The radical trap in atom transfer radical polymerization need not be thermodynamically stable.: A study of the MoX3(PMe3)3 catalysts

The molybdenum(lll) coordination complexes MoX3(PMe3)(3) (X = CI, 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 AI(OPri)(3) as a cocatalyst. Electrochemical and synthetic studies aimed at identifying the nature of the spin trap have been carried out. The cyclic voltammograrn Of MoX3(MAe(3))(3) (X = Cl, Br, I) shows partial reversibility (increasing in the order CI < 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 Mol(3)(PMe3)(3) complex; thus, the putative M-4(PMe3)(3) complex is redox unstable. Electrochemical studies Of Mol(3)(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 Mol(3)X(PMe3)(3) products are also redox unstable. The oxidation Of MoX3(PMe3)(3) with (1)/Br-2(2) 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 Mol(3)(PMe3)(3) by 12 slowly yields a tetraiodomolybdate(III) salt of iodotrimethylphosphonium, [Me(3)Pl][Mol(4)(PMe3)(3)], as confirmed by an X-ray study. This product has no controlling ability in radical polymerization. The redox instability of Mol3X(PMe3)3 can be reconciled with its involvement as a radical trapping species in the Mol(3)(PMe3)(3)-catalyzed ATRP, given the second-order nature of its decomposition rate.