Electrochemical behavior of phthalocyanines containing high oxidation state central metals: Titanium(IV), vanadium(IV), and tantalum(V)
Graphical abstract
The syntheses of titanium(IV), vanadium(IV) and tantalum(V) phthalocyanine complexes peripherally or non-peripherally substituted with mercaptopyridine groups is described. The non-peripherally substituted complexes are more difficult to reduce than peripherally substituted derivatives.
Highlights
► V, Ti and Ta phthalocyanines were synthesized. ► The complexes contain 2-mercaptopyridine at α or β positions. ► Ti and Ta phthalocyanines are reduced both at the ring and at the central metal. ► V complexes are reduced only at the ring.
Introduction
Metallophthalocyanines (MPcs) have attracted a lot of attention for a very long time due to their remarkable properties that include flexibility, chemical and thermal stabilities, semiconductivity and photoconductivity [1], [2]. The major drawback with MPcs is their poor solubility in most common solvents which consequently restricts them from being fully utilized. To overcome this problem, bulky substituents can be introduced onto the phthalocyanine ring causing a considerable decrease in the strong interactions between the phthalocyanine rings [3], [4]. Substitution with electron donating groups on the non-peripheral (α) or peripheral (β) positions results in a bathochromic shift of the Q band [5], [6], [7]. Sulfur containing substituents generally afford phthalocyanines that absorb in the near infrared region (NIR) [8], which are useful as optical data storage (ODS) devices and for security applications, particularly MPcs that absorb between 700 and 1000 nm region [8]. Thiol substituted MPcs may also be used to modify gold electrodes through the formation of self assembled monolayer (SAM) films [9], [10], [11]. The syntheses of MPcs containing substituents with sulfur groups are thus important for their use in a number of practical applications.
Even though many MPcs have been extensively studied, there has been limited attention paid to the synthesis and electrochemistry of titanium(IV), tantalum(V) and vanadium(IV) phthalocyanines due to the difficulty in synthesis of these complexes, especially TaPc and VPc derivatives. The non-peripherally substituted derivatives are particularly rare. MPcs containing high oxidation central metals should result in ease of reduction of the central metal and/or ring, therefore they have potential applications in electrocatalytic reactions that require reduction, hence our interest in their electrochemistry in this work. The size of the central metal (especially Ta) makes these complexes to be readily demetalated. This work reports on the syntheses of tetra 2,(3) and 1,(4) – (2-mercaptopyridine)phthalocyanine complexes of Ta, Ti and V Scheme 1 (complexes 5a to 9a). The mercaptopyride substituted MPc complexes have been reported before [12], [13], [14] for Zn and Mn central metals. This ligand is used for the first time for the Ta, Ti and V phthalocyanine complexes. The ligand lends itself to the possibility of quaternization and the presence of the sulfur allows for red shifting and of particular importance in this work, it allows for ease of oxidation. The electrochemistry of these complexes is compared with that of previously reported TiPc, VPc and TaPc derivatives [15], [16], [17], [18], [19], [20], [21], [22], [23], Fig. 1. The electrochemistry of TaPc, TiPc and VPc derivatives can show complicated behavior which depends on substituents. Multielectron transfers have been implicated as well as adsorption of the complexes on electrodes using cyclic voltammetry [15], [17]. In addition, MPcs with electron donating substituents, in particular sulfur substituted MPcs often show interesting electrochemistry that involves the central metal, the Pc ring and the substituents on the Pc ring. The mercaptopyridine substituent was chosen in this work in order to assess the effects of this ligand when compared to benzyl and phenyl substituents.
Section snippets
Materials
1-Pentanol, titanium butoxide, tantalum(V) butoxide, vanadium tetrachloride, tetrabutylammonium tetrafluoroborate (TBABF4) and 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) were from Sigma–Aldrich. Potassium carbonate, ethanol, methanol, tetrahydrofuran (THF), chloroform, deuterated chloroform (CDCl3) acetone, and dimethylformamide (DMF), were obtained from Merck. The synthesis of the 2-mercaptopyridine phthalonitrile (3a and 4a) has been reported [14]. Ultra pure water was obtained from a Milli-Q
Synthesis and characterization
Scheme 1 gives the synthetic pathways for TiPc (5a and 6a), VPc (7a and 8a) and TaPc (9a) complexes discussed in this work. Complexes 3a and 4a were treated with corresponding metal salts: titanium(IV) butoxide, anhydrous tantalum(V) butoxide or vanadium(IV) chloride in the presence of DBU as the catalyst to afford TiPc (5a and 6a), VPc (7a and 8a) and TaPc (9a) complexes (Scheme 1).
Titanium, vanadium and tantalum are large metals that do not to fit perfectly in a phthalocyanine core center.
Conclusion
TiPc, VPc and TaPc derivatives substituted with mercapto-pyridine on the α and β positions were successfully synthesized. The Q band of the complexes shift to the red with the change in the central metal as follows: Ta > V > Ti. The reduction couples for all the complexes were reversible to quasi-reversible and oxidation processes are irreversible. Spectroelectrochemistry confirmed one metal reduction, with the rest of the redox processes being centered on the phthalocyanine ring, except the VPc
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 as well as Rhodes University.
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Phthalocyanines: Properties and Applications
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