Assembly of a helical zinc(II) chain and a two-dimensional cadmium(II) coordination polymer using picolinate and sulfate anions as bridging ligands

Abstract

The initial use of the picolinate (pic −)/sulfate “blend” in metal chemistry has yielded two neutral coordination polymers. Complexes [Zn 2(SO 4)(pic) 2(H 2O) 3] n ( 1) and [Cd 2(SO 4)(pic) 2(H 2O) 3] n · nH 2O ( 2 · nH 2O) were prepared by the reaction of MSO 4 · xH 2O [M = Zn, x = 7; M = Cd, x = 8/3] with one equivalent of picolinic acid (picH) in aqueous solution in the absence of external base. The structures of both compounds have been determined by single-crystal X-ray crystallography. Complex 1 consists of helical chains which interact through hydrogen bonds and stacking interactions to form a 3D, 5-connected network. Complex 2 · nH 2O consists of ribbons that are crosslinked by the sulfato ligands to give a 2D, 4-connected network. Hydrogen bonds and stacking interactions lead to the formation of a 3D supramolecular architecture. In both coordination complexes, the pic − ligand adopts two different bridging coordination modes (2.111 and 2.201 in 1; 2.211 and 2.201 in 2 · nH 2O), while the SO 4 2 - behaves as a bidentate bridging 2.1100 (Harris notation) ligand. The Zn(II) complex has 5- and 6-coordinate metal ions, whereas 6- and 7-coordinate Cd II atoms are present in 2 · nH 2O. Characteristic vibrational (IR, Raman) bands are discussed in terms of the known structures, and the coordination modes of the pic − and SO 4 2 - ligands. The thermal decomposition of the Cd(II) compound has also been studied.