In this communication, Edwin Otten and Mu-Chieh Chang from the Stratingh Institute for Chemistry at the University of Groningen describe their work with formazanate ligands. They detail here an easier route to boron difluoride chelates with this type of ligand via an exchange transmetallation from a zinc complex with the same ligand in the presence of boron difluoride etherate (BF3.Et2O).
Formazan, or formazanate in its deprotonated form, is an example of a ‘non-innocent ligand’. Just like transition metals, it has an accessible redox chemistry all of its own, and can effectively store electrons by existing in several oxidation states stabilised by its structure. Chemically, formazanates, with a NNCNN backbone are nitrogen rich analogs of diketiminates, which can be represented by NCCCN. In this case, aryl substitution on the terminal nitrogen and central carbon atoms aid the electron stabilisation.
Crystal structure and cyclic voltammagram of formazanate boron difluoride complex
The mono formazanate boron difluoride complex was readily accessible by mixing a bis formazanate zinc complex with BF3 etherate in hot toluene. Zinc fluoride precipitated from solution and the air stable, crystalline material boron chelate was isolated in high yield. X-ray crystal structure determination was possible, showing a tetrahedral environment around the boron atom, and equal bond lengths in the NNCNN core of the ligand, proving its delocalised nature. An important intermediate of the process was also isolated and identified by this technique aiding mechanism elucidation.
Using cobaltocene as reducing agent, a reduced from of the complex was also isolated and characterised, and use of cyclic voltammetry quantified the redox potentials for formation of the further reduced forms of the material. All three redox states were observed. Applications of these materials in catalysis and further investigation in coordination chemistry are ongoing. The potential for application of such ligands in the area of sensors and devices, or even therapeutics poses many possibilities.
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Synthesis and ligand-based reduction chemistry of boron-difluoride complexes with redox-active formazanate ligands
M.-C. Chang and E. Otten
DOI: 10.1039/c4cc03244f
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