Sterically Induced Ligand Framework Distortion Effects on Catalytic Cyclic Ester Polymerizations
Joahanna A. Macaranas, Anna M. Luke, Mukunda Mandal, Benjamin D. Neisen, Daniel J. Marell, Christopher J. Cramer*, and William B. Tolman*
Macaranas, J. A.; Luke, A. M.; Mandal, M.; Neisen, B. D.; Marell, D. J.; Cramer, C. J.; Tolman, W. B. Sterically Induced Ligand Framework Distortion Effects on Catalytic Cyclic Ester Polymerizations. Inorg. Chem. 2018, 57, 3451–3457. DOI: 10.1021/acs.inorgchem.8b00250.
- Effectiveness of aluminum salen complexes with o-adamantyl substituents in initiating the polymerization of ε-caprolactone was compared to that of similar salen catalysts having o-tbutyl substituents.
- DFT modeling indicates that the reactivity of the catalyst is influenced by both the length of the backbone linker and the presence of o-aryl substituents on the ligand.
- Bulky o-substituents distorts the pre-catalyst geometry in a favorable way, making it easier to achieve the rate-limiting TS geometry, thus speeding up the reaction.
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 o-adamantyl substituents have been synthesized and investigated for the polymerization of ε-caprolactone. Geometric analysis of the catalysts used for the reaction reveals the metal coordination geometries to be intermediate between square-pyramidal and trigonal-bipyramidal. A detailed kinetic study accompanied by density functional theory modeling of key mechanistic steps of the reaction suggest that, in addition to the length of the backbone linker, the o-aryl substituents have a significant impact on the catalyst’s reactivity. Bulky ortho substituents favorably distort the precatalyst geometry and thereby foster the achievement of the rate-limiting transition-state geometry at low energetic cost, thus accelerating the reaction.
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