TRUSol aims to rationalize the factors controlling the electro- and photo-catalytic performance of a new family of qpy-based CO2 reduction catalysts by combining rational ligand design with catalytic and operando spectroscopic investigations. This combination has the general objective of ultimately revealing the key electronic and geometric features that optimize these molecular catalysts for facile CO2 reduction. The list of objectives, results and main achievements along with the general progress is outlined below:
1. Synthesis and characterization of a new family of quarterpyridine-based ligands complexed with iron.
In line with this objective, the previously reported [Fe(qpy)(H2O)2]2+ CO2R catalyst was prepared by modifying the preparation of the qpy (where qpy is quaterpyridine) ligand. Additionally, a series of derivatives were prepared using synthetic techniques and fully characterized. The family of qpy-based catalysts was extended with the preparation of the Fe complexes containing different electronic substituents to modify the electronic structures. Also, a pyrene-functionalized ligand was prepared, which allowed the immobilization of the system on graphitic supports for heterogeneous CO2R. Finally, the newly prepared ligands were used to prepare the corresponding Co-based derivatives, thereby extending the family to other metals.
2. Study of their redox properties and photo- as well as electro-catalytic performance.
The Fe- and Co-based systems prepared have been studied by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and controlled potential electrolysis (CPE) to investigate their redox properties and activity towards CO2RR in organic/aqueous solutions in the absence and presence of the substrate (CO2). Additionally, the light-driven activity of the complexes has been investigated in ternary mixtures of Catalyst/PS/SD (where PS is photosensitizer, and SD is sacrificial electron donor. These experiments were used to study the conditions used in our spectroscopic measurements and select an appropriate candidate. The selected candidate for the spectroscopic studies proposed in TRUSol was the [FeII(qpy)(L)2]2+ CO2R catalyst.
3. Characterization of the electronic structures of the reaction intermediates by steady state, operando and time-resolved spectroscopic techniques.
In order to gain mechanistic insights, the electronic structures of the selected candidate along with its corresponding reduced species and resting states were explored in WP3 and WP4 using a combination of steady-state and time-resolved spectroscopic techniques. This involved the use of Fe K-edge X-ray absorption spectroscopy to analyze the 1s to 3d pre-edge and 1s to 4p rising edge features sensitive to the oxidation state of the metal center, the coordination environment, and the local symmetry. Additionally, Mössbauer, EPR, and Kβ (metal 3p to 1s transition) XES spectroscopy were applied to further probe the electronic structure of these complexes by monitoring changes in the spin configuration. Overall, these studies concluded that the prepared [FeII(qpy)(L)2]2+ contains two aquo ligands in the solid state with an electronic configuration of S = 2. Upon dissolution, the system partially substitutes the aquo ligands for the corresponding solvent (e.g. MeCN), and a change in the spin configuration occurs from high spin (HS) to low spin (LS). Additionally, the one- and two-electron reduced steady-state intermediates were prepared and characterized as well, resulting in [FeI(qpy)(L)]+ and [FeI(qpy●-)] systems, with a mixed electronic configuration for the first (S = 3/2 and S = 1/2) and LS (S = 0) for the second intermediate.
Subsequently, laser/X-ray pump/probe time-resolved X-ray absorption spectroscopy was used to study the photocatalytic intermediates. We successfully identified two processes. The first was the reduction from [FeII(qpy)(L)2]2+ to [FeI(qpy)(L)]+, which takes place on the microsecond time scale (τ = 4.4 µs). During the CO2R reaction, [FeI(qpy)CO] species accumulate in the medium, and a second transient process, tentatively assigned to the [FeI(qpy)CO] to [FeI(qpy)] reaction, was monitored on the microsecond time scale (τ = 53 µs). Time-resolved Fourier transform infrared spectroscopy is currently being carried out to further confirm the nature of the second light-triggered process observed by time-resolved X-ray absorption spectroscopy.
