Gold(I), Gold(III), and Heterometallic Gold(I)−Silver(I) and Gold(I)−Copper(I) Complexes of a Pyridazine-Bridged NHC/Pyrazole HybridLigand and Their Initial Application in CatalysisJan Wimberg, Steffen Meyer, Sebastian Dechert, and Franc Meyer*

Institute of Inorganic Chemistry, Georg-August-University Gottingen, Tammannstrasse 4, D-37077 Gottingen, Germany

*S Supporting Information

ABSTRACT: The pyridazine-bridged NHC/pyrazole ligand L(HL = 3-[3-(2,4,6-trimethylphenyl)-3H-imidazolium-1-yl]-6-(3,5-dimethylpyrazol-1-yl)-pyridazine) that provides an organometallicand a classical N-donor compartment is shown to serve as aversatile scaffold for a variety of homo- and heterometallic gold(I)carbene complexes. Complexes [LAuX] (1Cl, X = Cl; 1Br, X = Br),[ L 2 A u ] ( P F 6 ) ( 2 ) , [ L 2 A u A g ] ( B F 4 ) ( P F 6 ) ( 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 casesAu(I) binds to the NHC site while the additional Ag(I) in 3 orCu(I) in 6 is accommodated in the pyrazole-derived site. Both 3 and 6 form two-stranded helical structures; racemization of theP 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 isrationalized on the basis of the different coordination chemistry preferences of these two metal ions. 3 may bind two furtherAg(I) ions to the central pyridazine N, giving 5. Treatment of 1Br with Br2 leads to bromination at the pyrazole C4 of the ligandbackbone, yielding [LBrAuBr] (8). In contrast, 1Cl could be successfully oxidized to the Au(III) complex [LAuCl3] (7) usingPhICl2; both 7 and the gold(I) complex 8 have been characterized crystallographically. Preliminary screening shows that 7, incombination with AgBF4, is a good catalyst for the etherification of 1-indanol with a variety of alcohol substrates and showssignificantly higher activity than the gold(I) catalyst 1Cl.

■ INTRODUCTIONN-heterocyclic carbenes (NHCs) have become an extremelypopular 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 withlittle or negligible π back-bonding, and they have shown theability to form robust complexes with various transition metalsin different oxidation states.3 As a further elaboration they areincreasingly incorporated in sophisticated multidentate ligandscaffolds that impart particular stability or constrainedstructures to the resulting complexes.4 Dinucleating ligands,which preorganize two metal ions in close proximity similar towhat is often observed in metalloenzyme active sites, offer greatprospects for cooperative effects in substrate activation andcatalysis.5 It is thus not surprising that the number of knowndinucleating ligands containing NHC donor sites is growingrapidly;6−8 this also includes some pyridazine-bridged bis-(NHC) ligands.9 While most of these systems are symmetric,however, examples of unsymmetrical compartmental ligandsthat provide two electronically distinct coordination sites, atleast one of which contains an NHC unit, are rare.10

We recently reported a new class of pyridazine-bridgedNHC/pyrazole hybrid ligands L that feature two topologicallysimilar yet electronically very different binding sites: anorganometallic {C/N} compartment involving the NHC andone of the pyridazine N atoms and a classical {N/N}

compartment involving the pyrazole N and the other pyridazineN.11 Formally the two compartments differ only in theswitching of adjacent C and N atoms in the five-membereddiazole heterocycles attached to the central pyridazine unit. Adetailed investigation of the silver(I) coordination chemistry ofthose ligands revealed the sequential binding of up to threesilver(I) ions, first to the NHC site, then to the pyrazole N, andfinally to the central pyridazine, with some ligand reshufflingfrom parallel to antiparallel ligand strands en route (Scheme 1).

In this work we report the first heterometallic complexes ofthis new ligand class. In particular we focus on gold(I)

Received: April 24, 2012Published: July 10, 2012

Scheme 1. Formation of Oligomeric Silver(I) Complexes ofPyridazine-Bridged NHC/Pyrazole Hybrid Ligands11

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complexes containing a second coinage-metal ion, either copperor silver. Also we evaluate whether gold(III) might be stabilizedby such an NHC/pyridazine scaffold. We anticipated that thedifferent preferences of gold(I), silver(I), and copper(I) for theorganometallic versus the classical N-donor sites as well as theparticular stability of the gold−NHC bond would preventligand reshuffling upon binding of the second and third metalions, 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 andis currently one of the most rapidly growing fields in transition-metal catalysis research.12 In homogeneous catalysis eithersimple gold(I) or gold(III) salts are often employed, or well-defined gold(I) or gold(III) complexes with phosphine andNHC ligands.13 NHC−gold complexes have especially gainedmuch popularity since the first report of gold catalysis involvingNHC ancillary ligands. Gold ions are usually assumed to behaveas π acids, mediating inter alia the activation of alkenes, allenes,and alkynes as well as the skeletal rearrangement of enynes. Incomparison to their gold(I) congeners, however, gold(III)compounds with their harder Lewis acidity are still rather rarelyused as catalysts.14 This report includes a preliminaryinvestigation of a new NHC/pyridazine−gold(III) complextoward the catalytic etherification of primary and secondaryalcohols with 1-indanol.

■ RESULTS AND DISCUSSIONThe pyridazine-bridged ligand L bearing a mesityl substituent atthe NHC side and methyl substituents at the pyrazole unit hasbeen used throughout this work.11

Reaction of the ligand precursor [HL]Cl (i.e., theimidazolium salt of L)11 with an excess of Ag2O in acetonesolution and subsequent transmetalation15 of the in situgenerated silver−carbene species with AuCl(SMe2) yielded,

after workup, the gold(I) complex LAuCl (1Cl; Scheme 2). Its1H NMR spectrum shows two characteristic doublets at 8.40and 9.01 ppm with a 3JHH coupling of 9.5 Hz for the pyridazineprotons (CHpdz), as well as the expected disappearance of theimidazolium C2 proton upon carbene formation and metalcoordination. Treatment of 1Cl with an excess of LiBr cleanlytransformed 1Cl into the analogous complex LAuBr (1Br),

evidenced by a shift of the 13C NMR resonance for the metal-bound 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 forpreviously reported NHC−gold(I) halide complexes16 and islikely a result of the lower acidity of the gold(I) ion in thebromide species, because the electronegativity of bromine islower than that of chlorine. Both 1Cl and 1Br could becrystallized by slow diffusion of diethyl ether into dichloro-methane solutions of the compounds. The molecular structuresof 1Cl (the asymmetric unit contains two crystallographicallyindependent 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−Xangles 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 pendantpyridazine groups in 1Cl and 1Br do not interact with thegold ion but their diazine moieties point in opposite directionof 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 theknown silver(I) analogue [L2Ag](PF6) (A)11 using stoichio-metric amounts of AuCl(SMe2) (Scheme 2). For this route itproved necessary, however, to first treat the gold precursor withAgBF4 in order to avoid formation of the NHC−gold chloridecomplex 1Cl (cf. Scheme 2). 2 shows characteristic 1H NMRsignals 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 thesilver complex A (182.3 ppm) and is in the typical range forbis(NHC)-ligated gold(I). Single crystals of 2 were obtainedupon slow diffusion of diethyl ether into a dichloromethanesolution of the crude product; the molecular structure of thecation of 2 is depicted in Figure 2. Coordination of the gold(I)ion again is quasi-linear with a CNHC−Au−CNHC angle of177.1°. This and the CNHC−Au bond lengths (2.02 Å) are wellwithin the range usually found for this kind of structuralmotif.18 While the pyridazine and pyrazole rings in 2 are almostcoplanar, the planes of the pyridazine and NHC heterocyclesare 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 acetonesolution 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 thepyridazine N in metal coordination. The titration curve (Figure3) suggests that 2 may accommodate up to three silver ions,though the effect of the third equivalent on δ(CHpdz) is onlymarginal. Addition of even more AgBF4 does not lead to anyfurther changes in δ(CHpdz).The sequential coordination of up to three silver(I) ions was

unambiguously confirmed by the targeted synthesis andcrystallographic characterization of complexes 3 and 5 that, inaddition to the single gold(I) ion, contain either one or threesilver ions, thus featuring heterobimetallic AuAg and hetero-tetrametallic AuAg3 cores (Scheme 3 and Figures 4 and 5). Inboth cases the quasi-linear [L2Au]

+ scaffold of 2 is retained (cf.Table 1), but rotation around an Au−CNHC bond has occurredand the two ligand strands, which are antiparallel in 2, are nowparallel. In complex 3 (Figure 4) a single silver(I) ion is nested

Chart 1. Ligand L Used in This Work

Scheme 2. Synthesis of Carbene Complexes A, 1Cl, 1Br, and 2

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in the resulting {N4} binding pocket with two five-memberedchelate rings that involve the pyrazole N (Npz) and the adjacentpyridazine N (Npdz). The coordination geometry of Ag1 isstrongly distorted from tetrahedral, and the Npz−Ag bonds(2.20 and 2.25 Å) are significantly shorter than the Npdz−Agbonds (2.39 and 2.47 Å). The Au···Ag separation is about 5.53Å, far beyond any possible interaction between the two metalions.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

[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

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

1Cl 1Br 2 3 5 6 7 8

Au−C 1.990(3) 1.988(4) 2.015(3) 2.013(5) 2.020(4) 2.013(5) 1.995(6) 1.978(5)1.990(3) 2.018(3) 2.020(6) 2.017(5) 2.000(6)

Au−X (trans) 2.280(1) 2.390(1) 2.309(1) 2.390(1)2.282(1) 2.304(1)

Au−X (cis) 2.288(1)2.308(1)2.286(1)2.302(1)

Au···M 5.530(1) 5.634(1) 5.202(1)3.436(1)

C−Au−C 177.1(1) 174.4(2) 174.5(2) 177.6(2)C−Au−X (trans) 175.3(1) 176.7(1) 178.0(1) 176.5(1)

173.8(1) 179.3(2)C−Au−X (cis) 89.5(2)

88.6(2)88.5(2)90.0(2)

δ(CNHC) 172.3 175.7 182.7 181.8 180.9 184.3 144.7

Figure 1. ORTEP plots (30% probability thermal ellipsoids) of themolecular structures of 1Cl (top) and 1Br (bottom). For the sake ofclarity, hydrogen atoms have been omitted. Only one of the twocrystallographically independent molecules of 1Cl is shown. Selectedbond 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 bondlengths (Å) and angles (deg) for 1Br: Au1−C1 = 1.988(4), Au1−Br1 =2.3904(4); C1−Au1−Br1 = 176.71(12).

Figure 2. ORTEP plot (30% probability thermal ellipsoids) of themolecular structure of 2. For the sake of clarity, hydrogen atoms andPF6

− 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).

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

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[Ag(MeCN)3]+ fragment at Npdz−Ag2 = 2.30 Å, which is even

shorter than the Npdz−Ag1 bonds. Thus, all four silver(I) ionsare found to be four-coordinate, somewhat distorted fromtetrahedral. The Au···Ag2 separation of 3.44 Å slightly exceedsthe sum of the van der Waals radii (3.38 Å),19 and anymetallophilic 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 spectrumreveals, however, that the two ligand strands are equivalent onthe NMR time scale. Assuming that one silver(I) ion is againnested in the {N4} site, it is likely that the second silver(I) ionspans the two remaining Npdz atoms (Scheme 4) or rapidlymigrates between them. Note that the related trisilver complex[L2Ag3]

3+ (Scheme 1) has a central silver(I) ion bound to bothavailable Npdz but has antiparallel ligand strands.Treatment of 2 with CuIOTf·1/2C6H6 gave the hetero-

bimetallic Au(I)/Cu(I) complex 6. Its 13C NMR carbene signalappears at 184.3 ppm, which is 2.5 ppm downfield from thesignal for the Au(I)/Ag(I) congener 3 (181.8 ppm; cf. Table1). The molecular structure of the cation of 6, determined byX-ray crystallography (Figure 6), is essentially similar to that of3 with a quasi-linear CNHC−Au−CNHC hinge (177.6°) andCNHC−Au bond lengths of 2.01 and 2.02 Å. The copper(I) ionis found to be in a roughly tetrahedral {N4} environment, but

due to the smaller ionic radius of copper(I) the Npz−Cu (1.97and 1.99 Å) and Npdz−Cu (2.03 and 2.05 Å) bonds aresignificantly 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 thanNpdz−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) toadopt 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 strandswith respect to each other is much more pronounced in 6(Figure 7), which is reflected by the much larger angle betweenthe NHC and pyridazine planes in 6 (62°) in comparison tothat in 3 (49°).The subtle structural differences between 3 and 6, originating

from the distinct coordination chemistry preferences ofcopper(I) versus silver(I), have drastic consequences for thedynamic behavior of the two systems. Individual molecules inboth 3 and 6 have apparent C2 symmetry (with the C2 axis

Scheme 3. Synthesis of Heterometallic AuAgx and AuCuComplexes

Figure 4. ORTEP plot (30% probability thermal ellipsoids) of themolecular structure of 3. For the sake of clarity, hydrogen atoms,counterions, and the solvent molecule have been omitted. Selectedbond 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).

Figure 5. ORTEP plot (30% probability thermal ellipsoids) of themolecular 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). Symmetryoperation used to generate equivalent atoms: (′) 1 − x, y, 1.5 − z.

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

Complex 4

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passing through the two metal ions) and thus are chiral withtwo helical enantiomers of P and M configuration; these arepresent as a racemic mixture in the crystal. In solution at roomtemperature the AuAg complex 3 shows single sharp 1H NMRresonances for the o-Me groups as well as for the aromatic CHof the mesityl substituents (δ 1.91 and 6.91 ppm, respectively),evidencing fast interconversion of the two helical enantiomerson the NMR time scale. We assume that this interconversionoccurs via the roughly planar intermediate 3′ (Scheme 5),whose formation is facilitated by the rather weak Npdz−Ag1interaction in 3. In contrast, the AuCu complex 6 in CD2Cl2solution at room temperature features two broad signals for theo-Me groups (δ 1.45 and 1.91 ppm) as well as for the aromaticCH of the mesityl substituents (δ 6.90 and 6.96 ppm). Thesecoalesce at higher temperatures but evolve into sharp separateresonances 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 ofthe temperature dependence gives a substantial energy barrierof around 65 kJ/mol for the racemization process, likely due tothe reluctance of the copper(I) to adopt the pseudolinearcoordination in 6′. In other words, hosting a copper(I) ion in

the {N4} pocket of the [L2Au]+ scaffold (2) severely hinders

rotation around the CNHC−Au−CNHC hinge in 6, while a moreflexible silver(I) ion imposes a much lower rotational barrier in3.Oxidation experiments to produce gold(III) carbene

complexes gave isolable products only in the case of 1Cl and1Br. While treatment of 1Cl with chlorine gas leads todecomposition, the reaction with 1.0 equiv of PhICl2

20 readilygave [LAuCl3] (7) (Figure 9). This causes a considerableupfield shift of the CHpdz 1H NMR resonances (from 8.40 and9.01 ppm in 1Cl to 8.27 and 8.52 ppm in 7) and a majordownfield 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 twocrystallographically independent molecules) is depicted inFigure 9. It confirms the expected square-planar geometry forthe 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 secondmolecule), respectively. The CNHC−Au distance (2.00 Å) isessentially unchanged from that of 1Cl, and little or no transinfluence is apparent in 7, since the bond lengths Au1−Cl2 andAu1−Cl1/Cl3 are similar (around 2.3 Å). Interestingly, incontrast to the case for 1Cl the pyridazine ring is rotated by180° in 7, with Npdz now pointing in the same direction as thecarbene donor. This brings the N1 atom relatively close to anaxial position of the gold(III) ion, d(Au1···N1) = 2.85 Å (2.99Å for the second molecule), suggesting some secondarybonding interaction.Reaction of 1Br with elemental bromine produced the new

complex 8, whose NMR data, however, are much like those of1Br except for the missing H4 signal of the pyrazole group. X-raydiffraction revealed that ligand bromination at that position hadoccurred (see the Supporting Information), with 8 still being agold(I) species. Ligand oxidation upon treatment of NHCgold(I) complexes with Br2 has been observed previously forcertain NHC derivatives.21 Selected bond lengths and angles aswell as carbene 13C NMR resonances for 1Cl, 1Br, 2, 3, and 5−8are summarized in Table 1.Having the new gold(III) complex 7 at hand, we tested its

performance as a catalyst for the synthesis of unsymmetricalethers (Scheme 6). Ethers are fundamental, widely usedcompounds in organic chemistry, but many procedures fortheir preparation suffer from limitations. This is particularlytrue for the synthesis of unsymmetrical ethers, where side

Figure 6. ORTEP plot (30% probability thermal ellipsoids) of themolecular structure of 6. For the sake of clarity, hydrogen atoms,counterions, and the solvent molecules have been omitted. Selectedbond 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).

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

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products from the competing formation of symmetrical ethersor from elimination reactions are often observed. Recently

NaAuCl4 has been reported to be an efficient and broad-scopedcatalyst for the etherification of benzylic (primary andsecondary) and tertiary alcohols under mild conditions.22 Wethus tested the new gold(III) complex 7 under similarconditions, using 1-indanol as a reasonably difficult testsubstrate in combination with an excess (10 equiv) of variousprimary and secondary alcohols ROH. Addition of AgBF4 ascocatalyst (1 equiv with respect to 7) proved necessary toachieve 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 therange 50−91% (entries 1−6). Most notably, the catalystderived from complex 7 appears to be stable even in thepresence of water and at elevated temperatures (80 °C). Thecorresponding gold(I) complex 1Cl is significantly less efficient(entries 7 versus 6) and shows activity comparable to that ofAgBF4, which itself catalyzes the reaction as well, even withoutany gold complex added. To rule out acid catalysis by HF(generated from the BF4

− anion), control experiments with

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).

Figure 9. ORTEP plot (30% probability thermal ellipsoids) of themolecular structure of 7. For the sake of clarity, hydrogen atoms havebeen omitted. Only one of the two crystallographically independentmolecules 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).

Scheme 6. Gold-Catalyzed Etherification of 1-Indanol

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AgOTf were performed, giving the same conversion as withAgBF4. Moreover, a mixture of AgBF4 and NaCl under thesame conditions (which leads to precipitation of AgCl butleaves 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 theunsymmetrical products. Since the Ag+ salt is used instoichiometric amounts with respect to 7, synergetic Au−Ageffects23 in the Au-catalyzed etherifications are unlikely. Incomparison to NaAuCl4 the catalyst derived from 7, in theetherification of 1-indanol, shows higher conversion for primaryalcohols (EtOH, entries 1 and 2) but lower conversion forsecondary alcohols (entries 3 and 4).22

■ CONCLUSIONSThe ditopic pyridazine-bridged NHC/pyrazole ligand L hasbeen shown to serve as a versatile scaffold for the controlledsynthesis of heterooligometallic coinage-metal complexes, usingboth its organometallic compartment (NHC side) and its N-donor compartment (pyrazole side). Au(I) preferably binds tothe NHC site, and the stability and inertness of the Au−carbene bond has allowed us to prepare and structurallycharacterize a series of complexes [LAuX] (X = Cl, Br) and[L2AuMx]

(x+1)+ (M = Cu, Ag). In the latter the quasi linearNHC−Au−NHC hinge preorganizes two ligand strands, whilethe first Ag(I) or Cu(I) ion is accommodated in the pyrazole-derived site. Additional Ag(I) is then bound to the centralpyridazine N. In the two-stranded [L2AuM]2+ systems thepyridazine acts as a hemilable ligand, modulating theracemization process that interconverts the P and M helicalenantiomers; racemization is much more rapid for M = Ag thanfor M = Cu.[LAuCl] could be successfully oxidized to the Au(III)

complex [LAuCl3] using PhICl2. Preliminary screening showsthat that [LAuCl3], in combination with AgBF4, is catalyticallyactive in the synthesis of unsymmetrical ethers of the

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

■ EXPERIMENTAL SECTIONGeneral Considerations. Pyridazine-bridged NHC/pyrazole

ligand precursor [HL]Cl and silver complex A were preparedaccording to the literature methods.11 1H and 13C NMR spectrawere recorded on a Bruker Avance 300 or Bruker Avance 500instrument at 25 °C unless stated otherwise; chemical shifts (δ) werereferenced internally to residual solvent signals. Mass spectrometrywas performed with an Applied Biosystems API 2000 instrument(ESI). Elemental analyses were performed by the analytical laboratoryof the Institute of Inorganic Chemistry at the Georg-August-UniversityGottingen using an Elementar Vario EL III instrument. Crystal dataand refinement details are given in the Supporting Information.

Synthesis of Complex 1Cl. [HL]Cl (427 mg, 1.08 mmol, 1.0equiv) was reacted with Ag2O (502 mg, 2.16 mmol, 2.0 equiv) inacetone. After 18 h of stirring at room temperature, AuCl(SMe2) (350mg, 1.19 mmol, 1.1 equiv) was added and the reaction mixture wasstirred for a further 18 h. After addition of activated carbon, filtrationover Celite, and evaporation of all volatile material under reducedpressure, an orange powder was obtained. Its 1H NMR spectrumindicates incomplete conversion of the crude product. According tothe intensity of unconverted material signals, additional AuCl(SMe2)was added and the solution stirred for 2 h more. After filtration overCelite the product was obtained as a yellow powder. Crystallization byslow diffusion of diethyl ether into a dichloromethane solution of 1Cl

at room temperature afforded colorless crystals suitable for X-raydiffraction. Yield: 470 mg (74%). 1H NMR (300 MHz, CD2Cl2): δ2.13 (s, 6 H, CH3

ar2,6), 2.30 (s, 3 H, CH3pz3), 2.39 (s, 3 H, CH3

ar4),2.76 (s, 3 H, CH3

pz5), 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). 13CNMR (75 MHz, CD2Cl2): δ 14.0 (CH3

pz3), 15.4 (CH3pz5), 18.3

(CH3ar2,6), 21.5 (CH3

ar4), 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 forC21H22AuClN6: 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 roomtemperature, and acetone was removed in vacuo. The resulting brightyellow residue was dissolved in DCM, and the solution was filteredover a plug of silica and dried over MgSO4. The solvent was thenremoved under reduced pressure, and the product was isolated as abright yellow powder. Crystallization by slow diffusion of diethyl etherinto a dichloromethane solution of 1Br at room temperature affordedcolorless crystals suitable for X-ray diffraction. Yield: 105 mg (81%).1H NMR (300 MHz, CD2Cl2): δ 2.13 (s, 6 H, CH3

ar2,6), 2.30 (s, 3 H,CH3

pz3), 2.39 (s, 3 H, CH3ar4), 2.76 (s, 3 H, CH3

pz5), 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,3J = 9.5 Hz, 1 H, CHpdz). 13C NMR (75 MHz, CD2Cl2): δ = 14.0(CH3

pz3), 15.4 (CH3pz5), 18.3 (CH3

ar2,6), 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 (171mg, 0.88 mmol, 1 equiv). A solution of [AgL2](PF6) (256 mg, 0.26mmol, 1 equiv) in acetone (10 mL) was added, and the reactionmixture was stirred at room temperature overnight. After addition of

Table 2. Etherification of 1-Indanol with Various AlcoholsROH

a10 equiv of ROH with respect to 1-indanol. bCatalyst loading, 2 mol%; additive, 1 equiv of AgBF4.

cYield determined by 1H NMR (1,3,5-trimethoxybenzene as internal standard), average of 2 trials; valueswith respect to the 1-indanol reactant. dCatalyst loading, 1 mol %. eNogold catalyst added.

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activated carbon the mixture was slowly filtered through Celite. Afterevaporation of the solvent the desired complex was obtained as acolorless powder. Crystallization by slow diffusion of diethyl ether intoa dichloromethane solution of 2 at room temperature affordedcolorless crystals suitable for X-ray diffraction. Yield: 232 mg (82%).1H NMR (300 MHz, CD2Cl2): δ = 1.92 (s, 6 H, CH3

ar2,6), 2.28 (s, 3H, CH3

ar4), 2.34 (s, 3 H, CH3pz3), 2.76 (s, 3 H, CH3

pz5), 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,3J = 9.5 Hz, 1 H, CHpdz). 13C NMR (75 MHz, CD2Cl2): δ 14.0(CH3

pz3), 15.6 (CH3pz5), 18.0 (CH3

ar2,6), 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) inacetone (5 mL) to a solution of complex 2 (50 mg, 0.47 mmol) inacetone (10 mL) and stirring for 2 h at room temperature. All volatilematerial was then removed under reduced pressure. Crystallization byslow diffusion of diethyl ether into a dichloromethane solution ofcrude 3 at room temperature afforded colorless crystals suitable for X-ray diffraction. Yield of crude product: 53 mg (89%). 1H NMR (300MHz, CD2Cl2): δ 1.91 (s, 6 H, CH3

ar2,6), 2.29 (s, 3 H, CH3ar4), 2.35 (s,

3 H, CH3pz3), 2.75 (s, 3 H, CH3

pz5), 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 (CH3

pz3), 15.6(CH3

pz5), 18.0 (CH3ar2,6), 21.2 (CH3

ar4), 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 solutionof 2 (50 mg, 0.47 mmol) in dichloromethane (5 mL) and stirring for 2h at room temperature. All volatile material was then removed underreduced pressure. Crystallization by slow diffusion of diethyl ether intoan acetonitrile solution of crude 5 at room temperature affordedcolorless crystals suitable for X-ray diffraction. Yield of crude product:58 mg (84%). 1H NMR (300 MHz, CD2Cl2): δ 1.68 (s, 6 H, CH3

ar2,6),2.42 (s, 3 H, CH3

ar4), 2.44 (s, 3 H, CH3pz3), 2.68 (s, 3 H, CH3

pz5), 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). 13CNMR (75 MHz, CD2Cl2): δ 14.3 (CH3

pz3), 15.2 (CH3pz5), 17.7

(CH3ar2,6), 21.4 (CH3

ar4), 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 complex2 (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 dichloro-methane solution of crude 6 at room temperature afforded red crystalssuitable for X-ray diffraction. Yield of crude product: 108 mg (90%).1H NMR (500 MHz, CD2Cl2, 243 K): δ (ppm) 1.39 (s, 3 H, CH3

ar2,6),1.86 (s, 3 H, CH3

ar2,6), 2.16 (s, 3 H, CH3pz3), 2.42 (s, 3 H, CH3

ar4),2.79 (s, 3 H, CH3

pz5), 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 = 1.8Hz, 1 H, CHim5), 8.32 (d, 3J = 9.5 Hz, 1 H, CHpdz), 8.53 (d, 3J = 9.5Hz, 1 H, CHpdz). 1H NMR (500 MHz, CD2Cl2, 293 K): δ (ppm) =1.46 (s, br, 3 H, CH3

ar2,6), 1.91 (s, br, 3 H, CH3ar2,6), 2.21 (s, 3 H,

CH3pz3), 2.45 (s, 3 H, CH3

ar4), 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 = 1.9Hz, 1 H, CHim4), 7.86 (d, 3J = 1.9 Hz, 1 H, CHim5), 8.37 (d, 3J = 9.5Hz, 1 H, CHpdz), 8.54 (d, 3J = 9.5 Hz, 1 H, CHpdz). 1H NMR (500MHz, CD2Cl2, 308 K): δ (ppm) 1.48 (s, br, 3 H, CH3

ar2,6), 1.91 (s, br,

3 H, CH3ar2,6), 2.22 (s, 3 H, CH3

pz3), 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 (125MHz, CD2Cl2, 243 K): δ (ppm) 14.2 (CH3

pz3), 15.0 (CH3pz5),

17.2(CH3ar2,6), 18.0 (CH3

ar2,6), 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/z4 8 8 . 1 [ M − OT f − P F 6 ]

2 + . A n a l . C a l c d f o rC42H44AuCuF12N12P2·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, 1equiv) and PhICl2 (106 mg, 0.35 mmol, 1.0 equiv) were dissolved inacetone (20 mL), and the reaction mixture was stirred at roomtemperature for 20 h. The solvent was then reduced to around half ofits volume, and hexane (50 mL) was added. The resulting precipitatewas collected by filtration and washed with hexane. The resulting solidwas dried to afford 7 as a yellow powder. Yellow single crystals suitablefor X-ray diffraction were grown by slow diffusion of diethyl ether intoa dichloromethane solution of the product. Yield: 202 mg (87%). 1HNMR (300 MHz, CD2Cl2): δ 2.24 (s, 6 H, CH3

ar2,6), 2.30 (s, 3 H,CH3

pz3), 2.40 (s, 3 H, CH3ar4), 2.80 (s, 3 H, CH3

pz5), 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,3J = 9.5 Hz, 1 H, CHpdz). 13C NMR (75 MHz, CD2Cl2): δ 14.0(CH3

pz3), 15.7 (CH3pz5), 18.9 (CH3

ar2,6), 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 dissolvedin dichloromethane (10 mL) and the solution cooled to −78 °C.Excess bromine (0.03 mL, 0.6 mmol, 3.8 equiv) was added, and thereaction mixture was stirred for 2 h and then warmed to roomtemperature. The solvent was removed under reduced pressure, andthe resulting orange residue was dried under vacuum to remove excessbromine. It was then dissolved in dichloromethane (5 mL) andpentane (100 mL) was added to precipitate 8 as an orange powder.Colorless single crystals suitable for X-ray diffraction were grown byslow diffusion of diethyl ether into a dichloromethane solution of thecrude product. Yield: 97 mg (85%). MS (ESI): m/z 439.0 [L1Br]+.

General Procedure for Etherification Reactions. A flask sealedwith a Teflon screw cap was loaded with catalyst (2 mol %), 2-indanol(0.5 mmol), and the respective aliphatic alcohol (5 mmol). AgBF4 (2mol %) was added, and the solution was heated with stirring to 80 °Cfor 3 h. The reaction mixture was cooled to room temperature anddiluted with ethyl acetate (6 mL). After filtration over Celite thesolution was evaporated under reduced pressure and 1,3,5-trimethoxybenzene (0.5 mmol) was added to the remaining oil/solidas an internal standard for determining the yields by 1H NMRspectroscopy.

In control experiments AgOTf (2 mol %) instead of AgBF4 wasused as catalyst, following the described procedure. Addition of NaCl(6 mol %) to these reaction mixtures led to complete inhibition of theetherification reactions.

■ ASSOCIATED CONTENT*S Supporting InformationText, figures, tables, and CIF files giving crystallographic data,an ORTEP plot of 8, and NMR spectra. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +49 551 393012. Fax: +49 551 393063. E-mail: franc.meyer@chemie.uni-goettingen.de.

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NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSFinancial support by the Fonds der Chemischen Industrie andthe Georg-August-University is gratefully acknowledged.

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