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

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}⊂​FeII8(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.