Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX
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Cluster−π Interactions Cause Size-Selective Reactivity of Cationic Silver Clusters with Acetylene: The Distinctive Ag7+[C2H2] Mengzhou Yang,†,‡ Haiming Wu,†,‡ Benben Huang,†,‡ and Zhixun Luo*,†,‡ †
Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China
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S Supporting Information *
ABSTRACT: Utilizing a customized multiple-ion laminar flow tube reactor in tandem with a triple quadrupole mass spectrometer, we report a study of the gasphase reactivity of Agn+ clusters with acetylene. Well-resolved Agn+ clusters (n = 1−20) are produced by a self-designed magnetron sputtering source (MagS); however, on their reactions with acetylene under sufficient collisional conditions, only Ag7+[C2H2] is produced with a reasonable intensity. DFT calculations reveal that Agn+ clusters do not form strong Ag−C bonds with C2H2 and Ag7+[C2H2] bears larger binding energy than the other Agn+[C2H2] although within similar cluster−π interactions. Besides gas-phase reaction rate estimation, the relatively large noncovalent cluster−π interaction in Ag7+[C2H2] is fully demonstrated via topological analysis and natural bonding orbital analysis. Also, we illustrate both thermodynamically and kinetically favored channels in producing the Ag7+[C2H2]. This study helps in understanding metal-involved noncovalent bonds and how such weak interactions are able to tune the material function and biological activity.
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INTRODUCTION Organometallic compounds have been a hot research area showing wide applications in gas storage, 1,2 magnetic memory,3 molecular recognition,4 selective catalysis,5,6 and functional materials.7,8 Organometallic complexes of ethene can be traced back to the historic example of Zeise’s platinum salt 200 years ago;9 in recent decades, metal-alkynes have also received reasonable research interest as they are key intermediates in abundant chemical transformations such as hydrogenation, cycloaddition, C−H bond functionalization, alkyne coupling, and addition of heteroatom−hydrogen bonds.10−13 In these systems, metal-alkyne interactions often dominate the property of coordination units giving rise to varied reaction processes and transformation,13,14 depending on the metal center and electron transport activity. It is anticipated that a metal cluster could largely alter the metalalkyne interactions, providing tunable valence electrons, ligancy, and oxidation state of the metal center simply by adding or reducing a single atom of the cluster.15−17 In recent years, ligand-protected metal clusters have attracted increasing attention enabling to develop functional materials of atomic precision, to design catalysts of specified active sites,18−21 to understand the charge-transfer mechanism in bioapplications, and even to dissect metal−organic coordination chemistry.22,23 It is notable that metal−organic interactions are not always associated with normal ionic or covalent coordination bonds depending on the Lewis acidity/ alkalinity. Some metastable metal−organic clusters could be © XXXX American Chemical Society
stabilized by noncovalent weak interactions, such as ion−π interactions.24−27 Profiting from the development of gas-phase chemistry and quantum chemical calculations, insights into the structure stability and reactivity of metal−organic clusters could be attained from isolated systems without the interruption of solvation or surface binding effect, enabling to fully unravel the nature of metal−organic interactions.
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EXPERIMENTAL AND THEORETICAL METHODS All the experiments in this study were conducted on a newly developed multiple-ion laminar flow tube reactor in tandem with a triple quadrupole mass spectrometer (MIFT-TQMS), combined with a fast flow tube reactor apparatus and a MagS cluster source (Scheme 1).28 A DC power supply (2 kW, Kurt Lesker) was used to provide the high voltages (ca. 240−300 V) needed for the MagS source. The C2H2 gas (Beijing Yalan Weiye Gas Co., Ltd., purity >99.99%, diluted 5% in He) was introduced to the cluster beam ∼30 cm downstream from the source, allowing reaction with the Ag clusters over a distance of 70 cm and a time of ∼10 ms. The pressure in the reaction flow tube was kept at 0.7 Torr, that is, the number of collisions for the clusters and reactant could be up to 105. Buffer gas helium (Beijing AP BAIF Gases Industry Co., Ltd., purity >99.996%) was introduced into the source chamber via an adjustable Received: July 9, 2019 Revised: July 15, 2019 Published: July 17, 2019 A
DOI: 10.1021/acs.jpca.9b06502 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Scheme 1. Drawing of the Multiple-Ion Flow Tube Reactor Apparatus Equipped with a Magnetron Sputtering Sourcea
From left to right: magnetron cluster source chamber (A), flow tube reactor (B), ion guide chambers consisting of a conical octupole ion focuser (C) and two linear octupole ion guides (D,E), and ion detection chamber with a quadrupole mass analyzer (F) and a pulse-counting electron multiplier (G). a
needle valve, with a flow rate of ∼10 slm to maintain ∼3 Torr background pressure in the source chamber. The sputtering gas argon (Beijing AP BAIF Gases Industry Co., Ltd., purity >99.999%) was introduced into the sputtering gun at an ∼20 sccm flow rate. The silver disk (99.99% pure, 50.8 mm diameter, 4 mm thickness) was obtained from Zhongnuo Advanced Material (Beijing) Technology Co., Ltd. The thermally stabilized clusters then went downstream to the flow tube reactor through a nozzle of 10 mm diameter. More experimental details are provided in the Supporting Information. DFT calculations were performed with the gradientcorrected Becke−Perdew (BP86) exchange correlation functional29,30 using the Gaussian 09 program.31 The geometries of the species were fully optimized using the small-core Los Alamos (“LANL2TZ(f)”) pseudopotentials; the basis sets that include the Dunning−Huzinaga full TZ and Los Alamos ECPs plus TZ were used for the Ag atoms with an extra f polarization function,32−34 while 6-311+g(d,p) was used for C and H atoms. Vibrational frequency calculations were performed on each of the optimized geometries. Zero-point vibrational corrections were performed for all the energy calculations. All the TSs were checked with the intrinsic reaction coordinate (IRC) scan to ensure a connection with both ends and only one imaginary frequency correlative to the reaction pathway. Quantum theory of atoms in molecules (AIM),35 electron localization function (ELF) analysis and noncovalent interaction plots based on independent gradient model (IGM), localized molecular orbital energy decomposition analysis (LMO-EDA),36 and natural bond orbital (NBO)37−39 analysis were conducted to fully demonstrate the bond nature and strength of cluster−π noncovalent interactions in Ag7+[C2H2] clusters. The NBO patterns are plotted via the software packages of visual molecular dynamics (VMD)40,41 and Multiwfn.42 The LMO-EDA is calculated at the MP2 level of theory by the GAMESS program.43,44
Figure 1. Mass spectra of silver cluster cations produced via a magnetron sputtering source in the absence (a) and presence of different quantities of C2H2 gas into the flow tube reactor (b,c).
model.45 Figure 1b,c displays the mass spectra after the clusters were exposed to different quantities of C2H2 gas (∼5% in helium). When reacting with C2H2, all the observed Ag cluster cations display decreased mass abundance but mostly do not form cationic products except Ag7+[C2H2]. As acetylene abundance increases, a little adducts of Ag 3 + [C2 H 2 ], Ag5+[C2H2], and Ag6+[C2H2] emerge, but their intensities are much weaker than that of Ag7+[C2H2]. Note that the intensity of Ag7+ decreases along with the increased mass abundance of Ag7+[C2H2], but the sum of the relative concentrations of Ag7+ and Ag7+[C2H2] is almost a constant (for details, see Figure S3), suggesting that the Ag7+ cluster mainly undergoes an addition reaction to form Ag7+[C2H2]. This is well consistent with the previous finding of distinctive reactivity of Al12− to produce Al12−[NH3].46
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RESULTS AND DISCUSSION As a simple model, herein, the interaction of silver clusters with acetylene (the smallest unsaturated hydrocarbon) is studied utilizing a customized MIFT-TQMS instrument. Figure 1a presents a typical mass spectrum of Agn+ clusters, which exhibit a nearly normal distribution along with a regular odd-even alternation of mass abundance with Ag9+ being the dominant species due to its closed electronic shell based on the jellium B
DOI: 10.1021/acs.jpca.9b06502 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A In order to understand the reactivity of Agn+ with C2H2, simply we have first estimated the pseudo-first-order rate constant (k) by the following equation47−49 ln I ij [A ] yz 0 zz = − k = −lnjjj j [A 0] zz ρ · t k {
() I
(1)
Here, I0 and I are the intensities of Agn+ before and after the reaction, respectively (more details in the Supporting Information). Based on this principle, the rate constant for the reaction between Ag7+ and C2H2 is about 6.7 × 10−10 cm3 molecule−1 mol−1, which is about 5 times those of Ag3+ and Ag5+ (for details, see the Supporting Information). The smaller reaction rates for Ag3+ and Ag5+ than that of Ag7+ are associated with the kinetics of Agn+[C2H2] adduct formation. The collisions of Agn+ with C2H2 give rise to vibrational excited transients (Agn+···C2H2), which may suffer from subsequent dissociation or simply thermalized by rich-pressure collisions with He gas to remove the excess vibrational and rotational energies. Because the larger Agn+···C2H2 cluster allows for more flexible vibrational structure relaxation while the small ones could exhibit a lower vibrational density of states leading to a shorter lifetime and smaller chance of collision cooling, the probability for the dissociation of small Agn+[C2H2] species (ca. n = 1−5) could be larger than those for the larger ones. On the other hand, thermal dissociation threshold values are usually smaller for much larger clusters (ca. 0.4 to 0.6 eV for such a flow tube reactor).50,51 Therefore, the very large Agn+ and Agn+[C2H2] clusters are usually unstable under richpressure collisions. Further, quantum chemical calculations are performed to investigate the stability and reactivity of Agn+ clusters toward acetylene. The optimized ground-state structures of Agn+ and Agn+[C2H2] clusters (n = 1−11) are shown in Figure 2a (more details are provided in Figures S4−S8). As is shown, the cationic clusters Agn+ with n ≤ 7 prefer planar structures, while those with n > 7 display three-dimensional structures. The ground-state structure of Ag7+ exhibits a quasi-planar hexagonal geometry, which is 0.12 eV lower in energy in comparison with its pentagonal bipyramid isomer as in the ground-state geometries of Ag7+[C2H2]. To measure the strength of the interactions between Agn+ and C2H2, we have checked the Agn+···C2H2 binding energies and Ag−C bond lengths for all these Agn+[C2H2] clusters, as shown in Figure 2b,c respectively. As a result, the rAg−C values and binding energies display slope regularity with increasing number of silver atoms, but Ag7+[C2H2] stands out of the slope tendency and finds the second smallest rAg−C and a relatively large binding energy. In comparison, the Agn+[C2H2] clusters with n > 7 display small binding energies (
