Unexpected Spin-Crossover and a Low-Pressure Phase Change in an Iron(II)/Dipyrazolylpyridine Complex Exhibiting a High-Spin Jahn鈥揟eller Distortion

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• 作者：Laurence J. Kershaw Cook ; Flora L. Thorp-Greenwood ; Tim P. Comyn ; Oscar Cespedes ; Guillaume Chastanet ; Malcolm A. Halcrow
• 刊名：Inorganic Chemistry
• 出版年：2015
• 出版时间：July 6, 2015
• 年：2015
• 卷：54
• 期：13
• 页码：6319-6330
• 全文大小：808K
• ISSN：1520-510X

文摘

The synthesis of 4-methyl-2,6-di(pyrazol-1-yl)pyridine (L) and four salts of [FeL₂]X₂ (X^{鈥?/sup> = BF₄鈥?/sup>, 1; X鈥?/sup> = ClO₄鈥?/sup>, 2; X鈥?/sup> = PF₆鈥?/sup>, 3; X鈥?/sup> = CF₃SO₃鈥?/sup>, 4) are reported. Powder samples of 1 and 2 both exhibit abrupt, hysteretic spin-state transitions on cooling, with T_1/2鈫?= 204 and T_1/2鈫?= 209 K (1), and T_1/2鈫?= 175 and T_1/2鈫?= 193 K (2). The 18 K thermal hysteresis loop for 2 is unusually wide for a complex of this type. Single crystal structures of 2 show it to exhibit a Jahn鈥揟eller-distorted six-coordinate geometry in its high-spin state, which would normally inhibit spin-crossover. Bulk samples of 1 and 2 are isostructural by X-ray powder diffraction, and undergo a crystallographic phase change during their spin-transitions. At temperatures below T_1/2, exposing both compounds to 10鈥? Torr pressure inside the powder diffractometer causes a reversible transformation back to the high-temperature crystal phase. Consideration of thermodynamic data implies this cannot be accompanied by a low 鈫?high spin-state change, however. Both compounds also exhibit the LIESST effect, with 2 exhibiting an unusually high T(LIESST) of 112 K. The salts 3 and 4 are respectively high-spin and low-spin between 3 and 300 K, with crystalline 3 exhibiting a more pronounced version of the same Jahn鈥揟eller distortion.}