📄 chemistry

Synthesis, characterization and crystal structures of four copper(II) complexes derived from tridentate Schiff bases with urease inhibitory activity

Four novel copper(II) complexes derived from tridentate Schiff bases were synthesized and structurally characterized, revealing that their urease inhibitory activities are significantly enhanced by coordination, with the polynuclear azide- and thiocyanate-bridged complexes exhibiting superior potency compared to their dinuclear halide-bridged counterparts.

Original authors: Yang-Lei Li, Zi-Han Pu, Fei-Yang Xie, Zhong-Lu You

Published 2026-09-08

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CC BY 4.0

Original authors: Yang-Lei Li, Zi-Han Pu, Fei-Yang Xie, Zhong-Lu You

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

Enzymes are the microscopic workhorses of life, specialized proteins that speed up chemical reactions essential for survival. One such enzyme, urease, acts as a molecular machine that breaks down urea, a common waste product found in urine and soil, into ammonia and carbon dioxide. While this process is vital for the nitrogen cycle in nature, an overabundance of urease activity can cause significant problems. In the human body, excessive ammonia production can damage tissues and contribute to infections, while in agriculture, it leads to the rapid loss of nitrogen from fertilizers, polluting waterways and wasting resources. Because of these consequences, scientists have long searched for effective ways to slow down or stop this enzyme. The most common strategy involves designing molecules that fit into the enzyme's active site like a key in a lock, blocking its function. However, finding inhibitors that are both powerful and safe remains a difficult challenge.

In a recent study, researchers explored a different approach by combining organic molecules with copper, a metal known for its ability to interact with biological systems. They focused on a class of organic compounds called Schiff bases, which are flexible structures capable of wrapping around metal ions to form stable complexes. The team hypothesized that by attaching these organic structures to copper, they could create new shapes that might fit into the urease enzyme more effectively than the organic parts alone. To test this, they synthesized four distinct copper-based structures, carefully varying the ingredients to see how small changes in the molecular architecture would affect the enzyme's activity. The goal was not just to make new compounds, but to understand how the specific arrangement of atoms within these molecules influences their ability to act as inhibitors.

The researchers began by creating two organic building blocks, which they then mixed with copper salts and other small helper molecules in a liquid solution. By changing the helper molecules, they guided the copper and organic parts to assemble into four different shapes. Two of the resulting structures formed long, repeating chains where copper atoms were linked together by nitrogen-based bridges. The other two structures formed isolated pairs, where two copper atoms were held together by both a halogen atom and a nitrogen bridge. Using advanced imaging techniques, the team mapped out the exact positions of every atom in these new complexes, confirming that the different helper molecules had successfully directed the assembly into these distinct patterns.

Once the structures were confirmed, the team tested how well each one could stop the urease enzyme from working. They measured the amount of each compound needed to reduce the enzyme's activity by half, a standard metric for potency. The results were striking. The two compounds that formed long chains proved to be exceptionally powerful, requiring only tiny amounts to shut down the enzyme. Specifically, one of these chain-like structures stopped half of the enzyme's activity at a concentration of just 0.2 micromoles per liter, while its partner in the series did so at 0.3 micromoles per liter. These numbers represent a level of effectiveness that far exceeds many existing inhibitors. In contrast, the two compounds that formed isolated pairs were much less potent, needing concentrations around 14 to 15 micromoles per liter to achieve the same effect. Even the free organic parts, before being attached to copper, showed very little ability to inhibit the enzyme on their own.

The study also compared these new copper complexes to a well-known inhibitor called acetohydroxamic acid, which is currently used in research but has limitations. The most effective chain-like copper complex was significantly more powerful than this standard reference, working at a concentration roughly one hundred times lower. The researchers also noted that the specific type of bridge holding the copper atoms together mattered less than the overall shape of the molecule. Whether the chain was held together by nitrogen or by a sulfur-containing group made little difference to the final result, as both performed with similar high efficiency. However, the difference between the chain-like structures and the isolated pairs was profound, suggesting that the way the copper atoms are arranged in space is the critical factor for success.

These findings highlight that the power of these inhibitors comes from the marriage of the organic framework and the copper metal. The copper does not simply act as a passive anchor; its presence and the specific geometry it creates are essential for the molecule to fit tightly into the enzyme's active site. While the study did not determine exactly how these molecules bind to the enzyme at the atomic level, the data clearly shows that the long-chain structures are far superior to the paired structures. This suggests that future designs for urease inhibitors should focus on creating extended, chain-like arrangements rather than isolated clusters. The work provides a clear roadmap for developing more effective tools to control urease activity, potentially leading to better treatments for infections or more efficient agricultural practices.

Technical Summary

Problem Statement
Schiff base ligands are versatile tools for constructing metal complexes with diverse structural architectures and bioactive properties. While transition metal Schiff base complexes are known potential enzyme inhibitors, the specific relationship between the coordination assembly directed by auxiliary anions and the resulting biological activity remains insufficiently explored. Specifically, there is a need to understand how variations in bridging modes (e.g., switching from $\mu$ -1,3 azide to $\mu$ -1,3 thiocyanate, or introducing simultaneous halide and azide bridges) influence the nuclearity and topology of Cu(II) assemblies and whether these structural changes correlate with urease inhibitory efficiency. Urease, a nickel-dependent metalloenzyme, is a significant target for inhibition due to its role in microbial pathogenicity and nitrogen metabolism, yet current inhibitors like acetohydroxamic acid have limitations that drive the search for more efficient alternatives.

Methodology
The study employed a synthetic and characterization approach to investigate four Cu(II) complexes derived from two tridentate Schiff bases: 2-bromo-6-(((2-(dimethylamino)ethyl)imino)methyl)phenol (HL1) and 2-bromo-6-(((3-(methylamino)propyl)imino)methyl)phenol (HL2).

Synthesis: Four complexes were synthesized under mild solution conditions in methanol at room temperature using a 1:1:1 molar ratio of Schiff base, Cu(II) salt, and auxiliary anion.
- Complexes 1 ( $[CuL1(\mu_{1,3}\text{-N}_3)]_n$ ) and 2 ( $[CuL1(\mu_{1,3}\text{-NCS})]_n$ ) were prepared using HL1, CuBr $_2$ , and either sodium azide (NaN $_3$ ) or ammonium thiocyanate (NH $_4$ SCN).
- Complexes 3 ( $[Cu_2(\mu\text{-Br})(L2)_2(\mu_{1,1}\text{-N}_3)]$ ) and 4 ( $[Cu_2(\mu\text{-Cl})(L2)_2(\mu_{1,1}\text{-N}_3)]$ ) were prepared using HL2, NaN $_3$ , and either CuBr $_2$ or CuCl $_2$ .
Characterization: The complexes were characterized by elemental analysis, FT-IR spectroscopy, UV-Vis spectroscopy, molar conductivity measurements, and single-crystal X-ray diffraction.
Biological Assay: Urease inhibitory activity was evaluated using a colorimetric assay with Jack bean urease, phenol red, and urea. Inhibition rates and IC $_{50}$ values were determined and compared against the free ligands (HL1, HL2), CuBr $_2$ , and the reference inhibitor acetohydroxamic acid.

Key Contributions and Results

Structural Diversity: Single-crystal X-ray diffraction revealed that the nature of the Schiff base and the auxiliary anion dictates the nuclearity and bridging topology of the complexes.
- Complexes 1 and 2 formed extended polynuclear Cu(II) architectures. In these structures, the azide (in 1) and thiocyanate (in 2) anions acted as $\mu_{1,3}$ bridges (end-to-end for azide; N,S-bridging for thiocyanate), linking Cu(II) centers into 1D chains. Each Cu(II) ion exhibited a distorted square-pyramidal geometry.
- Complexes 3 and 4 adopted discrete dinuclear Cu(II) structures. Here, the azide anion adopted an end-on ( $\mu_{1,1}$ ) bridging mode, coexisting with a $\mu_2$ -halide bridge (bromide in 3, chloride in 4). This combination resulted in isolated dinuclear units where each Cu(II) center was also five-coordinate with a distorted square-pyramidal geometry.
Spectroscopic Correlation: IR spectra confirmed the coordination modes; the azide stretching frequencies ( $\nu_{as}$ ) were lower for the $\mu_{1,3}$ bridged complex 1 (2044 cm $^{-1}$ ) compared to the $\mu_{1,1}$ bridged complexes 3 and 4 (2077 and 2072 cm $^{-1}$ ), consistent with X-ray data. UV-Vis spectra showed characteristic ligand-centered and ligand-to-metal charge transfer (LMCT) transitions typical of Cu(II)-Schiff base systems.
Urease Inhibitory Activity: Coordination to Cu(II) significantly enhanced urease inhibition compared to the free Schiff bases.
- Complexes 1 and 2 exhibited excellent inhibitory activity with IC $_{50}$ values of $0.2 \pm 0.1$ and $0.3 \pm 0.1$ $\mu$ mol L $^{-1}$ , respectively. These values are superior to the reference inhibitor acetohydroxamic acid (IC $_{50} = 23 \pm 1$ $\mu$ mol L $^{-1}$ ) and CuBr $_2$ (IC $_{50} = 5 \pm 1$ $\mu$ mol L $^{-1}$ ).
- Complexes 3 and 4 showed moderate activity with IC $_{50}$ values of $15 \pm 2$ and $14 \pm 2$ $\mu$ mol L $^{-1}$ , respectively.
- The activity of the free ligands was weak (15–21% inhibition at 100 $\mu$ mol L $^{-1}$ ).

Significance and Claims
The paper claims that the structural variations induced by different auxiliary anions and Schiff base frameworks play a critical role in modulating urease inhibition. Specifically:

Coordination Environment is Decisive: The dramatic difference in activity between the polynuclear complexes (1 and 2) and the dinuclear complexes (3 and 4) suggests that the Schiff base framework and the resulting Cu(II) coordination environment are more influential on inhibitory performance than the specific identity of the bridging anion (azide vs. thiocyanate or bromide vs. chloride).
Structure-Activity Relationship: The study demonstrates that aminoalkyl-substituted Schiff base Cu(II) complexes, particularly those derived from HL1 forming polynuclear architectures, represent highly potent urease inhibitors.
Strategic Insight: The work highlights that while changing bridging modes can generate structurally distinct assemblies, the retention or enhancement of biological activity is heavily dependent on the specific ligand scaffold and metal coordination geometry rather than the bridging anion alone.

The authors conclude that these findings support the development of aminoalkyl-substituted Schiff base Cu(II) complexes as promising candidates for further development as urease inhibitors.

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