Thermochromic and Photoresponsive Cyanometalate Fe/Co Squares: Toward Control of the Electron Transfer Temperature

Two structurally related and photoresponsive cyanide-bridged Fe/Co square complexes, {Fe₂Co₂}, are reported: {[(Tp^Me)­Fe­(CN)₃]₂[Co­(bpy)₂]₂[(Tp^Me)­Fe­(CN)₃]₂}·12H₂O (<b>2</b>) and {[(Tp^Me)­Fe­(CN)₃]₂[Co­(bpy)₂]₂[BPh₄]₂}·6MeCN (<b>3</b>), where Tp^Me and bpy are hydridotris­(3-methylpyrazol-1-yl)­borate and 2,2′-bipyridine, respectively. Through electrochemical and spectroscopic studies, the Tp^Me ligand appears to be a moderate σ donor in comparison to others in the [NEt₄]­[(Tp^R)­Fe^III(CN)₃] series [where Tp^R = Tp, hydridotris­(pyrazol-1-yl)­borate; Tp^Me = hydridotris­(3-methylpyrazol-1-yl)­borate; pzTp = tetrakis­(pyrazol-1-yl)­borate; Tp* = hydridotris­(3,5-dimethylpyrazol-1-yl)­borate; Tp*^Me = hydridotris­(3,4,5-trimethylpyrazol-1-yl)­borate]. The spectroscopic, structural, and magnetic data of the {Fe₂Co₂} squares indicate that thermally-induced intramolecular electron transfer reversibly converts {Fe^II_LS(μ-CN)­Co^III_LS} pairs into {Fe^III_LS(μ-CN)­Co^II_HS} units near ca. 230 and 244 K (<i>T</i>_1/2) for <b>2</b> and <b>3</b>, respectively (LS: low spin; HS: high spin). These experimental results show that <b>2</b> and <b>3</b> display light-induced {Fe^III_LS(μ-CN)­Co^II_HS} metastable states that relax to thermodynamic {Fe^II_LS(μ-CN)­Co^III_LS} ones at ca. 90 K. Ancillary Tp^R ligand donor strength appears to be the dominant factor for tuning electron transfer properties in these {Fe₂Co₂} complexes