Carbon Dioxide Utilisation -The Formate Route

UIDB/50006/2020 CEEC-Individual 2017 Program Contract.The relentless rise of atmospheric CO2 is causing large and unpredictable impacts on the Earth climate, due to the CO2 significant greenhouse effect, besides being responsible for the ocean acidification, with consequent huge impacts in our daily lives and in all forms of life. To stop spiral of destruction, we must actively reduce the CO2 emissions and develop new and more efficient “CO2 sinks”. We should be focused on the opportunities provided by exploiting this novel and huge carbon feedstock to produce de novo fuels and added-value compounds. The conversion of CO2 into formate offers key advantages for carbon recycling, and formate dehydrogenase (FDH) enzymes are at the centre of intense research, due to the “green” advantages the bioconversion can offer, namely substrate and product selectivity and specificity, in reactions run at ambient temperature and pressure and neutral pH. In this chapter, we describe the remarkable recent progress towards efficient and selective FDH-catalysed CO2 reduction to formate. We focus on the enzymes, discussing their structure and mechanism of action. Selected promising studies and successful proof of concepts of FDH-dependent CO2 reduction to formate and beyond are discussed, to highlight the power of FDHs and the challenges this CO2 bioconversion still faces.publishersversionpublishe

An organometallic building block approach to produce a multidecker 4 f single-molecule magnet

An organometallic building block strategy was employed to investigate the magnetic properties of a LnIII organometallic single-ion magnet (SIM) and subsequent single-molecule magnet (SMM) after coupling two of the monomeric units. New homoleptic DyIIICOT″2 and LnIII2COT″3 (Ln = Gd, Dy) complexes have been synthesized. DFT calculations of the bimetallic DyIII complex indicate strong metal–ligand covalency and uneven donation to the DyIII ions by the terminal and internal COT″2– (cyclooctatetraenide) rings that correlate with the respective bond distances. Interestingly, the studies also point to a weak covalent interaction between the metal centers, despite a large separation. The ac susceptibility data indicates that both DyIIICOT″2 and DyIII2COT″3 act as an SIM and an SMM, respectively, with complex multiple relaxation mechanisms. Ab initio calculations reveal the direction of the magnetic anisotropic axis is not perpendicular to the planar COT″ rings for both DyIIICOT″2 and DyIII2COT″3 complexes due to the presence of trimethylsilyl groups on the COT″ rings. If these bulky groups are removed, the calculations predict reorientation of the anisotropic axis can be achieved

A Cobalt Hydride Catalyst for the Hydrogenation of CO₂: Pathways for Catalysis and Deactivation

The complex Co­(dmpe)₂H catalyzes the hydrogenation of CO₂ at 1 atm and 21 °C with significant improvement in turnover frequency relative to previously reported second- and third-row transition-metal complexes. New studies are presented to elucidate the catalytic mechanism as well as pathways for catalyst deactivation. The catalytic rate was optimized through the choice of the base to match the p<i>K</i>_a of the [Co­(dmpe)₂(H)₂]⁺ intermediate. With a strong enough base, the catalytic rate has a zeroth-order dependence on the base concentration and the pressure of hydrogen and a first-order dependence on the pressure of CO₂. However, for CO₂:H₂ ratios greater than 1, the catalytically inactive species [(μ-dmpe)­(Co­(dmpe)₂)₂]²⁺ and [Co­(dmpe)₂CO]⁺ were observed

Understanding the Relationship Between Kinetics and Thermodynamics in CO₂ Hydrogenation Catalysis

Catalysts that are able to reduce carbon dioxide under mild conditions are highly sought after for storage of renewable energy in the form of a chemical fuel. This study describes a systematic kinetic and thermodynamic study of a series of cobalt and rhodium bis­(diphosphine) complexes that are capable of hydrogenating carbon dioxide to formate under ambient temperature and pressure. Catalytic CO₂ hydrogenation was studied under 1.8 and 20 atm of pressure (1:1 CO₂/H₂) at room temperature in tetrahydrofuran with turnover frequencies (TOF) ranging from 20 to 74 000 h^–1. The catalysis was followed by ¹H and ³¹P NMR spectroscopy in real time under all conditions to yield information about the rate-determining step. The cobalt catalysts displayed a rate-determining step of hydride transfer to CO₂, while the hydrogen addition and/or deprotonation steps were rate limiting for the rhodium catalysts. Thermodynamic analysis of the complexes provided information on the driving force for each step of catalysis in terms of the catalyst hydricity (Δ<i>G</i>°_H^–), acidity (p<i>K</i>_a), and free energy for H₂ addition (Δ<i>G</i>°_H₂). Linear free-energy relationships were identified that link the kinetic activity for catalytic hydrogenation of CO₂ to formate with the thermodynamic driving force for the rate-limiting steps of catalysis. The catalyst exhibiting the highest activity, Co­(dmpe)₂H, was found to have hydride transfer and hydrogen addition steps that were each downhill by approximately 6 to 7 kcal mol^–1, and the deprotonation step was thermoneutral. This indicates the fastest catalysts are the ones that most efficiently balance the free energy relationships of every step in the catalytic cycle