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 [FeL 2 ][BF 4 ] 2 ⋅ x  H 2 O (L=2,6‐bis{3‐methylpyrazol‐1‐yl}pyridine; x =0–1/3) at 1 K min −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 χ M T 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 [FeL 2 ][BF 4 ] 2 ⋅ x  H 2 O are isostructural ( C 2/ c , Z =8), but the spin‐crossover proceeds via a mixed‐spin intermediate phase that has a triple unit cell ( C 2/ 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. This 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 .