Phosphines are a three-valent phosphorus compound and with an unshared electron pair. Phosphine ligands are phosphines, compound of the formula PRR'R" (R, R', R" = H, alkyl, aryl, etc) that are used as "soft" σ-donating ligands in metal complexes. This gives solubilization and stabilization to organometallic complexes by forming complexes with various transition metal species including latter-period transition metals and others, and is also used for controlling the reactivity and selectivity of the transition metal promoted reactions by modulate the electronic properties of the metal center to which they are bound. Generally, the phosphine ligands with high electron density increase the reactivity of oxidative addition from a metal center, and their bulkiness improves the reductive elimination. Although, phosphine ligands are most notable for their remarkable electronic and steric tunability in designing metal complexes, they are most often spectator ligands, meaning that they don't participate in reactions, but hang on for the ride.
Fig.1 The general structural formula of phosphine ligands
According to their coordination number, phosphine ligands can be mainly divided into following several kinds including monodentate phosphine ligands, bidentate phosphine ligands and polydentate phosphine ligands.
- Monodentate phosphine ligands: Monodentate phosphine ligands refer to the ligand having only one coordinating atom, and this coordinating atom is a P atom. Compared with others, monophosphine ligands are easy to synthesize, flexible in structure, and excellent in catalytic performance and they have an easily predictable individual binding with the metal because only two modes exist: bound and not bound.
- Bidentate phosphines ligands: Bidentate phosphine ligands, sometimes called diphosphine ligands, are organophosphorus compounds most commonly used as 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 bidentate phosphine ligands have been synthesized with different linkers and R-groups.
- Polydentate phosphine ligands: A polydentate phosphine ligand refers to a ligand having a plurality of coordinating atoms in a ligand, and these coordinating atoms are P atoms. Usually, polydentate phosphine ligands with -(CH2)n- spacers between the donor atoms are useful backbones for the construction of bi- and polynuclear transition metal complexes. Polydentate phosphine ligands possess good coordination capability, versatile coordinated modes and provide several donor atoms and flexible molecular deformation. The resulting complexes have a wide range of applications.
Phosphorus containing ligands have been found to complex a large number of metal ions form a complex, often related to organometallic chemistry, catalysis, separations science and luminescence. These compounds are also used in other areas of chemistry.
- Catalysis: Phosphine ligands are important in homogenous catalysis and have been crucial for many reactions, such as cross-coupling (Suzuki, Heck, Kumada, Negishi couplings, reductive couplings, cycloadditions and C-H functionalizations), hydrofunctionalization, N-methylation, formylation, allyl reaction, ethylene oligomerization reaction, Michael addition, etc. These catalysis reactions are of great use in the following areas such as petrochemical, pharmaceutical catalysis agrochemicals reactions and fine chemical. Moreover, the chiral phosphine ligands often used in asymmetric catalysis. The chiral phosphine ligands based transition-metal catalysed reactions have played an important role in asymmetric organic synthesis by providing easy, selective, doable and environmentally friendly processes to produce a wide variety of organic product such as natural products, bioactive molecules and pharmaceuticals.
Fig.2 Schematic diagram of noncovalent interaction-assisted phosphine ligands in asymmetric catalysis
- Separations science: Because of the wide range of ionic selectivities possible with well-defined structural modifications, the phosphine ligands based complexes have great potential in separation science. For example, the ordered structures in complexes obtained from transition metal ions and phosphine ligands not only can be used in stereoselective syntheses but also their selectivity toward actinides and lanthanides can be utilized in the removal of radionuclides from aqueous solutions.
- Luminescence: Research on luminescent compounds has been actively pursued in the past two decades because of their numerous potential applications in light emitting devices. The d10 coinage metals or metals clusters coordinated by phosphine ligands present a great variety of structural forms associated with rich photophysical properties. For example, the complex incorporating [Cu4I4L4] clusters coordinated by phosphine ligands shows highly luminescent at room temperature, and this emission is strikingly sensitive to the temperature and the environment such as the rigidity of the host medium.
Fig.3 The molecular structure of [Cu4I4L4] clusters coordinated by phosphine ligands
- Others: Phosphines ligands are essential raw materials for the preparation of synthetic flavors, fragrances and flame retardants.
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