- Pyrazole-Based PCN Pincer Complexes of Palladium(II): Mono- and Dinuclear Hydroxide Complexes and Ligand Rollover C-H Activation
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Palladium complexes of the novel unsymmetrical phosphine pyrazole-containing pincer ligands PCNH (PCNH = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole) and PCNMe (PCNMe = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-5-methyl-1H-pyrazole) have been prepared and characterized through single-crystal X-ray diffraction and multinuclear 1H, 13C{1H}, and 31P{1H} NMR spectroscopy. In preparations of the monomeric hydroxide species (PCNH)Pd(OH), an unexpected N detachment followed by C-H activation on the heterocycle 5-position took place resulting in conversion of the monoanionic {P,C-,N} framework into a dianionic {P,C-,C-} ligand set. The dinuclear hydroxide-bridged species (PCNH)Pd(μ-OH)Pd(PCC) was the final product obtained under ambient conditions. The "rollover" activation was followed via 31P{1H} NMR spectroscopy, and dinuclear cationic μ-OH and monomeric PdII hydroxide intermediates were identified. DFT computational analysis of the process (M06//6-31G, THF) showed that the energy barriers for the pyrazolyl rollover and for C-H activation through a σ-bond metathesis reaction are low enough to be overcome under ambient-temperature conditions, in line with the experimental findings. In contrast to the PCNH system, no "rollover" reactivity was observed in the PCNMe system, and the terminal hydroxide complex (PCNMe)Pd(OH) could be readily isolated and fully characterized. (Chemical Equation Presented).
- Bailey, Wilson D.,Luconi, Lapo,Rossin, Andrea,Yakhvarov, Dmitry,Flowers, Sarah E.,Kaminsky, Werner,Kemp, Richard A.,Giambastiani, Giuliano,Goldberg, Karen I.
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- Iron complex-catalyzed N-arylation of pyrazoles under aqueous medium
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Commercially available FeCl3·6H2O with conformationally rigid diamine ligand is a highly effective catalyst for N-arylation of pyrazoles using aryl and heteroaryl iodides. It is notable to show that this complex is tolerable under aqueous medium and particularly the whole reaction utilizes water as the sole solvent without any additional organic co-solvents and surfactants. Attempted study using other nitrogen nucleophiles is described. This newly developed system provides an alternative protocol to Cu- and Pd-catalyzed N-arylation reactions.
- Lee, Hang Wai,Chan, Albert S.C.,Kwong, Fuk Yee
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experimental part
p. 5868 - 5871
(2010/01/11)
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