Eu(O(2)C‐C≡C‐CO(2)): An Eu(II) Containing Anhydrous Coordination Polymer with High Stability and Negative Thermal Expansion

Anhydrous Eu(II)–acetylenedicarboxylate (EuADC; ADC(2−) = (−)O(2)C‐C≡C‐CO(2) (−)) was synthesized by reaction of EuBr(2) with K(2)ADC or H(2)ADC in degassed water under oxygen‐free conditions. EuADC crystallizes in the SrADC type structure (I4(1)/amd, Z=4) forming a 3D coordination polymer with a diamond‐like arrangement of Eu(2+) nodes (msw topology including the connecting ADC(2−) linkers). Deep orange coloured EuADC is stable in air and starts decomposing upon heating in an argon atmosphere only at 440 °C. Measurements of the magnetic susceptibilities (μ (eff)=7.76 μ(B)) and (151)Eu Mössbauer spectra (δ=−13.25 mm s(−1) at 78 K) confirm the existence of Eu(2+) cations. Diffuse reflectance spectra indicate a direct optical band gap of E (g)=2.64 eV (470 nm), which is in accordance with the orange colour of the material. Surprisingly, EuADC does not show any photoluminescence under irradiation with UV light of different wavelengths. Similar to SrADC, EuADC exhibits a negative thermal volume expansion below room temperature with a volume expansion coefficient α (V)=−9.4(12)×10(−6) K(−1).