Electrical conductivity in two mixed-valence liquids

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 H-1 NMR spectroscopy to be 5.4 x 10(6) M-1 s(-1). The rate constant allowed calculation of an estimated electrical conductivity of 7.6 x 10(-5) Omega(-1) cm(-1) for BuFc-[BuFc(+)][NTf2-], twice the measured value of 3.8 x 10(-5) Omega(-1) cm(-1). Similarly, a previously reported self-exchange rate constant for TEMPO/TEMPO+ in CH3CN led to an estimated conductivity of 1.3 x 10(-4) Omega(-1) cm(-1) for TEMPO-[TEMPO+][NTf2-], a factor of about 3 higher than the measured value of 4.3 x 10(-5) Omega(-1) cm(-1).