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 PCN^H (PCN^H = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole) and PCN^Me (PCN^Me = 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 (PCN^H)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 (PCN^H)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 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 σ-bond metathesis reaction are low enough to be overcome under ambient-temperature conditions, in line with the experimental findings. In contrast to the PCN^H system, no "rollover" reactivity was observed in the PCN^Me system, and the terminal hydroxide complex (PCN^Me)Pd(OH) could be readily isolated and fully characterized. (Chemical Equation Presented).