The Power of Ferrocene, Mesoionic Carbenes, and Gold: Redox

Article pubs.acs.org/Organometallics

The Power of Ferrocene, Mesoionic Carbenes, and Gold: RedoxSwitchable Catalysis Sinja Klenk, Susanne Rupf, Lisa Suntrup, Margarethe van der Meer, and Biprajit Sarkar* Institut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin, Fabeckstraße 34-36, D-14195 Berlin, Germany S Supporting Information *

ABSTRACT: Catalysis with gold(I) complexes is a useful route for synthesizing a variety of important heterocycles. Often, silver(I) additives are necessary to increase the Lewis acidity at the gold(I) center and to make them catalytically active. We present here a concept in redox-switchable gold(I) catalysis that is based on the use of redox-active mesoionic carbenes, and of electron transfer steps for increasing the Lewis acidity at the gold(I) center. A gold(I) complex with a mesoionic carbene containing a ferrocenyl backbone is presented. Investigations on the corresponding iridium(I)−CO complex show that the donor properties of such carbenes can be tuned via electron transfer steps to make these seemingly electron rich mesoionic carbenes relatively electron poor. A combined crystallographic, electrochemical, UV−vis−near-IR/IR spectroelectrochemical investigation together with DFT calculations is used to decipher the geometric and the electronic structures of these complexes in their various redox states. The gold(I) mesoionic carbene complexes can be used as redox-switchable catalysts, and we have used this concept for the synthesis of important heterocycles: oxazoline, furan and phenol. Our approach shows that a simple electron transfer step, without the need of any silver additives, can be used as a trigger in gold catalysis. This report is thus the first instance where redox-switchable (as opposed to only redox-induced) catalysis has been observed with gold(I) complexes.

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INTRODUCTION The concept of redox-switchable catalysis (RSC) is based on the idea of influencing the catalytic activity of a transition metal by changing the electron-donating or -withdrawing nature of a coordinated ligand.1 This concept was first introduced in 1995. By combining the redox-active ligand 1,1′-bis(diphenylphosphino)cobaltocene with a rhodium(I) center, it was shown that the catalytic activity of this complex can be influenced depending on its redox-state. Thus, the same compound is a faster hydrosilylation catalyst in its oxidized form and a better hydrogenation catalyst in the reduced form.2 Inspired by this work, several groups applied the concept of RSC in homogeneous catalysis, e.g., in ring-opening polymerizations,3 ring-closing metatheses, and other reactions.4,5 The use of redox-switchable catalysts with a dendritic ferrocenylbased structure was reported in 2015.6 Recently, we reported on the redox-induced gold catalysis based on 1,2,3triazolylidene ligands with ferrocenyl substituents.7,8 These were the first examples in gold(I) catalysis, where an electrontransfer step was used as a trigger for increasing the catalytic efficiency at the gold center. Following this, the concept of redox-induced gold catalysis was shown for a hydroamination gold(I) catalyst in 20159 and for a ferrocenyl-imidazolylidene gold(I) complex in 2016.10 As ferrocene is a versatile molecule which is easy to functionalize and which exhibits a reversible oxidation process, this unit is often used as the redox-active component in catalysts. Only a few examples are reported where oxidation at © XXXX American Chemical Society

the ferrocene unit leads to an increase of the catalytic activity of the whole complex; usually it is the other way around.3b,11 Due to the fact that the catalytic activity depends partly on the donor ability of the coordinating ligand, the donating properties can be tuned by the ferrocene/ferrocenium couple. It was observed that in N-heterocyclic carbene complexes with a 1,1′ferrocenyl backbone the oxidation at the ferrocenyl unit results in an increase of the Tolman electronic parameter (TEP) in the range of 11 cm−1 which leads to more phosphine-like donor properties.4b,12−15 The TEP is usually taken as a measure to evaluate relative donor abilities of various ligands.16,17 1,2,3Triazolylidenes are prominent examples of mesoionic carbene (MIC) ligands, and are known to be strong σ-donors.18 Gold(I) complexes are useful catalysts,19,20 and their complexes with triazolylidenes19g−q have been used as catalysts in the presence of either silver(I) or other additives.19n,o We have recently shown that by using a reversible electron transfer step in ferrocenyl substituted triazolylidene ligands such strongly donating ligands can be converted into rather electron-poor ligands.7,8 In gold(I) catalysis, the catalytic activity is often correlated with the Lewis acidic nature of the gold(I) center, as these usually have a high affinity to π bonds.20−23 Thus, gold(I) centers possessing the right catalytic activity can be used as catalysts for the synthesis of important organic compounds such as oxazolines, furans, and phenols, among others.21 Having Received: April 10, 2017

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DOI: 10.1021/acs.organomet.7b00270 Organometallics XXXX, XXX, XXX−XXX

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Organometallics previously conclusively shown that electron transfer steps are sufficient to trigger catalysis at gold(I) centers by making them more Lewis acidic, we have now turned our attention to redoxswitchability in gold(I) catalysis. We mention here that to the best of our knowledge real redox-switchable catalysis (as opposed to redox-induced) has never been previously observed for any gold(I) catalysts. In the following, we present the synthesis, (spectro)electrochemical investigations, and the electronic properties of novel gold(I) and iridium(I) complexes with ferrocenyl substituted 1,2,3-triazolylidene ligands. The iridium(I)−CO complexes were synthesized to determine the TEP values for the MIC ligands. The concept of redox-switchable catalysis in gold(I) catalysis (Scheme 1) is presented by means of the

Chart 1. Ferrocenyl Triazole 1, Triazolium Salt 2[BF]4, and Ferrocenyl-Triazolylidene Complexes 3−5 Presented in This Work

Scheme 1. Concept of Redox-Switchable Gold(I) Catalysis Triggered by a Reversible Electron Transfer Step Influencing the Lewis Acidity at the Catalytic Gold Center

The ligands and the metal complexes were characterized by H and 13C{1H} NMR spectroscopy (Figures S1−S5) and with mass spectrometry. Formation of triazolylidene complexes 3 and 4 lead to the disappearance of the C−H proton signal of triazolium salt [2]BF4. The formation of triazolylidene complex 5, where the labile COD ligand is replaced by CO ligands, was shown by the disappearance of the proton signals of the COD ligand. All triazolylidene complexes display a relatively downfield-shifted signal of the carbene C atom (156.75 (3), 170.31 (4), and 182.33 (5) ppm) in the corresponding 13 C{1H} spectrum compared to the carbon C−H signal of the triazolium salt (122.13 ppm). We were able to obtain single crystals suitable for X-ray diffraction analyses for triazole 1, triazolium salt [2]BF4, and triazolylidene complexes 3 and 4. Crystallographic details as well as important bond lengths and angles are given in Tables S1−S4. In triazole 1 (Figure S6), the N2−N3 bond distance is shorter than the N1−N2 and the C1−N2 bond distances, indicating an “azo” character of the N2−N3 bond within the triazole unit. On methylating triazole 1 to triazolium salt [2]BF4 (Figure S7), these bond distances become more equal which points to a more delocalized situation within the triazolium unit. This delocalization can also be observed in the triazolylidene rings of complexes 3 and 4. In triazolylidene complex 3, the gold(I) center (d10) is linearly coordinated through the MIC-C and the Cl− donors with a deviation of 3° from linearity. In triazolylidene complex 4, the iridium(I) center (d8) displays a square-planar coordination through the MIC-C, the Cl−, and COD ligand, where the COD ligand coordinates in a η2,η2 fashion (Figure 1). Crystals of complex 5 were subjected to X-ray analysis and confirmed the substitution of COD by two carbonyl ligands (Figure S8) and the general connectivity in that metal complex. Unfortunately, the quality of the obtained data are not good enough to discuss bond distances. Cyclic Voltammetry, UV−Vis−NIR Spectroelectrochemistry, and (TD)DFT Calculations. The electrochemical properties of compounds 1−5 were investigated by cyclic voltammetry (Figures 2 and S9−S11). For compounds 1, [2]BF4, and 3, a reversible oxidation process was observed at 1

cyclization of N(2-propyn-1-yl)benzamide to 5-methylene-2phenyl-4,5-dihydrooxazole. Furthermore, we will extend this concept to the cyclization of 2-methylene-4-phenylbut-3-yn-1ol to 4-methyl-2-phenylfuran and of 2-methyl-5-((prop-2-ynyloxy)methyl)furan to 4-methyl-1,3-dihydroisobenzo-furan-5-ol. In all cases we will conclusively show that electron transfer as a perturbation is sufficient to trigger catalysis in redox-active gold(I) complexes and that one does not need any additional additives to increase the Lewis acidity at the catalytic gold center.

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RESULTS AND DISCUSSION Synthesis and Characterization. The reaction of ferrocenyl azide with phenylacetylene under typical Cu(I) catalyzed alkyne−azide cycloaddition (CuAAC) reaction conditions resulted in the formation of ferrocenyl triazole 1 in good yield. After methylation with Meerwein salt, resulting triazolium salt [2]BF4 was transmetalated by reacting with Ag2O followed by the reaction with [Au(SMe2)Cl] to obtain heterobimetallic ferrocenyl-triazolylidene-gold(I) complex 3. Ferrocenyl-triazolylidene-iridium(I)-COD complex 4 was obtained via direct deprotonation of [2]BF4 and addition of [Ir(COD)Cl]2 in THF in moderate yield. By bubbling CO gas through a solution of 4 in dichloromethane, ferrocenyltriazolylidene-iridium(I)-CO complex 5 could be obtained (Chart 1). B


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Organometallics

indirectly reveals a predominantly triazolium or triazolylidene based reduction process. For iridium(I) triazolylidene complexes 4 and 5, no reduction steps were observed within the solvent window. For iridium(I) triazolylidene complex 4, three irreversible oxidation steps are observed (Figure S11) which might be related to oxidation processes not only at the ferrocene moiety but also at the iridium(I) center. Iridium(I) triazolylidene complex 5 displays a reversible oxidation step at 0.25 V (Figure 2), which lies in the same range as the oxidation process of gold(I) triazolylidene complex 3. In contrast to 4, no further oxidation process is observed for complex 5. This might be related to the presence of the CO ligands which would make an oxidation at the iridium(I) center more difficult. The reversible nature of the oxidation processes prompted us to carry out UV−vis−NIR spectroelectrochemical measurements for compounds 1−3 and 5. All compounds mainly display absorptions in the UV region in their native states, whereas upon oxidation, a long wavelength band between 500 and 800 nm arises (Figures S12−S15 and Table S6) which is an indication for the oxidation at the ferrocenyl moiety.24 To get deeper insights into the nature of this long wavelength band, we performed (TD)DFT calculations of compounds 1−3. One-electron oxidized form [1]+ displays a long-wavelength band at 730 nm. (TD)DFT calculations show that this band corresponds to transitions localized on the oxidized ferrocenyl moiety (Figure S17 and Table S7). One-electron oxidized form [2]2+ displays a long-wavelength band at 647 nm. In this case, (TD)DFT calculations reveal this band to be a mixture of a predominant ligand-to-metal charge transfer (LMCT) and a minor d−d-type transition within the ferrocenyl moiety (Figure S20 and Table S8). The one-electron oxidized form of gold(I) complex 3 displays a long-wavelength band at 665 nm. (TD)DFT calculations show this band to be a Au−Cl to ferrocenyl metal-to-metal-charge transfer (MMCT) with a small contribution of a LMCT (Figure S23 and Table S9). The results from cyclic voltammetry, UV−vis−NIR spectroelectrochemistry, and (TD)DFT calculations thus confirm that all investigated compounds undergo a reversible oneelectron oxidation, and this process is centered on the ferrocenyl moiety for all cases. This information has strong consequences for the determination of TEP values of the MIC ligand through investigation of the iridium−CO complex and for the catalytic activity of the gold(I) complex (vide inf ra). IR Spectroelectrochemistry and Tolman Electronic Parameter. As could be seen by the (spectro)electrochemical data discussed above, the oxidation at the ferrocenyl moiety of all compounds is a reversible process. To get more insight into the electronic and donor properties of the triazolylidene complexes we prepared iridium(I) triazolylidene complex 5 with two CO ligands in its coordination sphere. By measuring the IR stretching frequencies of the CO ligands in the native and the oxidized state of the complex by IR spectroelectrochemistry, it is possible to calculate the modified TEP.25 This enables the possibility to compare the donor properties not only of recently reported triazolylidene ligands7,8 but also of well-established phosphine and carbene ligands25 with the triazolylidene ligand of this work. The IR spectrum of triazolylidene complex 5 displays two IR bands corresponding to the stretching frequencies of the CO ligands with an average value of 2020.1 cm−1 and a calculated TEP of 2047.0 cm−1. Upon oxidation, both IR bands are shifted to higher wavenumbers with an average value of 2028.7 cm−1 and a

Figure 1. Perspective view of 3 (top) and 4 (bottom). Ellipsoids are drawn at the 50% probability. Hydrogen atoms have been omitted for clarity.

Figure 2. Cyclic voltammograms of 3 (gray) in THF (25 mVs−1) and 5 (purple) in dichloromethane (25 mVs−1) containing 0.1 M Bu4NPF6 as the supporting electrolyte at room temperature.

0.17, 0.30, and 0.23 V in THF/0.1 M Bu4NPF6, respectively. The higher value for the oxidation potential of [2]BF4 compared to those of triazole 1 and gold(I) triazolylidene complex 3 is a result of the positive charge on the triazolium ring. In all three compounds, the oxidation step is considered to be localized at the ferrocenyl moiety. Spin population analyses of oxidized forms [1]+, [2]2+, and [3]+ (Figures S18, S21, and S24) calculated with structure-based DFT indirectly confirm a ferrocenyl based oxidation process. Additionally, an irreversible reduction step was observed for triazolium salt, [2]+ and gold(I) triazolylidene complex 3 at −2.02 and −2.47 V, respectively. The lower absolute value for the reduction process of the triazolium salt compared to that of the gold(I) triazolylidene complex is again a result of the positive charge at the triazolium ring. Spin population analyses (Figures S21 and S24) of reduced forms 2 and [3]− calculated with DFT C


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Organometallics calculated TEP of 2054.3 cm−1 (Figures 3 and S25 and Table S10). The same effect can be seen in iridium(I) triazolylidene

the substitution pattern at the triazolylidene unit. That opens up the possibility of fine-tuning of such ligands, resulting in triazolylidene complexes with tailored properties with regard to, for example, chemical reactivity and catalysis. Redox-Switchable Catalysis. Gold(I) triazolylidene complex 3 was employed in three different catalytic reactions: 1. Oxazoline formation 2. Furan formation 3. Phenol formation Oxazoline Formation. The concept of redox-switchable catalysis will be presented for the catalytic oxazoline formation (Scheme 2). We already show that it is possible to tune the Scheme 2. Gold-Catalyzed Cyclization of N(2-Propyn-1yl)benzamide (A) to 5-Methylene-2-phenyl-4,5dihydrooxazole (B)

Figure 3. Changes in the IR spectrum of 5 in the course of the oneelectron oxidation in dichloromethane/0.1 M Bu4NPF6.

catalytic activity of gold(I) triazolylidene complexes with a ferrocene backbone depending on the redox state of ferrocene. In this redox-induced approach,7,8 the oxidized form of complex 7 (Chart 3) displays a higher catalytic activity than

Chart 2. Complex 6 (Top), Triazolylidene Ligand with CBound Ferrocenyl Unit (Bottom Left), and Triazolylidene Ligand with N-Bound Ferrocenyl Unit (Bottom Right)

Chart 3. Previously Reported Gold Catalyst 77,8

the corresponding native form. Using gold(I) triazolylidene complex 3, we observed a slightly higher catalytic activity than that for comparable gold(I) triazolylidene complex 7 which is in accordance with the higher TEP value for 3. This indicates that gold(I) complex 3 with an N-bound ferrocenyl moiety contains a more Lewis-acidic gold(I) center both in the native and oxidized form than gold(I) complex 7 with a C-bound ferrocenyl moiety. The concept of redox-induced catalysis is thus again proven. Using complex 3 in its native form, a 30% conversion within 24 h from benzamide A to dihydrooxazole B can be observed. Addition of the easily accessible oxidizing agent [Fe(η5-C5H4COMe)Cp][BF4] leads to a significant increase of the conversion over the same period resulting in full conversion within 24 h (Figure S31). These results are in accordance with the assumption that oxidizing the ferrocenyl moiety in these gold(I) triazolylidene complexes leads to a decrease of the electron density at the gold center (resulting in higher TEP values for the corresponding iridium(I) triazolylidene complexes) and therefore to a more Lewis acidic gold center which can activate π bonds more easily. Control experiments using the oxidizing agent alone show no conversion to the product, so we can rule out that this additive itself catalyzes the reaction.

complex 6 (Chart 2) with a ferrocenyl unit, where the ferrocene is bound over the carbon C of the triazolylidene ring.8 Being attached to a less electronegative atom, this complex displays an average stretching frequency of the CO ligands of 2018.2 cm−1 and a calculated TEP of 2045.4 cm−1 for the native and an averaged CO stretching frequency of 2026.8 cm−1 and a calculated TEP of 2052.7 cm−1 for the oxidized form.8 Correlating the TEP values with the average CO stretching frequencies of triazolylidenes (Fc(N-MIC)Ph and Fc(CMIC)Mes) (Chart 2) and selected phosphine and carbene ligands (Figure S25 and Table S10) shows that the triazolylidene ligands in their native form display better donor properties than the widely used imidazolylidene NHCs. Upon oxidation, both triazolylidene ligands become poorer donors tending to lie in the range of phosphine ligands. As already mentioned, the triazolylidene ligand with a C-bound ferrocenyl unit displays lower CO stretching frequencies and TEP values resulting in better donor properties in the native as well as in the oxidized form. This fact clearly shows that the donor properties of these kind of triazolylidene ligands not only depend on the redox-state of the ferrocenyl moiety but also on D


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Organometallics

alkynyl alcohol C to furan D is the first example where no silver or copper additive is necessary to activate the gold(I) catalyst (Scheme 3).

Keeping in mind that the oxidation process is electrochemically reversible, we considered using a reducing agent to see if it is possible not only to turn the catalytic activity “on” but also to turn it “off” again. For that purpose, we chose decamethylferrocene (FeCp2*) as a reducing agent. In two independent NMR experiments, we first confirmed that the initially formed oxidized form of 3, which is paramagnetic, can indeed be reduced back to the diamagnetic form of 3 by addition of FeCp2* (Figures S26−S28). The slight paramagnetic broadening observed in the final spectrum obtained after the addition of the reducing agent (Figure S29) is related to the use of a slight excess of the oxidizing/reducing agents that is required for driving the electron transfer steps to completion. These experiments thus confirm the reversible nature of the chemically driven oxidation and reduction steps. Finally, under catalytic conditions, adding the oxidizing or reducing agent for conversion to dihydrooxazole B was determined at given times resulting in two scenarios: First, starting with the turned-on catalyst, FeCp2* was added after 2.5 h where 20% of the substrate already had been converted to product B (Figure S32 and Table S11). The catalytic activity of complex 3 stops at this point, and no further conversion can be observed over the next 17 h (Figure S32). By adding [Fe(η5-C5H4COMe)Cp][BF4], the catalytic activity can be turned on again, and the conversion increases to approximately 75% within 7 h. The maintained slope of percent conversion versus time during the activated catalysis might be an indication that the oxidized form of the complex is still intact after reducing it once (Figure S32). Second, the course of this redox-switchable catalysis can be extended by slotting a “turned off state” ahead resulting in the switching “off−on−off−on” (Figure 4).

Scheme 3. Gold-Catalyzed Cyclization of 2-Methylene-4phenylbut-3-yn-1-ol (C) to 4-Methyl-2-phenylfuran (D)

2-Alknylallyl alcohol C was prepared as previously reported. 27 Starting with the native form of gold(I) triazolylidene complex 3 under the same reaction conditions as for the oxazoline formation discussed above, we see no conversion to furan D within 24 h (Table S12). Adding [Fe(η5C5H4COMe)Cp][BF4] to generate the oxidized form of the catalyst we see a drastic change: Already 40% of substrate C is converted to the product within approximately 45 min (Figure S33 and Table S12). However, the reaction stops at that point, and only further addition of [Fe(η5-C5H4COMe)Cp][BF4] leads to an increase of the conversion of substrate C to furan D (Figure S33 and Table S12). A test reaction with [Fe(η5C5H4COMe)Cp][BF4] and substrate C alone shows no conversion, so we can again rule out that the oxidizing agent itself catalyzes the reaction (Table S12). It is not clear yet if the formation of product D inhibits the catalytic activity of [3]+ or if there are other reasons for the quenching of the reaction after a given times. Nevertheless, even for this case, the catalytic activity can be switched off by the addition of FeCp2* and switched on again by further addition of [Fe(η5-C5H4COMe)Cp][BF4] confirming the operation of RSC even in this case (Figure S34). Control experiments showed that the reducing agent FeCp2* does not itself react with C. This observation clearly shows that the concept of redox-switchable catalysis can indeed be used for the synthesis of furans. Phenol Formation. The gold(III)-catalyzed intramolecular reaction of a furan with a terminal alkyne (furan-alkyne system) in the phenol synthesis was reported first in 2000. During this reaction the furan ring system is opened, followed by an intramolecular oxygen migration and a rearrangement leading to phenolic system F (Scheme 4).28 The first gold(I)-catalyzed phenol synthesis was reported in 2006.29 Furan-yne E was synthesized as previously reported.30 Starting again with the native form of catalyst 3 we see a slight conversion of 12% in the cyclization of furanyne E to product F within 24 h, whereas addition of [Fe(η5-C5H4COMe)Cp][BF4] leads to a drastic increase of the conversion to 100% within 5

Figure 4. Plot of the percentage conversion of A to B versus time as catalyzed by 1 mol % of 3. The arrows indicate the time at which 1.25 equiv of said reagent with respect to 3 was added. The initial “off” state refers to the catalytic mixture containing only complex 3, without any oxidizing or reducing agents.

Scheme 4. Gold-Catalyzed Cyclization of 2-Methyl-5-((prop2-ynyloxy)methyl)furan (E) to 4-Methyl-1,3-dihydroisobenzofuran-5-ol (F)

To investigate the generality of redox-switchability in gold(I) catalysis, we extended the concept of redox-induced catalysis to two more gold-catalyzed cyclization reactions. Furan Formation. Several examples of the gold(I)- or gold(III)-catalyzed formation of furans derived from β-alkynyl alcohols are known.26 To the best of our knowledge, there are no examples where this kind of gold(I)-catalyzed transformation is carried out without activation by a silver(I) or copper(II) salt. In this regard, the redox-induced cyclization of E


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Organometallics

min, whereas the native form without [Fe(η5-C5H4COMe)Cp][BF4] shows just a slight conversion after 24 h. Additionally, a unique kind of switching as stated above is observed for this reaction. It should be mentioned here that for all three classes of reactions tested in this work, the oxidizing agent as a trigger is enough to activate the gold(I) center toward catalysis. The addition of other additives is not necessary. This is a somewhat unique observation in gold(I) catalysis, and to the best of our knowledge, there are just a few examples where no silver(I) salt was used in gold(I) catalysis when working with a cationic gold(I) complex.31 In all other cases either a silver(I) or a copper(II) additive is essential for effective catalysis with gold(I) complexes even when working with cationic ligands for synthesizing gold(I) catalysts. Our approach here thus delivers a way forward in sustainable gold(I) catalysis.

min (Table S13). The test reaction with substrate E and [Fe(η5-C5H4COMe)Cp][BF4] alone shows no conversion to phenol F. Thus, it is again proven that the oxidizing agent alone does not catalyze the reaction. Our attempts at establishing RSC in this case provided us with some intriguing observations. Addition of FeCp2* to the mixture above leads to the backconversion of the phenol F to starting material E. Further addition of [Fe(η5-C5H4COMe)Cp][BF4] led to the predominant formation of phenol F again. Thus, a different kind of switching is observed for this case (Scheme 5 and Figure 5). Scheme 5. Back and Forth Switching during the Gold(I)Catalyzed Phenol formation

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CONCLUSION

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EXPERIMENTAL SECTION

We have presented here a new ferrocenyl containing triazolylidene ligand and its gold(I) and iridium(I) complexes. All complexes were characterized by a battery of spectroscopic methods, and two of the complexes were characterized through single crystal X-ray diffraction. Electrochemical and spectroelectrochemical measurements showed that both the gold(I) and the iridium(I)−CO complexes can be reversibly oxidized at the ferrocenyl unit. IR spectroelectrochemical data on the iridium(I)−CO complexes was used to determine the TEP parameters for these redox active mesoionic carbene ligands in two different oxidation states. These measurements showed that the neutral mesoionic carbene ligands are strong donors, but their oxidized forms are rather electron-poor ligands. This concept was then used in gold(I) catalysis. The catalytic activity of the gold(I) triazolylidene complex presented here can be triggered toward the formation of a variety of heterocycles through a simple electron-transfer step. By using such a simple electron transfer trigger, we have shown that the oxidized forms of these complexes can be used as catalysts for the formation of oxazoline, furan, and phenol. No silver(I), copper(II) or any other additives are required for these catalytic reactions. The two main highlights of this approach are the observation of redox-switchable catalysis in oxazoline formation, a first in gold(I) catalysis, and the formation of phenol within less than 5 min by using the oxidized form of the complex. Additionally, a unique kind of redox-switchability is observed for the reaction involving phenol synthesis. These results thus open up new perspectives in gold(I) catalysis and show that a simple redox trigger can indeed be used for activating gold catalysts. We believe the observations made and the conclusions drawn here will contribute in a large way toward making gold catalysis more sustainable. Future studies will be directed toward the expansion of the nature of the catalysts, investigating catalytic reaction mechanisms, as well as expanding the substrate scope in redox-switchable gold catalysis.

Figure 5. From bottom to top: (a) Substrate E at the beginning of the reaction. (b) Formation of phenol product F within 5 min by addition of [Fe(η5-C5H4COMe)Cp][BF4]. (c) Subsequent addition of FeCp2* resulting in a mixture of substrate E and phenol F. (d) further addition of [Fe(η5-C5H4COMe)Cp][BF4] results in predominant formation of phenol F.

Independent control experiments showed that phenol F does not react with FeCp2*, with the mixture of FeCp2*/[Fe(η5C5H4COMe)Cp][BF4], or with the starting complex 3. Thus, the back and forth switching of the reaction seems to be inherently linked to the total catalytic mixture (Scheme 5). The results of all three catalytic reactions show that it is possible to activate gold(I) triazolylidene complex 3 for three different classes of reactions using [Fe(η5-C5H4COMe)Cp][BF4] as an oxidizing agent and perform RSC in each case. In the cases of the oxazoline and furan formations, a redoxswitchable approach is accessible, an observation that to the best of our knowledge has been made for the first time in gold(I) catalysis. In the case of furan formation, the quenching of the reaction after reaching about 40% product yield is a topic that will be subject of future investigations. Nevertheless, more than 90% conversion can be achieved by adding larger amounts of the oxidizing agent. The gold(I)-catalyzed phenol formation displays the most drastic change when adding [Fe(η5C5H4COMe)Cp][BF4]: Full conversion is reached within 5

Caution! Compounds containing azides are potentially explosive. Although we never experienced any problems during synthesis or analysis, all compounds should be synthesized only in small quantities and handled with great care! General Procedures, Materials, and Instrumentation. Unless otherwise noted, all reactions were carried out using standard Schlenkline-techniques under an inert atmosphere of nitrogen (Linde HiQ Nitrogen 5.9, purity ≥ 99.999%). Ferrocene azide,32 [Au(SMe2)Cl],33 F


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Organometallics and TBTA34 were prepared as described previously in the literature. Commercially available chemicals were used without further purification. The solvents used for metal complex synthesis and catalysis were available from MBRAUN MB-SPS-800 solvent system and degassed by standard techniques prior to use. The identity and purity of compounds 1, [2]BF4, and 3 were established via 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis. For compounds 4 and 5, a combination of 1H and 13C spectroscopy, mass spectrometry, and IR spectroscopy (only for compound 5) were used to establish the identity and purity of the substances. Solvents for cyclic voltammetry and UV/vis spectroelectrochemical measurements were dried and distilled under nitrogen and degassed by common techniques prior to use. Column chromatography was performed over silica 60 M (0.04−0.063 mm). 1H and 13C{1H} NMR spectra were recorded on a JEOL ECZ 400R spectrometer at 19−22 °C. Chemical shifts are reported in ppm referenced to the residual solvent peaks.35 The following abbreviations are used to represent the multiplicity of the signals: s (singlet), d (doublet), t (triplet). Mass spectrometry was performed on an Agilent 6210 ESI-TOF. X-ray data were collected on a BRUKER Smart AXS or BRUKER D8 Venture system. Data were collected at 100(2) or 140(2) K, respectively, using graphite-monochromated Mo Kα radiation (λα = 0.71073 Å). The strategy for the data collection was evaluated by using the APEX2 or Smart software. The data were collected by standard “ω scan techniques” or “ω−φ scan techniques” and were scaled and reduced using APEX2, SAINT+, and SADABS software. The structures were solved by direct methods using SHELXL-97 or intrinsic phasing using SHELXL-2014/7 and refined by full matrix least-squares with SHELXL-2014/7, refining on F2. Non-hydrogen atoms were refined anisotropically. If it is noted, bond length and angles were measured with Diamond Crystal and Molecular Structure Visualization, version 3.1.36 CCDC 1015654 (1), 1015655 ([2]BF4), 1043309 (3), and 1488875 (4) contain the supplementary crystallographic data for this paper. Cyclic voltammograms were recorded with a PAR VersaStat 4 potentiostat (Ametek) with a conventional three-electrode configuration consisting of a glassy carbon working electrode, a platinum auxiliary electrode, and a coiled silver wire as a pseudoreference electrode. The ferrocene/ferrocenium couple was used as internal reference. All measurements were performed at room temperature with a scan rate of 25 or 100 mV s−1. The experiments were carried out in absolute THF or CH2Cl2 containing 0.1 M Bu4NPF6 (Fluka, ≥99.0%, electrochemical grade) as the supporting electrolyte. UV/vis spectra were recorded with an Avantes spectrometer consisting of a light source (AvaLight-DH-S-Bal), a UV/vis detector (AvaSpecULS2048), and an NIR detector (AvaSpec-NIR256-TEC). IR spectra were recorded with a BRUKER Vertex 70 FT-IR spectrometer, and the program used was OPUS Version 7.5. UV/vis and IR spectroelectrochemical measurements were carried out in an optically transparent thin-layer electrochemical (OTTLE)37 cell (CaF2 windows) with a platinum-mesh working electrode, a platinum-mesh counter electrode, and a silver-foil pseudoreference. The experiments were carried out in absolute THF or CH2Cl2 containing 0.1 M Bu4NPF6 as the supporting electrolyte. The same solvents as for the CV measurements were used for each compound. The program package ORCA 3.0.0 was used for all DFT calculations.38 The geometry optimization and single-point calculations were performed by the DFT method with BP86 and B3LYP functionals, respectively,39 including relativistic effects in zero-order regular approximation (ZORA).40 Convergence criteria for the geometry optimization were set to default values (OPT), and “tight” convergence criteria were used for SCF calculations (TIGHTSCF). Triple-ζ valence quality basis sets (def2-TZVP) were used for all atoms.41 The resolution of the identity approximation (RIJCOSX) was employed42,43 with matching auxiliary basis sets. All spin populations were calculated according to Löwdin population analysis.44 Spin populations were visualized via the program Molekel.45 Preparation of 1-Ferrocenyl-4-phenyl-1,2,3-triazole (1). Ferrocene azide (1 equiv, 45 mg, 0.198 mmol), phenylacetylene (1.5 equiv, 30 mg, 0.297 mmol), copper(II) sulfate pentahydrate (0.42

equiv, 21 mg, 0.083 mmol), sodium ascorbate (0.84 equiv, 33 mg, 0.166 mmol), and TBTA (0.084 equiv, 9 mg, 0.016 mmol) were dissolved in a mixture of water/THF (4.5 mL/5 mL) under air. The reaction mixture was stirred for 3 days at room temperature under the exclusion of light. After stirring, THF was removed under reduced pressure, and a mixture of dichloromethane, water, and EDTA sodium salt in ammonia (4:2:1) was added. After additional stirring for 30 min, the product was extracted with dichloromethane (4 × 10 mL). The combined organic phases were washed with water and brine (1 × 10 mL, respectively), dried over sodium sulfate, filtered, and the solvent was evaporated. The crude product was then purified by column chromatography over silica gel by using dichloromethane as eluent. 1Ferrocenyl-4-phenyl-1,2,3-triazole (1) was obtained as an orange solid in a yield of 81% (53 mg, 0.160 mmol). Single crystals suitable for Xray diffraction analysis were obtained by slow vaporizing from a concentrated solution of the triazole in diethyl ether at 6 °C. 1H NMR (400 MHz, CD2Cl2, 21 °C) δ 8.06 (s, 1H, triazole-H5), 7.88 (m, 2H, phenyl-H), 7.47 (m, 2H, phenyl-H), 7.37 (m, 1H, phenyl-H), 4.90 (m, 2H, Fc-H), 4.34−4.30 (m, 2H, Fc-H), 4.24 (s, 5H, Fc-H) ppm. 13C NMR (101 MHz, CD2Cl2, 21 °C) δ 147.99 (triazole-Cq), 131.15, 129.44, 128.74, 126.16 (all phenyl-C), 119.72 (triazole-CH), 70.73, 67.35, 62.60 (all Fc-C) ppm. HRMS (ESI) Calcd for [C18H15FeN3] ([M + Na]+) m/z 352.0513. Found 352.0494. Anal. Calcd for C18H15FeN3·0.2 CH2Cl2: C, 63.15; H, 4.48; N, 12.14. Found: C, 63.15; H, 4.66; N, 12.36. Preparation of 3-Methyl-1-ferrocenyl-4-phenyl-1,2,3-triazolium Tetrafluoroborate ([2]BF4). To a solution of 1 (1 equiv, 60 mg, 0.182 mmol) in dichloromethane (5 mL) was added trimethyloxonium tetrafluoroborate (1.2 equiv, 32 mg, 0.219 mmol). After stirring the reaction mixture at room temperature for 3 days, methanol (1 mL) was added. The solution was precipitated in diethyl ether (100 mL) and filtered to give 3-methyl-1-ferrocenyl-4-phenyl1,2,3-triazolium tetrafluoroborate ([2]BF4) as a red solid in 97% (0.76 mg, 0.177 mmol) yield. Single crystals suitable for X-ray diffraction analysis were obtained by slow evaporation of dichloromethane at room temperature. 1H NMR (400 MHz, CD2Cl2, 19 °C) δ 8.78 (s, 1H, triazole-H5), 7.71−7.64 (m, 5H, phenyl-H), 5.16 (m, 2H, Fc-H), 4.52 (m, 2H, Fc-H), 4.37 (s, 5H, Fc-H), 4.28 (s, 3H, N−CH3) ppm. 13 C NMR (101 MHz, CD2Cl2, 19 °C) δ 144.60 (triazole-Cq), 132.71, 130.32, 129.99, 127.68 (all phenyl-C), 122.13 (triazole-CH), 92.41 (Fc-Cipso), 71.89, 69.47, 63.80 (all Fc-C), 39.40 (N−CH3) ppm. HRMS (ESI) Calcd for [C19H18FeN3]+ ([M-BF4−]+) m/z 344.0850. Found 344.0866. Anal. Calcd for C19H18FeN3BF4: C, 52.95; H, 4.21; N, 9.75. Found: C, 52.96; H, 4.54; N, 9.81. Preparation of the Gold Triazolylidene Complex (3). A mixture of [2]BF4 (1 equiv, 60 mg, 0.139 mmol), silver(I) oxide (2 equiv, 65 mg, 0.279 mmol), potassium chloride (2 equiv, 21 mg, 0.279 mmol), and cesium carbonate (3 equiv, 41 mg, 0.139 mmol) in absolute acetonitrile (10 mL) was stirred at room temperature under exclusion of light for 3 days. After this time, the reaction mixture was filtered over Celite under an N2 atmosphere, and the solvent was removed under reduced pressure. The residue was dissolved in absolute dichloromethane (10 mL), and [Au(SMe2)Cl] (1 equiv, 41 mg, 0.139 mmol) was added. After stirring at room temperature for an additional 3 days, the reaction mixture was filtered over Celite, and the solvent was evaporated. The crude product was purified by flash chromatography over silica gel using dichloromethane. The product was obtained as red crystals in a yield of 57% (46 mg, 0.079 mmol). Single crystals suitable for X-ray diffraction analysis were obtained by condensing hexane onto a concentrated solution of the complex in dichloromethane at room temperature. 1H NMR (400 MHz, CD2Cl2, 21 °C) δ 7.71 (m, 2H, phenyl-H), 7.58 (m, 3H, phenyl-H), 5.38 (m, 2H, Fc-H), 4.82 (m, 7H, Fc-H, Fc-H), 4.08 (s, 3H, N−CH3) ppm. 13C NMR (101 MHz, CD2Cl2, 22 °C) δ 156.75 (carbene-C), 147.91 (triazole-Cq), 130.91, 130.17, 129.63, 127.22 (all phenyl-C), 96.48 (Fc-Cipso), 71.58, 68.02, 64.79, 60.78 (all Fc-C), 38.43 (N−CH3) ppm. HRMS (ESI) Calcd for [C19H17FeN3AuCl]+ ([M]+) m/z 575.0126. Found 575.0130. Anal. Calcd for C19H17FeN3AuCl·0.4 CH2Cl2: C, 38.22; H, 2.94; N, 6.89. Found: C, 38.21; H, 3.06; N, 7.02. G


Article

Organometallics Preparation of the Iridium Triazolylidene Complex (4). A mixture of [2]BF4 (1 equiv, 52 mg, 0.12 mmol) and sodium bis(trimethylsilyl)amide (1.2 equiv, 27 mg, 0.14 mmol) in absolute THF was stirred for 10 min at room temperature. After adding [Ir(COD)Cl]2 (0.5 equiv, 40 mg, 0.06 mmol), the reaction mixture was stirred for 3 days. The following steps were done under air. The reaction mixture was filtered over Celite, and the solvent was evaporated. The resulting product was redissolved in ethyl acetate and passed through a short pad of SiO2. After evaporation of the solvent, the residue was dissolved in dichloromethane, precipitated with hexane, and filtered. The product was obtained as red solid in a yield of 44% (36 mg, 0.05 mmol). Single crystals suitable for X-ray diffraction analysis were obtained by slow evaporation of dichloromethane at room temperature. 1H NMR (400 MHz, CD2Cl2, 20 °C) δ 8.01 (m, 2H, phenyl-H), 7.53−7.51 (m, 3H, phenyl-H), 5.96 (m, 1H, Fc-H), 5.68 (m, 1H, Fc-H), 4.49−4.44 (m, 1H, COD-H), 4.33−4.31 (m, 2H,Fc-H), 4.26 (s, 5H, Fc-H), 4.05 (s, 3H, N−CH3), 2.76−2.72 (m, 1H, COD-H), 2.27−2.22 (m, 1H, COD-H), 2.17−2.02 (m, 2H, CODH), 1.95−1.85 (m, 1H, COD-H), 1.57−1.33 (m, 5H, COD-H), 1.19− 1.11 (m, 1H, COD-H) ppm. 13C NMR (101 MHz, CD2Cl2, 21 °C) δ 170.31 (carbene-C), 144.78 (triazole-Cq), 131.06, 129.72, 128.84, 128.63 (all phenyl-C), 96.47 (Fc-Cipso), 81.74, 80.12, 70.95, 67.31, 67.05, 66.65, 65.37, 51.70 (all COD-C and Fc-C), 38.15 (N−CH3), 34.34, 32.84, 30.39, 29.83 (all COD-C) ppm. HRMS (ESI) Calcd for [C27H29FeN3Ir]+ ([M − Cl]+) m/z 644.1340. Found 644.1328. Preparation of the Iridium Triazolylidene Complex (5). CO gas was run through a solution of iridium complex 4 (1 equiv, 18 mg, 0.027 mmol) in dichloromethane (30 mL) for 30 min. After additional stirring for 2 h under CO atmosphere, the solvent was evaporated under reduced pressure. To remove the free cyclooctadiene, the product was washed carefully several times with hexane ( CO complex 5 is partly soluble in hexane). The product was obtained as red solid in a yield of 70% (12 mg, 0.019 mmol). 1H NMR (400 MHz, CD2Cl2, 19 °C) δ 7.72 (m, 2H, phenyl-H), 7.56−7.54 (m, 3H, phenyl-H), 5.55 (m, 2H, Fc-H), 4.35 (m, 2H, Fc-H), 4.31 (s, 5H, Fc-H), 4.04 (s, 3H, N−CH3) ppm. 13C NMR (101 MHz, CD2Cl2, 22 °C) δ 182.33 (carbene-C), 169.42, 162.68 (all carbonyl-C), 147.92 (triazole-Cq), 131.20, 130.71, 129.25, 127.59 (all phenyl-C), 96.51 (Fc-Cipso), 71.09, 67.70, 66.26 (all Fc-C), 38.16 (N-CH3) ppm. HRMS (ESI) Calcd for [C21H17FeN3O2Ir]+ ([M − Cl]+) m/z 592.0299. Found 592.0248. NMR Experiments for the Confirmation of the Reversible Redox Process of Gold(I) Catalyst 3. Procedure A. Gold(I) triazolylidene complex 3 (1 equiv) was dissolved in 1 mL of dry and degassed CD2Cl2 and transferred into an NMR tube. After the first measurement, [Fe(η5-C5H4COMe)Cp][BF4] (1.25 equiv) dissolved in 0.5 mL of CD2Cl2 was added followed by a second measurement. Afterward, FeCp2* (1.25 equiv) dissolved in 0.5 mL of CD2Cl2 was added followed by a third measurement. Procedure B. Gold(I) triazolylidene complex 3 (1 equiv), [Fe(η5C5H4COMe)Cp][BF4] (1.25 equiv), and 1,2,4,5-tetrabromobenzene (1 equiv) as internal standard were dissolved in 5 mL of dry and degassed CH2Cl2. After stirring for 2 h, FeCp2* (1.25 equiv) was added. After additional 2 h, the solvent was removed under reduced pressure, and the remaining solids were dissolved in 0.5 mL of CD2Cl2. General Procedure for the Catalytic Cyclization Reaction of N-(2-Propyn-1yl)benzamide A to 5-Methylene-2-phenyl-4,5dihydrooxazole B. Catalyst 3 (1 mol %), N-(2-propyn-1yl)benzamide (0.2 mmol), and hexadecane (0.02 mmol) were dissolved in absolute dichloromethane (2 mL) and stirred at room temperature for 24 h. In the case of an additive ([Fe(η5-C5H4COMe)Cp][BF4] or FeCp2*), 1.25 mol % were added. Samples were taken after different reaction times. For that, 0.1 mL of reaction solution was taken with a syringe, filtered over a little plug of silica, and eluted with dichloromethane. The solvent was removed under reduced pressure, and the conversions to product 5-methylene-2-phenyl-4,5-dihydrooxazole were detected by 1H NMR spectroscopy with hexadecane as internal standard. General Procedure for the Catalytic Cyclization Reaction of 2-Methylene-4-phenylbut-3-yn-1-ol C to 4-Methyl-2-phenylfuran D. Catalyst 3 (1 mol %), 2-methylene-4-phenylbut-3-yn-1-ol

(0.3 mmol), and hexadecane (0.03 mmol) were dissolved in absolute dichloromethane (3 mL) and stirred at room temperature. In the case of an additive ([Fe(η5-C5H4COMe)Cp][BF4] or FeCp2*), 1.25 mol % were added. Samples were taken after different reaction times. For that, 0.1 mL of reaction solution was taken with a syringe, the solvent evaporated in a stream of argon, and the conversions to the product 4methyl-2-phenylfuran detected by 1H NMR spectroscopy with hexadecane as internal standard. General Procedure for the Catalytic Cyclization Reaction of 2-Methyl-5-((prop-2-ynyloxy)methyl)furan E to 4-Methyl-1,3dihydroisobenzofuran-5-ol F. Catalyst 3 (1 mol %), 2-methyl-5((prop-2-ynyloxy)methyl)furan (0.2 mmol) and 1,3,5-tribromobenzene (0.02 mmol) were dissolved in absolute dichloromethane (2 mL) and stirred at room temperature. In the case of an additive ([Fe(η5C5H4COMe)Cp][BF4] or FeCp2*), 1.25 mol % were added. Samples were taken after different reaction times. For that, 0.1 mL of reaction solution was taken with a syringe, the solvent evaporated in a stream of argon, and the conversions to product 4-methyl-1,3-dihydroisobenzofuran-5-ol were detected by 1H NMR spectroscopy with 1,3,5tribromobenzene as internal standard.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00270. Catalysis, electrochemistry, spectroelectrochemistry, and DFT calculations data; NMR spectra; crystallographic details and tables (PDF) Accession Codes

CCDC 1015654, 1015655, 1043309, and 1488875 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Biprajit Sarkar: 0000-0003-4887-7277 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Fonds der Chemischen Industrie (Kekuléstipendium for S.K.) and the Deutsche Forschungsgemeinschaft (SFB 765) for the financial support of this work. The high-performance computing facilities at ZEDAT, FU Berlin, are acknowledged for access to computing resources. Dr. S. Hohloch is kindly acknowledged for the determination of the structure of 1.

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REFERENCES

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