Solvent dependent anion dissociation limits copper(i) catalysed atom transfer reactions

Abstract

Atom transfer radical addition (ATRA) and polymerisation (ATRP) reactions are commonly catalyzed by copper(I) complexes which react, reversibly, with a dormant alkyl halide initiator (RX) releasing a reactive organic radical R˙. The copper catalyst bears a multidentate N-donor ligand (L) and the active catalyst is simply Cu^IL. The role of the catalyst in these reactions is to abstract a halogen atom from RX forming the corresponding higher oxidation state species Cu^IILX. However, in order to perform its catalytic function (in multiple turnovers) the halido ligand must be released from the copper ion en route to regenerating the active catalyst Cu^IL. In this work we investigate the kinetics of the Cu^ILX/Cu^IL equilibrium where L is the tridentate N,N,N′,N′′,N′′-pentamethyl-diethylenetriamine (PMDETA). Using electrochemical analysis we find that the rate of formation of the active catalyst Cu^IL is strongly dependent on solvent. We demonstrate that both the kinetics and thermodynamics of this simple ligand exchange reaction are critical in the overall reaction pathway.