Electrochemical Explorations of 9,10-Anthraquinone/Hafnium(IV) Ion Interactions in Nonaqueous Solvents

Abstract

Introduction The well-defined electrochemical behavior of quinones such as 9,10-anthraquinone (AQ) in nonaqueous solvents (1,2) suggests its use as a probe for interactions of metal cations with the AQ carbonyl oxygens. In previous work from this laboratory, the interaction of AQ with added Al(III) trifluoromethanesulfonate [Al(TfO)3] in adiponitrile has been shown to produce a shift in the initial AQ reduction process to more positive potentials (3). In contrast to the behavior seen for 9-fluorenone (4), however, the second AQ reduction process disappears even at the 0.4 : 1.0 Al3+ : AQ point. This observation suggests that AQ reduction products are good ligands for the Al3+ cation, with approximately three reduced AQ ligands coordinated to one Al3+. The present work explores whether this effect extends to other metals, particularly Hf4+. The general effect of Hf4+ on the cyclic voltammetry of AQ in adiponitrile is also treated. Experimental Adiponitrile (AN) and 9,10-anthraquinone (AQ) were obtained from Aldrich Chemical Co. Hafnium trifluoromethansulfonate [hafnium triflate, Hf(TfO)4] was obtained from Alfa-Aesar. Tetraethylammonium tetrafluoroborate [TEA BF4] was purchased from Southwestern Analytical Chemicals (SACHEM). Voltammograms were acquired with a PAR283 potentiostat using PowerSuiteTM software. Potentials are reported with respect to a Ag/AgCl (0.1M EMICl in EMI BF4) reference electrode (Cypress Systems). Vitreous carbon electrodes were obtained from Cypress Systems. All experiments were carried out in a Vacuum Atmospheres drybox. Results and Discussion A cyclic voltammogram of AQ in adiponitrile without added Hf(TfO)4 is shown in Figure 1a, which shows the usual two uncomplicated successive one-electron transfers. Upon addition of Hf(TfO)4 at the 0.4 : 1.0 Hf4+ : AQ level, however, a substantial positive potential shift is observed for AQ reduction (Figure 1b). This shift is due to complexation of one of the AQ carbonyl oxygens by Hf4+. As in the case of Al3+ addition, there is none of the original second AQ redox process at this point. This observation implies that Hf4+ is complexed by multiple AQ reduction products, as was also seen for Al3+ addition. Further additions of Hf(TfO)4 give only the shifted reduction process at the 1:1 Hf4+ : AQ point, with substantially more of the small reduction peak at +0.1 V in Figure 1b. The latter observation suggests the possibility that both AQ carbonyl oxygens can undergo complexation by Hf4+ in this system. The present results indicate extensive interaction of Hf4+ with AQ, including the role of reduced AQ species as multiple ligands for Hf4+. References N. A. Macıas-Ruvalcaba and D. H. Evans, J. Phys. Chem. C, 114 , 1285 (2010).L. Jeftic and G. Manning, J. Electroanal. Interfacial Echem, 26, 195 (1970).Electrochemical Society 226th Meeting, Fall 2014, Cancum, Mexico, ECS H1-1345.D. Canby, E. Sanders,and G. T. Cheek, J. Electrochem. Soc., 160(7), G3159 (2013). Figure 1