The Radical Trap in Atom Transfer Radical Polymerization Need Not Be Thermodynamically Stable. A Study of the MoX3(PMe3)3 Catalysts

International audienceThe molybdenum(III) coordination complexes MoX3(PMe3)3 (X = Cl, Br, and I) are capable of controlling styrene polymerization under typical atom transfer radical polymerization (ATRP) conditions, in conjunction with 2-bromoethylbenzene (BEB) as an initiator. The process is accelerated by the presence of Al(OPri)3 as a cocatalyst. Electrochemical and synthetic studies aimed at identifying the nature of the spin trap have been carried out. The cyclic voltammogram of MoX3(PMe3)3 (X = Cl, Br, I) shows partial reversibility (increasing in the order Cl < Br < I) for the one-electron oxidation wave. Addition of X- changes the voltammogram, indicating the formation of MoX4(PMe3)3 for X = Cl and Br. On the other hand, I- is more easily oxidized than the MoI3(PMe3)3 complex; thus, the putative MoI4(PMe3)3 complex is redox unstable. Electrochemical studies of MoI3(PMe3)3 in the presence of X- (X = Cl or Br) reveal the occurrence of facile halide-exchange processes, leading to the conclusion that the MoI3X(PMe3)3 products are also redox unstable. The oxidation of MoX3(PMe3)3 with 1/2Br2 yields MoX3Br(PMe3)3 (X = Cl, Br), whose molecular nature is confirmed by single-crystal X-ray analyses. On the other hand, the oxidation of MoI3(PMe3)3 by I2 slowly yields a tetraiodomolybdate(III) salt of iodotrimethylphosphonium, [Me3PI][MoI4(PMe3)3], as confirmed by an X-ray study. This product has no controlling ability in radical polymerization. The redox instability of MoI3X(PMe3)3 can be reconciled with its involvement as a radical trapping species in the MoI3(PMe3)3-catalyzed ATRP, given the second-order nature of its decomposition rate