Experimental and Computational Study of the Framework

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Organometallics 2009, 28, 3140–3151

Experimental and Computational Study of the Framework Fluxionality of Organometallic Derivatives of Polyoxometalates: Analysis of the Effect of the Metal and of the Solvent Danielle Laurencin,†,‡ René Thouvenot,† Kamal Boubekeur,† Françoise Villain,† Richard Villanneau,† Marie-Madeleine Rohmer,*,§ Marc Beńard,§ and Anna Proust*,† Institut Parisien de Chimie Molećulaire, UMR 7201, Case courrier 42, UniVersite´ Pierre et Marie Curie Paris 6, 4 place Jussieu, 75252 Paris Cedex 05, France, and Institut de Chimie, UMR 7177, UniVersite´ de Strasbourg, 4 rue Blaise Pascal, 67000 Strasbourg, France ReceiVed December 5, 2008

Organometallic oxides of the general formula [Mo4O16{Ru(arene)}4] display two isomeric forms, referred to as the “triple-cubane” and “windmill” structures. In a previous study (Laurencin et al. Chem.–Eur. J. 2004, 10, 208), we showed that the arene ligand plays an important role in the relative stability of the two isomers. In this work, new experiments and DFT calculations have been carried out to try to rationalize and further control the isomerization process in solution. The synthesis and spectroscopic study of [Mo4O16{Os(p-cymene)}4] showed that, when ruthenium is replaced by osmium, the equilibria in CHCl3 and CH2Cl2 can be altered. In the case of [Mo4O16{Ru(p-cymene)}4], 1H NMR experiments carried out in CD2Cl2/CD3OD solutions revealed that the solvent, through its polarity and molecular form, has a strong impact on the isomerization. DFT calculations were performed on [Mo4O16{M(arene)}4] (M ) Ru, Os; arene ) benzene, toluene, mesitylene, p-cymene, hexamethylbenzene). The change in relative energy of the computed structures upon switching from ruthenium to osmium was found to be in line with the experimental observations. Calculations also showed that the dielectric properties of the solvent, as well as the facility with which it can access the oxometallic core, directly influence the relative stabilities of the two isomers. This work should thus open the way to further studies of organometallic oxides, by allowing their rational preparation and a better understanding of their properties, through a precise analysis of their interaction with the solvation environment. Introduction Organometallic oxides are compounds in which an oxometallic core is surrounded by organometallic moieties.1 Over the past 15 years, organometallic derivatives of polyoxometalates have emerged as a specific class of organometallic oxides and have drawn the attention of many research groups, including ours.2-9 Indeed, these materials can be considered as discrete * To whom correspondence should be addressed. E-mail: anna.proust@ upmc.fr (A.P.), [email protected] (M.-M.R.). Fax: +33-(0)1-4427-38-41 (A.P.). † Universite´ Pierre et Marie Curie Paris 6. ‡ Current address: Institut Charles Gerhardt de Montpellier, UMR 5253, CNRS-UM2-ENSCM-UM1, Universite´ de Montpellier 2, CC 1701, Place Euge`ne Bataillon, 34095 Montpellier cedex 5, France. § Universite´ de Strasbourg. (1) (a) Bottomley, F.; Suttin, L. AdV. Organomet. Chem. 1988, 28, 239. (b) Bottomley, F. Polyhedron 1992, 11, 1707. (c) Herrmann, W. A. J. Organomet. Chem. 1995, 500, 149–174. (d) Herrmann, W. A.; Ku¨hn, F. E. Acc. Chem. Res. 1997, 30, 169–180. (2) (a) Day, V. W.; Klemperer, W. G. Science 1985, 228, 533. (b) Day, V. W.; Klemperer, W. G. Polyoxometalates: From Platonic Solids to AntiRetroViral ActiVity; Pope, M. T., Mu¨ller, A., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994; pp 87-104. (3) (a) Finke, R. G.; Rapko, B.; Domaille, P. J. Organometallics 1986, 5, 175. (b) Weiner, H.; Aiken, J. D., III; Finke, R. G. Inorg. Chem. 1996, 35, 7903. (c) Hayashi, Y.; Mu¨ller, F.; Lin, Y.; Miller, S. S.; Anderson, O. P.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 11401. (4) (a) Hayashi, Y.; Toriumu, K.; Isobe, K. J. Am. Chem. Soc. 1988, 110, 3666. (b) Hayashi, Y.; Ozawa, Y.; Isobe, K. Inorg. Chem. 1991, 30, 1025. (c) Isobe, K.; Nishioka, T.; Toriumi, K.; Ozawa, Y. Inorg. Chem. 1994, 33, 833. (5) Collange, E.; Metteau, L.; Richard, P.; Poli, R. Polyhedron 2004, 2605.

analogues of solid-oxide-supported heterogeneous catalysts, and they thus provide insight into the dynamics of organometallic fragments at the oxide surface.7b,10 Furthermore, they can serve as precursors for transition-metal nanoclusters,11 and they can display synergetic12 or bifunctional catalytic activity.13 Whereas d6-fac-{M(CO)3}+ (M ) Mn, Re) and d6-{M′(C5Me5)}2+ (M′ (6) (a) Su¨ss-Fink, G.; Plasseraud, L.; Ferrand, V.; Stoeckli-Evans, H. Chem. Commun. 1997, 1657. (b) Su¨ss-Fink, G.; Plasseraud, L.; Ferrand, V.; Stanislas, S.; Neels, A.; Stoeckli-Evans, H.; Henry, M.; Laurenczy, G.; Roulet, P. Polyhedron 1998, 17, 2817. (c) Plasseraud, L.; Stoeckli-Evans, H.; Su¨ss-Fink, G. Inorg. Chem. Commun. 1999, 2, 344. (7) (a) Gouzerh, P.; Proust, A. Chem. ReV. 1998, 98, 77. (b) Artero, V.; Proust, A.; Herson, P.; Thouvenot, R.; Gouzerh, P. Chem. Commun. 2000, 883. (c) Artero, V.; Proust, A.; Herson, P.; Gouzerh, P. Chem.sEur. J. 2001, 7, 3901. (d) Villanneau, R.; Artero, V.; Laurencin, D.; Herson, P.; Proust, A.; Gouzerh, P. J. Mol. Struct. 2003, 656, 67. (e) Laurencin, D.; Fidalgo, E.-G.; Villanneau, R.; Villain, F.; Herson, P.; Pacifico, J.; StoeckliEvans, H.; Beńard, M.; Rohmer, M.-M.; Su¨ss-Fink, G.; Proust, A. Chem.sEur. J. 2004, 10, 208. (f) Artero, V.; Laurencin, D.; Thouvenot, R.; Herson, P.; Gouzerh, P.; Proust, A. Inorg. Chem. 2005, 44, 2826. (g) Laurencin, D.; Villanneau, R.; Herson, P.; Thouvenot, R.; Jeannin, Y.; Proust, A. Chem. Commun. 2005, 5524. (h) Laurencin, D.; Villanneau, R.; Ge´rard, H.; Proust, A. J. Phys. Chem. A 2006, 110, 6345. (8) (a) Bi, L.-H.; Kortz, U.; Dickman, M. H.; Keita, B.; Nadjo, L. Inorg. Chem. 2005, 44, 7485. (b) Bi, L.-H.; Chubarova, E. V.; Nsouli, N. H.; Dickman, M. H.; Kortz, U.; Keita, B.; Nadjo, L. Inorg. Chem. 2006, 45, 8575. (c) Mal, S. S.; Nsouli, N.; Dickman, M. H.; Kortz, U. Dalton Trans. 2007, 2627. (9) (a) Sakai, Y.; Shinohara, A.; Hayashi, K.; Nomiya, K. Eur. J. Inorg. Chem. 2006, 163. (b) Kato, C. N.; Shinohara, A.; Moriya, N.; Nomiya, K. Catal. Commun. 2006, 7, 413. (c) Nomiya, K.; Hayashi, K.; Kasahara, Y.; Iida, T.; Nagaoka, Y.; Yamamoto, H.; Ueno, T.; Sakai, Y. Bull. Chem. Soc. Jpn. 2007, 80, 724. (d) Nomiya, K.; Kasahara, Y.; Sado, Y.; Shinohara, A. Inorg. Chim. Acta 2007, 360, 2313.

10.1021/om8011568 CCC: $40.75  2009 American Chemical Society Publication on Web 05/11/2009

Framework Fluxionality of Polyoxometalate DeriVatiVes

Figure 1. Molecular structures of the (left) triple-cubane7e and (right) windmill structures.6a

) Rh, Ir) units played a key role in the initial development of the organometallic chemistry of polyoxometalates,7a great interest has more recently been paid to d6-{Ru(arene)}2+ moieties, because of the potential applications of (arene)ruthenium complexes in catalysis.14 Among the numerous {Ru(arene)}2+ derivatives of polyoxometalates described in the literature, particular attention has been paid to the family of complexes with the general formula [M4O16{Ru(arene)}4] (M ) Mo, W).6b,7b,c,e In the solid state, two isomeric forms of these compounds have been evidenced. They are referred to as the “triple-cubane” (tc) and “windmill” (wm) forms,6a and they differ in the arrangement of the four {Ru(arene)}2+ fragments around the central {M4O16}8- core (Figure 1). In solution, some of the species maintain their molecular structure, whereas others undergo isomerization, during which coordination sites on the ruthenium are temporarily released (Figure 2). This isomerization reaction is of high interest. Indeed, we recently showed that the [Mo4O16{Ru(arene)}4] complexes that isomerize tend to evolve faster into a catalytically active species for the racemization of secondary alcohols.12b Furthermore, this framework fluxionality provides insight into the interactions between organometallic fragments and oxide surfaces and could (10) (a) Besecker, C. J.; Day, V. W.; Klemperer, W. G.; Thompson, M. R. J. Am. Chem. Soc. 1984, 106, 4125. (b) Abe, M.; Isobe, K.; Kida, K.; Yagasaki, A. Inorg. Chem. 1996, 35, 5114. (c) Nagata, T.; Pohl, M.; Weiner, H.; Finke, R. G. Inorg. Chem. 1997, 36, 1366. (d) Laurencin, D.; Thouvenot, R.; Boubekeur, K.; Proust, A. Dalton Trans. 2007, 1334. (11) (a) Lin, Y.; Finke, R. G. Inorg. Chem. 1994, 33, 4891–4910. (b) Aiken, J. D., III; Lin, Y.; Finke, R. G. J. Mol. Catal. A 1996, 114, 29. (c) ¨ zkar, S.; Finke, R. G. J. Am. Chem. Soc. 2002, 124, 5796–5810. (d) Finke, O ¨ zkar, S. Coord. Chem. ReV. 2004, 248, 135–146. (e) Boujday, S.; R. G.; O Blanchard, J.; Villanneau, R.; Krafft, J.-M.; Geantet, C.; Louis, C.; Breysse, M.; Proust, A. Chem. Phys. Chem. 2007, 8, 2636. (12) (a) Takahashi, A.; Yamagushi, M.; Shido, T.; Ohtani, H.; Isobe, K.; Ishikawa, M. J. Organomet. Chem. 1989, 373, C21. (b) Laurencin, D.; Villanneau, R.; Proust, A.; Brethon, A.; Arends, I. W. C. E.; Sheldon, R. A. Tetrahedron: Asymmetry 2007, 18, 367. (13) (a) Siedle, R.; Markell, C. G.; Lyon, P. A.; Hodgson, K. O.; Roe, A. L. Inorg. Chem. 1987, 26, 219. (b) Bar-Nahum, I.; Khenkin, A. M.; Neumann, R. J. Am. Chem. Soc. 2004, 126, 10236. (c) Branytska, O.; Shimon, L. J. W.; Neumann, R. Chem. Commun. 2007, 3957. (14) (a) Bennett, M. A.; Huang, T.-N.; Turney, T.; W., J. Chem. Soc., Chem. Commun. 1979, 312. (b) Cook, J.; Hamlin, J. E.; Nutton, A.; Maitlis, P. M. J. Chem. Soc., Chem. Commun. 1980, 144. (c) Lau, C. P.; Cheng, L. J. Mol. Catal. 1994, 464, 103. (d) Demonceau, A.; Stumpf, A. W.; Saive, E.; Noels, A. F. Macromolecules 1997, 30, 3127. (e) Hafner, A.; Mu¨hlebach, A.; van der Schaaf, P. A. Angew. Chem., Int. Ed. Engl. 1997, 36, 2121. (f) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413. (g) Fu¨rstner, A.; Liebl, M.; Lehmann, C. W.; Piquet, M.; Kunz, R.; Bruneau, C.; Touchard, D.; Dixneuf, P. H. Chem.sEur. J. 2000, 6, 1847. (h) Jan, D.; Delaude, L.; Simal, F.; Demonceau, A.; Noels, A. F. J. Organomet. Chem. 2000, 606, 55. (i) Rhyoo, H. Y.; Park, H.-J.; Chung, Y. K. Chem. Commun. 2001, 2064. (j) Su¨ss-Fink, G.; Faure, M.; Ward, T. R. Angew. Chem., Int. Ed. 2002, 41, 99. (k) Dijksman, A.; Elzinga, J. M.; Li, Y.-X.; Arends, I. W. C. E.; Sheldon, R. A. Tetrahedron: Asymmetry 2002, 13, 879.

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Figure 2. Schematic representation of the isomerization between the (left) triple-cubane and (right) windmill structures. For reasons of clarity, only the oxometallic architectures are represented.7b,e

thus help in the development of fields such as surface organometallic chemistry.15 To control such isomerization phenomena, it thus appears of great importance to understand in detail which parameters have a strong influence on them. In a previous study, we performed an experimental and computational analysis of the effects of the arene ligand on the formation and isomerization of [Mo4O16{Ru(arene)}4] complexes.7e Experimentally, we found that the arene plays an important but complex role. For instance, [Mo4O16{Ru(hmb)}4] (hmb ) hexamethylbenzene) crystallizes as the windmill form and does not display any framework fluxionality in solution, whereas [Mo4O16{Ru(tol)}4] (tol ) toluene) crystallizes as the triple-cubane form and isomerizes in methanol. Furthermore, in the case of [Mo4O16{Ru(p-cym)}4] (p-cym ) p-cymene), which crystallizes as the windmill structure and isomerizes in CHCl3 and CH2Cl2, the relative proportion between the isomers depends on the solvent.7b,e Density functional theory (DFT) calculations performed in the gas phase showed that, although the energy difference between the two isomers can be modulated by intramolecular interactions involving the arene, the windmill form is systematically more stable (by at least 0.4 eV), which means that the environment must play a key role in the stabilization of the triple-cubane form. In a continuation of our previous work, we have carried out further experimental and computational investigations of these systems, with the aims of better understanding the influence of the environment on the isomerization and better controlling the fluxionality of organometallic oxides. First, the experimental results are reported. Given that isomerization implies the breaking of RusO bonds, an osmium analogue was also prepared and studied in solution. Furthermore, NMR experiments were carried out to examine the influence of additional cations such as Na+ or protic solvents such as CH3OH on the equilibria. Then, the results of a DFT investigation are presented. The relative energies of [Mo4O16{M(arene)}4] species (M ) Ru, Os), both isolated and solvated (in CHCl3, CH2Cl2, and H2O), are compared, and the results are analyzed in view of the experimental observations.

Results and Discussion 1. Analysis of the Influence of the Group VIII Metal (Ru Ws Os). 1.1. Synthesis of [Mo4O16{Os(p-cym)}4] ([Mo4Os4]pcym)The previous studies performed on {Ru(arene)}2+ derivatives showed that two main synthetic routes can be used to prepare [Mo4O16{Ru(arene)}4] complexes: either by reacting [Ru(arene)Cl2]2 and Na2[MoO4] · 2H2O in water, in the presence of an excess of molybdenum to increase the yield, or by reacting (15) Cope´ret, C.; Chabanas, M.; Petroff Saint-Arroman, R.; Basset, J.M. Angew. Chem., Int. Ed. Engl. 2003, 42, 156.

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Laurencin et al. Table 1. Distribution of the MosO Distances (in Å) in the Windmill-Structure Compounds [Mo4Ru4]pcym and [Mo4Os4]pcym distance

[Mo4Os4]pcym

[Mo4Ru4]pcym

MosOt Mosµ2-O Mosµ2-O Mosµ4-O Mosµ4-O Mosµ4-O

1.717(6) 1.805(5) 1.824(5) 2.082(4) 2.118(5) 2.393(5)

1.706(3) 1.798(3) 1.804(3) 2.073(3) 2.122(3) 2.365(3)

Raman frequencies assigned to vibrations of the oxometallic frameworks of [Mo4Os4]pcym and [Mo4Ru4]pcym are very close (Table 2), thus underscoring the minor influence of the nature of the metal (Ru or Os) on these vibrations.

Figure 3. DIAMOND representation of [Mo4O16{Os(p-cym)}4].

[Ru(arene)Cl2]2 and (n-Bu4N)2[Mo2O7] in acetonitrile, in a Mo/Ru ratio of 1 or 2 to avoid the formation of insoluble oils or the crystallization of isopolyanions such as (nBu4N)4[R-Mo8O26]. Both of these procedures were explored for the preparation of [Mo4O16{Os(p-cym)}4] ([Mo4Os4]pcym). In water, the reaction between [Os(p-cym)Cl2]2 and Na2[MoO4] · 2H2O in a 5/1 Mo/Ru ratio led to the precipitation of the windmill complex [Mo4Os4]pcym. After slow diffusion of toluene in a CH2Cl2 solution of the precipitate, crystals of [Mo4Os4]pcym · 2C6H5CH3 that were suitable for X-ray diffraction analysis appeared. In acetonitrile, the reaction between [Os(p-cym)Cl2]2 and (n-Bu4N)2[Mo2O7] in a 2/1 Mo/Os ratio led to the cocrystallization of the windmill isomer [Mo4Os4]pcym and (n-Bu4N)4[R-Mo8O26], according to IR spectroscopy. Unfortunately, different attempts to purify these crystals were unsuccessful. Consequently, although both methods of synthesis lead to the formation of the windmill isomer of [Mo4O16{Os(p-cym)}4], higher yields and a better purity are obtained by reaction in water. 1.2. Spectroscopic Characterization in the Solid State. According to X-ray crystallography analysis, compound [Mo4Os4]pcym crystallizes in the P1j space group, in the presence of toluene. The unit cell contains two molecules of [Mo4Os4]pcym and four molecules of toluene. The thermal-ellipsoid representation of [Mo4Os4]pcym is shown in Figure 3. The structure of [Mo4Os4]pcym is typical of a windmill isomer: it consists of a central {Mo4O16}8- core to which four {Os(pcym)}2+ fragments are linked via one µ4-O and two µ2-O bridges. Each molybdenum atom has a six-coordinate environment, by coordination to three types of oxo ligands: one terminal (Ot), two µ2-O, and three µ4-O. As in the case of the windmill isomer of [Mo4O16{Ru(p-cym)}4] ([Mo4Ru4]pcym), the distribution in the MosO bond lengths around the molybdenum is rather continuous (Table 1). The IR and Raman spectra of solid samples of [Mo4Os4]pcym also allow the clear identification of a windmill oxometallic framework (Figure 4).7e Indeed, the IR spectrum displays one strong band at 925 cm-1 [ν(ModOt)]; two strong bands at 785 and 740 cm-1 [ν(Msµ2-O)]; and bands of weaker intensity at 646, 612, and 491 cm-1 [ν(Msµ4-O) and δ(MosOsM)]. In the Raman spectrum, three main bands appear, at 931 cm-1 [ν(ModOt)], 797 cm-1 [ν(Msµ2-O)], and 394 cm-1 [δ(MosOsM)].16 Moreover, it is noteworthy that the IR and

1.3. Spectroscopic Analysis of the Behavior in Solution. In the case of the ruthenium compounds [Mo4O16{Ru(arene)}4] (arene ) toluene, mesitylene, durene, p-cymene, hexamethylbenzene), there did not seem to be any particular link between the nature of the isomer isolated in the solid state and its behavior in solution: either, as in the case of [Mo4O16{Ru(hmb)}4], the complex maintains its molecular structure in solution or, as in the case of [Mo4O16{Ru(p-cym)}4]

Figure 4. IR (top) and Raman (bottom) spectra of solid [Mo4O16{Os(p-cym)}4]. Table 2. IR and Raman Wavenumbers (in cm-1) Assigned Mainly to the Oxometallic Framework of the Windmill-Structure in [Mo4Ru4]pcym and [Mo4Os4]pcym IR vibration

Raman

[Mo4Os4]pcym [Mo4Ru4]pcym [Mo4Os4]pcym [Mo4Ru4]pcym

ν(ModOt)

925

920

931

925

ν(MosOsM) and δ(MosOsM)

785 740 646 612 491

786 739 643 601 492

797

809 799

394

392


Figure 5. water.

17


O NMR spectrum of a solution of [Mo4Os4]pcym in CHCl3. The signal marked with an asterisk (*) corresponds to traces of Table 3.

95

Mo NMR Chemical Shifts (in ppm) of Different Triple-Cubane and Windmill Isomers

triple-cubane

windmill

[Mo4O16{Ru(tol)}4]

[Mo4O16{Ru(p-cym)}4]

[Mo4O16{Ru(hmb)}4]

[Mo4O16{Ru(p-cym)}4]

212

207

277

302

([Mo4Ru4]pcym), it isomerizes. For the latter compound, the presence of an equilibrium between the triple-cubane and windmill forms in CHCl3 and CH2Cl2 was previously demonstrated by a combination of multinuclear NMR, EXAFS and Raman spectroscopies.7b,e Moreover, NMR and EXAFS studies allowed the evaluation of the relative proportions between the two isomers of [Mo4Ru4]pcym: the triple-cubane/windmill ratio is equal to ∼40/60 in CHCl3 and ∼88/12 in CH2Cl2. (The latter value was previously reported as 80/20, but as shown below, we found in this work that 88/12 is more accurate.) We thus proceeded to a similar analysis of the behavior in solution of [Mo4Os4]pcym, using a combination of Raman and NMR spectroscopies. The 1H and 13C NMR spectra of [Mo4Os4]pcym in CDCl3 and CD2Cl2 display two sets of signals relative to p-cymene ligands, whose relative intensity varies from 80/20 in CDCl3 to 50/50 in CD2Cl2.17 When a few drops of CD2Cl2 were added to a solution of [Mo4Os4]pcym in CDCl3, an increase in the relative intensity of the peak corresponding to the minor species was observed, and a ratio of 50/50 was progressively reached by addition of more CD2Cl2. Reciprocally, the addition of a few drops of CDCl3 to a solution of [Mo4Os4]pcym in CD2Cl2 led to a decrease in intensity of the same set of signals, and the 80/20 ratio was progressively approached upon addition of more CDCl3. Furthermore, upon the complete evaporation of solutions of [Mo4Os4]pcym in CDCl3 and CD2Cl2, the IR spectra of the remaining solids were identical to that of the windmill isomer [Mo4Os4]pcym; therefore, when these solids were redissolved in CD2Cl2 and CDCl3, respectively, the 50/50 and 80/20 ratios were recovered.18 These observations show that there are two species in solution and that they are in equilibrium. By analogy with what was observed in the case of the ruthenium complex [Mo4Ru4]pcym, it is reasonable to suggest that there is an equilibrium between the windmill and triple-cubane isomers of [Mo4Os4]pcym. However, given that 1H and 13C NMR spectroscopies alone do not allow the attribution of the sets of signals to either isomer,17 we proceeded to further spectroscopic characterizations, in particular to identify the major isomer in CDCl3.

95 Mo NMR analyses of solutions of [Mo4Os4]pcym in CHCl3 and CH2Cl2 were performed. According to the molecular structures of the triple-cubane and windmill compounds, only one signal is expected for each isomer. However, previous work done on the ruthenium compounds showed that the molybdenum chemical shifts differ from one architectural type to the other (Table 3). The 95Mo NMR spectrum of [Mo4Os4]pcym in CHCl3 displays two broad signals, of relative intensity 80/20, located at 366 and 226 ppm, respectively (see Figure S1, Supporting Information).19 Given the values of the chemical shifts (by comparison with [Mo4Ru4]pcym), it is tempting to assign the most intense signal (at 366 ppm) to the windmill complex and to conclude that this isomer is thus the major species in chloroform. 17

O NMR studies were performed to confirm this hypothesis. Indeed, in contrast to the 1H, 13C, and 95Mo NMR spectra, the 17 O NMR spectra of triple-cubane and windmill complexes are different,4a,7b,c insofar as the connectivities of the oxygen atoms differ from one isomer to the other, as the triple-cubane isomer displays no doubly bridging oxygen atoms. The 17O NMR spectrum of a solution of [Mo4Os4]pcym in CHCl3 was recorded at natural abundance (Figure 5). Two very broad signals appeared at 440 and 530 ppm and were assigned to µ2-O bridges.7b,20 Because only the windmill compound contains such oxygen atoms, one can conclude that this isomer is the major species in CHCl3.21 (16) The bands at 797 and 394 cm-1 overlap with bands characteristic of the p-cymene ligand. For comparison, in the Raman spectrum of [Mo4Ru4]pcym, the former band is split into two bands (799 and 809 cm-1). (17) Given their molecular structure and symmetry, only one set of signals relative to the four arene ligands is expected in the 1H NMR spectrum of either the triple-cubane or windmill isomer (if one assumes fast rotation of the arene around the pseudo-C6 axis).7f (18) It is noteworthy that no significant influence of the temperature on the relative intensities or widths of the NMR signals was noticed between ∼5 and 40 °C. (19) In CH2Cl2, the two 95Mo NMR signals are observed at approximately the same chemical shifts as in CHCl3. However, in this case, they are of equal intensity.


Laurencin et al.

Figure 6. Raman analysis of solutions of [Mo4Os4]pcym in CHCl3 (left) and CH2Cl2 (right) by comparison with solid spectra of [Mo4Os4]pcym and [Mo4Ru4]tol. The band marked with an asterisk (*) is characteristic of a vibration of the toluene ligand (not of the triple-cubane oxometallic framework).

A comparison of the 17O NMR spectra of [Mo4Os4]pcym in the region characteristic of terminal oxygen atoms (between 800 and 1000 ppm) also leads to the same conclusion. In CH2Cl2, a solvent in which both species are present in equal quantities according to 1H NMR data, the integral of the signal at 880 ppm has twice the value of that at 845 ppm. Considering that there are eight terminal oxygen atoms in the triple-cubane form and four in the windmill, this means that the signals at 880 and 845 ppm correspond to the triple-cubane and windmill isomers, respectively (which also agrees with the relative chemical shifts of the terminal oxygen atoms in the wm and tc forms of [Mo4Ru4]pcym).7b In CHCl3, a solvent in which there is a major species, the signal at ∼845 ppm is much more intense (see Figure 5), meaning that the windmill form is indeed the predominant species in this solvent. Raman studies of solutions of [Mo4Os4]pcym in CHCl3 and CH2Cl2 were also performed. The Raman spectra of solutions of [Mo4Os4]pcym in CHCl3 and CH2Cl2 were compared to those of solid samples of [Mo4Os4]pcym (windmill isomer) and [Mo4O16{Ru(tol)}4] ([Mo4Ru4]tol, triple-cubane isomer),22 as shown in Figure 6. In the region characteristic of ModOt stretching vibrations, between 920 and 970 cm-1, two bands appear in the spectra of solutions of [Mo4Os4]pcym, and their relative intensity varies from one solvent to the other. By comparison with the spectra of the solid samples of [Mo4Os4]pcym (20) (a) Filowitz, M.; Ho, R. K. C.; Klemperer, W. G.; Shum, W. Inorg. Chem. 1979, 18, 93. (b) Besecker, C. J.; Klemperer, W. G.; Maltbie, D. J.; Wright, D. A. Inorg. Chem. 1985, 24, 1027. (21) It appears that, at the same temperature, namely, 300 K, the signals corresponding to µ2-O bridges are much broader for [Mo4Os4]pcym than for [Mo4Ru4]pcym.7b This might be due to fact that the dynamic process that leads to an exchange between the µ2-O ligands through a concerted motion of the four organometallic units,7b is faster for [Mo4Os4]pcym. (22) The choice of the ruthenium complex [Mo4O16{Ru(tol)}4] as a model for the Raman signature of the triple-cubane isomer of [Mo4Os4]pcym is legitimate. Indeed, in the Raman spectra of the windmill compounds [Mo4Ru4]pcym and [Mo4Os4]pcym, only very small variations in the intensities and wavenumbers of the oxometallic vibrations have been noticed, which tends to demonstrate that the nature of M′ (Ru or Os) has only a marginal influence on the vibrational features of the Mo4O16 core.

and [Mo4Ru4]tol, the band of lower wavenumber can be assigned to the windmill isomer of [Mo4Os4]pcym, and the one of higher frequency to the triple-cubane form. The relative intensity of the band of lower frequency is stronger in CHCl3 than in CH2Cl2, thus confirming that the windmill form is predominant in chloroform.23 The different spectroscopic studies performed on solutions of [Mo4Os4]pcym, which crystallizes in the windmill form, reveal that this compound partially isomerizes into the triple-cubane isomer in chlorinated solvents. This behavior is similar to that observed for the ruthenium analogue [Mo4Ru4]pcym. However, it is noteworthy that the relative proportions between the triplecubane and windmill isomers differ from one compound to the other: for [Mo4Os4]pcym, they are of 20/80 and 50/50 in CHCl3 and CH2Cl2, respectively, whereas for [Mo4Ru4]pcym, they are of 40/60 and 88/12, respectively. This finding emphasizes that the nature of the metal (ruthenium or osmium) has an influence on the equilibrium between the triple-cubane and windmill isomers. To be more specific, as shown in the scheme in Figure 7, osmium tends to increase the relative stability of the windmill form in nonpolar solvents such as CHCl3 and CH2Cl2, which means that, as in the case of the analogous ruthenium species, the solution becomes enriched in the triple-cubane form on going from CHCl3 to CH2Cl2. In this first section, we have shown that, by changing the nature of the group VIII metal in [Mo4O16{M(arene)}4] compounds, triple-cubane/windmill equilibria in solution can be affected. 2. Experimental Study of the Influence of External Factors on the Isomerization. 2.1. Temperature Effects. An 17O NMR study of the influence of the temperature on the behavior of [Mo4Ru4]pcym (23) The band characteristic of µ2-O bridges of the windmill isomer [Mo4Os4]pcym, which is located at 797 cm-1, clearly appears in the Raman spectra of both solutions but is more intense in chloroform. Given the difference in polarizability of the Mo)Ot bonds in the windmill and triplecubane structures, it is impossible to quantify the relative proportions of the two forms from the Raman spectra.



in CHCl3 has been reported.7b It showed that the main consequence of an increase in temperature is the disappearance of the µ2-O oxygen signals of the windmill, because of the additional fluxionality of this isomer at high temperatures. The spectra reported in that work tended to show that the changes in the relative quantities of the triple-cubane and windmill forms were minor, but this point was not precisely analyzed. We thus carried out a 1H NMR analysis of the temperature dependence of the triple-cubane/windmill equilibrium of [Mo4Ru4]pcym in CD2Cl2. At room temperature, the tc/wm ratio is equal to 88/12 (rather than 80/20, which was the value that we had previously reported). The concentration of the windmill form increases only slightly when the temperature rises: at 240 K, the tc/wm ratio is 94/6, whereas at 320 K, it is 80/20 (see Figure S3 in the Supporting Information). In the case of [Mo4O16{Os(p-cymene)}4], no temperature dependence of the equilibrium was observed between 278 and 313 K. Although this result could appear to be surprising, it should be noted that, similarly, no influence of temperature was observed for the isomerization of the organometallic derivative [Nb6O19{Ru(p-cymene)}2]4-.10d 2.2. Solvent Effects. [Mo4O16{M(arene)}4] complexes (M ) Ru and arene ) toluene, mesitylene, durene, p-cymene, hexamethylbenzene; M ) Os and arene ) p-cymene) are readily solubilized in nonpolar solvents such as CHCl3 and CH2Cl2. So far, it is only for [Mo4Ru4]pcym and [Mo4Os4]pcym that both the triple-cubane and windmill forms have been observed simultaneously in solution. As shown in Figure 7, in both cases, upon switching from CHCl3 to a slightly more polar solvent such as CH2Cl2, the triple-cubane form is noticeably favored. Other polar and protic solvents such as water and methanol also seem to play a role in the isomerization process and the stabilization of one structure with respect to the other. For example, in the case of [Mo4Ru4]tol, we observed that when a suspension of the triple-cubane isomer is refluxed in methanol, the compound partly transforms into the windmill form.7e Furthermore, in all of the crystal structures of triple-cubane forms reported thus far, the presence of water molecules has been noticed.24 In particular, in the case of [Mo4O16{Ru(mesitylene)}4] ([Mo4Ru4]mes), the positions of the water molecules in the crystal structure show that they form hydrogen bonds with the terminal oxygen atoms of the oxometallic core.7e Such interactions are likely to have a significant influence on the stabilization of the triple-cubane isomer in the solid state. However, the question of whether and how water molecules actually play a role in its stabilization in solution is not trivial. Indeed, the concentration of the triple-cubane form cannot be directly related to the small amount of water present in the

chlorinated solvents (see Table S1 in the Supporting Information), perhaps because of the very low solubility of water in these chlorinated solvents. A more detailed analysis of the influence of a polar protic solvent on the equilibrium was carried out with methanol, because it is soluble in dichloromethane. A solution of [Mo4O16{Ru(p-cymene)}4] in CD2Cl2 was prepared, and CD3OD was progressively added. As can be seen in Figure 8 (as well as Figure S4 in the Supporting Information), the 1H NMR spectra show that the windmill concentration increases and the tc/wm ratio reaches 70/30 after addition of 10 vol % of CD3OD. It should be noted that, after each addition of CD3OD, the new tc/wm equilibrium position was reached quasi-instantaneously and the only evolution of the NMR spectra was the progressive appearance over several hours of additional p-cymene signals, whose chemical shifts were consistent with the presence of {Ru(p-cymene)}2+ complexes, revealing a slow decomposition of the oxocluster in presence of methanol.26 Furthermore, beyond 10 vol % addition, no further change in the tc/wm ratio was observed. This study thus shows that methanol contributes to a relative stabilization of the windmill form with respect to the triple-cubane form in solution. This is all the more interesting because CH3OH is more polar than CH2Cl2: it thus appears that not only the polarity of a solvent but also its molecular nature (and notably its capacity to form hydrogen bonds) have an impact on the stabilization of one species with respect to the other. 2.3. Influence of Additional Cations on the Equilibrium. Apart from the triple-cubane/windmill family of compounds, there have been several other studies of isomerization reactions of organometallic oxides, and in the case of [Nb2W4O19{Rh(C5Me5)}2]2-, it was suggested that additional {Rh(C5Me5)}2+ fragments are involved in the isomerization process.10a Futhermore, cations such as H+ or Na+ have been shown to be able to interact reversibly with polyoxometalates,10c,25 and their coordination to the oxygen atoms of the polyoxometallic core might also induce the breaking of some of the RusO (or OssO) bonds in solution (and thus favor an isomerization as schematized in Figure 2). To further investigate these points, a 1H NMR study was carried out with [Mo4O16{Ru(pcymene)}4]. To a 0.01 mol L-1 solution of [Mo4Ru4]pcym in CD2Cl2, were successively added increasing amounts of [Ru(p-cymene)Cl2]2 (0.1, 0.2, 0.4, 1, and 4 equiv). No effect was observed on the equilibrium: the relative amounts of the two isomers remained the same, and only additional signals due to the dimeric unreacted complex were noticeable. These mixtures appear to be stable over time, which suggests that there is no strong driving force favoring the interaction of {Ru(p-cymene)}2+ with the triple-cubane and windmill oxometallic frameworks and, thus, that there is no dissociation of [Ru(p-cymene)Cl2]2. Similarly, when different amounts of a potential “H+ donor” such as (n-Bu4N)HSO4 (0.1, 0.2, 0.4, 1, and 4 equiv) were added to a 0.01 mol L-1 solution of [Mo4Ru4]pcym in CD2Cl2, no variation of the 1H NMR spectrum was observed, and the solutions were stable over time. Analysis of the potential influence of the small hard cations Na+ and NH4+ requires the use of a bulky anion unable to coordinate to Ru/Os metal centers. PF6- and SbF6- appeared to be good candidates for such a purpose; unfortunately, their

(24) In the case of the windmill isomer, the presence of water of crystallization has only been observed once, for [W4O16{Ru(hmb)}4]: Artero, V. Ph.D. Thesis, Universite´ Pierre et Marie Curie Paris VI, Paris, France, 2000.

(25) Finke, R. G.; Rapko, B.; Saxton, R. J.; Domaille, P. J. J. Am. Chem. Soc. 1986, 108, 2947. (26) It is noteworthy that a similar evolution has been observed upon dissolution of [Nb6O19{Ru(p-cymene)}4] in CD3OD.10d

Figure 7. Influence of the solvent and the metal on the isomerization of [Mo4O16{M(p-cym)}4] (M ) Ru, Os). The ratios represent the relative amounts of the triple-cubane and windmill forms in solution at 300 K.


Laurencin et al.

Figure 8. 1H NMR solution spectra of [Mo4O16{Ru(p-cym)}4] in CD2Cl2 (top) and in CD2Cl2/CD3OD (10:1 v/v) (bottom) with expansion of the aromatic parts, focusing on the variation of relative proportions of the windmill (wm) and triple-cubane (tc) isomers; signals from the solvents (CHDCl2 and CHD2OD), OH groups (residual H2O or CD3OH), and traces of silicon grease are identified by the symbols *, +, and ×, respectively.

Na+ and NH4+ salts are nearly insoluble in chlorinated solvents, and we had to turn again to methanol as the solvent for this study. We already noticed (see section 2.2) the organometallic oxide is not indefinitely stable in the presence of methanol; this is again examplified by the presence of some additional p-cymene signals in the 1H NMR spectrum of [Mo4Ru4]pcym in CD3OD. These signals can be assigned to {Ru(p-cymene)}2+ complexes issuing from a partial decomposition of the oxocluster [see Figure S5 (top) in the Supporting Information]. This process is slow enough at room temperature, however, for the planned study to be carried out. The progressive addition of up to 1 equiv of NH4PF6 to a 0.005 mol L-1 solution of [Mo4Ru4]pcym in CD3OD has no apparent effect on either the triple-cubane/windmill equilibrium or the relative quantities of the different species in solution. Similarly, no evolution was observed after addition of ca. 1.5 equiv of NaSbF6 to the methanolic solution of [Mo4Ru4]pcym, and the tc/wm ratio remained unchanged over time. Only a huge excess of the sodium salt (i.e., more than 100 equiv) was able to produce significant variations in the tc/wm equilibrium. When the solution became saturated in NaSbF6, the signals due to the windmill form nearly disappeared, and a significant increase in the decomposition products was noticed [see Figure S5 (bottom) in the Supporting Information]. In brief, it appears that the presence of additional cations in solution does not significantly affect the tc/wm ratio in solution (apart from the case where a very large excess of Na+ is added).

This is not fully surprising given that the triple-cubane and windmill compounds are neutral species and that their tendency to “attract” additional cations is thus expected to be limited. Nevertheless, it should be noted that these cations might have an effect on the kinetics of the interconversion. However, this point was not investigated here, because the equilibria were systematically reached quasi-instantaneously after each addition, thus rendering any kinetic NMR analysis very delicate. In this part, we have shown that, whereas the addition of cations has a minor effect on the position of the equilibrium, the nature of the solvent (its polarity, its bulkiness, and notably its capacity to form hydrogen bonds) is much more important, as it can lead to significant changes in the concentrations of the two isomers in solution. 3. DFT Analysis of the Influence of the Solvent. The experimental work described so far has shown that, in addition to the nature of the arene ligand, two key factors are likely to alter the triple-cubane/windmill equilibrium in solution: the nature of group VIII metal (Ru vs Os) and the solvent. To better rationalize the different experimental observations, a computational investigation was carried out. 3.1. Computational Approach. DFT calculations were performed on a series of [Mo4O16{M(arene)}4] derivatives (M ) Ru, Os; arene ) benzene, toluene, mesitylene, hexamethylbenzene, and p-cymene). Both the windmill (wm) and triplecubane (tc) configurations of each complex were studied. The equilibrium structures and energies of the isolated and solvated windmill and triple-cubane isomers were optimized using the

Framework Fluxionality of Polyoxometalate DeriVatiVes Table 4. Total Energy Differences (Etc - Ewm) for the Isolated [Mo4O16{M(arene)}4] Complexes (M ) Ru, Os)a arene

M ) Ru

M ) Os

benzene toluene mesitylene hexamethylbenzene p-cymene 1b p-cymene 2b

0.698 0.609 0.437 0.922 0.645 0.738

0.848 0.773 0.619 1.023 0.794 0.879

a All energies are in eV. b p-cymene 1 and p-cymene 2 forms are shown in the Supporting Information, Figure S6.

ADF software,27 at the DFT/BP86 level of theory,28 taking into account scalar relativistic effects by means of the ZORA approximation.29 The results are given in Table 4. In the particular case of p-cymene, the steric contacts occurring between the four nonsymmetric arene ligands hinder their possibility to rotate and, therefore, generate a large number of local energy minima. A search for the global minimum would require the use of annealing procedures, which is beyond the scope of the present work. Therefore, in line with our previous study,7e two different conformers were investigated, in which the initial positions of the p-cymene ligands were taken from observed triple-cubane and windmill geometries and then transposed to the other form and separately optimized, giving rise to different local minima in the two structures. These forms are referred to as p-cymene 1 and p-cymene 2 in Table 4 and are shown in Figure S6 in the Supporting Information. The analysis of solvation effects was carried out considering two nonpolar solvents (chloroform and dichloromethane) and water, using the conductor-like screening model (COSMO)30 of solvation. Within this model, solvents are described as continuum dielectrics and characterized by their dielectric constant ε (which was taken equal to 4.8, 8.9, and 78.0 for CHCl3, CH2Cl2, and H2O, respectively) and a radius rsolvent (which corresponds to a spherical model of the solvent molecule and defines the solvent excluding surface, SES).31 In the particular case of water, an ambiguity exists concerning the radius rsolvent in the COSMO formalism.31 Indeed, the default value available in the ADF implementation of COSMO is rwater ) 1.4 Å,30,27c but another set of parameters for water uses the same dielectric constant together with a larger radius value (rwater (27) (a) te Velde, G.; Bickelhaupt, F.; van Gisbergen, S.; Guerra, C.; Baerends, E. J.; Snijders, J.; Ziegler, T. J. Comput. Chem. 2001, 22, 931– 967. (b) Guerra, C.; Snijders, J.; te Velde, G.; Baerends, E. J. Theor. Chem. Acc. 1998, 99, 391–403. (c) ADF: Amsterdam Density Functional Software, version 2007.01; Scientific Computing & Modelling NV, Theoretical Chemistry, Vrije Universiteit: Amsterdam, The Netherlands, 2007; available at http://www.scm.com. (28) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Perdew, J. P. Phys. ReV. B 1986, 33, 8882; 1986, 34, 7406. (29) van Lenthe, E.; Ehlers, A. E.; Baerends, E. J. J. Chem. Phys. 1999, 110, 8943–8953. (30) (a) Klamt, A.; Schuü¨rmann, G. J. Chem. Soc., Perkin Trans 2 1993, 799. (b) Klamt, A. J. Chem. Phys. 1995, 99, 2224. (c) Model implemented in the ADF package by Pye, C. C.; Ziegler, T. J. Theor. Chem. Acc. 1999, 101, 396. (31) Even though the polarizable continuum method does not introduce explicit solvent molecules in the calculations, its implementation in ADF defines a solvent-excluding surface (SES), which consists of the path traced by the surface of a spherical solvent model molecule of specific radius, rolling about a van der Waals surface defined as the union of all atomic spheres. This implementation therefore preserves the concept of solvent accessibility, accounting for the ligand bulkiness and the molecular size of the various solvents. Default values, supposedly corresponding to water, fix the radius of the solvent sphere to 1.4 Å.27c However, a set of parameters specific to some typical solvents is also provided, such as for CHCl3 (ε ) 4.8, rchloroform ) 3.17 Å) and CH2Cl2 (ε ) 8.9, rdichloromethane ) 2.94 Å), and in the latter list, the solvent radius associated with water is now rwater ) 1.93 Å.27c

Organometallics, Vol. 28, No. 11, 2009 3147 Table 5. [Mo4O16{Ru(arene)}4]: Solvation Energies (Esolv) for the Triple-Cubane (tc) and Windmill (wm) Structures, Difference ∆Esolv ) Esolv(tc) - Esolv(wm), and Total Energy Differences Etc - Ewm for the Solvated Complexesa Arene

Esolv(tc)

Esolv(wm)

∆Esolv

[Etc - Ewm]solvated

Chloroform, ε ) 4.8, rsolvent ) 3.17 Å benzene -1.575 -1.432 -0.143 toluene -1.441 -1.292 -0.149 mesitylene -1.249 -0.980 -0.269 hexamethylbenzene -0.866 -0.675 -0.191 p-cymene 1 -1.169 -0.762 -0.407 p-cymene 2 -1.203 -0.999 -0.204 Dichloromethane, benzene -1.857 toluene -1.709 mesitylene -1.487 hexamethylbenzene -1.039 p-cymene 1 -1.377 p-cymene 2 -1.423

ε ) 8.9, rsolvent ) 2.94 Å -1.714 -0.143 -1.543 -0.166 -1.215 -0.272 -0.832 -0.207 -0.921 -0.456 -1.176 -0.247

0.555 0.460 0.168 0.731 0.238 0.534 0.555 0.443 0.165 0.715 0.189 0.491

benzene toluene mesitylene hexamethylbenzene p-cymene 1 p-cymene 2

Water, ε ) 78.4, rwater ) 1.93 Å -2.431 -2.246 -0.185 -2.165 -2.145 -0.020 -1.833 -1.694 -0.139 -1.351 -1.134 -0.216 -1.766 -1.481 -0.285 -1.793 -1.529 -0.264

0.513 0.589 0.298 0.706 0.360 0.474


Water, ε ) -2.549 -2.325 -1.935 -1.535 -1.853 -1.900

0.617 0.704 0.406 0.768 0.715 0.529

a

78.4, rwater -2.468 -2.420 -1.905 -1.381 -1.923 -1.690

) 1.4 Å -0.081 +0.095 -0.030 -0.154 +0.070 -0.210

All energies are in eV.

) 1.9 Å). Both of these options were therefore considered in order to investigate the influence of solvent accessibility.31 The bond energies and the sets of Cartesian coordinates corresponding to all optimized structures are given in the Supporting Information. 3.2. Computational Analysis of the Influence of the Group VIII Metal. Table 4 displays the total energy differences computed between the triple-cubane and windmill forms for all considered [Mo4O16{M(arene)}4] complexes, assumed isolated. In our previous work,7e the triple-cubane and windmill structures of a series of Ru(arene) complexes were optimized at a similar level of theory, except that relativistic effects were not taken into account. With all types of arene ligands, the windmill was found more stable by ∼0.4 to ∼1.0 eV, even with ligands such as toluene, mesitylene, and p-cymene, for which a triple-cubane structure was characterized experimentally. The energy difference computed between the two forms was shown to decrease as the arene became more bulky, with the notable exception of hexamethylbenzene (hmb), for which the difference was the largest. The general trend was interpreted by the presence of an excess of supposedly favorable intramolecular O · · · H contacts in the triple-cubane form of some substituted arene complexes (compared to the windmill). The anomalous behavior of [Mo4O16{Ru(hmb)}4] ([Mo4Ru4]hmb), on the other hand, was attributed to the overall increase in steric repulsions between the bulky hmb ligands, which indirectly appears through the significant elongation of the Ru-centroid distances in both isomers. These steric constraints strongly disfavor the triplecubane form, because in triple-cubane structures, the Ru · · · Ru distances are ∼3.3-3.4 Å,7e whereas in windmill structures, the Ru · · · Ru distances are longer than 5.3 Å, thus allowing a better spacing of bulky arene ligands.


Laurencin et al.

Table 6. [Mo4O16{Os(arene)}4]: Solvation Energies (Esolv) for the Triple-Cubane (tc) and Windmill (wm) Structures, Difference ∆Esolv ) Esolv(tc) - Esolv(wm), and Total Energy Differences Etc - Ewm for the Solvated Complexesa arene

Esolv(tc)

Esolv(wm)

∆Esolv

[Etc - Ewm]solvated

Chloroform, ε ) 4.8, rsolvent ) 3.17 Å benzene -1.335 -1.175 -0.161 toluene -1.211 -1.051 -0.160 mesitylene -1.113 -0.818 -0.295 hexamethylbenzene -0.759 -0.638 -0.121 p-cymene 1 -0.992 -0.685 -0.307 p-cymene 2 -0.996 -0.861 -0.135 Dichloromethane, benzene -1.571 toluene -1.424 mesitylene -1.313 hexamethylbenzene -0.881 p-cymene 1 -1.166 p-cymene 2 -1.181

ε ) 8.9, rsolvent ) 2.94 Å -1.379 -0.192 -1.277 -0.147 -1.008 -0.305 -0.730 -0.151 -0.810 -0.356 -1.008 -0.173

0.688 0.612 0.324 0.903 0.485 0.745 0.655 0.626 0.314 0.872 0.438 0.707


Water, ε ) 78.4, rwater ) 1.93 Å -2.001 -1.833 -0.168 -1.828 -1.768 -0.060 -1.615 -1.411 -0.204 -1.155 -0.989 -0.166 -1.490 -1.246 -0.245 -1.496 -1.293 -0.203

0.680 0.713 0.415 0.856 0.550 0.677


Water, ε ) -2.196 -1.945 -1.716 -1.312 -1.540 -1.635

0.658 0.846 0.516 0.890 0.823 0.706

a

78.4, rwater -2.005 -2.019 -1.613 -1.179 -1.569 -1.462

) 1.4 Å -0.191 +0.074 -0.103 -0.133 +0.029 -0.173

All energies are in eV.

In comparison to our previous study,7e no appreciable change in the energy differences of isolated species was obtained here when considering relativistic effects for the ruthenium and molybdenum (Table 4), which is not fully surprising for a second-period metal atom. Concerning the osmium analogues, the same trends were observed for the different arenes. However, the energy differences are consistently shifted toward a further increase of the relative stability of the windmill form, by ∼0.10-0.18 eV (i.e., ∼2.3-4.2 kcal mol-1). Notwithstanding the large energy gap obtained between the two isomers, this overall shift in favor of the windmill when replacing Ru by Os is in agreement with the shift of equilibrium observed experimentally upon substitution of Ru by Os in [Mo4O16{M(pcymene)}4] complexes (see Figure 7). 3.3. Computational Analysis of the Influence of the Solvent. Results of the computational study of the influence of solvation on the relative stability between the two isomers are summarized in Tables 5 and 6: the effects of CHCl3, CH2Cl2, and H2O solvents on both isomers of the Ru and Os clusters were considered. The difference between the energies computed for the free and solvated complexes provides the solvation energies, Esolv(tc) and Esolv(wm), associated with each isomer for a specific solvent. Solvation stabilizes the complexes, and the values of Esolv are therefore negative. The difference ∆Esolv ) Esolv(tc) - Esolv(wm) provides information on the influence of a specific solvent on the relative stabilities of the isomers. A negative value of ∆Esolv therefore means that solvation favors the triple-cubane form. In the last column of Tables 5 and 6, the energy difference Etc - Ewm (eV), including the solvation energies, is displayed for each arene ligand and each solvent

[Etc - Ewm]solvated ) [Etc - Ewm]isolated + ∆Esolv

(1)

The present results show that the continuum dielectric model of solvation is not sufficient to reverse the general trend in favor

of the windmill form, because the triple-cubane form remains higher in energy (Tables 5 and 6, last column). However, it appears that nonpolar solvents contribute to substantially reducing the energy difference between the two isomers, especially with substituted arenes such as mesitylene and p-cymene, for which the triple-cubane isomer has been observed experimentally. For example, in the case of ruthenium, the computed difference in stability between the two forms of the mesitylene derivative [Mo4Ru4]mes drops to ∼0.17 eV in CHCl3 and CH2Cl2. On the other hand, the energy difference in favor of the windmill form remains quite large (∼0.72 eV) for [Mo4Ru4]hmb, which has never been observed in the triple-cubane form. The case of p-cymene is more ambiguous, because the solvation effect appears to depend strongly on the relative position of the arene ligands, at least for the windmill form. A comprehensive scanning of the rotation of this arene would therefore be necessary to find the global minimum and investigate the dynamic behavior of solvated [Mo4Ru4]pcym complexes. Furthermore, the possibility for the nonsymmetric arene to rotate along the windmill T triple-cubane transformation pathway should be considered. Depending on these points, the advantage provided by solvation of the triple-cubane form of [Mo4Ru4]pcym could vary at least from ∼0.2 to ∼0.5 eV (Table 5, column 3: ∆Esolv). Finally, in the case of osmium, calculations carried out on [Mo4Os4]arene complexes showed that there is also a decrease in the energy difference between the two isomers when solvated in CH2Cl2 or CHCl3 (Table 6). However, as already observed in the gas phase (Table 4), there is an overall increase in stability of the windmill form for the osmium complexes upon solvation (compared to the Ru analogues), by ∼0.10-0.25 eV. A further analysis of the results displayed in Tables 5 and 6 reveals several distinctive trends: (1) On average, for both isomers, the opposite of the solvation stabilization energy, -Esolv, varies in the order -Esolv(CHCl3) < -Esolv(CH2Cl2) < -Esolv(H2O), that is, in the same order as the dielectric constants [ε(CHCl3) < ε(CH2Cl2) < ε(H2O)]. This suggests that the interaction of the solvent with the complexes, and more specifically with their polar Mo4O16 cores, must play an important role in the stabilization of these structures: the more polar the solvent, the stronger the stabilization. Furthermore, for both isomers, -Esolv decreases rapidly as the arene ligands become more bulky. This trend is observed in all three solvents and can be seen as a consequence of the loss of accessibility of the solvent to the Mo4O16 core31 and, thus, of a global decrease in the stabilizing interactions between the solvent and the central part of the complexes. It is noteworthy that, on this “ligand-shielding-effect” scale, p-cymene appears intermediate between mesitylene and hexamethylbenzene. (2) With either CHCl3 or CH2Cl2, the solvation effect is systematically more important for the triple-cubane form than for the windmill form (∆Esolv < 0). The difference increases with the size of the arene, with the exception of hexamethylbenzene, for which -∆Esolv goes back to values lower than those obtained for mesitylene (with M ) Ru, Table 5) or even for benzene (with M ) Os, Table 6). Once again, the key to this trend seems to be the accessibility of the oxometallic framework.31 In the triple-cubane form, the position of the metalarene moieties at both ends of the parallelepipedic Mo4O16 core leaves free access to the central part of the framework (Figure 1, left), whereas in the windmill form, the M-arene fragments are more regularly distributed around the oxometallic center and hinder its accessibility to a greater extent (Figure 1, right). The straightforward consequence of the better accessibility of the


triple-cubane core to CHCl3 and CH2Cl2 is thus the stronger stabilization of this isomer with respect to the windmill (i.e., ∆Esolv < 0). A limit to this favored access should exist, however, especially with arene ligands that are sufficiently bulky to constrain the access to both configurations of the complex core, and indeed, this limit seems to be reached with hexamethylbenzene, as well as with some conformations of [Mo4O16{M(pcym)}4]. (3) As expected from the polar character of water and from its high dielectric constant, the solvation energies are significantly larger than those obtained with organic solvents. The consistent decrease of the absolute value solvation energy obtained with a more extended radius of the solvent molecule (rwater ) 1.9 Å vs rwater ) 1.4 Å, Tables 5 and 6) is easily explained by the extension of the volume enclosed by the solvent exclusion surface, which results from the more difficult access of the solvent to the molecular core.31 The trend toward a strong decrease of the solvation energy with the bulkiness of the arene ligands is maintained, regardless of the value of rwater. However, the model solvent radius chosen appears to have a strong impact on the difference ∆Ewater ) Ewater(tc) - Ewater(wm). With the “small” value of rwater (1.4 Å), the advantage given to the triplecubane form when solvated with CHCl3 or CH2Cl2 becomes small and erratic and even vanishes for toluene and for one conformation of [Mo4M4]pcym (Tables 5 and 6). Increasing rwater to 1.9 Å restores the advantage provided by solvation to the triple-cubane configuration for all types of arene ligands, even though this advantage does not significantly increase with the bulkiness of the ligand, contrary to what was observed (with the exception of hexamethylbenzene) in CH2Cl2 and CHCl3. This dependence of ∆Esolv on the size taken for the spherical model of the solvent molecule emphasizes the importance of solvent accessibility to the Mo4O16 core of the complex. As shown for solvents with relatively large molecular sizes such as CH2Cl2 (rdichloromethane ) 2.94 Å) or CHCl3 (rchloroform ) 3.17 Å), this accessibility depends on the bulkiness of the ligands and, with nonsymmetric arenes such as p-cymene, on the orientation of the arene substituents. (4) When comparing the effects of the nonpolar solvents CH2Cl2 and CHCl3, the value of [Etc - Ewm]solvated, when averaged over all computed structures, is ∼0.02 eV smaller in CH2Cl2 than in CHCl3. In other words, the average energy difference between the triple-cubane and windmill forms in CH2Cl2 is ∼0.02 eV smaller. Such a small difference between both solvents is not fully surprising given their similar molecular sizes and dielectric properties of the two solvents, and the same trends are observed for Ru and Os complexes (Tables 5 and 6). Thus, these calculations suggest that even small changes in the structural or dielectric properties of the solvent are likely to affect the ratio between the isomers, as soon as an equilibrium is established between the two species in solution. In addition, calculations suggest that dichloromethane has a more pronounced tendency to reduce the energy difference in favor of the triple-cubane form for the hexamethylbenzene complex (∼0.02 eV) and the two p-cymene complexes (∼0.04 eV). Interestingly, this is in perfect agreement with experimental observations concerning [Mo4Ru4]pcym, for which the triplecubane form was found experimentally to be more abundant in CH2Cl2 than in CHCl3. In this simulation of the solvated [Mo4M4]arene complexes, we have demonstrated that the solvent, through its dielectric properties, has a clear influence on the relative stability of the triple-cubane and windmill isomers. To be more specific, on the molecular scale, the extent to which the solvent can interact


with the Mo4O16 core appears to play a key role in the energy difference between the compounds. Although roughly taken into account by means of a spherical model, the size of the solvent molecules was shown to play a crucial role through its interplay with the bulkiness of the arene ligands and the configuration of the complex in defining the solvent exclusion surface, which conditions the solvent access to the Mo4O16 core. The more open configuration of the triple-cubane complexes explains the larger solvation energy consistently attributed by the COSMO model to these isomers,30,31 which, however, is not sufficient to reverse the relative stability of the triple-cubane and windmill forms. Finally, calculations suggest that slight changes in the steric and/or dielectric properties of the solvent (such as when switching from CHCl3 to CH2Cl2) lead to small consistent variations in the energy difference between the solvated triplecubane and windmill forms. This explains why, in certain systems, the relative amounts of the two isomers in solution can be altered by using mixtures of solvents.

Conclusions The experiments and computations carried out in this work on the triple-cubane/windmill family of compounds provide clear evidence of the fact that the nature of both the metal bound to the arene (ruthenium or osmium) and the solvent (through its dielectric properties and its accessibility to the oxometallic core) play important roles in the relative stabilities of the two isomers in solution. The comparison of the behaviors of [Mo4Os4]pcym and [Mo4Ru4]pcym in CHCl3 and CH2Cl2 shows that the two systems are very similar, which is not fully surprising given our previous work on [PW11O39{M(p-cym)(H2O)}]5- (M ) Ru, Os) systems.7h Nevertheless, there is a small but noticeable influence of the metal, as osmium favors the stabilization of windmill structures. In contrast, more dramatic changes in the relative stability of the two isomers can be achieved by changing the nature of the arene7e or the solvent. In particular, calculations here show that, by slightly modifying the dielectric properties of the solvating medium (CHCl3 vs CH2Cl2), the relative stability of the two isomers is altered. This agrees with previous computational studies of polyoxometalates, which have shown that the dielectric medium surrounding polyoxometalate frameworks can noticeably affect their electronic properties.32 The proportion of the triple-cubane form is thus increased in CH2Cl2 relative to CHCl3, regardless of the identity of the metal, ruthenium or osmium. This effect can be ascribed to better access for solvent molecules to the central oxide core and stabilization thus increasing with the polarity of the solvent. The importance of small amounts of water or methanol molecules on the isomerization appears to be more complex. It seems that the capability of these solvents to form well-defined hydrogen bonds with the oxometallic framework is an important factor to consider. To verify this point, in line with our recent work on [XW11O39]7- polyoxometalates (X ) Si, P),33 a complete computational study would need to be performed, in which “explicit” water molecules were added around the polyoxometalate, with the rest of the solvent being modeled by a dielectric continuum. (32) (a) Lopez, X.; Fernandez, J. A.; Romo, S.; Paul, J. F.; Kazansky, L.; Poblet, J. P. J. Comput. Chem. 2004, 25, 1542. (b) Bridgeman, A. J. Chem.sEur. J. 2006, 12, 2094. (c) Bagno, A.; Bonchio, M.; Autschbach, J. Chem.sEur. J. 2006, 12, 8460. (33) Laurencin, D.; Proust, A.; Ge´rard, H. Inorg. Chem. 2008, 47, 7888.


In summary, the experimental and computational study presented here has shed light on the importance of the metal and the solvent in the triple-cubane/windmill equilibrium in solution. As a result, this study should open the way to further work on fluxional organometallic oxides in connection with surface organometallic chemistry and help determine how to finely tune their molecular form in solution, which should impact their catalytic activity.

Experimental Section Materials and Methods. [Os(p-cym)Cl2]234 and (n-Bu4N)2[Mo2O7]35 were prepared according to literature procedures. All other reagents and solvents were obtained from commercial sources and used as received. IR spectra were recorded from KBr pellets at room temperature on a Bio-Rad FT 165 spectrometer. The Raman spectra were obtained at room temperature on a Kaiser Optical Systems HL5R Raman spectrometer equipped with a near-IR laser diode working at 785 nm. The laser power was adjusted to 10-15 mW at the sample position for all spectra, and the resolution was 3 cm-1. The 1H and 13C NMR spectra were recorded in solution on a Bruker AvanceII 300 spectrometer operating at 300.13 MHz for 1H and 75.47 MHz for 13C. Solvent peaks were used as the internal reference relative to Me4Si for 1H and 13C chemical shifts (ppm). The 95Mo and 17O NMR spectra were recorded in 10-mmo.d. tubes on a Bruker DRX500 spectrometer, operating at 32.6 MHz for 95Mo and 67.8 MHz for 17O. 95Mo and 17O chemical shifts are given with respect to external references: an alkaline aqueous solution of [MoO4]2- and H2O, respectively. Coupling constants are given in hertz. The following abbreviations are used: s, singlet; d, doublet; m, multiplet. Elemental analyses were performed by the Analytical Service of the Universite´ Pierre et Marie Curie and by the Service Central d′Analyse of the CNRS (Vernaison, France). Preparation of [Mo4O16{Os(p-cym)}4] ([Mo4Os4]pcym). Method 1. A suspension of [Os(p-cym)Cl2]2 (0.539 g, 0.70 mmol, 1 equiv) and Na2[MoO4] · 2H2O (1.696 g, 7.00 mmol, 10 equiv) in distilled water (37 mL) was refluxed for 4 h. The orange precipitate that formed was separated by filtration36 and identified as the windmill isomer [Mo4O16{Os(p-cym)}4] (0.480 g, 71%).37 Crystals of composition [Mo4Os4]pcym · 2C6H5CH3 suitable for X-ray analysis were obtained by slow diffusion of toluene into a solution of [Mo4Os4]pcym in CH2Cl2. Anal. Calcd for C40H56O16Mo4Os4: C, 24.80; H, 2.91; Mo, 19.81. Found: C, 24.15; H, 2.89; Mo, 19.76. 1 H NMR (CDCl3):38 δ 1.31 [d, 3JHH ) 6.9 Hz, 6H, ArsCH(CH3)2], 1.38* [d, 3JHH ) 6.9 Hz, 6H, ArsCH(CH3)2], 2.36* (s, 3H, Ar-CH3), 2.46 (s, 3H, Ar-CH3), 2.87+ [m, 3JHH ) 6.7 Hz, 2 × (1H), Ar-CH(CH3)2], 6.01* (d, 3JHH ) 5.8 Hz, 2H, ArsH), 6.08+ [d, 3JHH ) 5.4 Hz, 2 × (2H), ArsH], 6.23 (d, 3JHH ) 5.4 Hz, 2H, ArsH). 1H NMR (CD2Cl2): δ 1.33 [d, 3JHH ) 6.8 Hz, 6H, ArsCH(CH3)2], 1.39* [d, 3JHH ) 6.8 Hz, 6H, ArsCH(CH3)2], 2.32* (s, 3H, Ar-CH3), 2.44 (s, 3H, Ar-CH3), 2.82+ [m, 3JHH ) 6.5 Hz, 2 × (1H), Ar-CH(CH3)2], 5.98* (d, 3JHH ) 5.9 Hz, 2H, ArsH), 6.06+ [d, 3JHH ) 5.6 Hz, 2 × (2H), ArsH], 6.18 (d, 3JHH ) 5.6 Hz, 2H, ArsH). 13C NMR (CDCl3): δ 19.5* (s, 1C), 19.9 (s, 1C), (34) (a) Arthur, T.; Stephenson, T. A. J. Organomet. Chem. 1981, 208, 369. (b) Cabeza, J. A.; Maitlis, P. M. J. Chem. Soc., Dalton Trans. 1985, 573. (35) Hur, N. H.; Klemperer, W. G.; Wang, R.-C. Inorg. Synth. 1990, 27, 79. (36) The filtrate contains the excess of unreacted Na2[MoO4] · 2H2O. (37) In some cases, the orange precipitate appeared to contain a small quantity of unreacted [Os(p-cym)Cl2]2. The purification of [Mo4Os4]pcym can be performed by silica-gel chromatography, as in the case of the synthesis of [Mo4Ru4]pcym: Therrien, B.; Plasseraud, L.; Su¨ss-Fink, G.; Laurencin, D.; Proust, A. Inorg. Synth. 2004, 34, 200. (38) Chemical shifts followed by an asterisk (*) symbol were assigned to the triple-cubane isomer; the others correspond to the windmill isomer. The plus (+) symbol means that, in the spectrum, the signals due to the two isomers overlap.

Laurencin et al. Table 7. Crystal Data and Structure Refinement for [Mo4Os4]pcym · 2C6H5CH3 [Mo4Os4]pcym · 2C6H5CH3 empirical formula Mr color temperature (K) crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (g cm-3) µ (cm-1) θmin - θmax (deg) octants collcd reflns measd unique reflns obsd reflns [I > 2σ(I)] refined params Rint R Rw (all data) goodness of fit S ∆F(max/min) (e Å-3)

C54H72O16Os4Mo4 2121.68 red 250 triclinic P1j 14.0580(19) 15.3150(13) 16.3040(11) 87.902(6) 79.099(8) 65.317(8) 3128.4(6) 2 2.252 89.24 2.75-30 -19, 19; -21, 21; -22, 22 64445 18117 10488 717 0.043 0.045 0.084 1.00 3.00/-1.58

22.8 (s, 2C), 23.3* (s, 2C), 32.4 (s, 1C), 32.6* (s, 1C), 66.0 (s, 2C), 67.6* (s, 2C), 71.4* (s, 2C), 72.3 (s, 2C), 83.1 (s, 1C), 84.8* (s, 1C), 92.4* (s, 1C), 93.5 (s, 1C). 13C NMR (CD2Cl2): δ 19.6* (s, 1C), 20.1 (s, 1C), 22.9 (s, 2C), 23.4* (s, 2C), 32.8 (s, 1C), 33.1* (s, 1C), 66.6 (s, 2C), 68.1* (s, 2C), 71.6* (s, 2C), 72.4 (s, 2C), 83.6 (s, 1C), 85.2* (s, 1C), 92.6* (s, 1C), 93.5 (s, 1C). 95Mo NMR (CDCl3): δ 226*, 366. 95Mo NMR (CD2Cl2): δ 225*, 367. 17O NMR (CDCl3):39δ 74-, 108-, 127-, 440, 530, 845, 880*. IR (KBr, cm-1): 3058 (w), 2960 (m), 2922 (w), 2872 (w), 1471 (w), 1386 (w), 1199 (w), 1149 (w), 1087 (w), 1056 (w), 925 (s), 894 (w), 877 (m), 785 (s), 740 (s), 646 (m), 612 (m), 491 (m), 351 (w). Raman (solid, cm-1): 1200 (w), 931 (m), 797 (s), 394 (m). Method 2. A solution of (n-Bu4N)2[Mo2O7] (0.550 g, 0.70 mmol, 2 equiv) and [Os(p-cym)Cl2]2 (0.271 g, 0.35 mmol, 1 equiv) in CH3CN (13 mL) was stirred at room temperature for 4 h. After slow evaporation of the red-orange solution at room temperature, the simultaneous formation of large colorless crystals and small red crystals was observed. These were identified as (n-Bu4N)4[RMo8O26] and [Mo4Os4]pcym, respectively, by IR spectroscopy. The crystals of [Mo4Os4]pcym (0.060 g, 18%) in this case were not suitable for XRD analysis. Crystal Structure Analysis. Diffraction data of [Mo4Os4]pcym · 2C6H5CH3 were collected, using a combination of φ and ω scans, on a Nonius Kappa-CCD diffractometer at 250 K (Table 7), with graphite-monochromated Mo KR (0.71073 Å) radiation. Unit cell parameter determination, data collection strategy, and integration were carried out with the Nonius Eval-14 suite of programs.40a The data were corrected from absorption by a multiscan method.40b The structure was solved by direct methods with SHELXS-86,40c refined by full least-squares on F2, and completed with SHELXL-97.40d Graphics were created with DIAMOND.40e All non-H atoms were (39) The minus (-) symbol means that the nature of the isomer to which the signal corresponds has not been identified. (40) (a) Duisenberg, A. J. M.; Kroon-Batenburg, L. M. J.; Schreurs, A. M. M. J. Appl. Crystallogr. 2003, 36, 220. (b) Blessing, R. H. Acta Crystallogr. A 1995, 51, 33. (c) Sheldrick, G. M. SHELX86: Computer Program for Structure Solution; University of Go¨ttingen: Go¨ttingen, Germany, 1986. (d) Sheldrick, G. M. SHELX97: Computer Program for Structure Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (e) Brandenburg, K.; Berndt, M. DIAMOND; Crystal Impact GbR: Bonn, Germany, 1999.

Framework Fluxionality of Polyoxometalate DeriVatiVes refined with anisotropic displacement parameters. The H atoms identified on difference Fourier maps were simply introduced in structure factors calculations (riding model). Supplementary crystallographic data for this structure have been deposited by the Cambridge Crystallographic Data Centre as supplementary publication No. CCDC-710190. These data can be obtained free of charge upon application to CCDC, 12 Union Road, Cambridge CB21EZ, U.K. [fax, (+44)1223-336-033; e-mail, [email protected]], or via www.ccdc.cam.ac.uk/conts/retrieving.html. Methods of Calculation. All calculations were carried out within the formalism of the density functional theory (DFT) within the generalized gradient approximation (GGA), as implemented in the ADF software,27 and using the functional due to Becke and Perdew that is currently referred to as BP8628 for exchange and correlation. The atomic basis sets were identical to those used in our previous work.7e For hydrogen, carbon, and oxygen, the Slater basis set used for the valence shell was of triple-ζ quality, supplemented with one p- or d-type polarization function.41 The 4s and 4p shells of metal atoms were described by a double-ζ Slater basis, the 4d and 5s shells by a triple-ζ basis, and the 5p shell by a single orbital. No polarization function was added for metal atoms. Scalar relativistic effects were taken into account by means of the ZORA approximation.29 Starting geometries were deduced from crystal structures, when available, and transposed to the configurations that were not characterized so as to minimize the ligand · · · ligand contacts. Calculations on the triple-cubane isomer were carried out (41) (a) Snijders, J. G.; Baerends, E. J.; Vernooijs, P. At. Data Nucl. Data Tables 1983, 26, 483. (b) Vernooijs, P.; Snijders, J. G.; Baerends, E. J. Slater-Type Basis Functions for the Whole Periodic System; Internal Report; Free University of Amsterdam: Amsterdam, The Netherlands, 1981.

Organometallics, Vol. 28, No. 11, 2009 3151 with the constraints of the D2d point group, whereas for the windmill isomers, calculations were performed using the Abelian subgroup of S4, namely, C2. Full geometry optimization was carried out for all species in the presence of the conductor-like screening model (COSMO),30 which accounts for the solvent effects. The dielectric constants used to model the solvent were ε ) 4.8, 8.9, and 78.0 for chloroform, dichloromethane, and water, respectively. The convergence criteria used for the geometry optimization process have been detailed in ref 7e.

Acknowledgment. DFT calculations were carried out at the IDRIS computer center (CNRS, Palaiseau, France) and at the CURRI center (ULP, Strasbourg, France). D.L. and A.P. thank Guillaume Stirnemann, a student of the premasters program of the Ecole Normale Supérieure in Paris, for his help in the experimental study of the osmium derivative. The CNRS and the Universities Pierre et Marie Curie and Louis Pasteur are acknowledged for their financial support. Supporting Information Available: 95Mo and 13C NMR spectra of [Mo4O16{Os(p-cym)}4] in CHCl3 and CH2Cl2, variable-temperature 1H NMR study of [Mo4O16{Ru(p-cym)}4] in CD2Cl2, 1H NMR spectra of [Mo4O16{Ru(p-cym)}4] in CD2Cl2 with addition of CD3OD, 1H NMR spectra of [Mo4O16{Ru(p-cym)}4] in CD3OD in the presence of NaSbF6, optimized geometries of the triple-cubane and windmill “p-cymene 1” and “p-cymene 2” species, table listing the amounts of water in CHCl3 and CH2Cl2 and the triple-cubane/ windmill ratio, cif file, and all of the xyz calculation files. This material is available free of charge via the Internet at http://pubs.acs.org. OM8011568

Experimental and Computational Study of the Framework

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