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.
Two rationally designed 4-nitrophenyl-based molecular clefts functionalized with thiourea (L1) and urea (L2) have been synthesized and studied for a variety of anions by UV-Vis and colorimetric techniques in DMSO. Results from the binding studies suggest that both L1 and L2 bind halides showing the order: fluoride > chloride > bromide > iodide; and oxoanions showing the order: dihydrogen phosphate > hydrogen sulfate > nitrate > perchlorate. Each receptor has been shown to form a 1:1 complex with an anion via hydrogen bonding interactions, displaying distinct color change for fluoride and dihydrogen phosphate in solution. As compared to the urea-based receptor L2, the thiourea-based receptor L1 exhibits stronger affinity for anions due the presence of more acidic thiourea functional groups.
A tripodal-based hexaurea receptor with two clefts (inner cleft and outer cleft) has been studied for spherical halides by 1 H NMR, UV-Vis titrations, and theoretical calculations using density functional theory (DFT). As demonstrated from experimental and computational results, the receptor exhibits cleftinduced binding for halides in a 1 : 2 binding mode, showing the binding strength in the order of fluoride > chloride > bromide > iodide. The strongest affinity for fluoride is attributed due to the best fit of two fluoride anions at the two clefts within the host's cavity. This is also supported by DFT calculations, suggesting that each anion is stabilized by six NH⋅⋅⋅F bonds -one at the inner cleft and other at the outer cleft. In addition, the synthesized receptor, as examined on HeLa cells, has been found to show good biocompatibility.
Results and DiscussionSynthesis: The receptor was synthesized by following the strategy reported earlier. [15] Attempts to obtain suitable crystals for X-ray analysis were unsuccessful.
In the title compound, C6H10N2
2+·2NO3
−, the dication lies on a crystallographic twofold rotation axis. The nitrate ions are linked to the dications though N—H⋯O hydrogen bonds, forming a three-dimensional network.
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