KAIST researchers overturn long-held rule on metal binding stability

- Team reverses Irving-Williams series using weak hydrogen bonds, without altering direct metal-binding structure

- Findings expected to advance selective metal separation, catalysis and biomimetic system design

An illustration showing how hydrogen bonds surrounding a metal restrict structural distortion in copper, selectively lowering its stability. [Provided by KAIST]

Copper, long regarded as the most stably binding metal in chemistry, has been knocked off the top of the rankings. Korean researchers succeeded in reversing the long-accepted order of metal binding stability — without modifying the structure that directly coordinates to the metal — by manipulating only the weak hydrogen bonds in the surrounding environment.

KAIST announced Sunday that a research team led by Professor Baek Yun-jung of the Department of Chemistry developed metal complexes using flavin — the core structure of vitamin B2 — and controlled hydrogen bonding around the metal to achieve a binding stability trend opposite to the established Irving-Williams series.

The Irving-Williams series is an empirical rule describing how stably transition metals — including manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) and zinc (Zn) — bind to surrounding molecules. Stability generally increases from manganese to copper, with copper in particular known to form exceptionally stable bonds.

This difference in stability has long been attributed to the intrinsic electronic structure of each metal. Previous attempts to alter the order therefore required either designing new ligands that directly coordinate to the metal or changing the coordination geometry between the metal and its surrounding molecules.

The research team took a different approach, turning their attention to weak hydrogen bonds acting on the outer environment rather than on the part that directly binds to the metal.

Using flavin derivatives with modified chemical structures, the team designed a system in which the coordinating molecules bound identically regardless of which metal — from manganese to zinc — was introduced. This allowed them to isolate the effect of hydrogen bonding on stability while keeping the basic coordination environment constant across all metals.

The hydrogen bonds were found to suppress the subtle structural distortion that copper relies on to gain additional stability. Copper normally achieves extra stability by deforming its surrounding coordination structure into a more favorable geometry, but the hydrogen bonds locked that structure in place, making such distortion difficult.

Copper ultimately lost the stability advantage it had previously enjoyed, and an anti-Irving-Williams trend — the reverse of the conventional series — was realized.

From left: Seo Chang-hyeon, an integrated master's-doctoral student at KAIST; Dr. Nitu Singh; Professor Baek Yun-jung; and Im Ha-neul, an integrated master's-doctoral student. [Provided by KAIST]

The study is significant in demonstrating that the binding stability hierarchy — in effect considered fixed for each metal — can be controlled through subtle changes in the surrounding environment alone.

The findings are expected to apply to the selective capture and recovery of specific metals from mixed-metal environments, as well as to catalyst design aimed at enhancing the reactivity of a target metal. The research is also expected to offer new design strategies for biomimetic systems that replicate the way proteins and enzymes select particular metals — such as iron, copper and zinc — from among a range of options.

"The core significance of this research lies not simply in lowering copper's stability, but in demonstrating that the binding stability order — long considered an intrinsic property of each metal — can be altered through the surrounding environment," Baek said. "We expect these findings to be applied to the design of new chemical systems capable of selectively binding or reacting with a desired metal."

The findings were published in the Journal of the American Chemical Society, an international peer-reviewed journal published by the American Chemical Society.

nbgkoo@heraldcorp.com