Two-Step Thermoinduced Metal-to-Metal Electron Transfer and ON/OFF Photoswitching in a Molecular [Fe₂Co₂] Square Complex

A cyanide-bridged [Fe₂Co₂] molecular square complex, {[Fe­(Tp)­(CN)₃]₂[Co­(L)₂]₂}­(BF₄)₂·2CH₃CN·6H₂O [1; Tp = hydrotris­(pyrazol-1-yl)­borate and L = bis­(1-ethyl-1H-imidazol-2-yl)­ketone], has been synthesized and characterized fully by single-crystal X-ray diffraction, (photo)­magnetic measurements, optical reflectivity, and other physical measurements. 1 exhibits a two-step metal-to-metal electron-transfer (MMET)-induced spin transition accompanied by thermal hysteresis (T_1/2↑ = 332 and 407 K and T_1/2↓ = 320 and 405 K, respectively), converting the low-temperature diamagnetic {Fe^II_LS–CN–Co^III_LS} ground state into the high-temperature paramagnetic {Fe^III_LS–CN–Co^II_HS} state. Additionally, 1 displays reversible photoinduced MMET under light irradiation (ON mode using 808 nm laser light and OFF mode using 532 nm laser light), as confirmed by optical reflectivity and (photo)­magnetic measurements. The photoinduced paramagnetic metastable state relaxes back to the diamagnetic ground state at 91 K (T_LIESST = 91 K). Astonishingly, 1 also exhibits a 27 K wide light-induced thermal hysteresis below 100 K. The overall results show that 1 is a multistimuli-responsive bistable material that exhibits reversible switching between the diamagnetic state, {Fe^II_LS–CN–Co^III_LS}, and the paramagnetic state, {Fe^III_LS–CN–Co^II_HS}, under the application of temperature and light.