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

A lariat anthraquinone macrocycle functionalized with catechol (HL) was synthesized via the Mannich reaction. The Mannich base HL can be partially decomposed into L1·3HO and HL1·NO·2HO in the presence of tetrabutylammonium hydroxide/Al(NO)·9HO in dimethyl sulfoxide (DMSO). Free L1·3HO is essentially coplanar, while protonated HL1·NO·2HO is highly distorted. Dark-green FeCl·HL·2HO powder and Fe(HL)Cl crystal can be isolated from ethanol (CHOH) in high/low HL concentration. Anthraquinone in HL is essentially coplanar but distorted in Fe(HL)Cl. The Fe(III) ion in Fe(HL)Cl 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. HL responds to chlorides of Li, Na, K, Cs, Mg, Ca, Sr, Ba, Fe, Cu, Zn, and Al in a DMSO solution, which can be observed by differential pulse voltammetry, UV-vis, and H NMR. All of these metal ions shift of anthraquinone to positive, especially the second reduction peak of anthraquinone. Fe, Zn, and Al change the reduction of catechol fundamentally. HL (0.50 mM) shows a chromogenic response to FeCl and Fe(NO) to form uncommon 2:1 and 3:2 (HL/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 Fe(HL)Cl in DMSO. Different from the fast reaction between HL and FeCl, Fe(NO) reacts with HL rather slowly in DMSO. Catechol can coordinate to FeCl without any deprotonation in CHOH and DMSO. HL also shows a chromogenic response to fluorides and hydroxides, which peak at 670 and 684 nm, respectively, in DMSO. The binding ratio between HL 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 CHOH solution in the presence of OH.