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

Compound Al(OiPr)(3) is shown to catalyze the halide-exchange process leading from [Mo(Cp)Cl-2-(iPrN=CH-CH=NiPr)] and CH3CH(X)COOEt (X=Br, 1) to the mixed-halide complexes [Mo(Cp)ClX(iPrN=CH-CH=NiPr)]. On the other hand, no significant acceleration is observed for the related exchange between [MoX3(PMe3)(3)] (X=Cl, 1) and PhCH(Br)CH3, by analogy to a previous report dealing with the Run complex [RUCl2(PPh3)(3)]. A DFT computation study, carried out on the model complexes [Mo(CP)Cl-2(PH3)(2)], [MoCl3(PH3)(3)], and [RuCl2(PH3)(]), and on the model initiators CH3CH(CI)COOCH3, CH3Cl, and CH3Br, reveals that the 16-electron Run complex is able to coordinate the organic halide RX in a slightly exothermic process to yield saturated, diamagnetic [RuCl2(PH3)(3)(RX)] adducts. The 15-electron [MoCl3(PH3)(3)] complex is equally capable of forming an adduct, that is, the 17-electron [MoCl3(PH3)(3)(CH3Cl)] complex with a spin doublet configuration, although the process is endothermic, because it requires an energetically costly electron-pairing process. The interaction between the 17-electron [MO(CP)Cl,(PH3)(2)] complex and CH3Cl, on the other hand, is repulsive and does not lead to a stable 19-electron adduct. The [RuCl2(PH3)(3)(CH3X)] system leads to an isomeric complex [RuClX(PH3)(3)(CH3Cl)] by internal nucleophilic substitution at the carbon atom. The transition state of this process for X=Cl (degenerate exchange) is located at lower energy than the transition state required for halogen-atom transfer leading to [RuCl3(PH3)(3)] and the free radical CH3. On the basis of these results, the uncatalyzed halide exchange is interpreted as the result of a competitive S(N)i process, whose feasibility depends on the electronic configuration of the transition-metal complex. The catalytic action of AI(OiPr)(3) on atom-transfer radical polymerization (and on halide exchange for the 17-electron half-sandwich Mo-III. complex) results from a more favorable Lewis acid-base interaction with the oxidized metal complex, in which the transferred halogen atom is bound to a more electropositive element. This conclusion derives from DFT studies of the model [Al(OCH3)(3)](n) (n=1,2,3,4) compounds, and on the interaction of AI(OCH3)(3) with CH3Cl and with the [Mo(Cp)Cl-3(PH3)(2)] and [RuCl3(PH3)(3)] complexes.