Mechanism and Chemoselectivity of Small Molecule (CO2, tBuNCO, and iPrNCNiPr) Activation by Homobimetallic Compounds: Cp*2Ae2 (Ae = Be, Mg, and Ca) vs Cp*2Zn2

The activation of small molecules is a growing area of research. Recently, studies have revealed that low-valent main group metal compounds exhibit reactivity similar to those of transition metal compounds in small molecule activation. In this study, the activation mechanisms of CO₂, ^tBuNCO, and ⁱPrNCNⁱPr by homobimetallic sandwich compounds Cp*₂M₂ (Cp* = C₅(CH₃)₅, M = Be, Mg, Ca, and Zn) have been investigated by density functional theory (DFT) calculations, and the chemo-selectivities have been discussed. The calculated results show that Cp*₂M₂ could activate the CO₂, ^tBuNCO, and ⁱPrNCNⁱPr at low or room temperature, except for Cp*₂Be₂ used in CO₂ activation. There are two different activation sites in Cp*₂M₂, the M–M bond insertion and the Cp*-M insertion mode. For Cp*₂Ae₂ (Ae = Be, Mg, and Ca), the activations of these small molecules are more inclined in the Ae–Ae bond insertion mode. Whereas for Cp*₂Zn₂, the Zn-Cp* insertion mode is preferred. The reactivity decreases in the sequence of Ae = Be, Mg, and Ca for the same small molecules, which aligns with the strength of the Ae–Ae bond. This research offers a theoretical framework to elucidate the reaction kinetics and chemoselectivity modulation. Furthermore, it highlights the promising utility of main group metal complexes featuring metal–metal bonds for small molecule activation.