Neutral and cationic aluminum complexes supported by acetamidate and thioacetamidate heteroscorpionate ligands as initiators for ring-opening polymerization of cyclic esters

摘要

The synthesis, structures, and ring-opening polymerization (ROP) activity of heteroscorpionate aluminum alkyl and aryloxide complexes are reported. The reactions of the acetamide and thioacetamide heteroscorpionate protio ligands pbptamH (pbptamH = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)thioacetamide), pbpamH (pbpamH = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide), sbpamH (sbpamH = N-sec-butyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide), and (S)-mbpamH ((S)-mbpamH = (S)-(-)-N-α-methylbenzyl-2,2-bis(3,5- dimethylpyrazol-1-yl)acetamide) with 1 equiv of AlR3 (R = Me, Et, iBu) proceed in very high yields to give the neutral heteroscorpionate dialkyl aluminum complexes [AlR2{κ ~2-pbptam}] (R = Me (1), Et (2)), [AlR2{κ~ 2-pbptam}] (R = Me (3), Et (4), iBu (5)), [AlR 2{κ~2-pbptam}] (R = Me (6), Et (7), iBu (8)), and [AlR2{κ~2-(S)-mbpam}] (R = Me (9), Et (10)). In the solid state, complexes 1-10 adopt a tetrahedral structure with the heteroscorpionate ligands arranged in a κ2 coordination mode; in the case of the thioacetamidate derivatives 1 and 2 a κ~2NN coordination mode is observed, whereas the acetamidate derivatives 3-10 present a κ~2NO coordination mode. The structures in solution of 1-10 were investigated by VT NMR spectroscopy, and fluxional exchange between coordinated and noncoordinated pyrazole rings was observed, producing interconversion between the different isomers. Compounds 7 and 10 were used as convenient starting materials for the synthesis of the aryloxide aluminum compounds [Al(OR)2{κ~2-sbpam}] (11) and [Al(OR) 2{κ~2-(S)-mbpam}] (12) (R = 2,6-Me_2C _6H_3O) by reaction with the corresponding 2,6-dimethylphenol. The complexes [AlMe{κ3-pbptam}][MeB(C _6F_5)_3] (13), [AlMe{κ~3-pbptam}] [B(C_6F_5)_4] (14), [AlEt{κ3- pbptam}][B(C6F5)4] (15), [AlEt{κ 3-sbpam}][B(C_6F_5)_4] (16), and [AlEt{κ~3-(S)-mbpam}][B(C_6F_5) _4] (17) are derived from the ionization of the neutral dialkyl aluminum complexes 1, 2, 7, and 10 with the alkyl abstracting reagent B(C _6F_5)_3 or [Ph_3C][B(C _6F_5)_4]. The NMR data were consistent with an overall Cs-symmetric structure for 13-15 and C1-symmetric structure for 16 and 17, which indicates an effective κ~3 coordination of the corresponding heteroscorpionate ligand to the cationic aluminum center. The structures of the complexes were determined by spectroscopic methods, and the X-ray crystal structures of 1, 2, and 7 were also established. Finally, exhaustive comparative catalytic studies of the aluminum complexes 1-10, 13, and 14 in the ring-opening polymerization of rac-lactide, l-lactide, and ε-caprolactone are also described.