Reversible quantitative guest sensing via spin crossover of an iron(ii) triazole

A new phenyl-triazole-pyrazine ligand, 4- -tolyl-3-(phenyl)-5-(2-pyrazinyl)-1,2,4-triazole ( ), was prepared in order to enforce pyrazine coordination of the iron(ii) centre in the resulting complex, [Fe ( ) (NCS) ]·THF ( ·THF). Structure determinations carried out on this discrete mononuclear complex, ·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 ·THF revealed that it shows highly sensitive and reversible solvent-dependent SCO, with ( ·THF) = 255 K ( ) = 212 K (with SCO of 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 ↔ ·THF can be considered an 'on-off' THF sensor, monitored by the reversibly shifting (by 43 K). Furthermore, of the fractional amount of THF present in · THF, 0 ≤ ≤ 1, is demonstrated. Monitoring the and using TGA to quantify (THF) revealed a linear dependence (25 data points; Pearson = 0.93): = 41.1 (THF) + 219. Finally, is also shown to take up CHCl [ ( ·CHCl ) = 248 K], with a logarithmic dependence on the fractional amount of CHCl present (10 data points; Pearson = 0.98): = 27.0 log [ (CHCl )] + 243. This study is a proof of principle that a (multi-use) material based on spin crossover is feasible.