Electrical conductivity in two mixed-valence liquids†
Abstract
Two different room-temperature liquid systems were investigated, both of which conduct a DC electrical current without decomposition or net chemical transformation. DC electrical conductivity is possible in both cases because of the presence of two different oxidation states of a redox-active species. One system is a 1 : 1 molar mixture of n-butylferrocene (BuFc) and its cation bis(trifluoromethane)sulfonimide salt, [BuFc+][NTf2−], while the other is a 1 : 1 molar mixture of TEMPO and its cation bis(trifluoromethane)sulfonimide salt, [TEMPO+][NTf2−]. The TEMPO–[TEMPO+][NTf2−] system is notable in that it is an electrically conducting liquid in which the conductivity originates from an organic molecule in two different oxidation states, with no metals present. Single-crystal X-ray diffraction of [TEMPO+][NTf2−] revealed a complex structure with structurally different cation–anion interactions for cis- and trans [NTf2−] conformers. The electron transfer self-exchange rate constant for BuFc/BuFc+ in CD3CN was determined by 1H NMR spectroscopy to be 5.4 × 106 M−1 s−1. The rate constant allowed calculation of an estimated electrical conductivity of 7.6 × 10−5 Ω−1 cm−1 for BuFc–[BuFc+][NTf2−], twice the measured value of 3.8 × 10−5 Ω−1 cm−1. Similarly, a previously reported self-exchange rate constant for TEMPO/TEMPO+ in CH3CN led to an estimated conductivity of 1.3 × 10−4 Ω−1 cm−1 for TEMPO–[TEMPO+][NTf2−], a factor of about 3 higher than the measured value of 4.3 × 10−5 Ω−1 cm−1.