Tetraosmium carbonyl clusters containing μ-NH2 amido ligands: syntheses, crystal structures and reactivities

Abstract

Treatment of [Os₄(μ-H)₄(CO)₁₂] with O-tert-butylhydroxylamine hydrochloride (^tBuONH₂·HCl) afforded two amino ligand containing osmium clusters [Os₄(μ-H)₄(CO)₁₁(η¹-NH₂O^tBu)] (1) and [Os₄(μ-H)₄(CO)₁₁{(μ-H)₃Os(CO)₂(η¹-NH₂O^tBu)₂Cl}] (2) in moderate yields. For the same reaction, in the presence of one equivalent of [Os₃(CO)₁₂], the novel heptaosmium carbonyl cluster [{Os₄(μ-H)₂(CO)₁₁(μ-NH₂)}{Os₃(μ-H)(CO)₁₁}] (3) was isolated together with 1 and 2. This heptaosmium cluster 3 displays an uncommon metal skeleton, where the tetraosmium tetrahedron and the triosmium triangle are connected by a metal–metal bond. Upon the addition of hydrobromic acid (HBr) to a solution of 1, a pair of geometric isomers, unsupported butterfly [Os₄(μ-H)₄(CO)₁₁(μ-NH₂)Br] (4a) and supported butterfly [Os₄(μ-H)₄(CO)₁₁(μ-NH₂)Br] (4b) clusters were formed. They are the first examples of μ-NH₂ amido tetraosmium carbonyl clusters, which serve as good models for the adsorbed nitrogen atoms on a stepped metal surface. Refluxing of 4a in toluene for several hours, afforded a new triosmium μ-NH₂ amido cluster [Os₃(μ-H)₂(CO)₉(μ-NH₂)Br] (5). Addition of an excess of triphenylcarbenium tetrafluoroborate [Ph₃C][BF₄] to a solution of 4a in dichloromethane gave another tetrahedral μ-NH₂ amido tetraosmium cluster [Os₄(μ-H)₂(CO)₁₁(μ-NH₂)Br] (6) in moderate yield upon elimination of two hydrides.