C-F and C-C bond activation by transition metals in low energy atomic ion/surface collisions

Abstract

The transition metal ions, Cr^+·, Mo^+·, W^+· and Re⁺, abstract one or more fluorine atoms or C_mF_n groups (m = 1,2; n = 1-5) in collisions with fluorocarbon self-assembled monolayers (F-SAMs). The number of atoms abstracted increases with collision energy, and with W^+· and Re⁺ it is possible to maximize a specific scattered product ion by selecting the appropriate collision energy. The collision energy dependence suggests that dissociation of the products of multiple abstractions is not an important source of any of the observed ion/surface reaction products. The ions W^+· and Re⁺ activate and insert into C-C as well as C-F bonds. In Re⁺ collisions, products of C-C bond activation are of comparable intensity to the C-F activation products. The reactivity of the ions towards fluorine abstraction is observed to be Cr^+· <Mo^+· <W^+· <Re⁺. The data are interpreted in terms of reaction at the surface and are rationalized by considering three factors (i) the electronic structures of the ions, (ii) the thermochemistry of fluorine abstraction, and (iii) the degree of orbital overlap of the metal ion and the F-SAM substrate.