Solvent Polarity Predictably Tunes Spin Crossover T_1/2 in Isomeric Iron(II) Pyrimidine Triazoles

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^2pyrimidine) and 4-(4-methylphenyl)-3-(4-pyrimidyl)-5-phenyl-4 H-1,2,4-triazole (L^4pyrimidine). When reacted with [Fe^II(pyridine)₄(NCE)₂], where E = S, Se, or BH₃, two families of mononuclear iron(II) complexes are obtained, including six solvatomorphs, giving a total of 12 compounds: [Fe^II(L^2pyrimidine)₂(NCS)₂] (1), [Fe^II(L^2pyrimidine)₂(NCSe)₂] (2), 2·1.5H₂O, [Fe^II(L^2pyrimidine)₂(NCBH₃)₂]·2CHCl₃ (3·2CHCl₃), 3 and 3·2H₂O, [Fe^II(L^4pyrimidine)₂(NCS)₂] (4), 4·H₂O, [Fe^II(L^4pyrimidine)₂(NCSe)₂] (5), 5·2CH₃OH, 5·1.5H₂O, and [Fe^II(L^4pyrimidine)₂(NCBH₃)₂]·2.5H₂O (6·2.5H₂O). Single-crystal X-ray diffraction reveals that the N₆-coordinated iron(II) centers in 1, 2, 3·2CHCl₃, 4, 5, and 5·2CH₃OH 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 3·2CHCl₃, which is low spin (LS). Solid-state magnetic measurements show that only 3·2CHCl₃ ( T_1/2 above 400 K) and 5·1.5H₂O ( T_1/2 = 110 K) undergo spin crossover (SCO); the others remain HS at 300-50 K. When 3·2CHCl₃ is heated at 400 K it desorbs CHCl₃ becoming 3, which remains HS at 400-50 K. UV-Vis studies in CH₂Cl₂, CHCl₃, (CH₃)₂CO, CH₃CN, and CH₃NO₂ solutions for the BH₃ analogues 3 and 6 led to a 6:1 ratio of L ^npyrimidine/Fe(II) being employed for the solution studies. These revealed SCO activity in all five solvents, with T_1/2 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_1/2 values in those solvents are observed for each complex, enabling predictable T_1/2 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.