Dissimilarity in the Chemical Behavior of Osmaoxazolium Salts and Osmaoxazoles: Two Different Aromatic Metalladiheterocycles

The preparation of aromatic hydride-osmaoxazolium and hydride-oxazole compounds is reported and their reactivity toward phenylacetylene investigated. Complex [OsH­(OH)­(CPh)­(IPr)­(PⁱPr₃)]­OTf (1; IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolylidene, OTf = CF₃SO₃) reacts with acetonitrile and benzonitrile to give [OsH­{κ²-C,O-[C­(Ph)­NHC­(R)­O]}­(NCR)­(IPr)­(PⁱPr₃)]­OTf (R = Me (2), Ph (3)) via amidate intermediates, which are generated by addition of the hydroxide ligand to the nitrile. In agreement with this, the addition of 2-phenylacetamide to acetonitrile solutions of 1 gives [OsH­{κ²-C,O-[C­(Ph)­NHC­(CH₂Ph)­O]}­(NCCH₃)­(IPr)­(PⁱPr₃)]­OTf (4). The deprotonation of the osmaoxazolium ring of 2 and 4 leads to the oxazole derivatives OsH­{κ²-C,O-[C­(Ph)­NC­(R)­O]}­(IPr)­(PⁱPr₃) (R = Me (5), CH₂Ph (6)). Complexes 2 and 4 add their Os–H and Os–C bonds to the C–C triple bond of phenylacetylene to afford [Os­{η³-C₃,κ¹-O-[CH₂C­(Ph)­C­(Ph)­NHC­(R)­O]}­(NCCH₃)₂(IPr)]­OTf (R = Me (7), CH₂Ph (8)), bearing a tridentate amide-N-functionalized allyl ligand, while complexes 5 and 6 undergo a vicarious nucleophilic substitution of the hydride at the metal center with the alkyne, via the compressed dihydride adduct intermediates OsH₂(CCPh)­{κ²-C,O-[C­(Ph)­NC­(R)­O]}­(IPr)­(PⁱPr₃) (R = Me (9), CH₂Ph (10)), which reductively eliminate H₂ to yield the acetylide-osmaoxazoles Os­(CCPh)­{κ²-C,O-[C­(Ph)­NC­(R)­O]}­(IPr)­(PⁱPr₃) (R = Me (11), CH₂Ph (12)).