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

Abstract

[EN] {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.