A Co(II)-Hydroxide Complex That Converts Directly to a Co(II)-Acetamide during Catalytic Nitrile Hydration

In exploring structural and functional mimics of nitrile hydratases, we report the synthesis of the pseudo-trigonal bipyramidal Co(II) complexes (K)[Co(II)(DMF)(LPh)] (1(DMF)), (NMe₄)₂[Co(II)(OAc)(LPh)] (1(OAc)), and (NMe₄)₂[Co(II)(OH)(LPh)] (1(OH)) (LPh = 2,2′,2’’-nitrilo-tris-(N-phenylacetamide; DMF = N,N-dimethylformamide; −OAc = acetate)). The complexes were characterized using NMR, FT-IR, ESI-MS, electronic absorption spectroscopy, and X-ray crystallography, showing the LPh ligand to bind in a tetradentate tripodal fashion alongside the respective ancillary donor. One of the complexes, 1(OH), is an unusual structural and functional mimic of the Co active site in Co nitrile hydratases. 1(OH) reacted with acetonitrile to yield the Co(II)-acetamide complex (NMe₄)₂[Co(II)(NHC(O)CH₃)(LPh)], 2, which was also thoroughly characterized. In the presence of excess hydroxide, 1(OH) was found to catalyze quantitative conversion of the added hydroxide into acetamide. Despite the differences in Co oxidation state in nitrile hydratases and 1(OH) (Co(III) versus Co(II), respectively), 1(OH) was nonetheless an effective nitrile hydration catalyst, selectively producing acetamide over multiple turnovers.