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

Two heterometallic coordination polymers {[NH(CH3)(2)(C2H5)](8)[Mn4Cl4Cr4(C2O4)(12)]}( n ) (1) and {[NH(CH3)-(C2H5)(2)](8)[Mn4Cl4Cr4(C2O4)(12)]}( n ) (2) were obtained by slow evaporation of an aqueous solutioncontaining the building block [A](3)[Cr(C2O4)(3)] [A = (CH3)(2)(C2H5)NH+ or (CH3)(C2H5)(2)NH+] and MnCl2 center dot 2H(2)O. The isostructural compounds comprise irregular two-dimensional(2D) oxalate-bridged anionic layers [Mn4Cl4Cr4(C2O4)(12)]( n ) ( 8n-) with a Shubnikov planenet fes topology designated as (4 center dot 8(2)),interleaved by the hydrogen-bonded templating cations (CH3)(2)(C2H5)NH+ (1) or (CH3)(C2H5)(2)NH+ (2). They exhibit remarkable humidity-sensingproperties and very high proton conductivity at room temperature [1.60x 10(-3) (omega center dot cm)(-1) at 90% relative humidity (RH) of 1 and 9.6 x 10(-4) (omega center dot cm)(-1) at 94% RH of 2]. The layered structure facilitates the uptake of watermolecules, which contributes to the enhancement of proton conductivityat high RH. The better proton transport observed in 1 compared to that in 2 can be tentatively attributedto the higher hydrophilicity of the cations (CH3)(2)(C2H5)NH+, which is closely relatedto their affinity for water molecules. The original topology of theanionic networks in both compounds leads to the development of interestingmagnetic phases upon cooling. The magnetically ordered ground statecan be described as the coupling of ferromagnetic spin chains in whichMn(2+) and Cr3+ ions are bridged by bis(bidentate)oxalate groups into antiferromagnetic planes through monodentate-bidentateoxalate bridges in the layers, which are triggered to long-range orderbelow temperature 4.45 K via weaker interlayer interactions. Two coordination polymers {[A](8)[Mn4Cl4Cr4(C2O4)(12)]}( n ) [A = (CH3)(2)(C2H5)NH+ (1) and (CH3) (C2H5)(2)NH+ (2)] show remarkable structural, electrical,and magnetic features:atypical 2D oxalate-bridged anionic layers with 4 center dot 8(2) topology, humidity-sensing properties, high proton conductivity,and long-range antiferromagnetic order.