Reversible Binding of Hydrogen and Styrene Coordination on a Manganese Phosphenium Complex

The reactions of two complexes [(^R NHP)Mn(CO)₄ ] (^R NHP=N-arylated N-heterocyclic phosphenium) with H₂ at elevated pressure (≈4 bar) were studied by NMR spectroscopy. Irradiation with UV light initialized in one case (5 a, R=Dipp) the unselective formation of (^R NHP-H)MnH(CO)₄ ] (6 a) via cooperative addition of H₂ across the Mn=P double bond. In the other case (5 b, R=Mes), addition of H₂ was unobservable and the reaction proceeded via decarbonylation to a dimeric species [(^R NHP)₂ Mn₂ (CO)₇ ] (7 b) that was isolated and identified spectroscopically. Taking into account the outcome of further reaction studies under various conditions in the absence and presence of H₂ , both transformations can be explained in the context of a common mechanism involving decarbonylation to 7 a,b as the first step, and the different outcome is attributable to the fact that 7 b is unreactive towards both H₂ and CO while 7 a is not. DFT studies relate this divergence to deviations in the molecular constitution and stability arising from a different level of steric congestion. Preliminary studies suggest further that 5 a/H₂ as well as 6 a enable the photo-induced hydrogenation of styrene to ethyl benzene, even if the mechanism and possibly catalytic nature of this process remain yet unknown.