Operando XAFS Studies on Rh(CAAC)-Catalyzed Arene

Mar 13, 2019 - (c) Pape, A. R.; Kaliappan, K. P.; Kündig, E. P. Transition-Metal-Mediated Dearomatization Reactions. Chem. Rev. 2000, 100, 2917– 20...
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Operando XAFS Studies on Rh(CAAC)-Catalyzed Arene Hydrogenation Ba L. Tran, John L. Fulton, John C. Linehan, Mahalingam Balasubramanian, Johannes A Lercher, and R. Morris Bullock ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04929 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019

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Operando XAFS Studies on Rh(CAAC)-Catalyzed Arene Hydrogenation Ba L. Tran, John L. Fulton, John C. Linehan, Mahalingam Balasubramanian,† Johannes A. Lercher, and R. Morris Bullock* Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352, United States †Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, United States ABSTRACT: Rh K-edge X-ray absorption fine structure (XAFS) spectroscopy was used to examine the Rh-catalyzed arene hydrogenation of diphenyl ether by a combination of stoichiometric reactions of [(CAACCy,Dipp)Rh(COD)Cl] (Rh-Cl) (CAACCy,Dipp = cyclic alkyl amino carbene) and operando XAFS kinetics studies. Our results unequivocally show that Rh nanoparticles, generated from the single-site Rh complex Rh-Cl, catalyze the arene hydrogenation. Operando XAFS studies illuminate the role of silver cation on the pre-catalyst reactivity, the effect of increasing H2 pressure on increasing the catalytic efficiency, the stabilizing influence of Ph2O on the relative rate of formation of active Rh nanoparticles, and the absence of soluble single-site Rh species that might leach from bulk heterogeneous Rh nanoparticles. We gained insights into the divergent deactivation pathways mediated by substoichiometric benzothiophene and excess KOtBu toward H2 activation, which is a key step en route to Rh nanoparticles for arene hydrogenation. Excess KOtBu leads to the formation of a Rh-OtBu complex that interferes with H2 activation, precluding the formation of Rh nanoparticles. Benzothiophene does not interfere with the activation of H2 at Rh in the CAACCy,Dipp complex while Rh nanoparticles are formed. Once Rh nanoparticles are formed, however, benzothiophene binds irreversibly to the Rh nanoparticles, preventing adsorption of H2 and diphenyl ether for arene hydrogenation. Keywords: operando XAFS, rhodium nanoparticles, arene hydrogenation, Rh(CAAC), benzothiophene poisoning, cationic rhodium

Introduction Aromatic molecules are a privileged class of organic compounds with profound fundamental and economic importance based on the key role of classical and contemporary chemical reactions to derivatize them for commodity chemicals, functional materials, agrochemicals, pharmaceutical ingredients,1 and hydrogen storage technology.2 Heterogeneous hydrogenation of benzene to cyclohexene and cyclohexane in the synthesis of Nylon is the prominent large-scale application of hydrogenative dearomatization. To hydrogenate benzene, which contains a high resonance stabilization energy among aromatic compounds, traditional heterogeneous systems generally require high H2 pressure at elevated temperature, leading to poor selectivity and low functional group tolerance. There has been growing interest in rapid conversion of simple, abundant aromatic compounds to saturated, complex intermediates under mild conditions with high selectivity and functional group tolerance.3

The application of metal complexes of N-heterocylic carbenes are well-documented catalysts for the asymmetric hydrogenation of heteroarenes and olefins.4 Cy,Dipp [(CAAC )Rh(COD)Cl] (abbreviated as Rh-Cl, where CAACCy,Dipp = 2-(2,6-diisopropylphenyl)-3,3-dimethyl-2azaspiro[4.5]decane), represents a recent advance in the mild, chemoselective hydrogenation of substituted arenes, yielding the corresponding saturated products that would otherwise require multi-step preparations (see Scheme 1). Initial work by Zeng and co-workers reported that Rh-Cl catalyzes the chemoselective hydrogenation of aryl carbonyls and phenols to the corresponding products of cyclohexyl carbonyl and cyclohexanone derivatives, respectively.5 Glorius and coworkers6 reported the chemoselective hydrogenation of aryl fluorides and fluorinated pyridines to the corresponding substituted cyclohexane and fluorinated piperidine derivatives, mitigating competing hydrodefluorination reactions typically encountered with Rh catalysis.7 In addition, aryl silanes and aryl boronic esters are also converted to the corresponding

Scheme 1. Rh(CAACCy,Dipp)-Catalyzed Chemoselective and Site-Selective Hydrogenation of Functionalized Arenes

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cyclohexyl-silane and cyclohexyl boronic ester products that are readily transformed into hydroxyl groups by oxidation, or as substrates for cross-coupling reactions.8 From structure-activity studies on arene hydrogenation, we found that Rh-Cl catalyzes site-selective arene hydrogenation under steric control, as demonstrated by detailed competition experiments (see Scheme 1, bottom).9 Given the increasing evidence of surface modification by N-heterocyclic carbenes10 that have led to improvements in functional materials11 and catalysis,12 we carried out studies that elucidated that the composition of the Rh catalyst is Rh nanoparticles (Rh NPs) coated with modified CAACCy,Dipp from Rh-Cl, as evidenced by a series of filtration, fractional poisoning, post-catalysis Rh Kedge XAFS spectroscopy, infrared spectroscopy, and microscopic measurements.9 These aforementioned studies are indirect experiments, in which the behavior of the Rh catalyst is inferred by the time-dependent progression of organic products, and, therefore, cannot provide direct evidence on the effect of H2 pressure, the role of poisons on the Rh species during catalysis, or the speciation of the Rh species prior to formation of the Rh NPs. Herein, we report detailed stoichiometric and operando studies using Rh K-edge XAFS experiments for the direct examination of the Rh species before addition of H2 and during hydrogenation of Ph2O. Specifically, operando XAFS studies were performed to understand the effect of H2 pressure, silver cation, Ph2O, and poisons (benzothiophene (BT) and KOtBu) on the formation of Rh NPs for arene hydrogenation (Scheme 2).

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Figure 1. k2-weighted |R)| plots of Rh-Cl in THF solution and a BN pellet with the fit for Rh-Cl in THF at 25 ºC. Rh-Cl adopts the same structure in solution and the solid state. The gray and orange circles represent the single scatters included in the XAFS fit. Here, and in all XAFS data herein, the k range for the Fourier transform is 1.2 – 16 Å-1; distances are not corrected for photoelectron phase shifts. The gray box indicates the Rrange of 1.0 – 3.0 Å applied for fitting of the XAFS data. From the XAFS fit for complex Rh-Cl, we assign the Rh-Cl scattering path to the tailing broad shoulder at 2.00 Å in the |R)| plot, and the sharp peak at 2.00 Å in the imaginary portion of the Fourier transform (Img [R)]) of the XAFS data (see Figure 2). With a clear assignment of the scattering paths of RhCl, we proceeded to use XAFS measurements to investigate the effect of H2, AgBF4, Ph2O, and poisons (benzothiophene (BT), KOtBu) with Rh-Cl by stoichiometric and catalytic reactions.

Scheme 2

Effect of Silver Cation in Stoichiometric and Catalytic Reactions We previously observed that AgBF4 is required in the hydrogenation of Ph2O by Rh-Cl. This led us to propose cationic Rh-solvento intermediates en route to Rh NPs that are active for arene hydrogenation. Formation of cationic [(NHC)Rh(COD)(solvent)] (NHC = N-Heterocyclic Carbenes)15 and Ir analogues16 by Ag-mediated chloride abstraction is well-documented. However, the instability of the cationic [(CAAC Cy, Dipp)Rh(COD)(solvent)] complexes in solution at extended times has precluded structural characterization by X-ray crystallography. Hence, XAFS measurements were performed on stoichiometric reactions of Rh-Cl and AgBF4 in THF or MeCN at 25 ºC (see Figure 2) for structural characterization in solution. The XAFS results of the halide-abstraction reactions of Rh-Cl by AgBF4 in THF and MeCN are compared to those of Rh-Cl in Figure 2.

Results and Discussion Benchmarking Rh K-Edge XAFS Studies with [(CAACCy,Dipp )Rh(COD)Cl] We initiated XAFS studies on the Rh-catalyzed hydrogenation of diphenyl ether (Ph2O) by benchmarking the XAFS of Rh-Cl. The magnitude portion of the Fourier transform of the XAFS data (|R)|) of Rh-Cl in THF solution and in a boron nitride (BN) pellet confirmed that the solution structure and solid-state structure of Rh-Cl are consistent with a mononuclear Rh species (see Figure 1). The previously reported13 solid-state data from single-crystal X-ray diffraction of Rh-Cl guided the fitting of the solution structure of Rh-Cl. To fit the XAFS data of Rh-Cl in THF, the theoretical phase and amplitude parameters were generated using FEFF914 from the crystallographic parameters of Rh-Cl (see Supporting Information, Table S1).

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ACS Catalysis

Figure 2. k2-weighted Img [R)] plots and theoretical fits of Rh-Cl, Rh-MeCN, and Rh-THF that confirm the loss of the chloride ligand from the Rh center. k2-weighted Img [R)] plots and theoretical fits of Rh-Cl and Rh-MeCN have been vertically shifted for clarity. The gray box indicates the R-range of 1.0 – 3.0 Å applied for fitting of the XAFS data. Analysis of the Img [R)] plot of Rh-Cl clearly shows a chloride ligand in the first shell of the Rh center (see Figure 2). Additionally, the Rh K-edge position (inflection point) for the XANES of Rh-Cl and the resulting species from the reaction Rh-Cl and AgBF4 are nearly the same, indicating that the oxidation state of the two Rh species are the same (see Supporting Information, Figure S4). This observation provides further direct evidence that the sole role of silver cation is to abstract Cl- and not to modify the oxidation state of the Rh. We fit the XAFS data for the reactions of Rh-Cl and AgBF4 in THF or MeCN to [(CAACCy,Dipp)Rh(COD)L]BF4 with L = THF (RhTHF) or MeCN (Rh-MeCN)] (see Figure 2). The scattering paths that fit Rh-THF and Rh-MeCN are mostly similar to those of Rh-Cl. In both cases, the Cl in Rh-Cl is replaced with either a Rh-O single scattering for Rh-THF, or for the fitting of Rh-MeCN, requiring both the Rh-N single-scattering and the Rh-N-C multiple scattering along the Rh-NCMe dative bond (see Supporting Information, Table S1).

Figure 3. k2-weighted Img [R)] plots to show effect of AgBF4 on Rh-Cl in Ph2O hydrogenation. Loss of the chloride ligand from Rh-Cl is a key activation step required for catalytic activity. To provide direct evidence for the effect of AgBF4 on the activation of Rh-Cl, we performed operando XAFS experiments during Ph2O hydrogenation by monitoring the reactions of Rh-Cl (3 mol% relative to Ph2O) in THF for 1 h under H2 with AgBF4 and without AgBF4 (see Figure 3). Without AgBF4, Rh-Cl remained unchanged under catalytic conditions, as evidenced by comparison to the XAFS measurement of parent Rh-Cl and the absence of conversion of Ph2O, determined by GC analysis. There is no evidence for RhCl interacting with Ph2O, THF, or H2, despite excess concentrations of these components in solution (see Figure 3, equation 2). Conversely, a representative Img [R)] plot from the operando XAFS catalytic reaction with AgBF4, conditions for active for Ph2O hydrogenation showed unequivocally that the loss of chloride from Rh-Cl is critical for catalytic activity. Effect of H2 Pressure and Ph2O Substrate on the Formation of Rh NPs Additional operando XAFS experiments were performed to understand the effect H2 pressure and Ph2O on the relative rate of formation of Rh NPs from Rh-Cl (see Figure 4, equation 3). These results are presented in Figure 4. The conversion of soluble Rh species to Rh NPs, which precipitated out of solution upon formation, were quantified by monitoring the decreasing height of the Rh K-edge signal over time by passing the beam through the solution phase of the catalytic reaction.

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More specifically, the hydrogenation of the COD ligand in cationic [(diphosphine)Rh(COD)]+ leads to a coordinatively unsaturated Rh center that binds an arene, forming [(diphosphine)Rh(-arene)]+.20a,20b,21 Equilibria for the olefin hydrogenation catalysts of cationic [(diphosphine)Rh(MeOH/THF)2]+ and [(diphosphine)Rh(arene)]+ (diphosphine = DIPHOS, Et-DuPhos, Me-DuPhos, BINAP) have been extensively studied.20a,20b,21-22 Scheme 3. Proposed Rh-Arene Interaction Delays Formation of Rh NPs Compared to Rh-THF Interaction O iPr

THF, 25 ºC H2 (6.8 atm)

Figure 4. Effect of Ph2O and H2 pressure on the formation of Rh NPs. Increasing H2 pressure led to faster formation of Rh NPs, while Ph2O reduces the rate of formation of Rh NPs.

N



N Rh

O

O Ph

Ph

slower formation of Rh NPs

iPr

N

iPr

O BF4

iPr



BF4

iPr

Rh

- COA

The reaction of Rh-Cl, AgBF4, and THF under H2 (6.8 atm), but without Ph2O, produced cationic Rh-THF that undergoes formation of Rh NPs with three different regimes of reactivity (see Figure 4, Condition A). In the first 12 min, 4% of the soluble Rh species is converted to Rh NPs. Between 12-20 min, there is a sharp decrease of soluble Rh by 8%, giving a total loss of 12% of soluble Rh. From 20-160 min, the relative rate of RhNPs formation follows a steady decay of soluble Rh for a total 24% loss of soluble Rh to form Rh NPs. When the hydrogenation was performed with Ph2O at 6.8 atm of H2 (see Figure 4, Condition B), the soluble Rh species are more stabilized than in the absence of Ph2O (Condition A). After 18 min, only 1% of the soluble Rh is lost, and after 156 min, 8% of soluble Rh is converted to Rh NPs under catalytic conditions. After 30 min, there has been a slow buildup of active Rh NPs. This observation of formation of Rh-NPs is important, and consistent with our previous kinetic studies on the consumption of Ph2O, which showed that hydrogenation of Ph2O was observed after 30 min.9 As the H2 pressure is increased to 20.4 atm (see Figure 4, Condition C), the conversion to Rh NPs exhibits clean decay with a negligible induction period. At 150 min, 17% of the Rh species have been converted to Rh NPs. In these studies, formation of the insoluble Rh NPs was confirmed by acquiring a spectrum of the precipitate at the bottom of the high-pressure reactor under operando conditions (see Supporting Information, Figure S7). Increasing H2 pressure leads to faster conversion of soluble Rh species to Rh NPs for arene hydrogenation by promoting faster activation of H2. In addition, direct evidence of faster generation of Rh NPs at higher pressure of H2 from this XAFS analysis is consistent with our experimental observations of faster arene hydrogenation at higher pressures of H2. The interpretation of Rh NP formation during the induction period has been directly corroborated by the operando XAFS data for Rh-Cl (3 mol%), AgBF4 (3 mol%), Ph2O, in THF under H2 (6.8 atm) (see Figure 4, Condition B). These operando XAFS results also suggest that Ph2O stabilizes the soluble Rh species via a Rh-arene interaction (see Supporting Information, Figure S6). In fact, the stabilizing effect of aromatic system originating from substrate,17 additive,18 or aromatic solvents (benzene, toluene)19 to form stable cationic Rh(-arene) intermediates is known to decrease the rates of hydrogenation of prochiral and achiral olefins.20

BF4 iPr

Rh O iPr

THF, 25 ºC H2 (6.8 atm) - COA

BF4 iPr

N

O Rh O

O

CAAC-stabilized Rh nanoparticles

faster formation of Rh NPs

In our catalytic system, we envision that after hydrogenation of COD to cyclooctane (COA), Ph2O can form cationic intermediates [(CAACCy,Dipp)Rh(-Ph)-OPh(THF)]BF4 or [(CAACCy,Dipp)Rh(-Ph)-OPh]BF4, or an equilibrium of the two cationic Rh species (see Scheme 3). Spectroscopic and structural data have been reported for closely related metal complexes containing an N-heterocyclic carbene ligand and an arene: [(IPr*)Ni(-C6H5Me)], [(IPr*)Ni(-C6H6)],  [(SIPr)Ni( -Ph)-OPh], and [(IPr)Cu(-C6H6)]SbF6.23,24 Whereas the hydrogenation of Rh-Cl with AgBF4 in THF without Ph2O leads to a less stable intermediate, [(CAACCy,Dipp)Rh(THF)3]BF4 provides faster formation of Rh NPs. Moreover, structurally related Rh complexes ligated by one carbene ligand and multiple solvent ligands, such as [(IPr)Rh(MeCN)3]OTf, [(IPr)Rh(C2H5)(MeCN)3]OTf2, and [(IPr)Rh(Et)(MeCN)4]OTf2 have been reported.25 Understanding the Deleterious Effect of Excess Alkoxides and Benzothiophene (BT) Treatment of Rh-Cl, AgBF4, and Ph2O with excess KOtBu (2 equiv to Ph2O) under H2 (6.8 atm) in THF showed no conversion of Ph2O over 24 h, as determined by GC-MS analysis. It is unclear if excess KOtBu competes with Ph2O for binding at the Rh NPs or if it prevents formation of Rh NPs. To understand the deactivating effect of KOtBu on this system, we performed operando XAFS on the catalytic reaction of Rh-Cl, AgBF4, Ph2O, and KOtBu (2 equiv. relative to Ph2O) in THF at 25 ºC (see Figure 5). We observed loss of Cl- from Rh-Cl in the presence of AgBF4, giving a new single-site Rh product, and we successfully fit the XAFS data to

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[(CAACCy,Dipp)Rh(COD)(OtBu)] (Rh-OtBu) (see Figure 6). Rh-OtBu does not transform to Rh NPs under the catalytic conditions, which suggests Rh-OtBu is relatively unreactive toward H2 in comparison to the catalytic reaction without KOtBu (see Scheme 4). This observation was corroborated by GC-MS analysis of the reaction mixture that showed no hydrogenation of COD at 6 h; after 24 h minimal hydrogenation of COD to COA was observed. In comparison, 90% of COD was converted to COA for the reaction of Rh-Cl, AgBF4, and Ph2O under H2 (6.8 atm) in THF after 6 h (see Supporting Information, pages S13-S14). These results suggest Rh-OtBu does not readily undergo oxidative addition of H2 under this reaction condition, and that the minor Rh species formed from Rh-OtBu after limited hydrogenation of COD to COA after 24 h is rapidly deactivated toward effective generation of Rh NPs for Ph2O hydrogenation.

We previously observed that benzothiophene (BT) poisons the hydrogenation of Ph2O to Cy2O and CyOH.9 We also observed the hydrogenation of the olefin group in the 5membered ring of BT to the corresponding 2,3dihydrobenzo[b]thiophene (DHBT). This observation indicates that hydrodesulfurization of benzothiophene does not occur under our catalytic conditions at 25 ºC.26 We proposed that BT does not interfere with the formation of Rh NPs because the hydrogenated products, COA and DHBT, were observed experimentally by GC-MS (see Scheme 4; see Supporting Information, pages S11-S12). We now have direct evidence that BT does not interfere with the formation of Rh NPs from RhCl. Analysis of operando XAFS kinetic studies on the reaction of Ph2O with Rh-Cl (3 mol%), AgBF4 (3 mol%), and BT (1.5 mol%) under H2 (6.8 atm) at 25 ºC for 1.5 h clearly showed the steady loss of soluble Rh as it is converted to Rh NPs (Figure 5). At 1.5 h, 13% of soluble rhodium species have transformed to Rh NPs. We also spectroscopically determined that the black precipitate in the operando XAFS kinetic studies is indeed Rh NPs. Additional analysis of the Rh NPs from the catalysis with BT by IR spectroscopy showed that the Rh NPs are coated with protonated CAACCy,Dipp ligands, which was previously observed for the isolated Rh NPs from reactions that underwent Ph2O hydrogenation. We sought to identify the possible interaction of benzothiophene (BT) with Rh-Cl prior to addition of H2 for catalysis. Analysis of the XAFS data on the stoichiometric reaction of Rh-Cl, AgBF4, and BT in THF at 25 ºC suggests that the Rh-BT species may not be the only Rh species in solution, because a Rh-BT complex would be expected to give a strong signal from the Rh-S scattering.27 An overlay of the XAFS data (see Figure 7) for Rh-Cl, AgBF4, and BT in THF versus Rh-Cl, AgBF4 in THF at 25 ºC appears similar to that of Rh-THF, with minor, yet noticeable, differences in the XAFS data from experiments containing BT. This result suggests that a possible equilibrium between cationic [(CAACCy,Dipp)Rh(COD)(THF)]BF4 (Rh-THF) and tentatively

Figure 5. Determining the deactivating effects of BT and excess KOtBu on Rh-Cl (3 mol%), AgBF4 (3 mol%), Ph2O, and THF by operando XAFS kinetic studies.

Figure 6. k2-weighted |R)| and Img [R)] plots and theoretical fits of experimental XAFS data for Rh-OtBu. The gray box indicates the R-range of 1.0 – 3.0 Å applied for fitting of the XAFS data. Scheme 4. Differentiating the Deactivating Pathways by BT and KOtBu on Rh-Catalyzed Arene Hydrogenation

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assigned [(CAACCy,Dipp)Rh(COD)(BT)]BF4 (Rh-BT) (see Figure 8, equation 5).

Figure 8. Evidence for the equilibrium between Rh-THF and Rh-BT, by variable temperature UV-vis spectroscopy. We cannot firmly assign the binding mode of BT at Rh from the XAFS data, due to the averaging of the Rh species from the existing equilibrium. However, it is possible that BT interconverts between Rh(-S-BT) and Rh (-2,3-BT), considering the observed hydrogenation of BT to DHBT under our catalytic reactions,29 the 18-electron rule, and literature precedent. For example, dissolution of structurally characterized [(C5Me5)Re-(CO)2(-S-BT)] leads to an equilibrium of [(C5Me5)Re-(CO)2(-S-BT) and  [(C5Me5)Re(CO)2( -2,3-BT) at 25 ºC.30 In a related study, [(nacnacMe2)Rh(-S-DBTP)2] (nacnacMe2 = ArNC(Me)CHC(Me)NAr, Ar = 2,6-Me2C6H3; DBT = dibenzothiophene) also undergoes a complex equilibrium with loss of a DBT ligand from [(nacnacMe2)Rh(-S-DBTP)2] to form [(nacnacMe2)Rh(-DBTP)] followed by additional loss of DBT to give [(nacnacMe2)Rh--(-DBTP)(nacnacMe2)Rh], in which the DBTP ligand bridges the two Rh centers (see Scheme 5).31

Figure 7. An overlay of k2-weighted Img [R)] plots for RhTHF and Rh-BT at 25 ºC in THF is presented for comparison. We obtained evidence for this equilibrium between Rh-THF and Rh-BT by investigating the reaction of Rh-Cl, AgBF4, and BT in THF at (-10 ºC to 55 ºC) by variable-temperature UV-vis spectroscopy (see Figure 8).28 At 25 ºC, the addition of BT (1 equiv.) to Rh-THF led to spectroscopic changes and diagnostic optical transitions of free BT at 290 nm and 298 nm, based on the independent UV-vis spectroscopic measurement of BT in THF, to indicate that not all BT coordinates to Rh (see Supporting Information, Figure S9). Cooling the reaction mixture from 25 ºC to -10 ºC led to a higher concentration of free BT, suggesting liberation of BT from Rh with a new broad Rh feature at 400 nm. Conversely, heating the reaction mixture (35 to 55 ºC) led to decreased free BT, suggesting BT is interacting with Rh. The results of the variable-temperature UV-vis spectroscopic studies support an equilibrium between Rh-THF and Rh-BT, which corroborates the XAFS data. The equilibrium between Rh-THF and Rh-BT explains the observation that BT does not inhibit H2 activation in the CAACCy,Dipp-Rh complex, because there is an appreciable concentration of Rh-THF present that allows for H2 activation and formation of Rh NPs. Moreover, detection of Rh NPs in the presence of BT by operando XAFS kinetic studies provides strong evidence that BT poisons Rh-catalyzed arene hydrogenation at the stage of Rh NPs, not at the single-site molecular Rh species.

Scheme 5. The Complex Equilibria of Metal-BT from the Dynamic Hapticity of BT

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ACS Catalysis studies, strongly suggest that benzothiophene poisons Rh NPs for arene hydrogenation of aryl boronic esters. Our Rhcatalyzed arene hydrogenation studies provide direct evidence that AgBF4 mediates chloride abstraction from Rh-Cl to generate cationic Rh intermediates for H2 activation that then generate the catalytically active Rh NPs. In the hydrogenation of aryl boronic esters, it is unclear how the Rh-Cl pre-catalyst is transformed into active Rh NPs under their reaction conditions: Rh-Cl (1-2 mol% relative to substrate), under 49.3 atm (50 bar) of H2 in the presence of 4 Å molecular sieves in CH2Cl2 or hexafluoroisopropanol at 25 ºC. It is unclear if the molecular sieves or solvent is assisting the activation of the Rh pre-catalyst toward formation of Rh NPs, or simply the higher H2 pressure of 49.3 atm, compared to our operating H2 pressure of 6.8 atm, is required to bypass the need for cationic, singlesite Rh species to activate H2 to form Rh NPs. While our mechanistic studies on Rh-Cl and AgBF4 for arene hydrogenation of Ph2O support that Rh NPs is the active species, it is important to note that the reaction conditions for the Rh(CAAC)-catalyzed chemoselective arene and heteroarene hydrogenation by Glorius and by Zeng5 are not identical to ours. Preliminary mechanistic studies by Glorius and co-workers on the hydrogenation of aryl boronic esters implicate formation of Rh NPs during catalysis. This observation does not unequivocally translate to formation of active Rh NPs in all Rh(CAAC)-catalyzed arene hydrogenations, as the active species could possibly be different under different reaction conditions.

Probing for the Interconversion to Single-Site Rh Species from Bulk Rh NPs Filtration results on Rh-catalyzed hydrogenation of Ph2O suggest that the active catalyst is a solid. However, the filtration test is a post-catalysis technique, and, thus cannot unequivocally rule out a possible dynamic process involving slow leaching of a single-site or cluster of a Rh(CAAC Cy,Dipp) species from bulk heterogeneous Rh during catalysis. Dynamic interconversion of bulk metal nanoparticles to soluble singlesite metal complexes has been observed for metal-catalyzed reactions.32 Operando XAFS was used, therefore, to probe for leaching single-site or clusters of Rh species from the bulk heterogeneous Rh material. We monitored the reaction of independently generated Rh NPs, prepared by refluxing RhCl3 in EtOH for 6 h, with protonated [CAACCy,Dipp]BF4, and with free CAACCy,Dipp, generated from [CAACCy,Dipp]BF4 and KOtBu, in THF with H2 (6.8 atm) (see Scheme 6). These experiments did not show single-site or molecular clusters of Rh species in the solution phase under catalytic conditions. Thus, the combination of filtration tests and operando XAFS allow us to conclude that generation of measurable concentrations molecular single-site or cluster Rh species from bulk Rh NPs is unlikely.

Conclusions Operando XAFS measurements provide direct evidence for formation of Rh NPs, via cationic Rh intermediates that are responsible for arene hydrogenation of Ph2O starting from welldefined [(CAACCy,Dipp)RhCl(COD)]. The induction period corresponds to the formation of Rh NPs for arene hydrogenation in this system. Increasing H2 pressure leads to the faster formation of Rh NPs and, in turn, to higher concentrations of Rh NPs for the enhanced rate of arene hydrogenation. While ex situ studies have shown that benzothiophene and KOtBu led to catalyst deactivation, the mode of deactivation remained unclear. Operando XAFS shows that the presence of excess KOtBu forms a soluble, single-site Rh-OtBu product that does not effectively activate H2; hence, it does not induce formation of Rh NPs. Conversely, reversible binding of benzothiophene to a single-site Rh species, which is supported by the observation of an equilibrium between cationic Rh-THF and cationic Rh-BT, allows for H2 activation, inducing formation of Rh NPs. Benzothiophene binds tightly to the Rh NPs, preventing H2 and substrate binding for subsequent arene hydrogenation under our reaction conditions. Operando XAFS studies are a powerful and direct method to determine the identity of the active species in question from single-site species to clusters to nanoparticles. Our detailed studies determined that Rh NPs decorated with protonated CAAC ligands are the active catalysts for arene hydrogenation of Ph2O from the well-defined molecular complex [(CAACCy,Dipp)Rh(COD)Cl]. This discovery provides implications and additional evidence for the use of highly modular carbene ligands to modify nanoparticles for rapid access to new catalytic and functional materials in this active field of research.

Scheme 6. Determination of leaching of molecular Rh species from bulk Rh NPs by operando XAFS

In related work on hydrogenation of aryl boronic esters, Glorius and co-workers performed fractional poisoning experiments with benzothiophene9 and filtration studies to conclude that Rh NPs formed from Rh-Cl, are the active species for the arene hydrogenation of aryl boronic esters.8b While Glorius and co-workers do not specify if benzothiophene poisons the single-site Rh or the Rh NPs for catalysis, their isolation of a black Rh precipitate, together with filtration

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Asymmetric Transition-Metal Catalysis. Chem. Soc. Rev. 2017, 46, 4845-4854. (b) Zhao, D.; Candish, L.; Paul, D.; Glorius, F., N‐Heterocyclic Carbenes in Asymmetric Hydrogenation. ACS Catal. 2016, 6, 5978-5988. 5. Wei, Y.; Rao, B.; Cong, X.; Zeng, X., Highly Selective Hydrogenation of Aromatic Ketones and Phenols Enabled by Cyclic (Amino)(Alkyl) Carbene Rhodium Complexes. J. Am. Chem. Soc. 2015, 137, 92509253. 6. Wiesenfeldt, M. P.; Nairoukh, Z.; Glorius, F., Hydrogenation of Fluoroarenes: Direct Access to All-cis(Multi)fluorinated Cycloalkanes. Science 2017, 357, 908912. 7. (a) Baumgartner, R.; Stieger, G. K.; McNeill, K., Complete Hydrodehalogenation of Polyfluorinated and Other Polyhalogenated Benzenes under Mild Catalytic Conditions. Environ. Sci. Technol. 2013, 47, 6545-6553. (b) Baumgartner, R.; McNeill, K., Hydrodefluorination and Hydrogenation of Fluorobenzene under Mild Aqueous Conditions. Environ. Sci. Technol. 2012, 46, 10199-10205. (c) Stanger, K. J.; Angelici, R. J., Hydrodefluorination of Fluorobenzene Catalyzed by Rhodium Metal Prepared from [Rh(COD)₂]BF₄ and Supported on SiO₂ and Pd-SiO₂. J. Mol. Cat. A 2004, 207, 59-68. (d) Yang, H.; Gao, H.; Angelici, R. J., Hydrodefluorination of Fluorobenzene and 1,2-Difluorobenzene under Mild Conditions over Rhodium Pyridylphosphine and Bipyridyl Complexes Tethered on a Silica-Supported Palladium Catalyst. Organometallics 1999, 18, 2285-2287. (e) Nairoukh, Z.; Wollenburg, M.; Schlepphorst, C.; Bergander, K.; Glorius, F., The Formation of All-cis-(Multi)fluorinated Piperidines by a Dearomatization–hydrogenation Process. Nat. Chem. 2019, 10.1038/s41557-018-0197-2. 8. (a) Wiesenfeldt, M. P.; Knecht, T.; Schlepphorst, C.; Glorius, F., Silylarene Hydrogenation — A Strategic Approach Enabling Direct Access to Versatile Silylated Saturated Carbo- and Heterocycles. Angew. Chem. Int. Ed. 2018, 57, 8297-8300. (b) Wollenburg, M.; Moock, D.; Glorius, F., Hydrogenation of Borylated Arenes. Angew. Chem. Int. Ed. 2018, 10.1002/anie.201810714. 9. Tran, B. L.; Fulton, J. L.; Linehan, J. C.; Lercher, J. A.; Bullock, R. M., Rh(CAAC)-Catalyzed Arene Hydrogenation: Evidence for Nanocatalysis and Sterically Controlled Site-Selective Hydrogenation. ACS Catal. 2018, 8, 8441-8449. 10. (a) Zhukhovitskiy, A. V.; MacLeod, M. J.; Johnson, J. A., Carbene Ligands in Surface Chemistry: From Stabilization of Discrete Elemental Allotropes to Modification of Nanoscale and Bulk Substrates. Chem. Rev. 2015, 115, 11503-11532. (b) Ott, L. S.; Cline, M. L.; Deetlefs, M.; Seddon, K. R.; Finke, R. G., Nanoclusters in Ionic Liquids: Evidence for N-Heterocyclic Carbene Formation from Imidazolium-Based Ionic Liquids Detected by ²H NMR. J. Am. Chem. Soc. 2005, 127, 5758-

Supporting Information Additional experimental setup and spectroscopic data are included in the Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Funding Sources We thank the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences for support. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy. This research used resources of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory and was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357, and the Canadian Light Source and its funding partners.

Notes

The authors declare no competing financial interest.

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2-ylidene; IPr* = 1,3-bis(2,6-bis(diphenylmethyl)4methylphenyl)imidazol-2-ylidene 25. (a) Palacios, L.; Di Giuseppe, A.; Opalinska, A.; Castarlenas, R.; Pérez-Torrente, J.; Lahoz, F. J.; Oro, L. A., Labile Rhodium(I)−N-Heterocyclic Carbene Complexes. Organometallics 2013, 32, 2768-2774. (b) Di Giuseppe, A.; Castarlenas, R.; Pérez-Torrente, J. J.; Lahoz, F. J.; Oro, L. A., Hydride-Rhodium(III)-N-Heterocyclic Carbene Catalysts for Vinyl-Selective H/D Exchange: A StructureActivity Study. Chem. Eur. J. 2014, 20, 8391-8403. 26. Weber, T.; Prins, R.; van Santen, R. A., Transition Metal Sulphides: Chemistry and Catalysis. Kluwer Academic Publishers: Dordrecht, 1998. 27. Attempts to fit the experimental XAFS data from the reaction of Rh-Cl, AgBF₄, and BT to a Rh-BT complex with a Rh-S scattering path did not match the XAFS data. 28. Independent UV-vis spectroscopic measurements for stoichiometric reactions of Rh-Cl, RhCl and AgBF₄ in THF at 25 ºC, and free BT have been conducted (see the Supporting Information, Figure S9). 29. Bianchini, C.; Meli, A., Hydrogenation and Hydrogenolysis of Thiophenic Molecules Catalysed by Soluble Metal Complexes. J. Chem. Soc., Dalton Trans. 1996, 801-814. 30. (a) Huckett, S. C.; Miller, L. L.; Jacobson, R. A.; Angelici, R. J., Ruthenium, Rhodium, and Iridium Complexes of .Pi.-Bound Thiophene and Benzo[b]thiophenes: Models for Thiophene Binding to HDS Catalysts. Organometallics 1988, 7, 686-691. (b) Choi, M.-G.; Robertson, M. J.; Angelici, R. J., Sulfur Versus 2,3-η² Coordination of Benzo[b]thiophene (BT) in Cp'(CO)₂Re(BT). J. Am. Chem. Soc. 1991, 113, 4005-4006. (c) Choi, M.-G.; Angelici, R. J., The Benzo[b]thiophene (BT) Rhenium Complexes Cp'(CO)₂Re(BT): Models for BT Adsorption on Hydrodesulfurization Catalysts. Organometallics 1992, 11, 3328-3334. (d) W., B. J.; Angelici, R. J., Equilibrium Studies of the Displacement of η¹-(S)-Thiophenes (Th) from Cp(CO)(PPh₃)Ru(η¹-(S)-Th). Organometallics 1992, 11, 922-927. (e) Robertson, M. J.;

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