Abstract
Anhydrous EuII–acetylenedicarboxylate (EuADC; ADC2− = −O2C‐C≡C‐CO2 −) was synthesized by reaction of EuBr2 with K2ADC or H2ADC in degassed water under oxygen‐free conditions. EuADC crystallizes in the SrADC type structure (I41/amd, Z=4) forming a 3D coordination polymer with a diamond‐like arrangement of Eu2+ nodes (msw topology including the connecting ADC2− 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 μB) and 151Eu Mössbauer spectra (δ=−13.25 mm s−1 at 78 K) confirm the existence of Eu2+ 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.
Highlights
Among the many ligands that have been used for the construction of coordination polymers (CPs) and metal-organic frameworks (MOFs) acetylenedicarboxylate (ÀO2C-CC-CO2À, ADC2À) belongs to the simplest ones
It crystallizes in the SrADC type structure (I41/amd, Z = 4)[12] and is stable under ambient conditions for at least several months
Measurements of the magnetic susceptibilities as well as 151Eu Mçssbauer spectroscopy data confirm that exclusively Eu2+ cations exist in EuADC
Summary
A tetragonal symmetry rpesffiuffi lts, but the metric of the unit cell is still close to cubic ( 2atet % ctet).[12] For SrADC, a negative thermal volume expansion (NTE) below room temperature was reported.[12] The resulting thermal expansion coefficient is rather small (aV = À4.7(13) x 10À6 KÀ1) and by a factor of % 6 smaller than the corresponding coefficients in other well-known NTE materials like ZrW2O8 (aV = À27.2 10À6 KÀ1).[16] The occurrence of NTE in SrADC was explained by a “guitar string vibration” of Sr···O···Sr units reducing the Sr···Sr distances with increasing temperature and increasing transverse vibrational motion of the bridging oxygen atom.[12] Kepert and co-workers developed a model for NTE in a number of Prussian Blue analogues, MIIPtIV(CN) with MII = Mn, Fe, Co, Ni, Cu, Zn, Cd.[17] Here, the absolute values of aV increase with increasing size of the MII cation This was correlated to the strengths of the metal-cyanide bonding interaction leading to different energies of the transverse vibration of the cyanide bridge (i.e., “guitar string vibrations”).[17] enhanced NTE behaviour is expected for more flexible structures. Because of the oxidation sensitivity of EuII, the synthetic procedure for SrADC being performed in water and air could not be transferred to the respective EuII compound and a new approach had to be developed
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