This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Copper has been knocked off the top of a stability ranking it had dominated for decades. Without altering the atoms directly bonded to the metal, a KAIST research team reversed the longstanding trend in which copper generally forms the most stable complexes by tuning only the weak hydrogen bonds in its surrounding environment.
The findings, which appear in the Journal of the American Chemical Society, could open new avenues for selective metal separation and recognition, as well as catalyst design. A research team led by Professor Yunjung Baek of the Department of Chemistry developed a "metal complex"—a structure in which several molecules surround and bond to a central metal—using a ligand based on the flavin framework found in vitamin B2. By tuning the hydrogen bonding around the metal, the team achieved a stability trend that runs opposite to the widely accepted Irving–Williams series.
The Irving–Williams series is an empirical rule that ranks how stably transition metals—such as iron, nickel and copper, which bond with other substances in a variety of ways—bind to surrounding molecules. Among manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) and zinc (Zn), stability is generally known to increase from manganese toward copper, with copper forming particularly stable bonds. This difference has been understood to arise from each metal's electronic structure, meaning how its electrons are arranged.
In other words, which metal forms the more stable complex has long been considered largely determined by the metal's inherent properties. Until now, changing this order typically required either designing a new ligand, the molecule that directly grips the metal, or altering the coordination structure, the way the metal bonds with surrounding molecules. The research team instead focused on hydrogen bonding, a force that acts outside the direct metal-bonding region.
Hydrogen bonds are relatively weak forces between molecules that help hold the surrounding structure in a fixed shape. Using flavin derivatives, versions of flavin with part of their chemical structure modified, the team incorporated different metals ranging from manganese to zinc while ensuring that all the metals shared the same basic coordination geometry. By keeping the basic conditions around each metal identical, the researchers were able to examine what difference hydrogen bonding alone made to each metal's stability.
The results showed that hydrogen bonding specifically blocks the structural change copper needs to become stable. Copper has a distinctive tendency to slightly reshape its surrounding bonding structure into a form that favors its stability, much like a person shifting slightly to find the most comfortable posture. In the structure developed by the team, however, the surrounding hydrogen-bonded framework constrained the geometry around copper, preventing it from adopting its preferred distorted structure.
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