Ru1 1.586 A˚) and an alkynyl ligand bridging the Ru1-Ru2 edge (C1-Ru1 2.064 A˚, C1-Ru2 2.472 A˚, C2Ru2 2.533 A˚). Therefore, this step represents an oxidative addition process in which the C1-H1 distance lengthens from 1.088 A˚ in 1 to a nonbonding distance, 2.501 A˚, in i1, and the C1-C2 distance shortens from 1.316 A˚ in 1 to 1.236 A˚ in i1. The transition state of this step, ts1, contains a κ2-C-H interaction of the alkyne with the Ru1 atom (H1-Ru1 1.669 A˚, C1-Ru1 2.149 A˚, C1-H1 1.400 A˚). As the hydride (one-electron donor) and bridging alkynyl (three-electron donor) ligands of (37) (a) Musaev, D. G.; Nowroozi-Isfahani, T.; Morokuma, K.; Rosenberg, E.; Abedin, J.; Hardcastle, K. I. Organometallics 2005, 24, 5973. (b) Musaev, D. G.; Nowroozi-Isfahani, T.; Morokuma, K.; Rosenberg, E. Organometallics 2006, 25, 203. (c) Nowroozi-Isfahani, T.; Musaev, D. G.; Morokuma, K.; Rosenberg, E. Inorg. Chem. 2006, 45, 4963. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, E. R.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M.; Gonzalez, W. C.; Pople, J. A. Gaussian-03 (Revision C2); Gaussian Inc.: Wallingford, CT, 2004.
3976
Organometallics, Vol. 29, No. 17, 2010
Cabeza and P erez-Carre~ no
Figure 2. Relative energy profile (kcal mol-1) and optimized structures of the stationary points involved in the isomerization of 1 into 2 via pathway 1. Table 1. Computed Distances (A˚) between Selected Atoms of the Stationary Points Involved in the Transformation of 1 into 2 via Pathway 1 atoms
1
ts1
i1
ts2
i2
ts3
i3
ts4
2
C1-C2 C1-H1 C2-H1 C2-H2 C1-Ru1 C1-Ru2 C2-Ru1 C2-Ru2 C3-Ru1 C3-Ru2 C4-Ru1 H1-Ru1 Ru1-Ru2
1.316 1.088 2.156 1.090 2.073 2.928 2.834 2.111 2.315 2.190 2.549 2.798 2.983
1.230 1.400 2.284 1.067 2.149 2.709 3.356 3.420 2.340 2.262 2.290 1.669 3.068
1.236 2.501 3.532 1.068 2.064 2.472 3.300 2.533 2.636 2.241 2.512 1.586 3.544
1.231 2.640 2.861 1.068 2.352 2.158 3.131 3.288 2.533 2.207 2.544 1.581 3.211
1.240 3.161 2.643 1.067 2.324 2.000 2.462 3.228 2.887 2.133 3.324 1.578 3.060
1.270 2.257 1.489 1.076 2.224 1.995 2.418 3.246 2.516 2.233 3.169 1.696 2.910
1.335 2.122 1.092 1.090 2.111 1.982 3.050 3.134 2.087 2.883 3.053 3.194 3.043
1.325 2.121 1.090 1.090 2.148 1.968 3.072 3.214 2.161 2.263 2.946 3.211 2.733
1.329 2.118 1.088 1.089 2.040 2.048 3.120 3.176 2.309 2.206 2.673 3.380 2.790
i1 contribute two electrons more than the bridging acetylene ligand of 1 (two-electron donor), in order to maintain a coordinative saturation of the metal atoms, the oxidative addition is accompanied by a lengthening of the Ru1-Ru2 distance (from 2.983 A˚ in 1 to a nonbonding distance, 3.544 A˚, in i1) and also by a lengthening of the distance between the Ru1 atom and the C3 atom of the face-capping vinyl ligand (from 2.315 A˚ in 1 to 2.636 A˚ in i1). The energy barrier of this step is high, 37.4 kcal mol-1. The second step of pathway 1 (i1 f i2) involves an easy rearrangement (4.0 kcal mol-1 from i1) by which the C2 atom of the bridging alkynyl ligand detaches from the Ru2 atom to end attached to Ru1 (C2-Ru2 2.533 A˚ in i1 and 3.228 A˚ in i2; C2-Ru1 3.300 A˚ in i1 and 2.462 A˚ in i2). While the Ru1-Ru2 distance shortens by 0.484 A˚ in this step, the C3 and C4 atoms of the face-capping vinyl ligand of i1 are not attached to Ru1 in i2, thus precluding a coordinative oversaturation of the Ru1 metal atom. In the third step of pathway 1, which has an energy barrier of 13.0 kcal mol-1, the terminal hydride H1 of i2 migrates to the C2 carbon atom to give a vinylidene ligand (C2-H1 2.643 A˚ in i2, 1.489 A˚ in ts3, and 1.092 A˚ in i3). This reductive elimination process is accompanied by a recoordination of the vinyl C3 to Ru1 (2.887 A˚ in i2 and 2.087 A˚ in i3).
At this point, it is worth noting that the two-step transformation i1 f i2 f i3 represents a 1,3-hydrogen-shift (the hydride H atom moves from the metal to the C2 carbon atom), comparable to those reported for mononuclear systems that transform hydrido-alkynyl intermediates into vinylidene derivatives.16-19,23-25 However, the energy barrier shown for this transformation in Figure 2 (13.1 kcal mol-1 from i1) is considerably lower than those found for mononuclear systems (20-50 kcal mol-1). It seems that in the latter the transfer of the H atom from the metal to the C2 carbon atom requires a bending of the originally straight M-C1-C2-H moiety that destabilizes the corresponding transition state. Such a bending is unnecessary in pathway 1 because the hydride and alkynyl ligands of i2 are σ-bound to different metal atoms and the alkynyl ligand is suitably oriented to receive the hydride ligand on the C2 atom. In other words, as a result of the collaboration of both metal atoms in the transformation, the barrier to the hydride migration onto the C2 carbon of the alkynyl ligand can be much lower in binuclear systems than in mononuclear ones. The fourth and final step of pathway 1 (i3 f 2) has a low energy barrier (5.3 kcal mol-1) and is thermodynamically favored, releasing 21.3 kcal mol-1. It involves the recoordination of the vinyl C4 atom to Ru1 (3.053 A˚ in i3 and 2.673 A˚
Article
Organometallics, Vol. 29, No. 17, 2010
3977
Figure 3. Relative energy profile (kcal mol-1) and optimized structures of the stationary points involved in the transformation of 1 into i2 via pathway 2.
Figure 4. Relative energy profiles (kcal mol-1) and optimized structures of the stationary points involved in the isomerization of 1 into 2 via pathways 3a and 3b.
in 2), the shortening of the Ru1-Ru2 distance (3.043 A˚ in i3 and 2.790 A˚ in 2), and the movement of the vinylidene ligand to a symmetric position bridging the Ru1-Ru2 edge (C1-Ru1 2.111 A˚ in i3 and 2.040 A˚ in 2; C1-Ru2 1.982 A˚ in i3 and 2.048 A˚ in 2). Figure 3 shows the initial two steps of a four-step mechanism (pathway 2) that was also found on the potential energy surface of the studied transformation, also involving the oxidative addition of an acetylene C-H bond. In this case, the oxidative addition of the alkyne C-H bond (1 f i4) begins with a movement of the alkyne that places the H1 atom close to Ru1 (H1-Ru1 2.125 A˚ in ts5) while the C1 and C2 atoms separate from the Ru1 and Ru2 atoms, respectively (C1-Ru1 3.107 A˚ and C2-Ru2 3.031 A˚ in ts5), to end in an intermediate (i4) that has a terminal hydride on Ru1 and a terminal alkynyl ligand on Ru2. This step has an energy barrier of 45.1 kcal mol-1. In the second step (i4 f i2), the coordination of the alkynyl C1 and C2 atoms to Ru1 induces the disconnection of the vinyl C3 and C4 atoms from the same metal atom. The last two steps of this reaction pathway (i2 f i3 f 2) are identical to those of pathway 1 (they are not represented in Figure 3). The fact that the energy of ts5 is 7.7 kcal mol-1 higher than that of ts1 implies that pathway 2 is kinetically disfavored with respect to pathway 1. Due to the asymmetry of compound 1, we devoted many efforts to look for reaction pathways in which the oxidative addition would occur over the Ru2 atom, thus involving intermediates having the terminal hydride attached to Ru2, but they all were unsuccessful. The way the face-capping vinyl ligand is attached to the metal atoms in compound 1 (it is σ-bound to Ru2, while it is π-bound to Ru1 and Ru3) may have implications in these results, because, according to the above-described reaction pathways, the Ru atom that undergoes the oxidative addition needs to avoid a coordinative oversaturation at some stage of the mechanism (e.g., formation of intermediate i2 in pathways 1 and 2), and this can be more easily achieved by Ru1 (by detaching the C3
and/or C4 atoms of the π-bound vinyl ligand) than by Ru2 (which is σ-bound to the vinyl ligand). 1,2-Hydrogen-Shift Pathways. Two different types of mechanisms were found to involve a 1,2-shift of a hydrogen atom of the acetylene ligand of compound 1. In one of them (Figure 4), the complete isomerization occurs in an elemental step in which the 1,2-hydrogen-shift is accompanied by the concomitant decoordination of the acetylene C atom that will finally be the C2 atom of the vinylidene ligand of compound 2. The asymmetry of compound 1 provokes that two alternative reaction pathways be possible for this type of mechanism (pathways 3a and 3b). The energy barriers of these two reaction pathways, 48.7 kcal mol-1 (pathway 3a) and 64.5 kcal mol-1 (pathway 3b), are much higher than that of pathway 1 (37.4 kcal mol-1). Therefore, pathways 3a and 3b are disfavored with respect to pathway 1. Four reaction pathways following another 1,2-hydrogenshift mechanism were also found on the potential energy surface of the studied reaction. In these cases, the 1,2-hydrogen shift proceeds through a transition state in which both acetylene C atoms are still attached to their corresponding ruthenium atoms (ts8a-ts8d, Figure 5). The shifted H atom can be that originally attached to C1 (as in ts8b and ts8d) or C2 (as in ts8a and ts8c) and be placed in the transition state on one side (as in ts8a and ts8b) or the other (as in ts8c or ts8d) of the C1-C2-Ru1-Ru2 plane. These options give rise to four possible reaction pathways (4a-4d), the energy barriers of which are very close to each other (they are in the range 55.057.6 kcal mol-1). As all these energy barriers are considerably higher than that of pathway 1 (37.4 kcal mol-1), their associated pathways are also disfavored with respect to pathway 1. As a representative example, Figure 5 contains only one of these reaction pathways (4a). In it, the transition state ts8a evolves toward complex 2 via the intermediate i3 and the transition state ts4, which also participate in pathway 1. It should be noted that the energy barriers to the 1,2hydrogen shifts shown in Figure 5 (55-58 kcal mol-1) are
3978
Organometallics, Vol. 29, No. 17, 2010
Figure 5. Relative energy profile (kcal mol-1) and optimized structures of the stationary points involved in the isomerization of 1 into 2 via pathway 4a. The energies and structures of transition states ts8b, ts8c, and ts8d are also shown.
higher than those reported for similar ligand rearrangements in mononuclear complexes (in the range 20-40 kcal mol-1).13-15,17,20-22 Therefore, in comparison with mononuclear systems, the coordination of the C1 and C2 alkyne carbon atoms to two metal atoms in a parallel mode seems to destabilize the transition state responsible for the 1,2hydrogen shift.
Concluding Remarks Pathway 1 has the lowest energy barrier of all investigated reaction pathways. Therefore, we conclude that the isomerization of 1 into 2 follows this reaction pathway, which is a four-step process that releases 10.7 kcal mol-1. The first step is rate-determining and involves the oxidative addition of a C-H bond of the acetylene ligand to Ru1. Its calculated energy barrier, 37.4 kcal mol-1, is coherent with the observa-
Cabeza and P erez-Carre~ no
tion that the reaction proceeds only at high temperatures (it was experimentally carried out in toluene at 110 °C).30 In addition, the experimental failure to observe intermediates during the reaction30 is also explainable if the first step is the slowest step of the entire process and if the energies of all intermediates are higher than those of 1 and 2, as is the case for pathway 1. We have shown that the collaboration of two metal atoms in the step that involves the migration of the hydride onto the C2 carbon of the alkynyl ligand results in an energy barrier much lower than those found for analogous steps in mononuclear systems. Additionally, the hemilabile character of the face-capping vinyl ligand of 1, which is able to detach from a metal one or two of its coordinated carbon atoms and recoordinate them later, facilitates some reaction steps because it helps the metal atoms to maintain their coordinative saturation. Therefore, the polymetallic (at least bimetallic) character of the system and the presence of the hemilabile vinyl ligand are important to the oxidative addition mechanism that transforms compound 1 into 2. Although the calculations reported herein confirm the participation of a mechanism of the C-H oxidative addition type in the transformation of 1 into 2 and show that mechanisms involving a 1,2-hydrogen-shift of an H atom of acetylene are energetically more costly than in mononuclear systems, the important role played by the face-capping vinyl ligand of compound 1 in the operating mechanism does not allow us to propose that a C-H oxidative addition mechanism will also be operating in other metal systems (with other ancillary ligands) in which an edgebridging alkyne to edge-bridging vinylidene rearrangement has also been observed.29 Therefore, additional mechanistic studies, concerning different metal systems, are necessary to gain a general view of the bridging alkyne to bridging vinylidene rearrangement in transition metal complexes.
Acknowledgment. This work has been supported by the European Union (FEDER grants) and the Spanish MICINN (projects CTQ2007-60865 and MAT2006-1997). Supporting Information Available: Electronic and Gibbs energies, atomic coordinates, and large structural figures of the stationary points involved in all studied reaction pathways. This material is available free of charge via the Internet at http:// pubs.acs.org.
