Spectroscopic Characterization of Nonconcerted [4 + 2] Cycloaddition

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Spectroscopic Characterization of Nonconcerted [4 + 2] Cycloaddition of 1,3-Butadiene with Lanthanacyclopropene To Form Lanthanum−Benzene in the Gas Phase Dilrukshi Hewage,† Wenjin Cao,† Jong Hyun Kim,† Ya Wang,‡ Yang Liu,*,‡ and Dong-Sheng Yang*,† †

Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, United States MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150080, China

‡

S Supporting Information *

ABSTRACT: The reaction between La atoms and 1,3-butadiene is carried out in a laservaporization molecular beam source. Metal−hydrocarbon species with formulas La(CnHn) (n = 2, 4, and 6) and La(CmHm+2) (m = 4 and 6) are observed with timeof-flight mass spectrometry and characterized with mass-analyzed threshold ionization spectroscopy. A lanthanum−benzene complex [La(C6H6)] is formed by 1,3-butadiene addition to lanthanacyclopropene [La(C2H2)] followed by molecular hydrogen elimination. Lanthanacyclopropene is an intermediate generated by the primary reaction between La and 1,3-butadiene. Two other intermediates produced by the La + 1,3butadiene reaction are La[η4-(1-buten-3-yne)] [La(C4H4)] and 1-lanthanacyclopent-3ene [La(C4H6)]. The La(benzene) complex exhibits distinctive metal−ligand bonding from that of the three intermediates as shown by the adiabatic ionization energies and ground electron configurations.

1. INTRODUCTION Not long ago, single-atom catalysis was a far-fetched idea because isolated atoms were thought to be chemically unstable or catalytically unreactive. Several recent studies, however, suggest that single-metal-atom catalysts supported on a solid surface are not only possible but also have numerous advantages.1−5 These latest studies shed new light on the relevance of gas-phase metal-atom chemistry to catalysis. Although they may not account for precise kinetics and thermodynamics in actual industrial reactions, gas-phase studies provide an efficient means to investigate intrinsic reactivity patterns, reaction pathways, and structure−reactivity relationships of crucial intermediates and to distinguish intrinsic chemistry without solvent and counterion interferences.6,7 Solvation may change the relative energies of the reactants, products, and transition states. If it alters the energy landscape of a reaction, differences are expected between the gas and solution phases; on the other hand, if the solvent effect does not change the overall shape of the potential energy surface of the reaction, parallels may exist between the two phases. Although connections remain to be found and rationalized for reactions in different environments, similarities have been reported for alkane reactions with radicals in the gas phase and solutions and on metal surfaces.8−11 A reasonable approach for searching for the connections is to examine systems in different phases to allow for detailed comparisons. Hydrocarbon compounds are ubiquitous in nature and the most abundant, low-cost feedstock for functionalized organic chemicals; yet many cannot participate in chemical reactions © XXXX American Chemical Society

under mild conditions because of their chemical inertness. Metal activation gets around this problem by stimulating inert hydrocarbons to react with other molecules and has continuously attracted extensive research activities in solution12−15 and gas7,16,17 phases. Although a large body of thermochemical and reactivity data for the gas-phase metalmediated hydrocarbon activation is available in the literature,6,7,16−25 spectroscopic measurements of intermediates and products formed in such reactions have only begun to emerge.26−33 Because of the lack of spectroscopic measurements, current knowledge about the structures and electronic states of the reaction intermediates and products is largely derived from theoretical predictions. However, the reliable prediction of the electronic states and geometric structures is complicated by the possibility of many low-energy structural isomers of each complex as well as a high density of low-lying electronic states or spin−orbit levels of each isomer. Therefore, a reliable identification of the molecular structures and electronic states generally requires confirmation by spectroscopic measurements. Several mass spectrometric studies have reported transition metal ion M+ (M = Cr, Mn, Fe, and Co)-mediated [4 + 2] cycloadditions of 1,3-butadiene with acetylene (or propyne) to yield 1,4-cyclohexadiene/M+, which subsequently dehydrogenates to produce benzene/M+ as the major product.34−37 Received: December 5, 2016 Revised: January 25, 2017 Published: January 26, 2017 A

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MATI scans. In the MATI experiment, the complexes were excited to high-lying Rydberg states in a single-photon process and ionized by a delayed pulsed electric field. The excitation laser was the same as that used for photoionization in the mass spectrometry and photoionization efficiency experiments and was the frequency-doubled output of a tunable dye laser (Lumonics HD-500), pumped by the third harmonic output (355 nm) of a Nd:YAG laser (Continuum Surelite II). The laser beam was collinear and counterpropagating with the molecular beam. The ionization pulsed field (320 V cm−1) was generated by two high-voltage pulse generators (DEI, PVX4140) and delayed by 10−20 μs from the laser pulse by a delayed pulsed generator (SRS, DG645). A small dc field (4.0 V cm−1) was applied to separate the ions produced by direct photoionization from the MATI ions generated by delayed field ionization. The MATI ion signal was obtained by scanning the wavelength of the tunable dye laser, detected by a dual microchannel plate detector, amplified by a preamplifier (SRS, SR445), averaged by a gated integrator (SRS, SR280), visualized by a digital oscilloscope (Tektronix TDS 3012), and stored in a laboratory computer. Laser wavelengths were calibrated against vanadium atomic transitions in the MATI spectral region.49 The Stark shift induced by the dc separation field was calculated using the relation of ΔIE = 6.1Ef1/2, where Ef is in V cm−1 and ΔIE is in cm−1.50 Geometry optimization and vibrational frequency calculations were carried out using the Gaussian 09 software package.51 In these calculations, we used the Becke’s threeparameter hybrid functional with the correlation functional of Lee, Yang, and Parr (B3LYP) and the 6-311+G(d,p) basis set for C and H and the effective-core-potential SDD basis set for La. We have extensively used the DFT method and found it generally produced adequate results for the spectral and structural assignments of organometallic complexes.44−46,52−55 No symmetry restrictions were imposed in the initial geometry optimizations, but appropriate point groups were used in the subsequent optimizations to help identify electronic symmetries. For each optimized stationary point, a vibrational analysis was performed to identify the nature of the stationary point (minimum or saddle point). In predicting reaction pathways, minima connected by a transition state were confirmed by intrinsic reaction coordinate calculations. To compare with the experimental MATI spectra, multidimensional FC factors were calculated from the equilibrium geometries, harmonic vibrational frequencies, and normal coordinates of the neutral and ionized complexes.56,57 In these calculations, the recursion relations from Doktorov et al.58 were employed, and the Duschinsky effect59 was considered to account for a possible axis rotation from the neutral complex to the cation. Spectral simulations were obtained using the experimental line width and a Lorentzian line shape. Transitions from excited vibrational levels of the neutral complex were considered by assuming thermal excitation at specific temperatures.

Without a dienophile, 1,3-butadiene reactions with metal ions (Fe+ and W+),38,39 metal oxide cluster ions (VxOy+ and TaxOy+),40−42 or neutral metal oxide clusters (VO3(V2O5)n)43 yielded association, dehydrogenation, or C−C bond cleavage products. In this work, we report spectroscopic characterization of the La atom reaction with 1,3-butadiene in a molecular beam source. In this reaction, we observed La(CnHn) (n = 2, 4, and 6) and La(CmHm+2) (m = 4 and 6) using time-of-flight (TOF) mass spectrometry and identified the structures of La(C2H2), La(C4H4), La(C4H6), and La(C6H6) using mass-analyazed threshold ionization (MATI) spectroscopy combined with density functional theory (DFT) calculations and multidimensional Franck−Condon (FC) simulations. La(C6H6) is identified as a metal−benzene complex formed by a nonconcerted [4 + 2] cycloaddition through multiple steps. La(C2H2), La(C4H4), and La(C4H6) are identified as lanthanacyclopropene, La[η 4 -(1-buten-3-yne)], and 1-lanthanacyclopent-3-ene, respectively. They are metal−hydrocarbon intermediates that may be involved in the formation of La(benzene) complex. This work continues our recent efforts on the spectroscopic characterization of metal-mediated hydrocarbon activation.44−47

2. EXPERIMENTAL AND COMPUTATIONAL METHODS The metal-cluster beam instrument used in this work consists of reaction and spectroscopy vacuum chambers and was described in a previous publication.48 The metal−hydrocarbon reaction was carried out in a laser vaporization metal cluster beam source. 1,3-Butadiene (≥99%, Sigma-Aldrich) was seeded in a He carrier gas with a concentration of ∼10−4 in a stainless steel mixing cylinder. Benzene (≥99%, Sigma-Aldrich) was introduced 3 cm downstream of the laser ablation point. La atoms were generated by pulsed-laser (Nd:YAG, Continuum Minilite II, 532 nm, ∼1.0 mJ/pulse) vaporization of a La rod (99.9%, Alfa Aesar) in the presence of the butadiene/He mixture (40 psi) delivered by a homemade piezoelectric pulsed valve. The metal atoms and gas mixtures entered into a clustering tube (2 mm diameter and 2 cm length) were then expanded into the reaction chamber, collimated by a coneshaped skimmer (2 mm inner diameter), and passed through a pair of deflection plates. Ionic species in the molecular beam that were formed during laser vaporization were removed by an electric field (100 V cm−1) applied on the deflection plates. The neutral products were identified by photoionization TOF mass spectrometry. A separate experiment was carried out to confirm that 1,3-butadiene was activated by neutral La atoms rather than La+ ions or the vaporization laser. In this experiment, a pair of electrodes was installed on the faceplate to remove metal ionic species by an electric field (200 V cm−1) before the metal−ligand reaction, and 1,3-butadiene was introduced 3 cm downstream of the laser vaporization point. The reaction products formed in the two experiments were identical, though a higher hydrocarbon concentration in the second experiment was required to produce comparable ion intensity in the TOF mass spectra. Because metal ionic species were removed before the reaction, the observed metal−hydrocarbons were formed by the reaction between the neutral metal atom and the organic ligand. Because 1,3-butadiene bypassed the vaporization region, the vaporization laser played no role in the hydrocarbon activation. Prior to the MATI measurements, photoionization efficiency spectra were recorded to locate the approximate ionization thresholds of the resultant La complexes to guide the survey

3. RESULTS AND DISCUSSION 3.1. Identification of La(benzene). Figure 1a presents the MATI spectrum of La(C6H6) formed in the reaction of La with 1,3-butadiene (∼10−4) seeded in 40 psi He carrier gas. The spectrum exhibits the origin band at 36820 (5) cm−1, a progression with vibrational intervals of 295 cm−1, a weak band at 527 cm−1 and its combination with a 295 cm−1 interval. It also shows satellite bands separated by 39 or 2 × 39 cm−1 from B

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Table 1. Adiabatic Ionization Energies (AIE, cm−1) and Vibrational Frequencies (cm−1) of Lanthanacyclopropene [La(C2H2)], La[η4-(butene-3-yne)] [La(C4H4)], 1Lanthanacyclopent-3-ene [La(C4H6)], and La(benzene) [La(C6H6)] from MATI Spectra and DFT/B3LYP Calculationsa lanthanacyclopropene (C2v), La(C2H2) AIE (1A1 ← 2A1) La−C2H2 stretch, ν4/ν4+ H scissor, ν3/ν3+ La[η4-(1-buten-3-yne)] (C1), La(C4H4) AIE (1A ← 2A) La−C2C4 stretch and C−H out-of-plane bend, ν19+ CH2 rock and La−C2C3 stretch, ν17+ C2C3C4 bend and CH2 rock, ν15+ C−H wag, ν12+ 1-lanthanacyclopent-3-ene (Cs), La(C4H6) AIE (1A′ ← 2A′) C2HC3H wag, ν13 /ν13+ La−C1C4 stretch and CH2 rock, ν12 /ν12+ CH2 twist, ν11+ CH2 rock (ν14+ − ν14) La(benzene) (C2v), La(C6H6) AIE (1A1 ← 2A1) La-benzene stretch, ν10/ν10+ C−H wag and in-plane ring deformation, ν18+ ring rock (ν24+ − ν24)

Figure 1. MATI spectra of La(C6H6) formed by the La + 1,3butadiene reaction in He (a), La(benzene) by the La + benzene association in He (b), and Ar:He = 1:1.5 (d) and 1A1 ← 2A1 simulations of La(benzene) (C2v) at 200 K (c) and 50 K (e).

the members of 295 cm−1 progression and a weak band at 152 cm−1. Although it has many stable isomers, we have identified C6H6 in the complex as a benzene molecule by comparing the MATI spectra of La(C6H6) and La(benzene) (Figure 1a,b). La(benzene) was produced by La association to benzene in the same molecular beam instrument. The two spectra are essentially identical, with the same origin-band energy and vibrational intervals, although they show slightly different signal-to-noise ratios. To assign the MATI spectra, we carried out spectral simulations based on the multidimensional FC factor calculations (Figure 1c,e). In these simulations, the theoretical 0−0 transition is aligned with the experimental origin band; the vibrational frequencies are unscaled in order to directly compare with the experimental data. The simulation of the 1A1 ← 2A1 transition of La(benzene) (C2v) (Figure 1c) reproduces almost all the observed bands, though the weak 152 cm−1 band is not shown. The 39 cm−1 satellite bands are identified to be transitions of thermally excited vibrational levels of the neutral state by comparing the MATI measurements with different carrier gases (Figure 1b,d) and spectral simulations at various temperatures (Figure 1c,e). In the spectrum of La(benzene) seeded in an Ar:He = 1:1.5 mixture (Figure 1d), the 39 cm−1 satellite bands either disappear or their intensities are dramatically decreased because of the lower molecular vibrational temperatures in the heavier Ar/He carrier.60 The vibrational temperatures of the complex are estimated to be ∼200 K with the He carrier and ∼50 K with Ar/He as the simulations at these two temperatures have the best agreement with the measured spectra. The spectral assignments based on the simulations are summarized in Table 1. The adiabatic ionization energy (AIE) of the complex corresponds to the energy of the origin band and is 36820 (5) cm−1 (4.5651(6) eV). The 295 and 527 cm−1 transitions are assigned to a La+-benzene stretch and a C−H wag coupled with an in-plane ring deformation, respectively. The 39 cm−1 satellite bands are sequence transitions of a ring rocking mode for which the vibrational frequency in the ion is 39 cm−1 higher than that in the neutral state. These observations are consistent with

MATI

DFT

41174 495/522 806/806

42107 498/528 833/833

41000 386 528 616 900

41791 373 516 626 901

39418 289/318 374/396 470 22

40164 280/310 360/391 492 24

36820 250/295 527 39

37472 263/287 535 50

The associated error in experimental AIEs is ±5 cm−1. νn and νn+ are vibrational modes in the neutral and ion states, respectively. The numbering of carbon atoms is shown in Figure 2.

a

those of the previous electron spectroscopic study of our group.61 Unlike a free benzene molecule, C6H6 in the La complex is bent away from the metal atom, and La(benzene) has a C2v structure (Figure 2d). The bending of the benzene

Figure 2. Structures of La(C2H2) (a), La(C4H4) (b), La(C4H6) (c), and La(C6H6) (d) formed by the La + 1,3-butadiene reaction.

ring is caused by a pseudo-Jahn−Teller distortion.61 The ground state of the neutral species is 2A1 with the La valenceelectron configuration 5d26s1. Ionization of the neutral doublet state yields the ground ion state 1A1 by removing the La 6s1 electron. 3.2. Reaction Intermediates and Formation of La(benzene). To investigate the formation of La(benzene), we first analyzed the chemical compositions of the metal−hydrocarbon species using TOF mass spectrometry and then determined their structures using MATI spectroscopy. Figure 3 shows a typical TOF mass spectrum of the molecular beam formed by the La reaction with 1,3-butadiene (∼10−4) seeded in a He carrier gas (40 psi). The mass spectrum displays Lahydrocarbon ions with formulas La(CnHn) (n = 2, 4, and 6) and La(CmHm+2) (m = 4 and 6), which are produced by singlephoton 230 nm (43 478 cm−1) laser ionization. Dissociative ionization is not possible because the incident photon energy is C

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MATI spectrum is 41 174 (5) cm−1, with the metal−ligand stretching frequencies 522/495 cm−1 and hydrogen scissor frequencies 806/806 cm−1 in the ion/neutral (1A1/2A1) states (Figure 4 and Table 1). The vibrational mode assignments are

Figure 3. TOF mass spectrum of the La + 1,3-butadiene reaction recorded at a laser ionization wavelength of 230 nm. The seeding concentration of 1,3-butadiene in He was ∼10−4.

only slightly above the AIEs of the La complexes (Table 1), and the excess photon energy is much smaller than the C−C or C− H bond dissociation energies. Although both La atoms and ions can be produced by laser vaporization of a La target, the observed metal−hydrocarbon species are formed by neutral La reactions, rather than La+ reactions or laser vaporization. This is confirmed by the experiments where 1,3-butadiene bypassed the laser ablation region and possible La+ ions were removed prior to the reaction as described in the Experimental section.

Figure 4. MATI spectrum of La(C2H2) formed by the La + 1,3butadiene reaction in He (a) and simulation (800 K) of the 1A1 ← 2A1 (C2v) transition (b).

based on the comparison of the experimental and simulated spectra. The bonding between La and C2H2 is an example of the classic Dewar−Chatt synergic interaction, which involves the donation of C2H2 π electrons to an empty La dσ orbital and the back-donation from a La dπ orbital to an empty C2H2 π* orbital. Because of the strong π electron-back-donation, the C− C triple bond in the free C2H2 molecule becomes a double bond in the complex and the carbon atomic orbitals rehybridize from sp to sp2 as indicated by the C−C distance (1.346 Å) and CCH angle (126.8°) in Table S1. The metallacycle is the most stable isomer of La(C2H2) and has also been observed from La reactions with acetylene and ethylene.45,46 The major difference is that the La(C2H2) complex formed by the C−C bond cleavage of 1,3-butadiene appears to be vibrationally much hotter than the same species formed by the simple association of acetylene or the dehydrogenation of ethylene. This is evident from the observation of the hydrogen scissor mode (806 cm−1) and the high intensity of the La−C2H2 stretch mode (495 cm−1) in the neutral 2A1 state (Figure 4). In contrast, the hydrogen scissor hot band was not observed, and the intensity of the metal−ligand stretch hot band was much lower in the MATI spectra of La(C2H2) formed by the La reactions with acetylene or ethylene. The formation of lanthanacyclopropene from La + trans-1,3-butadiene takes multiple steps (Figure S1a). The first step is the formation of a La(trans-C4H6) π complex. From the trans complex, we have found two possible pathways to form La(C2H2). The first path involves the trans− cis isomerization, followed by a multistep hydrogen migration to form an inserted species (IM4), and C2−C3 carbon bond cleavage to yield lanthanacyclopropene and ethylene. Alternatively, the La(trans-C4H6) complex could undergo a direct hydrogen migration to form the same inserted species prior to the C−C bond breakage. The overall reaction of La + 1,3butadiene → La(C2H2) + C2H4 is predicted to be barrierless and exothermic by 18.2 kcal mol−1. La(C2H2) could also be formed by La(C4H4) decomposition; however, the La(C4H4) → La(C2H2) + C2H2 dissociation is not only endothermic (49.2 kcal mol−1) but also has a very high energy barrier as shown in Figure S1b and Table S2.

Scheme 1. Overall Reactions with Predicted Reaction Energies (kcal mol−1)

Scheme 1 lists overall reactions for the formation of La(CnHn) (n = 2, 4, and 6) and La(CmHm+2) (m = 4 and 6). The primary reactions (1a−1c) include the association to form La(C4H6), dehydrogenation to La(C4H4), and C−C bond cleavage to La(C2H2); the secondary reactions involve the addition of the La(C2H2) to a second 1,3-butadine molecule (2a−2b) to form La(C6H8), which subsequently dehydrogenates to La(C6H6). These reactions account for all of the observed metal−hydrocarbon species and are predicted to be exothermic by the DFT calculations. In addition to the abovementioned reactions, La(C6H6) may be formed by La(C4H6) or La(C4H4) addition to a second 1,3-butadiene molecule followed by the C−C bond cleavages; however, these reactions can be excluded because the expected products La(C8H12) and La(C8H10) were not observed in our experiment. This is consistent with a previous study of reactions mediated by Fe+, which showed the preference of 1,3-butadiene + alkyne addition over butadiene dimerization.35 Similarly, La(C4H6) + C2H4 may be excluded because La(C6H10) was not observed. La(C6H6) may also be formed by La(C4H4) + C2H4; however, the amount of C2H4 produced by the primary reaction (1c) is expected to be too small to be significant for subsequent bimolecular reactions. In the following paragraphs, we will first discuss the MATI spectrum, bonding and structure, and formation of three intermediates La(C2H2), La(C4H4), and La(C4H6) and then the formation of La(benzene). La(C2H2) is identified as lanthanacyclopropene, a C2v structure with La 2-fold bonding to the carbon atoms (Figure 2a). The AIE of lanthanacyclopropene measured from the D

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The Journal of Physical Chemistry A Unlike lanthanacyclopropene, La(C4H4) is a nonplanar molecule without any symmetry element (except for the identity) and is identified as La[η4-(1-buten-3-yne) (Figure 2b). The AIE of the complex is measured to be 41 000 (5) cm−1; a metal−ligand stretching frequency is measured as 386 cm−1 for a largely La−C2,4 stretch; and CH2 rock, carbon skeleton bend, and C−H wag frequencies are determined to be 528, 616, and 900 cm−1, respectively (Figure 5 and Table 1). Like

Figure 6. MATI spectrum of La(C4H6) formed by the La + 1,3butadiene reaction in He (a) and simulation (200 K) of the 1A1 ← 2A1 (Cs) transition (b).

is calculated to be exothermic by 37.5 kcal mol−1 with a barrier of 18.6 kcal mol−1. However, the barrier is only 0.4 kcal mol−1 with respect to the energies of La + 2 1,3-butadiene (Table S2). Such a small barrier is easily overcome by molecular collision and thermal energies in the molecular beam source. Because it consists of multiple steps, the 1,3-C4H6 + La(C2H2) reaction can only been called a nonconcerted [4 + 2] cycloaddition, not a traditional Diels−Alder reaction. Furthermore, unlike previously reported metal ion-mediated [4 + 2] cycloadditions with both diene and dienophile as initial reactants,34−37 the dienophile in this reaction is a metallacycle produced as an intermediate by the primary reaction. The metal−ligand bonding of the three intermediates is considerably different from that of La(benzene) as indicated by their ground electron configurations and AIEs. Unlike La(benzene) that has a La-valence electron configuration of 5d26s1, the three intermediates have an electron configuration of La 6s1. The remaining two 5d electrons that are associated with the isolated La atom are spin paired in a molecular orbital that is a bonding combination between a 5d La orbital and a π* empty antibonding orbital of the hydrocarbons. The charge transfer from La to the hydrocarbons leads to a formal oxidation state of +2 for the ligated metal atom and higher AIEs of the three smaller species than that of the benzene complex (Table 1). We have also studied the reactions of La atoms with several other unsaturated hydrocarbons, including acetylene, propyne, and ethylene, and found many examples of metal-mediated hydrocarbon activation.44−46 The key step in these reactions is the formation of a metallacycle in the entrance channel that is formed at a much lower energy than the reactants and without any energy barriers. Successful reactions require access to a metal atomic low-energy dn−1s1 (or dn−2s0) configuration able to minimize long-range repulsive interactions and to form two new covalent bonds in the metallacycle (and the subsequent insertion complex). Although La has a nonreactive ground electron configuration 5d16s2, its reactivity benefits from the existence of low-energy, reactive configuration 5d26s1 (0.33 eV). We expect other rare-earth elements with similar reactive configurations, such as Y (4d25s1, 1.36 eV), Ce (4f15d26s1, 0.29 eV), and Gd (4f75d26s1, 0.79 eV), to be reactive with alkenes or alkynes as well. Sc also has a relative low energy 3d24s1 configuration (1.43 eV), but the first-row transition elements are generally less reactive than the second- and third-row counterparts.62,63

Figure 5. MATI spectrum of La(C4H4) formed by the La + 1,3butadiene reaction in He (a) and simulation (200 K) of the 1A ← 2A (C1) transition (b).

lanthanacyclopropene, the first step to form La[η4-(1-buten-3yne)] is the La association to trans-1,3-butadiene. Instead of subsequent isomerization, the trans-C4H6 motif undergoes metal insertion into C−H bonds, followed by H2 elimination to form the complex (Figure S2). The La + 1,3-butadiene → La(C4H4) + H2 reaction is exothermic by 26.2 kcal mol−1 and has no energy barriers. Depending on which two hydrogens are removed, the H2 elimination of La(trans-C4H6) could yield other isomers by different paths. However, those isomers are either at higher energies (5.3−31.0 kcal mol−1) than La[η4-(1buten-3-yne) or with significant energy barriers (9.1−24.4 kcal mol−1). La(cis-C4H6) is a five-membered Cs metallacycle, 1-lanthanacyclopent-3-ene, with the reflection plane passing through the La atom and the middle point of the C2−C3 bond (Figure 2c). The complex ionizes at 39 418 (5) cm−1. The La−C4H6 stretching, hydrogen wagging, and hydrogen twisting frequencies are measured to be 396, 318, and 470 cm−1 in the ion state, respectively (Figure 6 and Table 1). The two weak bands marked with an asterisk are combination bands of (318 + 396) and (396 + 476) cm−1. The metal−ligand stretching (374 cm−1) and hydrogen wagging (289 cm−1) frequencies are slightly lower in the neutral species. Upon La coordination, the carbon skeleton in the cis-C4H6 motif remains planar, the 1,3double bonds shift to the middle, and the two terminal carbon atoms bind with La (Figure 2c). The La + 1,3-butadiene → La(cis-C4H6) addition is exothermic by 57.6 kcal mol−1 (Figure S1a). The metallacyclopentene was also observed in the Lamediated C−C bond coupling of ethylene.45 La(benzene) is formed by the La(C2H2) + 1,3-butadiene (Figure S3) secondary reaction. The reaction begins with a simple association, followed by the C−C bond coupling and cyclization to form a La(cyclohexadiene) complex through a formal [4 + 2] cycloaddition. The metal−diene complex then undergoes H2 elimination via a metal-insertion intermediate to produce La(benzene). The La(C2H2) + 1,3-butadiene reaction E

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The Journal of Physical Chemistry A

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4. CONCLUSIONS Lanthanum−benzene is identified from the reaction between La atoms and 1,3-butadiene. The metal−benzene complex is formed by a nonconcerted cycloaddition of 1,3-butadiene to lanthanacyclopropene followed by dehydrogenation. The metallacyclopropene and two additional intermediates, La[η4(1-buten-3-yne)] and 1-lanthanacyclopent-3-ene, are formed in the primary reactions of La + 1,3-butadiene. Although all of these metal−hydrocarbon species prefer doublet ground electronic states, La(benzene) shows considerably different bonding from that of the three intermediates. Because cycloadditions are strongly influenced by the electronic and steric properties of hydrocarbon reactants, future studies using substituted dienes may offer opportunities to examine these effects.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b12239. Geometries of La(C2H2), La(C4H4), La(C4H6), and La(C6H6); relative energies of minima and transition states; and reaction pathways for the formations of La(C2H2), La(C4H4), and La(C6H6) from the La + 1,3butadiene reaction (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Y.L.) E-mail: [email protected]. *(D.-S.Y.) E-mail: [email protected]. ORCID

Dong-Sheng Yang: 0000-0001-9842-4343 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Foundation Division of Chemistry (Chemical Structure, Dynamics, and Mechanisms, Grant CHE-1362102 to D.S.Y.), the Kentucky Science and Engineering Foundation (D.S.Y.), the Natural Science Foundation of Heilongjiang province of China (Grant B2016004 to Y.L.), the Fundamental Research Funds for the Central Universities of China (Grant HIT.NSRIF.2017033 to Y.L.), and the National Natural Science Foundation of China (Grant 21203041 to Y.L).

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REFERENCES

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DOI: 10.1021/acs.jpca.6b12239 J. Phys. Chem. A XXXX, XXX, XXX−XXX