No abstract

Two isomeric pyrimidine-based Rdpt-type triazole ligands were made: 4-(4-methylphenyl)-3-(2-pyrimidyl)-5-phenyl-4 H-1,2,4-triazole (L) and 4-(4-methylphenyl)-3-(4-pyrimidyl)-5-phenyl-4 H-1,2,4-triazole (L). When reacted with [Fe(pyridine)(NCE)], where E = S, Se, or BH, two families of mononuclear iron(II) complexes are obtained, including six solvatomorphs, giving a total of 12 compounds: [Fe(L)(NCS)] (1), [Fe(L)(NCSe)] (2), 2·1.5HO, [Fe(L)(NCBH)]·2CHCl (3·2CHCl), 3 and 3·2HO, [Fe(L)(NCS)] (4), 4·HO, [Fe(L)(NCSe)] (5), 5·2CHOH, 5·1.5HO, and [Fe(L)(NCBH)]·2.5HO (6·2.5HO). Single-crystal X-ray diffraction reveals that the N-coordinated iron(II) centers in 1, 2, 3·2CHCl, 4, 5, and 5·2CHOH have two bidentate triazole ligands equatorially bound and two axial NCE co-ligands trans-coordinated. All structures are high spin (HS) at 100 K, except 3·2CHCl, which is low spin (LS). Solid-state magnetic measurements show that only 3·2CHCl ( T above 400 K) and 5·1.5HO ( T = 110 K) undergo spin crossover (SCO); the others remain HS at 300-50 K. When 3·2CHCl is heated at 400 K it desorbs CHCl becoming 3, which remains HS at 400-50 K. UV-Vis studies in CHCl, CHCl, (CH)CO, CHCN, and CHNO solutions for the BH analogues 3 and 6 led to a 6:1 ratio of L /Fe(II) being employed for the solution studies. These revealed SCO activity in all five solvents, with T values for the 2-pyrimidine complex (247-396 K) that were consistently higher than for the 4-pyrimidine complex (216-367 K), regardless of solvent choice, consistent with the 2-pyrimidine ring providing a stronger ligand field than the 4-pyrimidine ring. Strong correlations of solvent polarity index with the T values in those solvents are observed for each complex, enabling predictable T tuning by up to 150 K. While this correlation is tantalizing, here it may also be reflecting solvent-dependent speciation-so future tests of this concept should employ more stable complexes. Differences between solid-state (ligand field; crystal packing; solvent content) and solution (ligand field; solvation; speciation) effects on SCO are highlighted.