"Tail" tuning of iron(II) spin crossover temperature by 100 K

Abstract

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 [Fe II (py) 4 (NCS) 2 ] to give pinkish-red [Fe II (padpt) 2 (SCN) 2 ] (1) and purple-red [Fe II (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 T 1/2 for the amide-alkyl tailed 1 (182 K) than for the fluorocarbon tailed 2 (248 K), which in turn is far lower than the T 1/2 of 290 K previously reported for the nonamide-alkyl tailed analogue [Fe II (C 16 dpt) 2 (SCN) 2 ]· 2 / 3 H 2 O (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 (∑ = 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 T 1/2 values of 190, 243, and 285 K, respectively. Bright orange single crystals of the solvatomorph [Fe II (hpdpt) 2 (SCN) 2 ]·MeOH·H 2 O (2 solv) 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.