Sterically induced ligand framework distortion effects on catalytic cyclic ester polymerizations

Macaranas, J. A.; Luke, A. M.; Mandal, M.; Neisen, B. D.; Marell, D. J.; Cramer, C. J.; Tolman, W. B.
Inorg. Chem. 2018, 57, 3451 (doi:10.1021/acs.inorgchem.8b00250).

Aluminum alkoxide complexes supported by salen ligands (salen = N,N'-bis(salicylaldimine)-2-methylpropane-1,2-diamine or N,N'-bis(salicylaldimine)-2,2-dimethylpropane-1,3-diamine) with adamantyl ortho substituents have been synthesized and investigated for the polymerization of ε-caprolactone. Geometric analysis of the catalysts used for the reaction reveal the metal coordination geometries to be intermediate between square pyramidal and trigonal bipyramidal. Detailed kinetic study accompanied by density functional modeling of key mechanistic steps of the reaction suggest that in addition to the length of the backbone linker, the ortho-aryl substituents have significant impact on the catalyst's reactivity. Bulky ortho substituents favorably distort the precatalyst geometry and thereby achieve the rate-limiting transition-state geometry at low energetic cost, thus accelerating reaction.