What Factors Control the Reactivity of Cobalt–Imido Complexes in C

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What Factors Control the Reactivity of Cobalt-Imido Complexes in C-H Bond Activation via Hydrogen Abstraction? Lianrui Hu, and Hui Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02694 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 23, 2016

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What Factors Control the Reactivity of Cobalt-Imido Complexes in C-H Bond Activation via Hydrogen Abstraction? Lianrui Hu,†,‡ and Hui Chen*,† †

Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

‡

University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

Supporting Information Placeholder ABSTRACT: Metal-imido complexes are critical intermediates in transition metal-catalyzed C-H amination reactions. Discerning the factors that control their reactivity, however, remains largely open for exploration, particularly for the territory of cobalt-imido’s. Herein we describe a systematic computational exploration into this new frontier on the C-H activation mechanisms of typical well-defined cobalt-imido complexes, whose formal oxidation states cover an extremely wide range from Co(II) to Co(V). The hydrogen atom abstraction (HAA) is found to be the rate-limiting step in all these systems, with the open-shell electronic states of radical character consistently bearing kinetic advantage over the closed-shell ones. Surprisingly, there is no correlation found between the cobalt oxidation state and the HAA reactivity. To render a more accessible HAA channel, the dichotomous EER/anti-EER electron-shift scenarios for open-shell electronic structure are dependent on the cobalt oxidation states (Co(III) different from others), implying a paradigm shift from EER to anti-EER scenario in periodic table from Fe to Co. In contrast to the kinetic factor that determines the HAA reactivity, the reaction outcomes of C-H activation (amination or cyclometalation product) in cobalt-imido complexes are found to be controlled by the thermodynamic stabilities of the products. The current results for the cobalt-imido complexes imply that, in addition to HAA chemistry of metal-oxo’s, the HAA promoted by metal-imido species could also be subject to the radical-facilitated reactivity. From this work, it is predictable that the stabilization of less reactive closed-shell singlet state relative to other more reactive open-shell states is generally not beneficial to the HAA reactivity of cobalt-imido species.

KEYWORDS: hydrogen atom abstraction, cobalt-imido, C-H activation, amination, cyclometalation, DFT calculation, radical-facilitated reactivity, exchange-enhanced reactivity

1. INTRODUCTION In chemistry and biochemistry, hydrogen-atom abstraction (HAA) constitutes a fundamental way of C-H activation, which can lead to various C-H functionalizations.1 HAA processes exerted by the transition metal-heteroatom active species, wherein the electronegative heteroatom is O or N (eq. 1), have attracted immense research interests due to their relevance respectively to oxygenation and amination of C-H bond in transition metal (TM) catalysis.2 In HAA, since a

a Mes

= 2,4,6-trimethylphenyl, Dipp = 2,6-diisopropylphenyl, Ad = Adamantanyl, Dmp = 2,6-dimesitylphenyl.

Scheme 1. Typical Intramolecular C-H Activation Reactions Promoted by the Cobalt-Imido Complexesa [M]X + H-[C] → [M]X-H + [C•]

(X = O, NR)

(1)

hydrogen atom is abstracted from the C-H bond to generate a carbon radical, it is intriguing to elucidate the potential connection between the HAA reactivity and the radical character of the metal-heteroatom active species. For instance, Schwarz et al. had summarized that in gas-phase chemistry of metal-oxo clusters, all the reactive species promoting efficient HAA from methane at room temperature have a terminal oxygen-centered radical.3 Along these lines, HAA reactivity pattern of C-H bond was also rationalized by He et al. to qualitatively correlate with the O-centered radical character of some transition metal oxide clusters in gas-phase reactions.4 However, open-shell H-abstractor is not a prerequisite for HAA, since there are also HAA cases involving closedshell transition metal-oxo species such as CrO2Cl2 or MnO4-, which can abstract H from alkanes, even though the Habstractors have no unpaired electrons.2a,5 The recent debates about the HAAs by RuIV-oxo and MnV-oxo complexes also reflect the extensive attentions received by this issue of open-shell versus closed-shell H-abstractor for TM-oxo sys-

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tems,6 for which the understanding of the HAA reactivity pattern is highly desired.3,4,7,8 In comparison to TM-oxo systems, currently much less is known for the HAA promoted by TM-imido systems, especially for cobalt-imido’s. In this regard, Zhang, de Bruin, and their coworkers have inspiringly revealed that the key Co(III)-‘nitrene-radical’ intermediates ((Por)CoIII-N•Y) in Co(II)-porphyrin-catalyzed C-H amination with organic azides, although not isolable, have their unpaired spin density located almost entirely on the nitrogen atom of the nitrene moiety,9 which echoes the viewpoint of radical characterdependent reactivity for HAA in metal-oxo research field.3,4,8 Very recently, Deng et al. reported that a well-defined threecoordinate planar Co(IV) imido complex with N-heterocyclic carbene (NHC) ligand brings about intramolecular C-H activation, while its corresponding Co(V) imido complex with exactly the same coordination environment has no reaction in a similar reaction condition (Scheme 1a).10 This intriguing distinctive difference in C-H activation reactivity is somewhat counterintuitive, because higher valent Co(V) complex should be more oxidative and hence more reactive in oxidative C-H activation reactions than the lower valent Co(IV) one. The origin behind higher reactivity of Co(IV) imido complex over Co(V) imido one remains elusive. In addition to these well-defined Co(IV)/Co(V) imido systems, other Co(III) and Co(II) imido complexes capable of intramolecular C-H activation have been discovered in the groups of Theopold,11 Betley,12 and Deng.13 These complexes have rich structure varieties including two/three/fourcoordinate linear/triangle/tetrahedral geometries, and NHC and N-heteroaromatic supporting ligands, as summarized in Scheme 1b-d. All these cobalt-imido complexes with an extremely wide range of oxidation states from Co(II) to Co(V), provide an opportunity to systematically discern the factors that control their reactivity of C-H activation, a research territory still open for exploration.14 Particularly, to date, nobody has harnessed computational approach to gain insights into the electronic structure and reaction mechanism for elucidating the connection of their radical character and oxidation state to the HAA reactivity. To this end, herein we present our extensive density functional theory (DFT) theoretical study to explore the mechanisms and scenarios of electronic structure evolvements for C-H activations promoted by these typical cobalt-imido complexes. The insights gained from this work are expected to stimulate more studies on cobalt-imido systems for deeper understanding of their chemistry, which is currently premature but highly desired.

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el,18 taking the corresponding experimental one (shown in Scheme 1) as the solvent for each reaction. The reported free energies in this work were calculated at the TPSS/def2-TZVP level, including the Gibbs free energy thermal correction obtained from vibrational analysis in gas phase at the corresponding experimental reaction temperature (shown in Scheme 1), as well as DFT-D3 empirical dispersion correction.17 All DFT geometry optimizations and single point calculations were performed with Gaussian 09 program.19 We need to emphasize that although the commonly used approximate functionals in DFT method could have certain computational error in reaction barrier height calculation, it had been repeatedly found that their ability to follow the reactivity trend, which is directly related to relative barrier but not to absolute barrier height, is usually less functionaldependent and more reliable.20 Since the current computational study focuses on the relative reactivity and reactivity trend rather than the absolute barrier prediction, it tends to tolerate the variation of computational performance of various approximate functionals in current DFT method. To gauge the radical character, the computational details for population analysis based on the spin density natural orbitals can be found in the supporting information (SI) document. 3. RESULTS AND DISCUSSION 3.1. Co(IV)/Co(V) Imido Complexes The exactly same coordinate environment between Deng’s Co(IV) and Co(V) imido complexes10 (1 and 5), as shown in Scheme 1a, provides an ideal pair of systems for comparative exploration of the factors affecting and controlling the reactivity of C-H activation by cobalt-imido species. Due to the odd number of electrons of the Co(IV) imido complex 1, it inevitably bears an open-shell electronic structure. In contrast, Co(V) imido complex 5 with even number of electrons can have both open-shell and closed-shell electronic structures. How does this difference affect the C-H activation/amination reactions of 1 and 5?

2. METHODS All molecular geometries were optimized in gas phase, employing TPSS15 density functional combined with def2SVP basis set16 for all atoms, considering also Grimme’s DFT empirical dispersion correction DFT-D3 with the original short range damping.17 The optimized minima and transition states (TSs) were verified by harmonic vibrational analysis to have no and one proper imaginary frequency, respectively. To refine the calculated energies, single point calculations with larger basis set were then done based on these optimized structures, by using TPSS functional with def2-TZVP basis set.16 Solvent effect was modeled in these single point calculations by employing SMD continuum solvation mod-

Figure 1. DFT calculated reaction profile of intramolecular amination from 1. Due to the d7 electronic configuration of Co(II), at the product side (4), the sextet state is too high in energy to be involved in the last step of the profile. The key bond distances (R, in Å ) for TS1-2 and unpaired electron populations on N (ρN) for 1 are labelled. To reveal the origin of the reactivity difference between 1 and 5, we first investigated Deng’s Co(IV) imido complex 1. The calculated most favorable reaction pathway for the intramolecular C−H bond amination reaction of Co(IV) imido

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complex 1, is depicted in Figure 1. First, one imido N atom of 1 abstracts the benzylic H atom from the Mes group through HAA transition state TS1-2, resulting in the intermediate 2 bearing a benzylic radical. Then via a transition state TS2-3 the benzylic radical attacks cobalt center, leading to the benzylic Co(IV) intermediate 3 with the formation of Co-C σ bond. Finally, 3 undergoes a 1,2-migration process through TS3-4 to generate the final amination product 4. In addition to HAA, alternatively we also tried the [2π+2σ]-addition as the initial C-H activation step, which was recently found to be the possible mechanism for the Si-H bond activation by structurally similar Fe(IV) imido complex.21 However, the calculated barrier of exceeding 40 kcal/mol (see Figure S1 in the SI for more details) is much higher than that of the HAA mechanism in Figure 1, which therefore can hardly support the [2π+2σ]-addition pathway for C-H activation by Co(IV) imido species. From the benzylic radical 2 formed from HAA, we also explored the possibility of direct rebound of benzylic radical with the second imido N to form the final amination product 4. However, this direct rebound process, which was previously proposed tentatively,10 is less favorable in energy than the pathway involving the benzylic Co(IV) intermediate 3 shown in Figure 1 (see Figure S2 in the SI for more details). Inspecting Figure 1, one can see that except the last alkyl 1,2-migration step via TS3-4, which occurs preferably through a spin transition crossing from low-spin doublet state (S = 1/2) of 3 to medium-spin quartet state (S = 3/2) of TS3-4 by having a barrier of 23.3 kcal/mol, the lowest energy profile of all the other steps in this reaction pathway is on the low-spin doublet state. The rate-limiting step of the mechanism is the benzylic HAA (1 to 2), bearing a barrier of 24.2 kcal/mol via lowest-lying doublet state TS1-2. This calculated barrier value is in line with the experimental ambient reaction temperature. Notably, all spin states of Co(IV) imido complex are open-shell states, and the whole reaction has a small thermodynamic driving force of only about 8.3 kcal/mol from 1 to 4.

Figure 2. Electron-shift diagrams of the H-abstraction step in the intramolecular C-H bond activation of 1. In addition to the reaction pathway, the corresponding electronic structures are also of high interest. For the ratelimiting HAA step from 1 to 2, electron-shift diagrams are shown in Figure 2. For all three spin states from low-spin to high-spin one, during the HAA process, the β electron of the cleaved C-H bond transfers to Co atom to pair up with one unpaired α electron in the open-shell cobalt(IV) d-shell (into dxy orbital for doublet/quartet states, and into dyz for sextet state). In this way, HAA process from Co(IV) imido complex 1 effectively results in a radical relocation from cobalt-imido moiety to the C atom of the cleaved C-H bond. Interestingly, this type of electronic structure evolution to reduce the

number of unpaired electrons on the transition metal center in HAA process differs significantly from the recently emerging exchange-enhanced reactivity (EER) found from chromium-, manganese-, and iron-oxo species in HAA and other processes, since EER prefers to increase the number of unpaired electrons at metal site due to the beneficial stabilizing exchange interaction.22 This disparity of the electronic structure evolution behavior in HAA process is not likely to be attributed to the different character of imido versus oxo species, since previously in a structurally very similar Fe(IV) imido complex, HAA process was found to be of EER type, which was associated with increasing number of the unpaired electrons on iron center.21 As a result, this anti-EER behavior in electronic structure evolvement in HAA should reflect the difference between early/middle (Cr, Mn, Fe) and late (Co) first-row transition metals.

Figure 3. DFT calculated reaction profile of intramolecular amination from 5. The key bond distances (R, in Å ) for TS5-6 and unpaired electron population on N (ρN) for 5 are labelled. In contrast to Co(IV) imido complex 1, for Co(V) imido complex 5, results of calculated reaction kinetics are quite different. In the calculated most favorable amination reaction pathway from complex 5 as depicted in Figure 3, the rate-limiting benzylic HAA step via TS5-6 needs to overcome a barrier of at least 32.4 kcal/mol (from singlet 5 via triplet TS5-6), which is considerably higher than that (24.2 kcal/mol) for Co(IV) imido complex case. Interestingly, the net amination reaction energy from 5 to 8, i.e. 13.2 kcal/mol, is even larger than the corresponding reaction energy of 8.3 kcal/mol for Co(IV) imido system, which confirms the intuitively higher oxidation ability of Co(V) than Co(IV) complex. These results clearly demonstrate that it is HAA kinetic factor rather than the reaction thermodynamic driving force that accounts for the missing C-H activation reactivity of 5 in experiment.10 In the qualitatively similar mechanisms for the amination reactions of 1 and 5, both as low-spin states, closed-shell singlet 5 has quite higher HAA barrier (33.1 kcal/mol) than the open-shell doublet 1 (24.2 kcal/mol). However, the intrinsic HAA barriers (measured on one spin state from reactant to transition state) for medium-spin (triplet) and high-spin (quintet) states of 5, both of which are open-shell states, are only around 20 kcal/mol. These relatively low HAA barriers are comparable to the three relatively low HAA barriers on three open-shell spin states in Co(IV) imido system. These results strongly imply that it is openshell radical character of the cobalt-imido species that makes a difference in HAA barrier and hence affects the HAA reac-

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tivity (see the unpaired electron populations on imido-N atom labeled in Figures 1 and 3). The low HAA reactivity of the closed-shell singlet state of Co(V) imido complex 5 is reminiscent of Shaik’s viewpoint that the closed-shell need extra energy to prepare the abstractor for H-abstraction.8 The substantial spin gaps of open-shell states from the ground closed-shell singlet state of 5, as shown in Figure 3, effectively enhance their HAA barriers due to the required spin state transition. Besides the low reactivity of closed-shell singlet state, these large spin gaps constitute the second key factor leading to the missing HAA reactivity of Co(V) imido complex 5.

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imido N atom (see the unpaired electron population shown in Figure 5), this difference in HAA reactivity reinforces that the radical character of cobalt-imido complex impacts cobalt-imido HAA process. After the initial HAA step, the rebound of alkyl radical with the amido N atom proceeds via transition state TS10-11, preferably on medium-spin triplet state. Notably, from alkyl radical intermediate 10, this direct alkyl-N rebound process is more favorable in energy than the alternative pathway involving the cyclometalated alkyl Co(III) as intermediate (see Figure S3 in the SI for more details). This mechanistic preference is different from the Co(IV) imido complex 1.

Figure 4. Electron-shift diagrams of the H-abstraction step in the intramolecular C-H bond activation of 5. In line with the significantly different heights of HAA barriers on low-spin states of 1 and 5, their corresponding electron-shift diagrams on low-spin states are qualitatively different. As shown in Figure 4, during HAA process for the singlet state of 5, the α electron of the cleaved C-H bond transfers to the closed-shell Co(V) atom to form one unpaired α electron in the cobalt d-shell (into the empty dxy orbital). With the unpaired β electron left on the C atom of the cleaved C-H bond, this HAA process simply decouples the pair of electrons in the C-H bond and effectively creates two radicals from the closed-shell singlet state, which is in sharp contrast to the net radical relocation from cobaltimido moiety to the C atom of the cleaved C-H bond in the open-shell doublet state of 1. It is interesting to note that, by transferring the β electron to Co atom to pair up with one unpaired α electron in the open-shell cobalt(V) d-shell (antiEER behavior, electron into dxz for triplet state, and into dyz for quintet state), similar radical relocation picture of electronic structure evolvement as for 1 also holds true for openshell triplet and quintet states of 5. Collectively, for Co(IV) and Co(V) imido complexes 1 and 5, the closed-shell electronic structure of the singlet state of 5 apparently makes a difference in the HAA reactivity. Then, what is the situation with the Co(III) and Co(II) imido complexes? 3.2. Co(III) Imido Complexes The DFT-calculated most favorable pathway for the intramolecular C−H bond amination from Co(III) imido complex 9 is depicted in Figure 5. This mechanism is apparently a HAA-rebound type, with the HAA from the tert-butyl group of the TpR,R’ ligand (TpR,R’ = hydrotris(3-R,5-R’pyrazolyl)borate) via transition state TS9-10 as the ratelimiting step. Importantly, the intrinsic barrier of closedshell singlet state (26.5 kcal/mol) is significantly higher than those of open-shell triplet and quintet states (about 20 kcal/mol). Given the radical character of the H-abstracting

Figure 5. DFT calculated reaction profile of intramolecular amination from 9. Due to the d8 electronic configuration of Co(I), at the product side (11), the quintet state is too high in energy to be involved in the last step of the profile. The key bond distances (R, in Å ) for TS9-10 and unpaired electron population on N (ρN) for 9 are labelled.

Figure 6. DFT calculated reaction profile of intramolecular C-H cyclocobaltation from 12. The key bond distances (R, in Å ) for TS12-13 and unpaired electron population on N (ρN) for 12 are labelled. Similar to Co(III) imido complex 9, as depicted in Figure 6, the intramolecular C-H cyclocobaltation reaction from 12 is initiated from benzylic HAA by imido N atom, which is also the rate-limiting step. Here the closed-shell singlet state of 12 also exihibits lower reactivity than the open-shell triplet and quintet states, with HAA transition state TS12-13 lying 43.6, 31.9, and 30.5 kcal/mol for singlet, triplet, and quintet states, respectively, above the triplet ground state of 12. Hence for both Co(III) imido complexes of 9 and 12 bearing quite

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different geometries and coordinate numbers, open-shell states with radical character are uniformly found to have advantage over the closed-shell state in the HAA process. After the initial HAA step, the attack of alkyl radical to the cobalt center occurs via transition state TS13-14, preferably on low-spin singlet state. Interestingly, different from 9, the C-H activation product from 12 is not an amination one but a cyclometalation one (14), which was calculated to be more stable than the presumed amination product by 38.0 kcal/mol (see Figure S4 in the SI). In addition, compared to 12, the reversed relative stability between cyclometalation and amination species from 9 (see Figure S3a in the SI) indicates that the C-H activation outcomes of Co(III) imido complexes are controlled by thermodynamic stability, which is also the case in Co(IV) imido system as shown in Figure 1. Compared with the Co(V) imido complex 5, a notable difference of Co(III) imido complexes of both 9 and 12 is on the spin state gaps. The closed-shell singlet state in Co(V) case is substantially more stable over other open-shell states, while it is either close to or considerably higher than the other open-shell states in Co(III) cases. This difference effectively opens up accessible HAA channel via open-shell states for Co(III) complexes, which explains why Co(III) imido complexes exhibit HAA reactivity, whereas Co(V) one does not, even though their open-shell states all have advantage over closed-shell one in HAA reactivity.

states of both 9 and 12, which are less reactive than their corresponding open-shell states, exhibit a similar electronic structure evolution as the low-spin singlet state of Co(V) imido complex 5, in that HAA process simply decouples the pair of electrons in the C-H bond and effectively creates two radicals from the closed-shell singlet state. Intriguingly, medium-spin open-shell triplet states of both 9 and 12 hehave differently compared with all the open-shell states of 1 and 5, by having the EER type of electronic structure evolution with increasing number of the unpaired electrons on cobalt center. This distinctive electron-shift picture of Co(III) imido complexes different from Co(IV)/Co(V) ones implies that the electronic structure evolvement for cobaltimido complexes in HAA process is not only determined by the identity of the metal, but also affected by its oxidation state. We will see below that Co(III) imido complexes also differ from the lower Co(II) oxidation state in electronic evolution of HAA process. 3.3. Co(II) Imido Complex

Figure 9. DFT calculated reaction profile of intramolecular C-H cyclocobaltation from 15. The key bond distances (R, in Å ) for TS16-17 and unpaired electron population on N (ρN) for 16 are labelled.

Figure 7. Electron-shift diagrams of the H-abstraction step in the intramolecular C-H bond activation of 9.

Figure 8. Electron-shift diagrams of the H-abstraction step in the intramolecular C-H bond activation of 12. Concerning the electronic structure evolvement for the rate-limiting HAA step from Co(III) imido complexes 9 and 12, the electron-shift diagrams are shown in Figures 7 and 8, respectively. Despite both involving more complicted twoelectron shift, closed-shell singlet state and open-shell triplet state have different pictures in electronic structure evolution in HAA step. Apparently, the closed-shell low-spin singlet

For the two coordinate Co(II) imido complex 15, the most favorable intramolecular C-H cyclocobaltation reaction pathway from our calculations is shown in Figure 9. In line with the corresponding X-ray crystal structure of the cyclometallation product,13 we found that a η6-coordination of Mes phenyl ring of Dmp group to the Co(II) center is indispensible in the C-H activation reaction process, possibly due to the low coordinate number of 15. Interestingly, this phenyl π-ligation reverses the energy order of high-spin/lowspin Co(II) states in 15, and leads to a low-spin doublet ground state for 16. As shown in Figure 9, the rate-limiting step is benzylic HAA by imido N atom from the Mes group, with a relatively small barrier of 12.8 kcal/mol via transition state TS16-17 on the favorable low-spin doublet state. Similar to Co(IV) imido complex 1, the open-shell low-spin doublet state again provides an efficent HAA reaction channel. Following the initial HAA, benzylic radical attacks the cobalt center via TS17-18, preferably on high-spin quartet state with quite low barrier. Importantly, compared with the HAA pathway shown in Figure 9, our calculations do not support the previously proposed concerted [2π+2σ]-addition pathway13 for C-H activation by Co(II) imido species (see Figure S6 in the SI for details). In addition, our calculations indicate that the pathway featured by C-H activation via oxidative addition mechanism (see Figure S7 in the SI) is also

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unfavored in energy, despite the lowest central metal oxidtion state of Co(II) under study. Similar with the case of Co(III) imido complex 12, the cyclocobaltation reaction outcome rather than the amination one from 15 can be attributed to the instability of the corresponding amination product relative to cyclocobaltation one (18) by 12.9 kcal/mol (see Figure S5 in the SI). The electron-shift diagrams of the rate-limiting HAA process from Co(II) imido complex 16 are shown in Figure 10. It is indicative of an anti-EER type of electronic structure evolution with decreasing number of the unpaired electrons on cobalt center, similar to all open-shell electronic states of Co(IV) and Co(V) imido complexes 1 and 5, but different from the open-shell triplet state of Co(III) imido complexes 9 and 12. Combined with the previous EER type of electronic structure evolution in HAA by Fe(IV) imido complex,21 all these results imply that both oxidation state and metal identity would affect the electronic structure evolution behavior (EER or anti-EER) in HAA process in metal-imido complex.

Figure 10. Electron-shift diagrams of the H-abstraction step in the intramolecular C-H bond activation of 15. Summarizing all the cobalt imido complexes, their higher HAA reactivity of open-shell states indicates that radical character of the imido N atom in cobalt-imido complex is important in their HAA process. This turns out to be in line with Shaik’s viewpoint that closed-shell molecule in the normal HAA event will require a high barrier. Considering also Zhang and de Bruin’s Co(III)-‘nitrene-radical’ intermediates ((Por)CoIII-N•Y) supported by porphyrin type ligand in C-H amination, which was found to bear the unpaired spin density almost entirely on the nitrogen atom of the nitrene moiety,9 HAA seems to favor a N radical center in cobaltimido type of systems. Alternatively, it is notable that our calculations for C-H activation step in cobalt-imido systems under study do not support the electron transfer/proton transfer character as the crucial feature of the C-H activation, since there is no correlation between the charge population and the barrier height including all the HAA transition states (see Table S1 in the SI for details). This absence of the correlation also corroborates the experimental fact that the Co(V) imido complex 5, which is more oxidative than the Co(IV) imido complex 1, is less reactive in the C-H activation reaction. In total, we did not find any evidence in our calculations to support the alternative C-H activation mechanisms featured by electron transfer, such as proton-coupled electron transfer, or electron transfer followed by proton transfer.

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4. CONCLUSIONS Capable of activating C-H bond in C-H amination reaction, a myriad of cobalt-imido complexes with very rich coordination structures and a dazzling variety of metal oxidation states have recently emerged as a new frontier of metalimido chemistry. Discerning the factors that control their reactivity, however, remains a territory open for exploration. To this end, we herein systematically explored the C-H activation mechanisms of typical experimentally well-defined cobalt-imido complexes by employing computational approach. Associated with the reaction mechanism, the complicated electronic structures in the C-H activation processes by cobalt-imido complexes were also scrutinized to provide insights for the reactivity. Our key discoveries are as follows: (a) The HAA process is found to be the rate-limiting step in all the cobalt-imido systems under study, in which the open-shell states consistently have kinetic advantage over the closed-shell one. Inherent in the open-shell electronic structure, the radical character of the imido N atom plays a very crucial role to render an energetically more accessible HAA channel. Alternative C-H activation mechanisms such as [2π+2σ]-addition and oxidative addition are not supported by our calculations. The current results for the cobalt-imido complexes imply that, in addition to the well-investigated HAA chemistry of metal-oxo’s, the HAA promoted by metal-imido species could also be subject to the radical-facilitated reactivity. (b) Besides the low reactivity of closed-shell singlet state, the large spin gaps between closed-shell and open-shell states constitute the second key factor contributing to the missing HAA reactivity of Co(V) imido complex. Based on the factors controlling the reactivity revealed in this work, it is predictable that the stabilization of less reactive closed-shell singlet state relative to other more reactive open-shell states is generally not beneficial to the HAA reactivity of cobalt-imido species. Surprisingly, there is no correlation found between the formal oxidation state of cobalt and the HAA reactivity, which means that higher cobalt oxidation state is not necessarily of higher reactivity in HAA. (c) During the HAA process, the dichotomous EER/antiEER electron-shift scenarios in open-shell electronic structure depend on the cobalt oxidation states, in which Co(III) makes a difference from other ones from Co(II) to Co(V). Furthermore, compared with previously investigated iron-imido system, it seems likely that a paradigm transition of EER to anti-EER scenario for electronic structure evolution may exist in periodic table from Fe to Co for HAA promoted by metal-imido species. (d) Distinct from the kinetic factor that determines the reactivity of the initial rate-limiting HAA step, the reaction outcomes of C-H activation (amination or cyclometalation product) in all the cobalt-imido complexes are found to be controlled by the thermodynamic stability of the products. In view of the paucity of computational work for cobaltimido species, certainly more research work needs to be done to culminate in more general conclusions concerning these intriguing mechanistic and reactivity issues, which is crucial to further expand our understanding of chemistry in the metal-imido systems.

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ACS Catalysis

AUTHOR INFORMATION Corresponding Author [email protected]

ASSOCIATED CONTENT Supporting Information Seven figures and one table of supplementary computational results, Computational details, Cartesian coordinates of all the optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC, Nos. 21290194, 21521062, and 21473215), and Institute of Chemistry, Chinese Academy of Sciences.

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