l- and d‑[Ln(HCO<sub>2</sub>)(SO<sub>4</sub>)(H<sub>2</sub>O)]<sub><i>n</i></sub> (Ln = La, Ce, Pr, Nd, and Eu): Chiral Enantiomerically 3D Architectures Constructed by Double −[Ln–O]<sub><i>n</i></sub>– Helices
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A total of 10 three-dimensional chiral coordination compounds l- and d-[Ln(HCO2)(SO4)(H2O)]n (Ln = La, Ce, Pr, Nd, and Eu) have been synthesized without any chiral auxiliary and characterized by IR, thermogravimetric, and elemental analyses. Their structures were determined by single-crystal X-ray structural analysis, which shows that l-[Ln(HCO2)(SO4)(H2O)]n (Ln = La, Ce, Pr, Nd, and Eu) crystallize in space group P43 and are laevogyrate and isostructural. The chiral frameworks of l-[Ln(HCO2)(SO4)(H2O)]n are constructed from l-helical Ln–O cluster chains, while adjacent l-type helical −[Ln–O]n– chains are connected through O–Ln–O linkages to form chiral intertwined Ln–O double helices of left-handedness. d-[Ln(HCO2)(SO4)(H2O)]n crystallize in space group P41, and their chiral frameworks consist of d-helical Ln–O cluster chains. The observed second-harmonic-generation efficiencies of [La(HCO2)(SO4)(H2O)]n, Ce(HCO2)(SO4)(H2O)]n, [Pr(HCO2)(SO4)(H2O)]n, [Nd(HCO2)(SO4)(H2O)]n, and [Eu(HCO2)(SO4)(H2O)]n are 0.7, 0.8, 0.7, 0.5, and 0.7 times that of urea, respectively. It is particularly interesting that [Pr(HCO2)(SO4)(H2O)]n shows good two-photon absorption.
