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

The synthesis, characterization and catalytic activity of a new class of diruthenium hydrido carbonyl complexes bound to the ( t Bu ) PNNP expanded pincer ligand is described. Reacting ( t Bu ) PNNP with two equiv of RuHCl(PPh(3))(3)(CO) at 140 °C produces an insoluble air‐stable complex, which was structurally characterized as [Ru(2)(( tBu)PNNP)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 [Ru(2)(( tBu)PNNP(*))H(μ‐H)(μ‐OtBu)(CO)(2)] (3) featuring a partially dearomatized naphthyridine core. This enables metal‐ligand cooperative activation of H(2) analogous to the mononuclear analogue, [Ru(( tBu)PNP*)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 H(2) pressure with good functional group tolerance. Monitoring the semi‐hydrogenation of diphenylacetylene by (1)H 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.