Synthesis of Differently Substituted tacn-Based Ligands: Towards the Control of Solubility and Electronic and Steric Properties of Uranium Coordination Complexes

Starting from phenols R1,R2ArOH (5) and the anisole derivative 3,5-di-tert-butyl-2-methoxybenzyl bromide (13), a series of new tacn-based ligands (R1,R2ArOR3)3tacn (2) have been synthesized with substituents of varying bulkiness and electronic nature at the ortho and para positions with respect to the oxygen coordination site. It was observed that these groups not only determine the steric shielding and solubility properties of 2, but also deactivate the reactivity of the phenols in the modified Mannich reaction when electron-withdrawing groups are introduced at the para position in 5. Treatment of the ligands with UCl4 in thf led to the isolation of four uranium(IV) chloro complexes [{(R1,R2ArO)3tacn}UIVCl] (14), which were characterized by different spectroscopic and physical methods (e.g., 1H NMR, UV/Vis, SQUID), corroborating the +4 oxidation state in 14. Single-crystal X-ray structure analyses revealed that 14a center dot CH2Cl2 center dot CH3CN and 14b center dot 1.25CH2Cl2 crystallize in the chiral, orthorhombic space group P212121 [a = 24.934(3), b = 27.941(3), c = 9.045(1) angstrom, V = 6302(2) angstrom 3, Z = 4] and the chiral, hexagonal space group P63 [a = 22.097(3), c = 17.941(2) angstrom, V = 7587(2) angstrom 3, Z = 4], respectively. Interestingly, complexes 14a,b self-organize in the solid state into homochiral 1D polymeric superstructures as a result of weak intermolecular CH center dot center dot center dot Cl contacts.