Abstract
This contribution reports the first example of highly selective reduction of CO2 into CH4 via tandem hydrosilylation with mixed main-group organo-Lewis acid (LA) catalysts [Al(C6F5)3 + B(C6F5)3]
{[Al] + [B]}. As shown by this comprehensive experimental and
computational study, in this unique tandem catalytic process, [Al]
effectively mediates the first step of the overall reduction cycle,
namely the fixation of CO2 into HCOOSiEt3 (1)
via the LA-mediated C═O activation, while [B] is incapable of promoting
the same transformation. On the other hand, [B] is shown to be an
excellent catalyst for the subsequent reduction steps 2–4, namely the
hydrosilylation of the more basic intermediates [1 to H2C(OSiEt3)2 (2) to H3COSiEt3 (3) and finally to CH4]
through the frustrated Lewis pair (FLP)-type Si–H activation. Hence,
with the required combination of [Al] and [B], a highly selective
hydrosilylative reduction of CO2 system has been developed, achieving high CH4
production yield up to 94%. The remarkably different catalytic
behaviors between [Al] and [B] are attributed to the higher overall
Lewis acidity of [Al] derived from two conflicting factors (electronic
and steric effects), which renders the higher tendency of [Al] to form
stable [Al]–substrate (intermediate) adducts with CO2 as well as subsequent intermediates 1, 2, and 3. Overall, the roles of [Al] and [B] are not only complementary but also synergistic in the total reduction of CO2, which render both [Al]-mediated first reduction step and [B]-mediated subsequent steps catalytic.
{[Al] + [B]}. As shown by this comprehensive experimental and
computational study, in this unique tandem catalytic process, [Al]
effectively mediates the first step of the overall reduction cycle,
namely the fixation of CO2 into HCOOSiEt3 (1)
via the LA-mediated C═O activation, while [B] is incapable of promoting
the same transformation. On the other hand, [B] is shown to be an
excellent catalyst for the subsequent reduction steps 2–4, namely the
hydrosilylation of the more basic intermediates [1 to H2C(OSiEt3)2 (2) to H3COSiEt3 (3) and finally to CH4]
through the frustrated Lewis pair (FLP)-type Si–H activation. Hence,
with the required combination of [Al] and [B], a highly selective
hydrosilylative reduction of CO2 system has been developed, achieving high CH4
production yield up to 94%. The remarkably different catalytic
behaviors between [Al] and [B] are attributed to the higher overall
Lewis acidity of [Al] derived from two conflicting factors (electronic
and steric effects), which renders the higher tendency of [Al] to form
stable [Al]–substrate (intermediate) adducts with CO2 as well as subsequent intermediates 1, 2, and 3. Overall, the roles of [Al] and [B] are not only complementary but also synergistic in the total reduction of CO2, which render both [Al]-mediated first reduction step and [B]-mediated subsequent steps catalytic.
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