High Proton Conductivity of Magnetically Ordered 2D Oxalate-Bridged [Mn II Cr III ] Coordination Polymers with Irregular Topology.

Two heterometallic coordination polymers {[NH(CH ₃ ) ₂ (C ₂ H ₅ )] ₈ [Mn ₄ Cl ₄ Cr ₄ (C ₂ O ₄ ) ₁₂ ]} _n ( 1 ) and {[NH(CH ₃ )-(C ₂ H ₅ ) ₂ ] ₈ [Mn ₄ Cl ₄ Cr ₄ (C ₂ O ₄ ) ₁₂ ]} _n ( 2 ) were obtained by slow evaporation of an aqueous solution containing the building block [A] ₃ [Cr(C ₂ O ₄ ) ₃ ] [A = (CH ₃ ) ₂ (C ₂ H ₅ )NH ⁺ or (CH ₃ )(C ₂ H ₅ ) ₂ NH ⁺ ] and MnCl ₂ ·2H ₂ O. The isostructural compounds comprise irregular two-dimensional (2D) oxalate-bridged anionic layers [Mn ₄ Cl ₄ Cr ₄ (C ₂ O ₄ ) ₁₂ ] _n ^{8n -} with a Shubnikov plane net fes topology designated as (4·8 ² ), interleaved by the hydrogen-bonded templating cations (CH ₃ ) ₂ (C ₂ H ₅ )NH ⁺ ( 1 ) or (CH ₃ )(C ₂ H ₅ ) ₂ NH ⁺ ( 2 ). They exhibit remarkable humidity-sensing properties and very high proton conductivity at room temperature [1.60 × 10 ^-3 (Ω·cm) ^-1 at 90% relative humidity (RH) of 1 and 9.6 × 10 ^-4 (Ω·cm) ^-1 at 94% RH of 2 ]. The layered structure facilitates the uptake of water molecules, which contributes to the enhancement of proton conductivity at high RH. The better proton transport observed in 1 compared to that in 2 can be tentatively attributed to the higher hydrophilicity of the cations (CH ₃ ) ₂ (C ₂ H ₅ )NH ⁺ , which is closely related to their affinity for water molecules. The original topology of the anionic networks in both compounds leads to the development of interesting magnetic phases upon cooling. The magnetically ordered ground state can be described as the coupling of ferromagnetic spin chains in which Mn ²⁺ and Cr ³⁺ ions are bridged by bis(bidentate) oxalate groups into antiferromagnetic planes through monodentate-bidentate oxalate bridges in the layers, which are triggered to long-range order below temperature 4.45 K via weaker interlayer interactions.