Neutral and Cationic Aluminum Complexes Supported by Acetamidate and Thioacetamidate Heteroscorpionate Ligands as Initiators for Ring-Opening Polymerization of Cyclic Esters

Abstract

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{κ2-pbptam}] (R = Me (1), Et (2)), [AlR2{κ2-pbptam}] (R = Me (3), Et (4), iBu (5)), [AlR2{κ2-pbptam}] (R = Me (6), Et (7), iBu (8)), and [AlR2{κ2-(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 κ2NN coordination mode is observed, whereas the acetamidate derivatives 3−10 present a κ2NO 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{κ2-sbpam}] (11) and [Al(OR)2{κ2-(S)-mbpam}] (12) (R = 2,6-Me2C6H3O) by reaction with the corresponding 2,6-dimethylphenol. The complexes [AlMe{κ3-pbptam}][MeB(C6F5)3] (13), [AlMe{κ3-pbptam}][B(C6F5)4] (14), [AlEt{κ3-pbptam}][B(C6F5)4] (15), [AlEt{κ3-sbpam}][B(C6F5)4] (16), and [AlEt{κ3-(S)-mbpam}][B(C6F5)4] (17) are derived from the ionization of the neutral dialkyl aluminum complexes 1, 2, 7, and 10 with the alkyl abstracting reagent B(C6F5)3 or [Ph3C][B(C6F5)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 κ3 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.