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

Reacting the tBuPNNP expanded pincer ligand with two equiv of RuHCl(PPh3)3(CO) at high temperatures produces an insoluble air-stable complex, which was structurally characterized as [Ru2(tBuPNNP)H(μ-H)Cl(μ-Cl)(CO)2] 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 a soluble complex 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 produces an active catalyst for the E-selective semi-hydrogenation of alkynes under mild conditions (ambient temperature and pressure) with good functional group tolerance. Monitoring this reaction using 1H NMR spectroscopy for the hydrogenation of diphenylacetylene 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.