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Metal complexes as anion receptors

Final Activity Report Summary - ANION RECEPTORS (Metal complexes as anion receptors)

Coordination Chemistry studies compounds consisting of a central metal atom, usually bearing positive charge (a cation), bonded to a number of groups, either neutral molecules or negatively charged groups (anions), called ligands. These compounds are referred to as coordination compounds or complexes. Their number is very large, perhaps comparable to the number of organic compounds, and they exist for every metal known, making coordination chemistry one of the wider branches of Inorganic Chemistry. The phase of Coordination Chemistry that deals with ligands that bind the central metal through a carbon atom is known as Organometallic Chemistry, and bridges the traditional disciplines or Organic Chemistry and Inorganic Chemistry.

After decades of work, Coordination Chemistry and Organometallic Chemistry evolved from the initial stage of curiosity-driven, purely basic areas or research into mature disciplines. As a result, many coordination compounds were found to have practical applications as therapeutic agents, catalysts, etc., and some important naturally occurring substances (haemoglobin, chlorophyll) were recognised as coordination compounds.

Born in the late sixties, Supramolecular Chemistry has been defined as a generalised coordination chemistry, because it studies compounds containing not only bonds like those present in organic or coordination compounds, but also weak interactions, termed non-covalent interactions, such as hydrogen bonds. Hydrogen bonds have been long recognised to be crucial in biological systems (DNA, proteins, etc.), but using hydrogen bonds in the synthesis of molecules with a predetermined structure and function is one of the challenging goals of the young area of Supramolecular Chemistry. Some of these molecules, called anion hosts or receptors, are designed to bind anions using non-covalent interactions. Most anion receptors are purely organic molecules, and their synthesis may require tedious procedures. Recent studies showed that simple organic molecules able to act as ligands and at the same time containing groups able to form hydrogen bonds can be assembled around a metal centre resulting in a coordination compound that is also a supramolecular receptor of anions.

We have found that appropriate metal centers can play different structural roles in anion receptors:
(a) Pre-organise chelate ligands such as biimidazole or 1,2-benzenediamine, enforcing the convergence of two N-H bonds toward an external anion.
(b) Bind simple pyrazole molecules, suppressing pyrazole self-association and allowing the N-H bonds of two or three pyrazoles to converge toward external anions. The choice of the metal center was found to be crucial to afford stable receptors and avoid displacement of pyrazoles by the external anion.
(c) Act as templates for the coupling of organic molecules (e.g. coupling of a nitrile and a pyrazole) and as scaffolds to maintain the resulting new ligand (called pyrazolylamidino) and a second pyrazole in adjacent positions so that their N-H groups can simultaneously bind anions.

Distinctive features of our receptors are:
(1) The choice of tetrakis(3,5-bis(trifluoromethyl)phenyl)borate as counteranion of our cationic receptors. Although widely used in Organometallic Chemistry, it was never used before in Supramolecular Chemistry. The negative charge in this anion is highly delocalised, and this reduces the coulombic attraction with the cationic receptor, leaving the latter more available to interact with external anions. In addition, the resulting salts are highly soluble in moderately polar organic solvents.
(2) Using complexes with CO or CNR ligands allowed us to assess the stability of the resulting organometallic receptors in solution by IR spectroscopy, a simple and very sensitive technique.
(3) For the first time, organometallic complexes in which the metal plays a core structural role have been used as anion receptors, bringing together the fields of Supramolecular Chemistry and Organometallic Chemistry.