MetalâCyclopentadienyl Bond Energies in ... - ACS Publications

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Metal−Cyclopentadienyl Bond Energies in Metallocene Cations Measured Using Threshold Collision-Induced Dissociation Mass Spectrometry Tyson G. Rowland,† Bálint Sztáray,*,† and Peter B. Armentrout*,‡ †

Department of Chemistry, University of the Pacific, 3601 Pacific Ave, Stockton, California 95211, United States Department of Chemistry, University of Utah, 315 South 1400 East Room 2020, Salt Lake City, Utah 84112, United States

‡

S Supporting Information *

ABSTRACT: Metal−cyclopentadienyl bond dissociation energies (BDEs) were measured for seven metallocene ions (Cp2M+, Cp = η5-cyclopentadienyl = c-C5H5, M = Ti, V, Cr, Mn, Fe, Co, Ni) using threshold collision-induced dissociation (TCID) performed in a guided ion beam tandem mass spectrometer. For all seven room temperature metallocene ions, the dominant dissociation pathway is simple Cp loss from the metal. Traces of other fragment ions were also detected, such as C10H10+, C10H8+, C8H8+, C3H3+, H2M+, C3H3M+, C6H6M+, and C7H6M+, depending on the metal center. Statistical modeling of the Cp-loss TCID experimental data, including consideration of energy distributions, multiple collisions, and kinetic shifts, allow the extraction of 0 K [CpM+− Cp] BDEs. These are found to be 4.85 ± 0.15, 4.02 ± 0.14, 4.22 ± 0.13, 3.51 ± 0.12, 4.26 ± 0.15, 4.57 ± 0.15, and 3.37 ± 0.12 eV for Cp2Ti+, Cp2V+, Cp2Cr+, Cp2Mn+, Cp2Fe+, Cp2Co+, and Cp2Ni+, respectively. The measured BDE trend is largely in line with arguments based on a simple molecular orbital picture, with the exception of the anomalous case of titanocene, most likely attributable to its bent structure. The new results presented here are compared to previous literature values and are found to provide a more complete and accurate set of thermochemistry.

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INTRODUCTION For many scientists, the origin of organometallic chemistry stems from the discovery of ferrocene by Pauson and Kealy in 1951 and the “sandwich structure” characterization by Wilkinson in 1952.1−3 Through 60 years of research on ferrocene and other metallocene derivatives, significant applications have been discovered for these systems such as antiknocking reagents in fossil fuels,4 catalysts for olefin polymerization,5−8 and more recently their catalytic role in nanotube production.9 Despite the central role the sandwich complexes play in organometallic chemistry and especially textbook organometallic chemistry, the thermochemistry of these compounds is still not well established. The ionic bond dissociation energies (BDEs) found in the literature10−13 for the metal−cyclopentadienyl (Cp = c-C5H5) bond show variations of several electronvolts, and these ambiguities can be attributed largely to deficiencies in the models used to extract thermochemical data from the experiment. Similarly, conformations, molecular orbital occupations, and vibrational properties have been argued about for several decades.14−18 Over the years, many investigations have been performed to measure the metal−Cp bond strength in metallocene ions. Of these, one of the most extensive studies has been performed by Müller and D’Or who determined [CpM+−Cp] BDEs using © 2012 American Chemical Society

electron ionization mass spectrometry (EI-MS), obtaining 5.32 ± 0.15, 6.55 ± 0.15, 3.77 ± 0.15, 6.63 ± 0.15, 7.79 ± 0.15, and 5.43 ± 0.15 eV for M = V, Cr, Mn, Fe, Co, and Ni, respectively.10 In these measurements, the precursor Cp2M+ and fragment CpM+ ion appearance energies (AEs) were determined with their difference yielding the indicated BDE.19 Similar to our results, vide infra, multiple channels signifying either consecutive fragmentation or losses other than the C5H5 ligands were seen but were not analyzed because of the low ion intensities in such dissociation channels. Despite the rather impressive error bars reported, the accuracy of these numbers is questionable, not only because of the poor energy resolution of EI-MS but also because of the lack of attention in the analysis to kinetic and thermal energy shifts. Later, Opitz and Härter reexamined some of these metallocenes using a laser multiphoton ionization experiment but again resorted to electron ionization to determine thermochemistry, finding dissociation energies of 4.61 ± 0.43 eV for [CpFe+−Cp] (2 eV below that of Müller and D’Or), 2.34 ± 0.34 eV20 for [CpV+−Cp] (3 eV Special Issue: Peter B. Armentrout Festschrift Received: July 26, 2012 Revised: December 4, 2012 Published: December 5, 2012 1299

dx.doi.org/10.1021/jp307418c | J. Phys. Chem. A 2013, 117, 1299−1309

The Journal of Physical Chemistry A

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lower than Müller and D’Or), and 4.86 ± 0.42 eV21 for [CpCo+−Cp] (almost 3 eV lower than the Müller and D’Or). In this study, CpM+ threshold values were calculated by extrapolation to zero of a log-based product ion intensity (generated from metastable precursor ions) versus electron energy plot. In another EI-MS study, Pignataro and Lossing published bond energies of 5.81 ± 1.1, 6.33 ± 1.0, and 5.16 ± 1.1 eV for Cp2M+, M = Fe, Co, and Ni, respectively.22 The time-resolved photodissociation measurements of nickelocene ions in an ion cyclotron resonance (ICR) mass spectrometer by Lin and Dunbar23 took kinetic shifts into account by performing an extrapolation to threshold using Rice− Ramsperger−Kassel−Marcus (RRKM)24−27 rates. The RRKM rate curve, however, was only fitted to two data points, of which one of the rates is surprisingly slow, raising the question of interference from IR radiative relaxation. This method yielded a [CpNi+−Cp] dissociation energy of 3.24 ± 0.07 eV,23 2 eV below the EI-MS values. The most recent studies, performed by Baer, Sztáray, and co-workers, used threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. The analysis of the data utilized two rate theories (RRKM and the simplified statistical adiabatic channel model, SSACM28) to account for kinetic shifts. These studies gave ion BDEs of 4.61 ± 0.12 (5.04 ± 0.12), 3.43 ± 0.17, 5.59 ± 0.14 (5.77 ± 0.11), and 3.66 ± 0.16 (3.96 ± 0.11) eV for RRKM (SSACM) analysis of Cp2M+, M = Cr, Mn, Fe, and Ni, respectively.29,30 The only metallocene studied here for which we have not found previous ionic bond energy data is the titanocene cation, which is not entirely surprising as the neutral precursor, Cp2Ti, is not stable, rather forming a 14-electron η5-fulvalene-di-μ-hydrido species.31 In this paper, we present a comprehensive study on all available first-period metallocene ions and report ionic BDEs of [CpTi+−Cp], [CpV+−Cp], [CpCr+−Cp], [CpMn+−Cp], [CpFe+−Cp], [CpCo+−Cp], and [CpNi+−Cp] obtained using threshold collision-induced dissociation (TCID) on a guided ion beam tandem mass spectrometer (GIBMS), taking particular care to properly account for dissociation rates, ion energy distributions, and multiple collisions in the data analysis. In these studies, we also compare the present results that model the dissociation kinetics assuming loose (orbiting) transition states with earlier TPEPICO studies where the experimental data were modeled with either rigid activated complex (RAC)RRKM or SSACM. In addition, we consider whether the BDEs follow the trends derived from simple molecular orbital considerations, quantum chemical calculations, and literature experimental studies.

previously,29 together with counterpoise (cp) corrected39,40 single-point B3LYP/6-311++G(2df,2pd)41−45 calculations to derive theoretical BDE values. For the two metallocenes of titanium and vanadium, which were not included in the previous calculations, the ground state spin multiplicities of the Cp2M+ parent ions are 2 and 3, respectively, whereas for CpTi+ and CpV+ ions, they are 3 and 4, respectively. In addition, to provide computed values for comparison to the experimental BDEs and to include the two additional metallocenes in a consistent set of calculations, we have carried out two new sets of DFT calculations. First, test calculations show that diffuse basis functions have a strong effect on the total electronic energy and, potentially, the optimal geometry. Therefore, for all seven metal systems, 0 K CpM+−Cp bond energies were estimated by carrying out single-point B3LYP/6311++G(2df,2pd) calculations using geometries determined at the B3LYP/6-311+G(d,p) level. Zero-point vibrational energies (ZPVEs) were also taken from the B3LYP/6-311+G(d,p) calculations. The computed vibrational frequencies were not scaled with the usual scaling factors (determined for a set of organic molecules) as these frequency scaling factors may not be reliable for organometallic species and the difference introduced by scaling the zero-point vibrational energy differences is less than a few meVs. Comparison to solutionphase vibrational spectra of various metallocenium salts would be an option but the correction using the literature scaling factor (0.988) for zero-point vibrational energies46 is probably smaller than solvation effects on these frequencies. In contrast to the calculations in the TPEPICO paper,29 counterpoise corrections are omitted as these are computationally rather expensive and the cp corrections were found to be on the order of 50 meV or less, which is well within the expected accuracy of these theoretical BDE values. It has been noted recently that various DFT functionals can give wildly different homolytic BDEs for first-row metallocenes,47 with B3LYP systematically underestimating, and BP86 overestimating the BDEs, compared to experimental data. Therefore, we have repeated the same calculations outlined above, but with the BP86 functional, to get a theoretical “upper limit” on the BDEs at the BP86/6-311++G(2df,2pd) // BP86/ 6-311+G(d,p) level of theory. It is worth noting that the Cp2Mn+ and Cp2Fe+ systems required special handling as the default approach in Gaussian to finding the ground state wave function failed, requiring the use of the stable=opt keyword. In addition, geometry optimizations of the triplet state of Cp2Mn+ find a ground state having an imaginary frequency of 61 cm−1 corresponding to ring bends. Likewise, geometry optimizations of the doublet state of Cp2Fe+ find an unsymmetric conformation having an imaginary frequency of 7 cm−1 corresponding to a ring twist, as though pointing toward a D5h structure. In both cases, explicit explorations of the potential energy surfaces associated with these imaginary modes fail to find a lower energy conformation. As titanocene has a surprisingly large bond energy (vide infra), unexplained by the MO considerations, one could wonder whether radically different structures for Cp2Ti+ should also be considered. In our DFT calculations, we have found an isomer structure in which two hydrogen atoms are coordinated as an H2 unit (H−H bond length: 0.765 Å) and the Cp-rings fuse into a C10H8 fulvalene ligand with a ring-to-ring C−C distance of 1.48 Å. However, the energy of this isomer structure is calculated to lie 1.99 eV higher than the bent CpTiCp+ ion.

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QUANTUM CHEMICAL CALCULATIONS The modeling of the experimental data requires vibrational frequencies and rotational constants of the relevant neutral and ionic species. These were determined from quantum chemical calculations, which were performed using the Gaussian 03 and 09 packages.32,33 Initial calculations were carried out using density functional theory (DFT) with the B3LYP34−36 exchange−correlation functional and the 6-311G(d,p)37,38 basis set. Briefly, geometries of the relevant species were optimized at this DFT level, and the minima were confirmed by the absence of imaginary frequencies. Various spin states were considered for the neutral molecules and parent ions, and the lowest lying states were chosen to determine the vibrational frequencies used in the modeling. For Cp2M+ and CpM+, M = Cr, Mn, Fe, Co, and Ni, these calculations were performed 1300



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EXPERIMENTAL SECTION Instrumental Setup for GIBMS. The guided ion beam tandem mass spectrometer comprises five main sections: the ion source with a 1 m long flow-tube for ion thermalization, a magnetic sector for precursor ion selection, a radio frequency (rf) octopole ion guide48 with low-pressure collision cell, a quadrupole mass filter for mass analysis of the fragment ions, and a secondary electron/scintillator ion detector.49 Details of this instrument have been given previously.50−55 Briefly, metallocene cations were produced by microwave discharge in helium at flow rates between 4000−9000 standard cm3/min. Vapors of the commercially obtained (Strem Chemicals, Inc.) metallocenes (Cp2V−Cp2Ni) or Cp2Ti(CO)2 were introduced into the ionized/excited helium gas approximately 20 cm downstream with a flow of argon gas (∼10% of the helium flow rate) where the metallocenes or Cp2Ti(CO)2 were ionized and partially fragmented.56 Suitable precursors for other first-row transition metals are not available. The helium/argon flow was maintained at a pressure of ∼1 Torr in the 1 m long flow tube. This pressure allowed the ions to thermalize to room temperature both rotationally and vibrationally as they made ∼105 collisions with the buffer gas while traveling down the flow tube.57−59 Accordingly, a Maxwell−Boltzmann distribution of ground state ions at 298 K is assumed to be a good representation of the internal energy of the reactant metallocene ions. Following thermalization, the ions were focused into and mass selected by a magnetic sector mass analyzer. Upon leaving the magnetic field, the mass-selected precursor ions were decelerated in an exponential retarder and focused into a rf octopole ion guide. The octopole region provides ideal rf-only trapping of ions in the radial direction without perturbing the axial ion velocity, because of the steep increase (as r6) of the effective potential of the octopole’s trapping well.51,60,61 The octopole passes through a reaction gas cell with an effective length of 8.3 cm. In the experiments presented here, xenon was used as the collision partner given its large polarizability, which allows for efficient translational to internal energy transfer with the ion complex.52,62 Finally, product ions were focused into and mass analyzed with a linear quadrupole mass filter with high collection efficiency and then detected with a Daly type detector.49 The measured ion intensities were converted to absolute reaction cross sections as described previously.51 Absolute cross section magnitudes are estimated to be accurate within ∼20% and relative cross sections are accurate to 5%. In addition, laboratory (Lab) frame ion energies were converted to centerof-mass (CM) frame energies using ECM = ELab × M/(M + m), where M is the mass of the collision gas (Xe) and m is the mass of the reactant ion.51 All the energies listed below are in the CM frame unless otherwise noted. The absolute energy scale and kinetic energy distribution of the ion beam were determined by using the octopole as a retarding energy analyzer as described previously.51 This procedure limits uncertainties from contact potentials, space charge effects, and focusing aberrations. The full width half-maximum (fwhm) of the ion kinetic energy distribution is typically 0.2−0.4 eV (Lab frame). The absolute uncertainty of the energy scale in the Lab frame was 0.05 eV throughout the experiments.63,64 Another important consideration when performing and analyzing TCID experiments is pressure effects on reaction cross sections. Even when the collision cell pressure is maintained at low levels, the reactant ion can undergo multiple

collisions with the neutral collision gas. Therefore, the pressure dependence of all TCID experiments was explicitly measured as it is often difficult to predict how multiple collisions will effect a particular TCID experiment.65,66 In our experiments, data were collected using Xe pressures around 0.05 (low), 0.10 (medium), and 0.20 (high) mTorr. In the case of noticeable pressure effects, extrapolation to zero pressure was performed to obtain data that can be analyzed to yield accurate threshold energy values, as discussed further below. Modeling and Threshold Analysis. After the ion intensities are converted to reaction cross sections, plots of cross section (σ in Å2) versus center-of-mass collision energy (E in eV) data can be modeled using eq 1 to determine the 0 K threshold energy for metal−ligand dissociation, E0: σ(E) = σ0 ∑ gi

(E + Ei − E0)n E

(1)

where σ0 is an energy-independent scaling factor, n is an adjustable parameter that describes the efficiency of energy transfer in the collision with Xe,67 Ei is the ro-vibrational energy of the internal states of the reactant ion, i, and gi is the population of those states, where Σgi = 1. Derivation of this formula can be found in the literature.52,68 Furthermore, by integrating over an internal-energy-dependent dissociation probability, kinetic shifts are modeled using statistical rate theory as shown in eq 2.69−71 ⎛ σ ⎞ σ (E ) = ⎜n 0 ⎟ ∑ g i ⎝ E⎠ i

dε

E

∫E −E [1 − e−k(ε+E )τ] × (E − ε)n−1 i

0

i

(2)

Here, ε is the energy deposited in the metallocene ion upon collision with Xe, τ is the average experimental time available for dissociation (the ion time-of-flight from the collision cell to the quadrupole mass analyzer, on average ∼10−4 s), k(ε+Ei) is the unimolecular rate constant determined by RRKM calculations, and all other parameters are the same as eq 1.67 When the dissociation rate is fast compared to the time-offlight, eq 2 collapses to eq 1. By modeling TCID cross sections in terms of energy distributions and dissociation rates, the threshold energies for dissociation (E0) can be extracted from the experimental cross section curves using E0, n, and the scaling factor σ0 as fitting parameters.60 Because of the long-range ion-induced dipole attraction, the transition states for dissociation are expected to be loose and the threshold energies to correspond directly to the BDE. (As noted by a referee, it is possible that the covalent interactions between the Cp ligand and the open shell metal cations could alter this long-range behavior near threshold; however, as discussed further below, use of the rigid activated complex gave worse agreement with literature bond energies. Further, the TPEPICO experiments in which the absolute dissociation rates were explicitly measured definitely showed a loose TS.) Therefore, the dissociation rates were modeled using a loose phase space limit (PSL) transition state (TS) model for the Cploss dissociation of the metallocene ions. That is, the transitional degrees of freedom that become the translations and rotations of the dissociated products were treated as rotors.70 This approach has been shown to reproduce experimental TCID dissociation cross sections for a diverse set of systems including diatomic, small and large polyatomic species.62−73 In this model, the location of the TS is at the 1301



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activation at 1000 K as determined using methods outlined elsewhere.69,70

centrifugal barrier for the products and the vibrational frequencies of the TS are those associated with these products. Therefore, the B3LYP/6-311G(d,p) quantum chemical calculations used to obtain the geometries for the dissociation energy calculations were also used to provide the vibrational frequency sets and rotational constants of Cp2M+, CpM+, and the Cp neutral. The two-dimensional (2D) external rotations are treated adiabatically but include centrifugal effects.74 In the present work, the adiabatic 2D external rotational energy of the EM is calculated using a statistical distribution with an explicit summation over the possible values of the rotational quantum number.70 During the least-squares fitting process, the σ0, n, and E0 parameters were optimized until reasonable agreement with the experimental data was achieved. Furthermore, the uncertainty in E0 was established from variations over multiple data sets, by scaling the TS vibrational frequencies by ±10%, by increasing and decreasing the time window for dissociation by a factor of 2, as well as including the ±0.05 eV (Lab) energy uncertainty.

Table 1. Fitting Parameters of Eq 2 for Modeling Loss of Cp from Cp2M+ As Measured Using TCID σ0 +

Cp2Ti Cp2V+ Cp2Cr+ Cp2Mn+ Cp2Fe+ Cp2Co+ Cp2Ni+

12 14 3 6 14 9 19

n

± ± ± ± ± ± ±

1 2 1 2 7 1 2

1.7 1.7 1.3 1.5 1.2 1.1 1.2

± ± ± ± ± ± ±

E0 (eV) 0.1 0.1 0.1 0.1 0.1 0.2 0.1

4.85 4.02 4.22 3.51 4.26 4.57 3.37

± ± ± ± ± ± ±

ΔS‡1000 (J/K mol)

0.15 0.14 0.13 0.12 0.15 0.15 0.12

117 99 128 102 129 140 96

± ± ± ± ± ± ±

5 4 5 5 4 5 5

Nickelocene. The nickelocene data, Figure 2a, shows the least experimental noise and no low-energy tailing and, therefore it is the most easily analyzed system. Several energy ranges were considered for reproduction, with the general aim of reproducing the largest possible energy range while still reproducing the cross section onset with fidelity. The best results were obtained by fitting over the range of 3−8 eV, resulting in a reasonable value for the n parameter (1.2 ± 0.1) and a good fit at threshold, as shown in Figure 2a. Importantly, over this energy range, other competing dissociation channels account for less than 5% of the total fragment ion intensity. With this fitting range, the best value for the nickel− cyclopentadienyl BDE was found to be 3.37 ± 0.12 eV. This value is much lower than previous EI-MS values and somewhat lower than the TPEPICO values but agrees within the combined experimental uncertainties with the more recent photodissociation data of Dunbar and Lin,23 as summarized in Table 2. Although the primary aim of these studies is the acquisition of quantitative metal−cyclopentadienyl BDEs, the other fragmentation channels of the metallocene ions are also worth discussion. For nickelocene, C10H8+, H2Ni+, Ni+, C3H3Ni+, and C3H3+ ions were also observed and can be assigned to reactions 3−7.

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RESULTS AND DISCUSSION Cross Section and Threshold Analysis. The experimental dissociation cross sections versus center-of-mass collision energies for the cyclopentadienyl-loss process are shown in Figure 1 for all seven metallocene ions. This comparison

Figure 1. Experimental cross sections for the Cp2M+ → CpM+ + Cp dissociation with Xe as the collision gas (at ∼0.05 mTorr) as a function of kinetic energy in the center-of-mass frame.

Cp2 M+ + Xe → C10H8+ + MH 2 + Xe

(3)

→ H 2M+ + C10H8 + Xe

(4)

→ M+ + 2Cp + Xe

(5)

→ C3H3M+ + C2H 2 + C5H5 + Xe

(6)

+

→ C3H3 + C7H5 + MH 2 + Xe

suggests that the Cp-loss behaviors are very similar for all ions. In most cases, cross sections fade into instrumental noise below ∼3 × 10−18 cm2. In some cases, a low-energy tail can be observed (most notably in the case of vanadocene and manganocene). Figure 1 shows only the lowest pressure curve obtained for each metallocene ion; however, 3−6 different collision gas pressures were used to measure each reaction cross section. This not only allows evaluation of the reproducibility of the data but also establishes the pressure dependence of the cross sections and E0 values. In these studies, most of the metallocene systems did not show appreciable pressure effects except titanocene (vide infra), to some degree vanadocene, and at the highest experimental pressure, manganocene. The fitting parameters discussed below are summarized in Table 1, which also includes the entropy of

(7)

The observation of reactions 3 and 4 provides evidence for isomerization of the Cp2Ni+ ion into a C10H8NiH2+, fulvalenecontaining species, as previously discussed in the literature.29,31 Reactions 5 and 6 are clearly sequential dissociation processes of the primary CpNi+ product ion, losing either a second Cp ligand or acetylene. The C3H3+ product ion has the highest energy threshold and thus its assignment is the most tentative. Reaction 7 suggests it is a secondary decomposition of the primary C10H8+ product ion. Chromocene, Ferrocene, and Cobaltocene. The data for these three metallocene ions (Figure 2b−d) are similar to that of nickelocene: there is limited tailing and the energy ranges for fitting were also determined as discussed above. Threshold energies for the Cp-loss dissociations obtained by modeling are 4.22 ± 0.13, 4.26 ± 0.15, and 4.57 ± 0.15 eV for 1302



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Figure 2. Experimental cross sections for the collision-induced dissociation processes by Xe of (a) Cp2Ni+, (b) Cp2Cr+, (c) Cp2Fe+, and (d) Cp2Co+ in the center-of-mass frame (lower x-axis) and laboratory frame (upper x-axis) at medium collision-gas pressures. Solid lines show the best fit to the Cp-loss data using the model of eq 2 convoluted over the neutral and ion kinetic and internal energy distributions. Dashed lines show the model cross sections in the absence of experimental kinetic energy broadening for reactant ions with an internal energy of 0 K. Arrows indicate the derived average [CpM+−Cp] BDE values of 3.37 ± 0.12 eV for Cp2Ni+, 4.22 ± 0.13 eV for Cp2Cr+, 4.26 ± 0.15 eV for Cp2Fe+, and 4.57 ± 0.15 eV for Cp2Co+.

Table 2. [CpM+−Cp] Bond Dissociation Energies from TCID and TPEPICO Measurements, Theoretical Calculations, and the Literature TCIDa RRKM/PSL +

Cp2Ti Cp2V+ Cp2Cr+ Cp2Mn+ Cp2Fe+ Cp2Co+ Cp2Ni+

4.85 4.02 4.22 3.51 4.26 4.57 3.37

± ± ± ± ± ± ±

0.15 0.14 0.13 0.12 0.15 0.15 0.12

calcb

TPEPICO RAC-RRKM

SSACM

4.61 ± 0.12 3.43 ± 0.17f

5.04 ± 0.12

5.59 ± 0.14e 3.66 ± 0.16e

5.77 ± 0.11e 3.96 ± 0.11e

e

e

B3LYP

BP86

literature

3.93 3.41 4.07 2.95 3.67 4.11 3.13

4.37 3.96 4.71 3.32 4.86 5.57 3.88

5.32 ± 0.15,c 2.34 ± 0.34d 6.55 ± 0.15c 3.77 ± 0.15c 6.63 ± 0.15,c 4.61 ± 0.43,d 3.7 ± 0.3,g 3.95,h 6.7 ± 0.2,i 3.96 ± 0.13,j 5.81 ± 1.1k 7.79 ± 0.15,c 4.86 ± 0.42,d 6.33 ± 1.0k 5.43 ± 0.15,c 5.16 ± 1.1,k 3.24 ± 0.07l

a

This study. bBasis sets: 6-311G++(2df,2pd) (single point) and 6-311+(d,p) (geometry optimizations). cMuller and D’Or.10 dOpitz and Härter.20,21 Révész et al.29 fLi et al.30 gFaulk and Dunbar.11 hHan et al.12 iBarfuss et al.13 jLewis and Smith.75 kPignataro and Lossing.22 lLin and Dunbar.23

e

3.7−6.7 eV. The [CpFe+−Cp] TCID value of 4.26 ± 0.15 eV fits in the middle of these values, agreeing reasonably well with the electron ionization study of Opitz and Harter yielding 4.61 ± 0.43 eV20 and studies using UV photodissociation and lowpressure pyrolysis yielding 3.95 (no error given)12 and 3.96 ±

Cp2M+, M = Cr, Fe, and Co, respectively. As for nickelocene, n values for the best fits are similar: 1.3 ± 0.1, 1.2 ± 0.1, and 1.1 ± 0.2, respectively. Compared to the EI-MS BDEs of Müller and D’Or,10 our values are 2.3−3.2 eV lower in energy, Table 2. For ferrocene, there is a large set of BDE values, ranging from 1303



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Figure 3. Experimental cross sections for the collision-induced dissociation processes by Xe of (a) Cp2Mn+ at low collision gas pressure and (b) Cp2V+ at medium collision gas pressure. In both (a) and (b), open circles show the experimental Cp-loss cross sections. Also in (b), dotted lines are the model for the high pressure tail and the best fit to the data after subtracting the model of the tail using the model of eq 2 convoluted over the neutral and ion kinetic and internal energy distributions. In (a), this model is shown as the solid line, whereas in (b), the solid line shows the sum of the two models. For both (a) and (b), dashed lines show the model cross sections in the absence of experimental kinetic energy broadening for reactant ions with an internal energy of 0 K. Arrows indicate the derived [CpM+−Cp] BDE values of 3.51 ± 0.12 eV for Cp2Mn+ and 4.02 ± 0.14 eV for Cp2V+.

the “normal” part of the TCID curve and the resulting model was subtracted from the experimental data points. The resulting cross section is believed to be a reasonably good approximation for the ground state TCID curve and was modeled as the other metallocenes described above. For self-consistency, this model was subtracted from the raw TCID experimental data to yield data points that correspond to only the excited ions and extend beyond the limit where the ground state TCID curve begins. These data points were then modeled to obtain a better approximation of the tail function and again subtracted from the raw data. With this iterative procedure, a self-consistent set of cross sections for the tail and ground state was determined and used in the data analysis to obtain the final ground state E0 value. Figure 3b shows that the sum of these two models reproduces the experimental cross section throughout the energy range examined. In the case of manganocene, a similar procedure could be used to correct for the tail in the two higher-pressure data sets and generated cross sections that led to dissociation energies that matched those obtained from the lower pressure data. For Cp2Mn+, the average E0 value is 3.51 ± 0.12 eV, a value in surprising proximity with the EI-MS value of 3.77 ± 0.15 eV.10 In the case of vanadocene, the average ionic BDE is 4.02 ± 0.14 eV, which is much lower than the EI-MS value of 5.32 ± 0.15 eV,10 a discrepancy similar to those for the other metallocenes. The value of 2.34 ± 0.34 eV,20 obtained with electron ionization, is well below (by ∼1.7 eV) the present results for reasons that are not obvious. For both metallocene ions, the consecutive Cp-loss channel, given by reaction 5, to V+ and Mn+ was detected at high energies. Other dissociation channels in vanadocene include acetylene loss channels to C8H8V+ and C3H3V+, according to reactions 10 and 9, with magnitudes well below 2% of the Cploss channel. In the case of manganocene, the C10H10+ ion formed in reaction 8 was detectable at low and high energies. Traces of both C3H3Mn+ and C3H3+ (not shown for clarity) were also detectable at energies above 10 eV with maximum cross section magnitudes less than 2 × 10−18 cm2.

0.1375 eV. For cobaltocene, TCID yields a BDE again in reasonable agreement with the electron ionization study of Opitz.21 Compared to the TPEPICO values for Cp2Cr+ and Cp2Co+, the TCID values are lower and agree better with the RAC-RRKM interpretation of these data. These differences are discussed further below. With respect to other dissociation channels, reaction 5, loss of the second Cp ligand, is observed at higher energies in all three systems. The maximum magnitudes of these product ions are only ∼1 × 10−17 cm2 at 10.0 eV for M = Cr, Fe, and Co. Both ferrocene and cobaltocene yield a small amount of a C10H10+ product ion, presumably formed in reaction 8. Cp2 M+ + Xe → C10H10+ + M + Xe

(8)

In the case of ferrocene, C6H6Fe+ is clearly visible at low energy, and is presumably formed in reaction 9, although no C8H8Fe+ corresponding to loss of only a single acetylene molecule was observed. The low energy threshold for the reaction may indicate that it comes from an excited state or conformation of Cp2Fe+. Cp2 M+ + Xe → (C3H3)2 M+ + 2C2H 2 + Xe

(9)

Vanadocene and Manganocene. These two metallocene ions behave slightly differently than the four already discussed, as the TCID curves of both of these ions showed extensive tailing at low energies. For manganocene, only the two highpressure data sets taken at P(Xe) ∼ 0.2 mTorr displayed significant tailing, confirming that the tail is a result of multiple collisions with apparent thresholds lower than the main cross section feature by approximately a factor of 2. Lower pressure data sets look like the one in Figure 3a, similar to the other metallocenes discussed above. For vanadocene, all of the TCID curves showed a pronounced low-energy tail, such that a simple extrapolation to zero pressure does not remove the tailing. Hence, the tail is hypothesized to be the result of a small population of electronically excited vanadocene cations. In this case, the low-energy tail was modeled up to the energy onset of 1304



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Figure 4. (a) Experimental cross sections for the Cp-loss dissociation of Cp2Ti+ ions at various pressures. Pressures are, from left to right, 0.27, 0.24, 0.11, 0.11, 0.064, and 0.063 mTorr. The zero-pressure extrapolated cross sections are shown with open circles. (b) Extrapolated zero-pressure cross sections for the dissociation processes of Cp2Ti+ energized in collisions with Xe. The solid line shows the best fit to the Cp-loss data using the model of eq 2 convoluted over the neutral and ion kinetic and internal energy distributions. The dashed line shows the model cross section in the absence of experimental kinetic energy broadening for reactant ions with an internal energy of 0 K. The BDE for [CpTi+−Cp] was determined to be 4.85 ± 0.15 eV, as represented by the arrow.

Cp2 M+ + Xe → C8H8M+ + C2H 2 + Xe

on halobenzenes,76 that the rigid-activated-complex (RAC) RRKM model can underestimate the BDEs, whereas the SSACM model gives more accurate results.29 The present results support the opposite conclusion, although the TPEPICO values are typically higher, with the exception of the Cp2Mn+ value. Reasons for the discrepancy between these two approaches are unclear, but the trends in the data that overlap are similar except for the nearly isoenergetic Cp2Mn+/ Cp2Ni+ pair. Here TPEPICO finds a slightly stronger bond in the nickelocene ion, whereas TCID finds the reverse, although the differences are well within the uncertainties of both experiments. Simple molecular orbital (MO) theory considerations concerning the electronic structure of the metallocene Cp2M+ and half-sandwich CpM+ ions allow us to discuss the trends in [CpM+−Cp] BDEs in a qualitative fashion and compare the expected order to the TCID BDE data. For simplicity, Figure 5 shows only those MOs where the electron occupation varies upon bond formation (or dissociation).77−79 For consistency, the labels correspond to C5v and D5d symmetry for CpM+ and Cp2M+, respectively, even though, according to our DFT calculations, the most stable conformation of most of the metallocene ions did not show full D5d or D5h symmetry, with the exception of cobaltocene, which was D5h. We have found that the e2g MO of Cp2M+ has bonding character with respect to the CpM+−Cp bond, because of the favorable overlap of the Cp(π3) and the metal-centered lobes of CpM+(e2). The e*1g MO of Cp2M+ is an antibonding combination originating mainly from CpM+(e1) and Cp(π2). Both a1 and a1g orbitals are essentially nonbonding metal-centered dz2-type orbitals with little s character. Notably, the order of the molecular orbitals in Figure 5 varies among the metallocenes considered here such that no single diagram is suitable to accurately describe the bonding scheme in all metallocene ions. Furthermore, the order of the orbitals depends on the level of theory, which is not entirely surprising considering the fact that orbitals and orbital energies are themselves approximations, especially in density functional theory. As an example, the MO order in ferrocene is usually given with the a1g orbital above e2g,80 whereas recent

(10)

Titanocene. Titanocene is the least studied of these metallocene systems. In contrast to the other metallocenes, modeling any of the individual TCID cross sections for Cp2Ti+ resulted in n values that were unreasonably large (larger than 2) and both the energy dependence and the position of the TCID curves showed systematic variation with experimental pressure as shown in Figure 4a. Therefore, extrapolation of the cross sections to zero pressure at each individual energy value was performed to yield the zero-pressure data shown in Figures 1 and 4a,b. These data were modeled with the procedure outlined above and yield a threshold energy of 4.85 ± 0.15 eV and an n value of 1.7 ± 0.1. There is no literature comparison for this E0 value as previous studies for Cp ligand dissociation energy have not been performed on Cp2Ti+. There were numerous other channels observed in the dissociation of the titanocene ion, as shown in Figure 4b. At low energies, C8H8Ti+ formed in reaction 10 has intensity as much as 5% of the Cp-loss signal. Either Cp loss from this fragment ion or acetylene loss from CpTi+ leads to C3H3Ti+. It seems more likely that this ion results from Cp-loss from C8H8Ti+ as the shape of these two TCID curves resembles that of a consecutive dissociation; that is, the C8H8Ti+ cross section declines at the onset of the C3H3Ti+ signal. Some further ring fragmentation was also observed leading to traces of C7H6Ti+, possibly formed in reaction 11, and C6H6Ti+ ions, reaction 9. At high energies, sequential loss of the second Cp ligand in reaction 5 is also observed. Cp2 M+ + Xe → C7H6M+ + C3H4 + Xe

(11)

Bond Dissociation Energy Trends. Table 2 summarizes our threshold energies for the Cp-loss dissociation reaction Cp2M+ → CpM+ + Cp of all seven metallocene ions studied. In all cases, these can be equated with the desired metallocene BDE and other literature values. This table also compares these values with results from a recent TPEPICO metallocene study. In the TPEPICO work, it was assumed, on the basis of a study 1305



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of an electronvolt. For our MO considerations, we compare the high-spin manganocene ion to the other metallocenes and will return to the question of the triplet state when refining the qualitative BDE order. Our evaluation does not consider any correlation interaction among electrons, transition metal orbital energy differences, or Jahn−Teller distortions and is therefore not going to rigorously justify small differences in the BDE trends. Within this simple qualitative MO theory, we can present a comprehensive evaluation of metallocene bond energies from M = Ti through Ni, similar to that in the TPEPICO paper.29 In these arguments, it is simplest to consider changes in the electronic configuration upon bond formation between CpM+ and Cp, Table 3. A common motif for the metallocene ions Cp2Cr+ through Cp2Ni+ is that one metal e1 electron on CpM+ combines with the singly occupied π2 orbital of the Cp ring and forms a e1g stable bonding orbital (not included in Figure 5). The difference between these metallocenes is what happens to the other e1 electron(s). For Cp2Mn+ (quintet) and Cp2Ni+, the e1 electron is destabilized to the antibonding e*1g orbital, whereas for Cp2Mn+ (triplet), Cp2Fe+ and Cp2Co+, it is slightly stabilized to the a1g orbital. Formally, this stabilization requires a change in spin such that the reaction is spin forbidden. In TPEPICO30 and TCID90,91 experiments on first-row transition metal complexes, we have found that spin is not rigorously conserved, rather dissociation generally follows the lowestenergy pathway. Indeed, multiphoton ionization/dissociation experiments on the metallocenes of Fe, Co, and Ni reveal efficient spin-forbidden dissociations to the atomic metal cations.92 Therefore, the Fe and Co metallocenes should have higher BDEs than Cp2Mn+ (quintet) and Cp2Ni+. Cp2Cr+, starting out with only one e1 electron in CpCr+, should have a BDE in between these two groups. In Cp2Co+, there are four electrons stabilized from e2 to e2g, whereas Cp2Fe+ and Cp2Mn+ (triplet) have only three, therefore cobaltocene should have the highest BDE, completely in line with its very stable 18-electron configuration. The same argument should hold for manganocene and nickelocene, where Cp2Mn+ (quintet) has two electrons that stabilize in the e2g orbital and Cp2Ni+ has four, suggesting an expected higher BDE for Cp2Ni+. Indeed, this is the trend in the quoted TPEPICO study (although the manganocene data is older and it was not reanalyzed together with the new measurements in that work). The TCID results show a reversal, with a 0.14 ± 0.17 eV energetic advantage for Cp2Mn+ over Cp2Ni+. This relative stability of the manganocene ion, however, can come from the lower-lying low-spin triplet state, which is 0.24 eV more stable than the quintet state, according to our DFT calculations. Where do Cp2Ti+ and Cp2V+ come into this order? Formally, the latter can be compared to chromocene if the electron accounting for both metallocenes is done by assigning the a1 electron of CpM+ to combine with Cp(π2) to form the deeplying e1g bonding orbital of Cp2M+. For CpCr+, there is one extra e1 electron that formally stabilizes to the a1g orbital in Cp2Cr+. Therefore, the CpM+−Cp bond should be slightly weaker in the vanadocene ion than in the chromium analogue; the difference corresponding to the energy difference between the e1 and the a1g orbitals. This is in accord with the 0.20 ± 0.19 eV BDE difference measured in this study. Assuming that 5-fold symmetry is maintained, the MO considerations suggest that Cp2Ti+ should have the lowest bond energy because the e2g bonding orbital retains only one electron. Complicating this is the fact that DFT calculations indicate that the ground state

Figure 5. Molecular orbital diagram of metallocenes showing only the orbitals of varying electron occupation in the formation of Cp2M+ from the CpM+ and Cp fragments.

calculations at the B3LYP level81,82 reverse this order. At the B3LYP level, the a1g Kohn−Sham orbital is above the e2g orbital in chromocene and manganocene,83 whereas a1g is lower in vanadocene, ferrocene, and cobaltocene.81 Although this is certainly of interest to quantum chemists, the discrepancy in the MO order does not change our qualitative MO considerations, as the character of the MOs and their role in bonding stays largely the same between the systems. For example, the a1g orbital is below e2g in Cp2Fe+ and the a1 orbital is also lower than the e2 in CpFe+, such that the nonbonding character of both the a1 and a1g orbitals is expected to remain the same. Therefore, for the discussion here, a single MO scheme is adequate for our purposes. For the metallocene ions, the occupation numbers of the MOs are given in Table 3 and are taken from the Table 3. Molecular Orbital Occupations and Spin States in the Half-Sandwich and Sandwich Metallocene Ions As Taken from Elschenbroich,96 except for the Triplet State of Cp2M+ M

CpM+

2s + 1

Ti V Cr Mn

e22 e22a11 e22a11e11 e22a11e12

(3) (4) (5) (6)

Fe Co Ni

e23a11e12 e24a11e12 e24a12e12

(5) (4) (3)

→ → → → → → → →

Cp2M+

2s + 1

e2g1 e2g2 e2g2a1g1 e2g2a1g1e1g1 e2g3a1g1 e2g3a1g2 e2g4a1g2 e2g4a1g2e1g1

(2) (3) (4) (5) (3) forbidden (2) forbidden (1) forbidden (2)

literature.84−87 It is assumed that all CpM+ species are in a high-spin configuration, analogously with benzene complexes.88 This assumption may be incorrect for the manganocene ion, for which there seems to be some evidence that even though the first strong band in the photoelectron spectrum comes from the 6 A → 5E transition, there is a more stable low-spin triplet state in the cation.89 The calculated energy difference between the triplet and high-spin quintet state is on the order of a few tenths 1306



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geometry of Cp2Ti+ is bent (31° angle between the planes of the rings), which is also the case for Cp2V+ (angle of 24°), but none of the other metallocene cations. Therefore, it is hard to insert titanocene into the trend given for the other metallocene ions. The high BDE of 4.85 ± 0.15 eV may indicate some interaction between the two rings. Because of this unusually high metal−Cp bond energy, ring fragmentation in the form of acetylene loss is a more significant dissociation pathway than in any other metallocene system. To summarize the trend, the simple MO scheme (neglecting the bent Cp2Ti+ species and including the triplet-stabilization for Cp2Mn+) gives us a BDE order of Cp2Ni+ ≈ Cp2Mn+ < Cp2V+ < Cp2Cr+ < Cp2Fe+ < Cp2Co+, which agrees very well with the TCID results. This trend is also in reasonable agreement with the quantum chemical calculations, as can be seen in Figure 6. However, B3LYP results suggest a lower BDE for Cp2Ti+ and further indicate that the BDE of Cp2Cr+ is close to that of Cp2Co+ and well above that of the ferrocene cation. In contrast, the BP86 results largely keep this relative ordering but increase the Cp2Ni+ BDE and lower that for Cp2Ti+.

In general, for all of the metallocene ions, the B3LYPcalculated BDEs are systematically too low, with a mean absolute deviation (MAD) from experiment of 0.50 ± 0.25 eV. For comparison, the TPEPICO results have a MAD compared to B3LYP theory of 0.8 ± 0.5 eV for the four RRKM metallocene BDEs and 1.2 ± 0.4 eV for the three SSACM values. For the six metallocene BDEs available from the EI-MS results of Muller and D’Or,10 the MAD is 2.4 ± 1.0 eV. Figure 6a shows the average deviation (dashed line), a comparison that indicates that the trends in the experimental and theoretical values match reasonably well. Interestingly, the TPEPICO results for Cp2Cr+, Cp2Mn+, and Cp2Ni+ agree better with this displaced line as well. The large disagreement between the B3LYP calculated and experimental bond dissociation energies is not entirely a surprise. Predicting accurate geometries and bond energies in neutral metallocenes (where the vast majority of the calculations were done on ferrocene) has long been a difficult task for quantum chemists.17,18,93 High-level ab initio methods often give BDEs that are too high with results that are sensitive to the set of orbitals chosen for the active space in multiconfiguration calculations.94 Similarly, DFT calculations often have trouble predicting accurate thermochemistry with B3LYP BDEs on the low end and LDA on the high end.18,47,95 For instance, compared to experimental thermochemistry for four neutral metallocenes, B3LYP BDEs were systematically low by 0.37 ± 0.16 eV.47 On this basis, to find a reasonable upper limit for the theoretical BDE values, we have also carried out DFT calculations with the BP86 functional using the same basis sets as in the B3LYP calculations. Compared to the same four neutral metallocenes, this functional was found to give systematically higher BDEs than experiment with a MAD of 0.21 ± 0.16 eV.47 As shown in Figure 6b, this is also the case for the metallocene ions. The MAD between the TCID and BP86 values are 0.48 ± 0.30 eV, which is almost identical to what we found for B3LYP. The BP86 values are in quite good agreement with TPEPICO results, with MADs of 0.1 ± 0.1 eV for the four RRKM metallocene BDEs and 0.2 ± 0.1 eV for the three SSACM values; see Figures S1 and S2, Supporting Information. For the six metallocene BDEs available from the EI-MS results of Muller and D’Or,10 the MAD remains quite high at 1.5 ± 0.6 eV. One additional question of interest is whether the statistical rate model applied causes the differences between the TPEPICO and TCID numbers. Therefore, we also experimented with using a rigid activated complex (RAC)-RRKM model in the TCID data analysis, by using the TPEPICOoptimized transition state frequencies. Because the nickelocene dissociation was the cleanest data and easiest experiment to model, we chose this to compare the PSL and RAC-RRKM models. As expected, the tighter RAC-RRKM transition state lowers the threshold energy (in this case by 0.37 eV), thereby increasing the discrepancy between the TPEPICO and TCID numbers even further. Similar results should occur for all the metallocenes, hence a comprehensive evaluation of all systems using this approach was not pursued further. Another possible cause for the disagreement between the TCID and TPEPICO BDE values could be different spin states in the two experiments. However, it is unlikely that the various molecular ions that travel through the helium-filled flow cell of the TCID apparatus consistently end up primarily in excited electronic states. Such excited states may be possible in the TPEPICO experiment but their presence would only result in

Figure 6. (a) Comparison of TCID (blue circles), TPEPICO (RRKM) (red triangles), and TPEPICO (SSACM) (green inverted triangles) derived [CpM+−Cp] bond energies vs calculated B3LYP results. The full line shows exact agreement whereas the dashed line is displaced upward by 0.50 eV. (b) Comparison of the TCID bond energies vs calculated values with two different functionals: B3LYP (solid circles) and BP86 (open circles). The full line shows exact agreement. 1307



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the opposite difference of BDEs. Alternatively, the violation of spin conservation rules can have different efficiencies in the two experiments because of coupling with the very different angular momentum distributions in TCID versus photoionization. Interestingly, a relatively large difference in TCID-TPEPICO BDEs was found in the case of chromocene, where the dissociation is spin-allowed, whereas the TCID and TPEPICO BDEs agree rather well for manganocene, where the dissociation is formally spin-forbidden. Given the trend that the TPEPICO BDEs are higher than the TCID BDEs in all other cases, the result for Cp2Mn+ may indicate that the molecular ion indeed was trapped in the 5E excited state after photoionization, lowering the TPEPICO BDE with respect to the TCID value.

CONCLUSIONS Metal-cyclopentadienyl bond dissociation energies for seven Cp2M+ ions (M = Ti, V, Cr, Mn, Fe, Co, and Ni) were determined by threshold collision-induced dissociation in a guided ion beam tandem mass spectrometer. In the TCID experiments, the loss of the η5-cyclopentadienyl ligand was observed as the dominant dissociation pathway for all seven metallocene ions, generally followed by loss of the second Cp ring along with metal-specific ring-fragmentation competitive pathways, leading to such species as C10H10+, C10H8+, C8H8+, C3H3+, H2M+, C3H3M+, C6H6M+, C7H6M+, and C8H8M+. To measure the ionic [CpM+−Cp] dissociation energies, reaction cross sections were analyzed using a model that includes the internal energy distributions and dissociation kinetics determined using statistical rate theory. Although the absolute BDEs are systematically lower than those measured for some of the metallocenes in a recent TPEPICO study, the BDE trends in both studies correlate reasonably well with the results of B3LYP calculations, which are systematically lower than the present results. Furthermore, the expected order of the bond energies from simple molecular orbital theory, deduced to be roughly Cp2Ni+ ≈ Cp2Mn+ < Cp2V+ < Cp2Cr+ < Cp2Fe+ < Cp2Co+ is in good agreement with the TCID numbers. The significant differences between the TCID and TPEPICO values, however, certainly leaves room for further investigation into the thermochemistry of these most important organometallic species. ASSOCIATED CONTENT

S Supporting Information *

Tables of calculated geometries and DFT (B3LYP and BP86) ZPVE and electronic energies. Figures showing the comparison of the BP86 calculations with TCID and TPEPICO values. This information is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

(1) Kealy, T. J.; Pauson, P. L. Nature 1951, 168, 1039−1040. (2) Pauson, P. J. Organomet. Chem. 2001, 637, 3−6. (3) Wilkinson, G.; Rosenblum, M.; Whiting, M. C.; Woodward, R. B. J. Am. Chem. Soc. 1952, 74, 5764−5767. (4) Kasper, M.; Sattler, K.; Siegmann, K.; Matter, U.; Siegmann, H. C. J. Aerosol Sci. 1998, 30, 217−225. (5) Yang, Q.; Jensen, M. D.; McDaniel, M. P. Macromolecules 2010, 43, 8836−8852. (6) Kaminsky, W.; Funck, A. Macromol. Symp. 2007, 260, 1−8. (7) Napoli, M.; De Vita, R.; Immediata, I.; Longo, P.; Guerra, G. Polym. Adv. Technol. 2011, 22, 458−462. (8) Hlatky, G. G. Chem. Rev. 2000, 100, 1347−1376. (9) Hu, Z.; Liu, C.; Wu, Y.; Liu, R.; He, Y.; Luo, S. J. Polym. Sci., Part B: Polym. Phys. 2011, 49, 812−817. (10) Müller, J.; D’Or, L. J. Organomet. Chem. 1967, 10, 313−322. (11) Faulk, J. D.; Dunbar, R. C. J. Am. Chem. Soc. 1992, 114, 8596− 8600. (12) Han, S. J.; Yang, M. C.; Hwang, C. H.; Woo, D. H.; Hahn, J. R.; Kang, H.; Chung, Y. Int. J. Mass Spectrom. 1998, 181, 59−66. (13) Barfuss, S.; Emrich, K.-H.; Hirschwald, W.; Dowben, P. A.; Boag, N. M. J. Organomet. Chem. 1990, 391, 209−218. (14) Giordan, J. C.; Moore, J. H.; Tossell, J. a.; Weber, J. J. Am. Chem. Soc. 1983, 105, 3431−3433. (15) Nekrasov, Y. S.; Zverev, D. V.; Belokon, a. I. Russ. Chem. Bull. 1998, 47, 1336−1339. (16) Doman, T. N.; Landis, C. R.; Bosnich, B. J. Am. Chem. Soc. 1992, 114, 7264−7272. (17) Xu, Z.-F.; Xie, Y.; Feng, W.-L.; Schaefer, H. F. J. Phys. Chem. A 2003, 107, 2716−2729. (18) Mayor-López, M. .; Weber, J. Chem. Phys. Lett. 1997, 281, 226− 232. (19) Warren, J. W. Nature 1950, 810−811. (20) Opitz, J.; Harter, P. Int. J. Mass Spectrom. Ion Processes 1992, 121, 183−199. (21) Opitz, J. Int. J. Mass Spectrom. 2003, 225, 115−126. (22) Pignataro, S.; Lossing, F. G. J. Organomet. Chem. 1967, 10, 531− 534. (23) Lin, C.-Y.; Dunbar, R. C. J. Phys. Chem. 1995, 99, 1754−1759. (24) Kassel, L. S. J. Phys. Chem. 1928, 32, 225−242. (25) Marcus, R. A.; Rice, O. K. J. Phys. Chem. 1951, 55, 894−908. (26) Rice, O. K.; Ramsperger, H. C. J. Am. Chem. Soc. 1927, 49, 1617−1629. (27) Rice, O. K.; Ramsperger, H. C. J. Am. Chem. Soc. 1928, 50, 617− 620. (28) Troe, J.; Ushakov, V. G.; Viggiano, A. A. J. Phys. Chem. A 2006, 110, 1491−1499. (29) Révész, A.; Szepes, L.; Baer, T.; Sztáray, B. J. Am. Chem. Soc. 2010, 132, 17795−17803. (30) Li, Y.; Sztáray, B.; Baer, T. J. Am. Chem. Soc. 2002, 124, 5843− 5849. (31) Dias, A. R.; Veiros, L. F. J. Organomet. Chem. 2005, 690, 1840− 1844. (32) Frisch, M. J. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, Revision E.01; Gaussian Inc.: Wallingford, CT, 2004. (33) Frisch, M. J.; Trucks, ; G. W. Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision C.01; Gaussian Inc.: Wallingford, CT, 2009. (34) Becke, A. D. J. Chem. Phys. 1992, 97, 9173−9177. (35) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (36) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. 1988, B37, 785. (37) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650−654. (38) Blaudeau, J.-P.; McGrath, M. P.; Curtiss, L. A.; Radom, L. J. Chem. Phys. 1997, 107, 5016−5021.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: B.S., bsztaray@pacific.edu. P.B.A., armentrout@chem. utah.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Foundation, Grant No. CHE-1049580. We are also grateful to the ACS Petroleum Research Fund for financial support. 1308



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(39) Simon, S.; Duran, M.; Dannenberg, J. J. J. Chem. Phys. 1996, 105, 11024−11031. (40) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553−566. (41) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. V. R. J. Comput. Chem. 1983, 4, 294−301. (42) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265−3269. (43) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033−1036. (44) Hay, P. J. Chem. Phys. 1977, 66, 4377−4384. (45) Krishnan, R.; Trucks, G. W. J. Chem. Phys. 1989, 91, 2457− 2460. (46) Andersson, M. P.; Uvdal, P. J. Phys. Chem. A 2005, 109, 2937− 41. (47) Furche, F.; Perdew, J. P. J. Chem. Phys. 2006, 124, 044103. (48) Gerlich, D. Adv. Chem. Phys. 1992, 82, 1−176. (49) Daly, N. R. Rev. Sci. Instrum. 1960, 31, 264−267. (50) Loh, S. K.; Lian, L.; Hales, D. A.; Armentrout, P. B. J. Phys. Chem. 1988, 92, 4009−4012. (51) Ervin, K. M.; Armentrout, P. B. J. Chem. Phys. 1985, 83, 166− 189. (52) Armentrout, P. B. J. Anal. At. Spectrom. 2004, 19, 571−580. (53) Amicangelo, J. C.; Armentrout, P. B. J. Phys. Chem. A 2004, 108, 10698−10713. (54) Cooper, T. E.; O’Brien, J. T.; Williams, E. R.; Armentrout, P. B. J. Phys. Chem. A 2010, 114, 12646−12655. (55) Rodgers, M. T.; Armentrout, P. B. J. Phys. Chem. A 1997, 101, 1238−1249. (56) Schultz, R. H.; Armentrout, P. B. Int. J. Mass Spectrom. Ion Processes 1991, 107, 29−48. (57) Ye, S.; Moision, R.; Armentrout, P. Int. J. Mass Spectrom. 2005, 240, 233−248. (58) Schultz, R. H.; Crellin, K. C.; Armentrout, P. B. J. Am. Chem. Soc. 1991, 113, 8590−8601. (59) Gengeliczki, Z.; Sztáray, B.; Baer, T.; Iceman, C.; Armentrout, P. B. J. Am. Chem. Soc. 2005, 127, 9393−9402. (60) Armentrout, P. B. J. Am. Soc. Mass Spectrom. 2002, 13, 419−434. (61) Mark, S.; Gerlich, D. Chem. Phys. 1996, 209, 235−257. (62) Andersen, A.; Muntean, F.; Walter, D.; Rue, C.; Armentrout, P. B. J. Phys. Chem. A 2000, 104, 692−705. (63) Muntean, F.; Armentrout, P. B. J. Phys. Chem. B 2002, 106, 8117−8124. (64) Muntean, F.; Armentrout, P. B. J. Phys. Chem. A 2003, 107, 7413−7422. (65) Dalleska, N. F.; Honma, K.; Sunderlin, L. S.; Armentrout, P. B. J. Am. Chem. Soc. 1994, 116, 3519−3528. (66) Su, C.; Hales, D. A.; Armentrout, P. B. J. Chem. Phys. 1993, 99, 6613−6623. (67) Muntean, F.; Armentrout, P. B. J. Chem. Phys. 2001, 115, 1213− 1228. (68) Armentrout, P. B. Int. J. Mass Spectrom. 2000, 200, 219−241. (69) Khan, F. A.; Clemmer, D. E.; Schultz, R. J. Phys. Chem. 1993, 97, 7978−7987. (70) Rodgers, M. T.; Ervin, K. M.; Armentrout, P. B. J. Chem. Phys. 1997, 106, 4499−4508. (71) Armentrout, P. B.; Ervin, K. M.; Rodgers, M. T. J. Phys. Chem. A 2008, 112, 10071−10085. (72) Fisher, E. R.; Kickel, B. L.; Armentrout, P. B. J. Phys. Chem. 1993, 97, 10204−10210. (73) Dalleska, N. F.; Honma, K.; Armentrout, P. B. J. Am. Chem. Soc. 1993, 115, 12125−12131. (74) Waage, E. V; Rabinovitch, B. S. Chem. Rev. 1970, 70, 377−387. (75) Lewis, K. E.; Smith, G. P. J. Am. Chem. Soc. 1984, 4650−4651. (76) Stevens, W.; Sztáray, B.; Shuman, N.; Baer, T.; Troe, J. J. Phys. Chem. A 2009, 113, 573−582. (77) Albright, T. A. Tetrahedron 1982, 38, 1339−1388. (78) Crabtree, R. H. The organometallic chemistry of transition metals, 2nd ed.; Wiley-Interscience: New York, NY, 1994. (79) Lukehart, C. M. Fundamental transition metal organometallic chemistry; Brooks/Cole Publishing Company: Belmont, CA, 1985.

(80) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; Wiley-Interscience: New York, 1985. (81) Xu, Z.-F.; Xie, Y.; Feng, W.-L.; Schaefer, H. F. J. Phys. Chem. A 2003, 107, 2716−2729. (82) Bean, D. E.; Fowler, P. W.; Morris, M. J. J. Organomet. Chem. 2011, 696, 2093−2100. (83) Green, J. C.; Burney, C. Polyhedron 2004, 23, 2915−2919. (84) Rabalais, J. W.; Werme, L. O.; Bergmark, T.; Karlsson, L.; Hussain, M.; Siegbahn, K. J. Chem. Phys. 1972, 57, 1185−1192. (85) Evans, S.; Green, M. L. H.; Jewitt, B.; King, G. H.; Orchard, A. F. J. Chem. Soc., Faraday Trans. 2 1974, 70, 356−376. (86) Swart, M. Inorg. Chem. Acta 2007, 360, 179−189. (87) Cauletti, C.; Green, J.; Kelly, M.; Powell, P.; Vantilborg, J.; Robbins, J.; Smart, J. J. Electron Spectrosc. Relat. Phenom. 1980, 19, 327−353. (88) Meyer, F.; Khan, F. A.; Armentrout, P. B. J. Am. Chem. Soc. 1995, 117, 9740−9748. (89) Ryan, M. F.; Eyler, J. R.; Richardson, D. E. J. Am. Chem. Soc. 1992, 114, 8611−8619. (90) Schultz, R. H.; Elkind, J. L.; Armentrout, P. B. J. Am. Chem. Soc. 1988, 110, 411−423. (91) Fisher, E. R.; Schultz, R. H.; Armentrout, P. B. J. Phys. Chem. 1989, 93, 7382−7387. (92) Niles, S.; Prinslow, D. A.; Armentrout, P. B. J. Chem. Phys. 1992, 97, 3115−3125. (93) Phung, Q. M.; Vancoillie, S.; Pierloot, K. J. Chem. Theory Comput. 2012, 8, 883−892. (94) Vancoillie, S.; Zhao, H.; Tran, V. T.; Hendrickx, M. F. A.; Pierloot, K. J. Chem. Theory Comput. 2011, 7, 3961−3977. (95) Lein, M.; Frunzke, J.; Timoshkin, A.; Frenking, G. Chemistry 2001, 7, 4155−4163. (96) Elschenbroich, C. Organometallics, 3rd ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2006.

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