Complexation between decamethyl-3,3'-bis(dipyrrolylmethene) and zinc(II), copper(II), and cobalt(II) acetates

Decamethyl-3,3′-bis(dipyrrolylmethene) dihydrobromide H2L · 2HBr (H2L is bis(3,4,7,8,9-pentamethylpyrrol-3-yl)methane), which is the simplest representative of a novel class of oligo(dipyrrolylmethenes) belonging to chromophore chelating nonmacrocyclic ligands, were examined by 1H NMR, IR, and electronic absorption spectroscopy. Complexation reactions of H2L · 2HBr with M(AcO)2 (M = Zn(II), Cu(II), and Co(II)) in DMF at 298.15 K were monitored by electronic absorption spectroscopy and studied by the molar ratio method. The thermodynamic constants K 0 of these reactions were estimated. The d metal ions coordinate H2L to give the binuclear homoleptic complexes [M2L2]. The reactions proceed through the intermediate binuclear heteroleptic complex [M2L(AcO)2] detected by spectroscopic methods. The thermodynamic stabilities of [M2L2] and [M2L(AcO)2] increase when moving from Cu(II) to Zn(II) and Co(II). The probability of formation and stability of [M2L2] containing 3,3′-bis(dipyrrolylmethene) are substantially higher than those of analogous complexes with the 2,2′-isomer (decamethyl-2,2′-biladienea, c). The low K 0 values for the complexation between H2L and Cu(AcO)2 are due to slow oxidation of the biladiene ligand into a bilatriene with participation of Cu2+ ions.