Carbon dioxide reduction by early metal compounds: A propensity for oxygen atom transfer

While treatment of [(silox)₂TaH₂]₂ (1, silox = ^tBu₃SiO) with CO₂ provided evidence for myriad hydride transfer products, related hydrides (silox)₃TaH₂ (15-Ta), (silox)₃NbH (17), and [(silox)₂WH]₂ (20) afforded products of oxygen atom transfer (OAT) and CO reduction, such as (silox)₃Ta(?²-CH₂O) (16-Ta from 15-Ta) and [(silox)₂W]₂(μ-H)(μ-O)(μ-CH) (14, from 20). Low valent early metal derivatives (silox)₃Ta (7) and (silox)₃NbPMe₃ (7-NbPMe₃) generated (silox)₃TaO (8-Ta) and (silox)₃NbO (8-Nb), respectively, under high CO₂ concentrations. Under low CO₂ conditions, the products were the oxos and those derived from CO: (silox)₃TaCCO (9-Ta) and (silox)₃TaCCTa(silox)₃ (10-Ta); (silox)₃NbCCO (9-Nb) and (silox)₃NbCCNb(silox)₃ (10-Nb). On all occasions with CO present, preparations of 10-Nb yielded varying amounts of a similarly insoluble material tentatively characterized as (silox)₃NbCONb(silox)₃ (18-Nb). Metric parameters of a crystal structure determination purported to be of 10-Nb resemble those of 18-Nb according to calculations, but the bulk sample suggests cocrystallization of 9-Nb and 18-Nb. Calculations of 10-Ta, 10-Nb, and 18-Nb, including thermodynamics of dicarbide binding and CO scission help to understand the system. Since OAT is the major path for CO₂ activation, it is suggested that CO₂ reduction is best conducted by the water-gas shift reaction to produce CO, followed by Fischer-Tropsch processes.