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Download fileStructures and Chromogenic Ion-Pair Recognition of a Catechol-Functionalized 1,8-Anthraquinone Macrocycle in Dimethyl Sulfoxide
journal contribution
posted on 2021-03-12, 08:04 authored by Yongrong Zhang, Feifei Xing, Shourong ZhuA 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 CO is much higher
than that of outbound CO 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 Ep 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–.