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).