For the first time, the coordinative bonding forces in a transition metal complex have been investigated at the single-molecule level. The observations should add new insight to our understanding of the physicochemical nature of coordinative bonds. The method is expected to apply generally to a range of transition metal complexes and to make impacts on coordination chemistry and related areas broadly.
Transition metals have become highly important over the last few decades as catalysts in both organic and inorganic chemistry. This trend has renewed the scientific interest in coordination chemistry. Practically all transition metal containing compounds consist of coordination complexes, meaning complexes where a central atom is surrounded by an array of bound molecules (or ligands). A joint international effort led by DTU Chemistry has for the first time produced a method capable of studying the metal-ligand interactions at the single-molecule level directly aqueous media.
Associate Professor Qijin Chi, from the Nano Chemistry Group at DTU Chemistry, has recently reported the results in an article in Nature Communications (4(2013), 2121). The overall approach demonstrated in this work represents a remarkable advancement in studying coordination chemistry at a new level.
Osmium, an important transition metal used for many catalytic purposes, was chosen as the metal, while the ligand was terpyridine. Terpyridine, or “terpy” for short, is an organic compound known to be highly stable and therefore a well known ligand in coordination chemistry. The coordination and bond breaking between terpy and osmium was followed by electrochemically controlled atomic force microscopy (AFM) at the single-molecule level.
Surprisingly weak bonding
A new finding revealed by the innovative setup is the fact that the metal-ligand coordinative bonding strength was found to be only about 5 % of that for a covalent bond. A covalent bond, in which two atoms share two electrons simultaneously, is the strongest type of bond in chemistry.
“Traditionally, chemists have viewed coordinative bonds as a special type of covalent bonds. Thus, it would be surprising if they may in fact be much weaker. However, this could be explained by the coordinative bond breaking representing ligand substitution in aqueous solution rather than simple dissociation,” says Associate Professor Qijin Chi.
If, as it seems, coordinative bonds are quite weak at the single-molecule level, a natural question is what accounts for the well-known coordination chemistry strength at the larger scale.
“We could speculate that the strength comes from the compound being very stable or from the interface with the surrounding material where terpy is only partially coordinated, but really that should be the subject of further research. At this point the main thing is that we have shown some very basic chemistry to be quite different from what was anticipated. I think people need a little time to accept this. And once they do, I am sure we would begin to see new science that can explain the mechanisms behind our findings,” Qijin Chi remarks.
Academic resources were pooled
Another remarkable finding from the study was the fact that the redox state of osmium had a clear effect on the strength of the coordinative bonding to terpy. The setup allowed control of the redox state of the transition metal complex and record of force spectra simultaneously. The data showed higher bonding strengths for Os(III) than for Os(II).
“The DFT computation has proven the experimental observation. However, the exact reasons behind this difference are not yet fully understood. The overall approach demonstrated in the project has raised several interesting questions. After all, our experiments have shown what happens at the single-molecule level, but not why it happens. Hopefully, our observations will spur further insight into the physiochemical nature of coordinative bonds.”
Research groups from State Key Laboratory of Electroanalytical Chemistry (China), Chalmers University of Technology (Sweden), DTU Physics and DTU Chemistry have joined forces to achieve the breakthrough.
“It is hopeful that single-molecule studies would give you new findings. Therefore, several groups – including our own – have tried to accomplish such studies before, but the efforts were unsuccessful until we gained our academic resources together,” explains Qijin Chi.
“The method is expected to apply generally to other transition metal complexes and to impact coordination chemistry and related areas broadly. And while this is fundamental research, it is highly possible that such findings will spur development of more efficient compounds for use in catalysts, for instance in energy production, organic chemistry and other applications.”