Pincer-ligated nickel hydridoborate complexes: the dormant species in catalytic reduction of carbon dioxide with boranes.

Nickel pincer complexes of the type [2,6-(R(2)PO)(2)C(6)H(3)]NiH (R = (t)Bu, 1a; R = (i)Pr, 1b; R = (c)Pe, 1c) react with BH(3)·THF to produce borohydride complexes [2,6-(R(2)PO)(2)C(6)H(3)]Ni(η(2)-BH(4)) (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 η(2)-coordination mode, as suggested by IR spectroscopy and X-ray crystallography. Under the catalytic influence of 1a-c, reduction of CO(2) 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.