Elucidation of the Structure of a Highly Active Catalytic System for CO2/Epoxide Copolymerization: A salen-Cobaltate Complex of an Unusual Binding Mode
journal contributionposted on 02.11.2009, 00:00 by Sung Jae Na, Sujith S, Anish Cyriac, Bo Eun Kim, Jina Yoo, Youn K. Kang, Su Jung Han, Chongmok Lee, Bun Yeoul Lee
Salen-type ligands comprised of ethylenediamine or 1,2-cyclohexenediamine, along with an salicylaldehyde bearing a methyl substituent on its 3-position and a −[CR(CH2CH2CH2N+Bu3)2] (R = H or Me) on its 5-position, unexpectedly afford cobalt(III) complexes with uncoordinated imines. In these complexes, two salen−phenoxys and two 2,4-dinitrophenolates (DNPs), which counter the quaternary ammonium cations, coordinate persistently with cobalt, while two other DNPs are fluxional between a coordinated and an uncoordinated state in THF at room temperature. The complexes of this binding mode show excellent activities in carbon dioxide/propylene oxide copolymerization (TOF, 8 300−13 000 h−1) but with some fluctuation in induction times (1−10 h), depending on how dry the system is. The induction time is shortened (<1.0 h) and activity is increased ∼1.5 times upon the replacement of the two fluxional DNPs with 2,4-dinitrophenol-2,4-dinitrophenolate homoconjugation ([DNP···H···DNP]−). Imposing steric congestion either by replacing the methyl substituent on the salicylaldehyde with tert-butyl or by employing H2NCMe2CMe2NH2 instead of ethylenediamine or 1,2-cyclohexenediamine results in conventional imine-coordinating complexes, which show lower activities than uncoordinated imine complexes.