E-selective Semi-hydrogenation of Alkynes under Mild Conditions by a Diruthenium Hydride Complex

Abstract

The synthesis, characterization and catalytic activity of a new class of diruthenium hydrido carbonyl complexes bound to the tBuPNNP expanded pincer ligand is described. Reacting tBuPNNP with two equiv of RuHCl(PPh3)3(CO) at 140 °C produces an insoluble air-stable complex, which was structurally characterized as [Ru2(tBuPNNP)H(μ-H)Cl(μ-Cl)(CO)2] (1) using solid-state NMR, IR and X-ray absorption spectroscopies and follow-up reactivity. A reaction with KOtBu results in deprotonation of a methylene linker to produce [Ru2(tBuPNNP*)H(μ-H)(μ-OtBu)(CO)2] (3) featuring a partially dearomatized naphthyridine core. This enables metal-ligand cooperative activation of H2 analogous to the mononuclear analogue, [Ru(tBuPNP*)H(CO)]. In contrast to the mononuclear system, the bimetallic analogue 3 catalyzes the E-selective semi-hydrogenation of alkynes at ambient temperature and atmospheric H2 pressure with good functional group tolerance. Monitoring the semi-hydrogenation of diphenylacetylene by 1H NMR spectroscopy shows the intermediacy of Z-stilbene, which is subsequently isomerized to the E-isomer. Initial findings into the mode of action of this system are provided, including the spectroscopic characterization of a polyhydride intermediate and the isolation of a deactivated species with a partially hydrogenated naphthyridine backbone.