Home R & D Chem Recognition of Sulfate, Phosphate, and Arsenate Anions by Receptor 2

Recognition of Sulfate, Phosphate, and Arsenate Anions by Receptor 2

Recognition of Sulfate, Phosphate, and Arsenate Anions by Receptor 2

Receptor 2, synthesized through a series of chemical reactions, demonstrates the ability to recognize sulfate, phosphate, and arsenate anions in both the solid-state and solution state. Single-crystal X-ray diffraction analysis reveals the structural details of the complexes formed between receptor 2 and the anions.


  • 🔬 Receptor 2 is synthesized through a multistep process involving methylation of N,N′-di(4-nitro)phenylurea, followed by hydrogenation and conversion into isocyanates, and subsequent reaction with 4-n-butylaniline.
  • 🧪 The solid-state complex of receptor 2 with sulfate anion (SO42–) forms a trimeric assembly encapsulating the anion, with two tetra-n-butyl ammonium (TBA) cations present outside the complex for charge balance.
  • 📐 Single-crystal X-ray diffraction analysis reveals that receptor 2 adopts a cleft conformation and forms one-dimensional channels in the crystal lattice, with a pore diameter of approximately 5 × 5 Å.
  • 🧪 Receptor 2 also forms similar trimeric assemblies with phosphate (PO43–) and arsenate (AsO43–) anions in the solid state, with TBA cations present outside the complexes.
  • 💧 In solution state, receptor 2 shows affinity toward sulfate, phosphate, and arsenate anions as demonstrated by 1H NMR titration studies. The binding events are observed in different solvent systems (CDCl3/DMSO-d6, CD3CN/DMSO-d6, and DMSO-d6/H2O).

Receptor 2 is synthesized through a series of chemical reactions involving the methylation of N,N′-di(4-nitro)phenylurea to obtain the desired intermediate. The nitro groups are then hydrogenated into amines and further converted into isocyanates. Subsequent reactions with 4-n-butylaniline lead to the formation of receptor 2 in good yields.

In the solid-state, the crystal structure analysis reveals that receptor 2 forms trimeric assemblies with sulfate, phosphate, and arsenate anions. The structures show a propeller shape with an interior binding pocket surrounded by six urea groups. The oxyanions are stabilized through strong N–H···O hydrogen bonds from the urea groups, resulting in 3:1 host–guest complexes. The trimeric assemblies encapsulate the anions, and TBA cations are present outside the complexes for charge balance.

In the solution state, 1H NMR titration studies demonstrate the affinity of receptor 2 towards sulfate, phosphate, and arsenate anions. The urea N–H resonances in the NMR spectra shift and merge upon the addition of the anions, indicating the formation of host–guest complexes. The binding events are observed in different solvent systems, such as CDCl3/DMSO-d6, CD3CN/DMSO-d6, and DMSO-d6/H2O, although the affinities cannot be accurately measured due to intermediate-to-slow exchange and broadness of the urea N–H resonances.

The study provides structural evidence of receptor 2’s ability to recognize sulfate, phosphate, and arsenate anions, both in the solid-state and solution state, expanding the understanding of organic receptors for anion recognition. The differences in coordination around the anions are influenced by their ionic radius, with subtle variations in size leading to distinct binding modes.

Cleft Receptor for the Recognition and Extraction of Tetrahedral Oxyanions

Biswas; Ghorai; Paul; Maji; Natarajan 

Full text link: https://doi.org/10.1021/acs.cgd.3c00167

What this paper is about

  • Sulfate anions are formed in the aqueous nuclear waste stream by various processes, such as treating nuclear water material with sulfuric acid. This leads to a high concentration of sulfate anions in water, which are difficult to remove from the aqueous solution.
  • Furthermore, structural evidence for arsenate coordination through hydrogen bonding from charge-neutral organic receptors is still limited with one example for HAsO and two examples for AsO 3 anions.
  • Herein, we report the design and synthesis of a readily accessible bis-urea cleft receptor and its anion recognition chemistry with sulfate, phosphate, and arsenate anions.

What you can learn

  • Based on the similarity of the resonance positions of the urea NH groups to those observed in the titration with TBA salt of PO 3 which indicated the formation of a 3:1 complex, we infer that the current results also correspond to a 3:1 complex formation.
  • The binding affinity was found to be 3.43 10 7 M 3 Further, the resonances for the aromatic protons also shifted for the formation of the complex with one of the protons resonating at = 5.0 ppm.
  • Taken together, as summarized in, we performed the 1 H NMR binding analysis of 2 against TBA and Na salts of SO 2 PO 4 3 and AsO 4 3 in different solvent systems.
  • In conclusion, we have reported a synthetic receptor that selectively binds to tetrahedral oxyanions, such as sulfate, phosphate, and arsenate in the solid state.
  • The receptor forms a propeller-shaped trimeric assembly with three oxyanion guests, as confirmed by X-ray crystallography. This is a rare example of a synthetic receptor that recognizes the arsenate anion in particular.
  • We conducted further studies in solution to investigate the receptor’s ability to recognize with arsenate over phosphate and sulfate in a polar medium.

Q: What is Receptor 2?

A: Receptor 2 refers to a specific organic molecule that is synthesized through a series of chemical reactions.

Q: What are sulfate, phosphate, and arsenate anions?

A: Sulfate, phosphate, and arsenate anions are negatively charged ions commonly found in chemical compounds. They play important roles in various chemical processes and can be recognized and bound by Receptor 2.

Q: What is solid-state and solution state?

A: Solid-state refers to the physical state of matter where molecules are arranged in a fixed and ordered manner, typically forming a crystal lattice. Solution state refers to the state of matter where molecules are dispersed and dissolved in a liquid solvent.

Q: What is single-crystal X-ray diffraction analysis?

A: Single-crystal X-ray diffraction analysis is a technique used to determine the three-dimensional arrangement of atoms in a crystal. It involves exposing a single crystal to X-rays and analyzing the resulting diffraction pattern.

Q: What are complexes in this context?

A: Complexes refer to the structures formed when Receptor 2 interacts with sulfate, phosphate, or arsenate anions. These complexes involve the binding of the anions to Receptor 2.

Q: What is methylation of N,N′-di(4-nitro)phenylurea?

A: Methylation of N,N′-di(4-nitro)phenylurea is a chemical process where a methyl group (-CH3) is added to the N,N′-di(4-nitro)phenylurea molecule. It is a step in the synthesis of Receptor 2.

Q: What is hydrogenation and isocyanates?

A: Hydrogenation is a chemical process where hydrogen gas (H2) is added to a molecule. Isocyanates are a class of compounds that contain the functional group -NCO. In the synthesis of Receptor 2, hydrogenation and conversion into isocyanates are steps involved after methylation.

Q: What is 4-n-butylaniline?

A: 4-n-butylaniline is a specific organic compound that reacts with the isocyanates to form Receptor 2 in the synthesis process.

Q: What is a trimeric assembly?

A: A trimeric assembly refers to a structure composed of three identical or similar molecules (in this case, Receptor 2) that come together and interact with each other.

Q: What are tetra-n-butyl ammonium (TBA) cations?

A: Tetra-n-butyl ammonium (TBA) cations are positively charged ions that are present outside the complexes formed between Receptor 2 and the sulfate, phosphate, or arsenate anions. They help maintain charge balance in the system.

Q: What is charge balance?

A: Charge balance refers to the state where the total positive and negative charges in a chemical system are equal, ensuring overall neutrality.

Q: What is cleft conformation?

A: Cleft conformation refers to the specific shape or arrangement of Receptor 2, where it adopts a cleft-like structure with a groove or pocket.

Q: What are one-dimensional channels and crystal lattice?

A: One-dimensional channels are pathways or tunnels within the crystal lattice of Receptor 2, allowing the movement of molecules or ions. Crystal lattice refers to the repeating three-dimensional arrangement of molecules in a solid crystal.

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