Structures and Chromogenic Ion-Pair Recognition of a Catechol-Functionalized 1,8-Anthraquinone Macrocycle in Dimethyl Sulfoxide

A lariat anthraquinone macrocycle functionalized with catechol (H₂L) was synthesized via the Mannich reaction. The Mannich base H₂L can be partially decomposed into L1·3H₂O and HL1·NO₃·2H₂O in the presence of tetrabutylammonium hydroxide/Al­(NO₃)₃·9H₂O in dimethyl sulfoxide (DMSO). Free L1·3H₂O is essentially coplanar, while protonated HL1·NO₃·2H₂O is highly distorted. Dark-green FeCl₃·H₂L·2H₂O powder and Fe₂(HL)₂Cl₄ crystal can be isolated from ethanol (C₂H₅OH) in high/low H₂L concentration. Anthraquinone in H₂L is essentially coplanar but distorted in Fe₂(HL)₂Cl₄. The Fe­(III) ion in Fe₂(HL)₂Cl₄ adopts a less common five-coordination with three catecholate O and two Cl atoms in the dimer. The distortion of inbound CO is much higher than that of outbound CO in anthraquinone in all of these compounds. H₂L responds to chlorides of Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Fe³⁺, Cu²⁺, Zn²⁺, and Al³⁺ in a DMSO solution, which can be observed by differential pulse voltammetry, UV–vis, and ¹H NMR. All of these metal ions shift E_p of anthraquinone to positive, especially the second reduction peak of anthraquinone. Fe³⁺, Zn²⁺, and Al³⁺ change the reduction of catechol fundamentally. H₂L (0.50 mM) shows a chromogenic response to FeCl₃ and Fe­(NO₃)₃ to form uncommon 2:1 and 3:2 (H₂L/Fe) complexes, both peaking at 748 nm in DMSO. In the presence of 2 equiv of sodium hydroxide (NaOH), the 748 nm absorbance shifts to 777 nm, identical with Fe₂(HL)₂Cl₄ in DMSO. Different from the fast reaction between H₂L and FeCl₃, Fe­(NO₃)₃ reacts with H₂L rather slowly in DMSO. Catechol can coordinate to FeCl₃ without any deprotonation in C₂H₅OH and DMSO. H₂L also shows a chromogenic response to fluorides and hydroxides, which peak at 670 and 684 nm, respectively, in DMSO. The binding ratio between H₂L and F^–/OH^– is 1:2. In a higher concentration of hydroxides, a 684 nm greenish-blue 1:2 complex forms immediately, which gradually transforms to a red complex and peaks at ∼530 nm in minutes at room temperature. No color change can be observed in an C₂H₅OH solution in the presence of OH^–.