Note pubs.acs.org/Organometallics
[Ru4(CO)8(μ-OOCCH2CH3)4(THF)2] and [Ru3(μ3‑OH)(CO)6(μOOCtBu)4(OOCtBu)]: Novel Multinuclear Ruthenium Carbonyl Carboxylates Teresa K. Zimmermann,†,‡ Jennifer Ziriakus,†,‡,§ Eberhardt Herdtweck,† Alexander Pöthig,† and Fritz E. Kühn*,† †
Chair of Inorganic Chemistry/Molecular Catalysis and ‡WACKER Institut für Siliciumchemie, Catalysis Research Center, Department of Chemistry, Technische Universität München, Ernst-Otto-Fischer-Straße 1, 85747 Garching b. München, Germany § TUM Asia/German Institute of Science and Technology, 10 Central Exchange Green, Singapore 138649 S Supporting Information *
ABSTRACT: The reaction of Ru3(CO)12 with pivalic or propionic acid leads to the formation of two novel multinuclear ruthenium complexes, [Ru4(CO)8(μ-OOCCH2CH3)4(THF)2] (1) and [Ru3(μ3-OH)(CO)6(μ-OOCtBu)4(OOCtBu)] (2). Compound 1 is among the few examples of discrete tetranuclear ruthenium compounds showcasing a “dimer of dimers” paddlewheel structure, in which two “Ru2(CO)4(μOOCCH2CH3)2(THF)” fragments are linked via Ru−O interactions. Compound 2 features a unique structural motif in which a triangular Ru3 core is connected via a μ3-OH ligand.
uthenium carbonyl carboxylates were first described in the late 1960s by Crooks et al.,1 and over the past five decades, catalytic applications such as isomerization,2 hydrogenation,3 transvinylation,4 and C−C−bond formation reactions5 have been established. A variety of complexes with this structural motif have been reported, while the most diverse and best understood class of ruthenium carbonyl carboxylates consists of dinuclear ruthenium sawhorse type complexes.6 Accounts of tri- and tetranuclear complexes are scarce and often include unusual synthetic techniques. A series of tetranuclear Ru complexes have been described by the group of Petrukhina,5,7 and in general, these compounds are composed of two dimeric “Ru2(CO)4(μ-OOCR)2L” fragments linked via Ru−O5,7,8 or Ru−Ru7a,9 interactions (Figure 1). Discrete tetranuclear complexes are by far outnumbered by their numerous dinuclear counterparts, and only a handful of complexes of this type have been reported to date. Here the synthesis, structure, and electrochemical properties of the tetranuclear [Ru4(CO)8(μ-OOCCH2CH3)4(THF)2] (1) are presented. While tetranuclear ruthenium carbonyl carboxylates are rare, trinuclear complexes of this type have not been reported at all to date. Trinuclear ruthenium complexes with other ligand systems include complexes of the general formula [(μ2H)Ru3 (CO) 10(μ2 -L)] with bridging ligands L such as thiolates, 10 amino acids, 11 and formyl 12 as well as [HRu3(CO)9(μ3-L)] with bridges such as p-ethynylaniline13 and alkynoate acetyl salicylic acid esters.14 With precursors such
R
© 2014 American Chemical Society
Figure 1. Known tetranuclear “dimer of dimer” ruthenium carbonyl carboxylates with Ru−O5,7,8 (left) or Ru−Ru7a,9 (right) interactions intermittent between dimers.
as [RuCl2(η6-C6R6)]2 as starting materials,15 a number of complexes with a “(μ 3 -O)(μ-H)Ru 3 ” core have been reported.15b,c,16 To the best of our knowledge, μ3-bridging OH groups connecting three Ru centers are not known in ruthenium carbonyl carboxylates. Only one complex with a similar structural motif was described by the group of SüssFink, who found that in the tetranuclear cluster [Ru4(μ3Received: February 20, 2014 Published: May 13, 2014 2667
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OH)(μ2-H)3(η6-C6H6)4] three of the four Ru atoms are connected by a μ3-bridging OH.17 The facile synthesis of the trinuclear [Ru3(μ3-OH)(CO)6(μ-OOCtBu)4(OOCtBu)] (2) is reported in this work.
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RESULTS AND DISCUSSION The reaction of Ru3(CO)12 with propionic acid in toluene yields [Ru(CO)2(μ-OOCCH2CH3)]n,1 which forms highly pure 1 upon recrystallization from THF in 68% yield. According to 1H NMR, elemental analysis, and single-crystal XRD, the deep red complex is tetranuclear, which is quite remarkable for this synthetic approach. The dinuclear analogue [Ru2(CO)4(μ-OOCCH2CH3)2(THF)2] (1a) was described by Süss-Fink et al. in 1985.18 It was synthesized from Ru3(CO)12 with ethylene and water under pressure, giving oligomeric [{Ru2(CO)4(μ-OOCCH2CH3)2}6(THF)2] (1b), which reacted to give the dimer 1a upon addition of THF. Interestingly, the dinuclear adduct could not be isolated but was observed in situ in a THF-d8 solution of 1b. Similar reactivity was observed by our group: in THF solution, 1 dissociates into two identical dimeric fragments 1a. This could be evidenced via IR spectroscopy, revealing four CO absorption bands for 1, which merge into two absorption bands after dissolution in THF. This corresponds to the expected carbonyl absorption pattern. Subsequent solvent evaporation leads to the formation of oligomeric 1b (Figure S5, Supporting Information). 1 can be retrieved only from slow crystallization at 4 °C (Scheme 1).
Figure 2. ORTEP style plot of compound 1 in the solid state. Thermal ellipsoids are drawn at the 50% probability level. The centrosymmetric molecule is generated by a crystallographic inversion center (symmetry code: (_a) 1 − x, 2 − y, 1 − z) indicated by a star. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru1−Ru2 2.6295(2), Ru2−O6 2.1575(13), Ru2−O6_a 2.2784(13), Ru1−O9 2.2745(13), Ru1−O5 2.1138(14), Ru1−C1 1.842(2); O5− Ru1−O7 83.78(5), O5−Ru1−C2 95.79(7), O6−Ru2−O6_a 75.09(5), Ru2−O6−Ru2_a 104.91(5). A fully labeled figure is given in the Supporting Information.
Scheme 1. Relationship among 1, 1a, and 1ba
a Legend: (a) reaction with CH3CH2COOH followed by THF; (b) reaction with ethylene and water under pressure.18
Figure 3. HOMO (left) and LUMO (right) calculated for 1 (top) and 2 (bottom).
The molecular structure of 1 is shown in Figure 2. The tetranuclear core structure is confirmed, and the bridging Ru− O interaction between the dimeric fragments is evident. The propionate ligands constitute the four wheels of a paddlewheel structure, while the connecting Ru2, O6, Ru2a, and O6a lie together in one plane. The complex is centrosymmetric, and the Ru1−Ru2 bond length amounts to 2.6295(2) Å, which lies in the typical range for similar tetranuclear compounds.7b,8c,d The intermittent Ru−O interaction between the dimeric fragments is slightly weaker than direct μ2-coordination of propionate to the Ru1−Ru2 backbone, leading to a slightly lengthened Ru2−O6a bond distance of 2.2784(13) Å in comparison to Ru1−O5 (2.1138(14) Å) or Ru2−O6 (Ru2− O6 2.1575(13) Å). A cyclic voltammetric investigation of 1 was carried out to determine electronic communication effects between the Ru centers. However, only irreversible oxidation at 1.3 V vs ferrocene (Fc/Fc+) and no reduction processes were observed. A DFT study at the B3LYP/LANL2DZ level of theory reveals the shape of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). The HOMO is located mostly within the dinuclear fragments of the complex, i.e. between Ru1 and Ru2, while the LUMO is located
at the intersection between the two dinuclear fragments (Figure 3). This indicates that nucleophilic attack occurs at this position and corresponds well to experimental results. A theoretical investigation of the dissociation of 1 to 1a suggests a bond dissociation enthalpy ΔHdiss of −35.4 kJ mol−1 (ΔGdiss = −16.8 kJ mol−1), indicating that this process occurs under ambient conditions. 2 is synthesized from Ru3(CO)12 in excess pivalic acid. 2 is obtained in low yield (21%) as a colorless, hygroscopic crystalline solid. Single-crystal XRD revealed the molecular structure of 2 (Figure 4). The complex features an unusual “Ru3(μ3-OH)” structural motif where three Ru atoms are arranged in an almost equilateral triangle and are connected via a μ3-OH group. Addition of small amounts of water (up to 50 μL) accelerates the reaction significantly, indicating that water acts as the source of the bridging hydroxo ligand. Thus, the reaction probably proceeds according to Scheme 2. The oxygen atom of the “Ru3(μ3-OH)” moiety is slightly elevated over the Ru3 plane, and the proton sits even further above, completing a pyramidal structure. Two ruthenium atoms 2668
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
Note
Synthesis of 1. A 50 mg portion of Ru3(CO)12 (0.08 mmol, 0.75 equiv) was suspended in 2 mL of toluene, and 1.8 mL of propionic acid (0.24 mmol, 2.3 equiv) was added to the slurry. The mixture was stirred at 100 °C for 12 h, and all volatiles were removed in vacuo after cooling. Addition of 0.3 mL of THF and stirring at 70 °C for complete dissolution led to a deep red solution, which was stored at 4 °C for several weeks for crystallization. The red crystals were separated from excess THF and dried in vacuo. Yield: 68% (39 mg, 0.04 mmol). Anal. Calcd for C28H36O18Ru4: C, 31.58; H, 3.41. Found: C, 31.61; H, 3.41. 1 H NMR (400 MHz, MeOD-d4, δ): 1.10 (t, 3JH,H = 7.6 Hz, 3 H, CH3), 1.87 (m, 2 H, CH2CH2), 2.31 (q, 3JH,H = 7.6 Hz, 2 H, CH2CH3), 3.72 (t, 3JH,H = 6.6 Hz, 2 H, CH2O). 13C NMR (100.5 MHz, MeOD-d4, δ): 11.02 (CH3), 26.65 (CH2CH2), 31.03 (CH2CH3), 69.02 (CH2OCH2), 188.35 (COO), 203.59 (CO). Synthesis of 2. A 100 mg portion of Ru3(CO)12 (0.16 mmol, 1.0 equiv) was added to 3.4 g of pivalic acid, and the mixture was stirred at 100 °C for 14 h. The resulting dark green liquid was concentrated in vacuo, yielding a dark green solid, which was washed with ethanol (2 × 2 mL). The resulting colorless solid was recrystallized from toluene. Yield: 21% (34 mg, 0.03 mmol). Anal. Calcd for C31H46O17Ru3·H2O: C, 36.79; H, 4.78. Found: C, 36.91; H, 4.76. 1H NMR (400 MHz, CDCl3, δ): 0.99 (s, 9 H, CH3), 1.04 (s, 9 H, CH3), 1.17 (s, 18 H, CH3), 1.22 (s, 9 H, CH3), 14.37 (s, 1 H, OH). 13C NMR (100.5 MHz, CDCl3, δ): 27.84 (CH3), 27.85 (CH3), 27.95 (CH3), 28.01 (CH3), 28.15 (CH3), 40.80 (C(CH3)3), 41.13 (C(CH3)3), 41.78 (C(CH3)3), 42.73 (C(CH3)3), 43.59 (C(CH3)3), 195.2 (CO). Computational Details. All calculations were performed with GAUSSIAN-0919 using the density functional/Hartree−Fock hybrid model Becke3LYP20 and the LANL2DZ ECP basis set.21 No symmetry or internal coordinate constraints were applied during optimizations. All reported geometries were verified as being true minima by the absence of negative eigenvalues in vibrational frequency analysis. Crystallographic Details. For detailed information see the Supporting Information. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 986506 (1) and CCDC 986507 (2). These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_ request/cif.
Figure 4. ORTEP style plot of compound 2 in the solid state. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity, except for the hydroxyl hydrogen atom. Selected bond lengths (Å) and angles (deg): Ru1−O1 2.1361(10), Ru2−O1 2.1288(10), Ru3−O1 2.1077(11), O1−H1 0.84, Ru1···Ru2 3.7219(2), Ru1···Ru3 3.5959(2), Ru2···Ru3 3.5605(2), O14···H1 1.74; Ru1− O1−Ru2 121.54(5), Ru1−O1−Ru3 115.85(5), Ru2−O1−Ru3 114.37(5). A fully labeled figure is given in the Supporting Information.
Scheme 2. Proposed Reaction Pathway to 2 (R = tBu)
are connected via two bidentate pivalate bridges, while the third Ru atom is linked to each of these Ru atoms via one bidentate pivalate and bears the remaining fifth monodentate pivalate. All Ru atoms are coordinated in a distorted-octahedral manner by the central μ3-OH, two carbonyl, and three pivalate ligands. The OH proton can be unequivocally assigned to the bridging oxygen, considering the O1−H1 bond length of 0.84 Å in comparison to the 1.737 Å distance to the neighboring Hbonding monodentate pivalate. The OH proton is observed in 1 H NMR (CDCl3) at 14.37 ppm. Cyclic voltammetry reveals irreversible oxidation of 2 at 1.5 V vs ferrocene (Fc/Fc+). No reduction processes were observed. A DFT investigation of the localization of the LUMO of 2 (Figure 3) suggests a nucleophilic attack would be likely to open the trinuclear core structure of the complex. Both 1 and 2 were tested for their catalytic activity in transvinylation of propionic acid with vinyl acetate. However, their activities lie below those of previously reported systems.4a Details on the catalytic study are given in the Supporting Information.
S Supporting Information *
Text, figures, tables, and CIF and xyz files giving IR and NMR spectra, cyclic voltammograms, results of the catalytic study, crystallographic details for 1 and 2, and Cartesian coordinates for all computed molecules in a format for convenient visualization. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for F.E.K.:
[email protected]. Notes
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The authors declare no competing financial interest.
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CONCLUSION Two novel multinuclear ruthenium complexes are presented. Complex 1 is a new example of a tetranuclear ruthenium carbonyl carboxylate complex, and complex 2 features an unusual structural motif in which three Ru atoms are connected via a μ3-OH functional group. In both cases, DFT investigation of the respective HOMO and LUMO indicates that a nucleophilic attack would be likely to open the tri- or tetranuclear core structure, respectively.
ACKNOWLEDGMENTS T.K.Z. and J.Z. are grateful for the financial support of Wacker Chemie AG. Dr. Peter Gigler and Dr. Jürgen Stohrer are gratefully acknowledged for their scientific advice and helpful discussions. The authors thank Prof. Janos Mink for his help with IR interpretation. T.K.Z. acknowledges the support of TUM GS and the Bavarian Academy of Science for the provision of computing time. 2669
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Note
(21) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284−298. (d) Dunning, T. H. J.; Hay, P. J. Methods of Electronic Structure Theory; Plenum Press: New York, 1977; Vol. II.
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