Treatment of 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazol-1-yl)methyl)phenol (bpzmp-H) as tridentate ligand with aluminum trialkyls affords the corresponding heteroscorpionate aluminum complexes (bpzmp)-AlR2 (R ) Me (1); R ) Et (2); R ) iBu (3)) in high yield. In the solid state, complexes 1-3 were isolated as racemic mixtures in which each stereoisomer adopts a tetrahedral structure with the bpzmp ligand h2-coordinated to the metal via the phenoxy group and the imino nitrogen of one of pyrazolyl rings. The investigation of the solution structure of 1-3 by means of VT 1H NMR spectroscopy revealed fluxional exchange between coordinated and noncoordinated pyrazolyl rings, producing interconversion between the two enantiomers. The activation enthalpy for racemization processes was dependent on steric hindrance at the aluminum center. Reaction of 1 or 2 with B(C6F5)3 proceeds through net alkyl abstraction, forming the expected cationic aluminum complexes [(bpzmp)AlR]+[RB(C6F5)3]- (R ) Me (4); R ) Et (5)), in which the bpzmp fragment acts as a tridentate ligand. The ionization reaction of 3 with Lewis acidic compounds (B(C6F5)3 or [Ph3C][B(C6F5)4]) proceeds by net â-H abstraction from the isobutyl group, forming [(bpzmp)AliBu]+[HB(C6F5)3]- (6) along with isobutene; in contrast, reaction with the Bronsted acid [HNMe2Ph][B(C6F5)4] proceeds by protonolysis of the isobutyl group. Complex 4 is active in ringopening polymerization of e-caprolactone producing high-molecular-weight polymers. The 1H NMR monitoring of the reaction between 4 and e-CL in 1:1 molar ratio showed that the initiation involves monomer insertion into the Al-O bond of the bpzmp fragment, forming the intermediate [(bpzphe)AlMe]+- [MeB(C6F5)4]- (bpzphe ) 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazol-1-yl)methyl)phenyl 6-hydroxyhexanoate (8). The growth of the polymer chain occurs through continuous insertions of the monomer in the Al-alkoxide bond.

Neutral and Cationic Heteroscorpionate Aluminum Complexes: Synthesis, Structure and Ring Opening Polymerization of e-Caprolactone

MILIONE, Stefano;GRISI, Fabia;
2006

Abstract

Treatment of 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazol-1-yl)methyl)phenol (bpzmp-H) as tridentate ligand with aluminum trialkyls affords the corresponding heteroscorpionate aluminum complexes (bpzmp)-AlR2 (R ) Me (1); R ) Et (2); R ) iBu (3)) in high yield. In the solid state, complexes 1-3 were isolated as racemic mixtures in which each stereoisomer adopts a tetrahedral structure with the bpzmp ligand h2-coordinated to the metal via the phenoxy group and the imino nitrogen of one of pyrazolyl rings. The investigation of the solution structure of 1-3 by means of VT 1H NMR spectroscopy revealed fluxional exchange between coordinated and noncoordinated pyrazolyl rings, producing interconversion between the two enantiomers. The activation enthalpy for racemization processes was dependent on steric hindrance at the aluminum center. Reaction of 1 or 2 with B(C6F5)3 proceeds through net alkyl abstraction, forming the expected cationic aluminum complexes [(bpzmp)AlR]+[RB(C6F5)3]- (R ) Me (4); R ) Et (5)), in which the bpzmp fragment acts as a tridentate ligand. The ionization reaction of 3 with Lewis acidic compounds (B(C6F5)3 or [Ph3C][B(C6F5)4]) proceeds by net â-H abstraction from the isobutyl group, forming [(bpzmp)AliBu]+[HB(C6F5)3]- (6) along with isobutene; in contrast, reaction with the Bronsted acid [HNMe2Ph][B(C6F5)4] proceeds by protonolysis of the isobutyl group. Complex 4 is active in ringopening polymerization of e-caprolactone producing high-molecular-weight polymers. The 1H NMR monitoring of the reaction between 4 and e-CL in 1:1 molar ratio showed that the initiation involves monomer insertion into the Al-O bond of the bpzmp fragment, forming the intermediate [(bpzphe)AlMe]+- [MeB(C6F5)4]- (bpzphe ) 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazol-1-yl)methyl)phenyl 6-hydroxyhexanoate (8). The growth of the polymer chain occurs through continuous insertions of the monomer in the Al-alkoxide bond.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11386/1634799
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