Organometallics 2009, 28, 4217–4220 DOI: 10.1021/om900312p
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DFT Mechanistic Study of the Transformation of Cyclohexa-1,3-diene into a Bridging Allyl Ligand upon Reaction with a Triruthenium Hydrido Carbonyl Cluster Javier A. Cabeza,*,† Jose M. Fernandez-Colinas,† 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 April 23, 2009
Summary: Density functional theory calculations on the reaction that gives the edge-bridging allyl cluster [Ru3( μ-κ3C6H9)( μ3-κ2-HNNMe2)( μ-CO)2(CO)6] from [Ru3( μH)( μ3-κ2-HNNMe2)(CO)9] and cyclohexa-1,3-diene have shown that this reaction follows a three-step process. The first step, which is rate-limiting and involves an associative substitution of the diene for a CO ligand to give an η2-diene intermediate, is followed by the transfer of the hydride H atom to one of the coordinated C atoms of the η2-diene and by an accommodation of the resulting σ-allyl ligand that allows the final attachment of the allyl ligand as η3-edge-bridging. The first step of this reaction also provides a mechanistic insight into a more general reaction, i.e., the substitution of a twoelectron donor ligand for a CO ligand of carbonyl triruthenium clusters having bridging N-donor ligands.
Introduction The reactions of the hydrazido-bridged hydrido carbonyl triruthenium complex [Ru3( μ-H)( μ3-κ2-HNNMe2)(CO)9]1 (1; H2NNMe2 = 1,1-dimethylhydrazine) with a variety of terminal and internal alkynes without R-hydrogen atoms give trinuclear alkenyl derivatives that have their alkenyl ligands in edge-bridging or face-capping positions.2 However, when the alkyne reagents have R-hydrogen atoms, the products contain face-capping alkenyl (A and B in Scheme 1) or edge-bridging allyl ligands (C and D in Scheme 1).3 In all cases, the nature of the R groups of the alkyne reagents influences the regioselectivity of the reactions. DFT calculations have demonstrated that, for isomeric products, the allyl derivatives are thermodynamically more stable than their corresponding alkenyl isomers.3 In a recent work, studying the reactivity of compound 1 with conjugated dienes, we found that these unsaturated reagents also afford allyl cluster derivatives in high yields (Scheme 2).4 This synthetic approach represents
an interesting alternative to the use of alkynes having R-hydrogen atoms as precursors to allyl ligands, especially if the alkyne required to make a particular allyl cluster complex is unknown or difficult to obtain, as happens for cyclic alkynes. In the present paper we report that, using density functional theory (DFT) calculations, we have been able to model the mechanism of the reaction that gives the allyl cluster [Ru3( μ-κ3-C6H9)( μ3-κ2-HNNMe2)( μ-CO)2(CO)6] (2 in Scheme 2) from compound 1 and cyclohexa-1, 3-diene. This theoretical study not only sheds light on the reaction pathway by which cyclic 1,3-dienes are transformed into cyclic allyl ligands when they react with cluster 1 but also provides a mechanistic insight into a more general reaction, i.e., the substitution of a twoelectron donor ligand for a CO ligand of carbonyl triruthenium clusters having bridging N-donor ligands. Apart from those described in the above commented works,3,4 triruthenium cluster complexes having organic allyl ligands (R1R2CCR3CR4R5; Rn 6¼ M) are very rare, the only examples being [Ru3( μ-κ3-C3H5)( μ3-κ2-PPhCH2PPh2)(CO)8]5 and [Ru3( μ3-κ5-HabqCHCHCHR)( μ-CO)2(CO)6] (H2abqH = 2-amino-7,8-benzoquinoline; R = H, CtCSiMe3),6 which contain edge-bridging allyl ligands. Quite a few ruthenium cluster complexes containing 1,3-dimetalated allyl ligands (MR1CCR2CR3M) are known.7 One penta-8 and one hexaruthenium9 cluster have also been shown to contain edge-bridging allyl ligands.
*To whom correspondence should be addressed. E-mail: jac@uniovi. es. :: (1) Jenke, T.; Stoeckli-Evans, H.; Suss-Fink, G. J. Organomet. Chem. 1990, 391, 395. (2) Cabeza, J. A.; del Rı´ o, I.; Fernandez-Colinas, J. M.; Garcı´ aGranda, S.; Martı´ nez-Mendez, L.; Perez-Carre~ no, E. Chem.;Eur. J. 2004, 10, 6265. (3) Cabeza, J. A.; del Rı´ o, I.; Garcı´ a-Granda, S.; Martı´ nez-Mendez, L.; Perez-Carre~ no, E. Chem.;Eur. J. 2005, 11, 6040. (4) Cabeza, J. A.; del Rı´ o, I.; Gille, M.; Goite, M. C.; Perez-Carre~ no, E. Organometallics 2008, 27, 609.
(5) Bruce, M. I.; Williams, M. L. J. Organomet. Chem. 1985, 288, C55. (6) Cabeza, J. A.; del Rı´ o, I.; Garcı´ a-Granda, S.; Riera, V.; Suarez, M. Organometallics 2004, 23, 3501. (7) (a) Evans, M.; Hursthouse, M.; Randall, E. W.; Rosenberg, E. J. Chem. Soc., Chem. Commun. 1972, 545. (b) Castiglioni, M.; Milone, L.; Osella, D.; Vaglio, G. A.; Valle, M. Inorg. Chem. 1976, 15, 394. (c) Gervasio, G.; Osella, D.; Valle, M. Inorg. Chem. 1976, 15, 1176. (d) Beanan, L. R.; Rahman, Z. A.; Keister, J. B. Organometallics 1983, 2, 1062. (e) Beanan, L. R.; Keister, J. B. Organometallics 1985, 4, 1713. (f) Rao, K. M.; Angelici, R. J.; Young, V. G. Inorg. Chim. Acta 1992, 198, 211. (g) Doherty, S.; Corrigan, J. F.; Carty, A. J.; Sappa, E. Adv. Organomet. Chem. 1995, 37, 39. (h) Bruce, M. I.; Zaitseva, N. N.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 1999, 2777. (i) Wong, W. Y.; Chan, S.; Wong, W. T. J. Chem. Soc., Dalton Trans. 1995, 1497. (j) Gervasio, G.; Marabello, D.; King, P. J.; Sappa, E.; Secco, A. J. Organomet. Chem. 2003, 671, 385. (k) Cabeza, J. A.; da Silva, I.; del Rı´ o, I.; Garcı´ a-Granda, S.; Riera, V. Inorg. Chim. Acta 2003, 347, 107. (8) Chihara, T.; Yamazaki, H. J. Organomet. Chem. 1992, 428, 169. (9) Chihara, T.; Yamazaki, H. J. Chem. Soc., Dalton Trans. 1995, 1369.
r 2009 American Chemical Society
Published on Web 05/28/2009
pubs.acs.org/Organometallics
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Organometallics, Vol. 28, No. 14, 2009 Scheme 1
Computational Details DFT calculations were carried out using Becke’s three-parameter hybrid exchange-correlation functional10 and the B3LYP nonlocal gradient correction.11 The LanL2DZ basis set, with relativistic effective core potentials, was used for the Ru atoms.12 The basis set used for the remaining atoms was the 6-31G, with addition of (d,p)-polarization.13 It has been previously shown that the B3LYP/LanL2DZ/6-31G(d,p) approximation used in this paper provides reasonable agreement with available experiments and higher-level methods in analogous systems.14 No simplified model compounds were used for the calculations. Crystallographically determined structures provided initial geometries for the optimization of compounds 11 and 2.4 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, reactants, or products. All energies given in this article are potential energies calculated in the gas phase. It was verified that the gas phase potential energy profile is similar to the free energy profile, the Gibbs energies being slightly smaller than the corresponding potential energies. Atomic NPA (natural (10) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (11) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev., B 1988, 37, 785. (12) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (13) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (14) (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. (d) Cabeza, J. A.; del Rı´ o, I.; Garcı´ a-Granda, S.; Moreno, M.; Perez-Carre~ no, E.; Suarez, M. Organometallics 2004, 23, 5849. (e) Cabeza, J. A.; Perez-Carre~ no, E. Organometallics 2008, 27, 4697. (f) Cabeza, J. A.; del Rı´ o, I.; Perez-Carre~ no, E.; Sanchez-Vega, M. G.; V azquez-Garcı´ a, D. Angew. Chem., Int. Ed. 2009, 48, 555.
Notes Scheme 2
population analysis) charges were derived from the natural bond order (NBO) analysis of the data.15 All calculations were carried out without symmetry constraints utilizing the Gaussian03 package.16
Results and Discussion The energy profile of the reaction of the hydrido triruthenium cluster 1 with cyclohexa-1,3-diene, to give the μ-allyl derivative [Ru3( μ-κ3-C6H9)( μ3-κ2-HNNMe2)( μ-CO)2(CO)6] (2) + CO, is displayed in Figure 1, in which the given energies are relative to those of the reactants (energy of 1 + energy of cyclohexa-1,3-diene = 0.0 kcal mol-1) and include the energy of none (ts1) or one CO molecule (i1 to 2). 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). (15) (a) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (b) Reed, A. E.; Curtis, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (16) 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.
Note
Organometallics, Vol. 28, No. 14, 2009
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Figure 1. Optimized structures and relative energy profile (kcal mol-1) of the stationary points involved in the transformation of 1 + C6H8 into 2 + CO. Table 1. Computed Distances (A˚) between Selected Atoms of the Stationary Points Involved in the Transformation of 1 into 2 atoms
1
ts1
i1
ts2
i2
ts3
2
C1-Ru1 C4-C5 C4-H1 C4-Ru1 C4-Ru2 C5-C6 C5-Ru1 C6-C7 C6-Ru1 C6-Ru2 C7-Ru2 H1-Ru1 H1-Ru2
1.973
3.380 1.346 3.658 4.178 5.053 1.466 4.286 1.344 4.988 6.559 6.413 1.823 1.759
1.372 2.652 2.552 3.918 1.469 2.618 1.343 3.539 5.934 6.420 1.809 1.775
1.434 1.509 2.447 3.296 1.474 2.278 1.343 3.202 5.391 5.757 1.829 2.013
1.515 1.134 2.961 2.991 1.482 2.219 1.344 3.070 4.860 5.319 2.701 2.098
1.514 1.118 3.057 3.176 1.485 2.244 1.346 3.058 4.189 4.443 2.955 2.148
1.535 1.097 3.246 4.210 1.427 2.255 1.427 2.611 2.611 2.255 3.111 4.141
1.790 1.789
The overall reaction is a three-step process. The incorporation of the cyclic diene to the coordination shell of the trinuclear cluster occurs in an elementary step (1 + C6H8 f i1 + CO) in which the diene approaches the cluster Ru1 atom at the same time that the equatorial CO ligand C1-O1, which is cis to the bridging N-donor ligand (C1-Ru1=1.973 A˚ in 1), is being released (C1-Ru1= 3.380 A˚ in ts1, energy barrier = 24.6 kcal mol-1). The long C4Ru1 and C5-Ru1 distances of ts1, 4.178 and 4.286 A˚, are shortened to 2.552 and 2.618 A˚, respectively, in intermediate i1, which contains an η2-diene ligand in the same coordination site as that originally occupied by the released CO ligand. In the second step (i1 f i2), the diene ligand moves toward the bridging hydride H1 (in ts2: C4-H1=1.509 A˚, H1-Ru1 = 1.829 A˚, H1-Ru2 = 2.013 A˚) and separates from the Ru1 atom (H1-Ru1 = 2.701 A˚ in i2). In intermediate i2, the H1 atom is at bonding distance from C4 (C4-H1 = 1.134 A˚), while it still maintains some interaction with Ru2 (H1-Ru2=2.098 A˚). The energy requirement of this step is moderate, 17.1 kcal mol-1 from i1. The atomic charge of H1 changes from -0.109 in i1 to 0.034 in ts2 and 0.193 in i2, while that of C4 changes from -0.235 in i1 to -0.280 in ts2
and -0.450 in i2. These charges reflect the change of H1 from having a metal hydride character in i1 to having a hydrocarbon-H character in i2 and the change of C4 from having a sp2 character in i1 to having a sp3 character in i2. The third and final step (i2 f 2) is energetically little demanding (the energy of ts3 is only 2.5 kcal mol-1 above that of i2) and involves the detachment of H1 from Ru2 (H1-Ru2=2.148 A˚ in ts3) and an anticlockwise rotation of the cyclohex-2-enyl ligand about the C5-Ru1 axis that allows the coordination of the C6dC7 double bond to the Ru2 metal atom. The final complex 2 contains a cyclic allyl ligand bridging the Ru1 and Ru2 ruthenium atoms (C5-Ru1=2.255 A˚, C6-Ru1=2.611 A˚, C6-Ru2=2.611 A˚, C7-Ru2=2.255 A˚). The energy profile shown in Figure 1 indicates that, under standard conditions, the overall transformation is endothermic by 7.8 kcal mol-1. However, from a synthetic point of view, the relative energies shown for the stationary points i1 to 2 are not realistic because they include the energy of one CO molecule and, experimentally, the released CO is purged with an inert gas (the reaction was not carried out in a closed vessel).4 Therefore, it is more realistic to consider intermediate i1 as the starting point of an independent two-step process (i1 f i2 f 2). Consequently, the rate-determining step of the overall transformation (1 + C6H8 f 2 + CO) is the coordination of the cyclic diene (1 + C6H8 f i1 + CO). The energy barrier of this step, though relatively high (24.6 kcal mol-1), is not a problem when the reaction is carried out in toluene at reflux temperature (110 °C).4 Although we have not experimentally studied the kinetics of this reaction, the above-described theoretical mechanistic study suggests that the reaction should follow second-order kinetics, being first order in both metal cluster and diene concentrations. This suggestion is in complete agreement with a previous experimental kinetic study of the reaction of compound 1 with diphenylacetylene, which showed that the reaction is first order in the concentration of 1 and first order in alkyne concentration.2 Kinetic measurements have also shown that the reactions of phosphine ligands with the
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Organometallics, Vol. 28, No. 14, 2009
anionic clusters [Ru3( μ3-κ2-Xpy)( μ-CO)3(CO)6]- (X = S, NMe, NPh) to give CO-substituted products also follow an associative mechanism.17 Of particular interest is the way by which the diene replaces a CO ligand from complex 1 (Figure 1, step 1) because it sheds light on the mechanism operating in many CO-substitution reactions involving related carbonyl metal clusters. In addition to the mechanism shown in Figure 1, we also considered the possibility that the CO substitution is dissociative and studied the release of CO from different positions in compound 1 prior to the coordination of the diene. In addition, associative mechanisms in which the diene approaches Ru1 while a CO ligand is released from different positions of Ru1, Ru2, or Ru3 were also considered. However, all these alternative reaction pathways were either not found on the potential energy surface of the reaction or resulted in higher energy barriers than that depicted in Figure 1. (17) Shen, J. K.; Basolo, F.; Nombel, P.; Lugan, N.; Lavigne, G. Inorg. Chem. 1996, 35, 755.
Notes
In conclusion, the data reported in this paper not only shed light on the reaction mechanism by which cyclic 1, 3-dienes are transformed into cyclic allyl ligands when they react with cluster 1 but also strongly support the general statement that CO-substitution reactions of triruthenium cluster complexes having bridging N-donor ligands proceed through associative mechanisms that involve the concomitant release of a CO ligand cis to the bridging N atom and the attachment of the entering ligand to the metal atom in the coordination site previously occupied by the released CO ligand.
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: NPA atomic charges and atomic coordinates of the stationary points involved in the transformation of 1 into 2. This material is available free of charge via the Internet at http://pubs.acs.org.
