Guanidine-centred reactivity of CNN Pd(ii) pincer complexes including carboxylate-assisted N-H activation and -CH₂- -C(O)- oxygenation

Transition metal complexes of N-substituted guanidinates are widely studied due to their intriguing structures, reactivities, and bonding as homogeneous catalysts and precursors for chemical vapor deposition. In this manuscript, we have isolated a family of pincer complexes, [Pd{kappa(3)(CNN)(OC(O)R ')}] (CNN = N-picolyl-N ',N ''-diarylguanidinate(1-); aryl = 2-RC6H4; R = Me and CF3; R ' = CF3, Me and Bu-t; A) and studied their reactions with XylNC (Xyl = 2,6-Me2C6H3) and PPh3 (Ls ') with the objective of understanding the substituent effects of the pincer complexes upon the nature of the products. In this endeavour, we were able to isolate the respective cationic [Pd{kappa(3)(CNN)(L)}][OC(O)R '] (B) and neutral [Pd{kappa(3)(CNN)(CNXyl)}] (CNN = N-picolinoyl-N ',N ''-diarylguanidinate(2-); D) complexes with the latter complexes formed from the former through carboxylate-assisted N-H activation followed by -CH2- -> -C(O)- oxygenation of the 2-picolyl unit in the guanidinate(1-) ligand, which is induced by atm. O-2. An elusive intermediate, [Pd{kappa(3)(CNN)(CNXyl)}] (CNN = N-picolyl-N ',N ''-di(2-tolyl)guanidinate(2-); C) formed during B -> D transformation was successfully isolated. The new complexes were fully characterised and the molecular structures of nine complexes were determined by X-ray crystallography. Various intermediates formed during the oxygenation were mapped out using the model complexes through DFT calculations. The solution behaviour of A was studied by multinuclear NMR and their utility as catalysts in Suzuki-Miyaura coupling reactions and base-free cycloisomerization of 4-pentynoic acid was briefly explored. The carboxylate-assisted N-H activation and oxygenation of the Pd(II) bound guanidinate(1-/2-) ligands reported herein represent the redox active behaviour of the ligand towards atm. O-2 for the first time.