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, <b>1a</b>; R = ⁱPr, <b>1b</b>; R = ^cPe, <b>1c</b>) react with BH₃·THF to produce borohydride complexes [2,6-(R₂PO)₂C₆H₃]­Ni­(η²-BH₄) (<b>2a</b>–<b>c</b>), 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 <b>1a</b> 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 <b>1b</b> and <b>1c</b>, 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 <b>1a</b>–<b>c</b>, reduction of CO₂ leads to the methoxide level when 9-BBN or HBcat is employed as the reducing agent. The best catalyst, <b>1a</b>, involves bulky substituents on the phosphorus donor atoms. Catalytic reactions involving <b>1b</b> and <b>1c</b> 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