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.
