N-Heterocyclic Carbene-Phosphinidenide Complexes as Hydroboration Catalysts

The reactions of the N-heterocyclic carbene-phosphinidene adducts (NHC)PSiMe3 and (NHC)PH with the dinuclear ruthenium and osmium complexes [(eta(6)-p-cymene)MCl2](2) (M = Ru, Os) afforded the half-sandwich complexes [(eta(6)-p-cymene){(NHC)-P}MCl] and [(eta(6)-p-cymene){(NHC)PH}MCl2] with two- and three-legged piano-stool geometries, respectively (NHC = IDipp, IMes; IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). The complexes were initially tested as precatalysts for the hydroboration of benzonitrile, and the most active species, the ruthenium complex [(eta(6)-p-cymene){(IMes)P}RuCl], was further used for the efficient hydroboration of a wide range (ca. 50 substrates) of nitriles, carboxylic esters, and carboxamides in neat pinacolborane (HBpin) under comparatively mild reaction conditions (60-80 & DEG;C, 3-5 mol % catalyst loading). Preliminary mechanistic and kinetic studies are reported, and stoichiometric reactions with HBpin indicate the initial formation of the monohydride complex [(eta(6)-p-cymene){(IMes)P}RuH] as the putative catalytically active species.

Automated summary

The reactions of N-heterocyclic carbene-phosphinidene adducts with dinuclear ruthenium and osmium complexes resulted in the formation of half-sandwich complexes with different geometries. The ruthenium complex showed high activity in the hydroboration of various substrates under mild reaction conditions. Preliminary mechanistic and kinetic studies suggested the formation of a monohydride complex as the catalytically active species.