Catalytic CO2 Reduction with Boron- and Aluminum Hydrides

The previously reported dimeric NHI aluminum dihydrides 1 a,b, as well as the bis(NHI) aluminum dihydride salt 9(+)[OTs](-), the bis(NHI) boron dihydride salt 10(+)[OTs](-), and the free bis(NHI) ligand 12 were investigated with regard to their activity as a homogenous (pre)catalyst in the hydroboration (i. e. catalytic reduction) of carbon dioxide (CO2) in chloroform under mild conditions (i. e. room temperature, 1 atm; NHI=N-heterocyclic imine, Ts=tosyl). Borane dimethylsulfide complex and catecholborane were used as a hydride source. Surprisingly, the less sterically hindered 1 a exhibited lower catalytic activity than the bulkier 1 b. A similarly unexpected discrepancy was found with the lower catalytic activity of 10(+) in comparison to the one of the bis(NHI) 12. The latter is incorporated as the ligand to the boron center in 10(+). To elucidate possible mechanisms for CO2 reduction the compounds were subjected to stoichiometric reactivity studies with the borane or CO2. Aluminum carboxylates 4, 6, and 7(+) with two, four, and one formate group per two aluminum centers were isolated. Also, the boron formate salt 11(+)[OTs](-) was characterized. Selected metal formates were subjected to stoichiometric reactions with boranes and/or tested as a catalyst. We conclude that each type of catalyst (1 a,b, 9(+), 10(+), 12) follows an individual mechanistic pathway for CO2 reduction.