“Tail” Tuning of Iron(II) Spin Crossover Temperature by 100 K

Two
new Rdpt ligands featuring long “tails”, padpt (N-4H-1,2,4-triazole-3,5-di­(2-pyridyl)­palmitamide)
and hpdpt (4-(4-heptadecafluoroctylphenyl)-3,5-bis­(2-pyridyl)-4H-1,2,4-triazole),
were made and reacted with [FeII(py)4(NCS)2] to give pinkish-red [FeII(padpt)2(SCN)2] (1) and purple-red [FeII(hpdpt)2(SCN)2] (2) as solvent-free crystals. Magnetic measurements reveal
that both 1 and 2 exhibit complete and reproducible
spin crossovers, with a far lower T1/2 for the amide-alkyl tailed 1 (182 K) than for the fluorocarbon
tailed 2 (248 K), which in turn is far lower than the T1/2 of 290 K previously reported for the nonamide-alkyl
tailed analogue [FeII(C16dpt)2(SCN)2]·2/3H2O (3). Structure determinations
for 1 and 2 in both the high spin (HS) and
low spin (LS) states confirm the expected trans-NCS conformation and
reveal that (a) the “tails” interdigitate and (b) the
LS forms are less distorted than the HS forms (Σ = 58–70°
vs 47–54°). DSC and Raman spectroscopy confirmed the high
tail-dependence of the SCO events in 1 and 2, as well as in 3, with the Raman data giving T1/2 values of 190, 243, and 285 K, respectively.
Bright orange single crystals of the solvatomorph [FeII(hpdpt)2(SCN)2]·MeOH·H2O (2solv) were also structurally
and magnetically characterized and, in contrast to 2,
found to remain HS down to 4 K, providing further evidence of the
huge impact of crystal packing on SCO. Both 1 and 2 form stable Langmuir films at an air–water interface,
a single layer of which can be transferred to a solid support.