Neutral Dinuclear Silver(I)âNHC Complexes: Synthesis and

Article pubs.acs.org/Organometallics

Neutral Dinuclear Silver(I)−NHC Complexes: Synthesis and Photophysics Alexander Poethig and Thomas Strassner* Physikalische Organische Chemie, Technische Universität Dresden, 01062 Dresden, Germany S Supporting Information *

ABSTRACT: We report the synthesis and characterization of a new anionic pincer-type μ2-η2(1κCcarbene:2κC′carbene)−C*−N−C*) ligand with a triazine core. Hydrolysis of one chloro substituent of the cyanuric chloride leads to the hydroxy-2,4-dichloro-1,3,5-triazine and the corresponding bis(imidazolium) salts, which, after deprotonation, serve as monoanionic ligands. Treatment of the bis(imidazolium) precursor with silver(I) oxide leads to stable silver(I)−NHC complexes with interesting photophysical properties (phosphorescent emission in the deep blue region of the spectrum). Depending on the substitution pattern of the NHC ligand, the emission can be shifted between 417 and 480 nm. X-ray structure analysis reveals the dimeric structure of these complexes.

philic substitution of 2,6-dihalogenated pyridines with Nsubstituted imidazoles leads to precursors for pincer-type ligands (Chart 1, B), which have not only two but three donating electron pairs. The resulting complexes show even higher thermal stability and a relatively rigid conformation, which can possibly be used to induce asymmetric catalysis.27,28 Silver− NHC complexes with such a pyridyl-bridged ligand system have been reported to form dinuclear complexes (Chart 1, C).27−30 Although some ligands based on triazines with nitrogen heterocycles are known,31 to the best of our knowledge, triazine derivatives have not been used so far in (C*−N−C*) ligands, with C* denoting a carbene carbon atom. Cyanuric chloride (2,4,6-trichloro-1,3,5-triazine) is a well-known and commonly used reagent, which possesses three reactive positions that can be selectively substituted by nucleophiles, such as primary and secondary amines.32 Tertiary amines, like the substituted imidazoles, are known to activate the electrophilic positions toward nucleophilic attack. 32 In the presence of water, the hydrolysis of the first chlorine atom leads to a triazinol (and its tautomers) that allows the delocalization of the negative charge after deprotonation (Scheme 1). We present the synthesis and characterization of new bis(imidazolium) salts as precursors for a new class of anionic multidentate NHC−biscarbene ligands and their corresponding neutral silver−NHC complexes together with their analytical and photophysical data.

1. INTRODUCTION N-heterocyclic carbenes1−6 are an interesting class of ligands, which have been used for a large number of metal−organic complexes, being compatible with main group elements as well as transition metals. Among them, silver−NHC complexes are of increasing interest as their applications span a wide range from being potent antimicrobial agents to showing phosphorescent emission.7−11 Additionally, they are often used as precursors for transmetalation reactions.12 A large variety of possible structures of silver(I)−NHC complexes have been reported. Depending on the reaction conditions during the synthesis, variation of a counterion or a different alkyl chain can significantly change the resulting structure. Linear coordinated complexes as well as complicated cluster-like or supramolecular staircase-like structures have been observed.7,8,10 By varying the substitution pattern of the N-heterocycles (e.g., imidazoles), the ligands can be tuned by steric and electronic effects.13−15 Depending on the potential application, the requirements for the ligands are different; for photophysical applications, the stability and the emission properties are crucial.16,17 Metal−organic complexes with multidentate (benz)imidazolium-based NHC ligands have shown to be significantly more stable compared with those with monodentate ligands. 18 Therefore, they can be used to catalyze reactions, which take place under drastic conditions, for example, the CH-activation of methane.19,20 Furthermore, the multidentate ligands can also be anionic tethered NHCs, which are able to compensate charges of the coordinated metal.2 In the case of NHCbiscarbene ligands, various spacers between the two imidazolium heterocycles are known, for example, the methylene bridging motif (Chart 1, A), but also alkylene-,21 xylylene-,22,23 silylene-,24 or boron-containing25,26 spacers. Two-fold nucleo© 2011 American Chemical Society

2. RESULTS AND DISCUSSION Several N-substituted imidazoles with saturated33 and unsaturated34 substituents were synthesized according to known Received: September 13, 2011 Published: November 30, 2011 6674

dx.doi.org/10.1021/om200860y | Organometallics 2011, 30, 6674−6684

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Chart 1. Examples for the geometry of chelating NHC ligands and their metal complexes: methylene bridged bidentate NHCs (A), mononuclear pincer-type tridentate NHCs (B), and pyridyl-bridged dinuclear silver−NHC complexes (C)

Scheme 1. Retrosynthesis of the Neutral Silver−NHC Complex (Left), the Corresponding Triazine-Bridged Biscarbene ProLigand (Middle), Which Is Accessible from Cyanuric Chloride and Substituted Imidazoles (Right)

Scheme 2. Hydrolysis of One Chloro Substituent of Cyanuric Chloride, Induced by the Imidazole a

a

R = alkyl, aryl substituents.

procedures. In the reaction of cyanuric chloride with various imidazoles (in excess) dissolved in a CH3CN/H2O solvent mixture, we always observed the hydrolysis of one of the three chloro substituents at the triazine ring. The formation of the first imidazolium salt can been seen as an activation of an acid chloride, which can easily be hydrolyzed to form 2,4-dichloro-6hydroxytriazine (Scheme 2). Intermediate 3 is less reactive due to the possible delocalization of the partial positive charge at the carbon atom. Thus, the following substitution of the remaining two chlorine atoms with N-substituted imidazoles leads to stable bis(imidazolium) salts (Scheme 3). The in situ hydrolysis of the first chlorine atom is followed by the nucleophilic substitution of the second and third chlorine atoms by imidazoles 2a−2g (Scheme 3). An excess amount of the imidazoles is necessary to scavenge the generated HCl. The most important advantage of this method is the utilization of the different solubilities of the formed salts in acetonitrile. Species with only one positive charge, such as the protonated imidazolium species, and the monoimidazolium salts of the triazines are better soluble, whereas the bis(imidazolium) salts precipitate. When a mixture of protonated imidazolium salts and bis(imidazolium) salts occurred, we could wash the precipitate with hot acetonitrile and selectively separate the desired bis(imidazolium) salt.

Scheme 3. Formation of the Bis(imidazolium) Salts by in Situ Hydrolysis of the First Chlorine Atom

The reaction of the bis(imidazolium) salts 4a−4g with silver(I)-oxide is carried out in the dark using dichloromethane as the solvent and leads to stable silver−NHC complexes 5a− 5g (Scheme 4). After filtration of the crude reaction mixture through Celite, the resulting filtrate is concentrated and the product precipitated with diethylether. The complexes are sensitive toward acidic conditions; even the NMR solvent CDCl3 had to be filtrated through basic aluminum oxide before preparing the samples, since, otherwise, decomposition of the complex could be observed, due to protonation of the ligand. 6675


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dihedral angle, Table 1), which leads to the helical structure of the complexes. A closer look on the structure of 5a reported here shows the formation of a tubular network with cavities (Figure 4). Similar cavities could be found for each of our crystal structures, which explains the incorporation of solvent molecules in the accessible voids. As shown in Figure 5, the supramolecular assembly of the individual molecules of 5a is induced by hydrogen bonding among the complexes and between complexes and solvent molecules (Figure 5). A 2-fold hydrogen bond between a triazinone and an NHC backbone leads to the planar assembly along the b axis of the unit cell. Similar interactions can be found between the solvent molecules and the complexes, with one dichloromethane hydrogen atom interacting with the triazinone oxygen and one chlorine atom donating to a hydrogen atom at the aromatic ring of the mesityl backbone. This supports the suggestion of Manzano et al.,31 that the formation of the supramolecular assembly of the complexes is (partially) templated by weak interactions between complexes and solvent molecules, in our case, additionally between the complex molecules themselves. According to our data, the length of the silver−silver bond depends on the substitution pattern of the complexes as well as the crystal packing. The steric bulk of the mesityl-substituted complex 5a leads to a longer silver−silver bond compared with that in the other complexes. However, even the same compound can crystallize in different space groups, depending on the solvent. A different synthetic route and crystallization from DMSO had previously led to a different structure with a longer intermetallic bond (3.34 Å),37 caused by a different packing in the orthorhombic crystal system. In that structure, no hydrogen bonding between the complexes could be observed, only hydrogen bonds between the solvent molecules and the complexes. The dinuclear complex itself appears to possess a certain flexibility to adapt its geometry in the particular crystal system. Hydrolysis Experiments. The observation of the acid sensitivity of the complexes during the preparation of the

Scheme 4. Formation of the Silver−NHC Complexes

Five examples (5a, 5b, 5d, 5e, 5g) could be crystallized from dichloromethane or acetonitrile by slow diffusion of diethyl ether, and their structures could be unambiguously determined by X-ray single-crystal diffraction (Figures 1−3). For the silver complexes with p-MeO-phenyl (5c) and tert-butyl (5f) substituents, the 1H NMR, 13C NMR, and mass spectroscopic analyses confirm that they are structurally similar compared to those confirmed by single-crystal diffraction experiments. Selected bond lengths and angles are given in Table 1. The core structure is identical for all complexes 5. The central dinuclear silver−silver unit is coordinated by two ligands, forming a double helical structure with each ligand binding to both silver ions. In each case, the unit cell contains the racemate of P- and M-helical dinuclear silver complexes. For 5g, the asymmetric unit consists of both enantiomers, whereas for 5a, 5b, 5d, and 5e, the second enantiomer is generated by inversion of the enantiomer in the asymmetric unit. Argentophilic interactions could be observed for all of the complexes, that is, shorter silver−silver contacts than the sum of van der Waals radii,35 varying from 2.9815(18) (5d) to 3.1514(6) Å (5a) (Table 1). The lengths of the C−O bonds of the ligands are in a small range between 1.216(9) (5g″) and 1.244 Å (5d) and, therefore, more carbonyl type (Ø 1.230 Å for ureas36) than a single bond (C sp2−O: 1.362 Å for phenols36). The NHC rings are twisted out of the central triazinone plane by 7.5° (5d) to 41.2° (5b) (deviation from the ideal 180°

Figure 1. Molecular structure of compounds 5a in P1̅ (left) and 5b in P21/c. Hydrogen atoms and solvent molecules have been omitted for clarity. Thermal ellipsoids are drawn at 50% probability. 6676


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Figure 2. Molecular structure of compounds 5d in Iba2 (left) and 5e in Pbca. 5e shows a disordered cyclopentyl moiety. Hydrogen atoms and solvent molecules have been omitted for clarity. Thermal ellipsoids are drawn at 50% probability.

Figure 3. Molecular structure of compounds 5g in Pca21 showing two independent molecules (5g′ (Ag(1)/Ag(2), P chirality) and 5g″ (Ag(3)/ Ag(4), M chirality)) with different helical chiralities in the asymmetric unit. Hydrogen atoms and solvent molecules have been omitted for clarity. Thermal ellipsoids are drawn at 50% probability. To allow an easier comparison, the numbering of atoms of 5g″ corresponds to the numbering of the atoms in 5g′ plus 100. Ag3 corresponds to Ag1, Ag4 to Ag2.

probes for NMR spectroscopy led to a series of NMR titration experiments described below. We successively added trifluoro-

acetic acid (HOTFA) (Figure 6) and HCl (4N in dioxane) (Figure 7) to a solution of complex 5g in CDCl3. The signal of 6677


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Table 1. Selected Bond Lengths (in Å), Bond Angles (in °), and Dihedral Angles (in °) of Complexes 5 Ag(1)···Ag(2) Ag(1)−C(1) Ag(1)−C(10) Ag(2)−C(7) Ag(2)−C(16) C(5)−O(1) C(14)−O(2) C(1)−Ag(1)−C(10) C(7)−Ag(2)−C(16) N(4)−C(4)−N(2)−C(1) N(5)−C(6)−N(6)−C(7) N(11)−C(13)−N(9)−C(10) N(12)−C(15)−N(13)−C(16) a

5a

5b

5d

5e

5g′

5g″a

3.1514(6) 2.087(3) 2.085(4) 2.094(3) 2.092(3) 1.231(4) 1.230(4) 161.06(13) 168.67(12) 156.7(3) 162.9(3) 155.8(3) 155.7(3)

3.0503(6) 2.085(4) 2.087(4) 2.102(3) 2.110(3) 1.230(4) 1.226(5) 166.31(15) 160.45(15) 164.1(3) 154.6(3) 138.8(4) −171.8(4)

2.9815(18) 2.122(8) 2.112(7) 2.108(7) 2.089(7) 1.238(10) 1.244(10) 165.2(3) 170.2(3) 161.1(7) 165.8(7) −172.5(6) −151.3(7)

3.0381(7) 2.099(4) 2.102(4) 2.116(4) 2.102(4) 1.234(5) 1.235(5) 170.94(16) 169.32(16) −161.2(4) −156.9(4) −166.7(4) −152.5(4)

2.9885(11) 2.111(8) 2.101(8) 2.073(9) 2.107(8) 1.217(10) 1.230(10) 169.8(3) 169.2(3) −159.5(8) −157.9(7) −161.8(7) −159.7(8)

2.9712(10) 2.110(8) 2.105(8) 2.094(8) 2.108(7) 1.232(10) 1.216(9) 168.0(3) 167.5(3) 166.3(7) 154.7(8) 168.7(7) 158.6(6)

To allow an easier comparison, the numbering of atoms of 5g″ corresponds to the numbering of the atoms in 5g′ plus 100 (see Figure 3).

HOTFA. The isopropyl CH signal at 4.30 ppm decreases, whereas a signal at 5.12 ppm increases, which corresponds to the same group in the salt. During the titration, a third signal set occurs, which we believe belongs to an intermediate silver complex 6g, with only one of the silver atoms still being coordinated by carbene ligands, whereas the other two donating carbene ligand substituents are protonated (Scheme 5, top). Further proof comes from the fact that there is only one C2 proton signal and four signals for the imidazolium backbone protons are having the same integral. Nevertheless, our initial observation of the hydrolysis due to the HCl in CDCl3 showed a different spectrum of the decomposition product with two isopropyl CH signals with different chemical shifts, but equal integrals, comparable to that of the intermediate species seen in the HOTFA titration experiment. A possible reason for this could be the conjugated anion of the acid used, which, in the case of HOTFA, is less coordinating compared to HCl, which can, in principle, stabilize an intermediate monocarbene silver complex 7g (Scheme 5, bottom). We, therefore, carried out another titration experiment using HCl in dioxane as a protonating agent, which furthermore is an elegant way to determine the equivalents of hydrochloric acid by having the dioxane signal as an internal reference (Figure 7). The signal set, which can be assigned to 7g, is slowly evolving. Only when using 26.4 equiv or more of HCl does another signal at 12 ppm become visible, which belongs to 4g. We also checked whether this process is reversible by adding different bases to a solution of 5g in acidic CDCl3. In a first experiment, we added triethylamine, observing no change in the relative signal intensities. In a second experiment, we added solid aluminum oxide to the NMR probe and monitored the change of the signals with time (Figure 8). A decrease of the signal set of 7g could be observed, unfortunately accompanied by formation of the free imidazole 2g, caused by the nucleophilic character of the emerging base, which can undergo a nucleophilic attack on the triazine core. The decrease can be caused by the back-reaction to complex 5g or by degradation to the free imidazole. However, the significant decrease of 7g in the first 24 min, after which no noteworthy amount of 2g was detectable, as well as the fact that, after 22 h, a larger amount of 2g has been formed than was existent of 7g at the beginning are indications in favor of the back-reaction. To circumvent the nucleophilic degradation of the ligand, we finally added solid potassium bis(trimethylsilyl) amide

Figure 4. Crystal packaging of 5a with the unit cell. View along the c axis (left) and the b axis showing tubular cavities between the dinuclear complex units. Refined solvent molecules are left out for clarity.

Figure 5. Part of the unit cell of 5a (P1̅) showing the hydrogen bonding between the triazinone (O···H: 2.49 Å; N···H: 2.82 Å) and NHC backbone hydrogens of the complexes as well as between dichloromethane and the complexes (triazinone−O donating: 2.43 Å; Cl donating: 2.78 Å). The positions of the hydrogen atoms of the NHC backbone have been refined, whereas the positions of the solvent hydrogen atoms have been calculated.

the CH proton of the isopropyl substituent was taken as a probe for the superposed signal sets of the different species. As can be seen in Figure 6, the complex decomposes to the corresponding imidazolium salt by successive addition of 6678


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Figure 6. NMR titration experiment of 5g (orange layer) in CDCl3 with increasing equivalents of HOTFA showing the proposed intermediate 6g (green layer) and 4g-OTFA (blue layer).

Figure 7. NMR titration experiment of 5g (orange layer) in CDCl3 with increasing equivalents of HCl (4N in dioxane) showing the formation of the proposed intermediate 7g (red layer) and imidazolium salt 4g (blue layer).

Photoluminescence Measurements. The absorption and emission spectra of complexes 5a, 5b, and 5d−5g are shown in Figures 10 and 11. The absorption wavelengths of the complexes are almost exactly at 220 nm and seem to be independent from the substituent (Figure 10). In contrast, a significant influence of the substituent on the emission wavelength could be observed (Figure 11). The

(KHMDS) as a non-nucleophilic base to an NMR probe of 5g in acidic CDCl3, and the resulting spectrum shows only the signal set for complex 5g (Figure 9). The possibility to protonate the carbene carbon atoms can be interesting for a possible use in catalytic systems, for which a temporary opening of the relatively stable carbene−metal bond may be desirable. 6679


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Scheme 5. Possible Mechanisms of Hydrolysis of 5g with HOTFA and HCl

Figure 9. 5g in acidic CDCl3 prior to (bottom) and after (top) addition of KHMDS.

Figure 8. NMR experiment of 5g in acidic CDCl3 and addition of aluminum oxide showing the decrease of 7g (blue layer) relative to 5g (orange layer) and evolving signal for 2g (red layer).

complexes with aliphatic substituents (5d−5g) as well as 5b show emission wavelength maxima between 417 and 438 nm, whereas the aromatic-substituted complex 5a shows the maximum of the emission wavelength at 480 nm. Complex 5c showed a quantum yield below 3%, and therefore, the spectrum could not be measured. In Table 2, the photoluminescence data for complexes 5a, 5b, and 5d−5g are summarized. The CIE (Commission International de L′Eclairage) color coordinates of the photoluminescence emission show that the complexes emit in the blue area of the spectrum.

Figure 10. UV−vis absorption spectra of complexes 5a, 5b, and 5d− 5g in PMMA (2%).

A comparison of the emission wavelength maxima and the silver−silver distances reveals a fairly good correlation. Unfortunately, we could not grow a suitable crystal of 5f yet, but the remaining data are in good agreement with a linear 6680


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ligand and its silver complexes provide detailed information on the new anionic ligand type. It is one of the few multichelating anionic NHC ligands with potential applications in photophysics and catalysis, with the interesting feature of being able to reversibly open a carbene−metal bond. The emission data show that the dinuclear complexes presented here are promising emitters in the blue region of the spectrum. To gain a more detailed insight in the bonding situation of our system and to possibly correlate theoretical results to the spectroscopic measurements, we are currently performing theoretical studies using density functional theory methods.

4. EXPERIMENTAL SECTION General Experimental Methods. All chemicals used were obtained from commercial suppliers and used without further purification. Imidazoles 2a−2g were synthesized according to published procedures.33,34 The NMR spectra were recorded with a Bruker AC 300 P spectrometer and referenced internally to the references of the solvent. 1H NMR spectra were recorded at 300.13 MHz, 13C NMR spectra at 75.475 MHz. Elemental analyses were measured by the analytical laboratory of the department using a Eurovektor Hekatech EA-3000 elemental analyzer. General Procedure for the Synthesis of Bis(imidazolium) Salts 4. Up to 5 equiv of the according imidazole in acetonitrile (ca. 10 mL/mL imidazole) is added dropwise to a solution of cyanuric chloride in acetonitrile (5 mL/mmol cyanuric chloride at 0 °C), followed by formation of a precipitate. The suspension is stirred for 30 min at room temperature, 30 min at 60 °C, and refluxed for 4.5 h at 110 °C. The resulting precipitate is filtrated, washed with (hot) acetonitrile, and dried in vacuo. In most cases, the solvent could not be completely removed even under high vacuum and shows the corresponding solvent peaks in the NMR. Synthesis of 4,6-Bis(1-mesitylimidazolium-3-yl)-1,3,5-triazine-2olate-chloride Hydrochloride 4a. Cyanuric chloride (1.000 g, 5.4 mmol) is dissolved in 25 mL of MeCN. After addition of 5.050 g (27.1 mmol) of 2a in 25 mL of MeCN at 0 °C, the solution turns dark brown. The temperature program and work up, as described above, gives the product as a white powder. Yield: 1.153 g (43%). 1H NMR (DMSO-d6): δ 10.52 (s, 2H, NCHN), 8.83 (s, 2H, N-CHCH), 8.18 (s, 2H, N-CHCH), 7.18 (s, 4H, CHaromatic), 2.34 (s, 6H, CH3para), 2.10 (s, 12H, CH3ortho) ppm. 13C NMR (DMSO-d6): δ 165.21 (Cipso), 159.64 (Cipso), 140.51 (Cipso), 137.33 (CH), 134.17 (Cipso), 131.97 (Cipso), 129.22 (CH), 125.00 (CH), 119.65 (CH), 20.51 (CH3), 16.91 (CH3) ppm. Anal. Calcd for C27H30N7OCl2: C, 60.22; H, 5.43; N, 18.21. Found: C, 59.97; H, 5.79; N, 18.46. Synthesis of 4,6-Bis(1-(4-methylphenyl)imidazolium-3-yl)-1,3,5triazine-2-olate-chloride Hydrochloride 4b. Cyanuric chloride (1.113 g, 6.0 mmol) is dissolved in 30 mL of MeCN. After addition of 4.774 g (30.0 mmol) of 2b in 30 mL of MeCN at 0 °C, the temperature program and work up, as described above, as well as recrystallization from ethanol gives the product as a bright yellow powder. Yield: 0.850 g (32%). 1H NMR (DMSO-d6): δ 10.79 (s, 2H, N-CHN), 8.82 (s, 2H, N-CHCH), 8.53 (s, 2H, N-CHCH), 7.88 (d, J = 8.42 Hz, 4H, CHaromatic), 7.49 (d, J = 8.51 Hz, 4H, CHaromatic), 2.42 (s, 6H, CH3para) ppm. 13C NMR (DMSO-d6): δ 159.79 (Cipso), 140.29 (Cipso), 134.84 (CH), 132.25 (Cipso), 130.38 (CH), 122.54 (CH), 122.36 (CH), 119.64 (CH), 20.65 (CH 3) ppm. (The Cipso signal for the carbonyl carbon was not detected). Anal. Calcd for C23H21N7OCl2·2H2O·MeCN: C, 53.67; H, 5.04; N, 20.03. Found: C, 53.66; H, 4.89; N, 20.56. Synthesis of 4,6-Bis(1-(4-methoxyphenyl)imidazolium-3-yl)-1,3,5triazine-2-olate-chloride Hydrochloride 4c. Cyanuric chloride (1.844 g, 10.0 mmol) is dissolved in 50 mL of MeCN. After addition of 8.71 g (50.0 mmol) of 2c in 50 mL of MeCN at 0 °C, the temperature program and work up, as described above, as well as recrystallization from ethanol gives the product as a bright yellow powder. Yield: 2.998 g (62%). 1H NMR (DMSO-d6): δ 10.82 (s, 2H, N-CHN), 8.79 (s, 2H, N-CHCH), 8.49 (s, 2H, N-CHCH), 7.94 (d, J = 9.00 Hz, 4H, CHaromatic), 7.21 (d, J = 9.04 Hz, 4H,

Figure 11. Emission spectra of complexes 5a, 5b, and 5d−5g (excitation wavelength = 290 nm (300 nm for 5b and 5e), in PMMA 2%).

Table 2. Photoluminescence Data for Complexes 5a, 5b, and 5d−5g f 5a 5b 5d 5e 5f 5g

λexc [nm]a

XRGBb

YRGBb

λem [nm]c

ϕd

τM [μs]e

290 300 290 300 290 290

0.211 0.183 0.177 0.169 0.168 0.174

0.289 0.177 0.142 0.133 0.112 0.149

480 444 424 435 417 434

0.21 0.03 0.10 0.07 0.26 0.12

28.4 11.5 16.7 12.6

a

Excitation wavelength. bCIE coordinates at room temperature. Emission wavelength. dQuantum yield, radiant exposure under N2. e Average decay lifetime, not measured for ϕ < 0.10. fMeasured in 2% PMMA at RT. c

trend between the emission wavelength and the Ag···Ag distance (Figure 12).

Figure 12. Correlation between the Ag···Ag distance in the crystal (solid line) and the measured wavelength maxima (dotted line). For 5a, the average of the Ag···Ag distance of the crystal structure reported earlier37 and reported here was used. The coefficient of determination for a linear regression for a dependency of the wavelength against the Ag···Ag distance is R2 = 0.96, which indicates that the silver···silver interaction plays an important role in the excitation process.

3. CONCLUSION We were able to synthesize and fully characterize neutral dinuclear silver complexes with a new type of triazine-based bidentate NHC ligand. The structural and analytical data of the 6681


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CHaromatic ), 3.86 (s, 6H, CH3para ) ppm. 13C NMR (DMSO-d6): δ 165.59 (Cipso), 160.65 (Cipso), 160.08 (Cipso), 135.09 (CH), 127.88 (Cipso), 124.40 (CH), 122.99 (CH), 119.74 (CH), 115.26 (CH), 56.09 (CH3) ppm. Anal. Calcd for C23H21N7O3Cl2·H2O·MeCN: C, 52.36; H, 4.57; N, 19.54. Found: C, 52.24; H, 4.65; N, 19.94. Synthesis of 4,6-Bis(1-cyclohexylimidazolium-3-yl)-1,3,5-triazine2-olate-chloride Hydrochloride 4d. Cyanuric chloride (3.688 g, 20.0 mmol) is dissolved in 100 mL of MeCN. After addition of 12 g (80.0 mmol) of 2d in 100 mL of MeCN at 0 °C, a yellow precipitate occurs. The temperature program and work up, as described above, gives the product as a bright yellow powder. Yield: 3.096 g (33%). 1H NMR (DMSO-d6): δ 10.42 (s, 2H, N-CHN), 8.58 (s, 2H, N-CHCH), 8.15 (s, 2H, N-CHCH), 4.44 (m, 2H, CHcyclohexyl), 2.10 (m, 4H, CH2 cyclohexyl), 1.90 (m, 8H, CH2 cyclohexyl), 1.70 (m, 2H, CH2 cyclohexyl), 1.30 (m, 6H, CH2 cyclohexyl) ppm. 13C NMR (DMSO-d6): δ 165.24 (Cipso), 159.68 (Cipso), 135.05 (CH), 121.77 (CH), 118.87 (CH), 59.72 (CH), 32.08 (CH2), 24.56 (CH2), 24.15 (CH2) ppm. Anal. Calcd for C21H29N7OCl2·MeCN: C, 54.44; H, 6.36; N, 22.08. Found: C, 54.59; H, 6.43; N, 22.40. Synthesis of 4,6-Bis(1-cyclopentylimidazolium-3-yl)-1,3,5-triazine-2-olate-chloride Hydrochloride 4e. Cyanuric chloride (1.844 g, 10.0 mmol) is dissolved in 50 mL of MeCN. After addition of 6.81 g (50.0 mmol) of 2e in 50 mL of MeCN at 0 °C, a bright yellow precipitate occurs. The temperature program and work up, as described above, gives the product as a bright yellow powder. Yield: 0.598 g (14%). 1H NMR (DMSO-d6): δ 10.26 (s, 2H, N-CHN), 8.57 (s, 2H, N-CHCH), 8.08 (s, 2H, N-CHCH), 4.89 (m, 2H, CHcyclopentyl), 2.25 (m, 4H, CH2 cyclopentyl), 2.10−1.40 (m, 16H, CH2 cyclopentyl) ppm. 13 C NMR (DMSO-d6): δ 165.00 (Cipso), 159.48 (Cipso), 135.08 (CH), 121.66 (CH), 119.09 (CH), 61.10 (CH), 32.22 (CH2), 23.08 (CH2) ppm. Anal. Calcd for C19H26N7OCl2·H2O·0.5MeCN: C, 50.26; H, 6.22; N, 21.98. Found: C, 50.71; H, 6.73; N, 21.52. Synthesis of 4,6-Bis(1-tert-butylimidazolium-3-yl)-1,3,5-triazine2-olate-chloride Hydrochloride 4f. Cyanuric chloride (1.844 g, 10.0 mmol) is dissolved in 50 mL of MeCN. After addition of 5 mL (37.0 mmol, ρ = 0.92 g cm−3) of 2f in 50 mL of MeCN at 0 °C, a bright yellow precipitate occurs. The temperature program and work up, as described above, gives the product as a bright yellow powder. Yield: 3.102 g (82%). 1H NMR (DMSO-d6): δ 10.21 (s, 2H, N-CHN), 8.70 (s, 2H, N-CHCH), 8.26 (s, 2H, N-CHCH), 1.70 (s, 9H, CH3) ppm. 13C NMR (DMSO-d6): δ 165.29 (Cipso), 159.88 (Cipso), 134.30 (CH), 121.36 (CH), 119.29 (CH), 60.98 (C ipso), 28.91 (CH3) ppm. Anal. Calcd for C17H25N7OCl2·2MeCN: C, 50.81; H, 6.29; N, 25.39. Found: C, 50.80; H, 6.40; N, 25.25. Synthesis of 4,6-Bis(1-isopropylimidazolium-3-yl)-1,3,5-triazine2-olate-chloride Hydrochloride 4g. Cyanuric chloride (1.844 g, 10.0 mmol) is dissolved in 50 mL of MeCN. After addition of 5 mL (44.0 mmol, ρ = 0.96 g cm−3) of 2g in 50 mL of MeCN at 0 °C, a bright yellow precipitate occurs. The temperature program and work up, as described above, gives the product as a bright yellow powder. Yield: 3.254 g (93%). 1H NMR (DMSO-d6): δ 10.52 (s, 2H, N-CHN), 8.61 (s, 2H, N-CHCH), 8.18 (s, 2H, N-CHCH), 4.82 (sept, J = 6.7 Hz, 1H, CH), 1.60 (s, 6H, CH3), 1.58 (s, 6H, CH3) ppm. 13C NMR (DMSO-d6): δ 165.33 (Cipso), 159.77 (Cipso), 135.21 (CH), 121.43 (CH), 119.10 (CH), 53.38 (CH), 22.13 (CH 3) ppm. Anal. Calcd for C15H21N7OCl2·H2O·MeCN: C, 45.85; H, 5.88; N, 25.16. Found: C, 46.02; H, 5.95; N, 25.28. Synthesis of Di-[4,6-bis(1-mesitylimidazoline-2-ylidene-3-yl)-5H1,3,5-triazin-2-one-5-ide]-disilver(I) 5a. In a light-protected 25 mL round-bottom flask, 269 mg (0.5 mmol) of bis(imidazolium) salt 4a and 142 mg (0.6 mmol) of silver(I)-oxide are suspended in 15 mL of dichloromethane. After 24 h of stirring at room temperature, the suspension is filtered through Celite and concentrated to a volume of about 5 mL. The raw product is precipitated by addition of 20 mL of diethylether, afterward washed twice with 10 mL of diethylether, and then dried in vacuo to give the product as a white solid. Analytically pure material could be obtained upon crystallization from a solution in a dichloromethane diethylether mixture, and crystals suitable for X-ray diffraction could be grown by slowly condensing diethylether into a dichloromethane solution of 5a. Yield: 183 mg (64%). 1H NMR

(CDCl3): δ 8.26 (s, 4H, N-CHCH), 6.83 (s, 4H, N-CHCH), 6.80 (s, 4H, CHaromatic), 6.75 (s, 4H, CHaromatic), 2.26 (s, 12H, CH3 para), 1.75 (s, 4H, H2O), 1.60 (s, 12H, CH3 ortho), 1.54 (s, 12H, CH3 ortho) ppm. 13C NMR (CDCl3): δ 185.83 (dd, Ccarbene), 168.32 (Cipso), 163.08 (Cipso), 139.93 (Cipso), 135.58 (CH), 134.55 (Cipso), 133.37 (Cipso), 129.29 (CH), 129.11 (CH), 122.54 (CH), 119.68 (CH), 21.06 (CH3), 17.37 (CH3), 16.80 (CH3) ppm. ESI/MS (m/z): 1143.2 [M + H]+. Anal. Calcd for C54H52Ag2N14O2: C, 56.65; H, 4.58; N, 17.13. Found: C, 56.58; H, 4.33; N, 17.21. Synthesis of Di-[4,6-bis(1-(4-methylphenyl)imidazoline-2-ylidene-3-yl)-5H-1,3,5-triazin-2-one-5-ide]-disilver(I) 5b. In a light-protected 25 mL round-bottom flask, 241 mg (0.5 mmol) of bis(imidazolium) salt 4b and 139 mg (0.6 mmol) of silver(I)-oxide are suspended in 15 mL of dichloromethane. After 24 h of stirring at room temperature, the suspension is filtered through Celite and concentrated to a volume of about 5 mL. The raw product is precipitated by addition of 20 mL of diethylether, afterward washed twice with 10 mL of diethylether, and then dried in vacuo to give the product as a white solid. Crystals suitable for X-ray diffraction could be grown from slowly condensing diethylether into a dichloromethane solution of 5b. Yield: 123 mg (24%). 1H NMR (CDCl3): δ 8.38 (s, 4H, N-CH CH), 7.25 (s, 4H, N-CHCH), 7.01 (s, 4H, CHaromatic), 2.31 (s, 12H, CH3 para) ppm. 13C NMR (75 MHz; CDCl3): δ 183.1 (dd, Ccarbene), 168.8 (Cipso), 162.7 (Cipso), 139.8 (Cipso), 137.4 (Cipso), 130.3 (CH), 122.7 (CH), 121.9 (CH), 120.0 (CH), 21.1(CH 3) ppm. ESI/MS (m/z): 1033.3 [M + H]+. Anal. Calcd for C46H36Ag2N14O2·3.3H2O: C, 50.59; H, 3.93; N, 17.96. Found: C, 50.15; H, 3.93; N, 17.96. (The solvent could not be completely removed even after 2 days under high vacuum. The solid-state structure also shows cavities with an electron density of disordered solvent molecules.) Synthesis of Di-[4,6-bis(1-(4-methoxyphenyl)imidazoline-2-ylidene-3-yl)-5H-1,3,5-triazin-2-one-5-ide]-disilver(I) 5c. In a light-protected 25 mL round-bottom flask, 514 mg (1.0 mmol) of bis(imidazolium) salt 4c and 162 mg (0.7 mmol) of silver(I)-oxide are suspended in 15 mL of dichloromethane. After 24 h of stirring at room temperature, the suspension is filtered through Celite and concentrated to a volume of about 5 mL. The raw product is precipitated by addition of 20 mL of diethylether, afterward washed twice with 10 mL of diethylether, and then dried in vacuo to give the product as a white solid. Yield: 185 mg (17%). 1H NMR (300 MHz; CDCl3): δ 8.35 (s, 4H, N-CHCH), 7.24 (s, 4H, N-CHCH), 6.98 (d, J = 9.0 Hz, 4H, CHaromatic), 6.74 (d, J = 9.0 Hz, 4H, CHaromatic), 3.79 (s, 12H, OCH3 para) ppm. 13C NMR (75 MHz; CDCl3): δ 168.7 (Cipso), 162.8 (Cipso), 160.2 (Cipso), 132.8 (Cipso), 124.2 (CH), 122.2 (CH), 119.9 (CH), 114.8 (CH), 55.9 (CH3) ppm. ESI/MS (m/z): 1097.1 [M + H]+. Anal. Calcd for C46H36Ag2N14O6·1.55H2O: C, 49.13; H, 3.50; N, 17.44. Found: C, 48.71; H, 3.05; N, 17.19. (The solvent could not be completely removed even under high vacuum.) Synthesis of Di-[4,6-bis(1-cyclohexylimidazoline-2-ylidene-3-yl)5H-1,3,5-triazin-2-one-5-ide]-disilver(I) 5d. In a light-protected 25 mL round-bottom flask, 233 mg (0.5 mmol) of bis(imidazolium) salt 4d and 142 mg (0.6 mmol) of silver(I)-oxide are suspended in 15 mL of dichloromethane. After 24 h of stirring at room temperature, the suspension is filtered through Celite and concentrated to a volume of about 5 mL. The raw product is precipitated by addition of 20 mL of diethylether, afterward washed twice with 10 mL of diethylether, and then dried in vacuo to give the product as a white solid. Crystals suitable for X-ray diffraction could be grown from slowly condensing diethylether into a dichloromethane solution of 5d. Yield: 174 mg (70%). 1H NMR (CDCl3): δ 8.09 (s, 4H, N-CHCH), 6.96 (s, 4H, N-CHCH), 3.83 (m, 4H, CH), 1.75 (s, 8H, H 2O), 0.80−1.75 (m, 40H, CH3) ppm. 13C NMR (CDCl3): δ 182.44 (dd, Ccarbene), 168.79 (Cipso), 162.70 (Cipso), 118.83 (CH), 62.27 (CH), 35.23 (CH2), 32.18 (CH2), 25.27 (CH2), 25.20 (CH2), 24.85 (CH2) ppm. ESI/MS (m/z): 999.3 [M + H]+. Anal. Calcd for C42H52Ag2N14O2·0.8CH2Cl2: C, 48.10; H, 5.06; N, 18.35. Found: C, 48.32; H, 5.31; N, 18.09. (The solvent could not be completely removed even under high vacuum. The solid-state structure also shows cavities, with an electron density of disordered solvent molecules.) 6682


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Synthesis of Di-[4,6-bis(1-cyclopentylimidazoline-2-ylidene-3-yl)5H-1,3,5-triazin-2-one-5-ide]-disilver(I) 5e. In a light-protected 25 mL round-bottom flask, 44 mg (0.1 mmol) of bis(imidazolium) salt 4e and 28 mg (0.12 mmol) of silver(I)-oxide are suspended in 10 mL of acetonitrile. After 24 h of stirring at room temperature, crystals had formed that were suitable for X-ray diffraction and which were filtrated, washed with ether, and dried in vacuo. Yield: 22 mg (23%). 1H NMR (CDCl3): δ 8.12 (s, 4H, N-CHCH), 6.97 (s, 4H, N-CHCH), 4.41 (m, 4H, CH), 2.05−1.95 (m, 4H, CH2), 1.74−1.48 (m, 20H, CH2), 1.40−1.22 (m, 8H, CH2) ppm. 13C NMR (CDCl3): δ 182.8 (dd, Ccarbene), 169.0 (Cipso), 162.8 (Cipso), 119.4 (CH), 63.67 (CH), 34.63 (CH2), 33.10 (CH2), 24.58 (CH2), 24.53 (CH2) ppm. ESI/MS (m/z): 945.3 [M + H]+. Anal. Calcd for C38H44Ag2N14O2·H2O: C, 47.41; H, 4.82; N, 20.37. Found: C, 47.21; H, 4.84; N, 20.34. (The solvent could not be completely removed even under high vacuum.) Synthesis of Di-[4,6-bis(1-tert-butylimidazoline-2-ylidene-3-yl)5H-1,3,5-triazin-2-one-5-ide]-disilver(I) 5f. In a light-protected 25 mL round-bottom flask, 206 mg (0.5 mmol) of bis(imidazolium) salt 4f and 142 mg (0.6 mmol) of silver(I)-oxide are suspended in 15 mL of dichloromethane. After 24 h of stirring at room temperature, the suspension is filtered through Celite and concentrated to a volume of about 5 mL. The raw product is precipitated by addition of 20 mL of diethylether, afterward washed twice with 10 mL of diethylether, and then dried in vacuo to give the product as a white solid. Yield: 91 mg (40%). 1H NMR (CDCl3): δ 8.10 (s, 4H, N-CHCH), 7.14 (s, 4H, N-CHCH), 1.67 (s, 13.2H, H2O), 1.37 (d, 34H, CH3) ppm. 13C NMR (CDCl3): δ 182.11 (dd, Ccarbene), 168.62 (Cipso), 162.79 (Cipso), 118.77 (CH), 118.15 (CH), 57.88 (Cipso), 30.73 (CH3) ppm. ESI/MS (m/z): 895.2 [M + H]+. Anal. Calcd for C34H44Ag2N14O2·2H2O: C, 43.79; H, 5.19; N, 21.03. Found: C, 44.00; H, 4.88; N, 20.74. (The solvent could not be completely removed even under high vacuum.) Synthesis of Di-[4,6-bis(1-isopropylimidazoline-2-ylidene-3-yl)5H-1,3,5-triazin-2-one-5-ide]-disilver(I) 5g. In a light-protected 25 mL round-bottom flask, 193 mg (0.5 mmol) of bis(imidazolium) salt 4g and 142 mg (0.6 mmol) of silver(I)-oxide are suspended in 15 mL of dichloromethane. After 24 h of stirring at room temperature, the suspension is filtered through Celite and concentrated to a volume of about 5 mL. The raw product is precipitated by addition of 20 mL of diethylether, afterward washed twice with 10 mL of diethylether, and then dried in vacuo to give the product as a white solid. Crystals suitable for X-ray diffraction could be grown from slowly condensing diethylether into a dichloromethane solution of 5g. Yield: 115 mg (55%). 1H NMR (CDCl3): δ 8.10 (s, 4H, N-CHCH), 6.97 (s, 4H, N-CHCH), 4.30 (sept, 4H, CH), 1.30 (d, 12H, CH 3), 0.97 (d, 12H, CH3) ppm. 13C NMR (CDCl3): δ 182.10 (dd, Ccarbene), 168.91 (Cipso), 162.73 (Cipso), 119.39 (CH), 117.95 (CH), 54.71 (CH), 23.69 (CH 3), 23.01 (CH3) ppm. ESI/MS (m/z): 839.1 [M + H]+. Anal. Calcd for C30H36Ag2N14O2·3.9H2O: C, 39.56; H, 4.85; N, 21.53. Found: C, 40.14; H, 4.49; N, 20.96. (The solvent could not be completely removed even under high vacuum. The solid-state structure also shows cavities, with an electron density of disordered solvent molecules.) Structure Determination. Preliminary examination and data collection were carried out on a NONIUS κ-CCD device with an Oxford Cryosystems cooling system at the window of a sealed finefocus X-ray tube with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The reflections were integrated. Raw data were corrected for Lorentz and polarization effects and, arising from the scaling procedure, for latent decay. An absorption correction was applied using SADABS.38 After merging, the independent reflections were all used to refine the structure. The structures were solved by a combination of direct methods39,40 and difference Fourier synthesis.41 All non-hydrogen atom positions were refined with anisotropic displacement parameters. All hydrogen atoms were placed in calculated positions and those afterward refined using the SHELXL97 riding model; for denoted cases, the positions were refined. Fullmatrix least-squares refinements were carried out by minimizing ∑w(Fo2 − Fc2)2 with the SHELXL-97 weighting scheme and stopped at shift/err < 0.001. In the cases, for which the residual electron density could not be localized to specific atom positions, the PLATON-SQUEEZE 42,43 was used. Details of the structure

determinations are given in the Supporting Information. Neutral-atom scattering factors for all atoms and anomalous dispersion corrections for the non-hydrogen atoms were taken from the International Tables for Crystallography.44 All calculations were performed with the programs COLLECT,45 DIRAX,46 EVALCCD,47 SIR92,39 SIR97,40 SADABS,38 the SHELXL-97 package,41,48,49 and ORTEP-III.50 Graphics were generated and processed with the programs PLATON,42,43 ORTEPIII,50 MERCURY,51 and POV-RAY.52

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ASSOCIATED CONTENT * Supporting Information The crystallographic information files of complexes 5a, 5b, 5d, 5e, and 5g and the crystallographic details of the measurements. This material is available free of charge via the Internet at http://pubs.acs.org. S

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

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ACKNOWLEDGMENTS We thank Dr. Molt (BASF), Dr. Fuchs (BASF), and Dr. Lennartz (BASF) for helpful discussions and Dr. Wagenblast (BASF) for the measurement of the photoluminescence data. A.P. is grateful for financial support by the Studienstiftung des deutschen Volkes and the Fonds der Chemischen Industrie (FCI).

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REFERENCES

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Neutral Dinuclear Silver(I)âNHC Complexes: Synthesis and

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