The complex salts [FeL
2]X2 (1X
2
; L =
2,6-di{4-fluoropyrazol-1-yl}pyridine; X– = BF4
– or ClO4
–)
exhibit abrupt spin-transitions with narrow thermal hysteresis, at T
1/2 = 164 K (X– = BF4
–) and 148 K (X– = ClO4
–). The transition in 1[ClO
4
]
2
is complicated
by efficient thermally induced excited spin-state trapping (TIESST)
of its high-spin state below ca. 120 K, and the fully low-spin state
was achieved only inside the magnetometer at a scan rate of 0.5 K
min–1. Crystals of 1[BF
4
]
2
are tetragonal (P
21
c, Z = 2; phase 1) at 300 K but transform to a highly twinned
monoclinic phase 2 (P21, Z = 2) at 285 ± 5 K. These are forms of the “terpyridine
embrace” crystal lattice, which often affords cooperative spin-transitions
in iron/di(pyrazolyl)pyridine complexes. Phase 2 of high-spin 1[BF
4
]
2
shows a significant temperature dependence by powder diffraction,
which reflects increased canting of the monoclinic unit cell as the
temperature is lowered. In contrast, 1[ClO
4
]
2
retains phase
2 between 100 and 300 K, and was crystallographically characterized
in its thermally trapped metastable high-spin state at 100 K, as well
as its thermodynamic high- and low-spin forms at higher temperatures.
The spin-crossover transition temperature in 1[ClO
4
]
2
and related
compounds correlates well with a parameter describing angular changes
to the metal coordination sphere during the transition but not with
other commonly used structural indices. The TIESST metastable high-spin
state of 1[ClO
4
]
2
shows no single molecule magnet properties
at 2 K.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.