A Missing Piece of the Mechanism in Metal-Catalyzed Hydrogenation

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

A Missing Piece of the Mechanism in Metal-Catalyzed Hydrogenation: Co(−I)/Co(0)/Co(+I) Catalytic Cycle for Co(−I)Catalyzed Hydrogenation Song-Bai Wu,†,‡,∥ Tonghuan Zhang,†,‡,∥ Lung Wa Chung,*,‡ and Yun-Dong Wu†,§ Org. Lett. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 01/02/19. For personal use only.

†

Lab of Computational Chemistry and Drug Design, State Key Laboratory of Chemical Oncogenomics, Peking University Shenzhen Graduate School, Shenzhen 518055, China ‡ Department of Chemistry and Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055, China § College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: Hydrogenation catalyzed by unusually low-valent Co(−I) and Fe(−I) catalysts were recently reported. In contrast to the classical M(I)/M(III) (M = Rh or Ir) or Ir(III)/Ir(V) catalytic cycles in the singlet state (adiabatic reactions) for Rh- or Ir-catalyzed hydrogenation, our systematic DFT study elucidates a new Co(−I)/Co(0)/Co(+I) catalytic cycle involving both singlet and triplet states (nonadiabatic reaction). Also, the more electronrich cobalt center of the Co(−I) catalyst was found to contribute higher reactivity for alkene hydrogenation.

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Scheme 1. (a) Alkene Hydrogenations Catalyzed by [M(−I)L2]− Catalysts (M = Co (CAT1) or Fe (CAT2), L = Anthracene) and (b) Ketone Hydrogenations Catalyzed by CAT1

ydrogenation is one of the most important reactions due to its wide industrial applications.1 Precious metals, such as Pd, Pt, Rh, Ir, and Ru, have been commonly applied as the catalysts. 1 Driven by the principles of economy and sustainability, earth-abundant transition-metal catalysts2 have been attracting considerable attention from many chemists. Recently, some important advances in such hydrogenations have been achieved.3 Also, cobalt- and iron-catalyzed hydrogenations with well-defined ligands (such as pincer type) have been developed.4 Unlike most of Co and Fe catalysts with common oxidation states of +I or +II,2−4 Wolf, Wangelin and co-workers synthesized and applied cobaltate(−I) CAT1 and ferrate(−I) CAT2 complexes ([CoL2]− and [FeL2]−, L = anthracene) with a very low oxidation state (−I) for hydrogenation of alkenes, ketones (Scheme 1) and imines.5a A large excess of π-acidic ligands was proposed to facilitate ligand exchange and prolong the catalyst activity.5 Moreover, the Co(−I) catalyst was more reactive than the Fe(−I) catalyst in the alkene hydrogenation. Also, milder conditions were applied to the alkene hydrogenation than the ketone hydrogenation (Scheme 1). Computational chemistry has played an important role in elucidating the mechanisms of hydrogenation.6 For instance, the mechanism of hydrogenation using Wilkinson’s7 catalyst was supported by Morokuma’s studies.8 Mechanisms of hydrogenation catalyzed by first-row transition metals are more challenging9 due to the complex electronic structures © XXXX American Chemical Society

involved. In this letter, our extensive DFT (B3LYP-D3) study elucidates a unique mechanism for the Co(−I)- and Fe(−I)catalyzed hydrogenation which involves an unprecedented Co(−I)/Co(0)/Co(+I) catalytic cycle as well as both singlet and triplet states (with spin crossing). It is dramatically different from the classical Rh(I)/Rh(III) catalytic cycle for Rh(I) catalysts in a singlet state (adiabatic reaction). First, our DFT results suggested that the electronic ground state of the Co(−I) catalyst is a closed-shell singlet state Received: October 30, 2018

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DOI: 10.1021/acs.orglett.8b03463 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

Figure 1. Free energy profile of the most favorable pathway of the Co(−I)-catalyzed propene hydrogenation in different low-lying spin states (ΔEMECP1 = 1.5 kcal/mol; ΔE16Ad‑endo = 0.0 kcal/mol). The key distances are given in Å.

(1CAT1), which was lower in free energy than the open-shell singlet, triplet, and quintet10 states by ∼0.7, 1.9, and 12.4 kcal/ mol, respectively (Figure 1). When a propene molecule coordinates to CAT1, open-shell complexes OS11Aa‑exo and 3 1Aa‑exo are formed, and their formation is slightly exergonic by about 0.3−1.1 kcal/mol. Moreover, 1CAT1 or intermediates before oxidative addition (e.g., 11Aa‑exo) in the closed-shell singlet state formally possesses a 3d10 configuration (i.e., Co(−I), Scheme 2). Whereas, our spin density (ρ) analysis

charge-transfer state). These results revealed an unrecognized equilibrium between closed-shell 1CAT1 and open-shell diradical Co(0) intermediates before oxidative addition. Likewise, doublet Fe(−I) catalyst 2CAT2 is the most stable state and more stable than the quartet and sextet10 states with a formal Fe(0) form by 1.2 and 12.1 kcal/mol, respectively (Figure S3). In addition, the most favorable pathways catalyzed by CAT1 and CAT2 were very similar: (1) displacement of one anthracene by one substrate molecule and coordination of one H2; (2) oxidative addition of H2 to give metal(I)-dihydride species; (3) migratory insertion to give a metal(I)-alkyl (or metal(I)-alkoxyl) intermediate; (4) recoordination of an anthracene ligand followed by C−H forming reductive elimination to produce the alkane product for the alkene case (or followed by coordination of another H2, σ-bond metathesis to form the alcohol product and H−H forming reductive elimination after one anthracene coordination for the ketone case) and regenerate the catalyst (Scheme 3). As shown in Figure 1, the propene hydrogenation catalyzed by CAT1 is preferentially initiated by the coordination of propene to 1CAT1 to form 1Aa‑exo ([CoL2(alkene)]−) followed by dissociation of one anthracene ligand to form 2Ad ([CoL(alkene)]−) and coordination of H2 to give reactive intermediate 3Ad‑endo ([CoL(H2)(alkene)]−). Afterward, Co(−I) dihydrogen intermediate 13Ad‑endo undergoes facile oxidative addition to form slightly more stable Co(I) dihydride ([CoL(H)2(alkene)]−) intermediate 14Ad‑endo.11 Then, the rate-determining alkene insertion takes place by overcoming a barrier of ∼22.0 kcal/mol above OS11Aa‑exo and affords agostic Co(I)-alkyl intermediate 15Ad‑endo. Subsequent facile rearrangement forms a more stable Co(I)-alkyl isomer 16Ad‑endo without the agostic interaction. As a result, the formation of 1 6Ad‑endo from 14Ad‑endo was slightly exergonic by 1.8 kcal/mol. Interestingly, the reaction pathway in the triplet state is

Scheme 2. Schematic Electronic Configurations of CAT1 and Key Intermediates in Their Singlet and Triplet States

suggested that OS1CAT1, 3CAT1, OS11Aa‑exo, and 31Aa‑exo have about nine 3d electrons on the metal (ρ(Co): ∼1.28−1.55) and about one unpaired electron in a π* orbital on the ligands (ρ(ligands): −1.28 (OS1CAT1 and OS11Aa‑exo), 0.45 (3CAT1) and 0.55 (31Aa‑exo)), implying a formal Co(0) metal character with the monoanionic ligand (analogous to a metal−ligand B


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Organic Letters Scheme 3. Our Proposed Mechanism for the Co(−I)- and Fe(−I)-Catalyzed Hydrogenations with the Formal Oxidation States of the Metala

Table 1. Computed Overall Free Energy Barriers (in kcal/ mol) of the Key Steps for Four Systems CAT1 + propene

CAT2 + propene

CAT1 + acetone

CAT2 + acetone

TSOAa TSMIa TSREa

17.4b 22.0b 14.6d,g

22.6e 25.4e 18.1f,g

23.2c,j

TSSMa

−

−

23.7f 19.2f 36.9f,h 43.8f,i 29.5f

27.6d,h 32.3d,i 23.9d

a

OA: oxidative addition. MI: migratory insertion. RE: reductive elimination. SM: σ-bond metathesis. bClosed-shell singlet. cOpenshell singlet. dTriplet. eDoublet. fQuartet. gC−H forming RE. hH−H forming RE. iO−H forming RE. jA concerted transition state of OA with MI.

for the observed higher reactivities for CAT1 over CAT2.5a The higher barrier for the Fe catalyst can be attributed to the lower stability of its reactive low-spin metal-dihydride intermediate ([ML(H)2(alkene)]−; ΔG = 19.0 (M = Fe) vs 13.6 (M = Co) kcal/mol). Notably, the Mulliken charge of the metal in the M(−I) catalyst (q(Fe): 0.40; q(Co): 0.30) and the metal-dihydrogen intermediate (q(Fe) in 23Cd‑endo: 0.12; q(Co) in 13Ad‑endo: 0.05, see Scheme 4) reflects the more Scheme 4. Proposed Schematic Role of the Acidic πLigandsa a

The computed free energies (in kcal/mol) of the propene (top) and acetone (bottom) hydrogenation with the Co catalyst in the lowestenergy spin state are given.

generally quite similar in energy to that in the closed-shell singlet state and could also be competitive, but the alkene insertion in the triplet state requires a higher barrier (28.4 kcal/mol). After the migratory insertion, 16Ad‑endo ([CoL(H)(alkyl)]−) can recoordinate with an anthracene ligand to form 17Aa‑exo ([CoL2(H)(alkyl)]−)12 and undergo reductive elimination to form the stable Co(−I) alkane product (18Ad‑exo, [CoL2(alkane)]−) with a barrier of 14.1 kcal/mol relative to OS1 1Aa‑exo. Alternatively, 16Ad‑endo can first transform into its more stable triplet counterpart (36Ad‑endo) via a low-energy crossing point (MECP1, ΔΔE(MECP1 − 16Ad‑endo) = 1.5 kcal/ mol). Subsequently, the recoordination of an anthracene ligand to give 37Aa‑exo followed by the reductive elimination occurs to form stable Co(0) alkane product 38Aa‑exo by overcoming a barrier of 14.6 kcal/mol above 37Aa‑exo. Then, dissociation of the propane product regenerates the catalyst. Overall, our calculations show that the reaction catalyzed by CAT1 is highly exergonic by ∼23.4 kcal/mol, and the largest barrier (∼22.0 kcal/mol) is involved in the migratory insertion step. Furthermore, singlet and triplet states were found to be involved in the Co(−I)-catalyzed alkene hydrogenation for the first time. Analogous to the cobalt case, the propene hydrogenation catalyzed by CAT2 follows the same pathway (Scheme 3 and Figure S14). Again, the rate-determining step is migratory insertion from Fe(−I)-dihydride intermediate [FeL(H)2(alkene)]− to form an agostic five-coordinate Fe(I)-alkyl intermediate with a barrier of 25.4 kcal/mol (Table 1). This barrier for the Fe catalyst is 3.4 kcal/mol higher than that for the Co catalyst (22.0 kcal/mol), which qualitatively accounts

a

The Mulliken charge (q) of the metal and ligands are given.

electron-deficient nature of the iron center, which should disfavor the formation of the key low-spin oxidative-addition Fe(I)-dihydride intermediate and be a key factor in its lower reactivity. The electron-deficient iron center should be resulted from more back-donation (or charge transfer) from the less electronegative iron center to the acidic π-ligands: a more negative charge on the π-ligands (Scheme 4). Furthermore, two different spin states (doublet and quartet states) as well as three different oxidation states of the Fe metal (Fe(−I), Fe(0) and Fe(+I)) could also be involved in the Fe(−I)-catalyzed alkene hydrogenation reaction. The most favorable pathway of the acetone hydrogenation catalyzed by CAT1 is generally analogous to the abovediscussed propene hydrogenation except for the last process (Scheme 3). For instance, coordination of one acetone to CAT1 to form open-shell diradical intermediates OS11Ba‑exo (ΔG = −0.5 kcal/mol) and 31Ba‑exo (ΔG = 0.0 kcal/mol, Figure S15) is possible. These results suggest an equilibrium between closed-shell Co(−I) catalyst CAT1 and an open-shell Co(0)-acetone intermediate. However, an exergonic and concerted oxidative addition/migratory insertion step in the open-shell singlet state was found to preferentially operate to afford a Co(I)-alkoxyl diradical intermediate by overcoming a barrier of ∼23.2 kcal/mol, which is slightly higher than that for the propene hydrogenation (22.0 kcal/mol). Whereas, the insertion barriers in the triplet and closed-shell singlet states C


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are higher than that in the open-shell singlet state by 0.8−4.3 kcal/mol. Owing to a very high barrier for the O−H bond-forming reductive elimination step (32.3 kcal/mol for CAT1, Table 1),13 our calculations show a divergent pathway for the last process in which the σ-bond metathesis with one additional H2 to form a Co(I)-dihydride isopropanol intermediate followed by release of an isopropanol product, recoordination of the anthracene ligand and H−H bond-forming reductive elimination is the most favorable pathway. The H−H bond-forming reductive elimination step of this divergent pathway was computed to be the rate-determining step with a barrier of ∼27.6 kcal/mol (Table 1). Finally, the subsequent H2 dissociation regenerates the catalyst. Contrary to the exergonic final process in the propene hydrogenation, the final steps for the acetone hydrogenation were computed to be thermodynamically endergonic by 9.1−11.0 kcal/mol. In short, the higher barrier for the reductive elimination step (27.6 kcal/ mol) and the unfavorable thermodynamics should account for the lower observed reactivity of the Co(−I)-catalyzed acetone hydrogenation. Moreover, our calculations further suggest a much higher barrier (36.9 kcal/mol, see Table 1) for the acetone hydrogenation catalyzed by CAT2 than CAT1, mainly due to the very stable quartet Fe(I)-alkoxyl intermediate ([FeL(H)(alkoxyl)]−, ΔG = −17.1 kcal/mol). Furthermore, the basic oxygen’s lone pair in the metal-alkoxyl intermediate can facilitate the metal-mediated H−H heterolytic cleavage and, thus, change the mechanistic pathway in the acetone hydrogenation. Overall, our study suggests a mechanism involving three different oxidation states of the metal (M(−I), M(0), and M(+I)) and at least two different spin states (Scheme 3). Such a mechanism is different from the traditional catalytic cycles of M(I)/M(III) (M = Rh or Ir) and Ir(III)/Ir(V) for Rh(I), Ir(I), and Ir(III) catalysts, which involve only closed-shell singlet states.7,8,14 Previously, a π-acidic anthracene ligand was proposed to stabilize the catalyst.5a Additionally, our study further demonstrates the importance of the flexible coordination of an acidic anthracene ligand: the dissociation of one anthracene ligand (i.e., the more electron-rich and spacious metal coordination) facilitates the oxidative addition and migratory insertion steps (plus σ-bond metathesis step for the ketone system), while the subsequent recoordination of an acidic anthracene ligand (i.e., the more electron-poor and congested metal coordination) promotes the reductive elimination step. In summary, to the best of our knowledge, our DFT study suggested a novel mechanism involving an unprecedented M(−I)/M(0)/M(+I) (M = Co and Fe) catalytic cycle (Scheme 3) and at least two different spin states (nonadiabatic reaction with spin crossing)15 for the Co(−I)- and Fe(−I)catalyzed hydrogenation of alkenes and ketones for the first time. The more reactive Co(−I) catalyst can be attributed to the higher stability of the reactive low-spin Co(I)-dihydride intermediate. Moreover, flexible coordination modes of the acidic ligands can facilitate the key elementary steps. Our current study provides new mechanistic insights into hydrogenation, which could be helpful for designing next-generation catalysts for hydrogenations.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03463. Computational details, results and discussion (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.W.C.) ORCID

Lung Wa Chung: 0000-0001-9460-7812 Author Contributions ∥

S.-B.W. and T.Z. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is dedicated to Professor Chen-Ho Tung on the occasion of his 80th birthday. We are thankful for the financial support from the National Natural Science Foundation of China (21672096), SUSTech, and the Shenzhen Nobel Prize Scientists Laboratory Project (C17783101).

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A Missing Piece of the Mechanism in Metal-Catalyzed Hydrogenation

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