TEMPO-Mediated Aerobic

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Effects of Imidazole-Type Ligands in CuI/TEMPO-Mediated Aerobic Alcohol Oxidation Sven Adomeit, Jabor Rabeah, Annette E. Surkus, Ursula Bentrup, and Angelika Brückner* Leibniz-Institut für Katalyse, Universität Rostock, Albert-Einstein Straße 29A, 18059 Rostock, Germany S Supporting Information *

ABSTRACT: Selective aerobic oxidation of benzyl alcohol to benzaldehyde by a (bpy)CuI(IM)/TEMPO catalyst (IM represents differently substituted imidazoles) has been studied by simultaneous operando electron paramagnetic resonance/UV−vis/ attentuated total reflectance infrared spectroscopy in combination with cyclic voltammetry to explore the particular role of imidazole in terms of ligand and/or base as well as of its substitution pattern on the catalytic performance. For molar ratios of IM/Cu ≥ 2, a (bpy)CuI/II(IM)a(IM)b complex is formed, in which the Cu−N distances and/or angles for the two IM ligands a and b are different. The coordination of a second IM molecule boosts the oxidation of CuI to CuII and, thus, helps to activate O2 by electron transfer from CuI to O2. The rates of CuI oxidation and CuII reduction and, thus, the rates of benzaldehyde formation depend on R of the R−N moiety in the IM ligand. Oxidation is fastest for R = H and alkyl, while reduction is slowest for R = H. The CuI/CuII interplay leads to decreasing total benzaldehyde formation rates in the order R (I+ effect) > R (conjugated system) > R = H.

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INTRODUCTION The oxidation of alcohols to aldehydes belongs to the most important processes in the chemical industry because the resulting products are valuable intermediates in fine chemistry. Within the last decades, several routes of nitroxyl-radicalmediated aerobic oxidation have been used, such as a 2,2,6,6tetramethylpiperidinyl-1-oxyl (TEMPO)/CeIV/NaNO2 catalytic system,1 TEMPO in combination with VOSO4,2 and 9azabicyclo[3.3.1]nonane-N-oxyl together with Fe(NO3)3.3 Especially, the copper-catalyzed, nitroxyl-radical-mediated aerobic oxidation emerged as a promising method to oxidize aromatic alcohols, including secondary and sterically hindered ones4 as well as even more complex substrates such as polysaccharides.5 After the first report by Semmelhack et al., who used CuCl and TEMPO in combination with oxygen as the oxidant,6 Stack et al. introduced a model reaction mimicking the active site of the galactose oxidase enzyme consisting of a salen-type binaphthyl-containing ligand surrounding the Cu center.7 Thereby the need for suitable ligands to boost catalytic activity became evident, which has been addressed in subsequent studies. Thus, Sheldon et al. applied 2,2-bipyridine (bpy) as a ligand together with CuBr2, TEMPO, and KOtBu as a base, which is needed to activate the alcohol by deprotonation.8 The comprehensive work of Stahl et al. marked an important step forward in facilitating the aerobic alcohol oxidation by utilization of a CuI salt with noncoordinating anions and replacement of the inorganic base by 1-methylimidazole (NMI).9−11 They anticipated that NMI not only works as a base for deprotonating the alcohol but also serves as a ligand, and they postulated a (bpy)CuII(NMI) species as an © XXXX American Chemical Society

intermediate in the catalytic cycle. We could confirm this experimentally by electron paramagnetic resonance (EPR) data acquired with our recently developed simultaneous operando EPR/UV−vis/attentuated total reflectance infrared (ATR-IR) spectroscopy setup.12 On the basis of the results of this study, the main focus of which was exploring the role of TEMPO as a cocatalyst, we obtained strong evidence that an active (bpy)(NMI)CuII-O2•−/TEMPO species is formed by electron transfer from a CuI precursor, whereby TEMPO serves for fixation of the O2•− radical in the active intermediate Cu−OO• but does not undergo a direct redox reaction with CuI/CuII, as claimed in previous studies.6,10,11,13 Stahl et al. found that Cu/ TEMPO-catalyzed alcohol oxidation proceeds much faster when a NMI/Cu molar ratio of 2 was used instead of 1. On the basis of this result, they supposed that NMI acts both as a base and as a ligand. It is well-known that ligands in biological systems,14 catalysis,15,16 or materials science17 can interact with metal centers in different manners and modify their properties (e.g., catalytic activity/selectivity) by changing their electronic states and imposing a specific geometry.16,18,19 Therefore, sound knowledge on the role of a ligand is crucial for rational catalyst design.19 It is the aim of this work to explore the specific role of NMI in more detail by simultaneously coupled operando EPR/UV− vis/ATR-IR spectroscopy12 as well as by the individual methods. Moreover, the impact of imidazoles (IMs) bearing substituents in positions other than that at the nitrogen moiety has been analyzed to study the effect of the substitution pattern Received: December 6, 2016

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DOI: 10.1021/acs.inorgchem.6b02925 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

disk (d = 2 mm) working electrode, a platinum counter electrode, and an Ag/AgCl/LiClsat/EtOH reference electrode (all electrodes: Metrohm) were used. Potentials measured with respect to this reference were checked by using the ferrocenium/ferrocene internal reference system (potential of Fc+/Fc vs Ag/AgCl/LiClsat in EtOH: 0.505 V). CV scans were recorded three times at a scan rate of 100 mV/s. The initial measurement was performed with 0.05 mmol of CuIOTf dissolved in the electrolyte. Subsequently, 1.0 equiv of bpy was added, followed by a further addition of NMI in ratios ranging from 0.5−2.0 equiv. After each addition, CV scans were recorded. Differential pulse voltammograms of the same solutions were acquired in the anodic direction with a scan rate of 5 mV/s (step potential 2.5 mV, modulation amplitude 25 mV, modulation time 0.05 s, and interval time 0.5 s).

on the catalytic activity. ATR-IR allows one to follow product formation, while EPR and UV−vis spectroscopy give information about changes of the copper coordination sphere and valence state in solution. In addition, cyclic voltammetry (CV) has been used to monitor the redox properties of the Cu center during formation of the (bpy)CuI(IM) complex.

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EXPERIMENTAL SECTION

All chemicals were obtained from Sigma-Aldrich and used without further purification. A detailed description of our coupled operando EPR/UV−vis/ATR-IR spectroscopy setup is available from ref 12. Briefly, an EPR quartz flat cell, implemented in the rectangular cavity of an ELEXSYS 500 continuous-wave EPR spectrometer (Bruker), was equipped in its top cylindrical section with fiber-optical probes for recording ATR-IR and UV−vis spectra by a Nicolet Avatar 370 (Thermo Electron) and an AvaSpec-2048 UV−vis spectrometer (Avantes). O2 was supplied with a rate of 25 mL/min to the reaction mixture by a capillary located at the same position as those of the spectroscopic dip probes. The reaction solution was circulated with a rate of 2.5 mL/min through the cell by a syringe pump (DURATEC Analysentechnik GmbH). EPR spectra were recorded in X band with a microwave power of 6.9 mW, a modulation frequency of 100 kHz, and a modulation amplitude of up to 5 G. Spectral simulation was done by the EPRSIM32 program.20 Catalytic oxidation of benzyl alcohol (BzOH) to benzyl aldehyde (BA) in the presence of the (bpy)Cu complex and different IMs (Scheme 1) was conducted at room temperature in the following way:

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RESULTS AND DISCUSSION Ligand or Base: Influence of NMI. As mentioned above, Stahl et al. found that the catalytic reaction proceeds much more efficiently when a NMI/Cu molar ratio of more than 1 was used.9 It has been evidenced that the CuI starting species is oxidized to CuII by O2 and subsequently reduced again to CuI by the alcohol. It was supposed that, during these redox steps, one NMI coordinates to the active copper species, while the remaining NMI acts as a base abstracting a proton from the alcohol and, thus, facilitating its coordination to yield a copper(II) alkoxide.8,10,11 To gain deeper insight into the role of NMI in the catalytic cycle, we monitored the reaction by simultaneous operando EPR/UV−vis/ATR-IR spectroscopy using different amounts of NMI (0−3.0 equiv). Figure 1 shows

Scheme 1. Conditions of Aerobic Alcohol Oxidation Used for Investigations in This Work

To a solution of CuIOTf (0.05 mmol) in 5 mL of acetonitrile (ACN) were added 1.0 equiv of bpy (0.05 mmol), 0.5−3.0 equiv of NMI (0.025−0.15 mmol), or 2.0 equiv of other substituted IMs listed in Table S1 (0.10 mmol) and 1.0 equiv of TEMPO (0.05 mmol) under an argon atmosphere. After formation of the active (bpy)CuI(IM) complex (followed by UV−vis spectroscopy), 1.0 mmol of BzOH (20 equiv) was added and O2 was bubbled into the solution under continuous circulation. Conversion of BzOH to BA was determined from the area of the ν(CO) band of BA at 1702 cm−1 after previous calibration (Figure S1). Simultaneously, the formation of CuII was followed by EPR spectroscopy. For separate analysis of oxidation and reduction of the (bpy)Cu(NMI) complex, a solution containing 1.0 equiv (0.12 mmol) of CuIOTf, 1.0 equiv of bpy, and 1.0 equiv of TEMPO in 6 mL of ACN was freshly prepared before each experiment and flushed with argon for 15 min. Then, different equivalents of NMI were added to this solution. After formation of the active (bpy)CuI(NMI) complex (verified by UV−vis), O2 was supplied to the solution for 15 min, during which the TEMPO EPR signal disappeared and CuI was oxidized to CuII (evidenced by EPR and UV−vis). Subsequently, the solution was flushed with argon for 10 min until restoration of the TEMPO signal, before 1.0 equiv of BzOH was added, which reduced CuII to EPR-silent CuI. After vanishing of the CuII signal, O2 was again supplied for 15 min to investigate reoxidation of the active Cu site. All electrochemical studies were performed at room temperature in dried ACN p.A. (VWR) under an argon atmosphere with 0.1 mol/L tetrabutylammonium hexafluorophosphate (Fluka) as a conducting salt using an Autolab (PGSTAT 302N, Metrohm). A glassy carbon

Figure 1. Formation of BA as a function of time (monitored by ATRIR) without NMI and in the presence of different equivalents of NMI.

the yield of BA (derived from the ATR-IR band intensity at 1702 cm−1; Figure S1) as a function of the reaction time. The performance of the (bpy)Cu complex without NMI is very poor. Raising the amount of NMI from 0.5 to 2.0 equiv leads to a strong increase of the reaction rate, while the addition of a third 1 equiv of NMI did not have any further effect. This raises the question, what is the precise role of NMI during the reaction? If NMI acts as a ligand, an influence on the oxidation of CuI to CuII would be expected with increasing amount because there is additional coordination to the Cu center. Otherwise, if noncoordinated NMI acts as a base, deprotonates the alcohol, and hence facilitates the formation of a (bpy)(NMI)CuII-OR intermediate, the reduction of CuII to CuI is expected to proceed faster. Deeper insight into these oxidation and reduction steps during the catalytic cycle has been obtained from UV−vis and EPR spectra using a stepwise approach. First, the reaction mixture consisting of CuIOTf and bpy in ACN was studied in B

DOI: 10.1021/acs.inorgchem.6b02925 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry an inert atmosphere in the presence of 1.0 or 2.0 equiv of NMI. The UV−vis spectrum of the solution without NMI shows metal-to-ligand charge-transfer (MLCT) bands at 340 and 440 nm with a shoulder at 550 nm, arising from a (bpy)CuI complex (Figure 2).12 Upon the addition of 1.0 equiv of

because of dipolar interaction without being oxidized/reduced itself.12 As a measure of the CuII signal intensity, the relative amplitude change of the low-field component of the CuII hfs signal at 3158 G is plotted as a function of time in Figure 3B. Interestingly, this CuII EPR signal intensity achieved its maximum after about 9 min in the presence of 2.0 equiv of NMI but only after about 13 min when just 1.0 equiv of NMI was used, indicating that oxidation of the (bpy)CuI complex is faster with 2.0 equiv of NMI. After flushing with argon (to remove excess O2) and the addition of BzOH, the CuII signal intensity drops again because (bpy)CuII(NMI) is reduced to (bpy)CuI(NMI) (Figure 3B). However, now this occurs with the same rate for both 1.0 and 2.0 equiv of NMI, leading to complete decline after about 13 min. This process is accompanied by the reappearance of the corresponding MLCT bands of CuI and the decline of the CuII d−d band in the UV−vis spectra (Figure S3A,B). Simultaneously, the formation of benzaldehyde was detected by ATRIR during reduction. EPR and UV−vis spectra of (bpy)CuI + 2.0 equiv of NMI during reduction and spectra of (bpy)CuI + 1.0 equiv. NMI can be found in Figure S3. The repeated supply of O2 led to the same results as those during the initial oxidation, indicating that the active copper complex can reversibly cycle between valence states I and II (Figure S4). The fact that the CuII reduction rate does not depend on the presence of 1 or 2 equiv of NMI is surprising. We expected a faster reduction in excess of NMI when the latter acts as a base8,10,11 because deprotonation of the alcohol should facilitate the formation of a copper(II) alkoxide species, which might be faster converted to BA by CuII and form CuI. We cannot completely rule out the action of NMI as a base. However, our results suggest that it is the ligand sphere of the formed (bpy)Cu(NMI) complex rather than a pure base effect of NMI that is responsible for the higher catalytic performance compared to the presence of only one NMI. Further support for the negligible effect of NMI as a base is the fact that there was no direct evidence for the formation of imidazolium cations during the reaction. However, when 1.0 equiv of a separately prepared NMIH+OTf− salt was added to a solution containing Cu/bpy/2.0 equiv of NMI/TEMPO, fast oxidation to CuII was observed after the introduction of O2. Interestingly, the resulting solution was blue instead of green in the absence of NMIH+OTf− with distinct changes in the UV−vis spectrum (Figure S5), and this CuII could not be reduced again upon the addition of BzOH, as was the case without the addition of NMIH+OTf− (compare Figures 3B and S3). This suggests that a different CuII complex was formed by the addition of an imidazolium salt. Most likely, the resulting active (bpy)(NMI)CuII-O2•−/TEMPO complex12 is decomposed to catalytically inactive CuII-OH species.22−25 Considering that the first step of BzOH conversion should be the formation of an (adsorbed) alkoxide,10,12 we assume that proton transfer from BzOH to NMI occurs when both molecules are coordinated in the vicinity of the CuII center and adsorbed NMI acts as a proton shuttle (Scheme S1). Further evidence for weak interaction of more than one NMI ligand with the CuII center comes from a comparison of the CuII EPR and UV−vis signals formed in the presence of O2 and 1.0, 2.0, or 3.0 equiv of NMI but in the absence of TEMPO to avoid signal overlap (Figure 4). From Figure 4A, it can be seen that the width of the hfs lines at high field (B0 = 3289 and 3356 G) increase upon an increase in the amount of NMI, while the low-field components of the

Figure 2. UV−vis spectra of CuIOTf + bipy in ACN and in the presence of 1.0 and 2.0 equiv of NMI.

NMI, the MLCT band and shoulder at 440/550 nm rise, pointing to the coordination of one NMI to the CuI center. The addition of a second NMI leads to a minor blue shift to 423 nm and an intensity decrease of the MLCT bands, which suggests less electron density donation from CuI to coordinating ligands, in particular to bpy. We attribute these changes to weak interaction of the second NMI with the (bpy)CuI(NMI) complex. Under these conditions, negligible oxidation of CuI to CuII occurs. This is evidenced by the fact that no d−d transition band of CuII species around 650 nm21 is observed. However, such d−d bands are formed upon the introduction of O2 into these solutions (Figure 3A, inset). Simultaneously,

Figure 3. (A) EPR and UV−vis spectra of (bpy)CuI + 2.0 equiv of NMI during oxidation. (B) Change of the CuII EPR signal amplitude at B0 = 3158 G with time during the treatment of (bpy)CuI + 1.0 or 2.0 equiv of NMI in the presence of TEMPO with (1) O2 (oxidation) and (2) BzOH (reduction).

the charge-transfer bands of CuI decrease, indicating that (bpy)CuI(NMI) is oxidized to (bpy)CuII(NMI). This is also evident from the related EPR spectra in which the characteristic hyperfine structure (hfs) quartet of CuII is growing with time, which arises from spin coupling of the unpaired electron with the Cu nucleus (d9, S = 1/2, and I = 1/2; Figure 3A). This signal is partly superimposed on the high-field part by the hfs triplet (S = 1/2 and IN = 1) of the TEMPO radical at g = 2.004, which is explicitly shown in Figure S2. The TEMPO signal just broadens/narrows reversibly upon the addition/removal of O2 C

DOI: 10.1021/acs.inorgchem.6b02925 Inorg. Chem. XXXX, XXX, XXX−XXX

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BzOH (corresponding to the amount of BzOH in the reaction system) to a solution containing CuIOTf, bpy, TEMPO, and NMI virtually did not change the UV−vis spectrum of the formed (bpy)CuI(NMI) complex (Figure S8A). Concerning the effect of H2O, it is well-known that this deactivates the catalyst when it is in excess by forming copper hydroxide.11 However, the low H2O content formed by the catalytic reaction (only 1 equiv of H2O per 1 equiv of BzOH) might hardly influence the active copper complex because even the addition of a 5-fold amount (100 equiv of H2O) led to negligible changes of the UV−vis spectrum, indicating that the active (bpy)CuI(NMI) complex remains stable under catalytically relevant conditions (Figure S8B). To gain deeper insight into the redox behavior of the (bpy)CuI(NMI)x complex, CV was performed (Figure 5). One

Figure 4. (A) EPR spectra of (bpy)CuI after immersion of O2 for 30 min in the presence of 1.0, 2.0, or 3.0 equiv of NMI. The EPR spectrum for 1.0 equiv of NMI was calculated with giso = 2.124 and A = 63 G (dotted line). (B) Corresponding UV−vis spectra.

CuII hfs signal remain almost unchanged. Broadening of the individual hfs lines of a complex in solution may arise when its tumbling rate is slowed, e.g., by a higher viscosity of the solvent, hydrogen bonding, or a more rigid coordination sphere due to the coordination of bulky ligands. Such an effect has been observed for various vanadium cluster compounds26 and may occur also in (bpy)CuII(NMI) by weak interaction with a second NMI. This is also suggested by the EPR spectra recorded at 240 K (Figure S6) in which a slight increase of giso from 2.128 to 2.130 and a decrease of the hfs coupling constant from A = 65 G to 61 G are observed upon the addition of a second NMI. This is a clear indication that the second NMI interacts with the Cu center and modifies its local geometry. In the corresponding UV−vis spectra, such slight changes in the coordination sphere of (bpy)CuII(NMI) are reflected by a small change in the d−d band when more than 1.0 equiv of NMI is added (Figure 4B). This indicates weak coordination of an additional NMI molecule27−30 and is in accordance with the fact that IM-type ligands are known as labile ligands in biological systems.31−34 The labile coordination of NMI to the CuI center may be of vital importance for electron transfer from CuI to O2 because one coordinating NMI ligand is supposed to be temporarily displaced from the Cu center to enable O2 adsorption and subsequent superoxide formation. From in situ IR measurements at −40 °C, we obtained experimental support that this may indeed happen (Figure S7). Here, the deformation vibration of the IM ring35 at 661 cm−1 split into two bands at 657 and 664 cm−1 as soon as O2 was added to a solution of CuIOTf, bpy, and 2.0 equiv of NMI, which causes oxidation of CuI to CuII. The band at 664 cm−1 matches the one of free NMI in ACN, while the band at 657 cm−1 corresponds to NMI coordinated to CuII. Interestingly, when TEMPO and BzOH are added to this solution after flushing with argon to remove excess free O2, the band split disappears (Figure S7), suggesting consumption of the coordinated O2 and transient recoordination of NMI. This supports our assumption that NMI acts reversibly as a labile ligand. As mentioned above, the performance of a (bpy)Cu complex without NMI is very poor, meaning that the stability of the active (bpy)Cu(NMI) complex during reaction is crucial for high activity. This raises the question of whether other Lewis bases in the reaction system (e.g., BzOH or H2O formed as a reaction product) can displace NMI from Cu and, thus, deactivate the system. There is obviously no effect of the amount of BzOH because even the addition of 20 equiv of

Figure 5. Cyclic voltammograms of CuIOTf in ACN with the subsequent addition of bpy and different amounts of NMI. The cyclic voltammogram of NMI in ACN is shown for comparison.

CuI/II redox pair attributed to CuIOTf was detected at approximately 0.75 V [Fc+/Fc]. The subsequent addition of 1.0 equiv of bpy led to a decrease of this redox signal. In addition, a new redox event appeared from the (bpy)CuI complex at approximately −0.22 V [Fc+/Fc]. This shows that the bpy ligand lowers the CuI/II redox potential and facilitates the oxidation of CuI to CuII. The addition of 0.5 equiv of NMI shifted the redox potential to −0.25 V [Fc+/Fc], suggesting the formation of a (bpy)CuI(NMI) complex. This is in line with previous observations indicating that oxidation of the (bpy)CuI complex by O2 is promoted by NMI due to lowering of its oxidation potential.10,12 When the amount of NMI was raised to 1.0 equiv, the redox signal of CuIOTf vanished, suggesting the complete formation of the (bpy)CuI(NMI) complex. The further addition of NMI up to 1.5 and 2.0 equiv of NMI gave rise to a peak around 1.25 V [Fc+/Fc], which is close to that of pure NMI, suggests that the second NMI might be only very weakly bound, and hence is electrochemically accessible, in agreement with the UV−vis and EPR results discussed above. Effect of Substituents in the IM Scaffold. From Figure 6A,B, it is evident that the rate of BzOH oxidation depends sensitively on the substitution pattern of the IM used. In Table S1, all tested IMs are summarized, together with the time required for complete conversion of BzOH to BA. The reaction proceeds slowest with 1H-imidazole. When the H atom at the N1 atom is replaced by alkyl substituents, a much faster conversion of BzOH is observed (Figure 6A). This may be due to their I+ (inductive) effect36,37 by which the electron density is donated to the active CuI species via the IM ring and the D

DOI: 10.1021/acs.inorgchem.6b02925 Inorg. Chem. XXXX, XXX, XXX−XXX

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Exemplarily, operando EPR and UV−vis spectra are shown for 1-ethylimidazole (no NH group) and 2-methylimidazole (NH group present) in Figure S9. The corresponding EPR signal amplitude of the CuII hfs line at B0 = 3158 G, and the UV−vis absorbances at 550 nm (CuI) and 650 nm (CuII) are plotted as a function of time in Figure 6C,D. For 1ethylimidazole, a slow increase of the EPR signal due to oxidation of (bpy)CuI(1-ethylimidazole) to (bpy)CuII(1-ethylimidazole) by O2 is observed, going along with a slow drop of the corresponding CuI MLCT band (550 nm) and an increase of the CuII d−d band (650 nm) in the related UV−vis spectra (Figures 6C and S9A,B). In the catalytic cycle, this CuII oxidizes BzOH to BA and is reduced again back to CuI. Thus, as long as unconverted BzOH is still present in the solution, the CuII EPR and UV−vis signals reflect the equilibrium concentration of CuII. Consequently, a small CuII signal intensity under these conditions is an indication for fast BzOH oxidation, i.e., fast rereduction of CuII. This is the case for 1-ethylimidazole, for which complete BzOH conversion is reached already after 13 min (Table S1). In agreement with this, a rapid increase of the CuII EPR signal (accompanied by a drop of the UV−vis CuI MLCT signal at 550 nm) is observed after about 13 min because CuII reduction by BzOH is no longer possible (Figure 6C). In contrast, for 2-methylimidazole, a fast increase of the CuII EPR signal (accompanied by a drop of the related CuI MLCT and an increase of the CuII d−d transition band) occurs during the first few minutes (Figure 6D), while total conversion of BzOH to BA takes significantly longer (34 min, Figure 6B). Afterward, the CuII EPR signal and UV−vis d−d band slowly increase, while the CuI UV−vis MLCT slowly decreases. This suggests that reduction of the formed (bpy)CuII(2-methylimidazole) complex by BzOH is hindered. Similar results were found for other substituted IMs with an NH moiety, which agrees with the observed slower BA formation rates in the case of 1H-imidazoles (Figure 6B and Table S1). Because the relative equilibrium concentrations of CuI and CuII in the reaction solution are related to the ratio of the rates of CuI oxidation and CuII reduction, these two processes were analyzed by a stepwise procedure to gain deeper insight into the catalytic process, using the relative amplitude change of the low-field component of the CuII hfs signal at 3158 G as a monitor (Figure 7). First, a solution of (bpy)CuI in ACN

Figure 6. Formation of BA as a function of time (monitored by ATRIR) for various N-substituted IMs (A), IMs substituted at other positions (B), and corresponding EPR signal amplitude at B0 = 3158 G and UV−vis absorbance at 550 nm (CuI) and 650 nm (CuII) for 1ethylimidazole (C) and 2-methylimidazole (D).

coordinating second N atom.36 This might facilitate the ability of Cu to activate O2 in the first step of the reaction mechanism.12 We found almost no differences in the reaction rate depending on the chain length and the branching of the alkyl substituents because they have similar electronic properties (Table S1).38 However, with vinyl or aryl substituents the reaction was much slower. This is most probably due to a conjugation effect because the incorporation of unsaturated substituents like allyl or benzyl groups only led to a minor increase of the reaction time compared to NMI. We attribute this to a weaker electron density donation into the IM ring, leading, in turn, to slower electron transfer from CuI to O2 and, consequently, to slower CuII formation in the first reaction step. The reason for this might be the poor resonance donation of vinyl and phenyl substituents.39−41 To study the effect of an unprotected NH− moiety on the reaction progress, 1H-imidazoles substituted at other positions have been used (Figure 6B and Table S1). In all cases, markedly lower reaction rates were observed, with that of unsubstituted 1H-imidazole being the lowest. Differences in the reaction rates between 2-methylimidazole and 4(5)-methylimidazole might be due to both tautomeric42 and electronic effects43,44 because the inductive effect of a methyl substituent in the 2 position is slightly higher than that in the 4(5) position.37,44 The proton shifts easily between the two N atoms of the IM ring,42,45 but only in the case of asymmetric substitution like in 4 or 5 positions do isomers arise. Steric hindrance within the coordination sphere and weaker electron donation toward the CuI center might be a reason why (bpy)CuI(4(5)-methylimdiazole) is less active. Reaction times for 1,2- and 2,4(5)-dimethylimidazole isomers (Table S1) were considerably longer than for monosubstituted IMs, which we attribute to steric interactions. The faster rate for 1,2dimethylimidazole is very likely the result of the protected NH- moiety because 2,4(5)-dimethylimidazole can be deprotonated in situ and interact with other complex centers.46−50

Figure 7. Oxidation of (bpy)CuI(IM) by O2 in the presence of TEMPO (solid lines, filled symbols) and subsequent reduction of the formed (bpy)CuII(IM) by BzOH (dashed lines, open symbols) in the presence of 2.0 equiv of NMI, 1-vinylimidazole, or 1H-imidazole monitored by the amplitude of the CuII EPR signal. E

DOI: 10.1021/acs.inorgchem.6b02925 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry containing TEMPO and 2.0 equiv of NMI, 1-vinylimidazole, or 1H-imidazole was oxidized by adding O2 for 15 min, followed by flushing with argon for 15 min to remove excess O2. Then BzOH was added to monitor reduction of the active CuII species (Figure 7). The intensity change of the corresponding MLCT and d−d bands of CuI and CuII, respectively, is shown in Figure S10. Oxidation of the (bpy)CuI(IM) complex occurs with almost the same rate for both NMI and 1H-imidazole, while it is slightly slower for 1-vinylimidazole (Figure 7). As was found previously, donation of the electron density via the IM ring and the coordinating N atom to the active CuI species facilitates its ability to activate O2 by transfer of an electron and formation of an O2•− intermediate and CuII.10,12 In the case of NMI and 1vinylimidazole, this is caused by the I+ effect of the methyl group and, to a weaker extent, by the resonance effect of the vinyl group, respectively. When 1H-imidazole is used, the increase of electron density in the IM ring might be caused by deprotonation of its NH moiety (possibly by superoxide species or H2O formed during the reaction).10,12,46,51 In this case, the NH moiety serves as a proton source, which facilitates decomposition of the active CuII-O2•− adduct, as has been shown when the NMIH+OTf− salt was used.52,53 This leads to lower activity. In contrast, the reduction of (bpy)CuII(IM) proceeds with almost the same rate for both NMI and 1-vinylimidazole with no NH moiety present, while it is much slower for 1Himidazole. This confirms our assumption that reduction of the CuII species might be hindered when IMs contain an NH moiety. We suggest two effects to be responsible for this behavior. On the one hand, deprotonation of the NH moiety leads to a ligand with increased electron density known to stabilize the divalent state of Cu by increased electron donation.37,51,54 This makes the reduction to CuI more difficult. Furthermore, as was already mentioned, deprotonation could lead to decomposition of the active CuII-O2•− species. On the other hand, the NH moiety could interact with neighboring Cu complexes,53 ligand molecules, and/or −OTf counterions by hydrogen bonds38,46,47,55,56 and, thus, shield the active Cu center from access by BzOH. The latter aspect is also supported by the higher EPR line width of CuII complexes with 1H-imidazole ligands, which points to the presence of other paramagnetic copper species in close vicinity (compare Figures S6 and S11). Such an interaction could lead to less or nonactive CuII species. Interestingly, we observed by in situ EPR that the reduction of CuII species is not complete with 1Himidazole ligands, suggesting some remaining nonactive CuII species (Figure S11B). To further prove our hypothesis that electron density donation toward the CuI center is affected by the substituents at the IM scaffold in the (bpy)CuI(IM) complex, differential pulse voltammetry has been conducted to reveal possible shifts in the redox potentials. Because less electron density is donated by the vinyl substituent, a slightly higher potential would be expected for (bpy)CuI(1-vinylimidazole) compared to (bpy)CuI(NMI), while the redox potentials of (bpy)CuI(1Himidazole) and (bpy)CuI(NMI) should be similar, given that their observed EPR signal increases are comparable. The solution containing CuIOTf and bpy under argon shows a redox event at −0.22 V [Fc+/Fc] due to the formed (bpy)CuI complex (Figure 8). Upon the addition of 1.0 equiv of NMI, a peak shift to −0.33 V [Fc+/Fc] is caused by formation of the (bpy)CuI(NMI) complex. A very similar redox peak has been

Figure 8. Differential pulse voltammograms of (bpy)CuI in ACN and after the subsequent addition of either NMI, 1-vinylimidazole, or IM under argon.

obtained when 1.0 equiv of 1H-imidazole is added to the (bpy)CuI complex. In contrast, upon the addition of 1.0 equiv of 1-vinylimidazole, the redox peak shifts only to −0.28 V [Fc+/ Fc]. This clearly confirms the direct electronic effect of the substituent.57−59 The slightly higher potential of (bpy)CuI(1vinylimidazole) compared to (bpy)CuI(NMI) reflects its lower ability to be oxidized to its CuII analogue. This suggests that the resonance effect of the vinyl group might be weaker than the inductive effect of the methyl group of NMI and confirms the in situ EPR and UV−vis results.

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CONCLUSIONS In agreement with previous studies,9,10,12 it was confirmed that an active (bpy)CuI(IM) complex with one bpy and one IM ligand directly coordinated to the Cu center is formed in solution, which activates O2 by transfer of one electron and formation of the corresponding CuII complex, which, in turn, oxidizes the alcohol and is reduced again to the initial CuI complex. When a ratio of Cu/IM = 1/2 is used, the second IM molecule most probably coordinates weakly to the Cu ion forming a (bpy)CuI/II(IM)a(IM)b complex, in which the Cu−N distances and/or angles for the two IM molecules a and b are different. Coordination of the second IM molecule boosts the rate of electron transfer from CuI to O2 and, thus, helps to activate O2 by forming a reactive O2•− intermediate,10,12 This is reflected by the accelerated formation of CuII, as evidenced by EPR (Figure 3). The extent to which transfer of the electron density from the active CuI (i.e., oxidation) occurs depends on the nature of the substituents at the IM scaffold. It is fastest for 1-R substituents with a I+ effect and for R = H (in which deprotonation and/or hydrogen bonding could increase the electron density at the N1 atom and its transfer, via Cu, to O2) but slightly slower for 1-R substituents with a conjugated electron system: R with the I+ effect ≈ R = H > R with a conjugated system Interestingly, the rate of the (bpy)CuII(IM) complex to take up an electron from BzOH and to get reduced to CuI depends on the IM substituents as well, but in a different way: R with the I+ effect ≈ R with a conjugated system > R = H

In this case, the slow reduction of CuII is supposed to be due to the higher electron density (from deprotonation and/or hydrogen bonding of the NH moiety), which might hamper the uptake of electrons from BzOH. The interplay between the oxidation/reduction properties of the active copper complex F

DOI: 10.1021/acs.inorgchem.6b02925 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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and their modulation by the IM ligands governs the total formation rates of benzaldehyde (Scheme 2 and Table S1), which decrease in the order R with the I+ effect > R with a conjugated system > R = H. Scheme 2. Impact of Different Substituents at an IM Scaffold on the Rate of CuI Oxidation (Red) and CuII Reduction (Blue)

Apparently, the rates of Cu oxidation (O2 activation) differ only slightly depending on R, while the effect of R on the Cu reduction rates (oxidation of BzOH) is much stronger. Consequently, the total rates of BA formation are, in general, significantly slower for 1H-imidazoles (able to form hydrogen bonds and/or to undergo deprotonation) compared to 1Rimidazoles in which only the electron donating effect of R has an impact. Thus, as a tentative rule for the selection of effective IM cocatalysts with an RN moiety, it can be stated that those with R = alkyl lead to the best results, while the length and branching of the letter seems to be less important. IMs with an unprotected NH group are poorly effective in any case.

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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.inorgchem.6b02925. Detailed EPR and UV−vis spectra and additional data, including IMs used, low-temperature EPR and FTIR spectra, etc. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Thomas Werner for helpful discussion. REFERENCES

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H

DOI: 10.1021/acs.inorgchem.6b02925 Inorg. Chem. XXXX, XXX, XXX−XXX