Eu(O₂C‐C≡C‐CO₂): An Eu^II Containing Anhydrous Coordination Polymer with High Stability and Negative Thermal Expansion

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 (μeff=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 Eg=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 αV=−9.4(12)×10−6 K−1.

Oxidation of europium with ammonium perfluorocarboxylates in liquid ammonia: pathways to europium(ii) carboxylates and hexanuclear europium(iii) fluoridocarboxylate complexes

“…Possible explanations are: (i) a small band gap of the coordination polymer, that induces thermal ionization of the excited 5d electrons of Eu II into the conduction band. [42] This may be supported by the bulk yellow color of 1 that indicates strong absorption of blue light; (ii) the presence of Eu II next to Eu III allows energy transfer due to intervalence charge transfer (IVCT) processes; [43] (iii) quenching of the luminescence due to non-radiative relaxation of the excited states by the vibration of DMF ligands. Future detailed studies of the photoluminescence at temperatures below 100 K could help provide additional insights into the quenching mechanism of the preliminarily assigned Eu II -related blue luminescence at 458 nm.…”

Acetylenedicarboxylate as a linker in the engineering of coordination polymers and metal–organic frameworks: challenges and potential

“…105 It was rationalized by strong transverse M II… O … M II vibrations also known as guitar string vibrations. 44,50 For [Sr(ADC)] and [Eu II (ADC)] such a NTE behavior was reported for a large temperature range below room temperature (Fig. 37).…”

“…Reproduced from ref. 50 with permission of Wiley & Sons, copyright 2020. B) Types of coordination modes of carboxylate groups in the acetylenedicarboxylate dianion to metal cations (M n+ ) (For simplicity, only the coordination of one carboxylate is depicted.…”

“…10b). 50 ). 44 This seems to be a trend of ADC-based CPs with the diamond-like arrangement of metal cations.…”

UoC‐4: A MOF Based on Octahedral ScO6 Nodes and Fluorinated Trimesate Ligands

“…One interesting feature of the ORTEP plot shown in Figure 1 are large anisotropic displacement parameters of both oxygen atoms perpendicular to the C−O⋅⋅⋅Sc bonds. This might be an indication of so‐called “ guitar string ” vibrations and a possible negative thermal expansion (NTE), [29] which is quite frequently observed in MOFs [30] or coordination polymers [31,32] . However, a first view on the lattice parameters of as‐synthesized UoC‐4 obtained at room temperature and 150 K does not give any hint on such a NTE behaviour.…”