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Clean Production of Fine Chemicals, Shandong Normal University, Jinan 250014, P. R. China. Inorg. Chem. , Article ASAP. DOI: 10.1021/acs.inorgchem...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Comprehensive Mechanistic Insight into Cooperative Lewis Acid/ Cp*CoIII-Catalyzed C−H/N−H Activation for the Synthesis of Isoquinolin-3-ones Qiong Wang, Fang Huang,* Langhuan Jiang, Chuanxue Zhang, Chuanzhi Sun, Jianbiao Liu, and Dezhan Chen* College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Provincial Key Laboratory of Clean Production of Fine Chemicals, Shandong Normal University, Jinan 250014, P. R. China S Supporting Information *

ABSTRACT: The mechanism of B(C6F5)3 promoted Cp*CoIIIcatalyzed C−H functionalization was investigated in detail employing density functional theory (DFT). The formation free energy of every possible species in the multicomponent complex system was explored and the optimal active catalyst was screened out. The results uncover the role of B(C6F5)3 played in forming active catalyst is from the coordination with OAc−, but not from the formation of [I(C6F5)3B]−, and no acceleration effect is found in C−H activation as well as the formation of CoIII-carbene intermediate. Moreover, present theoretical results elucidate the Cp*CoIII-catalyzed C−H activation is mediated by imine N-coordination other than general proposed the sequence of N-deprotonation directed C−H activation. The metal-controlled C−H/N−H selectivity was then elucidated by insighting into [Cp*CoIIIOAc]+/[Cp*RhIIIOAc]+-catalyzed C−H and N−H activations, respectively.

1. INTRODUCTION Transition-metal-catalyzed C−H bond activation and functionalization provides a new strategy for improving the atom and step economy in chemical synthesis.1−3 Most of the achievements were accomplished with 4d and 5d transition metals.4−9 Very recently, the earth-abundant, low-cost, and eco-friendly first-row 3d transition metal complexes,10−13 with comptetent character and distinct reactivity, have drawn special attention in C−H activation reactions. For example, electron-rich CoI catalysts have previously been used as an alternative for traditional noble-metal catalysts in hydroarylation/hydroacylation/electrophile coupling reactions.14−18 However, such transformations generally require the use of a Grignard reagent to regenerate the catalyst and complete the catalytic cycle. As a consequence of the harsh reaction conditions, substrate scope is limited. In the ensuing time, high-valent Cp*CoIII catalysts have emerged to avoid these drawbacks.19−26 Breakthroughs in this field were reported by Lutz Ackermann,20,21 Frank Glorius22,23 and some other groups.24−26 They found that Cp*CoIII complexes, such as Cp*Co(CO)I2, [Cp*CoIII(arene](PF6)2, [Cp*Co(C6H6)][B(C6F5)4]2, and the dimeric [Cp*CoI2]2 adopted with AgSbF6 could catalyze effectively the C−H functionalization reactions, which not only emulates the similar Group 9 transition-metals (rhodium or iridium salt), but also exhibited their distinctive reactivity. Despite these prominent progresses, high-valent Cp*CoIII catalysts in C−H bond-activation reactions are © XXXX American Chemical Society

generally limited in substrate scope because of the requirement of strong directing groups.27 Of note, using the strongly electrondonating N-unsubstituted imine as directing group to make isoquinolins arises to be an important process.28,29 Recently, Glorius and co-workers reported a facile route toward the synthesis of isoquinolin-3-ones through a cooperative B(C6F5)3- and Cp*CoIII-catalyzed C−H bond activation of 1(phenyl)pentan-1-imine (1) with dimethyl 2-diazomalonate (2) (Scheme 1).30 The NH imines as the directing groups are an intrinsic part of the substrate and eventually transform into products. According to the report, it is the first example of Lewisacid-promoted Cp*CoIII-catalyzed C−H activation reaction for synthesis of isoquinolin-3-ones. The experimental result indicates that the addition to catalytic amounts of B(C6F5)3 dramatically accelerates the reaction rates. On the basis of the Scheme 1. Synthesis of Isoquinolin-3-ones through C−H Activation Catalyzed by Cp*CoIII Catalyst

Received: December 21, 2017

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DOI: 10.1021/acs.inorgchem.7b03216 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Formation thermodynamics of the catalysts from [Cp*Co(CO)I2], CsOAc, and B(C6F5)3. Free energies in kcal/mol. B3LYP for geometry optimizations and M06 for single-point energy calculations has been demonstrated by numerous studies to successfully produce energy profiles of reactions involving transition metal complexes.47−54 Natural bond orbital (NBO) analyses were performed at the B3LYP/[6-31G(d,p)+ LANL2DZ] level using NBO6w.55 In order to ensure that the lowest energy conformation of intermediates and transition states was presented and discussed in the text, extensive conformational searches were conducted. To correct the Gibbs free energies under pressure of 1 atm to the standard state in solution (1 mol/L), a correction of RTln(cs/cg) (about 2.71 kcal/mol) is added to energies of all species.56 Molecular structure figures were prepared using CYLView.57

experimental observations, a dual role, played by B(C6F5)3, is proposed by the experimenters. First, it may facilitate the generation of the active cationic CoIII species by forming a noncoordinating borate anion such as [I(C6F5)3B]−. The cationic CoIII species would be stabilized by this borate anion. Second, the inclusion of B(C6F5)3 may accelerate the rate of the C−H activation step and subsequent formation of the CoIIIcarbene intermediate. Nevertheless, the intrinsic role of B(C6F5)3 are ambiguous. Most of all, the mechanisms of Cp*CoIII-catalyzed C−H functionalization are still needed to be elucidated, though many improvements have been made very recently.31−34 We hence conducted a detailed theoretical insight into the mechanism of title reaction and explored the reaction thermodynamics of each species in the multicomponent complex system to figure out the role of B(C6F5)3 played in the reaction. We explored also [Cp*RhIIIOAc]+-catalyzed C−H and N−H activations of the reaction to clarify the catalytic effect of the firstrow and second-row transition metals on C−H/N-H functionalization. Our aim is to provide a comprehensive understanding of cooperative lewis acid/Cp*CoIII-catalyzed reaction from theoretical views.

3. RESULTS AND DISCUSSION 3.1. Formation Thermodynamics of the Catalyst in Multicomponent Complex System. Besides the substrates, there is also a multicomponent adduct in the solution. Experimentally, [Cp*CoIIIOAc]+ is supposed to be the active catalyst generated from cat1, CsOAc, and B(C6F5)3, of which B(C6F5)3 is proposed to facilitate the generation of CoIII species by forming a noncoordinating borate anion such as [I(C6F5)3B]−. The formation thermodynamics of the reaction for each species were explored first and the Gibbs formation free energies were calculated for every possible combination of cat1, CsOAc, and B(C6F5)3 based on our optimized structures (see Figure 1) to determine the role of every species played in reaction process and to screen out the optimal active catalytic system. On the basis of calculations, the free [I(C6F5)3B]− is difficult to be generated from cat1 and B(C6F5)3, because of their unfavorable thermodynamics which are endergonic by 29.4 and 38.1 kcal/mol, respectively (paths B and C). Of note, it is more favorable for CsOAc to abstract iodide from cat1, contrast to B(C6F5)3, because of its double coordination function (path F). Moreover, if a B(C6F5)3 coordinates to one acetate of cat7, a most stable cat8 was formed and it is a only thermodynamical spontaneous pathway with negative formation free energy. From the thermodynamic point of view, the most stable cat8 could be the active catalyst. We can come to a conclusion that the

2. COMPUTATIONAL METHODS All calculations were performed using the Gaussian 09 program package.35 The hybrid density functional B3LYP36−38 was employed for geometry optimizations. 6-31G(d,p)39,40 basis set was employed for nonmetal atoms and LANL2DZ41−43 was used for Co atom. Harmonic vibrational frequencies were computed at the same level to get the entropic corrections and to verify the nature of stationary points. The minimum energy structures have positive eigenvalues of the Hessian matrix, whereas the transition states have only one negative eigenvalue. When necessary, IRC calculations were performed to verify the right connections among a transition state and its forward and reverse minima.44 Because the M06 functional45 includes noncovalent interactions and can give accurate energies for transition metal systems, single-point calculations with solvation effects modeled by SMD46 in toluene solvent were applied for all gas-phase-optimized structures at the M06/[6-311++G(d,p)+ LANL2DZ] level. The effectiveness of B

DOI: 10.1021/acs.inorgchem.7b03216 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Four modes of cat8-mediated C−H and N−H activations.

Figure 3. Free energy profiles of cat4 mediated C−H and N−H activations. For the CYLVIEW structures, trivial hydrogen atoms are omitted for clarity. The distances are shown in Å.

facilitation effect of B(C6F5)3 in the generation of active CoIII catalyst is attributed to the coordination interaction between B(C6F5)3 and OAc−, but not the abstraction of iodide.58 To evaluate the effect of AgSbF6 on the generation of active catalyst and distinguish the difference between B(C6F5)3 and AgSbF6, we explored the formation thermodynamics of various cobalt complexes formed possibly from [Cp*Co(CO)I2], CsOAc, and AgSbF6 (see Figure S1). The most stable CoIII complex is cat11 (1.1 kcal/mol), which is higher by 2.3 kcal/mol

in free energy than cat8 (−1.2 kcal/mol). Hence, B(C6F5)3 is more beneficial than AgSbF6 in generating active catalyst. In the following discussions, the most stable CoIII species cat8 is set as the energy reference for the catalytic cycle. 3.2. Selectivity of C−H/N−H Activation. 3.2.1. Selectivity of CoIII-Catalyzed C−H/N−H Activations. At first, we optimized the geometrical conformation of imine 1 (see Figures S2 and S3), and located the most stable one, in which the imine H and aryl group are trans structure. Cat8, the most stable CoIII-complex in C

DOI: 10.1021/acs.inorgchem.7b03216 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry thermodynamics, was taken as the active catalyst first. We explored four possible activation modes, OAc-assisted C(sp2)-H activation of aryl group and N−H activation of imine (TS1 and TS2), B(C6F5)3OAc-assisted C(sp2)-H and imine N−H activations (TS3 and IntA) (Figure 2). The results indicate that the free energy barriers are very high both for B(C6F5)3OAcassisted C(sp2)-H and N−H activation. The pathway of OAcassisted N−H activation is favorable with a barrier of 16.4 kcal/ mol. The attempts to locate the transition state of B(C6F5)3OAcassisted N−H activation are failed. The intermediates IntA and [B(C6F5)3]HOAc formed in this step are located. It is much unfavorable in thermodynamics with an endergonic energy of 26.7 kcal/mol. Cat4, the second stable one (one acetate is abstracted completely by two molecular B(C6F5)3 from Co center of Cat8), has been considered to be active catalyst in previous experimental and theoretical reports.10,13,59 Therefore, we explored further its catalytic effect on the reaction system (Figure 3). We first explored the pathway that B(C6F5)3 directly capture H atom of C−H bond to evaluate the effect of B(C6F5)3 on C−H activation. Inevitably, it has to cross a considerable high barrier (TSB_H, 61.6 kcal/mol) because of the electron deficiency of B(C6F5)3. Next, we examined whether the coordination of B(C6F5)3 with imine N in 1 could influence the C−H activation. It leads to Int1 which is exergonic by 19.3 kcal/mol. Then, cat4 interacts with Int1 via an agostic bond between Co and meta-C−H of aryl group. C−H bond cleavage is assisted by OAc− ligand and via a concerted metalation-deprotonation (CMD) mechanism. The activation free energy of transition state TS2_3 is somewhat high (ΔG‡ = 36.5, relative to Int1). Instead of the coordination of B(C6F5)3 with imine N, Co center can coordinate with imine N forming Int4. Then it is followed by a CMD transition states (TS4_5 or TS4_6) to activate the C−H bond forming cobaltacycle intermediates Int5/Int6. The OAc-assisted six-membered transition state TS4_5 is 15.5 kcal/mol lower than that of the four-memebered transition state TS4_6. The releasing of HOAc from Int5/Int6 generates Int7 with a vacant coordination site for subsequent reactions. In addition, we explored the direct C(sp2)H activation by cat4(TS8_5). The result indicates that this process has to cross a much higher barrier. It has been reported from proximity-driven metalation that the consecutive N−H and C−H double activation is much more accessible than an alternative route based on the C−H activation of the neutral substrate.60 Our present calculated results show that the consecutive N-coordination and C−H activation is much more accessible for this reaction system. The difference of barriers between direct C−H activation and N-coordination driven C−H activation is 24.3 kcal/mol. Moreover, we explored the direct N−H activation strategy, and it shows that the N−H activation proceeds via six-membered CMD transition state (TS9_10) and the barrier (24.1 kcal/mol relative to Int4) is 6.0 kcal/mol higher than that of TS4_5 (18.1 kcal/mol). The Gibbs formation free energy of direct N−H activation is 13.9 kcal/mol higher than that of C−H one. It follows that the direct N−H activation is less favorable than N-coordination-driven the C−H activation both in thermodynamics and kinetics. Comparing the optimized structures of TS4_5 and TS9_10, the dihedral angles of N−C1-C2−C3 in TS4_5 and TS9_10 are −19.5° and −38.8°, respectively. Both are departed from the coplanar geometry in the reactant 1, especially TS9_10. The structural distortion will reduce the conjugation of the imine and phenyl group. On the other hand, the dihedral angle −2.3° of C3−Co-O1−C4 in TS4_5 is much close to a stable coplanar geometry. While, the

corresponding dihedral angle 32.4° in TS9_10 is more distorted. Additionally, cat7 and cat9 mediated C−H and N−H activation were investigated as well. The barrier heights are higher than those of cat4-mediated C−H process. The results are given in Figures S4 and S5. In brief, the present results suggest that CoIII-catalyzed C−H activation of 1 is via imine N- coordination followed by C−H activation other than general proposed N-deprotonation directed C−H activation. The cat4 is confirmed theoretically to be the active catalyst suggested by the experimenter. The introduction of B(C6F5)3 does not have an acceleration effect on the C−H activation. To understand the intrinsic preference for the C−H activation of the reaction, we analyzed two transition structures of C−H and N−H activations (TS4_5 and TS9_10) using the distortion/interaction model.61−64 The results in Figure 4

Figure 4. Distortion, interaction, and activation energies for the TS4_5 and TS9_10 (green, distortion energy of frag_cat4; blue, distortion energy of frag_1; red, interaction energy; black, activation energy, in kcal/mol).

show that the distortion energy of TS4_5 is similar to that of TS9_10. However, the interaction energy (−21.1 kcal/mol) is more favorable for TS4_5 compared to that of TS9_10 (−9.5 kcal/mol). Furthermore, the exploration of C−H/N−H activation reactivity was made also using energy decomposition analyses (EDA) methods.65,66 The Multiwfn and Grimme’s DFT-D3 are employed to evaluate the corresponding energies. In Multiwfn, the total energy variation of forming a complex is decomposed as eq 1: ΔEtot = E comlpex −

∑ Eifrag = (ΔEels + ΔEex ) + ΔEorb i

= ΔEsteric + ΔEorb

(1)

Here, ΔEels is the electrostatic interaction term and ΔEex is exchange repulsion term from Pauli repulsion effect. The two terms are combined as steric term (Esteric). Eorb represents orbital interaction term and is sometimes known as polarization term. Considering the deficiency of B3LYP functional in evaluating the dispersion energy, Grimme’s DFT-D3 program using zerodamping function67 is applied to evaluate the dispersion component (ΔEdisp) in total interaction energy. Then, the total interaction energy is determined using eq 2: ΔE′tot = ΔEtot + ΔEdisp = ΔEsteric + ΔEorb + ΔEdisp (2)

The calculated energies are given in Table 1. It can be seen from the table that the ΔE′tot of TS4_5 is lower by 7.95 kcal/mol than D

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breaking of C−H bond and formation of C−Co bond proceed synergistically. 3.2.2. Selectivity of RhIII-Catalyzed N−H/C−H Activations. We have reported previously that sequential N-deprotonation and C−H activation is more favorable for Cp*Rh-catalyzed N− H/C−H activation of benzamidederivatives.69 This is also supported by other groups’ reports.70,71 It is interesting that for Cp*Co-catalyzed C−H activation of 1 is via imine Ncoordination-driven C−H activation other than general proposed N-deprotonation. This mechanistic phenomenon motivates us to investigate the reason. Therefore, we explored the Cp*Rh- mediated N−H/C−H activations of 1. The results are given in Figure 6. Similar with Figure 3, Rhcat8 is selected as energy reference. For Cp*Rh-mediated N−H/C-H activation, the N-coordination and the N-deprotonation are synchronous, and has a lower barrier of 20.0 kcal/mol. To understand the difference in activation mechanism resulting from Co/Rh center, was also made the EDA for RhTS4_5 and RhTS9_10. The results (Table 1) show that the total interaction energy of RhTS9_10 is lower by 7.28 kcal/mol than that of RhTS4_5, of which orbital interaction plays a dominant role in N-coordination/N-deprotonation selection, as the ΔE orb (RhTS9_10) is 17.36 kcal/mol lower than ΔEorb(RhTS4_5). According to ECDA analysis (Figure 5), since the key orbital interaction results from the occupied orbitals of substrate with HOMO and LUMO of Rhcat4, E(2)(LPN → LVRh) is much smaller (28.00 kcal/mol). The dominating item of stabilization comes from E(2)(LPO → LVH)(Figure 7). Therefore, sequential N-deprotonation directed C−H activation is more favorable for Cp*Rh-catalyzed C−H activation.

Table 1. EDA Results for TS4_5, TS9_10, RhTS4_5, and RhTS9_10 (all energies in kcal/mol) TS4_5 TS9_10 RhTS4_5 RhTS9_10

ΔE′tot

ΔEsteric

ΔEorb

ΔEdisp

−113.05 −105.10 −99.22 −106.5

70.04 72.86 78.98 85.81

−165.15 −164.39 −161.13 −178.49

−17.95 −13.56 −17.07 −13.82

that of TS9_10 indicating TS4_5 is more favorable than TS9_10. This is in line with the reaction barriers we calculated (Figure 3). Moreover, the reduction of interaction energy mainly comes from the dispersion energy, while there is little difference in ΔEsteric and ΔEorb between TS4_5 and TS9_10. It suggests that both steric and orbital interaction are almost the same for CoIII-catalyzed C−H/N-H activation. The dominating item is resulted from the dispersion energy of the transition state. The charge decomposition analysis (CDA) and extended charge decomposition analysis (ECDA) of TS4_5 and TS9_10 were conducted as well employing Multiwfn program (Figure 5). The results indicate that the electrons primarily transfer from occupied HOMO−2 and HOMO orbitals of frag_1 to the unoccupied LUMO+1 and LUMO+2 orbitals of frag_cat4 for N-coordination-driven C−H activation. The stabilization energies E(2) from NBO analysis show that E(2)(LPN → LVCo) = 78.52 kcal/mol and E(2)(LPO → BD*C3−H) = 96.52 kcal/mol (Figure 7).68 These results reveal quantitatively N-coordinationdriven C−H activation resulted from the coordination of lone pair electron of imine N to CoIII center and the inflow of lone pair electron from acetate O to C3−H antibond. The latter makes

Figure 5. MO correlation digram of frag_1 and frag_cat4 for orbitals in TS4_5/TS9_10/RhTS4_5/RhTS9_10. The percentage given in red is orbital composition. E

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Figure 6. Free energy profiles of CoIII/RhIII-catalyzed C−H and N−H activations.

Figure 7. Key orbital interactions between substrate and catalyst in TS4_5/TS9_10/RhTS4_5/RhTS9_10.

thermodynamically with formation free energy −46.5 kcal/mol. Change in the conformation of substrate 2 leads to higher barriers (TS11_12′ and TS11_12″). According to the previous reports,72,73 the C−C coupling can proceed in a stepwise or concerted fashion. In stepwise path, a Co-carbene intermediate is

3.3. Denitrogenation Mechanism. Coordination of diazo compound 2 to the vacant Co site leads to Int11 (see Figure 8). Subsequent N2 extrusion proceeds through TS11_12 giving cobaltacycle intermediate Int12 with activation free energy about 9.7 kcal/mol relative to Int11. The transformation is favorable F

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Figure 9. Selected episode in the optimization process of IntB. The inset plot provides the IRC profile for TS11_12. Key structures are listed and trivial hydrogen atoms are omitted for clarity. The distances are shown in Å.

B(C6F5)3 in denitrogenation step, we further calculated B(C6F5)3 and cat1 mediated denitrogenation processes, respectively. The free energy profiles are delineated in Figure S6, it is obvious that the barriers of B(C6F5)3 (TS13_14, 31.1 kcal/mol) and cat1 (TS15_16, 45.0 kcal/mol) mediated denitrogenation are much higher than that of cat4 mediated one (TS11_12, 9.7 kcal/mol). It follows that based on our theoretical calculations, the cat4 mediated denitrogenation is the most favorable both in thermodynamics and kinetics, and no acceleration effect is provided by B(C6F5)3. 3.4. Nucleophilic Addition and Proton Transfer. After denitrogenation is nucleophilic addition and proton transfer steps. The corresponding free energy profiles and key structures are portrayed in Figure 10 and Figure S7. First, isomerization of Int12 leads to Int17 (Figure 10, black line), which is very important for the following nucleophilic addition as it makes N and C5 close to each other. Meanwhile, Co−C4 and C5O1 are slightly activated, with O1 closer to Co center. Nucleophilic addition then occurs via a transition state TS17_18, leading to a 6-membered ring intermediate Int18. The C5−O2 bond is cleaved with the aid of a coordinated HOAc (via TS19_20) with a low energy barrier. Finally, with the cleavage of H−O2, C5−O2 and Co−O1 bonds, the product isoquinolin-3-one (denoted as P) and MeOH are released from Co catalyst regenerating the active catalyst cat4. Another reaction path previously reported by Qu et al. in their study of Cp*Co-catalyzed C−H functionalization was also considered by us (the red path in Figure 10). Three steps (proto-demetalation, nucleophilic addition and methanol elimination) are involved in this mechanism. Comparing the two free energy surfaces in Figure 10, it is obvious that the path including nucleophilic addition and proton transfer is more favorable. 3.5. Overall Catalytic Reaction Mechanism. The overall free energy profile of the catalytic cycle is given in Figure 11. According to the energetic span model introduced by Kozuch and Shaik,74,75 Calculated apparent activation energy of the catalytic cycle is 31.6 kcal/mol. The rate constant k, which is actually a simple expression of the driving force between turnover-frequency(TOF)-determining transition state (TDTS) and the TOF-determining intermediate (TDI), is 2.2 × 10−5 s−1. Because the rate constant is not available in Glorius’s report, the value can only be estimated from the reported reaction profile and is about 1.9 × 10−5 s−1. Thus, our calculated rate constant k agrees well with the derived value, which also confirms the rationality of the mechanism we explored.

IRC, an unstable Co-carbene intermediate (IntB) is first located with distance 1.797 Å of Co−C4. We then optimized IntB leading to a stable cobaltacycle intermediate Int12. Along with the optimization (IntB → IntC → IntD), the distances of Co− C4 and Co−C3 are elongated, 1.797 Å → 1.818 Å → 2.014 and 1.954 Å → 2.035 Å → 2.258 Å, respectively. C3−C4 is strengthened as the distance getting shorter (2.813 Å → 1.733 Å → 1.498 Å) indicating the formation of C3−C4 bond. These results suggest the extrusion of N2 and migratory insertion proceed in concerted so that only the concerted path is located in the denitrogenation step. The experimenters proposed that B(C6F5)3 may accelerate the rate of the formation of CoIII-carbene intermediate. When 2 was treated with B(C6F5)3 experimentally, the consumption of 2 exceeded 95% in TFE solvent with a temperature of 120 °C after 6 h. However, when 2 was treated with cat1, the consumption of 2 is 55% under the same condition. To figure out the effect of

4. CONCLUSIONS The mechanism of cooperative Lewis acid/Cp*CoIIIcatalyzed C−H activation for the synthesis of isoquinolin-3-ones was explored in detail by utilizing density functional theory. The formation thermodynamics of the active catalyst were explored first to determine the role of each species played in the multicomponent complex system and to screen out the optimal active catalytic system. The present theoretical result verifies that the facilitation effect of B(C6F5)3 is to promote the generation of active CoIII catalyst and hence to enhance the rate of catalysis. B(C6F5)3, as additives, is more beneficial than AgSbF6 in generating active catalyst because of its stronger oxophilicity. Different from the experimenters’ deductions, the results uncover the role of B(C6F5)3 played in forming active catalys is from the coordination with OAc−, not from the formation of [I(C6F5)3B]−, and no acceleration effect is found in C−H activation as well as the formation of CoIII-carbene intermediate.

Figure 8. Free energy profile of the denitrogenation step.

formed after the denitrogenation, which is followed by a migratory insertion. In a report of Qu et al., both pathways are available, but the concerted one is slightly favorable.73 However, our attempts to optimize the cobalt-carbene species were failed for this system, and the optimizations inevitably converge to cobaltacycle intermediate Int12. To figure out the reason, intrinsic reaction coordinate (IRC) calculation of TS11_12 was conducted, as shown in Figure 9. Interestingly, following the

G

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Figure 10. Free energy profiles for the nucleophilic addition and proton transfer steps (black path) and proto-demetalation, nucleophilic addition, and methanol elimination steps (red path).

Figure 11. Free energy profile for the overall catalytic cycle (in kcal/mol).

We explored both C−H and N−H activations catalyzed by [Cp*CoIIIOAc]+ respectively. The optimal elementary reaction steps, C−H activation, denitrogenation, nucleophilic addition and proton transfer, are determined. To our surprise, unlike general proposed the sequence of N-deprotonation directed C−

H activation, such as for Cp*Rh-catalyzed C−H activation of benzamide derivatives, for CoIII-catalyzed system, N-coordination-driven C−H activation is more favorable than that of sequential N-deprotonation directed C−H activation. The mechanism of proximity-driven C−H activation is elucidated H

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Inorganic Chemistry by the comparative exploration for [Cp*Co III OAc] + / [Cp*RhIIIOAc]+ catalysis by means of EDA analyses. It is intrinsic preference of d orbitals originated from the first- and the second-row transition metals that resulted in the mechanistic difference of proximity-driven C−H activation, and the selectivity of C−H and N−H activation. Additionally, only a concerted C−C coupling and a barrier free migratory insertion are located in the denitrogenation process.

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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.7b03216. Additional computational results, Figures S1−S7 as described in the text, and Cartesian coordinates of all optimized structures involved in this study (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fang Huang: 0000-0003-4801-7111 Jianbiao Liu: 0000-0002-2550-3355 Dezhan Chen: 0000-0002-2192-4582 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundations of China (21403132, 21375082).

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REFERENCES

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