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International audienceA series of base-stabilized silylium species were synthesized and their reactivity toward CO 2 explored, yielding the characterization of a novel N/ Si + FLP-CO 2 adduct. These silicon species are active catalysts in the hydroboration of CO 2 to the methoxide level with 9-BBN, catecholborane (catBH), and pinacolborane (pinBH). Both experiments and DFT calculations highlight the role of the FLP-CO 2 adduct in the catalysis. Depending on the nature of the hydroborane reductant, two distinct mechanisms have been unveiled. While 9-BBN and catBH are able to reduce an intermediate FLP-CO 2 adduct, the hydroboration of CO 2 with pinBH follows a different and novel path where the B−H bond is activated by the silicon-based Lewis acid catalyst. In these mechanisms, the formation of a highly stabilized FLP-CO 2 adduct is found detrimental to the kinetics of the reaction. ■ INTRODUCTION Carbon dioxide is an abundant, inexpensive, low toxic, and renewable carbon feedstock and therefore an appealing C1-building block in organic chemistry. 1−3 The efficient reduction of CO 2 to important raw materials for the chemical industry or to value-added chemicals is, however, hampered by its high thermodynamic stability and kinetic inertness toward reduction. This drawback calls for efficient reduction catalysts able to synchronize C−O bond cleavage with C−H and C−C bond formation, at a high kinetic rate and with a low energy demand. While CO 2 activation and reduction at a metal center has been thoroughly investigated over the last four decades, 2,4−8 organic systems able to perform this task, based on main group elements (e.g., B, N, O, P, and Si), remain sporadic and underexplored. Many different coordination modes have been unveiled for the interaction of CO 2 with a transition metal or an f-element, based on the nature of the metal ion and the redox state of the CO 2 ligand. 9,10 In organic chemistry, CO 2 activation is quite monotonous and relies on the formation of stable adducts with organic Lewis bases, such as guanidines or N-heterocyclic carbenes (NHCs). 11−13 Nevertheless, most progress has been gained via the synergistic action of Lewis pairs, able to coordinate both the C and O centers of CO 2. Within the concept of frustrated Lewis pairs (FLPs), introduced by Stephan and Erker, 14−17 several CO 2 adducts have been successfully isolated over the last 6 years, in which the C atom is attached to a C, N, or P donor, while one or two O atoms coordinate a group XIII Lewis acid (B or Al). 18−2

acs catalysis

Implications of CO2 Activation by Frustrated Lewis Pairs in the Catalytic Hydroboration of CO2: A View Using N/Si+ Frustrated Lewis Pairs