Elsevier

Electrochimica Acta

Volume 220, 1 December 2016, Pages 347-353
Electrochimica Acta

Electrochemistry of the tris(2,2‘-bipyridine) complex of iron(II) in ionic liquids and aprotic molecular solvents

https://doi.org/10.1016/j.electacta.2016.10.126Get rights and content

Abstract

In this work, the electrochemistry of the tris(2,2‘-bipyridine) complex of iron(II) was investigated, in order to develop a better understanding of the behaviour of this complex. Two ionic liquids (ILs), based on the 1-ethyl-3-methylimidazolium cation and two ILs based on the 1-butyl-1-methylpyrrolidinium cation, as well as three conventional aprotic solvents were used. The semi-integral electroanalysis was used to aid the interpretation of the results. The iron(II) complex presented a reversible oxidation reaction and three reduction reactions, which are normally considered to be reversible, however our results demonstrate that this is not always the case, with the reversibility of the reactions being highly dependent on the solvent used. A novel method for synthesising iron(III) tris(2,2‘-bipyridine) is reported, and mixtures of the iron(II) and iron(III) complexes were used to determine the standard rate constants of the oxidation reaction. The long term stability of solutions of the iron(III) complex is also studied in this work, which is very important for possible electrochemical applications of this complex, such as in battery electrolytes.

Introduction

It is known that ligands can stabilize different oxidation states of metals [1], [2], and different ligands can stabilize a specific oxidation state of a metal [3], which influences the potentials of its redox reactions. For instance, the complex of cobalt (II) with 2,2‘-bipyridine (bpy) has a lower oxidation potential than the aquo complex of Co(II). Conversely, the Fe2+/3+ redox potential of the bpy complex is higher than that of the aquo complex [4]. Moreover, the ligands can take part in the redox reaction, donating or accepting electrons [5], [6], allowing the complex to have oxidation states that are not accessible to the metal [7]. Therefore, the appropriate combination of metals and ligands has led to a variety of electrochemical applications of metal complexes [8], [9], [10], [11], [12], [13], [14].

Bpy is one of the most widely used ligands, it was first synthesised in 1888 by Fritz Blau, who noticed that adding iron(II) to a solution of bpy generated an intense red colour [15], which he proved to be due to the formation of [Fe(bpy)3]2+ [16]. Many complexes of metals with bpy have been studied and various uses for these compounds have been found. The ruthenium(II) complex ([Ru(bpy)3]2+) can absorb visible light to generate an excited state with a lifetime that is suitable for several photochemical applications, such as photocatalysis [12] and electrochemiluminescence [17]; bpy complexes of osmium(II) [11] and iridium(III) [18] also have photochemical applications. Cobalt(II/III) bpy complexes proved to be efficient redox shuttles for both dye sensitised solar cells [19] and thermoelectrochemical cells [20].

One of the possible new uses of metal bpy complexes is in electrolytes for redox flow batteries (RFB), because of the many oxidation states that some of these complexes can have [21]; this allows the same discharged electrolyte to be used in the positive and negative electrodes, thus minimizing the problems associated with electrolyte crossover [22]. The use of bpy complexes of iron(II) ([Fe(bpy)3]2+) for RFB electrolytes has been suggested [23], [24]. However, there is evidence in the literature of low stability of [Fe(bpy)3]3+ [25], [26], which can be due to reaction with oxygen [27] or with water [28] and dissociation of the ligand[29]. It has been suggested that the use of some ionic liquids (ILs) can increase the stability of [Fe(bpy)3]3+ [30], [31], which could enable the use of this complexes for electrochemical applications. Furthermore, ILs generally present some useful properties such as low volatility, high thermal and electrochemical stabilities and intrinsic ionic conductivity [32]. Thus in this work, the electrochemistry of [Fe(bpy)3]2+ was investigated in four ILs and, for comparison, in three aprotic molecular solvents. Our goal was to better understand under which conditions this complex can be used, is stable and exhibits reversible redox processes. The results presented here demonstrate that under certain circumstances full reversibility of the oxidation and reduction of [Fe(bpy)3]2+ can be achieved.

Section snippets

Experimental

The synthesis of iron(II) tris(2,2‘-bipyridine) and a novel method for synthesizing iron(III) tris(2,2‘-bipyridine) are described in the supplementary information section.

The ionic liquids used: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (Solvionic, 99.9%), C2mim NTf2; 1-ethyl-3-methylimidazolium tetracyanoborate (Merck, high purity), C2mim B(CN)4; 1-butyl-1methylpyrrolidinium bis(trifluoromethanesulfonyl)amide (Solvionic, 99.9%), C4mpyr NTf2; 1-butyl-1-methylpyrrolidinium

Results and Discussion

The electrochemistry of the [Fe(bpy)3]2+ complex was investigated in three organic solvents – acetonitrile (ACN), 3-methoxypropionitrile (MPN) and propylene carbonate (PC) – and in four ionic liquids (ILs) – 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (C4mpyr NTf2), 1-Butyl-1-methylpyrrolidinium tetracyanoborate (C4mpyr B(CN)4), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C2mim NTf2) and 1-ethyl-3-methylimidazolium tetracyanoborate (C2mim B(CN)4); the

Conclusions

The electrochemistry of [Fe(bpy)3]2+ was investigated in four ILs – C2mim B(CN)4, C2mim NTf2, C4mpyr B(CN)4 and C4mpyr NTf2–and three aprotic molecular solvents – ACN, MPN and PC – with 200 mmol.l−1C4mim BF4 as supporting electrolyte. Four redox reactions were observed for [Fe(bpy)3]2+; one oxidation and three reductions. The oxidation is fully reversible, as demonstrated by the semi-integral analysis. Long term stability of [Fe(bpy)3]3+ is possible in some solvents, being increased by the

Acknowledgments

This work was financially supported by the Australian Research Council through the ARC Centre of Excellence for Electromaterials Science (GrantNo. CE140100012). DRM is grateful to the ARC for his Australian Laureate Fellowship. The authors also gratefully acknowledge the contribution of Dr. Elena Mashkina who developed the program used for the semi-integral analysis.

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