Interplay between kinetically slow thermal spin-crossover and metastable high-spin state relaxation in an iron(II) complex with similar T-1/2 and T(LIESST)

This paper describes the first material to show the well-known light-induced excited spin-state trapping (LIESST) effect, the metastable excited state of which relaxes at a temperature approaching its thermal spin-crossover. Cooling polycrystalline [FeL2]-[BF4](2)center dot xH(2)O (L=2,6-bis{3-methylpyrazol-1-yl)pyridine; X = 0-(1)/3) at 1 Kmin(-1) leads to a cooperative spin transition, taking place in two steps centered at 147 and 105 K, that is only 54% complete by magnetic susceptibility. Annealing the sample at 100 K for 2 h results in a slow decrease in chi T-M to zero, showing that the remainder of the spin-crossover can proceed, but is kinetically slow. The crystalline high- and fully low-spin phases of [FeL2]-[BF4](2)center dot xH(2)O are isostructural (C2/c, Z = 8), but the spin-crossover proceeds via a mixed-spin intermediate phase that has a triple unit cell (C2/c, Z = 24). The water content of the crystals is slowly lost on exposure to air without causing decomposition. However, the high-spin/mixed-spin transition in the crystal proceeds at 110 +/- 20 K when x = 1/3 and 155 +/- 5 K when x = 0, which correspond to the two spin-crossover steps seen in the bulk material. The high-spin state of the compound is generated quantitatively by irradiation of the low-spin or the mixed-spin phase at 10 K, and in approximately 70 % yield by rapidly quenching the sample to 10 K. This metastable high-spin state relaxes back to the low-spin ground state at 87 +/- 1 K in one, not two, steps, and without passing through the intermediate phase. Ibis implies that thermal spin-crossover and thermally activated high-spin-low-spin relaxation in this material become decoupled, thus avoiding the physical impossibility of T-(LIESST) being greater than T-1/2.