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

Treatment of [Os4(mu-H)4(CO)12] with O-tert-butylhydroxylamine hydrochloride (tBuONH2·HCl) afforded two amino ligand containing osmium clusters [Os4(mu-H)4(CO)11(1-NH2OtBu)](1) and [Os4(µ-H)4(CO)11{(µ-H)3Os(CO) 2(1-NH2OtBu)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(mu-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 (mu-NH2)Br](4a) and supported butterfly [Os4(mu-H)4(CO)11(µ-NH2)Br](4b) clusters were formed. They are the first examples of mu-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 mu-NH2 amido cluster [Os3(mu-H)2(CO)9(mu-NH2)Br](5). Addition of an excess of triphenylcarbenium tetrafluoroborate [Ph3C][BF4] to a solution of 4a in dichloromethane gave another tetrahedral mu-NH2 amido tetraosmium cluster [Os4(mu-H)2(CO)11(mu-NH2)Br](6) in moderate yield upon elimination of two hydrides