Chiral one- to three-dimensional uranyl–organic assemblies from (1R,3S)-(+)-camphoric acid

Abstract

Four complexes were obtained from reaction of uranyl nitrate with (1R,3S)-(+)-camphoric acid under solvo-/hydrothermal conditions with either acetonitrile or N-methyl-2-pyrrolidone (NMP) as the organic component. All complexes crystallize in chiral space groups and are enantiopure species. Complexes [(UO₂)₄(L)₃(OH)₂(H₂O)₄]·3H₂O (1) and [(UO₂)₈K₈(L)₁₂(H₂O)₁₂]·H₂O (2) were obtained in water–acetonitrile in the presence of LiOH or KOH in excess beyond or equal to that simply required to neutralize the acid, respectively. Whereas 1 is a 1D coordination polymer including hydroxide ions resulting from hydrolysis of the uranyl aqua-ion, 2 contains octanuclear uranyl camphorate cages analogous, but for their crystallographic symmetry, to those previously published; these cages are assembled into a 3D framework by bridging potassium ions. The two complexes obtained in water–NMP, [UO₂(L)(NMP)] (3) and [(UO₂)₂Cu(L)₃(NMP)₂] (4), are devoid both of water molecules and any solvent-derived anions, and they crystallize as 2D assemblies. The sheets in 4, with a thickness of ~14 Å, display a central layer of copper(II) ions surrounded by two layers of uranyl ions. These and previous results suggest that solvo-/hydrothermal conditions using NMP provide a new means of avoiding the formation of uranyl-containing oligomeric or 1D polymeric hydrolysis products which are frequent and often unpredictable outcomes in the synthesis of uranyl–organic assemblies under aqueous conditions, especially in the presence of cosolvents which in themselves are susceptible to hydrolysis. The emission spectrum of compound 3 under excitation at 350 nm displays the usual vibronic fine structure in the ~460–600 nm range, while uranyl luminescence is quenched by Cu(II) cations in complex 4.