Abstract
While [Ni(cyclam)]2+ and [Ni(dithiacyclam)]2+ complexes were shown to be potent electrocatalysts for the CO2 conversion, their respective Co complexes hitherto received only little attention. Herein, we report on the CoII complexes of the cyclam and dithiacyclam platform, describe their synthesis and reveal their rich solvent dependent coordination chemistry. We show that sulfur implementation into the cyclam moiety leads to a switch from a low spin CoII complex in [Co(cyclam)]2+ to a high spin form in [Co(dithiacyclam)]2+. Notably, while both complexes are capable to perform the reduction of CO2 to CO, H2 formation is generally preferred. Along this line, the complexes were shown to enable proton reduction from acetic acid. However, in comparison to [Co(cyclam)]2+, the altered electronics make [Co(dithiacyclam)]2+ complexes prone to deposit on the glassy carbon working electrode over time leading to an overall low faradaic efficiency for the reduction of protons or CO2.
Highlights
In order to replace fossil fuels as an energy source as well as a raw material for the chemical industry, intensive research is currently undertaken.[1,2,3,4,5] With its high energy density per kilogram,[6,7] hydrogen is an attractive energy storage molecule and can be obtained by water splitting
The successful complexation was supported by electrospray-ionization mass spectrometry (ESI-MS) revealing the characteristic mass peaks at m/z = 258.1 and 357.9 for the [Co(L1-H)]+ and [Co(L1)(ClO4)]+ fragments
Crystals of C1 were obtained from methanol solutions and further supported the molecular structure of C1 showing the expected [Co(L1)(ClO4)2] composition (Figure 1) that was already reported in the late 1970s.[39]
Summary
In order to replace fossil fuels as an energy source as well as a raw material for the chemical industry, intensive research is currently undertaken.[1,2,3,4,5] With its high energy density per kilogram,[6,7] hydrogen is an attractive energy storage molecule and can be obtained by water splitting. The macrocyclic aza-ligand cyclam L1, a 14-membered ring system with four nitrogen donor atoms (Scheme 1) is another promising ligand platform for CO2 and proton conversion processes.[25,26] Notably, one of the most prominent homogeneous transition-metal catalyst for electrochemical CO2 reduction is [Ni(cyclam)]2+.[27,28,29,30] it is astonishing that up to now only little is known about the corresponding Co complex and a majority of reports focuses on the cyclam-like diimine ligands L2 and L3 (Scheme 1).[25,31] The corresponding Co complexes were already investigated for their catalytic activity in the electrochemical reduction of CO2 in 1980 by Fisher and Eisenberg.[25] Under CO2 atmosphere in water/acetonitrile (2 : 1), controlled potential coulometry at –1.6 V (CoIIL2) and –1.5 V (CoIIL3) vs SCE utilizing a mercury working electrode afforded CO and H2 in a 1 : 1 ratio showing current efficienc-
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