Spectroscopic and Structural Properties of Bridge-Functionalized

Article pubs.acs.org/Organometallics

Spectroscopic and Structural Properties of Bridge-Functionalized Dinuclear Coinage-Metal (Cu, Ag, and Au) NHC Complexes: A Comparative Study Rui Zhong, Alexander Pöthig,* David C. Mayer, Christian Jandl, Philipp J. Altmann, Wolfgang A. Herrmann,* and Fritz E. Kühn* Chair of Inorganic Chemistry/Molecular Catalysis, Catalysis Research Center and Department of Chemistry, Technische Universität München, Ernst-Otto-Fischer-Straße 1, 85747 Garching b. München, Germany S Supporting Information *

ABSTRACT: Hydroxymethyl-functionalized methylene-bridged dinuclear N-heterocyclic bis(NHC) complexes with Ag(I) (2), Cu(I) (3) and Au(I) (4) have been synthesized and characterized by spectroscopic methods and X-ray diffraction studies. Depending on the metal employed, different isomers are formed. The structural properties and differences of all three new compounds are compared to those of previously reported nonfunctionalized methylene-bridged analogues. DFT calculations were conducted to gain a better understanding of the molecular arrangement observed.

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INTRODUCTION Coinage-metal NHC complexes have been widely investigated for their variable applications and diverse structural properties.1 In comparison to the intriguing progress of Ag- and Au-NHC complexes, the Cu-NHC complexes have been less explored, but their investigation has recently emerged as a burgeoning field of research, especially in homogeneous catalysis due to their potential catalytic activities and the relatively cheap price of copper.2 Since the first Ag-NHC complex reported by Arduengo in 1993,3 more than 100 articles regarding Ag-NHCs have been published for their various applications such as NHC transfer reagents, homogeneous catalysts, and luminescence materials.1c,d The respective Au-NHC complexes have achieved remarkable success in catalysis as well as in luminescent chemosensors over the past few years.2a,4 In addition, considerable effort has also been devoted to the medicinal applications of Ag- and Au-NHC complexes in recent years because of their high physiological stability and ease of modification toward desirable properties.5 Therefore, this attractive area of coinage-metal NHCs will continuously catch chemists’ attention, certainly with a focus on highly versatile NHC systems, which can further facilitate the applications of coinage-metal complexes. The bridge functionalization of the methylene-bridged bis(NHC) ligand provides facile access to tunable bis(NHC) complexes and extended applications of the latter in comparison to their nonfunctionalized analogues. Considering the wide use of coinage-metal NHC complexes, further investigation of bridge-functionalized bis(NHC) ligands with © XXXX American Chemical Society

different coinage metals would be of great interest. Recently, we reported a bis(NHC) ligand moiety featuring a hydroxymethylfunctionalized methylene bridge which allowed for successful synthesis of immobilized and water-soluble Pd catalysts (Chart 1).6 Chart 1. Recently Reported Hydroxymethyl-Functionalized Bis(NHC) Pd Motif and Its Applications6

To the best of our knowledge, no studies describing bis(NHC) complexes including all three coinage metals bound to the same bis(NHC) ligand moiety have been reported. Therefore, a comparative study between different coinage metals would provide further insight into the inherent properties of coinage-metal NHC complexes, hence facilitating their design and application in the future. In addition, it is Special Issue: Mike Lappert Memorial Issue Received: December 4, 2014

A

DOI: 10.1021/om5012402 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

respect to silver complex 2, the Ccarbene resonance cannot be observed. The absence of this resonance most probably results from poor relaxation of the quaternary Ccarbene of silver NHCs, as previously reported for some other Ag(I)-NHC complexes.1c,11 The 1H and 13C NMR spectra of all three dinuclear complexes present mostly similar signal distributions in comparison to the ligand precursor 1,6 indicating highly symmetrical structures for all three complexes. The NMR spectra also provide a potential tool to estimate the electronic properties of these three complexes. In the 1H NMR spectra, the signals for the NHC backbone (Figure 1b,c) of Au(I)

interesting to investigate what influence the bridge functionalization has in comparison to the nonfunctionalized analogues. In this work, we report the synthesis and characterization of three dinuclear coinage-metal complexes (Cu(I), Ag(I), Au(I)) bound to the bridge-functionalized bis(NHC) ligand shown in Chart 1. The spectroscopic properties and solid-state structures of all three bis(NHC) complexes are presented and compared to those of their nonfunctionalized analogues. Theoretical calculations allow for a better understanding of the differences in structures observed depending on the coinage metal used.

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RESULTS AND DISCUSSION Synthesis of Complexes. The hydroxyl-functionalized bis(imidazolium) salt 1 was prepared according to our previously reported procedure.6 The respective Ag(I) complex 2 could be synthesized using a modified protocol of Youngs et al. (Scheme 1),7 by reacting 1 with Ag2O (2 equiv) in water Scheme 1. Syntheses of Ag(I) Complex 2, Cu(I) Complex 3, and Au(I) Complex 4

Figure 1. Comparative 1H NMR spectra of complexes 2−4 in acetonitrile-d3.

complex 4 (7.32 and 7.61 ppm) are shifted further downfield as compared to those of complexes 2 and 3. This observation is in agreement with the downfield-shifted signal of the Ccarbene of complex 4 (74.3 ppm) in comparison to that of complex 3 in the 13C NMR spectra. This illustrates a strong deshielding effect of the Au(I) cation, leading to a low electron density on the NHC of complex 4. For further comparison, we synthesized the nonfunctionalized Cu(I), Ag(I), and Au(I) bis(NHC) complexes according to the literature7,8b,9 and compared their NMR spectra (see the Supporting Information). In the NMR spectra of the nonfunctionalized bis(NHC) complexes, the same trends could be observed in comparison to those signals of the hydroxymethyl-functionalized bis(NHC) complexes, suggesting that the introduced hydroxymethyl group has no detrimental effect to the electronic properties of the NHC ligands in complexes 2−4. However, for complex 4 the signal for the proton of the methylene bridge (Figure 1a, 7.48 ppm) is shifted downfield to a larger extent than in the nonsubstituted complex. This points toward a significant structural difference of 4 in comparison to 2 and 3, for which such a pronounced shift is not observed. The IR spectra of all three complexes 2−4 show similar absorption bands with moderately shifted wavelengths (mostly less than 10 cm−1; see the Supporting Information). In comparison to the analogue of ligand 1 with PF6− as counterion, all three complexes give rise to obvious red-shifted absorption bands, evidenced by the identifiable absorption bands attributed to the CC bond of the NHC ring of complexes 2−4 at 1398, 1399, and 1403 cm−1, respectively, whereas the ligand absorbs at 1548 cm−1.6 Additionally, the absorption and emission properties of complexes 2−4 were investigated. As shown in Figure 2a, all three complexes show intense absorption bands (in CH3CN at room temperature) in the range of 200−250 nm (228 nm for 2,

with exclusion of light at room temperature for 3 h, followed by addition of an excess amount of NH4PF6. On the basis of Ag(I) complex 2, the corresponding Cu(I) complex (3) and the Au(I) complex (4) were synthesized in good yields (97% and 96%, respectively) via transmetalation in acetonitrile using the coinage-metal precursors CuI and AuCl(SMe2), respectively.8 In addition, direct metalation of ligand 1 with AuCl(SMe2) in the presence of NaOAc was tested by adopting a described procedure9 and successfully gave complex 4 in 26% yield. It is worth noting that complex 3 is moisture and air sensitive, while complexes 2 and 4 are air and water stable and even exhibit limited solubility in water (approximately 1 mg/mL). Spectroscopic Studies. All three complexes 2−4 were characterized by NMR, mass, IR, and UV/vis spectroscopy. The ESI mass spectra show signals for [M − PF6−]+ at m/z 771.98 and 949.93 for complexes 2 and 4 and [M + H2O + H+]+ at m/z 849.94 for complex 3, respectively, and hence is the first evidence for the formation of a dimeric structure. The 1H NMR spectra of all three complexes show the absence of the carbenium proton resonance of ligand 1 at approximately 10 ppm. In the 13C NMR spectra, complexes 3 and 4 show the signals for the Ccarbene at 178.8 and 185.7 ppm, respectively, which are consistent with signals for the corresponding known metal NHC complexes.1a,4b,10 With B

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Figure 2. (a) UV/vis absorption spectra of complexes 2−4 (solid line) and the nonfunctionalized coinage-metal bis(NHC) complexes (Ag, Cu, and Au, dotted line) in acetonitrile solution. (b) Emission spectra of complexes 2−4 in CH3CN (solid line) and the nonfunctionalized coinage-metal bis(NHC) complexes (Ag and Au, dotted line), in acetonitrile solution at room temperature upon excitation at 368 nm. All spectra are normalized (for better comparison to a lower maximum for the nonsubstituted complexes).

complex in acetonitrile. The molecular structures in Figures 3 and 4 unambiguously confirm the constitution of the dinuclear complexes, each bearing two bridging bis(NHC) ligands. As shown in Figure 3, Ag(I) complex 2 is confirmed to be a dimeric structure, with the two hydroxymethyl groups lying on different sides of the mean Ccarbene plane defined by the Ccarbene and the Ag atoms. The C−Ag−C bond angles of 174.6(2) and 175.8(2)° slightly deviate from linearity. The bond distances Ag−Ccarbene, in the range of 2.065(5)−2.100(5) Å, are consistent with previously reported data for Ag(I) bis(NHC) complexes.1c,7,11 The Ag···Ag distances of 3.8964(8) and 3.4556(9) Å (two independent molecules in the asymmetric unit) exclude any intramolecular metal−metal interactions, as they are larger than the sum of the van der Waals radii of two silver atoms (3.4 Å) and the reported distance for the unsubstituted complex (3.226 Å).7 The molecular structure of Cu(I) complex 3 shows a similar configuration to that of the Ag(I) complex 2, in which the two bridging carbon atoms lie on different sides of the mean Ccarbene plane defined by the Ccarbene and the Cu atoms. The bond distances Cu−Ccarbene (1.904(3)−1.919(3) Å) lie within the range of the known values of other Cu(I)-NHC complexes.8b,10,13 The two Cu(I) atoms adopt an almost linear geometry, as deduced from the Ccarbene−Cu−Ccarbene bond angles (163.6(1)−174.9(1)°). Furthermore, the intramolecular Cu···Cu distance for one independent molecule (3.889(5) Å) is much longer than the sum of the van der Waals radii of two Cu atoms (2.8 Å) and those reported for analogous bis(NHC) Cu(I) complexes (2.903−3.350 Å),8b,14 suggesting no significant Cu···Cu interactions. However, for a second molecule in the asymmetric unit, a shorter distance is observed (3.1924(5) Å), suggesting that this contact is strongly influenced by packing effects in the crystal. The structure of Au(I) complex 4 (Figure 4) shows a symmetrical dimeric structure with plane symmetry (the mirror plane through the Au−Au axis), which is different from the case for complexes 2 and 3. Interestingly, a water molecule, originating from the solvent (moist NMR solvent of CD3CN), is found to be cocrystallized, bound to two hydroxyl groups via hydrogen bonding (Figure 4, right). It should be noted that the same moist solvent was also used for the crystallization of Ag(I) complex 2, the solid structure of which was found without any water molecules included. In contrast to those of complexes 2 and 3, the two bridging carbon atoms of complex 4 are located on the same side of the mean plane defined by the Ccarbene and the Au atoms.

211 nm for 3, and 248 nm for 4), which can be attributed to π−π* ligand-centered (LC) transitions.12 No further metalcentered (MC) bands could be detected for the Ag(I) and Au(I) complexes. However, for Cu(I) complex 3, a weaker absorption band shoulder could be observed at 323 nm, which may correspond to MC and MLCT electronic transitions according to a previous study by Tsubomura.8b The absorption maxima of complexes 2−4 are shifted toward larger wavelengths in the order Cu < Ag < Au, similarly to the absorption maxima of the nonfunctionalized Cu(I), Ag(I), and Au(I) bis(NHC) complexes (Figure 2a, dotted line). This phenomenon reveals that the shifted absorption bands among these bis(NHC) complexes may derive mainly from the perturbations of the different coordinated metals,12 while the hydroxymethyl group shows no obvious influence. The emission spectra of complexes 2−4 in solution are depicted in Figure 2b. All three complexes in acetonitrile exhibit a deep blue fluorescence with a broad band centered in the range of 425−480 nm upon excitation at 368 nm (Table 1). Table 1. Absorption and Emission Data for Complexes 2−4a complex

λem (nm)

λabs (nm)

complexb

λem (nm)

λabs (nm)

2 (Ag) 3 (Cu) 4 (Au)

461 477 447

228 211 248

bis(NHC)-Ag bis(NHC)-Cu bis(NHC)-Au

476

228 214 253

485

a

Spectra recorded in CH3CN solutions at room temperature; the excitation wavelength is 368 nm. bNonfunctionalized coinage-metal bis(NHC) complexes.

In comparison to Ag(I) complex 2 at 461 nm, the Au(I) complex exhibits a blue-shifted emission at 447 nm, which is similar to an observation made in a report by Hemmert,8a whereas the Cu(I) complex 3 shows a red-shifted emission at 477 nm (Figure 2b, solid line). No further MC emissions, attributed in the literature to metal−metal interactions, are observed, as the metal−metal distances of all three complexes are relatively long (see crystallographic studies). The spectra of the related coinage-metal bis(NHC) compounds without bridge functionalization show somewhat shifted emission maxima in comparison to complexes 2−4 (Figure 2b, dotted line), and the respective Cu(I) complex was observed to not emit in the range of 400−600 nm upon excitation at 368 nm. Crystallographic Studies. Single crystals of complexes 2− 4 suitable for X-ray diffraction spectroscopy were obtained by slow diffusion of diethyl ether into a solution of the respective C

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Figure 3. Molecular structures of complexes 2 (left) and 3 (right). Hydrogen atoms, counterions and solvent molecules have been omitted for clarity. Thermal ellipsoids are drawn at 50% probability. In both cases, two independent molecules can be observed in the asymmetric unit, of which one is shown.

Figure 4. (left) Molecular structure of compound 4. Hydrogen atoms, counterions and solvent molecules have been omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. (right) Hydrogen bonding from the ligand to a cocrystallized water molecule and one of the PF6− anions.

Moreover, the mean planes of the two NHC rings bonded to the same Au atom are slightly tilted with a dihedral angle of 15.88°. The bond angles Ccarbene−Au−Ccarbene show a slight deviation from linearity (175.5 and 171.0°). In addition, the bond distances Au−Ccarbene (2.013(9) and 2.022(8) Å) are comparable with those reported for analogous NHC-Au(I) complexes.8a,9,15 As observed for the Cu and Ag analogues, the Au···Au distance of 3.6401(5) Å is longer than typical distances for metal−metal interactions. The molecular structure of 4 also reveals the reason for the pronounced 1H NMR downfield shift of the methylene bridge. Due to the syn configuration, these protons are in close proximity to the Au(I) centers. With an Au···H distance of 2.85−2.86 Å and an C−H−Au angle of 119° this contact corresponds to an anagostic interaction,16 for which a downfield shift in the 1H NMR is expected. This also proves that the isomers are already formed during the synthesis, not upon crystallization. A closer look at the crystal structure of Au(I) complex 4 shows interesting packing effects in the solid-state structure (Figure 5). The stacking of the dimeric Au(I) complex leads to an intermolecular channel, in which solvent molecules cocrystallized and due to disorder had to be removed during the refinement using the PLATON/SQUEEZE procedure.17 The centroid−centroid distance between the two NHC rings

Figure 5. Crystal packing of complex 4. View along the NHC planes (a axis) showing the formation of tubular channels.

opposed to each other is 8.021 Å, and no obvious π−π stacking interactions between the parallel NHC rings of two neighboring complexes are present. However, this structural motif is highly interesting concerning the formation of cavities with accessible metal centers, giving promise for a possible application in supramolecular transition-metal catalysis. D

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Table 2. Selected Bond Lengths (Å) and Angles (deg) for 2−4 as well as the Reported Nonfunctionalized Bis(NHC) Ag,7 Cu,18 and Au4b Complexes M−Ccarbene M−M N−Cmethyl N−Ccarbene N−Cbridge CC N−Cbridge−N Cbridge−N−Ccarbene

2 (Ag)a

3 (Cu)a

4 (Au)a

bis(NHC)-Ag

bis(NHC)-Cu

bis(NHC)-Aub

2.065(5)-2.100(5) 3.4556(9)-3.8964(8) 1.456(7)-1.483(8) 1.334(6)-1.370(6) 1.446(6)-1.466(6) 1.327(7)-1.351(8) 110.1(4)-111.5(4) 122.5(4)-126.3(4)

1.904(3)-1.919(3) 3.1924(5)-3.8893(5) 1.462(4)-1.466(4) 1.349(4)-1.359(3) 1.459(3)-1.462(3) 1.339(4)-1.344(4) 110.5(2)-110.9(2) 122.6(2)-123.2(2)

2.013(9)-2.022(8) 3.6401(5) 1.47(1) 1.32(1)-1.39(1) 1.46(1) 1.30(1)-1.38(1) 110.0(6) 122.0(7)-123.9(7)

2.096−2.098 3.226 1.467 1.349−1.359 1.452−1.454 1.341−1.349 110.5 123.01−123.67

2.083−2.113 3.353 1.45−1.47 1.33−1.35 1.45−1.47 1.30−1.34 111.3 124.02−124.34

2.025−2.032 3.542 1.448−1.481 1.339−1.369 1.449−1.466 1.329−1.367 110.9 123.98−125.61

a

2 contains two molecules in the asymmetric unit, 3 contains two half-molecules in the asymmetric unit, and 4 contains an intramolecular mirror plane. bThe counteranion of the Au complex was iodide.

For better comparison, Table 2 highlights the selected bond lengths and angles of the structures of complexes 2−4 as well as those of the reported corresponding nonfunctionalized bis(NHC) Ag,7 Au,4b and Cu18 complexes. When the structures of complexes 2−4 are compared, it is found that the M−Ccarbene bond lengths increase in the order Cu < Au < Ag. Since the diameter of a Au atom is similar to that of a Ag atom,19 this observation may indicate that the Au−NHC bond is stronger than the Ag−NHC bond, which is in agreement with their NMR spectra as well as the reported theoretical studies.1a,20 The significant differences in the M−M bond lengths for the independent molecules in the solid-state structures of 2 and 3 show that this distance is influenced by packing effects in the crystal. In both cases an intermolecular contact between the hydroxylic oxygen and the dinuclear metal moieties can be observed, which most likely influences the metal−metal separation, depending on the proximity induced by the packing. Further structural comparison among complexes 3 and 4 and their nonfunctionalized analogues shows that the bond lengths and angles are quite similar with only slight deviations (e.g. N− Ccarbene 1.351(3) Å for 3 and 1.33(1)−1.347(9) Å for bis(NHC)Cu and N−Cbridge−N 110.0(6)° for 4 and 110.92° for bis(NHC)Au). This seems to demonstrate that the bridge functionalization does not have a significant effect on the coordinating properties of the bis(NHC) moiety. Computational Studies. For coinage-metal complexes it is known that bis(NHC) ligands adopt a bridging coordination mode, yielding dinuclear coinage-metal complexes.1a For the nonsubstituted methylene-bridged bis(NHC) ligands, these complexes generally consist of two metal centers, each of which is linearly coordinated by NHC moieties of two different bis(NHC) ligands. Hereby, two conformation isomers are possible, namely the syn or anti conformation (Chart 2a), which both have been reported in the literature4b,9,14,17 and correspond to a boat- or chairlike conformation of cyclohexane. Very recently, this isomerism has been studied in detail for a dinuclear Ag(I)-bis(NHC) with a nonsubstituted methylene bridge and it could be shown that the different isomers can be interconverted by changing the temperature.21 However, such a simple conformational change is not possible for the respective complexes with a monosubstituted methylene bridge. The introduction of the hydroxymethyl substituent reduces the symmetry of the ligand precursor from C2v to Cs. The bridging carbon atom is prochiral, and the imidazolium substituents are enantiotopic, which means, for dinuclear complexes with two ligands in the bridging coordination mode, the combination of two prostereogenic moieties directly results in diastereomers (configuration

Chart 2. Possible Stereoisomers of Dinuclear Coinage-Metal Complexesa,b

a

Nonfunctionalized methylene-bridged bis(NHC) ligands (conformation isomers). bHydroxymethyl (R = CH2OH) functionalized methylene-bridged bis(NHC) ligands (configuration isomers).

isomers). Consequently, the number of possible complex isomers that theoretically can be formed is increased (Chart 2b), in comparison to the complexes with a nonsubstituted methylene bridge, and is equivalent to the discussion of the numbers of stereoisomers in 1,4-disubstituted cyclohexane. Thus, as for the nonsubstituted ligand, the anti or syn configuration can be adopted. Additionally the hydroxymethyl substituents can both point toward the metal centers (“endo”) or away (“exo”) or each of them in the opposite direction (“meso”). As determined by the crystallographic studies, only the exo isomers are observed, most likely because in the endo configuration (and in the meso configuration) a hydroxmethyl substituent would point toward the central metal−metal unit, creating a significant steric repulsion. Using this definition, the Ag(I) complex 2 and Cu(I) complex 3 exhibit an anti,exo configuration (Figure 3), whereas with Au(I) the syn,exo isomer is formed (Figure 4). In order to gain better insight into the correlation between the structure configurations and different coinage metals with this bridge-functionalized ligand, theoretical calculations were carried out. Intermolecular interactions were ignored to conduct the calculations in a molecular theoretic model, which is justified since the formation of the configuration isomers is determined already during synthesis, not upon crystallization. Upon comparison of different theoretical approaches (see the Supporting Information), the ωB97X-D functional, which in terms of M−C and C−C bond lengths showed the best accordance with the respective crystal E

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water molecule stabilizes the syn configuration of the Au(I) complex 4 in acetonitrile via hydrogen bonding. Since the electronic properties are not influenced by the bridge functionalization, the complexes bear great promise for catalytic applications of their immobilized derivatives. Experiments exploring the catalytic properties and further applications of these compounds are currently in the focus of ongoing research in our laboratories.

structures, was then chosen to calculate the free energies of complexes 2−4 in both optimized anti or syn configurations. As expected from the geometrical considerations, the endo and meso configurations were calculated to be significantly higher in energy and therefore exist only virtually. Table 3 gives the free energy values of the calculated coinage complexes in both the gas phase and SMD model with MeCN.

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Table 3. Free Energy Values (kJ/mol) of the Calculated Syn Complexes Referenced to the Corresponding Anti Motif

a

complex

syn,exo

syn,exo (H2O)

syn,exo (H2O)

3 (Cu) 2 (Ag) 4 (Au)

+1.0 −3.6 −5.5

−7.8 −10.3 −16.2

+7.4 +5.9 −0.9

EXPERIMENTAL SECTION

General Methods. Dry and gassed solvents were obtained from a MBraun MB SPS purification system. All chemicals were purchased from commercial suppliers and used without further purification. Liquid NMR spectra were recorded on a Bruker Ultrashield 400 spectrometer (1H NMR, 400.13 MHz; 13C NMR, 100.53 MHz) at 298 K. The spectra were calibrated by using the residual solvent shifts as internal standards. Chemical shifts were referenced in parts per million (ppm). Abbreviations for signal multiplicities are as follows: singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), and broad (br). A Jasco VT-550 photometer was used to conduct UV/vis measurements, with cuvettes of 1 cm path length. Fluorescence spectra were recorded by a Tecan Infinite M200 PRO instrument at room temperature in MeCN solution. A Varian 670 FT-IR spectrometer was used to conduct IR measurements. ESI-MS analyses were performed on a Thermo Scientific LTQ Orbitrap XL by Thermo Fisher Scientific. Elemental analyses were performed by the microanalytical laboratory of the TUM. Synthesis of Ag(I) Complex 2. In a Schlenk tube covered with aluminum foil, 1 equiv of compound 1 (100 mg, 0.359 mmol) and 2.5 equiv of Ag2O (208 mg, 0.897 mmol) were suspended in water (2 mL) and stirred for 3 h at room temperature. After that, the reaction mixture was centrifuged and the solution obtained was then added to a solution of NH4PF6 (120 mg, 0.736 mmol) in water (2 mL) dropwise. After 0.5 h of stirring, the mixture was centrifuged and the precipitate was washed with isopropyl alcohol (2 × 5 mL) and diethyl ether (2 × 5 mL). The gray solid was then recrystallized from acetonitrile/diethyl ether to obtain complex 2 as an off-white solid. Yield: 64.3 mg, 40%. 1 H NMR (400 MHz, acetonitrile-d3, 25 °C): δ 7.57 (2 H, d, J = 4.0 Hz, NCHCHN), 7.28 (2 H, d, J = 4.0 Hz, NCHCHN), 7.03−7.07 (1 H, m, CHCH2OH), 4.39−4.41 (2 H, m, CHCH2OH), 3.80 (1 H, m, CHCH2OH), 3.85 (6 H, s, ImCH3). 13C NMR (100 MHz, acetonitrile-d3, 25 °C): δ 125.4 (NCCN), 119.4 (NCCN), 76.1 (CH), 61.9 (CH2), 39.7 (Im-CH3). ESI-MS ([M]+): m/z 771.98 [2 − PF6−]+. IR spectrum (ν/cm−1): 3181, 3150, 2952, 2894, 1574, 1452, 1399, 1328, 1263, 1223, 1125, 1073, 1055, 963, 832, 751, 737, 678, 661, 557, 521. Anal. Calcd for C20H28F12N8O2P2Ag2: C, 26.76; H, 3.07; N, 12.20. Found: C, 26.95; H, 3.39; N, 12.02. Synthesis of Cu(I) Complex 3. In a Schlenk tube, 1 equiv of complex 2 (50.0 mg, 0.055 mmol) and 2.1 equiv of CuI (22.2 mg, 0.117 mmol) were dissolved in dry acetonitrile (0.625 mL) under argon. The reaction mixture was stirred for 3 h at room temperature. The mixture was filtered under argon, and the obtained solution was concentrated to half volume. Further precipitation by addition of dry diethyl ether gave a brownish solid, which was then filtered and dried under vacuum to give complex 3. Yield: 44.1 mg, 97%. 1H NMR (400 MHz, acetonitrile-d3, 25 °C): δ (ppm) 7.42 (2H, d, J = 4.0 Hz, NCHCHN), 7.17 (2H, d, J = 4.0 Hz, NCHCHN), 6.78 (1H, s, CHCH2), 4.46 (2H, m, CHCH2), 3.88 (1 H, m, CHCH2OH), 3.75 (6H, s, ImCH3). 13C NMR (100 MHz, acetonitrile-d3, 25 °C): δ (ppm) 178.8(Ccarbene), 124.2 (NCHCHN), 120.5 (NCHCHN), 74.64 (CHCH2), 62.72 (CH2OH), 39.09 (ImCH3). ESI-MS ([M]+): m/z 849.94 [3 + H2O + H+]+. IR spectrum (ν/cm−1): 3177, 3148, 2952, 2895, 1573, 1450, 1399, 1328, 1263, 1223, 1166, 1071, 1054, 964, 829, 736, 680, 666, 556, 522. Anal. Calcd for C20H28F12N8O2P2Cu2· 0.7CH3CN: C, 29.95; H, 3.53; N, 14.20. Found: C, 29.98; H, 3.57; N, 14.20. Synthesis of Au(I) Complex 4. Silver Route. In a Schlenk tube, Au(SMe2)Cl (36.0 mg, 0.122 mmol) was added to a solution of silver

a

SMD model with MeCN.

When these gas-phase free energies are compared, it is found that the anti,exo configuration is more favored for the Cu(I) complex, while the Ag(I) and Au(I) complexes show lower free energies in the syn,exo configuration (−3.6 and −5.5 kJ/mol). This result is in accordance with the crystal structures of complexes 3 and 4 and indicates that the configuration is influenced by the metal coordinated. Since the water molecule was observed in the crystal structure of the Au(I) complex 4, it was interesting to see whether the hydrogen bonding with water would further stabilize the syn configuration of complexes 2−4. Thus, the free energies including hydrogen-bonded water molecules were calculated in the syn configurations of all three complexes. As expected, the water molecule indeed reduced the free energies in the syn,exo configuration of all three complexes, especially the Au(I) complex 4, which has the biggest energy gap of −16.2 kJ/mol in comparison to the anti configuration. In order to be closer to the real crystallization conditions, the calculation of the SMD model with MeCN was also conducted. This further calculation indicates that the syn configuration is exclusively favored for the Au(I) complex 4 with an exergonic energy gap of 0.9 kJ/mol, whereas the other two complexes have relatively larger endergonic energies in comparison to those of the syn configuration (2, 5.9 kJ/mol; 3, 7.4 kJ/mol). This result correlates with the crystal structures determined by X-ray diffraction, which shows that only Au(I) complex 4 is in the syn,exo configuration with molecular water cocrystallized. It is also in accordance with the observed isomers in solution, since the syn configuration is always preferred for Au(I), whereas the anti configuration is preferred for the other metals under the respective reaction conditions: i.e., in the absence of water for Cu(I) and in the presence of water for Ag(I) (intrinsically by using Ag2O).

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CONCLUSIONS In summary, three new dinuclear bridge-functionalized bis(NHC)-Ag(I), -Cu(I), and -Au(I) complexes have been synthesized and characterized. The spectroscopic properties of all three complexes do not give any evidence for an electronic influence of the bridge functionalization, while the structure configurations of these complexes were shown to be significantly affected. X-ray diffraction studies reveal that Ag(I) complex 2 and Cu(I) complex 3 adopt an anti configuration, whereas Au(I) complex 4 adopts a syn configuration. DFT calculations indicate that the coinage metals coordinated to this hydroxymethyl-functionalized methylene-bridged ligand determine the configuration of the respective dinuclear complexes. A F

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Organometallics

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complex 2 (50.0 mg, 0.0545 mmol) in dry acetonitrile (1 mL) under argon. A white precipitate formed immediately, and the reaction mixture was stirred for 20 h at 45 °C. The suspension was then centrifuged and filtrated through a pad of Celite. The obtained solution was under vacuum to remove the solvent. Recrystallization from acetonitrile/diethyl ether gave 4 as an off-white solid. Yield: 57.5 mg, 96%. Base Route. A mixture of the ligand 1 (50.0 mg, 0.18 mmol), sodium acetate (34.0 mg, 0.40 mmol), and AuCl(SMe2) (56.0 mg, 0.19 mmol) in dimethylformamide (3 mL) was stirred at 120 °C for 3 h under argon. After the mixture was cooled, n-hexane (15 mL) and dichloromethane (2 mL) were added to the reaction solution. The obtained precipitate was filtered and dried under vacuum. Methanol (2 mL) was then added to the solid, and a solution of KPF6 (148.0 mg, 0.80 mmol) in water (1 mL) was added to the suspension and stirred for 3 h at room temperature. The solid was centrifuged, washed with H2O (1 mL) and methanol (2 × 3 mL), and then dried under vacuum to give complex 4. Yield: 25.5 mg, 26%. 1H NMR (400 MHz, acetonitrile-d3, 25 °C): δ 7.58 (4 H, d, J = 4.0 Hz, NCHCHN), 7.48 (2 H, t = 4.0 Hz, CHCH2OH), 7.30 (2 H, d, J = 4.0 Hz, NCHCHN), 4.37 (2 H, m, CHCH2OH), 3.93 (1 H, m, CHCH2OH), 3.88 (6 H, s, ImCH3). 13C NMR: (100 MHz, acetonitrile-d3, 25 °C): δ (ppm) 185.7 (Ccarbene), 125.8 (NCCN), 119.5 (NCCN), 74.2 (CH), 62.0 (CH2), 39.0 (Im-CH3). ESI-MS ([M]+): m/z 949.93 [4 − PF6−]+; IR spectrum (ν/cm−1): 3175, 3149, 2953, 2879, 1574, 1457, 1403, 1328, 1273, 1222, 1069, 935, 1132, 834, 738, 685, 621, 557, 528. Anal. Calcd for C20H28F12N8O2P2Au2: C, 21.91; H, 2.57; N, 10.22. Found: C, 22.13; H, 2.76; N, 9.59.

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AUTHOR INFORMATION

S Supporting Information *

Text, figures, tables, and CIF and xyz files giving spectroscopic, crystallographic, and computational details. This material is available free of charge via the Internet at http://pubs.acs. org. CCDC 1037242 (2), CCDC 1037243 (3), and CCDC 1037244 (4) also contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc. cam.ac.uk/data_request/cif. Corresponding Authors

*E-mail for A.P.: [email protected]. *E-mail for W.A.H.: [email protected]. *E-mail for F.E.K.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.Z. thanks the TUM Graduate School for financial support and Dr. Lars-Arne Schaper for useful scientific discussions. We thank the Leibniz Rechenzentrum of the Bavarian Academy of Science for the provision of the computing time.

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DEDICATION Dedicated to the memory of Prof. Michael F. Lappert. REFERENCES

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DOI: 10.1021/om5012402 Organometallics XXXX, XXX, XXX−XXX