Synthesis and characterization of MoOI2(PMe3)3 and use of MoOX2(PMe3)3 (X = Cl, I) in controlled radical polymerization

Complex MoOCl2(PMe3)(3) smoothly reacts with NaI in acetone to produce MoOI2(PMe3)(3) in good yields. The geometry of the compound is mer-cis octahedral, that is, identical to that of the dichloride precursor, as shown by NMR spectroscopy and by an X-ray crystallographic study. Electrochemical investigations of MoOX2(PMe3)(3) show irreversible oxidation waves at E-p,E-a = +0.18 and +0.39 V for X = Cl and I, respectively. A study of the halide exchange between MoOCl2(PMe3)(3) and NaI, or between MoOI2(PMe3)(3) and Bu4NCl, shows two equilibrated isomers for the mixed halide intermediate MoOICl(PMe3)(3). The diiodide complex rapidly exchanges the iodo ligands with chloride upon dissolution in chloroform at room temperature, and with bromide from (1-bromoethyl)benzene (BEB) under more forcing conditions. The equilibrium favors the softer halide (1) on C and the harder one (Cl or Br) on Mo-IV Both oxido compounds catalyze the atom transfer radical polymerization (ATRP) of styrene in combination with the BEB initiator, yielding polymers with quite narrow molecular weight distributions (down to 1.11). The apparent polymerization rate constant is approximately doubled in the presence of 1 equiv. of the Al(OiPr)(3) cocatalyst. On the other hand, the system is not capable of efficiently controlling the radical chain growth for methyl acrylate polymerization.