Abstract
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.
Highlights
Nowadays, a chemical transformation of outstanding importance to the biosphere is the catalytic reduction of carbon dioxide
Research in the fields of lighter main group metal(loid) catalysis[2] and organocatalysis[3] has produced systems that are less harmful to the environment and contain less monetary expensive materials than comparable transition metal catalysts
With regard to mechanistic investigations one must note that the reaction of www.chemcatchem.org b with H3B · SMe2 (4 equivalents) had been reported to yield the aluminum borohydride 3 (Scheme 1).[14c]. In CDCl3 solution 3 does not convert when exposed to an atmosphere of CO2 which indicates that initial reaction between 1 b and the borane is not a viable pathway for the concerned catalytic reduction
Summary
A chemical transformation of outstanding importance to the biosphere is the catalytic reduction of carbon dioxide. Our group and others have reported the hydroboration of carbonyl functionalities promoted by aluminum hydride complexes as catalysts (Figure 2).[9] The successful implementation of this type of compounds for the catalytic reduction of CO2 has, not been described.
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