Thermodynamic and fluorescence emission studies on chemosensors containing anthracene fluorophores. Crystal structure of {[CuL1Cl]Cl}2·2H2O [L1 = N-(3-aminopropyl)-N′-3-(anthracen-9-ylmethyl)aminopropylethane-1,2-diamine]

Abstract

The co-ordination capabilities toward hydrogen ions, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ and Cd²⁺ of the novel receptor 2,6,9,13-tetraaza[14](9,10)anthracenophane (L) and of its open-chain counterpart N-(3-aminopropyl)-N′-3-(anthracen-9-ylmethyl)aminopropylethane-1,2-diamine (L¹) is described. Stepwise protonation constants of the cyclic receptor (L) are lower than those of the open-chain receptor (L¹) . Quenching effects of the fluorescence emission occur upon first and second deprotonation of L and upon second deprotonation of L¹. Stability constants of the Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ and Cd²⁺ complexes follow the Irving–Williams trend and are intermediate between those of triethylenetetraamine with terminal primary amino groups and those of the α,ω-dibenzylated receptor 1,12-dibenzyl-1,5,8,12-tetraazaciclododecane. Luminescence studies show that complexation of Cu²⁺ and Ni²⁺ by L¹ yield CHEQ effects (chelation enhanced quenching) while complexation of Zn²⁺ and Cd²⁺ produce CHEF effects (chelation enhanced fluorescence). The magnitude of these effects depends on the strength of the co-ordination. Crystals of {[CuL¹Cl]Cl}₂· 2H₂O are triclinic, space group P, with a = 13.307(1), b = 13.305(1), c = 16.538(2) Å, α = 104.94(1), β = 111.67(1), γ = 102.76(1)°, R₁ = 0.0885, wR₂ = 0.2667. The crystal structure of {[CuL¹Cl]Cl}₂·2H₂O shows a very strongly axially distorted square pyramidal co-ordination geometry in which the Cu²⁺ cation is co-ordinated by all the nitrogen donors of the receptor and a chloride ion disposed at the apical position of the square pyramid. The asymmetric unit is formed by two slightly different [CuL¹Cl]⁺ cations with an arrangement that shows π-stacking of their anthracene sub-units.