Heterobimetallic Coinage Metal‐Ruthenium Complexes Supported by Anionic N‐Heterocyclic Carbenes

Abstract The lithium complexes [(WCA‐NHC)Li(toluene)] of anionic N‐heterocyclic carbenes with a weakly coordinating borate moiety (WCA‐NHC, WCA=B(C6F5)3, NHC=IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were used for the preparation of silver(I) or copper(I) WCA‐NHC complexes. While the reactions in THF with AgCl or CuCl afforded anionic mono‐ and dicarbene complexes with solvated lithium counterions [Li(THF)n]+ (n=3, 4), the reactions in toluene proceeded with elimination of LiCl and formation of the neutral phosphine and arene complexes [(WCA‐NHC)M(PPh3)] and [(WCA‐NHC)M(η 2‐toluene)] (M=Ag, Cu). The latter were used for the preparation of chlorido‐ and iodido‐bridged heterobimetallic Ag/Ru and Cu/Ru complexes [(WCA‐NHC)M(μ‐X)2Ru(PPh3)(η 6‐p‐cymene)] (M=Ag, Cu, X=Cl; M=Ag, X=I). Surprisingly, these complexes resisted the elimination of CuCl, AgCl, or AgI, precluding WCA‐NHC transmetalation.

Schlosser base combinations of sodium or potassium bis-(trimethylsilyl)amides and n-BuLi leaves room for further optimisation. [18] Therefore, we turned our attention to WCA-NHC complexes of the lighter coinage metals, since silver(I) and as well as copper(I) NHC complexes have become well-established and widely used carbene transfer reagents. [19] As a result, we present, among other things, the synthesis and characterization of Ag(I) and Cu(I) complexes such as [(WCA-NHC)M(η 2 -toluene)] (5, M = Ag, Cu) and their attempted use for the preparation of ruthenium(II) WCA-NHC complexes ( Figure 1). To our surprise, however, it was found that the anticipated transmetalation reactions do not proceed with the elimination and precipitation of silver(I) or copper(I) halides but can be used for the controlled assembly of heterobimetallic Ag/Ru and Cu/Ru complexes.

Results and Discussion
We first studied the reaction of the lithium carbene complex 1·toluene with one equivalent of silver(I) and copper(I) chloride in THF solution, which afforded the ionic complexes 2·THF and 3 after filtration through Celite® and recrystallization from THF/ n-hexane solution (Scheme 1). Regardless of the 1 : 1 stoichiometry, the reaction with AgCl proceeded with formation of the anionic dicarbene silver complex with a linear CÀ AgÀ C angle of 178.08 (10) (2,4,6-trimethylphenyl)imidazolin-2-ylidene). The lithium counterion is solvated by four THF molecules and resides in a slightly distorted tetrahedral environment ( Figure 2). It is interesting to note that the 13 C NMR spectrum exhibits a nicely resolved doublet of doublets at 183.9 ppm, with coupling constants 1 J C,Ag of 195 and 224 Hz for C carbene bonding to the 107 Ag and 109 Ag nuclei. These values fall in the range observed for other homoleptic silver(I)-dicarbene complexes, [19a,22] for example, δ = 183.6 ppm, 1 J C,Ag = 188/209 Hz for [(IMes) 2 Ag][CF 3 SO 3 ]. [21a] The copper complex 3 crystallized with two independent molecular anions [(WCA-NHC)CuCl] À in the asymmetric unit, together with one [Li(THF) 4 ] + and another [Li(THF) 3 ] + unit that interacts with one of the chlorido ligands ( Figure 3). The copper atoms have linear coordination spheres with C1À Cu1À Cl1 = Scheme 1. Synthesis of silver(I) and copper(I) WCA-NHC complexes in THF solution.

Figure 2.
Molecular structure of 2·THF with thermal displacement parameters drawn at 50 % probability. All hydrogen atoms and one molecule THF are omitted for clarity. Pertinent structural data are assembled in Table 1. Figure 3. Molecular structure of 3 with thermal displacement parameters drawn at 50 % probability level. The two independent molecules in the asymmetric unit are shown. All hydrogen atoms are omitted for clarity. Pertinent structural data are assembled in Table 1. 175.59(9)°and C58À Cu-Cl2 = 179.18(10)°, and the coppercarbon bond lengths of 1.877(3) Å (Cu1À C1) and 1.879(3) Å (Cu2À C58) are in the same but slightly shorter range compared to the neutral analogues [(IDipp)CuCl] [23] and [(IMes)CuCl]. [24] Related copper(I) chloride complexes bearing anionic malonate or enolate functionalized anionic N-heterocyclic carbenes have also been reported. [25] It should be noted that we also obtained crystals of the complex [Li(THF) 4 ][{(WCA-NHC)Cu} 2 (μ-Cl)], in which the chlorido ligand is bridging two (WCA-NHC)Cu moieties; see Figure S11 in the Supporting Information for a presentation of the crystal structure. These findings prompted us to question our synthetic strategy, since carrying out the reactions in THF solution prevented the formation and precipitation of lithium chloride by solvation and resulted in ionic complexes of various, somewhat unreliable compositions. Moreover, transmetalation reactions consistently failed with 2 and 3, and for instance, the reaction of 3 with [(η 6 -p-cymene)RuCl 2 ] 2 afforded a bimetallic salt consisting of the non-interacting complex ions [{(η 6 -p-cymene)Ru} 2 (μ-Cl) 3 ] + and [(WCA-NHC) CuCl] -(see the Supporting Information, Figure S10). Therefore, we turned our attention to WCA-NHC transfer reactions in toluene solution, and initial experiments were carried out with chloro(triphenylphosphine)silver(I) and -copper(I). Thus, suspending 1 in toluene and addition of [(Ph 3 P)MCl] afforded the complexes [(WCA-NHC)M(PPh 3 )] (4) as colorless (4 a: M = Ag) and yellow (4 b: M = Cu) crystalline solids in 69 % and 63 % yield, respectively, after stirring for ca. 2 h, filtration through Celite® and recrystallization from dichloromethane/n-hexane or THF/n-hexane solutions (Scheme 2). The NMR spectroscopic characteristics are similar to those previously established for the corresponding gold(I) complex [(WCA-NHC)Au(PPh 3 )], however, the 31 P NMR resonances are found at significantly higher field, i.e., at 18.3 ppm (4 a) and 8.6 ppm (4 b) in comparison with 40.5 ppm reported for the gold congener. [7] For the silver complex 4 a, this signal is observed as a doublet of doublets with 1 J P,Ag = 462/533 Hz for phosphorus coupling with the 107 Ag/ 109 Ag nuclei. In contrast, the 13 C NMR signal for the carbene carbon atom in 4 a could not be resolved, whereas 4 b gave rise to doublet at 177.2 ppm with 2 J C,P = 69 Hz.
The complexes 4 were further characterized by single-crystal X-ray diffraction analysis; they are isotypic and crystallize in the monoclinic space group P2 1 /n. The molecular structure of the silver complex 4 a is shown in Figure 4, whereas the molecular structure of the copper complex 4 b is presented in the Supporting Information ( Figure S4). Selected bond lengths and angles are summarized in  6 ]. [26] Encouraged by the successful chloride substitution and carbene transfer from (WCA-NHC)Li·toluene (1·toluene) onto [(Ph 3 P)MCl] (M = Ag, Cu), the preparation of phosphine-free WCA-NHC silver(I) and copper(I) complexes was attempted.
Hence, the reaction of 1·toluene with silver(I) trifluoromethanesulfonate (AgOTf) afforded the toluene solvate [(WCA-NHC) Ag(toluene)] (5 a) in 82 % yield as a white crystalline solid after stirring for 10 min, filtration through Celite® and recrystallization from toluene/dichloromethane solution (Scheme 2). Longer reaction times produced significantly lower yields, which could tentatively be ascribed to ligand exchange and formation of ionic dicarbene-silver complexes as side products. [27] The corresponding copper complex 5 b could be isolated from the reaction of 1·toluene with CuCl in toluene solution and was isolated as a colorless crystalline solid in 91 % yield after stirring for 16 h, filtration through Celite® and recrystallization from toluene/diethyl ether solution. Gratifyingly, the 13 C NMR spectra of both complexes displayed the expected low-field signals for the carbene carbon atoms at 182.1 ppm (5 a, THF-d 8 ) and 175.1 ppm (5 b, CD 2 Cl 2 ). For the silver complex 5 a, this signal could be resolved as a doublet of doublets with 1 J C,Ag = 300/ 347 Hz for coupling with the 107 Ag/ 109 Ag nuclei. These coupling constants are significantly larger than usually found for silver(I) monocarbene complexes of the type ([(NHC)AgX], for example, [19a,22] , revealing a strong carbon-silver interaction in the solvated [(WCA-NHC) Ag] complex fragment of 5 a.
The molecular structures of both complexes 5 could be established by X-ray diffraction analysis (see Figure 5 for 5 a and Figure S6 in the Supporting Information for 5 b); they are again isotypic and crystallize in the monoclinic space group P2 1 /c. The metal-carbon bond lengths of 2.0839(14) Å (Ag1À C1 in 5 a) and 1.9016(17) Å (Cu1À C1 in 5 b) are virtually identical with those found in 4 a and 4 b ( Table 1). In addition, both structures confirm the presence of metal-arene interactions in the solid state, and a toluene ligand is additionally bound to the metal atoms in a η 2 -fashion with Ag1À C46/C47 = 2.3382(17)/ 2.4056(17) Å and Cu1À C46/C47 = 2.2013(19)/2.1220(19) Å. This interaction is best classified as charge-transfer bonding, with η 2coordination typically observed for π-complexes of the late transition metals. [28] Such interactions have been observed only for a small number of cationic silver(I) and copper(I) NHC complexes, which requires the presence of weakly coordinating counterions. Accordingly, the complex [(ITr)Ag(η 2 -C 6 H 5 F)][BAr F 4 ] was isolated from a fluorobenzene solution of [(ITr)Ag(OTf)] in the presence of Na[BAr F 4 ] (ITr = 1,3-bis(triphenylmethyl)-imidazolin-2-ylidene, Ar F = 3,5-bis(trifluoromethyl)phenyl). With silvercarbon distances of 2.115(3), 2.381(4) and 2.435(4) Å, this complex exhibits similar, but slightly longer AgÀ C bond lengths compared to 5 a. [29] Likewise, the copper(I) complexes [(IDipp) Cu(arene)][SbF 6 ] (arene = η 2 -benzene, η 3 -C 6 Me 5 , η 3 -toluene, η 3m-xylene) were isolated by reaction of [(IDipp)CuBr] with AgSbF 6 in CH 2 Cl 2 /arene solution and feature η 2 -or η 3 -coordination modes in the solid state. [30] Similar metal-arene interactions were also observed for coinage metal complexes of Nheterocyclic silylene ligands. [31] Originally, we envisaged that the silver(I) and copper(I) toluene complexes 5 might be ideally suited for WCA-NHC transfer to transition metals, with the ultimate goal to prepare ruthenium(II) WCA-NHC complexes for application in olefin metathesis. Such systems could serve as anionic analogues of recently developed and commercialized cationic ruthenium olefin metathesis catalysts bearing ammonium tags. [32] It was found, however, that these complexes resisted the elimination of silver(I) and copper(I) halides, presumably because of their exceptionally strong metal-carbene bonds. Accordingly, the reactions 5 a and 5 b with [(η 6 -p-cymene)RuCl 2 (PPh 3 )] in toluene solution afforded the heterobimetallic complexes 6 as orangered crystalline solids in 69 % (6 a) and 97 % (6 b) yield after filtration through Celite® and recrystallization from dichloromethane/n-hexane solutions (Scheme 2). The NMR spectra show the presence of both the (WCA-NHC)M (M = Ag, Cu) and the (η 6 -p-cymene)Ru units, with the carbene carbon atoms giving rise to a doublet of doublets at 185.0 ppm with 1 J C,Ag = 275/317 Hz and to a singlet at 180.9 ppm in the 13 C NMR spectra of 6 a and 6 b, respectively.

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The molecular structure of 7 was determined by X-ray diffraction analysis, confirming the formation of a heterobimetallic Ag/Ru complex with bridging iodido ligands (Figure 7). In contrast to 6 a, the four-membered Ag(μ-I) 2 Ru ring is close to planarity with a dihedral angle of 8.89(2)°between the AgI 2 and RuI 2 planes, which results in a significantly longer Ag1À Ru1 distance of 4.1057(5) Å. In contrast to 6 a and 6 b, the WCA-NHC ligand adopts a horizontal conformation and is almost perfectly aligned with the AgI 2 plane. However, the NMR spectra indicate fast rotation of the WCA-NHC ligand in solution on the NMR timescale, in agreement with time-averaged C s -symmetry. It should be noted that, to the best of our knowledge, no other crystal structure of an iodido-bridged Ag/Ru complex has been reported to date.

Conclusion
With the synthesis and characterization of the complexes [(WCA-NHC)M(η 2 -toluene)] (5, M = Ag, Cu), we have again successfully exploited this class of anionic N-heterocyclic carbenes with a weakly coordinating borate moiety (WCA-NHC) for the generation of neutral analogues of otherwise cationic transition metal complexes for applications in nonpolar solvents. [7][8][9] Accordingly, the high solubility of the complexes 5 in toluene and other aromatic hydrocarbons provides easy access to these and potentially numerous other silver(I) and copper(I) π-arene complexes. Attempts to use the complexes 5 as WCA-NHC transfer reagents were unsuccessful in the case of ruthenium(II); however, the observed transfer of the intact (WCA-NHC)M units enabled the isolation of the chlorido-and iodido-bridged heterobimetallic Ag/Ru and Cu/Ru complexes 6 and 7. While the latter complexes and related systems could find application in cooperative heterobimetallic catalysis, [35] in  (3). Other pertinent structural data are assembled in Table 1.   (4)

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Full Paper doi.org/10.1002/chem.202102553 view of the enormous importance of (NHC)Ag and (NHC)Cu in catalysis, [36] it appears particularly promising to further exploit the potential of the complexes 5 to serve as homogeneous catalysts, especially in nonpolar solvents.

Experimental Section
All operations with air-and moisture-sensitive compounds were performed in a glove box under a dry argon atmosphere (MBraun 200B) or on a vacuum line using Schlenk techniques. All solvents were distilled from Na/benzophenone or CaH 2 , degassed prior to use and stored over molecular sieves (4 Å). [(WCA-IDipp) Li(toluene)] [8] and [(η 6 -p-cymene)RuCl 2 (PPh 3 )] [37] were prepared according to literature procedures. Full details of all analytical methods and experimental procedures can be found in the Supporting Information.