Above room temperature spin transition in a series of iron(II) bis(pyrazolyl)pyridine compounds

We report on the synthesis of a series of iron(II) compounds of the general composition [FeII(L)2](ClO4)2·nCH3CN (where L is 4-iodo-2,6-bis(pyrazol-1-yl)pyridine (L1), 4-(4′-hydroxyphenyl)-2,6-bis(pyrazol-1-yl)-pyridine (L2), 4-(4′-trimethylsilylethynyl)-2,6-bis(pyrazol-1-yl)pyridine (L3), and 4-ethynyl-2,6-bis(pyrazol-1-yl)pyridine (L4)). The compounds [FeII(L)2](ClO4)2·nCH3CN (1–4; n = 0 for 1, 2, and 4; n = 2 for 3) were characterised by single crystal X-ray diffraction studies, MALDI-TOF mass spectrometry and elemental analysis. Temperature-dependent magnetic studies reveal different types of spin transition behaviour. Compounds [FeII(L1)2](ClO4)2 (1) and [FeII(L2)2](ClO4)2 (2) show above or near room temperature spin transition with T1/2 = 333 and 281 K. The investigations of compound [FeII(L3)2](ClO4)2·2CH3CN (3) elucidate a more complex magnetic behaviour with spin transition of half of the involved iron(II) centres at temperatures above 140 K and ferromagnetic exchange coupling of the remaining high-spin centres below 125 K. Finally, compound [FeII(L4)2](ClO4)2 (4) is completely blocked in the diamagnetic low-spin state in the temperature range from 5 to 380 K.