Water‐Solvation‐Dependent Spin Transitions in Cobalt(II)‐Octacyanidometallate Complexes

Abstract

The spin‐crossover (SCO) and charge‐transfer (CT) phenomena, the switching processes between two distinguishable magnetic states, are promising for developing materials capable of sophisticated memory and sensing functionalities. The majority of SCO systems are based on iron(II) complexes. However, cobalt(II)‐2,2′:6′,2′′‐terpyridine (terpy) systems emerge as a promising alternative. In this work, new complex salts [CoII(terpy)2]2[MoIV(CN)8] ⋅ 15H2O, Co2Mo(H2O), and [CoII(terpy)2]3[WV(CN)8]2 ⋅ 12H2O, Co3W2(H2O) were synthesized and physiochemically characterized. Structural studies for both compounds revealed [Co(terpy)2]2+ layers pillared by octacyanidometallate anions and completed with water molecules between them. Magnetic studies confirmed that the (de)solvated phases of both complexes exhibit partial SCO on the cobalt(II) centers: CoII−LS (SCo(II)‐LS=1/2)↔CoII−HS (SCo(II)‐HS=3/2). Moreover, handling dehydrated samples in a high‐humidity environment leads to partial recovery of previous magnetic properties via humidity‐induced SCO for Co2Mo: CoII−HS→CoII−LS, and the new phenomenon of isothermal humidity‐activated charge‐transfer‐induced spin transition, which we define here as HACTIST, for Co3W2: CoII−HS⋅⋅⋅WV (SCo(II)‐HS=3/2 and SW(V)=1/2)→CoIII−LS⋅⋅⋅WIV (SW(IV)=0 and SCo(III)‐LS=0). These comprehensive studies shed light on the water‐solvation‐dependent spin transitions in Co(II)‐octacyanidometallate(IV/V) complexes.