Acid-base interactions of tetrabromotetraazaporphin and its complex with zinc(II) in proton-donor media

The state of tetrabromotetraazaporphin and its complex with Zn(II) was stud ied by spectrophotometric titration in AcOH-H2SO4 and AcOH-antipyrine-H2SO4 acetic acid media. An analysis of the electronic absorption spectra was pe rformed to determine the stability constants of the acid forms of the compo unds and draw conclusions on their structure. The introduction of electron- acceptor bromine atoms into the macroring caused a sharp decrease in ligand basicity and an insignificant decrease in the basicity of the Zn complex; as a result, acid-base interactions were observed at higher medium aciditie s than with unsubstituted tetraazaporphin and its complexes with metals. As distinguished from their unsubstituted counterparts, the tetrabromo deriva tives participate in acid-base interactions by two meso-nitrogen atoms simu ltaneously.