1,2-Bis(diphenylphosphino)ethane

1,2-Bis(diphenylphosphino)ethane (dppe) is a commonly used bidentate ligand in coordination chemistry. Dppe is almost invariably chelated, although there are examples of unidentate (e.g., W(CO)5(dppe)) and of bridging behavior.

Preparation
The preparation of dppe is conducted via the alkylation of NaPPh2 which is typically prepared from triphenylphosphine (P(C6H5)3) as follows:
 * 1. P(C6H5)3 + 2 Na → NaP(C6H5)2 + NaC6H5

NaP(C6H5)2, which is readily air-oxidized, is treated with 1,2-dichloroethane (ClCH2CH2Cl) to give dppe:
 * 2. NaP(C6H5)2 + ClCH2CH2Cl → (C6H5)2PCH2CH2P(C6H5)2 + 2 NaCl

Reduction
The reduction of dppe by lithium to give PhHP(CH2)2PHPh has been reported.


 * 1. Ph2P(CH2)2PPh2 + 4 Li → PhLiP(CH2)2PLiPh + 2 PhLi

Hydrolysis by water gives:


 * 2. PhLiP(CH2)2PLiPh + 2 PhLi + 4H2O → PhHP(CH2)2PHPh + 4 LiOH + 2C6H6

Oxidation
Treatment of dppe with conventional oxidants such as hydrogen peroxide (H2O2), aqueous bromine (Br2), etc., always produces dppeO in low yield (e.g., 13%) as a result of non-selective oxidation leading to mixtures of the starting material, the monoxide, and dioxide. Selective mono-oxidation of dppe can be achieved by reaction with PhCH2Br to give dppeO.


 * 3. Ph2P(CH2)2PPh2 + PhCH2Br → Ph2P(CH2)2PPh2(CH2Ph)+Br-

This is followed by purification and alkaline catalyzed hydrolysis of the mono-phosphonium salt.


 * 4. Ph2P(CH2)2PPh2(CH2Ph)+Br- + NaOH + H2O → Ph2P(CH2)2P(O)Ph2

Coordination complexes of dppe
Coordination complexes of dppe, and diphosphine ligands in general, are almost entirely used as homogeneous catalysts for a wide range of reactions. Chiral diphosphines are especially important to the pharmaceutical industry for their ability to catalyze asymmetric reactions Two simple coordination complexes of dppe include Pd(dppe)2 and Ir(dppe)2. Pd(dppe)2 can be prepared by reduction of Pd(II) with NaBH4. It is most conveniently prepared, however, in situ from Pd(OAc)2.

dppe analogues
dppe is one compound of a larger class of ligands known as the diphosphines. The most widely used diphosphine ligands are the bis(diphenylphosphino)alkanes, Ph2P(CH2)nPPh2. These can be prepared from X(CH2)nX (X=halogen) and YPPh2 (Y=alkali metal) in THF. Bidentate phosphines with only one bridging group such as dppm tend to promote metal-metal interaction or bond formation because the two donor P atoms are so close together. The use of chelate phosphines with many bridging groups giving long flexible chains has quite a different effect. For example, the chelate phosphine Bu2tP(CH2)10PBu2t can give complexes that have as many as 72 atoms in a ring.

dppv
Dppv is the acronym for 1,2-bis(diphenylphosphino)ethylene, (C6H5)2PCH=CHP(C6H5)2. Both cis (m.p. 125 °C) and trans (m.p. 126°C) isomers are known, being derived from the respective isomers of ClCH=CHCl. The substitution of chloride is stereospecific. The cis isomer, however, is almost exclusively used for applications of a ligand.

dmpe
Dmpe is prepared in a similar way to that of dppe.


 * 1. (CH3)2P-P(CH3)2 + 2Na → 2NaP(CH3)2


 * 2. 2NaP(CH3)2 + ClCH2CH2Cl → (CH3)2PCH2CH2P(CH3)2 + 2NaCl

Dmpe should be handled with care. It is toxic and exposure to the air may result in ignition.

Dppf
1,1'-Bis(diphenylphosphino)ferrocene (dppf) is also used as a ligand. As its name implies, its backbone is made up of a ferrocene moiety, as opposed to the ethylene backbone of dppe.