Hydrogen-bonded networks of [Fe(bpp) 2 ] 2+ spin crossover complexes and dicarboxylate anions: structural and photomagnetic properties.

The paper reports the syntheses, crystal structures, thermal and (photo)magnetic properties of spin crossover salts of formula [Fe(bpp)2 ](C6 H8 O4 )·4H2 O (1·4H2 O), [Fe(bpp)2 ](C8 H4 O4 )·2CH3 OH·H2 O (2·2MeOH·H2 O) and [Fe(bpp)2 ](C8 H4 O4 )·5H2 O (2·5H2 O) (bpp = 2,6-bis(pyrazol-3yl)pyridine; C6 H8 O4 = adipate dianion; C8 H4 O4 = terephthalate dianion). The salts exhibit an intricate network of hydrogen bonds between low-spin iron(ii) complexes and carboxylate dianions, with solvent molecules sitting in the voids. Desolvation is accompanied by a low-spin (LS) to high-spin (HS) transformation in the materials. The dehydrated phase 2 undergoes a two-step transition with a second step showing thermal hysteresis (T1/2 ↑ = 139 K and T1/2 ↓ = 118 K). 2 displays a quantitative LS to HS photomagnetic conversion, with a T(LIESST) value of 63 K.