Proton dynamics of two-dimensional oxalate-bridged coordination polymers

Abstract

A two-dimensional porous coordination polymer (NH4)2{HOOC(CH2)4COOH}[Zn2(C2O4)3] (abbreviated as (NH4)2(adp)[Zn2(ox)3] (adp = adipic acid, ox = oxalate)), which accommodates water molecules between the [Zn2(ox)3] layers, is highly remarked as a new type of crystalline proton conductor. In order to investigate its phase behavior and the proton conducting mechanism, we have performed adiabatic calorimetry, neutron diffraction, and quasi-elastic neutron scattering experiments on a fully hydrated sample (NH4)2(adp)[Zn2(ox)3]·3H2O with the highest proton conductivity (8 × 10(-3) S cm(-1), 25 °C, 98% RH). Its isostructural derivative K2(adp)[Zn2(ox)3]·3H2O was also measured to investigate the role of ammonium ions. (NH4)2(adp)[Zn2(ox)3]·3H2O and K2(adp)[Zn2(ox)3]·3H2O exhibit higher order transitions at 86 K and 138 K, respectively. From the magnitude of the transition entropy, the former is of an order-disorder type while the latter is of a displacive type. (NH4)2(adp)[Zn2(ox)3]·3H2O has four Debye-type relaxations and K2(adp)[Zn2(ox)3]·3H2O has two similar relaxations above each transition temperature. The two relaxations of (NH4)2(adp)[Zn2(ox)3]·3H2O with very small activation energies (ΔEa < 5 kJ mol(-1)) are due to the rotational motions of ammonium ions and play important roles in the proton conduction mechanism. It was also found that the protons in (NH4)2(adp)[Zn2(ox)3]·3H2O are carried through a Grotthuss mechanism. We present a discussion on the proton conducting mechanism based on the present structural and dynamical information.

Proton-Conductive Metal–Organic Frameworks

“…It was also found that peak width is independent of scattering angle and that the fast proton motion can be attributed to local motion. 75 This result also indicates that the high proton conductivity of this sample can be attributed to Grotthuss-type motion, in which proton transfers occur in a localized motion.…”

Minerals With Metal-Organic Framework Structures

“…Although neither zhemchuzhnikovite nor stepanovite can exhibit permanent porosity, because of the presence of Mg(H 2 O) 6 2+ guests in the MOF pores, they exhibit similarities to previously reported analogous MOFs (10,11,(19)(20)(21). In particular, both minerals can undergo reversible loss and sorption of water guests, demonstrating the stability of individual hcb layers upon desolvation, and exhibit extended hydrogen-bonded architectures that suggest the potential for proton conduction (24). The open metal-organic architectures in zhemchuzhnikovite and stepanovite change our view of MOFs as strictly artificial materials and hint to the possibility that the future may unravel other MOF minerals, potentially even microporous ones.…”

“…The structural similarity of stepanovite and zhemchuzhnikovite to the proton-conducting oxalate MOFs is marked (11,21). Proton conductivity in these MOFs results largely from a Grotthuss-type proton-hopping mechanism, enabled by a 2D network of hydrogen bonds involving water molecules in the interlayer space and protic species located either between the MOF layers or lodged in the pores (11,24). On the basis of these considerations, stepanovite exhibits potential for proton conduction, because there is an uninterrupted 2D net of short hydrogen bonds in the interlayer space, involving interstitial water molecules and Mg(H 2 O) 6 2+ cations.…”

Neutron Instruments for Research in Coordination Chemistry

“…Exceptions are the monitoring of the Q dependence of rotational lines for adsorbed H 2 , and the characterization of the liquid‐like diffusion of the individual H 2 molecules in the pores of the MOFs Mg‐MOF74 and MIL‐53. [93a] On the atomic scale, the Grotthuss proton hopping mechanism in a new type of crystalline proton conductor composed of ammonium in a bridges zinc oxalate coordination polymer was characterized …”