Osmathiazole Ring: Extrapolation of an Aromatic Purely Organic System to Organometallic Chemistry

An osmathiazole skeleton has been generated starting from the cation of the salt [OsH­(OH)­(CPh)­(IPr)­(PiPr3)]­OTf (1; IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolylidene; OTf = CF3SO3) and thioacetamide; its aromaticity degree was compared with that of thiazole, and its aromatic reactivity was confirmed through a reaction with phenylacetylene. Salt 1 reacts with the thioamide to initially afford the synthetic intermediate [OsH­{κ2-N,S-[NHC­(CH3)­S]}­(CPh)­(IPr)­(PiPr3)]­OTf (2). Thioamidate and alkylidyne ligands of 2 couple in acetonitrile at 70 °C, forming a 1:1 mixture of the salts [OsH­{κ2-C,S-[C­(Ph)­NHC­(CH3)­S]}­(CH3CN)­(IPr)­(PiPr3)]­OTf (3) and [Os­{κ2-C,S-[CH­(Ph)­NHC­(CH3)­S]}­(CH3CN)3(IPr)]­OTf (4). Treatment of 3 with potassium tert-butoxide produces the NH-deprotonation of its five-membered ring and gives OsH­{κ2-C,S-[C­(Ph)­NC­(CH3)­S]}­(IPr)­(PiPr3) (5). The osmathiazole ring of 5 is slightly less aromatic than the osmathiazolium cycle of 3 and the purely organic thiazole. However, it is more aromatic than related osmaoxazoles and osmaoxazoliums. There are significant differences in behavior between 3 and 5 toward phenylacetylene. In acetonitrile, the cation of 3 loses the phosphine and adds the alkyne to afford [Os­{η3-C3,κ1-S-[CH2C­(Ph)­C­(Ph)­NHC­(CH3)­S]}­(CH3CN)2(IPr)]­OTf (6), bearing a functionalized allyl ligand. In contrast, the osmathiazole ring of 5 undergoes a vicarious nucleophilic substitution of hydride, by acetylide, via the dihydride OsH2(CCPh)­{κ2-C,S-[C­(Ph)­NC­(CH3)­S]}­(IPr)­(PiPr3) (7), which releases H2 to yield Os­(CCPh)­{κ2-C,S-[C­(Ph)­NC­(CH3)­S]}­(IPr)­(PiPr3) (8).