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

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.