Structures and Spin States of Iron(II) Complexes of Isomeric 2,6-Di(1,2,3-triazolyl)pyridine Ligands

Iron(II) complex salts of 2,6-di(1,2,3-triazol-1-yl)pyridine (L1) are unexpectedly unstable in undried solvent. This is explained by the isolation [Fe(L1)4(H2O)2][ClO4]2 and [Fe(NCS)2(L1)2(H2O)2]·L1, containing L1 bound as a monodentate ligand rather than expected tridentate fashion. These complexes associate into 44 grid structures through O–H···N hydrogen bonding; solvate related coordination framework, catena-[Cu(?-L1)2(H2O)2][BF4]2, also presented. The isomeric ligands 2,6-di(1,2,3-triazol-2-yl)pyridine (L2) 2,6-di(1H-1,2,3-triazol-4-yl)pyridine (L3) bind to iron(II) more typical Solvates [Fe(L3)2][ClO4]2 low-spin diamagnetic solid state solution, while [Fe(L2)2][ClO4]2 [Co(L3)2][BF4]2 fully high-spin. Treatment L3 with methyl iodide affords 2,6-di(2-methyl-1,2,3-triazol-4-yl)pyridine (L4) 2-(1-methyl-1,2,3-triazol-4-yl)-6-(2-methyl-1,2,3-triazol-4-yl)pyridine (L5). While [Fe(L5)2]2+ state, [Fe(L4)2][ClO4]2·H2O high-spin, [Fe(L4)2][ClO4]2·3MeNO2 exhibits hysteretic spin transition 50% completeness at T1/2 = 128 K (?T1/2 6 K). proceeds via symmetry-breaking phase an unusual low-temperature three unique cation sites low-spin, 1:1 mixed-spin populations. distribution states reflects “spin-state frustration” site equal number high-spin nearest neighbors. Gas-phase density functional theory calculations reproduce spin-state preferences these some complexes. highlight interplay between ?-basicity ?-acidity heterocyclic donors this type, which have opposing influences on molecular field. Brønsted basicities L1–L3 very sensitive linkage isomerism their triazolyl donors, explains why iron show variation better-known iron(II)/2,6-dipyrazolylpyridine system.