Spin-crossover in a trans-[FeL2(NCS)2] family (L = triaryltriazole): remote substituent effects on spin transition modes and temperature.

We report the syntheses, characterisations, and spin-crossover behaviours of four mononuclear trans-[Fe(L1-L4)2(NCS)2] complexes (L = 4-(p-R1-phenyl)-3,5-bis(2-pyridyl)-4H-1,2,4-triazole and 4-(p-methylphenyl)-3-(p-R2-phenyl)-5-(2-pyridyl)-4H-1,2,4-triazole, (1) R1 = Cl (L1); (2) R1 = Me (L2); (3) R2 = MeO (L3); (4) R2 = F (L4)). The X-ray single crystal structural analysis reveals that 1 and 2 crystallize in the triclinic space group P1, whereas 3·0.5H2O and 4 crystallize in the monoclinic space group P2(1)/c, and all complexes possess a similar distorted [FeN6] octahedron with two trans-positions NCS(-) anions. Changing of the distal substituent on the phenyl ring of the triaryltriazoles causes differences in the crystal packing of the complexes that are key to their magnetic properties. Magnetic measurements indicate that 1 shows an abrupt transition with T(1/2) = 235 K, 2 displays a 3 K hysteresis loop with T(1/2)(↓) = 217 K and T(1/2)(↑) = 220 K, 3 exhibits a gradual spin transition with T(1/2) = 156 K and 4 undergoes an incomplete (70%) spin transition with T(1/2) = 97 K. The substituted group effects and intermolecular interactions appear to be critical to spin transition modes and temperature in this family.