Fixation of CO_2 by Magnesium Cations: A Reinterpretation

摘要

Identifying the factors which determine the remarkable variety of bonding modes in the coordination of CO2 to metal centers[1] is central to controlling electron transfer and ultimately the reactivity.[2]-[4] In this respect, it seems legitimate to state that d electrons are of critical importance since CO2 coordination was never observed in d0- or main-group-metal systems.[1] Furthermore, the CO2 oxygen atoms do not possess sufficient basicity to form stable adducts with Lewis acids. The sole case of a genuine end-on-bound CO2 complex[5] involves a very strongly reducing trivalent uranium center, for which it seems likely that partial electron transfer plays a role. Nevertheless, CO2 is only weakly bound in this complex. As a rule with no exception until very recently, only low- and medium-valent metals have been used for CO2 coordination. For these reasons, the recent findings[6] describing the robust end-on coordination of CO2 to magnesium cations with formation of the unprecedented Mg/Al-containing cluster [{R2Al(-NSiMe3)(-OSiMe3)Mg(thf)2(-O2C)}3]2 thf (R=Me (1 a), Et (1 b)), certainly appear to be a major breakthrough. Not only do compounds 1 represent the first case of CO2 coordination to a non-transition-metal center in an unprecedented bridging end-on fashion, but also the irreversible fixation occurs in THF, the oxygen atom of which is normally regarded as a far better donor than the oxygen atoms of CO2.