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The Role of Hydrogen Bonds in Proton-Conductive Homo- and Heterometallic Compounds with Ammonium Cations

The oxalate group, C2O42-, used in the preparation of metal-organic coordination compounds, has the ability to bind to metal ions in various modes, which allows the synthesis of various structures, of different dimensionality and nuclearity. Its ability to mediate magnetic interaction between the paramagnetic metal centers, depending on the nature of the bridged metal ions, makes it a good ligand that can form compounds with ferro-, antiferro- or ferrimagnetic ordering. These compounds can combine several properties of the interest ; intrinsic properties of the host with additional functionalities originating from the selected guest molecules can result in materials with multifunctional properties. Oxalate polymers are mostly negatively charged and therefore can associate with different molecular cations, and combine cooperative magnetism with another property of interest.1 The proton conductivity of metal-organic networks has been intensively researched because of their crystallinity, high porosity, designability and tunability in structure and properties. As a new functionality of metal-organic compounds it can be achieved by incorporating a counterion such as hydronium (H3O+), ammonium [NH4+, (CH3)2NH2+, ...] or an anion (SO42−) into the anionic network during synthesis. The counterions form the hydrogen bonds with the guest water or other components of compound and create proton-conducting pathways consisting of hydrogen bonding networks. The oxygen atoms of the oxalate group can construct complex hydrogen-bonded networks with water, which are more than suitable for proton conduction.2 We investigated the proton conductivity behaviour of the novel oxalate-based coordination compounds: (NH4)2[Fe(H2O)Cl3(C2O4)]·H2O (1), {; ; ; [NH(CH3)2(C2H5)][FeCl2(C2O4)]·H2O}; ; ; n (2), {; ; ; [NH(CH3)(C2H5)2]8[Mn4Cl4Cr4(C2O4)12]}; ; ; n (3) and {; ; ; [NH(CH3)2(C2H5)]8[Mn4Cl4Cr4(C2O4)12]}; ; ; n (4). The discrete mononuclear compound 1 exhibits remarkable humidity-sensing properties and a very high proton conductivity of 2.17 × 10−3 (Ω cm)−1 at 298 K under 74% relative humidity (RH). The one-dimensional (1D) compound 2 containing infinite anionic zig-zag chains [FeCl2(C2O4)]nn− and cations (CH3)2(C2H5)NH+ located between them, shows an increase in conductivity of six orders of magnitude with increasing RH, reaching values of 2.00 × 10−4 (Ω cm)−1 at 298 K and 93% RH. Compounds 3 and 4 consisting of two-dimensional (2D) oxalate-bridged anionic layers [Mn4Cl4Cr4(C2O4)12]n8n− interleaved by the hydrogen-bonded templating cations (CH3)(C2H5)2NH+ (3) or (CH3)2(C2H5)NH+ (4), exhibit extraordinary humidity-sensing properties and very high proton conductivity of 9.6 × 10−4 (Ω·cm)−1 at 298 K under 94% RH and 1.60 × 10−3 (Ω·cm)−1 at 298 K under 90% RH, respectively.

coordination compounds ; oxalate group ; crystal structure ; proton conductivity ; mix metal oxide

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