Four CoII-GdIII mixed-metal phosphonate clusters as molecular magnetic refrigerants

Abstract

The building of 3d-Gd clusters are very essential in the fields of magnetic refrigerants. Three bulky phosphonic acids: (9-methyl-9H-fluoren-9-yl)phosphonic acid (RPO3H2), (2,7-di-tert-butyl-9-methyl-9H-fluoren-9-yl)phosphonic acid (R′PO3H2), and (9-(sec-butyl)-2,7-di-tert-butyl-9H-fluoren-9-yl)phosphonic acid (R″PO3H2) have been employed to construct four CoII-GdIII clusters [CoII2GdIII3(OH)(RPO3)2(tBuCO2)8(tBuCO2H)2(C5H5N)(H2O)2]·2H2O·tBuCO2H (1), [CoII4GdIII3(OH)2(RPO3)2(tBuCO2)11(py)(CH3CN)(H2O)2]⋅CH3CN (2), [CoII3GdIII3(OH)2(R′PO3)2(tBuCO2)9(tBuCO2H)(H2O)2]·tBuCO2H·2CH2Cl2 (3) and [CoII3GdIII(R″PO3)2(tBuCO2)5(CH3CN)(H2O)4]·tBuCO2H·3CH3CN (4) under ambient condition. Clearly, the varied hindrance of fluorenyl groups are responsible for the formation of various clusters. Single crystal analysis indicates that the metal ions arrange in one plane in each cluster. Magnetic studies show that the entropy changes are 18.3, 19.8, 20.0 and 9.6Jkg−1K−1 for 1–4, respectively, from which we can see these values are nearly proportional to the percentage of the gadolinium in the core.