Functionalised and cyclometallated N-Heterocyclic Carbene complexes of Pd, Ni, Rh and Ir : synthesis and applications

Stylianides, Neoklis (2007) Functionalised and cyclometallated N-Heterocyclic Carbene complexes of Pd, Ni, Rh and Ir : synthesis and applications. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

New imidazolinium salts have been synthesised with the following substitution: (i) 2-(3-methylpyridyl), 2-pyridyl; (ii) alkoxyphenyl (o-OMe-phenyl, p-OMe-phenyl, o,p-(OMe)₂-phenyl, o,p,o-(OMe)₃-phenyl, o-OⁱPr-phenyl, p-OⁱPr-phenyl); (iii) substituted aryl (o-Et-phenyl, o-ⁱPr-phenyl); (iv) fluoraryl (m, m-(CF₃)₂-phenyl, o-F-phenyl, o,p-F₂-phenyl). In addition, novel pyridyl functionalised imidazolium salts have been synthesised. The targeted imidazol(in)ium salts were hoped to give rise to metal complexes with hemilabile coordination sites (pyridine functionalised species) or to cyclometallated complexes which on heating may generate active catalysts. Deprotonation of two of the imidazolium salts led to the isolation of the corresponding free carbenes. Five novel pyridyl functionalised imidazoline-2-ylidene complexes of palladium, three novel imidazolin-2-ylidene palladacycle complexes, four novel aryl substituted imidazoline-2-ylidene palladium complexes and two novel alkoxyphenyl imidazolin-2-ylidene palladacycle complexes have been synthesised by reacting the in situ deprotonated salt with various palladium precursors or by transmetallation reactions from silver carbine complexes. The complexes were characterised by analytical, spectroscopic and diffraction techniques. Interestingly, the first palladium ‘C-C-C pincer’ type complex (3.14) has been isolated after facile double cyclometallation succeeding the ligand complexation. Selected palladium imidazoline-2-ylidene complexes have been tested as catalysts for Heck coupling reactions. [1,3-Bis(4-methoxyphenyl)imidazolin-2-ylidene] {2-[3-(4-methoxyphenyl) imidazolin-2-ylidene] -5- methoxy phenyl-κ²C,C’} methyl palladium(II) (3.16) and [1,3-bis(2,4-dimethoxyphenyl)imidazolin-2-ylidene] { 2-[3-(2,4-dimethoxy phenyl) imidazolin-2-ylidene] -3,5- dimethoxy phenyl- κ²C,C’} methyl palladium(II) (3.15) have been found to be excellent catalysts for the Heck couplings of aryl bromides, deactivated aryl bromides and activated aryl chlorides, while moderate results for the Heck coupling of chlorobenzene have been achieved. These complexes surpass the activity of standard palladium phosphine complexes and are amongst the most active palladium NHC complexes reported to date. Five novel picolyl-functionalised imidazol-2-ylidene complexes of Rh(I) and Ir(I) have been prepared and characterised by the reaction of [M(COD)C1]₂ (M = Rh, Ir) with 1-[2-(3-methyl)pyridyl]3-[2,6-diisopropyl)phenyl]imidazol-2-ylidene (2.1b). When M = Rh the nature of the products was found to be dependent on the reactant ratio. Furthermore, one novel Ir(III) picolyl functionalised imidazolin-2-ylidene complex featuring a 1-κ-4,5,6-η-C₈H₁₂ moiety (5,6) has been synthesised by reacting [Ir(COD)C1]₂ with, in situ formed, 1-[2-(3-picolyl)]-3- (2,6-diisopropylphenyl) imidazolin-2-ylidene.