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

Two new <b>Rdpt</b> ligands featuring long “tails”, <b>padpt</b> (<i>N</i>-4<i>H</i>-1,2,4-triazole-3,5-di­(2-pyridyl)­palmitamide) and <b>hpdpt</b> (4-(4-heptadecafluoroctylphenyl)-3,5-bis­(2-pyridyl)-4H-1,2,4-triazole), were made and reacted with [Fe^II(py)₄(NCS)₂] to give pinkish-red [Fe^II(<b>padpt</b>)₂(SCN)₂] (<b>1</b>) and purple-red [Fe^II(<b>hpdpt</b>)₂(SCN)₂] (<b>2</b>) as solvent-free crystals. Magnetic measurements reveal that both <b>1</b> and <b>2</b> exhibit complete and reproducible spin crossovers, with a far lower <i>T</i>_1/2 for the amide-alkyl tailed <b>1</b> (182 K) than for the fluorocarbon tailed <b>2</b> (248 K), which in turn is far lower than the <i>T</i>_1/2 of 290 K previously reported for the nonamide-alkyl tailed analogue [Fe^II(<b>C</b>_<b>16</b><b>dpt</b>)₂(SCN)₂]·²/₃H₂O (<b>3</b>). Structure determinations for <b>1</b> and <b>2</b> 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 <b>1</b> and <b>2</b>, as well as in <b>3</b>, with the Raman data giving <i>T</i>_1/2 values of 190, 243, and 285 K, respectively. Bright orange single crystals of the solvatomorph [Fe^II(<b>hpdpt</b>)₂(SCN)₂]·MeOH·H₂O (<b>2</b>_<b>solv</b>) were also structurally and magnetically characterized and, in contrast to <b>2</b>, found to remain HS down to 4 K, providing further evidence of the huge impact of crystal packing on SCO. Both <b>1</b> and <b>2</b> form stable Langmuir films at an air–water interface, a single layer of which can be transferred to a solid support