Osmium complexes synthesis

Synthesis. The most important startiug material for synthesis of osmium complexes is OsO. Other important complexes are disodium... 

OsCle- is a useful starting material for the synthesis of a range of osmium complexes (Figure 1.4). 

Although the above discussion is centered on the synthesis of polymeric osmium and ruthenium complexes, the methods employed are also very successful in the preparation of mononuclear complexes. In this context, the preparation of ruthenium or osmium complexes which are suitable for the formation of self-assembled monolayers (see Section 4.3 above) can be prepared by using the same approach. Starting from the precursor [M(bpy)2Cl2], one chloride atom can be replaced to yield complexes of the type [M(bpy)2Cl L]+, where L is the surface active ligand. In the presence of water, species of the type [M(bpy)2(L)2]2+ are obtained. 

The most widely utilized approach for the synthesis of nitrido osmium complexes proceeds by ligand exchange reactions of simple nitrido osmates. These are readily prepared from K0s03N, itself synthesized by treatment of 0s04 with KOH and ammonia (Eq. (66)) [91]. Subsequent treatment of K0s03N with KC1/HC1 then affords K20sCl5N (Eq. (67)) this and related salts are a commonly used precursors for the synthesis of osmium nitrides. 

The synthesis and chemistry of an 7] -selenophene osmium complex 50 has been studied <19990M1559>. Protonation and electrophilic substitution with acetaldehyde diethyl acetal occurred at C-2. Methylation of complex 50 with methyl triflate gave 51 which upon treatment with tetrabutylammonium borohydride (TBAB) led to the selenophene ring-opened complex 52 (Scheme 5). 

Because it forms stronger bonds with ligands, osmium complexes are generally less active catalysts than analogous ruthenium complexes. The exception is in oxidation chemistry where certain osmium complexes are better and more selective catalysts. Because of this, osmium tetroxide and other osmium 0X0 complexes are widely used in the synthesis of complex organic molecules. 

Figure 5.11 Scheme for the synthesis of a pyridinylimidazolyl ligand, its copolymerization with acrylic acid (AA) and butyl acrylate (BA), and subsequent ligand substitution reaction with an osmium complex to yield a redox polymer. From [147] with permission from Elsevier. 

Polymers containing all metal backbones of Ru-Ru or Os-Os bonds have been prepared via the electrochemical reduction of ruthenium and osmium complexes containing /ram-chloride ligands.81,82 Scheme 2.6 shows the synthesis of polymers with their backbones comprised solely of metal-metal bonds. The polymers were prepared by reducing [Mn(/ran.s-Cl2)(bipyXCO)2] (M = Ru, Os), 33, to M° complexes and forming the polymer after the loss of the chloride ligands. In both cases, the polymers were selective for the reduction of carbon dioxide. 

The relatively bulky osmium complex is sterically demanding and osmylation generally involves reaction at the less hindered face of an alkene. Osmylation of 254 to 255 by Rigby, in a synthesis of (+)-pancratistatin, shows the cis-delivery of the reagent and the selectivity for delivery from the less hindered face.357 in this case, the osmium complex formed on the top of the molecule despite the free hydroxyl group on the bottom face. A conformational drawing of 254 will show that the alkene unit and the lactam unit effectively flatten the two rings, and the top face is more accessible. 

There are considerable possibilities inherent in addition of organolithiums to osmium complexes of quinoline for the synthesis of 5-substituted quinolines. 

---