Ametal-phosphine complex is acoordination complex containing one or more phosphine ligands. Almost always, the phosphine is anorganophosphine of the type R3P (R = alkyl, aryl). Metal phosphine complexes are useful inhomogeneous catalysis.[1][2] Prominent examples of metal phosphine complexes includeWilkinson's catalyst (Rh(PPh3)3Cl),Grubbs' catalyst, andtetrakis(triphenylphosphine)palladium(0).[3]
Many metal phosphine complexes are prepared by reactions of metal halides with preformed phosphines. For example, treatment of a suspension ofpalladium chloride in ethanol with triphenylphosphine yields monomericbis(triphenylphosphine)palladium(II) chloride units.[4]
The first reported phosphine complexes werecis- andtrans-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 oftriphenylphosphine:[6]
Phosphines areL-type ligands. Unlike mostmetal ammine complexes, metal phosphine complexes tend to belipophilic, displaying goodsolubility inorganic solvents.
| L | ν(CO) cm−1 |
|---|---|
| P(t-Bu)3 | 2056.1 |
| PMe3 | 2064.1 |
| PPh3 | 2068.9 |
| P(OEt)3 | 2076.3 |
| PCl3 | 2097.0 |
| PF3 | 2110.8 |
Phosphine ligands are also π-acceptors. Theirπ-acidity arises from overlap of P-C σ*anti-bonding orbitals with filled metal orbitals. Aryl- and fluorophosphines are stronger π-acceptors than alkylphosphines.Trifluorophosphine (PF3) is a strong π-acid with bonding properties akin to those of thecarbonyl ligand.[8] 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.[9] The energy of the σ* orbitals is lower for phosphines withelectronegativesubstituents, and for this reasonphosphorus trifluoride is a particularly good π-acceptor.[10]
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, thesteric andelectronic properties of phosphine ligands can be manipulated.[11] The steric properties of phosphine ligands can be ranked by theirTolman cone angle[7] or percent buried volume.[12]
An important technique for the characterization of metal-PR3 complexes is31P 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 temperaturehydroformylation reactions, the scission of P-C bonds is observed however.[15] The thermal stability of phosphines ligands is enhanced when they are incorporated intopincer complexes.
One of the first applications of phosphine ligands in catalysis was the use oftriphenylphosphine in "Reppe" chemistry (1948), which included reactions ofalkynes,carbon monoxide, andalcohols.[16] In his studies, Reppe discovered that this reaction more efficiently produced acrylic esters using NiBr2(PPh3)2 as a catalyst instead ofNiBr2. 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 ofarsines 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 andoligomers:
Nickel(0) complexes of phosphites, e.g., Ni[P(OEt)3]4 are useful catalysts forhydrocyanation of alkenes. Related complexes are known forphosphinites (R2P(OR')) andphosphonites (RP(OR')2).
Due to thechelate effect, ligands with two phosphine groups bind more tightly to metal centers than do two monodentate phosphines. The conformational properties ofdiphosphines makes them especially useful inasymmetric catalysis, e.g.Noyori asymmetric hydrogenation. Several diphosphines have been developed, prominent examples include1,2-bis(diphenylphosphino)ethane (dppe) and1,1'-Bis(diphenylphosphino)ferrocene, thetrans spanningxantphos andspanphos. The complexdichloro(1,3-bis(diphenylphosphino)propane)nickel is useful inKumada coupling.
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