Metal-Dilution Effect on Spin Transition Behavior of Solvated/Desolvated Hydrogen-Bonded Cobalt(II)-Organic Frameworks

Terpyridine-based cobalt(II) complex, [Co^II(HL)₂] (<b>1</b>; H₂L = 2,2':6',2″-terpyridine-5,5″-diyl dicarboxylic acid), forms a hydrogen-bonded diamond framework with solvent absorption and desorption capabilities. The desolvated form (<b>1</b>·desolv) exhibits spin transition (ST) behavior accompanied by thermal hysteresis. To investigate the effect of metal-dilution, an Fe²⁺ center, which has a low-spin state (<i>S</i> = 0) and coordinates to two terpyridine moieties, was introduced. The resulting complexes, [Co^II _<i>x</i> Fe^II _1-<i>x</i> (HL)₂], where <i>x</i> = 0.88 (<b>2</b>), 0.55 (<b>3</b>), and 0 (<b>4</b>), demonstrated a significant influence of metal-dilution on the desolvated forms, but not on the solvated forms. Namely, the spin state is more strongly affected by the presence of solvent than by metal-dilution. However, in the absence of solvent, the Fe²⁺ ratio significantly impacts the ST behavior.