DFT mechanistic study of the selective terminal C–H activation of n pentane with a tungsten allyl nitrosyl complex Journal of Saudi Chemical Society (2017) xxx, xxx–xxx King Saud University Journal[.]
Journal of Saudi Chemical Society (2017) xxx, xxx–xxx King Saud University Journal of Saudi Chemical Society www.ksu.edu.sa www.sciencedirect.com ORIGINAL ARTICLE DFT mechanistic study of the selective terminal C–H activation of n-pentane with a tungsten allyl nitrosyl complex Richmond Lee a, Davin Tan a, Chaoli Liu a,b, Huaifeng Li a, Hao Guo b, Jing-Jong Shyue c, Kuo-Wei Huang a,* a KAUST Catalysis Center and Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia b Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, PR China c Research Center of Applied Sciences, Academia Sinica, 128 Academia Rd., Sec 2, Nankang, Taipei 115, Taiwan Received 25 November 2013; revised 26 December 2016; accepted 28 December 2016 KEYWORDS Tungsten; DFT; C–H bond activation; Nitrosyl complex Abstract Mechanistic insights into the selective C–H terminal activation of n-pentane with tungsten allyl nitrosyl complex reported by Legzdins were gained by employing density functional theory with B3LYP hybrid functional Using Bader’s atom in molecules (AIM) analysis on the elementary steps of the hydrogen transfer process, TS1 and TS2, it was observed that the calculated H-transfer models were closely similar to Hall’s metal-assisted r-bond metathesis through bond critical point (BCP) comparisons One distinguishable feature was the fact that the formal oxidation state of the W changed in the concerted H-transfer process To better differentiate, we term these processes as ‘Formal Reductive Hydrogen Transfer’ (FRHT) for TS1 and ‘Formal Oxidative Hydrogen Transfer’ (FOHT) for TS2 Ó 2017 King Saud University Production and hosting by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Introduction The selective functionalization of sp3 C–H bonds confers a facile and direct conversion of abundant hydrocarbon from * Corresponding author E-mail address: hkw@kaust.edu.sa (K.-W Huang) Peer review under responsibility of King Saud University Production and hosting by Elsevier petroleum sources to higher-valued products [1–5] Several theoretical studies have been undertaken in order to understand these elementary steps for sp3 C–H bond activation [6–21] Legzdins and co-workers have reported the synthesis of a tungsten methylallyl nitrosyl complex which possesses intriguing C–H bond activating properties that selectively activates the terminal C–H bond of linear n-pentane (Scheme 1) [22,23] The product as a stable tungsten pentyl methylallyl complex (4) was isolated and fully characterized by X-ray crystallography Further studies by the same group to activate branched alkanes, olefins, aromatics and heteroatoms with the same system have demonstrated its versatility and selectivity [24,25] http://dx.doi.org/10.1016/j.jscs.2016.12.004 1319-6103 Ó 2017 King Saud University Production and hosting by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Please cite this article in press as: R Lee et al., DFT mechanistic study of the selective terminal C–H activation of n-pentane with a tungsten allyl nitrosyl complex, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2016.12.004 R Lee et al Scheme C–H bond activation by It was proposed that a 16e intermediate 2a was formed from the extrusion of neopentane, corroborated by a trapping experiment with PMe3 When n-pentane-d12 was used, it was observed that deuterium was incorporated at the terminal C of the methyl allyl moiety, suggesting that the hydrogen migration originated from n-pentane The current interpretation of hydrogen atom transfer mediated by transition metals can be classified as a two-step or concerted process (Scheme 2) [5,26–30] Two-step processes involve an oxidative addition step (A), the formation of an Mn+2 intermediate, followed by a subsequent reductive elimination (B) [31–33], while concerted r-bond metathesis pathways proceed via an ‘‘oxidative” (C) or four-center transition state (D) [18] Herein, we attempted to correlate current understanding of r-bond metathesis process to elucidate this C–H activation mechanism [34] Although the hydrogen migration pathway has been reported by Legzdins and co-workers [22,23], the detailed mechanistic description for this process was not available In this study, we focused on the analysis of the H-transfer processes on the basis of Bader’s AIM description and two new terms (FRHT and FOHT) describing the processes were proposed Computational details Density functional theory (DFT) calculations were performed by employing the Gaussian 03 program [35] The Becke threeparameter functional with the nonlocal Lee–Yang–Parr correlation functional (B3LYP) theory was applied [36,37], LANL2DZ basis set including double-n valence basis set with the Hay and Wadt effective core potential (ECP) was used for the W atom [38–40], and 6-31G(d) Pople basis set for the rest Scheme Processes in r-bond metathesis Two-step process: A = oxidative addition; B = reductive elimination Concerted process: C = ‘‘oxidative” transition state; D = four-center transition state of atoms [41–43] Please see Supporting information for a summary of Cartesian coordinates and thermodynamic data For atoms in molecules quantum theory (AIM), the wavefunction was generated with Gaussian 09 package [44] B3LYP theory was applied, all electron Well-tempered basis set (WTBS) was used for W [45,46], and 6-31G(d) Pople basis set was used for the rest of atoms The wavefunction output was analyzed with the AIM2000 software for topological interpretation WTBS was obtained from the EMSL basis set library [47] Results and discussions The initial steps can be described as allylic isomerization, involving the change in the coordination mode of the methylallyl moiety The resulting orientation (1c) is fundamentally important for the terminal hydrogen transfer as the C–H bond must be close to the neopentyl group and the tungsten metal center [30] The hydrogen migration from the terminal methyl group of the methylallyl ligand to the neopentyl group proceeds through transition state TS1 [22,23] with an overall activation barrier of 26.5 kcal/mol As the formal oxidation state of W changes from +2 to 0, we termed this process as a ‘Formal Reductive Hydrogen Transfer’ (FRHT) route [48] The neopentane molecule then dissociates from the resulting 18e intermediate 2a to 2, which later r-coordinate with pentane to give the r-complex Subsequently, undergoes ‘Formal Oxidative Hydrogen Transfer’ (FOHT; W is formally oxidized from to +2) through transition state TS2 by hydrogen migration from the n-pentane to the coordinated olefin moiety overcoming an activation barrier of 23.2 kcal/mol (relative to 1) This process is similar in retrospect to r-complex assisted metathesis [49–54], whereby the incoming n-pentane forms a r-complex with the metal center The H-transferred 16e intermediate 4a is unstable and prefers to form g3 coordination with the allylic ligand to 4b Subsequent allylic isomerization through 4c forms the observed product (see Fig 1) A study on the various models of hydrogen transfer process by Vastine and Hall concisely categorized and summarized the various reaction models in literature according to Bader’s atoms in molecules (AIM) analysis [55,56] The electron density of the transition states and intermediates during the hydrogen transfer process provided valuable information about bond and ring critical points (BCP and RCP) that could be described using the AIM2000 software that analyzes electron density, gradient field and Laplacian of atoms according to AIM theory [57] These critical points are therefore pertinent in categorizing and characterizing the nature of hydrogen transfer during bond metathesis processes Using AIM2000 analysis on TS1 and TS2, we are able to identify the geometrical similarities of the critical points with Hall’s metalassisted r-bond metathesis type For both optimized transition Please cite this article in press as: R Lee et al., DFT mechanistic study of the selective terminal C–H activation of n-pentane with a tungsten allyl nitrosyl complex, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2016.12.004 Selective terminal C–H activation of n-pentane with a tungsten allyl nitrosyl complex Figure Reaction profile in relative energy, E (values are in italics) Dotted line for internal C–H activation W NO H C4H 10 W NO H Figure AIM analysis diagram and display of critical points of TS1 (top) and TS2 (bottom) Inset illustrates the whole 2D representation of molecule with zoom-in-area highlighted BCPs are red dots with bond path passing through while RCPs are yellow Ht refers to the transferring H and NO ligands are omitted for viewing clarity states TS1 and TS2, the W-H bond lengths are both about 1.8 A˚, which is similar to the neutron diffraction W–H bond length of 1.73 A˚ [58] The key BCP (Fig 2, BCP is red dot) between W and the transferring H was identified, suggesting that the transfer of H is mediated by the transient oxidativeadded W metal center Although the critical point features Please cite this article in press as: R Lee et al., DFT mechanistic study of the selective terminal C–H activation of n-pentane with a tungsten allyl nitrosyl complex, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2016.12.004 R Lee et al as FRHT and FOHT, which both involve a change in the formal oxidation of the metal center in the elementary step Acknowledgments This work is supported by King Abdullah University of Science and Technology Additional computing time from KAUST scientific cluster (Noor) and scholarships to R Lee, D Tan, and H.-F Li are gratefully acknowledged Appendix A Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jscs.2016 12.004 Figure Bond order analysis of TS2 closely resemble Hall’s metal-assisted r-bond metathesis, it is dissimilar in that our model involves changes in formal oxidation state from substrate to the hydrogen-transferred product Using Mayer’s bond order analysis of the transition state, we are able to further analyze the existence of bonding between the involved atoms in the transition state [59–62] In accordance with the principle of conservation of bond order, the sum of bond orders for forming and breaking a bond in a reaction must be close to unity [63] The sum of bond order of the terminal pentane C to both W and H is calculated to be 0.93, The hydrogen bond order between H–W, H–Cpentane and H–C1,allyl is computed to be 0.916, indicating that the hydrogen transfer proceeds through W with a significant W–H bond character Summation of all the W-ligand bond forming or breaking orders, W–Cpentane (0.617), W–C1,allyl (0.558), W–H (0.351) and W–C2,allyl (0.729) give a total bond order of 2.225, corresponding to the two newly formed W–C r-bonds of intermediate (Fig 3) Furthermore, our calculations on W activation of internal C2 of n-pentane reveal that this pathway incurs an additional 3.4 kcal/mol higher than that of TS2 (TS2-int 26.6 kcal/mol, relative to 1), which can be attributed to the more sterically demanding internal C–H environment These observations confirm the experimental results that only the terminal C–H of pentane is preferentially activated but not the internal C–H bond The activation entropies DSà, for both FRHT and FOHT routes leading to the transition states are 4.9 and 9.2 e.u., respectively, supporting the concerted mechanism model as the reaction proceeded through the more highly ordered transition states References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] Conclusion Our results 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Although the critical point features Please cite this article in press as: R Lee et al., DFT mechanistic study of the selective terminal C–H activation of n- pentane with a tungsten allyl nitrosyl complex, ... that analyzes electron density, gradient field and Laplacian of atoms according to AIM theory [57] These critical points are therefore pertinent in categorizing and characterizing the nature of. .. the terminal C–H bond of the pentane is more easily accessible than the internal C–H bonds of the molecule and hence the preference for activation at the terminal site as conforming to experiments