Article pubs.acs.org/Organometallics
Experimental Gas-Phase Thermochemistry for Alkane Reductive Elimination from Pt(IV) Erik P. A. Couzijn,† Ilia J. Kobylianskii, Marc-Etienne Moret,‡ and Peter Chen* Laboratorium für Organische Chemie, ETH Zürich, Vladimir-Prelog-Weg 2, 8093 Zürich, Switzerland S Supporting Information *
ABSTRACT: The gas-phase reactivity of the [(NN)PtIVMe3]+ (NN = α-diimine) complex 1 and its acetonitrile adduct has been investigated by tandem mass spectrometry. The only observed reaction from the octahedral d6 complex 1·MeCN is the simple dissociation of the coordinated solvent molecule with a binding energy of 24.5(6) kcal mol−1 measured by energy-resolved collision-induced dissociation experiments. Further reactions of 1 are observed. In addition to the expected reductive elimination of ethane from 1, competitive loss of methane occurs. Methane is generated from the initially formed ethane agostic complex via either C−H activation/ bond formation or σ-bond metathesis with the third methyl group. Energy-resolved collision-induced dissociation experiments indicate that the initial reductive C−C coupling step is rate limiting for both ethane and methane elimination, and afford a gas-phase barrier of 22.6(7) kcal mol−1 for this process. Density functional theory calculations confirm the reaction mechanisms, and a variety of functionals are benchmarked. The results at the M06-L/SDB-cc-pVTZ//mPW1K/SDD(d,p) level of theory agree well with the experiments and suggest that the generation of [(NN)PtH]+ at higher collision energy proceeds through sequential loss of methane and ethylene.
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INTRODUCTION Among elementary steps in organometallic chemistry, oxidative addition at a d8 center and its microscopic reverse, reductive elimination from a d6 center, play prominent roles.1 In particular, these elementary reactions are central to the bond activation of hydrocarbons by PtII and IrI complexes.2 We report a gas-phase mass spectrometric (MS) study of the collision-induced dissociation of the cationic [(NN)PtIVMe3]+ complex 1 and the solvent adduct 1·MeCN (Scheme 1) and density functional theory (DFT) calculations of the potential surface. With PtIV complexes as the starting materials, collisioninduced dissociation (CID) gives experimental access to the manifold of otherwise difficult to see intermediates that are presumed to intervene during the oxidative addition of hydrocarbons to a PtII center. The latter reaction, and the subsequent transformations of the metal-bound hydrocarbyl residues, have been the subject of much investigation3 due to their close mechanistic relationship to processes that may provide for the mild activation and conversion of methane.4 Using quantitative energy-resolved CID cross-section measurements,5,6 also called threshold CID (T-CID), we determine the activation energies for the gas-phase reactions of 1 and 1· MeCN. The system was found to undergo C−C as well as C− H bond formation and activation in the gas phase, ultimately eliminating ethane and methane. The study of the microscopic reverse of the oxidative addition offers practical advantages, © 2014 American Chemical Society
Scheme 1. MS Generation and CID Fragmentation
Received: May 5, 2014 Published: May 29, 2014 2889
dx.doi.org/10.1021/om500478y | Organometallics 2014, 33, 2889−2897
Organometallics
Article
mPW1K,12 M06-L, M06, and M06-2X15 density functionals in combination with the SDB-cc-pVTZ basis set,16 to which the SDD(d,p) ZPE corrections were added. Furthermore, D3 dispersion corrections were calculated with the DFT-D3 program of Grimme et al.17 using the appropriate coefficients for BP86-D3, BLYP-D3, M06L-D3, M06-D3, and M06-2X-D3. For the first two functionals, BeckeJohnson damping17b was applied, whereas for the M06 family only zero damping is currently available. B1 Diagnostic.18 The structures of all species involved in dissociation processes were reoptimized at the BLYP/SDD(d,p) level of theory, after which B1LYP/SDD(d,p) single-point energies were calculated. The differences between the BLYP and B1LYP// BLYP dissociation energies were divided by the number of bonds broken (being 1 in all cases) to afford the B1 diagnostic. These values are given in the Supporting Information. AIM Analysis. Atoms in molecules (AIM) topological analysis was performed with the AIM2000 program,19 which, however, cannot handle frozen-core wave functions. Therefore, the electron density for the mPW1K/SDD(d,p) structure of TS5−7 was recalculated at the mPW1K/D95+(d,p),Pt:mDZP level of theory. Here, mDZP is an allelectron double-ζ polarized basis set with an additional set of diffuse d functions.20 Attractors and bond and ring critical points were located using the default settings except for the rather steep cusp of Pt, which required relaxing the gradient convergence criterion to 1 × 10−5. Subsequently, bond gradient paths were calculated starting at the (3, −1) bond critical points.
being a unimolecular dissociation which is much easier to treat experimentally, while providing access to the same transition states as the oxidative addition itself. We also performed complementary DFT calculations on the proposed reaction mechanism and possible alternative pathways. It will be shown that reductive C−C coupling is rate determining and that branching into the two product channels occurs only afterward. We benchmark a series of popular density functionals against the experimental data.
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EXPERIMENTAL SECTION
General Considerations. Complexes and solutions were handled under a nitrogen atmosphere in a glovebox. Solvents were dried by distillation over appropriate drying agents. Anhydrous dichloroethane on molecular sieves, containing
