A Ferrocene-Quinoxaline Derivative as a Highly Selective Probe for Colorimetric and Redox Sensing of Toxic Mercury(II) Cations

A new chemosensor molecule 3 based on a ferrocene-quinoxaline dyad recognizes mercury (II) cations in acetonitrile solution. Upon recognition, an anodic shift of the ferrocene/ferrocenium oxidation peaks and a progressive red-shift (Δλ = 140 nm) of the low-energy band, are observed in its absorption spectrum. This change in the absorption spectrum is accompanied by a colour change from orange to deep green, which can be used for a “naked-eye” detection of this metal cation.


Introduction
The design and synthesis of chemosensors for environmentally and biologically relevant species have been actively investigated in recent years [1][2][3]. In this regard, chemosensors that can highly sensitively and selectively monitor heavy metal ions are especially important. Among heavy and transition metals, mercury, widely distributed in air, water and soil, is considered to be one of the highly toxic because both elemental and ionic mercury can be converted by bacteria in the environment to methyl mercury, which subsequently bioaccumulates through the food chain [4][5][6][7][8][9][10][11]. Mercury-induced toxicity can cause a number of severe health problems because it can damage the digestive organs, kidneys, central nervous system and endocrine system [12][13][14][15][16][17]. Given its high OPEN ACCESS toxicity and the increasing threat of global mercury release into the environment, considerable efforts are continuously made to develop highly selective and sensitive chemosensors for Hg (II). In this context, development of new and practical chemosensors which offer a promising approach for mercury ion detection is still a great challenge for the scientific community [18][19][20][21][22][23], triggering a large number of related investigations that have been recently reviewed [24][25][26].
Ferrocene is one of the favourite "building blocks" in the construction of sensing platforms based on redox-active units due to the availability, stability and tailorability of most of its derivatives. The sensing behaviour of these systems is mainly based on the potential shift shown upon their interaction with a variety of guest species. However, binding can also affect the UV-vis properties of the ferrocene unit when it is placed near the binding site. In general, metal complexation induces bathochromic shifts in the lower-energy, spin-allowed ferrocene absorption band, which is between 400 and 500 nm [27][28][29][30] On the other hand, quinoxaline derivatives are the subject of considerable interest from both academic and industrial perspectives because they are significant intermediates for the manufacture of pharmaceuticals and advanced materials [31][32][33][34] Moreover, the quinoxaline ring appropriately subtitued or fused to some other azaheterocyclic systems has also been studied as a putative binding subunit for the recognition and sensing of both anionic and cationic especies [35][36][37] The work presented here, forms part of our interest in designing chemosensors that are capable of reporting on the recognition of metal cations through a variety of physical responses, by combining various signalling units into an individual molecule. Toward this end, we report here a straightforward synthesis of the new 2,3-diferrocenylquinoxaline ligand which shows a selective, sensitive and reversible response to the Hg(II) ion through two different channels: redox and chromogenic

Experimental Section
All reactions were carried out using solvents which were dried by routine procedures. The melting point was determined on a hot-plate melting point apparatus and is uncorrected. 1 H-and 13 C-NMR spectra were recorded at 400 and 100 MHz, respectively on a Brucker AC 400. The following abbreviations for stating the multiplicity of the signals have been used: s (singlet), bs, d (doublet), t (triplet), st (pseudotriplet), and q (quaternary carbon atom). Chemical shifts refer to signals of tetramethylsilane in the case of 1 H-and 13 C-NMR spectra. The cyclic electrochemistry measurements were performed on a Bioanalytical Systems CV-50 W Voltammetric Analyzer potentiostat/galvanostat controlled by a personal computer and driven by dedicated software with a conventional three-electrode configuration consisting of platinum working and auxiliary electrodes and an SCE reference electrode. The experiments were carried out with a 10 −3 M solution of sample in dry CH 3 CN containing 0.1 M [(n-Bu) 4 N]ClO 4 as supporting electrolyte (Warning: Potential formation of highly explosive perchlorate salts of organic derivatives). Deoxygenation of the solutions was achieved by bubbling nitrogen for at least 10 min, and the working electrode was cleaned after each run. The cyclic voltammograms were recorded with a scan rate between 0.05 and 0.5 V s −1 . Linear sweep voltammetry (LSV), cyclic voltammetry (CV), and Osteryoung square wave voltammetry (OSWV) were recorded before and after the addition of aliquots of 0.1 equiv of 2.5 × 10 −2 M solutions of the corresponding cations in H 2 O. The following settings were used: pulse amplitude, 50 mV; pulse width, 50 ms; scan rate, 100 mV/s; sample width, 17 ms; pulse period, 200 ms. Decamethylferrocene (DMFe) (−0.07 V vs SCE) was used as an internal reference both for potential calibration and for reversibility criteria. UV-vis absorption spectra were regularly recorded after the addition a small aliquot of the corresponding cation (c = 2.5 × 10 −3 M) to a solution of the receptor (c = 1 × 10 −4 M) using a UV quartz cell.

Electrochemical and Optical Properties.
The redox properties of receptor 3 was investigated by linear sweep voltammetry (LSV), cyclic voltammetry (CV), and Osteryoung square wave voltammetry (OSWV) in a CH 3 CN solution containing 0.15 M [n-Bu 4 N]ClO 4 (TBAP) as supporting electrolyte. In spite of the symmetry of the receptor 3 it exhibited, in the range 0−0.9 V, two reversible one-electron redox wave at the half-wave potential value of 1 E 1/2 = 0.47 V and 2 E 1/2 = 0.58 V (E 1/2 = 110mV) versus decamethylferrocene (DMFc), demonstrating the existence of a weak interaction between the two iron centres (Figure 1). The criteria applied for reversibility was a separation of ~60 mV between cathodic and anodic peaks, a ratio of 1.0 ± 0.1 for the intensities of the cathodic and anodic currents Ic/Ia, and no shift of the half-wave potentials with varying scan rates. which are assigned to another localized excitations with a lower energy produced either by two nearly degenerate transitions, an Fe(II) d−d transition or by a metal−ligand charge transfer (MLCT) process (d π −π*) (LE band) [39] This assignment is in accordance with the latest theoretical treatment (model III) reported by Barlow et al. [40]. Such spectral characteristics confer an orange color to this species.

Cation Sensing Properties
One of the most interesting attributes of the new diferrocenylquinoxaline reported here is the presence of metal-ion binding sites on the quinoxaline ring close to a ferrocene redox-active moiety. Due to this structural feature metal recognition properties on the receptor 3 were evaluated by electrochemical, optical and 1 H-NMR techniques.
The electrochemical binding interactions of 3 towards cations of biological and environmental relevance, such as Li + , Na + , K + , Ca 2+ , Mg 2+ , Cu 2+ , Zn 2+ , Cd 2+ , Hg 2+ , Ni 2+ , and Pb 2+ , added as their perchlorate salts, were investigated in CH 3 CN (c = 1 × 10 −3 M). Titration studies with addition of the above-mentioned set of metal cations (2.5 × 10 −2 M in H 2 O) to an electrochemical solution of receptor 3 containing [n-Bu 4 N]ClO 4 (0.1 M) as supporting electrolyte, demonstrate that while addition of Cu 2+ and Hg 2+ ions promotes remarkable responses, addition of Li + , Na + , K + , Ca 2+ , Mg 2+ , Zn 2+ , Cd 2+ , Pb 2+ and Ni 2+ metal ions had no effect either on LSV or on the CV or OSWV of this receptor, even when present in a large excess. The results obtained on the stepwise addition of substoichiometric amounts of Hg 2+ revealed the appearance, in the OSWV, of a new oxidation peak at practically the same potential of the second redox peak in the free receptor (Ep = 0.55 V, Ep = 75 mV).This fact suggests that the complex is disrupted after the first monoelectronic oxidation of the complex 3 + ·Hg 2+ and the second oxidation really takes place on the uncomplexed mono-oxidized 3 + . The current intensity of this new peak increases until 1 equiv of the Hg 2+ cation is added [ Figure 2(a)]. Moreover, the CV analysis of the complex 3·Hg 2+ shows that one reduction process takes place at the same reduction potential showed by the uncomplexed ligand 3, indicating that the complex starts to be disrupted after its electronic oxidation [ Figure 2(b)]. This behaviour means that this receptor is not only able to monitor binding but it is also able to behave as an electrochemically induced switchable chemosensor for Hg 2+ through the progressive electrochemical release of these metal cations; as a result of a decrease of the corresponding binding constant upon electrochemical oxidation. Remarkably, LSV studies carried out upon addition of Cu 2+ to the CH 3 CN solution of this receptor showed a significant shift of the sigmoidal voltammetric wave toward cathodic currents, indicating that Cu 2+ cations promote the oxidation of the free receptor. On the other hand, the same experiments carried out upon addition of Hg 2+ revealed a shift of the linear sweep voltammogram toward more positive potentials, indicating the complexation process according to the previously observed by OSWV (Figure 3).
Previous studies on ferrocene-based ligands have shown that their characteristic low energy (LE) bands in the absorption spectra are perturbed upon complexation [41][42][43][44]. Therefore, the metal recognition properties of the ligand 3 toward metal ions were also evaluated by UV−vis spectroscopy. Titration experiments for CH 3 CN solutions of this ligand (c = 1 × 10 −4 M), and the corresponding cations were performed and analyzed quantitatively. [45] It is worth mentioning that no changes were observed in the UV−vis spectra upon addition of Li + , Na + , K + , Ca 2+ , Mg 2+ , Zn 2+ , Cd 2+ , and Ni 2+ and Pb 2+ metal ions, even in a large excess; however, significant modifications were observed upon addition of Hg 2+ .

Conclusions
We have successfully developed a new and easy-to-make quinoxaline-based molecular sensor 3 which shows selective response to Hg 2+ ions through a dual channel: Electrochemical and chromogenic. The reported quinoxaline-ferrocene sensor permits not only the naked-eye detection of this metal cation but also to monitor the recognition process through electrochemical measurements. Additionally, this receptor is also able to behave as an electrochemically induced switchable chemosensor for Hg 2+ . A combination of the UV-vis titration data and mass spectrometry has been successfully used to establish the 1:1 stoichiometry of the complex formed.