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 [Fe II ( L ) 2 ](ClO 4 ) 2 · n CH 3 CN (where L is 4-iodo-2,6-bis(pyrazol-1-yl)pyridine ( L 1 ), 4-(4′-hydroxyphenyl)-2,6-bis(pyrazol-1-yl)-pyridine ( L 2 ), 4-(4′-trimethylsilylethynyl)-2,6-bis(pyrazol-1-yl)pyridine ( L 3 ), and 4-ethynyl-2,6-bis(pyrazol-1-yl)pyridine ( L 4 )). The compounds [Fe II ( L ) 2 ](ClO 4 ) 2 · n CH 3 CN ( 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 [Fe II ( L 1 ) 2 ](ClO 4 ) 2 ( 1 ) and [Fe II ( L 2 ) 2 ](ClO 4 ) 2 ( 2 ) show above or near room temperature spin transition with T 1/2 = 333 and 281 K. The investigations of compound [Fe II ( L 3 ) 2 ](ClO 4 ) 2 ·2CH 3 CN ( 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 [Fe II ( L 4 ) 2 ](ClO 4 ) 2 ( 4 ) is completely blocked in the diamagnetic low-spin state in the temperature range from 5 to 380 K.