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.