An Ideal C₃-Symmetric Sulfate Complex: Molecular Recognition of Oxoanions by m-Nitrophenyl- and Pentafluorophenyl-Functionalized Hexaurea Receptors

Abstract

The anion-binding properties of two tripodal-based hexaureas appended with the m-nitrophenyl (1) and pentafluorophenyl (2) groups have been studied both experimentally and theoretically, showing strong affinities for sulfate over other inorganic oxoanions such as hydrogen sulfate, dihydrogen phosphate, bicarbonate, nitrate, and perchlorate. The structural analysis of the sulfate complex with 1 reveals that the receptor organizes all urea-binding sites toward the cavity at precise orientations around a tetrahedral sulfate anion to form an ideal C3-symmetric sulfate complex that is stabilized by 12 hydrogen-bonding interactions. The receptor and the encapsulated sulfate are located on the threefold axis passing through the bridgehead nitrogen of 1 and the sulfur atom of the anionic guest. The high-level density functional theory calculations support the crystallographic results, demonstrating that the C3-symmetric conformation of the sulfate complex is achieved due to the complementary NH···O between the receptor and sulfate.

Anion Complexation Studies of 3-Nitrophenyl-Substituted Tripodal Thiourea Receptor: A Naked-Eye Detection of Sulfate via Fluoride Displacement Assay

“…In an effort to design suitable chromogenic thiourea receptors for anion binding, we synthesized a 3-nitrophenyl-based tripodal tris-thiourea ( L ) by reacting tris(2-aminoethyl)amine and 3-nitrophenylisothiocyanate in CH 2 Cl 2 ( Scheme 1 ). A 3-nitrophenyl substituent was attached to the thiourea moiety due to its electron-withdrawing nature, which can provide enhanced H-bonding and chromogenic properties of the receptor, 22 making it suitable for the naked-eye detection of anions.…”

“…The binding constant for fluoride was calculated from the relative changes in the integrated intensity of NH signals for the free receptor and the complex, 22 yielding a binding constant larger than 10 4 M –1 . To determine the selectivity of the receptor, competition experiments were performed in which sulfate was added to the receptor containing 1 equiv of fluoride (C, Figure 4 ) or hydroxide (E, Figure 4 ) in DMSO- d 6 .…”

Cleft‐Induced Ditopic Binding of Spherical Halides with a Hexaurea Receptor

“…Since anions are crtical species in the environment and life, the study of anion binding with chemical receptors remains an active area of research in supramolecular chemistry . In particular, molecular compounds functionalized with urea groups have been shown to effectively bind inorganic anions under neutral conditions, allowing them to be utilized for potential applications such as extractions, transport, catalysis, and antiangiogenic activities relating to certain living cells . A number of tripodal‐based ureas derived from tris(2‐aminoethyl)amine (tren) have been reported recently, showing complementary binding for various anions in solution and solid states .…”

“…In particular, molecular compounds functionalized with urea groups have been shown to effectively bind inorganic anions under neutral conditions, allowing them to be utilized for potential applications such as extractions, transport, catalysis, and antiangiogenic activities relating to certain living cells . A number of tripodal‐based ureas derived from tris(2‐aminoethyl)amine (tren) have been reported recently, showing complementary binding for various anions in solution and solid states . Halides are of special interest because of their relevance in biological and environmental systems .…”

“…In particular, increasing the binding sites to a receptor could lead to an improved binding affinity for a guest . In order to increase the binding sites within a single host, we recently synthesized hexaurea‐based receptors that show an excellent selectivity for sulfate . It was structurally shown that the hexaurea receptor 1 functionalized with m ‐nitrophenyl substituents forms a C 3 symmetric sulfate complex in which the tetrahedral sulfate is encapsulated with twelve NH⋅⋅⋅O bonds.…”