Electrochemical Investigation of Benzil/Aluminum Ion Interactions

Introduction

       Benzil has been investigated as a model diketone in various electrochemical studies (1-4 ).  Previous work in this laboratory has dealt with the effect of metal ion addition on the electrochemical behavior of 9-fluorenone (5).  In the present work, the effect of aluminum ion additions to acetonitrile solutions of benzil has been studied.  The presence of adjacent carbonyl groups in benzil suggests that benzil reduction products may be effective ligands toward added metal ions.  The goal of this study is to determine the effects of these metal/ligand interactions upon the cyclic voltammetric characteristics of the benzil system.  

 Experimental

       Acetonitrile (AN), benzil, and aluminum trifluoromethansulfonate [aluminum triflate, Al(TfO)₃] were obtained from Aldrich Chemical Co. 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

     Benzil  undergoes two successive electron-transfers in acetonitrile/TEA BF₄ (Figure 1a), although the second process is somewhat irreversible due to proton donation from acetonitrile (2-4).  The addition of Al(TfO)₃ at the 1:1 Al(TfO)₃ : benzil  level produces a new reduction peak at -0.70V, which is assigned to the reduction of a benzil:Al³⁺ complex. The second reduction process has become shifted to slightly more positive potentials, suggesting interaction of the benzil radical anion with Al³⁺.  The reduction process at -1.80 in Figure 1a is due to the reduction of Al³⁺ in the solution.   Addition of further  Al(TfO)₃ to give 2:1 Al(TfO)₃ : benzil (Figure 1b) gives the -0.7 V reduction process as the predominant response. This general behavior supports a strong interaction of Al³⁺ with benzil reduction products, ultimately giving the benzil dianion with each oxygen complexed with one Al³⁺. 

       

References

1. H. E. Stapelfeldt and S. P. Perone,  Analytical Chemistry,  41(4), 623 (1969).
2. M. D. Ryan and D. H. Evans,  J. Electroanal. Chem.,  67, 333 (1976).
3. M. Chandrasekaran,  M. Noel and V. Krishnan,  J. Chem. Soc., Perkin Trans.2, 979 (1992).
4. G. T. Cheek, ECS Transactions, 6(6), 47 (2007).
5. D. Canby, E. Sanders,and G. T. Cheek, J. Electrochem. Soc., 160(7), G3159 (2013).