Structure Activity Relationships for Reversible O2 Chemisorption by the Solid Phases of Co(salen) and Co(3F-salen)

Mads Sondrup Møller, Christine J. McKenzie*

*Corresponding author for this work

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Abstract

The potential of solid-state materials comprising Co(salen) units for concentrating dioxygen from air was recognized over 80 years ago. While the chemisorptive mechanism at the molecular level is largely understood, the bulk crystalline phase plays important, yet unidentified roles. We have reverse crystal-engineered these materials and can for the first time describe the nanostructuring requisite for achieving reversible O2 chemisorption by Co(3R-salen) R = H or F, the simplest and most effective of the many known derivatives of Co(salen). Of the six phases of Co(salen) identified, α-ζ: α = ESACIO, β = VEXLIU, γ, δ, ϵ, and ζ (this work), only γ, δ, ϵ, and ζ are capable of reversible O2 binding. Class I materials (phases γ, δ, and ϵ) are obtained by desorption (40-80 °C, atmospheric pressure) of the co-crystallized solvent from Co(salen)·(solv), solv = CHCl3, CH2Cl2, or 1.5 C6H6. The oxy forms comprise between 1:5 and 1:3 O2:[Co] stoichiometries. Class II materials achieve an apparent maximum of 1:2 O2:Co(salen) stoichiometries. The precursors for the Class II materials comprise [Co(3R-salen)(L)·(H2O)x], R = H, L = pyridine, and x = 0; R = F, L = H2O, and x = 0; R = F, L = pyridine, and x = 0; R = F, L = piperidine, and x = 1. Activation of these depends on the desorption of the apical ligand (L) that templates channels through the crystalline compounds with the Co(3R-salen) molecules interlocked in a Flemish bond brick pattern. The 3F-salen system produces F-lined channels proposed to facilitate O2 transport through the materials through repulsive interactions with the guest O2. We postulate that a moisture dependence of the activity of the Co(3F-salen) series is due to a highly specific binding pocket for locking in water via bifurcated hydrogen bonding to the two coordinated phenolato O atoms and the two ortho F atoms.

Original languageEnglish
JournalJACS Au
Volume3
Issue number5
Pages (from-to)1484–1495
ISSN2691-3704
DOIs
Publication statusPublished - 2023

Keywords

  • chemisorption
  • Co(salen)
  • crystal engineering
  • dioxygen
  • gas−solid reaction

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