Water-free proton conduction in discotic pyridylpyrazolate-based Pt(II) and Pd(II) metallomesogens

Abstract

In this work we report on water-free proton conductivity in liquid-crystal pyridylpyrazolate-based Pt(II) and Pd(II) complexes [M(pzR(n,n)py)2] (pzR(n,n)py = 3-(3,5-dialkyloxyphenyl)-5-(pyridin-2-yl)pyrazolate, R(n,n) = C6H3(OCnH2n+1)2; n = 4, 12, 16, M = Pd; n = 12, M = Pt) with potential application as electrolyte materials in proton exchange membrane fuel cells. The columnar ordering of the complexes in the liquid-crystalline phase opens nanochannels, which are used for fast proton exchange as detected by impedance spectroscopy and NMR. The NMR spectra indicate that the proton conduction mechanism is associated with a novel C–H···N proton transfer, which persists above the clearing point of the material. The highest conductivity of ∼0.5 μS cm–1 at 180 °C with an activation energy of 1.2 eV is found for the Pt(II) compound in the mesophase. The Pd(II) complexes with different chain length (n = 4, 12, and 16) show lower conductivity but smaller activation energies, in the range of 0.74–0.93 eV.