Lanthanide contraction effect on crystal structures of lanthanide coordination polymers with cyclohexanocucurbit[6]uril ligand
Graphical abstract
The reaction of cyclohexanocucurbit[6]uril with lanthanide ions (La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+, Tb3+, Dy3+, Ho3+, Tm3+ and Yb3+) under hydrochloric acid in the presence of CdCl2 resulted in eleven compounds, which demonstrate interesting lanthanide contraction effect and provide a means of separating lanthanide ions.
Introduction
Recently, lanthanide coordination polymers have attracted increasing interest [1], [2], [3], [4], [5]. The interest is based on two reasons. From a structural viewpoint, the high coordination number and flexible coordination geometry of lanthanides may lead to diversity in crystal structures and provide opportunities to study fascinating topologies [2]. From a more practical standpoint, luminescent and magnetic properties of lanthanide cations may result in functional lanthanide coordination polymers with specific physical properties [3], [4], [5]. Over the past decade, a great variety of lanthanide coordination polymers have been synthesized, and in some cases novel physical properties have been discovered.
Cucurbit[n]urils (n=5–8, 10, abbreviated as Q[n]) are ideal macrocylic ligands for coordinating lanthanide ions, because their portals fringed by hard donor oxygen atoms [6]. Actually, the coordination chemistry of Q[n]s with lanthanide ions has been studied extensively by Fedin, [7] Thuery [8] and other groups [9], [10]. We are involved for many years in the study of cyclohexanocucurbit[6]uril (Cy6Q[6], Fig. 1) [11], [12], [13], a derivative of Q[6], which has been proven to possess stronger binding affinities than Q[6] itself. In previous work, we have systematically studied the coordination behavior of the lanthanide ions towards the Cy6Q[6] in neutral aqueous solution [13]. However, when the reaction of Cy6Q[6] with the Ln(NO3)3 (Ln=La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+, Tb3+, Dy3+, Ho3+, Tm3+ and Yb3+) was carried out under hydrochloric acid in the presence of CdCl2, eleven Cy6Q[6]-based compounds: {Ln(H2O)6Cy6Q[6]}·2(CdCl4)·H3O·xH2O [isomorphous with Ln=La (1), Ce (2), Pr (3) and Nd (4), x=11 (1), 11 (2), 10 (3) and 11 (4)], {Sm(H2O)5Cy6Q[6]}·2(CdCl4)·H3O·10H2O (5), {Ln(H2O)5(NO3)@Cy6Q[6]}·2(CdCl4)·2H3O·xH2O [isomorphous with Ln=Gd (6), Tb (7) and Dy (8), x=8 (6), 6 (7) and 6 (8)] and Cy6Q[6]·2(CdCl4)·2H3O·xH2O [isomorphous with x=10 (9), 10 (10) and 9 (11)], were obtained. Their X-ray determined structures, which are completely different from those obtained in neutral aqueous solution, demonstrate interesting lanthanide contraction effect. More importantly, their structural progression indicates the important potential utility of Cy6Q[6] in separating light and heavy lanthanide ions.
Section snippets
General
The macrocyclic ligand Cy6Q[6] was prepared according to literature methods [11]. All the other chemicals, such as lanthanide nitrate and CdCl2 were obtained from commercial sources and used without further purification. Elemental analyses (C, H, and N) were carried out on a PE 240C elemental analyzer.
Crystal structure determination
Single-crystal X-ray diffraction studies for compounds 1–11 were performed on a computer-controlled Bruker Smart Apex CCD diffractometer equipped with a graphite-monochromated Mo-Kα radiation
Results and discussion
Reaction of the macrocyclic ligand Cy6Q[6] with Ln(NO3)3·nH2O (Ln=La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Tm and Yb) under hydrochloric acid aqueous solution in the presence of CdCl2 resulted in block-shaped crystals of ten compounds. The eleven compounds can be categorized into three groups according to their solid state structures, which may attributed to the effect of lanthanide contraction. In the following, we describe the representatives for each group in detail.
Conclusions
In summary, we have described here ten Cy6Q[6]-based compounds 1–11 synthesized with lanthanide ions (La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+, Tb3+, Dy3+, Ho3+, Tm3+ and Yb3+) under hydrochloric acid aqueous solution in the presence of CdCl2. Compounds 1–8 demonstrate interesting structural progressions, which are mainly ascribed to the lanthanide contraction effect. In the case of compounds 9–11 (group III), no lanthanide ions (Ho3+, Tm3+ and Yb3+) were observed in their solid-state structures.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 21371004 and 20971002) and the Key scientific research projects in Colleges and Universities of Henan Province (Grant No.16A180026).
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