Neutral and Cationic Heteroscorpionate Aluminum Complexes: Synthesis, Structure, and Ring-Opening Polymerization of ε-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 κ2-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 (ΔH#) for racemization processes (10.1 ± 0.2
kcal·mol-1 (1); 11.6 ±0.3 kcal·mol-1 (2); 14.1 ± 0.2 kcal·mol-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 β-H abstraction from the isobutyl group,
forming [(bpzmp)AliBu]+[HB(C6F5)3]- (6) along with isobutene; in contrast, reaction with the Brönsted
acid [HNMe2Ph][B(C6F5)4] proceeds by protonolysis of the isobutyl group. Complex 4 is active in ring-opening polymerization of ε-caprolactone (ε-CL), producing high-molecular-weight polymers. The 1H
NMR monitoring of the reaction between 4 and ε-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.