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 (L-1) are unexpectedly unstable in undried solvent. This is explained by the isolation of [Fe(L-1)(4)(H2O)(2)][ClO4](2) and [Fe(NCS)(2)(L-1)(2)(H2O)(2)].L-1, containing L-1 bound as a monodentate ligand rather than in the expected tridentate fashion. These complexes associate into 4(4) grid structures through O-H center dot center dot center dot N hydrogen bonding; a solvate of a related 4(4) coordination framework, catena-[Cu(mu-L-1)(2)(H2O)(2)][BF4](2), is also presented. The isomeric ligands 2,6-di(1,2,3-triazol-2-yl)pyridine (L-2) and 2,6-di(1H-1,2,3-triazol-4-yl)pyridine (L-3) bind to iron(II) in a more typical tridentate fashion. Solvates of [Fe(L-3)(2)][ClO4](2) are low-spin and diamagnetic in the solid state and in solution, while [Fe(L-2)(2)][ClO4](2) and [Co(L-3)(2)][BF4](2) are fully high-spin. Treatment of L-3 with methyl iodide affords 2,6-di(2-methyl-1,2,3-triazol-4yl)pyridine (L-4) and 2-(1-methyl-1,2,3-triazol-4-yl)-6-(2-methyl-1,2,3-triazol-4-yl)pyridine (L-5). While salts of [Fe(L-5)(2)](2+) are lowspin in the solid state, [Fe(L-4)(2)][ClO4] (2)center dot H2O is high-spin, and [Fe(L-4)(2)][ClO4](2)center dot 3MeNO(2) exhibits a hysteretic spin transition to 50% completeness at T-1/2 = 128 K (Delta T-1/2 = 6 K). This transition proceeds via a symmetry-breaking phase transition to an unusual low-temperature phase containing three unique cation sites with high-spin, low-spin, and 1:1 mixed-spin populations. The unusual distribution of the spin states in the low-temperature phase reflects spin-state frustration of the mixed-spin cation site by an equal number of high-spin and low-spin nearest neighbors. Gas-phase density functional theory calculations reproduce the spin-state preferences of these and some related complexes. These highlight the interplay between the sigma-basicity and pi-acidity of the heterocyclic donors in this ligand type, which have opposing influences on the molecular ligand field. The Bronsted basicities of L-1-L-3 are very sensitive to the linkage isomerism of their triazolyl donors, which explains why their iron complex spin states show more variation than the better-known iron(II)/2,6-dipyrazolylpyridine system.

Automated summary

The study investigates the stability and spin states of iron(II) complex salts with different ligands through experimental and computational methods. It is found that the iron(II) complex salts exhibit different spin states under the influence of different ligands, which are related to the coordination mode and basicity of the ligands.