Tuning redox potentials of bis(imino)pyridine cobalt complexes: an experimental and theoretical study involving solvent and ligand
The structure and electrochemical properties of a series of bis(imino)pyridine CoII complexes (NNN)CoX₂ and [(NNN)₂Co][PF₆]₂ (NNN = 2,6-bis[1-(4-R-phenylimino)ethyl]pyridine, with R = CN, CF₃, H, CH₃, OCH₃, N(CH₃)₂; NNN = 2,6-bis[1-(2,6-(iPr)₂-phenylimino)ethyl]pyridine and X = Cl, Br) were studied using a combination of electrochemical and theoretical methods. Cyclic voltammetry measurements and DFT/B3LYP calculations suggest that in solution (NNN)CoCl₂ complexes exist in equilibrium with disproportionation products [(NNN)₂Co]²⁺ [CoCl₄]²⁻ with the position of the equilibrium heavily influenced by both the solvent polarity and the steric and electronic properties of the bis(imino)pyridine ligands. In strong polar solvents (e.g., CH₃CN or H₂O) or with electron donating substituents (R = OCH₃ or N(CH₃)₂) the equilibrium is shifted and only oxidation of the charged products [(NNN)₂Co]²⁺ and [CoCl₄]²⁻ is observed. Conversely, in nonpolar organic solvents such as CH₂Cl₂ or with electron withdrawing substituents (R = CN or CF₃), disproportionation is suppressed and oxidation of the (NNN)CoCl₂ complexes leads to 18e⁻ CoIII complexes stabilized by coordination of a solvent moiety. In addition, the [(NNN)₂Co][PF₆]₂ complexes exhibit reversible CoII/III oxidation potentials that are strongly dependent on the electron withdrawing/donating nature of the N-aryl substituents, spanning nearly 750 mV in acetonitrile. The resulting insight on the regulation of redox properties of a series of bis(imino)pyridine cobalt(II) complexes should be particularly valuable to tune suitable conditions for reactivity.
- Research Organization:
- Energy Frontier Research Centers (EFRC) (United States). Center for Electrocatalysis, Transport Phenomena and Materials for Innovative Energy Storage (CETM)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- SC0001055
- OSTI ID:
- 1065978
- Journal Information:
- Dalton Transactions, Vol. 41, Issue 12; Related Information: CETM partners with General Electric Global Research (lead); Lawrence Berkeley National Laboratory; Stanford University; Yale University; ISSN 1477-9226: ICHBD9
- Country of Publication:
- United States
- Language:
- English
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