Chromogenic reagents for iron(II): Studies in the 1,2,4-triazine series

Sixteen as-triazines with different substituents at the 3- and 5-/6-positions have been prepared as the first part of a comprehensive study ultimately to develop an improved means to determine whether an individual has handled a firearm. Their reaction with iron(II) in solution was studied spectrophotometrically, and sensitivity to iron(II) was compared with that of 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (PDT). Eight of the new compounds exhibited higher sensitivity than PDT. The substituents which brought about the greatest increase in sensitivity where 4-phenyl-2-pyridyl (at position 3) and 3,4-dimethoxyphenyl (at positions 5 and 6). Quite surprisingly, as-triazines containing the 2-quinolyl group at position 3 did not exhibit any visible reaction with iron(II) even after 12 h. 3-(4-Phenyl-2-pyridyl)-5,6-bis-(3,4-dimethoxyphenyl)-1,2,4- triazine was found to be the most sensitive ferroin-type chromogen reported to date (molar absorptivity = 3.81 x 10⁴ l mol^-1 cm^-1 at 571 nm).