Coinage metal complexes of the carbenic tautomer of Nitron

Journal of Organometallic Chemistry xxx (2016) 1e10

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Coinage metal complexes of the carbenic tautomer of Nitron Charlotte Thie, Sandra Hitzel, Lars Wallbaum, Clemens Bruhn, Ulrich Siemeling* Institute of Chemistry, University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 December 2015 Received in revised form 16 March 2016 Accepted 20 March 2016 Available online xxx Dedicated to Prof. Heinrich Lang on the occasion of his 60th birthday.

The reaction of the inexpensive analytical reagent Nitron (2) with coinage metal salts MX in dipolar aprotic solvents cleanly afforded complexes of its carbenic tautomer 1,4-diphenyl-3-phenylamino-1,2,4triazol-5-ylidene (2′) under mild conditions. With a stoichiometric ratio of 1:1, complexes of the type [MX(2′)] were obtained (M ¼ Cu, X ¼ Cl, Br, I; M ¼ Ag, X ¼ Cl, Br; M ¼ Au, X ¼ Cl). A stoichiometric ratio of 2:1 furnished complexes of the types [MX(2′)2] or [M(2′)2]X (M, X as above and M ¼ Ag, X ¼ [BF4], OTf). The structures of the 1:1 complexes [MX(2′)] (M ¼ Cu, X ¼ Cl, I; M ¼ Au, X ¼ Cl) and the 2:1 complexes [CuX(2′)2] (X ¼ Cl, Br, I) and [Ag(2′)2](OTf) were determined by single-crystal X-ray diffraction. The reaction of Nitron (2) with elemental selenium afforded the selenourea derivative 2′Se, which was also structurally characterized. © 2016 Elsevier B.V. All rights reserved.

Keywords: N-heterocyclic carbenes X-ray crystal structure Copper Silver Gold Selenium

1. Introduction The report of the first N-heterocyclic carbene (NHC) by Arduengo and co-workers in 1991 [1] triggered the development of such singlet carbenes from laboratory curiosities to key compounds for manifold applications in catalysis, materials science and medicinal chemistry [2]. A particularly prominent NHC is the 1,2,4-triazol-5ylidene derivative 1 (Fig. 1) introduced by Enders and co-workers in 1995 [3], since this compound soon proved to be extremely useful in organocatalysis and therefore was the first NHC to become commercially available [4]. We recently demonstrated that the mesoionic compound Nitron (2), which was introduced as an analytical reagent for gravimetric anion analysis in 1905 and has been commercially available since then at a comfortably low price [5], exhibits a reactivity in solution which is typical of an NHC [6]. This can be ascribed to the presence of the carbenic tautomer 2′ (Fig. 1), which is akin to the “Enders carbene” 1 both structurally and electronically, as is reflected by their essentially identical Tolman electronic parameter (TEP) values of ca. 2057 cm1 [6]. Nitron (2) belongs to the class of conjugated mesomeric betaines (CMBs) [7]. Related work on other five-membered CMBs in

equilibrium with their tautomeric NHCs was later published by the
sar and Lavigne [8], Braunstein and Danopoulos [9], groups of Ce and Schmidt [10], who also provided the first review of this burgeoning field [11]. In addition to these experimental studies, theoretical investigations addressing the relative stability of mes
szi and oionic and NHC tautomers have been performed by Nyula Streubel [12] and by Ramsden and Oziminski [13]. In our previous work with Nitron (2) we have observed the facile formation of RhI, RuII and RuIII complexes of its 1,2,4-triazol-5ylidene tautomer 2′ [6,14]. We have a general interest in pharmaceutically relevant carbene chemistry [15]. Consequently, in view of the current interest in 1,2,4-triazol-5-ylidene coinage metal complexes, which is due to their recently uncovered potential in the field of metallopharmaceuticals [16], we decided to extend our studies in terms of the coordination behavior of Nitron (2) towards the monovalent coinage metals. Although the field of coinage metal NHC complexes has been expanding rapidly [17], coinage metal complexes of the iconic “Enders carbene” 1 have not been reported to date. However, we note that Marichev et al. have investigated the copper(I) complexes [CuCl(2′)] and [CuX(2′)2] (X ¼ Cl, I) in terms of their antimicrobial activity [18].

* Corresponding author. E-mail address: [email protected] (U. Siemeling). http://dx.doi.org/10.1016/j.jorganchem.2016.03.023 0022-328X/© 2016 Elsevier B.V. All rights reserved.

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C. Thie et al. / Journal of Organometallic Chemistry xxx (2016) 1e10

Fig. 1. The 1,2,4-triazol-5-ylidene 1 (with numbering scheme for the heterocyclic ring), the conventional structure of Nitron (2) and the structure of its NHC-type tautomer 2′.

2. Results and discussion 2.1. Synthesis and spectroscopic characterization As a first step we have prepared the selenourea derivative 2′Se from Nitron (2) and elemental selenium, in order to gain additional information beyond the TEP value concerning the electronic similarity of 2′ and the “Enders carbene” 1. Ganter and co-workers recently established 77Se NMR spectroscopy of these compounds as a novel and highly convenient method for probing the electronic ligand properties of the corresponding carbenes, with the chemical shift value d(77Se) [19] and the coupling constant 1JCSe respectively being related to their p-accepting and s-donating abilities [20]. This work has been extended by Nolan and co-workers by utilizing a broad range of NHCs, confirming that d(77Se) can be deployed with confidence to quantify their p-accepting capability [21]. Large p-acceptor strengths are indicated by large downfield shifts of the 77 Se NMR signal. In the case of selenourea compounds derived from standard five-membered ring NHCs the chemical shifts lie between ca. 20 and 200 ppm [21a]. The selenourea derivative of the “Enders carbene”, 1Se, although long known [22], has apparently not been investigated by 77Se NMR spectroscopy so far. We have determined 77Se chemical shift values of 110 and 106 ppm for 1Se and 2′Se, respectively, in CDCl3. The similarity of these two values indicates almost identical p-accepting abilities of 1 and 2′. In conjunction with the essentially identical TEP values of 1 and 2′, this also points to almost identical s-donating abilities of both carbenes, since the TEP value provides a measure of the net electron donor capacity. This is corroborated by the identical values of the 1 JCH coupling constant (229 Hz) determined for the triazolium CeH unit of the corresponding protonated carbenes ([1H]ClO4 [23] and [2′H][BF4] [24] in CD3CN were used for this purpose). Ganter [20] and Kunz [25] have pointed out that such 1JCH coupling constants correlate inversely with the s-donor strengths of the respective carbenes, since the s-donor strength decreases with increasing scharacter of the s-orbital at the divalent carbon atom and large 1JCH coupling constants reflect high s-character of the carbon valence orbital involved in the CeH bond [26]. Not surprisingly, a very good correlation between the 1JCH and 1JCSe coupling constant values determined for protonated NHCs and the respective NHC-derived selenourea compounds has been observed [20]. We note that the 1 JCH coupling constant of Nitron (2) is significantly smaller than that of 2′H (219 vs. 229 Hz in CD3CN), indicating that the corresponding NHC bearing an anionic PhN substituent in the 3-position is a much better s-donor than 2′, which contains an electroneutral PhHN substituent. We now turn our attention to the coinage metal complexes of the carbenic tautomer of Nitron (2′) investigated in this study, which are schematically shown in Chart 1. Copper(I) and silver(I) complexes were obtained in good to excellent yields simply by reacting Nitron (2) with the corresponding metal halide in a stoichiometric ratio of 1:1 and 2:1, respectively. Dipolar aprotic solvents were used for these reactions. Analytically pure compounds of the compositions MX(2′) (named 1:1 complexes in the following) and MX(2′)2 (named 2:1 complexes in the following)

were thus obtained with CuCl, CuBr, CuI, AgCl, and AgBr. The compounds CuCl(2′)n (n ¼ 1, 2) and CuI(2′)2 had already been briefly described by Marichev et al. [18]. The use of AgI gave lower yields and did not afford analytically pure compounds so far. This can be ascribed to the tendency of silver iodide NHC complexes to aggregate to oligomers or polymers containing iodoargentate units of the type [AgmIn](n  m) [27,28]. The compounds obtained with AgI will not be considered any further here, since more work is needed to identify the species formed. The 1:1 and 2:1 gold(I) complexes of composition AuCl(2′) and AuCl(2′)2 were obtained from the reaction of Nitron (2) with the soluble complex [AuCl(THT)] [29] (THT ¼ tetrahydrothiophene) in the corresponding stoichiometric ratio. Some general trends concerning the solubilities of these coinage metal complexes can be made out. The solubility of 2:1 complexes is generally lower than that of 1:1 complexes. The solubilities are highest for M ¼ Cu and lowest for M ¼ Ag. At ambient temperature, dichloromethaneeacetonitrile mixtures turned out to work particularly well as solvent. The solubility in dimethyl sulfoxide, although significantly lower, was still sufficient in each case for obtaining satisfactory 13C NMR spectra in DMSO-d6. The complexes are hardly soluble in tetrahydrofuran and acetone, which were therefore not suitable for NMR purposes. In view of the rather poor solubilities of the complexes obtained with the silver halides, [Ag(2′)2][BF4] and [Ag(2′)2](OTf) were synthesized by reacting Nitron (2) with silver(I) tetrafluoroborate and silver(I) triflate, respectively. Indeed, these products containing non-nucleophilic anions exhibit comparatively high solubilities, particularly in dichloromethane. The metal-coordinated Ccarbene atom gives rise to a diagnostic low-field signal located at ca. 180 ppm in the 13C NMR spectrum of each complex in DMSO-d6. The exception is the 1:1 gold(I) complex [AuCl(2′)], whose carbenic 13C NMR signal is observed at 170.3 ppm. In the case of the silver complexes [Ag(2′)2]X (X ¼ [BF4], OTf), this signal is observed as a pair of doublets with nicely resolved couplings between carbon and silver of 1 13 109 J( C, Ag) z 220 Hz and 1J(13C,107Ag) z 190 Hz, indicating that the silverecarbon bond is not labile [30,31]. No such coupling was observed for the complexes obtained with AgCl and AgBr, pointing to a rapid exchange of the carbene ligands on the NMR time scale in these cases, as is frequently described for silver(I) complexes of five-membered ring NHCs [30,32]. In the case of complexes obtained from AgX and an NHC in a stoichiometric ratio of 1:1 such fluxional behavior is known to give rise to mixtures of [AgX(NHC)] and [Ag(NHC)2][AgX2] in solution [33]. The 1H and 13C NMR spectra of the 2:1 complexes obtained with AgCl and AgBr clearly show signals due to uncoordinated Nitron (2) (see Figs. S25eS28 of the Supplementary Information), indicating partial dissociation in dimethyl sulfoxide solution. In the case of the 2:1 complexes obtained with the copper halides CuX, two 13C NMR signals are observed at ca. 180 and 183 ppm in DMSO-d6 (see Figs. S16, S20 and S24 of the Supplementary Information), which are both distinct from the Ccarbene signal of the corresponding 1:1 complex [CuX(2′)] located at ca. 177 pm. However, with CDCl3 as solvent, only a single signal due to the Ccarbene atom is observed for the 2:1 complexes of CuBr and CuI (the solubility of the CuCl complex in CDCl3 was not sufficient for 13C NMR spectroscopy). Likewise, the 1H NMR spectra of the 2:1 complexes recorded in DMSO-d6 exhibit two signals due to the NH unit of 2′, whereas in CDCl3 only a single NH signal is observed. Signals due to uncoordinated Nitron (2) are essentially absent in each case. This behavior is compatible with a partial dissociation in the very polar solvent dimethyl sulfoxide (dielectric constant ε ¼ 47) [34] which concerns predominantly the halide ligand of [CuX(2′)2] and does not occur to a noticeable extent in the much less polar solvent chloroform (ε ¼ 5) [34].

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2.2. Crystal structures The crystal structures of the complexes [CuX(2′)] (X ¼ Cl, Br), [CuX(2′)2] (X ¼ Cl, Br, I), [Ag(2′)2](OTf), [Ag(2′)2][BF4] and [AuCl(2′)] as well as that of the selenourea derivative 2′Se were determined by single-crystal X-ray diffraction studies. Pertinent metrical data are collected in Table 1. Not surprisingly, in several cases a polar solvent molecule (acetone, acetonitrile or THF) is present per formula unit, which forms a hydrogen bond with a PhNH unit. The structure of [CuCl(2′)] (Fig. 2) is very similar to that reported for other 1,2,4-triazol-5-ylidene complexes of the type [CuCl(NHC)] [35] and exhibits no unusual features. The iodo homologue (Fig. 3) adopts a dimeric iodo-bridged structure in the solid state with a central diamond-shaped Cu2I2 moiety, which exhibits crystallographically imposed inversion symmetry (molecular point group Ci). This structural motif has been described before for [CuI(IAd)] and related imidazol-2-ylidene complexes [36]. The only structurally characterized 1:1 complex of CuI with a 1,2,4-triazol-5-ylidene ligand known to date contains a meta-phenylene-bridged bis(triazol-5-ylidene), whose two coordinated CuI units are aggregated to a kite-shaped Cu2I2 moiety [37]. The 2:1 complexes of 2′ with CuX (X ¼ Cl, Br, I; Figs. 4e6) contain tricoordinate CuI in a trigonal-planar coordination environment (sum of angles 360 ). The differences in the CueX bond lengths follow the trend expected from the different radii of Cl, Br and I [38]. Since complexes of the type [CuX(NHC)2] have not been reported to date with 1,2,4-triazol-5-ylidene ligands, the three structures are best compared with those of structurally characterized imidazol-2ylidene analogues [39]. Surprisingly, only a single example each is known for X ¼ Cl [39b], Br [39a], and I [39c], the respective NHC ligand being 1-[1-(2,6-dimethylphenylimino)ethyl]-3mesitylimidazol-2-ylidene, 1-allyl-3-mesitylimidazol-2-ylidene, and 1-isopropyl-3-methylimidazol-2-ylidene. In comparison with the corresponding complex of the type [CuX(2′)2], the imidazol-2ylidene complex exhibits a significantly longer CueX distance (Dd z 0.05, 0.15 and 0.15 Å for X ¼ Cl, Br and I, respectively) and a much wider CeCueC angle (D: z 12.5, 15.6 and 18.3 for X ¼ Cl, Br and I, respectively), while the CueC distances are indistinguishable within experimental error (ca. 1.93 Å). These structural data point to a less bulky nature of 2′ in comparison with the three imidazol-2ylidene ligands mentioned, which seems reasonable in view of the “flat” phenyl groups exclusively present in 2′. While the complexes obtained from Nitron (2) and the copper(I) halides could be structurally characterized in all cases except one, the analogous silver(I) complexes did not furnish crystals suitable for a single-crystal X-ray diffraction study, despite many attempts. We have been able, however, to determine the crystal structures of [Ag(2′)2](OTf) (Fig. 7) and [Ag(2′)2][BF4] (see Fig. S35 of the

Supplementary Information). In both cases, the cation has crystallographically imposed inversion symmetry and the bond parameters of the AgI atom are very similar to those reported for [Ag(IMes)2](OTf) (AgeC 2.067(4) and 2.078(4) Å, CeAgeC 176.3(2) ) [31b], which is the only structurally characterized complex of the type [Ag(NHC)2](OTf) (NHC ¼ imidazol-2-ylidene) known to date whose NHC ligands are not tethered together to form a chelate ligand. The last coinage metal complex to be considered here is [AuCl(2′)] (Fig. 8). Its structure is very similar to those reported for other 1,2,4-triazol-5-ylidene complexes of the type [AuCl(NHC)] [32,40], exhibiting essentially linear dicoordinate AuI atoms with AueC and AueCl distances of ca. 1.98 and 2.28 Å, respectively. As a sideline, we note that neighboring [AuCl(2′)] molecules are arranged as pairs in an antiparallel fashion in the crystal and exhibit an intermolecular Au/Au contact (3.57 Å) compatible with a weak aurophilic interaction [41]. We finally come to the structure of 2′Se (Fig. 9). The CeSe bond length of 1.826(3) Å is very similar to the values reported for the two structurally characterized 1,2,4-triazole-based selenourea derivatives known to date (viz. 1.808(2) and 1.844(5) Å) [42] and compares well with those of imidazole-based derivatives, which range from ca. 1.82e1.88 Å [21c,43]. These distances are in between the values typical of carboneselenium single and double bonds, which has been rationalized in terms of a significant contribution of zwitterionic structures that feature single N2CþeSe dative bonds [44].

Table 1 Selected bond lengths (Å) and angles ( ) for the structurally characterized compounds.

3. Conclusion

MeC

MeX

CeMeX

[CuCl(2′)] [{Cu(m-I)(2′)}2]a

1.866(3) 1.928(3)

[CuCl(2′)2]

1.915(6) 1.920(6) 1.937(3) 1.934(3) 1.9416(15) 2.099(3) 2.100(4) 1.984(6) 1.826(3)b

2.0987(9) 2.5841(4) 2.6159(5) 2.382(2)

2.6088(3)

177.88(10) 131.63(10) 123.37(10) 111.8(2) 105.4(2) 110.91(10) 113.56(10) 118.23(5)

2.2839(13)

179.37(15)

CuBr(2′)2] CuI(2′)2] [Ag(2′)2](OTf) [Ag(2′)2][BF4] [AuCl(2′)] 2′Se a b

2.4952(6)

IeCueI 104.861(15), CueIeCu 75.138(15). M ¼ Se.

CeMeC

142.8(3) 135.51(14) 123.54(9) 180.0 180.0

Fig. 2. Molecular structure of [CuCl(2′)] in the crystal. The solvent molecule (acetone) has been omitted for clarity.

We have shown that coinage metal complexes of the carbenic tautomer of Nitron (2′) can be prepared in high yields simply by reacting the inexpensive analytical reagent Nitron (2) with metal salts MX (X ¼ non-basic anion) under mild conditions in dipolar aprotic organic solvents. Together with our previous results from the coordination chemistry of rhodium [6] and ruthenium [14], these findings demonstrate that Nitron is a crypto-carbene. We have significantly expanded the number of structurally characterized 1,2,4-triazol-5-ylidene coinage metal complexes and, more generally, complexes of the type [CuX(NHC)2]. The results of studies addressing the bioactivity of these and related easily accessible complexes of the carbenic tautomer of Nitron will be reported in due course.

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Fig. 3. Molecular structure of [{Cu(m-I)(2′)}2] in the crystal. The solvent molecule (acetonitrile) has been omitted for clarity.

Fig. 4. Molecular structure of [CuCl(2′)2] in the crystal. Fig. 6. Molecular structure of [CuI(2′)2] in the crystal. The solvent molecule (THF) has been omitted for clarity.

tubes was used to separate precipitates which could not easily be removed by filtration. 1H and 13C NMR spectra were recorded with a Varian MR-400 spectrometer operating at 400 MHz for 1H. 77Se NMR spectra were recorded with a Varian NMRS-500 spectrometer operating at 500 MHz for 1H. Neat dimethylselenide was used as external standard (d ¼ 4 ppm) [45]. The 77Se NMR signal of 1Se (95 MHz, CDCl3, 25  C) was observed at d ¼ 110 ppm. Highresolution (HR) ESI mass spectra were obtained with a micrOTOF time-of-flight mass spectrometer (Bruker Daltonics, Bremen, Germany) using an Apollo™ “ion funnel” ESI source. Mass calibration was performed immediately prior to the measurement with ESI Tune Mix Standard (Agilent, Waldbronn, Germany). Elemental analyses were carried out with a HEKAtech Euro EA-CHNS elemental analyzer at the Institute of Chemistry, University of Kassel, Germany.

Fig. 5. Molecular structure of [CuBr(2′)2] in the crystal.

4. Experimental 4.1. General information All reactions were performed in an inert atmosphere (argon or dinitrogen) by using standard Schlenk techniques or a conventional glovebox. Gold and silver compounds were handled with exclusion of light. A Hettich ROTINA 46 RS centrifuge suitable for Schlenk

4.2. Synthetic procedures 4.2.1. Synthesis of 20 Se THF (10 mL) was added to 2 (500 mg, 1.60 mmol) and grey selenium (150 mg, 1.90 mmol). The suspension was stirred for 24 h. Volatile components were removed in vacuo. The remaining solid was extracted with toluene (3  10 mL) and the combined extracts filtered to remove traces of insoluble material. The filtrate was reduced to dryness in vacuo. The remaining light yellow solid was washed with acetone (3  5 mL), leaving the product as a colorless microcrystalline solid. Yield 370 mg (59%). 1H NMR (400 MHz,

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Fig. 7. Molecular structure of the cation of [Ag(2′)2](OTf) in the crystal. The solvent molecule (THF) and the triflate anion have been omitted for clarity.

CDCl3, 25  C): d ¼ 8.18 (d, 3J ¼ 8.3 Hz, 2 H), 7.70e7.63 (m, 3 H, Ph), 7.55e7.50 (m, 4 H, Ph), 7.44e7.39 (m, 3 H, Ph), 7.32 (t, 3J ¼ 8.0 Hz, 2 H, Ph), 7.06 (t, 3J ¼ 7.7 Hz, 1 H, Ph), 5.86 (s, 1 H, NH) ppm. 13C NMR (100 MHz, CDCl3, 25  C): d ¼ 159.6 (CSe), 148.5 (CN3), 138.9, 137.6, 132.7 (3  Cipso), 131.2, 130.7, 129.4, 129.1, 128.8, 128.3, 124.9, 123.4, 117.9 (9  CH) ppm. 77Se NMR (95 MHz, CDCl3, 25  C): d ¼ 106 ppm. HRMS/ESI (þ): m/z ¼ 415.04238 [MþNa]þ, 415.04379 calcd for [C20H16N4NaSe]þ. Elemental analysis calcd (%) for C20H16N4Se (391.33): C 61.39, H 4.12, N 14.32; found: C 61.40, H 4.82, N 14.72.

Fig. 8. Molecular structure of [AuCl(2′)] in the crystal.

Fig. 9. Molecular structure of 2′Se in the crystal.

4.2.2. Synthesis of [CuCl(20 )] A solution of 2 (400 mg, 1.28 mmol) in acetonitrile (30 mL) was added dropwise to a stirred solution of CuCl (129 mg, 1.30 mmol) in acetonitrile (20 mL). After 12 h the solution was stored at 10  C, leading to the formation of a colorless microcrystalline solid, which was isolated by centrifugation at 10  C, washed with a minimal amount of ice-cold acetonitrile and dried in vacuo. The volume of the mother liquor was reduced to ca. 5 mL in vacuo and subsequently cooled, which afforded a second crop of the product. Yield 505 mg (96%). 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 8.99 (s, 1 H, NH), 8.05, 7.74, 7.69e7.47, 7.32, 7.00 (5 m, 15 H, Ph) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 175.3 (Ccarbene), 150.6 (CN3), 139.6, 139.3, 134.7 (3  Cipso), 129.9, 129.5, 128.8, 128.5, 127.2, 122.2, 121.7, 118.3 (8  CH, 1 CH signal missing due to accidental signal overlap) ppm. HRMS/ESI (þ): m/z ¼ 433.02579 [MþNa]þ, 433.02572 calcd for [C20H16ClCuN4Na]þ. Elemental analysis calcd (%) for C20H16N4ClCu (411.37): C 58.39, H 3.92, N 13.62; found: C 58.88, H 4.10, N 13.74. 4.2.3. Synthesis of [CuBr(20 )] THF (20 mL) was added to 2 (267 mg, 0.85 mmol) and CuBr (101 mg, 0.70 mmol). The suspension was stirred for 2 h and subsequently filtered through a short Celite pad. The solid was washed with THF (3  5 mL) and then extracted with dichloromethane (3  5 mL) to dissolve the product. The combined extracts were layered with n-hexane (10 mL), which afforded the product as a colorless microcrystalline solid that was isolated by filtration and dried in vacuo. Additional product was obtained from the combined filtrate and THF washings by reducing the volume to ca. 10 mL in vacuo and subsequent layering with n-hexane (5 mL). Yield 244 mg (76%). 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 8.94 (s, 1 H, NH), 8.01, 7.69, 7.58, 7.53e7.43, 7.31, 6.99 (6 m, 15 H, Ph) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 177.1 (Ccarbene), 150.5 (CN3), 139.7,

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C. Thie et al. / Journal of Organometallic Chemistry xxx (2016) 1e10

Chart 1. Plausible structures of the coinage metal complexes of the present study. 1:1 complexes (left): M ¼ Cu, X ¼ , Cl, Br, I; M ¼ Ag, X ¼ Cl, Br; M ¼ Au, X ¼ Cl. 2:1 complexes (middle): M ¼ Cu, X ¼ Cl, Br, I; M ¼ Ag, X ¼ Cl, Br (in the latter two cases the ionic structure shown on the right is equally plausible). Ionic 2:1 complexes (right): M ¼ Ag, X ¼ [BF4], OTf; M ¼ Au, X ¼ Cl.

139.4, 134.9 7 (3  Cipso), 129.9, 129.9, 129.5, 128.9, 128.3, 127.2, 122.2, 121.5, 118.4 (9  CH) ppm. HRMS/ESI (þ): m/z ¼ 476.97485 [MþNa]þ, 476.97520 calcd for [C20H16BrCuN4Na]þ. Elemental analysis calcd (%) for C20H16N4BrCu (455.82): C 52.70, H 3.54, N 12.29; found: C 53.45, H 3.53, N 12.53. Although the result for carbon is outside the range viewed as establishing analytical purity, these data are provided to illustrate the best values obtained to date. 4.2.4. Synthesis of [CuI(20 )] The product was obtained from 2 (388 mg, 1.24 mmol) and CuI (196 mg, 1.03 mmol) in THF (20 mL) by a procedure analogous to that described for [CuBr(2′)]. Yield 422 mg (82%). 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 8.92 (s, 1 H, NH), 7.96, 7.68, 7.57, 7.44, 7.31, 6.99 (6 m, 15 H, Ph) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 178.4 (Ccarbene), 150.5 (CN3), 139.7, 139.4, 134.9 (3  Cipso), 129.9, 129.8, 129.4, 128.9, 128.1, 127.1, 122.2, 121.3, 118.3 (9  CH) ppm. HRMS/ESI (þ): m/z ¼ 524.95975 [MþNa]þ, 524.96134 calcd for [C20H16CuIN4Na]þ. Elemental analysis calcd (%) for C20H16N4CuI (502.82): C 47.77, H 3.21, N 11.14; found: C 47.26, H 2.95, N 10.74. 4.2.5. Synthesis of [CuCl(20 )2] THF (20 mL) was added to 2 (380 mg, 1.22 mmol) and CuCl (56 mg, 0.57 mmol). The suspension was stirred for 24 h and subsequently filtered through a short Celite pad. The solid was washed with THF (3  10 mL) and then extracted with warm dichloromethane (2  20 mL) to dissolve the product. The volume of the combined extracts was reduced to 10 mL in vacuo. Layering with nhexane (20 mL) afforded the product as a colorless microcrystalline solid, which was isolated by filtration and dried in vacuo. Yield 399 mg (97%). 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 8.75, 8.54 (2 br. s, 2 H, NH), 8.05, 7.62e7.20, 6.94 (3 m, 30 H, Ph) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 182.9, 179.8 (br.) (2  Ccarbene), 150.1, 149.9 (2  CN3), 139.9, 139.6, 139.1, 134.8, 134.5 (5  Cipso, 1 Cipso signal missing due to accidental signal overlap), 129.6, 129.5, 129.4, 129.3, 128.9, 128.8, 127.7, 127.4, 127.2, 126.9, 121.9, 121.5, 118.1, 117.9 (14  CH, 4 CH signals missing due to accidental signal overlap) ppm. HRMS/ESI (þ): m/z ¼ 687.20303 [MCl]þ, 687.20459 calcd for [C40H32CuN8]þ. Elemental analysis calcd (%) for C40H32N8ClCu (723.73): C 66.38, H 4.46, N 15.48; found: C 65.57, H 4.69, N 15.18. Although the result for carbon is outside the range viewed as establishing analytical purity, these data are provided to illustrate the best values obtained to date. 4.2.6. Synthesis of [CuBr(20 )2] THF (20 mL) was added to 2 (378 mg, 1.21 mmol) and CuBr (79 mg, 0.55 mmol). The suspension was stirred for 24 h and subsequently filtered through a short Celite pad. The solid was washed with THF (3  5 mL) and then extracted with dichloromethane (2  10 mL) to dissolve the product. The volume of the combined extracts was reduced to 5 mL in vacuo. Layering with n-hexane (10 mL) afforded the product as a colorless microcrystalline solid, which was isolated by filtration and dried in vacuo. Yield 407 mg (96%). 1H NMR (400 MHz, CDCl3, 25  C): d ¼ 7.97, 7.43, 7.28, 7.17,

7.11e7.04, 6.90 (6 m, 30 H, Ph) 5.82 (s, 2 H, NH) ppm. 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 8.71, 8.54 (2 br. s, 2 H, NH), 8.01, 7.58, 7.45e7.19, 7.01e6.92 (4 m, 30 H, Ph) ppm. 13C NMR (100 MHz, CDCl3, 25  C): d ¼ 180.7 (Ccarbene), 149.1 (CN3), 140.2, 138.2, 134.1 (3  Cipso), 130.3, 130.1, 129.3, 128.9, 127.4, 127.2, 122.9, 122.4, 117.5 (9  CH) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 182.9, 180.2 (2  Ccarbene), 150.2, 149.9 (2  CN3), 139.8, 139.5, 139.1, 134.7, 134.5 (5  Cipso, 1 Cipso signal missing due to accidental signal overlap), 129.7, 129.5, 129.4, 128.9, 128.8, 128.75, 128.7, 127.7, 127.4, 127.2, 127.0, 121.9, 121.5, 118.1, 117.9 (15  CH, 3 CH signals missing due to accidental signal overlap) ppm. HRMS/ESI (þ): m/z ¼ 687.20154 [MBr]þ, 687.20459 calcd for [C40H32CuN8]þ. Elemental analysis calcd (%) for C40H32N8BrCu (768.19): C 62.54, H 4.20, N 14.59; found: C 62.48, H 4.49, N 14.39. 4.2.7. Synthesis of [CuI(20 )2] The product was obtained from 2 (370 mg, 1.18 mmol) and CuI (100 mg, 0.53 mmol) in THF (20 mL) by a procedure analogous to that described for [CuBr(2′)2]. Yield 406 mg (95%). 1H NMR (400 MHz, CDCl3, 25  C): d ¼ 8.02, 7.51, 7.49e7.36, 7.28, 7.24e7.14, 7.01 (6 m, 30 H, Ph) 5.85 (s, 2 H, NH) ppm. 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 8.82, 8.52 (2 br. s, 2 H, NH), 7.82, 7.59e7.55, 7.48, 7.36e7.30, 6.99 (5 m, 30 H, Ph) ppm. 13C NMR (100 MHz, CDCl3, 25  C): d ¼ 181.3 (Ccarbene), 149.3 (CN3), 140.1, 138.1, 133.9 (3  Cipso), 130.4, 130.3, 129.3, 128.8, 127.4, 127.35, 122.9, 122.4, 117.5 ppm (9  CH). 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 182.9, 178.9 (2  Ccarbene), 150.5, 150.0 (2  CN3), 139.9, 139.8, 139.3, 134.8, 134.6 (5  Cipso, 1 Cipso signal missing due to accidental signal overlap), 129.8, 129.6, 129.2, 128.9, 128.8, 127.9, 127.2, 122.15, 122.0, 121.4, 118.3, 118.0 (12  CH, 6 CH signals missing due to accidental signal overlap) ppm. HRMS/ESI (þ): m/z ¼ 687.20416 [MI]þ, 687.20459 calcd for [C40H32CuN8]þ. Elemental analysis calcd (%) for C40H32N8CuI (815.19): C 58.94, H 3.96, N 13.75; found: C 59.09, H 4.19, N 13.39. 4.2.8. Synthesis of [AgCl(20 )] Dichloromethane (15 mL) was added to 2 (374 mg, 1.20 mmol) and AgCl (146 mg, 1.02 mmol). The suspension was stirred for 18 h. Volatile components were removed in vacuo. The remaining solid was washed with methanol (3  5 mL), taken up in dichloromethane (250 mL) and filtered through a short Celite pad to remove traces of insoluble material. The volume of the filtrate was reduced to 20 mL. Layering with n-hexane (30 mL) afforded the product as a colorless microcrystalline solid, which was isolated by filtration and dried in vacuo. Yield 387 mg (83%). 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 9.04 (s, 1 H, NH), 7.97, 7.73, 7.63e7.52, 7.31, 7.00 (5 m, 15 H, Ph) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 179.0 (Ccarbene), 151.1 (CN3), 139.6 (br.), 139.4, 134.9 (3  Cipso), 130.1, 130.0, 129.6, 128.9, 128.8, 127.2, 122.3, 122.25, 118.4 (9  CH) ppm. HRMS/ ESI (þ): m/z ¼ 476.99985 [MþNa]þ, 477.00122 calcd for [C20H16AgClN4Na]þ. Elemental analysis calcd (%) for C20H16N4AgCl (455.69): C 52.72, H 3.54, N 12.30; found: C 53.19, H 3.83, N 12.33. 4.2.9. Synthesis of [AgBr(20 )] Acetone (15 mL) was added to 2 (329 mg, 1.05 mmol), AgNO3

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C. Thie et al. / Journal of Organometallic Chemistry xxx (2016) 1e10

(166 mg, 0.98 mmol) and NaBr (107 mg, 1.04 mmol). Alternatively, AgBr may be used instead of AgNO3 and NaBr. The mixture was stirred for 48 h. Dichloromethane (300 mL) was added and the solution filtered through a short Celite pad to remove insoluble material (mainly NaBr). The volume of the filtrate was reduced to 10 mL, affording the product as a colorless, microcrystalline solid, which was isolated by centrifugation at 10  C, washed with acetone (3  5 mL) and dried in vacuo. Yield 545 mg (93%). 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 9.02 (s, 1 H, NH), 7.93, 7.71, 7.62e7.54, 7.31, 7.00 (5 m, 15 H, Ph) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 179.6 (Ccarbene), 151.2 (CN3), 139.7 (br.), 139.4, 134.9 (3  Cipso), 130.0, 129.95, 129.6, 128.9, 128.8, 127.1, 122.2, 118.4 (8  CH, 1 CH signal missing due to accidental signal overlap) ppm. HRMS/ESI (þ): m/z ¼ 520.94854 [MþNa]þ, 520.95070 calcd for [C20H16AgBrN4Na]þ. Elemental analysis calcd (%) for C20H16N4AgBr (500.14): C 48.03, H 3.23, N 11.20; found: C 49.03, H 3.13, N 11.08. Although the result for carbon is outside the range viewed as establishing analytical purity, these data are provided to illustrate the best values obtained to date. 4.2.10. Synthesis of [AgCl(20 )2] Dichloromethane (10 mL) was added to 2 (690 mg, 2.21 mmol) and AgCl (145 mg, 1.01 mmol). After stirring for 18 h, the volume of the suspension was reduced to 5 mL in vacuo. The solid was isolated by centrifugation at 10  C, suspended in a mixture of methanol (5 mL) and dichloromethane (1 mL) and again centrifuged off. Subsequently, the solid was taken up in dichloromethane (280 mL) and the solution filtered through a short Celite pad to remove traces of insoluble material. The volume of the filtrate was reduced to 30 mL in vacuo. Layering with n-hexane (50 mL) afforded the product as a colorless microcrystalline solid, which was isolated by filtration and dried in vacuo. Yield 718 mg (92%). 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 9.03 (br. s, 2 H), 7.96, 7.71, 7.58e7.52, 7.31, 6.99 (5 m, 30 H, Ph) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 179.0 (Ccarbene), 151.2 (CN3), 139.6, 139.4, 134.9 (3  Cipso), 130.1, 130.0, 129.6, 128.8, 127.2, 122.3, 118.4 (7  CH, 2 CH signals missing due to accidental signal overlap) ppm. HRMS/ESI (þ): m/z ¼ 731.17969 [MCl]þ, 731.18009 calcd for [C40H32AgN8]þ. Elemental analysis calcd (%) for C40H32N8AgCl (768.06): C 62.55, H 4.20, N 14.59; found: C 61.77, H 4.09, N 14.07. Although the result for carbon is outside the range viewed as establishing analytical purity, these data are provided to illustrate the best values obtained to date. The fact that the values determined for carbon, hydrogen and nitrogen are consistently lower than the calculated ones very likely indicates the presence of the 1:1 complex as impurity. 4.2.11. Synthesis of [AgBr(20 )2] Dichloromethane was added to 2 (324 mg, 1.04 mmol) and AgBr (96 mg, 0.51 mmol). The suspension was stirred for 72 h. The solid was isolated by centrifugation at 10  C, sequentially washed with dichloromethane, acetone and methanol (5 mL each) and dissolved in dichloromethane (300 mL). The solution was filtered through a short Celite pad to remove traces of insoluble material. The volume of the filtrate was reduced to 30 mL in vacuo. Layering with nhexane (50 mL) afforded the product as a colorless microcrystalline solid, which was isolated by filtration and dried in vacuo. Yield 333 mg (80%). 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 9.01 (br. s, 2 H, NH), 7.91, 7.68, 7.59, 7.51, 7.30, 6.98 (6 m, 30 H, Ph) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 179.9 (Ccarbene), 151.3 (CN3), 139.9, 139.4, 134.9 (3  Cipso), 129.9, 129.6, 128.8, 127.0, 122.2, 118.5 (6  CH, 3 CH signals missing due to accidental signal overlap) ppm. HRMS/ESI (þ): m/z ¼ 731.17637 [MCl]þ, 731.18009 calcd for [C40H32AgN8]þ. Elemental analysis calcd (%) for C40H32N8AgBr (812.51): C 59.13, H 3.97, N 13.79; found: C 58.43, H 3.67, N 13.18.

7

4.2.12. Synthesis of [Ag(20 )2][BF4] Dichloromethane (20 mL) was added to 2 (612 mg, 1.96 mmol) and Ag[BF4] (168 mg, 0.86 mmol). After stirring for 24 h, the precipitate was dissolved by addition of acetonitrile and dichloromethane (125 mL each). The solution was filtered through a short Celite pad to remove traces of insoluble material. The filtrate was reduced to dryness in vacuo and the residue washed with acetone (4  5 mL), leaving a colorless microcrystalline solid. The combined washings were reduced to dryness in vacuo and the remaining solid washed with a methanoledichloromethane (4:1) mixture (10 mL). The solids were combined and washed with methanol and diethyl ether (5 mL each), leaving the product as a colorless microcrystalline solid. Yield 618 mg (87%). 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 9.04 (s, 2 H, NH), 7.81, 7.64, 7.59e7.44, 7.30, 7.00 (5 m, 30 H, Ph) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 179.2 (2 d, 1 13 109 J( C, Ag/107Ag) ¼ 219/189 Hz, Ccarbene), 151.2 (d, 3 13 109 J( C, Ag/107Ag) z 7 Hz, CN3), 139.6, 139.3, 134.8 (3  Cipso), 130.1, 130.0, 129.7, 129.0, 128.9, 127.0, 122.4, 122.2, 118.5 (9  CH) ppm. HRMS/ESI (þ): m/z ¼ 731.17916 [MBF4]þ, 731.18009 calcd for [C40H32AgN8]þ. Elemental analysis calcd (%) for C40H32N8 AgBF4 (819.41): C 58.63, H 3.94, N 13.68; found: C 58.52, H 4.04, N 13.63. 4.2.13. Synthesis of [Ag(20 )2](OTf) Dichloromethane (15 mL) was added to 2 (468 mg, 1.50 mmol) and Ag(OTf) (180 mg, 0.70 mmol). After stirring for 24 h, volatile components were removed in vacuo. THF (15 mL) was added to the residue and the suspension filtered through a short Celite pad. The solid was washed with THF (3  5 mL) and then extracted with dichloromethane (25 mL) to dissolve the product. The volume of the extract was reduced to 5 mL. Layering with n-hexane (15 mL) afforded the product as a colorless microcrystalline sold, which was isolated by filtration and dried in vacuo. Yield 576 mg (93%). 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 9.03 (s, 2 H, NH), 7.80, 7.64, 7.58e7.44, 7.30, 7.00 (5 m, 30 H, Ph) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 179.2 (2 d, 1J(13C,109Ag/107Ag) ¼ 221/188 Hz, Ccarbene), 151.2 (d, 3J(13C,109Ag/107Ag) z 6 Hz, CN3), 139.6, 139.2, 134.8 (3  Cipso), 130.1, 130.0, 129.7, 129.0, 128.9, 127.0, 122.4, 122.2, 118.4 (9  CH) ppm. HRMS/ESI (þ): m/z ¼ 731.18243 [MOTf]þ, 731.18009 calcd for [C40H32AgN8]þ. Elemental analysis calcd (%) for C41H32N8AgF3O3S (881.67): C 55.85, H 3.66, N 12.71; found: C 55.80, H 3.14, N 12.65. 4.2.14. Synthesis of [AuCl(20 )] Acetonitrile (30 mL) was added to 2 (388 mg, 1.04 mmol) and [AuCl(THT)] (311 mg, 1.02 mmol). The solution was stirred for 12 h. The precipitate that had formed during this time was dissolved by addition of acetonitrile (60 mL). The solution was filtered through a short Celite pad. The volume of the filtrate was reduced to 20 mL in vacuo. Layering with n-hexane (30 mL) afforded the product as a colorless microcrystalline solid, which was isolated by centrifugation at 10  C and dried in vacuo. Additional product was obtained by reducing the mother liquor to dryness, washing the remaining solid with acetone (5 mL) and drying in vacuo. Yield 528 mg (95%). 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 9.01 (s, 1 H, NH), 8.05, 7.77, 7.67e7.55, 7.31, 7.00 (5 m, 15 H, Ph) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 170.3 (Ccarbene), 151.9 (CN3), 139.4, 138.9, 134.2 (3  Cipso), 130.5, 130.0, 129.6, 129.3, 128.9, 128.0, 123.7, 122.5, 118.7 (9  CH) ppm. HRMS/ESI (þ): m/z ¼ 567.060458 [MþNa]þ, 567.06269 calcd for [C20H16AuClN4Na]þ. Elemental analysis calcd (%) for C20H16N4AuCl (544.79): C 44.09, H 2.96, N 10.28; found: C 43.83, H 2.58, N 9.97. 4.2.15. Synthesis of [AuCl(20 )2] THF (20 mL) was added to 2 (446 mg, 1.38 mmol) and [AuCl(THT)] (205 mg, 0.64 mmol). The mixture was stirred for 12 h.

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8

C. Thie et al. / Journal of Organometallic Chemistry xxx (2016) 1e10

Table 2 X-ray crystallographic data.

Chemical formula Formula mass Crystal system Space group a/Å b/Å c/Å a/ b/ g/ Unit cell volume/Å3 T/K Crystal size/mm Z m/mm1 No. of refl. measured Independent reflections [Rint] Final R1 (wR2) [l > 2s(l)] Final R1 (wR2) [all data] Abs. Corr. Tmin/Tmax Goodness of fit on F2 CCDC no.

Chemical formula Formula mass Crystal system Space group a/Å b/Å c/Å a/ b/ g/ Unit cell volume/Å3 T/K Crystal size/mm Z m/mm1 No. of refl. measured Independent reflections [Rint] Final R1 (wR2) [l > 2s(l)] Final R1 (wR2) [all data] Abs. Corr. Tmin/Tmax Goodness of fit on F2 CCDC no.

2′Se$acetone

[CuCl(2′)]$acetone [CuCl(2′)2]

[AuCl(2′)]$THF

[{Cu(m-I)(2′)}2] $MeCN [CuBr(2′)2]

[CuI(2′)2] $THF

C23H22N4OSe 449.40 Monoclinic P21/n 5.7090(3) 30.839(2) 11.6973(5) 90 100.629(4) 90 2024.1(2) 100(2) 0.48  0.03  0.03 4 1.877 11,854 3926 [0.0574] 0.0341(0.0655) 0.0587(0.0730) Integration 0.6723/0.9648 0.962 1443079

C23H22ClCuN4O 469.43 Triclinic P-1 5.8590(5) 11.5026(11) 16.2487(14) 98.569(7) 95.696(7) 101.510(7) 1051.62(17) 100(2) 0.16  0.03  0.01 2 1.188 7766 3785 [0.0517] 0.0425(0.1103) 0.0521(0.1222) Integration 0.8754/0.9793 0.937 1443073

C24H24AuClN4O 616.89 Triclinic P-1 9.8877(8) 10.0738(7) 12.1263(10) 70.235(6) 88.397(7) 76.692(6) 1104.54(16) 100(2) 0.24  0.14  0.12 2 6.805 8932 4301 [0.0312] 0.0369(0.1013) 0.0375(0.1022) Integration 0.2769/0.4980 1.082 1443075

C42H35Cu2I2N9 1046.67 Monoclinic C2/c 14.9623(6) 12.9869(4) 21.4606(9) 90 107.711(3) 90 3972.4(3) 100(2) 0.13  0.11  0.02 4 2.669 9388 4511 [0.0617] 0.0291(0.0511) 0.0540(0.0556) Integration 0.7405/0.9373 0.869 1443076

C48H48CuIN8O2 959.38 Monoclinic C2/c 15.2574(6) 13.5172(4) 21.5577(9) 90 103.621(3) 90 4321.0(3) 100(2) 0.37  0.34  0.05 4 1.270 20,668 6277 [0.0310] 0.0289(0.0676) 0.0399(0.0716) Integration 0.6768/0.9576 1.028 1443077

C40H32ClCuN8 723.72 Orthorhombic P bca 14.2904(5) 24.1283(10) 20.1999(7) 90 90 90 6965.0(4) 100(2) 0.10  0.04  0.01 8 0.746 26,909 6481 [0.0900] 0.0693(0.1409) 0.1527(0.1849) Integration 0.9396/0.9920 1.111 443074

C40H32BrCuN8 768.18 Triclinic P-1 12.7994(8) 13.2379(9) 13.8349(10) 65.745(5) 65.283(5) 86.371(5) 1924.7(2) 100(2) 0.17  0.15  0.09 2 1.644 6611 6611 [0.0461] 0.0411(0.1068) 0.0560(0.1130) Integration 0.7857/0.9280 1.019 1443078

[Ag(2′)2](OTf) $THF

[Ag(2′)2][BF4]

C49H48AgF3N8O5S 1025.88 Triclinic P-1 8.5292(4) 9.6946(5) 15.1575(8) 72.115(4) 85.053(4) 69.185(4) 1114.55(10) 100(2) 0.60  0.41  0.05 1 0.571 25,141 5907 [0.0282] 0.0451(0.1203) 0.0471(0.1214) Integration 0.7739/0.9718 1.258 1443080

C40H32AgBF4N8 819.41 Monoclinic C2/c 18.403(2) 8.3388(10) 22.906(2) 90 98.909(8) 90 3472.7(6) 100(2) 0.17  0.02  0.02 4 5.210 7783 3199 [0.0523] 0.0392(0.0941) 0.0581(0.1005) Integration 0.6555/0.9256 0.961 1455560

The precipitate that had formed during this time was isolated by centrifugation at 10  C and subsequently dissolved in a dichloromethaneeacetonitrile (2:1) mixture (150 mL). The solution was filtered through a short Celite pad to remove traces of insoluble material. The filtrate was reduced to dryness in vacuo, leaving the product as a colorless microcrystalline solid, which was washed with acetone (15 mL) and dried in vacuo. Yield 509 mg (93%). 1H NMR (400 MHz, DMSO-d6, 25  C): d ¼ 9.06 (br. s, 2 H, NH), 7.80, 7.65e7.46, 7.29, 6.99 (4 m, 30 H, Ph) ppm. 13C NMR (100 MHz, DMSO-d6, 25  C): d ¼ 180.7 (Ccarbene), 151.3 (CN3), 139.6, 138.5, 133.9 (3  Cipso), 130.2, 129.8, 129.5, 129.2, 128.8, 127.5, 123.1, 122.3, 118.7 (9  CH) ppm. HRMS/ESI (þ): m/z ¼ 821.23827 [M]þ, 821.24156 calcd for [C40H32AuN8]þ. Elemental analysis calcd (%) for C40H32N8AuCl (857.16): C 56.05, H 3.76, N 13.07; found: C 55.98, H 3.79, N 13.18.

4.3. X-ray crystallography For each data collection a single crystal was mounted on a MiTeGen MicroMount using perfluoropolyether oil, rapidly transferred to a goniometer head and cooled to 100(2) K using an Oxford Cryosystems Cryostream open-flow nitrogen cooling device. Data collection was made either with a Stoe IPDS2 diffractometer equipped with a 2-circle goniometer and an area detector ([CuCl(2′)]$acetone, [CuCl(2′)2], [AuCl(2′)]$THF) or a Stoe StadiVari diffractometer equipped with a 4-circle goniometer and a Dectris Pilatus 2K detector ([{Cu(m-I)(2′)}2]$acetonitrile, [CuI(2′)2]$THF, [CuBr(2′)2], 2′Se$acetone, [Ag(2′)2](OTf)$THF, [Ag(2′)2][BF4]). CuKa radiation (l ¼ 1.54186 Å) was used for [Ag(2′)2][BF4]. MoKa radiation (l ¼ 0.71073 Å) was used in all other cases. The data sets were corrected for absorption, Lorentz and polarization effects. The structures were solved by direct methods (SIR 2008) [46] and refined using alternating cycles of least-squares refinements against F2 (SHELXL2014/7) [47]. H atoms were included to the

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