Ionic Liquid Media for Catalysis by NPs

Ionic liquids (ILs) were introduced in catalysis by Yves Chauvin in the 1990s [33a] and have received considerable attention in this field [33b,cj. Yves Chauvin introduced the imidazolium salts that are the most frequently used ILs in catalysis. ILs are valuable media for catalysis with PdNPs because the substituted imidazolium cation is bulky, favoring the electrosteric stabilization of NPs, as do the t-Bu N salts in Fig. 1.1. The size of the cation (that can eventually be tuned by the choice of the N-alkyl substituents) also has an important influence on the stabilization, size and solubUity of the NPs, these factors playing a role in catalysis. ILs are also non-innocent, however, as they readily produce Pd-N-heterocyclic carbene complexes upon deprotonation of the imidazoHum salt at sufficiently high temperature. Thus, these carbene ligands can be bound to the NP surface or give mononuclear mono- or bis-carbene complexes subsequent to leaching of Pd atoms from the PdNP surface (vide infra) [33]. 

The role of the IL is crucial in both the PdNP formation and stereospecifity of C—C coupling that could not be obtained in previous studies of PdNP-catalyzed Heck reactions [33e-g]. Salts of N-butyronitrile pyridinium cation react with PdCl2 to give dinitrile complexes that turn black upon addition of phenyltributyktan-nane, and the PdNPs formed catalyze Stille and Suzuki C—C coupling reactions. 

It is believed that the nitrile groups coordinate to the PdNP surface, which results in PdNP stabihzation [33h]. 

The selectivity of the reactions in such IL media also depends on the solubility, and the solubility difference can be used for the extraction of the product [33k-m]. 

In conclusion, IL are favorable media for the electrostatic stabilization of preformed NPs at room temperature and subsequent catalysis, whereas they give for instance Pd-carbene complexes upon deprotonation of the imidazolium cation, yielding, at high temperature, PdNP catalysts whose mechanism of action is discussed in the ligand-free catalysis section. 

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