Electrochemical Investigation of Quinone Complexation by Lewis Acids in a Chloroaluminate Ionic Liquid

Abstract

The electrochemical behavior of 9,10-anthraquinone (AQ) has been investigated in the initially neutral aluminum chloride : 1-ethyl-3-methylimidazolium chloride ionic liquid. It has been found that AQ is characterized by a single irreversible redox process in the neutral melt. Additions of AlCl3 cause a positive potential shift, first by the formation of two reversible redox processes corresponding to AlCl3 complexes of AQ under mildly acidic conditions, and then by the appearance of a shifted irreversible redox process in an acidic melt. HfCl4 additions cause similar behavior, except that the reversible systems persist at higher Lewis acid levels than for AlCl3. At the 11:1 Lewis acid: AQ point, AQ is still soluble as an orange solution for AlCl3 additions, but a yellow solid precipitates from a yellow (amber) solution for HfCl4 additions.

Electrochemical Studies of 9,10-Anthraquinone in Adiponitrile

“…Upon addition of Al(TfO) 3 to a solution of AQ (0.40 Al 3+ :AQ) in ADN/0.20 TEA BF 4 , a new reduction process appears at -0.40 V vs Ag/AgCl. As has been proposed for other metal -carbonyl systems (7,12), this process is assigned to reduction of a complex between Al 3+ and one of the AQ oxygen atoms. The potential shift is somewhat similar to that observed for complexation of 9-fluorenone by Al 3+ (12) in ADN.…”

“…Such results are not seen for the case of 9-fluorenone in ADN with added Al(TfO) 3 (12), implying that reduced fluorenone species are not as effective ligands toward Al 3+ as is the AQ dianion. This extensive interaction is also manifested by the large potential separation for the redox processes for AQ in ADN/TEA BF 4 , as is also observed in the AlCl 3 : 1-ethyl-3methylimidazolium chloride (EMIC) chloroaluminate ionic liquid (7).…”

“…This observation provides some support for the reduction of aluminum cation at -2.1 V. A comparison of the results for AQ and Al 3+ in ADN with those in the AlCl 3 : EMIC chloroaluminate ionic liquid ( 7) is appropriate at this point. As seen in Figure 2, the shift in reduction potential as the AQ carbonyl oxygen is complexed by Al 3+ is only +0.40V, whereas the corresponding shift in the chloroaluminate system is +1.25 V (7). This rather large difference apparently reflects the stronger degree of complexation by aluminum in the chloroaluminate system compared to the ADN system.…”

“…9,10anthraquinone (AQ) has been studied extensively in nonaqueous solvents (2)(3)(4)(5) and provides an convenient system to evaluate the effect of added cations and proton donors. Studies of AQ in chloroaluminate ionic liquid systems (6,7), in which Al 2 Cl 7 acts as the acidic species, have shown extensive interactions of the aluminum in the system with AQ. With this behavior in mind, it was thought to be of interest to compare the effect of added aluminum ions to AQ in a nitrile-based nonaqueous solvent.…”

Discovery of low energy pathways to metal-mediated BN bond reduction guided by computation and experiment

“…We note that the redox potential for the reduction of quinones is highly sensitive to hydrogen bonds and Lewis acid/base interactions with the quinone C]O bond. [71][72][73][74] Coordinated borazine likely acts as a Lewis acid in this regard, and consistent with a persistent interaction in solution, the anthraquinone wave is anodically shied by $400 mV, (Fig. S12 †) when 2 is also present in the solution.…”