How Does Fe⁺ Activate C−C and C−H Bonds in Ethane? A Theoretical Investigation Using Density Functional Theory

Abstract

The potential energy surface (PES) corresponding to the reaction of the iron cation with ethane, which represents a prototype of the activation of C−C and C−H bonds in alkanes by transition metal cations, has been investigated employing the recently suggested hybrid density functional theory/Hartree−Fock method (B3LYP) combined with reasonably large one-particle basis sets. The performance of this computational approach has been calibrated against experimentally known Fe+−R binding energies of fragments R relevant to the [Fe,C2,H6]+ PES and against the relative energies of the possible exit channels. Both the C−C and C−H bond activation branches of the PES are characterized by a low barrier for the first step, the insertion of the iron cation into a C−C and C−H bond, respectively. Rate determining are the second steps which in the C−C bond activation branch corresponds to an [1,3]-H shift leading to a complex between FeCH2 + and methane. Along the C−H activation reaction coordinate, no transition state corresponding to a β-hydrogen shift resulting in a dihydrido species could be located, even though such a step has been often postulated. The decisive step is rather a concerted saddle point connecting the C−H inserted species directly with a complex of Fe+ with molecular hydrogen and ethylene. The mechanistic scenario provided by our calculations is in concert with all experimental information and allows for the first time a detailed and consistent view on the mechanistic details of this import reaction sequence. It further demonstrates the usefulness of the B3LYP approach for describing even complex electronic situations such as present in open-shell transition metal compounds.

A theoretical study of the reaction of Ti+ with ethane

“…The C-H insertion TS ͑DFT B3LYP and MCQDPT2͒ or C-H insertion TS and H 2 elimination TS ͓CCSD͑T͔͒ are the highest energy points en route to the products, with a net energy requirement of 5-9 kcal/mol, depending on the level of theory. In comparison, for the Co ϩ -and Fe ϩ -mediated elimination of H 2 from C 2 H 6 studied recently with DFT, 39 the C-H insertion TS was located below the entrance channel and the H 2 loss TS was the rate determining step. The higher relative energy of the C-H insertion TS in the Ti ϩ case might result from the more shallow potential well of its initial complex as compared to the Co ϩ and Fe ϩ analogs.…”

An ab initio study of the reaction mechanism of Co++NH3

“…There have only been a few detailed studies of potential energy surfaces ͑PES's͒ of reactions of transition metal atoms 56,57 and ions [58][59][60][61][62][63] with alkanes and other prototypical species. Blomberg and co-workers 56 performed complete active space self-consistent field ͑CASSCF͒ plus CI calculations on the reactions of Fe, Co, Ni, Rh, and Pd with CH 4 and C 2 H 6 , and concluded that the second row transition metals have higher barriers to insertion than do their first row analogs.…”

A Comprehensive Theoretical Study on the Reactions of Sc+ with CnH2n+2 (n=1–3): Structure, Mechanism, and Potential‐Energy Surface

“…Double-dehydrogenation mechanisms in the reactions of M (M Fe, Co) with C 2 H 6 to form MC 2 H 2 2 H 2 were not reported in earlier studies by Holthausen et al [13,14] However, spontaneous double dehydrogenation was observed experimentally in the reactions of first-row transition-metal ions such as Sc and Ti with ethane. [55] Hence we also investigated double-dehydrogenation mechanism in the reaction of Sc with C 2 H 6 .…”

“…To explain the earlier experimental results and to generalize the mechanism postulated in Scheme 1 (middle), Holthausen et al reported two examples of theoretical investigations on the reactions of the late first-row transition-metal ions Fe and Co with ethane. [13,14] Their calculations indicated that: 1) These reactions process along both the CÀC and CÀH activation branches; 2) Each branch is composed of two elementary steps: C À C or C À H bond activation and subsequent isomerization of the inserted species by a b-H shift; 3) The rate-determining steps of the two branches are the isomerizations of the intermediate insertion products, rather than the initial CÀC or CÀH activation.…”