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The dehydrogenation of IrH 2 {C 6 H 3 -2,6-(CH 2 PBu t 2 ) 2 } (1) by tert-butylethylene followed by reaction with an excess of water leads to the isolation of IrH(OH){C 6 H 3 -2,6-(CH 2 PBu t 2 ) 2 } (2) in nearly quantitative yield. The hydrido hydroxo complex has been characterized by multinuclear NMR spectroscopy as well as a single-crystal X-ray structure determination. An isotopic labeling study with D 2 O indicates that 2 arises from the oxidative addition of water to the intermediate 14-electron complex Ir{C 6 H 3 -2,6-(CH 2 PBu t 2 ) 2 }. The title complex is an efficient catalyst for the transfer dehydrogenation of cyclooctane to cyclooctene but shows no catalytic activity for the hydroxylation of the alkane by water. The conversion of 1 to 2 can be reversed by placing a solution of 2 under 1 atm of H 2 at 25 °C.

Pincer complexes are useful tools for organic synthesis. Their high stability and easy functionalization have allowed the development of novel catalytic systems that have had a tremendous impact in different areas of chemistry. Thus, catalytic reactions are nowadays a fundamental part of several synthetic routes, as they allow “greener” procedures with high atom efficiency. In this context, pincer complexes have contributed to the establishment of novel and efficient catalytic reactions. Thus, herein we summarize the most recent relevant advances involving pincer complexes as catalysts.

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Pincer ligands have become ubiquitous in organometallic chemistry and homogeneous catalysis. Recently, new varieties of pincer ligands with non-symmetrical backbones and/or ligating groups have been reported and their application in transition metal complexes has been exploited in a variety of catalytic transformations. This non-symmetric approach vastly increases the structural and electronic diversity of this class of ligand. This approach has proven beneficial in a variety of ways, such as the use of a single weakly coordinating moiety, which can dissociate and thereby create a vacant coordination site to increase the catalyst activity. Additionally, this provides further access to chiral ligands and complexes for asymmetric induction. This perspective highlights recent, important examples of non-symmetric pincer ligands, which feature aryl or pyridine backbones, and the synthesis and use of subsequent complexes in catalytic transformations, and discusses the future potential of this type of ligand system.

Reaction of the bridged bisbenzimidazolium salts 2,6-bis(N 1-alkyl-N 3 -methylenebenzimidazolium)pyridine dibromide (1, alkyl = methyl; 2, alkyl = ethyl; 3, alkyl = n-propyl; 4, alkyl = n-butyl) with palladium acetate yields the palladium pincer complexes of type [Pd(L)Br]Br, 5−8 (L = 2,6-bis(N 1-alkyl-N 3-methylenebenzimidazolin-2-ylidene)pyridine). Compounds 1, 2·0.5MeOH, 4·CH2Cl2, and 6·MeOH (L = 2,6-bis(N 1-ethyl-N 3-methylenebenzimidazolin-2-ylidene)pyridine) were characterized by X-ray diffraction. The molecular structure of 6 shows a distorted square-planar coordination geometry for the palladium atom. In situ generated pincer complexes 5−8 have been tested as catalysts in Heck-type coupling reactions of different aryl halides with styrene.