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.