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

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Abstract
[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, T(1/2) approximately 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 E(a) approximately 280 cm(-)(1) and A(HL) approximately 10 s(-)(1), respectively. The lowest value of k(HL) around 1.2 x 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 (E(a) approximately 431 cm(-)(1) and A(HL) approximately 144 s(-)(1)) and tunneling (k(HL) approximately 1.7 x 10(-)(6) s(-)(1) at 5 K), were characterized. The crystal structure was solved at room temperature. It crystallizes in the triclinic P_1 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) A, Fe-N(2) = 2.121(2) A, and Fe-N(3) = 2.160(2) A for the pyridine, triazole, and dicyanamide ligands, respectively.