Spin-crossover compounds based on iron(ii) complexes of 2,6-bis(pyrazol-1-yl)pyridine (bpp) functionalized with carboxylic acid and ethyl carboxylic acid.

Four new salts of the iron(ii) complex of the 2,6-bis(pyrazol-1-yl)pyridine ligand functionalized with a carboxylic acid group (bppCOOH) of formulas Fe(bppCOOH)22 (1(BF4)2), Fe(bppCOOH)22·yMe2CO (1(CF3SO3)2·yMe2CO), Fe(bppCOOH)22·yMe2CO (1(AsF6)2·yMe2CO) and Fe(bppCOOH)22·yMe2CO (1(SbF6)2·yMe2CO) have been prepared and characterized together with a more complete structural and photomagnetic characterization of the previously reported Fe(bppCOOH)22 (1(ClO4)2). Furthermore, the iron(ii) complex of the ethyl ester derivative of bppCOOH (bppCOOEt) has been prepared and characterized (compound Fe(bppCOOEt)22·yMe2CO, 2(ClO4)2·yMe2CO). Isostructural 1(BF4)2 and 1(ClO4)2 show an abrupt and reversible spin transition with a much lower T1/2 for the BF4- salt. CF3SO3-, SbF6- and AsF6- counteranions and the bppCOOEt ligand lead to the incorporation of solvent molecules in the structures, which play an important role in the spin-crossover properties of these compounds. In the case of 1(CF3SO3)2·yMe2CO, a reversible spin transition is obtained after desolvation. All these compounds show a LIESST effect.