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 Na- ClO in acetic acid afforded 2,6-bis(4-chloropyrazol-1-yl)pyrazine (LCl). 2,6-Bis(4-bromopyrazol-1-yl)pyrazine (LBr), 2,6-bis(4-iodopyrazol-1- yl)pyrazine (LI), 2,6-bis(4-methylpyrazol- 1-yl)pyrazine (L Me), and 2,6-bis(4-nitropyrazol-1- yl)pyrazine (LNO ) were also prepared by reactions of the preformed 4-substituted pyrazoles with 2,6-dichloropyrazine. The reduction of LNO with iron powder gave 2,6-bis(4- aminopyrazol-1-yl)pyrazine (L NH) and LI was converted into 2,6-bis[4-(phenylethynyl)pyrazol-1-yl]pyrazine (LCCPh) by a Sonogashira coupling reaction. The salts [Fe(LMe)]X (X- = BF - and ClO -) underwent thermal spin-crossover abruptly at around 200 K in one and two steps, respectively. The [Fe(LMe)]X salts exhibited dif- ferent light-induced excited spin-state trapping (LIESST) behavior; the BF - salt behaves classically [T(LIESST) = 93 K], but the ClO - salt undergoes a multistep LIESST relaxation. In contrast, solid [Fe(L Cl)][BF]2 adopts a fixed 2:1 high/ low-spin-state population that does not change with temperature below 300 K, whereas [Fe(LBr)][BF]2 and [Fe(L I)][BF]2 form low-spin solvated crystals that are transformed into high-spin powders on drying. The pyrazinyl group in the LR ligands slightly stabilizes the low-spin state of the complexes, as determined by solution-phase magnetic measurements. The crystal structure of [Fe(LCCPh)(OH)z]- [BF]2 contains a disordered mixture of six- (z = 3) and sevencoordinate (z = 4) iron centers.