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}U^IVCl] (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.CH₂Cl₂.CH₃CN and 14b.1.25CH₂Cl₂ crystallize in the chiral, orthorhombic space group P2₁2₁2₁ [a = 24.934(3), b = 27.941(3), c = 9.045(1) Aa, V = 6302(2) Aa³, Z = 4] and the chiral, hexagonal space group P6₃ [a = 22.097(3), c = 17.941(2) Aa, V = 7587(2) Aa³, Z = 4], respectively. Interestingly, complexes 14a,b self-organize in the solid state into homochiral 1D polymeric superstructures as a result of weak intermolecular C-H..Cl contacts. (Copyright copyright 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)