N-Heterocyclic and Mesoionic Carbene Complexes of Rhodium and Palladium: Coordination Chemistry and Catalysis

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Keske, Eric
Organometallic , Inorganic , Organic , Catalysis
Dimeric rhodium N-heterocyclic carbene (NHC) complexes [Rh(NHC)(C2H4)Cl]2 react with a variety of other neutral donors to form heteroleptic complexes [(L)Rh(NHC)(C2H4)Cl] (L = phosphine, pyridine) or [(L)Rh(NHC)Cl] (L = 2,2’-bipyridine (bipy), 1,10-phenanthroline (phen)). The reactivity of the resulting complexes towards O2 was investigated. In particular, [(bipy)Rh(NHC)Cl] and [(phen)Rh(NHC)Cl] resulted in RhIII peroxo complexes. In contrast, [Rh(NHC)2(O2)Cl] display particularly short O-O bond lengths and are described as singlet oxygen species. Interestingly, the mode in which O2 binds is associated with the coordination number about the transition metal complex, which is related to its reducing power. [Rh(IPr)(C2H4)Cl]2 reacts with phenyl pyridine derivatives at room temperature resulting in formal C-H activations. Upon the treatment of phenyl pyridine with pinacol borane (HBPin) in the presence of a weak base and a catalytic amount of [Rh(IPr)(C2H4)Cl]2, C-H borylated products were obtained in high yield and selectivity. The borylated products can then be used as substrates in the palladium catalyzed Suzuki-Miyaura cross coupling with aryl halides. 1,2,3-Triazole mesoionic carbene (tMIC) ligands were generated upon treatment of corresponding triazolium salts with strong bases, and can be trapped in the presence of a transition metal. The synthesis of Ag-tMIC complexes proceeds by a facile and mild route upon treatment of the triazolium salt with Ag2O. The resulting Ag-tMIC complexes undergo facile transmetallation to both Pd and Rh under very mild conditions resulting in air and moisture stable metal complexes. Triazolium salts can further be metallated to Pd in the presence of weak bases, and the resulting Pd-tMIC complexes are active catalysts in the Mizoroki-Heck reaction with aryl iodides. Benzylic trifluoromethyl sulfones are competent electrophilic substrates in palladium catalyzed cross coupling reactions, resulting in the formation of triarylmethanes in high yields under mild conditions. These substrates are conveniently synthesized and are highly reactive starting materials with phenyl boronic acids in the presence of a Pd-NHC catalyst. The structure of the Pd-NHC precatalyst is crucial, as only [(NHC)Pd(allyl)Cl] type complexes appear to be effective. These complexes can be conveniently synthesized upon the treatment of the corresponding imidazolium salt with a strong base and [Pd(allyl)Cl]2.
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