Synthesis and metal complex equilibria studies of some binucleating ligands with a [rho]-Xylyl backbone

Abstract

The synthesis of a series of new binucleating ligands, 1,4-bis(2'-(3''-azapropyl)-2',5'-diazapentyl)benzene (PXBDE), 1,4-bis(2',5',8'-triazaoctyl)benzene (PXBEN), 1,4-bis-(2',5',8',11'-tetraazaundecyl) benzene (PXTRIEN), 1,4-bis- (2'-(3''-azaprophyl)-2',5',8'-triazaoctyl)benzene (PXTREN), 1,4-bis(10'-pyridyl-2',5',8'-triazanonyl)benzene (PXPYDE), 1,4-bis(10'-(2''-imidazolyl)-2',5',8'-triazanonyl)benzene (PX2IM), and 1,4-bis(2',5',5'-tricarboxymethyl-2',5'-diazapentyl)benzene (PXED3A), as well as the following polyamines, 1-pyridyl-2,5,8-triazaoctane (PYDE) and 1-(2'-imidazolyl)-2,5,8-triazaoctane (2IM), are reported. Aqueous equilibria of these liqands with Cu(II), Co(II), Ni(II), and Zn(II) have been studied by potentiometric methods. Complexation of Fe(II) and Fe(III) by PXED3A is also described. The ligand protonation constants, 1:1 and 2:1 metal ion to ligand chelate stability constants, and chelate protonation constants are reported herein. Protonation constants for the 1:1 and 2:1 hydrolyzed complexes have also been determined. Potentiometric equilibrium studies of the reaction of bis-Colbalt(II) complexes of binucleating polyamines with dioxygen indicate formation of intramolecular dioxygen complexes. Potentiometric equilibrium and spectrophotometric studies of the Fe(III)-PXED3A system indicate formation of an intramolecular (mu)-oxo bridged species above pH 3.5. Probably ligand-bonding sites in the complexes are proposed, and their stabilities are compared with those formed from analogous chelating agents.