Reactivity of [Os3 (μ-H) 2 (CO) 10] with N-Heterocyclic Carbenes: A

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Organometallics 2010, 29, 3828–3836 DOI: 10.1021/om100679a

Reactivity of [Os3( μ-H)2(CO)10] with N-Heterocyclic Carbenes: A Combined Experimental and DFT Computational Study§ Javier A. Cabeza,*,† Ignacio del Rı´ o,† Jos
e M. Fern
andez-Colinas,† Enrique P
erez-Carre~ no,‡ M. Gabriela S
anchez-Vega,† and Digna V
azquez-Garcı´ a† †

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 July 12, 2010

The unsaturated triosmium dihydrido cluster [Os3( μ-H)2(CO)10] (1) reacts at room temperature with N-heterocyclic carbenes (NHCs) of the 1,3-disubstituted imidazol-2-ylidene type (R1R2Im). While the addition product [Os3H( μ-H)(NHC)(CO)10] has been obtained only for NHC = Mes2Im, CO-substitution derivatives of the type [Os3( μ-H)2(NHC)(CO)9] have been obtained for NHC = Me2Im, MePhIm, Ph2Im, and Mes2Im. Small amounts of the hydroxo-bridged derivatives [Os3( μ-H)( μ-OH)(NHC)(CO)9] (NHC = Me2Im, MePhIm) are also formed in wet solvents. DFT mechanistic studies on the reactions of compound 1 with the small Me2Im and the very bulky Mes2Im have revealed that the substitution reactions follow an associative reaction pathway that starts with the addition of the NHC to compound 1 (in one elementary step that gives [Os3H( μ-H)(NHC)(CO)10]) and ends with the elimination of a CO ligand (in three steps from [Os3H( μ-H)(NHC)(CO)10]) to give the final substitution product [Os3( μ-H)( μ-OH)(NHC)(CO)9]. The relative energy of each stationary point depends upon the NHC ligand used, those of the Mes2Im system being higher than those of the Me2Im system. This computational study has also explained why the addition intermediate [Os3H( μ-H)(NHC)(CO)10] has been isolated only for NHC = Mes2Im.

Introduction The N-heterocyclic carbene (NHC) chemistry of transition metal carbonyl clusters, which was initiated by Lappert and Pye in 1977 (they reported the synthesis of [Ru3(Et2H2Im)(CO)11], Et2H2Im = 1,3-diethylimidazolin-2-ylidene),1 has awakened recently after being asleep for nearly 30 years. Our §

In memory of Professor Jos
e M. Concell
on. *To whom correspondence should be addressed. E-mail: jac@ uniovi.es. (1) Lappert, M. F.; Pye, P. L. J. Chem. Soc., Dalton Trans. 1977, 2172. (2) Cabeza, J. A.; del Rı´ o, I.; Miguel, D.; P
erez-Carre~ no, E.; S
anchezVega, M. G. Organometallics 2008, 27, 211. (3) Cabeza, J. A.; del Rı´ o, I.; Miguel, D.; S
anchez-Vega, M. G. Chem. Commun. 2005, 3956. (4) Cabeza, J. A.; del Rı´ o, I.; Miguel, D.; P
erez-Carre~ no, E.; S
anchezVega, M. G. Dalton Trans. 2008, 1937. (5) Cabeza, J. A.; P
erez-Carre~ no, E. Organometallics 2008, 27, 4697. (6) Cabeza, J. A.; del Rı´ o, I.; Miguel, D.; S
anchez-Vega, M. G. Angew. Chem., Int. Ed. 2008, 47, 1920. (7) Cabeza, J. A.; da Silva, I.; del Rı´ o, I.; S
anchez-Vega, M. G. Dalton Trans. 2006, 3966. (8) Cabeza, J. A.; del Rı´ o, I.; Fern
andez-Colinas, J. M.; S
anchezVega, M. G. Organometallics 2009, 28, 1243. (9) Cabeza, J. A.; Van der Maelen, J. F.; Garcı´ a-Granda, S. Organometallics 2009, 28, 3666. no, E.; S
anchez-Vega, (10) Cabeza, J. A.; del Rı´ o, I.; P
erez-Carre~ M. G.; V
azquez-Garcı´ a, D. Angew. Chem., Int. Ed. 2009, 48, 555. (11) Cabeza, J. A.; del Rı´ o, I.; Fern
andez-Colinas, J. M.; P
erezCarre~ no, E.; S
anchez-Vega, M. G.; V
azquez-Garcı´ a, D. Organometallics 2009, 28, 1832. (12) Ellul, C. E.; Mahon, M. F.; Saker, O.; Whittlesey, M. K. Angew. Chem., Int. Ed. 2007, 46, 6343. pubs.acs.org/Organometallics

Published on Web 08/02/2010

research group2-11 and those of Whittlesey,12-14 Cole,15 Wang,16 and Clyburne17-19 have importantly contributed to the blossoming of this chemistry. We have reported the reactivity of [Ru3(CO)12] and [Os3(CO)12] with a variety of NHCs in 1:1 mol ratio at room temperature,2 showing that [Ru3(CO)12] reacts easily with 1,3-dimethylimidazol-2-ylidene (Me2Im), more slowly with N-methyloxazol-2-ylidene (MeOx), and very slowly with 1,3-dimesitylimidazol-2-ylidene (Mes2Im) to give the corresponding normal CO-substitution products [Ru3(NHC)(CO)11] (NHC = Me2Im, MeOx, Mes2Im). The carbene Me2Im is the only NHC that reacts with [Os3(CO)12] at room temperature, giving [Os3(Me2Im)(CO)11]. The clusters [M3(Me2Im)(CO)11] (M = Ru,3-6 Os4,5) undergo facile methyl C-H bond activation processes upon heating. Levamisole derivatives have been used to prepare ruthenium and osmium clusters with ditopic NHC-thiolate ligands.7 Some reactivity8 (13) Critall, M. R.; Ellul, C. E.; Mahon, M. F.; Saker, O.; Whittlesey, M. K. Dalton Trans. 2008, 4209. (14) Ellul, C. E.; Saker, O.; Mahon, M. F.; Apperley, D. C.; Whittlesey, M. K. Organometallics 2008, 27, 100. (15) Bruce, M. I.; Cole, M. L.; Fung, R. S. C.; Forsyth, C. M.; Hilder, M.; Junk, P. C.; Konstas, K. Dalton Trans. 2008, 4118. (16) Zhang, C.; Luo, F.; Cheng, B.; Li, B.; Song, H.; Xu, S.; Wang, B. Dalton Trans. 2009, 7230. (17) Cooke, C. E.; Ramnial, T.; Jennings, M. C.; Pomeroy, R. K.; Clyburne, J. A. C. Dalton Trans. 2006, 3966. (18) Cooke, C. E.; Jennings, M. C.; Katz, M. J.; Pomeroy, R. K.; Clyburne, J. A. C. Organometallics 2008, 27, 5777. (19) Cooke, C. E.; Jennings, M. C.; Pomeroy, R. K.; Clyburne, J. A. C. Organometallics 2007, 26, 6059. r 2010 American Chemical Society

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Scheme 1

and an AIM theoretical analysis9 of the bonding in [Ru3( μ-H)( μ3-MeImCH)(CO)9] have been reported. Some triruthenium clusters formally derived from pyrid-2-ylidenes have been prepared by deprotonation of pyridinium cations in the presence of [Ru3(CO)12].10 Whittlesey et al. have reported that the very bulky carbenes 1,3-di-tert-butylimidazol-2-ylidene (tBu2Im) and 1,3-diadamantylimidazol-2-ylidene (Ad2Im) react with [M3(CO)12] (M = Ru, Os) in 1:1 mol ratio to give trinuclear CO-substitution products that contain abnormally bound (through C4) tBu2Im12 and Ad2Im ligands,12,13 respectively. These clusters are also prone to undergo intramolecular C-H bond activation processes. These authors have also shown that [Ru3(CO)12] reacts with an excess of some NHCs (at least 1:6 mol ratio) to give mononuclear products of the type [Ru(NHC)2(CO)3].14 Cole et al. have reported that [Ru3(CO)12] reacts with Mes2Im and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (Dipp2Im), in a 1:3 stoichiometric ratio, to give the mononuclear tetracarbonyls [Ru(NHC)(CO)4].15 These authors have also reported that some 1,3-disubstituted imidazole-2thiones and imidazolium chlorides react with [Ru3(CO)12] to give NHC cluster derivatives.15 A recent report by Wang et al. has described the reactivity of [Ru3(CO)12] with an indenyl-functionalized NHC ligand.16 The above-commented works have shown that the reactions of [Ru3(CO)12] and [Os3(CO)12] with NHCs are strongly influenced by the electronic properties and steric demands of the NHCs, the stoichiometry of the reagents, the reaction temperature, and also the intrinsic reactivity of the metal carbonyls. While the NHC chemistry of [Ru3(CO)12] and [Os3(CO)12] has already been studied to a considerable extent, that of other ruthenium and osmium carbonyl clusters has been less explored. Very recently, we have reported that the tetranuclear cluster [Ru4( μ-H)4(CO)12] reacts easily with NHCs to give the normal CO-substitution derivatives [Ru4( μ-H)4(NHC)CO)11] (NHC = Me2Im, Ph2Im, MePhIm, Mes2Im).11 Clyburne, Cooke, et al. have reported the reactions of [AgCl(Mes2Im)] with [Os3( μ-H)2(CO)10],17,18 [Os3(MeCN)2(CO)10],17,18 [Ru4( μ-H)4(MeCN)2(CO)10],18 and [Os4( μ-H)4(MeCN)2(CO)10];19 however, no simple Mes2Im for CO substitution products have been isolated from the reactions of the triosmium clusters, since the products contain chlorine and/or

the silver atoms arising from the [AgCl(Mes2Im)] reagent. The normal (through C2) and abnormal (through C4) coordination of the Mes2Im ligand have also been described for [Os4( μ-H)4(Mes2Im)(CO)11] clusters,18 which are also prone to undergo C-C and C-H bond activation processes at high temperatures.19 In this paper we describe the reactivity of the unsaturated 46-electron cluster [Os3( μ-H)2(CO)10] with a variety of NHC reagents, all of them of the N,N0 -disubstituted imidazol2-ylidene type (R1R2Im). While the addition product [Os3H( μ-H)(NHC)(CO)10] has only been obtained for NHC = Mes2Im, CO-substitution derivatives of the type [Os3( μ-H)2(NHC)(CO)9] have been obtained for NHC = Me2Im, MePhIm, Ph2Im, and Mes2Im. These results have been rationalized with the help of a computational mechanistic study, which, in addition to establishing the participation of the addition products as intermediates in the formation of the CO-substitution products, explains why the addition product has been observed only in the case of Mes2Im.

Results and Discussion Synthesis and Structural Characterization. The unsaturated cluster [Os3( μ-H)2(CO)10] (1) reacted readily with 1,3disubstituted imidazol-2-ylidenes, R1R2Im (prepared in situ from the corresponding imidazolium salts and potassium tert-butoxide), in THF at room temperature (Scheme 1). In all cases, the major products of these reactions were the redviolet CO-substitution derivatives [Os3( μ-H)2(NHC)(CO)9] (NHC = Me2Im, 3a; MePhIm, 3b; Ph2Im, 3c; Mes2Im, 3d). The yellow decacarbonyl product [Os3H( μ-H)(Mes2Im)(CO)10] (2d), which arises from the addition of the NHC to compound 1, was also obtained along with 3d in the reaction of 1 with Mes2Im, while compounds 3a and 3b were accompanied by small amounts of the hydroxo-bridged yellow derivatives [Os3( μ-H)( μ-OH)(NHC)(CO)9] (NHC = Me2Im, 4a; MePhIm, 4b). The product mixtures were separated by thin-layer chromatography (TLC) on silica gel. All isolated compounds gave satisfactory ESI mass spectra, which showed the group of lines corresponding to the molecular ion isotopomers, and elemental microanalyses (C, N, H). They all were also characterized by NMR and IR spectroscopy and, in some cases, by single-crystal X-ray diffraction.

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Figure 1. Molecular structure of compound 3a. Selected bond distances (A˚): Os(1)-Os(2) 2.6864(6), Os(1)-Os(3) 2.8138(6), Os(2)-Os(3) 2.8281(6), C(1)-Os(1) 2.088(9), C(1)-N(1) 1.35(1), C(1)-N(2) 1.35(1), C(2)-N(1) 1.37(1), C(3)-N(2) 1.38(1).

The structure shown in Scheme 1 for the only compound of type 2 (addition products) that was isolated, the Mes2Im derivative 2d, was strongly supported by a DFT structure optimization (see below) and by its 1H NMR spectrum, which, in addition to the signals of the Mes2Im ligand, contains two resonances at -7.95 and -12.16 ppm, both coupled to each other (J = 1.5 Hz), assignable to terminal and bridging hydride ligands, respectively. Such a situation has been previously reported, for example, for other derivatives of the type [Os3H( μ-H)(L)(CO)10]20 (L = CO,20a-c PPh3,20a-c PMe2Ph,20a-c AsMe2Ph,20a PhCN,20b amines,20d,e imines,20f,g t BuNC21), whose X-ray diffraction structures confirm that the terminal hydride and the added ligand occupy axial sites at the same side of the Os3 triangle.22,23 The molecular structures of the nonacarbonyl NHC derivatives 3a and 3c were determined by X-ray diffraction methods (Figures 1 and 2, respectively). They result from the formal substitution of the corresponding NHC ligand for one of the two equatorial CO ligands of the Os2( μ-H)2 moiety of compound 1. Their hydride-bridged Os-Os edge is ca. 0.13 A˚ shorter than the unbridged Os-Os edges, indicating that these clusters maintain the original unsaturation of their precursor 1 localized in the hydride-bridged edge.24 These structural features are comparable with those of [Os3( μ-H)2(CNtBu)(CO)9]22 and [Os3( μ-H)2(PR3)(CO)9].25 Their Os-CNHC bonds, Os(1)-C(1) = 2.088(9) A˚ for 3a and (20) (a) Shapley, J. R.; Keister, J. B.; Churchill, M. R.; De Boer, B. G. J. Am. Chem. Soc. 1975, 97, 4145. (b) Deeming, A. J.; Hasso, S. J. Organomet. Chem. 1975, 88, C21. (c) Deeming, A. J.; Hasso, S. J. Organomet. Chem. 1976, 114, 313. (d) Aime, S.; Dastr
u, W.; Gobetto, R.; Arce, A. J. Organometallics 1994, 13, 4232. (e) Aime, S.; Gobetto, R.; Valls, E. Organometallics 1997, 16, 5140. (f ) Aime, S.; F
erriz, M.; Gobetto, R.; Valls, E. Organometallics 1999, 18, 2030. (g) Cabeza, J. A.; del Río, I.; Grepioni, F.; Riera, V. Organometallics 2000, 19, 4643. (21) Adams, R. D.; Golembeski, N. M. J. Am. Chem. Soc. 1979, 101, 2579. (22) Adams, R. D.; Golembeski, N. M. Inorg. Chem. 1979, 18, 1909. (23) (a) Churchill, M. R.; De Boer, B. G. Inorg. Chem. 1977, 16, 2397. (b) Ehrenreich, W.; Herberhold, M.; Herrmann, G.; S€uss-Fink, G.; Gieren, A.; Hubner, T. J. Organomet. Chem. 1985, 294, 183. (c) Bruce, M. I.; Williams, M. L. J. Organomet. Chem. 1986, 314, 323. (d) Blake, A. J.; Ebsworth, E. A. V.; McIntosh, A. P.; Schroder, M. Acta Crystallogr. 1994, C50, 371. (24) (a) Churchill, M. R.; Hollander, F. J.; Hutchinson, J. P. Inorg. Chem. 1977, 16, 2697. (b) Allen, V. F.; Mason, R.; Hitchcock, P. B. J. Organomet. Chem. 1977, 140, 297. (c) Orpen, A. G.; Rivera, A. V.; Bryan, E. G.; Pippard, D.; Sheldrick, G. M.; Rouse, K. D. J. Chem. Soc., Chem. Commun. 1978, 723. (25) (a) Benfield, R. E.; Johnson, B. F. G.; Lewis, J.; Raithby, P. R.; Zuccaro, C.; Henrick, K. Acta Crystallogr. 1979, B35, 2210. (b) Adams, R. D.; Segmuller, B. E. Cryst. Struct. Commun. 1982, 11, 1971. (c) Tunik, S. P.; Khripun, V. D.; Haukka, M.; Pakkanen, T. A. Dalton Trans. 2004, 1775. (d) Farrugia, L. J. J. Organomet. Chem. 1990, 394, 515. (e) Mottalib, M. A.; Kabir, S. E.; Tocher, D. A.; Deeming, A. J.; Nordlander, E. J. Organomet. Chem. 2007, 692, 5007.

Figure 2. Molecular structure of compound 3c. Selected bond distances (A˚): Os(1)-Os(2) 2.6957(4), Os(1)-Os(3) 2.8165(4), Os(2)-Os(3) 2.8325(5), C(1)-Os(1) 2.081(7), C(1)-N(1) 1.36(1), C(1)-N(2) 1.38(1), C(2)-N(1) 1.41(1), C(3)-N(2) 1.40(1).

2.081(7) A˚ for 3c, are slightly shorter than that of [Os3(Me2Im)(CO)11], 2.116(9) A˚,2 reflecting that the steric hindrance between the NHC and the CO ligands is greater in [Os3(Me2Im)(CO)11] than in 3a and 3c. Curiously, the methyl H atoms of the Me2Im ligand of 3a are closer to the two axial CO groups of the Os(1) atom than the phenyl H atoms of the Ph2Im ligand of 3c, provoking a noticeable separation of these CO ligands from their ideal positions in the cluster. Such a different steric hindrance accounts for the different positions of the bridging hydride atoms (which are always trans to CO ligands) in these clusters, in which the dihedral angles between the Os(1)-H(100)-Os(2) and Os(1)-H(200)Os(2) planes are 130.99(5)o in 3a and 165.93(4)o in 3c. The solution IR spectra of the substitution products 3a-d contain a common band pattern in the carbonyl stretching region. This fact strongly suggests that these complexes are isostructural. An analogous suggestion can be inferred from their 1H NMR spectra, since they all contain, in addition to the resonances of the corresponding NHC ligand, a hydride resonance at approximately the same chemical shift (in the range -11.03 to -11.16 ppm).20a-c,21 The 1H NMR spectrum of the mesityl derivative 3d indicates that, in solution at room temperature, there is either free rotation of the bulky mesityl groups about the N-CMes bond or free rotation of the entire Mes2Im ligand about the Os-Ccarbene bond. The IR spectra in dichloromethane of the hydroxobridged products 4a and 4b display a common band pattern in the carbonyl stretching region. Their solid-state IR spectra (Nujol mulls) contain a band at 3600 cm-1 assignable to the OH stretching vibration. These facts indicate that both complexes are isostructural. An analogous hint can be inferred from their 1H NMR spectra, since they show, in addition to the resonances of the corresponding NHC ligand, a pair of singlets, one assigned to the OH group (at 4.02 ppm for 4a and 4.04 ppm for 4b) and the other to the hydride ligand (at -12.22 ppm for 4a and -12.16 ppm for 4b). The 1H NMR spectrum of the Me2Im derivative 4a indicates that there is free rotation of the Me2Im ligand about the Os-Ccarbene bond, since only two singlet resonances, with a 1:3 integral ratio, are observed for the Me2Im ligand. The atom connectivity of derivative 4a (depicted in

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Figure 3. Relative energy profile and structures of the stationary points involved in the reaction 1 þ Me2Im f 3a þ CO via pathway 1.

Scheme 1) was confirmed by an X-ray diffraction study. However, the low quality of the data we obtained precluded the publication of the structural parameters. All these analytical data (as appropriate) compare well with those of [Os3( μ-H)( μ-OH)(CO)10]26 and [Os3( μ-H)( μ-OH)(PPh3)(CO)9],27 whose structures have been determined by X-ray diffraction. Typical yields of 4a and 4b were 8-10% when the reactions were carried out under dry nitrogen, in Schlenk tubes, using 0.06-0.07 mmol of reagents in sodium-dried THF (10 mL). Under severely dry conditions (oven-dried glassware and all reagents weighed in a drybox, in addition to the previous conditions), the yields of 4a and 4b decreased considerably. Therefore, it is clear that the OH group of 4a and 4b arises from water. As we observed no trace of [Os3( μ-H)( μ-OH)(CO)10],28 4a, or 4b when compound 1 or the dihydrido derivatives 3a and 3b were stirred in wet THF at room temperature, the formation of the hydroxo-bridged products 4a and 4b in the reactions of 1 with Me2Im and MePhIm may be associated with the presence of a small amount of KOH in the reaction mixtures, probably arising from the hydrolysis of KOtBu, which was used for the in situ preparation of the NHCs. No attempts were made to investigate the mechanism of the processes that lead to the hydroxo-bridged products 4a and 4b. It is also unclear why no hydroxo-bridged products were obtained from the reactions of 1 with the bulkier Ph2Im and Mes2Im NHC ligands, which were also prepared in situ by deprotonation of the corresponding imidazolium salt with KOtBu. Computational Mechanistic Studies. In order to shed light on the mechanisms operating in the reactions of compound 1 (26) (a) Knoeppel, D. W.; Chung, J.-H.; Shore, S. G. Acta Crystallogr. 1995, C51, 42. (b) Karpov, M. G.; Tunik, S. P.; Denisov, V. R.; Starova, G. L.; Nikol'skii, A. B.; Dolgushin, F. M.; Yanovsky, A. I.; Struchkov, Y. T. J. Organomet. Chem. 1995, 485, 219. (c) Ho, W. G. Y.; Wong, W. T. Polyhedron 1995, 14, 2849. (27) Podberezskaya, N. V.; Maksakov, V. A.; Kedrova, L. K.; Korniets, E. D.; Gubin, S. P. Koord. Khim. 1984, 10, 919. (28) (a) Johnson, B. F. G.; Lewis, J.; Kilty, P. A. J. Chem. Soc. (A) 1968, 2859. (b) Dragonetti, C.; Lucenti, E.; Roberto, D.; Leong, W. K.; Lin, Q. Inorg. Synth. 2004, 34, 215. (c) Mottalib, M. A.; Begum, N.; Kabir, S. E J. Bangladesh Acad. Sci. 2005, 29, 135.

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with NHCs of imidazol-2-ylidene type and to explain why the behavior of Mes2Im differs from that of the remaining NHCs used in this work, the potential energy surfaces of the reactions of compound 1 with Me2Im and Mes2Im were explored using DFT methods at the B3LYP/LanL2DZ/ 6-31G(d,p) level of theory. Figures 3-6 contain the relative energy profiles and the structures of the intermediates (i) and transition states (ts) of each studied reaction pathway. The given energies are electronic potential energies and are relative to those of the reactants of each reaction (energy of Me2Im or Mes2Im (as appropriate) þ energy of 1 = 0.0 kcal mol-1). Relevant structural parameters of the stationary points involved in the reactions that lead to compounds 3a and 3d, through the pathway with lowest energy barrier in each case, are given in Tables 1 and 2, respectively. Three alternative reactions pathways (Figures 3-5) were found for the reaction 1 þ Me2Im f 3a þ CO. The first step, which is common to the three reaction pathways, involves the addition of Me2Im to one of the hydride-bridged Os atoms of 1. The approach of the entering ligand to the Os1 atom from one side of the Os3 plane induces the nearest hydride ligand (H1) to move from a bridging position, spanning the Os1-Os2 edge in 1 (Os1-H1 = Os2-H1 = 1.856 A˚), to a terminal position on Os2 in i1a (Os1-H1 = 3.403 A˚ and Os2-H1 = 1.668 A˚). In parallel, the Os1-Os2 distance lengthens 0.421 A˚ on going from 1 to i1a. This step releases 28.0 kcal mol-1 and has a very low energy barrier, 3.8 kcal mol-1, as expected for a process in which a coordinatively unsaturated reagent (1; 46-electron) adds a ligand to become a saturated species (i1a; 48-electron). Both the elimination of an equatorial (pathway 1, Figure 3) and that of an axial CO ligand (pathway 2, Figure 4) from the Os1 atom of intermediate i1a (2a in Scheme 1) were found to lead to compound 3a (in one step in pathway 1 or in two steps in pathway 2). However, in both cases, the relative energies calculated for the transition states associated with the COelimination steps (9.1 kcal mol-1 for ts2a1 and 4.6 kcal mol-1 for ts2a2) are higher than that of ts1a (3.8 kcal mol-1), and these data are not compatible with the experimental observation that intermediate i1a (2a in Scheme 1) was not found in the reaction mixture even at short reaction times. This experimental fact implies that the evolution of i1a toward the final product 3a should be faster than that of the reverse process toward 1 þ Me2Im, and, therefore, the transition state of the second step should have a lower relative energy than that of transition state ts1a. These data led us to look for an alternative mechanism more compatible with the experimental observations (pathway 3). The mechanism depicted in Figure 5 (pathway 3) shows that, prior to the CO-elimination step (i3a3 f 3a þ CO), intermediate i1a undergoes a two-step ligand rearrangement that implies (a) a ligand trigonal twist (ca. 60°), involving two CO ligands and the Me2Im ligand, that moves the latter from an axial to an equatorial position on the Os1 atom (i1a f i2a3), and (b) a rotation of the Me2Im ligand about the C1-Os1 bond of i2a3 (i2a3 f i3a3) in such a way that the N-methyl groups are placed away from the axial CO ligand C4O4, which is sited at the same side of the Os3 plane as the terminal hydride ligand H1. In the final step (i3a3 f 3a þ CO), the approach of the hydride H1 to the Os1 metal atom (Os1-H1 = 3.317 A˚ in i3a3, 2.516 A˚ in ts4a3, and 1.900 A˚ in 3a) is accompanied by the release of the C4O4 carbonyl ligand from Os1 (Os1-C4 = 1.960 A˚ in i3a3 and 2.924 A˚ in ts4a3) and the shortening of the Os1-Os2 edge (3.124 A˚ in

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Figure 4. Relative energy profile and structures of the stationary points involved in the reaction 1 þ Me2Im f 3a þ CO via pathway 2.

Figure 5. Relative energy profile and structures of the stationary points involved in the reaction 1 þ Me2Im f 3a þ CO via pathway 3. Table 1. Selected Interatomic Distances (A˚) in the Stationary Points Involved in the Reaction 1 þ Me2Im f 3a þ CO via Pathway 3 atoms

1

ts1a

i1a = 2a

ts2a3

i2a3

ts3a3

i3a3

ts4a3

3a

Os1-Os2 Os1-Os3 Os2-Os3 H1-Os1 H1-Os2 H2-Os1 H2-Os2 C1-Os1 C4-Os1

2.733 2.872 2.876 1.856 1.856 1.856 1.856

2.860 2.885 2.909 2.209 1.717 1.890 1.794 3.223 1.904

3.154 3.007 2.931 3.403 1.668 1.807 1.848 2.186 1.914

3.212 3.031 2.883 3.740 1.673 1.788 1.812 2.185 1.918

3.124 2.997 2.938 3.292 1.673 1.818 1.835 2.135 1.959

3.127 3.005 2.931 3.304 1.670 1.800 1.852 2.150 1.961

3.124 3.001 2.934 3.317 1.671 1.807 1.843 2.135 1.960

2.949 2.896 2.918 2.516 1.684 1.862 1.784 2.154 2.924

2.753 2.881 2.892 1.900 1.815 1.900 1.815 2.109

i3a3, 2.949 A˚ in ts4a3, and 2.753 A˚ in 3a). It should be noted that the relative energies associated with ts2a3 (-0.4 kcal mol-1), ts3a3 (-25.4 kcal mol-1), and ts4a3 (3.6 kcal mol-1) are low (close to that of the starting materials) and lower than that of ts1a (3.8 kcal mol-1). Although intermediates i1a,

i2a3, and i3a3 are more stable than 3a þ CO, the experimental irreversibility of the last step, in which CO gas is released, drives to the end the overall process, which releases 21.5 kcal mol-1. These facts are vital to explain the fact that no intermediates have been experimentally observed.

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Figure 6. Relative energy profile and structures of the stationary points involved in the reaction 1 þ Mes2Im f 3d þ CO via pathway 3. The energies and structures of the transition states ts2d1 (of pathway 1) and ts2d2 (of pathway 2) are also shown. Table 2. Selected Interatomic Distances (A˚) in the Stationary Points Involved in the Reaction 1 þ Mes2Im f 3d þ CO via Pathway 3 atoms

1

ts1d

i1d = 2d

ts2d3

i2d3

ts3d3

i3d3

ts4d3

3d

Os1-Os2 Os1-Os3 Os2-Os3 H1-Os1 H1-Os2 H2-Os1 H2-Os2 C1-Os1 C4-Os1

2.733 2.872 2.876 1.856 1.856 1.856 1.856

2.905 2.910 2.923 2.470 1.689 1.852 1.801 3.233 1.920

3.152 3.018 2.935 3.432 1.658 1.811 1.868 2.240 1.926

3.205 3.067 2.889 3.694 1.672 1.808 1.806 2.219 1.909

3.127 3.027 2.935 3.333 1.669 1.825 1.870 2.158 1.958

3.129 3.044 2.935 3.319 1.669 1.825 1.862 2.161 1.957

3.138 3.036 2.932 3.317 1.617 1.821 1.838 2.155 1.959

2.951 2.925 2.919 2.492 1.690 1.883 1.785 2.169 2.933

2.759 2.890 2.888 1.904 1.817 1.909 1.814 2.125

For the reaction of compound 1 with Mes2Im, three alternative reaction pathways were also found (Figure 6). They are entirely analogous to those described above for the reaction of 1 with Me2Im (Figures 3-5), but they differ in their relative energies, which, for intermediates and transition states that are equivalent at the molecular level (e.g., i1a vs i1d), are 13-22 kcal mol-1 higher in the case of the Mes2Im system than in the case of the Me2Im system. Again, the relative energies of the transition states associated with the second step of pathway 1 (ts2d1; 27.3 kcal mol-1) and pathway 2 (ts2d2; 21.4 kcal mol-1) were found to be higher than that of pathway 3 (ts2d3; 18.7 kcal mol-1), indicating that (29) Due to the long computing times required by these systems, the mechanistic studies on the disfavored pathways (1 and 2) of the reaction of 1 with Mes2Im were not studied further.

the latter pathway is the most favored one also in the reaction of compound 1 with Mes2Im.29 It is important to note that, in this case, the relative energy of transition state ts2d3 (18.7 kcal mol-1) is higher than that of ts1d (16.8 kcal mol-1). This implies that the rate of disappearance of intermediate i1d (2d in Scheme 1) toward compound 1 and Mes2Im should be faster than that of its evolution toward i2d3, and therefore, it explains the experimental observation of i1d (2d) in the reaction mixture. An inspection of the Os-CNHC bond distances calculated for the stationary points of pathway 3 of both reactions, 1 þ Me2Im (Figure 6, Table 1) and 1 þ Mes2Im (Figure 6, Table 2), reveals that, for all intermediates and transition states, the Os1-C1Me2Im distances are 0.01-0.05 A˚ shorter than the corresponding Os1-C1Mes2Im distances. This is due to the fact that the steric hindrance of Mes2Im is larger than that of Me2Im

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and accounts at least in part for the higher relative energies calculated for the stationary points of the Mes2Im system (Figure 6) with respect to the analogous stationary points of the Me2Im system (Figure 5), since a longer Os1-C1NHC distance implies a less efficient overlap between the orbitals involved in the Os1-C1NHC bond. For these substitution reactions, we also considered the possibility of a dissociative mechanism, in which a CO ligand was released from compound 1 prior to the addition of the NHC ligand, but all our calculations in this direction resulted in energy barriers for the first step (CO dissociation) much higher (>30 kcal mol-1) than that of the first step of the pathways discussed above (3.8 kcal mol-1).

Concluding Remarks The substitution of an NHC of the imidazol-2-ylidene family for a CO ligand of [Os3( μ-H)2(CO)10] (1) proceeds smoothly at room temperature to give the corresponding [Os3( μ-H)2(CO)9(NHC)] derivative (compounds 3a-d), even with the very bulky carbene Mes2Im, which, additionally, is the only NHC for which an addition product, [Os3H( μ-H)(CO)10(Mes2Im)] (2d), has also been isolated. Small amounts of hydroxo-bridged derivatives, [Os3( μ-H)( μ-OH)(NHC)(CO)9] (NHC = Me2Im, 4a; MePhIm, 4b), arising from the presence of moisture in the reaction mixtures, were also obtained from reactions of compound 1 with Me2Im and MePhIm. DFT mechanistic studies on the reactions of compound 1 with the small Me2Im and the very bulky Mes2Im have revealed that the substitution reactions follow a common associative reaction pathway that starts with the addition of the NHC to compound 1 (in one elementary step that gives 2a or 2d) and ends with the elimination of a CO ligand (in three steps from 2a or 2d) to give the final substitution product (3a or 3d). The relative energy of each stationary point depends upon the NHC ligand used, those of the Mes2Im system being higher than those of the Me2Im system. An inspection of the energy barriers associated with the transformation of the addition intermediates 2a and 2d also explains why the species 2a (NHC = Me2Im) has not been experimentally observed whereas 2d (NHC = Mes2Im) has been actually isolated. The mechanistic study described herein also sheds light on many previously reported reactions of compound 1 with neutral two-electron-donor ligands, which give nonacarbonyl substitution products in some cases and decacarbonyl addition products in other cases.30

Experimental Section General Data. The osmium cluster [Os3( μ-H)2(CO)10] was prepared by a published method.31 The remaining reagents were purchased from commercial suppliers. Imidazolium salts and KOtBu were stored, weighed, and transferred to the reaction Schlenk tube within an MBraun drybox. Solvents were dried over sodium diphenyl ketyl (hydrocarbons, THF) or CaH2 (dichloromethane) and distilled under nitrogen before use. For reactions carried out under “very dry” conditions, the glassware was dried in an oven at 120 °C for at least 12 h and was transferred hot to the drybox, where it was allowed to cool to (30) See, for example, the reactions mentioned in refs 20-25. (31) Knox, S. A. R.; Koepke, J. W.; Andrews, M. A.; Kaesz, H. D. J. Am. Chem. Soc. 1975, 97, 3942.

Cabeza et al. room temperature. The reactions were carried out under nitrogen, using Schlenk-vacuum line techniques, and were routinely monitored by solution IR spectroscopy (carbonyl stretching region) and spot TLC. IR spectra were recorded in solution on a Perkin-Elmer Paragon 1000 FT spectrophotometer. 1H NMR spectra were run on a Bruker DPX-300 instrument, using the dichloromethane solvent resonance as internal standard (δ = 5.30). Microanalyses were obtained from the University of Oviedo Analytical Service. ESI mass spectra were obtained from the University of A Coru~ na Mass Spectrometric Service; data given refer to the most abundant molecular ion isotopomer. [Os3( μ-H)2(Me2Im)(CO)9] (3a) and [Os3( μ-H)( μ-OH)(Me2Im)(CO)9] (4a). A mixture of potassium tert-butoxide (8 mg, 0.071 mmol) and 1,3-dimethylimidazolium iodide (16 mg, 0.071 mmol) in THF (10 mL) was stirred at room temperature for 30 min. Then, compound 1 (60 mg, 0.071 mmol) was added and the mixture was stirred for 2 h. The color changed from violet to yellow-brown. The solvent was removed under reduced pressure, and the resulting residue was redissolved in dichloromethane (10 mL). The filtered solution was concentrated to ca. 1 mL and was supported on preparative silica gel TLC plates. Dichloromethane-hexaneacetone (2:2:1) eluted several bands. The first one, yellow, contained compound 4a (8 mg, 12%). The second band was very weak and was discarded. The third band, red-violet, contained compound 3a (34 mg, 52%). Using oven-dried glassware, the yield of 4a decreased to 4%. Data for 3a: Anal. Calcd for C14H10N2O9Os3 (920.93): C, 18.26; H, 1.09; N, 3.04. Found: C, 18.21; H, 1.04; N, 2.99. (þ)-ESI-MS: m/z 922 [M]þ. IR (CH2Cl2): νCO 2088 (m), 2046 (s), 2003 (vs), 1996 (sh), 1982 (sh), 1933 (w). 1H NMR (CDCl3, 293 K): δ 7.06 (s, 2 H, CH), 3.68 (s, 6 H, Me), -11.16 (s, 2 H, μ-H). Data for 4a: Anal. Calcd for C14H10N2O10Os3 (936.92): C, 17.95; H, 1.08; N, 2.99. Found: C, 17.90; H, 1.03; N, 2.94. (þ)-ESI-MS: m/z 938 [M]þ. IR (CH2Cl2): νCO 2087 (m), 2046 (s), 2001 (vs), 1991 (sh), 1957 (sh), 1923 (w). 1H NMR (CDCl3, 293 K): δ 6.98 (s, 2 H, CH), 4.02 (s, br, 1 H, OH), 3.90 (s, 6 H, Me), -12.22 (s, 1 H, μ-H). [Os3( μ-H)2(MePhIm)(CO)9] (3b) and [Os3( μ-H)( μ-OH)(MePhIm)(CO)9] (4b). A mixture of potassium tert-butoxide (7 mg, 0.062 mmol) and 1-phenyl-3-methylimidazolium iodide (10 mg, 0.062 mmol) in THF (10 mL) was stirred at room temperature for 30 min. Then, compound 1 (50 mg, 0.059 mmol) was added and the mixture was stirred for 2 h. The color changed from violet to yellow-brown. The solvent was removed under reduced pressure, and the resulting residue was redissolved in dichloromethane (10 mL). The filtered solution was concentrated to ca. 1 mL and was supported on preparative silica gel TLC plates. Dichloromethane-hexane (1:1) eluted compounds 3b (first band, red-violet, 25 mg, 43%) and 4b (second band, yellow, 6 mg, 10%). Using oven-dried glassware, the yield of 4b decreased to 4%. Data for 3b: Anal. Calcd for C19H10N2O9Os3 (980.99): C, 23.22; H, 1.23; N, 2.85. Found: C, 23.17; H, 1.18; N, 2.80. (þ)-ESI-MS: m/z 982 [M]þ. IR (CH2Cl2): νCO 2085 (m), 2049 (s), 2000 (vs), 1995 (sh), 1954 (sh), 1929 (w). 1H NMR (CDCl3, 293 K): δ 7.47 (m, 4 H, CH), 7.31 (m, 1 H, CH), 7.22 (s, br, 1 H, CH), 7.19 (s, br, 1 H, CH), 3.81 (s, 3 H, Me), -11.15 (s, br, 2 H, μ-H). Data for 4b: Anal. Calcd for C19H10N2O10Os3 (996.99): C, 22.89; H, 1.01; N, 2.81. Found: C, 23.02; H, 1.05; N, 2.74. (þ)-ESI-MS: m/z 998 [M]þ. IR (CH2Cl2): νCO 2087 (m), 2046 (s), 2002 (vs), 1988 (sh), 1955 (sh), 1924 (w). 1H NMR (CDCl3, 293 K): δ 7.56 (m, 3 H, CH), 7.46 (m, 2 H, CH), 7.16 (d, J = 2.4 Hz, 1 H, CH), 7.14 (d, J = 2.4 Hz, 1 H, CH), 4.04 (s, br, 1 H, OH), 3.79 (s, 3 H, Me), -12.16 (s, 1 H, μ-H). [Os3( μ-H)2(Ph2Im)(CO)9] (3c). A mixture of potassium tertbutoxide (7 mg, 0.062 mmol) and 1,3-diphenylimidazolium iodide (13 mg, 0.062 mmol) in THF (10 mL) was stirred at room temperature for 30 min. Then, compound 1 (50 mg, 0.059 mmol) was added and the mixture was stirred for 2 h. The color changed from violet to yellow-brown. The solvent was removed under reduced pressure, and the resulting residue was redissolved in dichloromethane (10 mL). The filtered solution was concentrated to ca. 1 mL and was supported on preparative silica gel TLC plates. Dichloromethane-hexane-acetone

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Organometallics, Vol. 29, No. 17, 2010

Table 3. Crystal, Measurement, and Refinement Data for the Compounds Studied by X-ray Diffraction

formula fw cryst syst space group a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg V, A˚3 Z F(000) Dcalcd, g cm-3 μ(radiation), mm-1 cryst size, mm temp, K θ limits, deg min./max. h min./max. k min./max. l no. of collected reflns no. of unique reflns no. of reflns with I > 2σ(I) no. of params/restraints GOF (on F2) R1 (on F, I > 2σ(I)) wR2 (on F2, all data) max./min. ΔF, e A˚-3

3a

3c

C14H10N2O9Os3 920.84 monoclinic P21/c 8.8493(1) 17.5956(2) 14.0691(2) 90 102.775(1) 90 2136.45(5) 4 1632 2.863 33.391 (Cu KR) 0.06  0.04  0.02 293(2) 4.09 to 70.57 -10/10 -20/20 -16/16 11 938 4009 3080 259/0 1.065 0.0342 0.0809 0.782/-1.454

C24H14N2O9Os3 1044.98 monoclinic C2/c 35.5574(5) 7.8812(1) 22.7345(3) 90 109.310(1) 90 10118.0(3) 8 3776 2.309 23.850 (Cu KR) 0.04  0.03  0.02 293(2) 4.09 to 70.60 -43/43 -9/8 -27/27 24 272 5770 4760 349/0 1.022 0.0341 0.0977 2.492/-1.342

(2:2:1) eluted several bands. The third and major band, redviolet (the remaining bands were very weak), contained compound 3c (22 mg, 36%). Anal. Calcd for C24H14N2O9Os3 (1045.07): C, 27.58; H, 1.35; N, 2.68. Found: C, 27.53; H, 1.30; N, 2.63. (þ)-ESI-MS: m/z 1046 [M]þ. IR (CH2Cl2): νCO 2085 (m), 2044 (s), 2000 (vs), 1991 (sh), 1957 (sh), 1927 (w). 1H NMR (CDCl3, 293 K): δ 7.46 (m, 8 H, CH), 7.32 (m, 2 H, CH), 7.22 (s, 2 H, CH), -11.10 (s, 2 H, μ-H). [Os3H( μ-H)(Mes2Im)(CO)10] (2d) and [Os3( μ-H)2(Mes2Im)(CO)9] (3d). A mixture of potassium tert-butoxide (18 mg, 0.164 mmol) and 1,3-dimesitylimidazolium chloride (56 mg, 0.164 mmol) in THF (10 mL) was stirred at room temperature for 30 min. Then, compound 1 (140 mg, 0.164 mmol) was added and the mixture was stirred for 2 h. The color changed from violet to yellow-brown. The solvent was removed under reduced pressure, and the resulting residue was redissolved in dichloromethane (10 mL). The filtered solution was concentrated to ca. 1 mL and was supported on preparative silica gel TLC plates. Dichloromethane-hexane (1:1) eluted compound 2d (second band, yellow, 32 mg, 17%) and compound 3d (first band, redviolet, 40 mg, 21%). Data for 2d: Anal. Calcd for C31H26N2O10Os3 (1157.24): C, 32.17; H, 2.26; N, 2.42. Found: C, 32.22; H, 2.28; N, 2.31. (þ)-ESI-MS: m/z 1158 [M]þ. IR (CH2Cl2): νCO (32) 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.

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2087 (m), 2044 (s), 2000 (vs), 1975 (sh), 1960 (sh), 1926 (w). 1H NMR (CDCl3, 293 K): δ 7.07 (s, br, 2 H, CH), 7.04 (s, 2 H, CH), 7.02 (s, br, 2 H, CH), 2.36 (s, 6 H, Me), 2.24 (s, 6 H, Me), 2.11 (s, 6 H, Me), -7.95 (d, J = 1.5 Hz, 1 H, OsH), -12.16 (d, J = 1.5 Hz, 1 H, μ-H). Data for 3d: Anal. Calcd for C30H26N2O9Os3 (1129.05): C, 31.90; H, 2.32; N, 2.48. Found: C, 31.94; H, 2.36; N, 2.22. (þ)-ESI-MS: m/z 1130 [M]þ. IR (CH2Cl2): νCO 2085 (m), 2044 (s), 2004 (vs), 1977 (sh), 1955 (sh), 1936 (w). 1H NMR (CDCl3, 293 K): δ 7.16 (s, 2 H, CH), 6.97 (s, 4 H, CH), 2.29 (s, 12 H, Me), 2.13 (s, 6 H, Me), -11.03 (s, 2 H, μ-H). Computational Details. All optimized structures were calculated by hybrid DFT, within the GAUSSIAN-03 program suite,32 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 Os atoms.35 The basis set used for the remaining atoms was the standard 6-31G with addition of (d,p)-polarization. It has been previously shown that the B3LYP/LanL2DZ/6-31G(d,p) level of theory provides an acceptable balance between accuracy and computing time when working with large molecules containing heavy atoms.5,10,11,36 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 appropriate intermediates, reactants, or products. All energies given in this article are potential energies calculated in the gas phase. X-ray Diffraction Analyses. Diffraction data for 3a and 3c were collected on an Oxford Diffraction Xcalibur Nova single-crystal diffractometer, using Cu KR radiation. Empirical absorption corrections were applied using the SCALE3 ABSPACK algorithm implemented in the program CrysAlisPro RED.37 The structures were solved by Patterson interpretation using the program DIRDIF.38 Isotropic and full-matrix anisotropic least-squares refinements were carried out using SHELXL.39 All non-H atoms were refined anisotropically. The hydride H atoms of both structures were located in the corresponding Fourier maps and refined with restricted thermal parameters. All the remaining hydrogen atoms were set in calculated positions and refined riding on their parent atoms. A solvent accessible void was found in the final model of 3c. Although the mean residual peaks (all smaller than 0.25 C-atom) were located in that void, all attempts to model a disordered residual solvent molecule failed. Therefore, no solvent was included in the final refinement model. The molecular plots were made with the PLATON program package.40 The WINGX program system41 was used throughout the structure determinations. A selection of crystal, measurement, and refinement data is (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) (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 Río, I.; Goite, M. C.; P
erez-Carre~no, E.; Pruneda, V. Chem.-Eur. J. 2009, 7339. (e) Cabeza, J. A.; del Río, I.; Fern
andez-Colinas, J. M.; P
erezCarre~no, E.; V
azquez-García, D. Organometallics 2010, 29, in press (ASAP, DOI: 10.1021/om100148z). (37) CrysAlisPro RED, version 1.171.31.7; Oxford Diffraction Ltd.: Oxford, UK, 2006. (38) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Smits, J. M. M.; Garcı´ a-Granda, S.; Gould, R. O. The DIRDIF Program System, version 2008.3; Crystallography Laboratory, University of Nijmegen: Nijmegen, The Netherlands, 2008. (39) Sheldrick, G. M. SHELXL, version 2008. Acta Crystallogr. 2008, A64, 112. (40) Spek, A. L. PLATON: A Multipurpose Crystallographic Tool, version 1.15; University of Utrecht: Utrecht, The Netherlands, 2008. (41) Farrugia, L. J. WinGX, version 1.80.05 (2009). J. Appl. Crystallogr. 1999, 32, 837.

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Cabeza et al.

given in Table 3. CCDC deposition numbers: 777807 (3a) and 777806 (3c).

of Carabobo, Venezuela) and D.V.-G. (from Xunta de Galicia, Spain) are also acknowledged.

Acknowledgment. This work has been supported by the European Union (FEDER grants), the Spanish MICINN (projects CTQ2007-60865 and MAT2006-1997), and the Government of Principado de Asturias (project IB09-093). Fellowships to M.G.S.-V. (from the University

Supporting Information Available: Atomic coordinates of the structures optimized by DFT calculations and crystallographic data in CIF format for the compounds studied by X-ray diffraction. This material is available free of charge via the Internet at http://pubs.acs.org.