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Pyrazole-Based PCN Pincer Complexes of Palladium(II): Mono- and Dinuclear Hydroxide Complexes and Ligand Rollover C–H Activation

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posted on 24.08.2015, 00:00 by Wilson D. Bailey, Lapo Luconi, Andrea Rossin, Dmitry Yakhvarov, Sarah E. Flowers, Werner Kaminsky, Richard A. Kemp, Giuliano Giambastiani, Karen I. Goldberg
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.