Synthesis of 2,6-Di(pyrazol-1-yl)pyrazine derivatives and the spin-state behavior of their iron(II) complexes

Abstract

Chlorination of 2,6-bis(pyrazol-1-yl)pyrazine (bppz) with Na- ClO 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 (L 2Me), and 2,6-bis(4-nitropyrazol-1- yl)pyrazine (L2NO 2) were also prepared by reactions of the preformed 4-substituted pyrazoles with 2,6-dichloropyrazine. The reduction of L2NO 2 with iron powder gave 2,6-bis(4- aminopyrazol-1-yl)pyrazine (L 2NH2) 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]X 2 (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 dif- ferent 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(L 2Cl)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(L 2I)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 sevencoordinate (z = 4) iron centers. © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.