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

The synthesis of 4-methyl-2,6-di­(pyrazol-1-yl)­pyridine
(L) and four salts of [FeL2]­X2 (X– = BF4–, 1; X– = ClO4–, 2; X– = PF6–, 3; X– = CF3SO3–, 4) are reported. Powder samples of 1 and 2 both exhibit abrupt, hysteretic spin-state transitions on cooling,
with T1/2↓ = 204 and T1/2↑ = 209 K (1), and T1/2↓ = 175 and T1/2↑ = 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–Teller-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 T1/2, exposing both compounds to 10–5 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
→ 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–Teller distortion.