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

Terpyridine-based cobalt­(II) complex, [CoII(HL)2] (1; 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 (1·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 (S = 0) and coordinates to two terpyridine moieties, was introduced. The resulting complexes, [CoIIxFeII1–x(HL)2], where x = 0.88 (2), 0.55 (3), and 0 (4), 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.