Click Chemistry Derived Hexa-ferrocenylated 1,3,5-Triphenylbenzene for the Detection of Divalent Transition Metal Cations

Abstract

The 1,3-dipolar cycloaddition reaction (click chemistry approach) was employed to create a hexa-ferrocenylated 1,3,5-triphenylbenzene derivative. Leveraging the presence of metal-chelating sites associated with 1,2,3-triazole moieties and 1,4-dinitrogen systems (ethylenediamine-like), as well as tridentate chelating sites (1,4,7-trinitrogen, diethylene triamine-like) systems, the application of this molecule as a chemosensor for divalent transition metal cations was investigated. The interactions were probed voltammetrically and spectrofluorimetrically against seven selected cations: iron(II) (Fe²⁺), cobalt(II) (Co²⁺), nickel(II) (Ni²⁺), copper(II) (Cu²⁺), zinc(II) (Zn²⁺), cadmium(II) (Cd²⁺), and manganese(II) (Mn²⁺). Electrochemical assays revealed good detection properties, with very low limits of detection (LOD), for Co²⁺, Cu²⁺, and Cd²⁺ in aqueous solution (0.03-0.09 μM). Emission spectroscopy experiments demonstrated that the title compound exhibited versatile detection properties in solution, specifically turn-off fluorescence behavior upon the addition of each tested transition metal cation. The systems were characterized by satisfactory Stern-Volmer constant values (10⁵-10⁶ M^-1) and low LOD, especially for Zn²⁺ and Co²⁺ (at the nanomolar concentration level).

Figure 1

Synthesis of compounds 2 and 3 (reagents and reaction conditions: i. K₂CO₃, propargyl bromide, acetonitrile, 48 h, reflux, 97% yield; ii. (ferrocenylmethyl)azide, (CH₃CO₂)₂Cu, sodium ascorbate, H₂O, t-butanol, room temperature, 5 days, 42% yield). Structure of target compound 3 with the structural motifs marked with colors is also presented.

Figure 2

Representative characterization data for compound 3: (a) structure of 3 with atom labels for ¹H NMR analysis, inset of the (b) ¹H NMR (DMSO-d₆, 500 MHz) and (c) ¹H DOSY NMR spectra of 3 (DMSO-d₆, 500 MHz), (d) UV–vis (blue curve) and emission (λ_ex = 312 nm; red curve) spectra of 3 in DMSO (2 × 10^–5 M).

Figure 3

Cyclic voltammograms (CVs) of compound 3 recorded in the mixture of DMSO–DCM (v/v; 1:3) with the addition of 50 mM tetrabutylammonium hexafluorophosphate (TBAPF₆). Top inset: CV voltammogram for scan rate 0.05 V s^–1. Bottom insets: Dependencies of anodic peak currents vs square root of scan rate (left) and scan rate (right). Experimental conditions: C₃ = 0.17 mM, C_TBAPF6 = 50 mM, T = 21 °C.

Figure 4

DP voltammograms of the receptor (GC/compound 3-Nafion-TBAPF₆) recorded in 100 mM TBABF₄ aqueous solution (dashed lines) with different addition of Cd²⁺ as the representative analyte (solid lines). Insets: Dependencies of anodic peak currents vs Cd²⁺ concentration. Experimental conditions: C₃ = 1.7 mM, T = 21 °C, modulation time: 0.002 s, interval time: 0.1 s, modulation amplitude: 0.04995 V.

Figure 5

Possible binding sites for noncovalent interactions between compound 3 and cationic species together with exampled possible structures of the formed complexes (water molecules are not included for the clarity of the image; structures are presented with cadmium(II) as the representative cation). The PM6-optimized structure of 3 is also presented (hydrogen atoms are omitted for clarity).

Figure 6

DFT-predicted structures and binding energies of relevant fragments of 3 with Cd²⁺ cation and water molecules.

Figure 7

Changes in the emission spectra of 3 in the presence of Cd²⁺ as the representative analyte (C₃ = 2 × 10^–7 M, DMSO–H₂O 1:1 v/v, λ_ex = 270 nm).