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. 2018 Feb 6;9(10):2750-2755.
doi: 10.1039/c7sc04960a. eCollection 2018 Mar 14.

Intramolecular hydrogen-bonding in a cobalt aqua complex and electrochemical water oxidation activity

Juliet F Khosrowabadi Kotyk  1 , Caitlin M Hanna  1 , Rebecca L Combs  1 , Joseph W Ziller  1 , Jenny Y Yang  1
Affiliations

Intramolecular hydrogen-bonding in a cobalt aqua complex and electrochemical water oxidation activity

Juliet F Khosrowabadi Kotyk et al. Chem Sci. .

Abstract

Water oxidation is catalysed in Nature by a redox cofactor embedded in a hydrogen-bonded network designed to orchestrate proton transfer throughout the challenging 4 electron reaction. In order to mimic aspects of this microenvironment, [CoLDMA(CH3CN)2][BF4]2 (2) was synthesized, where LDMA is a dipyridyldiamine ligand with two dimethylamine bases in the secondary coordination sphere. Structural characterization of the corresponding aqua complexes establish hydrogen bonding between the bound water and pendant base(s). Cyclic voltammetry of [CoLDMA(CH3CN)2][BF4]2 (2) reveals enhanced oxidative current upon titration with water and controlled potential electrolysis confirms evolution of O2. The related complex [CoLH(CH3CN)2][BF4]2 (1), which has the same primary coordination environment as 2 but lacks pendant bases, is inactive. The structural and electrochemical studies illustrate the role positioned proton relays can play in promoting redox reactivity.

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Figures

Chart 1
Chart 1
Fig. 1
Fig. 1. ORTEP of the cobalt complexes (a) [CoLH(CH3CN)2][BF4]2 (1), (c) [CoLDMA(CH3CN)(H2O)][BF4]2, and (d) [CoLDMA(H2O)2][BF4]2. Thermal ellipsoids are drawn at the 50% probability level. Gray dashed lines indicate hydrogen-bonding interactions. (b) The quality of the structure for [CoLDMA(CH3CN)2][BF4]2 (2) was poor; a ball and stick diagram is used to depict connectivity. The BF4– counter anions, solvent molecules and hydrogen atoms other than those on the aqua ligands have been removed for clarity.
Fig. 2
Fig. 2. Cyclic voltammograms upon addition of water to 1 mM solutions of (a) [CoLH(CH3CN)2][BF4]2 (1) and (b) [CoLDMA(CH3CN)2][BF4]2 (2) in CH3CN at 100 mV s–1.
Fig. 3
Fig. 3. (a) Cyclic voltammograms in CPE cell. [CoLDMA(CH3CN)2][BF4]2 in 5 : 95 H2O/CH3CN (red), [CoLDMA(CH3CN)2][BF4]2 in 0 : 100 H2O/CH3CN (blue), and no catalyst present under the same conditions (gray and black). (b) Charge passed vs. time in the controlled potential electrolysis of a 1 mM solution of [CoLDMA(CH3CN)2][BF4]2 (2) in 95 : 5 CH3CN : H2O (v/v) at 1.07 V vs. Fe(C5H5)2+/0 (red trace). Equivalent electrolyses under the same conditions; [CoLH(CH3CN)2][BF4]2 (1, green trace), [Co(CH3CN)6][BF4]2 (lavender trace), and rinsed post-electrolysis electrode in fresh Bu4NPF6 solution (black trace).
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