In chemistry, a pincer ligand is a type of chelating agent that binds tightly to three adjacent coplanar sites, usually on a transition metal in a meridional configuration. The inflexibility of the pincer-metal interaction confers high thermal stability to the resulting complexes. This stability is in part ascribed to the constrained geometry of the pincer, which inhibits cyclometallation of the organic substituents on the donor sites at each end. In the absence of this effect, cyclometallation is often a significant deactivation process for complexes, in particular limiting their ability to effect C-H bond activation. The organic substituents also define a hydrophobic pocket around the reactive coordination site. Stoichiometric and catalytic applications of pincer complexes have been studied at an accelerating pace since the mid-1970s. Most pincer ligands contain phosphines. Reactions of metal-pincer complexes are localized at three sites perpendicular to the plane of the pincer ligand, although in some cases one arm is hemi-labile and an additional coordination site is generated transiently. Early examples of pincer ligands (not called such originally) were anionic with a carbanion as the central donor site and flanking phosphine donors and are referred to as PCP pincers.
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Scope of pincer ligands
Although the most common class of pincer ligands features PCP donor sets, variations have been developed where the phosphines are replaced by thioethers and tertiary amines. Many pincer ligands also feature nitrogenous donors at the central coordinating group position (see figure), such as pyridines.
An easily prepared pincer ligand is POCOP. Many tridentate ligands types occupy three contiguous, coplanar coordination sites. The most famous such ligand is terpyridine (“terpy”). Terpy and its relatives lack the steric bulk of the two terminal donor sites found in traditional pincer ligands.
History
The original work on PCP ligands arose from studies of the Pt(II) complexes derived from long-chain ditertiary phosphines, species of the type R2P(CH2)nPR2 where n >4 and R = tert-butyl. Platinum metalates one methylene group with release of HCl, giving species such as PtCl(R2P(CH2)2CH(CH2)2PR2).
Appendix: illustrative publications
Pincer complexes have been shown to participate in a variety of transformations, such as the activation of CO21, N2,2 carbon-halogen bond formation3 and oxidative addition of carbon-oxygen and carbon-fluorine bonds4, polymerization of alkenes5,6 and alkynes7, alkane dehydrogenation8-10 and alkane metathesis11, and transfer hydrogen catalysis.12 They have also recently been studied for their use as molecular sensors13,14 and switches.15