Chemical Communications Blog

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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Due to their low volatility, favourable solvent properties and tunable phase behaviour, ionic liquid (IL) technology continues to attract interest in a wide variety of applications. Over the last few decades, the term ‘task specific ionic liquid’ (TSIL) has appeared, the meaning of which is that the application has been considered more carefully in the design and structure of one or both of the constituent IL ions.

In this ChemComm communciation, researchers from the Los Alamos National Laboratory, New Mexico, USA, describe advances in the application of ILs in the area of nuclear fuel reprocessing. The authors describe how neptunium may exist in solution in +III, +IV, +V and + VI oxidation states. The pentavalent state is the most stable and common, but interestingly, usually the most reluctant to coordinate with ligands, or take part in ion exchange processes.

Ionic liquid molecular structure and stabilizing behaviour towards Np(V) in aqueous solution

In aqeuous solution, in the presence of the ionic liquid betaine bistriflimide, the lifetime of the Np(IV) solution species is significantly increased. This is followed by the presence of a characteristic electronic adsorption at 970nm. Without the IL, an anti-oxidant is required to keep the metal from converting to its +V form. Np(V) has a rich and complex coordination chemistry at room temperature, so the authors repeated experiments with sources of  Np(V) at 60^oC. After 3 days, 90% of the starting neptunium compound had been converted to the betaine complexed form, compared with a 15% conversion at room temperature. Again the material was stable in solution, not resulting in Np(IV) species unless a reducing agent such as hydrazine was deliberately added.

With this detailed study, the authors significantly add to the body of knowledge of competitive coordination chemistry of actinides in aqueous solutions of ionic liquids. This should, in turn, enable new separation technology R&D for the important task of nuclear fuel reprocessing.

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Unusual redox stability of neptunium in the ionic liquid [Hbet][Tf₂N]
Kristy Long, George Goff and Wolfgang Runde
DOI: 10.1039/C4CC01835D

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Graphene nanoribbons (GNRs) are ultra-thin strips of graphene which exhibit technologically relevant optical, electronic and magnetic properties. How well these properties can be defined, understood and exploited depends on how precisely these ‘GNRs’ can be prepared. Top-down approaches to realise these materials from, for example, graphite, graphene or carbon nanotubes have been developed over recent years. However, the resultant  materials are typically non-uniform, being wide, (relatively speaking) with a large amount of disorder.

In this ChemComm communication, Alexander Sinitskii and co-workers from the University of Nebraska and collaborators from the University of Puerto Rico report an elegant, controlled, bottom-up solution synthesis of well defined nitrogen doped graphene nano-ribbons. Though the materials produced are extremely insoluble, they have been extensively characterised via a range of surface analytical techniques.

The synthesis itself centres around the 3 step preparation of a multi-aryl containing dibromo pyrimidine monomer which is polymerised via a Yamamoto coupling using a nickel catalyst. The desired graphene  structure is delivered via a cyclodehydrogenation Scholl reaction using ferric chloride. The molecular control from this route yields a very well defined structure. The only disorder introduced is a symmetrical one due to alternate possible sites for the  four constituent nitrogen atoms present in each unit cell.

Synthetic route to well defined nitrogen doped graphene nanoribbons (4N-GNRs)

The 4N-GNR materials were deposited on a variety of supports to enable microscopic and surface analytics such as AFM, STM, TEM, XPS, EDX and Raman spectroscopy. The materials were observed to take on a ‘nanobelt’ conformation on deposition from sonicated dispersions. Particularly striking is the improvement in the fine structure reported in the Raman spectra, compared with typical signals for less well defined carbon materials, confirming the structural quality of the 4N-GNRs produced.

This communication will doubtless prove important to the development of sustainable synthetic routes to well defined graphene nanoribbons, enabling  further study of the fascinating properties of these exciting new materials.

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Bottom-up solution synthesis of narrow nitrgoen-doped graphene nanoribbons
Chem. Commun., 2014,50, 4172-4174
DOI: 10.1039/C4CC00885E

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