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)}.12H(2)O (2) and {[(Tp(Me))Fe(CN)(3)](2)[Co(bpy)(2)](2)[BPh4](2)}.6MeCN (3), 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 s 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 {(FeLS)-L-II(mu-CN)(CoLS)-L-III} pairs into {(FeLS)-L-III(mu-CN)(CoHS)-H-II} units near ca. 230 and 244 K (T-1/2) for 2 and 3, respectively (LS: low spin; HS: high spin). These experimental results show that 2 and 3 display light-induced {(FeLS)-L-III(mu-CN)(CoHS)-H-II} metastable states that relax to thermodynamic {(FeLS)-L-II(mu-CN)(CoLS)-L-III} 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.