Implications of C02 activation by Frustrated Lewis Pairs in the catalytic hydroboration of C02 : a view with N/Si+ Frustrated Lewis Pairs

International audienceln recent years, silylium species have found application in the design offrustrated Lewis pairs (FLPs) for the activation of small molecules. Highly electrophilic Si$^+$ species are strong $\sigma$ and $\pi$ acceptors and tend to undergo unwanted side-reactions in particular with solvents. Nevertheless, the use of base-stabilized silyl cations with tunable reactivity is of interest. Although C0$_2$ inserts readily into N-Si bonds, the reactivity of N/Si$^+$ FLPs for the reductive transformation of CO$_2$ and the activation of other small molecules has not yet been explored. Using complex 1 as a silyl cation stabilized by an intramolecular nitrogen base (Scheme 1), C0$_2$ can be inserted to form the isolable N/Si$^+$ FLP-C0$_2$ adduct 2. Complex 1 catalyzes hydroboration of C0$_2$ to methoxyboranes with 9-Borabicyclo(3.3.1)nonane (9-BBN), pinacolborane (pinBH) and catecholborane (catBH). The influence of the R alkyl groups on the silicon atom, the nature of the counter-ion X$^-$ on the stability of cation 2$^+$ and the catalytic performances were investigated. Experiments coupled with DFT calculations suggest that two different mechanisms proceed with 9-BBN and pinBH (Scheme 2). A novel electrophilic activation of pinacolborane that forms highly reactive "SiH" species is proposed, highlighting the role of C0$_2$ adducts during the catalytic cycle of C0$_2$ hydroboration