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(I)L. The role of the catalyst in these reactions is to abstract a halogen atom from RX forming the corresponding higher oxidation state species Cu(II)LX. 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(I)L. In this work we investigate the kinetics of the Cu(I)LX/Cu(I)L 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(I)L 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.