HOW DOES FE-C AND C-H BONDS IN ETHANE - A THEORETICAL INVESTIGATION USING DENSITY-FUNCTIONAL THEORY( ACTIVATE C)

The potential energy surface (PES) corresponding to the reaction of th e iron cation with ethane, which represents a prototype of the activat ion of C-C and C-H bonds in alkanes by transition metal cations, has b een investigated employing the recently suggested hybrid density funct ional theory/Hartree-Fock method (B3LYP) combined with reasonably larg e one-particle basis sets. The performance of this computational appro ach has been calibrated against experimentally known Fe+-R binding ene rgies of fragments R relevant to the [Fe,C-2,H-6](+) PES and against t he 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 barr ier for the first step, the insertion of the iron cation into a C-C an d 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 lead ing to a complex between Fe=CH2+ and methane. Along the C-H activation reaction coordinate, no transition state corresponding to a beta-hydr ogen shift resulting in a dihydrido species could be located, even tho ugh 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 mecha nistic scenario provided by our calculations is in concert with all ex perimental information and allows for the first time a detailed and co nsistent view on the mechanistic details of this import reaction seque nce. It further demonstrates the usefulness of the B3LYP approach for describing even complex electronic situations such as present in open- shell transition metal compounds.