Comparing B–H Bond Activation in Ni<sup>II</sup>X(NNN)-Catalyzed Nitrile Dihydroboration (X = Anionic N-, C-, O-, S-, or P-donor)
Saeed Ataie, R. Tom Baker
Abstract
One of the key steps in many metal complex-catalyzed hydroboration reactions is B–H bond activation, which results in metal hydride formation. Anionic ligands that include multiple lone pairs of electrons, in cooperation with a metal center, have notable potential in redox-neutral B–H bond activation through metal–ligand cooperation. Herein, using an easily prepared N pyridine N imine N pyrrolide ligand (L 2 ) −, a series of divalent Ni II X(NNN) complexes were synthesized, with X = bromide ( 2 ), phenoxide ( 3 ), thiophenoxide ( 4 ), 2,5-dimethylpyrrolide ( 5 ), diphenylphosphide ( 6 ), and phenyl ( 7 ). The complexes were characterized using 1 H and 13 C NMR spectroscopy, mass spectrometry, and X-ray crystallography and employed as precatalysts for nitrile dihydroboration. Superior activity of the phenoxy derivative ( 3 ) [vs thiophenoxy ( 4 ) or phenyl ( 7 )] suggests that B–H bond activation occurs at the Ni–X (vs ligand Ni–N pyrrolide ) bond. Furthermore, stoichiometric treatment of 2–7 with a nitrile showed no reaction, whereas stoichiometric reactions of 2–7 with pinacolborane (HBpin) gave the same Ni–H complex for 2, 3, and 5 . Considering that only 2, 3, and 5 successfully catalyzed nitrile dihydroboration, we suggest that the catalytic cycle involves a conventional inner sphere pathway initiated by substrate insertion into Ni–H.