Date of Award
Nolan, Steven P.; Fontaine, Laurent
N-heterocyclic carbenes (NHCs) have emerged as appropriate replacements for phosphines to synthesized highly active metal complexes in homogeneous catalysis. The advantages of NHCs over phosphines include ease of handling, minimal toxicity and powerful electron donating properties. They bind transition metals with no need of back-donation and yield complexes with enhanced stability. The ability of NHCs to bind all metals from the periodic table, in low and high oxidation states, creates an unprecedented opportunity for the design of new complexes. This dissertation is dedicated to the synthesis of well-defined organosilver and organogold complexes bearing NHC ligands. In addition, two short chapters describe briefly the synthesis of NHC acetylacetonate palladium(II) complexes, and the use of NHC ruthenium(II) indenylidene complexes in ring opening metathesis polymerization reactions. New silver(I) and gold(I) halide mono-NHC complexes are synthesized, structurally characterized, and compared to other existing silver(I) and gold(I) complexes. The silver cation trends to form thermodynamically favored bis-NHC complexes and the success for the synthesis of kinetically favored silver(I) mono-NHC complexes is strongly dependant of the carbene, the solvent, and the silver salts used. In contrast, the synthesis of gold(I) mono-NHC complexes appears easier specially by transmetalation starting from silver(I) NHC complexes. The reactivity of both metals NHC complexes is directly related to the strength of the metal-carbene bond. The gold(I) cation firmly bound to the carbene, undergoes oxidative addition with halogens to afford gold(III) NHC complexes, or dehalogenation in presence of inorganic silver salts to afford cationic gold(I) mono-NHC complexes in coordinating solvent. The coordinating solvent can be replaced by a neutral or anionic group to yield gold(I) NHC complexes with unusual ligands such as olefins, pyridines, sugars. In contrast, the silver(I) cation is weakly bound to the carbene, and silver(I) NHCs decompose in presence of halogens or inorganic silver salts. In conclusion the synthesis of the NHC complexes emphasizes deeply the similarities and the differences between silver and gold chemistries. While NHCs ideally stabilize gold(I) and(III) complexes, phosphines appears to be a more promising alternative than NHCs for the chemistry of silver.
De Fremont, Pierre, "Synthesis of Well-Defined N-Heterocyclic Carbene (NHC) Complexes of Late Transition Metals" (2008). University of New Orleans Theses and Dissertations. 829.