Low-Valent Molybdenum PNP Pincer Complexes as Catalysts for the Semihydrogenation of Alkynes.

Low-valent molybdenum PNP pincer complexes were studied as catalysts for the semihydrogenation of alkynes. For that purpose, t Bu-substituted PNP complexes PNP t Bu Mo(CO) 2 ( 6a ) and PNP t Bu Mo(CO) 3 ( 6c ) and the NNP complex NNP i Pr Mo(CO) 2 (PPh 3 ) (( rac )- 7 ) were synthesized and characterized. By utilizing the cyclohexyl-substituted complex PNP Cy Mo(CO) 2 (CH 3 CN) ( 5a ), several diphenylacetylene derivatives are transformed to the corresponding ( Z )-alkenes with good to very good diastereoselectivities (up to 91:9). Mechanistic experiments indicate an outer-sphere mechanism including metal-ligand cooperativity.