Chiral-at-Metal Osmium(VI) Phosphine Complexes

The chiral-at-metal osmium complex [N(n-Bu)4][cis-Os(N)(CH2SiMe3)PhCl2] results from
series of alkylation and protonation reactions from [N(n-Bu)4][Os(N)Cl4]. (S,S)-Chiraphos
displaces a chloride from this complex to produce two isomers of Os(N)(CH2SiMe3)PhCl((S,S)-chiraphos) with different configurations at osmium. 31P NMR spectroscopy indicates
that one isomer has a phosphorus donor from the chiraphos ligand trans to the nitride, while
the other has chloride in this position. Molecular mechanics calculations suggest that the
axial phosphine isomer has an S configuration at osmium. This isomer reversibly loses an
additional chloride ion and can be separated from the isomer with the opposite configuration
by the careful addition of AgSbF6. The diastereomers [SbF6][(S)-Os(N)(CH2SiMe3)Ph((S,S)-chiraphos)] and [SbF6][(R)-Os(N)(CH2SiMe3)Ph((S,S)-chiraphos)] can be isolated separately.
The two isomers of [SbF6][Os(N)(CH2SiMe3)Ph((R,R)-chiraphos)] are enantiomers to those
of [SbF6][Os(N)(CH2SiMe3)Ph((S,S)-chiraphos)]. The molecular structures of acetonitrile
adducts [SbF6][(S)-Os(N)(CH2SiMe3)Ph((S,S)-chiraphos)(NCMe)] and [SbF6][(R)-Os(N)(CH2SiMe3)Ph((S,S)-chiraphos)(NCMe)], from a crystal containing both isomers, show equatorial,
chelating (S,S)-chiraphos ligands with acetonitrile trans to the nitride in each.