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 ((LCl)-Cl-2). 2,6-Bis(4-bromopyrazol-1-yl)pyrazine ((LBr)-Br-2), 2,6-bis(4-iodopyrazol-1-yl)pyrazine ((LI)-I-2), 2,6-bis(4-methylpyrazol-1-yl)pyrazine ((LMe)-Me-2), and 2,6-bis(4-nitropyrazol-1-yl)pyrazine ((LNO2)-N-2) were also prepared by reactions of the preformed 4-substituted pyrazoles with 2,6-dichloropyrazine. The reduction of (LNO2)-N-2 with iron powder gave 2,6-bis(4-aminopyrazol-1-yl)pyrazine ((LNH2)-N-2) and (LI)-I-2 was converted into 2,6-bis[4-(phenylethynyl)pyrazol-1-yl]pyrazine ((LCCPh)-C-2) by a Sonogashira coupling reaction. The salts [Fe((LMe)-Me-2)(2)]X-2 (X- = BF4- and ClO4-) underwent thermal spin-crossover abruptly at around 200 K in one and two steps, respectively. The [Fe((LMe)-Me-2)(2)]X-2 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((LCl)-Cl-2)(2)][BF4](2) adopts a fixed 2:1 high/low-spin-state population that does not change with temperature below 300 K, whereas [Fe((LBr)-Br-2)(2)][BF4](2) and [Fe((LI)-I-2)(2)][BF4](2) form low-spin solvated crystals that are transformed into high-spin powders on drying. The pyrazinyl group in the (LR)-R-2 ligands slightly stabilizes the low-spin state of the complexes, as determined by solution-phase magnetic measurements. The crystal structure of [Fe((LCCPh)-C-2)(OH2)(z)][BF4](2) contains a disordered mixture of six- (z = 3) and seven-coordinate (z = 4) iron centers.