Dinuclear Coinage-Metal Complexes of Bis(NHC ... - ACS Publications

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Dinuclear Coinage-Metal Complexes of Bis(NHC) Ligands: Structural Features and Dynamic Behavior of a Cu−Cu Complex Verena Gierz,§ Alexander Seyboldt,§ Cac̈ ilia Maichle-Mössmer,§ Karl W. Törnroos,† Michael T. Speidel,‡ Bernd Speiser,‡ Klaus Eichele,§ and Doris Kunz* §

Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany Department of Chemistry, University of Bergen, Allégaten 41, 5007 Bergen, Norway. ‡ Institut für Organische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany. †

S Supporting Information *

ABSTRACT: Binuclear complexes of copper, silver, and gold bearing a 2,2′-bipyridine analogue, the pyridazine annelated bis(N-heterocyclic carbene) ligand (vegi) 1, were prepared and structurally characterized. They all feature the shortest metal− metal distances that have been measured so far in complexes with this structural motif bearing neutral bidentate ligands, indicative of d10−d10 interactions. While in the silver complex the linear coordination of each silver atom with two carbene ligands results in a planar complex, the ligand planes are twisted by 70° in the Cu complex 4 and by 31° in the gold complex 3. The copper complex shows a solvent-dependent equilibrium between the [Cu2L2]2+ complex and a [Cu2L3]2+ complex along with solvated CuPF6.

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INTRODUCTION N-heterocyclic carbene (NHC) complexes of the coinage metals bear interesting features, including homogeneous catalysis.1 While CuI(NHC) complexes are potential catalysts for various reactions, including [2 + 3] cycloadditions,2 nitrene and carbene transfer,3a and C−heteroatom bond formations,3b−d Ag(NHC) complexes are mostly used as carbene transfer reagents,4 but were also found to have interesting antiinflammatory properties,5 and Au(NHC) complexes were found to be active in various cyclization reactions.6 Binuclear NHC complexes of the coinage metals with short metal−metal contacts are still rare.7 Recently, we described the synthesis of the new bis-carbene ligand 1, the so-called “vegi” ligand,8 with directly N−N connected NHC moieties.9 Due to annelation at a pyridazine ring, rotation about the N−N bond, as is found for example in Crabtree’s bitz ligand I (Figure 1),10 is not possible. Rotation about the N−N bond could also be possible in a hypothetic binuclear complex of N−N connected acyclic biscarbene ligands, most likely the first carbene ligands ever synthesized already in 1915.11 Due to the restricted degree of freedom, we anticipated ligand 1 to obtain exclusively a 1,10phenanthroline-like chelating binding mode, as was found for the respective rhodium complexes II, but were surprised that the molecular structure of a silver complex revealed a bridging binding mode and realization of a binuclear complex III with a short Ag−Ag distance of only 2.74 Å.9b This shows that the ligand is quite stretchable within the ligand plane, and intraligand carbene−carbene distances between 2.70 Å in the rhodium complex II and 3.08 Å in the Ag−Ag complex III have been achieved so far. Although the mass spectra of the © 2012 American Chemical Society

respective copper and gold complexes also indicated dinuclear complexes with two vegiPr ligands, structural proof is still missing, as all attempts to obtain single crystals failed in our hands.9b We concluded that the n-propyl substituent of the vegi ligand, which enhances the solvation properties, might prevent the complexes from crystallization. Therefore, we chose a benzyl substituent, as it is expected not only to retain proper solubility of the complex but also to facilitate crystallization. Synthesis of the vegiBn ligand precursor 1·2HPF6 was quite straightforward using a synthesis protocol similar to that for the vegiPr ligand.12 We now report on the synthesis, structural properties, and dynamic features of coinage-metal complexes bearing the vegiBn ligand.

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RESULTS AND DISCUSSION Synthesis of the Silver Complex 2. Similar to the reported synthesis of III, the bisimidazolium salt 1·2HPF6 was reacted with Ag2O at 50 °C in acetonitrile. Filtration over a small column charged with neutral Alox yielded 64% of silver complex 2 in high purity (Scheme 1). The 1H NMR spectrum shows two singlets for the annelated heterocycle at 7.75 (3/10-H) and 7.22 ppm (1/2-H), indicating a symmetric ligand. In comparison with the imidazolium salt, the respective signals are shifted to high field. In the 13C NMR spectrum the signal for the carbene C atoms cannot be detected; however in a 1H,13C HMBC experiment a cross peak of the benzyl proton Special Issue: Copper Organometallic Chemistry Received: June 15, 2012 Published: August 30, 2012 7893

dx.doi.org/10.1021/om300544g | Organometallics 2012, 31, 7893−7901

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Figure 1. The literature known bitz ligand I and reported Rh (II) and Ag (III) complexes of the veginPr ligand.

Scheme 1. Synthesis of Coinage Metal Complexes of vegiBn

of a d10−d10 interaction.14 However, as found also for complex III, the silver−carbene bonds do not lie on the bisecting line of the N−C−N angle. It seems that the silver atoms are pushed away from each other, forming a 7° angle with the N−C−N bisecting line. This means that, from the geometric environment of the ligand, the Ag−Ag distance could be even shorter, indicating that any attractive d10−d10 interactions are already counterbalanced by repulsive interactions. Characteristic bond lengths of the annelated heterocycles of the bis-carbene ligand are identical with those of silver complex III. The intraligand carbene distance is 3.075 Å, and the mean of the N−C−N angles is 102.4°, a rather small value for N-heterocyclic carbene silver complexes.15 Short contacts of the PF6− anions are found with the carbene and nitrogen atoms (F1−C8 = 3.144 Å, F1−N7 = 2.960 Å) as well as the benzylic CH2 hydrogen atoms (F2−H17 = 2.310 Å, F3−H7 = 2.950 Å, F4−H6 = 2.870 Å, and F3−H6 = 2.668 Å) and the heterocyclic hydrogen atoms (F1−H18 = 2.424 Å and F1−H9 = 2.837 Å). Synthesis of the Gold Complex 3. In contrast to the synthesis of silver carbene complexes, the analogous basic oxide of gold is not available. Therefore, gold complexes are generated either from the free carbene or by transmetalation.16 We chose the transmetalation from the in situ generated

signal at 5.33 ppm and the carbene signal at 169.9 ppm confirms formation of a silver complex. Particularly, resolution of the direct Ag−C coupling was observed by a clear doublet structure of the cross peak with an estimated mean 1J(107/109AgC) coupling constant of 160 Hz (Figure 2). The chemical shift and coupling constant of the carbene signal are in a typical range for cationic [Ag(NHC)2]+ complexes (168−186 Hz), while for neutral complexes higher values (219−253 Hz) are observed.13 All peaks are similar to those of the Ag−Ag complex III bearing the vegiPr ligand.9b In the high-resolution mass spectrum a peak at m/z 1 035.080 444 (calcd 1 035.080 111) already indicates the binuclear nature of this complex. From a solution of 2 in dichloromethane we obtained large colorless single crystals suitable for X-ray structure analysis. The molecular structure (Figure 3) confirms the binuclear Ag−Ag structure and the bridging binding mode of the vegiBn ligand. The metal cations and ligands are related via an inversion center, with the benzyl substituents in each ligand oriented in opposite directions relative to the coordination plane. The planes of the phenyl rings are tilted toward each other, leading to a short contact between the ortho hydrogen H16 and the ortho carbon of the opposing phenyl ring of 2.867 Å. The Ag−Ag distance of only 2.7391 Å is identical with that found for complex III (2.7405 Å),9b indicative 7894


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complex 2, the differences of the chemical shift for all remaining signals are less than 1 ppm. In the high-resolution FT-ICR mass spectrum of complex 3 the signal at m/z 1 215.202 336 (calcd 1 215.203 029) is in accordance with a [[Au2(vegiBn)2]PF6]+ fragment, indicating again a binuclear structure related to that of silver complex 2. Structural evidence for the binuclear complex 3 with two bridging vegiBn ligands was obtained from X-ray single-crystal analysis. Suitable crystals were grown from slow diffusion of diethyl ether into a solution of 3 in acetonitrile. The molecular structure (Figure 4)

Figure 2. 1H,13C HMBC spectrum of the silver complex 4, showing the doublet structure 1J(107/109AgC) of the cross peak of the benzylic protons with the carbene carbon atoms. Figure 4. Molecular structure of gold complex 3. Anisotropic displacement parameters are given at the 50% probability level; counterions and hydrogen atoms have been omitted for clarity.

of the binuclear complex also shows a bridging binding mode of the vegiBn ligands, with the benzyl substituents pointing in the same direction in one ligand and in opposite directions in the second ligand, as governed by the crystallographic 2-fold axis running along the Au−Au bond. In contrast to silver complexes III and 2, the ligand planes are not coplanar but are twisted by 31°. The distance between the gold cations (2.779 Å) is short in comparison with the Au−Au distance of elemental gold (2.88 Å).14 It is the shortest bond found for binuclear bis(NHC) Au complexes so far and to our knowledge the shortest distance for this structural motif with neutral ligands.15 The Au−Au distance lies more in the range of anionic dithiocarbamato ligands.17 The N−C−N angles (103.2°, mean) are larger than in the Ag complexes III (102.5°, mean) and 2 (102.4°, mean), and the deviation of the Ag−carbene bond from the ideal bisecting line of the N−C−N angle is similar (6°) to that of the Ag complexes (9° (III) and 7° (2)). It seems that the ligand can avoid any additional deviation by an even larger carben−carbene distance of 3.102 Å (III, 3.06 Å; 2, 3.075 Å). Therefore, the short Au−Au distance is rather a result of the steric properties of the ligand and tentatively also its strong electron donating character. Ligand-unsupported aurophilic interactions are usually found at Au−Au lengths of about 3.14 Å or higher.18 The Au−carbene distances (2.021 Å, mean) fit very well into the range of literature known Au(NHC) complexes (2.008−2.043 Å)15 and are 0.08 Å shorter than those of the respective Ag−carbene distances (complex 2: 2.103 Å, mean), as expected, due to increased relativistic effects.19 However, this does not explain the longer Au−Au distance in 3 in comparison to the respective Ag−Ag distance. The remaining bond lengths and angles of the annelated ring moiety are similar to those in silver complexes III and 2.

Figure 3. Molecular structure of silver complex 2. Anisotropic displacement parameters are given at the 50% probability level; counterions and hydrogen atoms have been omitted for clarity.

binuclear silver complex 2 to [AuCl(SMe)2], which was carried out in acetonitrile at room temperature. The AgCl that formed was filtered off over neutral Alox and the filtrate concentrated to dryness. The oily residue solidified upon stirring with dichloromethane, and the product 3 was isolated by filtration in 69% yield and high purity. The 1H NMR spectrum of 3 shows only marginal differences of the chemical shifts in comparison with the spectrum of silver complex 2. All signals are shifted downfield by only 0.03 ppm. In the 13C NMR spectrum the signal of the carbene carbon atom can be detected at 172.7 ppm, slightly shifted downfield in comparison with the respective silver complex 2. The value is typical for Au(NHC) complexes. In comparison with silver 7895


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Figure 5. Aromatic region of the 1H NMR spectrum of copper complex 4 (*) in CD3CN in equilibrium with complex 5 (○).

Synthesis of Copper Complex 4. For the preparation of Cu(NHC) complexes, two routes are commonly applied: transmetalation from Ag(NHC) complexes16 and direct reaction of the imidazolium salts with basic Cu2O, in analogy to the preparation of Ag complexes.20 For the preparation of a vegiBn complex we reacted the bis-imidazolium salt 1·2HPF6 with Cu2O at 100 °C in acetonitrile overnight. As expected from the analogous reaction with the vegiPr ligand, the 1H NMR spectrum of the raw material in DMSO-d6 shows one signal set for the ligand and chemical shifts in the same region as for the respective Ag and Au complexes 2 and 3. After removal of residual salts by extraction with degassed water and filtration over neutral Alox a second signal set was observed in the NMR spectrum in CD3CN. The coupling pattern indicates a symmetric ligand, but in contrast to the species found in the spectrum of the raw material (and the minor product after aqueous workup), all signals are shifted to high field, especially those of the phenyl protons, which are in addition strongly separated: in particular the o-H signal has a lower chemical shift than those of m-H and p-H (6.47 ppm, o-H; 6.90 ppm, m-H; 7.00 ppm, p-H) (Figure 5). A possible decomposition of the Cu complex during the aqueous workup procedure could be excluded from the mass spectrum, which does not show any oxidized or hydroxylated species. However, an intense signal at m/z 570.160 428 can be assigned according to the calculated isotope pattern to the dicationic species [Cu2(1)3]2+ (calcd 570.159 045) consisting of a dinuclear species with three coordinated vegiBn ligands, while for the respective veginPr complex only a [Cu2(veginPr)2]2+ fragment was observed. The [M2L3]2+ coordination pattern is known in the literature for gold, silver, and copper complexes bearing the sterically less demanding bis(dimethylphosphino)methane ligand (dmpm).21 Attempts to purify the complex without using water for workup resulted in an identical 1H NMR spectrum. The elemental analysis reveals the presence of 1.5 equiv of [Cu(CH3CN)3]PF6. Another explanation for the formation of the unexpected complex 5 together with 4 might be an equilibrium between the two species (eq 1).

3[Cu 2(1)2 ](PF6)2 + 8CH3CN 4

⇌ 2[Cu 2(1)3 ](PF6)2 + 2[Cu(CH3CN)4 ]PF6 5

(1)

While we observe 4 and 5 in a 1:1.6 to 1:4.9 ratio in acetonitrile-d3 (a variation of the ratio is observed also in different samples of the same compound and might be due to concentration effects). Dissolving the isolated complex in dichloromethane-d2 also showed the signals of the two species 4 and 5, but in a different ratio (4:5 = 1:0.2 to 1:0.3). In DMSO-d6 the ratio lies between those of dichloromethane-d2 and acetonitrile-d3 (1:1.6). Evaporation of dichloromethane-d2 and measurement of the same sample in acetonitrile-d3 showed again the ratio for 4 and 5 in acetonitrile-d3 and vice versa. These observations give proof for a solvent-dependent equilibrium of the species. As Cu(I) is very well coordinated by acetonitrile, we suspect that this enhances this position of the equilibrium. Formation of the dinuclear copper carbene complex 5 bearing three vegiBn ligands is also supported by the 13 C NMR spectrum. The carbene signal of complex 4, in which two NHC moieties are coordinated to Cu, is detected at 166.7 ppm. Considering that the influence of the copper on the NHC moiety is reduced if three NHC moieties are coordinated to Cu, a shift of the carbene signal toward the direction of the signal of free NHC is expected. Indeed, the carbene signal of complex 5 is shifted to low field and detected at 175.1 ppm. The difference in the chemical shift of about 8 ppm is also found in the literature for N-heterocyclic carbene ligands in a Cu(NHC)3 and Cu(NHC)2 coordination mode.22 To corroborate that the proposed structure of complex 5 and the fragment found in mass spectrometry are identical, we also conducted a diffusion ordered (DOSY) NMR experiment and determined the 1H spin−lattice relaxation times via inversion recovery. The peaks at δ 7.73 (3/10-H) and 7.22 (1/2-H) of compound 4 and the respective signals at δ 7.19 and 6.67 of compound 5 show T1 values of 3.94 and 3.61 s (4) and 2.87 7896


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Figure 6. (left) Molecular structure of copper complex 4. Anisotropic displacement parameters are given at the 50% probability level; counterions, hydrogen atoms, and the N-benzyl substituents of one carbene ligand have been omitted for clarity. (right) View along the b axis shows the torsion between the two ligand planes and short contacts between one cocrystallized CH2Cl2 molecule and two benzylic protons.

cations (eq 1). The back reaction would be transmetalation of one vegiBn ligand of complex 5 to [Cu(CH3CN)4]PF6. However, a detailed mechanistic analysis of the equilibrium requires further experimental and theoretical studies, which goes beyond the scope of this work. Single crystals were obtained from a solution of the product in dichloromethane. X-ray structure analysis reveals the molecular structure of only the dinuclear complex 4 (Figure 6). In contrast to the molecular structures of Ag complexes III and 2, and like that of Au complex 3, the two carbene ligand planes are significantly twisted relative to each other, but this time by as much as 70°. As for 3, complex 4 has a 2-fold axis along the Cu−Cu bond. To maintain an almost linear coordination at the copper atoms (C−Cu−C = 174.59 and 177.19°), the tricyclic pyridazine moiety is twisted by 14.91° (C5−N6−N7−C8). This arrangement might be additionally stabilized by short contacts of the Cu atoms to one ipso carbon atom of each ligand (2.865 Å). The very short Cu−Cu distance of 2.4923 Å is one of the shortest distances found for this structural motif, the shortest distance measured for an N-heterocyclic carbene complex, and to our knowledge also the shortest for any neutral ligand. 15 This distance could be indicative of d 10 −d 10 interactions, but as the Cu−carbene bond lies perfectly on the bisecting line of the N−C−N carbene angles, the ligand geometry rather forces the Cu cations into this short distance,25 tentatively with the help of the strong electron donating character of the ligand by reducing Coulomb repulsion. The N−C−N angles of 101.60(19)° (N4−C5−N6) and 102.53(18)° (N7−C8−N9) are smaller than those found in the respective Ag or Au complexes. The Cu−C bonds of 1.898 Å are short in comparison with other binuclear complexes of the type [Cu2(NHC)4]2+.15 In accordance with complexes III, 2, and 3, the N6−N7 bond length is rather long (1.373(3) Å) and the C1−C2 bond rather short (1.341(4) Å). This is the reverse of the situation for the vegi ligand in a chelating fashion: the N−N bonds are shorter and the C1−C2 bonds longer.9b,12 In addition, short contacts are found between cocrystallized dichloromethane and one set of benzylic hydrogen atoms as well as one m-H (Cl2−H41B = 2.797 Å, Cl2− H94 = 2.853 Å), which might be an explanation for the larger torsion angle of 70°.

and 2.68 s (5). The faster relaxation of compound 5 is an additional hint for its higher molecular weight and thus for the likely structure [Cu2(vegiBn)3](PF6)2. We also tried to synthesize complex 4 by transmetalation from the Ag complex 2 generated in situ. Filtration over Alox and a small layer of Celite removed all Ag salts. After evaporation of the solvent the product was obtained in 65% yield without any additional copper salt or solvent according to elemental analysis. Also in this case, the 1H NMR spectrum of the product (CD3CN) showed the equilibrium between 4 and 5 in a 1:0.9 ratio (differences from the ratio found above might be due to concentration effects). Proof for an equilibrium was also obtained by variable-temperature 1H NMR measurement of a sample in acetonitrile in the range between room temperature and 60 °C. The initial 4:5 ratio of 1:1.1 at 26 °C shifts to 1:0.9 at 40 °C and 1:0.6 at 60 °C. Upon cooling to back to 26 °C the concentration of 4 and 5 equilibrates back to the initial ratio of 1:1.1 (see the Supporting Information). As already mentioned above, [M2(L)3]2+ complexes are known in the literature for the sterically less demanding dmpm ligand with coinage metals.21 Interestingly, also the [Ag2(dmpm)2]2+ complex was prepared21c and structurally characterized.23 Although there is no report on an equilibrium, the mentioned light sensitivity21c of [Ag2(dmpm)2]2+ in contrast to [Ag2(dmpm)3]2+ might be a hint for the formation of Ag(0) from free Ag+ that could result from a reaction similar to that found above for the Cu-vegiBn equilibrium (eq 1). Another hint for an equilibrium also in dmpm complexes might be the fact that Schubert crystallographically revealed the [Ag2(dmpm)2]2+ fragment while the elemental analysis fits perfectly a [Ag2(dmpm)3](PF6)2 complex.23 The more sterically demanding bis(dicyclohexylphosphino)methane (dcpm) ligand is known to form [Cu2(dcpm)2(acetonitrile)2]2+ complexes from [Cu2(dcpm)2]2+,24a while in the even more strongly shielded, planar [Cu2(dtbpm)2]2+ complex (dtbpm = bis(tert-butylphosphino)methane) further coordination to copper is blocked.24b As Cu(NHC) complexes are well-known to act as carbene transfer reagents, any free copper cation must be in equilibrium with its Cu(NHC) complex. Accordingly, in our case, NHC transmetalation occurs between two Cu(NHC) complexes of 4, most probably with the help of acetonitrile that forms the complex [Cu(CH3CN)4]PF6 upon release of the copper 7897


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Figure 7. Excitation and emission spectra of the Cu (4/5), Ag (2), and Au (3) complexes in dichloromethane. There is no emission/excitation below/above the presented limits.

Likely due to the observed equilibrium, we failed to obtain single crystals of complex 5 even at low temperature, where the concentration of 5 should be increased. Electronic Spectra of Complexes 2, 3, and 4/5. We have measured the excitation and emission spectra of complexes 2 and 3 and the copper complexes 4/5 in dichloromethane (Figure 7). The silver and copper complexes show one broad excitation maximum at 354 nm (2) and 381 nm (4/5), which most likely stems from π−π* transitions of the ligand, and emission bands at 445 nm (broad, 2) and 448, 542, and 592 nm (4/5). The respective imidazolium salt 1·2HPF6 shows a similar excitation band at 324 nm corresponding to a broad emission band at 405 nm (Supporting Information). In addition, the imidazolium salt has a blue-shifted excitation band at 251 nm, which leads to an additional emission at 284 nm with lower intensity, which is not observed in the spectra of the complexes. In comparison with the silver and copper complexes, the excitation and emission spectra of gold complex 3 are different: in addition to an excitation band at 369 nm a second band with lower intensity is found at 300 nm. In gold complexes d10−d10 interactions are known to result in a characteristic absorption in the UV region whose wavelength was found to be dependent on the Au−Au distance.26 The related bis(dicyclohexylphosphino)methane complex with a Au−Au distance of 2.926 Å shows an absorption maximum at λ 277 nm,27a,b and [Au2(CS3)](nBu4N)2 with a Au−Au distance of 2.799 Å absorbs at 314 nm. For both absorption bands a 1(5dσ* → 6pσ) excitation is considered.27c Therefore, the excitation at 300 nm of gold complex 3 is a reasonable candidate for this metal−metal transition. In contrast to the emission spectra of silver complex 2 and copper complexes 4/5, the emission bands of complex 3 are red-shifted and are more narrow with maxima at 521 and 563 nm and a shoulder at 610 nm causing a light orange luminescence of the solution. Further assignment of the electronic spectra would require special spectroscopic methods and/or theoretical calculations and goes beyond the scope of this work. Cyclic Voltammetry of Complexes 2, 3, and 4/5. In a CH3CN/0.1 M NBu4PF6 electrolyte, only the copper compounds 4/5 exhibit well-defined cyclic voltammetric peaks within the accessible potential window, while such signals are absent for complexes 2 and 3. The voltammograms of 4/5 at a

Figure 8. Cyclic voltammogram of complexes 4/5 in CH3CN/0.1 M NBu4PF6 (c = 7.7 × 10−5 M, v = 0.1 V/s), with background correction by current potential data in the pure electrolyte.

Pt-disk electrode show two oxidation signals of different height and shape, a broad one at ∼+0.25 V and a more intense and narrow one at ∼+0.55 V (potentials are given vs the ferrocene/ ferrocenium redox couple; Figure 8). This could be consistent with the presence of several redox-active species as predicted by equilibrium (1). Fast follow-up reactions are coupled to the oxidations, as indicated by the absence of respective reverse peaks on the backward scan. Only at ∼−0.44 V a broad ill-defined reduction wave is observed as a consequence of the oxidations. The chemical irreversibility of the oxidation processes is accompanied by a successive degradation of the electrode activity.

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CONCLUSION We were able to synthesize coinage-metal complexes with the benzyl-substituted vegiBn ligand and determine their molecular structures by X-ray analyses. All complexes show very short M−M bond lengths. The molecular structures show a planar orientation of the ligand planes in silver complex 2, a medium twist of 31° in the Au complex 3, and a strong twisting (70°) in the copper complex 4. In solution copper complex 4 reveals an equilibrium with the dinuclear complex 5, bearing three vegiBn ligands and solvent-coordinated CuPF6. 7898


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Synthesis of [Cu2(1)2](PF6)2 (4) and [Cu2(1)3](PF6)2 (5). In a Schlenk tube, the benzyl-substituted bis-imidazolium salt 1·2HPF6 (50.0 mg, 79.3 μmol) and Cu2O (11.4 mg, 79.3 μmol) were suspended in dry and degassed CH3CN (6 mL). The suspension was stirred at 100 °C in a closed Schlenk tube overnight. After 16 h the mixture was filtered and the solution was evaporated to dryness. The residue was dissolved in CH3CN and filtered over a short column of neutral Alox (flame-dried). The column was extracted with CH3CN (20 mL combined), and the filtrate was evaporated to dryness. The residue was washed with small amounts of CH2Cl2 and pentane and dried in vacuo to give the product as an off-white solid (25.8 mg, the product contains 1.5 equiv of CuPF6 and 4.5 equiv of CH3CN). In solution the product [Cu2(1)2](PF6)2 (4) shows a solvent-dependent equilibrium with [Cu2(1)3](PF6)2 (5). Suitable crystals for X-ray diffraction analysis of [Cu2(1)2](PF6)2 (4) were obtained from a saturated CH2Cl2 solution at 4 °C. 1H NMR (CD3CN, 400.13 MHz): [Cu2(1)2](PF6)2 (4), δ 7.69 (s, 2H, 3/10-H), 7.18 (s, 2H, 1/2-H), 7.15−7.17 (m, 4H, 13/13′-H or 14/14′-H; overlaid by the signal of the protons 3/10-H of species 5), 6.95−7.04 (m, 6H, 15/15′-H and 13/13′-H or 14/14′-H; overlaid by the signal of the protons 15/15′-H of species 5), 5.23 (s, 4H, CH2); [Cu2(1)3](PF6)2 (5), δ 7.16 (s, 2H, 3/10-H), 6.95−7.04 (m, 2H, 15/15′-H; overlaid by the signal of the aryl protons of complex 4), 6.90 (t, br, 4H, 3JHH = 7.6 Hz, 14/14′-H), 6.64 (s, 2H, 1/2-H), 6.47 (d, br, 4H, 3JHH = 7.6 Hz, 13/13′-H), 5.24 (d, 2H, 2JHH= 15.8 Hz, CH2), 5.17 (d, 2H, 2JHH = 15.8 Hz, CH2). 13C{1H} NMR (CD3CN, 100.61 MHz): [Cu2(1)2](PF6)2 (4), δ 166.7 (C5/8), 136.6 (C12/12′), 129.8 (2 signals, C13/13′ and C14/14′), 129.2 (C15/15′), 126.5 (C2a/10a), 118.8 (C3/10), 114.4 (C1/2), 56.6 (CH2); [Cu2(1)3](PF6)2 (5), δ 175.1 (C5/8), 137.7 (C12/12′), 129.2 (C14/14′), 128.4 (C15/15′), 126.0 (C2a/10a), 125.7 (C13/13′), 117.3 (C3/10), 113.9 (C1/2), 55.9 (CH2). Anal. Calcd for C44H36Cu2F12N8P2 + 1.5(CuPF6) + 4.5(CH3CN): C, 40.00; H, 3.14; N, 11.00. Found: C, 40.07; H, 3.14; N, 10.82. FT-ICR HR-ESI+ (CH3CN): m/z 570.160 428 [M]2+, calcd 570.159 045 [Cu2(1)3]2+. Species containing the [Cu2(1)2]2+ fragment are not observed. Synthesis of [Cu2(1)2](PF6)2 (4) and [Cu2(1)3](PF6)2 (5) via Transmetalation. In a Schlenk tube, the benzyl-substituted bisimidazolium salt 1·2HPF6 (70.0 mg, 111 μmol) and Ag2O (25.7 mg, 111 μmol) were suspended in dry and degassed CH3CN (7 mL). The suspension was stirred at 50 °C for 16 h. After filtration CuCl (10.4 mg, 106 μmol) was added to the filtrate. The suspension was stirred at room temperature and then filtered over a short column of neutral Alox (flame-dried) with a small layer of Celite. The column was extracted with 40 mL of CH3CN and the filtrate evaporated to dryness. The oily residue was stirred overnight with pentane and the solidified residue filtered and washed with pentane (5 mL). After drying in vacuo the product was obtained as an off-white solid (37.7 mg, 65% based on CuCl). In solution the product [Cu2(1)2](PF6)2 (4) shows a solventdependent equilibrium with [Cu2(1)3](PF6)2 (5). NMR and FT-ICR HR-ESI+: see above. Anal. Calcd for (C44H36Cu2F12N8P2): C, 48.31: H, 3.32; N, 10.24. Found: C, 48.39; H, 3.10; N, 10.35. Cyclic Voltammetry. Experiments were performed at room temperature with an ECO-Autolab PGSTAT101 (Metrohm, Filderstadt, Germany) with the NOVA software package 1.7.8 in a gastight full glass three-electrode cell under an argon atmosphere. As the working electrode a Pt disk (Metrohm, Fildestadt, Germany) with an electroactive area of A = 0.0077 cm2 was used. The electroactive surface area was calibrated by fc cyclic voltammetry in CH3CN/0.1 M NBu4PF6 assuming Dfc = 2.5 × 10−6 cm2 s−1.29 The counter electrode was a 1 mm Pt wire. As the potential standard, a Haber-Luggin double reference electrode with Ag/AgClO4 (0.01 M in CH3CN/0.1 M NBu4PF6) was used. Potential values were rescaled to an external fc/fc+ reference standard determined in the same system (E°(fc/fc+) = 0.075 ± 0.002 V). Electrolytes were degassed by argon bubbling. In all CV experiments, NBu4PF6 was used as the supporting electrolyte (c = 0.1 M). Different concentrations and scan rates were used (c = 26.1− 76.7 μM, v = 0.05−1.0 V/s) for the cyclic voltammetric characterization of 4/5.

EXPERIMENTAL SECTION

Unless otherwise noted, all reactions were carried out under an argon atmosphere in dried and degassed solvents using Schlenk techniques. Pentane, acetonitrile, toluene, and dichloromethane were purchased from Sigma Aldrich, dried, and degassed using an MBraun SPS-800 solvent purification system. All other chemicals used were purchased from commercial suppliers and used without further purification. Deuterated solvents were purified according to general procedures.28 Air- and moisture-sensitive compounds were handled in an MBraun glovebox under argon. The imidazolium salt precursor (1·2HPF6) of the vegiBn ligand (1) was synthesized according to the literature.12 NMR spectra were recorded using a Bruker AVII+400 spectrometer. 1 H and 13C{1H} NMR spectra were calibrated to TMS on the basis of the relative chemical shift of the solvent as an internal standard. DOSY 1H (Bruker pulse program ledbpgp2s) and 1H inversion recovery NMR experiments were obtained on a Bruker AVII+500 NMR spectrometer. Elemental analyses were carried out at the Institut für Anorganische Chemie at Tübingen University. Mass spectra were recorded using a Bruker Daltonics APEX II FT-ICR instrument. UV/vis excitation and emission spectra were recorded on a PerkinElmer LS55 fluorescence spectrometer in dichloromethane. All measurements were carried out at Tübingen University. Synthesis of [Ag2(1)2](PF6)2 (2). In a Schlenk tube the benzylsubstituted bis-imidazolium salt 1·2HPF6 (50.0 mg, 79.3 μmol) and Ag2O (18.4 mg, 79.3 μmol) were suspended in dry and degassed CH3CN (6 mL). The suspension was stirred at 50 °C overnight. Then, the reaction mixture was filtered over a short column of neutral flamedried Alox. The column was extracted several times with CH3CN (25 mL combined) and the filtrate evaporated to dryness. The oily residue was extracted with CH2Cl2 (10 mL) and the solution again evaporated to dryness to give the product as a white solid which was washed with pentane (5 mL) and dried in vacuo for several hours (29.9 mg, 64%). Suitable crystals for X-ray diffraction analysis were obtained by slow diffusion of pentane into a solution of the silver complex in CH2Cl2. 1H NMR (CD3CN, 400.13 MHz): δ 7.75 (s, 2H, 3/10-H), 7.20−7.28 (m, 6H, 14/14′/15/15′-H), 7.22 (s, 2H, 1/2-H), 7.14−7.19 (m, 4H, 13/13′-H), 5.33 (s, 4H, 11/11′-H). 13C{1H} NMR (CD3CN, 100.61 MHz): δ 169.9 (from 1H,13C-HMBC: d, 1J107/109AgC ≈ 160 Hz, C5/8), 136.8 (C12/12′), 130.1 (C14/14′), 129.6 (C15/15′), 128.8 (C13/13′), 128.3 (C2a/10a), 119.1 (C3/10), 114.5 (C1/2), 58.2 (C11/11′). Anal. Calcd for C44H36Ag2F12N8P2: C, 44.69; H, 3.07; N, 9.48. Found: C, 44.74; H, 3.04; N, 9.38. FT-ICR HR-ESI+ (CH3CN): m/z 1 035.080 444 [M + PF6]+, calcd 1 035.080 111. Synthesis of [Au2(1)2](PF6)2 (3). In a Schlenk tube the benzylsubstituted bisimidazolium salt 1·2HPF6 (50.0 mg, 79.3 μmol) and Ag2O (18.4 mg, 79.3 μmol) were suspended in dry and degassed CH3CN (4 mL). The black suspension was stirred overnight at 50 °C. After 15 h the mixture was filtered and [AuCl(SMe2)] (22.2 mg, 75.4 μmol) was added. A white suspension was formed which turned gray upon stirring at room temperature for 4 h. The gray solid was filtered off, and the solution was dried in vacuo. The residue was dissolved in CH3CN (10 mL) and filtered over a short column of neutral Alox (flame-dried). The column was washed with CH3CN (20 mL), and the colorless filtrate was evaporated to dryness. The oily residue was stirred with CH2Cl2 (2 mL), and a solid was formed, which was filtered, washed with pentane (5 mL), and dried in vacuo to give the product as a white solid (35.5 mg, 69% based on [AuCl(SMe2)]). 1H NMR (CD3CN, 400.13 MHz): δ 7.77 (s, 2H, 3/10-H), 7.26−7.33 (m, 6H, 14/14′/15/15′-H), 7.24 (s, 2H, 1/2-H), 7.16−7.21 (m, 4H, 13/13′-H), 5.48 (s, 4H, 11/11′-H). 13C{1H} NMR (CD3CN, 100.61 MHz): δ 172.7 (C5/8), 136.3 (C12/12′), 130.1 (C14/14′), 129.7 (C15/15′), 129.2 (C2a/10a), 128.3 (C13/13′), 119.0 (C3/10), 115.0 (C1/2), 58.4 (C11/11′). Anal. Calcd for C44H36Au2F12N8P2: C, 38.84; H, 2.67; N, 8.24. Found: C, 38.72; H, 2.76; N, 8.10. FT-ICR HR-ESI+ (CH3CN): m/z 1 215.202 336 [M + PF6]+, calcd 1 215.203 029; m/z 535.131 467 [M]2+, calcd 535.119 432 (internal calibration on the [M + PF6]+ peak explains the larger deviation of the [M]2+ peak from the calculated value). 7899


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Crystallographic Data. Data collection was carried out on a Bruker ApexII CCD diffractometer using Mo Kα radiation (λ = 0.710 73 Å) and a graphite monochromator. Corrections for absorption effects were applied using SADABS.30 All structures were solved by direct methods using SHELXS and SHELXL for structure solution and refinement.31 Further details of the refinement and crystallographic data are given in the Supporting Information. The structure of 3 has one disordered PF6− anion, tilted some 17.7° out of both sides of a crystallographic mirror plane. Furthermore, there is a noninterpretable solvent disorder in the structure, which has been eliminated from the structure factor file using the SQUEEZE option in the program PLATON.32

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(6) (a) Nolan, S. P. Acc. Chem. Res. 2011, 44, 91−100. (b) Hashmi, A. S. K. Pure Appl. Chem. 2010, 82, 657−668. (c) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1176−1986. (7) McKie, R.; Murphy, J. A.; Park, S. R.; Spicer, M. D.; Zhou, S.-z. Angew. Chem. 2007, 119, 6645−6648; Angew. Chem., Int. Ed. Engl. 2007, 34, 6525−6528. (8) “vegi” is named after Verena Gierz, the person who first synthesized this bis-carbene. (9) (a) Merck Patent GmbH (Appl.) Kunz, D.; Gierz, V. (Invs.) DE102011106849 (A1), priority date 15.12.2010. (b) Gierz, V.; MaichleMössmer, C.; Kunz, D. Organometallics 2012, 31, 739−747. (10) (a) Poyatos, M.; McNamara, W.; Incarvito, C.; Peris, E.; Crabtree, R. H. Chem. Commun. 2007, 2267−2269. (b) Poyatos, M.; McNamara, W.; Incarvito, C.; Clot, E.; Peris, E.; Crabtree, R. H. Organometallics 2008, 27, 2128−2136. (c) Sanz, S.; Azua, A.; Peris, E. Dalton Trans. 2010, 39, 6339−6343. (11) (a) Tschugajeff, L.; Skanawy-Grigorjewa, M.; Posnjak, A. Z. Anorg. Allg. Chem. 1925, 148, 37−42 and references therein. The biscarbene structure of the ligand was recognized 45 years later by: (b) Burke, A.; Balch, A. L.; Enemark, J. H. J. Am. Chem. Soc. 1970, 92, 2555−2557. (c) Rouschias, G.; Shaw, B. L. Chem. Commun. 1970, 183. (12) Gierz, V.; Urbanaite, A.; Seyboldt, A.; Kunz, D. Rh complexes bearing 1,10-phenanthroline analogous bis(NHC) ligands are active catalysts for transfer hydrogenation of ketones. Submitted for publication. (13) Su, H.-L.; Pérez, L. M.; Lee, S.-J.; Reibenspies, J. H.; Bazzi, H. S.; Bergbreiter, D. E. Organometallics 2012, 31, 4063−4071. (14) Pyykkö, P. Chem. Rev. 1997, 97, 597−636. (15) Cambridge Structural Database: Allen, F. H. Acta Crystallogr. 2002, B58, 380−388. (16) Lin, I. J. B.; Vasam, C. S. Can. J. Chem. 2005, 83, 812−825. (17) (a) Heinrich, D. D.; Wang, J.-C.; Fackler, J. P. Acta Crystallogr. Sect. C 1990, 46, 1444−1447. (b) Mansour, M. A.; Connick, W. B.; Lachicotte, R. J.; Gysling, H. J.; Eisenberg, R. J. Am. Chem. Soc. 1998, 120, 1329−1330. (c) Tzeng, B.-C.; Liu, W.-H.; Liao, J.-H.; Lee, G.-H.; Peng, S.-M. Cryst. Growth Des. 2004, 4, 573−577. (18) (a) Nakamoto, M.; Hiller, W.; Schmidbaur, H. Chem. Ber. 1993, 126, 605−610. (b) Fackler, J. P.; Staples, R. J.; Elduque, A.; Grant, T. Acta Crystallogr. 1994, C50, 520−523. (c) Onaka, S.; Katsukawa, Y.; Shiotsuka, M.; Kanegawa, O.; Yamashita, M. Inorg. Chim. Acta 2001, 312, 100−110. (d) De la Riva, H.; Pintado-Alba, A.; Nieuwenhuyzen, M.; Hardacre, C.; Lagunas, M. C. Chem. Commun. 2005, 4970−4972. (e) Pyykkö, P.; Runeburg, N.; Mendizabal, F. Chem. Eur. J. 1997, 3, 1451−1457. (19) Tripathi, U. M.; Bauer, A.; Schmidbaur, H. J. Chem. Soc., Dalton Trans. 1997, 2865−2868. (20) First direct synthesis with Cu2O: Tulloch, A. A. D.; Danopoulos, A. A.; Kleinhenz, S.; Light, M. E.; Hursthouse, M. B.; Eastham, G. Organometallics 2001, 20, 2027−2031. (21) (a) Bensch, W.; Prelati, M.; Ludwig, W. J. Chem. Soc., Chem. Commun. 1986, 1762−1763. (b) Mao, Z.; Chao, H.-Y.; Hui, Z.; Che, C.-M.; Fu, W.-F.; Cheung, K.-K.; Zhu, N. Chem. Eur. J. 2003, 9, 2885− 2897. (c) Fournier, E.; Decken, A.; Harvey, P. D. Eur. J. Inorg. Chem. 2004, 4420−4429. (22) (a) Hu, X.; Castro-Rodriguez, I.; Meyer, K. Organometallics 2003, 22, 3016−3018. (b) Hu, X.; Castro-Rodriguez, I.; Meyer, K. J. Am. Chem. Soc. 2003, 125, 12237−12245. (c) Hu, X.; CastroRodriguez, I.; Olsen, K.; Meyer, K. Organometallics 2004, 23, 755− 764. (23) Karsch, H. H.; Schubert, U. Z. Naturforsch. 1982, 37B, 186−189. (24) (a) Fu, W.-F.; Gan, X.; Che, C.-M.; Cao, Q.-Y.; Zhou, Z.-Y.; Zhu, N. N.-Y. Chem. Eur. J. 2004, 10, 2228−2236. (b) Straub, B. F.; Rominger, F.; Hofmann, P. Inorg. Chem. 2000, 39, 2113−2119. (25) (a) Merz, K. M.; Hoffmann, R. Inorg. Chem. 1988, 27, 2120− 2127. (b) Cotton, F. A.; Feng, X.; Timmons, D. J. Inorg. Chem. 1998, 37, 4066−4069. (26) Mendizabal, F.; Olea-Azar, C. Int. J. Quantum Chem. 2005, 103, 34−44.

ASSOCIATED CONTENT

S Supporting Information *

CIF files giving experimental and crystal data for 2−4 and figures giving NMR spectra of all new compounds. This material is available free of charge via the Internet at http:// pubs.acs.org. CCDC Nos. 896687 (2), 896688 (3), and 896689 (4) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +49 7071 29-72063. Fax: +49 7071 29-2436. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Deutsche Forschungsgemeinschaft (KU1437/2-3), the BMBF/MWK-BW (Professorinnenprogramm), and the Landesgraduiertenförderung BW (fellowship for V.G.) for financial support. We also thank Uwe Monkowius for valuable discussions and Aurelija Urbanaite for assisting in the synthesis of starting material.

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ABBREVIATIONS vegi (bis-carbene 1), named after Verena Gierz, the person who first synthesized this compound. REFERENCES

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