Mixed-Valence Iron(II, III) Trimesates with Open Frameworks Modulated by Solvents

Solvothermal reactions with different solvents produced two iron trimesates [Fe2(H2O)2(BTC)4/3]Cl x 4.5(DMF) (1) and [Fe4Cl(BTC)8/3]Cl2 x H2O x 2.5(DEF) (2) (BTC = 1,3,5-benzenetricarboxylate, DMF = N,N'-dimethylformamide, DEF = N,N'-diethylformamide). The framework of 1 is a (3,4)-connected net constructed from mixed-valence paddlewheel Fe2(II, III) units and BTC linkers, while the framework of 2 is a (3,8)-connected net built from mixed-valence square-planar Fe4(III, III, III, II) units and BTC linkers. The large volume inside the framework of 1 (or 2) is occupied by disordered Cl- anions and guest DMF (or DEF) molecules. The mixed-valence character of the frameworks of 1 and 2 was confirmed by Mössbauer spectroscopy studies. The active electronic property of iron cations may be the origin of the variability of the iron-organic frameworks, which are readily affected by some synthetic factors, such as solvents. Magnetic studies reveal that there are antiferromagnetic exchange interactions among the Fe atoms in 1 and 2. Ion-exchange studies for 1 show that the Cl- anions inside the framework of 1 can be exchanged by CNS- anions.