Supramolecular Iron Metallocubanes Exhibiting Site-Selective Thermal and Light-Induced Spin-Crossover

Treatment of Fe[BF 4 ] 2 ·6H 2 O with 4,6-di(pyrazol-1-yl)-1 H -pyrimid-2-one (HL 1 ) or 4,6-di(4-methylpyrazol-1-yl)-1 H -pyrimid-2-one (HL 2 ) affords solvated crystals of [{Fe III (OH 2 ) 6 }⊂Fe II 8 (μ-L) 12 ][BF 4 ] 7 ( 1 , HL = HL 1 ; 2 , HL = HL 2 ). The centrosymmetric complexes contain a cubic arrangement of iron(II) centers, with bis-bidentate [L] - ligands bridging the edges of the cube. The encapsulated [Fe(OH 2 ) 6 ] 3+ moiety templates the assembly through 12 O-H···O hydrogen bonds to the [L] - hydroxylate groups. All four unique iron(II) ions in the cages are crystallographically high-spin at 250 K, but they undergo a gradual high → low spin-crossover on cooling, which is predominantly centered on one iron(II) site and its symmetry-related congener. This was confirmed by magnetic susceptibility data, light-induced excited spin state trapping (LIESST) effect measurements, and, for 1 , Mössbauer spectroscopy and diffuse reflectance data. The clusters are stable in MeCN solution, and 1 remains high-spin above 240 K in that solvent. The cubane assembly was not obtained from reactions using other iron(II) salts or 4,6-di(pyrazol-1-yl)pyrimidine ligands, highlighting the importance of hydrogen bonding in templating the cubane assembly.