Pincer ligand

In chemistry, pincer ligand is a type of chelating agent that binds tightly to three adjacent coplanar sites, usually on a transition metal. The inflexibility of the pincer-metal interaction confers high thermal stability to the resulting complexes. This stability is 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 the pincer effect, cyclometallation is a significant deactivation process for complexes that often limits their ability for C-H bond activation. The organic substituents also define a hydrophobic pocket around the reactive coordination site. Reflecting their high reactivity, pincer catalysts are often poisoned by dinitrogen. Stoichiometric and catalytic applications of pincer complexes have been studied at an accelerating pace since the mid 1970's, especially for C-H activation, processes that promise to functionalize simple hydrocarbons. Most pincer ligands contain phosphines.Jensen, C. M., "Iridium PCP pincer complexes: highly active and robust catalysts for novel homogeneous aliphatic dehydrogenations", Chemical Communications, 1999, 2443–2449. doi|10.1039/a903573g.] Reactions of metal-pincer complexes are localized at three site 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.

cope of pincer ligands

Although most pincer ligands feature PCP donor sets, variations have been developed where the phosphines are replaced by thioethers and tertiary amines. More recent pincer ligands (see figure) feature nitrogenous donors, such as pyridines. [Gunanathan, C.; Ben-David, Y. and Milstein, D., "Direct Synthesis of Amides from Alcohols and Amines with Liberation of H₂", Science, 2007, 317, 790-792.doi|10.1126/science.1145295.]

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).

References

Appendix: illustrative publications

Pincer complexes have been shown to participate in a variety of transformations, such as the activation of CO₂¹, N₂², halogen-carbon bond formation,³ polymerization of alkenes^4,5 and alkynes⁶, alkane dehydrogenation^7,8, and transfer hydrogen catalysis.⁹ They have also recently been studied for their use as molecular sensors^10,11 and switches.¹²

# Lee, D. W.; Jensen, C. M.; Morales-Morales, D. "Organometallics" 2003, "22," 4744-4749.
# Vigalok, A.; Ben-David, Y.; Milstein, D. "Organometallics" 1996, "15," 1839-1844.
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#Yao, J.; Wong, W.T.; Jia, G. "J. Organomet. Chem." 2000, "598," 228-234.
# Liu, F.;Pak, E.B.; Sigh, B.; Jensen, C. M.; Goldman, A. S. "J. Am. Chem. Soc."1999, "121," 4086-4087.
#Jensen, C. "Chem. Commun."1999, 2443-2449.
#Dani, P.; Karlen, T.; Gossage, R. A.; Gladiali, S.; van Koten, G. "Angew. Chem., Int. Ed." 2000, "39," 743-745.
#Albrecht, M.; van Koten, G. "Angew. Chem., Int. Ed." 2001, "40," 3750-3781.
#Albrecht, M./; Gossage, R.A.; Lutz, M.; Spek, A. L.; van Koten, G. "Chem.-Eur. J." 2000, "6," 1431-1445.
#Steenwinkel, P.; Grove, D. M.; Veldman, M.; Spek, A. L.; van Koten, G. "Organometallics" 1998, "17," 5647-5655.