Synthesis of 2,6-Di(pyrazol-1-yl)pyrazine Derivatives and the Spin-State Behavior of Their Iron(II) Complexes



Chlorination of 2,6-bis(pyrazol-1-yl)pyrazine (bppz) with NaClO in acetic acid afforded 2,6-bis(4-chloropyrazol-1-yl)pyrazine (L2Cl). 2,6-Bis(4-bromopyrazol-1-yl)pyrazine (L2Br), 2,6-bis(4-iodopyrazol-1-yl)pyrazine (L2I), 2,6-bis(4-methylpyrazol-1-yl)pyrazine (L2Me), and 2,6-bis(4-nitropyrazol-1-yl)pyrazine (L2NO2) were also prepared by reactions of the preformed 4-substituted pyrazoles with 2,6-dichloropyrazine. The reduction of L2NO2 with iron powder gave 2,6-bis(4-aminopyrazol-1-yl)pyrazine (L2NH2) and L2I was converted into 2,6-bis[4-(phenylethynyl)pyrazol-1-yl]pyrazine (L2CCPh) by a Sonogashira coupling reaction. The salts [Fe(L2Me)2]X2 (X = BF4 and ClO4) underwent thermal spin-crossover abruptly at around 200 K in one and two steps, respectively. The [Fe(L2Me)2]X2 salts exhibited different light-induced excited spin-state trapping (LIESST) behavior; the BF4 salt behaves classically [T(LIESST) = 93 K], but the ClO4 salt undergoes a multistep LIESST relaxation. In contrast, solid [Fe(L2Cl)2][BF4]2 adopts a fixed 2:1 high/low-spin-state population that does not change with temperature below 300 K, whereas [Fe(L2Br)2][BF4]2 and [Fe(L2I)2][BF4]2 form low-spin solvated crystals that are transformed into high-spin powders on drying. The pyrazinyl group in the L2R ligands slightly stabilizes the low-spin state of the complexes, as determined by solution-phase magnetic measurements. The crystal structure of [Fe(L2CCPh)(OH2)z][BF4]2 contains a disordered mixture of six- (z = 3) and seven-coordinate (z = 4) iron centers.