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

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)2 L3 ]Br·0.5H2 O (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.