Saccharified Uranyl Ions: Self‐Assembly of UO₂²⁺ into Trinuclear Anionic Complexes by the Coordination of Glucosamine‐Derived Schiff Bases

Abstract

The reaction of UO2(OAc)2 ⋅ 2H2O with the biologically inspired ligand 2‐salicylidene glucosamine (H2L1) results in the formation of the anionic trinuclear uranyl complex [(UO2)3(μ3‐O)(L1)3]2− (12−), which was isolated in good yield as its Cs‐salt, [Cs]21. Recrystallization of [Cs]21 in the presence of 18‐crown‐6 led to formation of a neutral ion pair of type [M(18‐crown‐6)]21, which was also obtained for the alkali metal ions Rb+ and K+ (M=Cs, Rb, K). The related ligand, 2‐(2‐hydroxy‐1‐naphthylidene) glucosamine (H2L2) in a similar procedure with Cs+ gave the corresponding complex [Cs(18‐crown‐6)]2[(UO2)3(μ3‐O)(L2)3 ([Cs(18‐crown‐6)]22). From X‐ray investigations, the [(UO2)3O(Ln)3]2− anion (n=1, 2) in each complex is a discrete trinuclear uranyl species that coordinates to the alkali metal ion via three uranyl oxygen atoms. The coordination behavior of H2L1 and H2L2 towards UO22+ was investigated by NMR, UV/Vis spectroscopy and mass spectrometry, revealing the in situ formation of the 12− and 22−dianions in solution.

Insights at the molecular level into the formation of oxo-bridged trinuclear uranyl complexes

“…27 It is slightly longer than that observed in the μ 3 –O complex of a related ligand from our previous studies, which has an average U–O bond length of 2.237(12) Å. 18 Although the proton of the central μ 3 –OH ligand of H 2 − could not be observed unambiguously by X-ray diffraction, the bridging oxygen is raised 0.7116(8) Å above the (UO 2 ) 3 -plane to result in an average U–O–U bond angle of 111.59(3)°. This clearly indicates sp 3 -hybridization of the oxygen, as expected for a bridging hydroxo-ligand.…”

“…In a previous study we investigated the coordination behaviour of selected glucosamine-derived Schiff bases towards uranyl ions. 18 While we were able to reliably isolate trinuclear anionic uranyl complexes for these ligand systems, no further species were isolated and details of the precise formation pathways for these trinuclear complexes remained unsolved. As an extension of our previous study, in the present work we investigated the coordination behaviour of the glucosamine-derived Schiff base 1,3,4,6-tetra- O -acetyl- N -(2-hydroxy)-naphthylidene glucosamine (H L (Ac)), an acetylated derivative of the ligand employed in our previous study, 18 towards the uranyl ion.…”

“…18 While we were able to reliably isolate trinuclear anionic uranyl complexes for these ligand systems, no further species were isolated and details of the precise formation pathways for these trinuclear complexes remained unsolved. As an extension of our previous study, in the present work we investigated the coordination behaviour of the glucosamine-derived Schiff base 1,3,4,6-tetra- O -acetyl- N -(2-hydroxy)-naphthylidene glucosamine (H L (Ac)), an acetylated derivative of the ligand employed in our previous study, 18 towards the uranyl ion. From the reaction of H L (Ac) with uranyl acetate we successfully isolated the dinuclear uranyl complex [(UO 2 ) 2 ( L ) 2 ] ( 1 , see Scheme 1).…”

“…Trinuclear uranyl complexes with a variety of ligand systems are well established in the literature and commonly incorporate a central μ 3 –oxo bridge in their structures. 18,24–28 In contrast, only a few examples are known that contain a μ 3 –OH binding motif of the type observed in [(UO 2 ) 3 (μ 3 –OH)( L ) 3 ] − (H 2 − ). 27,29,30 The average U–O bond length in H 2 − of 2.3977(10) Å is comparable to the values reported by Yoshimura et al for their μ 3 –OH complex, that has an average U–O bond length of 2.429(8) Å.…”

“…Similar binding of cations is well established for other anionic trinuclear uranyl-containing complexes. 18,27,28,32…”