An Efficient Iridium Catalyst for Reduction of Carbon Dioxide to Methane with Trialkylsilanes

Cationic silane complexes of general structure (POCOP)Ir(H)(HSiR3) {POCOP = 2,6-[OP(tBu)(2)](2)C6H3} catalyze hydrosilylations of CO2. Using bulky silanes results in formation of bis(silyl)acetals and methyl silyl ethers as well as siloxanes and CH4. Using less bulky silanes such as Me2EtSiH or Me2PhSiH results in rapid formation of CH4 and siloxane with no detection of bis(silyl)acetal rind methyl silyl ether intermediates. The catalyst system is long-lived, and 8300 turnovers can be achieved using Me2PhSiH with a 0.0077 mol % loading of iridium. The proposed mechanism for the conversion of CO2 to CH4 involves initial formation of the unobserved HCOOSiR3. This formate ester is then reduced sequentially to R3SiOCH2OSiR3, then R3SiOCH3, and finally to R3SiOSiR3 and CH4.