Iron complexes of 3-(pyrazinyl)-1,2,4-triazole ligands

The iron complex chemistry of the ligands 3-(pyrazinyl)-1,2,4-triazole ((LH)-H-2) and 1-methyl-3-(pyrazinyl)-1,2,4-triazole (L 2 Me) has been investigated. The complexes [Fe((LH)-H-2)(3)](ClO4)(2), [Fe((LMe)-Me-2)(3)]X-2 (X- = BF4- or PF6-), [Fe((LMe)-Me-2)(3)](ClO4)(2) . xH(2)O (x = 0 or 1) and [Fe((LMe)-Me-2)(3)]SO4 . 2H(2)O have been isolated by complexation of the appropriate hydrated iron(II) salt by 3 equiv. of an (LR)-R-2 ligand. Attempts to prepare other salts of these complexes gave intractable oil products; the autooxidation product [{FeCl(L-2-Me)(2)}(2)(mu-O)](PF6)(2) slowly crystallised from one such reaction mixture, and was identified by crystallography. The single crystal structure of [Fe((LMe)-Me-2)(3)](ClO4)(2).H2O contains mer-six-coordinate Fe(II) centres, that are essentially fully low-spin at 150 and 296 K. The anhydrous powders [Fe((LH)-H-2)(3)](ClO4)(2) and [Fe((LMe)-Me-2)(3)]Y-2 (Y- = BF4-, ClO4- or PF6-) undergo gradual spin-state transitions with 182 less than or equal to T-1/2 less than or equal to ca. 400 K, that range from 61% to 100% completeness at 50 K. [Fe((LMe)-Me-2)(3)]SO4 . 2H(2)O is high-spin above 200 K, but shows a decline in T-ZM at lower temperatures whose origin is uncertain. The sin le crystal X-ray structure of 1,4-dihydro-3,6-dipyrazinyl-1,2,4,5-tetrazine, a byproduct from an attempted synthesis of 5-methyl-3-(pyrazinyl)-1,2,4-triazole, is also reported. (C) 2004 Elsevier Ltd. All rights reserved.