Hysteretic Spin Crossover above Room Temperature and Magnetic Coupling in Trinuclear Transition-Metal Complexes with Anionic 1,2,4-Triazole Ligands

The reaction of 4‐(1,2,4‐triazol‐4‐yl)ethanesulfonate (L) with Zn2+, Cu2+, Ni2+, Co2+, and Fe2+ gave a series of analogous neutral trinuclear complexes with the formula [M3(μ‐L)6(H2O)6] (1–5). These compounds were characterized by single‐crystal X‐ray diffraction, thermogravimetry, and elemental analysis. The magnetic properties of compounds 2–5 were studied. Complexes 2–4 show weak antiferromagnetic superexchange, with J values of −0.33 (2), −9.56 (3), and −4.50 cm−1 (4) (exchange Hamiltonian H=−2 J (S1S2+S2S3)). Compound 5 shows two additional crystallographic phases (5 b and 5 c) that can be obtained by dehydration and/or thermal treatment. These three phases exhibit distinct magnetic behavior. The Fe2+ centers in 5 are in high‐spin (HS) configuration at room temperature, with the central one exhibiting a non‐cooperative gradual spin transition below 250 K with T1/2=150 K. In 5 b, the central Fe2+ stays in its low‐spin (LS) state at room temperature, and cooperative spin transition occurs at higher temperatures and with the appearance of memory effect (T1/2↑=357 K and T1/2↓=343 K). In the case of 5 c, all iron centers remain in their HS configuration down to very low temperatures, with weak antiferromagnetic coupling (J=−1.16 cm−1). Compound 5 b exhibits spin transition with memory effect at the highest temperature reported, which matches the remarkable features of coordination polymers. Memory over room temperature: Five neutral trinuclear complexes [M3(μ‐L)6(H2O)6] (M=Zn2+, Cu2+, Ni2+, Co2+, Fe2+, and L=4‐(1,2,4‐triazol‐4‐yl)ethanesulfonate) were synthesized and characterized. For the FeII compound, three phases with different magnetic behaviors were isolated. One of them showed spin crossover with a wide hysteresis over room temperature (see figure).