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Synthesis, crystal structure and Hirshfeld surface analysis of Schiff base Bis ((1-chlorophenyl) methylene) carbonohydrazide

This study reports the synthesis and crystal structure of the Schiff base bis((1-chlorophenyl)methylene)carbohydrazide, revealing a non-planar monoclinic arrangement stabilized by dominant O...H-N hydrogen bonds and specific halogen interactions, which suggests its potential as an effective chelating agent in coordination chemistry and materials science.

Original authors: Christian Jelsch, Abdoulaye Diatta, Mbaye Diagne Mbaye, Ibrahima Goudiaby, Insa Badiane, Emmanuel Wenger, Magatte Camara

Published 2026-07-21

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CC BY 4.0

Original authors: Christian Jelsch, Abdoulaye Diatta, Mbaye Diagne Mbaye, Ibrahima Goudiaby, Insa Badiane, Emmanuel Wenger, Magatte Camara

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the microscopic world of chemistry not as a collection of boring formulas, but as a giant, bustling dance floor where molecules are constantly looking for a partner. Some molecules are like shy introverts who only want to hold hands with specific types of people, while others are the life of the party, sticking to anyone nearby. This is the world of crystallography, the science of figuring out exactly how atoms arrange themselves into solid structures. To understand this, you need to know about Schiff bases. Think of these as molecular "snap-together" toys made by smashing a specific type of amine (a nitrogen-rich molecule) into an aldehyde (a carbon-based molecule). When they click, they form a strong double bond, creating a new shape that is often very stable and good at grabbing onto metal atoms, like a magnet. Scientists love these because they can be used to clean up toxic waste, fight bacteria, or even help build new materials. But here's the tricky part: just because you know what a molecule is doesn't mean you know exactly how it stands when it freezes into a crystal. Does it lie flat? Does it twist? Who is hugging whom? That's the mystery this paper sets out to solve.

In this study, a team of chemists from France and Senegal decided to take a closer look at a specific Schiff base called bis((1-chlorophenyl)methylene) carbonohydrazide. They started by mixing two ingredients: a molecule called carbonohydrazide and another called 1-chlorobenzaldehyde. They heated this mixture up in a flask for a full day, creating a white powder. Then, they dissolved that powder in a solution containing nickel chloride and let it sit quietly for a week. Slowly, beautiful, clear crystals grew, ready for a deep dive. Using a powerful X-ray machine, the researchers peered inside these crystals to see exactly how the atoms were arranged. They found that the molecule doesn't lie flat like a pancake; instead, it twists and turns into a 3D shape. This happens because of the chlorine atoms attached to the rings, which act like little bumpers, forcing the molecule to twist out of alignment.

The real magic of the paper comes from a special technique called Hirshfeld surface analysis. Imagine wrapping the molecule in a transparent, invisible balloon that shows exactly where it touches its neighbors. By looking at this "balloon," the scientists could map out every single handshake, hug, and bump between the molecules in the crystal. They discovered that the most important hugs are hydrogen bonds, specifically between oxygen and hydrogen atoms. These are the "super-glue" holding the crystal together. The paper suggests that these strong bonds, along with some weaker "van der Waals" forces (which are like gentle, fuzzy touches between atoms), create a stable structure. Interestingly, the crystal organizes itself into layers: the "hydrophobic" (water-fearing) parts of the molecule stick to other water-fearing parts, while the "hydrophilic" (water-loving) parts cluster together. It's like a dance floor where the cool kids only dance with the cool kids, and the energetic kids only dance with the energetic kids, keeping the groups separate but the whole party stable.

The researchers also looked at the electrical charges on the surface of the molecules to see if they fit together like puzzle pieces. They found that for the most part, the positive and negative areas lined up perfectly, creating a strong attraction. However, they noticed one specific interaction involving a chlorine atom and a hydrogen atom that was a bit odd. The angle between them was about 164.4°, which is close to a straight line but not quite perfect. Because of this, the positive side of the chlorine was facing the positive side of the hydrogen, which isn't the most attractive combination. Despite this, the overall structure remains very stable. The paper concludes that this specific Schiff base is a robust molecule with a clear separation between its different chemical "personalities." While the authors suggest this structure could make it a good candidate for grabbing onto metal ions (acting as a chelating agent) or being used in new materials, they present these as promising possibilities based on the structural data, rather than proven industrial applications. The study successfully maps out the molecular architecture, showing us exactly how these twisted, chlorine-wearing molecules hold hands to build a solid crystal.

Technical Summary: Synthesis, Crystal Structure, and Hirshfeld Surface Analysis of Bis((1-chlorophenyl)methylene)carbonohydrazide

Problem and Context
Schiff bases, formed by the condensation of primary amines with aldehydes or ketones, are widely studied for their stability, ease of synthesis, and coordination properties, particularly in pharmaceutical applications and heavy metal recovery. While their potential as chelating agents and catalysts is well-recognized, industrial application is often limited by issues such as ligand degradation, demetallization, and the need for more efficient, low-cost processes. Understanding the precise intermolecular interactions that stabilize these molecules in the solid state is crucial for predicting their behavior in coordination chemistry and materials science. This study focuses on the specific Schiff base, bis((1-chlorophenyl)methylene)carbonohydrazide, to evaluate its structural characteristics and interaction profile.

Methodology
The research employed a combination of synthetic chemistry, single-crystal X-ray diffraction, and advanced computational surface analysis:

Synthesis: The target ligand was synthesized via the condensation of carbonohydrazide with 2-chlorobenzaldehyde in ethanol under reflux for 24 hours. Colorless crystals suitable for analysis were obtained through slow evaporation of a solution containing the ligand and nickel chloride over one week.
Crystallography: X-ray diffraction data were collected at 298 K using a Bruker SMART X2S diffractometer with MoKα radiation. The structure was solved by direct methods (SHELXT) and refined by full-matrix least-squares (SHELXL).
Surface Analysis: The intermolecular interactions were analyzed using Hirshfeld surface calculations (via CrystalExplorer17 and MoProViewer). This included 2D fingerprint plots, contact enrichment ratios ( $E_{XY}$ ), and electrostatic potential mapping ( $V_{int}$ and $V_{ext}$ ) to visualize electrostatic complementarity.

Key Results

Crystal Structure: The compound crystallizes in the monoclinic system with space group $P2_1/c$ . The molecule adopts a non-planar conformation due to the ortho-chlorine substituents on the aromatic rings, which induce significant torsion angles (dihedral angles between 60° and 80° relative to the rest of the molecule).
Intermolecular Interactions:
- Hydrogen Bonding: The crystal packing is primarily stabilized by strong O...H-N hydrogen bonds. Specifically, bifurcated interactions occur between the carbonyl oxygen ( $C=O$ ) acceptor and two $N-H$ donors. A secondary $N-H...N$ interaction is also present but less dominant.
- Hirshfeld Surface Analysis: The 2D fingerprint plots reveal that $O...H$ contacts are the most abundant and strongly enriched ( $E > 10$ ). Other significant contacts include $Cl...H$ (17.4% of the surface) and $C...C$ (14.39%), the latter attributed to aromatic stacking.
- Hydrophobic/Hydrophilic Separation: The analysis shows a clear segregation of contacts. Hydrophilic interactions ( $O, N, H-n$ ) and hydrophobic interactions ($C, H-c, Cl$) are largely separated in the crystal stacking, with hydrophilic contacts showing a high enrichment ratio ( $E = 5.94$ ).
Electrostatic Complementarity: Electrostatic potential mapping indicates that electronegative regions (around oxygen and non-protonated nitrogens) face electropositive regions (around $N-H$ groups). However, the study notes a specific interaction, $Cl1B...H4A$ (distance 2.828 Å), which appears as a reciprocal spike in the electrostatic energy density plot but falls in the $(+,+)$ quadrant. This indicates the interaction is not electrostatically attractive, as the electropositive sigma-hole of the chlorine faces the electropositive hydrogen, despite the angle ( $164.4^\circ$ ) being close to linear.

Significance and Claims
The paper claims that the synthesized Schiff base exhibits a robust crystal structure stabilized by a combination of strong hydrogen bonds, van der Waals forces, and specific halogen interactions. The detailed characterization of the electrostatic complementarity and contact enrichment provides a clear picture of how the molecule organizes in the solid state.

The authors suggest that these structural characteristics—specifically the stability of the ligand and its defined interaction profile—indicate that the compound could serve as an effective chelating agent. The clear separation of hydrophobic and hydrophilic regions within the crystal highlights potential for selective molecular recognition. Consequently, the study posits that this Schiff base shows promise for applications in coordination chemistry and the design of functional materials, contributing to the broader understanding of structure-property relationships in this class of compounds.

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