Abstract
A novel one-dimensional (1D) oxalate-bridged coordination polymer of iron(III), {[NH(CH3)(C2H5)2][FeCl2(C2O4)]}n (1), exhibits remarkable humidity-sensing properties and very high proton conductivity at room temperature (2.70 × 10−4 (Ω·cm)−1 at 298 K under 93% relative humidity), in addition to the independent antiferromagnetic spin chains of iron(III) ions bridged by oxalate groups (J = −7.58(9) cm−1). Moreover, the time-dependent measurements show that 1 could maintain a stable proton conductivity for at least 12 h. Charge transport and magnetic properties were investigated by impedance spectroscopy and magnetization measurements, respectively. Compound 1 consists of infinite anionic zig-zag chains [FeCl2(C2O4)]nn− and interposed diethylmethylammonium cations (C2H5)2(CH3)NH+, which act as hydrogen bond donors toward carbonyl oxygen atoms. Extraordinarily, the studied coordination polymer exhibits two reversible phase transitions: from the high-temperature phase HT to the mid-temperature phase MT at T ~213 K and from the mid-temperature phase MT to the low-temperature phase LT at T ~120 K, as revealed by in situ powder and single-crystal X-ray diffraction. All three polymorphs show large linear thermal expansion coefficients.
Highlights
Metal-organic coordination polymers, which have a wealth of multifunctional applications due to their immense structural diversity and flexibility, are currently being extensively studied
Yellow rod-shaped crystals of compound 1 were obtained via slow evaporation of an aqueous solution containing a mixture of FeCl3 (1 mmol), H2C2O4 ·H2O (1 mmol), and (C2H5)2(CH3)N
Phases HT and MT crystallized in a P21 /c space group, while LT crystallized in P21 /n (Table 1), with all three comprising [FeCl2 (C2 O4 )]n n − chains packed in the same fashion and diethylmethylammonium cations (CH3 )(C2 H5 )2 NH+ located between them (Figure 2)
Summary
Metal-organic coordination polymers, which have a wealth of multifunctional applications due to their immense structural diversity and flexibility, are currently being extensively studied Multifunctional properties of these materials can be achieved by combining the intrinsic properties of the host, especially the magnetic ones, with additional functionalities derived from the selected guest molecules [1,2]. Coordination polymers and metal–organic frameworks providing additional proton-conducting pathways were established, thereby opening new avenues for improving proton conductivity [7,8,9,10,11] These systems can generally be obtained by introducing (i) the guest molecules (such as water) and counterions or acids into the voids, which helps to create intricate hydrogen-bonded networks and, improve the proton conductivity, or (ii) other functional groups, such as –COOH, –PO3 H, –SO3 H, and –OH, which can better the acidity and hydrophilicity of the organic ligands and, form an efficient proton transport pathway [8]. The title compound shows great structural versatility with several single-crystal-to-single-crystal transformations induced by temperature
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