Carbon dioxide activation by cobalt(I) macrocycles: Factors affecting CO2 and CO binding

Etsuko Fujita, Carol Creutz, Norman Sutin, David J. Szalda

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Abstract

The cobalt(I) complexes of several 14-membered tetraazamacrocycles were prepared in CH3CN by either electrochemical or sodium amalgam reduction. The electronic absorption spectra and other physical properties of the CoI, CoI-CO2 and CoI-CO complexes are reported. The CO2 and CO binding constants were determined by spectroscopic and/or electrochemical methods. The binding constants range from 5 × 104 to ≥3 × 108 M-1 for CO and from ≤0.5 to >106 M-1 for CO2 at 25 °C. Both binding constants increase as the CoL2+/+ reduction potentials (which range from -0.34 to -1.65 V vs SCE in CH3CN) become more negative. Thus charge transfer from CoI to CO2 or CO is an important factor in stabilizing these adducts. However, hydrogen-bonding interactions between the bound CO2 and amine macrocycle N-H protons may serve to additionally stabilize the adduct in some cases, while steric repulsion by the macrocycle methyl groups may destabilize the adducts, depending upon the complex. The equilibrium ratios of N-meso and N-rac isomers of (5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene cobalt(I) and -(II) complexes were determined by 1H NMR; the N-rac isomers of both predominate in CD3CN at room temperature. The crystal and molecular structure of the perchlorate salt of (3,5,7,7,10,12,14,14-octamethyl-1,4,8,1 l-tetraazacyclotetradeca-4,11-diene cobalt(I) was determined from single-crystal X-ray diffraction data collected with use of Mo Kα radiation. Crystallographic data: space group Pl with a = 8.433 (2) Å, b = 18.333 (4) Å, c = 7.257 (2) Å α = 100.22 (2)°, β = 91.29 (2)°, γ = 87.68(2)°, V = 1103 (1) Å3, Z = 2 (Rw = 0.085, Rw = 0.105). The two square-planar cobalt atoms in the asymmetric unit are situated on crystallographic inversion centers.

Original languageEnglish
Pages (from-to)343-353
Number of pages11
JournalJournal of the American Chemical Society
Volume113
Issue number1
Publication statusPublished - Jan 2 1991

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ASJC Scopus subject areas

  • Chemistry(all)

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