Neutral and cationic heteroscorpionate aluminium complexes: syntesis, structure, and ring-opening polymerization of e-caprolactone

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 2-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 (¢H#) for racemization processes (10.1 ( 0.2 kcalâmol-1 (1); 11.6 (0.3 kcalâmol-1 (2); 14.1 ( 0.2 kcalâmol-1 (3)) 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 Bro¨nsted acid [HNMe2Ph][B(C6F5)4] proceeds by protonolysis of the isobutyl group. Complex 4 is active in ringopening polymerization of -caprolactone (-CL), producing high-molecular-weight polymers. The 1H NMR monitoring of the reaction between 4 and -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.