Remarkable Stability of Copper(II)–N-Heterocyclic Carbene

Article pubs.acs.org/Organometallics

Remarkable Stability of Copper(II)−N-Heterocyclic Carbene Complexes Void of an Anionic Tether Benjamin R. M. Lake and Charlotte E. Willans* School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K. S Supporting Information *

ABSTRACT: A library of pyridyl- and picolyl-substituted imidazolium salts have been synthesized and coordinated to copper, via transmetalation from silver(I)− N-heterocyclic carbenes (NHCs), to prepare several copper(I)− and copper(II)−NHC complexes. The copper(I)−NHCs are complexes of the type Cu(NHC)Br, with the solid-state structures revealing a variety of coordination environments around the copper centers. The stability of the copper(II) complexes is particularly unusual, given the absence of a “hard” anionic tethering group appended to the ligands. The stability has been attributed to the pyridyl substituent, with the complexes being extremely stable, while those with an appended anionic group tend to be more sensitive to air/moisture. The ligands and complexes have been examined in an Ullmann-type etherification reaction and exhibit improved activity in comparison to copper in the absence of a ligand or the common Cu(I)−NHC complexes Cu(IMes)Cl and [Cu(IMes)2]PF6, indicating stabilization of higher oxidation state species by the ligands during the catalytic cycle.

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INTRODUCTION The chemistry of organocopper(I) complexes is relatively well explored and understood in the literature. Numerous examples of coordination to copper(I) by alkyl,1 aryl,2 alkenyl,3 alkynyl,4 carbonyl,5 cyano,6 and carbenic7 functional groups have been reported. The prevalence of copper(I)−carbon bonds is attributable to the complementary nature of the “soft−soft” interaction between the copper(I) cation and the carbon donor. This favorable interaction between copper(I) cations and carbon donors is utilized widely. For example, in nature, it is believed that the formation of a copper(I)−ethylene adduct in the ETR 1 plant protein is a key step in the regulation of several aspects of the plant life cycle.8 In addition, synthetic chemistry often takes advantage of copper(I)−carbon bonds: for example, as catalysts for Ullmann-type coupling reactions9,10 and conjugate addition reactions11 and also in CO binding and sequestration.12 Well-characterized higher oxidation state (copper(II) and copper(III)) organocopper chemistry is somewhat more scarce, due to the obvious mismatch between the intermediate/hard copper(II)/copper(III) cations and soft carbon donors. A few examples of copper(II)−N-heterocyclic carbene (NHC) complexes have been reported, with the first appearing in 2003 (though this complex was not structurally characterized by X-ray diffraction analysis).13 In this instance, the copper(II) center was thought to be tetracoordinated by a chelating nitrogen-anchored tris−NHC ligand. Since then, other copper(II)−NHC complexes have appeared where typically the NHC ligand possesses an anionic ancillary donor such as an alkoxide, phenoxide, or amide.14−17 Alternatively, acetate ligands have been shown to stabilize copper(II)−NHC complexes sufficiently to allow characterization, though such complexes tend to undergo ring-opening upon exposure to moisture.18,19 © 2014 American Chemical Society

We have recently been working with imidazolium salts and NHC ligands with an appended pyridyl group, to examine the effect of further donors on copper−ligand stability.20 Pyridyland picolyl-substituted NHC ligands have been described in the literature previously and coordinated to palladium(II),21−23 nickel(II),24,25 ruthenium(II),26 iron(II),27 and copper(I).28−30 In addition, pyridyl- and picolyl-tethered NHC ligands have been coordinated to copper(II), though these ligands also contain an anionic pyrazolate substituent on the second nitrogen of the NHC (i.e., a stabilizing anionic ancillary donor).17,31 Herein, we report a family of pyridyl and picolyl Nsubstituted imidazolium salts void of an anionic tether and their coordination to copper to form both copper(I)− and copper(II)−NHC complexes. We have observed a wide range of structures and copper coordination environments and explored these ligands and complexes in copper-catalyzed Ullmann-type etherification reactions.

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RESULTS AND DISCUSSION Silver(I)−NHC complexes were prepared from imidazolium salts using the Ag2O route (Scheme 1). The NHC ligands were transferred from silver(I) onto either copper(I) or copper(II), with the stoichiometry of silver(I)−NHC to CuBrx (x = 1, 2) being critical in determining the outcome of the reaction. In addition to transferring the ligand from silver(I) onto copper(II), stable copper(II) complexes were also observed upon oxidation of the copper(I)−NHCs in the air. Imidazolium salts 1a−f (Figure 1) were prepared20 and fully characterized, with the solid-state structures of 1c,f being determined using X-ray crystallography (Figure 2). The crystal Received: February 19, 2014 Published: April 9, 2014 2027

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Scheme 1. Synthesis of copper(I)−NHC complexes 3a−f and copper(II)−NHC complexes, via transmetalation from silver(I)− NHCs 2a−f

Figure 1. Imidazolium salts 1a−f.

structure of 1c shows two imidazolium bromide moieties and one water molecule in the asymmetric unit. One of the bromide ions (Br2) and the water molecule form a 1D hydrogen-bonded zigzag which propagates throughout the structure. The other bromide (Br1) appears to be involved in two anion−π interactions with the neighboring π clouds of the imidazolium rings (N1−C4−N2 and N4−C16−N5) and two hydrogen bonds with neighboring imidazolium protons (H4 and H16). In contrast to 1c, the solid-state structure of imidazolium salt 1f does not show any evidence of anion−π interactions between the bromide anion and the electron-deficient imidazolium N− C−N π cloud. Such interactions are likely precluded by the presence of the bulky mesityl group adjacent to the imidazolium ring. However, a hydrogen bond between the imidazolium proton and bromide ion exists, with such interactions being ubiquitous features of structurally characterized imidazolium halides. Copper(I)−NHC complexes were prepared using a route which proceeds via silver(I)−NHCs (Scheme 1). The solidstate structure of the silver(I)−NHC complex 2c was elucidated using X-ray crystallography (Figure 3). The complex crystallized as a [Ag(NHC)2]AgBr2-type complex, with a possible weak argentophilic interaction (3.04 Å) between the two independent silver(I) centers, though a simple electrostatic interaction between the silver cation and the AgBr2 anion cannot be ruled out. The silver(I)−carbene bond length, at 2.089(3) Å, is typical of silver(I) bis(carbene) complexes.32,33 Copper(I)−NHC complexes 3a−f (Scheme 1) were fully characterized using NMR spectroscopy, mass spectrometry, and elemental analysis and confirmed as complexes of the type Cu(NHC)Br. We have previously reported the structures of complexes 3a,b,d and found that the nature of the pyridyl substituent has a significant effect on copper−alkene coordination.20 Complexes 3c,e,f were further examined using X-ray crystallography (Figure 4). Complex 3c exhibits a

Figure 2. Molecular structures of imidazolium salts 1c,f. Ellipsoids are shown at the 50% probability level.

distorted T-shaped geometry around the copper(I) center, while complexes 3e,f exhibit Cu−Cu distances that are in accord with cuprophilic interactions (3e, 2.4010(8) Å; 3f, 2.5247(5) Å). It is clear that all of the complexes are devoid of a copper(I)−alkene interaction, which further confirms our previous observation that copper(I)−alkene binding is more complicated than simply considering ligand basicity. Complex 3e crystallized as a dimer containing unusual bridging NHCs, 2028

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Figure 3. Molecular structure of silver(I)−NHC complex 2c. Ellipsoids are shown at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Ag1−C4 = 2.089(3), Ag1−Ag2 = 3.0405(6), Ag2−Br1 = 2.4443(3), C4−Ag1−C4 = 179.0(1), C4−Ag1−Ag2 = 90.51(9), Br1− Ag1−Br1 = 175.61(2).

where the bridging NHCs are presumably supported by the presence of the ancillary pyridyl donor group. Bridging NHCs are rather rare in copper(I) chemistry, with only a few examples having been reported to date.34−38 As a result of the bridging nature of the NHC ligands, the copper(I)−carbene bond lengths (2.011(4) and 2.284(5) Å) are much longer than those of terminally bound NHCs, which are typically between 1.87 and 1.96 Å,36 with the carbenic center of each NHC being quite notably closer to one copper(I) center than the other. Also, as a result of the bridging NHCs, the two copper(I) centers have been brought into close proximity (2.40 Å), which results in a possible cuprophilic interaction. It is likely that, in solution, the complex is labile, as there is no evidence of resonances attributable to the methylene protons being diastereotopic in the 1H NMR data. Bridging NHCs were the subject of recent DFT work by Hor et al.35 It was observed that the energetic difference between the addition of a bridging or terminal NHC to a [Cu2(μ-I)2(NHC)2]-type system was within experimental error, suggesting that bridging NHCs can form very stable complexes. It was also found that bridging NHCs are more similar to CO than they are to phosphines in their interactions with metal centers. They are better π acceptors and can therefore benefit from the increased π back-bonding from the two metal centers that they bridge. Complex 3f also crystallizes as a dimeric structure, though the NHCs are terminally bound rather than bridging. It was found that, upon exposure to low oxygen levels, a dichloromethane solution of complex 3b yielded some green crystals suitable for X-ray diffraction analysis, which were found to be an interesting mixed copper(I)−copper(II)−NHC complex (4b′; Figure 5). The structure illustrates the presence of two chelating NHC units and a bromide atom around the copper(II) center, which is charge-balanced by an alkenecoordinated CuBr2− unit. The coordination geometry around the copper(II) center can be described as slightly distorted trigonal bipyramidal, with the NHC donors occupying the

Figure 4. Molecular structures of copper(I)−NHC complexes 3c,e,f. Ellipsoids are shown at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): 3c, Cu1−C4 = 1.899(4), Cu1−Br1 = 2.2527(6), Cu1−N3 = 2.338(3), C4−Cu1−Br1 = 171.78(12), C4−Cu1−N3 = 78.23(14), N3−Cu1−Br1 = 109.99(8), N1−C4−N2 = 103.3(3); 3e, Cu1−C4 = 2.011(4), Cu1−C4′ = 2.284(5), Cu1−Br1 = 2.4623(8), Cu1−N3 = 2.053(3), Cu1−Cu1′ = 2.4010(8), Br1−Cu1−C4 = 109.5(1), Br1− Cu1−N3 = 106.0(1), N3−Cu1−C4 = 125.4(2), C4−Cu1−C4′ = 112.4(2); 3f, Cu1−C6 = 1.933(2), Cu1−Br1 = 2.4142(3), Cu1−N1 = 2.037(2), Cu1−Cu1′ = 2.5247(5), Br1−Cu1−C6 = 113.48(5), Br1− Cu1−N1 = 109.83(4), C6−Cu1−N1 = 127.47(7). 2029

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Figure 5. Molecular structure of the mixed copper(I)−copper(II)− NHC complex 4b′. Ellipsoids are shown at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Cu1−C4 = 1.991(10), Cu1−C15 = 1.994(9), C1−Br1 = 2.5229(15), C1−N3 = 2.181(8), C1−N6 = 2.175(7), C12−C13 = 1.356(13), Cu2−alkene = 1.964, C4−Cu1− C15 = 176.1(4), Br1−Cu1−N3 = 118.4(2), Br1−Cu1−N6 = 134.6(2), N3−Cu1−N6 = 107.0(3).

Figure 6. Molecular structure of copper(II)−NHC complex 4b. Ellipsoids are shown at the 50% probability level. Hydrogen atoms and four molecules of chloroform have been omitted for clarity. Selected bond distances (Å) and angles (deg): Cu1−C4 = 1.957(5), C1−Br1 = 2.4826(8), Cu1−N3 = 2.115(4), Cu1−Br3 = 2.587(1), Cu1−Br4 = 2.346(1), Cu2−C15 = 1.959(5), C2−Br2 = 2.4805(8), C2−N6 = 2.126(4), C4−Cu1−Br4 = 169.5(1), Br1−Cu1−N3 = 136.6(1), Br1− Cu1−Br3 = 107.67(3), Br3−Cu1−N3 = 115.6(1).

apical positions and the pyridyl donors and a bromide atom occupying the equatorial plane. This distortion from idealized trigonal bipyramidal geometry probably arises, in part, as a result of the relatively narrow bite angle of the chelating pyridyl−NHC ligand (approximately 78°). The copper− carbene bond lengths (1.991(10) Å) are at the upper end of the small number of values reported in the literature for copper(II)−NHCs, which fall between 1.91 and 2.01 Å.14−16,18,19,31,39 This relative lengthening is potentially a result of the trans influence imparted by the strong σ-donor NHC ligands on each other. Upon exposure of a solution of complex 3b in chloroform to atmospheric conditions, followed by crystallization from the vapor diffusion of pentane into the solution, a small number of green needles were again produced. On this occasion, the complex was found to have crystallized as a copper(II)−(μhalide)2-bridged dimer (4b; Figure 6). The bridging halides are substitutionally disordered (labeled as Br3 and Br4 in Figure 6), containing a mixture of chloride and bromide. It is anticipated that the chloride originates from impurities in the wet chloroform solvent. Interestingly, the terminally bound halides were not found to be substitutionally disordered, comprising only bromide. The geometry around the two copper(II) centers can again be described as slightly distorted trigonal bipyramidal. The coordination sphere of the copper(II) center comprises the NHC and a bridging halide in the apical positions, with the equatorial positions being occupied by a terminal halide, a bridging halide, and the pyridyl donor. The copper−carbene bond lengths of 1.957(5) Å compare well with literature precedent and are slightly shorter than those observed in complex 4b′, possibly reflecting the weaker trans influence of the bridging halide in comparison to the NHC donor. Exposure of an acetonitrile solution of complex 3c to low oxygen levels, followed by crystallization by the vapor diffusion of diethyl ether, led to blue crystals of suitable quality for X-ray diffraction analysis. The structure was found to be a mixed copper(I)−copper(II)−NHC complex, similar to that of 4b′ (4c′; Figure 7). The coordination sphere of the copper(II)

Figure 7. Molecular structure of the mixed copper(I)−copper(II)− NHC 4c′. Ellipsoids are shown at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Cu1−C4 = 1.981(6), Cu1−C16 = 1.982(6), Cu1−Br1 = 2.5057(9), Cu1−N3 = 2.127(5), Cu1−N26 = 2.192(5), C4−Cu1− C16 = 175.2(2), Br1−Cu1−N3 = 134.3(1), Br1−Cu1−N26 = 119.6(1).

center contains two chelating NHC units and a bromide atom and is charge-balanced by a near-linear (174°) CuBr2− anion. In this case, interaction between the alkene and a copper center is not observed. The adventitious isolation and X-ray crystallographic analysis of copper(II)−NHC complexes 4b,b′,c′ illustrates that the pyridyl-appended NHC ligands are highly effective in stabilizing higher oxidation states of copper. We therefore attempted the deliberate synthesis of copper(II)−NHC complexes derived 2030

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Figure 8. Molecular structure of copper(II)−NHC complex 5b. Ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvent molecules have been omitted for clarity. Selected bond distances (Å) and angles (deg): Cu1−C4 = 1.981(4), Cu1−N3 = 2.133(4), Cu1−Br1 = 2.4607(7), Cu1−Br1 = 2.6653(8), Cu1−Br2 = 2.4473(7), Cu2−C15 = 1.934(5), Cu2−N6 = 2.135(4), Cu2−Br3 = 2.4492(7), Cu2−Br4 = 2.4325(7), Cu2−Br4 = 2.5600(7), C4−Cu1−Br1 = 171.00(13), C4−Cu1−Br1 = 92.27(14), C4−Cu1−Br1 = 92.27(14), C4−Cu1−Br2 = 96.61(13), C4−Cu1−N3 = 79.44(16).

from the ligand precursors displayed in Figure 1. The silver(I)− NHC complex 2b was reacted with one equivalent of CuBr2 in dichloromethane (Scheme 1). The resulting dark green solid was characterized using elemental analysis, which confirmed the presence of Cu(NHC)Br2 (5b). Analysis of the complex by NMR spectroscopy is precluded due to the paramagnetism of the complex. Single crystals suitable for X-ray diffraction analysis were grown via the vapor diffusion of diethyl ether into a concentrated solution of the complex in dichloromethane (Figure 8). The asymmetric unit contains two halves of two independent copper(II)−(μ-Br)2-bridged dimers, along with 1.5 molecules of cocrystallized dichloromethane. Each copper(II) center resides in a slightly distorted trigonal bipyramidal coordination environment. The NHC donor and a bridging bromide atom occupy the apical positions, while the pyridyl donor, the terminally bound bromide atom, and the second bridging bromide atom occupy the equatorial plane. In one of the (μ-Br)2-bridged dimers, the pyridyl donors are in a trans orientation with respect to each other, similarly to complex 4b (Figure 6). However, in the other (μ-Br)2-bridged dimer the pyridyl groups are in a cis orientation with respect to one another, with the terminal bromide atoms also being cis oriented. The copper−carbene bond lengths compare well with literature precedent, with those in the trans-oriented dimer at 1.981(4) Å being quite notably longer than those in the cisoriented dimer (1.934(5) Å). Another complex of the type Cu(NHC)Br2 was obtained when silver(I)−NHC 2c was reacted with CuBr2, with elemental analysis confirming the molecular formula of complex 5c. Crystals suitable for X-ray diffraction analysis were grown via the vapor diffusion of diethyl ether into a concentrated solution of the product in dichloromethane (Figure 9). The presence of a methyl group in the para position of the pyridyl ring results in a monomeric unit, rather than a μ-halide-bridged dimer as observed in complex 5b. This may be a packing effect as a result of the added methyl group, or perhaps an electronic effect due to the slightly increased basicity of the pyridyl donor (copper−pyridyl bond length shortens from 2.133(4) Å in 5b to 2.064(3) Å in 5c). The coordination geometry about the

Figure 9. Molecular structure of copper(II)−NHC complex 5c. Ellipsoids are shown at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Cu1−C4 = 1.972(3), Cu1−N3 = 2.064(3), Cu1−Br1 = 2.3879(5), Cu1−Br2 = 2.3667(6), C4−Cu1−Br2 = 159.39(10), C4− Cu1−N3 = 80.66(12), C4−Cu1−Br1 = 97.34(10), N3−Cu1−Br1 = 146.28(8).

four-coordinate copper center is best described as tetragonally distorted tetrahedral, resulting from loss of degeneracy of the t2 orbitals and thus a distortion of the tetrahedral geometry toward square planar.40 The copper−carbene bond length, at 1.972(3) Å, is within the expected range for a copper(II)− NHC. Single crystals suitable for X-ray diffraction analysis were also grown from pyridine solutions of 5b,c, which revealed structures of the type Cu(NHC)Br2(pyridine) (Figure 10). Similarly to the previous copper(II)−NHC complexes, the coordination geometry around the copper(II) centers can be described as slightly distorted trigonal bipyramidal. The apical positions are occupied by the NHC donor and a pyridine ligand, with the equatorial planes being occupied by the pyridyl donor and two bromide atoms. The copper−carbene bond length in the complex 5b·(pyridine), at 1.950(3) Å, compares well with literature precedent and complex 4b, though it is 2031

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being obtained on standing of a dilute solution of the product in acetonitrile/diethyl ether (1/4). The structure obtained displays a copper(II) center in a four-coordinate environment, with the NHC, pyridyl donor, and two bromide atoms (Figure 11). The coordination geometry about the four-coordinate

Figure 11. Molecular structure of copper(II)−NHC complex 5d. Ellipsoids are shown at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Cu1−C4 = 1.966(2), Cu1−N3 = 2.0366(19), Cu1−Br1 = 2.3769(4), Cu1−Br2 = 2.3747(3), C4−Cu1−Br2 = 152.11(7), N3− Cu1−Br1 = 143.03(6), C4−Cu1−Br1 = 100.40(7).

copper center is best described as tetragonally distorted tetrahedral, similar to that of complex 5c. Again, it is surprising that the complex does not increase its electronic and steric saturation through formation of a μ-bromide-bridged dimer, in analogy to complex 5b (Figure 8). Sterically this would be achievable; hence, it is likely that the increased basicity of the methoxy-substituted pyridyl donor (Cu−N = 2.0366(19) Å in 5d in comparison to an average of 2.134 Å in 5b) renders the complex electronically satisfied. Crystals of complex 5d grown from a pyridine solution illustrate a structure of the type Cu(NHC)Br2(pyridine)· (pyridine) (Figure 12). The copper−carbene bond length, at 1.961(5) Å, remains similar to that of complexes 5d and 5c· (pyridine). However, the copper−pyridyl bond length increases significantly to 2.207(4) Å. This is likely due to a crystal packing effect, as the units assemble into a 2D network based on six Cu(NHC)Br2(pyridine) molecules, revealing wide solvent-accessible channels running parallel to the crystallographic c axis. One molecule of cocrystallized pyridine solvent per Cu(NHC)Br2(pyridine) unit, located in the solventaccessible channel, could be observed in the diffraction map and refined to convergence. However, more diffuse electron density within the solvent-accessible channels was removed from the structure using the SQUEEZE routine in PLATON. The diameter of the pores is approximately 13 Å, with a remarkable solvent accessible void volume of 36.5% of the total unit cell volume. Crystals of complex 5d·dmso could also be obtained on standing of a solution of 5d in a DMSO/DCM/diethyl ether (1/1/4) mixture. The structure illustrates a distorted trigonal bipyramidal coordination environment around the copper(II)

Figure 10. Molecular structure of copper(II)−NHC complexes 5b· (pyridine) and 5c·(pyridine). Ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvent molecules have been omitted for clarity. Selected bond distances (Å) and angles (deg): 5b· (pyridine), Cu1−C4 = 1.950(3), Cu1−N3 = 2.169(3), Cu1−Br1 = 2.5154(6), Cu1−Br2 = 2.5267(5), Cu1−N4 = 2.035(3), C4−Cu1− N4 = 169.14(14), C4−Cu1−N3 = 78.81(13), C4−Cu1−Br1 = 90.18(10), C4−Cu1−Br2 = 91.40(10); 5c·(pyridine), Cu1−C4 = 1.963(5), Cu1−N3 = 2.148(3), Cu1−Br1 = 2.4782(7), Cu1−Br2 = 2.5844(7), Cu1−N4 = 2.031(3), C4−Cu1−N4 = 168.67(17), C4− Cu1−N3 = 79.03(16), C4−Cu1−Br1 = 94.49(13), C4−Cu1−Br2 = 90.19(13).

shorter than that observed in complex 4b′ (1.991(10) Å), reflecting the weaker trans influence of the pyridine ligand in comparison to an NHC donor. The copper−carbene bond length in 5c·(pyridine), at 1.963(5) Å, is longer than that in 5b· pyridine (1.950(3) Å). This is perhaps due to the increased basicity of the pyridyl donor and the resulting shorter copper− pyridyl bond length (2.148(3) Å in 5c·(pyridine) in comparison to 2.169(3) Å in 5b·(pyridine)). Reaction of silver(I)−NHC complex 2d with 1 equiv of CuBr2 also results in the formation of a copper(II)−NHC complex of the type Cu(NHC)Br2 (5d), confirmed through elemental analysis. The complex was examined using X-ray crystallography, with yellow pleochroic crystals of the complex 2032

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Figure 12. (a) Molecular structure of the copper(II)−NHC complex 5d·(pyridine). Ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvent molecules have been omitted for clarity. Selected bond distances (Å) and angles (deg): Cu1−C4 = 1.961(5), Cu1−N3 = 2.207(4), Cu1−Br1 = 2.4875(8), Cu1−Br2 = 2.5434(8), Cu1−N4 = 2.023(4), C4−Cu1−N4 = 168.62(18), C4−Cu1−N3 = 78.03(16), N3−Cu1− Br1 = 124.40(10). (b) Space-filling diagram revealing one of the channels running down the crystallographic c axis in the absence of solvent (left) and several of the channels occupied by pyridine molecules (right).

for X-ray diffraction analysis were obtained via the vapor diffusion of diethyl ether into a concentrated solution of the complexes in acetonitrile. The coordination geometry around the copper(II) centers is distorted trigonal bipyramidal, with the NHC donors in apical positions and the pyridyl donors and bromide atom in the equatorial planes (Figure 14). The copper−carbene bond lengths (2.014(8) and 1.970 (8) Å in 6b and 1.990(2) and 1.984(2) Å in 6c) are at the upper end of the values reported in the literature for copper(II)−NHCs, which fall between 1.91 and 2.01 Å. This is likely a result of the trans influence imparted by the strong σ-donor NHC ligands on each other. Due to the demonstrated ability of pyridyl-substituted NHC ligands to stabilize both copper(I) and copper(II), we were keen to test the complexes in catalysis. As many processes are thought to proceed via a copper(I)/copper(II) or copper(I)/ copper(III) pathway, the ability to stabilize higher oxidation state copper species is likely to reduce catalyst deactivation and hence increase activity. Carbon−carbon and carbon−heteroatom bond-forming reactions represent one of the most important developments in chemical synthesis.41−43 The significantly lower cost of copper in comparison to the

center, with the bromide atoms and the pyridyl donor in the equatorial plane and the NHC and dmso occupying the apical positions (Figure 13). The dmso coordinates, as expected, through the oxygen atom, which results in a lengthening of the copper−pyridyl bond from 2.052(3) Å in complex 5d to 2.129(3) Å in the complex 5d·(dmso). Attempts were made to prepare a copper(II)−NHC complex derived from the m-nitropyridyl-substituted ligand precursor 1a, through the reaction of 2 equivalents of the silver−NHC 2a with CuBr2. However, the expected color changes were not observed, and no identifiable product could be obtained from the reaction mixture. The decreased basicity of the pyridyl donor, due to the nitro substituent, is likely to decrease the stability of higher oxidation state copper complexes in comparison to the other pyridyl-substituted NHC ligands. Bis(NHC) complexes derived from ligand precursors 1b,c were also synthesized by reaction of the corresponding silver(I)−NHC complex with 0.5 equivalents of CuBr2 and AgPF6, which was added to these reactions to prevent the formation of silver halide counterions. Mass spectrometry and elemental analysis confirmed molecular formulas of [Cu(NHC)2Br]PF6 for both complexes (6b,c). Crystals suitable 2033

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2b−e (entries 5−8) provide an improved yield in comparison to the ligand-free reaction, which is doubled when using ligand 1e. This is likely due to the stabilizing effect of the pyridyl or picolyl substituent of higher oxidation state species, with the added flexibility of the picolyl group in 2e providing the greatest stabilization. Using the most efficient ligand precursor 1e, the effect of altering the amount of base was examined. Increasing the amount of Cs2CO3 in the reaction increases the efficiency, with a yield of 58% being achieved when using a Cs2CO3:CuI ratio of 40:1 (entry 10). To the best of our knowledge, this is the highest yield achieved when using deactivated aryl halides for the arylation of phenols catalyzed by copper−NHCs.9,10,48 Increasing the amount of base may help recycle the active catalyst during the reaction, potentially by removing halide anions more effectively from the copper centers. The reactions reported in Table 1 were performed under a flow of nitrogen. Entry 10 was repeated using strict inert conditions (thorough degassing of solvents, reagents, and reaction vessel), though this did not improve the performance (57% yield achieved). This is encouraging when considering these complexes as catalysts for industrial processes, as the performance is as efficient under non-inert conditions.

Figure 13. Molecular structure of the copper(II)−NHC complex 5d· (dmso). Ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvent molecules have been omitted for clarity. Selected bond distances (Å) and angles (deg): Cu1−C4 = 1.946(4), Cu1−Br1 = 2.4930(7), Cu1−Br2 = 2.5883(7), Cu1−N3 = 2.129(3), Cu1−O2 = 1.972(3), C4−Cu1−O2 = 168.16(15), N3−Cu1−Br1 = 126.59(9), N3−Cu1−Br2 = 116.01(9), Br1−Cu1−Br2 = 117.39(3), C4−Cu1− N3 = 79.06(15).

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CONCLUSION In summary, we have reported the synthesis of several copper(I)−NHC complexes bearing pyridyl or picolyl substituents. Rare copper(II)−NHC complexes devoid of an anionic tether have been isolated and show remarkable stability. This stability is attributed to the pyridyl substituent preventing dissociation of the NHC from the copper(II) center. The copper(I) complexes exhibit reasonable activity in the arylation of 3,5-dimethylphenol using an electron-rich aryl halide. A yield of 58% has been achieved using imidazolium salt 1e together with CuI and Cs2CO3, which is a significant improvement on similar reactions reported in the literature, particularly when using NHC ligands. The higher activity is attributed to the picolyl substituent improving the stability of higher oxidation state copper−NHC species during the catalytic cycle. Although higher yields have been achieved in the literature for this specific reaction when using different ligands (e.g., when using 1,1,1-tris(hydroxylmethyl)ethane a yield of 82% was obtained following heating at 110 °C for 48 h),50 the insight into copper−NHC complexes using this ligand system is likely to be of significant interest in developing copper−NHCs further for industrially relevant transformations. We are currently examining the effect of the second nitrogen substituent (i.e., the allyl group) in our laboratory.

transition metals that are currently used in these reactions (e.g., palladium) is making this metal an attractive alternative for industrially relevant transformations.44−47 The complexes synthesized during the course of these studies were examined in an Ullmann-type etherification reaction; the use of NHCs in this area is currently very limited,9,10,48 likely due to dissociation of the NHC ligand from the metal center during catalysis leading to deactivation. Pyridyl- and picolylsubstituted ligands have previously been examined in coppercatalyzed aryl amination reactions, though the complexes were not fully elucidated in the solid-state.49 Therefore, we wanted to determine if our ligands, which we have shown to stabilize copper(II), enhance the catalytic activity of copper. We specifically chose a difficult transformation, the arylation of 3,5-dimethylphenol using the electron-rich aryl halide 4iodoanisole, which was known to be sluggish when using either copper iodide (in the absence of a ligand) or the common IMes NHC complexes, as exemplified by entries 1−3 (Table 1). In situ generated catalysts were also investigated using imidazolium salts 1a−f and copper iodide, with Cs2CO3 as a base to promote NHC formation. It is clear from Table 1 that the in situ formed complexes behave almost identically to the preformed complexes, resulting in very similar yields (entries 4−9). Complex 2a (entry 4) does not improve the yield in comparison to copper iodide in the absence of a ligand precursor (entry 1). This is likely due to the NO2 substituent weakening coordination of the pyridyl to the metal center, resulting in negligible stabilization of the higher oxidation state copper−NHC species during the reaction. Indeed, this is further indicated by the inability to prepare a discrete copper(II)−NHC complex using this ligand. Complex 2f (entry 9) also gives a yield similar to that for copper iodide alone, probably as a result of the bulky mesityl group preventing efficient catalysis. This is further implied by low yields when using the common copper(I)−NHC complexes Cu(IMes)2PF6 and Cu(IMes)Cl (entries 2 and 3). Complexes

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EXPERIMENTAL SECTION

General Methods. Where stated, manipulations were performed under an atmosphere of dry argon by means of standard Schlenk line or glovebox techniques. The gas was dried by passing through a twincolumn drying apparatus containing molecular sieves (4 Å) and P2O5. Anhydrous solvents were prepared by passing the solvent over activated alumina to remove water, copper catalyst to remove oxygen, and molecular sieves to remove any remaining water, via the Dow− Grubbs solvent system. Deuterated chloroform, acetonitrile, and DMSO were dried over CaH2, cannula-filtered or distilled, and then freeze−pump−thaw degassed prior to use. All other reagents and solvents were used as supplied. 1 H and 13C{1H} NMR spectra were recorded on either a Bruker DPX300 or a Bruker AV500 spectrometer. The values of chemical shifts are given in ppm and values for coupling constants (J) in Hz. 2034

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Figure 14. Molecular structure of copper(II)−bis(NHC) complexes 6b,c. Ellipsoids are shown at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): 6b, Cu1−C4 = 2.014(8), Cu1−C15 = 1.970(8), Cu1−N3 = 2.172(7), Cu1− N6 = 2.189(7), Cu1−Br1 = 2.5030(15), C4−Cu1−C15 = 177.3(4), N3−Cu1−N6 = 113.4(3), C4−Cu1−N3 = 99.2(3); 6c, Cu1−C4 = 1.990(2), Cu1−C16 = 1.984(2), Cu1−N3 = 2.1639(19), Cu1−N6 = 2.139(2), Cu1−Br1 = 2.5141(4), C4−Cu1−C16 = 177.21(10), N3−Cu1−N6 = 107.77(7), C4−Cu1−N6 = 98.63(8). prepared using modified synthetic procedures20 and can be found in the Supporting Information. 1-Allyl-3-(4-methyl-2-pyridyl)imidazolium Bromide (1c). 2(1-Imidazolyl)-4-methylpyridine (0.80 g, 5.0 mmol), allyl bromide (2.0 mL, 23 mmol), and acetonitrile (50 mL) were placed in a small roundbottomed flask and heated to reflux for 16 h. After this time, the mixture was cooled to room temperature and the volume of solvent reduced in vacuo (to approximately 15 mL). Slow addition of diethyl ether (35 mL) to the stirring acetonitrile solution led to the precipitation of the product as an off-white crystalline solid, which was collected by vacuum filtration, washed repeatedly with diethyl ether, and dried in vacuo. Yield: 1.25 g, 4.46 mmol, 89%. 1H NMR (300 MHz, CDCl3): δ 11.34 (s, 1H, NCHN), 8.42 8.21 (m, 3H, pyH and imH), 7.67 (s, 1H, imH), 7.20 (d, J = 4.9 Hz, 1H, pyH), 6.09 (ddt, J = 16.9, 10.1, 6.5 Hz, 1H, CHCH2), 5.56 (d, J = 16.9 Hz, 1H, CHCHH(trans)), 5.44 (d, J = 10.1 Hz, 1H, CHCHH(cis)), 5.21 (d, J = 6.5 Hz, 2H, NCH2), 2.48 (s, 3H, CH3). 13C{1H} NMR (75 MHz, CDCl3): δ 152.9, 148.6, 146.1, 135.2, 129.8, 126.2, 123.2, 122.5, 119.2, 115.5, 52.6, 21.3. HRMS (ESI+): m/z 200.1180 [M − Br]+, calcd for [M − Br]+ 200.1182. Anal. Calcd for C12H14BrN3·H2O: C, 49.84; H, 5.23; N, 14.53. Found: C, 49.60; H, 5.10; N, 14.53. Mp: 158.6−160.4 °C.

Assignment of some 1H NMR spectra was aided by the use of 2D 1 1 H H COSY experiments, and the assignment of some 13C{1H} NMR spectra was aided by 13C{1H} DEPT 135 experiments. Mass spectra were collected on a Bruker Daltonics (micro TOF) instrument operating in the electrospray mode. Microanalyses were performed using a Carlo Erba Elemental Analyzer MOD 1106 spectrometer. Xray diffraction data were collected on either a Bruker Nonius X8 diffractometer fitted with an Apex II detector with Mo Kα radiation (λ = 0.71073 Å) or an Agilent SuperNova diffractometer fitted with an Atlas CCD detector with Mo Kα radiation (λ = 0.71073 Å) or Cu Kα radiation (λ = 1.5418 Å). Crystals were mounted under oil on glass or nylon fibers. Data sets were corrected for absorption using a multiscan method, and the structures were solved by direct methods using SHELXS-97 and refined by full-matrix least squares on F2 using ShelXL-97, interfaced through the program X-Seed. Molecular graphics for all structures were generated using POV-RAY in the XSeed program. Cu(IMes)2PF6 and Cu(IMes)Cl were prepared using electrochemical methods.7 Imidazolium salts 1a,b,d and complexes 3a,b,d were prepared as previously reported.20 Imidazolium salt 1f was 2035

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Table 1. Results of Copper-Catalyzed Arylation Reactionsa

this time, the dark suspension was filtered and the solvent removed in vacuo to yield the crude product as a white oily solid. Recrystallization from a minimum amount of acetone/pentane gave the pure product as a white crystalline solid. Yield: 0.39 g, 1.1 mmol, 80.9%. Single crystals suitable for X-ray diffraction analysis were grown by the vapor diffusion of diethyl ether into a concentrated solution of the product in chloroform. 1H NMR (300 MHz, CDCl3): δ 8.50 (dd, J = 4.9, 1.4 Hz, 1H, pyH), 8.13 (d, J = 7.9 Hz, 1H, pyH), 7.88 (dd, J = 7.9, 1.4 Hz, 1H, pyH), 7.85 (d, J = 1.9 Hz, 1H, imH), 7.36 (ddd, J = 7.6, 4.9, 0.8 Hz, 1H, pyH), 7.16 (d, J = 1.9 Hz, 1H, imH), 6.01 (ddt, J = 16.3, 10.3, 6.0 Hz, 1H, CHCH2), 5.39 5.27 (m, 2H, CHCH2), 4.89 (dt, J = 6.0, 1.4 Hz, 2H, NCH2). 13C{1H} NMR (75 MHz, CDCl3): δ 181.6, 151.0, 149.1, 139.5, 132.4, 123.9, 121.7, 120.4, 120.3, 115.6, 55.4. HRMS (ESI +): m/z 477.0953 [Ag(NHC)2 ]+, calcd for [Ag(NHC) 2] + 477.0951. Anal. Calcd for C11H11AgBrN3·1/3MeCN: C, 36.24; H, 3.13; N, 12.07. Found: C, 36.50; H, 3.00; N, 11.50. [1-Allyl-3-(4-methyl-2-pyridyl)imidazol-2-ylidene]silver(I) Bromide (2c). To freshly activated 4 Å molecular sieves were added imidazolium bromide 1c (0.40 g, 1.4 mmol) and Ag2O (0.22 g, 0.9 mmol). To these was added anhydrous dichloromethane (30 mL), and the mixture was stirred at room temperature in the absence of light for 2.5 h. After this time, the mixture was filtered and the solvent removed in vacuo to give the crude product. Recrystallization from chloroform/ pentane gave the pure product as a white solid. Yield: 0.43 g, 1.1 mmol, 78%. Single crystals suitable for X-ray diffraction analysis were grown by the vapor diffusion of pentane into a concentrated solution of the product in chloroform. 1H NMR (300 MHz, CDCl3): δ 8.38 (d, J = 5.0 Hz, 1H, pyH), 7.89 (s, 1H, pyH), 7.81 (d, J = 1.9 Hz, 1H, imH), 7.19 (d, J = 5.0 Hz, 1H, pyH), 7.14 (d, J = 1.9 Hz, 1H, imH), 6.01 (ddt, J = 16.3, 10.4, 6.0 Hz, 1H, CHCH2), 5.41 5.28 (m, 2H, CHCH2), 4.86 (d, J = 6.0 Hz, 2H, NCH2), 2.48 (s, 3H, CH3). 13 C{1H} NMR (75 MHz, CDCl3): δ 181.1, 151.5, 151.1, 148.8, 132.3, 125.1, 121.5, 120.5, 120.4, 116.2, 55.4, 21.5. HRMS (ESI+): m/z 505.1267 [Ag(NHC)2]+, calcd for [Ag(NHC)2]+ 505.1264. Anal. Calcd for C12H13AgBrN3: C, 37.24; H, 3.39; N, 10.86. Found: C, 37.55; H, 3.30; N, 10.70. [1-Allyl-3-(4-methoxy-2-pyridyl)imidazol-2-ylidene]silver(I) Bromide (2d). Imidazolium bromide 1d (0.1600 g, 0.54 mmol) and Ag2O (0.0814 g, 0.35 mmol) were placed in a Schlenk flask along with freshly activated 4 Å molecular sieves. To these was added anhydrous dichloromethane (25 mL), and the mixture was stirred in the absence of light at room temperature for 2.5 h. After this time, the mixture was filtered through Celite and the solvent removed in vacuo to yield the crude product as an off-white, oily solid. Recrystallization from acetone/pentane gave the pure product as a white solid. Yield: 0.17 g, 0.43 mmol, 80%. 1H NMR (300 MHz, CDCl3): δ 8.30 (d, J = 5.8 Hz, 1H, pyH), 7.87 (d, J = 1.9 Hz, 1H, imH), 7.67 (d, J = 2.2 Hz, 1H, pyH), 7.13 (d, J = 1.9 Hz, 1H, imH), 6.89 (dd, J = 5.8, 2.2 Hz, 1H, pyH), 6.01 (ddt, J = 16.3, 10.2, 6.0 Hz, 1H, CHCH2), 5.40 (d, J = 10.2 Hz, 1H, CHCHHcis), 5.34 (d, J = 17.0 Hz, 1H, CH CHHtrans), 4.85 (d, J = 6.0 Hz, 2H, NCH2), 4.00 (s, 3H, OCH3). 13C NMR (75 MHz, CDCl3): δ (C−Ag not observed), 168.2, 152.6, 149.8, 132.3, 121.4, 120. 7, 120.5, 111.8, 100.5, 56.5, 55.4. HRMS (ESI+): m/ z 537.1167 [Ag(NHC)2]+, calcd for [Ag(NHC)2]+ 537.1163. Anal. Calcd for C12H13AgBrN3O: C, 35.76; H, 3.25; N, 10.43. Found: C, 35.85; H, 3.15; N, 10.10. [1-Allyl-3-((2-pyridyl)methyl)imidazol-2-ylidene]silver(I) Bromide (2e). To freshly activated 4 Å molecular sieves were added imidazolium bromide 1e (0.15 g, 0.54 mmol) and Ag2O (0.21 g, 0.91 mmol). To these was added anhydrous dichloromethane (40 mL), and the resultant dark suspension was stirred at room temperature in the absence of light for 16 h. After this time the mixture was filtered and the organics were removed in vacuo to yield the product as a white oily solid. Yield: 0.16 g, 0.41 mmol, 76%. 1H NMR (300 MHz, CDCl3): δ 8.57 (d, J = 4.2 Hz, 1H, pyH), 7.70 (td, J = 7.8, 1.7 Hz, 1H, pyH), 7.34 (d, J = 7.8 Hz, 1H, pyH), 7.30 7.22 (m, 1H, pyH), 7.20 (d, J = 1.7 Hz, 1H, imH), 6.99 (d, J = 1.7 Hz, 1H, imH), 5.95 (ddt, J = 16.6, 10.3, 5.9 Hz, 1H, CHCH2), 5.39 (s, 2H, py−CH2), 5.32 (dd, J = 10.3, 0.8 Hz, 1H, CHCHHcis), 5.24 (dd, J = 16.6, 0.8 Hz, 1H, CHCHHtrans), 4.73 (d, J = 5.9 Hz, 2H, NCH2). 13C{1H} NMR (75 MHz, CDCl3): δ

GC yield (%)b entry

catalyst (or imidazolium salt + CuI)

preformed complex

1 2 3 4 5 6 7 8 9 10

CuI Cu(IMes)2PF6 Cu(IMes)Cl 2a (1a + CuI) 2b (1b + CuI) 2c (1c + CuI) 2d (1d + CuI) 2e (1e + CuI) 2f (1f + CuI) (1e + CuI)c

20 6 15 12 33 27 33 38 21

in situ generated complex

14 32 27 34 42 17 58

a

Conditions: 1.2 mmol of 3,5-dimethylphenol, 1.0 mmol of 4iodoanisole, 0.1 mmol of copper complex or imidazolium salt + CuI, 2.0 mmol of Cs2CO3, 5 mL of MeCN, 90 °C, 24 h. bYields determined by GC using p-cymene as an internal standard. cCs2CO3 increased to 4.0 mmol.

1-Allyl-3-((2-pyridyl)methyl)imidazolium Bromide (1e). 2Bromomethylpyridine·(HBr) (0.51 g, 2.0 mmol), allylimidazole (0.23 g, 2.1 mmol), K2CO3 (1.4 g, 10 mmol), and acetonitrile (50 mL) were placed in a round-bottomed flask and stirred vigorously at room temperature for 24 h. After this time, the mixture was filtered and the solvent removed in vacuo to give a pale orange oil. Dissolution in acetonitrile (20 mL) followed by reprecipitation with diethyl ether (50 mL) (twice) gave the pure product as a pale yellow oil. Yield: 0.56 g, 2.0 mmol, quantitative. 1H NMR (300 MHz, CDCl3): δ 10.60 (s, 1H, NCHN), 8.52 (d, J = 4.7 Hz, 1H, pyH), 7.82 (d, J = 7.7 Hz, 1H, pyH), 7.74 (td, J = 7.7, 1.7 Hz, 1H, pyH), 7.66 (s, 1H, imH), 7.33 7.26 (m, 2H, pyH and imH), 6.01 (ddt, J = 16.6, 10.0, 6.5 Hz, 1H, CH CH2), 5.78 (s, 2H, NCH2-pyridyl), 5.53 5.41 (m, 2H, CHCH2), 4.95 (d, J = 6.5 Hz, 2H, NCH2). 13C{1H} NMR (75 MHz, CDCl3): δ 152.5, 150.0, 137.9, 137.6, 129.6, 124.3, 124.2, 123.1, 121.3, 110.1, 54.2, 52.4. HRMS (ESI+): m/z 200.1181 [M − Br]+, calcd for [M − Br]+ 200.1182. Due to the oily nature of the product, microanalytical data were not obtained. [1-Allyl-3-(5-nitro-2-pyridyl)imidazol-2-ylidene]silver(I) Bromide (2a). To freshly activated 4 Å molecular sieves were added imidazolium bromide 1a (0.16 g, 0.51 mmol) and Ag2O (0.070 g, 0.30 mmol). To these was added anhydrous dichloromethane (50 mL), and the resultant dark suspension was stirred at room temperature in the absence of light for 18 h. After this time the mixture was filtered and the solid washed repeatedly with hot acetonitrile. The combined organics were reduced in volume in vacuo to 30 mL. Slow addition of diethyl ether induced the precipitation of the product as off-white needles, which were collected and dried. Yield: 0.14 g, 0.34 mmol, 68%. 1H NMR (300 MHz, DMSO-d6): δ 9.17 (d, J = 2.7 Hz, 1H, pyH), 8.85 (dd, J = 9.0, 2.7 Hz, 1H, pyH), 8.39 8.26 (m, 2H, pyH and imH), 7.79 (d, J = 1.9 Hz, 1H, pyH), 6.12 (ddt, J = 17.1, 10.5, 5.6 Hz, 1H, CHCH2), 5.30 (dd, J = 10.5, 0.9 Hz, 1H, CHCHH(cis)), 5.23 (dd, J = 17.1, 0.9 Hz, 1H, CHCHH(trans)), 4.94 (d, J = 5.6 Hz, 2H, NCH2). 13C{1H} NMR (75 MHz, DMSO-d6): δ 182.3, 153.5, 144.5, 143.7, 135.6, 133.7, 124.1, 120.6, 118.8, 115.6, 54.45. HRMS (ESI+): m/z 567.0622 [Ag(NHC)2]+, calcd for [Ag(NHC)2]+ 567.0653. Anal. Calcd for C11H10AgBrN4O2: C, 31.61; H, 2.41; N, 13.40. Found: C, 31.80; H, 2.20; N, 13.20. [1-Allyl-3-(2-pyridyl)imidazol-2-ylidene]silver(I) Bromide (2b). Imidazolium bromide 1b (0.34 g, 1.28 mmol) and Ag2O (0.19 g, 0.83 mmol) were placed in a Schlenk flask along with 4 Å molecular sieves. To these was added anhydrous dichloromethane (30 mL), and the dark suspension was stirred for 18 h in the absence of light. After 2036

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Article

C17H17BrCuN3: C, 50.19; H, 4.21; N, 10.33. Found: C, 50.15; H, 4.20; N, 10.30. [1-Allyl-3-(2-pyridyl)imidazol-2-ylidene]copper(II) Dibromide (5b). Silver(I)−NHC 2b (0.21 g, 0.57 mmol) and CuBr2 (0.13 g, 0.57 mmol) were placed in a small Schlenk flask. To these was added anhydrous dichloromethane (35 mL), and the resultant suspension was stirred at room temperature in the absence of light for 24 h. After this time, the dark green suspension was filtered to remove the precipitated AgBr and the dichloromethane removed in vacuo to give the product as a green oily solid. Trituration with tetrahydrofuran gave the pure product as a dark green solid. Yield: 0.13 g, 0.32 mmol, 57%. Very dark green single crystals suitable for X-ray diffraction analysis were grown via the vapor diffusion of diethyl ether into a concentrated solution of the complex in dichloromethane. Anal. Calcd for C11H11Br2CuN3·1/3C4H8O: C, 34.24; H, 3.18; N, 9.71. Found: C, 34.60; H, 3.20; N, 9.40. [1-Allyl-3-(4-methyl-2-pyridyl)imidazol-2-ylidene]copper(II) Dibromide (5c). Silver(I)−NHC 2c (0.10 g, 0.26 mmol) and CuBr2 (0.058 g, 0.26 mmol) were placed in a small Schlenk flask. To these was added anhydrous dichloromethane (25 mL), and the resultant suspension was stirred at room temperature in the absence of light for 18 h. After this time, the dark green suspension was filtered to remove the precipitated AgBr and the dichloromethane removed in vacuo to give the product as a green solid. Yield: 0.091 g, 0.22 mmol, 83%. Black single crystals suitable for X-ray diffraction analysis were grown via the vapor diffusion of diethyl ether into a concentrated solution of the complex in dichloromethane. Anal. Calcd for C12H13Br2CuN3: C, 34.11; H, 3.10; N, 9.94. Found: C, 34.00; H, 3.10; N, 9.60. [1-Allyl-3-(4-methoxy-2-pyridyl)imidazol-2-ylidene]copper(II) Dibromide (5d). Silver(I)−NHC 2d (0.022 g, 55 μmol) and CuBr2 (0.012 g, 55 μmol) were placed in a small Schlenk flask. To these was added anhydrous dichloromethane (15 mL), and the resultant suspension was stirred at room temperature in the absence of light for 18 h. After this time, the dark yellow suspension was filtered to remove the precipitated AgBr and the dichloromethane removed in vacuo. Recrystallization of the crude product from dichloromethane/ diethyl ether gave the pure product as a green-yellow crystalline solid. Yield: 0.021 g, 47 μmol, 86%. Single crystals suitable for X-ray diffraction analysis could be obtained on standing of a solution of the product in acetonitrile/diethyl ether (1/4). Anal. Calcd for C12H13Br2CuN3O·1/2CH2Cl2: C, 31.21; H, 2.93; N, 8.73. Found: C, 31.40; H, 2.70; N, 8.50. Bis[1-allyl-3-(2-pyridyl)imidazol-2-ylidene]copper(II) Bromide Hexafluorophosphate (6b). Silver(I)−NHC 2b (0.11 g, 0.28 mmol), CuBr2 (0.032 g, 0.14 mmol), and AgPF6 (0.036 g, 0.14 mmol) were placed in a small Schlenk flask. To these was added anhydrous dichloromethane (30 mL), and the resultant suspension was stirred at room temperature in the absence of light for 16 h. After this time, the blue suspension was filtered to remove the precipitated AgBr and the dichloromethane removed in vacuo to give the crude product as a blue solid. Recrystallization from acetonitrile/diethyl ether gave the pure product as a blue crystalline solid. Yield: 0.059 g, 90 μmol, 63%. Single crystals suitable for X-ray diffraction analysis could be grown via the vapor diffusion of diethyl ether into a concentrated solution of the product in acetone. HRMS (ESI+): m/z 216.5596 [Cu(NHC)2]2+ calculated for [Cu(NHC)2]2+ 216.5595. Anal. Calcd for C22H22BrCuF6N6P: C, 40.10; H, 3.37; N, 12.76. Found: C, 40.30; H, 3.30; N, 12.70. Bis[1-allyl-3-(4-methyl-2-pyridyl)imidazol-2-ylidene]copper(II) Bromide Hexafluorophosphate (6c). Silver(I)−NHC 2c (0.051 g, 0.13 mmol), CuBr2 (0.015 g, 0.07 mmol), and AgPF6 (0.017 g, 0.07 mmol) were added to a small Schlenk flask. To these was added anhydrous dichloromethane (15 mL), and the resultant suspension was stirred at room temperature in the absence of light for 4 h. After this time, the blue suspension was filtered to remove the precipitated AgBr and the dichloromethane removed in vacuo to give the crude product as a blue solid. Recrystallization from acetonitrile/ diethyl ether gave the pure product as a blue crystalline solid. Yield: 0.035 g, 51 μmol, 77%. Single crystals suitable for X-ray diffraction analysis could be grown via the vapor diffusion of diethyl ether into a

181.7, 155.2, 149.8, 137.4, 132.8, 123.4, 122.7, 122.1, 121.4, 119.6, 57.1, 54.4. HRMS (ESI+): m/z 505.1263 [Ag(NHC)2]+, calcd for [Ag(NHC)2]+ 505.1264. Due to the oily nature of the product, microanalytical data were not obtained. [1-Mesityl-3-(2-pyridyl)imidazol-2-ylidene]silver(I) Bromide (2f). To freshly activated 4 Å molecular sieves were added imidazolium bromide 1f (0.21 g, 0.61 mmol) and Ag2O (0.093 g, 0.45 mmol). To these was added anhydrous dichloromethane (30 mL), and the resultant dark suspension was stirred at reflux in the absence of light for 16 h. After this time the mixture was filtered and the organics were removed in vacuo to give an oily white solid. Recrystallization from acetone/pentane gave the product as a white solid. Yield: 0.19 g, 0.42 mmol, 69%. 1H NMR (300 MHz, CDCl3): δ 8.54 (d, J = 4.6 Hz, 1H, pyH), 8.28 (d, J = 8.2 Hz, 1H, pyH), 8.10 (d, J = 1.7 Hz, 1H, imH), 7.92 (td, J = 8.2, 1.7 Hz, 1H, pyH), 7.40 (dd, J = 8.2, 4.6 Hz, 1H, pyH), 7.09 (d, J = 1.7 Hz, 1H, imH), 6.97 (s, 2H, ArH). 13C{1H} NMR (75 MHz, CDCl3): δ 150.8, 149.0, 139.8, 139.6, 135.7, 134.7, 129.7, 124.0, 123.3, 120.3, 115.7, 21.2, 18.0. HRMS (ESI+): m/z 633.1912 [Ag(NHC)2]+, calcd for [Ag(NHC)2]+ 633.1890. Anal. Calcd for C17H17AgBrN3: C, 45.26; H, 3.80; N, 9.31. Found: C, 45.50; H, 3.80; N, 9.15. [1-Allyl-3-(4-methyl-2-pyridyl)imidazol-2-ylidene]copper(I) Bromide (3c). Silver(I)−NHC 2c (0.39 g, 1.0 mmol) and CuBr (0.22 g, 1.5 mmol) were placed in a small ampule. To these was added anhydrous dichloromethane (35 mL), and the resultant yellow suspension was stirred at room temperature in the absence of light for 3 days. After this time, the yellow suspension was filtered to remove AgBr and unreacted CuBr and the solvent removed in vacuo to give the product as a yellow solid. Yield: 0.31 g, 0.91 mmol, 91%. Single crystals suitable for X-ray diffraction analysis were grown on standing of a solution of the product in a chloroform/hexane mixture (1/2). 1H NMR (300 MHz, CDCl3): δ 7.81 (br s), 7.08 (br s), 5.98 (br s), 5.32 (br s), 4.77 (br s), 2.31 (br s). A 13C{1H} spectrum was not obtained due to broadening. HRMS (ESI+): m/z 303.0665 [M − Br + MeCN]+, calcd for [M − Br + MeCN]+ 303.0665. Upon exposure to air, complex 3c oxidizes to give complex 4c. Anal. Calcd for C12H13CuBrN3: C, 42.06; H, 3.82; N, 12.26. Found: C, 42.00; H, 3.80; N, 12.10. [1-Allyl-3-((2-pyridyl)methyl)imidazol-2-ylidene]copper(I) Bromide (3e). Silver(I)−NHC 2e (0.43 g, 1.1 mmol) and CuBr (0.29 g, 2.0 mmol) were placed in a small Schlenk flask. To these was added anhydrous dichloromethane (30 mL). The suspension that formed was stirred at room temperature in the absence of light for 24 h. After this time, the suspension was filtered to remove AgBr/CuBr and the yellow filtrate collected. The solvent was removed in vacuo to give the product as a pale yellow crystalline solid. Yield: 0.22 g, 0.64 mmol, 33%. 1H NMR (300 MHz, CDCl3): δ 8.76 (br s, 1H), 7.74 (t, J = 7.3 Hz, 1H), 7.42 (d, J = 7.3 Hz, 1H), 7.30 (br s, 1H), 7.05 (br s, 1H), 6.91 (br s, 1H), 5.91 (m, 1H), 5.55 (s, 2H), 5.21 (m, 2H), 4.67 (d, 2H). A 13C{1H} spectrum was not obtained due to broadening. HRMS (ESI+): m/z 262.0397 [M − Br]+, calcd for [M − Br]+ 262.0400. Anal. Calcd for C12H13CuBrN3: C, 42.06; H, 3.82; N, 12.26. Found: C, 41.80; H, 3.75; N, 11.80. [1-Mesityl-3-(2-pyridyl)imidazol-2-ylidene]copper(I) Bromide (3f). Silver(I)−NHC 2f (0.73 mmol) was dissolved in anhydrous dichloromethane (25 mL). To this was added solid CuBr (0.11 g, 0.80 mmol). The yellow suspension that formed was stirred at room temperature in the absence of light for 16 h. After this time, the suspension was filtered and the volatiles were removed in vacuo to give the crude product as a yellow solid. Recrystallization from THF/ pentane gave the pure product as a yellow crystalline solid. Yield: 0.26 g, 0.65 mmol, 89%. Single crystals suitable for X-ray diffraction analysis were grown by vapor diffusion of pentane into a concentrated solution of the product in chloroform. 1H NMR (501 MHz, CDCl3): δ 8.55 (br s, 1H), 8.35 (br s, 1H), 8.08 (br s, 1H), 7.94 (s, 1H), 7.42 (br s, 1H), 7.07 (br s, 1H), 6.97 (s, 2H, mesH), 2.33 (s, 3H, p-CH3), 2.07 (s, 6H, o-CH3). Assignments of the aromatic resonances are hampered by spectral broadening. A 13C{1H} NMR spectrum was not obtained due to broadening. HRMS (ESI+): m/z 367.0977 [M − Br + MeCN]+, calcd for [M − Br + MeCN] + 367.0978. Anal. Calcd for 2037

dx.doi.org/10.1021/om500178e | Organometallics 2014, 33, 2027−2038

Organometallics

Article

concentrated solution of the product in acetonitrile. HRMS (ESI+): m/z 230.5750 [Cu(NHC)2]2+ calculated for [Cu(NHC)2]2+ 230.5752. Anal. Calcd for C24H26BrCuF6N6P: C, 41.96; H, 3.82; N, 12.23. Found: C, 42.10; H, 3.80; N, 12.00. General Procedure for Catalytic Reactions. 3,5-Dimethylphenol (0.15 g, 1.2 mmol), 4-iodoanisole (0.23 g, 1.0 mmol), “catalyst” (0.10 mmol), and Cs2CO3 (0.65 g, 2.0 mmol) were placed in a carousel tube and dried in vacuo. To this was added anhydrous acetonitrile (5 mL) via syringe. The resulting mixture was heated with stirring at 90 °C for 24 h. After this time, the mixture was cooled and a 100 μL aliquot was withdrawn from the reaction mixture and added to 2 mL of a 10 mM stock solution of p-cymene (internal standard) in dichloromethane. The solution containing the reaction mixture and internal standard was filtered through Celite and subsequently analyzed by GC.

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ASSOCIATED CONTENT

* Supporting Information S

Text, figures, tables, and CIF files giving crystallographic data for 1c,f, 2c, 3c,e,f, 4b,b′,c′, 5b−d, 5b·(pyridine), 5c·(pyridine), 5d·(pyridine), 5d·(dmso), and 6b,c and synthetic procedure for 1f. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for C.E.W.: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by BP and the University of Leeds. REFERENCES

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dx.doi.org/10.1021/om500178e | Organometallics 2014, 33, 2027−2038