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

Abstract

The molybdenum(III) coordination complexes MoX(3)(PMe(3))(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(OPr(i))(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 MoX(3)(PMe(3))(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 MoX(4)(PMe(3))(3) for X = Cl and Br. On the other hand, I(-) is more easily oxidized than the MoI(3)(PMe(3))(3) complex; thus, the putative MoI(4)(PMe(3))(3) complex is redox unstable. Electrochemical studies of MoI(3)(PMe(3))(3) in the presence of X(-) (X = Cl or Br) reveal the occurrence of facile halide-exchange processes, leading to the conclusion that the MoI(3)X(PMe(3))(3) products are also redox unstable. The oxidation of MoX(3)(PMe(3))(3) with (1)/(2)Br(2) yields MoX(3)Br(PMe(3))(3) (X = Cl, Br), whose molecular nature is confirmed by single-crystal X-ray analyses. On the other hand, the oxidation of MoI(3)(PMe(3))(3) by I(2) slowly yields a tetraiodomolybdate(III) salt of iodotrimethylphosphonium, [Me(3)PI][MoI(4)(PMe(3))(3)], as confirmed by an X-ray study. This product has no controlling ability in radical polymerization. The redox instability of MoI(3)X(PMe(3))(3) can be reconciled with its involvement as a radical trapping species in the MoI(3)(PMe(3))(3)-catalyzed ATRP, given the second-order nature of its decomposition rate.

An Experimental and Computational Study on the Effect of Al(OiPr)3 on Atom‐Transfer Radical Polymerization and on the Catalyst—Dormant‐Chain Halogen Exchange

“…[5,14] Complexes [MoX 3 (PMe 3 ) 3 ] (X= Cl, Br, I) are also capable of controlling the ATRP of styrene. [6,15] For both families of ATRP catalysts, the addition of compound Al(OiPr) 3 to the catalytic system was found to speed up the polymerization process by a factor between 3 and 13, depending on the nature of the halides. [5,6] To probe the possible effect of the Al additive, the interactions of this compound with catalyst, monomer, and initiator were carried out.…”

“…Halide exchange for the 15-electron [MoX 3 (PMe 3 ) 3 ] system: Since the styrene polymerizations catalyzed by the [MoX 3 -(PMe 3 ) 3 ] complexes were carried out with the (1-bromoethyl)benzene (BEB) initiator, [6] halide-exchange tests on this system were carried out with the same initiator. The exchange process was monitored by using both the trichloride and the triiodide complex, in the absence and in the presence of one equivalent of Al(OiPr) 3 per equivalent of Mo.…”

“…[3,4] Of particular interest for the present study, the addition of certain Lewis acids, most notably Al(OiPr) 3 , in conjunction with a number of catalysts has been shown to result in faster polymerization. This effect seems rather general, since it was shown for many different complexes based on a variety of different transition metals from Groups 6 to 11, including [Mo(Cp)Cl 2 (iPrN=CHÀCH=NiPr)], [5] [MoX 3 (PMe 3 ) 3 ], [6] [ReIO 2 (PPh 3 ) 2 ], [7] [Fe(Cp)X(CO) 2 ] (X= Br, I), [8] [RuCl 2 -(PPh 3 ) 3 ], [9] [NiBr 2 (PPh 3 ) 2 ], [10] [Ni(PPh 3 ) 4 ], [11] and [CuBr/ bipy]. [12] The detailed mechanism of action of this additive, however, is not completely understood.…”

“…We have recently observed similar phenomena for another system. [6] At any rate, the nature of the radical trap in the ATRP process (i.e., MX in Scheme 1) is different from the one-electron oxidation product observed in the cyclic voltammogram (M + ). Thus, the cyclic voltammetric results for complex [RuCl 2 (PPh 3 ) 3 ] are not directly relevant for the rationalization of the accelerating effect by the Al(OiPr) 3 additive.…”

From UV to NIR light, photo-triggered 1,3 dipolar cycloadditions as a modern ligation method in solution and on surface

“…The employed metal must possess the ability to expand the coordination sphere and change the oxidation number. A variety of metals have been found to be suitable for ATRP such as Mo, 70 Fe, 71 Ni, 72 and Cu. 73 The corresponding nitrogen containing ligands (L) are required in order to stabilize the low oxidation state of the metal complex.…”