Reactions of Tp–Os nitrido complexes with the nucleophiles hydroxide and thiosulfate

The reaction between TpOs(N)Cl2 (1) [Tp = hydrotris(1-pyrazolyl)borate] and aqueous (nBu4N)(OH) in THF-d8 forms the nitrosyl complex TpOs(NO)Cl2 (5) among other products, suggesting an initial hydroxide attack at the nitrido ligand. In contrast, the reaction of the acetate complex TpOs(N)(OAc)2 (2) with NaOH in Me2CO/H2O yields the osmium bis-hydroxide complex TpOs(N)(OH)2 (3), which has been structurally characterized by single-crystal X-ray diffraction. Acetate for hydroxide exchange could occur by ligand substitution or by nucleophilic attack at the carbonyl carbon of the acetate ligands (saponification). Reacting 2 with Na18OH in H218O/CD3CN yields predominantly doubly 18O-labeled TpOs(N)(18OH)2 (3-18O2) and unlabeled acetate, by ESI/MS and 13C{1H} NMR. This indicates that hydroxide reacts by substitution rather than by attack at the ligand. The reaction of 2 with the softer nucleophile thiosulfate occurs at the nitrido ligand, giving the thionitrosyl complex TpOs(NS)(OAc)2 (4). Reacting 4 with NaOH in (CD3)2CO/D2O also generates the bis-hydroxide complex 3.

Graphical abstractReaction of TpOs(N)(OAc)2 (2) with thiosulfate occurs at the nitrido ligand to yield TpOs(NS)(OAc)2 (4). Reaction with hydroxide, however, gives TpOs(N)(OH)2 (3) by a direct substitution of OH− for OAc−, as confirmed by 18O-labeling. The contrasting reactivity of hydroxide with TpOs(N)Cl2 (1), 2, 3, and 4, at the metal versus at a ligand, is described.Figure optionsDownload full-size imageDownload as PowerPoint slide