L- and d-[Ln(HCO₂)(SO₄)(H₂O)]_n (Ln = La, Ce, Pr, Nd, and Eu): Chiral enantiomerically 3D architectures constructed by double-[Ln-O]_n-helices

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(HCO ₂)(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(HCO ₂)(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(HCO ₂)(SO₄)(H₂O)]_n, [Pr(HCO ₂)(SO₄)(H₂O)]_n, [Nd(HCO ₂)(SO₄)(H₂O)]_n, and [Eu(HCO ₂)(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.