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

摘要

The reaction of 4-(1,2,4-triazol-4-yl)ethanesulfonate (L) with Zn~(2+), Cu~(2+), Ni~(2+), Co~(2+), and Fe~(2+) gave a series of analogous neutral trinuclear complexes with the formula [M_3(μ-L)_6(H_2O)_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 antiferro magnetic superexchange, with J values of -0.33 (2), -9.56 (3), and -4.50 cm~(-1) (4) (exchange Hamiltonian H=-2J(S_1S_2 + S_2S_3)). Compound 5 shows two additional crystal logra phic phases (5 b and 5 c) that can be obtained by dehydration and/or thermal treatment. These three phases exhibit distinct magnetic behavior. The Fe~(2+) 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 Fe~(2+) 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 an T_(1/2)↓ = 343 K). In the case of 5c, all iron centers remain in their HS configuration down to very low temperatures, with weak antiferro magnetic coupling (J=-1.16 cm~(-1)). Compound 5b exhibits spin transition with memory effect at the highest temperature reported, which matches the remarkable features of coordination polymers.