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Jul 10, 2012 - The pyridazine-bridged NHC/pyrazole ligand L (HL = 3-[3-(2,4 ..... of the gold(III) ion, d(Au1···N1) = 2.85 Å (2.99 Å for the seco...
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Gold(I), Gold(III), and Heterometallic Gold(I)−Silver(I) and Gold(I)− Copper(I) Complexes of a Pyridazine-Bridged NHC/Pyrazole Hybrid Ligand and Their Initial Application in Catalysis Jan Wimberg, Steffen Meyer, Sebastian Dechert, and Franc Meyer* Institute of Inorganic Chemistry, Georg-August-University Göttingen, Tammannstrasse 4, D-37077 Göttingen, Germany S Supporting Information *

ABSTRACT: The pyridazine-bridged NHC/pyrazole ligand L (HL = 3-[3-(2,4,6-trimethylphenyl)-3H-imidazolium-1-yl]-6-(3,5dimethylpyrazol-1-yl)-pyridazine) that provides an organometallic and a classical N-donor compartment is shown to serve as a versatile scaffold for a variety of homo- and heterometallic gold(I) carbene complexes. Complexes [LAuX] (1Cl, X = Cl; 1Br, X = Br), [L2Au](PF6) (2), [L2AuAg](BF4)(PF6) (3), [L2AuAg3(MeCN)6](BF4)4 (5), and [L2AuCu](OTf)0.75(PF6)1.25 (6) have been characterized by X-ray crystallography. In all cases Au(I) binds to the NHC site while the additional Ag(I) in 3 or Cu(I) in 6 is accommodated in the pyrazole-derived site. Both 3 and 6 form two-stranded helical structures; racemization of the P and M enantiomers is much more facile in the Ag(I) case 3 but has a barrier of around 65 kJ/mol in the Cu(I) case 6, which is rationalized on the basis of the different coordination chemistry preferences of these two metal ions. 3 may bind two further Ag(I) ions to the central pyridazine N, giving 5. Treatment of 1Br with Br2 leads to bromination at the pyrazole C4 of the ligand backbone, yielding [LBrAuBr] (8). In contrast, 1Cl could be successfully oxidized to the Au(III) complex [LAuCl3] (7) using PhICl2; both 7 and the gold(I) complex 8 have been characterized crystallographically. Preliminary screening shows that 7, in combination with AgBF4, is a good catalyst for the etherification of 1-indanol with a variety of alcohol substrates and shows significantly higher activity than the gold(I) catalyst 1Cl.

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INTRODUCTION N-heterocyclic carbenes (NHCs) have become an extremely popular and powerful ligand class in organometallic chemistry, with many beneficial properties for homogeneous catalysis.1,2 NHCs are usually viewed as strong σ-donating ligands with little or negligible π back-bonding, and they have shown the ability to form robust complexes with various transition metals in different oxidation states.3 As a further elaboration they are increasingly incorporated in sophisticated multidentate ligand scaffolds that impart particular stability or constrained structures to the resulting complexes.4 Dinucleating ligands, which preorganize two metal ions in close proximity similar to what is often observed in metalloenzyme active sites, offer great prospects for cooperative effects in substrate activation and catalysis.5 It is thus not surprising that the number of known dinucleating ligands containing NHC donor sites is growing rapidly;6−8 this also includes some pyridazine-bridged bis(NHC) ligands.9 While most of these systems are symmetric, however, examples of unsymmetrical compartmental ligands that provide two electronically distinct coordination sites, at least one of which contains an NHC unit, are rare.10 We recently reported a new class of pyridazine-bridged NHC/pyrazole hybrid ligands L that feature two topologically similar yet electronically very different binding sites: an organometallic {C/N} compartment involving the NHC and one of the pyridazine N atoms and a classical {N/N} © 2012 American Chemical Society

compartment involving the pyrazole N and the other pyridazine N.11 Formally the two compartments differ only in the switching of adjacent C and N atoms in the five-membered diazole heterocycles attached to the central pyridazine unit. A detailed investigation of the silver(I) coordination chemistry of those ligands revealed the sequential binding of up to three silver(I) ions, first to the NHC site, then to the pyrazole N, and finally to the central pyridazine, with some ligand reshuffling from parallel to antiparallel ligand strands en route (Scheme 1). Scheme 1. Formation of Oligomeric Silver(I) Complexes of Pyridazine-Bridged NHC/Pyrazole Hybrid Ligands11

In this work we report the first heterometallic complexes of this new ligand class. In particular we focus on gold(I) Received: April 24, 2012 Published: July 10, 2012 5025

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evidenced by a shift of the 13C NMR resonance for the metalbound NHC C2 from 172.3 ppm (in 1Cl) to 175.7 ppm (in 1Br; cf. Table 1). This shift is in good agreement with data for previously reported NHC−gold(I) halide complexes16 and is likely a result of the lower acidity of the gold(I) ion in the bromide species, because the electronegativity of bromine is lower than that of chlorine. Both 1Cl and 1Br could be crystallized by slow diffusion of diethyl ether into dichloromethane solutions of the compounds. The molecular structures of 1Cl (the asymmetric unit contains two crystallographically independent molecules) and 1Br are shown in Figure 1. In both cases the gold(I) ion is coordinated in a quasi-linear fashion by the NHC and halide ligands, with CNHC−Au−X angles of 174−175° (X = Cl) and 176.7° (X = Br; cf. Table 1). The CNHC−Au bond lengths are 1.99 Å for both 1Cl and 1Br. These values as well as d(Au−Cl) = 2.28 Å (1Cl) and d(Au−Br) = 2.39 Å (1Br) are comparable with values for similar gold(I)− NHC motifs reported in the literature.17 The pendant pyridazine groups in 1Cl and 1Br do not interact with the gold ion but their diazine moieties point in opposite direction of the NHC-unit, away from the gold. The cationic complex [L2Au](PF6) (2) with bis(NHC) ligated gold(I) could be prepared via transmetalation of the known silver(I) analogue [L2Ag](PF6) (A)11 using stoichiometric amounts of AuCl(SMe2) (Scheme 2). For this route it proved necessary, however, to first treat the gold precursor with AgBF4 in order to avoid formation of the NHC−gold chloride complex 1Cl (cf. Scheme 2). 2 shows characteristic 1H NMR signals for the pyridazine protons at 8.22 and 8.29 ppm (3JHH = 9.5 Hz) and a 13C NMR carbene resonance at 182.9 ppm (cf. Table 1); the latter is almost identical with the value for the silver complex A (182.3 ppm) and is in the typical range for bis(NHC)-ligated gold(I). Single crystals of 2 were obtained upon slow diffusion of diethyl ether into a dichloromethane solution of the crude product; the molecular structure of the cation of 2 is depicted in Figure 2. Coordination of the gold(I) ion again is quasi-linear with a CNHC−Au−CNHC angle of 177.1°. This and the CNHC−Au bond lengths (2.02 Å) are well within the range usually found for this kind of structural motif.18 While the pyridazine and pyrazole rings in 2 are almost coplanar, the planes of the pyridazine and NHC heterocycles are rotated by around 31 and 45° with respect to each other, with the pyridazine N directed away from the metal ion. Since 2 features a number of N-donor atoms available for metal ion binding, it was titrated with AgBF4 in acetone solution and the reaction monitored by 1H NMR spectroscopy. Changes in the chemical shifts of the pyridazine protons, δ(CHpdz), are a good indicator for the involvement of the pyridazine N in metal coordination. The titration curve (Figure 3) suggests that 2 may accommodate up to three silver ions, though the effect of the third equivalent on δ(CHpdz) is only marginal. Addition of even more AgBF4 does not lead to any further changes in δ(CHpdz). The sequential coordination of up to three silver(I) ions was unambiguously confirmed by the targeted synthesis and crystallographic characterization of complexes 3 and 5 that, in addition to the single gold(I) ion, contain either one or three silver ions, thus featuring heterobimetallic AuAg and heterotetrametallic AuAg3 cores (Scheme 3 and Figures 4 and 5). In both cases the quasi-linear [L2Au]+ scaffold of 2 is retained (cf. Table 1), but rotation around an Au−CNHC bond has occurred and the two ligand strands, which are antiparallel in 2, are now parallel. In complex 3 (Figure 4) a single silver(I) ion is nested

complexes containing a second coinage-metal ion, either copper or silver. Also we evaluate whether gold(III) might be stabilized by such an NHC/pyridazine scaffold. We anticipated that the different preferences of gold(I), silver(I), and copper(I) for the organometallic versus the classical N-donor sites as well as the particular stability of the gold−NHC bond would prevent ligand reshuffling upon binding of the second and third metal ions, in contrast to the case of the homometallic silver(I) complexes shown in Scheme 1. It should be noted in this context that organogold chemistry has experienced an amazing upsurge during the past decade and is currently one of the most rapidly growing fields in transitionmetal catalysis research.12 In homogeneous catalysis either simple gold(I) or gold(III) salts are often employed, or welldefined gold(I) or gold(III) complexes with phosphine and NHC ligands.13 NHC−gold complexes have especially gained much popularity since the first report of gold catalysis involving NHC ancillary ligands. Gold ions are usually assumed to behave as π acids, mediating inter alia the activation of alkenes, allenes, and alkynes as well as the skeletal rearrangement of enynes. In comparison to their gold(I) congeners, however, gold(III) compounds with their harder Lewis acidity are still rather rarely used as catalysts.14 This report includes a preliminary investigation of a new NHC/pyridazine−gold(III) complex toward the catalytic etherification of primary and secondary alcohols with 1-indanol.

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RESULTS AND DISCUSSION The pyridazine-bridged ligand L bearing a mesityl substituent at the NHC side and methyl substituents at the pyrazole unit has been used throughout this work.11 Chart 1. Ligand L Used in This Work

Reaction of the ligand precursor [HL]Cl (i.e., the imidazolium salt of L)11 with an excess of Ag2O in acetone solution and subsequent transmetalation15 of the in situ generated silver−carbene species with AuCl(SMe2) yielded, Scheme 2. Synthesis of Carbene Complexes A, 1Cl, 1Br, and 2

after workup, the gold(I) complex LAuCl (1Cl; Scheme 2). Its 1 H NMR spectrum shows two characteristic doublets at 8.40 and 9.01 ppm with a 3JHH coupling of 9.5 Hz for the pyridazine protons (CHpdz), as well as the expected disappearance of the imidazolium C2 proton upon carbene formation and metal coordination. Treatment of 1Cl with an excess of LiBr cleanly transformed 1Cl into the analogous complex LAuBr (1Br), 5026

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Table 1. Selected Bond Lengths (Å), Bond Angles (deg) and δ(CNHC) Values (ppm) for 1Cl, 1Br, 2, 3, and 5−8 Au−C Au−X (trans)

1Cl

1Br

2

3

5

6

7

8

1.990(3) 1.990(3) 2.280(1) 2.282(1)

1.988(4)

2.015(3) 2.018(3)

2.013(5) 2.020(6)

2.020(4)

2.013(5) 2.017(5)

1.995(6) 2.000(6) 2.309(1) 2.304(1) 2.288(1) 2.308(1) 2.286(1) 2.302(1)

1.978(5)

2.390(1)

Au−X (cis)

Au···M C−Au−C C−Au−X (trans)

5.530(1) 177.1(1) 175.3(1) 173.8(1)

176.7(1)

172.3

175.7

174.4(2)

5.634(1) 3.436(1) 174.5(2)

5.202(1) 177.6(2)

C−Au−X (cis)

δ(CNHC)

182.7

181.8

180.9

2.390(1)

184.3

178.0(1) 179.3(2) 89.5(2) 88.6(2) 88.5(2) 90.0(2) 144.7

176.5(1)

Figure 2. ORTEP plot (30% probability thermal ellipsoids) of the molecular structure of 2. For the sake of clarity, hydrogen atoms and PF6− have been omitted. Selected bond lengths (Å) and angles (deg): Au1−C1 = 2.015(3), Au1−C31 = 2.018(3); C1−Au1−C31 = 177.07(13).

[L2Au]+ metalloligand with Npz−Ag1 (2.26 Å) and Npdz−Ag1 (2.46 Å) bond lengths that are similar to those in 3. In addition, however, the remaining Npdz atoms are each bound to a Figure 1. ORTEP plots (30% probability thermal ellipsoids) of the molecular structures of 1Cl (top) and 1Br (bottom). For the sake of clarity, hydrogen atoms have been omitted. Only one of the two crystallographically independent molecules of 1Cl is shown. Selected bond lengths (Å) and angles (deg) for 1Cl: Au1−C1 = 1.990(3), Au1− Cl1 = 2.2802(7), Au2−C31 = 1.990(3), Au2−Cl2 = 2.2819(7); C1− Au1−Cl1 = 175.25(8), C31−Au2−Cl2 = 173.81(8). Selected bond lengths (Å) and angles (deg) for 1Br: Au1−C1 = 1.988(4), Au1−Br1 = 2.3904(4); C1−Au1−Br1 = 176.71(12).

in the resulting {N4} binding pocket with two five-membered chelate rings that involve the pyrazole N (Npz) and the adjacent pyridazine N (Npdz). The coordination geometry of Ag1 is strongly distorted from tetrahedral, and the Npz−Ag bonds (2.20 and 2.25 Å) are significantly shorter than the Npdz−Ag bonds (2.39 and 2.47 Å). The Au···Ag separation is about 5.53 Å, far beyond any possible interaction between the two metal ions. Complex 5, which has 2-fold crystallographic symmetry, contains one gold(I) and three silver(I) atoms (Figure 5). Again one silver ion (Ag1) is hosted in the {N4} pocket of the

Figure 3. Changes in the 1H NMR chemical shift of a selected pyridazine proton (CHpdz) upon addition of AgBF4 to a solution of 2 in acetone. 5027

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Scheme 3. Synthesis of Heterometallic AuAgx and AuCu Complexes

Figure 5. ORTEP plot (30% probability thermal ellipsoids) of the molecular structure of 5. For the sake of clarity, hydrogen atoms, disorder, counterions, and the solvent molecule have been omitted. Selected bond lengths (Å) and angles (deg): Au1−C1 = 2.020(4), Ag1−N6 = 2.258(3), Ag1−N4 = 2.457(3), Ag2−N8 = 2.181(5), Ag2− N1 = 2.297(3), Ag2−N7 = 2.383(5), Ag2−N9 = 2.395(6), Au1···Ag1= 5.6343(5), Au1···Ag2 = 3.4362(5), Ag1···Ag2 = 3.9659(6), Ag2···Ag2′ = 4.7794(7); C1−Au1−C1′ = 174.5(2), N6−Ag1−N6′ = 131.40(18), N6−Ag1−N4 = 69.06(11), N6−Ag1−N4′ = 149.06(12), N4−Ag1− N4′ = 105.13(15), N8−Ag2−N1 = 144.46(15), N8−Ag2−N7 = 114.90(16), N1−Ag2−N7 = 91.48(13), N8−Ag2−N9 = 94.3(2), N1− Ag2−N9 = 106.59(17), N7−Ag2−N9 = 96.33(19). Symmetry operation used to generate equivalent atoms: (′) 1 − x, y, 1.5 − z.

Figure 4. ORTEP plot (30% probability thermal ellipsoids) of the molecular structure of 3. For the sake of clarity, hydrogen atoms, counterions, and the solvent molecule have been omitted. Selected bond lengths (Å) and angles (deg): Au1−C22 = 2.013(5), Au1−C1 = 2.020(6), Ag1−N12 = 2.197(4), Ag1−N6 = 2.253(4), Ag1−N4 = 2.386(4), Ag1−N10 = 2.470(4), Au1···Ag1 = 5.5296(7); C22−Au1− C1 = 174.4(2), N12−Ag1−N6 = 135.54(16), N12−Ag1−N4 = 153.22(16), N6−Ag1−N4 = 69.29(15), N12−Ag1−N10 = 68.87(16), N6−Ag1−N10 = 139.15(17), N4−Ag1−N10 = 98.76(15).

Scheme 4. Proposed Structure of the Cation [L2AuAg2]3+ of Complex 4

[Ag(MeCN)3]+ fragment at Npdz−Ag2 = 2.30 Å, which is even shorter than the Npdz−Ag1 bonds. Thus, all four silver(I) ions are found to be four-coordinate, somewhat distorted from tetrahedral. The Au···Ag2 separation of 3.44 Å slightly exceeds the sum of the van der Waals radii (3.38 Å),19 and any metallophilic interactions are unlikely. Unfortunately we were not successful in isolating, in crystalline form, the putative intermediate between 3 and 5, namely [L2AuAg2](PF6)(BF4)2 (4). Its 1H NMR spectrum reveals, however, that the two ligand strands are equivalent on the NMR time scale. Assuming that one silver(I) ion is again nested in the {N4} site, it is likely that the second silver(I) ion spans the two remaining Npdz atoms (Scheme 4) or rapidly migrates between them. Note that the related trisilver complex [L2Ag3]3+ (Scheme 1) has a central silver(I) ion bound to both available Npdz but has antiparallel ligand strands. Treatment of 2 with CuIOTf·1/2C6H6 gave the heterobimetallic Au(I)/Cu(I) complex 6. Its 13C NMR carbene signal appears at 184.3 ppm, which is 2.5 ppm downfield from the signal for the Au(I)/Ag(I) congener 3 (181.8 ppm; cf. Table 1). The molecular structure of the cation of 6, determined by X-ray crystallography (Figure 6), is essentially similar to that of 3 with a quasi-linear CNHC−Au−CNHC hinge (177.6°) and CNHC−Au bond lengths of 2.01 and 2.02 Å. The copper(I) ion is found to be in a roughly tetrahedral {N4} environment, but

due to the smaller ionic radius of copper(I) the Npz−Cu (1.97 and 1.99 Å) and Npdz−Cu (2.03 and 2.05 Å) bonds are significantly shorter than in the silver(I) case. In particular, Npz−Cu and Npdz−Cu bond lengths are roughly similar in 6 (Δ ≈ 0.06 Å), while Npz−Ag distances are much shorter than Npdz−Ag distances in 3 (2.20/2.25 versus 2.39/2.47 Å; Δ ≈ 0.20 Å). This likely reflects the higher propensity of silver(I) to adopt low coordination numbers and a linear geometry, whereas copper(I) more strongly prefers a tetrahedral {N4} geometry. As a consequence, twisting of the two ligand strands with respect to each other is much more pronounced in 6 (Figure 7), which is reflected by the much larger angle between the NHC and pyridazine planes in 6 (62°) in comparison to that in 3 (49°). The subtle structural differences between 3 and 6, originating from the distinct coordination chemistry preferences of copper(I) versus silver(I), have drastic consequences for the dynamic behavior of the two systems. Individual molecules in both 3 and 6 have apparent C2 symmetry (with the C2 axis 5028

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the {N4} pocket of the [L2Au]+ scaffold (2) severely hinders rotation around the CNHC−Au−CNHC hinge in 6, while a more flexible silver(I) ion imposes a much lower rotational barrier in 3. Oxidation experiments to produce gold(III) carbene complexes gave isolable products only in the case of 1Cl and 1Br. While treatment of 1Cl with chlorine gas leads to decomposition, the reaction with 1.0 equiv of PhICl220 readily gave [LAuCl3] (7) (Figure 9). This causes a considerable upfield shift of the CHpdz 1H NMR resonances (from 8.40 and 9.01 ppm in 1Cl to 8.27 and 8.52 ppm in 7) and a major downfield shift of the carbene 13C NMR signal to 144.7 ppm; the latter is a reasonable value for (NHC)AuCl3 complexes.17 The molecular structure of 7 (the asymmetric unit contains two crystallographically independent molecules) is depicted in Figure 9. It confirms the expected square-planar geometry for the gold(III) atom with trans angles CNHC−Au1−Cl2 and Cl1− Au1−Cl3 of 178.6 and 178.0° (179.3 and 178.4° for the second molecule), respectively. The CNHC−Au distance (2.00 Å) is essentially unchanged from that of 1Cl, and little or no trans influence is apparent in 7, since the bond lengths Au1−Cl2 and Au1−Cl1/Cl3 are similar (around 2.3 Å). Interestingly, in contrast to the case for 1Cl the pyridazine ring is rotated by 180° in 7, with Npdz now pointing in the same direction as the carbene donor. This brings the N1 atom relatively close to an axial position of the gold(III) ion, d(Au1···N1) = 2.85 Å (2.99 Å for the second molecule), suggesting some secondary bonding interaction. Reaction of 1Br with elemental bromine produced the new complex 8, whose NMR data, however, are much like those of 1Br except for the missing H4 signal of the pyrazole group. X-ray diffraction revealed that ligand bromination at that position had occurred (see the Supporting Information), with 8 still being a gold(I) species. Ligand oxidation upon treatment of NHC gold(I) complexes with Br2 has been observed previously for certain NHC derivatives.21 Selected bond lengths and angles as well as carbene 13C NMR resonances for 1Cl, 1Br, 2, 3, and 5−8 are summarized in Table 1. Having the new gold(III) complex 7 at hand, we tested its performance as a catalyst for the synthesis of unsymmetrical ethers (Scheme 6). Ethers are fundamental, widely used compounds in organic chemistry, but many procedures for their preparation suffer from limitations. This is particularly true for the synthesis of unsymmetrical ethers, where side

Figure 6. ORTEP plot (30% probability thermal ellipsoids) of the molecular structure of 6. For the sake of clarity, hydrogen atoms, counterions, and the solvent molecules have been omitted. Selected bond lengths (Å) and angles (deg): Au1−C1 = 2.013(5), Au1−C31 = 2.017(5), Cu1−N6 = 1.966(4), Cu1−N16 = 1.989(4), Cu1−N14 = 2.030(4), Cu1−N4 = 2.045(5), Au1···Cu1 = 5.2020(7); C1−Au1− C31 = 177.6(2), N6−Cu1−N16 = 127.41(17), N6−Cu1−N14 = 139.40(19), N16−Cu1−N14 = 79.96(16), N6−Cu1−N4 = 79.46(18), N16−Cu1−N4 = 129.85(19), N14−Cu1−N4 = 106.80(18).

passing through the two metal ions) and thus are chiral with two helical enantiomers of P and M configuration; these are present as a racemic mixture in the crystal. In solution at room temperature the AuAg complex 3 shows single sharp 1H NMR resonances for the o-Me groups as well as for the aromatic CH of the mesityl substituents (δ 1.91 and 6.91 ppm, respectively), evidencing fast interconversion of the two helical enantiomers on the NMR time scale. We assume that this interconversion occurs via the roughly planar intermediate 3′ (Scheme 5), whose formation is facilitated by the rather weak Npdz−Ag1 interaction in 3. In contrast, the AuCu complex 6 in CD2Cl2 solution at room temperature features two broad signals for the o-Me groups (δ 1.45 and 1.91 ppm) as well as for the aromatic CH of the mesityl substituents (δ 6.90 and 6.96 ppm). These coalesce at higher temperatures but evolve into sharp separate resonances at −30 °C (δ 1.38/1.85 and 6.88/6.93 ppm); selected NMR spectra for 6 are shown in Figure 8. Analysis of the temperature dependence gives a substantial energy barrier of around 65 kJ/mol for the racemization process, likely due to the reluctance of the copper(I) to adopt the pseudolinear coordination in 6′. In other words, hosting a copper(I) ion in

Figure 7. Side views along the M···Au axis of 3 and 6, illustrating the different twisting of the two ligand strands. 5029

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Scheme 5. Proposed Dynamic Process Interconverting the P and M Helical Enantiomers in 3 (M = Ag) and 6 (M = Cu)

Figure 8. Aromatic region of the 1H NMR spectra of 6 at selected temperatures (CD2Cl2, 500 MHz).

Scheme 6. Gold-Catalyzed Etherification of 1-Indanol

NaAuCl4 has been reported to be an efficient and broad-scoped catalyst for the etherification of benzylic (primary and secondary) and tertiary alcohols under mild conditions.22 We thus tested the new gold(III) complex 7 under similar conditions, using 1-indanol as a reasonably difficult test substrate in combination with an excess (10 equiv) of various primary and secondary alcohols ROH. Addition of AgBF4 as cocatalyst (1 equiv with respect to 7) proved necessary to achieve good conversion. The results are collected in Table 2. At 2 mol % catalyst loading moderate to good conversions of 1-indanol and no formation of the symmetric bis(1-indanyl) ether were found, with yields of the unsymmetrical ethers in the range 50−91% (entries 1−6). Most notably, the catalyst derived from complex 7 appears to be stable even in the presence of water and at elevated temperatures (80 °C). The corresponding gold(I) complex 1Cl is significantly less efficient (entries 7 versus 6) and shows activity comparable to that of AgBF4, which itself catalyzes the reaction as well, even without any gold complex added. To rule out acid catalysis by HF (generated from the BF4− anion), control experiments with

Figure 9. ORTEP plot (30% probability thermal ellipsoids) of the molecular structure of 7. For the sake of clarity, hydrogen atoms have been omitted. Only one of the two crystallographically independent molecules is shown. Selected bond lengths (Å) and angles (deg): Au1−C1 = 1.995(6), Au1−Cl3 = 2.2882(14), Au1−Cl1 = 2.3078(14), Au1−Cl2 = 2.3092(13), Au2−C31 = 2.000(6), Au2−Cl13 = 2.2855(14), Au2−Cl11 = 2.3020(14), Au2−Cl12 = 2.3041(14); C1−Au1−Cl3 = 89.46(15), C1−Au1−Cl1 = 88.60(15), Cl3−Au1− Cl1 = 178.00(5), C1−Au1−Cl2 = 178.63(16), Cl3−Au1−Cl2 = 90.31(5), Cl1−Au1−Cl2 = 91.64(5), C31−Au2−Cl13 = 88.48(15), C31−Au2−Cl11 = 89.95(15), Cl13−Au2−Cl11 = 178.36(5), C31− Au2−Cl12 = 179.32(16), Cl13−Au2−Cl12 = 90.97(5), Cl11−Au2− Cl12 = 90.61(5).

products from the competing formation of symmetrical ethers or from elimination reactions are often observed. Recently 5030

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reasonably challenging substrate 1-indanol. It will be interesting to probe whether the presence of a second metal ion, nested in the proximate N-donor site of the ditopic ligand, affects the catalytic performance of [LAuX] and [LAuCl3] in this and other reactions.

Table 2. Etherification of 1-Indanol with Various Alcohols ROH

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EXPERIMENTAL SECTION

General Considerations. Pyridazine-bridged NHC/pyrazole ligand precursor [HL]Cl and silver complex A were prepared according to the literature methods.11 1H and 13C NMR spectra were recorded on a Bruker Avance 300 or Bruker Avance 500 instrument at 25 °C unless stated otherwise; chemical shifts (δ) were referenced internally to residual solvent signals. Mass spectrometry was performed with an Applied Biosystems API 2000 instrument (ESI). Elemental analyses were performed by the analytical laboratory of the Institute of Inorganic Chemistry at the Georg-August-University Göttingen using an Elementar Vario EL III instrument. Crystal data and refinement details are given in the Supporting Information. Synthesis of Complex 1Cl. [HL]Cl (427 mg, 1.08 mmol, 1.0 equiv) was reacted with Ag2O (502 mg, 2.16 mmol, 2.0 equiv) in acetone. After 18 h of stirring at room temperature, AuCl(SMe2) (350 mg, 1.19 mmol, 1.1 equiv) was added and the reaction mixture was stirred for a further 18 h. After addition of activated carbon, filtration over Celite, and evaporation of all volatile material under reduced pressure, an orange powder was obtained. Its 1H NMR spectrum indicates incomplete conversion of the crude product. According to the intensity of unconverted material signals, additional AuCl(SMe2) was added and the solution stirred for 2 h more. After filtration over Celite the product was obtained as a yellow powder. Crystallization by slow diffusion of diethyl ether into a dichloromethane solution of 1Cl at room temperature afforded colorless crystals suitable for X-ray diffraction. Yield: 470 mg (74%). 1H NMR (300 MHz, CD2Cl2): δ 2.13 (s, 6 H, CH3ar2,6), 2.30 (s, 3 H, CH3pz3), 2.39 (s, 3 H, CH3ar4), 2.76 (s, 3 H, CH3pz5), 6.13 (s, 1 H, CHpz4), 7.09 (s, 2 H, CHar3,5), 7.19 (d, 3J = 2.1 Hz, 1 H, CHim4), 8.22 (d, 3J = 2.1 Hz, 1 H, CHim5), 8.39 (d, 3J = 9.5 Hz, 1 H, CHpdz), 9.00 (d, 3J = 9.5 Hz, 1 H, CHpdz). 13C NMR (75 MHz, CD2Cl2): δ 14.0 (CH3pz3), 15.4 (CH3pz5), 18.3 (CH3ar2,6), 21.5 (CH3ar4), 111.3 (Cpz4), 121.2 (Cim5), 122.8 (Cpdz), 123.9(Cim4), 124.6 (Cpdz), 130.1 (Car3,5), 135.3 (Car2,6), 135.6 (Car1), 140.9 (Car4), 143.2 (Cpz5), 152.6 (Cpdz), 152.7 (Cpz3), 157.8 (Cpdz), 172.3 (CNHC). MS (ESI): m/z 613.1 [M + Na]+. Anal. Calcd for C21H22AuClN6: C, 42.69; H, 3.75; N, 14.22. Found: C, 42.37; H, 3.67; N, 14.22. Synthesis of Complex 1Br. LiBr (176 mg, 2.00 mmol, 10 equiv) was added to a solution of complex 1Cl (120 mg, 0.20 mmol, 1 equiv) in acetone. The resulting solution was stirred for 18 h at room temperature, and acetone was removed in vacuo. The resulting bright yellow residue was dissolved in DCM, and the solution was filtered over a plug of silica and dried over MgSO4. The solvent was then removed under reduced pressure, and the product was isolated as a bright yellow powder. Crystallization by slow diffusion of diethyl ether into a dichloromethane solution of 1Br at room temperature afforded colorless crystals suitable for X-ray diffraction. Yield: 105 mg (81%). 1 H NMR (300 MHz, CD2Cl2): δ 2.13 (s, 6 H, CH3ar2,6), 2.30 (s, 3 H, CH3pz3), 2.39 (s, 3 H, CH3ar4), 2.76 (s, 3 H, CH3pz5), 6.13 (s, 1 H, CHpz4), 7.09 (s, 2 H, CHar3,5), 7.19 (d, 3J = 2.1 Hz, 1 H, CHim4), 8.23 (d, 3J = 2.1 Hz, 1 H, CHim5), 8.39 (d, 3J = 9.5 Hz, 1 H, CHpdz), 9.02 (d, 3 J = 9.5 Hz, 1 H, CHpdz). 13C NMR (75 MHz, CD2Cl2): δ = 14.0 (CH3pz3), 15.4 (CH3pz5), 18.3 (CH3ar2,6), 21.5 (CH3ar4), 111.3 (Cpz4), 121.0 (Cim5), 122.8 (Cpdz), 123.8 (Cim4), 124.4 (Cpdz), 130.0 (Car3,5), 135.3 (Car2,6), 135.5 (Car1), 140.9 (Car4), 143.2 (Cpz5), 152.6 (Cpz3), 152.6 (Cpdz), 157.8 (Cpdz), 175.7 (CNHC). MS (ESI): m/z 554.9 [M − Br]+. Anal. Calcd for C21H22AuBrN6·CH2Cl2: C, 36.69; H, 3.36; N, 11.67. Found: C, 36.53; H, 3.36; N, 11.55. Synthesis of Complex 2. A solution of AuCl(SMe2) (258 mg, 0.88 mmol, 1 equiv) in acetone (20 mL) was treated with AgBF4 (171 mg, 0.88 mmol, 1 equiv). A solution of [AgL2](PF6) (256 mg, 0.26 mmol, 1 equiv) in acetone (10 mL) was added, and the reaction mixture was stirred at room temperature overnight. After addition of

a

10 equiv of ROH with respect to 1-indanol. bCatalyst loading, 2 mol %; additive, 1 equiv of AgBF4. cYield determined by 1H NMR (1,3,5trimethoxybenzene as internal standard), average of 2 trials; values with respect to the 1-indanol reactant. dCatalyst loading, 1 mol %. eNo gold catalyst added.

AgOTf were performed, giving the same conversion as with AgBF4. Moreover, a mixture of AgBF4 and NaCl under the same conditions (which leads to precipitation of AgCl but leaves BF4− present) does not give any observable conversion. These experiments corroborate that the Lewis acidic metal ions (Ag+ or the Au catalyst) are the catalytically active species, which is also in line with the high selectivity for the unsymmetrical products. Since the Ag+ salt is used in stoichiometric amounts with respect to 7, synergetic Au−Ag effects23 in the Au-catalyzed etherifications are unlikely. In comparison to NaAuCl4 the catalyst derived from 7, in the etherification of 1-indanol, shows higher conversion for primary alcohols (EtOH, entries 1 and 2) but lower conversion for secondary alcohols (entries 3 and 4).22

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CONCLUSIONS The ditopic pyridazine-bridged NHC/pyrazole ligand L has been shown to serve as a versatile scaffold for the controlled synthesis of heterooligometallic coinage-metal complexes, using both its organometallic compartment (NHC side) and its Ndonor compartment (pyrazole side). Au(I) preferably binds to the NHC site, and the stability and inertness of the Au− carbene bond has allowed us to prepare and structurally characterize a series of complexes [LAuX] (X = Cl, Br) and [L2AuMx](x+1)+ (M = Cu, Ag). In the latter the quasi linear NHC−Au−NHC hinge preorganizes two ligand strands, while the first Ag(I) or Cu(I) ion is accommodated in the pyrazolederived site. Additional Ag(I) is then bound to the central pyridazine N. In the two-stranded [L2AuM]2+ systems the pyridazine acts as a hemilable ligand, modulating the racemization process that interconverts the P and M helical enantiomers; racemization is much more rapid for M = Ag than for M = Cu. [LAuCl] could be successfully oxidized to the Au(III) complex [LAuCl3] using PhICl2. Preliminary screening shows that that [LAuCl3], in combination with AgBF4, is catalytically active in the synthesis of unsymmetrical ethers of the 5031

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activated carbon the mixture was slowly filtered through Celite. After evaporation of the solvent the desired complex was obtained as a colorless powder. Crystallization by slow diffusion of diethyl ether into a dichloromethane solution of 2 at room temperature afforded colorless crystals suitable for X-ray diffraction. Yield: 232 mg (82%). 1 H NMR (300 MHz, CD2Cl2): δ = 1.92 (s, 6 H, CH3ar2,6), 2.28 (s, 3 H, CH3ar4), 2.34 (s, 3 H, CH3pz3), 2.76 (s, 3 H, CH3pz5), 6.16 (s, 1 H, CHpz4), 6.96 (s, 2 H, CHar3,5), 7.18 (d, 3J = 2.1 Hz, 1 H, CHim4), 8.10 (d, 3J = 2.1 Hz, 1 H, CHim5), 8.22 (d, 3J = 9.5 Hz, 1 H, CHpdz), 8.29 (d, 3 J = 9.5 Hz, 1 H, CHpdz). 13C NMR (75 MHz, CD2Cl2): δ 14.0 (CH3pz3), 15.6 (CH3pz5), 18.0 (CH3ar2,6), 21.2 (CH3ar4), 111.6 (Cpz4), 121.9 (Cim5), 123.0 (Cpdz), 124.2 (Cpdz), 124.7 (Cim4), 130.1 (Car3,5), 135.1 (Car2,6), 135.1 (Car1), 140.8 (Car4), 143.3 (Cpz5), 152.1 (Cpdz), 152.9 (Cpz3), 157.8 (Cpdz), 182.9 (CNHC). MS (ESI): m/z 913.1 [M − PF6]+. Anal. Calcd for C42H44AuF6N12P: C, 47.64; H, 4.19; N, 15.87. Found C, 47.05; H, 4.10; N, 15.67. Synthesis of Complex 3. Complex 3 was prepared by the addition of a solution of AgBF4 (92 mg, 0.47 mmol, 1 equiv) in acetone (5 mL) to a solution of complex 2 (50 mg, 0.47 mmol) in acetone (10 mL) and stirring for 2 h at room temperature. All volatile material was then removed under reduced pressure. Crystallization by slow diffusion of diethyl ether into a dichloromethane solution of crude 3 at room temperature afforded colorless crystals suitable for Xray diffraction. Yield of crude product: 53 mg (89%). 1H NMR (300 MHz, CD2Cl2): δ 1.91 (s, 6 H, CH3ar2,6), 2.29 (s, 3 H, CH3ar4), 2.35 (s, 3 H, CH3pz3), 2.75 (s, 3 H, CH3pz5), 6.17 (s, 1 H, CHpz4), 6.96 (s, 2 H, CHar3,5), 7.18 (d, 3J = 2.1 Hz, 1 H, CHim4), 8.08 (d, 3J = 2.1 Hz, 1 H, CHim5), 8.22 (d, 3J = 9.5 Hz, 1 H, CHpdz), 8.29 (d, 3J = 9.5 Hz, 1 H, CHpdz). 13C NMR (75 MHz, CD2Cl2): δ 14.1 (CH3pz3), 15.6 (CH3pz5), 18.0 (CH3ar2,6), 21.2 (CH3ar4), 111.7 (Cpz4), 121.9 (Cim5), 123.1 (Cpdz), 124.3 (Cim4), 124.7 (Cpdz), 130.1 (Car3,5), 135.1 (Car1), 135.1 (Car2,6), 140.8 (Car4), 143.4 (Cpz5), 152.2 (Cpdz), 153.0 (Cpdz), 157.6 (Cpz3), 183.0 (CNHC). Anal. Calcd for C42H44AuAgBF10N12P: C, 40.24; H, 3.54; N, 13.41. Found: C, 39.76; H, 3.76; N, 13.10. Synthesis of Complex 5. This was prepared by adding an acetone solution (5 mL) of AgBF4 (276 mg, 1.41 mmol, 3 equiv) to a solution of 2 (50 mg, 0.47 mmol) in dichloromethane (5 mL) and stirring for 2 h at room temperature. All volatile material was then removed under reduced pressure. Crystallization by slow diffusion of diethyl ether into an acetonitrile solution of crude 5 at room temperature afforded colorless crystals suitable for X-ray diffraction. Yield of crude product: 58 mg (84%). 1H NMR (300 MHz, CD2Cl2): δ 1.68 (s, 6 H, CH3ar2,6), 2.42 (s, 3 H, CH3ar4), 2.44 (s, 3 H, CH3pz3), 2.68 (s, 3 H, CH3pz5), 6.46 (s, 1 H, CHpz4), 6.92 (s, 2 H, CHar3,5), 7.25 (d, 3J = 2.1 Hz, 1 H, CHim4), 8.09 (d, 3J = 2.1 Hz, 1 H, CHim5), 8.51 (s, 2 H, CHpdz). 13C NMR (75 MHz, CD2Cl2): δ 14.3 (CH3pz3), 15.2 (CH3pz5), 17.7 (CH3ar2,6), 21.4 (CH3ar4), 113.1 (Cpz4), 123.3 (Cim5), 126.2 (Cpdz), 126.6 (Cim4), 129.5 (Cpdz), 129.9 (Car3,5), 134.7 (Car1),135.1 (Car2,6), 140.5 (Car4), 145.6 (Cpz5), 153.6 (Cpdz), 154.5 (Cpdz), 155.1 (Cpz3), 180.7 (CNHC). Anal. Calcd for C42H44Ag3AuB4F16N12·4MeCN: C, 34.34; H, 3.23; N, 12.81. Found: C, 34.85; H, 3.51; N, 12.36. Synthesis of Complex 6. Complex 6 was prepared by adding Cu(OTf)·C6H6 (24 mg, 0.94 mmol, 1 equiv) to a solution of complex 2 (100 mg, 0.94 mmol) in dry acetone (10 mL) and stirring overnight. All volatile material was then removed under reduced pressure. Crystallization by slow diffusion of diethyl ether into a dichloromethane solution of crude 6 at room temperature afforded red crystals suitable for X-ray diffraction. Yield of crude product: 108 mg (90%). 1 H NMR (500 MHz, CD2Cl2, 243 K): δ (ppm) 1.39 (s, 3 H, CH3ar2,6), 1.86 (s, 3 H, CH3ar2,6), 2.16 (s, 3 H, CH3pz3), 2.42 (s, 3 H, CH3ar4), 2.79 (s, 3 H, CH3pz5), 6.44 (s, 1 H, CHpz4), 6.89 (s, 1 H, CHar3,5) 6.93 (s, 1 H, CHar3,5), 7.11 (d, 3J = 1.8 Hz, 1 H, CHim4), 7.80 (d, 3J = 1.8 Hz, 1 H, CHim5), 8.32 (d, 3J = 9.5 Hz, 1 H, CHpdz), 8.53 (d, 3J = 9.5 Hz, 1 H, CHpdz). 1H NMR (500 MHz, CD2Cl2, 293 K): δ (ppm) = 1.46 (s, br, 3 H, CH3ar2,6), 1.91 (s, br, 3 H, CH3ar2,6), 2.21 (s, 3 H, CH3pz3), 2.45 (s, 3 H, CH3ar4), 2.82 (s, 3 H, CH3pz5), 6.45 (s, 1 H, CHpz4), 6.90 (s, 1 H, CHar3,5) 6.96 (s, 1 H, CHar3,5), 7.11 (d, 3J = 1.9 Hz, 1 H, CHim4), 7.86 (d, 3J = 1.9 Hz, 1 H, CHim5), 8.37 (d, 3J = 9.5 Hz, 1 H, CHpdz), 8.54 (d, 3J = 9.5 Hz, 1 H, CHpdz). 1H NMR (500 MHz, CD2Cl2, 308 K): δ (ppm) 1.48 (s, br, 3 H, CH3ar2,6), 1.91 (s, br,

3 H, CH3ar2,6), 2.22 (s, 3 H, CH3pz3), 2.45 (s, 3 H, CH3ar4), 2.83 (s, 3 H, CH3pz5), 6.45 (s, 1 H, CHpz4), 6.93 (s, 2 H, CHar3,5) 7.11 (d, 3J = 2.0 Hz, 1 H, CHim4), 7.87 (d, 3J = 2.0 Hz, 1 H, CHim5), 8.37 (d, 3J = 9.5 Hz, 1 H, CHpdz), 8.55 (d, 3J = 9.5 Hz, 1 H, CHpdz). 13C NMR (125 MHz, CD2Cl2, 243 K): δ (ppm) 14.2 (CH3pz3), 15.0 (CH3pz5), 17.2(CH3ar2,6), 18.0 (CH3ar2,6), 21.3 (CH3ar4), 113.3 (Cpz4), 122.0 (Cim5), 122.1 (Cpdz), 124.6 (Cim4), 129.0 (Cpdz), 129.7 (Car3,5), 134.3 (Car1), 134.6 (Car2,6), 134.8 (Car2,6), 139.8 (Car4), 143.1 (Cpz5), 151.3 (Cpz3), 152.4 (Cpdz), 153.0 (Cpdz), 183.3 (CNHC). MS (ESI): m/z 488.1 [M − OTf − PF6]2+. Anal. Calcd for C42H44AuCuF12N12P2·CH2Cl2: C, 38.22; H, 3.36; N, 12.44. Found: C, 38.11; H, 3.39; N, 12.04. Synthesis of Complex 7. Complex 1Cl (205 mg, 0.35 mmol, 1 equiv) and PhICl2 (106 mg, 0.35 mmol, 1.0 equiv) were dissolved in acetone (20 mL), and the reaction mixture was stirred at room temperature for 20 h. The solvent was then reduced to around half of its volume, and hexane (50 mL) was added. The resulting precipitate was collected by filtration and washed with hexane. The resulting solid was dried to afford 7 as a yellow powder. Yellow single crystals suitable for X-ray diffraction were grown by slow diffusion of diethyl ether into a dichloromethane solution of the product. Yield: 202 mg (87%). 1H NMR (300 MHz, CD2Cl2): δ 2.24 (s, 6 H, CH3ar2,6), 2.30 (s, 3 H, CH3pz3), 2.40 (s, 3 H, CH3ar4), 2.80 (s, 3 H, CH3pz5), 6.15 (s, 1 H, CHpz4), 7.10 (s, 2 H, CHar3,5), 7.41 (d, 3J = 2.1 Hz, 1 H, CHim4), 8.07 (d, 3J = 2.1 Hz, 1 H, CHim5), 8.27 (d, 3J = 9.5 Hz, 1 H, CHpdz), 8.52 (d, 3 J = 9.5 Hz, 1 H, CHpdz). 13C NMR (75 MHz, CD2Cl2): δ 14.0 (CH3pz3), 15.7 (CH3pz5), 18.9 (CH3ar2,6), 21.5 (CH3ar4), 111.8 (Cpz4), 123.4 (Cim5), 123.9 (Cpdz), 124.7 (Cim5), 127.2 (Cpdz), 130.5 (Car3,5), 133.0 (Car1), 135.9 (Car2,6), 142.0 (Car4), 143.8 (Cpz5),144.9 (CNHC), 151.2 (Cpdz), 153.2 (Cpz3), 158.2 (Cpdz). MS (ESI): m/z 555.0 [M − Cl]+. Anal. Calcd for C21H22AuCl3N6: C, 38.11; H, 3.35; N, 12.70. Found: C, 37.70; H, 3.30; N, 12.70. Synthesis of 8. 1Br (100 mg, 0.16 mmol, 1.0 equiv) was dissolved in dichloromethane (10 mL) and the solution cooled to −78 °C. Excess bromine (0.03 mL, 0.6 mmol, 3.8 equiv) was added, and the reaction mixture was stirred for 2 h and then warmed to room temperature. The solvent was removed under reduced pressure, and the resulting orange residue was dried under vacuum to remove excess bromine. It was then dissolved in dichloromethane (5 mL) and pentane (100 mL) was added to precipitate 8 as an orange powder. Colorless single crystals suitable for X-ray diffraction were grown by slow diffusion of diethyl ether into a dichloromethane solution of the crude product. Yield: 97 mg (85%). MS (ESI): m/z 439.0 [L1Br]+. General Procedure for Etherification Reactions. A flask sealed with a Teflon screw cap was loaded with catalyst (2 mol %), 2-indanol (0.5 mmol), and the respective aliphatic alcohol (5 mmol). AgBF4 (2 mol %) was added, and the solution was heated with stirring to 80 °C for 3 h. The reaction mixture was cooled to room temperature and diluted with ethyl acetate (6 mL). After filtration over Celite the solution was evaporated under reduced pressure and 1,3,5trimethoxybenzene (0.5 mmol) was added to the remaining oil/solid as an internal standard for determining the yields by 1H NMR spectroscopy. In control experiments AgOTf (2 mol %) instead of AgBF4 was used as catalyst, following the described procedure. Addition of NaCl (6 mol %) to these reaction mixtures led to complete inhibition of the etherification reactions.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving crystallographic data, an ORTEP plot of 8, and NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +49 551 393012. Fax: +49 551 393063. E-mail: franc. [email protected]. 5032

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Notes

(14) (a) Casado, R.; Contel, M.; Laguna, M.; Romero, P.; Sanz, S. J. Am. Chem. Soc. 2003, 125, 11925. (b) Hashmi, A. S. K.; Weyrauch, J. P.; Rudolph, M.; Kurpejović, E. Angew. Chem., Int. Ed. 2004, 43, 6545. (c) Gaillard, S.; Slawin, A. M. Z.; Bonura, A. T.; Stevens, E. D.; Nolan, S. P. Organometallics 2010, 29, 394. (15) Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Chem. Rev. 2009, 109, 3561. (16) (a) de Frémont, P.; Singh, R.; Stevens, E. D.; Petersen, J. L.; Nolan, S. P. Organometallics 2007, 26, 1376. (b) Jahnke, M. C.; Pape, T.; Hahn, F. E. Z. Anorg. Allg. Chem. 2010, 636, 2309. (17) Pažický, M.; Loos, A.; Ferreira, M. J.; Serra, D.; Vinokurov, N.; Rominger, F.; Jäkel, C.; Hashmi, A. S. K.; Limbach, M. Organometallics 2010, 29, 4448. (18) (a) Baker, M. V.; Barnard, P. J.; Berners-Price, S. J.; Brayshaw, S. K.; Hickey, J. L.; Skelton, B. W.; White, A. H. Dalton Trans. 2006, 3708. (b) Gaillard, S.; Nun, P.; Slawin, A. M. Z.; Nolan, S. P. Organometallics 2010, 29, 5402. (19) Bondi, A. J. Phys. Chem. 1964, 68, 441. (20) Zhao, X.-F.; Zhang, C. Synthesis 2007, 551. (21) Samantaray, M. K.; Pang, K.; Shaikh, M. M.; Ghosh, P. Dalton Trans. 2008, 4893. (22) Cuenca, A. B.; Mancha, G.; Asensio, G.; Medio-Simón, M. Chem. Eur. J. 2008, 14, 1518. (23) Weber, D.; Gagné, M. R. Org. Lett. 2009, 11, 4962.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Fonds der Chemischen Industrie and the Georg-August-University is gratefully acknowledged. REFERENCES

(1) (a) Herrmann, W. A.; Köcher, C. Angew. Chem., Int. Ed. 1997, 36, 2161. (b) Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39. (c) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (d) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122. (e) Frémonta, P.; Marion, N.; Nolan, S. P. Coord. Chem. Rev. 2009, 253, 862. (2) (a) Nolan, S. P. N-Heterocyclic Carbenes in Synthesis, 1st ed.; Wiley-VCH: Weinheim, Germany, 2006. (b) Glorius, F. N-Heterocyclic Carbenes in Transition Metal Catalysis; Springer: Berlin/Heidelberg, 2007; Topics in Organometallic Chemistry Vol. 21. (c) Cazin, C. S. J. N-Heterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis, 1st ed.; Springer: Heidelberg/London/New York, 2010; Catalysis by Metal Complexes Vol. 32. (d) Díez-González, S. NHeterocyclic Carbenes: From Laboratory Curiosities to Efficient Synthetic Tools, 1st ed.; Royal Society of Chemistry: Cambridge, U.K., 2011. (3) (a) Green, J. C.; Scurr, R. G.; Arnold, P. L.; Cloke, F. G. N. Chem. Commun. 1997, 1963. (b) Termaten, A. T.; Schakel, M.; Ehlers, A. W.; Lutz, M.; Spek, A. L.; Lammertsma, K. Chem. Eur. J. 2003, 9, 3577. (c) Crudden, C. M.; Allen, D. P. Coord. Chem. Rev. 2004, 248, 2247. (d) Cavallo, L.; Correa, A.; Costabile, C.; Jacobsen, H. J. Organomet. Chem. 2005, 690, 5407. (4) (a) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239. (b) Pugh, D.; Danopoulos, A. A. Coord. Chem. Rev. 2007, 251, 610. (c) Lee, H. M.; Lee, C.-C.; Cheng, P.-Y. Current Org. Chem. 2007, 11, 1491. (d) Poyatos, M.; Mata, J. A.; Peris, E. Chem. Rev. 2009, 109, 3677. (e) Edwards, P. G.; Hahn, F. E. Dalton Trans. 2011, 40, 10278. (5) (a) Gavrilova, A. L.; Bosnich, B. Chem. Rev. 2004, 104, 349. (b) Klingele, J.; Dechert, S.; Meyer, F. Coord. Chem. Rev. 2009, 253, 2698. (6) (a) Zhou, Y.; Chen, W. Organometallics 2007, 26, 2742. (b) Jeon, S.-J.; Waymouth, R. M. Dalton Trans. 2008, 437. (c) Scheele, U. J.; John, M.; Dechert, S.; Meyer, F. Eur. J. Inorg. Chem. 2008, 373. (d) Scheele, U. J.; Georgiou, M.; John, M.; Dechert, S.; Meyer, F. Organometallics 2008, 27, 5146. (e) Zhou, Y.; Xi, Z.; Chen, W.; Wang, D. Organometallics 2008, 27, 5911. (f) Georgiou, M.; Wöckel, S.; Konstanzer, V.; Dechert, S.; John, M.; Meyer, F. Z. Naturforsch., B 2009, 64, 1542. (g) Liu, B.; Liu, B.; Zhou, Y.; Chen, W. Organometallics 2010, 29, 1457. (7) (a) Hahn, F. E.; Radloff, C.; Pape, T.; Hepp, A. Chem. Eur. J. 2008, 14, 10900. (b) Jahnke, M. C.; Hussain, M.; Hupka, F.; Pape, T.; Ali, S.; Hahn, F. E.; Cavell, K. J. Tetrahedron 2009, 65, 909. (c) Chang, Y.-H.; Liu, Z.-Y.; Liu, Y.-H.; Peng, S.-M.; Chen, J.-T.; Liu, S.-T. Dalton Trans. 2011, 40, 489. (8) (a) Ye., J.; Jin., S.; Chen., W.; Qiu, H. Inorg. Chem. Commun. 2008, 11, 404. (b) Willians, C. E.; Anderson, K. M.; Paterson, M. J.; Junk, P. C.; Barbour, L. J.; Steed, J. W. Eur. J. Inorg. Chem. 2009, 2835. (c) Chang, Y.-H.; Liu, Z.-Y.; Liu, Y.-H.; Peng, S.-M.; Chen., J.-T.; Liu, S.-T. Dalton Trans. 2011, 40, 489. (9) (a) Lee, K.-M.; Chen, J. C. C.; Lin, I. J. B. J. Organomet. Chem. 2001, 617−618, 364. (b) Scheele, U. J.; Dechert, S.; Meyer, F. Inorg. Chim. Acta 2006, 359, 4891. (c) Scheele, U. J.; Dechert, S.; Meyer, F. Tetrahedron Lett. 2007, 48, 8366. (10) Gu, S.; Xu, D.; Chen, W. Dalton Trans. 2011, 40, 1576. (11) Wimberg, J.; Scheele, U. J.; Dechert, S.; Meyer, F. Eur. J. Inorg. Chem. 2011, 3340. (12) (a) Fürstner, A.; Davies, P. W. Angew. Chem., Int. Ed. 2007, 46, 3410. (b) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180. (c) Arcadi, A. Chem. Rev. 2008, 108, 3266. (d) Jiménez-Núñez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326. (13) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776. 5033

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