Article pubs.acs.org/IC
Copper Chloride Catalysis: Do μ4‑Oxido Copper Clusters Play a Significant Role? Sabine Becker,†,∥ Maximilian Dürr,‡,∥ Andreas Miska,† Jonathan Becker,† Christopher Gawlig,† Ulrich Behrens,§ Ivana Ivanović-Burmazović,‡ and Siegfried Schindler*,† †
Institut für Anorganische und Analytische Chemie, Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany ‡ Lehrstuhl für Bioanorganische Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany § Institut für Anorganische und Angewandte Chemie, Universität Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany S Supporting Information *
ABSTRACT: Copper chloride catalysis is a well-established field in organic and inorganic chemistry. However, in most cases a detailed mechanistic understanding of the individual reaction steps and identification of reactive intermediates are still missing. The present study reports the results of spectroscopic and spectrometric measurements that support formation of copper agglomerates during catalytic processes. The composition of CuCl2·2H2O in several coordinating solvents and the influence of basic coreagents such as NaOtBu and K2CO3 on the structure in the solid state as well as in solution were investigated. Several experiments involving crystal structure determination, IR spectroscopy, and ultra-high-resolution cryospray-ionization mass spectrometry were performed. The crystal structures of [CuCl2(H2O)]·0.5(CH3)2CO (1), [Cu2(CH3CN)2Cl4] (2), [Cu3(CH3CN)3Cl6] (3), [Cu3Cl6(THF)4] (4), [Cu(DMSO)2Cl2] (5), (H2N(CH3)2)2[CuCl3] (6), and [Cu4OCl6(THF)(urea)3]·3THF·urea (8) are reported herein. It can be clearly demonstrated that μ4-oxido copper clusters of the formula [Cu4OCl6(solvent)4] are the main product from the reactions of CuCl2·2H2O and basic coreagents. As a final result of these experiments, it can be stated that μ4-oxido copper clusters most likely play an important role in the mechanism of copper chloride-catalyzed reactions.
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INTRODUCTION CuCl2·2H2O and CuCl are simple copper salts that are widely used as catalysts for oxidative coupling as well as oxygenation reactions in organic syntheses.1,2 Consequently, a large number of oxygenation reactions using CuCl2·2H2O as a catalyst have been reported over the past couple decades.3−5 Although they have been employed as catalytic reagents for a long time, little is known about the detailed reaction mechanisms. The electron transfer between the catalyst and the substrates is a key step in understanding the reactivity of this system; however, most attempts fail to explain why simple copper salts such as CuCl2· 2H2O are an efficient catalyst in multi-electron-transfer reactions. There obviously is a discrepancy between the necessity of the catalyst to allow a transfer of several electrons and the missing capability of simple copper salts to accomplish this. Due to these uncertainties, reactions of this kind often only are referred to as shown in the following general equation:
CH bonds by the use of multinuclear catalysts such as cubane9−12 and μ4-oxido clusters.13−17 The electron transfer for both catalyst types can be explained by reasonable mechanisms; however, none of these reaction pathways can be assigned to simple copper salts such as CuCl2·2H2O. Better understanding of these catalytic systems not only would provide more insight into catalytic redox reactions in general but furthermore should allow the design and synthesis of second-generation copper catalysts. With this study, we provide new evidence that multinuclear copper species are involved in the key steps of copper catalysis with simple copper salts such as CuCl2·2H2O. This finding might guide the scientific discussion in a new direction.
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CuCl 2
substrate ⎯⎯⎯⎯⎯⎯→ product solvent
Copper complexes are also widely used as catalysts in oxidation reactions in combination with noninnocent ligand systems.3,6 Another possibility to employ effective copper-based catalysts is the combination of multiple copper centers within one molecule, which ensures the electron transfer as a “multielectron package” as required for this reaction type.7,8 Progress in the latter regard has been made in the activation of © XXXX American Chemical Society
EXPERIMENTAL SECTION
All chemicals used were of p.a. quality and were purchased from Acros, Aldrich, Fluka, or Merck, if not mentioned otherwise. Dry solvents for air-sensitive reactions were redistilled under argon. Preparations and handling of air-sensitive compounds were performed under an argon atmosphere using a glovebox from MBraun (equipped with water and dioxygen sensors; H2O and O2 concentrations less than 0.1 ppm), or standard Schlenk techniques were used. Crystallography. Single crystals suitable for X-ray diffraction were mounted on the tip of a glass rod using inert perfluoropolyether oil. Received: November 7, 2015
A
DOI: 10.1021/acs.inorgchem.5b02576 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry The X-ray crystallographic data were collected on a Bruker Nonius KappaCCD equipped with a low-temperature system. In addition, a Bruker D8 Venture system equipped with dual IμS microfocus sources, a PHOTON100 detector, and an Oxford Cryosystems 700 lowtemperature system was used for data collection. Data collection was performed using Mo Kα radiation with a wavelength of 0.71073 Å and a collimating Quazar multilayer mirror. Empirical absorption corrections were calculated with SADABS.18 The space groups were determined by XPREP19 through analysis of metric symmetry and systematic absences. The structures were solved with SHELXS9720 or SHELXT201421 and refined by using full-matrix least-squares in SHELXL9720 or SHELXL 2014.23 Each structure was checked for higher symmetry using PLATON.22 All non-hydrogen atoms were located and refined anisotropically. Hydrogen atoms were assigned to idealized positions and given thermal parameters equal to either 1.5 (methyl hydrogen atoms) or 1.2 (non-methyl hydrogen atoms) times the thermal parameters of the atoms to which they were attached if not otherwise stated. Crystallographic data of the reported structures have been deposited with the Cambridge Crystallographic Data Centre. For the corresponding numbers please see the tables of crystal data and structure refinement for the specific compound in the Supporting Information. Copies of the data can be obtained, free of charge, from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. The crystallographic data for some compounds were not deposited because the crystals diffracted rather weakly or the structures are already known in the literature. Mass Spectrometry. Cryospray-ionization MS (CSI-MS) measurements were performed on a UHR-TOF Bruker Daltonik (Bremen, Germany) maXis plus, an ESI-qToF mass spectrometer capable of resolution of at least 60 000 fwhm, which was coupled to a Bruker Daltonik cryospray unit. Detection was in both ion modes, and the source voltage was 3.9 kV in positive ion mode and 2.1 kV in negative ion mode. The flow rates were 280 μL/h. The drying gas (N2), to aid solvent removal, was held at −35 °C, and the spray gas was held at −40 °C. The machine was calibrated prior to every experiment via direct infusion of the Agilent ESI-TOF low-concentration tuning mixture, which provided an m/z range of singly charged peaks up to 2700 Da in both ion modes. Infrared Spectroscopy and Elemental Analyses. All IR measurements were performed on a Jasco FT/IR 4100. If not mentioned otherwise, all samples were measured as KBr pellets. All measurements concerning elemental analysis were carried out using a Carlo-Erba 1106 CHN analyzer. Analytical data of compounds 1−9 are reported in the Supporting Information. Preparation of 1. Ca. 0.5 g of CuCl2·2H2O was suspended in acetone (ca. 20 mL) and heated under reflux for ca. 1 h. The solution was then allowed to stand until crystallization occurred. The resulting pale-orange-colored needles could not be isolated because of their high sensitivity toward air and moisture, but it was still possible to pick some crystals for a crystal structure determination. Preparation of 2 and 3. As described for 1 the same reaction was performed in acetonitrile. Red-colored crystals of 3 were obtained. Storing the red crystals under atmospheric conditions caused decomposition to the yellow-colored compound 2 within hours. Preparation of 4. Ca. 0.75 g of CuCl2·2H2O was suspended in THF (ca. 20 mL) and heated to 100 °C for 1 h. A part of the solvent was removed by evaporation. The solution was then allowed to stand until crystallization occurred. Orange-colored crystals were obtained. Preparation of 5 and 6. Ca. 0.5 g of CuCl2·2H2O was suspended in either DMSO or DMF (ca. 20 mL) and heated to 100 °C for ca. 1 h. The solution was then allowed to stand until crystallization occurred. Pale-green-colored crystals were filtered from the DMSO solution and dried to give [Cu(DMSO)2Cl2]. Crystals were obtained from DMF solutions as well. Preparation of 7. Synthesis was carried out under inert conditions. CuCl2·2H2O (0.60 g, 3.5 mmol), CuCl2 (3.2 g, 24 mmol), and NaOtBu (0.75 g, 7.5 mmol) were dissolved in absolute methanol (ca. 10 mL). The mixture was heated to reflux for ca. 2 h. The unreacted parts of the reactants were filtered off. Then the solvent was
evaporated to give 2.0 g (3.1 mmol, 95%) of the yellow-colored compound 7. Preparation of 8. Ca. 0.2 g of CuCl2·2H2O was dissolved in acetonitrile (ca. 10 mL). The solution was stirred for 10 min, and then an excess of NaOtBu was added to the stirred solution. The solution then was allowed to stand until crystallization occurred. Preparation of 9. The “methanol cluster” [Cu4OCl6(MeOH4)] was synthesized according to the literature13,43 and used as a template. The cluster (116 mg, 0.19 mmol) was dissolved in dry THF (ca. 10 mL), and urea (46 mg, 0.77 mmol) was slowly added under inert conditions. After 2 h the red-orange precipitate was filtered, washed with THF, and dried under vacuum. The product was recrystallized in dry THF. Sample Preparation for CSI-MS Measurements. Solutions of CuCl2·2H2O (1, 10, 20, 50, 100, and 200 mM) in the chosen solvent were prepared, and 2 (or 1) equiv of solid KOtBu/K2CO3 was added. The solutions were shaken until a color change was observed (usually ca. 1−2 min).
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RESULTS AND DISCUSSION Formation of μ4-oxido cluster complexes is a common phenomenon when copper(II) chloride is used (only a few selected examples are given in the references).24−31 In this context we previously reported several μ4-oxido clusters that had been prepared from CuCl2·2H2O with simple ligands.13,32 Within this work we observed a strong tendency of some of these copper clusters to be involved in redox reactions.13 Guided by the idea that cluster formation in general could play an important role in simple copper salt catalysis, we investigated the composition of CuCl2·2H2O in common coordinating solvents and its structural change upon addition of a basic coreagent such as NaOtBu. The latter is owed to the fact that CuCl2·2H2O with an additional (strong) base is often used as a catalytic system for a number of oxidative coupling and oxygenation reactions.33−35 Solid-State Structures. Investigations Concerning the Structure of CuCl2·2H2O in Organic Solvents. Several spectroscopic techniques such as crystal structure analysis, IR spectroscopy, and ultra-high-resolution cryospray ionization mass spectrometry (UHR CSI-MS) were applied to determine the structure of CuCl2·2H2O in common organic solvents such as methanol, acetonitrile, acetone, THF, DMF, and DMSO. (Some selected examples for the use of coordinating solvents in catalysis are given in the references.5,36−38) Noncoordinating solvents such as dichloromethane were neglected because they are less used in the reactions of interest. First, the structure of CuCl2·2H2O was determined by the use of X-ray diffraction in the solid state. CuCl2·2H2O was recrystallized from the solvents named above, and the molecular structures were determined to be [CuCl2(H2O)]· 0.5(CH3)2CO (1), [Cu2(CH3CN)2Cl4] (2), [Cu3(CH3CN)3Cl6] (3), [Cu3Cl6(THF)4] (4), [Cu(DMSO)2Cl2] (5), and (H2N(CH3)2)2[CuCl3] (6) (see Figure 1 and Tables S1−S12). In methanol crystallization of pure CuCl2·2H2O without incorporation of any solvent was observed; however, when working under exclusion of water [Cu(MeOH)2Cl2] was obtained, which had been reported previously.13 The structures of 2, 3, and 5 have been reported previously as well and thus are not discussed herein.39,40 The pale orange needles that had been obtained for 1 consist of polymeric [CuCl2·H2O] chains with a distorted square pyramidal geometry for each copper ion. The copper ions are surrounded by four chloride ions in the equatorial plane and one water molecule in a distorted axial position. Noncoordinating acetone molecules are observed in B
DOI: 10.1021/acs.inorgchem.5b02576 Inorg. Chem. XXXX, XXX, XXX−XXX
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case of DMF is hardly comparable to the other experiments because the system CuCl2·2H2O/solvent was altered due to a reaction between CuCl2·2H2O and DMF. In all other cases linear chains formed of [CuCl2] units were found. For CH3CN this tendency was observed only to a minor degree. Adducts consisting of only two or three [CuCl2] units were formed. Applying THF and acetone as the solvent led to the formation of long linear chains of [CuCl2] units. Solvent molecules were incorporated in all molecular structures determined. They were directly bound to copper centers in all complexes except for [CuCl2(H2O)]·0.5(CH3)2CO. Here stronger coordinating water molecules impede the coordination of acetone to the copper centers. The tendency of CuCl2·2H2O to form multinuclear compounds in many solvents is an important hint for its role as a multielectron shuttle in catalytic redox reactions. Influence of a Base on the Structure of CuCl2·2H2O in Organic Solvents. As described above, CuCl2·2H2O in combination with a (strong) base is a widely employed catalytic system for a number of oxidative coupling and oxygenation reactions.33−35 Recrystallization experiments of CuCl2·2H2O from several solvents were repeated with the addition of NaOtBu to investigate the influence of the base on the molecular structure, which was still unknown at this point. Adding NaOtBu to a solution of CuCl2·2H2O led to an immediate color change. A yellow-colored precipitate was obtained from a methanol solution (7) and red crystals from an acetonitrile solution (8). 7 was stable in solution; however, it decomposed within seconds once separated from the solution. Using IR spectroscopy, the characteristic ν(Cu4O) absorption band of the μ4-oxido cluster [Cu4OCl6(MeOH)4] (methanol cluster) was detected at 591 cm−1.13,43 The crystal structure of this cluster has been reported previously by some of us.13 (For a comparison of the IR data see Figure S27 and Table S34.) Figure 2 shows a schematic representation of the general
Figure 1. Molecular structures of the compounds that were obtained by the recrystallization of CuCl2·2H2O from several solvents. Thermal ellipsoids are set at the 50% probability level.
the lattice as well. Although the composition of 1 resembles that of CuCl2·2H2O, their molecular structures differ: 1 shows long parallel strains of CuCl2 units, while CuCl2·2H2O is formed of discrete [Cu(H2O)2Cl2] complexes. 1 was not stable if in contact with air; decomposition was observed leading to a colorless powder, presumably due to the loss of solvent (probably acetone) molecules. After a while, regeneration of the starting material, CuCl2·2H2O, was observed. Recrystallization of CuCl2·2H2O in THF resulted in orangebrown needles of 4. Similar to 1, 4 was not stable in air, leading to a green solid, again most likely as a consequence of the loss of solvent molecules. The molecular structures of 4 and 1 are quite similar. 4 also shows long strains of CuCl2 units; however, the coordination geometry alternates for the copper ions between octahedral (two coordinated THF molecules) and square planar pyramidal (only one coordinated THF molecule). When DMF was used as the solvent, the complex formation of (H2N(CH3)2)2[CuCl3] was observed. The structure shows a trigonal planar coordinated copper(I) anion, [CuCl3]2−, and two dimethylammonium cations. Obviously the cation derived from DMF, whose cleavage was promoted by CuCl2·2H2O that seems to facilitate this known reactivity of DMF; however, the reduction of Cu(II) to Cu(I) remains unclear. Similar compounds with the copper(II) anion [CuCl4]2− have been reported previously.41,42 As described above, discrete [Cu(H2O)2Cl2] complexes are observed in the molecular structure of CuCl2·2H2O. The formation of such discrete complex units was observed as well when applying DMF and DMSO as the solvent. However, the
Figure 2. Schematic representation of μ4-oxido clusters.
structure of these clusters. Their characteristic feature is the central [Cu4OX6] motif with the central oxide, tetrahedrally coordinated by four copper ions that are connected via six bridging halide ions (Cl− or Br−). Four additional ligands (either organic ligands or halide anions such as Cl−) are coordinated to the μ4-oxido unit. Worth mentioning are the socalled “solvent clusters”, where solvent molecules are coordinated to the cluster unit. Molecular structures of several solvent clusters [Cu4OCl6L4] (L = methanol, acetonitrile, DMF, and DMSO) have been reported previously by some of us.13 C
DOI: 10.1021/acs.inorgchem.5b02576 Inorg. Chem. XXXX, XXX, XXX−XXX
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(methanol, acetonitrile, acetone, THF, DMF, and DMSO). Consistent results were obtained for each of the solvents, but for clarity we here solely discuss the results for the measurements in acetonitrile in detail. The mass spectra and results for the other solvents are presented in the Supporting Information. We investigated CuCl2·2H2O/CH3CN solutions in the concentration range of 1−200 mM and added 1−2 equiv of KOtBu to the solutions after the measurement of pure CuCl2·2H2O. Figure 3 shows a representative mass spectrum of a 50 mM CuCl2·2H2O/CH3CN solution.
Until now preparation of these solvent clusters under atmospheric conditions has been excluded because of their high sensitivity against hydrolysis. Accordingly, all solvent clusters reported so far had been prepared and stored under inert conditions.13,43 However, these clusters seem to tolerate small amounts of water under certain conditions as indicated by some crystal structures, demonstrating the coexistence of such clusters and H2O.44,45 Addition of a base not only stabilizes μ4oxido clusters but enforces their formation through facilitating the generation of the central O 2− anion from H 2 O. Furthermore, a weak buffer (e.g., NaOtBu/HOtBu) which prevents hydrolysis of the μ4-oxido unit is presumably generated. It is well-known that water molecules (if not in excess), molecular oxygen, or bases such as NaOH can act as an oxygen donor for the formation of such cluster units.43,46−48 Thus, our results are in line with previous findings. Nevertheless, it had been unknown that μ4-oxido clusters with such labile ligands as solvent molecules can be stabilized to such an extent that they become stable under atmospheric conditions until now. This is essential for potential applications as a catalyst under atmospheric conditions (i.e., that such solvent clusters play an important role in copper chloride catalysis). The high stability under atmospheric conditions was clearly demonstrated by the “acetonitrile cluster” [Cu 4 OCl 6 (CH3CN)4]·2CH3CN (8), which had been obtained by adding a small portion NaOtBu to a solution of CuCl2·2H2O in acetonitrile under atmospheric conditions. Crystals suitable for X-ray crystal structure analysis (see Figure S7 and Tables S13 and S14) could be grown from this solution. A similar crystal structure of this cluster has recently been published by some of us.13 Analysis of the IR spectrum of 7 and the crystal structure of 8 shows that the role of the base indeed is confined to an indirect stabilization effect, because it does not bind itself to the cluster unit. This stabilization effect could further be observed by regeneration of solvent clusters after hydrolysis/decomposition. Adding NaOtBu to an aged solution caused reformation of the cluster units. In general, solvent clusters containing methanol or acetonitrile bind their ligands very loosely; hence, cluster formation with stronger coordinating ligands is quite likely as well. Thus, the results obtained for the systems CuCl2·2H2O/MeOH and CuCl2·2H2O/CH3CN can be assigned to many other systems of the kind CuCl2·2H2O/ ligand as long as a base is added. Structures in Solution. Although μ4-oxido solvent clusters were obtained as solid compounds, their formation in solution was still questionable. Previous investigations have shown that the structure of solid copper(II) chloride dihydrate mainly persists in aqueous solutions,49 and depending on the concentration, oligomeric structures such as Cu3Cl6(H2O)8 and Cu5Cl10(H2O)12 were observed as well.50 In comparison, little has been reported on the structure of copper(II) chloride in organic solvents. Golubeva et al. showed that in nonpolar solvents, besides mononuclear species, oligomeric compounds [Cu2Cl6]2− and [Cu3Cl8]2− were observed as well.51 In contrast, polar solvents such as DMF, DMSO, and acetonitrile rather support the formation of mononuclear chlorido complexes such as [CuCl4]2− and [CuCl3]−.52−55 There are no reports about the structure of CuCl2·2H2O in a basic solution. Therefore, we investigated the structure of CuCl2· 2H2O in organic solvents with and without addition of a base with the help of ultra-high-resolution CSI-MS experiments. CSI-MS spectra were collected for solutions of CuCl2·2H2O in the same solvents described in the previous paragraph
Figure 3. CSI mass spectrum of a 50 mM solution of CuCl2·2H2O in acetonitrile. The data were collected in the negative ion mode.
The spectrum shows different adducts with the general formulation [CunCl2n+1]− (see Table S22 for the exact assignment). The species observed in the MS experiments correspond to the anions of the actual neutral clusters with the formulation of [CunCl2n]x. Measurements in the positive ion mode revealed the appearance of solvent clusters with the general formulation [CunCl2n(solvent)]x. These results are in agreement with our determined crystal structures and indicate that oligomeric structures of this kind are not restricted only to nonpolar solvents.50,51 Our MS studies expose the existence of several species such as [CunCl2n+1]−, [CunCln+1]−, [CunCl2n]−, and [CunCl2n−1(solvent)]+, depending on the solvent system. In contrast to our findings, calorimetric and spectrophotometric studies by Ohtaki et al. suggested the presence of solely mononuclear species such as [CuCl4]2− and [CuCl3]− in polar solvents such as DMF, DMSO, and acetonitrile.52−55 Altering the detection range of the mass spectrometer, [CuCl3]− and [CuCl2]− become visible as the dominating ions, confirming the existence of mononuclear species and indicating the coexistence of Cu(I) and Cu(II) in solution. Their relative quantities seem to depend on the concentration of CuCl2· 2H2O and vary from 100:50 [Cu(I):Cu(II)] for a 10 mM solution to 25:100 for a 200 mM solution in acetonitrile (see Figure S16). As previously reported, μ4-oxido clusters might exist as complex redox equilibrium mixtures of diverse copper(I) and copper(II) species.13 The observation of mixed-valence species (such as [Cu3Cl3]− and [Cu4OCl4]+; see Figures S9 and S21) provides further evidence for this redox equilibrium. To investigate the influence of a basic coreagent, the MS experiments were repeated upon addition of KOtBu. Typical reaction conditions for reacting CuCl2·2H2O with KOtBu required an excess of 2 equiv. A representative mass spectrum of CuCl2·2H2O + 2 equiv of KOtBu in acetonitrile (50 mM) is shown in Figure 4. D
DOI: 10.1021/acs.inorgchem.5b02576 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 4. CSI mass spectrum of a 50 mM solution of CuCl2·2H2O + 2 equiv of KOtBu in acetonitrile. The data were collected in the negative ion mode.
Figure 6. Isotopic patterns of the species observed for CuCl2·2H2O (50 mM) and KOtBu (2 equiv) in acetonitrile in comparison with simulated spectra of the species [Cu5OCl6(CH3CN)2]+.
Fewer species were observed compared with the samples that did not contain a base. Among them two dominating species were identified as fragments of μ4-oxido clusters: [Cu3OCl5]− (m/z 383.6255, m/z calculated 383.6225) and [Cu4OCl7]− (m/ z 518.4906, m/z calculated 518.4873). Figure 5 shows a comparison of their isotopic patterns and the simulated mass spectra.
Figure 7. CSI mass spectrum of a 50 mM solution of CuCl2·2H2O + 2 equiv of K2CO3 in acetonitrile. The data were collected in the negative ion mode.
Figure 5. Isotopic patterns of the species observed for CuCl2·2H2O and KOtBu in acetonitrile in comparison with simulated spectra of the species [Cu3OCl5]− and [Cu4OCl7]−.
As for mere CuCl2·2H2O, these experiments were repeated in the positive ion mode and revealed the coexistence of a large number of clusters. In accordance with the results for the negative ion mode, several species could be identified that contain a μ4-oxido copper cluster moiety in the form of [Cu5OCl6(CH3CN)2]+. The measured and calculated isotopic patterns are compared in Figure 6. Mixed-valent copper species were also obtained as already observed for mere CuCl2·2H2O. We repeated the experiments with K2CO3, which is also commonly applied in conjunction with CuCl2·2H2O.56,57 Data collected in the negative ion mode resulted in well-identified fragments of the μ4-oxido cluster: [Cu3OCl5]− (m/z 383.6256, m/z calculated 383.6225) and [Cu4OCl7]− (m/z 518.4910, m/z calculated 518.4873) (Figure 7). Their calculated isotopic patterns are shown in Figure 8. These results are in agreement with those obtained in the presence of KOtBu and clearly demonstrate that μ4-oxido clusters are formed in acetonitrile (even in the presence of water, if a common base is added). The results obtained in the other solvents are consistent with those discussed here (see the
Figure 8. Isotopic patterns of the species observed for CuCl2·2H2O and K2CO3 in acetonitrile in comparison with simulated spectra of the species [Cu3OCl5]− and [Cu4OCl7]−.
Supporting Information for further information and mass spectra). In all cases (MeOH, acetone, acetonitrile, THF, DMF, and DMSO), the pure CuCl2·2H2O oligomeric compounds of the general formula [CunCl2n−1(solvent)x]+ (x = 1, 2, 3) were observed. However, it cannot be excluded that the agglomeration to higher oligomeric compounds is owed to the nature of the electrospray ionization process. To clarify this issue, a MS experiment using in-source collision-induced decomposition (isCID) was performed. The results show a significant decrease in the relative quantity of oligomeric compounds with an E
DOI: 10.1021/acs.inorgchem.5b02576 Inorg. Chem. XXXX, XXX, XXX−XXX
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Table 1. Overview of All Cluster Fragments Observed When Adding KOtBu (2 equiv) to a CuCl2·2H2O Solution of a Given Concentration acetone
acetonitrile
THF
DMF DMSO
20 mM
50 mM
[Cu3OCl5]− [Cu4OCl7]− [Cu4OCl5(L)]+ [Cu4OCl5(L)2]+ [Cu4OCl5(L)3]+ [Cu4OCl5(L)4]+ [Cu3OCl5]− [Cu4OCl7]− [Cu4OCl4(L)]+ [Cu5OCl5(L)]+ [Cu3OCl5]− [Cu4OCl7]− [Cu8O2Cl11(L)2]+ [Cu8O2Cl11(L)3]+ [Cu4OCl5(L)3]+ [Cu4OCl5(L)3]+
[Cu3OCl5]− [Cu4OCl7]− [Cu4OCl5(L)]+ [Cu4OCl5(L)2]+ [Cu4OCl5(L)3]+ [Cu4OCl5(L)4]+ [Cu3OCl5]− [Cu4OCl7]− [Cu4OCl4(L)]+ [Cu5OCl6(L)2]+ [Cu3OCl5]− [Cu4OCl7]− [Cu8O2Cl11(L)2]+ [Cu8O2Cl11(L)3]+ [Cu4OCl5(L)3]+ [Cu4OCl5(L)3]+
increased potential step. This fact implies that the majority of oligomeric clusters are formed during the ionization process. When KOtBu was added, μ4-oxido clusters were immediately formed. In each case cluster fragments could be identified as follows: [Cu 4 OCl 5 (acetone) n ] + , [Cu 4 OCl 5 (DMF) 3 ] + , [Cu4OCl5(DMSO)3]+, [Cu4OCl5(THF)n]+, and [Cu8O2Cl11(THF)n]+. However, as stated above the dimeric compounds most likely are formed during the ionization process. Concerning methanol, only pure cluster fragments without solvent molecules were observed: [Cu4OCl4]+, [Cu5OCl5]+, and [Cu7O2Cl12]+. As one might expect, the cluster formation is concentration dependent. Thus, μ4-oxido clusters were not observed when using very low concentrations of CuCl2·2H2O. However, at concentrations >10 mM μ4-oxido cluster were the predominant species observed. This is important to note, because concentrations of 20−200 mM are common in organic syntheses, and thus, the results reported herein clearly correlate to the standard copper chloride catalysis conditions. An overview of all species observed is given in Table 1. Our experiments show that formation of μ4-oxido clusters is quite favorable, even in the presence of water. Moreover, traces of water or oxygen presumably support a μ4-oxido cluster formation. As already mentioned, in most cases when CuCl2· 2H2O is used in organic syntheses, coordinating solvents are used5,36−38 and a basic coreagent (e.g., NaOtBu) is added as well.33−35 As demonstrated here, cluster formation is fairly easy under these conditions and could be observed in most cases. However, depending on the configuration mode of the instrument, mononuclear species such as [CuCl3]− still could be determined in the solutions (see Figure S16). Probably the use of coordinating solvents is not absolutely necessary for the cluster formation itself, because the central motif [Cu4OCl6] can be formed without solvent molecules as additional ligands. This is supported by the observation of solely “bare” [Cu3OCl5]− and [Cu4OCl7]− cluster fragments when operating the mass spectrometer in the negative mode. Furthermore, a negatively charged cluster with four additional chloride anions such as [Cu4OCl10]4− could form in noncoordinating solvents as well. This cluster type has been known for a long time58 and moreover occurs in the natural mineral ponomarevite.59
100 mM [Cu4OCl7]− [Cu4OCl5(L)3]+ [Cu4OCl5(L)4]+ [Cu4OCl5(L)5]+
[Cu3OCl5]− [Cu4OCl7]− [Cu5OCl6(L)2]+ [Cu8O2Cl11(L)2]+ [Cu8O2Cl11(L)3]+
[Cu4OCl5(L)3]+ [Cu4OCl5(L)3]+
Even if CuCl instead of CuCl2 is used, a cluster formation still is very likely, because of the redox equilibrium between copper(I) and copper(II) species with μ4-oxido clusters involved.13 In addition, we observed a mixture of copper(I) and copper(II) species in several of the CSI-MS measurements as described above, which gives further evidence for a redox equilibrium of these two species in solution. Furthermore, CuCl purchased from commercial suppliers often is contaminated with copper(II) chloride, which is revealed by its green color. Thus, in most reactions with commercial CuCl (if used without further purification) there is a mixture of copper(I) and copper(II) species. In summary, the reaction conditions of most copper-catalyzed reactions are ideal for a cluster formation, e.g., μ4-oxido clusters. This emphasizes the important role these clusters may have in catalysis. The formation of copper clusters most likely explains why copper chloride itself shows catalytic activity in many oxygenation reactions that require several electron transfers, although it cannot store/provide more than one electron at once as described in the Introduction. Due to ligand exchange reactions, the substrate can be bound and multiple electrontransfer steps can occur. It is well-known that especially labile solvent clusters such as [Cu4OCl6(solvent)4] (solvent = MeOH, acetone, THF, etc.) undergo ligand substitution processes if a stronger coordinating ligand is added.13,43 In that regard it is interesting to note that during our attempted synthesis of a urea copper cluster unit a “crystallographic snapshot” of this ligand exchange process was obtained when the “THF cluster”, whose crystal structure already had been reported,44,60 was reacted with urea. Figure 9 shows the crystal structure of this “ligand exchange” process. The structure shows a [Cu4OCl6(THF)(urea)3] cluster (9) surrounded by three THF molecules and one urea molecule. Three THF molecules of the four former coordinated molecules have been replaced by three urea molecules; the exchange process was “frozen” before the exchange of all four solvent molecules could be completed. The three substituted THF molecules remain in a close proximity to the cluster itself arranged in the form of a secondary coordination sphere. F
DOI: 10.1021/acs.inorgchem.5b02576 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
reactions should be reviewed. These conclusions not only affect CuCl2·2H2O catalysis but include CuCl catalysis as well, because both copper compounds can be in a redox equilibrium. The detailed understanding of the mechanism of CuCl2·2H2O catalysis is fundamental for the development of new, potent copper catalysts. The facile formation and handling of such clusters might allow development of a number of new copper clusters that are able to catalyze multiple electron-transfer steps. As we demonstrated, ligand substitution reactions can easily take place, and offer the possibility to prepare a large number of clusters with different ligand systems. However, as recently reported, not all ligands seem to support a cluster formation.32 This fact also has to be investigated in more detail in the future. Despite the fact that many open questions concerning the detailed mechanisms of copper chloride-catalyzed reactions remain, our results provide an important insight into the catalytic activity of simple copper salts and should lead the scientific discussion on that topic in a new direction.
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Figure 9. Crystallographic “snapshot” of the ligand exchange process of the “THF cluster” with urea molecules. Thermal ellipsoids are set at the 50% probability level.
ASSOCIATED CONTENT
* Supporting Information S
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02576. Materials and Methods, crystallographic data, IR spectra, and Elemental Analyses (PDF) X-ray crystallographic data for compound 1 (CIF) X-ray crystallographic data for compound 4 (CIF) X-ray crystallographic data for compound 5 (CIF) X-ray crystallographic data for compound 2 (CIF) X-ray crystallographic data for compound 9 (CIF) X-ray crystallographic data for compound 3 (CIF) X-ray crystallographic data for compound 6 (CIF)
CONCLUSION The molecular structure(s) of CuCl2 ·2H2 O in several coordinating solvents was investigated in the presence and absence of a basic coreagent such as NaOtBu and K2CO3. This was accomplished by crystal structure determination, IR spectroscopy, and UHR CSI-MS. Our experiments demonstrated that the presence of basic coreagents facilitates formation of μ4-oxido clusters of the type [Cu4OCl6(solvent)4] (solvent = methanol, acetone, acetonitrile, THF, DMF, DMSO). These clusters, which are extremely sensitive toward moisture, could be stabilized due to addition of base and thus could be easily handled and analyzed under atmospheric conditions. Unfortunately still many chemists tend to speak of mere and naked CuCl2 without ligands/complexes as a catalyst, e.g., as recently proposed by Reddy et al.,61 although they perform their reactions in coordinating solvents and/or use coordinating additives or substrates. However, our results clearly demonstrate that in most cases naked “CuCl2·2H2O” does not exist and hence is not the active catalyst. Instead solvent clusters will be formed that are either already the active species or a precursor of it. It is likely that due to ligand exchange reactions facile formation of other copper clusters is possible, and they can then act as the catalytically active species. These results provide important new insights into CuCl2·2H2O catalysis, because until now the possibility of CuCl2·2H2O to form multinuclear compounds as active species was more or less rejected when a reaction mechanism was postulated. To the best of our knowledge and apart from our own work,13 there are only two other examples reported: Sun et al. previously proposed a μ4-oxido cluster derived from CuCl2·2H2O as an active catalyst in the oxidation of 2,3,6-trimethylphenol to trimethyl-1,4-benzoquinone.15 Furthermore, Liang et al. previously reported a μ3-hydroxido copper cluster as an active species in selective oxidation reactions of alcohols to aldehydes via CuCl2·2H2O.62 As a result of this rejection, most reaction mechanisms proposed cannot explain sufficiently how CuCl2· 2H2O enables a multiple-electron transfer, which is required in most catalytic redox reactions. The observed formation of multinuclear copper clusters does explain this indeed! As a consequence, mechanisms postulated in the past for such
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions ∥
S.B. and M.D. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.B. and S.S. are thankful to the Stiftung Stipendienfonds der Chemischen Industrie e.V. for providing a Ph.D. scholarship to S.B. M.D. and I.I.-B. gratefully acknowledge support from the “Solar Technologies Go Hybrid” initiative of the State of Bavaria.
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