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

• Published in: Journal of the American Chemical Society 2019-11, Vol.141 (47), p.18759-18770
• Main Authors: Berdiell, Izar Capel, Hochdörffer, Tim, Desplanches, Cédric, Kulmaczewski, Rafal, Shahid, Namrah, Wolny, Juliusz A, Warriner, Stuart L, Cespedes, Oscar, Schünemann, Volker, Chastanet, Guillaume, Halcrow, Malcolm A
• Format: Article
• Language: English
• Subjects: Chemical Sciences; Material chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.9b08862

Treatment of Fe­[BF4]2·​6H2O with 4,6-di­(pyrazol-1-yl)-1H-pyrimid-2-one (HL1) or 4,6-di­(4-methylpyrazol-1-yl)-1H-pyrimid-2-one (HL2) affords solvated crystals of [{FeIII(OH2)6}⊂​FeII 8(μ-L)12]­[BF4]7 (1, HL = HL1; 2, HL = HL2). 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(OH2)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.