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) are unexpectedly unstable in undried solvent. This is explained by the isolation of [Fe(L)(HO)][ClO] and [Fe(NCS)(L)(HO)]·L, containing L bound as a monodentate ligand rather than in the expected tridentate fashion. These complexes associate into 4 grid structures through O-H···N hydrogen bonding; a solvate of a related 4 coordination framework, -[Cu(μ-L)(HO)][BF], is also presented. The isomeric ligands 2,6-di(1,2,3-triazol-2-yl)pyridine (L) and 2,6-di(1-1,2,3-triazol-4-yl)pyridine (L) bind to iron(II) in a more typical tridentate fashion. Solvates of [Fe(L)][ClO] are low-spin and diamagnetic in the solid state and in solution, while [Fe(L)][ClO] and [Co(L)][BF] are fully high-spin. Treatment of L with methyl iodide affords 2,6-di(2-methyl-1,2,3-triazol-4-yl)pyridine (L) and 2-(1-methyl-1,2,3-triazol-4-yl)-6-(2-methyl-1,2,3-triazol-4-yl)pyridine (L). While salts of [Fe(L)] are low-spin in the solid state, [Fe(L)][ClO]·HO is high-spin, and [Fe(L)][ClO]·3MeNO exhibits a hysteretic spin transition to 50% completeness at = 128 K (Δ = 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 σ-basicity and π-acidity of the heterocyclic donors in this ligand type, which have opposing influences on the molecular ligand field. The Brønsted basicities of L-L 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.