Effect of coordinating (–CN) vs. non-coordinating (–F) substituents in 3-pyridyl urea receptors toward second sphere sulfate recognition: selective crystallisation of CuSO4 from mixtures of competing anions/cations

Abstract

4-Fluorophenyl and 4-cyanophenyl substituted 3-pyridyl urea, L1111 and L2222 respectively, were synthesized and explored extensively to demonstrate SO42− binding via second sphere coordination in their respective self-assembled structures assisted by Cu2+, Cd2+ and Co2+. A single crystal X-ray diffraction study depicts second sphere SO42− recognition in the C4v symmetric cleft formed by the assembly of four molecules of the Cu2+ complex of L1111, [CuL11114(H2O)2]SO4, (1), via eight N–H⋯O interactions from four urea moieties. Similarly, second sphere SO42− recognition via twelve hydrogen bonding interactions (N–H⋯O and C–H⋯O) is also demonstrated in the assembly of the Cd2+ complex of L1111 i.e. [CdL11114(H2O)2]SO4, (2). Detailed structural analysis of 1 and 2 shows the formation of the coordination complexes of Cu2+ and Cd2+ with four units of pyridyl urea, which further assemble to generate a suitable coordination environment for the recognition of SO42− via second sphere coordination. On the other hand, L2222 with a coordinating substituent shows second sphere SO42− coordination with eleven hydrogen bonding interactions (N–H⋯O and C–H⋯O) via the formation of a Cu2+ assisted 1D coordination polymer [{CuL22224}SO4]α, (3). The coordinating property of the –CN of L2222 is reflected in 3, as it directly coordinates to the Cu2+ that assists the formation of the 1D coordination polymer. Second sphere coordination of SO42− in the assembly of L2222 is also established with Co2+ in its complex, [CoL22222(H2O)4]SO4, (4). However, in this case the metal ion prefers to form a coordination complex, as observed in 1 and 2, instead of the 1D-polymeric network in 3. The solution state UV-Vis studies of L1111 with various copper salts, such as Cu(ClO4)2, CuSO4, Cu(NO3)2, CuCl2 and their mixtures show the selective formation of 1. Cu2+ selective second sphere coordination of SO42− in solution is also demonstrated by UV-Vis studies of the complex isolated from the mixtures of various Cu2+ salts or SO42− salts of different metal ions. Furthermore, the selective formation of complex 1 is also demonstrated when complexation of CuSO4 is carried out with mixtures of L1111 and L2222 in MeOH–H2O.