Synthesis and Catalytic Activity of N-Heterocyclic Carbene Dinuclear Metal Complexes

Abstract
In this thesis, we focused on the preparation of binuclear transition metal carbene complexes and the study of their catalytic activities. Modification of 2,7-dichloro-1,8- naphthyridine with N-mesityl imidazole yielded 7, which reacted with Ag2O to yield the corresponding silver complex 9. Treatment of 9 with [Pd(PhCN)2Cl2] in CH3CN provided the palladium complex 10. X-ray crystal structural analysis show that two NHC ligands are seated in trans positions around palladium center. In addition, treatment of 9 with [ClAu(Me2S)] yielded the corresponding Au(I) complexes 11. Oxidative addition of the C–H bond at 2-position of pyridinyl-imidazolium salt 8 with [Pd2(dba)3] generated the desired mono-carbene complex 12.
Complex 11 appears to be fairly stable toward sulfur, trifluoroacetic acid and triphenylphosphine. Complex 11 can act as an acid-catalyst on the hydrolysis of ketal. In particular, 5-(Hydroxymethyl)-2,2,5-trimethyl-1,3-dioxane was completely hydrolyzed within 8 hours under mild conditions. Furthermore, complex 11 can be easily recycled with the similar catalytic activity.
The use of complex 10 as a catalyst in Kumada coupling reactions were investigated. Reaction of 4-bromoanisole and cyclohexylmagnesium bromide catalyzed by 12 only provided cyclohexene and anisole. On the other hand, 85% cross coupling product was obtained with the use of 10 as the catalyst for the same reaction. The possible reason may due to the rigid arrangement of NHC ligand in structure 10. The naphthyridine-based system readily hinders the isomerization of NHC moieties at cis position, thus reduces the possibility of B-H elimination. A series of coupling reactions between a wide range of aromatic or aliphatic Gringnard reagents with various alkyl bromides were studied.