Indium Complexes of Fluorinated Dialkoxy-Diimino Salen-like Ligands for Ring-Opening Polymerization of rac-Lactide: How Does Indium Compare to Aluminum?

The reaction between InCl3 and {(ONNO)-N-Et}K-2 (1a), prepared from (CF3)(2)(OH)CCH2C(CH3)=N-R-N=C(CH3)-CH2C(OH)(CF3)(2) ({(ONNO)-N-Et}H-2, R = C2H4 (a); {(ONNO)-N-Cy}H-2, R = rac-1,2-cyclohexylene (b)) and PhCH2K, gave {(ONNO)-N-Et}InCl (2a). The reaction between InCl3 and 3 equiv of MeLi led to a mixture of [InMe3] and Li[InMe4] which upon further treatment with {(ONNO)-N-R}H-2 proligands gave the ate complexes [{(ONNO)-N-R}Li]InMe2 (4a,b); the methyl complex {(ONNO)-N-Et}InMe (3a) was also isolated from this reaction. Hydrocarbyl complexes {(ONNO)-N-R}In(CH2SiMe3) (5a,b) were prepared cleanly from the 1:1 reactions between In(CH2SiMe3)(3) and {ONRNO}H-2. The solid-state molecular structures of mononuclear 2a, 3a, 4b, and 5b and of ((1a)(2)center dot(mu-H2O)](n), a distorted c-ubane-like core made up of four potassium atoms and two ligands, were determined. Compounds 5a,b are moderately active initiators/catalysts for the ring-opening polymerization of rac-lactide, giving polymers with controlled molecular weights and narrow polydispersities, especially in the presence of added isopropyl alcohol (1-10 equiv) as exogenous initiator. Isotactic-enriched (P-m = 0.62-0.69) PLAs were obtained from 5a, while 5b gave atactic materials. The heterobimetallic compounds 4a,b are also active and afforded slightly heterotactic-enriched PLAs (P-r = 0.57-0.62), but with broader polydispersities. Those results allowed us to discuss the initiation mechanisms according to the constitution of the systems (alkyl vs alkyl/iPrOH) and also stereoselective abilities as a function of the nature (In vs Al) and coordination environment (kappa(4)-ONNO vs. kappa(2)-NN) of the metal center.