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

Compound Al(O i Pr) 3 is shown to catalyze the halide‐exchange process leading from [Mo(Cp)Cl 2 ( i PrNCHCHN i Pr)] and CH 3 CH(X)COOEt (X=Br, I) to the mixed‐halide complexes [Mo(Cp)ClX( i PrNCHCHN i Pr)]. On the other hand, no significant acceleration is observed for the related exchange between [MoX 3 (PMe 3 ) 3 ] (X=Cl, I) and PhCH(Br)CH 3 , by analogy to a previous report dealing with the Ru II complex [RuCl 2 (PPh 3 ) 3 ]. A DFT computation study, carried out on the model complexes [Mo(Cp)Cl 2 (PH 3 ) 2 ], [MoCl 3 (PH 3 ) 3 ], and [RuCl 2 (PH 3 ) 3 ], and on the model initiators CH 3 CH(Cl)COOCH 3 , CH 3 Cl, and CH 3 Br, reveals that the 16‐electron Ru II complex is able to coordinate the organic halide RX in a slightly exothermic process to yield saturated, diamagnetic [RuCl 2 (PH 3 ) 3 (RX)] adducts. The 15‐electron [MoCl 3 (PH 3 ) 3 ] complex is equally capable of forming an adduct, that is, the 17‐electron [MoCl 3 (PH 3 ) 3 (CH 3 Cl)] 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 2 (PH 3 ) 2 ] complex and CH 3 Cl, on the other hand, is repulsive and does not lead to a stable 19‐electron adduct. The [RuCl 2 (PH 3 ) 3 (CH 3 X)] system leads to an isomeric complex [RuClX(PH 3 ) 3 (CH 3 Cl)] 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 [RuCl 3 (PH 3 ) 3 ] and the free radical CH 3 . 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 Al(O i Pr) 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(OCH 3 ) 3 ] n ( n =1,2,3,4) compounds, and on the interaction of Al(OCH 3 ) 3 with CH 3 Cl and with the [Mo(Cp)Cl 3 (PH 3 ) 2 ] and [RuCl 3 (PH 3 ) 3 ] complexes.