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: {[(TpMe)­Fe­(CN)3]2[Co­(bpy)2]2[(TpMe)­Fe­(CN)3]2}·12H2O (2) and {[(TpMe)­Fe­(CN)3]2[Co­(bpy)2]2[BPh4]2}·6MeCN (3), where TpMe and bpy
are hydridotris­(3-methylpyrazol-1-yl)­borate and 2,2′-bipyridine,
respectively. Through electrochemical and spectroscopic studies, the
TpMe ligand appears to be a moderate σ donor in comparison
to others in the [NEt4]­[(TpR)­FeIII(CN)3] series [where TpR = Tp, hydridotris­(pyrazol-1-yl)­borate;
TpMe = 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 {FeIILS(μ-CN)­CoIIILS} pairs into {FeIIILS(μ-CN)­CoIIHS} units near
ca. 230 and 244 K (T1/2) for 2 and 3, respectively (LS: low spin; HS: high spin).
These experimental results show that 2 and 3 display light-induced {FeIIILS(μ-CN)­CoIIHS} metastable states that relax to thermodynamic
{FeIILS(μ-CN)­CoIIILS} ones at ca. 90 K. Ancillary TpR ligand donor strength
appears to be the dominant factor for tuning electron transfer properties
in these {Fe2Co2} complexes.