Gas Phase Reactions of Second-Row Transition Metal Atoms with Small Hydrocarbons: Experiment and Theory

Abstract

For reactions of gas phase, ground state, neutral transition metal atoms from the 4d series with alkanes and alkenes, we combine 300 K kinetics measurements with ab initio electronic structure calculations to infer mechanisms in some detail. The theoretical method PCI-80 with zero-point energy corrections to the bare potential surface apparently produces bond energies, reaction exothermicities, and even saddle point energies accurate to within 2-3 kcal/mol, provided that the correct ground state has been located, which is sometimes difficult. The reactions fall into two general categories: termolecular stabilization of long-lived M(hydrocarbon) complexes and bimolecular elimination of Hz. By using the ab initio energies and vibrational frequencies in a statistical unimolecular rate theory (RRKM theory), we can model the lifetimes of M(hydrocarbon) complexes to assess the plausibility of a saturated termolecular mechanism at 1 Torr He. Termolecular examples include the reactions of Pd with alkanes to form long-range v2 complexes; the reactions of Rh and Pd with alkenes to form n complexes; and probably the reactions of Y, Zr, Nb, Rh, and Pd with cyclopropane to form CH or CC insertion complexes. In other reactions, all of the evidence indicates a bimolecular H2 elimination mechanism. Rhodium is unique among the 4d metal atoms in effecting HZ elimination from ethane and larger alkanes. Yttrium, zirconium, and niobium almost surely insert in CH bonds of ethylene and larger alkenes, ultimately eliminating H2. We discuss the general requirements on the pattern of atomic electronic states that pennit efficient CH bond activation and H2 elimination. The good agreement between the observed reaction rates and the PCI-80 calculations lends confidence to future efforts to apply ab initio techniques to more complicated catalytic systems, including condensed phase reactions involving ligated metal centers.

Ab initio benchmark study for the oxidative addition of CH4 to Pd: Importance of basis-set flexibility and polarization

“…3 The insertion of the palladium atom into the C-H bond in alkanes has therefore received considerable attention, both experimentally 4 -8 and theoretically. 5,7,[9][10][11][12][13][14][15][16] In this work the insertion of the Pd-d 10 atom into the C-H bond of methane as an important example of this type of reactions has been surveyed, see Chart 1. Experimental investigations on the kinetics of the reaction of palladium with alkanes have been carried out by Weisshaar and co-workers 6,7 using laser-induced fluorescence ͑LIF͒ techniques and, most recently, by Campbell.…”

“…II͒. The existing computational benchmark for oxidative addition of methane to palladium was obtained by Blomberg and co-workers 7 with the parametrized configuration interaction method PCI-80, 18 in which the effect of correlation is estimated by an extrapolation procedure. The PCI-80 study arrives at a Pdϩmethane complexation energy of 5.1 kcal/mol, an activation energy of 3.6 kcal/mol with respect to separate reactants, and a reaction energy of 2.3 kcal/mol ͑see Table I͒.…”

“…Note also that, in addition to the extrapolation procedure associated with PCI-80, the computation of these benchmark values involves a further approximation, namely the final scaled MCPF energies of the PCI-80 study were not computed at the MCPF but, instead, the HF optimum geometry. 7,14 In the present study, we use a consistent set of equilibrium and transition-state structures that have been fully optimized at the BLYP/TZ2P ͑see Sec. II A͒ level of the generalized gradient approximation ͑GGA͒ of density functional theory ͑DFT͒.…”

Vibrational vs. translational energy in promoting a prototype metal–hydrocarbon insertion reaction

“…Unfortunately, the presence of multiple ligands in transition metal complexes makes theoretical calculations difficult. Consequently, substantial effort has been devoted to carrying out calculations on model systems involving insertion of isolated transition metal atoms into C-H and C-C bonds (14,15). Interestingly, recent theoretical work on the dynamics of the dissociative adsorption of methane on Ir(111) (16) and Ni(111) (17) surfaces indicate that the metal lattice undergoes reconstruction during reaction, with a local surface metal atom undergoing significant (0.6 Å) displacement outward from the surface.…”

“…2 4d 1 for Y), yet form strong M-H and M-C bonds (14,15). However, neutral metal atoms encounter substantial potential energy barriers for insertion into C-H bonds of saturated hydrocarbon molecules (14,15).…”