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(ƞ2-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.