Remarkable enhancement of Ca²⁺ affinity using a redox-switchable coordinating group†

Juan Pedro Merino ^ab, Adrián M. Abelairas ^a, Javier Hernández-Ferrer ^c, Ana M. Benito ^c, Wolfgang K. Maser ^c, José L. Vilas-Vilela ^bd, David Esteban-Gómez ^a, Alejandro Criado ^a, Jesús Mosquera *^a and Carlos Platas-Iglesias *^a
aCentro Interdisciplinar de Química e Bioloxía (CICA) and Departamento de Química, Facultade de Ciencias, Universidade da Coruña, 15071 A Coruña, Spain. E-mail: carlos.platas.iglesias@udc.es
^bMacromolecular Chemistry Research Group (labquimac), Department of Physical Chemistry, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), 48940 Leioa, Spain
^cInstituto de Carboquímica (ICB-CSIC), C/Miguel Luesma Castán 4, 50018 Zaragoza, Spain
^dBC Materials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain

First published on 2nd August 2024

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Introduction

Responsive materials are poised at the forefront of scientific research and technological innovation,¹ offering the potential to revolutionize domains like medicine, electronics, and energy storage.^2–4 These versatile materials exhibit adaptability in response to external stimuli like pH, light, or redox potential. Notably, redox-responsive materials are particularly significant in industry for their capability to employ electrons as a versatile trigger, offering promising applications across various domains.⁵

A specific category of redox-responsive materials encompasses those equipped with redox-switchable coordinating groups.⁶ These groups exhibit a remarkable ability to selectively bind or release metals in direct response to changes in electrical potential. To date, the structural diversity of redox-switchable coordinating groups is quite limited. Most examples are grounded in metal complexes, where the oxidation state of the metallic center significantly influences the donor capacity of adjacent atoms for the target metal.^6–11 A variation of this strategy has been recently published where an ortho-substituted closo-carborane was applied for the reversible capture of uranyl ions.¹² Other notable examples include redox-responsive groups capable of releasing metals, such as Ca²⁺, upon oxidation.^13–15

Building upon the pioneered work by Kaifer et al., where nitrobenzyl groups were employed as redox-switchable entities to enhance the affinity of crown ethers for sodium,¹⁶ herein we explore the application of 2-nitropyridine as a redox-switchable coordinating group for recognizing Ca²⁺. The Ca²⁺ ion plays a key role in several physiological processes including the signaling pathways of the nervous system.¹⁷ Electrochemical sensors represent the most promising alternative to overcome several limitations of luminescence sensors, to allow real-time monitoring of Ca²⁺ in live brains.^18,19

In its native state, the nitro moiety manifests limited coordinating capacity, accompanied by its electron-withdrawing characteristics, which concurrently diminish the Lewis basicity of the pyridine. However, upon reduction, the nitro moiety might undergo a transformative process, evolving into its radical anion form. This transition would endow 2-nitropyridine with the attributes of a robust bidentate coordinating group, facilitating effective metal binding. Significantly, thanks to its chemical simplicity and remarkable stability, this group can be effortlessly incorporated into the targeted chemical framework, offering a direct pathway for the development of novel redox-switchable ligands.

To check our hypothesis, we decided to couple 2-nitropyridine to a diaza-crown ether (Fig. 1). Crown ethers are extensively applied as building blocks in the field of supramolecular chemistry due to their unique capabilities to recognize metal cations.²⁰ They have a distinct cavity size and charge distribution that allows for selective recognition of a metal cation with a specific dimension. This ligand was inspired in MACROPA, a well-known chelator containing pyridinecarboxylate groups, which was developed for selective complexation of large metal ions.^21–24

Results and discussion

Design, synthesis and electrochemical characterization of receptor L₁

A primary advantage of 2-nitropyridine is its chemical simplicity. Numerous commercially available analogs of this molecule simplify their direct incorporation into the desired platform. Specifically, L₁ was prepared in quantitative yield by treating 4,13-diaza-18-crown-6 with 2-(bromomethyl)-6-nitropyridine in acetonitrile in the presence of sodium carbonate as base (reflux, 48 h, Fig. 1a). We envisaged that the poor coordinating ability of the pendant arms will result in weak binding to alkaline and alkaline-earth cations, while electrochemical reduction will result in strong coordination of the pendant arms after formation of the radical anion form. DFT studies supported this design, as the CaL₁²⁺ complex is characterized by a weak coordination of the 2-nitropyridine units, with Ca–N and Ca–O distances of 2.763 and 2.563 Å, respectively. These distances decrease dramatically upon reduction of the system, amounting to 2.543 (Ca–N) and 2.385 Å (Ca–O) in CaL₁²˙ (Fig. 1b, see ESI† for computational details).

The cyclic voltammogram (CV) of an acetonitrile solution of L₁ (0.1 M NBu₄PF₆) shows a single electrochemically quasi-reversible reduction occurring at a E_1/2 value of −864 mV versus the silver couple (Ag⁺/Ag⁰, Fig. 2). According to literature, the reduction of the nitro group attached to aromatic compounds is strongly influenced by the nature of the solvent.²⁵ In aprotic solvents, nitrobenzene can undergo up to two consecutive one-electron reduction steps, yielding the radical anion and the dianion successively (Scheme S1†).²⁶ Both steps are reversible, however the dianion tends to react rapidly with solvent, electrolytes or impurities. Analyzing (eqn (S1)†) the CV of L₁ with the Randles–Sevcik equation, gives an electron number of 1.6 e⁻ for the standard equation, and 1.9 e⁻ for the equation with parameters corrected by Matsuda.²⁷ This is consistent with the expected behavior of nitro groups in acetonitrile,²⁸ and thus a two-electron reaction can be unequivocally assumed, with each nitropyridine group contributing one electron. With this insight, we can infer that the observed quasi-reversible process is attributed to the formation of a dianion with one electron on each nitrobenzene. Additionally, the presence of a singular redox couple suggests that both active redox groups are sufficiently separated in space, ensuring that the reduction of the first group, yielding L₁˙⁻, does not influence the subsequent reduction leading to the formation of L₁²˙⁻.

Electrochemical response of L₁ in the presence of Ca²⁺ and affinity constants

The CV of L₁ was replicated following the addition of 1 equiv. of calcium trifluoromethanesulfonate [Ca(OTf)₂]. Strikingly, the resulting CV profile significantly deviates from that of L₁ in the absence of metal (Fig. 3a). The primary distinction arises from the observation of two quasi-reversible redox couples rather than one. We propose that, unlike the scenario in the absence of the metal, the interaction with the Ca²⁺ ion forces the two nitropyridine groups into proximity, leading to repulsion between the radical anions and hindering the second reduction.

The second notable distinction pertains to the E_1/2 values, which exhibit a noteworthy anodic shift potential. Specifically, the E_1/2 value for the initial reduction is −208 mV, and for the subsequent reduction, it is −416 mV. Thus, the initial reduction shifted by 0.656 V respect to L₁ in the absence of metal. This substantial variance is associated with the distinct affinity constants of L₁²˙⁻, L₁˙⁻, and L₁˙ for Ca⁺². Thus, a UV-vis absorption spectrophotometric titration was applied to determine the affinity constant of L₁ for Ca²⁺ in the conditions applied in the CV experiments, specifically 0.1 M NBu₄PF₆ in acetonitrile at room temperature. The absorption spectrum of L₁ experiences significant changes upon addition of Ca²⁺, with the absorption maximum at 278 nm undergoing a blue shift to 274 nm, with well-defined isosbestic points developing during the titration (Fig. S9, ESI†). The spectrophotometric data were successfully fitted to a 1:1 binding isotherm with a binding constant of K = 3.6(1) × 10⁶ M⁻¹ using standard methods (see ESI† for details).²⁹ This value, along with the variation in reduction potentials observed in the presence of calcium, was utilized to derive the affinity constants of the reduced ligands with eqn (1):

	(1)

An analogous expression was employed to estimate the association constant of L₁²˙⁻ (see ESI† for details). Previous studies used an analogous expression to estimate the stability of complexes when the metal ion is the redox-active species.^30,31

L₁˙⁻ showed the highest affinity constant for Ca²⁺, attaining an extraordinary logK value of 17.65. The Ca²⁺ complexes with the highest stabilities are formed with anionic ligands such as DOTA⁴⁻, reaching a logK value of 17.2.³² To our knowledge, this signifies the highest reported affinity constant for this cation. For comparison, complexes with crown ethers and cryptates are characterized by logK values <7.0, although some Ca²⁺ cryptates were found to be particularly inert.³³ The structurally related MACROPA²⁻ forms a Ca²⁺ complex with logK value of 5.25 in aqueous medium.³⁴ Regarding other redox-switchable hemilabile ligands, information on their affinities is frequently unreported. However, it is anticipated that the reduced form of these ligands would demonstrate a significantly lower affinity for this ion, attributed to their dependence on redox-active metals. In contrast to L₁, where the reduction induces the formation of strong coordinating groups, the reduction in other ligands occurs over a metal that is not directly coordinated with Ca⁺², leading to a reduction in electrostatic interaction.

Interestingly, the fully reduced form of L₁, denoted as L₁²˙⁻, displays a lower affinity compared to the aforementioned intermediate, with a logK value of 10.07. The unexpected lower affinity of L₁²⁻ could be ascribed, at least in part, to the repulsion that arises between the two radical anions when they are in proximity and interacting with the metal. Indeed, the angle defined by the least square planes of the aromatic units in the DFT structure of CaL₁²⁺ (35.8°) decreases to 22.4° in CaL₁˙⁺, suggesting attractive interactions among the two aromatic units in the latter. This interaction energy between the two arms was estimated by DFT to be ∼−35 kJ mol⁻¹ in CaL₁˙⁺. However, DFT predicts a very important repulsive interaction between the pendant arms in CaL₁²˙ (+237 kJ mol⁻¹, Fig. 3b). DFT provides values for the first and second reduction processes of −286 and −259 kJ mol⁻¹, in line with the CV data that shows that the first reduction is more favorable than the second. The energy difference among the two processes corresponds to a difference in potential of ΔE = 270 mV, in reasonable agreement with the experimental data of 208 mV. The analysis of kinetic electrochemical parameters (Table S1†) also agrees with a process involving significant changes in conformational and solvation energies.³⁵

The former findings suggest that a modified version of the L₁ ligand, featuring a single redox-switchable coordinating group, is anticipated to display a solitary quasi-reversible redox couple upon coordination with Ca⁺², aligning closely with the initial reduction potential of L₁. To test this hypothesis, we synthesized ligand L₂, differing from L₁ by the absence of one of the 2-nitropyridine moieties. As expected, L₂ exhibited a single redox couple for both the molecule in the absence and presence of Ca⁺² with E_1/2 values of −811 and −225 mV respectively (Fig. 4).

Electrochemical response of L₁ in the presence of different cations

To get further insights on the performance of L₁ as redox-switchable ligand, we recorded CV experiments in the presence of equimolar quantities of additional redox-inactive mono- and di-positive metal ions, namely, Na⁺, K⁺, Ca²⁺, Sr²⁺ and Ba²⁺. All the cations exhibited a behavior akin to Ca²⁺, with two distinct redox couples observed at potentials more positive than those recorded for the ligand in its original state (Fig. 5). As depicted in Fig. 5, among the Group 2 cations, Sr²⁺ exhibited E_1/2 values that closely resembled those of Ca²⁺ at −284 and −451 mV, followed by Ba²⁺, with values of −341 and −494 mV. In contrast, alkali metals displayed values much more similar to the original ligand, with Na⁺ at −580 and −811 mV and K⁺, at −757 and −888 mV. On this basis, it can be concluded that, (i) in all cases, L₁˙⁻, has a larger affinity for the metals than the fully reduced L₁²˙⁻, and (ii) the impact of reduction on the affinity for the metals follows the order Ca⁺² > Sr⁺², Ba⁺² ≫ Na⁺ > K⁺. Akin to previously reported redox-switchable coordinating groups, this order correlates with the Lewis acidity of the cations, directly linked to the charge density of the metal ions. It has been shown that the Lewis acidity of metal ions correlates very well with the pK_a of the aquated ions,³⁶ which have been demonstrated to be very good descriptors of Lewis acidity.³⁷ Indeed, the plots of the observed E_1/2 values versus the pK_a data of the corresponding aqua-ions are linear, with the potential becoming more negative as the pK_a increases (Fig. S8, ESI†). The quality of the linear regression is particularly good for the first reduction wave (R² > 0.995). The poorer linear correlation of the second reduction wave (R² > 0.95) suggests that the repulsive effects among the two reduced nitropyridine groups depend significantly on the size of the metal ion.

Conclusions

In conclusion, our findings show that 2-nitropyridine effectively functions as a redox-switchable coordinating group. Its limited coordinating capabilities change dramatically upon reduction, forming a robust bidentate ligand. Leveraging its chemical simplicity, we integrated this group into a diaza-crown ether framework, yielding a redox-responsive ligand finely tuned for the biorelevant Ca²⁺ cation. This ligand allows attaining unprecedented affinity for Ca²⁺ upon its reduction (logK value of 17.65). The principles presented here will allow the design of electrochemical sensors for Ca²⁺ and other metal ions, which requires anchoring the electroactive ligand on the surface of electrodes.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge financial support from MCIN/AEI/10.13039/501100011033 and “ESF Investing in your future”: RYC2020-030183-I and PID2021-127002NA-I00 (A. C.) RYC2019-027842-I (J. M.), PID2022-138335NB-I00 (C. P.-I.). Financial support from Xunta de Galicia (ED431C 2023/33 and ED431F 2022/02), Gobierno de Aragón (DGA, T03_23R), and Government of the Basque Country (Grupos de Investigación IT1756-22) are also acknowledged. J. P. M. acknowledges a Margarita Salas postdoctoral grant under the “Convocatoria de ayudas para la recualificación del sistema universitario español para 2012–2023”.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01612b

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