Our research makes use of physical-chemical methods (spectroscopy, kinetics, X-ray diffraction, computational modelling) at room temperature or under cryogenic in order to understand the electronic ground state of the synthesised complexes. The spectroscopic approaches we use include (paramagnetic) NMR, parahydrogen-induced polarisation (PHIP, collab. with Prof Torsten Gutmann, University of Darmstadt/Padderborn) vibrational spectroscopy (Raman, IR), 57Fe-Mössbauer spectroscopy, which is complemented by other techniques such as Cyclic Voltammetry, X-ray diffraction, kinetic profiling and investigations (for more details see the Techniques section). The obtained experimental data are used to “anchor” the computational data to reality. Computational approaches used include Potential Surface Energy scans using DFT, Broken Symmetry Approaches, post-HF approaches (CASSCF), calculation of Mössbauer spectra, simulation/calculation of NMR and EPR spectra, calculation of temperature dependence of magnetic susceptibility (using CASSCF). A few applications of these approaches summarised below:

Spin-state change following N2 dissociation in PNNFe-complexes. The dissociation of terminal N2 ligands in PNN-based iron complexes determines a strengthen of activation of the remaining bridging N2 ligand – a first time such an effect was observed just using N2 pressure gradients. Using a combined Mössbauer, NMR and Raman approach and supported by computational modelling, we have determined that this activation is due to a change from ls-Fe(I) to hs-Fe(I). Interestingly, the magnetic ground state remains an open-shell singlet due to the antiferromagnetic (AF) magnetic coupling between the PNN-redox active ligand, the iron centre, as well as the AF coupling of the two iron centres via the N2 bridge.

Parahydrogen induced polarisation (PHIP) using Fe and polarisation transfer to 19F (collab. with Prof Torsten Gutmann). While PHIP is a very useful mechanistic and imagistic method, it remains rather limited when first row transition metals are involved. This due to rapid conversion of para-hydrogen to its equilibrium state through the low-lying states of higher multiplicity of the metal centres. Using our PNN-based systems, we could circumvent this issue and moreover show for the first time that polarisation transfer from para-hydrogen to 19F is possible in the presence of a Fe catalyst.

Mechanistic investigations on reactions mediated by catalysts which possess low-lying excited states. Pyrimidinediimine iron complexes are the most active systems known for the intramolecular [2+2]-cycloaddition of olefins. In order to establish direct ligand structure-catalytic activity relationships a combined approach relying on kinetic studies, Mössbauer spectroscopy and computational modelling was undertaken. This revealed that multiple potential-energy surface crossings occur withing the catalytic cycle owing to the low-lying excited states of the active intermediates. This allowed to design ligands that can promote this transformation with catalyst loading under 1 mol%.