Hydroboration Reduction of CO₂ Catalyzed by a Doubly Reduced Arylborane: DFT Insight into Double Active-Site and CO₂ Self-Promoting Single Active-Site Mechanisms and Counterion Effects

Hydroboration reduction of CO2 by HBpin catalyzed by ambiphilic boron centers, a doubly reduced 9,10-dimethyl-9,10-dihydro-9,10-diboraanthracene [DBA-Me-2](2-) (A0) and its metal salts ((M)A0) reported by ), has been explored by density functional theory (DFT) calculations. Unexpectedly, the cycloadduct (A1(a)) from the barrier-free activation of CO2 and bridging across the two B atoms of A0 may serve as a highly active catalyst to boost the CO2 hydroboration reduction through a single active-site mechanism. The catalytic hydroboration of CO2 by dianion A0 and its metal salts M-n-[DBA-Me-2] (n = 2 for M = Li+, Na+, K+; and n = 1 for M = Mg2+, Ca2+) follows the double active-site mechanism, where the diboron site acts as the active center. Generally, the CO2 hydroboration reduction by HBpin may take place through three independent and cascade subcycles, leading to primary HCOOBpin, secondary HCHO, and reduction product CH(3)OBpin, respectively. Counter cations and THF solvent molecules remarkably influence the activation of HBpin and H-2 by A0, but the relative energetics of the overall hydroboration process mediated by A1(a) is less changed. The present results show that the doubly reduced arylborane and its CO2 adduct, as well as their metal salts are quite promising for the hydroboration reduction of CO2.