Quinone Oxygen-Coordinated Palladium(II) Complexes with Anthraquinone Ligands BearingN-Heterocyclic Coordination Sites

Abstract

The coordination properties of anthraquinone ligands bearing N-heterocyclic coordination sites to divalent palladium, and structural characterization of the quinone oxygen-coordinated palladium(II) complexes are described. The intramolecular NH···O (quinone) hydrogen bond [N(1)···O(2), 2.617 Å] was observed in the crystal structure of 1-chloro-8-[2-(2-pyridyl)ethylamino]-9,10-anthraquinone (1a). The anthraquinone moieties of 1a are oriented in a face-to-face manner to form π-stacks in the crystal, with an interplanar distance of ca. 3.5 Å between the neighboring anthraquinone moieties. Complexation of 1a with Pd(OAc)2 afforded the palladium(II) complex 2a. The quinone oxygen, which is bent out of the anthraquinone plane, is coordinated to the palladium(II) center, which is substituted by the amino nitrogen in a position trans to the pyridyl nitrogen in the crystal structure 2a. 1-(2-Phenylethylamino)-8-[2-(2-pyridyl)ethylamino]-9,10-anthraquinone (1b) was also found to form the 1:1 quinone oxygen-coordinated palladium(II) complex 2b. The single-crystal X-ray structure determination of 2b shows the existence of two independent molecules in the asymmetric unit. These two molecules are present in a good mirror image relationship, indicating conformational enantiomers. Furthermore, they are overlapped in a face-to-face manner to form the π-stack dimer, which arranges in a herringbone motif with the virtues of offset stacking and edge-to-face contacts in the crystal packing. Starting from 1,5-bis[2-(2-pyridyl)ethylamino]-9,10-anthraquinone (1c) bearing two Nheterocyclic coordination sites, the conjugated palladium(II) homobimetallic complex 2c was obtained. Two palladium atoms are bridged by the quinone moiety to afford a redox-active conjugated homobimetallic complex system. The crystal structure of 2c reveals that two palladium atoms are disposed on opposite faces of the anthraquinone plane, accompanied by the bend of the quinone oxygens out of the anthraquinone plane. The anthraquinone moieties of 2c are arranged in an edge-to-face orientation in the crystal packing. In the palladium(II) complexes 2b−c, the d,π-conjugated system is interconnected through a π-π interaction between the π-conjugated systems. The cyclic voltammograms of these quinone oxygen-coordinated palladium(II) complexes 2a−c show two redox couples based on the quinone moiety, which are positively shifted relative to the free quinone ligands.