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

Abstract

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{κ³(CNN)(OC(O)R′)}] (CNN = N-picolyl-N′,N′′-diarylguanidinate(1−); aryl = 2-RC₆H₄; R = Me and CF₃; R′ = CF₃, Me and ^tBu; 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{κ³(CNN)(L)}][OC(O)R′] (B) and neutral [Pd{κ³(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. O₂. An elusive intermediate, [Pd{κ³(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. O₂ for the first time.