Mesogenic enaminoketone Ni(II) complexes of phenazine and quinoxaline derivatives

Article abstract of DOI:10.1080/15421406.2011.653714

The new Ni(II) metallomesogenes were synthesized. Their mesogenic cores are constructed from tetradental enaminoketone unit connected to 1,2:3,4-di(4,5-dioctyloxybenzo)phenazine or 2,3-di(3,4-dioctyloxyphenylo) quinoxaline. The described compounds form columnar hexagonal phase (Colhd) in very broad temperature range. The high stability of the columnar phase results from presence of a permanent dipol moment in enaminoketone moieties created by strong electron withdrawing properties of carbonyl groups.

Full text of DOI:10.1080/15421406.2011.653714

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Mesogenic Enaminoketone Ni(II)
Complexes of Phenazine and Quinoxaline
Derivatives
Paulina Krzyczkowska a , Adam Krówczyński a , Joanna Kowalewska a
, Jerzy Romiszewski a , Damian Pociecha a & Jadwiga Szydłowska a
a Faculty of Chemistry , University of Warsaw , Żwirki i Wigury 101,
02-089 , Warsaw , Poland
Published online: 18 Apr 2012.
To cite this article: Paulina Krzyczkowska , Adam Krówczyński , Joanna Kowalewska , Jerzy
Romiszewski , Damian Pociecha & Jadwiga Szydłowska (2012) Mesogenic Enaminoketone Ni(II)
Complexes of Phenazine and Quinoxaline Derivatives, Molecular Crystals and Liquid Crystals, 558:1,
93-101, DOI: 10.1080/15421406.2011.653714
To link to this article: http://dx.doi.org/10.1080/15421406.2011.653714
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Mol. Cryst. Liq. Cryst., Vol. 558: pp. 93–101, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.653714
Mesogenic Enaminoketone Ni(II) Complexes
of Phenazine and Quinoxaline Derivatives
∗
´
´
PAULINA KRZYCZKOWSKA, ADAM KROWCZYNSKI,
JOANNA KOWALEWSKA, JERZY ROMISZEWSKI,
DAMIAN POCIECHA, AND JADWIGA SZYDŁOWSKA
˙
Faculty of Chemistry, University of Warsaw, Zwirki i Wigury 101,
02-089 Warsaw, Poland
The new Ni(II) metallomesogenes were synthesized. Their mesogenic cores are
constructed from tetradental enaminoketone unit connected to 1,2:3,4-di(4,5-
dioctyloxybenzo)phenazine or 2,3-di(3,4-dioctyloxyphenylo)quinoxaline. The described
compounds form columnar hexagonal phase (Colhd) in very broad temperature range.
The high stability of the columnar phase results from presence of a permanent dipol
moment in enaminoketone moieties created by strong electron withdrawing properties
of carbonyl groups.
Keywords Claisen formylation; dipol moment; liquid crystals; metallomesogenes
1. Introduction
Self organization of a substance into liquid crystalline phases depends mainly on its molec-
ular shape anisotropy. A rod-like or disk-like shape, is mainly determined by mesogenic
core structure. One of the first molecules that was reported forming columnar phases based
on hexasubstituted benzene [1]. The next were constructed, for example, from condensed
aromatic ring systems like triphenylenes [2–4] or perylenes [5]. Also metaloorganic cores
like porphyrins and phthalocyanines [6] were applied to create compounds showing LC
columnar phases. The structure of this types of mesogenic molecules were completed
by symmetrical attachment of various chains at the core. Another group of symmetric
molecules are dendrimers with fractal structure containing many molecular fragments that
induce mesogenity. These can be organized into columnar or calamitic phases in depen-
dance on the number of peripheral paraffinic chains [7]. It is well known that not only
discoid cores are essential to generate the columnar phases. There are many examples of
substances having the core distorted from the perfect shape [8] like half-discoid [9, 6c],
elliptical [10], rectangular [11], triangular [12] and so on. Non perfect shape is compensated
by adjusted number of lateral chains attached at proper position [13]. When the number
of side chains is too small the compounds with the same mesogenic core can also form
lamellar phases [8b, 11, 12]. The circular cross section of columns in Dh phase can be
reached by alternating arrangement of molecules. It is the case of half-discs, rectangles or
triangules as building blocks [8a, 9a]. Also columnar cylindrical shape can be ashured by
∗Address correspondence to P. Krzyczkowska, Faculty of Chemistry, University of Warsaw,
Zwirki i Wigury 101, 02-089 Warsaw, Poland. E-mail: pkrzyczkowska@chem.uw.edu.pl
˙
93

94
Paulina Krzyczkowska et al.
molecular tilt to the columnar axis. It is the way when the longest molecular dimension
is effectively shortened in projection [3, 10]. Substances exhibiting LC columnar phases
and having the most disadvantageous shape of mesogenic centres are polycatenars. These
are constructed from a long chain of aromatic rings [4, 14]. To fill the columnar diameter
with calamitic molecules the one columnar slice is occupied by a few mesogenic cores
[15]. Similar as previously the circular cross section of the columns can be achieved due
to molecular tilt [16]. Despite the intermolcecular dispersion forces that induce LC phase
organization the molecular self-assambling can be strengthened by additional interaction
like metal-metal [8a, 9a] or hydrogen bonds [7] directed along the columnar axis. Another
important factor stabilizing columnar phases is intermolecular dipol-dipol interaction [17].
Thanks to this forces even half disc molecules or bend core policatenares can create colum-
nar phases [9b, 18]. A permanent dipol moment can be introduced into molecules as an
electron shifting moiety like carbonyl, cyano, chloride or other group. As it was reported
by Foster, Lavigueur, and Ichihara for compounds with triphenylene or dibenzophenazine
cores, substitution of electron-withdrawing fragments increase temperature range of LC
phases [19]. Their higher stability results from antiferroelectric interaction appearing be-
tween neighboring molecules that stabilize columnar stack. The intermolecular forces can
even result in creation of molecular dimers that broaden the columnar diameter and give
larger lattice parameters [20].
In described here newly synthesized compounds the polarizing moiety was inserted
into molecular core and tetradental enaminoketone chelating center was applied as an
electron withdrawing group. The Ni(II) metallic center was connected to dibenzophenazine
or diphenyloquinixaline units. The nickel complexes of Series I were constructed from
1,2:3,4-di(4,5-dioctyloxybenzo)-7,8-diaminophenazine moiety expanded by two acylvinyl
groups which give chelating plane (Scheme 1a, I a–d). To acylvinyl fragments alkyl chains
R1 and R2 were attached at carbonyl and vinyl carbons, respectively. The diaminophenazine
unit with enaminoketone rings constitute a condensed polycyclic aromatic system. In the
next series of complexes, to make core structure more loose and flexible, the bond between
phenyl groups in the phenatrene moiety was broken. In Series II the benzophenazine moiety
was replaced by 2,3-di(3,4-dioctyloxyphenylo)-6,7-diaminoquinoxaline group (Scheme 1b,
II a–d). The both series of the described complexes exhibit liquid crystalline hexagonal
columnar disordered phase (Colhd).
Experimental
Measurements
The mesophase identification was based on microscopic examination of liquid crystalline
texture. A Zeiss Imager A2m polarizing microscope equipped with a Linkam hot stage was
used. The microscopic identification was confirmed by the X-ray studies, performed with
Bruker diffractometer D8 GADDS (CuKα radiation, Vantec 2000 area detector) equipped
with temperature hot stage. The X-ray diffraction patterns were obtained for partially
oriented samples. The detected diffraction signals were indexed with Miller indices of two
dimensional hexagonal lattice. The spacing ratio of the first three visible sharp peaks ((100):
√
(110): (200)) fulfilled the known relation 1 : 1/ 3 : 1/2. Phase transition temperatures and
enthalpy changes were determined by calorimetric measurements performed with a TA DSC
Q200 set-up, in inertnitrogen atmosphere. The molecular dimensions were estimated with
HyperChem software package.

New Ni(II) Metallomesogenes
95
Scheme 1. Synthetic route to the complexes of.

96
Paulina Krzyczkowska et al.
Synthesis
The synthetic procedure leading to the complexes of Series I and II is sketched in
Scheme 1.
It starts from preparation 2,3,6,7-tetraoctyloxyphenantrene-9,10-dione, 1 and 3,3ꢀ,4,4ꢀ-
tetraoctyloxydibenzoyl, 1ꢀ that was described many times previously [12].
The derivatives of diamines, 3 or 3ꢀ were received in the condensation reaction of 1 or
1ꢀ with 1,2,4,5-benzenetetramine tetrahydrochloride, 2. In compounds 3 and 3ꢀ two amino
groups were substituted by various formyl ketone sodium salts that determines the structure
of final complexes. To the obtained ligands 5 or 5ꢀ without their purification nickel acetate
solution was added. It gave the designed final Ni(II) complexes I a–d and II a–d.
1,2:3,4-di(4,5-octyloxybenzo)-7,8-diaminophenazine (3). 2,3,6.7-tetraoctyloxy-
phenantrene-9,10-dione, 1 (3.6 g, 1.2 mmol) and anhydrate sodium acetate were dissolved
in warm ethanol (100 ml). To this solution 1,2,4,5-benzenetetramine tetrahydrochloride, 2
(2.8 g, 3.8 mmol) dissolved in water-ethanol solvent was added and the reaction mixture
was refluxed for several hours in inert atmosphere. After that to the reaction mixture water
(500 ml) was poured and contents was alkalized with sodium carbonate. The precipitate
was filtrated under reduced pressure, washed with water and purified in crystallization from
toluene to give dark orange crystals of 3 (4 g). Yield: 70%.
Elemental Analysis for 3 C52H78N4O4 (823.20 g/mol); Calculated: C, 75.87; H, 9.55;
N, 6.81%; Found: C, 75.94; H, 9.76; N, 6.94%; 1H NMR –(500 MHz, CDCl3): δ = 0.84 –
2.20 (60H, m); 4.27 (4H, t, J = 6.6 Hz); 4.34 (4H, t, J = 6.6 Hz);7.43 (H, s); 7.76 (H, s);
8.71 (H, s).
2,3-di(3,4-dioctyloxyphenylo)-6,7-diaminoquinoxaline (3ꢀ). 3,3ꢀ,4.4ꢀ-tetraoctyloxy
dibenzoil, 1 (3.6 g, 1.2 mmol) and anhydrate sodium acetate were dissolved in warm
ethanol (100 ml). To this solution 1,2,4,5-benzenetetramine tetrahydrochloride, 2 (2.8 g,
3.8 mmol) dissolved in water-ethanol solvent was added and the reaction mixture was
refluxed for several hours in inert atmosphere. After that water (500 ml) was poured to the
reaction mixture and it was alkalized with sodium carbonate. The crude product 3ꢀ was
extracted with hexane and the solvent was evaporated to give amorphous residue (4 g). The
diamine derivative, 3ꢀ was used in the next reaction step without purification. Yield: 70%.
Elemental Analysis for 3ꢀ C52H78N4O4 (823.20 g/mol); Calculated: C, 75.68; H,
9.77; N, 6.79%; Found: C, 75.75; H, 9.82; N, 6.87%; 1H NMR –(500 MHz, CDCl3): δ =
0.85–1.84 (60H, m); 3.79 (2H, t, J = 6.5 Hz); 3.95-4.00 (8H, m);6.81 (2H, d, J = 8.4 Hz);
6.97 (2H, d, J = 1.9 Hz); 7.04 (2H, dd, J = 1.9 and 8.4 Hz); 7.28 (2H, s).
Ligand (5). 1,2:3,4-di(4,5-octyloxybenzo)-7,8-diaminophenazine, 3 (0.2 g, 0.25
mmol) was dissolved in DMF on heating. To the warm solution of an appropriate formyl
ketone sodium salt 4 a–d (1 mmol) dissolved in DMF was added. Then the solution was
acidified with acetic acid (to pH = 6). The mixture was stirred and heated at 80◦C for 15
min. After cooling the residue was isolated by filtration under reduced pressure and washed
with ethanol. The crude product 5 was used to the next step.
Ligand (5ꢀ). 2,3-di(3,4-dioctyloxyphenylo)-6,7-diaminoquinoxaline, 3ꢀ (0.2 g, 0.25
mmol) was dissolved in ethanol on heating. To the warm solution of an appropriate formyl
ketone sodium salt 4 a–d (1 mmol) dissolved in ethanol was added. Then the solution was
acidified with acetic acid (to pH = 6). The mixture was boiled and stirred for 15 min. After
cooling the residue was isolated by filtration under reduced pressure and washed with cold
ethanol. The crude product 5ꢀ was used to the next step.
Ni(II) complexes I a–d and II a–d. The crude ligands 5 or 5ꢀ were dissolved in
isopropanol or ethanol, respectively. Then a small excess of nickel acetate (0.2 g) dissolved
in methanol was added and the mixture was boiled under stirring for 10 min. After cooling

New Ni(II) Metallomesogenes
97
the precipitate was filtered under reduced pressure, washed with ethanol and purified by
preparative thin layer chromatography (silicagel, hexane/methylene chloride 1:2).
Elemental Analysis for I a C84H136N4NiO6 (1356.69 g/mol); Calculated: C, 74.36;
H, 9.55; N, 4.13%; Found: C, 74.46; H, 9.68; N, 4.21%.
1H NMR –(500 MHz, CDCl3): δ = 0.88–2.02 (108 H, m); 2.40 (4H, t, J = 7.7 Hz);
2.48 (4H, t, J = 7.6 Hz); 4.26 (4H, t, J = 6.6 Hz); 4.34 (4H, t, J = 6.6 Hz); 7.66 (2H, s);
7.73 (2H, s); 8.13 (2H, s); 8.73 (2H, s).
Elemental Analysis for I b C72H112N4NiO6 (1188.37 g/mol); Calculated: C, 72.77;
H, 9.50; N, 4.71%; Found: C, 72.85; H, 9.63; N, 4.81%.
1H NMR –(500 MHz, CDCl3): δ = 0.88–2.00 (86H, m); 2.43 (4H, t, J = 7.8 Hz); 4.24
(4H, t, J = 6.6 Hz); 4.28 (4H, t, J = 6.6 Hz); 5.55 and 7.52 (4H, AB, J = 6.7 Hz); 7.64
(2H, s), 7.92 (2H, s); 8.56 (2H, s).
Elemental Analysis for Ic C76H120N4NiO6 (1244.48 g/mol); Calculated: C, 73.35; H,
9.72; N, 4.50%; Found: C, 73.56; H, 9.84; N, 4.62%.
1H NMR –(500 MHz, CDCl3): δ = 0.88–2.00 (94H, m); 2.43 (4H, t, J = 7.8 Hz); 4.24
(4H, t, J = 6.6 Hz); 4.28 (4H, t, J = 6.6 Hz); 5.55 and 7.52 (4H, AB, J = 6.7 Hz); 7.64
(2H, s), 7.92 (2H, s); 8.56 (2H, s).
Elemental Analysis for Id C66H100N4NiO6 (1104.21 g/mol); Calculated: C, 71.66; H,
9.13; N, 5.07%; Found: C, 71.75; H, 9.24; N, 5.15%.
1H NMR –(500 MHz, CDCl3): δ = 0.88–0.93 (12H, m); 1.23 (18H, s); 1.28-2.10
(48H, m); 4.26 (4H, t, J = 6.6 Hz); 4.32 (4H, t, J = 6.6 Hz); 5.75 and 7.75(4H, AB, J =
6.8 Hz), 7.70 (2H, s); 8.09 (2H, s); 8.69 (2H, s).
Elemental Analysis for IIa C84H138N4NiO6 (1358.71 g/mol); Calculated: C, 74.26;
H, 10.24; N, 4.12%; Found: C, 74.45; H, 10.37; N, 4.19%.
1H NMR –(500 MHz, CDCl3): δ = 0.86–1.86 (108 H, m); 2.35 (4H, t, J = 7.6 Hz);
2.48 (4H, t, J = 7.6 Hz); 3.80 (4H, t, J = 6.6 Hz); 3.99 (4H, t, J = 6.6 Hz); 6.82 (2H, d,
J = 8.3 Hz), 7.00 (2H, d, J = 2.1 Hz); 7.08 (2H, dd, J = 2.1 and 8.3); 7.58 (2H, s); 8.00
(2H, s).
Elemental Analysis for IIb C72H114N4NiO6 (1190.39 g/mol); Calculated: C, 72.65;
H, 9.65; N, 4.70%; Found: C, 72.77; H, 9.81; N, 4.86%.
1H NMR –(500 MHz, CDCl3): δ = 0.86–1.87 (86H, m); 2.43 (4H, t, J = 7.8 Hz); 3.80
(4H, t, J = 6.6 Hz); 3.99 (4H, t, J = 6.6 Hz); 5.60 and 7.54 (4H, AB, J = 6.8 Hz); 6.82
(2H, d, J = 8.3), 7.01 (2H, d, J = 2.1); 7.06 (2H, dd, J = 2.1 and 8.3 Hz); 7.98 (2H, s).
Elemental Analysis for IIc C76H122N4NiO6 (1246.49 g/mol); Calculated: C, 73.23;
H, 9.86; N, 4.49%; Found: C, 73.36; H, 9.94; N, 4.57%.
1H NMR –(500 MHz, CDCl3): δ = 0.86–1.86 (94H, m); 2.43 (4H, t, J = 7.8 Hz); 3.80
(4H, t, J = 6.6 Hz); 3.99 (4H, t, J = 6.6 Hz); 5.60 and 7.54 (4H, AB, d, J = 6.8 Hz);.6.82
(2H, d, J = 8.3 Hz), 7.01 (2H, d, J = 2.1 Hz); 7.06 (2H, dd, J = 2.1 and 8.3 Hz); 7.98 (2H, s).
Elemental Analysis for IId C66H102N4NiO6 (1106.23 g/mol); Calculated: C, 71.66;
H, 9.29; N, 5.06%; Found: C, 71.74; H, 9.35; N, 5.13%.
1H NMR –(500 MHz, CDCl3): δ = 0.86–0.92 (12H, m); 1.22 (18H, s); 1.24–2.10
(48H, m); 3.80 (4H, t, J = 6.6 Hz); 3.99 (4H, t, J = 6.6 Hz); 5.75 and 7.62 (4H, AB, J =
7.0 Hz), 6.82 (2H, d, J = 8.3 Hz); 7.00 (2H, d, J = 2.1); 7.06 (2H, dd, J = 2.1 and 8.3 Hz);
7.99 (2H, s)
Results and Discusion
The phase sequence, phase transition temperatures and enthalpy changes for the studied
compounds of series I and II are gathered in Table 1.

98
Paulina Krzyczkowska et al.
Table 1. Structure of Ni(II) complexes of Series I and II, phase transition temperatures and
their thermal effects
R1
R2
Phase sequence, (enthalpy changes)
Ia
Ib
Ic
Id
IIa
IIb
IIc
IId
C7H15
C7H15
C9H19
C(CH3)3
C7H15
C7H15
C9H19
C(CH3)3
C6H13
H
H
H
C6H13
H
H
Cr 117.1 (34.0) Colhd 171.9 (0.7) I
Cr 65.1 (33.6) Colhd 265.5 (1.7) I
Cr 78.1(30.7) Colhd 260.0∗ I
Cr 106.2 (35.6) Colhd 253.3 (4.9) I
Cr 75.7 (50.3) Colhd 87.8 (2.4) I
m.p. below r.t. Colh 109.0 (5.1) I
Cr 73.5 (33.7) Colh 112.4 (5.2) I
Cr 82.8 (5.9) I
H
• Taken from microscopy.
Figure 1 shows, as an example, a microscopic texture obtained for the compound Ia near
clearing temperature. Beside the dark isotropic phase, the dendrite-like dark domains were
detected confirming formation of hexagonal columnar phase that have columns alignment
homeotropic to the glass plate. Similar dendritic textures were observed for other complexes.
All the complexes of Series I having condensed polycyclic aromatic rings form liquid
crystalline columnar hexagonal disordered phase (Colhd). The temperature range of liquid
crystallinity is very broad and, in the case of complex Ib, with heptyl chains in positions R1,
reaches 200◦C. However, substitution of longer terminal chains e.i. nonyl chains makes the
liquid crystalline properties slightly weaker. Broadening the molecules due to attachment
of hexyl side chains at vinyl carbons (R2, compound Ia) raises the melting point and makes
lower the clearing points, respectively. Less condensed aromatic rings system of diamino-
quinoxaline in Series II does not give so rigid mesogenic core as it is for diaminophenazine
derivatives of Series I. The compounds IIa–IIc show less stable hexagonal columnar phase
and have much lower clearing points. The complex IId with the bulky tertbutyl substituents
does not form any LC phase.
Figure 1. Microscopic texture of Ia complex at 172◦C at crossed polarizers.

New Ni(II) Metallomesogenes
99
Figure 2. X-ray pattern for partially oriented sample of IIb complex at room temperature.
The X-ray studies confirmed hexagonal structure of the detected liquid crystalline
phase (Figs. 2 and 3). The X-ray diffraction signals of all compounds well matched to the
Colhd phase structure; unit cell parameter, a, is presented in Table 2 for all compounds. The
quinoxaline derivatives (Series II) have larger intercolumnar distances and form narrower
liquid crystalline phases than it is observed for the phenazine derivatives (Series I). This
can be related to more flexible mesogenic core of molecules of Series II and more free
molecular motions. In both series, compounds having the largest number of attached chains
(Ia and IIa) exhibit highest density. One of the reasons can be related to the effective size
of molecular discs and the dimensions of the created LC columns that do not depend on the
side chains R2. The discussed molecules are elongated and when they form the columns
in LC phase the following molecules are probably aligned more or less perpendicular to
one another. In this arrangement the circular cross section of the column is formed by two
or more molecules and its dimension is mainly determined by the molecular length. So
Figure 3. Intensity distribution of X-ray diffraction signals of (IIb) complex at room temperature.
Signal indexed as (001) reflects distance between mesogenic cores of neighboring molecules along
the column, while the strong diffused signal, centered at ∼20 deg. reflects mean distance between
aliphatic chains.

100
Paulina Krzyczkowska et al.
Table 2. Hexagonal lattice parameters (in Å) taken from X-ray measurements performed at
given temperature (◦C) and calculated density, ρ ( g·cm−3) for Colhd phase of compounds
of Series I and II
T
a
Chains
Disc distance (001)
ρ
Ia
Ib
Ic
Id
IIa
IIb
IIc
170
100
200
245
80
24.9
25.75
26.6
24.3
26.1
27.2
28.3
4.6
4.55
4.7
4.7
4.5
4.5
4.6
3.6
3.5
3.5
3.5
3.5
3.5
3.5
1.18
1.00
0.98
1.05
1.11
0.90
0.87
100
100
the presence of side chains R2 in the complexes Ia and IIa only increase the molecular
weight. This growth is about several percent that contributes to the phase density growth.
However, the value of the lattice parameter a of the complexes Ia and IIa which was found
smaller than for related complexes Ib and IIb is hard to explain. The size of the molecules
when modelled in the most extended conformation is equal for all the complexes Ia, Ib,
IIa and IIb, being ∼33 Å. Comparison of this value to the a distances taken from X-ray
measurements shows that molecular chains of neighbouring molecules interdigitate by 3–4
carbon atoms.
It is worth noticing that semiproduct 1 exhibit also LC properties with a phase sequence
Cr 83.6 (80.0) Colhd 92.7 (3.4) I.
Conclusions
Two series of LC Ni(II) complexes with new structure of mesogenic cores were synthesized
and studied. The phenazine and quinoxaline cores were extended by dienaminoketone moi-
ety forming chelating plane. The oxygen atoms in the carbonyl groups of enaminoketones
withdraw electrons from the conjugated π bond system of the molecular core and cre-
ate strongly non-uniform electron distribution resulting in permanent dipol moment. The
dipol-dipol interaction between molecules promote Colhd phase with very high clearing
temperature especially in the case of rigid phenazine mesogenic core.
Acknowledgment
This work was supported by FNP Program TEAM/2010-5/4.
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