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]pheny1]-1H-pyrazole) and PCNMe (PCNMe = 1-[3-(di-tert-butylphosphino)methyl]pheny1]-5-methyl-1H-pyrazole) have been prepared and characterized through single-crystal X-ray diffraction and multinudear H-1, C-13{H-1}, and P-31{H-1} 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(mu-OH)Pd(PCC) was the final product obtained under ambient conditions. The rollover activation was followed via P-31{H-1} NMR spectroscopy, and dinuclear cationic mu-OH and monomeric Pd-II 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 sigma-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.