Comparing B-H Bond Activation in NiIIX(NNN)-Catalyzed Nitrile Dihydroboration (X = Anionic N-, C-, O-, S-, or P-donor).

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.

Tetracopper(I) thiolate- and amido-(SNS) complexes and copper-catalyzed azide-alkyne cycloaddition in water.

Two tetranuclear Cu(I) complexes bearing thiolate- and amido-SNS ligands were characterized by X-ray diffraction and mass spectrometry. Although the amido ligand undergoes irreversible N-protonation by the copper-bound alkyne, the thiolate complex demonstrates good activity in the copper-catalyzed azide-alkyne cycloaddition reaction with a variety of substrates. The base-free reactions are performed in water and afford excellent yields over 2 h at 70 °C. DFT calculations suggest a proton-shuttle role for the thiolate donor in formation of the initial dicopper σ,π-alkynyl intermediate.

Coinage metal amido and thiolate SNS complexes: consequences of catalyst speciation in Cu(I)-catalysed carbonyl hydroboration.

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). Although these complexes showed no catalytic activity, treatment of the IPr-Ag-SNS amido complex with pinacolborane released the N-borylated ligand, SMeNBpinSMe, (L1-Bpin). This finding led us to reinvestigate the IPr-Cu-SNS amido precatalyst, revealing that immediate loss of L1-Bpin converts our catalyst system to [CuH(IPr)]2.

Solvent-free Zn (NSNO) complex-catalysed dihydroboration of nitriles.

N-donors are the most commonly employed Lewis bases in ligand-assisted catalysis. A dimeric zinc complex (Zn-1) employing a tetradentate pyridine-thioether-anilido-aryloxide NSNO ligand (L) effects the quantitative conversion of nitriles to the corresponding double hydroborated products at 1 mol% catalyst loading. Variable Time Normalization Analysis kinetic studies showed a first-order dependence with respect to the nitrile, pinacolborane and zinc and clear evidence for catalyst deactivation. A plausible ligand-assisted reaction pathway involves B-H bond activation by the aryloxide (vs. anilido) donor.

SNS ligand-assisted catalyst activation in Zn-catalysed carbonyl hydroboration.

Ligands that include Lewis acid/base functionality have extensive applications in bifunctional catalysis using first row metals. In this work, zinc bis(amido), bis(thiolate) and amido-thiolate SNS complexes were prepared and compared as precatalysts for carbonyl hydroboration using pinacolborane. Mechanistic studies revealed two different ligand-assisted precatalyst activation pathways, both leading to an active and robust zinc alkoxide catalyst. This work furthers our understanding of metal-ligand cooperation in first-row metal catalysis.