Abstract
Four new molecular Co(II)tetrapyridyl complexes were synthesized and evaluated for their activity as catalysts for proton reduction in aqueous environments. The pyridine groups around the macrocycle were substituted for either one or two pyrazine groups. Single crystal X-ray analysis shows that the pyrazine groups have minimal impact on the Co(II)–N bond lengths and molecular geometry in general. X-band EPR spectroscopy confirms the Co(II) oxidation state and the electronic environment of the Co(II) center are only very slightly perturbed by the substitution of pyrazine groups around the macrocycle. The substitution of pyrazine groups has a substantial impact on the observed metal- and ligand-centered reduction potentials as well as the overall H2 catalytic activity in a multimolecular system using the [Ru(2,2′-bipyridine)3]Cl2 photosensitizer and ascorbic acid as a sacrificial electron donor. The results reveal interesting trends between the H2 catalytic activity for each catalyst and the driving force for electron transfer between either the reduced photosensitizer to catalyst step or the catalyst to proton reduction step. The work presented here showcases how even the difference of a single atom in a molecular catalyst can have an important impact on activity and suggests a pathway to optimize the photocatalytic activity and stability of molecular systems.
Highlights
Catalysts that can transform typically inert but sustainable reagents, such as light and water, to generate high-value fuels are absolutely critical to mitigating atmospheric CO2 accumulation and climate disruption [1,2,3,4,5]
The synthesis of 1–4 followed a similar strategy of macrocycle formation, folfollowed by metalation metalationwith withCo(II), Co(II), introduction of pyrazine groups necessilowed by thethe introduction of pyrazine groups necessitated tated an additional coupling to complete the macrocycle
We demonstrated that O-CAT has very high activity for light-activated proton reduction from water when compared with similar Co(II)-based molecular catalysts
Summary
Catalysts that can transform typically inert but sustainable reagents, such as light and water, to generate high-value fuels are absolutely critical to mitigating atmospheric CO2 accumulation and climate disruption [1,2,3,4,5]. The development of such catalytic systems can be accelerated using molecular complexes, which provide the opportunity to investigate the impact of chemical and electronic structures on catalytic mechanisms and activities with atomic-level resolution [6,7,8,9,10]. In transformations that are more demanding than proton reduction, such as CO2 reduction, the addition of amine groups around the periphery of a tetraazine
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