Near-infrared-photoinduced metamagnet based on a layered cyanido-bridged Co–W assembly with π–π interactions

Abstract

A newly synthesized photomagnet based on a cyanido-bridged Co–W assembly, [{Co(isoquinoline)₄}₃{W(CN)₈}₂]·2EtOH (CoWisoq), exhibits a controllable photoinduced (PI) phase with a metamagnetic behavior. The PI phase shows a transition from paramagnetic to ferromagnetic with a coercive field of 550 Oe at 2 K. It has a monoclinic (P2₁/n) crystal structure and is comprised of two-dimensional cyanido-bridged Co–W coordination layers with two crystallographically independent Co sites (Co1, Co2) and one W site. Both Co sites adopt a pseudo-octahedral six-coordinated geometry involving four isoquinoline and two cyanide ligands. π–π interactions are induced at half of the isoquinoline ligands in Co1 and all of those in Co2 sites, suggesting that the coordination environment at the Co1 site has structural flexibility. CoWisoq displays a thermal phase transition around 140 K with a small thermal hysteresis due to the charge transfer–induced spin transition between Co^II–W^V of the high temperature (HT) phase and Co^III–W^IV of the low temperature (LT) phase. This charge transfer, which is also responsible for a drastic color change between purple (HT) and yellow (LT), involves only the structurally flexible Co1 sites. The ligand field, which is composed of four isoquinoline and two cyanide ligands, contributes to a unique metal-to-metal charge transfer absorption band of the LT phase at 1025 nm. Irradiation of the LT phase with near–infrared light produces a PI phase with the Co^II–W^V state.