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DFT Calculations on the Mechanism of Transition Metal Catalyzed Reaction of Diazo Compounds with Phenols: O-H Insertion versus C-H Insertion Yuan Liu, Zhoujie Luo, John Zenghui Zhang, and Fei Xia J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b05735 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on August 2, 2016

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The Journal of Physical Chemistry

DFT Calculations on the Mechanism of Transition Metal Catalyzed Reaction of Diazo Compounds with Phenols: O-H Insertion versus C-H Insertion Yuan Liu,† Zhoujie Luo,‡ John Zenghui Zhang,†,§ Fei Xia*,‡,§ †

School of Physics and Materials Science, East China Normal University, ‡School of Chemistry and Molecular Engineering, East China Normal University, and §NYU-ECNU Center for Computational Chemistry at New York University Shanghai, East China Normal University, Shanghai 200062, China

ABSTRACT The reaction of diazo compounds with transition metal carbenes is an efficient way to achieve the functionalization of chemical bonds in organic molecules, especially for the C-H and O-H bonds. However, the selective mechanisms of C-H and O-H bond insertions by various metal carbenes such as Rh and Cu complexes are not quite clear. In this work, we performed a comprehensively theoretical investigation of the phenol C-H and O-H bonds inserted by Rh and Cu carbenes by using DFT calculations. The calculated results reveal that the nucleophilic additions of phenols to the Rh and Cu carbenes in the C-H bond insertions are the rate-determining steps of whole reactions, which are higher than the barriers in the O-H insertions. In the process of intramolecular [1,3]-H transfer, the Rh and Cu ligands in their carbenes tend to dissociate into solution rather than the intramolecular migration due to their weak metal-carbon bonds. A deeply theoretical analysis on the electronic structures of Rh, Cu and Au carbenes as well as their complexes elucidated their differences in the chemo-selectivity of C-H and O-H insertion products, which accords with the experimental observations well.

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1. INTRODUCTION Small organic molecules such as phenols are the important natural resources in organic synthesis due to their wide availability and low prices. The specific functionalization of O-H or C-H bonds in organic molecules1-8 is a severe challenge for organic chemists, especially for the selective functionalization of inert C-H bonds. One way to functionalize the C-H and O-H bonds is to employ the metal carbenes generated from the mixture of diazo compounds and transtion metal catalysts.9-16 The generated metal carbenes possess high reactity toward the C-H and O-H bonds so that the methology of employing metal carbenes was rapidly developed these years. In most cases, the widely used transition metal catalysts include the copper,17-23 rhodium24-29 and gold30-34 complexes. The generated transition metal carbenes exhibit remarkable catalysis in activating the C-H and O-H bonds, which arouse the interests of theoretical researchers to explore their mechanisms. The previous theoretical studies35-39 mainly focus on the mechanisms of insertion reactions of C-H bonds in alkanes and O-H bonds in water or alcohol. One hand, for the mechanism of alkyl C-H bond insertion, Nakamura et al.35 firstly carried out a comprehensive study of alkyl C-H bonds catalyzed by dirhodium tetracarboxylate complexes and proposed that the C-H bond activation follows a concerted mechanism via a three-centered transition state. Afterwards, Braga et al.36 performed a further study of alkane C-H bond insertion by copper and silver complexes. Recently, we reported a detailed comparison of the mechanisms of aromatic C-H bonds of anisole and alkyl C-H bonds inserted by gold carbenes,40 which reveals that the aromatic C(sp2)-H bond activation follows a stepwise mechanism actually. Xie et al.41 studied the mechanism of Rhodium-catalyzed C-H functionalization of indole, they concluded the enol pathway is always favored for the phenyl-substituted carbene. On the other hand, Liang et al.37 explored the mechanism of O-H bond insertion of water molecules catalyzed by copper and rhodium complexes. They concluded that the O-H bond insertion catalyzed by copper complex follow the ligand-associated pathway, while the rhodium complex prefers the ligand-free pathway in the reaction process.


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Xie et al.39 calculated the mechanism of O-H bonds in allyl alcohol inserted by rhodium carbenes and suggested that the O-H insertion products could be obtained through a [1,3]-proton shift in the enol intermediate or a [1,2]-proton shift in the oxonium ylide, which were regulated by the orientation of the ester groups in carbenes. The most of current experiments attempt to improve the efficiency of C-H and O-H bond insertions catalyzed by metal carbenes. In particular, how to enhance the chemo-selectivity toward C-H bond insertion remains a severe challenge for organic chemists. Very recently, Zhang42 and Shi’s group43 independently found that the triphenylphosphine oxide gold (Au-TPPO) carbenes have an unprecedented selectivity toward the C-H bond insertion in phenols. The prominent catalysis of gold catalysts stimulated us to perform a detailed theoretical and experimental study toward its selective mechanisms.44 Our recently published results reveal that the stability of enol intermediates play an important role in the selectivity. It is the first case in which the selective mechanisms of C-H and O-H bond insertions were detailedly elucidated by theoretical calculations.44 However, the mechanisms of selective C-H and O-H bond insertions catalyzed by other important transition metal Rh and Cu carbenes remain unclear, especially for the pivotal question how the effects of electronic structures of the coordinated transition metals affect the selectivity.20, 42, 44 In this work, we carried out a comprehensively theoretical investigation on the C-H and O-H bonds inserted by Rh and Cu carbenes. The reactions of phenols and methyl α-diazophenylacetate catalyzed by the catalysts with the ligands copper bisoxazoline (Cu-box), copper pyridine bisoxazoline (Cu-pybox) and dirhodium tetraacetate (Rh-TC) were chosen as the research targets, since the three complexes above were widely used in lab to generate the related carbenes for reacting with the diazo compounds.20, 27 By using the density functional theory(DFT) methods, we calculated the detailed pathways and free energy profiles of C-H and O-H bonds inserted by Rh and Cu carbenes. Based on a deeply theoretical analysis and comparison of electronic structures of key intermediates of Rh, Cu and



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Au carbenes, we elucidate the reason why the Rh, Cu and Au carbenes give rise to different C-H and O-H bond insertion products. 2. COMPUTATIONAL METHODS All the DFT calculations throughout this work were carried out using the Gaussian 09 software package.45 The geometric structures of intermediates and transition states were optimized using the M06 functional.46,47 The M06 functional has been demonstrated that it could give accurate descriptions for the intramolecular interactions in organometallic systems, especially for the noncovalent interactions between aromatic groups. The Lanl2dz basis set48,49 combined with the effective core potential was used to describe the heavy elements Rh, Cu and Au, and the 6-31G* basis set50 was utilized to describe the nonmetallic elements C, N, O, P and H, as the same as the computational levels that we used in the previous studies.40,44 The frequency analyses were performed on the optimized structures to verify that the intermediates are local minima and transition states are stationary points that possess only one imaginary frequency. The solvent effect of CH2Cl2 in the reactions was evaluated using the integral equation formalism model (IEFPCM)51 with a dielectric constant ε = 8.93, based on the structures obtained in gas phase. All the calculated energies of intermediates and transition states discussed here refer to the standard Gibbs free energies ∆Gsol in kcal/mol calculated at the temperature 298 Kelvin, including the solvation corrections evaluated from the IEFPCM51 model. The natural bond orbital (NBO)52 analysis and Wiberg bond order53 were performed at the same computational level. The contour images of molecular orbitals of key structures were plotted using the visualized package GaussView.54 The Cartesian coordinates of the intermediates and transition states along the calculated lowest energy pathways are provided in the section 1 of Supporting Information. 3. RESULTS AND DISCUSSION


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3.1 Pathways of O-H Bond Insertion Catalyzed by Rh Carbenes The DFT calculations performed by Liang37 and Liu et al.40 demonstrate that the metal catalysts could react with diazo compounds to release dinitrogen molecules and form active metal carbenes as the precursors for the insertion reactions. The optimized structures of Rh-TC, Cu-pybox, Cu-box and Au-TPPO carbenes as well as the phenol are displayed in Figure 1. The transition metal carbenes and phenols are regarded as the reactants of the whole O-H insertion reactions, with their sum of free energies being 0.0 kcal/mol. Figure 2 shows the calculated reaction pathways and free energy profiles of the O-H bond insertion by the Rh-TC carbenes. The experiments performed by Zhang et al.55 have demonstrated that the attacking of the hydroxyl oxygen of benzyl alcohol to the carbene carbon leads to the species of oxonium ylide. By means of DFT calculations, we found that the oxonium ylide Int-o1 forms through the transition state TS-o1 with a modest barrier 14.4 kcal/mol. However, the energy of ylide Int-o1 is higher than the reactants by 13.8 kcal/mol and quite unstable in thermodynamics. There exist three possible reaction pathways37,44 connecting the ylide Int-o1 on the energy surfaces. One of pathways is that Int-o1 transforms to the ligand-free ylide Int-o2* by releasing the Rh-TC ligand into solution. To reach the product Pro-o1, the hydroxyl hydrogen in Int-o2* needs to transfer to the carbene carbon, through a water mediated TS-o2* with an endothermicity of 23.3 kcal/mol. The second pathway involves the formation of the metal-associated ylide Int-o2** with an endothermicity of 20.4 kcal/mol, where the Rh-TC ligand migrates to the aromatic ring in carbenes. The third pathway is the direct [1,2]-proton transfer in TS-o2**. However, the barriers of three pathways relative to Int-o1 above are too high and should be ruled out from the possible mechanisms. In our recent study of the O-H bonds inserted by Au-TPPO carbenes, we proposed that the hydroxyl hydrogen in ylide tends to transfer to the adjacent carboxyl oxygen and form enol complexes.44 Here, we optimized the enol complex Int-o2 as well as TS-o2 and found that Int-o2 is more stable than Int-o1 by 16.3 kcal/mol. Subsequently, the Int-o2 transforms to the enol Int-o3 by



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liberating the Rh-TC ligand into solution instead of the migration to form Int-o3*. Finally, the [1,3]-H transfer occurs via TS-o3, which leads to the final stable product Pro-o1. In addition, in order to compare to the pathway from the carbonyl side shown in Figure 2, we have calculated the pathway of nucleophilic addition from the methoxy group side and the corresponding energy profiles are shown in Figure S1 of Supporting Information. Obviously, the lowest energy reaction pathway of that from the methoxy group side is less favorable in energetics than that from the carbonyl side. 3.2 Pathways of C-H Bond Insertion Catalyzed by Rh Carbenes The energy profiles of C-H insertion catalyzed Rh-TC carbene are shown in Figure 3. Since the para site of phenol is the most favorable site for the phenol C-H bond insertion, we only calculated the reaction pathway occurred at the para-position of phenol. Our recent theoretical study40 indicates that the first step of the aromatic C(sp2)-H insertion is the direct addition of the C(sp2)-C(sp2) atoms between the phenol and metal carbene. The barrier of C(sp2)-C(sp2) addition via the transition state TS-c1 is calculatedly 19.2 kcal/mol, which is obviously higher than 14.4 kcal/mol of the O-H bond nucleophilic attack in Figure 2. The addition intermediate Int-c1 is unstable in thermodynamics, with the energy 12.2 kcal/mol higher than the reactants. The [1,3]-migration intermediate Int-c2* can be hardly obtained because of its high endothermicity. Similar to the process of O-H insertion, the carbonyl oxygen atom is ready to abstract the proton to form the enol complex Int-c2, with a small barrier of 3.8 kcal/mol via TS-c2. The enol complex Int-c2 could be further stabilized by releasing the Rh-TC ligand into solution, leading to a free enol Int-c3. Through the TS-c3, the hydrogen transfer occurs in Int-c3 and the final product Pro-c1 is produced. The rate-determining step for the C-H bond insertion is the C(sp2)-C(sp2) addition with the barrier of 19.2 kcal/mol. It is higher than that of [1,3]-hydrogen transfer in the O-H insertion by 2 kcal/mol. Based on the kinetic viewpoint, the difference of 2 kcal/mol in free energy will leads to a large discrepancy in the reaction


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rates. Therefore, the O-H insertion in phenol is more facile than the C-H insertion catalyzed by the Rh-TC carbene, which accords with the experimental results. 3.3 Pathways of O-H and C-H Bond Insertion Catalyzed by Cu-Carbenes Figure 4 shows the calculated lowest energy profiles of phenol O-H and C-H bond insertions catalyzed by Cu-pybox carbenes, denoted by the red and black curves, respectively. For the O-H insertion, the O-H nucleophilic attack of phenol to the Cu-pybox carbene is still facile, requiring an activation free energy of 16.1 kcal/mol via transition state TS-o4. Through TS-o5, the enol complex Int-o5 is generated and further transforms to a more stable enol Int-o3 by liberating the Cu-pybox ligand into solution. Finally, the product Pro-o1 is formed through a water-assisted transition state TS-o3. The crucial step in the O-H bond insertion is the [1,3]-hydrogen transfer and the total energy barrier is 17.2 kcal/mol. For the C-H insertion, the reaction process is similar to that in Figure 3. The rate-determining step is the first step of nucleophilic attack via TS-c4, with the barrier 19.3 kcal/mol. Figure 5 shows the lowest energy pathways of C-H and O-H bond insertions catalyzed by Cu-box carbenes. As for the first steps of nucleophilic attack of C-H and O-H bond insertions, the energy difference between the transition states TS-c6 and TS-o6 is 3.5 kcal/mol. For the C-H insertion, it is still the rate-determining step in the whole reaction. The crucial barriers of [1,3]-H transfer for TS-o3 and TS-c3 in the O-H and C-H bond insertions are almost the same, 17.2 and 17.5 kcal/mol, respectively. Nevertheless, the [1,3]-H transfer is still the rate-determining step for the O-H insertion, lower than the crucial step of C-H insertion by 3 kcal/mol. Thus, the O-H bond insertion by Cu-box carbenes is also more favorable than C-H insertion in kinetics. The calculated results explain the experimental selectivity of C-H and O-H bonds of phenol well. 3.4 Nucleophilic Addition of C-H Bond Insertion



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Based on the pathways calculated for the Rh-TC, Cu-pybox and Cu-box carbenes shown above, we found that the first addition steps in the C-H bond insertions always possess higher barriers, with 19.2, 19.3 and 20.2 kcal/mol for the three carbenes respectively. Why the first steps in the C-H insertions have such high barriers? The answer could be obtained by analyzing the electronic structures of transition metal carbenes. Figure 1(b) shows that the Wiberg bond order of Rh-C bond in Rh-TC carbene is 0.83, which is larger than the values of other three metal-carbon bonds in Figure 1(c)-(e). The molecular orbital of Rh-TC carbene in Figure.6(a) shows that a typical π orbital of Rh-TC carbene is formed by the dxy orbital of rhodium and the px orbital of carbon, and meanwhile Figure 6(b) shows the sdx2 hybrid orbital of rhodium overlaps with the sp2 orbital of carbon and form the σ bond between them. Therefore, we conclude that the Rh-C bond in the Rh-TC carbene is actually a double bond. Figure 6(c) and Figure 6(d) show the occupied orbitals of the Cu-box and Au-TPPO carbenes. The Cu-C and Au-C bonds exhibit the similar orbital composition, which consist of sd hybrid orbital of gold and the sp2 hybrid orbital of carbon. Due to the lack of the π conjugation, the Cu-C and Au-C bonds are actually single bonds that are weaker than the Rh-C double bond. Even though the bond orders of Cu-C bonds in Cu-pybox carbene and Cu-box carbene are lower than that in Rh-TC carbene, we noticed that the electronic effects of the coordinated ligands have an important influence on the barriers of addition. The Cu-pybox and Cu-box ligands are typical electron-donating groups in experiments, while the Au-TPPO ligand is an electron-withdrawing one. The electron-donating effect makes the carbene carbons more electronegative so that the nucleophilic addition becomes more difficult and vice versa. It explains the reason why the addition in the C-H insertion catalyzed by Au-TPPO carbenes has a lower barrier of 15.6 kcal/mol,44 lower than that of Cu-pybox and Cu-box carbenes by 3.7 and 4.6 kcal/mol, respectively. 3.5 Analysis of Key Enol Intermediates


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Apart from the nucleophilic additions in the C-H insertions, the steps of [1,3]-H transfers also play an important role in the pathways of O-H bond insertions. The [1,3]-H transfers in the O-H insertions involve the crucial precursors, namely, the free enols which were generated from the corresponding enol complexes. Whether it is easy for the enol complexes to liberate the attached metal ligands has an important influence on the subsequent barriers of [1,3]-H transfers. Moreover, it also determines whether the enol complexes prefer a ligand-associated mechanism or ligand-free along the reaction pathways. Thus, a deeply theoretical analysis upon the stability of enol complexes is necessary. We evaluated the NBO charges and the Wiberg bond orders of four enol complexes, Int-o2, Int-o5, Int-o7 and Au-enol, as shown in Figure 7. The bond orders of the metal-carbon bonds in Int-o2, Int-o5, Int-o7 and Au-enol are 0.26, 0.15, 0.21 and 0.38 respectively, which reveals that Au-enol has the strongest Au-C bond among the enol complexes. The NBO analysis shows that the charge of the rhodium atom in the Rh-TC ligand is 0.68, while the charge of the rhodium atom in the complex reduced to 0.50. The reduction of charge with the 0.18 unit indicates that the rhodium atom accepts the electron donation from the carbon atom through the orbital interactions between them. Similarly, the NBO charges in Cu-pybox, Cu-box and Au-TPPO decrease by 0.04, 0.15 and 0.23, respectively. Obviously, the Au-C bond has the largest bond order value 0.38, which means that the stronger interaction exits between the Au and C atoms. This explains the reason why the Au-TPPO ligand prefers the migration to the liberation into the solution in the C-H and O-H bond insertions.44 In contrast, the Rh and Cu carbenes tend to liberate the ligands into solution due to the weak interactions between the metal and carbon atoms in the enol complexes. 3.6 Composition of Metal-Carbon Bonds In a bonding analysis further, Figure 8 shows a schematic comparison of the orbital composition of metal-carbon bonds in Int-o2, Int-o7 and Au-enol. Figure 8(a)



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indicates that there exists a weak σ interaction between the dxy orbital of rhodium and the px orbital of the carbene carbon. Compared to the Rh-C double bond in Figure 6(a), the Rh-C interaction in Int-o2 has decreased dramatically since a NBO analysis reveals that the π conjugation between them was broken. It means that the dissociation of the Rh-TC ligand from Int-o2 is a quite facile, which explains the reason why Int-o2 has a low barrier for dissociation. Figure 8(b) and Figure 8(c) display the HOMO orbitals of Int-o7 and Au-enol, respectively. Although the Cu and Au atoms in the carbenes adopt a pattern of sd hybridization to overlap with the sp2 orbitals of carbon atoms, it is evident that the overlap in Figure 8(c) is more effective than that in Figure 8(b). The reason lies in the fact that the 6s orbital in Au atom has larger orbital radius so that it improves the overlaps of atomic orbitals between the gold an carbon atoms. In our previous study,44 the dissociation of Au-TPPO ligand of Au-enol in Figure.7(g) into solution needs an endothermic energy of 7.1kcal/mol. Thus, the Au-TPPO ligand prefers to migrate to the aromatic ring in Au-enol and form new interactions to compensate for the energy loss in the dissociation. 4. CONCLUTIONS In this work, we performed a comprehensively theoretical investigation on the mechanisms of selective functionalization of phenol O-H and C-H bond insertions catalyzed by the Rh-TC, Cu-pybox and Cu-box carbenes by using the DFT methods. By comparison of the mechanisms of Rh-TC, Cu-pybox and Cu-box carbenes to that of Au-TPPO carbene, we found that two remarkable differences exist in the mechanisms of selectivity between the Au-TPPO carbene and others. For the C-H and O-H bond insertions catalyzed by Rh-TC, Cu-pybox and Cu-box carbenes, the first steps of nucleophilic additions of phenols to carbenes in the C-H insertions have considerably high barriers with the values of 19.2, 19.3 and 20.2 kcal/mol, respectively. They are higher than the barriers of the nucleophilic additions in the O-H insertions as well as that of [1,3]-H transfers in the C-H and O-H insertions. Therefore, the nucleophilic additions in the C-H bond insertions are


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rate-determining steps and unfavorable in kinetics. The calculated pathways account for the experimental observations why the major O-H bond insertion products catalyzed by Rh-TC, Cu-pybox and Cu-box carbenes have been obtained. Secondly, the Rh-TC, Cu-pybox and Cu-box ligands tend to dissociate from the enol complexes to from free enols which are followed by the [1,3]-H transfers. The Au-TPPO ligand in Au-carbene prefers the migration to the dissociation so that the subsequent [1,3]-H transfer occurs in ligand-associated complex. Based on a detailed electronic structure analysis on the metal-carbon bonds in complexes, we elucidated the reason why the differences in the ligand dissociation and migration exist between the Rh, Cu and Au carbenes. The mechanistic insights presented above give an unambiguous explanation in the chemo-selectivity of O-H and C-H bonds of phenols inserted by the Rh-TC, Cu-pybox, Cu-box and Au-TPPO carbenes, especially for the electronic structure effects of various transition metals on the reaction mechanisms. This will help us to take deep insight into the chemo-selectivity for different metal catalysts and find the most efficient catalyst for the functionalization of chemical bonds in organic molecules. Supporting Information The Cartesian coordinates of intermediates and transition states are listed in section 1 and the calculated pathways of O-H insertion from the methoxy group side are shown in Figure S1 of section 2. Corresponding Authors *E-mail: [email protected]

Tel: +86-(0)21-20596009.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grants No. 21433004 and 21473056) and Natural Science Foundation of Shanghai



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(14ZR1411800). We acknowledge the support of the NYU-ECNU Center for Computational Chemistry at NYU Shanghai. We also thank the supercomputer center of ECNU for providing computer time.

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Figures and Captions: Figure 1. Optimized structures of (a) Phenol, (b) Rh-TC carbene, (c) Cu-pybox carbene, (d) Cu-box carbene and (e) Au-TPPO carbene, where the values of NBO charges and Wiberg bond orders are shown in black and red, respectively. Figure 2. Calculated free energy profiles ∆Gsol of the phenol O-H bond insertion catalyzed by Rh-TC carbene as well as the corresponding structures of intermediates and transition states along different pathways. The lowest energy pathway is shown in red and the other marked in black. All the values of free energies are in the units of kcal/mol.

Figure 3. Calculated free energy profiles ∆Gsol of the phenol C-H bond insertion catalyzed by Rh-TC carbene as well as the corresponding structures of intermediates and transition states along different pathways. The lowest energy pathway is shown in red and the other marked in black. All the values of free energies are in the units of kcal/mol.

Figure 4. Calculated competitive pathways of the phenol O-H and C-H insertions catalyzed by Cu-pybox carbene. The red and black curves denote the calculated lowest-energy pathways of O-H and C-H bond insertions, respectively.

Figure 5. Calculated competitive pathways of the phenol O-H and C-H insertions catalyzed by Cu-box carbene. The red and black curves denote the calculated lowest-energy pathways of O-H and C-H bond insertions, respectively.

Figure 6. Contour maps and schematic representations for (a) the 116th occupied molecular orbital of Rh-TC carbene, (b) the 115th occupied molecular orbital of Rh-TC carbene, (c) the HOMO of Cu-box carbene and (d) the 118th occupied molecular orbital of Au-TPPO carbene.


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Figure 7. Optimized structures of intermediates and metal ligands, (a) Int-o2, (b) Rh-TC, (c) Int-o5, (d) Cu-pybox, (e) Int-o7, (f) Cu-box, (g) Au-enol and (h) Au-TPPO, where the values of NBO charges and Wiberg bond orders are shown in black and red. Figure 8. Contour maps of (a) the 142th occupied molecular orbital of Int-o2, (b) the HOMO of Int-o7 and (c) the HOMO of Au-enol.



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Figure 1.


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Figure 2.



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Figure 3.


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Figure 4.



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Figure 5.


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Figure 6.



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Figure 7.


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Figure 8.



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