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 )AlR 2 (R = Me ( 1 ); R = Et ( 2 ); R = i Bu ( 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 κ 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 1 H NMR spectroscopy revealed fluxional exchange between coordinated and noncoordinated pyrazolyl rings, producing interconversion between the two enantiomers. The activation enthalpy (Δ H # ) 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(C 6 F 5 ) 3 proceeds through net alkyl abstraction, forming the expected cationic aluminum complexes [( bpzmp )AlR] + [RB(C 6 F 5 ) 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(C 6 F 5 ) 3 or [Ph 3 C][B(C 6 F 5 ) 4 ]) proceeds by net β-H abstraction from the isobutyl group, forming [( bpzmp )Al i Bu] + [HB(C 6 F 5 ) 3 ] - ( 6 ) along with isobutene; in contrast, reaction with the Brönsted acid [HNMe 2 Ph][B(C 6 F 5 ) 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 1 H 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(C 6 F 5 ) 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.