ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 12, pp. 2470–2477. © Pleiades Publishing, Ltd., 2010. Original Russian Text © V.Ya. Fain, B.E. Zaitsev, M.A. Ryabov, 2010, published in Zhurnal Obshchei Khimii, 2010, Vol. 80, No. 12, pp. 2008–2016.

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Role of Tautomerism and Rotational Isomerism in the Interaction of α-Hydroxyanthraquinones

with Boric Acid V. Ya. Fain, B. E. Zaitsev, and M. A. Ryabov

Russian University of Peoples’ Friendship, ul. Miklukho-Maklaya 6, Moscow, 117198 Russia e-mail: vfain@mail.ru

Received January 28, 2010

Abstract―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.

The interaction of α-hydroxy-substituted anthra-quinones with boric acid is widely used for the synthesis of new anthraquinone derivatives, including industrially important products [1], as well as in analytical chemistry for the spectroscopic and photo-metric determination of boron [2]. The anthraquinone derivatives have very wide and diverse practical applications: highly stable dyes, pigments, phosphors, medical and physiologically active preparations, chemicals for the information processing and storing, analytical reagents, indicators, catalysts and inhibitors of industrially important chemical processes, sen-sitizers of photochemical reactions, etc. [3]. Therefore, the knowledge of the structure of boron complexes is of great importance.

Typically, the structure ascribed to the α-hydroxyl-anthraquinones is a compound where the hydroxy hydrogen atom is connected with oxygen atoms of the carbonyl groups by intramolecular hydrogen bond to form a six-membered chelate ring, for example, I.

It is believed that at the interaction with boric acid a boron complex is formed where the boron atom is bound by coordination bond with the oxygen atom of the carbonyl group. This boron atom becomes negat-ively charged, while the ligand bears a positive charge delocalized over different carbon atoms of anthracene framework, and the complex structure is described by a scheme with the delocalized charge [4, 5], for example, II. These complexes were isolated in the crystalline state in the reaction of α-hydroxyanthra-quinones with boroacetic anhydride in acetic anhydride [4].

Meanwhile, the existing assumptions on the struc-ture of α-hydroxyanthraquinones and their boron complexes cannot explain the known absorption spec-tra. Quantum-chemical calculations indicate that in these compounds only one πl–π*-transition is allowed, responsible for its color [6, 7]. However, the experi-mental absorption spectra contain several πl–π*-bands, and their number and positions measured by different investigators for the same compounds vary con-siderably. The use of α-hydroxyanthraquinones as reagents for boron is based on the concept of the color deepening at the formation of boron complexes. It was suggested that the lack of color change at the addition of boric acid or its salt indicates that in these con-ditions no boron complex is formed [8, 9]. It appears, however, that the electron absorption spectra of reagents and their boron complexes in the respective

DOI: 10.1134/S1070363210120121

O

O

O

9

10

1

HO

O

OB

HO OH−

I II

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environments are virtually identical [4, 6]. Therefore it is impossible to explain the color deepening in the framework of the existing concepts.

The multiplicity of the πl–π*-bands and significant variations in their number and position, even for the same compounds in the same environment measured by different researchers is typical for many quinones, but until our work they did not find any explanation, being one of the mysteries of modern organic chemistry. It became apparent that existing notions about the structure of quinones and their derivatives did not correspond to reality and should be reviewed.

In previous studies (e.g., [7, 10–12]) we have shown that α-hydroxy-substituted anthraquinones exist as a dynamic equilibrium mixture of quinoid tautomers and their conformers resulting from the rotation of one or more of hydroxy groups with the cleavage of the intramolecular hydrogen bonds. The structure of these compounds cannot be described by a single structural formula. The equilibrium of tautomers and conformers can be shifted under the influence of external factors. This leads to a plurality of πl–π*-bands and to the difference in the absorption spectra of different samples of the same compound. The commonly ac-cepted idea of necessary identity of different samples of compounds obtained or purified by different me-thods, is misleading: they may differ in content of tautomers and conformers.

Each tautomer and conformer can interact with boric acid and its salts. It can form two types of boron-containing derivatives: (1) boron complexes where the boron atom is bound by a coordination bond with the carbonyl group oxygen atom and (2), borate esters without a coordination bond. It was suggested that these complexes are characterized by the existence of tautomers, differing by the distribution of double bonds and by partial localization of positive charge on different carbon atoms [6]. This is well consistent with the NMR data evidencing redistribution of the bonds typical of the isomeric anthraquinones [4, 5].

A perfect tool for the study of isomerism is the correlation analysis of πl–π*-absorption bands [10]: each isomer is characterized by a single πl–π*-band, which allows judging about its structure. We have developed two independent methods of correlation analysis. The assignment of each experimental πl–π*-band to the corresponding tautomer can be made by comparing the values of λmax with the values of λcalc. obtained by a quantum-chemical method. The criterion

for the reliability of the assignment is not the maximal closeness of these values but their linear correlation [13]. The π-electron method of Pariser–Parr–Pople (PPP) in the Dewar’s modification [14] with the use of approximation of β variable [15] still remains the only semiempirical quantum-chemical method, which on many examples demonstrated its ability to simulate adequately and accurately the results of structural changes in the hydroxyanthraquinones. Insufficient accuracy of modern nonempirical calculations does not allow us to apply them for the reliable assignment of a λmax value to the corresponding tautomer [16].

Since the PPP method is insufficient for the calculation of λcalc. of individual conformers, we sug-gested to assign the πl–π*-bands by means of correlation of the experimental νmax values with the sum of σA-constants of free hydroxy groups (OH) and those involved into H-bonding (OH*) of the cor-responding isomers [10]. The legitimacy of using the σA-constants for the protonated forms of the hydroxyanthraquinones has been shown earlier [17]. The σA constants allowing distinguishing between the boron-bearing ester groups involved and not involved in the formation of coordination bonds should be proportional to σA(OH)- and σA(OH*)-constants, since the values of λmax of two series of compounds struc-turally differing by only one attribute are always proportional [13]. Therefore, the use for this purpose of σA(OH)- and σA(OH*)-constants is quite justified.

The methodology of correlation analysis of electron absorption spectra has been proved for various compounds and their reactions. Hundreds of examples with extremely high r values and low s values, the similarity in assignment obtained and confirmed by independent methods leave no doubt about the authenticity of the found patterns, although some cor-relations were performed with the objectively minimum number of points. Their credibility can be judged from the fact that all the known values of λmax of all the investigated compounds can be assigned to the respective tautomers or conformers.

While analyzing absorption spectra, it is very important to go beyond one’s own measurements and use the data of other authors obtained with different samples of compounds, which can vary by the con-tribution of tautomers. The existence of different spec-tra for the same compound in the same or similar media for long time remained undetected, probably be-cause the researchers trust only their own measure-ments, discarding those differing from their data as

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C

O

O

O

9

10

1H

HH

H

BHO OH−

OB

HO OH−

C+

C+

O

OB

HO OH

OB

HO OH−

−

C+

C+

O

OH

HH

H

BHO OH

OB

HO OH

−

−

III IV V

580 570 560 550 540 530 520 510 500 490 480

440 460 480 500 520 540 460

λ max

of b

orat

e co

mpl

ex, n

m

λcalc of dications, nm

III

IV

V

Fig. 1. Correlation of experimental λmax values of borate-acetate complex of 1,5-dihydroxyanthraquinone with λcalc. of the 1,5-dihydroxyanthraquinone dications.

erroneous. Use of a large number of measurements of different researchers makes it possible to identify the maximum number of really formed tautomers.

1,5-Dihydroxyanthraquinone is the simplest of the studied group of compounds, whose absorption spectra of boron derivatives showed 3 πl–π*-bands, the minimum number required for correlation analysis. Various measurements were virtually identical: for

diborate complex in sulfuric acid 493, 528 and 570 nm [4], for the boroacetate complex in acetanhydride 491, 530, and 572 nm [18], 491, 527, and 569 nm [19]. Only two of these πl–π*-bands was found in [20]: 527 and 570 nm.

There are three possible tautomeric diborate complexes of 1,5-dihydroxyanthraquinone: with 9,10- (III), 1,10- (IV) and 1,5-quinoid structure (V).

By the PPP method we obtained the following λcalc. values for the corresponding 1,5-dihydroxyanthra-quinone dications: 444, 492 and 543 nm [21]. All the experimental values of λmax perfectly correlate with these values {Fig. 1, Eqs. (1)–(3), obtained by analysis of the measurements [4, 18, 19], respectively}.

λmax(borate complex) = (0.778 ± 0.027)λcalc + (147 ± 13) nm. (1)

The number of bands N = 3, the correlation coef-ficient r = 0.9994, standard deviation s = 1.9 nm.

λmax(boroacetate complex) = (0.8182 ± 0.0032)λcalc + (127.6 ± 1.6) nm, (2)

N = 3, r = 1.0000, s = 0.2 nm,

λmax(boroacetate complex) = (0.788 ± 0.021)λcalc + (140.4 ± 10.5) nm, (3)

N= 3, r = 0.9996, s = 1.5 nm.

The values of λmax measured in [20] should be attributed to 1,10- and 1,5-quinoid tautomers.

A much greater variety of absorption spectra is known for boron derivatives of 1,4-dihydroxyanthra-quinone. For the diborate complex in different studies were found the πl–π*-bands at 510 [22] or 540 nm [23], three πl–π*-band at 475, 508, or 547 nm [4], and according to our measurements four πl–π*-bands, at 440sh, 468, 505, and 546 nm. For the boroacetate complex in acetic anhydride two πl–π*-bands, at 520 and 550 nm [24] or three πl–π*-bands, at 486, 520, and 560 nm were described [4].

Formally, three tautomers of the diborate complex of 1,4-dihydroxyanthraquinone can exist: with 9,10- (VI), 1,10- (VII), and 1,4-quinoid structure (VIII). However, the presence of four πl–π*-bands indicates that into the equilibrium not only the three tautomers, but in addition at least one borate ester may be involved. To each of the tautomers (VI–VIII) may correspond two borate ester: monoester, for example, (VIa) or (VIIa-9), and diester, for example, (VIIb).

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C+O

O

O

9

10

1H

H

BHO OH−

O

CHC+

O

O

O

VIa VIIa-9 VIIb

4

BOH

OH

9

OB

OHHO

BHO

OH

O

O

O

9

O

BHO

OH

BOH

OH

23000

22000

21000

20000

19000

18000

17000

–1.6 –1.5 –1.4 –1.3 –1.2 –1.1 –1.0 –0.9

VII

VIIa-9

VI

VIa

XI

XII IX

X

1

2

νmax, cm–1

ΣσА

Fig. 2. Correlation of νmax of the boron complexes of (1) 1,4- and (2) 1,8-dihydroxyanthraquinones with the sum of σА constants.

The assignment of four πl–π*-bands was carried out using the correlation of the νmax values with the sums of σA-constants of the tautomers [Fig. 2, Eq. (4)]. It turns out that in equilibrium participate diborate complexes VI and VII, and monoborate esters VIa and VIIa-9 with 9,10- and 1,10-quinoid structures.

νmax = (10719 ± 289)ΣσA + (34656 ± 386) cm–1, (4) N = 4, r = 0.9993, s = 93 cm–1.

The λmax values of the three πl–π*-bands of borate and boroacetate complexes of 1,4-dihydroxyanthra-quinone [4] did not correlate with the corresponding λcalc. values of. dications. This means that these systems do not consist of a mixture of three quinoid tautomers. Correlation analysis of the νmax values with the sums of σA-constants showed that two long-wavelength bands correspond to the diborate complex of 1,10-quinoid structure (VII) and to the monoesters VIIa-9, and the short-wavelength πl–π*-band could equally belong to 9,10-tautomer VI [Eq. (5)] or to diborate ether VIIb [Eq. (6)].

νmax = (9371 ± 244) ΣσA + (32216 ± 340) cm–1, (5) N = 3, r = 0.9997, s = 50 cm–1,

νmax = (10440 ± 399) ΣσA + (33802 ± 559) cm–1, (6) N = 3, r = 0.9993, s = 73 cm–1.

Similarity of ΣσA values and the small difference in the correlation parameters of the two equations do not permit an unambiguous choice between the two structures.

Thus, the product of reaction of 1,4-dihydroxy-anthraquinone with boric acid is a dynamic mixture of the diborate complexes and borate esters with 9,10- and 1,10- (but not 1,4-) quinoid structure.

In the absorption spectrum of boron complex of 1,8-dihydroxyanthraquinone in sulfuric acid or acetic anhydride a πl–π*-band at 580 nm was registered [25]. Other authors give for the boracetate complex signify-

cantly different values: 530 [18], 481, and 508 nm [19]. It is obvious that such differences cannot be ex-plained solely in terms of 9,10-quinoid structures.

As a result of tautomeric and conformational transformations the structure of this compound may be described by two 9,10- (IX, X) and two 1,10- (XI, XII) quinoid structures:

C+O

O

O

9

10

1H

H

BHO OH−

IX X

8

OB

HO

OH

O

O

OOB

HO

OHB

OH

OH

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FAIN et al. 2474

C+O

O

O

9H

H

BHO OH−

XI XII

8

OB

HO

OH

O

O

OOBOH

The formation of the diborate ester of 8,9-dihydroxy-1,10-anthraquinone seems hardly probable because of significant steric strains.

The correlation analysis with the sums of σA-constants allowed unambiguous attribution of the bands at 481, 508 and 580 nm to the respective struc-tures IX–XI [Fig. 2, Eq. (7)].

νmax = (8668 ± 55) ΣσA + (29293 ± 64) cm–1, (7) N = 3, r = 0.99998, s = 16 cm–1.

The point at 530 nm deviates from the straight line to the shorter wavelengths, which is consistent with

assigning the corresponding πl–π*-band to the mono-borate ester XII. Compound XII does not belong to the isostructural series IX–XI, and therefore does not obey the found pattern.

The absorption spectrum of the 1,4,5,8-tetrahyd-roxyanthraquinone boron complex in sulfuric acid contains five πl–π*-bands: 527, 541, 568, 585, and 618 nm [26]. The biprotonated 1,4,5,8-tetrahydroxyanthra-quinone formally can form four tautomers: derivatives of 9,10-, 1,10-, 1,4- and 1,5-anthraquinones. The values of λmax and λcalc.do not correlate: the points do not fit a straight line. Hence, no more than two πl–π*-bands belong to the tautomeric diborate complexes. The correlation analysis with the sums of σA-constants confirmed this result and allowed us to assign each of five πl–π*-bands to boron-substituted 9,10- (XIII, XIV), 1,10- (XV), 1,4- (XVI) and 1,5-anthraquinones (XVII) respectively [Fig. 3, Eq. (8)]:

νmax = (1774 ± 1961) ΣσA + (22423 ± 168) cm–1, (8) N = 5, r = 0.998, s = 76 cm–1.

O

O

O

XIII XIV

OB

HO

OH

C+

C+

O

O

OOB

OH

OH

BOH

OH

O OB B

OH

HO

OH

OH

H

H

BHO OH

O

H

HO

BHO

OHB

HO OH

C+O

O

OOB

HO

OH

O OB B

OH

HO

OH

OH

H

H

H

H

BHO OH−

C+

C+O

O

OOB

HO

OH

O OBOH

HO

BHO OH−

BHO OH

−

−

−

XVII

CHC+O

O

OOB

HO

OH

O O

BHO OH−

B

OHXV XVI

Two well-known absorption spectra of the boron complex of 1-hydroxyanthraquinone differ noticeably by the position of the long-wavelength πl–π*-bands: 504, 552 nm [26] and 505, 540 nm [4]. Formally the formation of 9,10- and 1,10-quinoid tautomers XVIII

and XIX, and the corresponding borate esters XVIIIa and XIXa is possible.

The correlation analysis of 3 different πl–π*-bands [Eq. (9)] allowed us to assign them to the boron

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complex of 9-hydroxy-1,10-anthraquinone (XIX) and borate ester of 1-hydroxy-9,10- (XVIIIa) and 9-hydroxy-1,10-anthraquinone (XIXa).

νmax = (12848 ± 1054)ΣσA + (26149 ± 604) cm–1, (9) N = 3, r = 0.997, s = 104 cm–1.

Hence, the compound to which the structure of boron complex of 1-hydroxy-9,10-anthraquinone XVIII is ascribed is not this one, because in its known samples this structure is absent.

Reliability of assignment of the experimental πl–π*-bands of boron derivatives of α-hydroxyanthra-quinones to borate esters and boron complexes with 9,10-, 1,4-, 1,5- and 1,10-tautomeric structures is con-firmed by a number of independent correlations. Here are just some of them.

The values of νmax of the boroacetate complex of 1,5-dihydroxyanthraquinone [18] correlates with the sum of σA-constant of the bound hydroxy groups [Eq. (10)]:

νmax = (4345 ± 280)ΣσA + (25800 ± 451) cm–1, (10) N = 3, r = 0.998, s = 131 cm–1.

A linear relationship of πl–π*-bands of α-hydroxyl-anthraquinones and related boron derivatives, for ex-ample, the λmax values of the diborate complex of 1,5-dihydroxyanthraquinone and λmax of 1,5-dihydroxy-3-metilanthraquinone in ethanol, for which the presence of the same 3-quinoid structure was evidenced [Eq. (11)]:

λmax(diborate complex) = (1.097 ± 0.033) ·λmax(1,5-dihydroxy-3-methylanthraquinone)

+ (55 ± 14) nm, (11) N = 3, r = 0.9996, s = 1.1 nm.

For the values νmax of 1,4,5,8-tetrahydroxy-9,10-anthraquinone in hexane [12] and its boron derivatives in sulfuric acid the similar relation is found [Eq. (12)]:

νmax(1,4,5,8-tetrahydroxy) = (0.3615 ± 0.0220) ·νmax(borate derivatives) + (11112 ± 450) cm–1, (12)

N = 5, r = 0.995, s = 44 cm–1.

These and other similar relations evidence that the fine structure of the πl–π*-bands of the boron deriva-tives is of the same nature as in the initial α-hydroxyl-anthraquinones, that is, are a consequence of quinoid tautomerism and rotational isomerism. Consequently, the existing concepts of delocalization of the charge over different carbon atoms are inaccurate: the presence of tautomers is a result, at least partly, of its localization.

Equations (7) and (8) make it possible to calculate the values of νmax of the yet not found borate esters and boron complexes tautomers of 1,4-di- and 1,4,5,8-tetrahydroxyanthraquinones. It turned out that the values of νmax of these di- and tetrahydroxyanthra-quinones are connected linearly with each other [Eq. (13)].

νmax(1,4,5,8) = (0.40939 ± 0.00009)νmax(1,4) + (10431 ± 1.4) cm–1, (13)

N = 6, r = 1.0000, s = 0.4 cm–1.

Similar patterns of proportional response [13] connect the νmax values of 1,4,5,8-tetrahydroxyanthra-qunone with the corresponding values for 1,5- and 1,8-dihydroxyanthraquinones.

For each quinoid tautomer of the boron derivatives of di- and tetrahydroxyanthraquinones the νmax value is linearly dependent on the number of coordination bonds n in the molecule. For example, this dependence in the case of boron derivatives of 1,4,5,8-tetra-hydroxy-9,10-anthraquinone is illustrated by Fig. 4 and Eq. (14). νmax(1,4,5,8) = (18946 ±0.2) – (230.7±0.1)n cm–1, (14)

N 5, r 1.00000, s 0.3 cm–1.

C+

O

O OH

H

BOHHO −

C+

O

O OB

OHHO −

H H

XVIII XIX

O

XVIIIa XIXa

O O

O

O OB B

OH

HOOH

OH

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0 1 2 3 4

19000

18800

18600

18400

18200

18000

0 1 2 3 4

νmax, cm–1

n

XVII

XVI

XV

XIV

XIII19000

18500

18000

17500

17000

16500

16000

–3.6 –3.4 –3.2 –3.0 –2.8 –2.6 –2.4 –2.2 –2.0 –1.8 ΣσА

νmax, cm–1

XVII

XVI

XV

XIV

XIII

Fig. 3. Correlation of νmax of the boron complex of 1,4,5,8-tetrahydroxyanthraquinone with the sum of σА constants.

Fig. 4. Dependence of the position of πl–π*-bands of the boron derivatives of 1,4,5,8-tetrahydroxy-9,10-anthraquinone on the number (n) of coordination bonds.

It is noticed that the tautomeric compositions of hydroxyanthraquinones and their boron complexes usually do not coincide. This implies that the complex formation is accompanied by a shift of tautomeric and conformational equilibria.

The results of this study agree well with those obtained in the study of the structure of metal com-plexes of α-hydroxy-substituted anthraquinones [27–29]. They provide an opportunity to formulate new ideas about the structure of the studied boron-substituted derivatives.

Boron derivatives of α-hydroxyanthraquinones are not individual compounds, but are the dynamic equilib-rium mixtures of tautomers, and their structures cannot be represented by a single structural formula. They are not exclusively the derivatives of 9,10-anthraquinone: The tautomeric 1,10-, 1,4- and 1,5-quinoid structures are also involved into the equilibrium.

Typically, an equilibrium mixture contains not only borate complexes characterized by the presence of coordination C=O→B bonds, but also compounds with borate ester groups not associated with the carbonyl groups by coordination bond.

Using hydroxyanthraquinones in analytical chem.-istry as reagents for the determination of boron is based not on the complexation with boron itself, but on the shift of tautomeric equilibria accompanying the complexation.

Hence, a number of important general conclusions can be drawn. First, it is confirmed that the modern

chemistry of anthraquinones as a very extensive class of organic compounds cannot be developed solely in the frame of the 9,10-quinoid structures. Second, it is confirmed that the chemical reactions of compounds that are the mixtures of tautomers and conformers, usually are accompanied by a shift of tautomeric and conformational equilibria. The study of such reactions separately from the study of the equilibria is incorrect. Third, the concepts that the molecule capable to have a geometry appropriate for the formation of a coor-dination bond is certainly characterized by this geometry is not correct. The correlation analysis of electron absorption spectra shows that a dynamic equilibrium mixture of conformers can exist differing by the presence or absence of coordination bonds.

The phenomenon of tautomerism is very common, and electron absorption spectra serve as a tool for the study of many reactions. The approaches developed in our work may be useful not only for the derivatives of quinones, but also for other classes of organic com-pounds. Many of the reactions of compounds charac-terized by the conformational and tautomeric equilibria are unreasonably studied separately from the study of these equilibria leading probably to erroneous con-clusions. Studying the reactions of these substances should begin with determining the tautomeric structure of the initial sample and completed with the determination of the same for the final products.

The main goal of our work we see in drawing attention to the great role of tautomerism and rotational isomerism, which they play in organic chemistry, the

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need for some serious changes in the conventional notions about the structure of these compounds. So far only initial steps were made in this direction. Beyond still remain, for example, specific studies aimed at finding out the changes in the conditions of obtaining substances caused by the tautomeric shifts and procedures to obtain samples with a given tautomeric composition. It is expectable that on the basis of such studies the processing of some industrially important products can be greatly improved.

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