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

A pentafluorophenyl (–C6F5) substituted 3-pyridyl urea, L(1), is explored extensively to demonstrate SO4(2-) binding exclusively via second sphere coordination in the cavity of a 1D polymeric self-assembly of L(1), selectively assisted by Cu2+. A single crystal X-ray diffraction study depicts SO4(2-) encapsulation in the C2 symmetric cleft via nine hydrogen bonding interactions contributed by eight urea protons of four L(1) moieties in [CuL(1)4(DMF)2]SO4 (1). To revalidate the importance of Cu2+ selective anion coordination via exclusive second sphere coordination, a complex of L(1) and Cu(NO3)2, i.e. [CuL(1)4(H2O)2](NO3)2 (2), is also isolated and characterized by a single crystal X-ray diffraction study. When SO4(2-) salts of different metal ions such as Co2+/Ni2+ are employed, the first sphere coordination of SO4(2-) is observed in cases of complexes [CoL(1)3(DMF)2SO4] (3) and [NiL(1)3(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 L(1) towards such recognition of SO4(2-) in 1, the phenyl (–C6H5) analogue of L(1), i.e.L(2), is allowed to complex with the SO4(2-) salt of Cu2+. Interestingly, L(2) shows first sphere SO4(2-) coordination in the complex [CuL(2)2(DMF)(H2O)2SO4] (5). Solution state UV-Vis experiments of L(1) 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, Cu(2+) selective second sphere coordination of SO4(2-) in solution is also demonstrated by UV-Vis studies of complexes isolated from the mixtures of various Cu(2+) salts and/or SO4(2-) salts of different metal ions.