Preparation and characterization of chemically bonded argon–boroxol ring cation complexes
were prepared via a laser vaporization supersonic ion source in the gas phase. Their vibrational spectra were measured via mass-selected infrared photodissociation spectroscopy. Spectroscopy combined with quantum chemical calculations revealed that the [ArB 3 O 5 ] + , [ArB 4 O 6 ] + and [ArB 5 O 7 ] + cation complexes have planar structures each involving an aromatic boroxol ring and an argon-boron covalent bond. In contrast, the [ArB 3 O 4 ] + cation is characterized to be a weakly bound complex with a B 3 O 4 + chain structure.
Introduction<!>Experimental and theoretical methods<!>Results and discussion
<p>The rst synthesis of the stable xenon-containing compounds XePtF 6 , XeF 4 and XeF 2</p><p>1 inspired great interest in searching for new noble gas compounds to improve our understanding of noble gas chemistry. Since then, many chemically bound compounds containing heavier noble gas atoms have been prepared and structurally characterized, most of them being xenon compounds. [2][3][4][5] The lighter rare gas argon is even more chemically inert than the heavier noble gas atoms due to its stable valence shell. Although a number of theoretical studies indicate that chemical bonding between argon and other atoms is possible to form some metastable argon-containing compounds, 6 HArF is presently the only experimentally known neutral molecule containing a chemically bound argon atom that is stable in a low temperature argon matrix. 7 Theoretical calculations indicated that the H-Ar bond in HArF is covalent, whereas the Ar-F bond is mainly electrostatic. 8 The bare ArH + ion has been detected in cosmic dust associated with the Crab Nebula supernova, and is the rst noble-gas molecule detected in outer space. 9 Argon can act as a Lewis base that is able to coordinate to positively charged metal centers. Complexes containing argon such as ArBeX (X ¼ O, S, or CO 3 ), 10,11 ArW(CO) 5 , 12 ArMX (M ¼ Cu, Ag, Au, etc., and X ¼ F, Cl, Br or O), [13][14][15] CUO(Ar) 16 and Ar-mixed gold-silver trimer cations 17 have been experimentally observed in the gas phase or in lowtemperature matrices. Most of the Ar-M interactions in these complexes can be regarded as Lewis acid-base interactions with quite large binding energies (about 5-10 kcal mol À1 ), but the electron density distribution analyses indicate that they have some dative bonding property instead of normal dative or electron-sharing covalent bonds. 18 Signicant efforts have also been made in searching for chemical bonding between argon and more challenging elements such as boron. Argon-boron containing compounds including FArBO, FArBF 2 and FArBN À were theoretically studied, and were predicted to be thermally metastable species with covalent Ar-B bonding. 19 Argon coordination complexes including ArBBAr, ArB + , ArBF 2 + and B 3 Ar 3 + have also been studied computationally, [20][21][22][23] and some of them exhibit remarkably strong Ar-B bonding. 22,23 Only the BAr + and ArBF 2 + cations as well as the metastable ArBF n 2+ dications have been experimentally detected using mass spectrometry in the gas phase. 24,25 Here we report the experimental preparation of a group of boron oxide-argon cation complexes in the gas phase. Mass-selected infrared photodissociation spectroscopy and high-level quantum chemical calculation studies reveal strong argon-boron chemical bonding in these cation complexes featuring an aromatic boroxol ring.</p><!><p>The experiment was performed using a collinear tandem timeof-ight mass spectrometer equipped with a pulsed laser vaporization/supersonic expansion cluster ion source that was described in detail previously. 26 The cation clusters were produced by pulsed laser (532 nm, Spectral Physics, 10 Hz repetition rate) ablation of a target made of 10 B-depleted or 10 Benriched powders (Alfa Aesar, 90-97%). The clusters were entrained by a helium carrier gas seeded with 0.1-1.0% O 2 and 5-10% argon at a backing pressure of 0.6-1.2 MPa and underwent a supersonic expansion (General Valve Series 9) to form a collimated and cold molecular beam. The composition and cooling of the cation clusters were controlled by the time delay between the carrier gas pulse and the ablation laser. The cation clusters were extracted and analysed with a time-of-ight mass spectrometer (TOFMS). The ions of interest were mass selected, decelerated and then subjected to infrared photodissociation. ] + cation clusters was performed at the B3LYP-D3/aug-cc-pVTZ 27-29 level of theory using the Gaussian 09 program 30 with the empirical dispersion correction added to increase the accuracy on the long-range force. 31 The lowest energy structures were then re-optimized at the CCSD(T)/cc-pVTZ 32 level of theory using the MOLPRO 2010 program. 33 Chemical bonding analyses were performed by the adaptive natural density partitioning (AdNDP) method, 34 quantum theory of atoms in molecules (QTAIM) 35 and energy decomposition analysis with natural orbitals of chemical valence (EDA-NOCV). 36 AdNDP is a theoretical tool for obtaining patterns of chemical bonding based on the concept of the electron pair as the main element of chemical bonding models. It achieves seamless description of systems featuring both localized and delocalized bonding without invoking the concept of resonance. 34 The AdNDP analyses were performed using the Multiwfn program. 37 The wave function les generated from CCSD(T) calculations were employed to analyse the topology of the electron density with the AIMALL package. 38 The critical points (atom, bond and ring), bond paths, and Laplace distribution of density provided a comprehensive view of the electron density between the boron and argon. The EDA-NOCV bonding was analysed at the BP86/TZ2P [39][40][41] level with the geometries optimized at the CCSD(T) level using the ADF2014.10 program package. 42 The EDA-NOCV bonding analysis focuses on the instantaneous interaction energy, DE int , of a bond A-B between two fragments, A and B, in the particular electronic reference state and in the frozen geometry of AB. This DE int is divided into three main components: the quasiclassical electrostatic interaction energy (DE elstat ), the Pauli repulsion energy (DE Pauli ) and the orbital interaction (DE orb ). The DE orb accounts for the charge transfer and polarization effects, and is decomposed into contributions from each irreducible representation of the point group for the system.</p><!><p>The mass spectrum using a 10 B-depleted boron target in expansions of helium gas seeded with oxygen in the m/z range of 40-250 is shown in Fig. 1 1) with 10 B/ 11 B isotopic ratios in the range of 1.023-1.036 indicate that all of these bands are BO stretching vibrations. The bands above 2050 cm À1 are assigned as stretching vibrations of terminally bonded B^O fragments; the bands in the range of 1400-1700 cm À1 can be ). These isomers are isoenergetic with similar infrared spectra (Fig. S9 and S10 †). Thus, the experimentally observed cations may be due to mixtures of these conformational isomers. As shown in Table 1 and Fig. S7-S10, † the calculated vibrational frequencies and boron isotopic shis of the most stable structures for these cations were in quite good agreement with the experimental values.</p><p>The Ar-B bond distances were predicted to be 2.004, 2.002 and 2.001 Å for the [ArB 3 O 5 ] + , [ArB 4 O 6 ] + and [ArB 5 O 7 ] + cation complexes, respectively, at the CCSD(T)/cc-pVTZ level. These Ar-B bond distances are much shorter than the sum of the van der Waals radii of argon and boron (Ar + B ¼ 1.88 + 2.08 ¼ 3.96 Å), 46 and are close to the sum of their single-bond covalent radii (Ar + B ¼ 1.81 Å). 47 This suggests strong interaction between argon and boron in these cation complexes. The Wiberg bond orders (Table 2) were predicted to be 0.543, 0.545 and 0.547, respectively, slightly smaller than that of the H-Ar bond in HArF.    To understand the bonding in these cation complexes, AdNDP analysis was carried out (Fig. 5). This method has been successfully used to analyze chemical bonding in boron clusters, aromatic molecules and gold clusters. 49 The analyses of [ArB 3 O 5 ] + , [ArB 4 O 6 ] + and [ArB 5 O 7 ] + revealed one 2c-2e s bond with an occupation number (ON) of 1.99 for each cation complex, which was formed between the in-plane 2p atomic orbitals of the argon and boron. No such Ar-B bond can be found in the [ArB 3 O 4 ] + cation complex with the chain structure. For each boroxol ring cation complex, there are three 3c-2e delocalized p bonds within the boroxol ring, satisfying the 4n +   2 rule for aromaticity, similar to the B 4 O 4 + cations reported previously. 50 Thus, all three boroxol ring cation clusters are p aromatic.</p><p>We investigated the topology of the electron density distribution of the species using the quantum theory of atoms in molecules (QTAIM) method. 35 Fig. 6 shows the Laplacian of the electron density (V 2 r(r)) of the cation complexes in the molecular plane (the side view is shown in Fig. S11 †). There is a bond critical point (Vr(r) ¼ 0 and V 2 r(r) < 0 in one direction) and an associated bond path for the Ar-B moiety in each cation complex. The Laplacian electron density distribution clearly shows a depletion region (V 2 r(r) > 0) between argon and boron in [ArB 3 O 4 ] + and charge concentration "islands" (V 2 r(r) < 0, red dashed lines) between boron and argon in the boroxol ring complexes, suggesting a noncovalent Ar-B interaction in [ArB 3 O 4 ] + and a genuine Ar-B dative bond in the boroxol ring cation complexes. Natural bond orbital analysis indicated that the argon atom carries positive charges of around 0.36e in the argon-boroxol ring species, but positive charges of only 0.04e in [ArB 3 O 4 ] + (Table 2).</p><p>The nature of the Ar-B interactions was further analyzed with the EDA-NOCV method, 36    covalent or electrostatic properties. 18a Fig. 7</p>