Six cobalt(ii), zinc(ii), nickel(ii) and copper(ii) complexes based on bis-benzimidazolyl bidentate ligands with phenolyl ether linkers: synthesis, structural studies and recognition of HSO4−†

Three bis-benzimidazolyl bidentate ligands with phenolyl ether linkers, 1,n-bis[2-(1H-1,3-benzimidazol-1-ylmethyl)phenoxy]alkane (L₁: n = 1, L₂: n = 3, L₃: n = 4), and their six metal complexes were synthesized by the reactions of the respective ligands with metal salts. The six metal complexes included two cobalt(II) complexes ([Co(L₁)(NO₃)₂] (1) and [Co(L₃)(L_A)]_n (6) (L_A = maleate)), two copper(II) complexes ([Cu(L₁)(SO₄)]₂ (4) and [Cu(L₃)(SO₄)]₂ (5)), one zinc(II) complex ([Zn(L₁)(NO₃)₂] (2)) and one nickel(II) complex ([Ni(L₂)(NO₃)₂] (3)). Complexes 1 and 2 had one 16-membered macrometallocycle, and complex 3 had one 18-membered macrometallocycle. In complexes 4 and 5, two SO₄²⁻ joined two macrometallocycles together (16-membered macrometallocycle for 4 and 19-membered macrometallocycle for 5). In complexes 1–5, each macrometallocycle was constructed from one ligand and one metal ion. In complex 6, 19-membered macrometallocycles were connected together by maleate groups (L_A) to form a 1D polymeric chain, where each 19-membered macrometallocycle was constructed from one L₃ and one Co(II) ion. In crystal packings of complexes 1–6, H-bonds, π–π interactions and C–H⋯π contacts participated in the formation of 2D supramolecular layers and 3D supramolecular architectures. The conformations of complexes 1–6 were also analyzed. Additionally, the recognition performance of complex 1 as a chemosensor for HSO₄⁻ was studied through fluorescence spectra, UV/vis spectra, ¹H NMR titrations, HRMS spectra and IR spectra. The large binding constant (1.81 × 10⁴ M⁻¹) showed that there was a strong acting force between 1 and HSO₄⁻, and the low detection limit (1.40 × 10⁻⁷ mol L⁻¹) indicated that the detection of 1 for HSO₄⁻ was sensitive. The experimental results showed that complex 1 can effectively differentiate HSO₄⁻ from other anions.