Crystal Structure, Charge-Transfer-Induced Spin Transition, and Photoreversible Magnetism in a Cyano-Bridged Cobalt−Tungstate Bimetallic Assembly

This paper describes the crystal structure, magnetic properties, and photoreversible magnetic properties of Co II 3 [W V (CN) 8 ] 2 (pyrimidine) 4 ·6H 2 O. We found that complexes of this formula had two types of crystal structures ([{Co II (pyrimidine) 2 } 2 {Co II (H 2 O) 2 }{W V (CN) 8 } 2 ]·4H 2 O ( 1 ) and [{Co II (pyrimidine)(H 2 O)} 2 {Co II (H 2 O) 2 }{W V (CN) 8 } 2 ](pyrimidine) 2 ·2H 2 O ( 2 )). These two structures had similar metal−organic frameworks but differed in the coordination environment around Co1, i.e., Co1(NC) 4 (pyrimidine) 2 in 1 and Co1(NC) 4 (pyrimidine)(H 2 O) in 2 . In 1, a temperature-induced phase transition from the Co II ( S = 3/2)−NC−W V ( S = 1/2) [high-temperature (HT)] phase to the Co III ( S = 0)−NC−W IV ( S = 0) [low-temperature (LT)] phase was observed due to a charge-transfer-induced spin transition. However, 2 did not exhibit such a phase transition. When the LT phase of 1 was irradiated by 840 nm light, ferromagnetism with a Curie temperature of 40 K and magnetic coercive field of 12 kOe were observed. UV–vis reflectance and infrared measurements suggested that the LT phase optically transited to the photoinduced (PI) phase, which had a similar valence state as the HT phase, through the metal-to-metal charge-transfer (W IV → Co III ) band. In contrast, when the back metal-to-metal charge transfer (Co II → W V ) band of the PI phase was excited by 532 nm light, the reverse phase transition from the PI phase to the LT phase occurred, and the spontaneous magnetization decreased.