Theoretical Study of σ-Bond Activation Reactions and Catalytic Reactions by Transition Metal Complexes

Abstract

This chapter reports mini-review of theoretical and computational studies of σ-bond activation reactions by transition metal complexes. Because the σ-bond activation reaction through oxidative addition has been theoretically investigated well for long, I wish to focus on the different types of σ-bond activation reactions here. One good example is the C–H σ-bond activation of benzene and methane by Pd(II)–formate complex. The σ-bond activation of methane by Ti(IV)-imido complex is another good example. Their theoretical and computational studies clearly indicate that these reactions are typical heterolytic σ-bond activation without any change of metal oxidation state. The orbital interaction diagram is completely different between the heterolytic σ-bond activation and the oxidative addition reactions; in the heterolytic σ-bond cleavage, the mixing of the C–H σ-bonding orbital into the bonding overlap between the C–H σ*-antibonding and the M–X bonding orbitals plays crucial roles. Theoretical study of oxidative addition to the M–L moiety, which is a new type of σ-bond activation, is also discussed based on theoretical study and recent experimental report. In this reaction, not only metal center but also organic ligand participates in the σ-bond activation reaction like the heterolytic σ-bond activation. However, the metal oxidation state increases by 2 in a formal sense like the usual oxidative addition, which is different from the heterolytic σ-bond activation. The heterolytic σ-bond activation is involved in many catalytic reactions. One of such good examples is Ru-catalyzed hydrogenation of CO2 to formic acid, in which the H–H bond cleavage with the Ru-(η1-OCOH) moiety is involved as a key step. This reaction is essentially the same as the C–H σ-bond activation of benzene with the Pd-(η1-OCOH) moiety. Another example is Pd-catalyzed cross-coupling reaction in which transmetallation is involved as a key step. This transmetallation is understood to be the heterolytic σ-bond activation. I wish to discuss how to accelerate this reaction based on theoretical and computational studies. Recently reported direct-cross coupling reaction occurs via heterolytic C–H σ-bond activation reaction, which is also discussed based on theoretical study.