Investigation of electrocatalytic behaviour of low symmetry cobalt phthalocyanines when clicked to azide grafted carbon electrodes

https://doi.org/10.1016/j.jelechem.2020.113896Get rights and content

Highlights

  • The glassy carbon electrodes were azide functionalized followed by clicking of low symmetry Co phthalocyanines.

  • The constructed electrodes showed excellent electrocatalytic activity towards hydrazine oxidation.

  • Hydrazine oxidation peaks occurred at low potentials of 0.21 V and 0.26 V with a low detection limit of 0.94 μM.

Abstract

This work describes the electrochemical properties of low symmetry cobalt phthalocyanines namely, tris-[(4-tert-butylphenoxy)-4-(pent-4-yn-1-yloxy) phthalocyaniato] cobalt (II) (3) and tris-[(4-tert-butylphenoxy)-4-(4-ethybylbenzyl-oxy) phthalocyaniato] cobalt (II) (5). The complexes were characterized by a number of techniques including UV–Vis, mass, and infrared spectra, as well as elemental analysis. The glassy carbon electrodes were first azide functionalized then clicked to low symmetry phthalocyanines. The click reaction was confirmed using X-ray photoelectron spectra. The constructed electrodes showed excellent electrocatalytic activity towards hydrazine oxidation. Oxidation peaks with low potentials of 0.21 V and 0.26 V, for complexes 3 and 5, respectively were obtained. Complex-5 gave a better detection limit of 0.94 μM and electrocatalytic rate constant of 5.6 × 106 M−1 s−1.

Introduction

Metallophthalocyanines (MPcs) are macrocyclic 18π electron conjugated molecules. They are used in various fields such as dye sensitized solar cells [1], photodynamic therapy [2], non-linear optics [3] and electrocatalysis [4] due to their excellent stability to light, heat and harsh chemical environments. MPc's structure can be finely tuned to enhance the electrocatalytic activity [5].

MPcs containing Co, Ni, Fe and Mn as central metals have been reported to possess good electrocatalytic activity [[6], [7], [8]]. MPcs have been used for electrocatalysis of different analytes such as hydrazine [9], hydrogen peroxide [10], amitrole [11] and cysteine [12]. In this manuscript hydrazine is used as a test analyte.

Click chemistry is the reaction of terminal alkynyl containing complexes with azide moiety in the presence of Cu(I) catalyst resulting in triazole linkage [13]. Click chemistry has been used for electrode modification via two steps, the first step involves grafting of 4-azidobenzene diazonium salt followed by attachment of alkynyl containing complexes on the grafted electrode [[14], [15], [16]].

We have reported on electrode modification using click chemistry and symmetric alkynyl terminated MPcs [9,17,18]. In this work, we report on the electrode modification using asymmetric Co phthalocyanines since asymmetry is known to improve electrocatalytic activity of phthalocyanines and related porphyrins [19,20]. This is the first time that an electrode is modified with a mono alkynyl phthalocyanine via click chemistry for electrocatalysis. The tert-butyl substituents enhance solubility of the phthalocyanines due to their bulky nature, making purification easier. In addition tert-butyl ligands can donate electron density into the Pc ring improving electrocatalytic activity [21]. Cobalt is employed as a central metal due to the known electrocatalytic activity of CoPc derivatives [6,8,22]. The mechanism proposed for the oxidation of hydrazine suggests the involvement of the CoIIPc(−2)/CoIPc (−2) couple of the adsorbed CoPc [22], this will be discussed below.

We compare asymmetric CoPc containing one ethynylbenzyl group with the one containing one pentynoxy group, the former may improve the redox property of Pcs due to the presence of electron rich benzene groups.

Section snippets

Materials

Cobalt (II) chloride, N,N dimethylformamide (DMF), iron ferricyanide, iron ferrocyanide, tetrabutyl ammonium tetrafluoroborate (TBABF4), 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU), bromo tris (triphenylphosphine) copper (I) (Cu(PPh3)3Br), trimethylamine, 4-azidoaniline hydrochloride, and hydrazine monohydrate were obtained from Sigma-Aldich. Acetonitrile (ACN) was obtained from Minima. Type II water was obtained from an Elga PURELAB Chorus 2 (RO/DI) system. Alumina (~0.05, 0.3, 1, 10 μm) was

Syntheses of complexes 3 and 5

Scheme 1 shows the synthetic route of target asymmetric MPcs (3 and 5).

Novel mono alkynyl terminated Co phthalocyanines were synthesized by the statistical condensation reactions of 1 and 2 for 3, 1 and 4 for 5 with the anhydrous cobalt chloride in 1-pentanol. After work-up, the mixtures were purified by chromatography over silica gel, eluting with CHCl3 and a gradient of CHCl3–methanol up to 5% methanol to give pure CoPcs (3 and 5).

The CoPcs were characterized by the combination of FT-IR,

Conclusions

New low symmetry cobalt phthalocyanine were successfully synthesized and characterized. Glassy carbon electrodes were azide functionalized and clicked with cobalt phthalocyanines, the modified electrodes were characterized with SECM and XPS. The probes showed a good sensing properties with LOD of 0.94 μM. These types of electrodes offer some advantages over conventional electrodes in their ease of fabrication, sensitivity, simplicity and good electrocatalytic activity.

CRediT authorship contribution statement

Lekhetho S. Mpeta: Conceptualization, Investigation, Writing - original draft. Pinar Sen: Methodology, Data curation. Tebello Nyokong: Funding acquisition, Resources, Supervision, Project administration, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Department of Science and Technology (DST) and National Research Foundation (NRF), South Africa through DST/NRF South African Research Chairs Initiative for Professor of Medicinal Chemistry and Nanotechnology (UID 62620) as well as Rhodes University/DST Institute for Nanotechnology Innovation, Rhodes University, South Africa.

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