In the past research on metal-radical compounds has focused on the nitronyl nitroxide radicals as suitable bridging ligands between transition metal ions. Being interested in metal-radical systems of higher dimensionality, we have recently shown that the introduction of a phenolate moiety in the para-position of phenyl-nitronyl nitroxides leads to a new coordination mode and moreover a small but significant exchange interaction between the organic radical ligands and phenolate-coordinated transition metals is proven.
Contributing electronic structure of Bis(2-(dimethylaminomethyl)-4-NIT-phenolato)nickel(II)
We investigated the electronic structure and spin distribution of the para-substituted phenolate nitronyl nitroxides and other closely related para-substituted phenyl-nitronyl nitroxides with the aim of understanding the reasons for their electronic and magnetic behaviour. Finally this project has lead to a combined experimental and theoretical study with the group of Prof. J.J. Novoa, searching for para-substituted phenyl-nitronyl nitroxides capable of showing large exchange interaction through the para-functional group. By theoretical calculations on fifteen potentially metal-coordinating nitronyl nitroxides we have shown that the spin density distribution can be tuned by a careful choice of the peripheral functional group attached in the para-position. In the majority of cases, the unpaired electron is almost entirely localized on the five-atomic unit, ONCNO, of the imidazolidine ring. However, using some doubly negatively charged substitutents as in the NIT-phosphonate and NIT-boronate, it is possible to shift the unpaired electron towards the para group. This allows the existence of new magnetic interaction pathways between the para-substituted NIT-radical and an open-shell metal coordinated to the radical in the para position.
Calculated spin density distribution in para-substituted phenyl-nitronyl nitroxides. Contribution of the radical character to the resulting SOMO.
The boronate substituted nitronly nitroxide ligand was successfully coordinated to a manganese(IV) dimeric centre, providing one of the best model complexes to simulate the EPR spectra of the S2Y2* state of Photosystem II (PSII) due to long distance magnetic interactions between two S=1/2 spin centers. The phosphinate and phosphonate substituted radicals are currently investigated in collaboration with Prof. Sutter in Bordeaux and Prof. Köhler in Munich by means of NMR studies to observe directly the spin density distributions in combination with DFT calcluations. The spin distribution of mono-, bi- and tri-radical phosphine derivatives has been investigated by high-resolution fluid solution EPR and 1H and 31P MAS-NMR spectroscopy, and the results compared to DFT computations.