Synthesis and Structural Characterization of Water-Soluble Gold(I) N

Article pubs.acs.org/Organometallics

Synthesis and Structural Characterization of Water-Soluble Gold(I) N‑Heterocyclic Carbene Complexes. An X‑ray Absorption Fine Structure Spectroscopy (XAFS) Study ́ ‡,§ Alicia B. Chopa,†,∥ Gabriela A. Fernández,† Agustıń S. Picco,‡ Marcelo R. Ceolın, and Gustavo F. Silbestri*,†,§ †

Instituto de Quı ́mica del Sur (INQUISUR), Departamento de Quı ́mica, Universidad Nacional del Sur, Av. Alem 1253, B8000CPB Bahı ́a Blanca, Argentina ‡ Instituto de Investigaciones Fisicoquı ́micas Teóricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET. CC. 16 Suc. 4, 1900 La Plata, Argentina S Supporting Information *

ABSTRACT: Water-soluble gold(I) N-heterocyclic carbene complexes were synthesized and characterized using X-ray absorption spectroscopy (XAFS) in combination with traditional analytical techniques such as NMR, mass spectrometry, and UV−vis spectroscopy. XANES and EXAFS regions are sensitive to coordination number, ligand electronic structure, and distance around the metal center, providing information on the oxidation state and bonding structure of gold, which allows discrimination between mono- and bis-carbene species. Preliminary results showed that the catalysts are active in the hydration of terminal alkynes in aqueous media; in addition, they are highly recyclable.

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INTRODUCTION One of the key aspects of organometallic catalysis, regardless of the solvent used, is the improvement of catalytic processes through the development and testing of new ligands and complexes. In the past decade, N-heterocyclic carbenes (NHCs) have contributed to significant advances in catalytic processes such as, for example, ruthenium-mediated olefin metathesis, palladium-promoted cross-coupling, and other transition-metal-catalyzed reactions.1 It is well-known that they are more powerfully σ-donating than the closely related phosphine ligands, forming stronger bonds to transition metals and thereby also leading to an electron-rich metal center.2 This neutral monodentate ligand has a series of advantages, such as stability and tolerance to a variety of functional groups. It is worth mentioning that there are few references related to water-soluble NHC complexes, which show their effectiveness in aqueous media.3 It is also worth noting that the insolubility in water of many organic products facilitates the processes of separation and, in the case of water-soluble catalysts, opens the way for their recycling and reuse.4 Even less information has © 2013 American Chemical Society

been reported regarding, for example, their stability to the hydrolysis of metal−carbene bonds.3a,5 On the other hand, in the last two decades, the use of gold catalysts has increased substantially due to their high efficiency in chemical transformations, expanding the application of transition metals. For example, Schmidbaur provided detailed and extensive information on almost all molecular prototypes with Au0, Au+, and Au3+ bound to alkenes, alkynes, and even arenes with low η2 hapticity;6 Akai generated a powerful catalyst for the intramolecular cyclizations of readily available γhydroxyalkynones under mild conditions by a combination of (p-CF3C6H4)3PAuCl and AgOTf,7 rendering the substituted 3(2H)-furanones in 55−94% yields. This method was also applicable to the preparation of 2,3-dihydro-4H-pyran-4-ones; Espinet and Echavarren explored the Sonogashira coupling reaction catalyzed by AuI/dppe in the presence of 0.1 mol % of Pd(0) complexes,8 and Sahoo reported a general and simple Received: July 5, 2013 Published: October 25, 2013 6315

dx.doi.org/10.1021/om400663a | Organometallics 2013, 32, 6315−6323

Organometallics

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Figure 1. Structure and solubility (in parentheses) of synthesized gold(I)−NHC complexes (solubility at 25 °C; (w) water, (d) DMSO).

of AuI- and AuIII-containing phosphines, phosphites, and monoand bidentate ligands and excluded the presence of nanoparticles during the catalytic oxidation of aldehydes.28 More recently, Nguyen et al. reported studies of two different goldcatalyzed reactions using solution EXAFS at the L3 edge (11.919 keV) in combination with traditional analytical tools.29 We now report the synthesis and full characterization of seven AuI−NHC complexes (C1−C7) shown in Figure 1. Preliminary results show that C1, C4, C5, and C7 are active catalysts in the hydration of terminal alkynes in aqueous media; in addition, C1 and C4 are highly recyclable. This recyclability may probably be associated with the stability of the gold− carbene bond under aqueous reaction conditions.

strategy for the synthesis of functionally diverse arylvinyl ethers through gold-catalyzed intermolecular addition of electronically and sterically substituted phenols with unactivated alkynes.9 It is known that hydration of alkynes is an appropriate reaction for the synthesis of aldehydes and ketones; first, it was accomplished through the mediation of Hg compounds as catalysts.10 Efficient methodologies, using other transitionmetal catalysts, have been developed in order to replace the toxic metal. Thus, catalysts containing Rh,11 PdII,12 RuIII,13 CuII,14 or PtII3d,15 have been reported. These reactions lead to the synthesis of ketones following the Markovnikov rule. In addition, Wakatsuki reported the first anti-Markovnikov hydration of alkynes catalyzed by ruthenium(II) complexes, yielding mainly aldehydes.16 In addition to these complexes, gold(I) and gold(III) are effective catalysts for Markovnikov alkyne hydration. Since 1991, when Fukuda and Utimoto reported the hydration of alkynes catalyzed by Na[AuCl4],17 a rapid development of research on the use of gold catalysts has been initiated. For example, Teles reported the addition of methanol catalyzed by Au(I) complexes together with acidic cocatalysts;18 in addition, Tanaka has described the usefulness of AuI acidic systems in these reactions,19 and Laguna and coworkers have reported gold(III) complexes that are highly active in acidic media.20 More recently, Mohr and Laguna were able to hydrate phenylacetylene using water-soluble gold(I) complexes containing sulfonated phosphane ligands as catalysts.21 In addition, gold complexes with NHC ligands have gained great significance in catalysis, due to the aforementioned stability.22 Recently, Joó et al. isolated different sulfonated imidazolium salts in order to get the corresponding water-soluble AuI−NHC−Cl complexes to be used as catalysts in terminal alkyne activation.23 Surprisingly, little is understood about the real catalytic species, and most gold-catalyzed mechanisms are proposed by analogy to other transition-metal-catalyzed processes in a conventional solvent.24,25 Leyva and Corma has revealed that with an [Au(2-(dicyclohexylphosphino)-2′,6′dimethoxybiphenyl)bis(trifluoromethanesulfonyl)imidate] catalyst in a water/methanol mixture, hydratation of phenylacetylene involved the active role of methanol, and this favors the enol ether/ketal route.26 In this context, solution-based XAFS offers an alternative tool to study both the catalyst and catalytic intermediates. XANES and EXAFS regions are sensitive to coordination number and to different types of ligands around the metal center;27 consequently, it should be possible to discern the oxidation state and bonding state of gold. For example, Hashmi used EXAFS to monitor the oxidation state of a series of compounds

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RESULTS AND DISCUSSION Synthesis and Characterization. On the basis of our previous experience,3a complexes C1−C7 were initially synthesized from [AuCl(tht)]30 (tht = tetrahydrothiophene) and the corresponding imidazolium compounds L1−L7 (see the Experimental Section) (Scheme 1, method A). UnfortuScheme 1. General Procedures for Preparation of Gold(I)− NHC Complexes

nately, this route, simple and direct, was not adequate for the formation of the desired gold(I) species, giving mixtures of mono- and bis-carbene. Next, we synthesized C1−C7 through the silver oxide route developed by Lin and co-workers31 and based on [Ag−NHC−Cl] complexes as NHC transfer agents32 (Scheme 1, method B); this route turned out to be more effective (90−95% of isolated complex), avoiding the formation of the respective bis-carbene complexes [AuI−(NHC)2]+. All gold(I) complexes were found to be stable in air and could be stored for prolonged periods when protected from light (over 1 month). 6316


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Scheme 2. Temperature-Dependent Dimerization from Mono- to Bis-Carbene

Figure 2. TEM images of C4 in water: (A) 25 °C; (B) 100 °C, 10 min; (C) 100 °C, 96 h.

Sulfonated Gold(I)−NHC Complexes [AuI−NHC−Cl]. The water-soluble gold(I)−carbene complexes C1−C4 were fully characterized by UV−visible, FT-IR, and 1H and 13C NMR spectroscopy, elemental analysis, and mass spectrometry (see the Experimental Section for details). The NMR data confirm the metal coordination by the disappearance of the proton signal of the imidazole ligands (L1−L4). Electrospray ionization mass spectra (ESI-TOF), obtained in methanol, showed in all cases the molecular anion corresponding to the loss of a sodium(1+) ion. The presence of chromophore and auxochrome groups allowed us to perform UV−visible spectroscopy studies. Taking into account that, in most cases, the absorption bands shifted to higher wavelengths when the ligands were coordinated to the metal center, we were able to determine the partitioning constant of gold complexes in mixtures of water and organic solvents by this technique. Moreover, information obtained by 1H NMR spectroscopy showed that all complexes were completely dissolved in the aqueous phase in mixtures of this solvent with diethyl ether or toluene. On the other hand, the IR spectra of the complexes presented a characteristic absorption, in KBr, at 590 cm−1 (ν(Au−C2)). The studies reported so far and summarized above have shown that the metal−NHC bond is stable on dissolution in water and even the formation of such bonds is sometimes possible in this solvent.23a,33 However, the literature gives scarce information about the long-term hydrolytic stability of the metal−carbene bond, especially at high temperatures or under catalytic conditions.3a Our studies show that sulfonated gold(I)−NHC complex C4 was stable in D2O for more than 1 month at room temperature and up to 3 days at 80 °C. However, partial hydrolysis was observed when the solutions were heated at 100 °C. Thus, after 24 h the presence of the corresponding free ligand was detected in the solution (1H NMR). At longer heating times the bis-carbene was generated

and after 96 h the dimerization from mono- to bis-carbene was complete (13C NMR, based on signals of Au−C2) (Scheme 2b).34 The generation of bis-carbene was accompanied by a violet coloration; it is known that this indicates the presence of Au ions in the medium.23a Moreover, the generation of colloidal metallic gold in the mixture was confirmed by TEM microscopy (Figure 2). The same tests were performed with C2 and C3. In both cases, the behavior was similar to that of C4. On the other hand, C1 was not stable even at room temperature. Thus, the water dissolution of C1 was accompanied by a violet coloration and, after 48 h, the initial mixture evolved to a pure compound resulting from the dimerization of mono- to bis-carbene (Scheme 2a); also, the presence of colloidal metallic gold was observed by TEM microscopy.35 13C NMR signals of Au-C2 of monocarbene complexes C1−C4 were found between 167.7 and 171.6 ppm and in respective bis-carbene complexes between 183.4 and 184.1 ppm. The value is in good agreement with data found in the literature.23a,36 Cationic Gold(I)−NHC Complexes [Au−NHC−L]+ (L = Water, Acetonitrile, Pyridine). Cationic gold is a soft Lewis acid in the third row of the periodic table that, furthermore, experiences relevant relativistic effects over its 6s valence electron. This makes gold a high π-acid with a low oxophilicity,37 and consequently, cationic gold can activate unsaturated C−C bonds in the presence of water, alcohols, or any other oxygen-containing functionality. We have found the conditions for the generation of ionic species I by treatment of the neutral complex with AgSCN (Scheme 3). Addition of AgSCN to an aqueous solution of complexes C5,31a,38 C6,31a,38 and C739 resulted in precipitation of AgCl and generation of [Au]+ species by exchange of chloro ligand with a less coordinating molecule. It should be mentioned that, when the reaction was performed in CH3CN, 13C NMR signals of Au−C2 6317


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Scheme 3. General Procedure for Preparation of AuI−NHC Ionic Complexes

shifted to higher field (172 to 176 ppm), indicating a new coordination to the metal center. This path allows the synthesis of soluble catalysts, regardless of the presence of sulfonic groups. All ionic compounds were soluble and stable in water (even by heating at 100 °C for 24 h). The synthesized complexes were fully characterized by 1H and 13C NMR spectroscopy, elemental analysis, and mass spectrometry (see the Experimental Section for details). Electrospray ionization mass spectra (ESI-TOF), obtained in methanol, showed the molecular anion corresponding to the gain of a sodium(1+) ion or ammonium(1+) ion. X-ray Absorption Fine Structure. XAFS is a powerful local spectroscopic technique that provides structural and electronic information on the local environment of selected atoms usually called absorbers.26 In this paper we performed XAFS spectra around the L3 (11919 eV) gold absorption edge in order to obtain structural and electronic information on gold atoms. The analysis of the edge position and shape of the spectra near the absorption edge (XANES) is used to assess valuable information on local symmetry, electronic structure, and formal valence of the absorber atom.40 Typically, the position of the absorption edge (the position of the first sharp maximum on the first-derivative curve obtained numerically from the XAFS spectrum around the nominal absorption threshold) was correlated to the oxidation state of the absorber being shifted to higher energies for absorbers with higher nominal valence. However, L3 lines in 4d and 5d compounds may have unexpected behaviors.41 In fact, L3-XANES spectroscopy from AuIII arises mainly from 2p to 5d transitions and low L3XANES for Au0 and AuI reflects mainly effects due to 2p to 6s transitions. Therefore, the general trend of increasing XANES edge energy with oxidation state is not observed, and we adopted the following approach: the oxidation state of gold in a compound can be deduced by the shape of the Au L3 edge.41 Metallic gold (Au0) is identified by two broad peaks located at 11945 and 11967 eV, and this feature is retained even when the gold metal particle is as small as 10 Ǻ . Compounds containing Au3+ are clearly identified by a sharp spike (or “white line”) on the edge rise. Remembering that L-edge lines are associated with 2p → 5d transitions, sharp white lines are clearly associated with holes into the d electron shell as expected for AuIII (5d8). In contrast, Au+ spectra do not have as many features and present characteristics between those observed for Au0 and Au3+. Figure 3 shows the XANES spectrum of compound C4 together with those of metallic gold, i.e. Au0, and AuIIICl3 as references. C4 presents characteristics that are similar to those observed for very well known AuI compounds.41,42 Miyamoto et al. explained the intensity of the silver L3 white line in Ag+ compounds as originating not only in s−d hybridization but also in core polarization due to the softacid nature of the Ag+ ion. Although the Lewis acidity hardness of gold is somewhat higher than the value for silver, a similar reasoning can be followed to explain the origin of the B1 line in gold.43

Figure 3. Au L3-edge XANES spectra for Au0, compound C4, and AuIIICl3. The inset depicts the B1 line present on C4.

Together with the inflection point in the spectra, the B1 line can be used to determine the oxidation state of gold centers; its intensity or area correlates very well with the oxidation state of Au and is indicative of the Au 5d vacancy. Figure 4 displays a clear correlation between the position of the absorption edge and the intensity of the B1 line obtained

Figure 4. Correlations between the position of the absorption edge and the intensity of the B1 line obtained from XANES.

from XANES experiments for all the compounds reported here. Black (●) and white (○) points correspond to the values obtained from XANES spectra of metallic gold (Au0) and AuIIICl3 references, respectively. Clearly, gold atoms in one of the compounds (C7 (△) in Figure 4) display spectral characteristics very similar to those of Au0, compatible with low charge transfer to its ligands. On the other hand, compounds C1 and C4−C6 (▲) display spectral features between those expected for Au0 and AuIII. They can be assigned as AuI with some degree of vacancy in the 5d shell, as deduced from the intensity of the B1 line. The metallic character displayed by compound C7 (as deduced from its spectral similarities with Au0) suggest the absence of empty 5d levels for C7. Spectral oscillations above the absorption edge define the extended X-ray absorption fine structure (EXAFS) region of the absorption spectrum.40 XAFS experiments performed on 6318


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Table 1. Fitting Parameters, Error Bars, and Figures of Merit of C4 and C7

a

Coordination number obtained for a model including the atom indicated in parentheses. The typical error for coordination numbers was 25%.

Figure 5. (A) Fourier transform EXAFS spectrum obtained for compound C4 (dashed line) and its corresponding fitting (solid line; see text for details). (B) k3 weighted EXAFS oscillations of C4 (dashed line) and their corresponding fit (solid line).

Figure 6. (A) Fourier transform EXAFS spectra obtained for compound C7 (solid line) and C4 (dashed line). (B) k3 weighted EXAFS oscillations of C7 (dashed line) and its corresponding fit (solid line).

samples, fitted with models using simple scattering paths since multiple paths were found to be negligible, showed that all

complexes have a two-coordinate gold(I) atom, in an essentially linear environment with the C2−Au−Cl bond. 6319


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For all compounds, the Au−C2 average distance found was 1.97 Å and for the Au−Cl bond was 2.25 Å, which are consistent with the values observed in other organic compounds containing gold atoms.44 Details on the fits, for C4 and C7, can be found in Table 1. A summary of fitting parameters, error bars, and figures of merit for the structural analyses of all compounds is given in Table S1 (Supporting Information). A close inspection to the values obtained for AuC2 and Au−Cl coordination length shows that bulky carbene ligands like those in C4 and C7 display shorter Au-C2 and Au− Cl bond lengths than smaller ligands. Figures 5 and 6 show details of the individual contributions to the Fourier transform EXAFS spectra for C4 and C7, respectively. The local maximum around Reff = 2.5 Ǻ can be associated with the single-leg Au−N−Au scattering path arising from the imidazole ligand. This path is hard to fit to the experimental data. This fact can be associated with the presence of solvent molecules trapped near the absorber. Attempts to reproduce the experimental data using two carbon atoms in place of one chloro atom (bis-carbene) were not successful. Nolan et al. prepared single crystals of compounds similar to those examined in this work by slow diffusion of Et2O in a saturated solution of complexes in CH2Cl2 or CHCl3.36 They examined the compounds using X-ray diffraction (XRD), obtaining Au− C2 and Au−Cl distances in close agreement with our results. Altogether, XAFS results point to the formation of monocarbene species for all the compounds investigated. Preliminary Study in Hydration of Terminal Alkynes in Water. The NHC complexes C1, C4, C5, and C7 were examined in the hydration of phenylacetylene in a water/ methanol mixture. The selection of complexes was performed in order to evaluate the influence of structural characteristics (symmetrical and asymmetrical) as well as of solubility in aqueous medium. The complexes were active after 12 h at 80 °C with an Au loading of 1 mol % in the absence of acid as cocatalyst, the activity being comparable to the Au− phosphane−base catalyst for the same reaction.21 It is noteworthy that for an efficient activation of the complex and to prevent the generation of bis-carbene, the optimal addition order of reagents was: H2O−MeOH/alkyne/KPF6 or AgSCN/ Cx. The activities of complexes C1 and C4 were similar, reaching quantitative conversions of phenylacetylene to benzophenone at 80 °C after 12 h (Table 2, entries 1 and 2), whereas the ionic complexes C5 and C7 showed lower activity: 60% and 55%, respectively. Longer reaction times (48 h) produced an increase in the yield of ketone to 90% and 82%, respectively (Table 2, entries 3 and 4).

Catalyst Recycling. Each recycling experiment was repeated until the catalyst was almost inactive. The number of cycles that attained this conversion can be found in Figure 7.

Figure 7. Recycling experiment: number of cycles vs conversion percentage of different [Au] complexes.

Recovery experiments showed that cationic gold(I)−NHC complexes (C5, C7) suffered from significant activity losses after the initial cycles, whereas the sulfonated analogues complexes (C1, C4) have attained high levels of recyclability. Although the stability of these complexes during the catalytic experiments is debatable, the absence of colloidal Au particles (TEM microscopy) in aqueous solutions at the end of the catalyzed reactions would suggest that the complexes are stable under the reaction conditions used. The significant decrement of activity shown by complexes C5 and C7 (Figure 7) could be explained by the fact that their concentrations (ICP-OES analysis) decay significantly in the aqueous solutions after extraction with organic solvents (40% in the first extraction), indicating the low affinity of cationic gold species for the aqueous medium. On the other hand, the percentage of gold retained in the aqueous solutions at the end of the recycling experiments was high for both C1 and C4 complexes (96% in the first extraction; 50% after nine cycles), showing the efficiency of the sulfonated NHC ligands in trapping the active gold species in this phase.

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CONCLUSIONS In summary, we have synthesized, in excellent yields (90− 95%), a series of symmetric and asymmetric AuI−NHC complexes (C1−C7). These complexes have been completely characterized by complementary techniques (1H and 13C NMR, FT-IR, UV−vis, mass spectrometry, elemental analysis, and XAFS). The oxidation state of the absorber and the nature and structure of its first neighbors were elucidated by XAFS techniques (XANES and EXAFS) on the gold L3 edge. The coordination environments of the metal atom in the complexes were fairly similar, although differences among Au0, AuIII, AuI monocarbene species (C1−C7) and AuI bis-carbene species (C8 and C9) were important for confirming the results obtained by mass spectrometry. The complexes were soluble in water, catalytically active, and reusable in the hydration of terminal alkynes. This is a very competitive protocol in comparison to other aqueous alkyne hydrations,21 even in the absence of acid, which is a common cocatalyst for most hydration reactions. Our results support the stability of the Au−C2 bonds under these reaction conditions.

Table 2. Comparison of Catalysts for the Hydration of Phenylacetylene in Watera

entry

Au catalyst

yieldb,c

1 2 3 4

C1 C4 C5 C7

100 100 60 (90) 55 (82)

a

Conditions for a typical experiment: 0.5 mmol of phenylacetylene; 1 mol % of [Au] catalyst; 0.005 mmol of KPF6; H2O−MeOH (1.5 mL− 1.5 mL); 80 °C (oil bath); 12 h. bIsolated yield. cYield in parentheses refers to 48 h. 6320


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1H, Imz), 7.06 (s, 2H, Ar), 7.03 (d, 3JH−H = 2.0 Hz, 1H, Imz), 4.40 (t, 3 JH−H = 6.0 Hz, 2H, NCH2), 2.91 (t, 3JH−H = 6.0 Hz, 2H, CH2S), 2.26 (s, 3H, p-MeAr), 2.08 (q, 2H, CH2CH2CH2), 1.98 (s, 6H, o-MeAr). 1H NMR (300 MHz, DMSO-d6): δ 7.81 (s, 1H, Imz), 7.47 (s, 1H, Imz), 7.07 (s, 2H, Ar), 4.29 (t, 3JH−H = 6.0 Hz, 2H, NCH2), 2.41 (t, 3JH−H = 6.0 Hz, 2H, CH2S), 2.31 (s, 3H, Ar p-Me), 2.15 (q, 2H, CH2CH2CH2), 1.95 (s, 6H, Ar o-Me). 13C NMR (75 MHz, DMSOd6): δ 169.7 (s, Imz C2), 138.5 (s, Ar C1), 134.2 (s, Ar C2), 134.1 (s, Ar C4), 128.7 (s, Ar C3), 122.5 (s, Imz C4), 122.0 (s, Imz C5), 49.4 (s, NCH2), 47.8 (s, SCH2), 27.1 (s, CH2CH2CH2), 20.3 (s, p-MeAr), 16.9 (s, o-MeAr). ESI-MS (negative ion, MeOH): m/z 539.05 [M − Na]−; UV−vis (H2 O): 218, 248, and 267 cm −1 . Anal. Calcd for C15H19AuClN2NaO3S: C, 30.29; H, 3.22; N, 4.71; S, 10.78. Found: C, 30.76; H, 3.30; N, 5.07; S, 10.82. [1-(2,6-Diisopropylphenyl)-3-(3-sulfonatopropyl)imidazol-2ylidene]gold(I) chloride (C2): brown solid (0.332 g, 0.55 mmol, 92%); mp 240−243 °C. Complex C2 is soluble in water, methanol, tert-butyl alcohol, and DMSO, partially soluble in isopropyl alcohol, and insoluble in tetrahydrofuran, diethyl ether, and acetone. Water solubility at 25 °C: 80 g/L. 1H NMR (300 MHz, D2O): δ 7.79 (s, 1H, Imz), 7.68 (s, 1H, Imz), 7.51 (m, 1H, Ar H4), 7.40 (m, 2H, Ar H3), 4.39 (t, 3JH−H = 6.0 Hz, 2H, NCH2), 2.90 (t, 3JH−H = 6.0 Hz, 2H, CH2S), 2.21 (h, 3JH−H = 6.0 Hz, 2H, CHMe2), 1.92 (q, 2H, CH2CH2CH2), 1.09 (t, 3JH−H = 6.0 Hz, 12H, CHMe2). 1H NMR (300 MHz, DMSO-d6): δ 6.95 (s, 1H, Imz), 6.75 (s, 1H, Imz), 6.63 (t, 3 JH−H = 7.7 Hz, 1H, Ar H4), 6.46 (d, 3JH−H = 7.6 Hz, 2H, Ar H3), 3.41 (t, 3JH−H = 6.2 Hz, 2H, NCH2), 1.61 (t, 3JH−H = 6.5 Hz, 2H, CH2S), 1.39 (h, 3JH−H = 6.5 Hz, 2H, CHMe2), 1.29 (q, 3JH−H = 6.5 Hz, 2H, CH2CH2CH2), 0.31 (d, 3JH−H = 6.7 Hz, 6H, CHMe2), 0.21(d, 3JHH= 6.7 Hz, 6H, CHMe2). 13C NMR (75 MHz, DMSO-d6): δ 170.7 (s, Imz C2), 144.4 (s, Ar C1), 134.9 (s, Ar C2), 130.2 (s, Ar C4), 125.5 (s, Ar C3), 124.4 (s, Imz C4), 120.6 (s, Imz C5), 52.2 (s, NCH2), 49.3 (s, SCH2), 30.1 (s, CHMe2), 26.5 (s, CH2CH2CH2), 22.3 (s, CHMe2), 22.0 (s, CHMe2). ESI-MS (negative ion, MeOH): m/z 581.10 [M − Na]−. UV−vis (H2O): 260, 271.5, and 409 (weak) cm−1. Anal. Calcd for C18H25AuClN2NaO3S: C, 35.74; H, 4.16; N, 4.63; S, 5.30. Found: C, 35.76; H, 4.12; N, 4.87; S, 5.56. [1-Methyl-3-(3-sulfonatopropyl)imidazol-2-ylidene]gold(I) chloride (C3): 23a white solid (0.106 g, 0.22 mmol, 89%); mp 143−145 °C. The complex is soluble in water, methanol, tert-butyl alcohol, and DMSO, partially soluble in isopropyl alcohol, and insoluble in tetrahydrofuran, diethyl ether, and acetone. Water solubility at 25 °C: 645 g/L (lit. 680 g/L). 1H NMR (300 MHz, D2O): δ 7.15 (d, 3 JH−H = 1.4, 1H, Imz), 7.09 (d, 3JH−H = 1.4, 1H, Imz), 4.16 (t, 3JH−H = 7.0, 2H, NCH2), 3.67 (s, 3H, NCH3), 2.82 (t, 3JH−H = 7.5, 2H, CH2S), 2.17 (m, 2H, CH2CH2CH2). 13C NMR (75 MHz, D2O): δ 167.7 (s, Imz C2), 122.8 (s, Imz C5), 121.2 (s, Imz C4), 49.3 (s, NCH2), 47.9 (s, SCH2), 37.8 (s, NCH3), 26.1 (s, CH2CH2CH2). ESI-MS (negative ion, MeOH): m/z 435.03 [M − Na]−; UV−vis (H2O): 222.5, 231, and 245 cm−1. Anal. Calcd for C7H11AuClN2NaO3S: C, 18.33; H, 2.42; N, 6.11; S, 6.99. Found: C, 18.22; H, 2.38; N, 6.18; S, 7.05. [1,3-Bis(2,6-diisopropyl-4-sodiumsulfonatophenyl)imidazol-2ylidene]gold(I) chloride (C4): white solid (0.465 g, 0.57 mmol, 95%); mp >300 °C. The complex is soluble in water, methanol, tert-butyl alcohol, isopropyl alcohol, and DMSO and insoluble in tetrahydrofuran, diethyl ether, and acetone. Water solubility at 25 °C: 111 g/L. 1H NMR (300 MHz, D2O): δ 7.76 (s, 4H, Ar), 7.67 (s, 2H, Imz), 2.57 (h, 3 JH−H = 7.0 Hz, 4H, CHMe2), 1.28 (d, 3JH−H = 7.0 Hz, 12H, CHMe2), 1.20 (d, 3JHH= 7.0 Hz, 12H, CHMe2). 1H NMR (300 MHz, DMSOd6): δ 8.00 (s, 2H, Imz), 7.58 (s, 4H, Ar), 2.17 (h, 3JH−H = 6.0 Hz, 4H, CHMe2), 1.22 (t, 3JH−H = 6.5 Hz, 24H, CHMe2). 13C NMR (75 MHz, D2O): δ 171.6 (s, Imz C2), 147.4 (s, Ar C3), 144.7 (s, Ar C4), 136.1 (s, Ar C2), 124.3 (s, Ar C1), 121.4 (s, Imz C4,5), 28.8 (s, CHMe2), 23.5 (s, CHMe2), 22.9 (s, CHMe2). 13C NMR (75 MHz, DMSO-d6): δ 173.0 (s, Imz C2), 149.8 (s, Ar C4), 144.8 (s, Ar C2) 134.0 (s, Ar C1), 124.6 (s, Ar C3), 121.1 (s, Imz C4,5), 28.4 (s, CHMe2), 23.9 (s, CHMe2), 23.5 (s, CHMe2). ESI-MS (negative ion, MeOH): m/z 801.12 [M − Na]−, 489.06 [M − 2Na]2−. UV−vis (H2O): 269, 277 cm−1. Anal. Calcd for C27H34AuClN2Na2O6S2: C, 39.31; H, 4.15; N, 3.40; S, 7.70. Found: C, 39.21; H, 4.26; N, 3.45; S, 7.67.

Further work is in development in our laboratories focusing on the reactivity of these water-soluble gold(I)−NHC complexes.

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EXPERIMENTAL SECTION

General Procedures. All reactions were carried out under a dry nitrogen atmosphere by using Schlenk techniques. Deionized water (type II quality) was obtained with a Millipore Elix 10 UV Water Purification System. Organic solvents were dried and distilled under nitrogen and degassed prior to use. Unless otherwise stated, reagents were obtained from commercial sources and used as received. [Au(tht)Cl] was prepared according to reported procedures.30 1H and 13C NMR spectra were recorded with a Bruker Advance 300 spectrometer. Chemical shifts (δ, ppm) are quoted relative to SiMe4 (1H, 13C). Coupling constants (J) are given in hertz. The Analytical Services of the Universidad de Alcalá performed the C, H, S, and N analyses with a Heraeus CHN-O-Rapid microanalyzer and the ESI mass spectra with an Automass Multi ThermoQuest spectrometer. Xray absorption experiments were performed at the D08B-XAFS2 beamline of Laboratorio Nacional de Luz Sincrotron (LNLS), Campinas, Brazil (Imax = 250 mA, E = 1.37 GeV). The line setup was optimized for absorption experiments around the L3 edge of gold (11919 eV), and energy selection was performed using a Si(111) double crystal monochromator. Gas-filled (Ar/N2) ionization chambers were used for photon detection. Energy calibration and continuous checks for monochromator stability were performed using a gold metal foil. EXAFS spectra were performed on thin sample + boron nitride disks at room temperature (22 °C). All spectra were aligned using metallic gold spectra as reference (E0 = 11919 eV) prior to analysis. EXAFS analysis was performed using the procedures implemented in the program ARTEMIS from the program package IFEFFIT.45 The fits were performed in Fourier transform R space, using simultaneous k weighting (k = 1−3) within an Reff range from 1.0 to 2.25 Ǻ using a Hanning window. Reaction mixtures were analyzed by gas−liquid chromatography (GLC) with an instrument equipped with a flame-ionization detector and a HP5 capillary column (30 m × 0.25 mm × 0.25 μm). The LANAQUI laboratories of CERZOS-CONICET-UNS (Bahı ́a Blanca) performed ICP analysis using an atomic emission spectrometer inductively coupled plasma (simultaneous Shimadzu 9000 as EPA Standard 200.7). Determinations were performed in all cases by using external calibration pattern/gold standard certificate fromChem-Lab, B-8210 Zedelgem, Belgium. TEM images were obtained by the Transmission Electron Microscope Services of CCT-CONICET (Bahı ́a Blanca) using a JEOL 100 CX II microscope operating at an accelerating voltage of 100 kV. Samples were prepared by placing 2 drops of the aqueous solutions on a holey-carbon-coated grid and allowing the solvent to evaporate in air. Synthesis of the Imidazolium Salts. All imidazolium salts (L1− L7) were prepared according to reported procedures. Here we give their water solubilities at 25 °C: L1,5,46 171 g/L; L2,5,46 220 g/L; L3,23b 350 g/L; L4,47 160 g/L; L5,48 385 g/L; L6,48 400 g/L; L7,39 230 g/L. General Procedure for Preparation of Sulfonated Gold(I)− NHC Complexes [AuI−NHC−Cl]. In a 25 mL Schlenk tube was prepared a solution of [AgI−NHC−Cl], from the imidazolium salt (0.60 mmol), silver oxide (0.083 g, 0.36 mmol), and sodium chloride (0.035 g, 0.60 mmol), in MeOH or DMSO (5 mL). [Au(tht)Cl] (0.192 g, 0.60 mmol) was added, and the mixture was stirred at room temperature for 4 h. Then NaCl (0.035 g, 0.60 mmol) was added and the final solution was filtered through a plug of Celite; the solvent was partially removed under vacuum to a remaining volume of 2 mL. The gold complex was then precipitated with dry acetone (20 mL), separated by filtration, washed with dry acetone (3 × 10 mL), and dried under vacuum. [1-Mesityl-3-(3-sulfonatopropyl)imidazol-2-ylidene]gold(I) chloride (C1): white solid (0.302 g, 0.54 mmol, 90%); mp 197−199 °C. Complex C1 is soluble in water, methanol, tert-butyl alcohol, and DMSO, partially soluble in isopropyl alcohol, and insoluble in tetrahydrofuran, diethyl ether, and acetone. Water solubility at 25 °C: 180 g/L. 1H NMR (300 MHz, D2O): δ 7.57 (d, 3JH−H = 2.0 Hz, 6321


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Article

183.7 (s, Imz C2), 140.0 (s, Ar C1), 135.3 (s, Ar C2), 134.8 (s, Ar C4), 129.0 (s, Ar C3), 123.2 (s, Imz C4), 122.0 (s, Imz C5), 48.9 (s, NCH2), 47.8 (s, SCH2), 26.2 (s, CH2CH2CH2), 20.4 (s, p-MeAr), 16.7 (s, oMeAr). ESI-MS (negative ion, MeOH): m/z 811.19 [M − 2Na]2−. Anal. Calcd for C30H38AuN4Na2O6S2: C, 42.00; H, 4.46; N, 6.53; S, 7.47. Found: C, 41.98; H, 4.52; N, 6.50; S, 7.45. Bis[1,3-bis(2,6-diisopropyl-4-sulfonatephenyl)imidazol-2ylidene]gold(0) (C9). The complex is soluble in water, methanol, tertbutyl alcohol, isopropyl alcohol, and DMSO and insoluble in tetrahydrofuran, diethyl ether, and acetone. 1H NMR (300 MHz, D2O): δ 7.60 (s, 8H, Ar H3), 7.44 (s, 4H, Imz), 2.31 (h, 3JH−H = 7.0 Hz, 8H, CHMe2), 1.01 (d, 3JH−H = 7.0 Hz, 24H, CHMe2), 0.82 (d, 3 JHH= 7.0 Hz, 24H, CHMe2). 13C NMR (75 MHz, D2O): δ 183.4 (s, Imz C2), 146.8 (s, Ar C4), 145.0 (s, Ar C1), 136.0 (s, Ar C2), 125.5 (s, Ar C3), 121.8 (s, Imz C4,5), 28.8 (s, CHMe2), 23.5 (s, CHMe2), 22.7 (s, CHMe2). ESI-MS (negative ion, MeOH): m/z 1335.35 [M − Na]−, 656.16 [M − Na]2. Anal. Calcd for C54H68AuN4Na4O12S4: C, 46.92; H, 4.96; N, 4.05; S, 9.28. Found: C, 46.95; H, 5.00; N, 4.08; S, 9.26. General Method for the Alkyne Hydration Reactions in Water. A 0.02 mmol portion of KPF6 and 0.005 mmol of catalyst were added to a solution of phenylacetylene (0.5 mmol, 55 μL) in water− methanol (1.5 mL−1.5 mL). The mixture was vigorously stirred for 12 h at 80 °C (oil bath) in an ampule tube equipped with a PTFE valve. After the mixture was cooled to room temperature, the organic product was extracted with diethyl ether (3 × 15 mL), the combined ethereal layers were dried over MgSO4, and the volatiles were removed under vacuum to give the desired product. The aqueous phase was reused for several cycles, as specified in the Results and Discussion.

General Procedure for Preparation of Cationic Gold(I)−NHC Complexes [Au−NHC−L]+. In a 25 mL Schlenk tube was prepared a solution of [AgI−NHC−Cl], from the imidazolium salt (0.60 mmol) and silver oxide (0.083 g, 0.36 mmol), in CH2Cl2. [Au(tht)Cl] (0.192 g, 0.60 mmol) was added, and the mixture was stirred at room temperature for 4 h and then filtered through a plug of Celite to finally eliminate the solvent by vacuum, giving the neutral complex. By addition of 3 equiv of KPF6 or AgSCN the complexes were solubilized in water. [1-Mesityl-3-propylimidazol-2-ylidene]gold(I) chloride (C5):..31a,39 yellowish solid (0.267 g, 0.57 mmol, 93%); mp 150−153 °C. The complex is soluble in tetrahydrofuran, diethyl ether, acetone, and DMSO and insoluble in water, methanol, and tert-butyl alcohol. DMSO solubility at 25 °C: 200 g/L. 1H NMR (300 MHz, CDCl3): δ 7.14 (d, 3JH−H = 1.9 Hz, 1H, Imz), 6.94 (s, 2H, Ar), 6.87 (d, 3JH−H = 1.9 Hz, 1H, Imz), 4.29 (t, 3JH−H = 7.2 Hz, 2H, NCH2), 2.32 (s, 3H, pMeAr), 2.00 (s, 6H, o-MeAr), 1.90 (q, 2H, CH2CH2CH2), 1.39 (h, 2H, CH2CH2CH3), 0.99 (t, 3JH−H = 7.2 Hz, 3H, CH2CH2CH3). 13C NMR (75 MHz, CDCl3): δ 171.7 (s, Imz C2), 139.6 (s, Ar C1), 134.7 (s, Ar C2), 129.6 (s, Ar C3), 129.2 (s, Ar C4), 122.1(s, Imz C4), 120.3 (s, Imz C5), 51.2 (s, NCH2), 33.1 (s, CH2CH2CH2), 21.1 (s, p-MeAr), 19.6 (s, o-MeAr), 17.8 (s, CH2CH2CH3), 13.7 (s, CH2CH2CH3); ESI-MS (positive ion): m/z 497.10 [M + Na]+. UV−vis (CH2Cl2): 230 and 281 cm−1. Anal. Calcd for C16H22AuClN2: C, 40.48; H, 4.67; N, 5.90. Found: C, 40.41; H, 4.40; N, 5.89. [1-(2,6-Diisopropylphenyl)-3-propylimidazol-2-ylidene]gold(I) chloride (C6):.31a,38 brown solid (0.278 g, 0.54 mmol, 90%); mp 107− 110 °C. The complex is soluble in tetrahydrofuran, diethyl ether, and acetone and insoluble in water, methanol, tert-butyl alcohol, and DMSO; DMSO solubility at 25 °C: 440 g/L. 1H NMR (300 MHz, CDCl3): δ 7.39 (t, 3JH−H = 7.8 Hz, 1H, Ar H4), 7.19 (d, 3JH−H = 7.0 Hz, 2H, Ar H3), 7.09 (d, 3JH−H = 1.8 Hz, 1H, Imz), 6.85 (d, 3JH−H = 1.8 Hz, 1H, Imz), 4.25 (t, 3JH−H = 7.2 Hz, 2H, NCH2), 2.40 (q, 2H, CH2CH2CH2), 2.30 (h, 3JH−H = 6.8 Hz, 2H, CHMe2), 1.34 (m, 3JH−H = 7.5 Hz, 2H, CH2CH2CH3), 1.21 (d, 3JH−H = 6.8 Hz, 6H, CHMe2), 1.04 (d, 3JH−H = 6.8 Hz, 6H, CHMe2), 0.94 (t, 3JH−H = 7.3 Hz, 3H, CH2CH2CH3). 13C NMR (75 MHz, CDCl3): δ 172.7 (s, Imz C2), 145.7 (s, Ar C1), 130.5(s, Ar C2), 124.3 (s, Ar C4), 124.0 (s, Ar, C3), 123.3 (s, Imz C4), 120.0 (s, Imz C5), 51.2 (s, NCH2), 33.2 (s, CH2CH2CH2), 33.0 (s, CHMe2), 24.3 (s, CHMe2), 22.3 (s, CHMe2), 19.6 (s, CH2CH2CH3), 13.7(s, CH2CH2CH3). ESI-MS (positive ion): m/z 498.22 [(M − Cl) + NH4]+. UV−vis (CH2Cl2): 281, 235.5, and 248.5 cm−1. Anal. Calcd for C19H28AuClN2: C, 44.15; H, 5.46; N, 5.42. Found: C, 48.04; H, 5.53; N, 5.76. [1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene]gold(I) chloride (C7):.39,49 black solid (0.345 g, 0.56 mmol, 94%); mp 91−93 °C. The complex is soluble in water, methanol, tert-butyl alcohol, and DMSO, partially soluble in isopropyl alcohol, and insoluble in tetrahydrofuran, diethyl ether, and acetone. DMSO solubility at 25 °C: 330 g/L. 1H NMR (300 MHz, CDCl3): δ 7.45 (t, 3JHH= 8.0 Hz, 2H, Ar H4), 7.24 (d, 3JHH= 8.0 Hz, 4H, Ar H3), 7.13 (s, 2H, Imz), 2.34 (h, 3JHH= 7.0 Hz, 4H, CHMe2), 1.26 (d, 3JHH= 7.0 Hz, 12H, CHMe2), 1.09 (d, 3JHH= 7.0 Hz, 12H, CHMe2). 13C NMR (75 MHz, CDCl3): δ 173.0 (s, Imz C2), 142.7 (s, Ar C1), 134.6 (s, Ar C2), 126.2 (s, Ar C3), 123.4 (s, Ar C4), 121.8 (s, Imz C4,5), 33.1 (s, CHMe2), 26.4 (s, CHMe2) 24.2 (s, CHMe2). ESI-MS (positive ion): m/z: 638.05 [M + NH3]+. UV−vis (CH2Cl2): 231.5, 247, 254.5, and 315.5 (weak) cm−1. Anal. Calcd for C27H36AuClN2: C, 52.21; H, 5.84; N, 4.51. Found: C, 52.32; H, 5.79; N, 4.56. Bis[1-mesityl-3-(3-sulfonatepropyl)imidazol-2-ylidene]gold(0) (C8). The complex is soluble in water, methanol, tert-butyl alcohol, and DMSO, partially soluble in isopropyl alcohol, and insoluble in tetrahydrofuran, diethyl ether, and acetone. 1H NMR (300 MHz, D2O): δ 7.46 (s, 2H, Imz), 7.15 (s, 2H, Imz), 7.07 (s, 4H, Ar H3), 4.02 (t, 3JH−H = 6.0 Hz, 4H, NCH2), 2.58 (t, 3JH−H = 6.0 Hz, 4H, CH2S), 2.36 (s, 6H, p-MeAr), 2.11 (q, 4H, CH2CH2CH2), 1.83 (s, 12H, oMeAr). 1H NMR (300 MHz, DMSO-d6): δ 7.74 (s, 2H, Imz), 7.07 (s, 4H, Ar H3), 7.03 (s, 2H, Imz), 4.03 (t, 3JH−H = 6.0 Hz, 4H, NCH2), 2.40 (t, 3JH−H = 6.0 Hz, 4H, CH2S), 2.36 (s, 6H, p-MeAr), 2.25 (q, 4H, CH2CH2CH2), 1.78 (s, 12H, o-MeAr). 13C NMR (75 MHz, D2O): δ

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ASSOCIATED CONTENT

S Supporting Information *

Figures and a table giving 1H and 13C NMR spectra for compounds C1−C9, a summary of fitting parameters, error bars, and figures of merit for the structural analyses, and EXAFS spectra and Fourier transforms obtained from the samples investigated. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for G.F.S.: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. § Member of CONICET. ∥ Member of CIC.

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ACKNOWLEDGMENTS This work was partially supported by the CONICET, CIC, ANPCYT, LNLS (Campinas-Brazil) and the Universidad Nacional del Sur, Bahı ́a Blanca, Argentina. CONICET is thanked for a research fellowship to G.A.F. and A.S.P. Special thanks are due to Dr. E. de Jesús for obtaining the ESI mass spectra and elemental analyses.

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REFERENCES

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Synthesis and Structural Characterization of Water-Soluble Gold(I) N

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