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Hydrodefluorination of Carbon&Ndash ARTICLE Received 22 Jul 2013 | Accepted 4 Sep 2013 | Published 9 Oct 2013 DOI: 10.1038/ncomms3553 Hydrodefluorination of carbon–fluorine bonds by the synergistic action of a ruthenium–palladium catalyst Sara Sabater1, Jose A. Mata1 & Eduardo Peris1 Catalytic hydrodefluorination of organic molecules is a major organometallic challenge, owing to the strength of C–F sigma bonds, and it is a process with multiple industrial applications. Here we report a new heterodimetallic ruthenium–palladium complex based on a triazolyl- di-ylidene ligand. The complex is remarkably active in the hydrodefluorination of aromatic and aliphatic carbon–fluorine bonds under mild reaction conditions. We observe that both metals are required to promote the reaction process. The overall process implies that the palladium fragment facilitates the C–F activation, whereas the ruthenium centre allows the reduction of the substrate via transfer hydrogenation from isopropanol/sodium t-butoxide. The activity of this heterodimetallic complex is higher than that shown by a mixture of the related homo- dimetallic complexes of ruthenium and palladium, demonstrating the catalytic benefits of the heterodimetallic complex linked by a single-frame ligand. 1 Departamento de Quı´mica Inorga´nica y Orga´nica, Universitat Jaume I, Avda. Sos Baynat s/n, 12071 Castello´n, Spain. Correspondence and requests for materials should be addressed to J.A.M. (email: [email protected]) or to E.P. (email: [email protected]). NATURE COMMUNICATIONS | 4:2553 | DOI: 10.1038/ncomms3553 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3553 wing to the high industrial demand of organic molecules arenes is focused on finding effective catalysts that show high that contain carbon–fluorine bonds1–3, both C–F bond activity under the mildest reaction conditions. Oformation and selective C–F bond activation have During the last few years, we have reported efficient methods become subjects of active investigation4–6. Carbon–fluorine for the preparation of homo- and heterodimetallic complexes bonds are considered as the most passive organic functional- based on a 1,2,4-triazolyl-3,5-di-ylidene ligand, with metals of the ities7–9, and their selective activation under mild conditions platinum group including gold (Fig. 1)33–39. The preparation of remains poorly achieved. Although there has been considerable such types of heterodimetallic complexes allowed us to study progress in the stoichiometric activation of C–F bonds, the metal- several catalytic processes in which each metal fragment catalysed versions of the process remain relatively scarce5,10–21. facilitated a mechanistically different cycle33,35,36,40. We also The simplest modification of the C–F bond is its transformation confirmed that the presence of the two different metals furnish to the simple C–H bond (hydrodefluorination, HDF), a process catalytic benefits compared with the cases in which mixtures of that has been mainly limited to fluoroarenes, with very few homodimetallic species are used. Exploiting our experience in the examples referring to trifluoromethylarenes and other perfluor- preparation of Pd-based catalysts for arene-halogen activation, oalkyl groups1,22–26. HDF is an interesting process not only and Ru-based catalysts for borrowing-hydrogen reactions, here because of the fundamental importance on the C–F bond we combine these two metals into one catalyst and explore its activation chemistry involved but also because of the potential performance in the more challenging HDF of fluoroarenes and applications in the activation and disposal of the chlorofluor- trifluoromethylarenes. ocarbons, considered as ‘super greenhouse gases’27–31. Metal hydrido complexes are often chosen for HDF, but C–F activation Results can produce stable fluoride complexes, which may hamper 4,15 Description of the catalysts used. Figure 2 shows the complexes catalytic turnover . Consistent with the decreasing metal– under study in this work. All three complexes were obtained from fluoride bond energies, electron-rich metal centres are preferred, 1,2,4-trimethyl-triazolium tetrafluoroborate, following similar but still strong reductants such as high-pressure H2 or silanes are synthetic procedures as those that we have previously reported33–39. needed to promote effective HDF, thus often producing over- The preparation of the Pd-Ru heterodimetallic complex 3 was reduced products. For C6F6 À nHn substrates, it has been achieved by the stepwise deprotonation of the triazolium salt, demonstrated that the C–F bond dissociation energies show a 32 followed by the subsequent addition of the corresponding metal general increase with larger n ; therefore, although the research source, as schematically shown in Fig. 2 (below). The preparation on the activation of highly fluorinated arenes is mainly focused on of the homodimetallic complex of ruthenium 1 was recently selectivity, the search for catalysts that activate low fluorinated described by us41. Complexes 2 and 3 were fully characterized by NMR, high-resolution mass spectroscopy (HRMS) and elemental analysis. N N MLn M'L'n Catalytic dehalogenation of aryl halides. To explore the catalytic N activities of 1–3, we first decided to test the three complexes in the dehalogenation of several phenyl halides (Table 1). The reactions Figure 1 | Complex formation by the triazolyl-di-ylidene ligand. were performed in 2-propanol in the presence of tBuONa at Schematic representation of homo- and heterodimetallic complexes 80 °C with a catalyst loading of 1 mol%. In the optimization of the attached by the triazolyl-di-ylidene ligand. reaction conditions (Supplementary Table S1), we observed that a N N Ru Ru N Cl Cl Cl Cl 1 – (BF4 )2 N N b H + H N Cl N N ii N Pd Pd N N Cl Ac N O – (BF4 ) 2 Acetone Pd NaCl N N 2 50°, 15h Ru + H N Cl Cl N N i Cl iii Ru Pd N N Cl Cl 3 Figure 2 | Catalysts used in this work. (a) Structure of previously reported complex 1.(b) Schematic protocol for the preparation of 2 and 3. 2 NATURE COMMUNICATIONS | 4:2553 | DOI: 10.1038/ncomms3553 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3553 ARTICLE Table 1 | Catalytic dehalogenation of phenyl halides using Table 2 | Hydrodefluorination of fluoroarenes using the catalyst 1–3*. complexes 1–3*. [Cat] tBuONa i-PrOH, 80° 1h Entry Catalyst X Yield (%)w Entry Catalyst Substrate Product Yield (%) 1 — Br 0 2 1 Br 0 1 1 0 3 2 Br 80 2 2 3 4 3 Br 100 3 3 100 4† 1+2 49 5 — Cl 0 5 0 6 1 Cl 0 1 6 2 3 7 2 Cl 65 7 3 ‡ 8 3 Cl 100 70 (96) 9 — F08† 1+2 68 10 1 F09 1 0 11 2 F310 2 15 12 3 F 100 11 3 93 † 13z 1 þ 2 F49 12 1+2 45 13 1 0 *Reactions were carried out with 0.3 mmol of fluoroarene, tBuONa (0.3 mmol), catalyst 14 2 0 (1 mol%) and 2 ml of 2-propanol for 1 h at 80 °C. w 15 3 63 Yields determined by gas chromatography analyses using anisole as an internal standard. † zCatalyst 1 (0.5 mol%) þ catalyst 2 (0.5 mol%). 16 1+2 13 17 3 85 the best results were obtained when carrying out the reactions in the presence of tBuONa, although weaker bases such as Cs2CO3 18 3 100 were also effective. We also carried out the reactions in the absence of catalysts (entries 1, 5 and 9) and observed that all the starting substrate was recovered, indicating that a nucleophilic aromatic 19 3 85 substitution does not occur under these reaction conditions. As expected, the bis-ruthenium complex 1 is completely ineffective in 20 3 100 the dehalogenation of any of the substrates used. The bis-palla- dium complex 2, is capable of moderately dehalogenating bro- 21 3 30 (55)‡ mobenzene (entry 3) and chlorobenzene (entry 7), but is completely ineffective in the HDF of fluorobenzene (entry 11). ‡ The heterodimetallic complex 3 is very active in the hydro- 22 3 76 (96) dehalogenation of all three phenyl halides, affording quantitative conversions to benzene (entries 4, 8 and 12). Interestingly, equi- 23 3 100§ molecular mixtures of 1 and 2 afforded moderate yields of benzene coming from the HDF of fluorobenzene (entry 13), therefore # suggesting that the two different metals have a role in the overall 24 3 100 process, but also that the presence of these two metals in a single catalyst provides a significant benefit (compare entries 12 and 13). 25 3 100# To prove that the reaction is homogeneously catalysed, we also carried out the HDF of fluorobenzene using 3, in the presence ¶ of a mercury drop42, and observed that the activity of the catalyst 26 3 100 did not change. Although we are aware that the Hg(0) may occasionally lead to inconclusive results43, we believe that this *Reactions were carried out with 0.3 mmol of fluoroarene, tBuONa (0.3 mmol), catalyst experiment supports the idea that the process is homogeneous. (1 mol%) and 2 ml of 2-propanol for 1 h at 80 °C. Yields determined by gas chromatography analyses using anisole as an internal standard. w Catalyst 1 (0.5 mol%) þ catalyst 2 (0.5 mol%). zYields in parenthesis are after 2 h. HDF of fluoroarenes. To survey the generality and scope of the yIsolated yield after 3 h. ||Two equivalents of tBuONa (0.6 mmol) and 2 h reaction. heterodimetallic Ru/Pd complex, a variety of fluoroarenes was zThree equivalents of tBuONa (0.9 mmol) and 2 h reaction. examined (Table 2). As can be seen from the results shown, the Ru/Pd catalyst 3 is highly active in the HDF of a series of arenes with different functional groups, affording good to excellent differences in activity, suggesting that the rate-determining step is yields.
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