Silver and Palladium Complexes Containing Ditopic N-Heterocyclic

Article pubs.acs.org/Organometallics

Silver and Palladium Complexes Containing Ditopic N‑Heterocyclic Carbene−Thione Ligands Miriam Slivarichova, Eamonn Reading, Mairi F. Haddow, Hafiizah Othman, and Gareth R. Owen*,‡ The School of Chemistry, University of Bristol, Bristol, BS8 1TS, U.K. S Supporting Information *

ABSTRACT: The mixed donor N-heterocyclic carbene (NHC)/thione ligand precursors [1-(3-R-2H-imidazol-1-yl-2thione)methyl-3-R-2H-imidazol-2-ium]X, [HCSR]X (R = methyl, benzyl; X = Br, I), have been utilized to prepare a range of silver and palladium complexes. The coordination of CSR to silver(I) salts has been explored, providing dimeric complexes of the type [AgX(CSR)]2 (where R = methyl, benzyl; X = Br, I). Structural characterization of [AgX(CSBn)]2 revealed a bidentate coordination mode for the mixed donor ligand and dinuclear structures where the silver centers are bridged by two bromido centers. Palladium complexes bearing one or two CSR ligands have additionally been prepared either directly, utilizing [Pd(OAc)2] as precursor, or via transmetalation strategies. The dicationic complexes [Pd(CSR)2][X]2 and neutral complexes [PdX2(CSR)] (where R = methyl, benzyl; X = Br, I, PF6) have been synthesized and fully characterized. The CSR ligand in the aforementioned complexes does not undergo transformation of the NHC unit to a urea function, which had been found to occur in the previously reported copper complexes. Palladium complexes containing both NHC/thione and bis-phosphine ligands were also prepared. Complexes of the type [Pd(CSMe)(L2)][X]2 and [PdX(CSMe)(L2)][X] (where L2 = dppe, dppp; X = Br, OAc, I, PF6) were obtained. The presence of the bis-phosphine appears to destabilize the coordination of the NHC/thione ligand and as a consequence leads to its elimination from the complex.

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INTRODUCTION

RESULTS AND DISCUSSION Synthesis of Silver Complexes. The coordination chemistry of NHC ligands and their derivatives has been extensively investigated using a wide range of transition metal centers. In particular, there has been great interest in silver complexes primarily due to their widespread use as transmetalating agents.9,10 More recent applications have also included medicinal applications,11 for example as potential anticancer12 and antibacterial13 drugs. Following the successful preparation of copper(I) complexes 1−4 using copper(I) acetate as metal precursor, we moved our attention to the synthesis of the analogous silver complexes. The utilization of silver acetate as a precursor was first reported by Bertrand in 1997,14 and surprisingly this metal precursor has been used only a few times since.4a,15 This reagent has the benefit that it acts as a base that can deprotonate the imidazolium proton and is the metal to which the NHC coordinates. By using this protocol, the complexes [AgBr(CSMe)]2 (5) and [AgBr(CSBn)]2 (6) were readily prepared via addition of one equivalent of the appropriate imidazolium precursor to an acetonitrile solution containing silver(I) acetate (Scheme 2). The reaction mixtures were stirred for 18 h, after which time the resulting white solid was isolated by filtration and washed with acetonitrile to provide the products in moderate yields. Complexes 5 and 6 were characterized by NMR spectroscopy

N-Heterocyclic carbene (NHC) ligands have featured prominently in the chemical literature since the isolation of the first stable carbene and their popularization by Arduengo in 1991.1 Since this time, these strong and robust ligands have been comprehensively explored.2 It is without doubt that they are outstanding supporting ligands for an extensive range of homogeneous catalytic applications.3,4 Some investigations have focused on NHC ligands containing additional functional groups based on donors such as phosphorus, nitrogen, and oxygen.5 In the past few years, there has been a sharp rise of interest in sulfur-based functionalization (Chart 1).6−8 We recently added to this range of ligands by preparing a new family of mixed NHC/thione ligands.8 The monoimidazolium salt ligand precursors [HCSR]X (where R = methyl, benzyl; X = Br, I) were prepared by addition of one equivalent of sulfur to the corresponding methylene-bridged bis-imidazolium salts [(H)2CCR]X2 in the presence of a base. In the report we outlined the synthesis of a series of copper(I) complexes containing the CSR ligands (Scheme 1).8 Herein, we wish to extend the range of complexes to those based on silver and palladium centers. We additionally report the utilization of both copper and silver complexes as effective transmetalating agents in the synthesis of the first palladium complexes to feature the CSR ligands. © 2012 American Chemical Society

Received: July 3, 2012 Published: August 31, 2012 6595

dx.doi.org/10.1021/om300615e | Organometallics 2012, 31, 6595−6607

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Chart 1. Selected Sulfur-Functionalized N-Heterocyclic Carbene Ligands

Scheme 1. Synthesis of Methylene-Bridged Mixed Bidentate NHC/Sulfur Ligand Precursors and Their Corresponding Copper Complexes from Methylene-Bridged Bis-Imidazolium Salts

form 1-(3-R-2H-imidazol-1-yl-2-thione)-methyl-3-R-2H-imidazole-2-one (OSR), which had previously been observed with the copper complexes 1−4.8 Structural Characterization of Complexes 6 and 5·HBr. In order to further confirm our assignment of the above complexes, attempts were made to structurally characterize them. Single crystals of complex 6 were obtained by layering a cooled saturated DMF solution with diethyl ether. The crystal structure of 6 revealed that the complex forms a dimeric motif in the solid state involving two silver atoms, two CSBn ligands, and two bridging halides (Figure 1). The structure contains a four-membered AgBrAgBr ring resembling a parallelogram motif with the two supplementary angles Ag(1)−Br(1)−Ag(1′) and Br(1)−Ag(1)−Br(1′), 84.889(12)° and 95.111(12)° (∑ = 180°), respectively. The Br(1)−Ag(1)−Br(1′)−Ag(1′) torsion angle is 0°, confirming the planarity of the AgBrAgBr core. The corresponding Ag(1)−Br(1) and Ag(1)−Br(1′) distances are

Scheme 2. Syntheses of Dinuclear Silver Complexes [AgX(CSMe)]2 and [AgX(CSBn)]2

and mass spectrometry. While satisfactory elemental analysis was obtained for complex 6, the corresponding analysis of isolated samples of 5 always gave lower values than those expected. It is likely that the product was contaminated with insoluble silver impurities, which could not be separated from the desired complex. The 1H NMR spectra of 5 and 6, recorded in DMSO-d6, were consistent with the coordination of the new ligands to the silver centers, which was corroborated by the disappearance of the characteristic signals corresponding to the imidazolium protons. The methylene resonances, bridging the two heterocycles of the ligand, were located as broad singlet resonances at 6.34 and 6.38 ppm, respectively, a small upfield chemical shift relative to their respective ligand precursors, suggestive of a bidentate coordination mode (κ2-CS) for the CSR ligands. Following their isolation, complexes 5 and 6 were highly insoluble even in polar solvents such as DMSO. Such observations are typical of Ag-NHC complexes.9b,c We also attempted to prepare silver complexes via [Ag2O], a wellknown precursor for the preparation of silver-NHC compounds.16 In the case of [HCSMe]I, the reaction between Ag2O and this ligand precursor was limited, with only approximately 10% conversion to the target complex, [AgI(CSMe)]2 (7), even after extended periods in refluxing DCM.17,18 On the other hand, the reaction of [HCSBn]I with Ag2O proceeded to completion after 24 h. The new complex, [AgI(CSBn)]2 (8), was isolated via standard workup in 63% yield and fully characterized. In contrast to complexes 5 and 6, complex 8 was found to be more soluble. The silver complexes were found to be only moderately light sensitive in both solution and the solid state. As a precaution, the isolated solids were protected from light. They showed no indication of reactivity with oxygen to

Figure 1. Crystal structure of [AgBr(κ2-CS-CSBn)]2 (6). The hydrogen atoms have been removed for clarity. Selected bond distances (Å) and angles (deg): Ag(1)−Br(1) 2.5995(4), Ag(1)−Br(1′) 2.8645(4), Ag(1)−S(1) 2.7334(4), Ag(1)−C(6) 2.132(3), Ag(1)···Ag(1′) 3.6927(3), Br(1)···Br(1′) 4.0361(5), C(1)−S(1) 1.693(3), Ag′−Br− Ag 84.889(12), Br′−Ag−Br 95.111(12), S(1)−Ag(1)−C(6) 100.57(8), S(1)−Ag(1)−Br(1) 105.670(19), S(1)−Ag(1)−Br(1′) 103.808(19), C(6)−Ag(1)−Br(1) 139.12(8), C(6)−Ag(1)−Br(1′) 108.49(8), N(2)−C(5)−N(3) 112.5(2), C(6)−Ag(1)−S(1)−C(1) −2.78(13). 6596


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2.5995(4) and 2.8645(4) Å, respectively. A number of complexes featuring a AgBrAgBr core have previously been reported. Within the four-membered ring, the Ag(1)···Ag(1′) distance is 3.6927(3) Å [cf. ∑r(AgAg) = 2.90 Å].19 The silver centers in 6 adopt highly distorted tetrahedral geometries with angles ranging between 95.111(12)° and 139.12(8)°; the largest angle corresponds to C(6)−Ag(1)−Br(1), while the angle corresponding to the bidentate ligand, C(6)−Ag(1)− S(1), is 100.57(8)°. Attempts to obtain single crystals of complex 5 led to the isolation of an unexpected compound, [Ag(HCSMe)Br2]2 (5·HBr) (Figure 2). The structure was inconsistent with the

Figure 3. Comparison of the three coordination modes of CSR (a schematic representation of the ligand has been used, and the other substituents have been removed for clarity).

depending on the metal center, the nature of the R group, and the coligands (i.e., the halide). The previously reported copper complexes feature “Cu2(μ-S)2” cores while in 6 (and most likely 5, 7, and 8), the silver centers are bridged by the halide ligands, thereby possessing a “Ag2(μ-halide)2” core. There are a number of copper(I) and silver(I) halide complexes containing thione groups which exhibit μ-S coordination modes.22,23 At the same time there are also some related examples that feature “M2(μ-halide)2” (M = Cu, Ag) cores.23,24 The balance between the two structural motifs is currently not well understood.23,25 Synthesis of Palladium Complexes. Bis-ligand Complexes. Palladium complexes are the most widely studied group of metal-NHC complexes due in part to their outstanding performance as supporting ligands in cross-coupling catalysis.26 A large number of synthetic routes to such complexes have been achieved.27 The use of bidentate ligands has attracted much attention because of the increased stability upon coordination to the metal center. The first palladium complexes to contain bidentate NHC ligands were reported by Fehlhammer28 and Herrmann.29 It was of interest to prepare the analogous complexes containing our mixed NHC and thione ligands. Accordingly, palladium complexes featuring two CSR ligands were initially targeted via the metal precursor, palladium(II) acetate. The complexes [bis{1-(3-R-2H-imidazol-1-yl-2-thione)methyl-3-R-2H-imidazol-2-ylidene}palladium][X]2, [Pd(CSR)2][X]2 (R = Me, X = Br, I, PF6, 9−11 and R = Bn, X = Br, I, PF6, 12−14), were synthesized by addition of two equivalents of the corresponding ligand precursor [HCSR]X to palladium acetate in acetonitrile (Scheme 3). The reaction mixtures were stirred for prolonged periods of time. The extent of the reactions was monitored by taking aliquot samples of the mixtures, evaporating the solvent, and recording the 1H NMR spectra of the resulting residues. In all cases, the NMR spectra revealed the complete disappearance

Figure 2. Crystal structure of [Ag(κ1-S-HCSMe)(Br)2]2 (5·HBr). The hydrogen atoms, with the exception of H(6) and H(6′), have been removed for clarity. Ag(1)−Br(1) 2.6626(3), Ag(1)−Br(2) 2.6779(3), Ag(1)−Br(2′) 2.7868(3), Ag(1)−S(1) 2.6010(5), Ag(1)···Ag(1′) 3.2951(3), Br(2)···Br(2′) 4.3608(4), C(1)−S(1) 1.700(2), Ag(1)− Br(2)−Ag(1′) 74.138(8), Br(2)−Ag(1)−Br(2′) 105.861(8), S(1)− Ag(1)−Br(1) 107.070(14), S(1)−Ag(1)−Br(2′) 100.609(13), Br(1)− Ag(1)−Br(2) 116.563(9), Br(1)−Ag(1)−Br(2′) 111.695(9), N(2)− C(5)−N(3) 111.75(16).

spectroscopic data, and it was therefore concluded that this was not representative of the bulk material.20 It is likely that small quantities of 5·HBr were present in the mixture, and this species crystallizes in preference to 5. Complex 5·HBr might be formed by an anion exchange process as shown in eq 1. [HCSMe]Br + [Ag(OAc)] → [Ag{κ 1‐S ‐HCSMe}(OAc)Br] → 1/2[Ag{HCSMe}(OAc)2 ] + 1/2 5·HBr

Scheme 3. Synthesis of [Pd(CSR)2][X]2 [R = Me; X = Br (9), I (10), PF6 (11); R = Bn; X = Br (12), I (13), PF6 (14)]a

(1)

The structure of 5·HBr is different since the imidazoliumbased ligand [HCSMe] binds to the silver centers via the sulfur group only, in a monodentate fashion.21 Nevertheless, a fourmembered AgBrAgBr core that features a parallelogram motif [Ag(1)−Br(2)−Ag(1′) angles 74.138(8)° and Br(2)−Ag(1)− Br(2′) angles 105.861(8)°] is also observed (Figure 2). Here, the Br(2)−Ag(1)−Br(2′)−Ag(1′) angle is also 0°. The two bridging silver−bromine distances, Ag(1)−Br(2) and Ag(1)− Br(2′), are 2.6779(3) and 2.7868(3) Å, while the Ag(1)−Br(1) terminal distance is 2.6626(3) Å. In 5·HBr, the Ag(1)···Ag(1′) distance [3.2951(3) Å] is significantly shorter than that found in 6 [cf. 3.6927(3) Å]. This is consistent with a much smaller Ag(1)−Br(2)−Ag(1′) angle. The CSR ligands in the copper and silver complexes 1−8 exhibit various different coordination modes (Figure 3)

a

Hydrogen atoms HA and HB highlight the different chemical environments of the methylene protons. When R = Bn, the benzyl protons are also in different chemical environments; see Figure 4 and text for details. 6597


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Monoligand Complexes. Our initial attempts to prepare the corresponding monoligand complexes using the above deprotonation methodology proved unsuccessful. Reaction of one equivalent of the [HCSR]X salts with palladium(II) acetate gave significant conversion to the bis-ligand complexes and was therefore not a viable route to the complexes containing only one ligand. Accordingly, alternative synthetic routes were investigated.7i,31 As described above, silver NHC complexes have been found to be excellent transmetalating agents, providing one of the key methodologies for the syntheses of a wide range of late transition metal NHC complexes including those with palladium.9,10 Furthermore, many researchers have shown that it is possible to prepare the silver complexes in situ and react it directly with the palladium precursor in a one-pot procedure. This was indeed an effective method for the synthesis of our target monoligand complex, [PdBr2(CSMe)] (15). Reaction of an equimolar mixture of [HCSMe]Br and silver acetate, forming one-half equivalent of [AgBr(CSMe)]2 in situ, followed by addition of one equivalent of [PdBr2(COD)] resulted in >95% conversion to 15, as determined by 1H NMR spectroscopy (Scheme 4). Complex 15 was isolated via a standard workup as a yellow solid in moderate yield and was fully characterized by standard analytical and spectroscopic techniques.32

of the imidazolium signal. While the conversions to the [Pd(CSMe)2][X]2 complexes were essentially complete within 18 h, the conversions to the corresponding [Pd(CSBn)2][X]2 complexes proceeded more slowly. In the case of 12, the reaction mixtures went to completion after 72 h at room temperature (or within 5 h when the mixtures were heated to reflux). While the 1H NMR data confirmed complete disappearance of the ligand precursor, a minor side product (typically 4 h) some signs of decomposition were apparent. When the crystal structure of the new complex 20 was determined, it was surprising to find that only one arm of the CSMe ligand was coordinated. Furthermore, the crystal is more properly described as a solid solution since there was disorder in the nature of the fourth coordination site and the counterion. The crystal structure of 20 was consistent with the formula [Pd(Br) 0.13 (OAc) 0.87 (κ 1 -C-CS Me )(dppp)][Br] 0.55 [OAc] 0.45 , such that the total ratio of Br:AcO is 0.68:0.34 The two different coordination modes of CSMe, κ1-C-CSMe and κ2-CSCSMe, are discussed in more detail below. X-ray Crystal Structures of 17 and 20. An acetonitrile solution of 17, which was prepared by adding two equivalents of the ligand, was left standing overnight in a Young's NMR tube, providing crystals suitable for X-ray crystallography (Figure 10). Crystals of 20 were grown by slow diffusion of diethyl ether into a saturated acetonitrile solution (Figure 11). As described above, there was disorder in the location of the bromide and acetate groups in 20; both species are shown. 6601


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Table 3. Comparison of Selected Bond Lengths (Å) and Bond Angles (deg) for 17 and 20 bond length (Å)/angle (deg)

Figure 10. Crystal structure of [Pd(κ2-CS-CSMe)(dppe)][Br][OAc] (17). The counterions and hydrogen atoms have been removed for clarity.

Selected bond lengths and bond angles of the two complexes are presented in Table 3. In both structures the palladium center adopts distorted square planar geometries with cis angles in the ranges 82.03(6)−95.32(15)° and 88.23(5)−92.5(2)° for 17 and 20, respectively. The bite angle of the CSMe ligand in 17 [(C(6)− Pd(1)−S(1)] is 93.09(14)°. The C(1)−S(1) bond distance in 17 is 1.712(6) Å and is comparable to all the other structurally characterized palladium complexes described above [i.e., 1.707(2)−1.719(4) Å], whereas the corresponding distance in 20 is 1.676(2) Å. This is much closer to that of the free ligand {cf. 1.6753(14) Å for [HCSMe]Br} as expected since it is not coordinated to the metal center. The Pd(1)−C(6) bond distances, 2.051(5) and 2.0443(18) Å for 17 and 20, respectively, are significantly longer than the corresponding distances found in 9 [1.991(4) Å] and 15 [1.957(4) Å], where the NHC unit is trans to thione and bromido ligands, respectively. The distances Pd(1)−P(1) and Pd(1)−P(2) [2.2695(15) and 2.3037(15) Å] in 17 reflect the stronger trans influence of carbene unit over thione. In 20, the Pd(1)− P(1) distance is 2.2472(5) Å, while the Pd(1)−P(2) distance is

17

Pd(1)−S(1) Pd(1)−C(6) Pd(1)−P(1) Pd(1)−P(2) Pd(1)−Br(1)/Pd(1)−O(1)

2.3775(14) 2.051(5) 2.3037(15) 2.2695(15)

C(1)−S(1) S(1)−Pd(1)−C(6) P(1)−Pd(1)−P(2) P(1)−Pd(1)−S(1) P(2)−Pd(1)−Br(1)/P(2)−Pd(1)−O(1)

1.712(6) 93.09(14) 82.03(6) 89.87(5)

P(1)−Pd(1)−C(6) C(6)−Pd(1)−Br(1)/C(6)−Pd(1)−O(1)

95.32(15)

N(2)−C(5)−N(3) C(1)−S(1)−Pd(1)−C(6)

108.5(4) −21.8(3)

20 2.0443(18) 2.2472(5) 2.3183(5) 2.612(10)/ 2.0634(16) 1.676(2) 92.075(18) 92.5(2)/ 88.23(5) 90.31(5) 85.9(3)/ 88.97(7) 112.92(16)

2.3183(5) Å; note the latter distance will be affected by the disordered coordinating bromido/acetate ligands. Nevertheless, these data are consistent with the fact that the NHC has a stronger trans influence than phosphine, while the phosphine has stronger trans influence than sulfur and bromido/oxygen. There is a particularly short distance between the atoms H(5b) and O(4) (where the acetate ligand is the counterion), 2.210(5) Å. Such interactions might be responsible for determining the different coordination mode in this case. The P(1)−Pd(1)−P(2) angles in 17 and 20 are 82.03(6)° and 92.075(18),° respectively, and are consistent with the reported “natural bite angles” of the dppe and dppp ligands.50 The larger bite angle of the dppp ligand means that its phenyl substituents are pushed toward the metal center. These steric factors may well contribute to the fact that CSMe adopts a κ1-C coordination mode in 20 (vide inf ra). Interconversion between the κ2-CS and κ1-C Coordination Modes of CSMe. The two crystals structures show different coordination modes for the CSMe ligand. In 17 the ligand

Figure 11. Crystal structure of [PdX(κ1-C-CSMe)(dppp)][X] (X = Br/OAc) (20). Both [Pd(OAc)(κ1-C-CSMe)(dppp)][Br] (left) and [PdBr(κ1-CCSMe)(dppp)][OAc] (right) are shown. The counterions and hydrogen atoms have been removed for clarity. 6602


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adopts a κ2-CS coordination mode, whereas in 20 it adopts a κ1C coordination mode, suggesting potential interconversion between the two modes in solution (Figure 12). There appears

structures that are bridged via the halides, forming structures with four-membered cores. Palladium complexes of the type [Pd(κ2-CS-CSR)2][X]2 and [PdX2(κ2-CS-CSR)] (X = Br, I, PF6) have been synthesized in addition to the complexes [Pd(κ2-CS-CSMe)(L2)][X]2 and [PdX(κ1-C-CSMe)(L2)]X (X = Br/OAc, I, PF6; L2 = dppe, dppp). These complexes were synthesized either directly from palladium(II) acetate or via transmetalation from their corresponding silver and copper NHC complexes. The CSR ligands exhibit strong robust κ2-CS coordination modes in the non-phosphine complexes (9−15) and are stable under aerobic conditions. On the other hand, the presence of bis-phosphine ligands appears to promote some demetalation of the CSR ligands by altering the coordination mode from κ2-CS to κ1-C. None of the silver or palladium complexes reported herein exhibit the same reactivity with oxygen that leads to the transformation of the carbene fragment into a urea functionality as previously reported in the case of the copper complexes (1−4). The strength and coordination properties of the CSR ligands are currently being investigated at other transition metal centers.

Figure 12. Different coordination modes of CSMe. In [Pd(κ2-CSCSMe)(dppe)][X]2 (left) the methylene protons are restricted due to the seven-membered ring, whereas in [PdX(κ1-C-CSMe)(dppe)][X] (right) these protons can exchange positions rapidly on the NMR time scale and appear as a singlet resonance in the spectrum.

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to be a number of different factors determining which coordination mode is observed including solvent, temperature, and anion interactions. As described above, the 1H NMR spectrum of 17, in DMSO-d6, reveals only one chemical environment for the methylene protons in solution. However, when the 1H NMR spectrum of 17 was repeated in MeCN-d3, the signal appeared as the expected pair of doublets at 5.68 and 5.74 ppm (1JHH = 13.6 Hz). This behavior suggests that the coordination mode of the ligand might be disrupted in the highly polar solvent DMSO (Figure 12). Furthermore, the 1H NMR spectrum of 20 in DMSO-d6 revealed that the bridging methylene protons had two well-separated signals (5.89 and 6.64 ppm), which suggests distinct chemical environments for these protons. This observation may be the result of shortrange CH···X interactions as highlighted in the crystal structure discussion above, providing different chemical environments for the methylene protons. Previous investigations focusing on palladium complexes bearing related neutral sulfur-functionalized NHC ligands have revealed a low propensity for sulfur coordination (hemilability) to the metal center.6,7 In fact, in all other examples featuring neutral sulfur donors reported to date, the presence of a coordinating anion will preferentially substitute the sulfur function.51 This is not the case for the non-phosphinecontaining complexes reported herein even when the sulfur donor is trans to a NHC. Note that the CSR ligands in complexes 9, 10, 12, and 13, where this is indeed the case, all exhibit [Pd(κ2-CS-CSR)2][X]2 motifs rather than [PdX2(κ1-CCSR)2] or [PdX(κ1-C-CSR)(κ2-CS-CSR)][X] conformations. Interestingly, when one of the CSR ligands is replaced by a bis-phosphine such as dppe or dppp, the propensity for thione coordination over coordinating anion is reduced and there is evidence to suggest that the ligand can readily interchange between the two coordination modes. Furthermore, this change in coordination mode appears to affect the stability of the NHC complexes and can lead to its elimination from the coordination sphere. This is clear in the side reactions involving complex 17 and 18 {leading to the formation of [PdBr2(dppe)] and [PdI2(dppe)], respectively}.

EXPERIMENTAL SECTION

General Remarks. All syntheses were carried out under a nitrogen atmosphere using standard Schlenk techniques. All chemicals were used as received. The solvents MeCN, DCM, hexane, and Et2O were dried using a Grubbs’ alumina system and were kept in Young’s ampules under N2 over molecular sieves (4 Ǻ ). 1H NMR spectra were recorded at room temperature on a JEOL Lambda 300 spectrometer operating at 300 MHz and a JEOL ECP 400 spectrometer operating at 400 MHz. 13C{1H} NMR spectra were recorded at room temperature on a JEOL ECP 400 spectrometer operating at 101 MHz or a Varian VNMR 500 spectrometer operating at 125 MHz (1H). Where given, assignments of proton and carbon nuclei were determined by a combination of HMQC, HMBC, and COSY NMR experiments (see Figure 13). Electrospray mass spectra (ESI+) were recorded on a

Figure 13. Numbering scheme for the 1H and 13C{1H} NMR spectroscopy assignments of the silver and palladium complexes. Bruker Daltonics Apex 4e 7.0T FT-MS mass spectrometer. Infrared spectra were recorded in the region 4000−650 cm−1 on a PerkinElmer Spectrum 100 FT-IR spectrometer (solid state, powder film). Elemental analyses were performed by the microanalytical laboratory, School of Chemistry, University of Bristol. Experimental details for complexes 10, 11, 13, 14, 18, and 19 have been provided in the Supporting Information. Synthesis of Bromido{1-(3-methyl-2H-imidazol-1-yl-2thione)methyl-3-methyl-2H-imidazol-2-ylidene}silver(I), [AgBr(CSMe)]2 (5). Silver(I) acetate (60.0 mg, 0.36 mmol) was suspended in acetonitrile (40 mL) under a nitrogen atmosphere, and [1-(3-methyl-2H-imidazol-1-yl-2-thione)methyl-3-methyl-2H-imidazol-2-ium] bromide (104.0 mg, 2.40 mmol) was added. The mixture was stirred for 18 h at RT. The solvent was then filtered and the product washed twice with MeCN (2 × 10 mL). Yield: 71.0 mg, 0.18 mmol, 50%. NMR δ ppm: 1H (DMSO-d6, 400 MHz), 3.53 (s, 3H, NCH3), 3.75 (s, 3H, NCH3), 6.34 (s, 2H, NCH2N), 7.31 (d, 3JHH = 2.3 Hz, 1H, CHCH), 7.47 (d, 3JHH = 1.7 Hz, 1H, CHCH), 7.60 (d, 3 JHH = 2.3 Hz, 1H, CHCH), 7.79 (d, 3JHH = 1.7 Hz, 1H, CHCH). MS (ESI)+: 209 [(HCSMe)]+, 479 [Ag(HCSMe)][(Br)2]+. Synthesis of Bromido{1-(3-benzyl-2H-imidazol-1-yl-2thione)methyl-3-benzyl-2H-imidazol-2-ylidene}silver(I), [AgBr-

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CONCLUSIONS In summary, we have reported the synthesis of a number of new silver and palladium complexes featuring the recently reported CSR ligands. The silver complexes form dimeric 6603


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(CSBn)]2 (6). Silver(I) acetate (30.0 mg, 0.18 mmol) was suspended in acetonitrile (10 mL) under a nitrogen atmosphere, and [1-(3-benzyl2H-imidazol-1-yl-2-thione)methyl-3-benzyl-2H-imidazol-2-ium] bromide (79.1 mg, 0.18 mmol) was added. The mixture was stirred for 18 h in RT. The solvent was reduced, diethyl ether (10 mL) added, and the resulting solution filtered. Yield: 40.0 mg, 0.07 mmol, 40%. NMR δ ppm: 1H (DMSO-d6, 400 MHz), 5.23 (s, 2H, CH2), 5.28 (s, 2H, CH2), 6.38 (s, 2H, NCH2N), 7.28−7.34 (m, 10H, Ph), 7.36 (d, 3 JHH = 2.3 Hz, 1H, CHCH), 7.54 (d, 3JHH = 1.8 Hz, 1H, CHCH), 7.64 (d, 3JHH = 2.3 Hz, 1H, CHCH), 7.83 (d, 3JHH = 1.8 Hz, 1H, CHCH). MS (ESI)+: 361 [HCSBn]+, 975 [{Ag(CSBn)}2 + K]+. Anal. Calcd for C21H20AgBrN4S: C 46.01, H 3.68, N, 10.22. Found: C 45.91, H 3.64, N 9.80. Synthesis of Iodido{1-(3-benzyl-2H-imidazol-1-yl-2-thione)methyl-3-benzyl-2H-imidazol-2-ylidene}silver(I), [AgI(CSBn)]2 (8). Silver(I) oxide (53 mg, 0.23 mmol) was suspended in dichloromethane (100 mL) under a nitrogen atmosphere and [1-(3benzyl-2H-imidazol-1-yl-2-thione)methyl-3-benzyl-2H-imidazol-2ium] iodide (220 mg, 0.45 mmol) was added. The mixture was heated to 40 °C for 24 h and filtered. The volume of the filtrate was reduced to ca. 25 mL, hexane (40 mL) added, and the resulting solid isolated by filtration. Total yield: 168 mg, 0.28 mmol, 63%. NMR δ ppm: 1H (DMSO-d6, 400 MHz), 5.23 [s, 2H, NCH2Ph (C2)], 5.30 [s, 2H, NCH2Ph (C7)], 6.41 [s, 2H, NCH2N (C5)], 7.26−7.35 [m, 11H, 2 × Ph + CHCH (C3)], 7.51 [d, 3JHH = 1.9 Hz, 1H, CHCH (C9)], 7.59 [d, 3 JHH = 2.3 Hz, 1H, CHCH (C4)], 7.82 [d, 3JHH = 1.9 Hz, 1H, CHCH (C8)]; 13C{1H} (DMSO-d6, 100 MHz), 50.4 [NCH2Ph (C2)], 54.3 [NCH2Ph (C7)], 59.5 [NCH2N (C5)], 117.9 [CHCH (C4)], 119.2 [CHCH (C3)], 122.0 [2 × CHCH (C8 + C9)], {127.8, 127.9, 128.0 (2 overlapping signals), 128.5 and 128.6 [o-, p-, and m-Ph]}, 136.1 [i-Ph (thione)] and 136.8 [i-Ph (NHC)], 160.7 [CS (C1)], 185.8 [NCN (C6)]. MS (ESI)+: 361 [CSBn]+, 927 [AgI(CSBn)2 − S]+. Anal. Calcd for C21H20AgIN4S: C 42.37, H 3.39, N, 9.41, S, 5.39. Found: C 42.23, H 3.67, N 9.17, S, 5.34. Synthesis of Bis{1-(3-methyl-2H-imidazol-1-yl-2-thione)methyl-3-methyl-2H-imidazol-2-ylidene}palladium(II) Dibromide (9). Palladium(II) acetate (100.0 mg, 0.45 mmol) was suspended in acetonitrile (30 mL) under a nitrogen atmosphere, and [1-(3-methyl-2H-imidazol-1-yl-2-thione)methyl-3-methyl-2H-imidazol-2-ium] bromide (257.0 mg, 0.90 mmol) was added. The mixture was stirred for 72 h. The solution was transferred, solvent reduced, and diethyl ether (20 mL) added. The resulting solution was then filtered to give a yellow-colored product. Yield: 267.0 mg, 0.39 mmol, 87%. NMR δ ppm: 1H (DMSO-d6, 400 MHz), 3.21 [s, 3H, NCH3 (C2)], 3.64 [s, 3H, NCH3 (C7)], 6.65 [d, 2JHH = 14.3 Hz, 1H, NCH2N (C5)], 6.87 [d, 2JHH = 14.3 Hz, 1H, NCH2N (C5)], 7.48 [d, 3JHH = 1.8 Hz, 1H, CHCH (C3)], 7.58 [d, 3JHH = 2.0 Hz, 1H, CHCH (C8)], 7.99 [2 overlapping signals, 2H, CHCH (C4 and C9)]; 13C{1H} (DMSO-d6, 100 MHz), 35.2 [NCH3 (C7)], 37.1 [NCH3 (C2)], 59.2 [NCH2N (C5)], 120.0 [CHCH (C9)], 121.6 [CHCH (C4)], 122.2 [CHCH (C8)], 126.1 [CHCH (C3)], 151.2 [NCN (C6)], 159.1 [CS (C1)]. IR: 3394, 3089, 2160, 1568, 1467, 1398, 1201, 1071, 675 cm−1. MS: MS (ESI)+: 603 [{Pd(CSMe)2} + Br]+. Anal. Calcd after recrystallization from DCM/hexane for C18H24Br2N8PdS2·CH2Cl2: C 29.72, H 3.41, N, 14.60. Found: C 29.83, H 3.80, N 14.12. Synthesis of Bis{1-(3-benzyl-2H-imidazol-1-yl-2-thione)methyl-3-benzyl-2H-imidazol-2-ylidene}palladium(II) Dibromide (12). Palladium(II) acetate (150.0 mg, 0.67 mmol) was suspended in acetonitrile (60 mL) under a nitrogen atmosphere, and [1-(3-benzyl-2H-imidazol-1-yl-2-thione)methyl-3-benzyl-2H-imidazol-2-ium] bromide (589.3 mg, 1.34 mmol) was added. The mixture was refluxed for 5 h. The solution was then transferred, solvent reduced to minimum, and Et2O (30 mL) added. The resulting mixture was then filtered to give a white/yellow-colored product. Yield: 242.0 mg, 0.25 mmol, 37%. NMR δ ppm: 1H (DMSO-d6, 400 MHz), 4.99 [d, 2JHH = 16.0 Hz, 1H, NCH2Ph (C2)], 5.21 [d, 2JHH = 16.0 Hz, 1H, NCH2Ph (C2)], 5.26 [d, 2JHH = 15.4 Hz, 1H, NCH2Ph (C7)], 5.34 [d, 2JHH = 15.4 Hz, 1H, NCH2Ph (C7)], 6.66 [d, 2JHH = 14.0 Hz, 1H, NCH2N (C5)], 6.72 [d, 2JHH = 14.0 Hz, 1H, NCH2N (C5)], 6.98−7.00 [m, 2H, o-Ph (thione)], 7.20−7.22 [m, 2H, o-Ph

(NHC)], 7.27−7.31 [m, 3H, m/p-Ph (thione)], 7.34−7.39 [two overlapping signals m, 3H, m/p-Ph (NHC) + unresolved d, 1H, CHCH (C3)], 7.71 [d, 3JHH = 2.2 Hz, 1H, CHCH (C9)], 7.77 [d, 3JHH = 1.8 Hz, 1H, CHCH (C4)], 8.01 [d, 3JHH = 2.2 Hz, 1H, CHCH (C8)]; 13C{1H} (DMSO-d6, 100 MHz), 50.8 [NCH2Ph (C7)], 53.3 [NCH2Ph (C2)], 59.6 [NCH2N (C5)], 120.7 [CHCH (C8)], 121.4 [CHCH (C9)], 122.1 [CHCH (C4)], 125.6 [CHCH (C3)], 126.2 [oPh (thione)], 127.5 [o-Ph (NHC)], 127.8 [p-Ph (thione)], 128.2 [p-Ph (NHC)], 128.7 [m-Ph (thione)], 128.8 [m-Ph (NHC)], 134.8 [i-Ph (NHC)], 135.1 [i-Ph (thione)], 151.2 [NCN (C6)], 159.1 [CS (C1)]. IR: 3080, 1568, 1453, 1357, 1228, 1166, 1075, 731, 692 cm−1. MS (ESI)+: 413 [Pd(CSBn)2]2+ (90%), 907 [{PdBr(CSBn)2}]+ (100%). Anal. Calcd for C42H40Br2N8PdS2: C 51.10, H 4.08, N, 11.35. Found: C 51.41, H 4.40, N 11.12. Synthesis of Bis-bromido{1-(3-methyl-2H-imidazol-1-yl-2thione)methyl-3-methyl-2H-imidazol-2-ylidene}palladium(II) (15). Silver acetate (150.0 mg, 0.90 mmol) was suspended in DCM (60 mL), and [1-(3-methyl-2H-imidazol-1-yl-2-thione)methyl-3-methyl-2H-imidazol-2-ium] bromide added (259.7 mg, 0.90 mmol). The mixture was stirred for 2 h, and then CODPdBr2 (337.0 mg, 0.90 mmol) added. The resulting mixture was stirred for a further 2 h. Then the solvent was removed under reduced pressure and redissolved in MeCN (60 mL), and Et2O (25 mL) added. The solvent was then filtered off to give a yellow-colored product. Yield: 224 mg, 0.47 mmol, 53%. NMR δ ppm: 1H (DMSO-d6, 400 MHz), 3.54 [s, 3H, NCH3 (C7)], 3.89 [s, 3H, NCH3 (C2)], 6.39 [br, 1H, NCH2N (C5)], 6.70 [br, 1H, NCH2N (C5)], 7.40 [br d, unresolved, 1H, CHCH (C3)], 7.42 [br d, unresolved, 1H, CHCH (C8)], 7.61 [2 overlapping signals, 2H, CHCH (C4 and C9)]; 13C{1H} (DMSO-d6, 100 MHz), 35.0 [NCH3 (C7)], 38.6 [NCH3 (C2)], 59.2 [NCH2N (C5)], 119.5 [CHCH (C9)], 120.7 [CHCH (C4)], 121.6 [CHCH (C8)], 125.9 [CHCH (C3)], 152.9 [NCN (C6)], 155.7 [br, CS (C1)]. IR: 3014, 1571, 1465, 1399, 1224, 1197, 1069, 740, 675 cm−1. MS (ESI)+: 394 [PdBr(CSMe)]+ (25%), 472 [PdBr2(CSMe)]+ (100%). Anal. Calcd for C9H12Br2N4PdS: C 22.78, H 2.55, N, 11.81. Found: C 22.97, H 2.68, N 11.20. Synthesis of Bis-bromido{1-(3-benzyl-2H-imidazol-1-yl-2thione)methyl-3-benzyl-2H-imidazol-2-ylidene}palladium(II) (16). Bromido{1-(3-benzyl-2H-imidazol-1-yl-2-thione)methyl-3-benzyl-2H-imidazol-2-ylidene]copper(I) dimer8 (336.6 mg, 0.67 mmol) was dissolved in MeCN (20 mL), and the solution added slowly (dropwise over a period of 10 min) into a solution of palladium dibromo(1,5-cyclooctadiene) (250 mg, 0.67 mmol) in MeCN (20 mL). The mixture was left stirring for 2 h and the solvent reduced. DCM (10 mL) was then added, and the solvent filtered to yield a yellow-colored product. Yield: 381.5 mg, 0.61 mmol, 91%. NMR δ ppm: 1H (DMSO-d6, 500 MHz), 5.28−5.39 (br d, 3H, NCH2Ph), 6.03 (br s, 1H, NCH2Ph), 6.51 [br s, 1H, NCH2N (C5)], 6.86 [br s, 1H, NCH2N (C5)], 7.24−7.26 [m, 2H, o-Ph (thione)], 7.33−7.38 [two overlapping signals, 2H, o-Ph (NHC) + unresolved d, 1H, CHCH (C3)], 7.39−7.44 (m, 6H, m/p-Ph), 7.59 [d, 3JHH = 2.0 Hz, 1H, CHCH (C9)], 7.71 [d, unresolved, 1H, CHCH (C4)], 7.74 [d, 3JHH = 2.0 Hz, 1H, CHCH (C8)]; 13C{1H} (DMSO-d6, 125 MHz), 50.5 [NCH2Ph (C7)], 53.6 [NCH2Ph (C2)], 59.6 [NCH2N (C5)], 120.4 [CHCH (C8)], 120.8 [CHCH (C9)], 121.3 [CHCH (C4)], 125.7 [CHCH (C3)], 127.5 (Ph), 127.8 (Ph), 128.0 (Ph), 128.1 (Ph), 128.5 (Ph), 128.7 (Ph), 134.8 (i-Ph), 136.1 (i-Ph), 152.7 [NCN (C6)], 156.9 [br, CS (C1)]. IR: 3118, 1566, 1415, 1357, 1225, 1187, 1073, 733, 694 cm−1. MS (ESI)+: 465 [Pd(CSBn)]+ (100%), 547 [PdBr(CSBn)]+ (75%). Anal. Calcd for C21H20Br2N4PdS: C 40.18, H 3.37, N, 8.93. Found: C 40.02, H 3.39, N 8.86. Synthesis of {1-(3-Methyl-2H-imidazol-1-yl-2-thione)methyl3-methyl-2H-imidazol-2-ylidene}(1,2-bis(diphenylphosphino)ethane)}palladium(II) Acetate Bromide (17). Palladium(II) acetate (50.0 mg, 0.22 mmol) was suspended in acetonitrile, and dppe (1,2-bis(diphenylphosphino)ethane) (88.8 mg, 0.22 mmol) added.52 After 10 min stirring [1-(3-methyl-2H-imidazol-1-yl-2thione)methyl-3-methyl-2H-imidazol-2-ium] bromide (64.3 mg, 0.22 mmol) was added. The mixture was stirred at RT for 24 h. The solution was then filtered, the solvent reduced, and Et2O added. The 6604


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mixture was filtered again to give a yellow-colored product. Yield: 172.7 mg, 0.20 mmol, 92%. NMR δ ppm: 1H (DMSO-d6, 500 MHz), 2.19−2.33 (m, 1H, PCH2CH2P), 2.81−3.12 (m, 3H, PCH2CH2P), 3.34 (s, 3H, NCH3), 3.49 (s, 3H, NCH3), 5.95 (s, 2H, NCH2N), 7.13 (br s, 1H, CHCH), 7.26 (br s, 1H, CHCH), 7.33−7.37 (m, 3H, Ph + CHCH), 7.41−7.65 (m, 15H, Ph + CHCH), 7.73−7.77 (m, 1H, Ph + CHCH), 7.89−7.97 (m, 3H, Ph + CHCH); 13C{1H} (DMSO-d6, 125 MHz), 21.4 (CH2CH2), 29.5 (CH2CH2), 34.6 (CH3), 37.6 (CH3), 58.5 (NCH2N), 116.8 (CHCH), 120.1 (CHCH), 122.7 (CHCH), 125.5 (CHCH), 127.6−133.4 (Ph), 160.0 (NCN), 168.7 (CS), 173.1 (CO); 31P{1H} (DMSO-d6, 120 MHz), 52.04 (d, 2JPP = 13.0 Hz, PPh2), 59.67 (br d, PPh2). IR: 3388, 3074, 1568, 1452, 1409, 1229, 1166, 1076, 691 cm−1. MS (ESI)+: 793 [{Pd(dppe)(CSMe)}Br]+ (100%), 711 [Pd(dppe)(CSMe)]+ (17%), 584 [Pd(dppe)Br]+ (48%), 563 [Pd(dppe)(OAc)]+ (30%). Anal. Calcd for C37H39BrN4O2P2PdS: C 52.16, H 4.61, N, 6.58. Found: C 52.10, H 5.09, N 6.37. NMR δ ppm: 1H (MeCN-d3, 300 MHz), 2.08−2.29 (m, 1H, PCH2CH2), 2.66−3.22 (m, 3H, PCH2CH2), 3.28 (s, 3H, NCH3), 3.57 (s, 3H, NCH3), 5.68 (d, 2JHH = 13.6 Hz, 1H, NCH2N), 5.74 (d, 2JHH = 13.6 Hz, 1H, NCH2N), 6.69 (d, 3JHH = 2.57 Hz, 1H, CHCH), 6.69 (d, 3JHH = 2.57 Hz, 1H, CHCH, 7.11−7.16 (m, 1H, Ph + CHCH), 7.25−7.33 (m, 2H, Ph + CHCH), 7.38−7.44 (m, 4H, Ph + CHCH), 7.49−7.68 (m, 13H, Ph + CHCH), 7.87−8.01 (m, 4H, Ph + CHCH); 31P{1H} (MeCN-d3, 121 MHz), 51.66 (d, 2JPP = 10.9 Hz, PPh2), 59.68 (d, 2JPP = 10.9 Hz, PPh2). Synthesis of Bromido{1-(3-methyl-2H-imidazol-1-yl-2thione)methyl-3-methyl-2H-imidazol-2-ylidene}(1,2-bis(diphenylphosphino)propane)}palladium(II) Acetate (20). Palladium(II) acetate (100.0 mg, 0.45 mmol) and 1,3-bis(diphenylphosphino)propane (dppp) (185.6 mg, 0.45 mmol) were suspended in acetonitrile (30 mL)53 and [1-(3-methyl-2H-imidazol-1yl-2-thione)methyl-3-methyl-2H-imidazol-2-ium] bromide (128.7 mg, 0.45 mmol) added. The mixture was stirred at RT for 24 h. The solution was then filtered, solvent reduced, Et2O (20 mL) added, and the mixture filtered to yield an light-yellow-colored product. Yield: 201.0 mg, 0.23 mmol, 52%. NMR δ ppm: 1H (DMSO-d6, 400 MHz), 1.49 (br s, 4H, PCH2CH2CH2P), 2.91 (br s, 2H, PCH2CH2CH2P), 3.55 (s, 3H, NCH3), 3.85 (s, 3H, NCH3), 5.89 (d, 2JHH = 13.7 Hz, 1H, NCH2N), 6.64 (d, 2JHH = 13.4 Hz, 1H, NCH2N), 7.22−7.38 (m, 9H, Ph + CHCH), 7.44−7.56 (m, 6H, Ph + CHCH), 7.68−7.77 (m, 7H, Ph + CHCH), 8.22 (2 overlapping d, 2H, Ph + CHCH); 13C{1H} (DMSO-d6, 125 MHz), 22.0 (PCH2CH2CH2P), 24.0 (PCH2CH2CH2P), 24.8 (PCH2CH2CH2P), 34.6 (CH3), 37.9 (CH3), 58.4 (NCH2N), 117.0 (CHCH), 119.5 (CHCH), 122.0 (CHCH), 124.4 (CHCH), 126.1−134.6 (20C, Ph), 161.4 (br NCN), 169.3 (br CS), 173.6 (CO); 31P{1H} (DMSO-d6, 120 MHz), −1.54 (unresolved d), 10.82 (unresolved d). IR: 3131, 2984, 2176, 1567, 1432, 1397, 1376, 1224, 1199, 1149, 1097, 975, 835, 798, 744, 693 cm−1. MS (ESI)+: m/z 807 [PdBr(CSMe)(dppp)]+ (100%), 363 [Pd(CSMe−S)Br]+ (55%). Anal. Calcd for C36H38Br2N4P2PdS: C 48.75, H 4.32, N, 6.32. Found: C 49.06; H 4.48; N 5.79.54 NMR δ ppm: 1H (MeCN-d3, 400 MHz), 1.42−1.55 (m, 1H, PCH2CH2CH2P), 1.70 (br, 1H, PCH2CH2CH2P), 2.59−2.76 (m, 3H, PCH2CH2CH2P), 3.46 (s, 3H, NCH3), 3.49 (s, 1H, PCH2CH2CH2P), 3.76 (s, 3H, NCH3), 5.29 (d, 2JHH = 13.8 Hz, 1H, NCH2N), 6.18 (d, 2JHH = 13.8 Hz, 1H, NCH2N), 6.76 (d, 3JHH = 2.57 Hz, 1H, CHCH), 6.95 (m, 1H, Ph), 7.00 (d, 3JHH = 2.57 Hz, 1H, CHCH), 7.10−7.19 (m, 5H, Ph), 7.22−7.29 (m, 2H, Ph + CHCH), 7.33−7.45 (m, 8H, Ph + CHCH) 7.59−7.75 (m, 5H, Ph), 8.02−8.09 (m, 2H, Ph); 31P{1H} (MeCN-d3, 121 MHz), −6.06 (d, 2JPP = 35.3 Hz, PPh2), 7.98 (d, 2JPP = 35.3 Hz, PPh2).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. ‡ G.R.O. is a Royal Society Dorothy Hodgkin Research Fellow.

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ACKNOWLEDGMENTS The authors thank the Royal Society for funding a Royal Society Dorothy Hodgkin Research Fellowship for G.R.O. REFERENCES

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S Supporting Information *

Further experimental details for complexes 10, 11, 13, 14, 18, and 19 together with crystallographic details and structural parameters for the structurally characterized complexes are available free of charge via the Internet at http://pubs.acs.org. 6605


Organometallics

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Danopoulos, A. A.; Winston, S.; Kleinhenz, S.; Eastham, G. Dalton Trans. 2000, 4499. (c) Zinner, S. C.; Herrmann, W. A.; Kühn, F. E. J. Organomet. Chem. 2008, 693, 1543. (11) (a) Marcs, L.; Albrecht, M. Chem. Soc. Rev. 2010, 39, 1903. (b) Kascatan-Nebioglua, A.; Panznera, M. J.; Tessiera, C. A.; Cannonb, C. L.; Youngsa, W. J. Coord. Chem. Rev. 2007, 251, 884. (12) (a) Teyssot, M.-L.; Jarrousse, A.-S.; Manin, M.; Chevry, A.; Roche, S.; Norre, F.; Beaudoin, C.; Morel, L.; Boyer, D.; Mahioue, R.; Gautier, A. Dalton Trans. 2009, 6894. (b) Monteiro, D. C. F.; Phillips, R. M.; Crossley, B. D.; Fielden, J.; Willans, C. E. Dalton Trans. 2012, 41, 3720. (13) Roland, S.; Jolivalt, C.; Cresteil, T.; Eloy, L.; Bouhours, P.; Hequet, A.; Mansuy, V.; Vanucci, C.; Paris, J.-M. Chem.Eur. J. 2011, 17, 1442. (14) Guerret, O.; Solé, S.; Gornitzka, H.; Teichert, M.; Trinquier, G.; Bertrand, G. J. Am. Chem. Soc. 1997, 119, 6668. (15) The related compound Ag(O2CCF3) has also been utilized as silver precursor; see: Maishal, T. K.; Basset, J.-M.; Boualleg, M.; Copéret, C.; Veyre, L.; Thieuleux, C. Dalton Trans. 2009, 6956. (16) Hayes, J. M.; Viciano, M.; Peris, E.; Ujaque, G.; Lledós, A. Organometallics 2007, 26, 6170. (17) Selected data for 7: NMR δ ppm: 1H (DMSO-d6, 300 MHz), 3.47 (s, 3H, CH3), 3.79 (s, 3H, CH3), 6.37 (s, 2H, NCH2N), 7.25 (d, 3 JHH = 2.4 Hz, 1H, CHCH), 7.47 (d, 3JHH = 1.7 Hz, 1H, CHCH), 7.55 (d, 3JHH = 2.4 Hz, 1H, CHCH), 7.79 (d, 3JHH = 1.7 Hz, 1H, CHCH). MS (ESI)+: 209 [(CSMe + H)]+ (100%), 775 [Ag(CSMe)2][(I)2]+ (55%). (18) Complex 6 could also be prepared from Ag2O and its respective ligand salt precursor, showing >90% conversion to the product after 24 h in refluxing DCM. Unfortunately, the product could not be separated from the silver impurities due to its poor solubility. Accordingly, analytically pure solids could not be obtained via this methodology. (19) Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.; Echeverría, J.; Cremades, E.; Barragán, F.; Alvarez, S. Dalton Trans. 2008, 2832. (20) The sample from which the crystal structure of 5·HBr was obtained contained both both 5·HBr and 5 in an approximate 1:1 ratio, as determined by NMR spectroscopy. Selected data for 5·HBr: 1 H NMR (DMSO-d6): 3.47 (s, 3H, CH3), 3.87 (s, 3H, CH3), 6.25 (s, 2H, NCH2N), 7.24 (br, 1H, CHCH), 7.45 (br, 1H, CHCH), 7.71 (br, 1H, CHCH), 7.90 (br, 1H, CHCH), 9.39 [br, 1H, NC(H)N]. (21) For examples of complexes bearing pendent imidazolium ligands see: Zamora, M. T.; Ferguson, M. J.; McDonald, R.; Cowie, M. Dalton Trans. 2009, 7269 , and references therein. (22) (a) Bowmaker, G. A.; Hanna, J. V.; Pakawatchai, C.; Skelton, B. W.; Thanyasirikul, Y.; White, A. H. Inorg. Chem. 2009, 48, 350 , and references therein. (b) Lobana, T. S.; Sultana, R.; Castineiras, A.; Butcher, R. J. Inorg. Chim. Acta 2009, 362, 5265. (c) Creighton, J. R.; Gardiner, D. J.; Gorvin, A. C.; Gutteridge, C.; Jackson, A. R. W.; Raper, E. S.; Sherwood, P. M. A. Inorg. Chim. Acta 1985, 103, 195. (d) Mentzafos, D.; Terzis, A.; Karagiannidis, P.; Aslanidis, P. Acta Crystallogr. 1989, C45, 54. (23) (a) Lobana, T. S.; Khanna, S.; Hundal, G.; Liaw, B.-J.; Liu, C. W. Polyhedron 2008, 27, 2251. (b) Lobana, T. S.; Khanna, S.; Hundal, G.; Kaur, P.; Thakur, B.; Attri, S.; Butcher, R. J. Polyhedron 2009, 28, 1583. (c) Lobana, T. S.; Khanna, S.; Hundal, G.; Kaur, P.; Thakur, B.; Attri, S.; Butcher, R. J. Polyhedron 2009, 28, 1103. (24) (a) Bowmaker, G. A.; Chaichit, N.; Pakawatchai, C.; Skelton, B. W.; White, A. H. Can. J. Chem. 2009, 87, 161. (b) Bonamartini, A. C.; Gasparri, M. G. F.; Belicchi, M. F.; Nardelli, M. Acta Crystallogr. C 1987, 43, 407. (c) Cox, P. J.; Aslanidis, P.; Karagiannidis, P.; Hadjikakou, S. Inorg. Chim. Acta 2000, 310, 268. (25) (a) Hadjikakou, S. K.; Antoniadis, C. D.; Aslanidis, P.; Cox, P. J.; Tsipis, A. C. Eur. J. Inorg. Chem. 2005, 1442. (b) Lobana, T. S.; Sharma, R.; Hundal, G.; Butcher, R. J. Inorg. Chem. 2006, 45, 9402. (c) Lobana, T. S.; Khanna, S.; Butcher, R. J.; Hunter, A. D.; Zeller, M Inorg. Chem. 2007, 46, 5826. (26) Nolan, S. P.; Fortman, G. C. Chem. Soc. Rev. 2011, 40, 5151.

(27) Díez-González, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612. (28) Fehlhammer, W. P.; Bliss, T.; Kernbach, U.; Brüdgam, I. J. Organomet. Chem. 1995, 490, 149. (29) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 2371. (30) Selected examples of methylene-bridged NHC ligands showing an AB pattern in the 1H NMR spectrum: (a) Taige, M. A.; Zeller, A.; Ahrens, S.; Goutal, S.; Herdtweck, E.; Strassner, T. J. Organomet. Chem. 2007, 692, 1519. (b) Heckenroth, M.; Neels, A.; Stoeckli-Evans, H.; Albrecht, M. Inorg. Chem. Acta. 2006, 359, 1929. (31) One strategy involved the synthesis of the zwitterion complexes [Pd(HCSMe)Br3] via addition of the [HCSMe]Br to [PdBr2(COD)] and monitoring their subsequent reactivity with various bases. Unfortunately, the reactions did not provide the desired products, and mixtures were obtained. Selected data for [Pd(HCSMe)Br3]: [PdBr2(COD)] (30 mg, 0.08 mmol) was suspended in DCM (30 mL), and [1-(3-methyl-2H-imidazol-1-yl-2-thione)methyl-3-methyl2H-imidazol-2-ium] bromide (23.1 mg, 0.08 mmol) was added. The mixture was stirred for 2 h, and the solvent was then filtered off to obtain a brown-orange-colored product. NMR δ ppm: 1H (DMSO-d6, 400 MHz), 3.73 (s, 3H, NCH3), 3.90 (s, 3H, NCH3), 6.57 (s, 2H, NCH2N), 7.76 (s, 2H, CHCH), 7.96 (s, 2H, CHCH), 9.48 (s, 1H, NCHN). MS (ESI)+: 209 (HCSMe)+ 100%, 394 [PdBr(HCSMe)]+ (26%), 474 [PdBr2(HCSMe)]+ (12%), 764 [Pd(HCSMe)2 + 3Br]+ (14%). (32) The corresponding complex [PdCl2(CSMe)] was also prepared via a transmetalation reaction involving PdCl2(COD) and copper complex 1. Selected data for [PdCl2(CSMe)]: NMR δ ppm: 1H (DMSO-d6, 400 MHz), 3.54 (s, 3H, NCH3), 3.91 (s, 3H, NCH3), 6.52 (d, 2JHH = 13.7 Hz, 1H, NCH2N), 6.87 (d, 2JHH = 14.3 Hz, 1H, NCH2N), 7.47 (br s, 2H, CHCH), 7.61 (br s, 2H, CHCH). MS (ESI)+: 350 [PdCl(CSMe)]+ (100%), 634 [{Pd(CSMe)}2 + H]+ (55%). (33) (a) Venkatachalam, G.; Heckenroth, M.; Neels, A.; Albrecht, M. Helv. Chim. Acta 2009, 92, 1034. (b) Furst, M. R. L.; Cazin, C. S. J. Chem. Commun. 2010, 46, 6924. (34) These complexes are the first structurally characterized examples where a carbene moiety and a thiourea unit are coordinated to the same transition metal center. (35) Fey, N.; Orpen, A. G.; Harvey, J. N. Coord. Chem. Rev. 2009, 253, 704. (36) For an example where a phosphorus ligand has been substituted by a NHC ligand see: Douthwaite, R. E.; Haüssinger, D.; Green, M. L. H.; Silcock, P. J.; Gomes, P. T.; Martins, A. M.; Danopoulos, A. A. Organometallics 1999, 18, 4584. (37) For an examples where a NHC ligand has been substituted by a bis-phosphine ligand see: Doyle, M. J.; Lappert, M. F.; Pye, P. L.; Terreos, P. J. Chem. Soc., Dalton Trans. 1984, 2355. (38) (a) Herrmann, W. A.; Böhm, V. P. W.; Gstöttmayr, C. W. K.; Grosche, M.; Reisinger, C.-P.; Weskamp, T. J. Organomet. Chem. 2001, 617−618, 616. (b) Chan, K.-T.; Tsai, Y.-H.; Lin, W.-S.; Wu, J.-R.; Chen, S.-J.; Liao, F.-X.; Hu, C.-H.; Lee, H. M. Organometallics 2010, 29, 463. (39) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953. (40) The isolated solid was sometimes contaminated with small quantities (generally

Silver and Palladium Complexes Containing Ditopic N-Heterocyclic

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