Article pubs.acs.org/Organometallics
Interaction of Ruthenium(II) with Terminal Alkynes: Benchmarking DFT Methods with Spectroscopic Data Anton Škríba, Juraj Jašík, Erik Andris, and Jana Roithová* Department of Organic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030/8, 12843 Prague 2, Czech Republic S Supporting Information *
ABSTRACT: Helium tagging infrared photodissociation (IRPD) spectroscopy for the characterization of organometallic complexes is presented. The IRPD spectrum of the [RuCp(PPh3)(PhCCH)]+ complex reveals that more than 80% of the detected ions correspond to a structure with π-coordinated phenylacetylene, and the rest are complexes with the alkyne probably isomerized to its vinylidene form. The detected CC and C−H stretches of the terminal alkyne reflect the degree of activation of the triple bond. They are used to benchmark the popular DFT functionals used in theoretical studies of ruthenium catalysis. It is shown that there are only small differences between the methods. GGA methods (e.g., BP86 or PBEPBE) and (hybrid) meta GGA functionals (e.g., M06) provide slightly better descriptions of this system than hybrid DFT methods such as B3LYP. A notable exception is M06-2X, which significantly underestimates the activation of the CC bond by the coordination to the ruthenium complex.
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INTRODUCTION Density functional theory (DFT) methods are the dominant theoretical approach for the investigation of transition-metalcatalyzed reactions. Many reaction mechanisms have been suggested based on the calculation of relevant potential energy surfaces.1,2 Therefore, it is imperative to know the reliability of this approach.3 Here, we will concentrate on the interaction of a cationic ruthenium(II) catalyst with the terminal alkyne phenylacetylene. Ruthenium catalysts are some of the most exploited in organic synthesis, and activation of a triple carbon−carbon bond (Scheme 1) is the initial step in many important reactions such as cycloisomerizations,4,5 cyclotrimerizations,6 addition reactions,7,8 and coupling reactions.1
species in the gas phase that can be directly compared with theoretical calculations.12 Sensitive parameters that reflect the activation of a terminal carbon−carbon triple bond are the respective CC and C−H stretching vibration frequencies. The required data for isolated ionic complexes in the gas phase can be obtained using mass spectrometry and infrared photodissociation spectroscopy.13−19 Our experiments are based on storing mass-selected ions in an ion trap that is cooled to 3 K.20 Ions are trapped using a helium buffer gas that results in both relaxation of their internal energy and the formation of helium-tagged complexes. The ion cloud is then irradiated by IR photons. Absorption of a photon by a helium-tagged complex leads to an increase of its internal energy and subsequent elimination of the helium atom. The depletion of helium-tagged complexes as a function of the IR photon energy thus gives the helium tagging infrared photodissociation (IRPD) spectrum. Helium is the least reactive element, and it can be assumed that its attachment to a complex such as the one investigated here does not influence the infrared spectrum of the given complex.21−24 It was shown that its effect was negligible for more reactive systems where a stronger interaction with helium is expected.25,26 This method is therefore ideal for obtaining experimental data for the benchmarking of theoretical models.27 We present here, to the best of our knowledge, the first helium tagging IRPD spectra of organometallic complexes.
Scheme 1
Relevant experimental data to benchmark the theoretical models are very scarce.9 Geometry parameters can be obtained for example from gas-phase electron diffraction experiments or microwave spectroscopy. This approach was successfully used for benchmarking of a series of DFT methods for transition metal complexes.10 Alternatively, the DFT methods can be tested with respect to the geometries of metal complexes obtained by classical X-ray spectroscopy.11 Some of the best data originate from thermodynamic experiments on isolated © XXXX American Chemical Society
Received: January 12, 2016
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DOI: 10.1021/acs.organomet.6b00021 Organometallics XXXX, XXX, XXX−XXX
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RESULTS AND DISCUSSION The complexes of interest were generated from a CH2Cl2 solution of phenylacetylene and a prototypical cationic ruthenium catalyst, [CpRu(PPh3)(PF6)] (Figure S1 in the Supporting Information). We have shown previously that the [RuCp(PPh3)(PhCCH)]+ complex transferred to the gas phase by electrospray ionization (ESI) can bear either a π-bonded alkyne or an alkyne that has rearranged to its vinylidene isomer (Scheme 1).28,29 Figure 1a shows the helium tagging IRPD
represents C−H stretching of the terminal alkyne by exchanging this hydrogen with deuterium, which caused the respective C−H stretching mode to disappear (blue trace in Figure 1a). In order to estimate the ratio between the π-coordinated alkyne and its vinylidene isomer in the mixture of the ions transferred to the gas phase, we have performed attenuation experiments (Figure 2).30 If we have an ideal overlap of the ion
Figure 2. Depletion of the helium-tagged [RuCp(PPh3)(PhCCH)]+ complexes by irradiation at (a) 3091 cm−1 and (b) 3140 cm−1 as a function of the photon beam power.
cloud and the photon beam, at an absorption band with a sufficient photon flux we can deplete all of the helium complexes.30 If only part of the ions absorb the photons, the photon power dependence of the helium complex depletion will converge to the number of ions that are not interacting with the given photons (cf. Figure 2). When the complexes were irradiated at the C−H absorption bands corresponding to the triphenylphosphine ligand (ν̃ = 3091 cm−1, Figure 2a), i.e., all of the trapped ions should theoretically interact with the photons, only 77% of the stored helium complexes were depleted. This is taken as the maximum number of ions that can be depleted. We note in passing that the experimental data were fitted with a single-exponential function. We thus assume the same photofragmentation cross section for all isomers contributing to this band. It is justified given the quality of the experimental data and the fact that we are mainly interested in the determination of the number of noninteracting ions (i.e., 23% of the persisting helium complexes in Figure 2a). The same experiment performed at ν̃ = 3140 cm−1 (Figure 2b) gave 64% interacting ions. This band characterizes only the complexes with π-coordinated phenylacetylene, and it means that these isomers represent about 83% among the generated complexes. The remaining 17% probably correspond to complexes bearing the vinylidene ligand (green trace in Figure 1a). We have also measured the attenuation curve of the peak at 3272 cm−1 (Supporting Information, Figure S3a), which showed a linear depletion dependence with less than 10% attenuation at 12 mJ. This is characteristic for nonfundamental features such as overtones or combinational bands. Figure 1b−f show the theoretically calculated spectra of the π-complex and the vinylidene isomer of the [RuCp(PPh3)(PhCCH)]+ complex. On the basis of the fingerprint region, we can exclude the isomer with the hydrogen atom bound to the ruthenium center (Figure S12 in the Supporting Information). The vinylidene isomer should be revealed by the C−H vibration below 3000 cm−1, which was not detected. This may be due to the low abundance of this isomer and a low IR cross section of this band (see also Figure S12 in the Supporting Information). The theoretical B3LYP-D331 spectrum of the isomer with πcoordinated phenylacetylene shows a nice agreement with the
Figure 1. (a) He tagging IRPD spectrum of [RuCp(PPh3)(PhCCH)]+ (red trace; the gray spectrum was acquired with 7 times lower beam power) and [RuCp(PPh3)(PhCCD)]+ (blue trace; offset: 0.2). Green trace (offset: 0.4) corresponds to the He tagging IRPD spectrum of the ions surviving hard ionization conditions, presumably the vinylidene isomers. (b−f) Theoretical IR spectra. SF denotes the applied scaling factor.
spectrum of the generated [RuCp(PPh3)(PhCCH)]+ complex. Most of the bands are associated with the phenyl and cyclopentadienyl groups (1444, 1486, 1583, 3076, 3091, 3121 cm−1; the assignment is based on comparison with the theoretical IR spectra). The expected characteristic vibrations of the π-coordinated alkyne can be found at 1746 and 3140 cm−1. We have confirmed that the band at 3140 cm−1 B
DOI: 10.1021/acs.organomet.6b00021 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
The first group is composed of different variants of the B3LYP method and other hybrid GGA methods (first two groups in Figure 3, methods are listed in the left column below the graph; for more methods see Figures S7 and S8 in the Supporting Information). All of these methods predict that the C−H vibration is blue-shifted by about 30−50 cm−1. The C C vibration is usually also blue-shifted, but the shift can be dramatically reduced if a triple-ζ basis set is used. The effect of applying a dispersion correction term is not very significant and is largely outweighed by the basis set effect. The tested GGA functionals (third group in Figure 3; upper right column lists the methods) again show systematic blueshifting of the C−H stretching mode in a similar fashion to the hybrid functionals. Notably, however, they predict the CC stretch is in very good agreement with the experiment. When we look at the geometry parameters obtained from the calculations, we can see that the GGA methods systematically predict longer CC as well as C−H bond lengths and the binding (CC−H) angle is somewhat smaller than predicted by the hybrid methods (Figure S6c−e in the Supporting Information). The best analysis is to compare the bond-length changes upon the complexation of phenyl acetylene with the [RuCp(PPh3)]+ complex obtained at the given level of theory (Figure 3b,c). GGA methods predict slightly larger prolongation of the CC bond and a larger change of the binding angle (Figure 3d) than the hybrid methods. This results in a correct prediction of the CC stretching mode. Finally, we have also investigated meta GGA and hybrid meta GGA methods. As far as the prediction of the C−H stretching mode is concerned, these perform best from all of the tested methods. The CC stretching vibration is blue-shifted by about 30−50 cm−1, as was found for the hybrid DFT methods. The correct prediction of the C−H stretch is complimented by the corresponding change of C−H bond length upon πcoordination, which is larger than with all other groups of DFT functionals (Figure 3c). In comparison to the GGA functionals the change of the CC bond length is similar, but the absolute CC bond length is somewhat shorter (see Figure S6 in the Supporting Information). This might be the origin of the blueshift of νCC. The local M06-L functional combines the results for the C−H stretch obtained by the hybrid meta GGA functionals and the CC stretch description by the GGA methods. As a result, both followed stretches are even slightly red-shifted. Contrary to other (hybrid) meta GGA methods the M06-2X variant completely fails (the last point in Figure 3). This functional was not parametrized for organometallic compounds, and its failure is thus anticipated.32 We are pointing it out, because despite its unsuitability it is quite often applied in theoretical treatment of organometallic reactions. We have also tested the MP2 method (see the Supporting Information). The MP2 method predicts a much larger CC bond elongation and a binding CC−H angle of 141°. The CC stretch is therefore considerably red-shifted compared to experiment (Figure 1d). We have also tested to what extent the deviations in prediction of the CC and C−H stretch are already present in theoretical IR spectra of free phenylacetylene. The CC stretch is predicted to be blue-shifted with respect to the experimental value (2120 cm−1)33 in a similar manner to that in the ruthenium complex (Figures S4 and S5 in the Supporting Information). The deviations in the CC stretching vibration imposed by the coordination of the alkyne to ruthenium are
experiment in the fingerprint region and in the range of the C− H vibrations of the phenyl rings (Figure 2b). The CC stretching mode is theoretically predicted to be blue-shifted by about 30 cm−1. The alkyne C−H vibration is predicted to be blue-shifted by 60 cm−1. We have tested whether the blueshifting of the alkyne bands is associated with the harmonic approximation of the theoretical IR spectra. The anharmonic frequencies calculated at the B3LYP/6-31G(d,p):LanL2DZ level, however, agree well with the scaled harmonic frequencies (Figure S13 in the Supporting Information), and the C−H and CC stretching modes remain significantly blue-shifted. We have tested various functionals in order to exclude that the effect is an error associated with the B3LYP method (Figure 3; results of more methods can be found in the Supporting
Figure 3. (a) Theoretical ν̃ of the C−H and CC stretching modes predicted at different levels of DFT (indicated at the bottom of the graph; the notations of the functionals are used as in the Gaussian program). (b, c) Phenylacetylene bond length changes upon coordination to [RuCp(PPh3)]+. (d) Phenylacetylene CC−H bonding angle in the [RuCp(PPh3)(PhCCH)]+ complex.
Information). In order to compare the results on the same grounds, we have scaled the harmonic frequencies in the region of the C−H stretching vibrations so that the C−H stretches of the phenyl rings fit the bands at 3076 and 3091 cm−1 (all plotted spectra can be found in the Supporting Information). The theoretical spectra in the fingerprint region were scaled so that the two composite bands at 1444 and 1486 cm−1 are in agreement. The obtained results can be divided into three subgroups. C
DOI: 10.1021/acs.organomet.6b00021 Organometallics XXXX, XXX, XXX−XXX
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thus rather small (except for the M06-2X method as mentioned above). The evaluation of the prediction of the C−H stretch of free phenylacetylene is complicated by the fact that it is involved in a Fermi resonance and therefore cannot be relevantly compared to the results obtained at the harmonic approximation.31,34
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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.organomet.6b00021. Theoretical IR spectra and further experimental details (PDF) Cartesian coordinates in a format for convenient visualization (XYZ)
CONCLUSIONS
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In conclusion, we present a helium tagging infrared spectrum of the [RuCp(PPh3)(PhCCH)]+ complex. The spectrum clearly shows the CC and C−H stretching vibrations of πcoordinated phenylacetylene. It is used not only for the structural characterization of the complex but also for benchmarking the DFT methods used in theoretical calculations of reactions catalyzed by ruthenium complexes. We have shown that hybrid meta GGA methods such as M06 and GGA methods (PB86, PBEPBE, etc.) provide a better description of the system than hybrid DFT methods (e.g., B3LYP). The differences between the methods are however rather subtle, and the basis set selection often matters more than the functional itself. A notable exception is the M06-2X functional, which is not suitable for this type of complex.
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Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by European Research Council (ERC-StG ISORI, No. 258299) and the Grant Agency of the Czech Republic (14-20077S). Computational resources were provided by the MetaCentrum under the program LM2010005 and the CERIT-SC under the program Centre CERIT Scientific Cloud, part of the Operational Program Research and Development for Innovations, reg. no. CZ.1.05/3.2.00/ 08.0144. The authors would like to thank Dr. Andrew Gray for his help in preparation of the manuscript.
EXPERIMENTAL SECTION
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He Tagging IRPD Spectra. The He tagging IRPD spectra were measured with the ISORI instrument described in detail in refs 20 and 30. The ions were generated from a CH2Cl2 solution of phenylacetylene and [CpRu(PPh3)(PF6)]. The [RuCp(PPh3)(PhCCH)]+ complex was then mass selected (m/z 531) by the first quadrupole and transferred by an octopole toward a cryo-cooled wire quadrupole ion trap operated at 3 K and 10 Hz trapping cycle. The ions were trapped with a helium buffer gas pulse (30 ms wide). During their interaction with helium, about 10% of the trapped ions were transformed to helium tagged [(RuCp(PPh3)(PhCCH)(He)]+ complexes (m/z 535). After a 85 ms time delay, the ion cloud was irradiated by a photon pulse (8 ns) generated in an optical parametric oscillator (OPO) operating at 10 Hz frequency. At 90 ms, the exit electrode of the trap was opened, the ions were mass-analyzed by the second quadrupole, and their number (Ni) was determined by a Dalytype detector operated in ion-counting mode. In the following cycle the light from the OPO was blocked by a mechanical shutter, giving the number of unirradiated ions (N0). The IRPD spectra are constructed as the wavenumber dependence of 1 − Ni/N0. OPO wavenumbers in the region 1300−1800 cm−1 were calibrated using water absorption lines observed due to humidity in air. Positions of the lines were pinned down using the HITRAN database.35 For region 2900−3300 cm−1, a polystyrene film (Analytical Technology Inc.) was used for calibration. Line positions from its absorbance spectrum were compared to frequencies set by NIST SRM 1921 polystyrene film. Wavenumber accuracy of the reported spectra is better than ±2 cm−1. DFT Calculations. Calculations have been performed using different density functional theory methods (listed in Figure 3 and in the Supporting Information) as implemented in the Gaussian 09 suite.36 At each level of theory, the relevant minima were fully optimized and confirmed as minima on the potential energy surface by computation of the Hessian matrix at the same level of theory as used for the optimization. This also provided theoretical IR spectra. For comparison of the theoretical and experimental spectra, frequency scaling was used as explained in the text. Preparation of Deuterium-Labeled Phenylacetylene (PhCCD). PhCCD was prepared according to a published procedure.37
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