A metal-phosphine complex is a coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R3P (R = alkyl, aryl). Metal phosphine complexes are useful in homogeneous catalysis.[1] [2] Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh3)3Cl), Grubbs' catalyst, and tetrakis(triphenylphosphine)palladium(0).[3]
Many metal phosphine complexes are prepared by reactions of metal halides with preformed phosphines. For example, treatment of a suspension of palladium chloride in ethanol with triphenylphosphine yields monomeric bis(triphenylphosphine)palladium(II) chloride units.[4]
[PdCl<sub>2</sub>]n + 2PPh3 → PdCl2(PPh3)2The first reported phosphine complexes were cis- and trans-PtCl2(PEt3)2 reported by Cahours and Gal in 1870.[5]
Often the phosphine serves both as a ligand and as a reductant. This property is illustrated by the synthesis of many platinum-metal complexes of triphenylphosphine:[6]
RhCl3(H2O)3 + 4PPh3 → RhCl(PPh3)3 + OPPh3 + 2HCl + 2H2O
See also: π-backbonding. Phosphines are L-type ligands. Unlike most metal ammine complexes, metal phosphine complexes tend to be lipophilic, displaying good solubility in organic solvents.
| L | ν(CO) cm-1 | |
|---|---|---|
| P(t-Bu)3 | 2056.1 | |
| 2064.1 | ||
| 2068.9 | ||
| 2076.3 | ||
| 2097.0 | ||
| 2110.8 |
220px|thumb|right|Cone angle is a common and useful parameter for evaluating the steric properties of phosphine ligands.In contrast to tertiary phosphines, tertiary amines, especially arylamine derivatives, are reluctant to bind to metals. The difference between the coordinating power of PR3 and NR3 reflects the greater steric crowding around the nitrogen atom, which is smaller.
By changes in one or more of the three organic substituents, the steric and electronic properties of phosphine ligands can be manipulated.[11] The steric properties of phosphine ligands can be ranked by their Tolman cone angle or percent buried volume.[12]
An important technique for the characterization of metal-PR3 complexes is 31P NMR spectroscopy. Substantial shifts occur upon complexation. 31P-31P spin-spin coupling can provide insight into the structure of complexes containing multiple phosphine ligands.[13] [14]
Phosphine ligands are usually "spectator" rather than "actor" ligands. They generally do not participate in reactions, except to dissociate from the metal center. In certain high temperature hydroformylation reactions, the scission of P-C bonds is observed however.[15] The thermal stability of phosphines ligands is enhanced when they are incorporated into pincer complexes.
One of the first applications of phosphine ligands in catalysis was the use of triphenylphosphine in "Reppe" chemistry (1948), which included reactions of alkynes, carbon monoxide, and alcohols.[16] In his studies, Reppe discovered that this reaction more efficiently produced acrylic esters using NiBr2(PPh3)2 as a catalyst instead of NiBr2. Shell developed cobalt-based catalysts modified with trialkylphosphine ligands for hydroformylation (now a rhodium catalyst is more commonly used for this process).[17] The success achieved by Reppe and his contemporaries led to many industrial applications.[18]
The popularity and usefulness of phosphine complexes has led to the popularization of complexes of many related organophosphorus ligands.[5] Complexes of arsines have also been widely investigated, but are avoided in practical applications because of concerns about toxicity.
Most work focuses on complexes of triorganophosphines, but primary and secondary phosphines, respectively RPH2 and R2PH, also function as ligands. Such ligands are less basic and have small cone angles. These complexes are susceptible to deprotonation leading to phosphido-bridged dimers and oligomers:
2 LnM(PR2H)Cl → [L<sub>n</sub>M(μ-PR<sub>2</sub>)]2 + 2 HCl
Nickel(0) complexes of phosphites, e.g., Ni[P(OEt)<sub>3</sub>]4 are useful catalysts for hydrocyanation of alkenes. Related complexes are known for phosphinites (R2P(OR')) and phosphonites (RP(OR')2).
See main article: Diphosphines. Due to the chelate effect, ligands with two phosphine groups bind more tightly to metal centers than do two monodentate phosphines. The conformational properties of diphosphines makes them especially useful in asymmetric catalysis, e.g. Noyori asymmetric hydrogenation. Several diphosphines have been developed, prominent examples include 1,2-bis(diphenylphosphino)ethane (dppe) and 1,1'-Bis(diphenylphosphino)ferrocene, the trans spanning xantphos and spanphos. The complex dichloro(1,3-bis(diphenylphosphino)propane)nickel is useful in Kumada coupling.