Pincer-Ligated Nickel Hydridoborate Complexes: the Dormant Species in Catalytic Reduction of Carbon Dioxide with Boranes

Nickel pincer complexes of the type [2,6-(R2PO)2C6H3]­NiH (R = tBu, 1a; R = iPr, 1b; R = cPe, 1c) react with BH3·THF to produce borohydride
complexes [2,6-(R2PO)2C6H3]­Ni­(η2-BH4) (2a–c), as confirmed by NMR and IR spectroscopy, X-ray crystallography,
and elemental analysis. The reactions are irreversible at room temperature
but reversible at 60 °C. Compound 1a exchanges its
hydrogen on the nickel with the borane hydrogen of 9-BBN or HBcat,
but does not form any observable adduct. The less bulky hydride complexes 1b and 1c, however, yield nickel dihydridoborate
complexes reversibly at room temperature when mixed with 9-BBN and
HBcat. The dihydridoborate ligand in these complexes adopts an η2-coordination mode, as suggested by IR spectroscopy and X-ray
crystallography. Under the catalytic influence of 1a–c, reduction of CO2 leads to the methoxide level
when 9-BBN or HBcat is employed as the reducing agent. The best catalyst, 1a, involves bulky substituents on the phosphorus donor atoms.
Catalytic reactions involving 1b and 1c are
less efficient because of the formation of dihydridoborate complexes
as the dormant species as well as partial decomposition of the catalysts
by the boranes.