Pincer-Type Ligands

Pincer ligands were originally typified by (8.152a) where E was a group containing N, P, O, S, Se. 

Most known ligands contain P and are symmetrical , but data are available on unsymmetrical varieties as well [28] (see also Section 8.20). 

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In 2008, Gade and coworkers reported that the asymmetric hydrosilylation of ketones was catalyzed by the Fe complex with a highly modular class of pincer-type ligand (Scheme 22) [71]. This Fe catalyst system showed both moderate to good... 

Scheme 22 Asymmetric hydrosilylation catalyzed by the Fe complex with the pincer-type ligand...

Interestingly, the activity of the corresponding cobalt catalyst possessing a pincer-type ligand is higher than that of the iron complex. In addition, the cobalt complex also acts as a catalyst in asymmetric mtermolecular cyclopropanations. 

Pincer ligand scaffolds have been shown to possess an extraordinarily useful potential for directing the properhes and reactivity patterns of coordinated metal (M) centers. This is illustrated not only by the sheer number of publications on this topic, but also by the various applications that have been disclosed by using pincer-type ligands, such as highly efficient sensors, switches and catalysts [1]. 

The results of some initial experiments indicated that the catalytic activity of 1 is strongly temperature-dependent. For instance, at 100 °C, the rates are relatively low but there is an appreciable turnover [turnover frequency (TOE) = 20.5 h" j. Increasing the reaction temperature to 200 °C increases the TOE to 720 h hence, the thermal robustness of the iridium catalyst is pivotal for optimal catalyst performance. The tridentate pincer-type ligands provide a particularly stable platform for metal confinement through covalent Ir—C bonding combined with the terden-tate chelating coordination. 

Cyclometallation, which provides multidentate ligands containing carbon donors, has been known as a useful synthetic method for preparing organometallic complexes. A noticeable extension in such a category, which substantially facilitates the recent development of organoplatinum chemistry, is the pincer-type ligands, which allow tridentate meridional chelation with various donor combinations to the metal. ... 

Extensive studies using pincer-type ligands (130) in metal-catalyzed C-H and C-C bond activations have been performed by Milstein s group and by van Koten and coworkers." "" ... 

Reasonable success in these areas has been achieved primarily via pincer -type ligands using group viii metal-j 40,40a,40f Kaska reported that the Ir complex, (PCP )Ir, catalytically dehydrogenates cycloalkanes at... 

As illustrated by other chapters of this book, a diverse array of pincer and pincer-type ligands are known in the literature and can be incorporated to make new nickel complexes. Their catalytic activity in various cross-coupling reactions is likely to be the focus of future research. Another research direction that deserves more attention is carbon-heteroatom bond-forming reactions very few nickel pincer complexes have been studied for these catalytic applications. 

Magnesium does not form phosphine complexes as readily as most other metals but pincer-type ligands (8.116) are exceptions [27]. (See (8.152) below.)... 

Examples of P and N coordination are provided by many pincer-type ligands... 

Over the past decade, many efficient catalysts have been derived from cyclometallated pincer-type ligands (8.144) [55]. 

It was shown that palladacydes 1 [3c, 24] prepared from palladium] I) acetate and tris(o-tolyl)- or trimesitylphosphine are excellent catalysts for the Heck coupHng of triflates and halides including certain aryl chlorides. In some of these cases, a possible involvement of oxidation states +II and +IV in the catalytic cycle has been considered [25]. Similarly, other palladacydes such as 3 [26e,h] or 6 [27] have been used in the Heck reactions (Figure 8.1) [24, 26, 28]. It has been proposed that, at least for NC palladacydes, the reaction proceeds through the classical phosphine-free Pd(0)/Pd(II) catalytic cycle and that the active catalysts are actually slowly formed palladium clusters [29]. Besides classical palladacydes, complexes with pincer-type ligands such as 2 [30] have become very popular in palladium catalysis [31]. 

A novel 0,C,S-coordinating pincer-type ligand was made available that was used in the synthesis of a triorganotin cation stabilized by sulfur and oxygen donor atoms (Scheme 56). ... 

A chloro complex 599 with an optically active NGN pincer-type ligand, having two chiral imidazole groups ((, )-Phebox), has been prepared via the reaction of the stannyl derivative of the ligand with Zeise s complex (Equation (131))V ... 

A 4-methylanilidoplatinum(ii) complex with a PGP pincer-type ligand 615 reacts with acrylonitrile to form an alkylplatinum complex 616 via insertion of the G=G bond into the Pt-N bond (Scheme 86). Protonation of the complex releases A-(2-cyanoethyl)-4-methylaniline. The Pt complex also catalyzes addition of the aniline to the G=G bond of acrylonitrile. 

A hydride bridged Pt(ll) -Pt(ll) bond was reported in compound 197, with a Pt-Pt bond length of 2.8324(1) A (Entry 8, Table 10.13). Complex 197 was a product of hydrogenolysis of a Pt(II) chloride with pincer-type ligand (Scheme 10.82) [139]. 

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