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Title: Evaluation of the coordination preferences and catalytic pathways of heteroaxial cobalt oximes towards hydrogen generation

Three new heteroaxial cobalt oxime catalysts, namely [Co III(prdioxH)( 4tBupy)(Cl)]PF 6 (1), [Co III(prdioxH)( 4Pyrpy)(Cl)]PF 6 (2), and [Co III(prdioxH)( 4Bzpy)(Cl)]PF 6 (3) have been studied. These species contain chloro and substituted tert-butyl/pyrrolidine/benzoyl-pyridino ligands axially coordinated to a trivalent cobalt ion bound to the N 4-oxime macrocycle (2 E,2' E,3 E,3' E)-3,3'-(propane-1,3-diylbis(azanylylidene))bis(butan-2-one)dioxime, abbreviated (prdioxH)– in its monoprotonated form. Emphasis was given to the spectroscopic investigation of the coordination preferences and spin configurations among the different 3d 6 Co III, 3d 7 Co II, and 3d 8 Co I oxidation states of the metal, and to the catalytic proton reduction with an evaluation of the pathways for the generation of H 2 via Co III–H or Co II–H intermediates by mono and bimetallic routes. The strong field imposed by the (prdioxH)– ligand precludes the existence of high-spin configurations, and 6-coordinate geometry is favored by the LSCo III species. Species 1 and 3 show a split Co III/Co II electrochemical wave associated with partial chemical conversion to a [Co III(prdioxH)Cl 2] species, whereas 2 shows a single event. The reduction of these Co III complexes yields LSCo II and LSCo I species in which the pyridine acts as the dominantmore » axial ligand. In the presence of protons, the catalytically active Co I species generates a Co III–H hydride species that reacts heterolytically with another proton to generate dihydrogen. The intermediacy of a trifluoroacetate-bound Co III/Co II couple in the catalytic mechanism is proposed. Finally, these results allow for a generalization of the behavior of heteroaxial cobalt macrocycles and serve as guidelines for the development of new catalysts based on macrocyclic frameworks.« less
Authors:
 [1] ;  [1] ;  [2] ;  [1] ;  [1] ;  [1] ;  [3] ;  [2] ;  [1] ;  [1]
  1. Wayne State Univ., Detroit, MI (United States)
  2. Argonne National Lab. (ANL), Argonne, IL (United States)
  3. Michigan State Univ., East Lansing, MI (United States)
Publication Date:
Grant/Contract Number:
AC02-06CH11357
Type:
Accepted Manuscript
Journal Name:
Chemical Science
Additional Journal Information:
Journal Volume: 7; Journal Issue: 5; Journal ID: ISSN 2041-6520
Publisher:
Royal Society of Chemistry
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); Chemical Sciences; Geosciences; and Biosciences Division
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
OSTI Identifier:
1351546