Subscriber access provided by UNIV OSNABRUECK
Article 3
–
Oxidative Addition of CHI to Au in the Gas Phase Satoru Muramatsu, Kiichirou Koyasu, and Tatsuya Tsukuda J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b10168 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 6, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 21
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Oxidative Addition of CH3I to Au– in the Gas Phase Satoru Muramatsu,† Kiichirou Koyasu,†, ‡ Tatsuya Tsukuda*,†, ‡ †
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyoku, Tokyo 113-0033. ‡
Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520.
ABSTRACT: Reaction of the atomic gold anion (Au–) with CH3I under high-pressure helium gas affords the adduct AuCH3I–. Photoelectron spectroscopy and density functional theory calculations reveal that in the AuCH3I– structure, the I and CH3 fragments of CH3I are bonded to Au in a linear configuration, which can be viewed as an oxidative addition product. Theoretical studies indicate that oxidative addition proceeds in two steps: nucleophilic attack of Au– on CH3I, followed by migration of the leaving I– to Au. This mechanism is supported by the formation of an ion-neutral complex, [Au–…t-C4H9I], in the reaction of Au– with t-C4H9I because of the activation barrier along the SN2 pathway resulting from steric effects. Theoretically studies are conducted for the formation mechanism of AuI2–, which is observed as major product. From the thermodynamic and kinetic viewpoints, we propose that AuI2– is formed via sequential oxidative addition of two CH3I molecules to Au– followed by reductive elimination of C2H6. The results suggest that Au– acts as a nucleophile to activate C(sp3)–I bond of CH3I and induces the C–C coupling reaction of CH3I.
1 Environment ACS Paragon Plus
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1. Introduction The discovery of Au catalysis by Haruta et al.1 stimulated the practical development of highly active and selective Au catalysts.2–7 This discovery also led to fundamental studies on model systems to gain insights into size-specific catalysis. Previous experimental and theoretical studies on bare Au clusters with defined size and charge states revealed that electronic structure governs its “magic” stability8,9 and chemical reactivity with O2, which is related to oxidation catalysis.10–12 It has been shown that O2 is activated by electron transfer from the Au cluster anions, and that superoxo- or peroxo-like species thus formed oxidizes CO on the cluster surface.13 Although these results may not directly explain the origin of oxidation catalysis of Au catalysts in the real world2–7 due to significant differences in the surrounding environments, they provide a design principle for Au-based oxidation catalysts: Au clusters must be sufficiently small and negatively charged for catalytic use in aerobic oxidation.14 Using this simple concept, we have studied oxidation catalysis of small and monodisperse (1.2 ± 0.2 nm) negatively charged Au clusters stabilized by polyvinylpyrrolidone (PVP).15 The Au:PVP clusters with a diameter of 1–2 nm showed size-specific catalysis for aerobic oxidation of alcohols.15 The activation of O2 through electron transfer by a negatively charged Au:PVP cluster is thought to be the key step in this oxidation.15 The close similarity in the key step for aerobic oxidation for both bare and PVPstabilized Au clusters demonstrates that cluster-based catalysts can be rationally designed and developed. Recently, it has been reported that Au complexes and nanoparticles catalyze a variety of important and useful coupling reactions forming carbon–carbon bonds, such as the Sonogashira, Ullmann, and Suzuki-Miyaura coupling reactions.16–20 These findings suggest that C(sp or sp2)–X (X = halogen) bonds can be activated by Au catalysts via oxidative addition. In contrast, catalytic
2 Environment ACS Paragon Plus
Page 2 of 21
Page 3 of 21
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
activation of C(sp3)–X bonds has gained less attention because of difficulties resulting from the more electron-rich sp3 carbon compared with sp or sp2 carbons.21 It is important to understand whether Au clusters can activate the C(sp3)–X bonds in organohalides for future development of Au-based catalysts for coupling reactions. As an initial step toward this goal, we focused on the simple gas-phase reaction between the atomic gold anion (Au–) and methyl iodide (CH3I) with the expectation that Au– is more nucleophilic than Au(0), Au(I), and Au(III) species commonly used for gold-catalyzed coupling reactions. The first question we addressed was whether Au– can activate the C–I bond of CH3I. Wilkins previously reported the production of I– by the nucleophilic attack of Au– on CH3I.22 However, it was not clear whether Au–CH3 bond was formed because neutral products were not characterized. In this study, we conducted the reaction under high-pressure helium and successfully isolated the adduct of Au– and CH3I by collisional cooling. Photoelectron spectroscopy and theoretical calculations demonstrated that Au is inserted in the C–I bond of CH3I. The second question concerns the mechanism of insertion of Au into the C–I bond of CH3I. Two mechanisms have been proposed for the oxidative addition of haloalkanes to a metal atom (Scheme 1):23–25 Equation (1) in Scheme 1 is the direct oxidative insertion into the C–X bond, and eq. (2) is the SN2 reaction followed by migration of the leaving X–. The mechanism is important from the viewpoint of asymmetric synthesis because the stereochemistry of the central carbon is retained in eq. (1), while it is inverted in eq. (2). A theoretical study based on the global reaction route mapping (GRRM) method26–29 revealed that formation of the adduct [I–Au–CH3]– occurred via the SN2 route (eq. 2). We finally propose that another CH3I molecule undergoes oxidative addition to the [I–Au–CH3]– adduct to yield the coupling product C2H6 and AuI2–. Scheme 1. Proposed mechanism of oxidative addition of haloalkanes to a metal atom
3 Environment ACS Paragon Plus
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
2. Methods 2.1. Experimental The experimental apparatus used in this study consisted of a laser ablation ion source, a reaction cell (Figure 1), a time-of-flight mass spectrometer (TOF-MS), and a magnetic-bottle photoelectron spectrometer (MB-PES). Details are described elsewhere.30 In brief, the atomic gold anion was generated by irradiating a focused second harmonic output from a Nd:YAG laser (10–50 mJ/pulse) onto a rotating and translating gold target rod (purity: 99.95%, diameter: 2 mm, length: 20 mm). The generated atomic gold anion was introduced into a channel (diameter: 2 mm, length: 80 mm) by a flow of helium gas (purity: 99.999%) supplied through a pulsed valve (backing pressure: 4.0–10 atm, injection orifice: 0.8 mm, pulse width 200–300 μs). Then, the atomic gold anion was allowed to react with haloalkane molecules injected into the reaction cell by a pulse valve with/without dilution with helium gas (0–1.5 atm). After the reaction, the anionic species were extracted perpendicularly to the initial beam direction, accelerated to 4.0 keV, and analyzed by a linear TOF-MS with a flight path of 1.59 m. The anion of interest was mass-selected and introduced to the MB-PES after momentum deceleration,31 where it was irradiated by an unfocused 3rd (355 nm, 3.50 eV) or 4th (266 nm, 4.66 eV) harmonics output from another Nd:YAG laser. The photoelectrons were detected by MB-PES with a flight path of 1.07 m. The photoelectron spectra were obtained by accumulations of 10,000–20,000 laser shots,
4 Environment ACS Paragon Plus
Page 4 of 21
Page 5 of 21
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
after background subtraction in case of measurements at 266 nm. The spectra were smoothed by averaging the photoelectron counts of neighboring three points. The photoelectron binding energies were calibrated against the known spectra of I– (ref. 32) and Au–.33 The resolution of the MB-PES was
