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

Abstract

Abstract Chlorination of 2,6‐bis(pyrazol‐1‐yl)pyrazine (bppz) with NaClO in acetic acid afforded 2,6‐bis(4‐chloropyrazol‐1‐yl)pyrazine ( L 2 Cl). 2,6‐Bis(4‐bromopyrazol‐1‐yl)pyrazine ( L 2 Br), 2,6‐bis(4‐iodopyrazol‐1‐yl)pyrazine ( L 2 I), 2,6‐bis(4‐methylpyrazol‐1‐yl)pyrazine ( L 2 Me), and 2,6‐bis(4‐nitropyrazol‐1‐yl)pyrazine ( L 2 NO 2 ) were also prepared by reactions of the preformed 4‐substituted pyrazoles with 2,6‐dichloropyrazine. The reduction of L 2 NO 2 with iron powder gave 2,6‐bis(4‐aminopyrazol‐1‐yl)pyrazine ( L 2 NH 2 ) and L 2 I was converted into 2,6‐bis[4‐(phenylethynyl)pyrazol‐1‐yl]pyrazine ( L 2 CCPh) by a Sonogashira coupling reaction. The salts [Fe( L 2 Me) 2 ]X 2 (X – = BF 4 – and ClO 4 – ) underwent thermal spin‐crossover abruptly at around 200 K in one and two steps, respectively. The [Fe( L 2 Me) 2 ]X 2 salts exhibited different light‐induced excited spin‐state trapping (LIESST) behavior; the BF 4 – salt behaves classically [ T (LIESST) = 93 K], but the ClO 4 – salt undergoes a multistep LIESST relaxation. In contrast, solid [Fe( L 2 Cl) 2 ][BF 4 ] 2 adopts a fixed 2:1 high/low‐spin‐state population that does not change with temperature below 300 K, whereas [Fe( L 2 Br) 2 ][BF 4 ] 2 and [Fe( L 2 I) 2 ][BF 4 ] 2 form low‐spin solvated crystals that are transformed into high‐spin powders on drying. The pyrazinyl group in the L 2 R ligands slightly stabilizes the low‐spin state of the complexes, as determined by solution‐phase magnetic measurements. The crystal structure of [Fe( L 2 CCPh)(OH 2 ) z ][BF 4 ] 2 contains a disordered mixture of six‐ ( z = 3) and seven‐coordinate ( z = 4) iron centers.