Abstract
Discrete mononuclear [Fe
II
(
tolpzph
)
2
(NCS)
2
]·THF (
1
·THF), shows highly sensitive, robust and reversible solvent-dependent spin crossover, enabling it to act as a
quantitative
small molecule sensor.
A new phenyl-triazole-pyrazine ligand, 4-
p
-tolyl-3-(phenyl)-5-(2-pyrazinyl)-1,2,4-triazole (
tolpzph
), was prepared in order to enforce pyrazine coordination of the iron(
ii
) centre in the resulting complex, [Fe
II
(
tolpzph
)
2
(NCS)
2
]·THF (
1
·THF). Structure determinations carried out on this discrete mononuclear complex,
1
·THF, at 273 K (mostly high spin) and 100 K (mostly low spin) demonstrate this was successful, and that spin crossover (SCO) occurred on cooling. Subsequent magnetic measurements on
1
·THF revealed that it shows highly sensitive and reversible solvent-dependent SCO, with
T
1/2
(
1
·THF) = 255 K
vs. T
1/2
(
1
) = 212 K (with SCO of
1
more abrupt and occurring with a 4 K hysteresis loop), a drop of 43 K due to THF loss. This is reversible over at least 10 cycles of re-solvating with THF followed by re-drying, so
1
↔
1
·THF can be considered an ‘on–off’ THF sensor, monitored by the
T
1/2
reversibly shifting (by 43 K). Furthermore,
quantitative sensing
of the fractional amount of THF present in
1
·
n
THF, 0 ≤
n
≤ 1, is demonstrated. Monitoring the
T
1/2
and using TGA to quantify
n
(THF) revealed a linear dependence (25 data points; Pearson
r
2
= 0.93):
T
1/2
= 41.1
n
(THF) + 219. Finally,
1
is also shown to take up CHCl
3
[
T
1/2
(
1
·CHCl
3
) = 248 K], with a logarithmic
T
1/2
dependence on the fractional amount of CHCl
3
present (10 data points; Pearson
r
2
= 0.98):
T
1/2
= 27.0 log
10
[
n
(CHCl
3
)] + 243. This study is a proof of principle that a (multi-use)
quantitative sensor
material based on spin crossover is feasible.