Behind the Science: Cobaltocenium Carboxylate as a Ligand

Behind the Science: Cobaltocenium Carboxylate as a Ligand

Author: Danielle Flemming, Benno Bildstein

Danielle Flemming, Zeitschrift für anorganische und allgemeine Chemie, talks to Professor Benno Bildstein about his article on sandwich complexes as ligands.

Professor Benno Bildstein and his group investigated cobaltocenium carboxylate transition metal complexes. In contrast to ferrocene and its derivatives, much less is known of the isoelectronic cobaltocenium salts despite of their advantageous properties e.g., high chemical stability and reversible redox chemistry.

Compared with neutral ferrocenes, cobaltocenium salts are positively charged and are thus water soluble, which makes them potentially useful for applications in green chemistry or as functional hybrid materials. They are, however, difficult to synthesize due to their cationic core and are only scarcely reported on. Cobaltocenium carboxylate is especially interesting because it represents a unique zwitterionic carboxylate. The results show that its transition metal complexes have a rich structural and medicinal chemistry, different to normal carboxylates.

Professor Bildstein, what was the inspiration behind this study?

This research is based on our general interest in functionalized cobaltocenium compounds, a rather neglected class of stable metallocenes. Cobaltocenium salts share many advantageous properties with their isoelectronic ferrocenyl derivatives, e.g., high chemical stability, diamagnetism, reversible redox behavior. However, due to the cationic charge of cobaltocenium salts, their synthesis is more challenging. For this reason, their chemistry is much less developed in comparison to that of ferrocene.

Cobaltocenium carboxylate is an especially interesting compound because it represents a unique zwitterionic carboxylate that should exhibit different coordination chemistry and reactivities compared with “normal” uninegative carboxylates.

Why was your attention focused on cobaltocenium?

We have a long-standing interest in functionalized metallocenes and other sandwich complexes of early to late transition metals, aiming at generation of new organometallic compounds with useful new chemical or physical properties. For this study, we wanted to exploit the cobaltocenium moiety as a highly stable, strongly electron-accepting substituent that alters normally observed properties and reactivities of attached functional groups, in this specific case carboxylate.

How long did this investigation take?

Overall, it took us over one year to synthesize and characterize these compounds, as well as to investigate their reactivity and cytotoxicity.

What is the broader impact of this paper for the scientific community?

Hopefully, this paper will foster the development of cobaltocenium chemistry. We strongly believe that there are many opportunities in metallocene chemistry besides the very well explored ferrocene chemistry.

How will you follow up on this discovery?

We are currently exploring alternative approaches to the so far elusive cobaltocenium-carbene complexes. In parallel work, we are investigating the formally analogous chemistry of rhodocenium carboxylate, a compound with distinctly different redox properties that might prove useful for our goals.



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