Proton conductivity as a function of the metal center in porphyrinylphosphonate-based MOFs

Dalton Transactions. 2021. Vol. 50. P. 6549–6560.

Enakieva Y., Zhigileva E., Fitch A., Chernyshev V., Stenina I., Yaroslavtsev A. B., Sinelshchikova A., Kovalenko K., Gorbunova Y., Tsivadze A.

The rational design of metal–organic frameworks (MOFs) is highly important for the development of new
proton conductors. Porphyrinylphosphonate-based MOFs, providing the directed tuning of physical and
chemical properties of materials through the modification of a macrocycle, are potentially high-conducting
systems. In this work the synthesis and characterization of novel anionic Zn-containing MOF based on
palladium(II) meso-tetrakis(3-(phosphonatophenyl))porphyrinate, IPCE-2Pd, are reported. Moreover, the
proton-conductive properties and structures of two anionic Zn-containing MOFs based on previously
described nickel(II) and novel palladium(II) porphyrinylphosphonates, IPCE-2M (M = Ni(II) or Pd(II)), are
compared in details. The high proton conductivity of 1.0 × 10−2 S cm−1 at 75 °C and 95% relative humidity
(RH) is revealed for IPCE-2Ni, while IPCE-2Pd exhibits higher hydrolytic and thermal stability of the
material (up to 420 °C) simultaneously maintaining a comparable value of conductivity (8.11 × 10−3 S
cm−1 at 95 °C and 95% RH). The nature of the porphyrin metal center is responsible for the features of
crystal structure of materials, obtained under identical reaction conditions. The structures of IPCE-2Pd
and its dehydrated derivative IPCE-2Pd-HT are determined from the synchrotron powder diffraction data.
The presence of phosphonic groups in compared materials IPCE-2M affords a high concentration of
proton carriers that together with the sorption of water molecules leads to a high proton conductivity.

Keywords: X-ray analysis, Metal-organic frameworks, proton conductivity, porphyrinylphosphonate