Inorganic Chemistry, Vol.54, No.2, 682-693, 2015
Iron(II) Complexes of Tridentate Indazolylpyridine Ligands: Enhanced Spin-Crossover Hysteresis and Ligand-Based Fluorescence
Reaction of 2,6-difluoropyridine with 2 equiv of indazole and NaH at room temperature affords a mixture of 2,6-bis(indazol-1-yl)pyridine (1-bip), 2-(indazol-1-yl)-6-(indazol-2-yl)pyridine (1,2-bip), and 2,6-bis(indazol-2-yl)pyridine (2-bip), which can be separated by solvent extraction. A two-step procedure using the same conditions also affords both 2-(indazol-1-yl)-6-(pyrazol-1-yl)pyridine (1-ipp) and 2-(indazol-2-yl)-6-(pyrazol-1-yl)pyridine (2-ipp). These are all annelated analogues of 2,6-di(pyrazol-1-yl)pyridine, an important ligand for spin-crossover complexes. Iron(II) complexes [Fe(1-bip)(2)](2+), [Fe(1,2-bip)(2)](2+), and [Fe(1-ipp)(2)](2+) are low-spin at room temperature, reflecting sterically imposed conformational rigidity of the 1-indazolyl ligands. In contrast, the 2-indazolyl complexes [Fe(2-bip)(2)](2+) and [Fe(2-ipp)(2)](2+) are high-spin in solution at room temperature, whereas salts of [Fe(2-bip)(2)](2+) exhibit thermal spin transitions in the solid state. Notably, [Fe(2-bip)(2)][BF4](2).2MeNO(2) adopts a terpyridine embrace lattice structure and undergoes a spin transition near room temperature after annealing, resulting in thermal hysteresis that is wider than previously observed for this structure type (T-1/2 = 266 K, Delta T = 16-20 K). This reflects enhanced mechanical coupling between the cations in the lattice through interdigitation of their ligand arms, which supports a previously proposed structure/function relationship for spin-crossover materials with this form of crystal packing. All of the compounds in this work exhibit blue fluorescence in solution under ambient conditions. In most cases, the ligand-based emission maxima are slightly red shifted upon complexation, but there is no detectable correlation between the emission maximum and the spin state of the iron centers.