Metal Complexes with 1,5- and 1,8-Dihydroxy-9,10-Anthraquinones: Electronic Absorption Spectra and Structure of Ligands

Metal complexes with 1,5-dihydroxy-9,10-anthraquinone are studied by the spectrophotometric, quantum-chemical, and correlation methods. It was established that the ligand in these complexes can occur in seven excited states that differ not only in the ionization degree but also in the prevailing contribution of the tautomeric 9,10-, 1,10-, and 1,5-anthraquinoid structures. In all known complexes with 1,8-dihydroxy-9,10-anthraquinone and singly ionized ligand, this ligand has the 1,10-anthraquinoid structure; in complexes with the doubly ionized ligand, the latter ligand most often has the 9,10-anthraquinoid structure. The known complexes are classified according to the ligand structures.     

Metal Complexes with Alizarin Complexone AC: Electronic Absorption Spectra and Ligand Structure

The character of the electronic absorption spectra of metal complexes with alizarin complexone AC is determined by the ionization degree of the ligand and the ratio between its excited states with different contributions of tautomeric 9,10-, 1,10-, 2,9-, and 1,2-anthraquinoid resonance structures. It was found by the spectrophotometric, quantum-chemical, and correlation methods that the ligand in metal complexes can exist in three forms, namely, neutral and two ionized forms (containing one or two deprotonated hydroxy groups). For each of the latter two forms, four excited states with the dominating contribution of the 9,10-, 1,10-, 2,9-, or 1,2-anthraquinoid resonance structures are possible. The formation of red monometallic complexes involves the peri- or ortho-hydroxycarbonyl group in anthraquinoid tautomers (mostly, 1,2- and 2,9-structures). The color of bimetallic complexes is determined by four anthraquinoid structures of the ligand (from red 9,10- to blue 1,10-anthraquinones). Fluorine-containing complexes exist only as 1,2- and 1,10-anthraquinoid structures, which are responsible for their blue color. The known metal complexes with Alizarin Complexone AS were classified by their structures.     

A Quantum-Chemical and Correlation Study of the Ionization of Purpurin

Anthraquinoid tautomers participate in the ionization of purpurin. The tautomerism takes place in both ground and excited states of the molecules; the excited state is more sensitive to the tautomerism. The solvation and ionization shift the tautomeric equilibria. In the experimental absorption spectra of purpurin, the major bands correspond to the 9,10-, 1,4-, and 1,10-anthraquinoid tautomers; anions with the 9,10-anthraquinoid structure are not detected. The position and intensity of the π,π* bands, and also the quantum-chemical parameters linearly correlate with the degree of ionization of purpurin.     

Electronic Absorption Spectra and Tautomerism of Quinizarin and Its Substituted Derivatives

According to the data of quantum-chemical calculations and correlation analysis, the experimental electronic absorption spectra of quinizarin and its substituted derivatives contain bands corresponding to 9,10-, 1,10-, and 1,4-anthraquinoid tautomers. Analysis of the absorption spectra revealed specific features of the tautomeric structure of substituted quinizarins.     

Tautomerism of metal complexes with 1,8-dihydroxy-3-R1-6-R2-9,10-anthraquinones

The structures of metal complexes with 1,8-dihydroxyanthraquinones studied by spectrophotometric, quantum-chemical, and correlation methods were found to be determined by tautomeric 9,10–1,10-anthraquinoid structures of the ligands and by the degree of the ligand ionization. Complex formation is accompanied by the shifts in tautomeric equilibria influencing both excited and ground states of the ligands. The 1,10-anthraquinoid forms of complexes are the most characteristic. The known complexes are classified in accordance with the ligand structures.     

Quantum-chemical and correlation study on the tautomerism and ionization of 1,4,5,8-tetrahydroxy-9,10-anthraquinone and its alkyl-substituted derivatives

1,4,5,8-Tetrahydroxy-9,10-anthraquinone and its alkyl derivatives exist as equilibrium mixtures of prototropic tautomers and rotational isomers differing in the mode of intramolecular hydrogen bonding. Their electronic absorption spectra contain π_l,π* bands corresponding to 9,10-, 1,10-, 1,4-, and (more rarely) 1,5-anthraquinoid structures. Introduction of substituents, solvation, ionization, and complex formation lead to displacement of tautomeric and conformational equilibria, which are responsible for the observed diversity of their absorption spectra.     

Tautomerism and ionization of carminic acid

Carminic acid exists as an equilibrium mixture of 9,10-, 1,4-, 1,10-, 2,9-, and 1,7-anthraquinoid tautomers. Its anions have 9,10-, 1,4-, 1,10-, and 2,9-anthraquinoid structures. No conformational equilibria were detected for carminic acid anions. Variation of the solvent and pH and ionization are accompanied by displacements of tautomeric equilibria. Shifts of the long-wave absorption maxima due to tautomeric transformations are determined mainly by change in the energy of the ground rather than excited states of molecules.     

Tautomerism of Anthraquinones: I. Purpurin and Anions Derived Therefrom

A procedure was proposed for quantitative analysis of tautomeric equilibria of organic compounds. Purpurin was found to exist mainly as 9,10-, 1,4-, and 1,10-anthraquinoid tautomers, its monoanion, as 1,10-anthraquinoid tautomer, the dianion, as 1,10- and 2,9-anthraquinoid tautomers, and the trianion, as 1,10-, 1,4-, and 2,9-anthraquinoid tautomers. Tautomeric transformations occur both in the ground and in the excited states, and the corresponding changes of quantum-chemical parameters in these states are essentially different. The excited states are more sensitive to tautomeric transformations than the ground states.__________Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 1, 2005, pp. 43–50.Original Russian Text Copyright © 2005 by Fain, Zaitsev, Ryabov.     

Quantum-chemical and correlation study on tautomerism and ionization of Quinalizarin

Quinalizarin and anions derived therefrom exist as equilibrium mixtures of different tautomers and conformers, whose structure depends on the conditions. Quinalizarin was shown to have 9,10-, 1,10-, 1,4-, 1,5-, 1,7-, and 2,9-quinoid structures, but not 1,2-quinoid structure; and its anions in ethanol media were identified as 9,10-, 1,10-, 2,9-, and 1,5-quinoid tautomers. Interactions with solvents and ionization could give rise to displacement of tautomeric and conformational equilibria, leading to considerable change in the number and position of π _l ,π* bands in the electronic absorption spectra, which are responsible for the color.     

Quantum-chemical and correlation study of ionization of Alizarin

According to the results of quantum-chemical and correlation studies, ionized states of Alizarin and Alizarin Red are contributed mainly by tautomeric anthraquinoid structures. The probability of their formation increases due to solvation. The experimental electronic absorption spectra of the corresponding anions contain bands typical of the 2,9-, 1,10-, and 1,2-anthraquinoid tautomers. The 9,10-anthraquinoid structure of the anions is less probable.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 10, 2004, pp. 1681–1686.Original Russian Text Copyright © 2004 by Fain, Zaitsev, Ryabov.This revised version was published online in April 2005 with a corrected cover date.     

Metal complexes with 1-hydroxyanthraquinone and its derivatives: Electronic absorption spectra and ligand structures

Reactions of metal salts with 1-hydroxyanthraquinone and its derivatives gave tautomeric 9,10-and 1,10-quinoid complexes and compounds containing no C=0 → M coordination bond. Each form is characterized by a single π₁,π*-band. The absorption bands were assigned by using correlations with the σ^A-constants of the hydroxy and oxido groups for tautomeric anthraquinones. Complexes with nonionized ligands have particularly the 9,10-anthraquinoid structures; complexes with ionized ligands can form both 9,10-and 1,10-quinoid structures.     

Study of tautomeric transformations of 1,4,5,8-tetraaminoanthraquinone by electronic spectroscopy

Based on the example of 1,4,5,8-tetraamino-9,10-anthraquinone, a method is described for establishing the tautomeric compositions and sequences of tautomeric transformations that occur in known samples of quinones. An analysis of electronic absorption spectra shows that this substance is a dynamic equilibrium mixture of 4,5,8,9-tetraamino-1,10-anthraquinone and the imines tautomeric to it. The tautomeric compositions of different samples of matter differ and can contain 1,4,8-triamino-5-hydroxy-9,10-anthraquinone-10-imine, 4,8,9-triamino-10-hydroxy-1,5-anthraquinone-5-imine, 5,8-diamino-4,9-dihy-droxy-1,10-antraquinonediimine, and 8,10-diamino-5,9-dihydroxy-1,4-antraquinonediimine. Tautomeric equilibrium shifts can accompany not only chemical reactions but physical processes as well, so any investigation of the properties of substances without establishing their tautomeric compositions would be incorrect.     

Anthraquinones tautomerism: VI. Substituted 1,4,5-trihydroxyanthraquinones

1,4,5-Trihydroxy-9,10-anthraquinone and its substituted derivatives exist in equilibrium of structures distinguished by quinoid tautomerism and rotational isomerism. Their electron absorption spectra contain π₁, π*-bands corresponding to 9,10-and 1,10-, more seldom to 1,5-and 1,4-anthraquinoid structures. Of three isomeric 1,10-anthraquinones only 4,8,9-trihydroxy-1,10-antraquinones were found. All tautomer may exist as conformers with contiguous CO and OH groups not bound by an intramolecular hydrogen bond. The considerable difference in color of structurally similar substituted compounds is due to tautomerism and conformer transformations.     

Competing tautomeric transformations and the structure of 1-(alkyl,aryl)amino-4-hydroxyanthraquinones

Keto-enol and amine-imine tautomerism and equilibria with trans-conformers are characteristic of 1-(alkyl,aryl)amino-4-hydroxy-9,10-anthraquinones. Amino forms possess 1,10- and 1,4-, but not 9,10-quinoid structure. Various tautomeric and conformeric transformations are in competition. The outcome of this competition may be studied by the correlation analysis of electron absorption spectra but it would be possible to understand the causes of changes in the direction of the competing transformation only in the case when each action on the substance would be accompanied with establishing the corresponding alterations in its tautomeric composition.     

A Quantum-Chemical and Correlation Study of Ionization of α,α'-Dihydroxyanthraquinones

The participation of states with the predominant contribution of tautomeric anthraquinoid resonance structures in ionization of 1-hydroxy- and 1,4-, 1,5-, and 1,8-dihydroxy-9,10-anthraquinones was demonstrated by quantum-chemical and correlation methods. Solvation increases the probability of formation of such structures. The bands corresponding to anthraquinoid tautomers were revealed in the experimental absorption spectra of the ionized compounds. Published data on the pH dependence of the ratio of neutral molecules, monoanions, and dianions, obtained without taking into account tautomeric structures, require revision.     

Quantum-chemical and correlation study of deprotonation and complexation of 1-amino-4-hydroxyanthraquinone

Existing views on the deprotonation and complexation of 1-amino-4-hydroxyanthraquinone are wrong. This compound, its anions, and complexes with metals are not individual substances, but they form a dynamic equilibrium mixture of keto-enol (keto-oxide) and amino-imine tautomers. Different samples of the same compound differ by the tautomeric composition, the respective information is contained in their electron absorption spectra. In weak alkaline solutions the deprotonation occurs exclusively at the hydroxy group. Most typical structure of 1-amino-4-hydroxyanthraquinone anions is 1,10-quinoid, its metal complexes have 9,10-and 1,10-quinoid structures. The ground states of molecules are more responsible for the tautomeric transformations than the excited states. Quantum-chemical calculations of tautomeric anthraquinones by semiempirical PPP methods are more reliable than modern ab initio calculations.     

Synthesis and spectroscopic studies of mixed-ligand and polymeric dinuclear transition metal complexes with bis-acylhydrazone tetradentate ligands and 1,10-phenanthroline

Two types of dinuclear copper(II) and nickel(II) complexes with two tetradentate N2O2 donor ligands 1,4-bis(1-anthranoylhydrazonoethyl)benzene (L1), 1,4-bis(1-salicyloylhydrazonoethyl)benzene (L2) and N,N'-bidentate heterocyclic base [1,10-phenonthroline (phen)] have been synthesized and characterized by elemental analysis, infrared spectra, UV-vis electronic absorption spectra and magnetic susceptibility measurements. The reaction of metal(II) acetates with the solution containing ligand and 1,10-phenonthroline in methanol gives mixed-ligand dinuclear metal(II) complexes with general formula [M2L(phen)2]Cl2 (L=L1 or L2), whereas, the ligands react with metal(II) acetates to form polymeric dinuclear complexes with general formula [(M2L2)n] (L=L1 or L2). In the complexes, the ligands act as dianionic tetradentate and coordination takes place in the enol tautomeric form with the enolic oxygen and azomethine nitrogen atoms while the phenolic hydroxyl and amino groups of aroylhydrazone moiety do not participate in coordination. The effect of varying pH and solvent on the absorption behavior of both ligands and complexes has been investigated.     

Role of tautomerism and rotational isomerism in the interaction of α-hydroxyanthraquinones with boric acid

The products of reaction of α-hydroxyanthraquinones with boric acid are mixtures of 9,10-, 1,10 -, 1,4- and 1,5-quinoid tautomeric complexes of boric acid and borate esters differing by the coordination bonds with carbonyl groups existing in the dynamic equilibrium. The deepening of the reagents color in the presence of boron does not a result only of the complexation, but in the accompanying shift of the tautomeric equilibria.     

Tautomerism of the Natural Anthraquinones Physcion and Emodin and Their Analogs

The 9,10–1,10-anthraquinoid tautomer was found to be characteristic of physcion and emodin and their analogs in solutions using spectrophotometric, quantum-chemical, and correlation methods. Ionization of these compounds was accompanied by a shift in tautomeric equilibrium. In alkaline solutions 1,10-anthraquinoid anions with a single α-hydroxy that were stabilized by an intramolecular H-bond were formed. Tautomerism occurred in both the ground and excited states of the molecules. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 411–415, September–October, 2005.     

Tautomerism of the Natural 1,8-Dihydroxy-9,10-anthraquinones Chrysophanol,Aloe-emodin,and Rhein

Existence of chrysophanol, aloe-emodin, and rhein in solutions as primarily the 9,10-anthraquinoid forms was established by quantum-chemical and correlation methods. However, the 1,10-anthraquinoid tautomers are observed in certain media. As a rule, these compounds in alkaline solutions form 1,10-anthraquinoid anions that are stabilized by intramolecular H-bonds and have a single α-hydroxy group. Tautomers exist in both the ground and excited states of the molecule. Quantitative rules that control changes in the ionization and tautomerization parameters of chrysophanol were found.__________Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 119–124, March–April, 2005.