Selective recognition of sulphate in a Cu(II) assisted 1D polymer of a simple pentafluorophenyl substituted pyridyl-urea via second sphere coordination

Abstract

A pentafluorophenyl (–C₆F₅) substituted 3-pyridyl urea, L¹, is explored extensively to demonstrate SO₄²⁻ binding exclusively via second sphere coordination in the cavity of a 1D polymeric self-assembly of L¹, selectively assisted by Cu²⁺. A single crystal X-ray diffraction study depicts SO₄²⁻ encapsulation in the C₂ symmetric cleft via nine hydrogen bonding interactions contributed by eight urea protons of four L¹ moieties in [CuL¹₄(DMF)₂]SO₄ (1). To revalidate the importance of Cu²⁺ selective anion coordination via exclusive second sphere coordination, a complex of L¹ and Cu(NO₃)₂, i.e. [CuL¹₄(H₂O)₂](NO₃)₂ (2), is also isolated and characterized by a single crystal X-ray diffraction study. When SO₄²⁻ salts of different metal ions such as Co²⁺/Ni²⁺ are employed, the first sphere coordination of SO₄²⁻ is observed in cases of complexes [CoL¹₃(DMF)₂SO₄] (3) and [NiL¹₃(DMF)₂SO₄] (4) respectively. These results clearly suggest the importance of Cu²⁺ towards anion recognition via purely second sphere coordination in the case of complexes 1 and 2. To understand the importance of (–C₆F₅) substitution in the design of L¹ towards such recognition of SO₄²⁻ in 1, the phenyl (–C₆H₅) analogue of L¹, i.e.L², is allowed to complex with the SO₄²⁻ salt of Cu²⁺. Interestingly, L² shows first sphere SO₄²⁻ coordination in the complex [CuL²₂(DMF)(H₂O)₂SO₄] (5). Solution state UV-Vis experiments of L¹ with various copper salts such as Cu(ClO₄)₂, CuSO₄, Cu(NO₃)₂, CuCl₂ and CuBr₂ in DMF show the formation of a binary complex corresponding to 1. Further, Cu²⁺ selective second sphere coordination of SO₄²⁻ in solution is also demonstrated by UV-Vis studies of complexes isolated from the mixtures of various Cu²⁺ salts and/or SO₄²⁻ salts of different metal ions.