Electrochemical Co2 and O2 reduction using nickel and cobalt macrocyclic complexes
Date
2016
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Electrochemical CO2 reduction was first tested on N-methylated nickel cyclen derivatives. The tetramethylated Ni(TMcyclen) displays the best Faradaic efficiency for CO production followed by the dimethylated Ni(DMcyclen). Ni(cyclen) has no catalytic activity. The catalytic active species are Ni(0) species as verified by cyclic voltammetry. The CO2 reduction undergoes a proton coupled electron transfer in which protons come from the trace amount water in the acetonitrile solvent. The order of reactivity for Ni(cyclen) derivatives drastically contrast the literature reported Ni(cyclam) and its N-methylated derivatives. DFT calculation revealed that the difference in reactivity originates from different HOMOs of the complexes interacting with CO2, which is related to the steric strain caused by macrocyclic ring size. The analogous palladium complexes were first time synthesized and structurally characterized. They are catalytic inactive for CO2 reduction due to severe electrode deposition.
Bis and mono amide substituted nickel cyclen two-arm and one-arm complexes were designed, synthesized and characterized. X-ray crystallography demonstrated the six-coordinated nickel center. The oxygen atoms from the amide arms bind with nickel strongly with metal to ligand back-donation interaction as indicated by IR spectroscopy. Zinc derivatives were synthesized to reveal the non-innocent ligand reduction property of the nickel complexes. CV in combination of UV-vis and NMR spectroscopy indicates the ligand based electrocatalytic CO2 reduction for the nickel two-arm complexes. As the functional groups on the amide arms become more electron donating, the catalytic reactivity for CO2 reduction improves. The tBu and mesityl derivatized nickel complexes show the best Faradaic efficiency and current density for CO production in contrast to the fluorinated nickel complexes by CPE experiments. The loss of Faradic efficiency was due to the complex degradation which causes the electrode deposition. The one-arm derivatives were improved in CO2 catalytic reduction as they open the coordination site for substrate binding, which was illustrated by the doubled to tripled Faradaic efficiency for CO production.
Novel tetrapyrrolic nickel and cobalt complexes with sp 3 hybridized meso carbons were also developed for catalytic applications. Nickel and cobalt dimethylbiladiene complexes were created through a convenient synthetic procedure. The oxidative instability of nickel dimethylbiladiene leads to the discovery of nickel dimethylisocorrole. The first example of cobalt dimethylisocorrole was also demonstrated. These new tetrapyrrole nickel and cobalt complexes display strong light absorptivity and rich multielectron redox chemistry. The unique photophysical and redox properties of the complexes were shown due to great metal ligand cooperativity by DFT and TDDFT calculations.
The cobalt dimethylbiladiene and dimethylisocorrole complexes were especially active for ORR catalysis. In addition with the similar cobalt tetrapyrrole complex, cobalt dimethylphlorin, these cobalt tetrapyrroles were examined for their electrocatalytic O2 reduction reactivity and compared to the literature reported cobalt porphyrin. The sp 3-carbon containing cobalt tetrapyrroles show comparable selectivity for water production as cobalt porphyrin. The cobalt dimethylisocorrole stands out as the most selective and efficient four-electron O2 reduction catalyst by both electrochemical study and homogeneous study. Its catalysis goes through a two-step process which contains a two-electron O2 to H2O2 reduction and a two-electron H2O 2 to H2O reduction. The remarkable O2 reduction catalytic reactivity of cobalt dimethylisocorrole is linked with its multielectron redox engendering ligand scaffold, which implicates great potential for applications in other multielectron redox processes.