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

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·nTHF, 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 r² = 0.93): T_1/2 = 41.1n(THF) + 219. Finally, 1 is also shown to take up CHCl₃ [T_1/2(1·CHCl₃) = 248 K], with a logarithmic T_1/2 dependence on the fractional amount of CHCl₃ present (10 data points; Pearson r² = 0.98): T_1/2 = 27.0 log₁₀[n(CHCl₃)] + 243. This study is a proof of principle that a (multi-use) quantitative sensor material based on spin crossover is feasible.