A stepwise strategy to form tris(bidentate)-Osmium(II) complexes enclosing three different bidentate ligands

Abstract

The cleanest synthetic strategy for the preparation of M–NHC complexes (M = transition metal) is the direct metalation. This method requires the presence at the starting complex of a strong Brønsted base as ligand, which becomes labile by deprotonating the imidazolium salt. Neutral transition-metal polyhydride complexes are basic enough to produce this deprotonation. In addition, the complex OsH6(PiPr3)2 has demonstrated to activate σ bonds of a wide range of organic molecules.1 In the light of these precedents, we reacted this hexahydride with N-heterocyclic carbenes in order to obtain the corresponding neutral trihydride derivatives OsH3(C,C'–L)(PiPr3)2 (C is referred to ortho-metalated phenyl carbon atom; C' to N-heterocyclic carbene carbon atom; L to chelate NHC ligand). Protonation of the formed trihydrides with tetrafluoroboric acid leads to the formation of the corresponding cationic bis(dihydrogen) derivatives [Os(C,C'–L)(μ2–H2)2(PiPr3)2]BF4. Herewith, we show how these novel bis(dihydrogen) complexes act as precursors for the formation of tris(bidentate)-osmium(II) derivatives, enclosing bidentate ligands as acetylacetonate (acac) and cis-1,2-bis(diphenylphosphino)ethylene (bdppe).