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 C_4v 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(H₄O)₂]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.