Isatin acylhydrazones with sterically hindered phenolic fragments: Synthesis and structures

Article abstract of DOI:10.1007/s11172-009-0264-3

Isatin acylhydrazones with sterically hindered phenolic fragments were obtained. Their structures were determined using 1D and 2D ¹H and ¹³C NMR spectroscopy and X-ray diffraction. The products synthesized by condensation of isatin and 1-(3,5-di- tert-butyl-4-hydroxybenzyl)-l H-indole-2,3-dione with 3-(3,5-di- tert-butyl-4-hydroxyphenyl)propiono-hydrazide exist as E- and Z-isomers about the C=N bond. E- Z isomerization takes place in boiling ethanol. The E-isomer of the condensation product from isatin and 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionohydrazide was structurally characterized using X-ray diffraction: this isomer is a cis-conformer about the amide group and has a E-configuration about the N-N bond. According to data on the concentration dependence of the signals for the NH protons in the ¹H NMR spectra, the acylhydrazones obtained are stabilized in solution by intra-and intermolecular hydrogen bonding.

Full text of DOI:10.1007/s11172-009-0264-3

Russian Chemical Bulletin, International Edition, Vol. 58, No. 9, pp. 1934—1938, September, 2009
1934
Isatin acylhydrazones with sterically hindered
phenolic fragments: synthesis and structures
G. N. Nugumanova,a R. G. Tagasheva,a S. V. Bukharov,a D. B. Krivolapov,b
I. A. Litvinov,b V. V. Syakaev,b N. A. Mukmeneva,a and A. R. Burilovb
aKazan State Technological University,
68 ul. K. Marksa, 420015 Kazan, Russian Federation.
Fax: +7 (843) 231 4111. Eꢀmail: guliang1@rambler.ru
bA. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843) 272 7573
Isatin acylhydrazones with sterically hindered phenolic fragments were obtained. Their
structures were determined using 1D and 2D 1H and 13C NMR spectroscopy and Xꢀray
diffraction. The products synthesized by condensation of isatin and 1ꢀ(3,5ꢀdiꢀtertꢀbutylꢀ
4ꢀhydroxybenzyl)ꢀ1Hꢀindoleꢀ2,3ꢀdione with 3ꢀ(3,5ꢀdiꢀtertꢀbutylꢀ4ꢀhydroxyphenyl)propionoꢀ
hydrazide exist as Eꢀ and Zꢀisomers about the C=N bond. E—Z isomerization takes place
in boiling ethanol. The Eꢀisomer of the condensation product from isatin and 3ꢀ(3,5ꢀdiꢀ
tertꢀbutylꢀ4ꢀhydroxyphenyl)propionohydrazide was structurally characterized using Xꢀray
diffraction: this isomer is a cisꢀconformer about the amide group and has a Eꢀconfiguration
about the N—N bond. According to data on the concentration dependence of the signals for
the NH protons in the 1H NMR spectra, the acylhydrazones obtained are stabilized in solution
by intraꢀ and intermolecular hydrogen bonding.
Key words: isatin, isatin acylhydrazones, sterically hindered phenolic fragment, synthesis,
molecular structures, crystal structures, Xꢀray diffraction analysis, NMR spectroscopy.
Isatin acylhydrazones containing sterically hindered
ableꢀvalence metal ions. Interest in such compounds
is due to their possible use as polyfunctional polymer
stabilizers tending toward “intramolecular” synergism.1
In addition, such compounds are of considerable interest
phenolic fragments are polyfunctional compounds that
can terminate the kinetic oxidation chains by deactivating
peroxide radicals and form chelate complexes with variꢀ
Scheme 1
R = H (1, 4),
(2, 5)
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1873—1877, September, 2009.
1066ꢀ5285/09/5809ꢀ1934 © 2009 Springer Science+Business Media, Inc.

Isatin acylhydrazones
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 9, September, 2009 1935
for the study of their biological activity because both isatin
hydrazones2 and sterically hindered phenols3 find appliꢀ
cation as drug components.
In the present work, we carried out reactions of isatin
1 and 1ꢀ(3,5ꢀdiꢀtertꢀbutylꢀ4ꢀhydroxybenzyl)ꢀ1Hꢀindoleꢀ
2,3ꢀdione (2) with 3ꢀ(3,5ꢀdiꢀtertꢀbutylꢀ4ꢀhydroxyphenyl)ꢀ
propionohydrazide (3). The resulting acylhydrazones 4
and 5 contain sterically hindered phenolic fragments
(Scheme 1).
Compound 2 was prepared from isatin 1 and 3,5ꢀdiꢀ
tertꢀbutylꢀ4ꢀhydroxybenzyl acetate (6) in DMF or formic
acid (Scheme 2).
Scheme 2
Table 2. 13C NMR spectra of compounds 4 and 5 (δ)
Atom
4´
4″
5″
CDCl3 DMSOꢀd6
CDCl3
Acylhydrazone 4 crystallizes as two isomers 4´ and 4″
with different solubilities, melting points, and Rf values.
Their structures were proved using 1D and 2D H and
13C NMR spectroscopy (Tables 1 and 2). The key HMBC
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(17)
CMe3
CMe3
C(18)
C(19)
C(20)
C(21)
C(22)
CMe3
CMe3
—
—
164.6
—
162.6
132.9
120.8
123.2
131.1
111.0
120.6
140.9
175.8
34.3 (37.8)a
30.7
160.9
132.8
120.6
1
124.3
123.1
132.7
111.4
116.5b
142.5b
176.3b
34.3b
30.4b
132.0b
125.2
136.5b
153.0b
34.3
30.4
—
126.0
121.6
132.3
110.4
115.3
143.6
—
123.0 (124.4)a
130.8
correlations for compounds 4 and 5 are given below.
109.9
120.2
143.1
Table 1. 1H NMR spectra of compounds 4 and 5 in CDCl3
175.7 (170.0)a
34.5 (38.0)a
30.7
Atom
δ
—
30.4
131.7
124.3
139.1
152.0
34.4
30.4
—
—
—
—
—
4´
4″
5″
131.5
124.9
136.1
152.2
34.3
30.3
—
—
—
—
—
—
—
131.7
125.0
136.0
152.2
34.4
30.4
43.8
125.9
124.7
136.4
153.6
34.3
H(1)
7.83
7.62
7.12
7.39
6.94
9.36
3.19
2.98
7.06
5.05
1.43
—
8.13 (7.90)
7.60 (7.80)
—
7.61 (7.85)
7.10
7.34
6.92
12.59 (13.14)
3.14 (2.74)
2.99
H(4)
H(5)
H(6)
H(7)
H(10)
H(12)
H(13)
H(15)
ОH
CMe3
H(18)
H(20)
OH
7.12
7.35
6.94
12.43 (12.95)
3.16 (2.74)
—
—
—
—
—
—
3.01
7.11
5.05
1.47
—
7.12
5.08
1.45
4.83
7.18
5.20
—
—
30.22
—
—
—
—
a Hereafter, the signals for the minor form are enclosed in paꢀ
rentheses.
b The δ values are determined from the positions of the cross
peaks in the HMBC spectra.
CMe3
—
—
1.40
Note. The signals for the minor form are enclosed in parentheses.

1936
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 9, September, 2009
Nugumanova et al.
Aldehyde acylhydrazones are known4 to form difꢀ
ferent spatial structures due to cis—trans isomerism of the
amide group, hindered rotation about the N—N bond,
and E—Z isomerism at the C=N bond. The ZN—N conforꢀ
mation of hydrazones is usually unfeasible for steric hinꢀ
drances that make the fragment C=N—NH nonplanar
and, accordingly, violate the n—πꢀconjugation.5,6 Thereꢀ
fore, the following four spatial structures can be expected
for compounds 4 and 5.
Table 3. Selected bond lengths d and bond angles ω in the crystal
structure 4´ according to Xꢀray diffraction data
Bond
d/Å
Angle
ω/deg
N(1)—C(2)
N(1)—C(9)
O(2)—C(2)
N(11)—N(10) 1.348(2)
N(11)—C(12) 1.378(2)
1.358(3)
N(10)—N(11)—C(12) 119.4(2)
1.395(3) C(3)—N(10)—N(11) 119.7(2)
1.226(2)
N(10)—C(3)—C(4)
N(10)—C(3)—C(2)
C(4)—C(3)—C(2)
137.1(2)
116.2(2)
106.6(2)
N(10)—C(3)
C(3)—C(4)
1.294(2)
O(12)—C(12)—N(11) 118.4(2)
1.462(3) O(12)—C(12)—C(13) 123.4(2)
O(18)—C(18) 1.386(2)
N(11)—C(12)—C(13) 118.3(2)
C(4)—C(9)
1.407(3) C(2)—N(1)—C(9) 111.6(2)
C(12)—C(13) 1.503(3)
O(2)—C(2)—N(1)
O(2)—C(2)—C(3)
N(1)—C(2)—C(3)
C(8)—C(9)—N(1)
N(1)—C(9)—C(4)
125.8(2)
128.3(2)
105.9(2)
127.2(2)
110.1(2)
with the heterocycle (the deviation from its plane is
0.023(2) Å). Thus, the hydrazine fragment is coplanar
with the isatin imine system, which can be attributed to
the conjugation of the C=N bond, the lone electron pair
of the N(11) atom, and the C=O bond. The geometrical
parameters of structure 4´ have standard values. Selected
bond lengths and bond angles are given in Table 3.
Molecular packing in the crystal is primarily due to interꢀ
molecular hydrogen bonds of the type NH...O forming
infinite zigzag chains along the principal diagonal
(Fig. 2). The parameters of the hydrogen bonds are as
follows. The bond N(1)—H(1)...O(2)´ (3 – x, 2 – y, 1 – z):
N(1)—H(1), 0.86(2) Å; H(1)...O(2)´, 1.99(2) Å; N(1)...O(2)´,
2.857(2) Å; angle N(1)—H(1)...O(2)´, 179(2)°. The
bond N(11)—H(11)...O(12)″ (1 – x, 1 – y, 1 – z):
N(11)—H(11), 0.90(2) Å; H(11)...O(12)″, 2.19(2) Å;
N(11)...O(12)″, 3.034(2) Å; angle N(11)—H(11)...O(12)″,
158.0(17)°.
According to Xꢀray diffraction data (Fig. 1), isomer 4´
is cis,E with the Eꢀconfiguration about the N—N bond.
In its molecule, the isatin imine and phenol rings are
linked by the bridging fragment CH2CH2C(O)NHN,
which is in the trans,trans,cis,transꢀconformation with the
torsion angles τ listed below.
Angle
τ/deg
C(12)—C(13)—C(14)—C(15)
N(11)—C(12)—C(13)—C(14)
N(10)—N(11)—C(12)—C(13)
C(3)—N(10)—N(11)—C(12)
170.3(2)
176.5(2)
2.6(2)
We believe that isomer 4″ is Z with respect to the
C=N bond. Intramolecular hydrogen bonding in this
178.7(2)
The isatin imine and phenol rings are virtually orthogoꢀ
nal: the dihedral angle between their planes is 87.2(5)°.
The heterocycle N(1)C(2)C(3)C(4)C(9) is planar to
within 0.002(2) Å; the exocyclic N(11) atom is coplanar
C(21)
O(2)
C(2)
C(16)
C(15)
C(13)
C(17)
N(1)
C(6)
O(18)
C(18)
C(3)
N(10)
N(11)
C(8)
C(7)
C(4)
C(5)
C(14)
C(20)
C(12)
O(12)
C(25)
Fig. 1. ORTEP diagram of structure 4´ in the crystal with anisoꢀ
tropic thermal displacement ellipsoids (50% probability).
Fig. 2. Intermolecular hydrogen bonds in the crystal strucꢀ
ture 4´.

Isatin acylhydrazones
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 9, September, 2009 1937
isomer results in a strong downfield shift of the signal
for the NH proton of the acylhydrazone fragment7 (see
Table 1). The H NMR spectrum of isomer 4″ shows a
concentrationꢀcaused change in the chemical shift of the
signal for the H(10) proton in Eꢀisomer 5´.
It should be noted that variation in the concentrations
of Zꢀisomers 4″ and 5″ does not change the ratio of their
cisꢀ and transꢀforms.
1
double set of signals for some protons with an intensity
ratio of 3.3 : 1. Since, according to Refs 7 and 8, the
signals for the protons of the CH2C(O) fragment in
the cisꢀconformer are shifted downfield by ~0.5 ppm
compared to analogous signals for the transꢀconformer,
we assigned the more intense signals to cis,Zꢀ4″ and the
minor signals to trans,Zꢀ4″. This assignment is confirmed
by the δC value of the carbonyl C atom of the fragment
C(O)NHN=. This signal for the cisꢀconformers of acylꢀ
hydrazones is shifted downfield by 5—6 ppm compared
to an analogous signal for the transꢀconformers7,9 and
appears at δ 170 for the acylhydrazones described in
Ref. 7. Therefore, acylhydrazone 4″ is a 3.3 : 1 mixture
of the cis,Zꢀ and trans,Zꢀisomers.
Compound 5 also consists of Eꢀ and Zꢀisomers about
the C=N bond in a nearly equal proportion. ZꢀIsomer 5″
was isolated and characterized in the individual state (see
Tables 1 and 2). In turn, this isomer is a 2.7 : 1 mixture of
cisꢀ and transꢀisomers with respect to the amide bond. In
boiling ethanol, a mixture of the Eꢀ and Zꢀisomers of
compound 5 or Eꢀisomer 4´ isomerize into Zꢀisomers 5″
or 4″, respectively.
The fact that the positions of the signals for the N(10)H
protons in the 1H NMR spectra of the compounds obtained
are insensitive to their concentrations (Table 4) suggests
the presence of the aforesaid intramolecular hydrogen
bonds in structures 4″ and 5″ and intermolecular hydroꢀ
gen bonds involving the H(1) proton in structures 4´ (also
detected in its crystal, see Fig. 2) and 4″. One can assume
that intermolecular hydrogen bonding through the H(10)
proton (also detected in the crystal structure 4´) occurs
in more concentrated solutions and is undetectable for
1H NMR spectroscopy because of the low solubility of
this compound. This is evident from an appreciable
Experimental
1H and 13C NMR spectra were recorded on a Bruker
Avanceꢀ600 instrument (600 (1H) and 150.9 MHz (13C)).
Signals for the residual protons of deuterated solvents were used
as standards.
1ꢀ(3,5ꢀDiꢀtertꢀbutylꢀ4ꢀhydroxybenzyl)ꢀ1Hꢀindoleꢀ2,3ꢀdione
(2). A. A solution of isatin 1 (1.47 g, 0.01 mol) and acetate 6
(2.78 g, 0.01 mol) in DMF (25 mL) was stirred at 70 °C for 3 h.
The reaction mixture was cooled to room temperature. The
precipitate that formed was filtered off, washed with water, and
dried in air to a constant weight. The yield was 1.32 g (36.3%),
orange crystals, m.p. 245—246 °C (from acetone). An additional
crop of compound 2 (1.91 g, 52.4%) was isolated from the mother
liquor. 1H NMR (CDCl3), δ: 1.40 (s, 18 H, CMe3); 4.81 (s, 2 H,
CH2); 5.23 (s, 1 H, OH); 6.91 (d, 1 H, H(7), 3J = 7.5 Hz);
7.09 (t, 1 H, H(5), 3J = 7.2 Hz); 7.16 (s, 2 H, HAr); 7.55 (t, 1 H,
H(6), 3J = 7.5 Hz); 7.59 (d, 1 H, H(4), 3J = 7.2 Hz). 13C NMR
(CDCl3), δ: 30.45 (CMe3), 34.54 (CMe3), 44.61 (CH2), 111.18,
118.02, 123.83, 124.90, 125.51, 125.61, 136.86, 138.33, 151.45,
153.89 (CAr), 158.50 (NC=O), 183.74 (C=O). Found (%):
C, 75.99; H, 7.57; N, 3.54. C23H27NO3. Calculated (%):
C, 75.62; H, 7.40; N, 3.82.
B. A solution of isatin 1 (1.47 g, 0.01 mol) and acetate 6
(2.78 g, 0.01 mol) in formic acid (15 mL) and acetone (15 mL)
was stirred at 70 °C for 20 h. The reaction mixture was cooled
to room temperature. The precipitate that formed was filtered
off, washed with water, and dried in air to a constant weight.
The yield was 1.47 g (40.3%), orange crystals. A red resinous
product (1.6 g, 43.8%) was isolated from the mother liquor. The
content of compound 2 in the product was 20% (1H NMR).
N´ꢀ(2ꢀOxoꢀ1,2ꢀdihydroindolꢀ3ꢀylidene)ꢀ3ꢀ(3,5ꢀdiꢀtertꢀbutylꢀ
4ꢀhydroxyphenyl)propionohydrazide (4). A solution of isatin 1
(1.47 g, 0.01 mol) and hydrazide 3 (2.92 g, 0.01 mol) in EtOH
(50 mL) was refluxed for 4 h. The reaction mixture was cooled
to room temperature. The precipitate that formed was filtered
off, washed with ethanol, and dried in air to a constant weight.
The yield of compound 4´ was 2.56 g (60.8%), yellow crystals,
m.p. 214—216 °C (from MeCN). Found (%): C, 71.58; H, 7.60;
N, 9.64. C25H31N3O3. Calculated (%): C, 71.26; H, 7.36; N, 9.98.
Compound 4″ was isolated from the mother liquor. The
yield was 0.7 g (16.8%), yellow solid, m.p. 223—224 °C (from
MeCN). Found (%): C, 71.44; H, 7.58; N, 9.68. C25H31N3O3.
Calculated (%): C, 71.26; H, 7.36; N, 9.98.
Table 4. Positions of the signals for the NH protons in the
1H NMR spectra of compounds 4 and 5 in CDCl3
Comꢀ
pound
C/mol L–1
δ
H(1)
H(10)
4´
1.1•10–3
4.1•10–3
2.0•10–3
5.0•10–2
2.0•10–3
5.0•10–2
3.1•10–3
8.1•10–2
3.1•10–3
8.1•10–2
3.1•10–3
8.1•10–2
7.60
7.83
7.53
8.40
7.53
8.12
—
—
—
—
—
9.37
9.35
cisꢀ4″
transꢀ4″
5´
12.42
12.41
12.93
12.95
9.35
N´ꢀ[1ꢀ(3,5ꢀDiꢀtertꢀbutylꢀ4ꢀhydroxybenzyl)ꢀ2ꢀoxoꢀ1,2ꢀdihydroꢀ
indolꢀ(3E)ꢀylidene]ꢀ3ꢀ(3,5ꢀdiꢀtertꢀbutylꢀ4ꢀhydroxyphenyl)ꢀ
propionohydrazide (5). A solution of compound 2 (3.65 g,
0.01 mol), hydrazide 3 (2.92 g, 0.01 mol), and AcOH (2 mL) in
EtOH (50 mL) was refluxed for 3 h. The reaction mixture was
cooled to room temperature. The precipitate that formed
was filtered off, washed with ethanol, and dried in air to a
constant weight. The yield of compound 5 was 5.64 g (88.2%),
yellow crystals (a mixture of Eꢀ and Zꢀisomers).
9.50
cisꢀ5″
transꢀ5″
12.56
12.56
13.11
13.11
—

1938
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 9, September, 2009
Nugumanova et al.
Table 5. Crystallographic parameters and the data collection
and refinement statistics for structure 4´
performed with the WinGX12 and APEX2 programs.13 The
diagram was drawn with the ORTEP3 program.14 The hydrogen
bonds were drawn and analyzed with the PLATON program.15
Parameter
Value
References
Crystal color
Crystal shape
Molecular formula
Molecular weight
Crystal system
Space group
a/Å
b/Å
c/Å
α/deg
β/deg
Transparent, colorless
Prisms
1. N. M. Emanuel´, A. L. Buchachenko, Khimicheskaya fizika
molekulyarnogo razrusheniya i stabilizatsii polimerov [The
Chemical Physics of the Degradation and Stabilization of
Polymer Molecules], Nauka, Moscow, 1988, 258 pp. (in
Russian).
2. G. I. Zhungietu, M. A. Rekhter, Izatin i ego proizvodnye
[Isatin and Its Derivatives], Shtiintsa, Chisinau, 1977, 225
pp. (in Russian).
3. V. V. Ershov, G. A. Nikiforov, A. A. Volod´kin, Prostranstvenno
zatrudnennye fenoly [Sterically Hindered Phenols], Khimiya,
Moscow, 1972, 328 pp. (in Russian).
4. S. A. Flegontov, Z. S. Titova, B. I. Buzykin, Yu. P. Kitaev,
Izv. Akad. Nauk SSSR, Ser. Khim., 1976, 559 [Bull. Acad.
Sci. USSR, Div. Chem. Sci. (Engl. Transl.), 1976, 25, 541].
5. I. A. Litvinov, O. N. Kataeva, L. V. Ermolaeva, G. A. Vagina,
T. V. Troepol´skaya, V. A. Naumov, Izv. Akad. Nauk SSSR,
Ser. Khim., 1991, 75 [Bull. Acad. Sci. USSR, Div. Chem. Sci.
(Engl. Transl.), 1991, 40, 62].
C25H31N3O3
421.53
Triclinic
P–1
5.7302(10)
9.6897(17)
21.076(4)
86.742(2)
85.999(2)
87.418(2)
1164.5(4)
2
γ/deg
V/Å3
Z
dcalc/g cm–3
1.202
0.80
452
μ/cm–1
F(000)
θ range/deg
Rint
1.94 ≤ θ ≤ 26.00
0.0447
–7 ≤ h ≤ 7,
–11 ≤ k ≤ 11,
–25 ≤ l ≤ 25
11451/4490
Ranges of h, k, l indices
6. S. A. Flegontov, Z. S. Titova, A. P. Stolyarov, B. I. Buzykin,
Yu. P. Kitaev, Izv. Akad. Nauk SSSR, Ser. Khim., 1979, 1014
[Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.), 1979,
28, 948].
Number of measured/
independent reflections
7. V. V. Syakaev, S. N. Podyachev, B. I. Buzykin, S. K. Latypov,
W. D. Habicher, A. I. Konovalov, J. Mol. Struct., 2006,
788, 55.
Number of reflections
with I > 2σ(I)
R
Rw
Rall
Rw,all
GOOF
Δ/σ
2851
0.0467
0.0975
0.0885
0.1141
1.022
0.00
8. U. Himmelreich, F. Tschwatschal, R. Borsdorf, Monatsh.
Chem., 1993, 124, 1041.
9. J. B. Stothers, 13CꢀNMR Spectroscopy, Academic Press,
New York, 1972, 559 pp.
10. A. Altomare, G. Cascarano, C. Giacovazzo, D. Viterbo,
Acta Crystallogr., Sect. A, 1991, 47, 744.
Number of parameters refined
404
11. G. M. Sheldrick, SHELXLꢀ97, Program for the Crystal
Structure Refinement, Göttingen University, Göttingen
(Germany), 1997.
12. L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837.
13. APEX2 (Version 2.1), SAINTPlus. Data Reduction and
Correction Program (Version 7.31A), Bruker AXS Inc.,
Madison, Wisconsin (USA), 2006.
Compound 5″ was isolated from the mother liquor. The
yield was 0.07 g (1.1%), light yellow crystals, m.p. 188—190 °C
(from ethanol). Found (%): C, 71.44; H, 7.58; N, 9.68.
C25H31N3O3. Calculated (%): C, 71.26; H, 7.36; N, 9.98.
Xꢀray diffraction analysis was carried out on a Bruker Smart
APEX2 automatic diffractometer (graphite monochromator,
λ(MoꢀKα), ω scan mode, 293 K). Crystallographic parameters
and the data collection and refinement statistics for structure 4´
are summarized in Table 5. Absorption correction was not
applied. The structures were solved by the direct methods with
the SIR program10 and refined first isotropically and then anisoꢀ
tropically with the SHELXLꢀ97 program.11 The coordinates of
the hydrogen atoms were determined from difference electronꢀ
density maps and refined isotropically. All calculations were
14. L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
15. A. L. Spek, Acta Crystallogr., Sect. A, 1990, 46, 34.
Received October 30, 2008