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Pincer ligand
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 Chemistry of Pincer Compounds; Morales-Morales, D.; Jensen, C., Eds.; Elsevier Science: Amsterdam, 2007. ISBN 0444531386〕 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.〔Jensen, C. M., "Iridium PCP pincer complexes: highly active and robust catalysts for novel homogeneous aliphatic dehydrogenations", Chemical Communications, 1999, 2443–2449. .〕 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.
==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.〔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..〕
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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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