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 T_1/2=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 (T_1/2↑=357 K and T_1/2↓=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.