Intrinsic Reactivity of Diatomic 3d Transition-Metal Carbides in the Thermal Activation of Methane: Striking Electronic Structure Effects

Mechanistic aspects of the C-H bond activation of methane by metal-carbide cations MC⁺ of the 3d transition-metals Sc-Zn were elucidated by NEVPT2//CASSCF quantum-chemical calculations and verified experimentally for M = Ti, V, Fe, and Cu by using Fourier transform ion-cyclotron resonance mass spectrometry. While MC⁺ species with M = Sc, Ti, V, Cr, Cu, and Zn activate CH₄ at ambient temperature, this is prevented with carbide cations of M = Mn, Fe, and Co by high apparent barriers; NiC⁺ has a small apparent barrier. Hydrogen-atom transfers from methane to metal-carbide cations were found to proceed via a proton-coupled electron transfer mechanism for M = Sc-Co; wherein the doubly occupied _xz/yz-orbitals between metal and carbon at the carbon site serve as electron donors and the corresponding metal-centered vacant_xz/yz-orbitals as electron acceptors. Classical hydrogen-atom transfer transpires only in the case of NiC⁺, while ZnC⁺ follows a mechanistic scenario, in which a formally hydridic hydrogen is transferred. CuC⁺ reacts by a synchronous activation of two C-H bonds. While spin density is often so crucial for the reactions of numerous MO⁺/CH₄ couples, it is much less important for the C-H bond activation by carbide cations of the 3d transition-metals, in which one notes large changes in bond dissociation energies, spin states, number of d-electrons, and charge distributions. All these factors jointly affect both the reactivity of the metal carbides and their mechanisms of C-H bond activation.