Structural changes in Salophen versus Pyrophen actinide complexes –a case study

Understanding how structural changes of the ligand scaffold influence the coordination chemistry of early actinides is indispensable for the development of selective ligands for e.g. decontamination purposes. To obtain a profound insight into the electronic und bonding properties of early actinides, Schiff base ligands based on the salen type, have been extensively used in our group and others.[1–4] Schiff bases have shown to be excellent model ligands due to their accessible synthesis and easy functionalization and their ability to coordinate to the 5f-elements. In this respect, we have been exploring the coordination chemistry of the early actinides (Th – Pu) with pyrophen, a pure nitrogen donor ligand formed by a condensation reaction of 2-formylpyrrole with o-phenylendiamine.[5,6] A series of homoleptic 2:1 complexes has been synthesized, allowing the comparison of the hard oxygen donor of the phenolate group in the salophen system versus the soft nitrogen donor of the pyrrolide group in pyrophen. Quantum chemical calculations along with spectroscopic studies by nuclear magnetic resonance (NMR) and single crystal x-ray diffraction (SC-XRD) have been performed to elucidate the influence of hard and soft donors on the bonding strength. In solution, as confirmed by NMR, we encountered just one set of signals corresponding to a 2:1 homoleptic complex in the pyrophen system compared to two sets of signals corresponding to 2:1 homoleptic isomers present in the salophen case. In the solid state, crystallographic analysis shows that the salophen ligand adopts a sandwich like structure whereas in the pyrophen system a pincer like arrangement is preferred (see figure below).

Figure 1 Uranium(IV) complexes of salophen (left) and pyrophen (right)
References
[1] T. Radoske et al., Dalton Transactions 2020, 49, 17559–17570.
[2] R. Kloditz et al., Inorg. Chem. 2021, 60, 2514–2525.
[3] L. Köhler et al., Chemistry – A European Journal 2021, 27, 18058–18065.
[4] B. E. Klamm et al., Inorg. Chem. 2018, 57, 15389–15398.
[5] C.D. Bérubé et al., Organometallics 2003, 22, 434–439.
[6] T. Duckworth et al. manuscript in preparation.

Acknowledgment
This work was supported by the German Federal Ministry of Education and Research (BMBF) under project number 02NUK059 (f-char).