Influence of different aryl substitution on the crystal structures of benzil monohydrazone and dibenzil azine parent compounds

Article abstract of DOI:10.1007/s11224-011-9817-9

A series of benzil monohydrazones (1a, 1d) and corresponding dibenzil azines (2a-2d) featuring different p-aryl substituents have been synthesized and studied in regard of their crystal structures. Configurational and conformational properties as well as non-covalent interaction and packing behaviour of the molecules connected with the different substitution are discussed. The dibenzil azine 2a was found as a polymorph with reference to a known crystal structure.

Full text of DOI:10.1007/s11224-011-9817-9

Struct Chem (2011) 22:1267–1279
DOI 10.1007/s11224-011-9817-9
ORIGINAL RESEARCH
Influence of different aryl substitution on the crystal structures
of benzil monohydrazone and dibenzil azine parent compounds
•
Marcel Wieland Wilhelm Seichter
•
•
Anke Schwarzer Edwin Weber
Received: 10 February 2011 / Accepted: 31 May 2011 / Published online: 21 June 2011
Ó Springer Science+Business Media, LLC 2011
Abstract A series of benzil monohydrazones (1a, 1d)
and corresponding dibenzil azines (2a–2d) featuring dif-
ferent p-aryl substituents have been synthesized and stud-
ied in regard of their crystal structures. Configurational and
conformational properties as well as non-covalent interac-
tion and packing behaviour of the molecules connected
with the different substitution are discussed. The dibenzil
azine 2a was found as a polymorph with reference to a
known crystal structure.
and identification of carbonyls or other colorimetric sensing
[10]. Similarly, systems containing two adjacent hydrazone
units, called osazones [11], are particular important in car-
bohydrate chemistry. Hydrazones have also been found
physiologically active showing antiviral [12], antibacterial
[13], fungicidal [14] and anticarcinogenic [15] behaviour.
Compared with the hydrazones, the azines are less useful in
chemical synthesis [2, 16] but show useful properties as
fluorescent optical materials including sensor [17] and pig-
ment applications [18]. A typical example is the commercial
pigment Yellow 101 (P.Y.101) which is broadly used for
dyeing of viscose and as pigment for printing and painting
[18]. A recent study proved that in the solid P.Y.101 and its
derivatives a specific mode of hydrogen bond interaction
seems to be essential for the fluorescence [19]. Moreover,
coordination polymers based on the Agꢁ complexation both
of hydrazones and azines have demonstrated a promising
approach to the design of hybrid organic–inorganic materials
[20].
Keywords Benzil monohydrazones ꢀ Dibenzil azines ꢀ
X-ray analysis ꢀ Conformational behaviour ꢀ
Supramolecular interactions
Introduction
Hydrazones [1] and azines [2] are long-known nitrogen
containing derivatives of carbonyl compounds. They are
produced from condensation of hydrazine and aldehydes or
ketones with either one or a second mole of the carbonyl
compound, respectively [3, 4]. In particular the hydrazones
are much-used in chemical synthesis, e.g. as an intermediate
for the Wolff-Kishner reduction [5, 6] or the stereoselective
alkylation [7] of a carbonyl compound. Arylhydrazones [8],
especially 2,4-dinitrophenylhydrazones [9], are usually
solids and are commonly employed for analytical separation
However, aside from few monohydrazones of aliphatic
1,2-diketones [21, 22] crystal structures of corresponding
monohydrazones of simple benzils are not described in the
literature. Also, crystal structures of aliphatic bis(1,2-
diketone) azines have only rarely been studied, mostly with
reference to their conformational behaviour [23, 24], and
only one single structure of a corresponding dibenzil azine
is demonstrated in the recent literature [25], which is a
polymorph of one of the present structures. With a view of
the interaction motifs expected of the functional subunits
typical of compounds pertinent to the benzil monohyd-
razone and dibenzil azine substance classes for the poten-
tial employment in crystal engineering, we have studied
and present here crystal structures of respective com-
pounds. They include the benzil monohydrazones 1a and
1d as well as the dibenzil azines 2a–2d (Scheme 1)
M. Wieland ꢀ W. Seichter ꢀ A. Schwarzer ꢀ E. Weber (&)
¨
Institut fu¨r Organische Chemie, Technische Universitat
Bergakademie Freiberg, Leipziger Str. 29, 09596 Freiberg/
Sachs, Germany
e-mail: edwin.weber@chemie.tu-freiberg.de
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Struct Chem (2011) 22:1267–1279
Synthesis
Benzil monohydrazones 1a–1d. General procedure [30]
Into a saturated hot solution of the respective benzil
(25 mmol) in dry ethanol was slowly dropped hydrazine
hydrate (80 p.c., 1.62 g, 25.5 mmol). The mixture was
heated under reflux for 5 min and then cooled to 0 °C. The
precipitate which has formed was separated, washed twice
with little icy-cold ethanol and dried. Specific data for the
compounds are given below.
Benzil monohydrazone (1a): 89% colourless crystals,
m.p. 151 °C (Ref. [31] 148–150 °C).
4,40-Dimethylbenzil monohydrazone (1b): 93% colour-
less crystals, m.p. 140 °C (Ref. [32] 139–140 °C).
4,40-Difluorobenzil monohydrazone (1c): 78% colour-
less crystals (plates), m.p. 194–196 °C (Ref. [33]
190–191 °C).
4,40-Dichlorobenzil monohydrazone (1d): 75% colourless
crystals (needles), m.p. 149 °C (Ref. [34] 149.5–151.5 °C).
Dibenzil azines 2a and 2b
The corresponding benzils 1a and 2a (20 mmol) were
dissolved under heating in aqueous ethanol (80 p.c.,
50 mL). Hydrazine hydrochloride (wetted with little water,
0.8 g, 10 mmol) was added and the reaction mixture was
heated to reflux for 30 min. The yellow precipitate which
has formed was collected and crystallized from toluene.
Specific data for the compounds are given below.
Scheme 1 Chemical structures of the compounds studied in this
article
featuring different substitution in the p-position of the aryl
rings. It is also a point in question, how will these sub-
stituents influence the crystal structures of the parent
compounds.
Dibenzil azine (2a): 90% bright yellow crystals, m.p.
200 °C (Ref. [35] 201–202 °C).
4,40-Dimethylbenzil azine (2b): 86% deep yellow crys-
tals, m.p. 258–259 °C. Anal. Calc. (%) for C32H28N2O2
(472.58): C, 81.33; H, 5.97: N, 5.93. Found (%): C, 81.17;
H, 6.03; N, 5.89; m/z (GC–MS): 472 [M]?. IR (KBr): 3033,
2919, 2857, 1677, 1606, 1574, 1513, 1449, 1414, 1328,
1299, 1228, 1210, 1181, 1121, 1056, 1021, 902, 828, 789,
726, 693, 498 cm-1. 1H NMR (CDCl3): 2.32 (s, 6H, CH3),
2.40 (s, 6H, CH3), 7.04 (d, 3JHH = 8.6 Hz, 4H, ArH), 7.19
(d, 3JHH = 8.6 Hz, 4H, ArH), 7.39 (d, 3JHH = 8.7 Hz, 4H,
ArH), 7.84 (d, 3JHH = 8.7 Hz, 4H, ArH). 13C NMR = 21.5
(CH3), 21.8 (CH3), 128.1, 129.3, 129.6, 129.8, 133.4,
142.0, 144.9 (Ar), 166.6 (C=N), 197.3 (C=O).
Experimental
General remarks
Melting points (uncorrected) were measured on a hot-stage
microscope PHMK (Rapido Dresden). IR spectra were
determined on a Nicolet 510 FT-IR spectrometer. NMR
spectra were recorded on a Bruker DPX 400 spectrometer
(1H 400, 13C 100, 19F 376 MHz) in solutions using TMS as
internal standard. MS spectra were obtained using a Hewlett-
Packard MS 5989A (GC–MS) instrument. Solvents were
purified and dried using standard laboratory procedures. The
benzils as starting compounds (benzil [26], 4,40-dimethyl-
benzil [27], 4,40-difluorobenzil [28], 4,40-dichlorobenzil
[29]) for the synthesis of the monohydrazones (1a–1d) and
dibenzil azines (2a–2d) were prepared according to the lit-
erature procedures.
Dibenzil azines 2c and 2d
To a stirred solution of the corresponding benzil mono-
hydrazone 1c and 1d (10 mmol) in dichloromethane
(100 mL) was added copper(II) acetate (0.4 g, 2 mmol).
The mixture was stirred for 30–50 min under admission of
air to cause a colour change from pale blue to yellow. After
filtration of the mixture, the filtrate was evaporated and the
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Struct Chem (2011) 22:1267–1279
1269
remaining solid crystallized. Specific data for the com-
pounds are given below.
procedure [30]. The dibenzil azines 2a and 2b were syn-
thesized by condensation of a 2:1 mixture of the corre-
sponding benzils with hydrazine hydrochloride. The
dibenzil azines 2c and 2d were prepared from the benzil
monohydrazones 1c and 1d, respectively, by oxidation with
copper(II) acetate.
4,40-Difluorobenzil azine (2c): Crystallization from
acetone yielded 87% yellow crystals, m.p. 225 °C. Anal.
Calc. (%) for C28H16F4N2O2 (488.43): C, 68.85; H, 3.30;
N, 5.74. Found (%): C, 68.79; H, 3.37; N, 5.73; m/z (GC–
MS) 488 [M]?. IR (KBr): 3072, 1682, 1597, 1505, 1413,
1318, 1230, 1157, 903, 850, 805, 755, 736, 606, 508 cm-1
.
=
=
Structural study
3
3
1H NMR (CDCl3): 6.96, 6.98 (2d, JHH = 9.0 Hz, JFH
3
8.4 Hz, 4H, ArH), 7.18, 7.20 (2d, JHH = 8.9 Hz, JFH
3
Crystal structures of the benzil monohydrazones 1a and 1d,
of dibenzil azine (2a) and its substituted derivatives 2b–2d
have been determined by X-ray diffraction. Crystal data
and details of the refinement of the structures are summa-
rized in Table 1. The conformation of the molecules in
their solid state structures can be described by the torsion
angles around the a-ketohydrazone (1a, 1d) or the
bis(ketazine) fragment (2a–2d) as well as the dihedral
angle between the planes of the aromatic rings, designated
as A and B in the illustrations of the molecular structures.
These geometric parameters together with selected bond
lengths are listed in Table 2. Information regarding non-
covalent interactions is given in Table 3. ORTEP plots of
the molecular structures including atom labelling are
shown in Figs. 1, 4, 6 and 9, while illustrations of the
packing structures are presented in Figs. 2, 3, 5, 7, 8
and 10.
3
8.3 Hz, 4H, ArH), 7.48, 7.49 (2d, JHH = 9.0 Hz,
3
4JFH = 5.3 Hz, 4H, ArH), 7.95, 7.96 (2d, JHH = 9.0 Hz,
4JFH = 5.3 Hz, 4H, ArH). 13C NMR (CDCl3): 116.0 (d,
2JFC = 22.1 Hz, Ar), 116.4 (d, 2JFC = 22.6 Hz, Ar), 128.0
4
(d, JFC = 2.8 Hz, Ar), 130.1 (d, JFC = 9.2 Hz, Ar),
3
4
131.6 (d, JFC = 2.7 Hz, Ar), 131.8 (d, JFC = 9.5 Hz,
3
1
Ar), 164.6 (d, JFC = 174.2 Hz, Ar), 165.6 (C=N), 166.6
(d, JFC = 177.5 Hz, Ar), 195.2 (C=O). 19F NMR
1
(CDCl3): -102.9, -106.9.
4,40-Dichlorobenzil azine (2d): Crystallization from
toluene yielded 76% yellowish orange crystals, m.p.
191 °C. Anal. Calc. (%) for C28H16Cl4N2O2 (554.52): C,
60.68; H, 2.91; N, 5.05. Found (%): C, 60.54; H, 2.85; N,
4.92; m/z (GC–MS) 554 [M]?. IR (KBr): 3088, 1676, 1587,
1486, 1401, 1318, 1220, 1176, 1093, 1013, 900, 840, 758,
739, 489 cm-1. 1H NMR (CDCl3): 7.26 (d, 3JHH = 8.8 Hz,
3
4H, ArH), 7.40 (d, JHH = 8.7 Hz, 4H, ArH), 7.49 (d,
3
3JHH = 8.7 Hz, 4H, ArH), 7.84 (d, JHH = 8.6 Hz, 4H,
Crystal structures of the benzil monohydrazones 1a and 1d
ArH). 13C NMR (CDCl3): 129.2, 129.3, 129.6, 130.1,
130.5, 133.5, 138.6, 141.1 (Ar), 166.0 (C=N), 195.5 (C=O).
The benzil monohydrazone 1a crystallizes in the mono-
clinic space group P21/c with one molecule in the asym-
metric part of the unit cell. A perspective view of the
molecular structure is presented in Fig. 1a, showing
E-configuration around the C=N bond. Not only the C=N
X-ray crystallography
The intensity data of 1a, 1d and 2a–2d were collected on a
Bruker APEX II diffractometer with Mo Ka radiation
˚
[1.307(1) A], N–N [1.321(1) A] and C=O bond lengths
˚
˚
˚
[1.235(1) A] are within the range observed for other dione
(k = 0.71073 A) using x- and /-scans. Reflections were
corrected for background, Lorentz and polarization effects.
Preliminary structure models were derived by application
of direct methods [36] and were refined by full-matrix
least-squares calculation based on F2 for all reflections
[36]. With the exception of the amino hydrogens H(2A)
and H(2B) in 1a und 1d, all other hydrogen atoms were
included in the models in calculated positions and were
refined as constrained to bonding atoms.
monohydrazones [21, 22] but also the C(7)–C(8) bond
˚
distance of 1.467(1) A, being approximately the value
expected for a C(sp2)–C(sp2) single bond. As depicted in
Fig. 1a, the carbonyl and hydrazone groups adopt a tran-
soid conformation with a torsion angle of 172.9(1)° for the
atomic sequence O(1)–C(7)–C(8)–N(1), which indicates
approximate planarity of the a-ketohydrazone moiety. The
carbonyl and hydrazone group are inclined at angles of
40.9(1)° and 54.2(1)° with respect to the aromatic rings to
which they are attached. The dihedral angle between the
phenyl rings is 79.5(1)°. The crystal structure of 1a (Fig. 2)
is constructed of supramolecular strands extending along
the crystallographic a-axis. Within a given strand, one of
the amino hydrogens is connected with the carbonyl oxy-
Results and discussion
Synthesis
˚
The benzil monohydrazones 1a–1d were prepared by
simple 1:1 condensation reaction of the corresponding
benzils with hydrazine hydrate following the standard
gen of a consecutive molecule [N(2)–H(2A)ꢀꢀꢀO(1) 1.91 A,
174°], whereas the second amino hydrogen H(1A) is
excluded from non-covalent bonding. Interstrand
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Struct Chem (2011) 22:1267–1279
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Struct Chem (2011) 22:1267–1279
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Table 2 Relevant structural parameters of the compounds studied
Compound
1a
1b
2a
2b
2c
2d
Interplanar anglesa
A/B
A0/B0
79.5(1)
63.8(1)
88.1(1)
82.1(1)
86.3(1)
84.1(1)
86.0(1)
68.7(1)
Bond lengths
C(1)–C(7)
1.501(2)
1.496(1)
1.235(1)
1.307(1)
1.321(1)
1.499(2)
1.498(2)
1.237(2)
1.302(2)
1.324(2)
1.476(2)
1.467(2)
1.219(2)
1.291(2)
1.477(3)
1.471(2)
1.225(2)
1.292(2)
1.479(2)
1.474(2)
1.219(1)
1.289(1)
1.487(2)
1.470(2)
1.215(2)
1.292(2)
C(8)–C(9)
C(7)–O(1)
C(8)–N(1)
N(1)–N(2)
N(1)–N(1)
1.414(2)
1.523(2)
1.484(2)
1.466(2)
1.219(2)
1.292(2)
1.425(3)
1.524(2)
1.423(2)
1.522(2)
1.405(2)
1.530(2)
1.489(2)
1.469(2)
1.215(2)
1.289(2)
C(7)–C(8)
1.467(2)
1.468(2)
C(1A)–C(7A)
C(8A)–C(9A)
C(7A)–O(1A)
C(8A)–N(1A)
N(1A)–N(2A)
N(1A)–N(1A)
1.415(2)
1.520(2)
1.407(2)
1.527(2)
C(7A)–C(8A)
Torsion angles
C(2)–C(1)–C(7)–O(1)
C(10)–C(9)–C(8)–N(1)
O(1)–C(7)–C(8)–N(1)
C(2)–C(1)–C(7)–C(8)
C(1)–C(7)–C(8)–C(9)
C(2A)–C(1A)–C(7A)–O(1A)
C(10A)–C(9A)–C(8A)–N(1A)
O(1A)–C(1A)–C(8A)–N(1A)
C(2A)–C(1A)–C(7A)–C(8A)
C(1A)–C(7A)–C(8A)–C(9A)
a
-39.5(1)
-53.6(2)
172.9(1)
138.9(1)
177.8(1)
-43.7(2)
-64.0(2)
174.3(2)
-1.6(2)
5.6(2)
1.9(3)
8.6(2)
-1.5(2)
-12.7(2)
94.7(1)
-13.1(2)
1.8(2)
87.4(2)
-179.1(1)
86.8(2)
-19.5(2)
2.4(2)
-101.5(2)
-179.3(2)
-102.1(2)
100.3(2)
167.8(1)
101.7(1)
-0.1(2)
1.5(2)
133.4(2)
-178.5(1)
94.0(2)
-177.6(2)
90.0(2)
161.0(1)
90.9(2)
104.6(2)
-179.6(1)
109.3(1)
Means angle between the best plane through atoms of the rings. Ring A: C(1)ꢀꢀꢀC(6), ring B: C(9)ꢀꢀꢀC(14), ring A0: C(1A)ꢀꢀꢀC(6A), ring B0:
C(9A)ꢀꢀꢀC(14A)
association is accomplished by weak C–HꢀꢀꢀO hydrogen
to the bound aromatic rings A and B, respectively. The
crystal structure of 1d is stabilized by a variety of inter-
molecular interactions of different strength (Fig. 3). In a
similar fashion as in 1a, the carbonyl oxygen is hydrogen
bonded to one of the amino hydrogens of an adjacent
˚
bonding [37] C(14)–H(14)ꢀꢀꢀO(1) 2.63 A, 172°] and
˚
C–Hꢀꢀꢀp contacts [38] [C(10)–H(10)ꢀꢀꢀcentroid(A) 2.81 A,
139°].
4,40-Dichlorobenzil monohydrazone (1d), the molecular
structure of which also demonstrates E-configuration
around the C=N bond (Fig. 1b), crystallizes in the mono-
clinic space group P21/n with Z = 4. Taking into account
experimental error, the bond lengths are identical with
those of 1a. The presence of chloro substituents hardly
influences the molecular conformation (see Table 2). The
a-ketohydrazone part of the molecule is approximately
planar with maximum atomic distances of -0.038(1) and
˚
molecule [N(2)–H(2A)ꢀꢀꢀO(1) 2.03 A, 153°], resulting in
the formation of infinite molecular strands running along
the crystallographic b-axis. Moreover, the oxygen is
involved in interstrand connection via C–HꢀꢀꢀO bonding
˚
[37] [C(5)–H(5)ꢀꢀꢀO(1) 2.54 A, 169°]. As is obvious from
the packing diagram, the chlorine atom Cl(2) acts as a
bifurcated site by forming a weak N–HꢀꢀꢀCl hydrogen bond
˚
[39] with the amino hydrogen H(2A) [N–HꢀꢀꢀCl 2.85 A,
˚
0.060(1) A for O(1) and C(7) from the mean plane of this
141.0°] as well as a ClꢀꢀꢀCl contact [40]. The Cl(1)ꢀꢀꢀCl(2)
˚
distance of 3.40 A, which is distinctly less than twice of the
fragment, respectively. The carbonyl and hydrazone groups
˚
van der Waals radius of chlorine (3.52 A) [41], and the
are twisted at angles of 45.2(2)° and 63.8(2)° with respect
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Struct Chem (2011) 22:1267–1279
Table 3 Selected hydrogen bond and halogenꢀꢀꢀhalogen interactions in the compounds 1a, 1b and 2a–2d
˚
Distance (A)
Atoms
Symmetry
Angle (°)
D–H
C–X
DꢀꢀꢀA
CꢀꢀꢀX
HꢀꢀꢀA
XꢀꢀꢀX
D–HꢀꢀꢀA
C–XꢀꢀꢀX
1a
N(2)–H(2A)ꢀꢀꢀO(1)
C(14)–H(14)ꢀꢀꢀO(1)
C(10)–H(10)ꢀꢀꢀcentroid(A)
1b
1 ? x, y, z
0.94
0.95
0.95
2.535(2)
3.575(2)
3.576(3)
1.91
2.63
2.81
174
172
139
-x, 1 - y, -z
1 - x, 1 - y, 1 - z
N(2)–H(2A)ꢀꢀꢀO(1)
C(5)–H(5)ꢀꢀꢀO(1)
N(2)–H(2B)ꢀꢀꢀCl(2)
C(4)–Cl(1)ꢀꢀꢀCl(2)
C(12)–Cl(2)ꢀꢀꢀCl(1)
2a
x, 1 ? y, z
0.90
0.95
0.85
1.74
1.74
2.861(2)
3.481(2)
3.563(2)
4.985(3)
5.038(3)
2.03
2.54
2.85
3.40
3.40
153
0.5 - x, 0.5 ? y, 1.5 - z
0.5 - x, 0.5 ? y, 0.5 - z
x, y, 1 ? z
169
141
150.2
155.8
x, y, -1 ? z
C(14A)–H(14A)ꢀꢀꢀO(1)
C(2)–H(2)ꢀꢀꢀO(1A)
2b
x, y, z
x, y, z
0.95
0.95
3.247(2)
3.268(2)
2.66
2.52
120
136
C(13)–H(13)ꢀꢀꢀO(1)
C(3)–H(3)ꢀꢀꢀcentroid(B)
2c
2 - x, 2 - y, 2 - z
x, 0.5 - y, 0.5 ? z
0.95
0.95
3.309(3)
3.769(3)
2.45
2.85
150
163
C(5)–H(5)ꢀꢀꢀO(1)
C(14)–H(14)ꢀꢀꢀO(1)
C(3)–H(3)ꢀꢀꢀF(1)
C(10)–H(10)ꢀꢀꢀF(2)
2d
x, 1.5 - y, 0.5 ? z
0.95
0.95
0.95
0.95
3.330(2)
3.226(2)
3.318(2)
3.247(2)
2.70
2.63
2.54
2.51
125
121
139
135
2 - x, 0.5 ? y, 0.5 - z
3 - x, 2 - y, 1 - z
1 - x, -0.5 ? y, 0.5 - z
C(2A)–H(2A)ꢀꢀꢀO(1)
C(13)–H(13)ꢀꢀꢀO(1A)
C(10)–H(10)ꢀꢀꢀCl(1)
C(4A)–Cl(1A)ꢀꢀꢀCl(1)
C(6)–H(6)ꢀꢀꢀCl(2A)
C(4)–Cl(1)ꢀꢀꢀCl(1A)
-1 ? x, y, z
0.95
0.95
0.95
1.74
0.95
1.73
3.420(2)
3.125(2)
3.606(2)
5.215(2)
3.360(2)
5.215(2)
2.67
2.55
2.88
3.50
2.86
3.50
136
1 ? x, y, z
119
1 - x, 1 - y, 2 - z
1 - x, 2 - y, 2 - z
-x, 1 - y, 1 - z
1 - x, 2 - y, 2 - z
134
77.6
114
170.2
Fig. 1 ORTEP plot of the
E-isomers of benzil
monohydrazone (1a) (a) and
4,40-dichloro benzil
monohydrazone (1d) (b),
including atom labelling.
Thermal ellipsoids are drawn at
50% probability level
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Struct Chem (2011) 22:1267–1279
1273
Fig. 2 Packing diagram of the
E-isomer of benzil
monohydrazone (1a) viewed
down the crystallographic
b-axis. Oxygens are displayed
as dotted and nitrogens as
hatched circles. Broken lines
represent hydrogen bond type
interactions
˚
almost perfect type I geometry (C–ClꢀꢀꢀCl 150.2 A, 155.8°)
[42, 43] furnishes sure proof of the existence of this par-
ticular kind of interaction.
the crystal structures of related compounds [23, 24, 44, 45].
It should be noted at this point that a previously obtained
crystal structure of 2a [25] (Refcode MOZZEH) has
the monoclinic space group P21/c with one molecule in the
asymmetric unit cell. In the crystal of this polymorph, the
molecules are C–HꢀꢀꢀO bonded to form supramolecular
strands with one of the ortho-hydrogens of the benzoyl
moieties acting as a donor. Molecules of adjacent strands
are associated by C–Hꢀꢀꢀp arene interactions [38] involving
the phenyl rings of the benzoyl units. Also the crystal
structure of the present polymorph of 2a is composed of
supramolecular strands in which, however, ortho-hydro-
gens of the dibenzylidene hydrazine part contribute to
hydrogen bonding with the carbonyl oxygen [C(14A)–
Crystal structures of the dibenzil azines 2a–2d
The dibenzil azine 2a, the molecular structure of which is
presented in Fig. 4, belongs to the space group P-1 with the
asymmetric part of the unit cell containing two halves of
the molecule, i.e. the molecules exhibit inversion symme-
try. In each of the molecules, the two benzoyl residues and
the dibenzylidene hydrazine fragments are planar and ori-
ented nearly orthogonal [87.1(1)°, 81.1(1)°] to one another.
The C=N bond lengths are 1.291(2) [1.292(2)]°, while the
˚
˚
N–N distances are 1.414(2) [1.415(2)] A. The bond lengths
˚
C(7)–C(8) [C(7A)–C(8A)] are 1.523(2) [1.520(2)] A.
H(14A)ꢀꢀꢀO(1) 2.66 A, 120°]. Another difference between
the two polymorphs is that in the present structure inter-
actions between molecular strands are dominated by arene
These values are largely in accordance with those found in
123

1274
Struct Chem (2011) 22:1267–1279
Fig. 3 Packing diagram of the
E-isomer of 4,40-dichlorobenzil
monohydrazone (1d) viewed
down the crystallographic
a-axis. Oxygens are displayed
as dotted, nitrogens as hatched
and chlorine atoms as cross-
hatched circles. Broken lines
represent hydrogen bond type
interactions
Fig. 4 ORTEP drawing of
dibenzil azine (2a) including
atom labelling. Thermal
ellipsoids are drawn at 50%
probability level
˚
stacking [46, 47] (centroidꢀꢀꢀcentroid distance 3.84 A)
involving the aromatic rings of the dibenzylidene hydra-
zine part of the molecules. Figure 5 shows a comparison
of the two polymorphs demonstrating the structural
differences.
angle between the pairs of aromatic rings A and B is
86.2(1)°. The crystal structure of 2b is composed of
˚
C–HꢀꢀꢀO bonded strands [C(13)–H(13)ꢀꢀꢀO(1) 2.45 A,
149°] running along the crystallographic a-axis (Fig. 7).
Unlike the structure 2a, in the present case the molecular
strands feature centrosymmetry. Interstrand association is
accomplished by C–Hꢀꢀꢀp arene interactions [38] [C(3)–
The methyl substituted derivative 2b (Fig. 6a) crystal-
lizes in the monoclinic space group P21/c with the asym-
metric unit cell containing one half of the molecule. The
benzoyl moieties are nearly planar, whereas the aromatic
rings within the dibenzylidene hydrazine fragment are
inclined at an angle of 10.4(1)° with respect to the plane of
the C=N–N=C unit. The torsion angle given by the atomic
sequence O(1)–C(7)–C(8)–N(1) is 101.5(2)°; the dihedral
˚
H(3)ꢀꢀꢀcentroid(B) 2.85 A, 163°].
The introduction of halogen substituents as in 2c and 2d
affects the molecular conformation of the azine framework
to a less degree but has a marked influence on the packing
behaviour of the molecules in their solid state structures.
It is evident from the crystal structures of these two
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Struct Chem (2011) 22:1267–1279
1275
shown in Fig. 6b. The crystal structure is characteristic of a
three-dimensional network of hydrogen bonds (Fig. 8),
which include C–HꢀꢀꢀO type interactions [37] with the
oxygen acting as a bifurcated acceptor [C(5)–H(5)ꢀꢀꢀO(1)
˚
˚
2.70 A, 125°; C(14)–H(14)ꢀꢀꢀO(1) 2.63 A, 121°]. Also
C–HꢀꢀꢀF–C contacts [48–50], albeit weaker, contribute to
˚
stabilize the packing structure [C(3)–H(3)ꢀꢀꢀF(1) 2.54 A,
˚
139°; C(10)–H(10)ꢀꢀꢀF(2) 2.51 A, 135°].
A structural situation different from 2c is found in the
crystal of the chlorine substituted analogous compound 2d,
which has the triclinic space group P-1 with two crys-
tallographically independent molecules within the unit cell
(Fig. 9), i.e. the molecules are located on symmetry cen-
tres. In the packing structure (Fig. 10), the molecules are
connected via ClꢀꢀꢀCl interactions of type II geometry
[42, 43] [h1 = C(4)–Cl(1)ꢀꢀꢀCl(1A) = 170.2°, h2 = C(4A)–
Cl(1A)ꢀꢀꢀCl(1) = 77.6°]. Moreover, the chlorine atoms
Cl(1) and Cl(2A) take part in formation of C–HꢀꢀꢀCl con-
˚
tacts [51, 52] [d(HꢀꢀꢀCl) 2.86, 2.88 A]. Compared with 2c,
the degree of molecular association based on C–HꢀꢀꢀO
hydrogen bonds is reduced because here the oxygens give
rise only to the formation of single hydrogen bonding.
Conclusion
Fig. 5 Packing diagrams of the present (a) and reported [25]
(b) polymorph of dibenzil azine 2a viewed down the a-axis. Oxygens
are displayed as dotted and nitrogens as hatched circles. Broken lines
represent hydrogen bond type interactions
The crystal structures of the E-stereoisomer of benzil
monohydrazone (1a) and its p-chloro-substituted derivative
1d reveal related molecular geometries, which are char-
acterized by planarity of the a-ketohydrazone fragments.
The transoid arrangement of the polar groups is determined
by intramolecular dipole–dipole interaction and the ten-
dency of molecules to form infinite N–HꢀꢀꢀO=C bonded
strands in the solid state. These 1D chains can be
compounds, being either fluorine (2c) or chlorine substi-
tuted (2d), that the crystal symmetry is determined by the
nature of the halogen. Crystals of 2c yield the monoclinic
space group P21/c with one half of the molecule in the
asymmetric unit cell. A perspective view of the molecule is
Fig. 6 ORTEP drawings of the dibenzil azines 2b (a) and 2c (b) including atom labelling. Thermal ellipsoids are drawn at 50% probability level
123

1276
Struct Chem (2011) 22:1267–1279
Fig. 7 Packing diagram of 2b.
Oxygens are displayed as dotted
and nitrogens as hatched circles.
Broken lines represent hydrogen
bond type interactions, broken
double lines C–Hꢀꢀꢀp contacts
Fig. 8 Packing diagram of 2c
viewed down the a-axis.
Oxygens are displayed as
dotted, nitrogens as hatched and
fluorine atoms as grey circles.
Broken lines represent hydrogen
bond type interactions
characterized by the graph set descriptor C11 (6) [53].
Nevertheless, marked structural differences are found
regarding the packing of molecules. The binding behaviour
of the chloro substituents in 1d, comprising N–HꢀꢀꢀCl
interactions and type I ClꢀꢀꢀCl contacts, extensively inter-
links molecular strands, whereas in 1a only weak associ-
ation exists. On the other hand, in their crystal structures,
the dibenzil azines 2a–2d exist in the Z,Z-configuration
(Ci-symmetry). The molecular structures are characterized
by a nearly orthogonal arrangement of the planar 1,4-
diarylazine part and the benzoyl fragments. A possible
reason for this unique conformational behaviour may be
seen in intramolecular steric effects between the extended
molecular entities as well as dipolar interactions between
123

Struct Chem (2011) 22:1267–1279
1277
Fig. 9 Perspective view of the
dibenzil azine 2d including
atom labelling. Thermal
ellipsoids are drawn at 50%
probability level
Fig. 10 Packing diagram of
2d viewed down the
crystallographic a-axis.
Oxygens are displayed as
dotted, nitrogens as hatched and
chlorine atoms as cross-hatched
circles. Broken lines represent
hydrogen bond type interactions
˚
significantly shorter than those of 1a and 1d (1.497 A),
the carbonyl groups and the central diimine unit. Moreover,
˚
the average values of 1.416 A for the N–N bond and
˚
1.469 A for the Caryl–Cimino bond, the latter being
indicate delocalization of the p-electron system within the
1,4-diarylazine unit. For 2a–2d, this means that the nature
123

1278
Struct Chem (2011) 22:1267–1279
of the p-substituents hardly affects the molecular confor-
mation. However, dependent on the substitution, the modes
of intermolecular association are changed, i.e. from strand
to three-dimensional assembly, on going from non-halo-
genated 2a and 2b to the halogen substituted compounds 2c
and 2d. In particular, the crystal of 2d is found to feature a
type II ClꢀꢀꢀCl contact which is different from type I ClꢀꢀꢀCl
interaction in the case of 1d.
Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ,
UK; fax: ?44(0)1223-336033; e-mail: deposit@ccdc.cam.
ac.uk].
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