ATRP catalysts of tetradentate guanidine ligands – do guanidine donors induce a faster atom transfer?

Abstract

Tripodal tetradentate N donor ligands stabilise the most active ATRP catalyst systems. Here, we set out to synthesise the new guanidine ligand TMG-4NMe₂uns-penp, inspired by p-substituted tris(2-pyridylmethyl)amine (TPMA) ligands. The impact of changing pyridine against guanidine donors was examined through solid state and solution experiments and density functional theory (DFT) calculations. In the solid state, the molecular structures of copper complexes based on the ligands TMG-4NMe₂uns-penp, TMG-uns-penp and TMG₃tren were discussed concerning the influence of a NMe₂ substituent at the pyridines and the guanidine donors. In solution, the TMG-4NMe₂uns-penp system was investigated by several methods, including UV/Vis, EPR and NMR spectroscopy indicating similar properties to that of the highly active TPMA^NMe2 system. The redox potentials were determined and related to the catalytic activity. Besides the expected trends between these and the ligand structures, there is evidence that guanidine donors in tripodal ligand systems lead to a better deactivation and possibly a faster exchange within the ATRP equilibrium than TPMA systems. Supported by DFT calculations, it derives from an easier cleavable Cu–Br bond of the copper(II) deactivator species. The high activity was stated by a controlled initiator for continuous activator regeneration (ICAR) ATRP of styrene.