Elucidation of the structure of a highly active catalytic system for CO2/epoxide copolymerization: A salen-cobaltate complex of an unusual binding mode

Sung Jae Na, S. Sujith, Anish Cyriac, Bo Eun Kim, Jina Yoo, Youn K. Kang, Su Jung Han, Chongmok Lee, Bun Yeoul Lee

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Abstract

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+Bu 3)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, 8300-13000 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 H2NCMe2CMe 2NH2 instead of ethylenediamine or 1,2-cyclohexenediamine results in conventional imine-coordinating complexes, which show lower activities than uncoordinated imine complexes.

Original languageEnglish
Pages (from-to)10455-10465
Number of pages11
JournalInorganic Chemistry
Volume48
Issue number21
DOIs
StatePublished - 2 Nov 2009

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