Pincer-Ligand

A pincer ligand is a tridentate ligand system, which is usually bound to a transition metal.

History

The first pincer complexes were published in the 1970s by BL Shaw and CJ Moulton. Since then, many different ligand systems have been synthesized and published.

Properties

Pincer ligands are constructed so that they can bind ( as phosphorus, nitrogen, or sulfur ) and a σ - bond to the ( transition ) metal with two donor atoms. Often phosphines are used as donors, since they can be easily changed. For example, the steric be easily changed. If the pincer ligand is bound via two phosphorus donors and with a carbon-metal σ - bond to the metal, it is called a PCP pincer ligands. Also, the binding by means of a nitrogen - metal σ - bond is possible ( PNP). Furthermore, NCN (nitrogen - carbon-nitrogen ) and SCS (sulfur -carbon, sulfur) ligand systems often. As ligand backbone usually serve phenyl systems. However, aliphatic systems are known. If the ligand bound to a metal is coplanar systems, which are very rigid arise. This leads to an increased thermal stability of the system.

PNN pincer ruthenium complex

Aliphatic palladium PCP pincer complex

Applications

Pincer ligands can be used in many catalytic processes: activation of carbon dioxide ( CO2), nitrogen ( N2), polymerization of alkenes and alkynes, alkane dehydrogenation and transfer hydrogenation.