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)(P i Pr 3 )]OTf ( 1 ; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene; OTf = CF 3 SO 3 ) 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{κ 2 - N,S -[NHC(CH 3 )S]}(≡CPh)(IPr)(P i Pr 3 )]OTf ( 2 ). Thioamidate and alkylidyne ligands of 2 couple in acetonitrile at 70 °C, forming a 1:1 mixture of the salts [OsH{κ 2 - C,S -[C(Ph)NHC(CH 3 )S]}(CH 3 CN)(IPr)(P i Pr 3 )]OTf ( 3 ) and [Os{κ 2 - C,S -[CH(Ph)NHC(CH 3 )S]}(CH 3 CN) 3 (IPr)]OTf ( 4 ). Treatment of 3 with potassium tert -butoxide produces the NH-deprotonation of its five-membered ring and gives OsH{κ 2 - C,S -[C(Ph)NC(CH 3 )S]}(IPr)(P i Pr 3 ) ( 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{η 3 - C 3 , κ 1 -S -[CH 2 C(Ph)C(Ph)NHC(CH 3 )S]}(CH 3 CN) 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 OsH 2 (C≡CPh){κ 2 - C,S -[C(Ph)NC(CH 3 )S]}(IPr)(P i Pr 3 ) ( 7 ), which releases H 2 to yield Os(C≡CPh){κ 2 - C,S -[C(Ph)NC(CH 3 )S]}(IPr)(P i Pr 3 ) ( 8 ).