Anion-Dependent Electron Transfer in the Cyanide-Bridged [Fe₂Co₂] Capsules

A family of molecular capsules, {[(Tp*)Fe(CN)₃Co(bpy^C═N(CHbpy)]₂[X]₂}·sol (1, X = ClO₄, sol = 6DMF; 2, X = PF₆, sol = 6DMF; 3, X = OTf, sol = 6DMF; 4, X = BPh₄, sol = 2DMF; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate; bpy = 2,2'-bipyridine), were prepared via the Schiff-base condensation of the aldehyde-substituted bpy (bpy^CHO) and 1,7-diaminoheptane (H₂N(CH₂)₇NH₂). All the complexes contain the same cyanide-bridged cationic square cores ([Fe₂Co₂]²⁺), which are encapsuled by the flexible alkyl chains. Variable-temperature single-crystal X-ray diffraction, FT-IR spectra, and magnetic studies reveal the abrupt and complete, thermo- and photo-induced electron-transfer-coupled spin transition for 1-3, while the pure high-spin phase for 4. Such distinct behavior is attributed to the effective long-range cooperative interactions mediated by the intercluster π-π couplings in 1-3, which, however, are significantly blocked in 4 due to the steric effect of interstitial BPh₄^- anions. Furthermore, the shift in the thermally induced transition temperatures of 254 K for 1, 233 K for 2, and 187 K for 3, respectively, is likely correlated to the variable H···O and H···F interactions between the solvent molecules, anions, and the bipyridine ligands of the [Fe₂Co₂] squares, suggesting the significant anion-dependent effect in such a system.