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An Efficient Molecular Tool with Ferrocene Backbone: Discriminating Fe3+ from Fe2+ in Aqueous Media Sushil Ranjan Bhatta,† Varun Bheemireddy,‡ Gonela Vijaykumar,§ Sashi Debnath,§ and Arunabha Thakur*,† †

Department of Chemistry, National Institute of Technology Rourkela, Odisha 769008, India Department of Physical Science and §Department of Chemical Science, Indian Institute of Science Education and Research Kolkata, Mohanpur, West Bengal 741246, India

‡

S Supporting Information *

ABSTRACT: Two novel molecular probes with ferrocene backbone have been designed and synthesized for the first time, and they were subsequently found capable of distinguishing Fe3+ and Fe2+ ion in aqueous media. The discrimination of both the oxidation states (II/III) of iron by these receptors can be established either from a striking shift in redox potential (1: ΔE1/2 ≈ 90 mV and 2: ΔE1/2 ≈ 59 mV) for Fe2+ ion or from UV−vis absorption studies (using light-absorption ratio variation approach (LARVA)). Moreover, the discrimination of Fe2+ and Fe3+ cations could be performed by naked-eye observation because of the development of different colors upon interaction with these probes which act as indicators for the in situ qualitative detection of Fe3+ and Fe2+. The limits of detection of Fe2+ and Fe3+ cations with receptor 2 were found to be as low as 30 and 15 parts per billion (ppb), respectively. The probable binding modes of these receptors with Fe2+ have also been suggested on the basis of the 1H NMR spectroscopic titration, electrospray ionization mass spectrometry (ESI-MS), Job’s plot, and computational (DFT) studies along with electrochemical and spectro-photochemical data. Single crystal X-ray diffraction analysis of 1 revealed that its solid-state structure was stabilized via intermolecular C−H/O and O−H/N hydrogen bonds and by C−H/π interactions. Interestingly, detailed theoretical calculations (DFT) indicated that hydroxymethyl (−CH2OH) group attached to naphthalene unit plays a pivotal role in sensing Fe2+/3+ ion selectively and in the stabilization of 2 in unusual eclipsed configuration through C−H···O type hydrogen bonding.

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INTRODUCTION Iron, one of the most essential trace elements in biological systems, and one of the most abundant transition metals present in the human body, performs a vital role in cellular metabolism and in the function of hemoglobin and various enzymes, proteins, and transcriptional events.1 In biological systems, iron plays a vibrant role as a cofactor in various proteins because of its ferrous/ferric (Fe2+/Fe3+) states which are one of the most important redox pairs in biological systems.2 The behavior of iron in biological and environmental contexts depends strongly on its oxidation state. Solubility in water and the ability to form complexes are two important characteristics that are species dependent. Distinction between the two iron species, Fe2+ and Fe3+, is thus necessary to assess bioavailability, as Fe2+ is more soluble and therefore more readily available for phytoplankton uptake and growth. Thus, the discrimination between +2 and +3 oxidation states of iron is important in order to understand the biological functions regulated by iron.3 Due to high reactivity and reversibility as well as involvement in various secondary chemical equilibria including complexation with naturally occurring ligands, oxidation by aerial oxygen, and © XXXX American Chemical Society

hydrolysis, the direct quantitative determination of iron species at trace levels is a genuine challenge.4 The species Fe2+ is generally the most soluble form of iron, but it is also very unstable, especially in solutions containing high levels of dissolved oxygen, being readily oxidized to Fe3+.5 Therefore, it is still a challenge to determine whether the system contains both Fe3+ and Fe2+ or either of them individually. Several methods for the determination of Fe2+/3+ have been reported by different analytical techniques. Among the various chemosensing techniques, sensors based on a naked-eye response (colorimetric) have many advantages because of their ability to provide a simple, sensitive, selective, precise, and economical method for the detection of a target analyte without the use of specific/sophisticated instrumentation.6 Electrochemical methods such as voltammetry and potentiometry also provide an alternate platform for the detection of iron at low level due to their high sensitivity.7 A variety of sensors specific for Fe3+ have also been designed,8 and literature have been reported demonstrating discrimination between Fe2+ and Fe3+ Received: March 15, 2017

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DOI: 10.1021/acs.organomet.7b00199 Organometallics XXXX, XXX, XXX−XXX

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Organometallics by fluorescence9 including ratiometric fluorescence10 spectroscopy, scanner electrochemistry,11 and visual strip method.12 However, to the best of our knowledge a simple ferrocenebased chromogenic and electrochemical sensor capable of distinguishing the two oxidation states of iron is still unexplored. There are very few ferrocene-based receptors for Fe2+ available in the literature.13 Nevertheless, those sensors were not only selective for Fe2+/3+ ions but also sensed a range of other metal ions.13 Our ongoing research, aimed at the synthesis of new inexpensive and easy-to-make optical and electrochemical sensors of biologically and/or chemically important ions, leads us to explore unprecedented examples of a couple of simple ferrocene-based sensor molecules which can detect and differentiate both the oxidation states (+2/+3) of iron electrochemically as well as colorimetrically with a very low limit of detection in aqueous media.

Furthermore, computational studies based on density functional theory (DFT) were performed to gain an insight into the properties of the sensor molecules. X-ray Structure Analysis of Compound 1. Single crystal X-ray diffraction analysis revealed that organometallic compound 1 crystallized in the monoclinic centrosymmetric space group P21/c. As shown in Figure 1, all the C−N bond distances in the five-membered triazole ring (avg ≈ 1.347 Å) are normal. The bond distance between N2−N3 is slightly shorter in comparison with N1−N2, which may be due to the doublebond character between N2−N3. The crystal structure of 1 was stabilized via intermolecular C−H/O and O−H/N hydrogen bonds. In addition, the packing is further stabilized by C−H/π interactions. The oxygen of the CH2OH group of naphthalene ring is involved in hydrogen bonding with CH unit of the triazole ring via a C− H···O interaction (C···O = 3.053(10) Å; ∠C−H···O = 134.00°), and the nitrogen of the triazole ring is further involved in hydrogen bonding with OH of another unit of CH2OH group via O−H···N interaction (O···N = 2.822(9) Å; ∠O−H···N = 176.00°) that leads to the formation of a 1D hydrogen-bonded network (Figure 2a,b). Such 1D networks are further self-assembled via two C−H···π (C···π = 3.817(2) and 3.705(4) Å) interactions (Figure 2c). A similar kind of 1D networks was observed for ferrocene-based organometallics compounds15 where dicarboxylate unit was found to be hydrogen-bonded with three ammonium cations via N−H/O interactions [N···O = 2.694(3)−2.779(3) A; ∠N−H···O = 159.5−169.5°]. Furthermore, such 1D networks were observed in many organic ionic solids16 and N-heterocyclic compounds17 where all the hydrogen bonding parameters (bond angles and bond lengths) are comparable with our 1D features. Electrochemical Studies. The electrochemical signal, which can be easily read out on-site, is another essential parameter for ferrocene-containing chemosensors in the recognition process.18 The interactions of ferrocene moiety with different metal ions have been studied by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) to investigate the reversibility and the change in their redox potentials in the presence of different metal cations, such as Li+, Na+, K+, Ag+, Ca2+, Mg2+, Cr3+, Al3+, Zn2+, Fe3+, Ni2+, Co2+, Fe2+, Cu2+, Cd2+, Hg2+, and Pb2+ as perchlorate salts in CH3CN/H2O (2/8, v/v) solution containing 0.1 M [(nBu)4N]ClO4 as supporting electrolyte. No perturbation of the CV and LSV voltammograms of 1 and 2 were observed in the presence of several metal ions, except for Fe3+ and Fe2+ ions (Figures S9 and S10). As shown in Figures 3a and 4a, upon addition of 1 equiv of Fe3+ into a CH3CN/H2O (2/8, v/v) solution of 1 and 2, the voltametric wave shifted toward a more cathodic current with a slight shift toward anodic potential (ΔE1/2 = 20 mV for 1 and 11 mV for 2). The shift of the voltammetric wave toward cathodic current indicated that Fe3+ metal cation promoted the oxidation of the Fc unit to Fc+ ion with concomitant reduction to Fe2+ and that the resultant Fe2+ ion further formed a chelate complex with the oxidized ligand which induced a slight anodic shift. Further addition of Fe3+ ion beyond 1 equiv did not produce any significant change in CV voltammograms which indicates that no ferrocene unit remained to be oxidized to ferrocenium ion. In contrast, as shown in Figures 5a and 6a, stepwise addition of Fe2+ ion (0−1 equiv) to the receptors 1 and 2 led to a significant voltametric shift toward anodic potential (ΔE1/2 ≈ 90 mV and ΔE1/2 ≈ 59 mV for 1 and 2,

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RESULTS AND DISCUSSION Synthesis of Compounds 1 and 2. Designing a suitable probe is one of the most important aspects in developing chromogenic, electrochemical, and fluorogenic sensors for target analytes. Both Fe2+ and Fe3+ cations are relatively hard centers over transition metal backgrounds; thus, they have a tendency to coordinate with harder center oxygen over the nitrogen center. Keeping this idea in mind, the probe having 1(hydroxylmethyl) 2-naphthol containing oxygen as hard donor centers was chosen for chelation. Phenolic −OH or hydroxymethyl moiety has a strong affinity to coordinate with Fe2+ metal. The critical role of −OH groups in iron chelation has been reported.14 As shown in Scheme 1, copper-catalyzed Scheme 1. Synthesis of Receptors 1 and 2

azide−alkyne [2 + 3] cycloaddition (CuAAC) reaction of (2(prop-2-yn-1-yloxy) naphthalene-1-yl) methanol with mono (azidomethyl) ferrocene and 1,1′-bis (azidomethyl) ferrocene afforded compounds 1 and 2 in 71 and 47% yields, respectively. Both compounds have been characterized by 1H and 13C NMR spectroscopy, electrospray ionization mass spectrometry (ESIMS), and elemental analysis. In addition, the solid-state structure of compound 1 has been unequivocally established by single crystal X-ray diffraction analysis. To understand the metal complexation properties of 1 and 2, studies were carried out by absorption and emission spectroscopy, electrochemical study, 1H NMR spectroscopic titration, and mass spectrometry. B


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Figure 1. Molecular structure of 1 with thermal ellipsoid plot drawn at 50% probability level. Selected bond length (Å) and angles (deg): C2−C11 1.50(1), O2−C11 1.41(1), O1−C1 1.374(9), O1−C12 1.42(1), C13−C12 1.496(8), N1−C15 1.485(8), C16−C15 1.480(8), [triazole ring: N1− N2 1.329(8), N3−N2 1.318(8), N1−C14 1.336(8), C13−C14 1.37(1)]; O2−C11−C2 112.8(7), O1−C1−C2 114.6(6), C12−O1−C1 119.2(6), O1−C12−C13 105.6(5), C14−C13−C12 129.8(6), N1−C15−C16 113.0(5), N3−C13−C12 121.6(6), [triazole ring: N3−C13−C14 108.6(6), N2−N3−C13 108.7(6), N1−N2−N3 106.8(5), N2−N1−C14 111.6(5), N1−C14−C13 104.3(6)].

Figure 2. Crystal structure description of 1: (a) formation of 1D hydrogen bonding network, (b) molecular packing viewed along the a-axis, (c) 1D hydrogen bonded network formed via C−H···O, O−H···N, and C−H···π interactions.

Figure 3. Evolution of CV (a) and LSV (b) of 1 (10−4 M) in CH3CN/H2O (2/8) using [(n-Bu)4N]ClO4 as supporting electrolyte when [Fe(ClO4)3] is added up to 0−1 equiv.

Fe2+ ion beyond 1 equiv did not produce any significant shifts in CV diagrams. The LSV experiments also corroborate the

respectively), which is due to the formation of new complex species between the receptors and Fe2+ ion. Further addition of C


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chelation with Fe2+. This experiment clearly indicates that these probes can selectively discrminate Fe3+ from Fe2+. UV−Visible Absorption Studies. The selective and sensitive signal responses of probes 1 and 2 toward Fe2+ and Fe3+ ions were preserved in absorption. The solutions of different cations were added separately to the prepared solution of compounds 1 and 2, and UV−vis absorption spectra were recorded. Addition of low concentrations of Fe3+ and Fe2+ led to a significant change in UV−vis spectra, whereas no other metals exhibited any kind of significant effect on the absorption

results obtained from CV studies (Figures 3b, 4b, 5b, and 6b). These experiments clearly demonstrated that sensor molecules 1 and 2 can discriminate the two oxidation states of iron (+2 and +3) by two different phenomena (complexation with Fe2+ ion whereas oxidation for Fe3+ ion). It is evident that Fe2+ ion did not produce any significant offset toward Fe3+ detection. In contrast, addition of Fe3+ ion into a solution containing ligands and Fe2+ further led to voltammetric wave toward cathodic current, indicating that Fe3+ metal cation can promote the oxidation of the Fc unit to Fc+ ion even when the ligand is in D


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Figure 7. Changes in the absorption spectra of 1 (a) and 2 (b) in CH3CN/H2O (2/8) (10−4 M) upon addition of increasing amounts of Fe3+ up to 1 equiv; inset: expanded region.

Figure 8. Changes in the absorption spectra of 1 (a) and 2 (b) in CH3CN/H2O (2/8) (10−4 M) upon addition of increasing amounts of Fe2+ up to 1 equiv; inset: expanded region.

Figure 8b, with increasing the concentration of Fe2+ ion into the solution of 2 both the absorption bands at 320 and 332 nm decreased. As a result, the absorbance ratio of A320 nm and A332 nm of the 1 or 2 solutions varied differently with increasing concentration of Fe2+ and Fe3+ ions. On the basis of these characteristics, the light-absorption ratio variation approach (LARVA) has been applied to the determination of Fe3+ and Fe2+ ions selectively, and both probes 1 and 2 can be applied to discriminate Fe3+ from Fe2+ ion by simple UV−vis spectroscopic measurements. Well-defined isosbestic points at ca. 339 and 392 nm for 1 and 316 and 341 nm for 2 were observed during the titration with Fe2+ ion, indicating that a spectrally distinct complex was formed between ligands and Fe2+ ion. The stoichiometry of the complexes formed between ligands and Fe2+/Fe3+ are 1:1 as determined based on Job’s plot (Figures S13 and S14). These results have also been confirmed by ESI-MS, where peaks at m/z 707 and 775 correspond to the 1:1 complex of 1 and 2 with Fe2+, respectively (Figures S15 and S16). No changes were observed in the UV−visible spectrum upon addition of the above-mentioned several metal cations even in a little excess (2 equiv).

spectrum (Figures S11 and S12). To get further insight, absorption titrations of receptors 1 and 2 were performed with Fe2+ and Fe3+ ions. These titration experiments were accomplished through a stepwise addition of metal salt solutions (c = 10−4 M) to the solution of receptors 1 and 2. The UV−vis spectra of these receptors showed two absorption bands with a maximum at λmax ≈ 320 and 332 nm which are in accordance with the UV−vis data of most ferrocenyl chromophores.19 These high-energy bands (HE band) can be attributed to a π−π* electronic transition (L−π*). As shown in Figure 7a, upon gradual addition of 1 equiv of Fe3+ into aqueous solution of 1, both the high-energy (HE) absorption bands at λ = 320 nm (ε = 10807 M−1 cm−1) and 332 nm (ε = 10779 M−1 cm−1) increase simultaneously along with the appearance of a new low-energy absorption band at λ = 629 nm (ε = 1167 M−1 cm−1). In contrast, as shown in Figure 8a, with increasing concentration of Fe2+ ion, both the bands at 320 and 332 nm decreased. Similarly, in the case of probe 2, with increasing concentration of Fe3+ from 0 to 1 equiv in the solution, both the bands (Figure 7b) at 320 and 332 nm rose simultaneously along with a new low-energy absorption band appearing at 644 nm (ε = 758 M−1 cm−1), whereas as shown in E


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Figure 9. (a) Visual color changes observed for 1 in CH3CN/H2O (2/8) (10−4 M) after addition of 1 equiv of Fe2+ and Fe3+ ions. (b) In situ qualitative detection of Fe3+ and Fe2+.

Figure 10. Fluorescence emission titration of compounds 1 (a) and 2 (b) (1.5 × 10−6 M) upon addition of Fe3+ ion up to 1 equiv in CH3CN/H2O solution.

Colorimetric Discrimination of Fe3+ and Fe2+ Ions. In colorimetric experiments, significant color changes of CH3CN/ H2O solutions of probes 1 and 2 were observed upon addition of Fe3+ and Fe2+ ions over the other tested metal cations (Figure 9a). A distinct color change of receptors 1 and 2 from yellow to deep greenish blue and light green were observed in the presence of Fe3+ and Fe2+ ions, respectively, which indicates the sensitive and selective naked-eye detecting ability for these cations. When 1 equiv of different metal cations of perchlorate salts (Li+, Na+, K+, Ag+, Ca2+, Mg2+, Mn2+, Cr3+, Al3+, Co2+, Cu2+, Zn2+, Cd2+, Ni2+, Hg2+, and Pb2+) were added to the solutions of ligands 1 and 2 in CH3CN/H2O (2/8), no significant color change was observed (Figure S17). Taking advantage of the naked-eye results, we have applied both the receptors as indicators for the in situ qualitative detection of Fe3+ and Fe2+ ions (Figure 9b). A solution of receptor 1 (1.5 mL, 1 × 10−3 M, in CH3CN/H2O (2/8)) is yellow. After addition of 1 equiv of Fe2+, it turns green. When 1 equiv K2Cr2O7 is added to this green solution, immediately the color changes to deep blue which is due to the formation of ferrocenium ion (Fc+). K2Cr2O7 promoted the oxidation of Fc to Fc+ ion. On further addition of 1 equiv of sodium L-ascorbate (LAS) as a reducing agent, the observed deep blue color was converted reversibly into green color. Upon addition of excess LAS into the deep blue color Fe3+ solution, the yellow color solution of receptor 1 reappeared. These results indicated that we can monitor the presence of both the oxidation states of iron (Fe2+ and Fe3+) by using our receptors. This strategy is

convenient and inexpensive. In this case, both difficult ligand synthesis and complex signal transduction are avoided. Fluorescence Studies. The cation binding behavior of 1 and 2 was also investigated by emission spectroscopic measurements. The amount by which the emission intensity of receptors 1 and 2 was affected in the presence of several cations was tested by fluorescence spectroscopy in CH3CN/ H2O (2/8; v/v). Both the free receptors show a moderate emission band around 355 nm in CH3CN/H2O (2/8; v/v) solution (1.5 × 10−6 M) upon excitation at λexc = 330 and 320 nm for 1 and 2, respectively. Iron(III) is known to be an efficient quencher of excited state fluorophores due to the paramagnetic nature of the ferric ion.20,21 Indeed, when Fe3+ ions were added to a solution of 1 or 2, fluorescence was drastically quenched. However, addition of Fe2+ also led to the diminution of emission band significantly due to the mixing of charge-transfer excitations with the π−π* transition (as revealed by TDDFT calculation) leading to radiationless decay of excited state. Furthermore, in [2·Fe2+], the presence of unoccupied Fe centered orbital (LUMO) (Figure S18) in between the fluorophore π−π* transition orbitals possibly results in the decay of excited state through ligand-to-metal charge transfer (LMCT). Furthermore, we have performed titration experiments to determine the amount of Fe3+ and Fe2+ ions required for complete quenching of the fluorescence signals of 1 and 2 (Figure 10). After gradual addition of 1 equiv of Fe3+ and Fe2+, the fluorescence signal of 1 and 2 were completely “turned-off”. Further addition of analytes leads to F


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Figure 11. Fluorescence emission titration of compounds 1 (a) and 2 (b) (1.5 × 10−6 M) upon addition of Fe2+ ion up to 1 equiv in CH3CN/H2O solution.

Table 1. Comparison of the Analytical Parameters of C25H23FeN3O2 (1) and C40H36FeO4N6 (2) with Those of Other Reported Fe2+/Fe3+ Sensors performance sample no. 1

ligand molecular formula C10H8N2

analyte Fe(II)/(III)

method of detection

2

C18H12N3S

Fe(II)/(III)

ratiometric fluorescent

3 4 5 6

C36H25N5 C39H35N5O2 C6H7NS C39H43N5O2

Fe(II) Fe(II) Fe(II) Fe(II)

7

C30H26N4O2S

Fe(II)/(III)

electrochemical ratiometric fluorescent electrochemical electrochemical chromogenic and colorimetry

8

C12H8N2

Fe(II)

visual strip sensor

9 10 11 12 13

C12H8N2 C50H30O2Fe C21H13N4SCl C33H21NFe C33H32N5O2

Fe(II) Fe(II)/(III) Fe(III) Fe(III) Fe(III)

14

C25H23FeN3O2

Fe(II)/(III)

15

C40H36FeO4N6

Fe(II)/(III)

color change

electrochemical turn off fluorescence turn off fluorescence turn on fluorescence turn off fluorescence electrochemical and colorimetry electrochemical and colorimetry

detection limit

binding constant

0.6 × 10−6 M

electrochemical colorless to orange colorless to violet colorless to pink yellow to pink pale yellow to dark green colorless to orange red

3.88 × 10 M 5

2.0 mM

−1

60 × 10−9 mL−1 1.9 mM 1.6 × 10−7 M 1.1 × 104 M−1

0.21 μM 50 ng mL−1 −8

3.6 × 10

−1

mol L

1.4 × 10−6 M colorless to pink yellow to green yellow to deep blue

0.05 μmol L−1 Fe2+ = 60 × 10−9 M, Fe3+ = 45 × 10−9 M Fe2+ = 30 × 10−9 M, Fe3+ = 15 × 10−9 M

sensitivity

ref

medium

7a

medium

10a

medium high high high

7b 10b 7c 7d

low

8h

high

12

high medium high very low high

7e 8g 8d 8e 8c this work this work

5.2 × 105 M−1

high

7.5 × 105 M−1

high

Alternatively, to evaluate limit of detection (LOD), we fitted a straight line (data extracted from Figures 10 and 11) and applied the widely used criterion for LOD: LOD = 3.3σ/S (where σ is the standard deviation of the blank and S is the slope of the calibration curve). The detection limits (taken as 3.3σ/S) are in the same range as those also obtained from other methods. The quantification limits for probe 1 (LOQ) are 180 and 135 nM for Fe2+ and Fe3+, respectively (taken as 10σ/S). The present probes for Fe2+/Fe3+ are compared with the other iron sensor molecules in Table 1. Table 1 shows that the DLs of our present probes are similar or even lower than those obtained with other molecular sensors for Fe2+ or Fe3+ ion; this is the first example of ferrocene-based sensor which can differentiate the two oxidation states of iron efficiently. The binding constant values of Fe2+ with 1 and 2 have been determined from the emission intensity data following the modified Benesi−Hildebrand equation,22,23 1/F − F0 versus [C]−1 where F and F0 are the emission intensities of 1 and 2 considered in an intermediate Fe2+ concentration and in the

no significant changes of the emission signal (Figure 10). Notably, addition of other coexisting metal ions, even in excess amounts, caused insignificant change in the emission intensity of the receptors (Figure S19). In order to determine the sensitivity of receptors 1 and 2, we have further performed fluorescence titration of 1 (1.5 × 10−6 M) and 2 (1.5 × 10−6 M) with Fe2+ and Fe3+ ions (1.5 × 10−6 M) in CH3CN/H2O (2/8; v/v). No detectable change was observed in the fluorescence spectra up to the analyte concentration of 0.03 equiv of Fe2+ and 0.02 equiv of Fe3+ for 1. An appreciable diminution in the signal intensity was observed in the presence of 0.04 equiv of Fe2+ and 0.03 equiv of Fe3+, whereas minimum fluorescence intensity was observed in the presence of 1 equiv of each Fe2+ and Fe3+ ions. These experiments show low detectable limit (DL) with 1 is 60 × 10−9 M (60 nM) and 45 × 10−9 M (45 nM) for Fe2+ and Fe3+, respectively. A similar method was adopted for the calculation of DL for ligand 2 with Fe2+ and Fe3+, and they are found to be 30 × 10−9 M (30 nM) and 15 × 10−9 M (15 nM), respectively. G


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Figure 12. 1H NMR spectroscopic titration of 1 upon addition of increasing amount of Fe2+ up to 1 equiv in DMSO-d6 solution.

absence of Fe2+, respectively. From the plot of [1/F − F0] against [C]−1, the value of binding constants extracted from the slope are 5.2 × 105 and 7.5 × 105 M−1 for 1 and 2, respectively (Figure S20). From Table 1, it is evident that compound 2 is more efficient than compound 1 in terms of the detection limit and binding constant value. This is may be due to the fact that in compound 2 both arms are held in an eclipsed conformation to provide a suitable cavity where iron can bind strongly in a tetra-coordinated fashion. Competitive Metal Ion Titrations. In order to show the practical utility of the probes for the detection of Fe3+ and Fe2+ ions selectively, competitive metal ion titrations were carried out in CH3CN/H2O (2:8, v/v) solution on the basis of emission output. The emission intensity of 1 was almost unaffected by the addition of several equiv of competing metal ions like (Li+, Na+, K+, Ag+, Ca2+, Mg2+, Mn2+, Cr3+, Al3+, Co2+, Cu2+, Zn2+, Cd2+, Ni2+, Hg2+, and Pb2+) except for Fe2+ and Fe3+ ions (Figure S21). Therefore, compounds 1 and 2 could be used for detection of Fe3+ and Fe2+ ions in the presence of all other competing metal ions. Furthermore, to ascertain the selectivity of probes 1 and 2 toward Fe2+ and Fe3+ ions, a competition experiment between Fe3+ and Fe2+ has been performed on the basis of absorption spectra (as both the probes showed quenching of emission intensity). We have taken the mixtures of Fe2+ and Fe3+ in different ratios such as Fe3+/Fe2+ = 9:1, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9. The absorption spectra of 1 and 2 were found to be dominated by the presence of Fe3+ ion up to 2:8 ratio (Fe3+/ Fe2+) (Figure S22). Up to a 2:8 ratio of Fe3+/Fe2+, the appearance of the new low energy band at 630 nm was observed due to the formation of ferrocenium ion formed by the oxidation of ferrocene unit by Fe3+ ion. This experiment clearly showed that Fe2+ ion did not produce any significant offset toward the detection of Fe3+ ion, whereas the presence of Fe3+ ion even in lesser amounts obstructed the detection of Fe2+ ion. Therefore, these probes can selectively discriminate Fe3+ ion from Fe2+ ion. Reversibility of Probes 1 and 2. Reversibility is an important aspect of a good sensor for its wide and practical applications. The interaction between 1 and Fe2+ was reversible,

which was verified by the introduction of EDTA as a complexing agent into the system containing 1 (1 μM) and Fe2+ (1 equiv) since EDTA has a strong tendency to chelate Fe2+ ion. The experiment showed that the introduction of EDTA led to the recovery of the fluorescence. When Fe2+ was further added to the system, the fluorescence of 1 was again quenched (Figure S23). This process could be repeated at least three times without loss in sensitivity, which clearly demonstrates the high degree of reversibility of the complexation/decomplexation process. Similarly, the reversibility of the oxidation process by Fe3+ for probes 1 and 2 was verified by the introduction of sodium L-ascorbate as a reducing agent into the oxidized solution. Furthermore, for the analytical applicability of sensors 1 and 2, we have investigated their action in solid support. The sensing of Fe2+ by 1 and 2 worked very well on a solid support. In this experiment, the silica gel (100−200 mesh, 5.0 g, colorless) was soaked with 1 and 2 (10−3 M) in 15 mL of CH3CN solution and then dried to afford a faint yellow color silica gel due to the adsorption of the sensor on the surface. When the treated silica gel was added to an aqueous solution of Fe2+ and Fe3+ ions (10−3 M) separately, the faint yellow color instantly turned to a light green and dark greenish blue color, respectively (Figure S24). The instantaneous color change of the solid silica gel in aqueous solution clearly indicates the practical application of these sensors for the qualitative detection of Fe2+ and Fe3+ in aqueous media. The results indicate that we can use this silica-supported method not only in the determination of Fe2+ and Fe3+ ions from water but also in the extraction/separation of Fe2+ and Fe3+ ions from water. 1 H NMR Spectroscopic Titration. To obtain additional information about the coordination mode of receptor 1 with Fe2+ metal cation and to further support the results obtained by electrochemical and spectroscopy experiments, we have performed the 1H NMR spectroscopic titration of receptor 1 in DMSO-d6 (Figure 12). The most significant spectral changes observed upon addition of Fe2+ ion to a solution of free receptor 1 are the following: (i) The Hc proton attached to triazole unit became upfield-shifted by 0.15 ppm and broadened. (ii) Similarly, the Ha proton became upfield-shifted H


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Organometallics by 0.11 ppm, whereas no significant shift or splitting was observed for Hb proton. Chemical shifts of all other protons did not show any significant shift upon addition of Fe2+ ion. From the 1H NMR chemical shifts, it can be concluded that the plausible binding sites of Fe2+ are the N atom of the triazole ring and O atom of the pendant CH2OH group attached with naphthalene ring. Computational Details. Geometry optimization of both receptors 1 and 2 (Figure S25) was done at B3LYP/def2-SVP level24 (Figure 13). The calculated structural features of 1 are in

Figure 14. DFT optimized hydrogen bonding interactions in 2.

bonding orbitals involved in hydrogen bonds. NBO analysis predicts HB 1 in 2 arising from the interaction of oxygen atom lone pairs with the empty antibonding (σ*) C−H orbital and computed second order perturbation energy (E2) of the hydrogen bond is 0.17 eV (assuming additivity of interactions). But in case of HB 2 and HB 3 any type of appreciable covalent interaction (as low as 0.01 eV) could not be found, suggesting that they are weak electrostatic (or might be even weaker van der Waals) interactions. Therefore, from the combined NBO and structural analysis, it could be pointed out that hydroxymethyl group (−CH2OH) plays a pivotal role in stabilization of 2 in unusual eclipsed configuration through C− H···O type hydrogen bond. We think that if we remove the −CH2OH moiety then the potential binding site will be only N atom of triazole unit and maybe the O atom of the ether linkage. Therefore, selectivity will change, and it may sense other metal ions like Hg2+ or Pb2+ ion.25a,f It should be noted that the discussion is focused on inter-arm hydrogen bonds (albeit, other very weak intra-arm hydrogen bonds exist) since these interactions are primarily responsible for the nontrivial configuration of 2. Similarly in the case of 1, a weak C−H···O type hydrogen bond exists with dH···O = 2.272 Å, ∠C−H···O = 120.6° (the angle is very bent, but the distance is quite small, resulting in a weak H-bond also suggested by NBO). NBO analysis suggests that the interaction is between oxygen lone pair orbitals and empty antibonding (σ*) C−H orbital with E2 energy of 0.06 eV. Geometry optimization of complexes [1·Fe](ClO4)2 and [2· Fe2+] was done at B3LYP/def2-SVP level, and further NBO analysis was done at same level but with improved basis set def2-TZVP on N and O atoms. The optimized structures and frontier Kohn−Sham orbitals of complexes are presented in Figure 13. The relaxed structures of [1·Fe](ClO4)2 is quite similar to the metal center surrounded by oxygen and nitrogen atoms in distorted octahedral type environment. The optimized geometry of [2·Fe2+] depicts a binding core with near-perfect and distorted square pyramidal complex. The selected structural and bonding features of complexes are tabulated in Table S1.

Figure 13. Frontier molecular orbitals (isovalue 0.03) of 1, [1· Fe2+](ClO4)2 (top), and 2, [2·Fe2+] (bottom) as obtained from DFT calculations.

good agreement with the crystal structure data. The geometry optimization of 2 resulted in a structure where both the arms are held in an eclipsed conformation, which is unusual for most of the disubstituted ferrocene-triazole derivatives.25 The stabilization of this configuration can be attributed to the C− H···O and C−H···N type hydrogen bonding interaction between the arms of 2. The structural features of the plausible hydrogen bonding interactions are as follows: hydrogen bond (HB) 1: dH···O = 2.277 Å, ∠C−H···O = 166.2°; HB 2: dH···N = 2.540 Å, ∠C−H···N = 155.9°; HB 3: dH···N = 2.448 Å, ∠C−H··· N = 143.4° (Figure 14). On the basis of the preliminary structural features, both HB 2 and HB 3 indicate weaker interactions as compared to those of HB 1. Furthermore, natural bond orbital (NBO) analysis at B3LYP/def2-SVP (def2-TZVP on N and O) level was performed to gain insight into the interaction between donor and acceptor natural

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CONCLUSIONS We have developed a couple of easy-to-synthesize ferrocenebased receptors 1 and 2 for the selective on-site detection and discrimination of the two oxidation states of iron. The developed sensor molecules are capable of discriminating the two oxidation states of iron colorimetrically and electrochemically. Moreover, the discrimination of Fe3+ from Fe2+ has been well-demonstrated by UV−vis spectroscopy as well as colorimetrically by providing conditions suitable for oxidation and reduction of iron in in situ qualitative determination of Fe3+ and Fe2+ experiment. To best of our knowledge, this is the first I


Article

Organometallics

Then, the crude product was purified by silica gel column chromatography. Elution with EtOAc/hexane (7:3, v/v) yielded yellow solid compound 1 (0.2 g, 71%). Compound 2 was synthesized by following the procedure adopted for 1 using the following chemicals: 1,1′-bis (azidomethyl) ferrocene (0.150 g, 0.506 mmol), 2.1 equiv of (2-(prop-2-yn-1-yloxy)naphthalene-1-yl) methanol (0.225 g, 1.062 mmol), CuI (0.202 mmol), and DBU (0.253 mmol). The reaction was continued for 5 h at 65 °C. The crude product was purified by silica gel column chromatography. Elution with EtOAc/hexane (9:1, v/v) yielded yellow solid 2 (0.170 g, 47%). Compound 1. 1H NMR (CDCl3, 400 MHz): δ = 8.13 (d, 1H, Hnaphthalene, J = 8.8 Hz), 7.81−7.78 (m, 2H, Hnaphthalene), 7.53−7.50 (m, 1H, Hnaphthalene), 7.49 (s, 1H, Htriazole), 7.40−7.35 (m, 2H, Hnaphthalene), 5.34 (s, 2H, OCH2), 5.27 (s, 2H, CH2OH), 5.14 (s, 2H, NCH2), 4.21 (t, 2H, HCp, J = 1.6 Hz), 4.18 (t, 2H, HCp, J = 1.6 Hz), 4.15 (s, 5H, HCp), 2.45 (s, 1H, CH2OH); 13C NMR (CDCl3, 100 MHz): δ = 153.9, 143.9, 133.0, 130.2, 129.8, 128.5, 127.1, 124.2, 123.4, 123.3, 122.2, 115.4, 80.7, 69.2, 69.0, 68.9, 63.8, 55.6, 50.2. ESI-MS, m/z (relative intensity): 453 (M+ + 1), 475 (M+ + 23); Anal. Calcd for C25H23FeN3O2 C, 66.24; H, 5.11; N, 9.27. Found: C, 66.05; H, 4.94; N, 9.14. Compound 2. 1H NMR (CDCl3, 400 MHz): δ = 8.07 (d, 2H, Hnaphthalene, J = 8.4 Hz), 7.78−7.75 (m, 4H, Hnaphthalene), 7.59 (s, 2H, Htriazole), 7.49−7.45 (m, 2H, Hnaphthalene), 7.38−7.33 (m, 4H, Hnaphthalene), 5.36 (s, 4H, OCH2), 5.12 (s, 4H, CH2OH), 5.05 (s, 4H, NCH2), 4.08 (t, 4H, HCp, J = 2 Hz), 4.02 (t, 4H, HCp, J = 2 Hz), 3.52 (s, 2H, CH2OH); 13C NMR (DMSO-d6, 100 MHz): δ = 153.6, 143.5, 133.5, 129.7, 129.5, 128.5, 126.7, 124.7, 124.4, 124.0, 123.5, 116.4, 83.5, 69.9, 69.7, 63.6, 53.7, 49.1. ESI-MS, m/z (relative intensity): 721 (M+ + 1); Anal. Calcd for C40H36FeO4N6 C, 66.67; H, 5.04; N, 11.66. Found: C, 66.29; H, 4.92; N, 11.52. X-ray Crystallographic Analysis. Suitable X-ray quality crystals of 1 were grown by slow diffusion of a hexane/EtOAc (4:6, v/v) solution. Single crystal X-ray diffraction studies were undertaken, and intensity data were collected on a Super Nova, Dual, Cu at zero, Eos diffractometer. The crystal was kept at 100.00(10) K during data collection. Using Olex2,29 the structure was solved with the Superflip30 structure solution program using charge flipping and refined with the SHELXL31 refinement package and least squares minimization. The hydrogen atoms were refined isotropically at calculated positions using a riding model. Crystal Data for 1. Formula, C25H23FeN3O2; crystal system, space group: monoclinic, P21/c; unit cell dimensions, a = 27.6493(16) Å, b = 7.5364(4) Å, c = 10.2449(6) Å, β = 92.225(5); Z = 4; density (calcd) 1.410 mg/m3; Final R indices [I > 2σ(I)] R1 = 0.0930, wR2 = 0.2288 (all data); θ range (deg) 3.20−66.76; total reflections collected 3720, independent reflections 2410; goodness-of-fit on F2 1.004. Computational Details. Gaussian 0932 software was used to perform all theoretical calculations at DFT level with B3LYP hybrid functional33 in gas phase. All-electron def2-SVP24 basis set (def2-ECP on Hg)34 was employed for optimizing all molecular structures in singlet state without any symmetry constraints. Furthermore, NBO were calculated with similar basis set except for usage of triple-ζ quality def2-TZVP on nitrogen and oxygen atoms for an improved analysis of binding core. The real harmonic vibrational wave numbers obtained for each relaxed structure confirmed the optimized geometry. An iso surface value of 0.03 has been used to visualize the frontier Kohn− Sham orbitals. Natural population analysis was performed with NBO 3.035 as implemented in Gaussian 09. Bond strengths were evaluated with Wiberg Bond Index.36

example of ferrocene-based sensor molecule which can selectively discriminate Fe3+ from Fe2+ through multiple channels. The events of complexation with Fe2+ have been further supported by computational calculations. Detailed theoretical calculations indicated that the hydroxymethyl group (−CH2OH) plays a pivotal role in sensing Fe2+/3+ ions selectively and in stabilization of 2 in unusual eclipsed configuration through C−H···O type hydrogen bonding. These sensors can detect Fe2+ and Fe3+ up to 30 and 15 ppb level by fluorescence spectroscopy, suggesting its applicability to detect Fe2+/3+ in the nanomolar range. In addition to the very low detection limit (LOD), our strategy is convenient and inexpensive, and both difficult ligand synthesis and complex signal transduction are avoided. This kind of ferrocene-based molecular tools for selective and sensitive detection and discrimination of Fe3+ and Fe2+ may find their use in biological assays in the real-time applications.

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EXPERIMENTAL SECTION

Materials and Methods. All reagents used were of analytical grade and used without any further purification. The perchlorate salts of Ag+, Cr3+, Al3+, Mg2+, Fe3+, Mn2+, Zn2+, Pb2+, Fe2+, and Co2+ were purchased from Alfa Aesar. The perchlorate salts of Li+, Na+, K+, Cu2+, and Hg2+ and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and CuI were purchased from Sigma-Aldrich. Ferrocene, N,N,N′,N′-tetramethyl ethylene diamine (TMEDA), POCl3, n-butyllithium (2.5 M in hexane), propargyl bromide, NaN3, and NaBH4 were purchased from Loba Chemie. DMF and acetonitrile (HPLC) were purchased from Thermo Fisher scientific and freshly distilled prior to use. Chromatography was carried out using 100−200 mesh silica gel in a column of 2.5 cm diameter. All the necessary solvents were dried by conventional methods and distilled under N2 atmosphere before use. Mono (azidomethyl) ferrocene and 1,1′-bis (azidomethyl) ferrocene were synthesized as per the literature procedure.26,27 All electrochemical measurements were carried out with an electrochemical workstation with a three-electrode cell, including a glassy carbon as working electrode, Pt wire as counter electrode, and Ag/Ag+ as reference electrode, separated from the solution by a plug. The cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were carried out at a scan rate of 0.05 V s−1, in the potential range of 0−1 V vs Ag/Ag+. The CH3CN/H2O (2/8) was used as a solvent, and 0.01 M [(n-C4H9)4NClO4] (TBAP) was used as a supporting electrolyte for all the voltammetric measurements. The working electrode was cleaned after each run. The UV−vis absorption spectra were carried out in CH3CN/H2O (2/8, v/v) solutions at c = 2.5 × 10−4 M as it is stated in the corresponding figure captions, and the fluorescence spectra were carried out at c = 1.5 × 10−6 M as stated in the corresponding figure captions. Instrumentation. The 1H and 13C NMR spectra were recorded on BRUKER 400 MHz FT-NMR spectrometers, using tetramethyl silane as an internal reference and CDCl3 as a deuterated solvent. The absorption spectra were recorded with an Agilent Technologies Cary Series UV−visible spectrophotometer at room temperature. Fluorescence was recorded with HORIBA Scientific Fluoromax-4 Spectrophotometer. CV performed on a CH Instruments Electochemical Analyzer (Model 6003E). ESI-MS measurement was carried out on PerkinElmer Flexar SQ 300 MS Detector. Caution! Metal pechlorate salts are potentially explosive in certain conditions. All due precautions should be taken while handling perchlorate salts!28 Synthesis of 1 and 2. Mono (azidomethyl) ferrocene (0.150 g, 0.622 mmol) and 1.1 equiv of (2-(prop-2-yn-1-yloxy) naphthalene-1yl) methanol (0.145 g, 0.684 mmol) were taken in a dry Schlenk flask in distilled DMF. It was thoroughly degassed with argon for 30 min. Catalytic amounts of CuI (0.248 mmol) and DBU (0.311 mmol) were added to it, and the resulting solution was heated at 65 °C for 4 h. The reaction mixture was diluted in dichloromethane and washed several times with an excess of methanol/water (1:1, v/v). The organic phase was separated, and the solvent was removed under reduced pressure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00199. 1 H, 13C, and ESI-MS data of 1 and 2; electrochemical data for 1 and 2 upon titration with different metal ions; J


Article

Organometallics UV−vis spectra and fluorescence spectra upon titration with different metal ions; ESI-MS spectrum of [1·Fe2+] and [2·Fe2+]; Job’s plot; detailed computational studies (PDF) Cartesian coordinates (XYZ)

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Accession Codes

CCDC 1476857 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Phone: 0661-2462986, +918895012676. ORCID

Arunabha Thakur: 0000-0003-4577-3683 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support of the Department of Science and Technology, DST, New Delhi for Inspire Faculty fellowship is gratefully acknowledged. V.B. is grateful to Department of physical Sciences, IISER Kolkata for the Institute Fellowship.

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REFERENCES

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