Light- and thermal-induced spin crossover in {Fe(abpt)2[N(CN)2]2}.: Synthesis, structure, magnetic properties, and high-spin ⇆ low-spin relaxation studies

{Fe(abpt)2[N(CN)2]2} (abpt = 4-amino-3,5- bis(pyridin-2-yl)-1,2,4-triazole) represents the first example of an iron(II) spin-crossover compound containing dicyanamide ligand, [N(CN)2]-, as a counterion. It shows an incomplete two-step spin transition with around 37% of HS molecules trapped in the low-temperature region when standard cooling or warming modes, i.e., 1-2 K min-1, were used. The temperature, T1/2 ≈ 86 K, at which 50% of the conversion takes place, is one of the lowest temperatures observed for an iron(II) spin-crossover compound. Quenching experiments at low temperatures have shown that the incomplete character of the conversion is a consequence of slow kinetics. The quenched HS state relaxes back to the LS state displaying noticeable deviation from a single-exponential law. The rate of relaxation was evaluated in the range of temperatures 10-60 K. In the upper limit of temperatures, where thermal activation predominates, the activation energy and the pre-exponential parameter were estimated as Ea ≈ 280 cm-1 and AHL ≈ 10 s-1, respectively. The lowest value of kHL around 1.2 × 10-4 s-1 (T = 10 K) was obtained in the region of temperatures where tunneling predominates. A quantitative light induced excited spin state trapping (LIESST) effect was observed, and the HS → LS relaxation in the range of temperatures 5-52.5 K was studied. From the Arrhenius plot the two above-mentioned characteristic regimes, thermal-activated (Ea ≈ 431 cm-1 and AHL ≈ 144 s-1) and tunneling (kHL ≈ 1.7 × 10-6 s-1 at 5 K), were characterized. The crystal structure was solved at room temperature. It crystallizes in the triclinic P1 space group, and the unit cell contains a centrosymmetric mononuclear unit. Each iron atom is in a distorted octahedral environment with bond distances Fe-N(1) = 2.216(2) Å, Fe-N(2) = 2.121(2) Å, and Fe-N(3) = 2.160(2) Å for the pyridine, triazole, and dicyanamide ligands, respectively.