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Rare-Earth Supported Nickel Catalysts for Alkyne Semihydrogenation: Chemo- and Regioselectivity Impacted by the Lewis Acidity and Size of the Support

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posted 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-PhCCPh)­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.