Article pubs.acs.org/JPCA
Coordination versus Solvation in Al+(benzene)n Complexes Studied with Infrared Spectroscopy Kimberly N. Reishus, Antonio D. Brathwaite, Jonathan D. Mosley, and Michael A. Duncan* Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States S Supporting Information *
ABSTRACT: Singly charged aluminum−benzene cation complexes are produced by laser vaporization in a pulsed supersonic expansion. The Al+(benzene)n (n = 1−4) ions are mass selected and investigated with infrared laser photodissociation spectroscopy. Density functional theory (DFT) is employed to investigate the structures, energetics and vibrational spectra of these complexes. Spectra in the C−H stretching region exhibit sharp multiplet bands similar to the pattern known for the Fermi triad of the isolated benzene molecule. In the fingerprint region, strong bands are seen corresponding to the ν19 C−C ring motion and the ν11 out-of-plane hydrogen bend. The hydrogen bend is strongly blue-shifted compared to this vibration in benzene, whereas the ν19 carbon ring distortion is only slightly shifted to the red. Computed structures and energetics, together with experimental fragmentation and vibrational patterns, indicate a primary coordination of three benzene molecules around the central Al+ cation. The n = 4 complex contains one second-sphere solvent molecule.
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INTRODUCTION
electrostatic in nature. It is less clear what the coordination and structures will be for these systems. Gas phase metal ion−benzene complexes have been studied extensively to probe the fundamental bonding interactions in these systems. Formation processes and reactions have been investigated under different conditions in mass spectrometry.13,18−22 Collision-induced dissociation,23−27 radiative association,28 and photodissociation29−37 measurements have explored cation−benzene bond energies. Spectroscopy studies have included tunable laser electronic photodissociation32−34,38,39 and photoelectron spectroscopy.40−45 Perhaps the most informative data about structures and bonding come from infrared photodissociation studies of mass-selected ions.46−52 Free electron lasers (FELs) have generally been employed for resonance-enhanced multiphoton dissociation (REMPD) studies in the fingerprint region,47−49 whereas IR optical parametric oscillator (OPO) laser systems have investigated the C−H stretching region,46,50−52 usually using the method of tagging with rare gas atoms. Computational work has complemented many of these spectroscopic studies.53−67 Unfortunately, the infrared spectrum of isolated benzene in the C−H stretching region is complicated by the occurrence of a Fermi triad in the vibrational structure.68 This feature and its response to metal cation binding are not handled with standard harmonic computational methods, limiting the insight that can be gained in this region. Likewise, REMPD vibrational bands in the fingerprint region are often broadened
Metal−benzene complexes provide prototypical examples of organometallic bonding and cation−π interactions.1−8 The spectroscopy and chemistry of these systems have been studied for many years in the condensed phase, due to their relevance for homogeneous catalysis.1−3 A renewed interest in these complexes has been stimulated by the importance of cation−π interactions in supramolecular chemistry and biological systems.4−8 The fundamental details of bonding patterns, charge transfer, and progressive solvation have been studied in gas phase systems, which can be compared more directly with computational results.9−18 In the present report, we investigate gas phase aluminum cation-benzene complexes using infrared photodissociation spectroscopy and computational chemistry. The most common structure for metal benzene complexes is the dibenzene sandwich.1,2 Cr(benzene)2 is the classic example of this structure, and its stability is attributed at least in part to its 18-electron configuration. Ionized analogs that achieve this same closed-shell electronic structure have also been investigated in traditional condensed phase chemistry.2 Although symmetric sandwich structures are most familiar, when the metal center is electron-rich or electron-poor, deviations from this common motif may occur. Complexes with different numbers of ligands may form, as well as those in which the metal binds in sites with less than the usual 6-fold coordination.18 These issues have been investigated for transition metal systems, but there is less information for main group metal complexes. In the present study, we explore benzene complexes with the aluminum cation. Unlike transition metal systems, the 3s2 valence configuration of Al+ is expected to lead to bonding interactions with benzene that are primarily © XXXX American Chemical Society
Special Issue: Kenneth D. Jordan Festschrift Received: January 22, 2014 Revised: March 11, 2014
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RESULTS AND DISCUSSION The mass spectrum of complexes produced in this experiment (see Supporting Information) contains primarily ions of the form Al+(benzene)n and Al+(benzene)nAr. Infrared excitation in the C−H stretching region does not cause photodissociation of the Al+(benzene)1,2 complexes, consistent with their relatively high dissociation energies (see below). However, larger complexes beginning with the n = 3 species photodissociate by eliminating intact benzene molecules. The efficiency of this process is quite low for the n = 3 complex, but greater for the larger systems. Figure 1 shows the photodissociation mass
by multiphoton effects, shifting their positions and relative intensities. Benzene vibrations should shift in systematic ways depending on the nature of the bonding and the metal−ligand charge transfer.57 However, improved spectroscopy on these systems in the fingerprint region is needed to see these effects. In the present study, we investigate the infrared spectroscopy of Al+(benzene)n complexes (n = 1−4). The binding of aluminum cation to small molecules including benzene has been the subject of several previous studies.28,69−75 The IR spectroscopy of the monoligand Al+−benzene ion has been reported using REMPD with the FELIX free electron laser.48 Here, we apply photodissociation and the method of rare gas atom tagging in the C−H stretching region, extend these studies into the fingerprint region and to multiligand complexes. Spectral patterns, complemented by computational studies, reveal the unusual coordination in this system.
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Article
EXPERIMENTAL SECTION
Aluminum−benzene ion−molecule complexes, and those with attached argon atoms, are produced by laser vaporization in a pulsed nozzle molecular beam source using the “cutaway” source configuration.76 Benzene vapor above the liquid at room temperature is entrained in mixtures of helium, neon, and argon to produce an expansion gas mixture used at a nozzle backing pressure of about 10 atm. Ions are sampled into a differentially pumped time-of-flight mass spectrometer, where they are mass analyzed and selected for spectroscopy. Ions are mass selected with pulsed deflection plates and then irradiated at the turning point in a reflectron instrument with a tunable infrared laser system. The instrument configuration for these experiments has been described in detail previously.77 Mass spectra are recorded with a digital oscilloscope connected to a PC with an IEEE-488 interface. Different configurations of the optical parametric oscillator (OPO) laser system cover the IR regions 2000−4500 and 600−2300 cm−1.78,79 The laser line width is about 1 cm−1; wavelengths are calibrated with photoacoustic measurements of the spectrum of methane. Spectra are collected by recording the yield of a specific fragment ion versus the laser energy. In the case of Al+(benzene)1−3 complexes, which do not fragment efficiently with IR excitation, we use the method of argon tagging.80−85 The corresponding Al+(benzene)1−3Ar complex is produced and mass selected, and then the fragment detected following infrared excitation is that corresponding to the loss of the argon. The Al+(benzene)4 complex spectrum is detected in the mass channel corresponding to the loss of benzene. Computational studies employed density functional theory (DFT). To determine structures and vibrational frequencies, the B3LYP functional was employed as implemented in Gaussian 03.86 This method has been documented in previous work to provide the most reliable predictions for infrared spectra. To account for dispersion forces influencing molecular geometries and to determine more accurate binding energies, the B97-D functional was employed as implemented in GAMESS.87,88 The 6-311+G(d,p) basis set was used for both sets of calculations. The energies are not ZPVE or BSSE corrected. For comparison to the experimental infrared spectra, harmonic frequencies were scaled on a mode-by-mode basis using the computed versus experimental frequencies for benzene.68 Bands in simulated spectra were given a 10 cm−1 fwhm Lorentzian line shape.
Figure 1. Photodissociation mass spectra of Al+(C6H6)n complexes, showing the tendency for each cluster to eliminate benzene molecules upon IR excitation.
spectra for these ions, when the laser is tuned to the most intense band of the C−H stretch Fermi triad. The negative peak indicates the depletion of the parent ion and the positive peaks indicate the resulting photofragments. As shown, Al+(benzene)3 loses one benzene to produce Al+(benzene)2. Likewise, Al+(benzene)4 also loses primarily one benzene, producing an intense signal for Al+(benzene) 3 as the fragmentation product. Al+(benzene)5,6 each lose a series of benzene molecules terminating at the Al+(benzene)3 ion. This kind of behavior, in which one particular ion survives preferentially in the dissociation processes from different starting ions, has been seen in the past for many metal ion− ligand complexes. Clearly, the details of ligand elimination depend on the ligand binding energy, the photon energy employed, and the number of photons absorbed. However, the behavior here suggests that the coordination around the aluminum cation is complete with three benzene molecules. To investigate the energetics of these fragmentation processes and the corresponding Al+(benzene)n structures, we have employed DFT computations. We first used the standard B3LYP functional, which has been shown to be reliable for the determinations of structures and the prediction of vibrational spectra for many transition metal cation−ligand complexes.89,90 B
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acting essentially as a solvent molecule. The external benzene is bound primarily by bifurcated π-hydrogen bonds involving C− H groups of two of the inner-sphere molecules directed into the π system of the outer one. This configuration is reminiscent of those seen for the neutral91 or protonated92 benzene dimers or for the external benzenes in Ni+(benzene)n complexes.65 The binding energies determined with the two functionals (not corrected for ZPVE or BSSE) are presented in Table 1. The values obtained with the B97-D functional are all significantly higher than those determined with B3LYP, consistent with a significant dispersion component in the bonding. The B3LYP data also exhibit a much steeper falloff in binding energies for the larger complexes. As shown in the table, the B97-D bond energy for the Al+(benzene) complex (35.0 kcal/mol) agrees well with previous computational results54 and with the experimental value.28 There are no previous results for the larger complexes. We have done all these calculations at the B3LYP level for complexes with and without argon. As shown in the Supporting Information, argon has virtually no effect on the structures or predicted vibrational frequencies. The predicted spectra shown in the figures throughout the paper are those for complexes without argon. The bent-sandwich structure for Al+(benzene)2 and the nonplanar configuration of Al+(benzene)3 are perhaps a little surprising, but in fact not unexpected on the basis of previous studies of aluminum ion−molecule complexes. As first described by Bauschlicher,70 the occupied 3s orbital for Mg+, Al+, and Si+ is highly polarizable, and electron repulsion causes a lobe of electron density to form in the space opposite ligands that bind to these ions. Because of this polarized orbital, second ligands do not bind exactly opposite the first but instead bind in bent positions, and third ligands also avoid the electron density to produce nonplanar complexes. These bent structures have been documented in experiments on the carbonyl and CO2 complexes of Mg+, Al+, and Si+.93−96 The same effect is seen for the Al+ and Zn+ cations in complexes with water, which are bent in the same way.70,74,90 The wavelength dependence of photodissociation processes such as these can provide their vibrational spectroscopy, and test these predictions of cluster structures. However, as noted above, the small Al+(benzene)n complexes do not photodissociate, suggesting that their bond energies are greater than the energy of the IR photons used to excite the vibrations. Multiphoton processes are possible in principle, but they are not usually efficient with the pulse energies (
