A tripodal pyrrol imine schiff base ligand: synthesis and coordination chemistry with early actinides

Our laboratory has been actively engaged in exploring the fundamental behavior of the early 5f elements in solution and solid state, for instance with tetradentate coordinating Schiff base ligands.[1–4] Schiff base ligands are of special interest since they can easily be prepared by a condensation reaction and are known to coordinate to different metal centers in various oxidation states. To study the reactivity and complexation differences from the tetradentate to a hexadentate binding motif, with regards to bonding trends and electronic properties, we recently extended our palette of Schiff base chelators to the hexadentate pure N-donor (tris-((1H-pyrrol-2-ylmethyl-ene)ethane)-amine (trenpy) and began to explore its coordination chemistry with the early tetravalent actinides.
Applying a salt metathesis reaction of one equivalent of AnCl4(dme)2 (An = Th, U, Np, and Pu) and one equivalent of the trianionic trenpy ligand led to the formation of the respective complexes. SC-XRD measurements confirmed the coordination of three imine and three pyrrolide nitrogens of one trenpy ligand to the metal center in the predicted hexadentate fashion. In addition, a chloro ligand binds to the An(IV), forming a 7-coordinate distorted monocapped octahedron. Analysis of SC-XRD data and quantum chemical calculations revealed different bond lengths and strengths for the different nitrogen donors (Npyrrolide and Nimine) which are slightly longer for the first and slightly shorter for the later case compared to the reported homoleptic bispyren complexes.[3] Remarkably, quantum chemical calculations for Pa(IV)-N revealed no exceptional backbonding effects as seen in [Pa(pyren)2]. We can speculate that the chloride atom indicating an influence of the chlorine atom contributes to the suppression of this effect.

Acknowledgement: This work is supported by the German Federal Ministry of Education and Research (BMBF) under project number 02NUK059B (f-Char).

[1] T. Radoske et al., Chemistry – A European Journal 2020, 26, 16853–16859.
[2] T. Radoske et al., Dalton Transactions 2020, 49, 17559–17570.
[3] L. Köhler et al., Chemistry – A European Journal 2021, 27, 18058–18065.
[4] R. Kloditz et al., Inorganic Chemistry 2021, 60, 2514–2525.