Anion-directed self-assembly synthesis, crystal structures and antibacterial activity of nickel(II) complexes derived from 1-(((2-(pyrrolidin-1-yl)ethyl)imino)methyl)naphthalen-2-ol
This study reports the anion-directed self-assembly synthesis, crystal structures, and enhanced antibacterial activities of four novel nickel(II) Schiff base complexes, demonstrating that auxiliary anions effectively regulate coordination motifs and that the resulting trinuclear complex exhibits superior antimicrobial efficacy compared to the free ligand.
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
In the world of chemistry, scientists often study how tiny building blocks called molecules stick together to form larger structures. One popular type of building block is known as a Schiff base. These are organic molecules created by joining an amine, which contains nitrogen, with a carbonyl compound, which contains carbon and oxygen. When they connect, they form a specific bond that acts like a hook, ready to grab onto metal atoms. Metals like nickel are particularly interesting because they can hold these hooks in different shapes and arrangements, creating complex structures that can act as catalysts, sensors, or even medicines. A key question in this field is how to control exactly how these pieces assemble. Researchers have found that the invisible partners accompanying the metal, known as anions, play a surprisingly large role. These anions are not just passive bystanders; they can act like traffic directors, guiding the metal and the organic hooks into specific patterns, deciding whether the final structure will be a single unit or a cluster of several units linked together.
A team of researchers led by Zhong-Lu You at Sichuan University of Arts and Science set out to explore this directing power using a specific organic molecule derived from naphthalene, a substance found in mothballs, and a flexible ring called pyrrolidine. They mixed this organic molecule with nickel salts in a liquid solution, but they changed the type of salt used in each experiment. By swapping the anion that came with the nickel, they watched how the structure of the resulting solid changed. When they used a salt containing acetate, the nickel atoms linked up to form a chain of three metal centers, creating a trinuclear complex. However, when they introduced salts containing thiocyanate or azide, the nickel atoms stayed alone, forming single, isolated units. In a third variation using nitrate, the nickel atoms paired up with two organic molecules each, but the nitrate ions stayed outside, acting only as counterweights to balance the electrical charge rather than touching the metal directly. This simple change in the starting ingredients proved that the anion dictates the final architecture, forcing the nickel into either a multi-atom cluster or a solitary square shape.
The researchers then examined these new crystals under powerful microscopes to see exactly how the atoms were arranged. The first structure, formed with acetate, revealed a symmetrical line of three nickel atoms. The two nickel atoms at the ends were connected to the central one by oxygen atoms from the organic molecules and by oxygen atoms from the acetate groups, which acted like bridges holding the trio together. The central nickel atom sat perfectly in the middle, surrounded only by oxygen atoms, while the outer ones were held by a mix of oxygen and nitrogen. In contrast, the structures formed with thiocyanate and azide showed a single nickel atom in the center of a flat, square arrangement. Here, the organic molecule wrapped around the metal using three points of contact, while the thiocyanate or azide ion filled the fourth spot. The structure made with nitrate was different again; it consisted of a single nickel atom holding two organic molecules, but the nitrate ions did not touch the metal at all. Instead, they floated nearby, held in place by weak electrical attractions to the hydrogen atoms on the organic rings.
Beyond just building these structures, the team wanted to know if they could fight bacteria. They tested the original organic molecule and all four new nickel complexes against four types of bacteria: two that are Gram-positive, which have a thick outer wall, and two that are Gram-negative, which have a thinner wall protected by an extra outer layer. The results showed that the organic molecule alone was not very effective at stopping the bacteria. However, once the nickel was added, the ability to kill bacteria improved significantly. The most powerful agent was the trinuclear complex with the three linked nickel atoms. It stopped the growth of the bacteria at very low concentrations, requiring only 4.7 micromolar to halt the most sensitive strain. The single-unit complexes were also effective, but generally needed higher amounts to achieve the same result. Interestingly, all the compounds worked better against the Gram-positive bacteria than the Gram-negative ones, likely because the extra outer layer of the Gram-negative bacteria made it harder for the chemicals to get inside.
This work demonstrates that scientists can design the shape and size of metal complexes simply by choosing the right anion to start with. The acetate ion acted as a glue, pulling three nickel atoms into a single cluster, while the other ions allowed the nickel to remain solitary. The findings suggest that the number of metal atoms in a structure and the specific way they are held together are crucial for their ability to fight infection. The three-atom cluster proved to be the most potent, hinting that having multiple metal centers working together might be a key factor in biological activity. By understanding how these anions direct the assembly process, researchers can now plan better ways to create new materials with specific shapes and improved medical properties, moving closer to designing custom-made compounds for future use.
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