Control of Crystalline Proton-Conducting Pathways by Water-Induced Transformations of Hydrogen-Bonding Networks in a Metal–Organic Framework

Structure-defined
metal–organic frameworks (MOFs) are of
interest because rational design and construction allow us to develop
good proton conductors or possibly control the proton conductivity
in solids. We prepared a highly proton-conductive MOF (NH4)2(adp)­[Zn2(ox)3]·nH2O (abbreviated to 1·nH2O, adp: adipic acid, ox: oxalate, n = 0, 2, 3) having
definite crystal structures and showing reversible structural transformations
among the anhydrate (1), dihydrate (1·2H2O), and trihydrate (1·3H2O) phases. The crystal structures
of all of these phases were determined by X-ray crystallography. Hydrogen-bonding
networks consisting of ammonium ions, water molecules, and carboxylic
acid groups of the adipic acids were formed inside the two-dimensional
interlayer space in hydrated 1·2H2O and 1·3H2O. The crystal system of 1 or 1·2H2O (P21/c, No. 14) was changed into
that of 1·3H2O (P1̅, No. 2), depending on water content
because of rearrangement of guests and acidic molecules. Water molecules
play a key role in proton conduction as conducting media and serve
as triggers to change the proton conductivity through reforming hydrogen-bonding
networks by water adsorption/desorption processes. Proton conductivity
was consecutively controlled in the range from ∼10–12 S cm–1 (1) to ∼10–2 S cm–1 (1·3H2O) by the humidity. The relationships among the
structures of conducting pathways, adsorption behavior, and proton
conductivity were investigated. To the best of our knowledge, this
is the first example of the control of a crystalline proton-conducting
pathway by guest adsorption/desorption to control proton conductivity
using MOFs.