Three helical chain-based 3D coordination polymers: solvent-induced syntheses, tunable structures and catalytic properties for the Strecker reaction

Abstract

Three three-dimensional (3D) helical chain-based coordination polymers [Sc(aip)(OH)·(H₂O)]_n (1), [Sc₂(aip)₃]_n (2) and [Sc₂(nip)₃]_n (3) (H₂aip = 5-aminoisophthalic acid, H₂nip = 5-nitroisophthalic acid) have been synthesized through solvent induction without any chiral auxiliary systematically. We first synthesized CP 1, a 3D network, which contains intertwined left- and right-hand helical chains {–ScO₆–OH–}_n. By changing the polarity of the solvents from CH₃CN/H₂O to H₂O, a genuine single helix of 2 was constructed, in which adjacent Sc(COO)₆ polyhedra are connected alternately by aip²⁻ ligands to form a 3D framework with intertwined parallel left-hand helical chains {–Sc1(COO)–Sc2(COO)–}_n. Then by changing the substituent of the ligand, CP 3 was obtained with right-handed helical chains. As for 3, the aip²⁻ ligands bridge Sc(III) ions alternately into infinite right-handed helical chains {–Sc(COO)–}_n, which serve as secondary building units (SBU) to generate a 3D framework. Solid-state circular dichroism spectrum confirms that 3 is optically active. We also studied the catalytic properties of 1–3 in the Strecker reaction. Importantly, 1 shows excellent catalytic activity under mild conditions and can be reused at least four times without significant reduction in catalytic ability.