Anion binding by metallo-receptors of 5,5′-dicarbamate-2,2′-bipyridine ligands

Abstract

Three 5,5′-dicarbamate-2,2′-bipyridine ligands (L = L¹–L³) bearing ethyl, isopropyl or tert-butyl terminals, respectively, on the carbamate substituents were synthesized. Reaction of the ligands L with the transition metal ions M = Fe²⁺, Cu²⁺, Zn²⁺ or Ru²⁺ gave the complexes ML_nX₂·xG (1–12, n = 1–3; X = Cl, NO₃, ClO₄, BF₄, PF₆, ½SO₄; G = Et₂O, DMSO, CH₃OH, H₂O), of which [Fe(L²)₃ ⊃ SO₄]·8.5H₂O (2), [Fe(L¹)₃ ⊃ (BF₄)₂]·2CH₃OH (7), [Fe(L²)₃ ⊃ (Et₂O)₂](BF₄)₂·2CH₃OH (8), [ZnCl₂(L¹)][ZnCl₂(L¹)(DMSO)]·2DMSO (9), [Zn(L¹)₃ ⊃ (NO₃)₂]·2H₂O (10), [Zn(L²)₃ ⊃ (ClO₄)(Et₂O)]ClO₄·Et₂O·2CH₃OH·1.5H₂O (11), and [Cu(L¹)₂(DMSO)](ClO₄)₂·2DMSO (12) were elucidated by single-crystal X-ray crystallography. In the complexes ML_nX₂·xG the metal ion is coordinated by n = 1, 2 or 3 chelating bipyridine moieties (with other anionic or solvent ligands for n = 1 and 2) depending on the transition metal and reaction conditions. Interestingly, the carbamate functionalities are involved in hydrogen bonding with various guests (anions or solvents), especially in the tris(chelate) complexes which feature the well-organized C₃-clefts for effective guest inclusion. Moreover, the anion binding behavior of the pre-organized tris(chelate) complexes was investigated in solution by fluorescence titration using the emissive [RuL₃]²⁺ moiety as a probe. The results show that fluorescent recognition of anion in solution can be achieved by the Ru^II complexes which exhibit good selectivities for SO₄²⁻.