Diphosphines

Diphosphines, sometimes called bisphosphanes, are organophosphorus compounds most commonly used as bidentate phosphine ligands in inorganic and organometallic chemistry. They are identified by the presence of two phosphino groups linked by a backbone, and are usually chelating. A wide variety of diphosphines have been synthesized with different linkers and R-groups. Alteration of the linker and R-groups alters the electronic and steric properties of the ligands which can result in different coordination geometries and catalytic behavior in homogeneous catalysts. Many widely used diphosphine ligands have the general formula Ar2P(CH2)nPAr2. These compounds can be prepared from the reaction of X(CH2)nX (X=halogen) and MPPh2 (M = alkali metal): Cl(CH2)nCl + 2 NaPPh2 → Ph2P(CH2)nPPh2 + 2 NaCl Diphosphine ligands can also be prepared from dilithiated reagents and chlorophosphines: XLi2 + 2 ClPAr2 → X(PAr2)2 + 2 LiCl (X = hydrocarbon backbone) This approach is suitable for installing two dialkylphosphino groups, using reagents such as chlorodiisopropylphosphine. Another popular method, suitable for preparing unsymmetrical diphosphines, involves the addition of secondary phosphines to vinylphosphines: Ph2PH + 2 CH2=CHPAr2 → Ph2PCH2-CH2PAr2 (2-Lithiophenyl)diphenylphosphine can be used also to give unsymmetrical diphosphines. The lithiated reagent is available from (2-bromophenyl)diphenylphosphine: Ph2P(C6H4Br) + BuLi → Ph2P(C6H4Li) + BuBr Ph2P(C6H4Li) + R2PCli → Ph2P(C6H4PR2) + LiCl Many diphosphines are prepared from compounds of the type X(PCl2)2 where X = (CH2)n or C6H4. The key reagents are 1,2-bis(dichlorophosphino)ethane and 1,2-bis(dichlorophosphino)benzene. The short-chain diphosphine dppm tends to promote metal-metal interactions as illustrated by A-frame complexes. When the two phosphine substituents are linked by two to four carbon centres, the resulting ligands often chelate rings with a single metal. A common diphosphine ligand is dppe, which forms a five-membered chelate ring with most metals.