Iron(ii) complexes of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine and related ligands with annelated distal heterocyclic donors

Abstract

Following a published synthesis of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine (L¹), treatment of α,α′-dibromo-2,6-diacetylpyridine with 2 equiv. 2-aminopyrimidine or 2-aminoquinoline in refluxing acetonitrile respectively gives 2,6-bis(imidazo[1,2-a]pyrimidin-2-yl)pyridine (L²) and 2,6-bis(imidazo[1,2-a]quinolin-2-yl)pyridine (L³). Solvated crystals of [Fe(L¹)₂][BF₄]₂ (1[BF₄]₂) and [Fe(L²)₂][BF₄]₂ (2[BF₄]₂) are mostly high-spin, although one solvate of 1[BF₄]₂ undergoes thermal spin-crossover on cooling. The iron coordination geometry is consistently distorted in crystals of 2[BF₄]₂ which may reflect the influence of intramolecular, inter-ligand N⋯π interactions on the molecular conformation. Only 1 : 1 Fe : L³ complexes were observed in solution, or isolated in the solid state; a crystal structure of [FeBr(py)₂L³]Br·0.5H₂O (py = pyridine) is presented. A solvate crystal structure of high-spin [Fe(L⁴)₂][BF₄]₂ (L⁴ = 2,6-di{quinolin-2-yl}pyridine; 4[BF₄]₂) is also described, which exhibits a highly distorted six-coordinate geometry with a helical ligand conformation. The iron(II) complexes are high-spin in solution at room temperature, but 1[BF₄]₂ and 2[BF₄]₂ undergo thermal spin-crossover equilibria on cooling. All the compounds exhibit a ligand-based emission in solution at room temperature. Gas phase DFT calculations mostly reproduce the spin state properties of the complexes, but show small anomalies attributed to intramolecular, inter-ligand dispersion interactions in the sterically crowded molecules.