Counter-ligand control of the electronic structure in dinuclear copper-tetrakisguanidine complexes

Abstract

The redox-active GFA (Guanidino-Functionalized Aromatic compound) 1,4,5,8-tetrakis(tetramethylguanidino)-naphthalene (6) is used to synthesize new dinuclear copper complexes of the formula [6(CuX₂)₂] with different electronic structures. With X = OAc, a dinuclear Cu^II complex of the neutral GFA is obtained (electronic structure [Cu^II-GFA-Cu^II], two unpaired electrons), and with X = Br a diamagnetic dinuclear Cu^I complex of the dicationic GFA (electronic structure [Cu^I-GFA²⁺-Cu^I], closed-shell singlet state). The different electronic structures lead to significant differences in the optical, structural and magnetic properties of the complexes. Furthermore, the complex [6(CuI)₂]²⁺ (electronic structure [Cu^I-GFA²⁺-Cu^I], closed-shell singlet state) is synthesized by reaction of 6²⁺ with two equivalents of CuI. Slow decomposition of this complex in solution leads to the fluorescent dye 2,7-bis(dimethylamino)-1,3,6,8-tetraazapyrene. In an improved synthesis of this tetraazapyrene, 6 is reacted with CuBr in the presence of dioxygen. Quantum chemical calculations show that the addition of counter-ligands to the trigonal planar Cu^I atoms of [6(CuI)₂]²⁺ favors or disfavors one of the electronic structures, depending on the nature of the counter-ligand.