Zinc, cobalt and copper coordination polymers with different structural motifs from picolyl-triazole hybrid ligands

Abstract

A series of one-dimensional coordination polymers, viz. [ZnCl₂L¹]_n (1a: 1a-I and 1a-II), [ZnCl₂L²]_n (1b), [ZnCl₂L³]_n (1c), {[Co(L¹)₂(OH₂)₂]·(ClO₄)₂·(H₂O)₂}_n (2a), {[Co(L²)₂(OH₂)₂]·(ClO₄)₂·(H₂O)₂}_n (2b), {[Cu(L¹)₂(OH₂)₂]·(ClO₄)₂·(H₂O)₂}_n (3a), {[Cu(L²)₂(OH₂)]·(ClO₄)₂·(H₂O)·(MeOH)}_n (3b), and {[Cu(L³)₂(OH₂)]·(ClO₄)₂·(H₂O)₂}_n (3c), and a three-dimensional coordination polymer [Cu₂I₂L³]_n (4c) have been synthesized by self-assembly of 4-picolyl substituted 1,2,3-triazoles, viz.1-(4-picolyl)-4-butyl-1H-1,2,3-triazole (L¹), 1-(4-picolyl)-4-pentyl-1H-1,2,3-triazole (L²) and 1-(4-picolyl)-4-hexyl-1H-1,2,3-triazole (L³), with appropriate metal salts. These coordination polymers have been characterized by single-crystal and powder X-ray diffraction (XRD) and thermogravimetric analyses (TGA). Complexes 3a–c have been subject to EPR analysis. Complexes 1a–c are 1D coordination polymers formed by singly bridging L¹–L³ using their picolyl and 3-positioned nitrogen. Their modes of propagation (zig-zag, helical and wave-like) vary with the length of the alkyl pendant at the 4-position on the triazole moiety. Centro-symmetric polymers 2a and 2b are formed by doubly bridging spacers as 1D chains of 18-membered metallocycles fused at octahedral Co(II) centers with coordinated aqua ligands with extensive water–ClO₄⁻ H-bonding. Cu(II) complex 3a (space groupP2₁/n) is isostructural with 2a, but 3b and 3c crystallize with a chiral space group (P2₁) with square pyramidal Cu(II) doubly bridged by L² and L³ to give 1D macrocyclic chiral chains. The spacer in 4c shows uniquely high coordination ability by engaging the donor functions of the nitrogen, not only at the picolyl and 3-position but also at the 2-position of the triazole. The resultant 3D polymer network is neutral and solvate free and has higher symmetry (space groupI4₁/a) than 4a and 4b. The spontaneous resolution of 3b and 3c is traced to the configurational characteristics of the four ligands and its transfer to the crystal network through space chiral packing. The 1D coordination polymers 1 are thermally most stable, whereas the MeOH-solvated and hydrated perchlorate salt 3b decomposes violently upon heating. Variable temperature photoluminescence (VT-PL) measurements revealed strong low energy (LE) rt emissions for 4a and 4b but not for 4c. The high energy (HE) emissions of 4a–c however increase significantly as temperature decreases. The remarkable variety of structural motifs in these coordination polymers is the result of (a) flexible bonding modes of the picolyl–triazole hybrid ligands, (b) different metal geometry options, (c) halide participation as bridging or capping ligands, (d) possibility of hydrate or solvate coordination and (e) extensive H-bonding involving anionic perchlorate methanol solvate as well as hydrates that are coordinated directly (viz. primary) and indirectly (viz. secondary) to the metal.