Atmospheric pressure chemical ionization mass spectrometric and visible spectrophotometric studies of copper(I) and copper(II) complexes with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol.

Complexes of copper(II) with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) formed in aqueous methanol in a wide range of pH (from acidic to alkaline) as well as copper(I)-5-Br-PADAP species formed in methanolic solutions were investigated by spectrophotometry and mass spectrometry. Pseudomolecular and fragment ions created in the atmospheric pressure chemical ionization (APCI) source confirmed the molecular masses of the complexes existing in the solvents and their structures. The structure of the Cu(II) complex with 5-Br-PADAP formed in acidic medium was proposed as CuR(R - H) (where R is the undissociated molecule of the reagent). The binding sites of the two bound reagent molecules were different: in one of them the oxygen atom of the dissociated phenolic group and the nitrogen atom from the azo (-N=N-) group took part in complex formation, whereas in the other only nitrogen atoms from the pyridyl ring and azo group were involved. The complex was stable and could not be reduced to Cu(I) species by use of standard reducing agents (ascorbic acid, hydroxylamine). In alkaline solutions the complex tended to polymerize and precipitated in media containing less than 80% of methanol. The copper(I)-5-Br-PADAP complex was extremely unstable and could be obtained (as a mixture with Cu(II) species) in media free of water or oxygen. For this complex, CuR(2) was proposed as the most probable structure. According to this proposal copper(I) reacted exclusively with nitrogen-containing binding sites and the undissociated phenolic group was not engaged in complex formation. In this system Cu(I)/Cu(II) electron transfer is very rapid, accelerated by a polar environment, e.g. in the presence of water molecules or dissolved oxygen.
(Copyright 2001 John Wiley & Sons, Ltd.)