14.1.5: Application of Pincer Ligands

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Pincer ligands are chelating agents that bind tightly to three adjacent coplanar sites of a metal complex. Stoichiometric and catalytic applications of pincer complexes have been studied at an accelerating pace since the mid-1970s.

Pincers often include one anionic, two-electron donor flanked by two neutral, two-electron donor groups. It consists of a rigid, planar backbone, usually consisting of aryl frameworks. 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 ligand exchange and cyclometallation of the pincer. In contrast, cyclometallation is often a significant deactivation process in the case of other types of chelates, in particular limiting their ability to effect C-H bond activation. The pincer ligand also creates a hydrophobic pocket around the reactive coordination site.

There are various types of pincer ligands that are used in transition metal catalysis. Often, they have the same two-electron donor flanking the metal centre, but this is not a requirement. Early examples of pincer ligands were anionic with a carbanion as the central donor site and flanking phosphine donors; these compounds are referred to as PCP pincers. 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. By altering the properties of the pincer ligands, it is possible to significantly alter the chemistry at the metal centre. Changing the hardness/softness of the donor, using electron-withdrawing groups (EWGs) in the backbone, and the altering the steric constraints of the ligands are all methods used to tune the reactivity at the metal centre.

Figure \(\PageIndex{1}\): Examples of transition metal pincer complexes. (Scott0, Examples pincer complexes, CC BY-SA 3.0)

The Role of Pincer Complexes in Catalysis

Suzuki-Miyaura coupling

Pincer complexes have been shown to catalyse Suzuki-Miyaura coupling reactions, a versatile carbon-carbon bond forming reaction. Suzuki-Miyaura coupling (or Suzuki coupling) is a metal catalyzed reaction, typically with Pd, between an alkenyl (vinyl), aryl, or alkynyl organoborane (boronic acid or boronic ester, or special cases with aryl trifluoroborane) and halide or triflate under basic conditions. This reaction is used to create carbon-carbon bonds to produce conjugated systems of alkenes, styrenes, or biaryl compounds (Scheme 1).

Modified conditions have demonstrated reactivity with less reactive substrates such as alkyl boranes (BR₃) or aryl or alkenyl chlorides by amending the base and ligands employed.

Figure \(\PageIndex{3}\): Suzuki coupling proceeds via three basic steps: oxidative addition, transmetallation, and reductive elimination. (Suzuki-Miyaura Coupling)

Sonogashira Coupling

The Sonogashira reaction (also called the Sonogashira-Hagihara reaction) is the cross coupling of aryl or vinyl halides with terminal alkynes to generate conjugated enynes and arylalkynes (Scheme 1). The reaction typically proceeds in the presence of a palladium(0) catalyst, a copper(I) cocatalyst, and an imine base. Alternative procedures describe Sonogashira coupling reactions performed without the Cu(I) cocatalyst.

Dehydrogenation of alkanes

Alkanes undergo dehydrogenation at high temperatures. Typically this conversion is promoted heterogeneously because typically homogeneous catalysts do not survive the required temperatures (~200 °C) The corresponding conversion can be catalyzed homogeneously by pincer catalysts, which are sufficiently thermally robust. Proof of concept was established in 1996 by Jensen and co-workers. They reported that an iridium and rhodium pincer complex catalyze the dehydrogenation of cyclooctane with a turnover frequency of 12 min⁻¹ at 200 °C. They found that the dehydrogenation was performed at a rate two orders of magnitude greater than those previously reported. The iridium pincer complex was also found to exhibit higher activity than the rhodium complex. This rate difference may be due to the availability of the Ir(V) oxidation state which allows stronger Ir-C and Ir-H bonds.

The homogeneously catalyzed process can be coupled to other reactions such as alkene metathesis. Such tandem reactions have not been demonstrated with heterogeneous catalysts.

Figure \(\PageIndex{5}\): Iridium pincer complex catalyzing the dehydrogenation of cyclooctane to cyclooctene. (Paul Gray, Iridium Pincer Catalysis, CC BY-SA 3.0)

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