Metal phosphine complex
Phosphines are used as ligands for many metal complexes. Perhaps the most popular phosphine ligand used is triphenylphosphine, a shelf-stable solid that undergoes oxidation in air relatively slowly. Unlike most metal ammine complexes, metal phosphine complexes tend to be lipophilic, displaying good solubility in organic solvents. They also are compatible with metals in multiple oxidation states. Because of these two features, metal phosphine complexes are useful in homogeneous catalysis. Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh3)3Cl), Grubbs' catalyst, and tetrakis(triphenylphosphine)palladium(0).
Phosphines are L-type ligands. They are both σ-donors and π-acceptors. Phosphine ligands' π-acidity is due to P-C σ* anti-bonding orbitals. Arylphosphines are much stronger π-acceptors than alkylphosphines, which are poor π-acceptors or possibly π-donors. The phosphine with the strongest π-acidity is trifluorophosphine (PF3); its π-acidity approaches that of the carbonyl ligand.
As L-type ligands, phosphines do not change the overall charge of the metal complex. These complexes may be prepared by addition of phosphines to a coordinatively unsaturated metal precursor, or by ligand displacement of another L-type complex, such as a solvent molecule acting as a ligand. For example, palladium chloride forms infinite chains due to the bridging chloride ligands; when treated with triphenylphosphine, the coordination polymer degrades, yielding monomeric bis(triphenylphosphine)palladium(II) chloride units.
- [PdCl2]n + 2n PPh3 → n PdCl2(PPh3)2
The first phosphine complexes were cis- and trans-PtCl2(PEt3)2 reported by Cahours and Gal in 1870.
Phosphines primarily function as Lewis bases, interacting with metals as σ donor ligands. They also can accept electron density from metal into P–C σ* antibonding orbitals that have π symmetry. In early work, phosphine ligands were thought to utilize 3d orbitals to form M-P pi-bonding, but it is now accepted that d-orbitals on phosphorus are not involved in bonding. The energy of the σ* orbitals is lower for phosphines with electronegative substituents, and for this reason phosphorus trifluoride is a particularly good π-acceptor.
By changes in one or more of the three organic substituents, the steric and electronic properties of phosphine ligands can be manipulated. These properties are manifested in the catalytic properties of the host metal center. The Tolman cone angle and Tolman electronic parameter were first used to systematically categorize phosphines by their steric and electronic properties. These parameters have since been extended to other ligand types.
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. The thermal stability of phosphines ligands is enhanced when they are incorporated into pincer complexes.
Applications to homogeneous catalysis
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. 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). The success achieved by Reppe and his contemporaries led to many industrial applications.
Complexes of other organophosphorus ligands
The popularity and usefulness of phosphine complexes has led to the popularization of complexes of many related organophosphorus ligands. Complexes of arsines have also been widely investigated, but are avoided in practical applications because of concerns about toxicity.
Complexes of primary and secondary phosphines
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 → [LnM(μ-PR2)]2 + 2 HCl
Complexes of PRx(OR')3-x
Due to the chelate effect, ligands with two phosphine groups bind more tightly to metal centers than do two monodentate phosphines. The conformation of diphosphines is restricted, and for this reason many are used in asymmetric catalysis, e.g. Noyori asymmetric hydrogenation. Several diphosphines have been developed; many of these diphosphines have two arylphosphines linked by a short alkylidene bridge, e.g. 1,2-bis(diphenylphosphino)ethane (dppe). 1,1'-Bis(diphenylphosphino)ferrocene is interesting, because it uses a ferrocene unit as the linker. Several trans-spanning ligands like xantphos and spanphos are known as well. The complex dichloro(1,3-bis(diphenylphosphino)propane)nickel is useful in Kumada coupling.
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