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Abstract Dinuclear manganese hydride complexes of the form [Mn₂(CO)₈(μ‐H)(μ‐PR₂)] (R=Ph,1; R=iPr,2) were used inE‐selective alkyne semi‐hydrogenation (E‐SASH) catalysis. Catalyst speciation studies revealed rich coordination chemistry and the complexes thus formed were isolated and in turn tested as catalysts; the results underscore the importance of dinuclearity in engendering the observedE‐selectivity and provide insights into the nature of the active catalyst. The insertion product obtained from treating2with (cyclopropylethynyl)benzene contains acis‐alkenyl bridging ligand with the cyclopropyl ring being intact. Treatment of this complex with H₂affords exclusivelytrans‐(2‐cyclopropylvinyl)benzene. These results, in addition to other control experiments, indicate a non‐radical mechanism forE‐SASH, which is highly unusual for Mn−H catalysts. The catalytically active species are virtually inactive towardscistotransalkene isomerization indicating that theE‐selective process is intrinsic and dinuclear complexes play a critical role. A reaction mechanism is proposed accounting for the observed reactivity which is fully consistent with a kinetic analysis of the rate limiting step and is further supported by DFT computations.