Double helical and monomeric Ag(i) and Zn(ii) complexes of 1,2-cyclohexanediyl-bis(iminophenanthridine)ligands

Abstract

The tetradentate ligands tcp and ccp, made of two bidentate iminophenanthridine halves, separated, respectively, by trans- and cis-cyclohexanediyl spacers, have been studied in acetonitrile as regards their ability of coordinating the d¹⁰ cations Ag⁺ and Zn²⁺. Spectrophotometric and spectrofluorimetric titrations revealed the tendency to form 1 ∶ 2 and 2 ∶ 2 metal–ligand complexes for Ag⁺, when metal–ligand molar ratios lower than 1 ∶ 1 are chosen. Determination of the formation constants of the 1 ∶ 2 and 2 ∶ 2 complex species demontrate that at metal–ligand molar ratio = 1 ∶ 1 the preferred species are the helicates [Ag^I₂(tcp)₂]²⁺ and [Ag^I(ccp)₂]²⁺, whose nature was been further supported by ¹H NMR and ESI-MS spectra. The crystal and molecular structure of [Ag₂(ccp)₂](CF₃SO₃)(Et₂O) has also been determined by X-ray diffraction, confirming the double helical nature of the molecular cation. The same ligands tend to form 1 ∶ 2 and 1 ∶ 1 metal–ligand complexes with Zn²⁺ in acetonitrile, at molar ratios lower than 1 ∶ 1. However, only the monomeric [Zn(tcp)]²⁺ and [Zn(ccp)]²⁺ complexes are formed at 1 ∶ 1 stoichiometry, as indicated by calculation of formation constants, and by ¹H NMR and ESI-MS spectra. The luminescent properties of the ligands have been also studied, which are connected to the presence of the luminescent phenanthridine heterocycle. In the free ligands, intramolecular excimer formation between the two phenanthridine halves of the molecule occurs. Complexation of the ligands with Ag⁺ affords non-luminescent helicates, while the monomeric complexes obtained with Zn²⁺ are strongly luminescent.