posted on 2022-12-01, 19:51authored bySaeed Ataie, R. Tom Baker
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 NpyridineNimineNpyrrolide ligand (L2)−,
a series of divalent NiIIX(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 1H and 13C 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–Npyrrolide) 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.