Zinc amido and thiolate SNS complexes exhibit distinct ligand-assisted pre-catalyst activation pathways for carbonyl hydroboration using pinacolborane.

A zinc complex uses its aryloxide (vs. amido) donor to generate a Zn–H catalyst that effects quantitative, solvent-free hydroboration of nitriles to the corresponding diborylamines at 1 mol% catalyst loading.

Three new IPr-Ag- and -Au-SNS amido and thiolate complexes were synthesized and compared to their previously reported Cu analogues as carbonyl hydroboration catalysts (IPr = bis(2,6-diisopropylphenyl)imidazol-2-ylidene).

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.

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