Tetracyanidoborates of Divalent Eu and Yb 3D Metal–Organic Frameworks with Cubic Structure. Thermochromic Luminescence and Slow Relaxation of Magnetization of Eu[B(CN)₄]₂

Tetracyanidoborates of divalent lanthanides, Ln[B(CN)4]2·THF (Ln = Eu (1), Yb (2)), were obtained by a salt metathesis reaction of lanthanide diiodides and ammonium tetracyanidoborate. Unlike known tetracyanidoborates of alkaline-earth metals and trivalent lanthanides, compounds 1 and 2 are 3D metal–organic frameworks (MOFs) with a cubic structure. MOF 1 demonstrates air stability and thermal stability up to 400 °C. At 298 K, 1 shows bright broadband photoluminescence (PL) of Eu2+ ions that peaked at 650 nm with 80% absolute quantum yield (QY). When the temperature increases from 77 to 500 K, the maximum of PL spectra of 1 exhibit a blue shift from 13900 to 16350 cm–1. Quantum chemical calculations and X-ray diffraction studies of 1 at 100 and 390 K reveal that observed PL thermochromism is accompanied by changes in the geometry of the Eu–N–C fragment and electronic structure of the [B(CN)4]− anion. Europium-containing MOF 1 exhibits slow relaxation of magnetization at temperatures up to 4 K. The magnetic relaxation dynamics of MOF 1 is dominated by an Orbach-like mechanism with an effective energetic barrier estimated as 16.7 K. Removal of THF from the pores of MOF 1 leads to the same changes in structure and luminescent behavior as heating but significantly reduces its stability in air. The singularity of structural, magnetic, and luminescent behavior, as well as extraordinary stability of europium MOF 1, are due to specifics in europium–tetracyanidoborate bonding. The reported tetracyanidoborates 1 and 2 are the porous 3D MOFs based on divalent lanthanide ions: europium and ytterbium.