Substituent effect on metal-to-metal charge transfer behavior of cyanide-bridged {Fe2Co2} square

Abstract Using different cyanometallate building blocks ([(TpR)Fe(CN)3]–) and pyrazino[2, 3-f][1,10]phenanthroline (dpq) ligand, three cyanide–bridged {Fe2Co2} tetranuclear complexes {[(Tp*)Fe(CN)3]2[Co(dpq)2]2[ClO4]2}·4H2O (1), {[(Tp4-Me)Fe(CN)3]2[Co(dpq)2]2[ClO4]2}·4H2O (2) and {[(Tp)Fe(CN)3]2[Co(dpq)2]2 [(Tp)Fe(CN)3][ClO4]}·2CH3OH·5H2O (3) were synthesized. Single crystal X-ray diffraction study showed that all compounds shared the similar square tetranuclear structures. Magnetic study indicated that 1 maintained the high-spin state for the CoII ion in the whole temperature range, and showed the ferromagnetic interaction between low-spin FeIII and high-spin CoII ions. In contrast, 2 and 3 showed thermally and photoinduced intramolecular metal-to-metal charge transfer (MMCT) between the FeII/III and CoII/III redox pairs. The comparison study suggested that the electronic and steric effects of the substituents in cyanometallate building blocks as well as the intermolecular interactions can affect the redox potential of metal ions, thereby resulting in different MMCT properties.