Synthesis, chemical properties, and crystal structure of 2,4,6,8-tetra( tert-butyl)-9-hydroxyphenoxazin-1-one

Article abstract of DOI:10.1007/s11172-009-0182-4

The reaction of di(tert-butyl) derivatives of pyrocatechol with 2,6-dihydroxyaniline afforded 2,4,6,8-tetra(tert-butyl)-9-hydroxyphenoxazin-1- one. The chemical properties of the reaction product and its ability to form complexes with metal salts as the tridentate ligand were investigated. The structure of hydroxyphenoxazinone was established by X-ray diffraction.

Full text of DOI:10.1007/s11172-009-0182-4

Russian Chemical Bulletin, International Edition, Vol. 58, No. 7, pp. 1361—1370, July, 2009
1361
Synthesis, chemical properties, and crystal structure
of 2,4,6,8ꢀtetra(tertꢀbutyl)ꢀ9ꢀhydroxyphenoxazinꢀ1ꢀone
V. I. Simakov,a Yu. Yu. Gorbanev,b T. E. Ivakhnenko,b V. G. Zaletov,b K. A. Lyssenko,c
Z. A. Starikova,c E. P. Ivakhnenko,b and V. I. Minkinb
aSouthern Federal University,
7 ul. Zorge, 344090 RostovꢀonꢀDon, Russian Federation
bInstitute of Physical and Organic Chemistry, Southern Federal University,
194/2 prosp. Stachki, 344090 RostovꢀonꢀDon, Russian Federation.
Fax: +7 (863) 224 3466. Еꢀmail: E.Ivakhnenko@ipoc.rsu.ru
cA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation
The reaction of di(tertꢀbutyl) derivatives of pyrocatechol with 2,6ꢀdihydroxyaniline affordꢀ
ed 2,4,6,8ꢀtetra(tertꢀbutyl)ꢀ9ꢀhydroxyphenoxazinꢀ1ꢀone. The chemical properties of the reacꢀ
tion product and its ability to form complexes with metal salts as the tridentate ligand were
investigated. The structure of hydroxyphenoxazinone was established by Xꢀray diffraction.
Key words: 2,4,6,8ꢀtetra(tertꢀbutyl)ꢀ9ꢀhydroxyphenoxazinꢀ1ꢀone, tautomerism, metal cheꢀ
lates, tridentate ligands, ESR spectroscopy, Xꢀray diffraction study, quantum chemical calꢀ
culations.
Scheme 1
Phenoxazine systems and their sterically hindered
structural analogs can be involved in reversible oneꢀelecꢀ
tron redox reactions to form paramagnetic intermediates,
the presence of the tertꢀbutyl groups leading to a substanꢀ
tial increase in the stability of such freeꢀradical species.1
The development of methods for the synthesis of steriꢀ
cally hindered quinoneimine systems2,3 made it possible to
prepare 2,4,6,8ꢀtetra(tertꢀbutyl)phenoxazinꢀ1ꢀone (1).4—6
It appeared that the reaction of 3,5ꢀdi(tertꢀbutyl)ꢀ1,2ꢀ
benzoquinone (2) with 3,5ꢀdi(tertꢀbutyl)ꢀ2ꢀhydroxyaniline
3 gives sterically hindered tricyclic system 1 (see Ref. 6)
instead of the expected hydroxyphenyliminobenzoquiꢀ
none 4 (see Ref. 7) (Scheme 1).
It was shown5 that the reaction of 3,5ꢀdi(tertꢀbutyl)ꢀoꢀ
benzoquinone (2) with 3,5ꢀdi(tertꢀbutyl)pyrocatechol (5)
in aqueous ethanolic ammonia (under anaerobic condiꢀ
tions) gives the diamagnetic crystalline adduct A. Accordꢀ
ing to the Xꢀray diffraction data, the adduct A consists of
two phenoxazinone molecules 1 linked to the molecule of
bis[3,5ꢀdi(tertꢀbutyl)ꢀ2ꢀhydroxyphenyl]amine (6) by hyꢀ
drogen bonds.
The oxidation of benzene solutions of the adduct A
with atmospheric oxygen affords 2,4,6,8ꢀtetra(tertꢀbutyl)ꢀ
phenoxazinꢀ1ꢀone (1) in quantitative yield. This confirms
the instability of compound 6 in oxidation reactions, which
are accompanied by the intramolecular homolytic cyclizaꢀ
tion giving phenoxazinone 1.5
Previously, it has been shown that compound 6 can
serve as the paramagnetic tridentate ligand stabilized by
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1323—1331, July, 2009.
1066ꢀ5285/09/5807ꢀ1361 © 2009 Springer Science+Business Media, Inc.

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Simakov et al.
the metal chelation in complexes prepared by the temꢀ
plate synthesis.7
2,4,6,8ꢀtetra(tertꢀbutyl)ꢀ9ꢀhydroxyphenoxazinꢀ1ꢀone (12)
(Scheme 2).
The chemistry of sterically hindered phenoxazinones
has attracted interest primarily due to the ability of pheꢀ
noxazinone 1 to be involved in reversible oneꢀelectron reꢀ
dox processes to form a radical anion, whose reactions
with metal salts give paramagnetic bidentate metal comꢀ
plexes 7—9 with different chemical structures.8
Based on the structures of compounds 7—9 established
by Xꢀray diffraction and the character of the intramolecuꢀ
lar spin exchange in these metal complexes studied by
ESR spectroscopy, such compounds can be divided into
three main groups.8,9
Scheme 2
One group includes freeꢀradical allylꢀtype organomeꢀ
tallic compounds (monoradicals) 7. Two other groups inꢀ
clude compounds 8 and 9, which are biradical systems
that differ in the type of the electron shell (closed (8) or
open (9)) of the central metal atom in the bidentate cheꢀ
late unit of the metal complexes.
Results and Discussion
Product 12 is a violet crystalline compound readily
soluble in lowꢀpolarity aprotic solvents (hexane and benꢀ
zene) and poorly soluble in alcohols. The UV spectrum of
compound 12 is shown in Fig. 1.
The aim of the present study was to synthesize the
sterically hindered phenoxazine system capable of
forming the tridentate metal chelate unit upon the coordiꢀ
nation.
←
←
The intramolecular character of the O1 → N → O2 proꢀ
totropy (Scheme 3) in molecule 12 is confirmed by the
fact that the polarity of the solvent has no effect on the
character of the UV spectrum of compound 12 (see Fig. 1,
curves 1 and 2). Thus the spectra recorded in hexane and
ethanol are identical in the shortꢀwavelength region and
are only slightly different in the longꢀwavelength region.
The addition of an equimolar amount of an ethanolic
NaOH solution to an ethanolic solution of compound 12
resulted in the appearance of an additional absorption band
at λmax = 695 cm–1 (see Fig. 1, curve 3), which is apparꢀ
ently associated with the formation of symmetric mesomꢀ
eric anion 12a. The subsequent addition of an equimolar
amount of MeCOOH to the ethanolic solution of anion
12a results in the regeneration of the starting 2,4,6,8ꢀtetraꢀ
For this purpose, we studied the reaction of sterically
hindered pyrogallol with ammonia. It was found that
4,6ꢀdi(tertꢀbutyl)pyrogallol (10) (which was prepared by
the alkylation of pyrogallol with tertꢀbutyl alcohol in sulꢀ
furic acid)2 easily undergoes amination under the action
of aqueous ammonia. However, the reaction gives the symꢀ
metric product 3,5ꢀdi(tertꢀbutyl)ꢀ2,6ꢀdihydroxyaniline
(11) instead of the expected 3,5ꢀdi(tertꢀbutyl)ꢀ2,3ꢀdihyꢀ
droxyaniline. This result suggests that, as opposed to pyꢀ
rogallol, the central hydroxy group is involved in the amiꢀ
nation in the sterically hindered analog. The condensaꢀ
tion of 3,5ꢀdi(tertꢀbutyl)ꢀ2,6ꢀdihydroxyaniline 11 with
3,5ꢀdi(tertꢀbutyl)pyrocatechol (5) in toluene in the presꢀ
ence of catalytic amounts of pꢀtoluenesulfonic acid affords

2,4,6,8ꢀBut4ꢀ9ꢀHOꢀphenoxazinꢀ1ꢀone
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 7, July, 2009
1363
A
a tautomer with the hydrogen atom located at one of the
oxygen atoms (O(2)) (Fig. 2). The ethanol molecule
forms relatively strong hydrogen bonds with the C=O
(O(1S)...O(3), 2.761(2) Å; O(1S)H(1S)O(3), 169.8°) and
OH (O(2)...O(1S), 2.680(2) Å; O(2)H(102)O(1S), 161°)
groups serving both as the hydrogen atom donor and acꢀ
ceptor. Based on the N(1)...O(2) distance (2.714(2) Å),
the formation of the intramolecular O(2)H(2)...N(1)H
hydrogen bond cannot be ruled out as well, although the
O(2)H(102)N(1) angle (110.2°) essentially deviates from
180°. It should also be mentioned that there is the forced
shortened O(1S)...N(1) contact (2.876(2) Å), which,
among other factors, can stabilize this solvate.
3
0.8
2
0.6
3
3
1
0.4
0.2
0
1
2
1
2
An analysis of the molecular geometry of compound
12 (Table 1) showed that the bond length distribution in
the quinone ring C(7)—C(12) (1.365(2)—1.488(2) Å) is
virtually identical to that found previously10 in 2,4,6,8ꢀtetraꢀ
(tertꢀbutyl)phenoxazinꢀ1ꢀone (1). In the second hydroxyꢀ
substituted ring, the bond lengths are more equalized
(1.389(2)—1.427(2) Å) (see Table 1). The fused ring in
compound 12 is planar (the average deviation of the atoms
is no larger than 0.0158 Å); the deviations of the central
C(13), C(17), C(21), and C(25) atoms of the tertꢀbutyl
groups from this plane are in the range of 0.029—0.09 Å.
All tertꢀbutyl groups deviate in the same direction, whereꢀ
as the C(25) atom deviates in the opposite direction, which
is apparently attributed to the steric crowding of the moleꢀ
cule. Actually, the presence of four bulky tertꢀbutyl subꢀ
stituents results in the formation of numerous shortꢀ
ened intermolecular contacts, such as two C—H...O(2)
contacts (H(20C)...O(2), 2.24 Å; H(18A)...O(2), 2.33 Å),
two C—H...O(3) contacts (H(23B)...O(3), 2.36 Å;
H(22c)...O(3), 2.33 Å), and four C—H...O(2) contacts
(the H...O distances vary in the range of 2.34—2.47 Å). In
addition to the C—H...O interactions, there are also
abnormally short intramolecular H...H contacts
H(15B)...H(26A) and H(14B)...H(26A) (1.83 and 1.89 Å,
respectively). In all tertꢀbutyl groups, the CCC bond anꢀ
gles at the central atom are distorted in the same manner,
viz., the CCC angles at the carbon atom of the methyl
groups (C(16), C(19), C(28), or C(24), respectively),
which is involved in none of the aboveꢀmentioned inꢀ
tramolecular contacts, are decreased.
300
400
500
600
700 λ/nm
Fig. 1. UVꢀVis spectra of compound 12 in hexane (1) and ethaꢀ
nol (2, 3) before (2) and after the addition of NaOH (3);
C = 4•10–5 mol L–1).
(tertꢀbutyl)ꢀ9ꢀhydroxyphenoxazinꢀ1ꢀone (12). This is inꢀ
dicative of the reversibility of the reaction under considerꢀ
ation. As opposed to 2,4,6,8ꢀtetra(tertꢀbutyl)phenoxazinꢀ
1ꢀone (1),6 the low basicity of the nitrogen atom in the
phenoxazine ring of compound 12 makes it impossible to
prepare protonated derivatives by the treatment with minꢀ
eral acids or free HCl in solvents of different polarity (benꢀ
zene or ethanol).
Scheme 3
To estimate whether the tautomer that is found in the
crystal corresponds to the energy minimum or results from
the stabilization by specific solvation and to investigate
the character of the aboveꢀmentioned intramolecular inꢀ
teractions, we carried out quantum chemical calculations
(B3LYP/6ꢀ31G**).
The geometry optimization of two tautomers containꢀ
ing the hydroxy group (A) and the protonated nitrogen
atom (B) showed that the tautomer A in the gas phase also
corresponds to the global minimum and is 4.33 kcal mol–1
more stable. A comparison of the experimental and calcuꢀ
lated geometry reveals that the chosen level of theory and
The Xꢀray diffraction study showed that compound 12
crystallizes as the solvate with ethanol. Molecule 12 is

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Simakov et al.
H(2SA)
C(2S)
H(2SB)
H(2SC)
H(1SA)
H(1SB)
C(1S)
O(1S)
H(10S)
O(3)
H(22C)
H(23B)
H(22B)
H(18A)
H(20C)
H(1O2)
O(2)
H(18B)
H(23C)
C(22)
H(20B)
H(18C)
C(23)
H(22A)
C(9)
C(20)
H(23B)
N(1)
C(8)
C(5)
H(24B)
C(17)
H(20A)
C(21)
C(7)
H(19C)
H(19B)
C(6)
C(24)
H(24C)
C(4)
C(3)
C(19)
H(24A)
C(10)
H(10A)
H(19A)
C(12)
H(27C)
C(1)
C(11)
H(3A)
C(2)
O(1)
H(27A)
C(25)
H(28A)
H(14C)
C(13)
H(16C)
H(14B)
C(14)
H(28C)
C(28)
H(28B)
C(27)
H(27B)
H(26B)
C(26)
H(16A)
C(16)
H(15A)
H(14A)
H(26A)
H(15B)
C(15)
H(16B)
H(15C)
H(26C)
Fig. 2. Molecular structure of compound 12 represented by displacement ellipsoids (p = 50%).
the basis set adequately reproduce the main structural feaꢀ
tures of compound 12 (see Table 1). Apparently, the tauꢀ
tomer A is additionally stabilized due to the essential alꢀ
ternation of the bonds in two quinonoid rings in the tauꢀ
tomer B (C—C, 1.373—1.474 Å) resulting in the loss of
aromaticity compared to the hydroxyꢀsubstituted tautomer
A. In addition, the intramolecular N—H...O hydrogen
bond can occur in the tautomer A, whereas the presence of
intramolecular hydrogen bonds in the tautomer B is unꢀ
likely. To study the character of interactions in detail, we
carried out the topological analysis of the electron density
distribution ρ(r) for the tautomers A and B. According to
the results of the analysis, the degree of aromaticity in the
cyclic system of the tautomer B is, on the whole, lower.
This can be illustrated, for example, with the use of the
absolute value of the negative eigenvector of the Hessian
(|λ1|) at the critical points (3, +1) of the rings as the quanꢀ
titative criterion. This value characterizes the curvature
ρ(r) in the direction perpendicular to the plane of the ring
and, consequently, reflects the degree of πꢀdensity deloꢀ
calization.11
Actually, the |λ1| values for the tautomer B are 0.3277
and 0.4338 e Å–5 in the quinonoid and central rings, reꢀ
spectively, whereas the |λ1| values for both the hydroxyꢀ
substituted and central rings in the tautomer A are someꢀ
what larger (0.34463 and 0.4579 e Å–5, respectively), and
a slight decrease in |λ1| (0.328 e Å–5) is observed only for
the quinonoid ring. In addition, there is the intramolecuꢀ
lar H(1O2)...N(1) hydrogen bond in the tautomer A.
The energy of this bond estimated based on Espinosa´s
correlation scheme12 is 7.25 kcal mol–1. The topological
analysis of ρ(r) also showed that all the aboveꢀmentioned
intramolecular C—H...O and H...H contacts do correꢀ
spond to bonding interactions, their number and strength
in both tautomers being virtually identical (Fig. 3). Thus
the energy of the C—H...O interactions evaluated accordꢀ
ing to the same correlation scheme for the hydroxy and
C=O groups is 2.9—3.0 kcal mol–1, and this energy for the
endocyclic oxygen atom is 2 kcal mol–1. The energy of the
H...H interactions is substantially lower (0.76 kcal mol–1).
As can be seen from the molecular graphs of the tautomers
A and B, there are not only the aboveꢀmentioned H...H

2,4,6,8ꢀBut4ꢀ9ꢀHOꢀphenoxazinꢀ1ꢀone
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 7, July, 2009
1365
Table 1. Selected bond lengths (d) and bond angles (ω) in the crystal of compound 12 and in the isolated tautomers A and B calculated
at the B3LYP/6ꢀ31G* level of theory
Parameter
Bond
12
12A
12B*
Parameter
Angle
12
12A
12B*
d/Å
ω/deg
O(1)—C(12)
O(1)—C(1)
O(2)—C(5)
O(3)—C(8)
N(1)—C(6)
N(1)—C(7)
C(1)—C(2)
C(1)—C(6)
C(2)—C(3)
C(3)—C(4)
C(4)—C(5)
C(5)—C(6)
C(7)—C(12)
C(7)—C(8)
C(8)—C(9)
C(9)—C(10)
C(10)—C(11)
C(11)—C(12)
1.386(2)
1.388(2)
1.336(2)
1.255(2)
1.364(2)
1.303(2)
1.389(2)
1.412(2)
1.408(2)
1.395(2)
1.397(2)
1.427(2)
1.444(2)
1.488(3)
1.444(3)
1.365(2)
1.439(2)
1.365(2)
1.380
1.386
1.335
1.246
1.355
1.305
1.398
1.413
1.413
1.399
1.399
1.429
1.449
1.498
1.453
1.373
1.447
1.376
1.396
—
—
1.246
1.355
—
—
—
—
—
O(2)—C(5)—C(6)
C(4)—C(5)—C(6)
120.2(2)
119.6(2)
122.4(2)
116.8(2)
120.9(2)
123.3(2)
115.7(2)
121.0(2)
123.7(2)
119.5(2)
116.7(2)
116.8(2)
128.6(2)
115.5(2)
121.8(2)
121.2(2)
117.0(2)
111.3(2)
107.2(2)
106.5(2)
110.1(2)
107.4(2)
107.9(2)
110.1(2)
107.6(2)
107.9(2)
110.7(2)
107.0(2)
107.4(2)
120.6
118.8
121.6
116.8
121.6
124.7
114.6
120.7
124.6
119.5
116.0
117.1
128.8
115.5
122.3
120.9
116.8
110.7
107.1
106.8
109.9
107.5
107.6
109.5
107.8
107.9
110.7
106.6
107.0
—
—
—
—
N(1)—C(6)—C(1)
N(1)—C(6)—C(5)
C(1)—C(6)—C(5)
N(1)—C(7)—C(12)
N(1)—C(7)—C(8)
C(12)—C(7)—C(8)
O(3)—C(8)—C(9)
—
118.7
115.9
125.3
127.3
118.1
114.6
117.0
128.5
115.6
122.9
118.9
118.2
—
O(3)—C(8)—C(7)
C(9)—C(8)—C(7)
—
—
C(10)—C(9)—C(8)
C(9)—C(10)—C(11
C(12)—C(11)—C(10)
C(11)—C(12)—O(1
C(11)—C(12)—C(7)
O(1)—C(12)—C(7)
C(15)—C(13)—C(14)
C(15)—C(13)—C(16)
C(14)—C(13)—C(16)
C(20)—C(17)—C(18)
C(20)—C(17)—C(19)
C(18)—C(17)—C(19)
C(23)—C(21)—C(22)
C(23)—C(21)—C(24)
C(22)—C(21)—C(24)
C(26)—C(25)—C(27)
C(26)—C(25)—C(28)
C(27)—C(25)—C(28)
1.422
1.474
1.457
1.373
1.449
1.380
—
—
—
—
Angle
ω/deg
C(12)—O(1)—C(1)
C(7)—N(1)—C(6)
O(1)—C(1)—C(2)
O(1)—C(1)—C(6)
C(2)—C(1)—C(6)
C(1)—C(2)—C(3)
C(4)—C(3)—C(2)
C(3)—C(4)—C(5)
O(2)—C(5)—C(4)
120.6(2)
118.8(2)
120.8(2)
117.9(2)
121.3(2)
114.8(2)
127.3(2)
116.1(2)
120.2(2)
121.5
120.4
121.5
117.5
121.0
114.6
127.6
116.3
120.6
121.1
125.0
—
—
—
—
—
—
—
—
109.4
108.1
108.2
110.5
106.8
107.2
* The atomic numbering scheme for the tautomers A and B is identical to that given in Fig. 2. For the tautomer B, only the independent
parameters are given (the molecule has the symmetry C2).
and C—H...O contacts in the structure, but also unusual
C—H...C interactions between the hydrogen atoms of the
aromatic system and the aliphatic carbon atoms of the
tertꢀbutyl groups. These contacts are characterized by
a relatively high energy (3—3.4 kcal mol–1) compared to
the H...H interactions. However, unlike the aboveꢀmenꢀ
tioned contacts, these contacts are not stable because the
distance between the critical points (CP) (3, –1) and
(3, +1) is relatively small and, consequently, even a slight
change in the distance between the interacting atoms will
lead to the disappearance of the bond path. It should be
noted that the presence of H...H bonding interactions is
manifested not only in the topological parameters but also
in the geometric parameters of the C—H bonds. Thus the
distances for the pair of the interacting hydrogen atoms
are 0.006—0.008 Å shorter than the corresponding disꢀ
tances with noninteracting atoms.
with the experimental parameters of the hydrogen bonds.
Thus, the O(1S)...O(3) distance (2.655 Å) is slightly shortꢀ
er, whereas the O(2)...O(1) distance (2.799 Å) is, on the
contrary, elongated. The topological analysis of ρ(r) of the
solvate showed that the ethanol molecule forms, in addiꢀ
tion to the expected O—H...O bonds, the shortened
N(1)...O(1S) contact at which the CP (3, –1) is also loꢀ
cated. It should be noted that the intramolecular hydroꢀ
gen bond that is observed in the isolated tautomer A is
absent in the associate. Taking into account that the
N(1)...O(1S) distance in the associate is somewhat longer
(2.919 Å) compared to that in the crystal, it can be
concluded that this distance is retained in the crystal.
The estimation of the interaction energy according to
the aboveꢀmentioned scheme gave the energies of 9.09
and 13.45 kcal mol–1 for the O—H...O bonds with the
C=O and COH groups, respectively, and the energy of
3.13 kcal mol–1 for the O...N interactions. The validity of
the estimation of the energy of the hydrogen bonds in the
The results of the calculations for the complex of comꢀ
pound 12 with an ethanol molecule are in good agreement

1366
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 7, July, 2009
Simakov et al.
O
O
CP (3, –1)
N
CP (3, +1)
O
A
O
O
N
CP (3, +1)
O
CP (3, –1)
B
Fig. 3. Molecular graphs of the tautomers A and B (compound 12) calculated at the B3LYP/6ꢀ31G* level of theory.
associate is additionally confirmed by the comparison of
the total energy of the interaction between the methanol
molecule and compound 12 (25.67 kcal mol–1) with the
difference between the energy of the associate and the sum
of the energies of the noninteracting tautomer A and the
ethanol molecule (17.83 kcal mol–1). Actually, the forꢀ
mation of the associate is accompanied by the disappearꢀ
ance of the intramolecular N—H...O bond and, conseꢀ
ed that this is favorable for the proton transfer from the
hydroxy group to the carbonyl group, thus lowering the
barrier to tautomeric transformations.
Product 12 is, on the one hand, a close structural anaꢀ
log of 2,4,6,8ꢀtetra(tertꢀbutyl)phenoxazinꢀ1ꢀone 1 (see
Ref. 10) and, on the other hand, a tricyclic analog of the
wellꢀknown tridentate freeꢀradical ligand, which is formed
by the oxidation of metastable bis[3,5ꢀdi(tertꢀbutyl)ꢀ2ꢀ
hydroxydiphenyl]amine.4
quently, the energy of the complex is 18.4 kcal mol–1
,
1
which is in good agreement with the aboveꢀmentioned value.
Taking into account the high energy of the O—H...O
interactions with the ethanol molecule, it can be suggestꢀ
The H NMR spectra of compound 12 in solvents of
different polarity (CDCl3, DMSOꢀd6, and tolueneꢀd8)
show signals for the tertꢀbutyl groups and the hydrogen

2,4,6,8ꢀBut4ꢀ9ꢀHOꢀphenoxazinꢀ1ꢀone
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 7, July, 2009
1367
atoms at the carbocycles pairwise averaged due to the
benzene affords the phenylmercury derivative of phenoxꢀ
azine 13. The 1H NMR spectrum of the latter compound
(CDCl3, tolueneꢀd8) shows pairwiseꢀaveraged signals of
the tertꢀbutyl groups (δ 1.47 (18 H), 1.38 (18 H), CDCl3)
and the aromatic protons of the phenoxazine ring (δ 7.45
←
←
threeꢀcenter prototropy O1 → N → O2 (δ 7.45 (2 H), 1.44
(18 H), 1.39 (18 H), CDCl3). A decrease in the temperaꢀ
ture to 207 K does not lead to a change in the spectral
pattern, which indicates that this tautomeric process is
fast on the 1H NMR time scale.
←
←
(2 H), CDCl3), which is indicative of the O1 → N → O2
migration of the phenylmercury moiety in compound 13
fast on the NMR time scale.
Since 2,4,6,8ꢀtetra(tertꢀbutyl)ꢀ9ꢀhydroxyphenoxazinꢀ
1ꢀone (12) is the first representative of the threeꢀcenter
←
←
tautomeric system (O1 → N → O2) (Scheme 4) in the seꢀ
ries of sterically hindered phenoxazinones, we investigatꢀ
ed the dependence of the character of the tautomeric proꢀ
cess on the nature of the substituent at position 9 of the
phenoxazine ring.
The substantially different spectral patterns are obꢀ
served for compounds 14 and 15. The NMR spectra of
these compounds are indicative of the absence of the tauꢀ
tomeric process. The acylation of phenoxazinꢀ1ꢀone 12
with acetic anhydride affords 9ꢀacetoxyꢀ2,4,6,8ꢀtetra(tertꢀ
butyl)phenoxazinꢀ1ꢀone (14) in high yield. OꢀMethyl deꢀ
rivative 15 of phenoxazinone 12 was synthesized by the
reaction of compound 12 with a threefold excess of diꢀ
methyl sulfate in acetone in the presence of anhydrous
potassium carbonate.
Scheme 4
As opposed to 2,4,6,8ꢀtetra(tertꢀbutyl)phenoxazinꢀ1ꢀ
one (1),9,13 the reaction of phenoxazinꢀ1ꢀone 12 with
a powder of zinc metal in benzene gives diamagnetic coorꢀ
dination compound 16 instead of the expected ZnII
oꢀiminobenzosemiquinone complex. The symmetrical
1H NMR spectral pattern (δ 7.41 (2 H), 1.45 (18 H), 1.24
(18 H), CDCl3) of compound 16 corresponds to the triꢀ
dentate ligand stabilized due to the metal chelation. The
structurally identical sixꢀcoordinate ZnII complex 16 was
obtained by the reaction of compound 12 with ZnII aceꢀ
tate in methanol.
The reaction of 2,4,6,8ꢀtetra(tertꢀbutyl)ꢀ9ꢀhydroxyꢀ
phenoxazinꢀ1ꢀone 12 with CuCl2 in methanol gave comꢀ
plex 17, which was studied by ESR spectroscopy. We inꢀ
vestigated powders and toluene solutions of compound 17
at room temperature and in lowꢀtemperature glasses
(60% of toluene and 40% of chloroform) at liquid nitrogen
temperature.
The most informative spectra in lowꢀtemperature
glasses have a set of lines characteristic of the chelate unit,
in which the nearest environment of the central atom
has a nearly axial symmetry. In the g|| region, there
are hyperfine lines assigned to the interaction between the
unpaired electron spin and the spin of the 63.65Cu nucleꢀ
us with the g factor g|| = 2.0035 and the hyperfine couꢀ
pling constant A|| = 145•10–4 cm–1. The intense line
in the g⊥ region is split into two components, which
is attributed to a small deviation of the structure of
the chelate unit in compound 17 from the axial symmetry
(g2 = 2.224, g3 = 2.260). The g|| value is similar to
the g factor of the free electron (g0 = 2.0023) and the g2
and g3 values are larger than g||. These facts provide
evidence that the orbital of the unpaired electron in
Due to the high lability of the hydrogen atom of the
hydroxy group in compound 12 associated with the threeꢀ
←
←
center prototropic tautomerism (O1 → N → O2), it can be
replaced by substituents with different chemical properꢀ
ties (HgPh, COMe, or Me) (see Scheme 4).
2
compound 17 is close to the 3dz orbital. This is characꢀ
teristic of sixꢀcoordinate chelate units, in which the
nearest environment of the copper atom can be described
The reaction of 2,4,6,8ꢀtetra(tertꢀbutyl)ꢀ9ꢀhydroxyꢀ
phenoxazinꢀ1ꢀone (12) with phenylmercury chloride in
2
as a flattened octahedron. The coefficients k1 and k0

1368
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 7, July, 2009
Simakov et al.
1
vents. The H NMR spectra were recorded in CDCl3, tolueneꢀ
d8, and DMSOꢀd6 on a Varian Unityꢀ300 instrument (300 MHz)
with the accuracy of 0.1 ppm; the signals of the residual protons
of these solvents were used as the internal standard. The ESR
spectra were measured on a Bruker EMX 10/12 spectrometer.
The IR spectra were recorded on a Specordꢀ75IR spectrometer
in Nujol mulls. Column chromatography was performed on aluꢀ
mina (Reakhim). The course of the reactions was monitored and
the purity of the reaction products was checked by TLC on plates
coated with alumina (Brockman activity IV).
4,6ꢀDi(tertꢀbutyl)pyrogallol (10). Concentrated H2SO4 (5 mL)
was added with stirring to a solution of pyrogallol (12 g, 0.093 mol)
in tertꢀbutyl alcohol (35 mL, 0.367 mol) at 50 °C. The reaction
mixture was stirred for 0.5 h. Then the organic layer was separatꢀ
ed and washed with hot water (3×15 mL). The solidified subꢀ
stance was recrystallized from hexane and dried under reduced
pressure. White crystals of compound 10, which turned pink in
air, were obtained in a yield of 12.28 g (55%), m.p. 110 °C.
1H NMR (CDCl3), δ: 6.72 (s, 1 H, arom.); 5.12 (br.s, 3 H, OH);
1.3 (s, 18 H, But). Found (%): C, 90.31; H, 9.30. C14H22O3.
Calculated (%): C, 90.72; H, 9.24.
were evaluated from the spectral parameters according
to the equations
A|| = k12P0(–k0 + 4/7) + P0(g|| – g0) – 1/7P0(g⊥ – g0),
A⊥ = k12P0(–k0 – 2/7) + 15/14[P0(g⊥ – g0),
suitable for the 3dz2 ground state.14 In these equations, the
2
coefficient k1 characterizes the degree of delocalization
of the unpaired electron orbital, the coefficient k0 deterꢀ
mines the isotropic component of the hyperfine coupling,
P0 = 360•10–4 cm–1, and g⊥ = 1/2(g2 + g3). It was asꢀ
sumed that A⊥ = 0 because of the absence of the hyperfine
splitting of the spectral lines in the g region. The evaluated
k1 = 0.81 is indicative of the 80% localization of the unꢀ
paired electron on the CuII atom. The lower k0 = 0.033
compared to k0 = 0.43 for the planar complexes with the
3dx2y2 ground state can be attributed to an admixture of the
4s state in the 3dz2 ground state.15
The spectra of powders of compound 17 are similar to
those measured in lowꢀtemperature glasses. The g|| reꢀ
gion shows the hyperfine coupling with g|| = 2.012, A|| =
= 139•10–4 cm–1; the g⊥ region has a broad line with
g⊥ = 2.23. The hyperfine coupling observed in powders is
apparently associated with the large size of the molecules
of complex 17, resulting in an increase in the distances
and, consequently, in a weakening of the dipoleꢀdipole
interaction between the spins of CuII of the adjacent molꢀ
ecules in the crystal lattice, which hinders the observation
of the hyperfine coupling. In toluene solutions, the hyperꢀ
fine coupling is not observed. The spectrum consists of
a single asymmetric line with g = 2.19. Apparently. the
rate of the random rotation of the bulky molecules in
toluene solutions is insufficient to average the anisotroꢀ
pic spectrum, which is observed in the spectra of free radꢀ
icals or copper complexes with small ligands characterꢀ
ized by isotropic spectra with the average parameters
gaver = 1/3g|| + g⊥, Aaver = 1/3A|| + 2/3A⊥.
To conclude, we found the route to the synthesis of the
first representative of the threeꢀcenter tautomeric systems
in the series of sterically hindered phenoxazines, which
are widely used as ligands in studies of intramolecular
redox reactions in metal complexes. A comparison of the
chemical behavior of 2,4,6,8ꢀtetra(tertꢀbutyl)phenoxazinꢀ
1ꢀone (1) and 2,4,6,8ꢀtetra(tertꢀbutyl)ꢀ9ꢀhydroxyphenoxꢀ
azinꢀ1ꢀone (12) in the complex formation with metal salts
leads to the conclusion that the presence of the hydroxy
group in position 9 of the sterically hindered phenoxazine
ring results in a change in the coordination mode from
bidentate (iminobenzosemiquinolate) for compound 1 to
tridentate (diamagnetic) for compound 12.
2,6ꢀDihydroxyꢀ3,5ꢀdi(tertꢀbutyl)aniline (11). Compound 10
(10 g, 0.042 mol) was stirred with aqueous ammonia (300 mL)
for 3 h. The precipitate that formed was filtered off, washed with
hexane, and dried under reduced pressure. White crystals of comꢀ
pound 11, which rapidly turned dark in air, were obtained, m.p.
147 °C. The yield was 9.66 g (97%). 1H NMR (CDCl3), δ:
6.95 (s, 1 H, arom.); 1.4 (s, 18 H, But). Found (%): C, 90.50;
H, 9.62; N, 5.99. C14H23NO2. Calculated (%): C, 91.10; H, 9.71;
N, 5.91.
2,4,6,8ꢀTetra(tertꢀbutyl)ꢀ9ꢀhydroxyphenoxazinꢀ1ꢀone (12).
A catalytic amount of TsOH was added to a solution of 3,5ꢀdiꢀ
(tertꢀbutyl)pyrocatechol (5) (6.3 g, 0.028 mol) and compound 11
(8 g, 0.034 mol) in benzene (200 mL). The reaction mixture was
refluxed with a Dean—Stark trap for 7 h until 0.5 mL of water
was distilled off. Then the mixture was concentrated to dryness
and chromatographed on alumina using chloroform as the eluꢀ
ent, and the violet fraction with Rf = 0.35 was collected. The
solvent was evaporated and the product was washed with meꢀ
thanol. Violet crystals of compound 12 were obtained, m.p.
1
190 °C. The yield was 4.66 g (38%). H NMR (CDCl3), δ: 7.45
(s, 2 H, arom.); 1.44 (s, 18 H, But); 1,39 (s, 18 H, But). IR,
ν/cm–1: 3430 (OH), 1640 (C=O), 1630 (C=N). Found (%):
C, 76.78; H, 8.90; N, 3.25. C28H39NO3. Calculated (%): C, 76.89;
H, 8.92; N, 3.20.
2,4,6,8ꢀTetra(tertꢀbutyl)ꢀ9ꢀphenylmercuroxyphenoxazinꢀ1ꢀ
one (13). Phenylmercury chloride (0.22 g, 0.6 mmol) and an
excess of K2CO3 were added to a solution of phenoxazinꢀ1ꢀone
12 (0.2 g, 0.4 mmol) in benzene (15 mL). The reaction mixture
was refluxed for 10 h and then chromatographed on alumina
using benzene as the eluent. The solvent was evaporated. Darkꢀ
green crystals of compound 13 were obtained, m.p. 130 °C. The
yield was 0.23 g (82%). 1H NMR (CDCl3), δ: 7.64—7.36 (m, 7 H,
arom.); 1.47 (s, 18 H, But); 1.38 (s, 18 H, But). Found (%):
C, 57.09; H, 5.97; N, 1,99. C34H43NO3Hg. Calculated (%):
C, 57.22; H, 6.03; N, 1.96.
9ꢀAcetoxyꢀ2,4,6,8ꢀtetra(tertꢀbutyl)phenoxazinꢀ1ꢀone (14).
Triethylamine (2 mL) was added to a solution of compound 12
(1 g, 0.002 mol) in acetic anhydride (20 mL). The reaction mixꢀ
ture was kept for 1 h. Then water (20 mL) was added, and the
reaction mixture was kept for 12 h. The blue precipitate that
Experimental
The UVꢀVis spectra were measured on an APEL PDꢀ303
UV spectrometer in polar (ethanol) and nonpolar (hexane) solꢀ

2,4,6,8ꢀBut4ꢀ9ꢀHOꢀphenoxazinꢀ1ꢀone
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 7, July, 2009
1369
formed was filtered off and washed with water. Blue needleꢀlike
The calculations were carried out with the use of the SHELXTL
5.10 program package.17
crystals of compound 14 were obtained, m.p. (decomp.) 151 °C.
1
The yield was 1.07 g (97%). H NMR (CDCl3), δ: 7.57 (s, 1 H,
Quantum chemical calculations were performed by the denꢀ
sity functional theory with the use of the B3LYP hybrid funcꢀ
tional and the 6ꢀ311G(d,p) basis set using the Gaussian 03W
program.18 The topological analysis of the theoretical elecꢀ
tron density distribution was performed with the MORPHY98
program.19
arom.); 7.38 (s, 1 H, arom.); 2.58 (s, 3 H, Ac); 1.49 (s, 9 H, But);
1.44 (s, 9 H, But); 1.39 (s, 9 H, But); 1.30 (s, 9 H, But). IR,
ν/cm–1: 1780 (w ether), 1640 (C=O), 1620 (C=N). Found (%):
C, 72.69; H, 8.19; N, 2.85. C30H41NO4. Calculated (%): C, 72.73;
H, 8.28; N, 2.83.
2,4,6,8ꢀTetra(tertꢀbutyl)ꢀ9ꢀmethoxyphenoxazinꢀ1ꢀone (15).
Dimethyl sulfate (0.75 mL, 0.006 mol) and an excess of dry
K2CO3 were added to a solution of compound 12 (1 g, 0.002 mol)
in acetone (20 mL). The reaction mixture was refluxed for 3 h.
Then the solvent was evaporated and the residue was dried at
room temperature. The mixture was chromatographed on aluꢀ
mina using benzene as the eluent, and the fraction with Rf = 0.55
was collected. The solvent was evaporated. Black crystals of comꢀ
pound 15 were obtained, m.p. 178 °C. The yield was 0.72 g (80%).
1H NMR (CDCl3), δ: 7.44 (s, 1 H, arom.); 7.40 (s, 1 H, arom.);
4.22 (s, 3 H, OMe); 1.49 (s, 9 H, But); 1.47 (s, 9 H, But); 1.40
(s, 9 H, But); 1.33 (s, 9 H, But). IR, ν/cm–1: 1640 (C=O), 1620
(C=N). Found (%): C, 82.35; H, 8.97; N, 3.11. C29H41NO3.
Calculated (%): C, 82.48; H, 9.09; N, 3.10.
Zinc complex of 2,4,6,8ꢀtetra(tertꢀbutyl)ꢀ9ꢀhydroxyphenoxꢀ
azinꢀ1ꢀone (16). Zinc chloride (54.4 mg, 0.4 mmol) was added
with heating to a solution of phenoxazinꢀ1ꢀone 12 (0.4 g,
0.8 mmol) in methanol. The reaction mixture was kept for 12 h.
Then the solvent was evaporated. The reaction product was twice
washed with methanol. Blueꢀgreen crystals of compound 16 were
obtained, m.p. 295 °C. The yield was 0.28 g (77%). 1H NMR
(CDCl3), δ: 7.41 (s, 2 H, arom.); 1.45 (s, 18 H, But); 1.24 (s, 18 H,
But). Found (%): C, 71.63; H, 8.05; N, 3.07. C56H76N2O6Zn.
Calculated (%): C, 71.72; H, 8.11; N, 2.99.
This study was financially supported by the Russian
Foundation for Basic Research (Project No. 06ꢀ03ꢀ32557)
and the Southern Federal University (Grant Kꢀ07ꢀTꢀ80).
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Received January 15, 2008;
in revised form November 10, 2008