15N NMR spectroscopy of 5-(2-heteroaryl)formazans and their zinc and cadmium chelates

The15N NMR shifts of 5-heteroarylformazans (Het = 3-chloro-2-quinoxalyl and 2-benzothiazolyl) and their complexes with zinc and cadmium and also some model compounds were measured. Procedures were developed for the use of15NMR in structural investigations of formazans and their complexes, and the formation of pseudooctahedral bisformazanates with metals of the zinc subgroup was confirmed. The increments of complexation in the chemical shifts of the15N nitrogens and13C carbons of the tridentate heteroarylformazans are not independent quantities. There are algebraic combinations of the increments in the chemical shifts that are practically independent of the substitution of the zinc by cadmium, substitution of substituents at the 1-aryl and the meso-carbon atom, and even substitution of the heteroaryl. This property is reminiscent of the analogous relations between the increments of the substituents in such π-conjugated systems as benzene derivatives.