Tuning the electron transfer events in a series of cyanide-bridged [Fe 2 Co 2 ] squares according to different electron donors.

It has been recognized that both the ligand fields and intermolecular interactions may greatly impact the electron-transfer-coupled spin transition (ETCST) events in switchable magnetic materials; however, the engineering of these factors within a given system is still challenging. In this article, we chose the 4,4'-substituent 2,2'-bipyridine derivatives as chelating ligands according to their increasing electron-donating strength and incremental potential for forming hydrogen bonds (bpy ^{CHO,CH ₃} (L1) < bpy ^{CH ₂ OH,CH ₃} (L2) < bpy ^{CH ₂ OH,CH ₂ OH} (L3)), and prepared three new [Fe ₂ Co ₂ ] complexes, {[(Tp*)Fe(CN) ₃ Co(L) ₂ ] ₂ [ClO ₄ ] ₂ }·Sol (1, L = L1, Sol = 4MeCN·2H ₂ O; 2, L = L2, Sol = 3MeCN; 3, L = L3, Sol = 4MeOH; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate). X-ray crystallography studies revealed that all the complexes share similar cyanide-bridged [Fe ₂ Co ₂ ] square compositions except for the different substituted groups of L ligands, which led to the clearly evidenced intercluster hydrogen bonds between the neighbouring hydroxyl groups in 2 and 3. As a result, 1 remained in the paramagnetic [FeIII,LS2CoII,HS2] state over the whole temperature range, while 2 and 3 showed complete ETCST behaviour with the transition temperatures ( T _1/2 ) being 221 and 294 K, respectively.