pubs.acs.org/IC Article Transmetalation Reactions Triggered by Electron Transfer between Organocopper Complexes Olmo Lozano-Lavilla, Pablo Gómez-Orellana, Agustí Lledós,* and Juan A. Casares* Cite This: Inorg. Chem. 2021, 60, 11633−11639 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: [Cu(bipy)(C6F5)] reacts with most aryl iodides to form heterobiphenyls by cross-coupling, but when Rf−I is used (Rf = 3,5-dicholoro-2,4,6-trifluorophenyl), homocoupling products are also formed. Kinetic studies suggest that, for the homocoupling reaction, a mechanism based on transmetalation from [Cu(bipy)- (C6F5)] to Cu(III) intermediates formed in the oxidative addition step is at work. Density functional theory calculations show that the interaction between these Cu(III) species and the starting Cu(I) complex involves a Cu(I)−Cu(III) electron transfer concerted with the formation of an iodine bridge between the metals and that a fast transmetalation takes place in a dimer in a triplet state between two Cu(II) units. ■ INTRODUCTION Copper occupies a prominent place among the metals used in Scheme 1. Accepted Catalytic Cycle for Copper-Catalyzed Cross-Coupling Reactions and the Hypothetical Interaction between Cu(III) and Cu(I) Species organic synthesis, and its compounds are extremely versatile and efficient homogeneous catalysts. The Ullmann reaction for the homocoupling of arenes using metallic copper as the catalyst was reported more than a century ago, and during this time milder conditions have been developed by using copper complexes instead of the bulk metal.1,2 Simultaneously, methodologies for cross-coupling based on copper have been developed and complement palladium and nickel catalysts.3−6 Quite surprisingly, it is difficult to correlate mechanistically both reactions, leading to homocoupling and cross-coupling. By analogy with palladium, the most accepted mechanistic proposal for copper-catalyzed cross-coupling reactions includes the oxidative addition of R−X to a Cu(I), forming a Cu(III) intermediate in which reductive elimination takes place (Scheme 1). However, mechanisms involving radical oxidation Surprisingly, the interaction between Cu(III) and Cu(I) to produce Cu(II) intermediates seem to operate in a species remains almost unexplored, although the reversal significant number of reactions, and Cu(II) intermediates process involving the disproportionation of Cu(II) organo- have been invoked in several mechanistic proposals for the metallics has been proposed as a critical step in several catalytic Ullmann reaction.2,7,8 systems.21,22 Also, even though transmetalation between The two-electron oxidative addition reaction has been copper species is implicitly assumed in several coupling addressed in computational and experimental studies.2,9−19 It processes and connects the Ullmann homocoupling reaction entails an oxidation state change between Cu(I) (the resting with the cross-coupling reaction, transmetalation between state of the catalyst) and Cu(III). This is usually the slowest step on the cycle, and unlike what happens in palladium- Received: May 27, 2021 catalyzed reactions, the oxidized state, Cu(III), is often Published: July 14, 2021 considered to be unstable and therefore a very reactive intermediate.20 Hence, the reaction between the Cu(I) species, very abundant in the reaction medium, and the very reactive Cu(III) intermediates cannot be disregarded in these systems. © 2021 American Chemical Society https://doi.org/10.1021/acs.inorgchem.1c01595 11633 Inorg. Chem. 2021, 60, 11633−11639 Inorganic Chemistry pubs.acs.org/IC Article Cu(I) complexes and other copper species has not been tendency (red numbers in Figure 1). These results suggest that explicitly considered in experimental- or computational-based the reaction behaves as a nucleophilic attack of the copper mechanistic proposals. This is striking considering the known complex to the ipso-carbon of the aryl iodide, in accordance ability of Cu(I) organometallics to transmetalate to other with previous studies in other Cu(I) systems.12,26 The trend of metals, particularly d8 complexes such as Pd(II) deriva- computed atomic charges at the ipso-carbon agrees with this tives.23,24 This lack of fundamental chemical information view (see the Supporting Information, SI). prompted us to address the study of these reactions. As reported for analogous systems, 1 suffers a self-ionization ■ RESULTS AND DISCUSSION Kinetic Studies. We have pursued to induce the equilibrium in solution to give the cation [Cu(bipy)2]+ and the cuprate [Cu(Pf)2]− (eq 1).12,27,28 interaction between Cu(I) and Cu(III) complexes in trans- 2[Cu(bipy)(Pf)] F [Cu(bipy)2 ]+ + [Cu(Pf)2 ]− (1) metalation reactions between [Cu(bipy)(Pf)] (1; Pf = C6F5) and the putative Cu(III) intermediates formed after the From a van’t Hoff plot, the values ΔH = −2.7 ± 0.3 kcal mol−1 oxidative addition of aryl iodides to 1. If the reductive and ΔS = 23 ± 2 cal mol−1 K−1 were obtained for this reaction. elimination takes place very fast, the cross-coupling product is Using these parameters, Keq at 25 °C takes a value of 9 × 10−4, obtained; otherwise, the Cu(III) intermediate could be meaning that about 97% of the copper in the system is in the intercepted by reacting with Cu(I) species (Scheme 2). form of the neutral complex 1. The participation of cuprates in the oxidative addition reaction has been previously discussed, Scheme 2. Expected Pathways for Cu(III) Intermediates both computationally and experimentally, and in a very general Formed by Oxidative Addition way, it has been found that they react at a much lower rate than the parent neutral tricoordinated complexes.12,29 To verify this fact in our system, the tetrabutylammonium salt of the organocuprate (NBu4)[CuPf2] was synthesized (see the synthesis and X-ray structure in the SI), and the reaction of pentafluorophenylation of iodobenzene was attempted in the same conditions as those for compound 1. Only traces of the heterocoupling product were obtained from this experiment. The same result was obtained by using [Cu(bipy)2]+ or The reactions of 1 with nonfluorinated aryl iodides R−I [Cu(bipy)I] as the copper reagent.14 follow only the first path, giving the cross-coupling products Thus, in principle, the cross-coupling can be pictured as an R−Pf without any detectable intermediate and without oxidative addition initiated by a nucleophilic attack of complex homocoupling products other than those formed by the 1 on the C−I bond to give an intermediate of the type basal decomposition of 1. The kinetics show a first-order [Cu(bipy)I(Pf)(Ar)], followed by fast reductive elimination.30 dependence on the copper complex. To explore the influence In an attempt to stabilize the Cu(III) species after the of the electronic properties of the aryl halides in this reaction, nucleophilic attack step, we tested the oxidative addition of 1- competitive experiments with different para-substituted aryl iodo-3,5-dichloro-2,4,6-trifluorobenzene (Rf−I). The reaction iodides along with iodobenzene were carried out. For the would lead to a Cu(III) intermediate with two different reaction with Ph−I at 50 °C, kobs = 2.14 × 10−4 s−1 M−1 and fluoroaryl substituents that would benefit from the large ΔG⧧318.16 = 24 kcal mol−1.25 The larger relative rates were stabilization of the C−M bonds that fluoroaryls provide.31 The obtained for the aryl iodides bearing the most electron- presumed intermediate was not stable enough to be withdrawing substituents, and the Hammett plots of the spectroscopically observed. Instead, in addition to the expected relative reaction rates displayed a very good correlation with cross-coupling product Rf−Pf, the homocoupling biaryls Pf−Pf the σp− values of the para substituents (Figure 1). The density and Rf−Rf were formed along with [Cu(bipy)(Rf)] and Pf−I functional theory (DFT)-computed barriers reproduce this (Scheme 3). Two pathways can be envisioned to explain the formation of homocoupling products: a set of double oxidative addition− fast reductive elimination reactions (model I in Scheme 4) or a pathway involving Cu(III) intermediates that react with 1 to Scheme 3. Products Obtained from the Reaction of Complex 1 with Aryl Iodidesa Figure 1. Hammett plot for the reaction rates of the pentafluor- ophenylation reaction of aryl iodides with the para substituents indicated in the figure. The numbers in red are the computed Gibbs energy barriers in THF, in kcal mol−1. a See the SI for reaction conditions and yields. 11634 https://doi.org/10.1021/acs.inorgchem.1c01595 Inorg. Chem. 2021, 60, 11633−11639 Inorganic Chemistry pubs.acs.org/IC Article form species containing two Pf groups (model II in Scheme homocoupling/heterocoupling depends competitively on the 4).32 rates of the reductive elimination of C−C and C−I. On the contrary, in model II, the selectivity depends on the relative Scheme 4. Simplified Kinetic Modelsa kinetic rates of the reductive elimination and transmetalation from the intermediate [Cu(bipy)I(Pf)(Rf)] (I1) and then on the concentration of 1. Figure 3 represents the evolution of the a See the SI for a more detailed description. In model I, the Figure 3. Homocoupling/heterocoupling ratio during the course of homocoupling product Pf−Pf is formed after accumulation in the the reaction of 1 with Rf−I (7.1 × 10−2 M) in THF at 25 °C in medium of Pf−I, which can then react with Cu(I) complexes. reactions with different initial concentration of 1. The homocoupling accounts for the sum of Pf−Pf plus Rf−Rf: (red dots) [1]0 = 1.07 × 10−1 M; (green dots) [1]0 = 7.1 × 10−2 M; (blue dots) [1]0 = 2.4 × Because [Cu(bipy)(Rf)] is also a reaction product, Pf−Pf 10−2 M. and Rf−Rf should be formed at the same pace.33 Figure 2 shows the formation of products starting from an equimolar mixture of 1 and Rf−I and the best nonlinear fittings obtained homo/heterocoupling ratio in reactions with different initial using models I and II (see the SI for further details) showing concentrations of 1. Consistent with model II, the amount of the different rates of formation of these two products. Also, the homocoupling products increases with [1]0. selectivity of the reaction agrees with the involvement of a DFT Calculations. Thus, from these results, it seems clear second copper unit: In model I, the overall selectivity that some kind of transmetalation reaction is involved in the Figure 2. Experimental (dots) and best least-squares fitting (solid lines) of the products formed in the reaction of 1 (7.1 × 10−2 M) with Rf−I (7.1 × 10−2 M) in THF at 25 °C: (purple) Rf−Pf; (pink) Rf−Rf; (blue) Pf−Pf; (green) [Cu(bipy)(Rf)]; (red) Pf−I; (dark-gray) Rf−H; (light-gray) Pf−H. The upper figure shows the fitting with model I and the lower that with model II. Note the bad fitting on the upper figure, particularly for the homocoupling products Pf−Pf and Pf−I (blue and red). 11635 https://doi.org/10.1021/acs.inorgchem.1c01595 Inorg. Chem. 2021, 60, 11633−11639 Inorganic Chemistry pubs.acs.org/IC Article Figure 4. DFT-computed (B3LYP-D3/BS2 functional in THF) pathways for the competitive reactions of the formation of heterocoupling and homocoupling products. Relative Gibbs energies are given in kcal mol−1. Blue line: singlet potential energy surface. Red line: triplet potential energy surface. S1, S2, and S3 represent intermediates in the singlet state and T1, T2, and T3 intermediates in the triplet state. Optimized structures of all of the species in the profile are displayed in the SI. Figure 5. Optimized geometries of the singlet (S1) and triplet (T1) intermediates and the corresponding minimum-energy crossing point connecting both (MECP1). Distances are in angstroms. formation of homocoupling products, although the intimate Cu(III) intermediate I1 that leads to a very low barrier (3.9 mechanism cannot be established with the available exper- kcal mol−1) for the reductive elimination step. imental data. To disclose the mechanism of such a trans- Because homocoupling requires the presence of a second formation, we have carried out DFT calculations (B3LYP-D3 copper center, we included in the calculation one molecule of functional with a SMD (solvation model based on density)/ the Cu(I) reagent 1, present in the medium in a large excess. continuum description of the tetrahydrofuran (THF) solvent The presence of 1 has no effect in the stability of I1 in the (see Computational Details in the SI). The calculated Gibbs singlet state (S1 in Figure 4 at 9.1 kcal mol−1). However, the optimization of S1 (the couple I1·1) in the triplet state causes energy profile for the reaction between 1 and Rf−I is shown in dramatic changes in the system: a much more stable Figure 4 intermediate T1 (at −2.6 in the Gibbs energy profile; Figure Heterocoupling follows the expected pathway. The oxidative 4) is formed as a consequence of a comproportionation addition of Rf−I to 1 takes place with an energy barrier of 21.3 reaction between the Cu(III) and Cu(I) centers. The jump kcal mol−1 (TS_OA), yielding the Cu(III) pentacoordinate from the singlet S1 [Cu(IIII)−Cu(I)] to the triplet T1 intermediate I1,34 which affords the heterocoupling product [formally a Cu(II)−Cu(II) system, see copper atomic charges Rf−Pf after crossing a low barrier (3.9 kcal mol −1 ; in I0, S1, and T1 and the spin-density plot for T1 in the SI] TS_RE_Het) for the reductive elimination step. An important takes place through a minimum-energy crossing point point evidenced by the calculation is the instability of the (MECP1) at 12.9 kcal mol−1. The electron transfer between 11636 https://doi.org/10.1021/acs.inorgchem.1c01595 Inorg. Chem. 2021, 60, 11633−11639 Inorganic Chemistry pubs.acs.org/IC Article the two copper complexes happens with the simultaneous I1, which is consistent with a kinetic reaction order of 1 on 1 exchange of an iodine atom. The optimized geometries of S1, for this reaction. The selectivity for the formation of homo- or MECP1, and T1 are depicted in Figure 5. heterocoupling products depends on the relative energies of To assess the reliability of the singlet−triplet crossing, the MECP1 and TS_RE_Het and also on the concentration of 1, relative energies of the singlet S1 and triplet T1 intermediates but because the selection takes place after the rate-limiting step have been computed with 12 functionals containing variable (oxidative addition), the overall kinetic order on 1 is 1. percentages of Hartree−Fock exchange, using an extended Homocoupling requires the presence of a Cu(III) basis set (BS2; see the SI). Although this energy difference is intermediate that can cross to the triplet potential energy very dependent on the functional, all of the functionals agree surface by a comproportionation reaction with a Cu(I) center. that T1 is more stable than S1. As mentioned above, heterocoupling Pf−Rf is the only From T1, an easy transmetalation of Pf happens (through observable reaction for nonfluorinated aryls.35 We have TS-Transmet, Figure 6), placing both Pf groups bonded to the computed the energy profiles for a series of nonfluorinated para-substituted aryl iodides (R = H, OMe, Br, NO2; see Figure 1). The computed Gibbs energy profiles for all of these nonfluorinated Ar−I (see the SI) show that no metastable Cu(III) [Cu(bipy)I(Pf)(Ph)] intermediate is formed in this reaction, with the Ar−Pf product being reached in only one step. Therefore, the comproportionation reaction with a Cu(I) intermediate cannot occur. Accordingly, transmetalation does not happen either and homocoupling products are not observed. When I−Pf or I−Rf are used, the Cu(III) intermediates benefit from the large stabilization of the C− M bonds that fluoroaryls provide,31 and intermediate I1 is stable enough to be captured by 1, allowing electron transfer between both copper centers to occur, yielding a biradical Cu(II)−Cu(II) system. A fast bimolecular scrambling of aryls may take place between the Cu(II) centers, leading to a statistical formation of copper complexes with two Pf groups or with one Pf group and one Rf group. From this fast-exchange system, Pf−Pf and Pf−Rf can be formed and also [Cu(bipy)- (Rf)] through the reductive elimination of Pf−I. Finally, it is worth noting the stability of the triplets T1, T2, and T3 compared with the singlet Cu(III)/Cu(I) pairs. All of them are neutral species with bridging iodine, readily accessible from Cu(III) intermediates. These Cu(II) intermediates are kinetically labile and open new reaction pathways that cannot be predicted when only one molecule of the catalyst is considered, and they correlate with the reaction mechanisms of single copper species with binuclear complexes and with bioinorganic structures.36 In our case study, the pathway available in Cu(II) is transmetalation, which apparently is not accessible if only Cu(I) and Cu(III) species are considered. Figure 6. Optimized structure of the transmetalation transition state (TS_Transmet) (upper figure) and its spin-density plot (lower ■ CONCLUSION Our study presents a case in which the abundant Cu(I) figure). In black are the distances (in angstroms), and in blue are the reagents intercept Cu(III) intermediates in an electron-transfer Mulliken spin populations. comproportionation reaction, followed by a fast trans- metalation. This comproportionation reaction can also be same copper center, although mutually trans. Then, through a described as an inner-sphere electron transfer coupled with a low barrier isomerization (TS_t-c, Figure 4), the cis-Pf spin surface crossing. From a kinetic point of view, the intermediate is reached (T3). Recrossing from the triplet participation of two copper molecules in the reaction is (T3) to singlet (S2) potential energy surfaces through MECP2 somewhat hidden by the fact that it takes place after the rate- entails, in addition to electron transfer between the two copper limiting step, the oxidation of Cu(I) to Cu(III), although it has centers, the detachment of an iodide. In S2, Cu(I) [Cu(bipy)- a clear effect on the selectivity. DFT calculations clearly show (Rf)], Cu(III) cis-[Cu(bipy)(Pf2)]+, and I− are present, and the easiness of the electron transfer and transmetalation Pf−Pf homocoupling occurs in the Cu(III) center with a between copper complexes. We think that this model barrier of only 1.2 kcal mol−1 (TS_RE_Homo). A practically represents well reactive systems that go through Cu(III) barrierless replacement of Pf−Pf by I− in the coordination intermediates. In this regard, some of us have recently reported sphere of one copper ion (TS_I) yields the homocoupling comproportionation reactions between Cu(III), Cu(0), and product (Pf−Pf) and two copper(I) complexes, [Cu(bipy)- Cu(I) centers, yielding Cu(II) species in oxygen-bridged (Rf)] and [Cu(bipy)I]. polynuclear copper complexes.37 In these systems, the The computed profile depicts a situation in which the concentration of Cu(I) species accounts for most of the heterocoupling is a unimolecular process from the intermediate overall copper, making very probable the event of its reaction 11637 https://doi.org/10.1021/acs.inorgchem.1c01595 Inorg. Chem. 2021, 60, 11633−11639 Inorganic Chemistry pubs.acs.org/IC Article with complexes in other oxidation states. In this case, the (3) Beletskaya, I. P.; Cheprakov, A. V. The Complementary bimolecular comproportionation has to be taken into Competitors: Palladium and Copper in C−N Cross-Coupling consideration in mechanistic proposals. Reactions. Organometallics 2012, 31, 7753−7808. ■ (4) Hirano, K.; Miura, M. Recent Advances in Copper-Mediated Direct Biaryl Coupling. Chem. Lett. 2015, 44, 868−873. ASSOCIATED CONTENT (5) (a) Thapa, S.; Shrestha, B.; Gurung, S. K.; Giri, R. Copper- * sı Supporting Information Catalysed Cross-Coupling: An Untapped Potential. Org. Biomol. The Supporting Information is available free of charge at Chem. 2015, 13, 4816−4827. (b) Cheng, L.-J.; Mankad, N. P. C−C https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01595. and C−X coupling reactions of unactivated alkyl electrophiles using copper catalysis. Chem. Soc. Rev. 2020, 49, 8036−8064. Experimental section, control experiments, kinetic data, (6) Hickman, A. J.; Sanford, M. S. High-valent organometallic kinetic simulations and NMR spectra, crystallographic copper and palladium in catalysis. Nature 2012, 484, 177−185. data for (NBu4 )[Cu(C6 F5) 2 ], and computational (7) Casitas, A.; Ribas, X. The Role of Organometallic Copper(Iii) methods and extended description of the computational Complexes in Homogeneous Catalysis. Chem. Sci. 2013, 4, 2301− results (PDF) 2318. (8) Li, S.-J.; Lan, Y. Is Cu(III) a Necessary Intermediate in Cu- Accession Codes Mediated Coupling Reactions? A Mechanistic Point of View. Chem. CCDC 2072143 contains the supplementary crystallographic Commun. 2020, 56, 6609−6619. data for this paper. These data can be obtained free of charge (9) Zhang, S.-L.; Liu, L.; Fu, Y.; Guo, Q.-X. Theoretical Study on via www.ccdc.cam.ac.uk/data_request/cif, or by emailing Copper(I)-Catalyzed Cross-Coupling between Aryl Halides and data_request@ccdc.cam.ac.uk, or by contacting The Cam- Amides. Organometallics 2007, 26, 4546−4554. bridge Crystallographic Data Centre, 12 Union Road, (10) Giri, R.; Brusoe, A.; Troshin, K.; Wang, J. Y.; Font, M.; Cambridge CB2 1EZ, UK; fax: +44 1223 336033. Hartwig, J. F. Mechanism of the Ullmann Biaryl Ether Synthesis ■ Catalyzed by Complexes of Anionic Ligands: Evidence for the Reaction of Iodoarenes with Ligated Anionic CuI Intermediates. J. AUTHOR INFORMATION Am. Chem. Soc. 2018, 140, 793−806. Corresponding Authors (11) Yu, H.-Z.; Jiang, Y.-Y.; Fu, Y.; Liu, L. Alternative Mechanistic Agustí Lledós − Departament de Química, Edifici C.n. Explanation for Ligand-Dependent Selectivities in Copper-Catalyzed Universitat Autònoma de Barcelona, Cerdanyola del Vallès, N- and O-Arylation Reactions. J. Am. Chem. Soc. 2010, 132, 18078− Catalonia 08193, Spain; orcid.org/0000-0001-7909- 18091. 422X; Email: agusti@klingon.uab.es (12) Kalkman, E. D.; Mormino, M. G.; Hartwig, J. F. Unusual Juan A. Casares − IU CINQUIMA/Química Inorgánica, Electronic Effects of Ancillary Ligands on the Perfluoroalkylation of Facultad de Ciencias, Universidad de Valladolid, Valladolid Aryl Iodides and Bromides Mediated by Copper(I) Pentafluoroethyl Complexes of Substituted Bipyridines. J. Am. Chem. Soc. 2019, 141, 47011, Spain; orcid.org/0000-0001-8833-2066; 19458−19465. Email: casares@qi.uva.es (13) Tye, J. W.; Weng, Z.; Johns, A. M.; Incarvito, C. D.; Hartwig, J. Authors F. Copper Complexes of Anionic Nitrogen Ligands in the Amidation and Imidation of Aryl Halides. J. Am. Chem. Soc. 2008, 130, 9971− Olmo Lozano-Lavilla − IU CINQUIMA/Química 9983. Inorgánica, Facultad de Ciencias, Universidad de Valladolid, (14) Tye, J. W.; Weng, Z.; Giri, R.; Hartwig, J. F. Copper(I) Valladolid 47011, Spain; orcid.org/0000-0002-5752- Phenoxide Complexes in the Etherification of Aryl Halides. Angew. 1144 Chem., Int. Ed. 2010, 49, 2185−2189. Pablo Gómez-Orellana − Departament de Química, Edifici (15) Huang, Z.; Hartwig, J. F. Copper(I) Enolate Complexes in α- C.n. Universitat Autònoma de Barcelona, Cerdanyola del Arylation Reactions: Synthesis, Reactivity, and Mechanism. Angew. Vallès, Catalonia 08193, Spain Chem., Int. Ed. 2012, 51, 1028−1032. (16) Konovalov, A. I.; Lishchynskyi, A.; Grushin, V. V. Mechanism Complete contact information is available at: of Trifluoromethylation of Aryl Halides with CuCF3and the Ortho https://pubs.acs.org/10.1021/acs.inorgchem.1c01595 Effect. J. Am. Chem. Soc. 2014, 136, 13410−13425. (17) Lin, X.; Hou, C.; Li, H.; Weng, Z. Decarboxylative Notes Trifluoromethylating Reagent [Cu(O2CCF3)(Phen)] and Difluor- The authors declare no competing financial interest. ocarbene Precursor [Cu(Phen)2][O2CCF2Cl]. Chem. - Eur. J. 2016, ■ ACKNOWLEDGMENTS The authors thank the Spanish MCIN (Projects PID2019- 22, 2075−2084. (18) Lu, Z.; Liu, H.; Liu, S.; Leng, X.; Lan, Y.; Shen, Q. 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The role of chelating diamine ligands in the Goldberg reaction: A kinetic study on the copper- catalyzed amidation of aryl iodides. J. Am. Chem. Soc. 2005, 127, 4120−4121. (d) Dubinina, G. G.; Ogikubo, J.; Vicic, D. A. Structure of Bis(Trifluoromethyl)Cuprate and Its Role in Trifluoromethylation Reactions. Organometallics 2008, 27, 6233−6235. (30) This agrees with most of the proposals for oxidative addition reactions on trigonal Cu(I) intermediates. See, for instance, refs 5, 10, and 12. (31) Clot, E.; Mégret, C.; Eisenstein, O.; Perutz, R. N. Exceptional Sensitivity of Metal-Aryl Bond Energies to Ortho-Fluorine Sub- stituents: Influence of the Metal, the Coordination Sphere, and the Spectator Ligands on M-C/H-C Bond Energy Correlations. J. Am. Chem. Soc. 2009, 131, 7817−7827. (32) Other possible pathways, such the formation of intermediates of the type [Cu(bipy)(Pf)(Rf)2], have been considered and are discussed in the SI. (33) We have shown that C6F5 and C6Cl2F3 behave as electronically equivalent groups. For instance, see: Pérez-Iglesias, M.; Lozano- Lavilla, O.; Casares, J. A. [Cu(C6Cl2F3)(tht)]4: An Extremely Efficient Catalyst for the Aryl Scrambling between Palladium Complexes. Organometallics 2019, 38, 739−742. and references cited therein. (34) This value fits nicely with the experimental value ΔG⧧ = 20.8 kcal mol−1 obtained from the kinetic data. (35) Very small amounts of Pf−H and Pf−Pf are always formed when complex 1 is in solution for some hours due to adventitious amounts of water and oxygen in solution. See the SI. 11639 https://doi.org/10.1021/acs.inorgchem.1c01595 Inorg. Chem. 2021, 60, 11633−11639