Rare-Earth Supported Nickel Catalysts for Alkyne Semihydrogenation: Chemo- and Regioselectivity Impacted by the Lewis Acidity and Size of the Support
journal contributionposted on 06.03.2020, 15:47 by Bianca L. Ramirez, Connie C. Lu
Bimetallic catalysts of nickel(0) with a trivalent rare-earth ion or Ga(III), NiML3 (where L is [iPr2PCH2NPh]−, and M is Sc, Y, La, Lu, or Ga), were investigated for the selective hydrogenation of diphenylacetylene (DPA) to (E)-stilbene. Each bimetallic complex features a relatively short Ni–M bond length, ranging from 2.3395(8) Å (Ni–Ga) to 2.5732(4) Å (Ni–La). The anodic peak potentials of the NiML3 complexes vary from −0.48 V to −1.23 V, where the potentials are negatively correlated with the Lewis acidity of the M(III) ion. Three catalysts, Ni–Y, Ni–Lu, and Ni–Ga, showed nearly quantitative conversions in the semihydrogenation of DPA, with NiYL3 giving the highest selectivity for (E)-stilbene. Initial rate studies were performed on the two tandem catalytic reactions: DPA hydrogenation and (Z)-stilbene isomerization. The catalytic activity in DPA hydrogenation follows the order Ni–Ga > Ni–La > Ni–Y > Ni–Lu > Ni–Sc. The ranking of catalysts by (Z)-stilbene isomerization initial rates is Ni–Ga ≫ Ni–Sc > Ni–Lu > Ni–Y > Ni–La. In operando 31P and 1H NMR studies revealed that in the presence of DPA, the Ni bimetallic complexes supported by Y, Lu, and La form the Ni(η2-alkyne) intermediate, (η2-PhCCPh)Ni(iPr2PCH2NPh)2M(κ2-iPr2PCH2NPh). In contrast, the Ni–Ga resting state is the Ni(η2-H2) species, and Ni–Sc showed no detectable binding of either substrate. Hence, the mechanism of Ni-catalyzed diphenylacetylene semihydrogenation adheres to two different kinetics: an autotandem pathway (Ni–Ga, Ni–Sc) versus temporally separated tandem reactions (Ni–Y, Ni–Lu, Ni–La). Collectively, the experimental results demonstrate that modulating a base-metal center via a covalently appended Lewis acidic support is viable for promoting selective alkyne semihydrogenation.