**2.7 Silica-supported N-heterocyclic carbene-Pd**

*Solvents, Ionic Liquids and Solvent Effects*

30,000 h<sup>−</sup><sup>1</sup>

**Figure 10.**

average yield in reuse studies up to five times though catalyst activity was gradually lost. High efficiency of the catalyst was exhibited by a TON of 2 million and TOF of

bonded in a hexagonal lattice. They have extremely large surface area, fast charge mobility, remarkably high mechanical strength, Young's modulus and chemical stability; they are inexpensive and thus are excellent candidate to disperse or immobilize catalytically active species. Silylation modification technique on graphene oxide provides graphene nanocomposites with catalytic activities. To prepare the catalyst, first, a sub-stoichiometric amount of Pd(OAc)2 was reacted with an excess of 1-methyl-3-(3-(trimethoxysilyl)propyl)-1H-imidazol-3-ium chloride to afford the (NHC) N-heterocyclic carbene Pd IL. This NHC-Pd was grafted on the surface of the graphene oxide (GO) in refluxing ethanol (**Figure 11**). The catalyst was characterized by FT-IR, SEM, TEM, Raman, XPS, TGA, and EDS measurements. The catalyst provided high yields with aryl iodide and bromides but required the addition of terabutylammonium bromide (TBABr) for chloride substrates. The

From the structural point of view, graphene is a single layer of sp2

*Suzuki-Miyaura reaction catalyzed by immobilized Pd catalyst on reversed phase alumina [24].*

*2.6.3 Graphene oxide-based heterogeneous ionic liquid*

in the reaction of 4-bromo acetophenone and phenyl boronic acid [24].

C atoms

**136**

**Figure 11.**

*HIL on graphene oxide [25].*

N-heterocyclic carbenes (NHCs) have been found to be excellent ligands for several aryl-X activation reactions due to the high dissociation energies of NHCmetal complexes and making them good ligands for heterogeneous systems. Polymers, silica, or NPs have been used as supports. Immobilization of Pd complexed with NHCs is a much wanted technology for the recycling of the catalyst. The immobilization was carried out by the reaction of appropriately functionalized imidazolium chlorides, triethoxysilylpropylimidazolium chloride with silica gel, or variations. The catalyst Pd(OAc)2 was then anchored on this silica-supported ionic liquid. Other catalysts were supported on polymers like polystyrene, surfacegrafted polystyrene resins (**Figure 12**). These could be recycled for over 10 successive runs with very high TONs exceeding 50,000 and TOFs of 5200 (**Figure 13**). Heterogeneous silica NHC-Au (I) gave good results for the Suzuki coupling of aryl iodides [26]. Pd-NHCs were immobilized on the surface of polymer or silica-coated

**Figure 12.** *Silica-supported NHC Pd complex [27].*

**Figure 13.** *Silica-supported N-heterocyclic carbene-Pd-Suzuki coupling [27].*

MNPs to generate MNP-Pd-NHC complexes. Catalysts could be easily separated by external magnets [27].

The Suzuki coupling of aryl iodides was catalyzed by this heterogenized ionic liquid transition metal catalysts (HIL-TMCs), and reactions were complete in 0.5 h. Bromides were equally active while substituted aryl chlorides gave lower yields and required longer reaction times mixed with some homocoupling of the aryl boronic acids as byproduct. The catalyst could be reused for six times without significant loss of activity. Atomic absorption spectroscopy (AAS) showed no leaching of Pd [28].
