Comparative Analysis of Tetravalent Actinide Schiff Base Complexes: Influence of Donor and Ligand Backbone on Molecular Geometry and Metal Binding

A series of isostructural early actinide An complexes was synthesized in order to investigate the influence of a conjugated framework in the ligand backbone on An bonding. Therefore, the An complexes [An(pyrophen)] (An = Th, U, Np, and Pu) with the pure N-donor ligand bis(2-pyrrolecarbonylaldehyde)-o-phenylenediamine referred to as pyrophen, were synthesized and characterized. Solid state analysis via single-crystal X-ray diffraction (SC-XRD) reveals two sets of ligands binding in an almost orthogonal arrangement to the actinide center. For the larger actinides Th and U, the coordination sphere allows for additional coordination by a solvent molecule. Nuclear magnetic resonance spectroscopy (NMR) studies show the presence of highly symmetrical complexes in solution in good agreement with the solvent-free solid structures. While SC-XRD suggests mainly ionic binding, an analysis of paramagnetic NMR contributions and quantum chemical bond analysis hint towards significant covalency in the U, Np, and Pu compounds. This series of An complexes allowed for a thorough structural and theoretical comparison of a conjugated system to a closely related N-donor ligand (pyren), as well as to the mixed N,O Schiff base ligands salophen (conjugated) and salen (non-conjugated).