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

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 PowerSuite^TM  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

1. N. A. Macıas-Ruvalcaba and D. H. Evans,  J. Phys. Chem. C, 114, 1285 (2010).
2. L.  Jeftic and G. Manning,  J. Electroanal. Interfacial Echem, 26, 195 (1970).
3. Electrochemical Society 226^th Meeting, Fall 2014, Cancum, Mexico, ECS Abstract H1-1345.
4. D. Canby, E. Sanders,and G. T. Cheek, J. Electrochem. Soc., 160(7), G3159 (2013).