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The Mutable Properties of Nonheme Iron(III)-Iodosylarene Complexes Result in the Elusive Multiple-Oxidant Mechanism Yiran Kang, Xiao-Xi Li, Kyung-Bin Cho, Wei Sun, Chungu Xia, Wonwoo Nam, and Yong Wang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 19, 2017
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The Mutable Properties of Nonheme Iron(III)-Iodosylarene Complexes Result in the Elusive Multiple-Oxidant Mechanism Yiran Kang,†,‡,# Xiao-Xi Li,†,# Kyung-Bin Cho,§ Wei Sun,† Chungu Xia,† Wonwoo Nam*,§ and Yong Wang*,† †
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea Supporting Information Placeholder ABSTRACT: Although nonheme iron(III)-iodosylarene complexes present amazing oxidative efficiency and selectivity, the nature of such complexes and related oxidation mechanism are still unsolved after decades of experimental efforts. Density functional calculations were employed to explore the structurereactivity relationship of the iron(III)-iodosylbenzene complex, [FeIII(tpena-)(PhIO)]2+ (1), in thioanisole sulfoxidation. Our theoretical work revealed that complex 1 can evolve into two resonance valence-bond electronic structures (a high-valent iron– oxo species and a monomeric PhIO species) in thioanisole sulfoxidation to present different reaction mechanisms (the novel bond-cleavage coupled electron transfer mechanism or the direct oxygen-atom transfer mechanism) as a response to different substrate attack orientations.
Scheme 1. The schematic plots of a) the zigzag structure of iodosylbenzene polymer, b) the one-oxidant mechanism and c) the multiple-oxidant mechanism
I
odosylarenes are considered as the most versatile and efficient terminal oxidants used in organic synthesis, biological and biomimetic oxidation reactions.1 Generally, they are insoluble amorphous powder, in a linear polymeric, asymmetrically bridged zigzag structure (Scheme 1a).2 Catalysts (metal complexes, Lewis or Brønsted acids, etc.) are usually employed to make them soluble before introducing iodosylarenes into the catalytic systems.3 According to the consensus one-oxidant mechanism (Scheme 1b),4 once iodosylarenes are mixed with metal complexes, metaliodosylarene adducts are formed. Subsequently, these complexes are converted to high-valent metal-oxo species, which are believed to act as the oxidants to perform various oxidations, such as C-H hydroxylation, C=C epoxidation, sulfoxidation, etc. For instance, Groves and co-workers reported that when an iron(III) porphyrin complex was mixed with iodosylbenzene to oxidize substrates, alkenes were preferentially oxidized to the corresponding epoxides.4a Similarly, alcohols were obtained as major products in alkane hydroxylation.4a The oxo-iron(IV) porphyrin cation radical complex was therefore proposed as the sole oxidant to do those oxidations via the well-known “oxygen-rebound” mechanism.5 Combined with Shaik’s “two-state reactivity” (TSR) mechanism,6 most mechanistic problems in P450 chemistry and related biomimetic nonheme chemistry could be well explained and solved with the one-oxidant mechanism, which makes it popular.7 However, in the 1990s, Valentine and co-workers reported that olefin epoxidation by iodosylbenzene could be significantly promoted by redox-innocent metal ions, such as zinc(II) and
aluminum(III), in which the metal-iodosylbenzene complexes cannot be converted to high-valent metal-oxo oxidants.8 This experimental observation casts the one-oxidant mechanism into doubt. An electrophilic iodine(III)-attacking mechanism, using the metal-iodosylbenzene complex instead of the high-valent metaloxo species as the oxidant, was postulated. In 2000, Collman, Brauman and co-workers carried out competitive alkane hydroxylation with various iodosylarenes in an iron porphyrin system and found that the reaction rate ratios for each pair of substrates varied when different iodosylarenes were employed as terminal oxidants.9 Further, Nam and co-workers reported that iron(III)-iodosylbenzene species could reversibly convert to high-valent iron-oxo complex and iodobenzene,10 unambiguously demonstrating that at least one more oxidant is formed besides the high-valent metal-oxo oxidant. Thus, a multiple-oxidant mechanism (Scheme 1c) was proposed.9,10 According to this mechanism, if the O-I bond cleavage occurs fast, then the metal-iodosylarene complex will convert to a high-valent metaloxo oxidant to oxidize substrates. On the other hand, if the O-I bond cleavage is slow and the metal-iodosylarene complex has enough oxidizing power, then the metal-iodosylarene complex becomes the oxidant. To explore the elusive nature of metaliodosylarene complexes, extensive experimental efforts using
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competitive reaction kinetics and various spectroscopic methods (e.g. ESI-MS, UV-vis, rRaman, X-ray diffraction etc.) were performed.11 However, despite these efforts, the mechanistic mystery of the structure-reactivity relationship of the metal-iodosylarene complex remains elusive. Herein, based on the crystal structure of a nonheme iron(III)iodosylarene complex , [FeIII(tpena-)OIPh]2+ (1, tpena- = N,N,N'tris(2-pyridylmethyl)ethylendiamine-N'-acetate), reported by McKenzie and co-workers,11b we have performed for the first time a comprehensive theoretical study on the conversion of 1 and its reaction with thioanisole. All calculations were performed in the acetonitrile solvent using the conductor-like polarizable continuum model. The benchmark on the reliability and stability of the employed theoretical methods is presented in Supporting Information. Surprisingly, we found that during the substrate approaches, the iron(III)-iodosylarene complex 1 evolves into two resonance valence-bond structures (a high-valent iron-oxo species and a monomeric iodosylbenzene species). These species then oxidize thioanisole to the corresponding product. The conversion of 1 to a high-valent iron-oxo species 2 was studied at the UB3LYP/lanl2dz(Fe)-lanl2dzdp(I)-6-31+G*(rest) computational level (Figure 1). For 1, the ground state is a highspin sextet state (S = 5/2). The excited quartet/doublet spin states lie at 14.3/21.7 kcal mol-1 higher. In 61, the average Fe-N distance is 2.293 Å, the Fe-O(carboxylate) distance is 2.080 Å, and the FeO(PhIO) distance is 2.001 Å (Figure S2). Thus, the iron core is in a heptacoordinate state, which is consistent with McKenzie’s experimental results.11b During O-I bond elongation, spin reversion occurs and the reaction path switches from the sextet to the doublet spin state (S = 1/2) on the transition state; therefore, it is a two-state reactivity (TSR) process.6 The lowest activation energy is 24.5 kcal mol-1. In 2TS12, the Fe-O distance is 1.630 Å, the O-I distance is 2.756 Å and the Fe-O(carboxylate) distance is 3.860 Å. Therefore, the iron core becomes hexacoordinated. The formed hexacoordinated iron-oxo complex 2 on the quartet ground state lies 18.1 kcal mol-1 higher than 1. In 2, the spin of the Fe=O moiety is ca. 2.3 and the PhI moiety is ca. 0.6, thus the O-I bond cleavage is a homolysis process to form an iron(IV)(O)(tpena-)(PhI cation radical) species 2 (Table S4). Interestingly, further enlogation of the O-I distance from 4.7 Å to 7.7 Å in 2 triggers an intermolecular electron transfer from the tpena- ligand to the PhI to form an iron(IV)(O)(tpena radical)(PhI) species 3 (Figure S2 and Table S4), which lies 8.6 kcal mol-1 higher than 2. In short, the conversion of 1 to the high-valent iron-oxo species 2 and 3 is both kinetically and thermodynamically difficult. When the substrate thioanisole is introduced in the reaction system, we found two orientation modes of the reagent complex (Figure 2). In the halogen-bonding mode RC (Figure 2a), thioanisole lies trans to the oxygen of 1. The S-I-O angle is nearly 180° and the phenyl ring of PhIO forms a T-shape with this linear S-IO moiety. In the other normal substrate attack mode RC' (Figure 2b), thioanisole is oriented toward the oxygen of 1. RC is more stable than RC', by 4.1 kcal mol-1. For the halogen-bonding case, surprisingly, the activation barrier of O-I bond cleavage dramatically decreases to 19.4 kcal mol-1 (Figure 3), compared to that of O-I cleavage without the substrate (24.5 kcal mol-1 in Figure 1). In 6 TSBCCET, the O-I distance is 2.628 Å and the Fe-O distance is 1.659 Å. One pyridine ligand decoordinates from the iron core (the Fe-N distance is 4.325 Å), thereby generating a hexacoordinate complex. The halogen bond interaction between the sulfur atom of thioanisole and the leaving iodine atom still persists with an S-I distance of 3.184 Å. Molecular orbital analysis also shows the strong conjugation interaction between the PhSMe--PhI-O=Fe triad (Figure S8), which stabilizes the electronic structure to lower the reaction energy barrier. Inspection of the electronic
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I N
I O
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FeIII N
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e O
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2.756 1.630
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18.1
2 Fe-N = 2.051 Fe-O-I = 143.1 υimg = i 32
Figure 1. Energy profiles for the conversion of the ferriciodosylbenzene complex 1 to the iron(IV)(O)(tpena radical)(PhI) species 3 via the iron(IV)(O)(tpena-)(PhI cation raidcal) species 2. The key geometric information of the transition state 2TS12 is presented. Hydrogen atoms are omitted for clarity. Energies are in kcal mol-1 units, lengthes are in Å units, angles are in degree units, and the imaginary frequency is in cm-1 unit.
Figure 2. Geometric information of a) the halogen-bonding reagent complex RC and b) the normal reagent complex RC'. Hydrogen atoms are omitted for clarity. Lengths are in Å units, angles are in degree units. structure of TSBCCET and IM reveals that during the O-I bond cleavage, an intermolecular electron transfer from thioanisole to 1 occurs to form an iron(IV)-oxo species with an one-electron oxidized thioanisole cation radical. Thus, this step is a novel halogenbond promoted electron transfer process. The lowerest energy state of the intermediate IM is now a degenerated doublet/quartet pair. For 2/4IM, The Fe-O distance is 1.624 Å, and the iron is in a hexacoordinate state with one pyridine free. Subsequently, 2IM undergoes a transition state 2TSOT with a tiny barrier of 0.7 kcal mol-1, and transfers the oxygen to the one-electron oxidized thioanisole cation radical to yield the iron(III)(tpena-) complex and sulfoxide as products. At the product complex state, the free pyridine ligand recoordinates to the iron core and generates the sixcoordinate iron(III)(tpena-) complex in preparation for the next catalytic cycle. In short, the reaction proceeds stepwise via an initially concerted but asynchronous process of halogen-bondpromoted O-I bond cleavage, coupled to an electron transfer from the substrate thioanisole to the iron-oxo moiety. This is followed
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Figure 3. Energy profiles for thioanisole sulfoxidation via the bondcleavage-coupled electron transfer mechanism. Key geometric information of transition states 6TSBCCET and 2TSOT is presented. Hydrogen atoms are omitted for clarity. Energies are in kcal mol-1 units, lengths are in Å units, angles are in degree units, and imaginary frequencies are in cm-1 units.
by an oxygen rebound step to form the sulfoxide product. Despite the role of iodine, this mechanism is not the same as Valentine’s mechanism8b which was deemed not possible in this reaction due to a large barrier (> 40 kcal mol-1, Figure S17). Such a novel intermolecular electron transfer event is similar to the metal-ion coupled electron transfer (MCET) mechanism proposed by Nam, Fukuzumi and co-workers in the thioanisole sulfoxidation by the Sc3+-binding nonheme iron(IV)-oxo complexes12 and we term this mechanism as the bond-cleavage coupled electron transfer (BCCET) mechanism. For thioanisole sulfoxidation by the heme/nonheme high-valent metal-oxo species, the widely accepted mechanism is the direct oxygen-atom transfer (DOT) mechanism.13 Thus, thioanisole sulfoxidation by 1 via the DOT mechanism was investigated. As shown in Figure 4, for the normal reagent complex (RC' in Figure 2), the activation energy barrier of the DOT process is 18.7 kcal mol-1, which is competing with that of the BCCET process (19.4 kcal mol-1, taking the halogen-bonding reaction complex RC as the reference point). Investigation of the geometry of 6TSDOT shows that the Fe-O bond is 3.097 Å, the O-I bond is 1.886 Å, and the O-S bond is 3.003 Å. This TS geometry demonstrates that the direct DOT process proceeds in a concerted way, i.e., initially substrate-induced iodosylbenzene deligation from the iron(III) core to some extent, then this iodosylbenzene transfers its oxygen atom to thioanisole to form sulfoxide. This indicates that the monomeric iodosylbenzene may be a potent oxidant. To investigate this, the thioanisole sulfoxidation oxidized by one iodosylbenzene molecule was investigated. The activation barrier is only 11.0 kcal mol-1, thus the iodosylbenzene monomer is a robust oxidant, which is consistent with previous reports.1,14 However, after taking the PhIO-Fe bond dissociation energy into account (16.0 kcal mol-1, Table S13), the overall barrier of such stepwise PhIO-deligation/sulfoxidation process is 27.0 kcal mol-1, which is much higher than that of the concerted process (18.7 kcal mol-1).
Figure 4. Energy profiles for thioanisole sulfoxidation via the direct oxygen-atom transfer mechanism. The top-right inset shows the energy profile for thioanisole sulfoxidation by monomeric PhIO. Key geometric information of transition states is presented. Hydrogen atoms are omitted for clarity. Energies are in kcal mol-1 units, lengths are in Å units, angles are in degree units, and imaginary frequencies are in cm-1 units.
Based on our theoretical findings in addition to earlier postulated multiple-oxidant mechanism (Scheme 1c), we propose the following scenario (Scheme 2). After mixing the metal catalysts with the iodosylarene polymer, a metal-iodosylarene complex 1 is formed. If the O-I bond cleavage is faster, the metal-iodosylarene complex preferentially self-decays to form a high-valent metaloxo species 2 (oxidant 1) to oxidize the substrate to form the product. If the O-I bond cleavage is slower and the oxidizing power of the metal-iodosylarene complex is robust, the metaliodosylarene complex 1 acts as the oxidant (oxidant 2). It will evolve into two resonance valence-bond structures (a high-valent metal–oxo species and a monomeric PhIO species shown in Scheme 2)15 as a response to different substrate orientation. If the substrate forms a halogen-bond with 1, 1 will evolve into the
Scheme 2. The proposed catalytic cycle of oxidation involving the metal-iodosylarene complexes
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high-valent iron-oxo species to trigger an international electron transfer from the substrate to the high-valent metal-oxo species (e.g., the BCCET process in Figure 3). Thus, the oxidation occurs in a stepwise fashion; If on the other hand the substrate attacks directly the oxygen of 1, then 1 will evolve into the monomeric PhIO species, which will directly transfer the oxygen atom to the substrate. Thus, this process is concerted. After the oxidation, the metal catalyst is regenerated and joins in the next catalytic cycle. Obviously, the substrate-induced process in Scheme 2 is more interesting, because we can potentially modulate the nature of the metal-iodosylarene complex to generate the specific oxidation species at will. For instance, we can employ more electron-rich nonheme ligands and electron-deficient iodosylarenes to facilitate the O-I bond cleavage via the “push-pull” mechanism.16 In summary, the structure-reactivity relationship of the iron(III)-iodosylbenzene complex, [FeIII(tpena-)OIPh]2+ (1), was investigated by means of density functional calculations. Our theoretical work demonstrated for the first time that the metaliodosylarene complex 1 can evolve into two resonance valencebond structures (a high-valent iron–oxo species and a monomeric PhIO species) in thioanisole sulfoxidation to present different reaction mechanisms (the novel bond-cleavage coupled electron transfer mechanism or the direct oxygen-atom transfer mechanism) as a response to different substrate attack orientations. Based on these findings and the multiple-oxidant mechanism, the whole catalytic cycle of oxidation involving the metal-iodosylarene complex is postulated (Scheme 2) to present a profound understanding on the elusive metal-iodosylarene chemistry, which will greatly stimulate the development of rational nonheme catalyst design.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. The detailed computational methodology, energetic and geometric data and Cartesian coordinates (PDF)
AUTHOR INFORMATION Corresponding Author
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*
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[email protected] Author Contributions #
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These authors contributed equally to this work.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT The authors acknowledge financial support from National Natural Science Foundation of China to Y.W. (grants 21003116 and 21173211) and to W.S. (grant no. 21473226), the NRF of Korea through the CRI (NRF-2012R1A3A2048842) and GRL (NRF2010-00353) to W.N and MSIP (2013R1A1A2062737) to K.B.C.).
REFERENCES (1) (a) Stang, P. J.; Zhdankin, V. V. Chem. Rev. 1996, 96, 1123. (b) Zhdankin, V. V.; Stang, P. J. Chem. Rev. 2002, 102, 2523. (c) Yoshimura, A.; Zhdankin, V. V. Chem. Rev. 2016, 116, 3328. (2) (a) Carmalt, C. J.; Crossley, J. G.; Knight, J. G.; Lightfoot, P.; Martin, A.; Muldowney, M. P.; Norman, N. C.; Orpen, A. G. J. Chem. Soc.
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