Catalysis Pincer ligands

Chapter 14 Organometallic Reactions and Catalysis FIGURE 14.9 Pincer Ligands. 

Pincer ligands, that is, tridentate Hgands that enforce meridional geometry upon complexation to transition metals, result in pincer complexes which possess a unique balance of stability versus reactivity [3]. Transition-metal complexes of bulky, electron-rich pincer ligands have found important appHcations in synthesis, bond activation, and catalysis [4, 5]. Among these, pincer complexes of Pr-PNP (2,6-bis-(di-iso-propylphosphinomethyl)pyridine), Bu-PNP (2,6-bis-(di-terPbutyl-phosphinomethyl)pyridine), and PNN ((2-(di-tert-butylphosphinomethyl)-6-diethyl-aminomethyl)pyridine), PNN-BPy (6-di-tert-butylphosphinomethyl-2,2 -bipyridine) ligands exhibit diverse reactivity [6-8]. These bulky, electron-rich pincer ligands can stabilize coordinatively unsaturated complexes and participate in unusual bond activation and catalytic processes. 

Reductive elimination reactions are responsible for the final C-C/C-X bond formation step in catalytic processes. In principle, C-bound or any other anionic pincer ligands could engage in reductive couphng reactions, bringing about the decomposition of the pincer complex. In order to enable catalysis by such complexes, it is mandatory to avoid reductive elimination reactions involving the pincer ligand itself Fortunately, this seems to be usually the case. For example, reductive elimination of ethane from the Pd(IV) pincer complex 18 (Scheme 2.3) is preferred... 

In contrast to unmodified pincer ligands, comparatively few examples of pincer ligand frameworks appended with polar functionaHty have been reported. The overarching goals associated with such modifications are diverse and span the fields of catalysis, supramolecular assembly, and molecular recognition. 

Another class of chelating phosphines has received considerable attention in recent times for potential application in homogeneous catalysis. These are called pincer ligands. As shown in structure 232, here two phosphorous and one carbon atom occupy three coordination sites of the metal atom. Pincer ligands are therefore tridentate ligands, where two phosphorous atoms form two dative bonds, but with the carbon atom there is a formal covalent bond. 

The wide apphcation of NHCs to catalysis places them along with phosphines and cyclopentadienyls in their utility for organometaUic catalysis. Being much more donor in nature than phosphines, the NHCs occupy a distinct region of the Tolman ligand map [32]. A number of pincer NHC derivatives are catalyticaUy active, but are not discussed here because they have been fully described elsewhere [33]. Some other important recent reviews discuss catalysis by NHC complexes in general [10, 34, 35]. 

Pyridylphosphinoalkane ligands are typical chelate-forming compounds, 386, extremely popular in homogenous catalysis when sometimes dearomatization of one, 387, or both arms in the pincer occurs. The chelate function may combine with the bridging mode, 388. In some specific ligands P,C,C-coordination 389 is possible. 

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