Dinuclear ortho-metallated palladium(II) complexes with acetato and chloro bridges as liquid crystals

Article abstract of DOI:10.1080/15421406.2011.536473

In this article we present the preparation and the investigation of the liquid crystal properties of a series of dinuclear acetato- and chloro-bridged ortho-metallated Pd(II) complexes bearing six alkoxy peripheral chains in the molecule. Their structures were assigned based on elemental analysis, Infrared (IR), and ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, whereas the thermal behavior was investigated by polarizing optical microscopy and differential scanning calorimetry. All these complexes show mesomorphic behavior with smectic A phases being displayed. The acetato complexes exhibit a monotropic smectic A phase with transition temperatures from the isotropic state depending on the alkyl chain lengths, whereas the chloro complexes decompose on melting from the crystalline state to an SmA phase. Copyright Taylor & Francis Group, LLC.

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Dinuclear ortho-Metallated Palladium(II)
Complexes with Acetato and Chloro
Bridges as Liquid Crystals
Viorel Cîrcu a & Florea Dumitraşcu b
a Inorganic Chemistry Department, University of Bucharest,
Bucharest, Romania
b Centre of Organic Chemistry “C. D. Nenitzescu,” Romanian
Academy, Bucharest, Romania
Published online: 18 Jan 2011.
To cite this article: Viorel Crcu & Florea Dumitracu (2011) Dinuclear ortho-Metallated Palladium(II)
Complexes with Acetato and Chloro Bridges as Liquid Crystals, Molecular Crystals and Liquid Crystals,
534:1, 41-49, DOI: 10.1080/15421406.2011.536473
To link to this article: http://dx.doi.org/10.1080/15421406.2011.536473
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Mol. Cryst. Liq. Cryst., Vol. 534: pp. 41–49, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.536473
Dinuclear ortho-Metallated Palladium(II)
Complexes with Acetato and Chloro Bridges
as Liquid Crystals
1
VIOREL CIRCU AND FLOREA DUMITRAS¸CU
2
ˆ
1Inorganic Chemistry Department, University of Bucharest,
Bucharest, Romania
2Centre of Organic Chemistry ‘‘C. D. Nenitzescu,’’ Romanian Academy,
Bucharest, Romania
In this article we present the preparation and the investigation of the liquid crystal
properties of a series of dinuclear acetato- and chloro-bridged ortho-metallated
Pd(II) complexes bearing six alkoxy peripheral chains in the molecule. Their struc-
1
tures were assigned based on elemental analysis, Infrared (IR), and H and 13C
nuclear magnetic resonance (NMR) spectroscopy, whereas the thermal behavior
was investigated by polarizing optical microscopy and differential scanning calor-
imetry. All these complexes show mesomorphic behavior with smectic A phases
being displayed. The acetato complexes exhibit a monotropic smectic A phase with
transition temperatures from the isotropic state depending on the alkyl chain
lengths, whereas the chloro complexes decompose on melting from the crystalline
state to an SmA phase.
Keywords Liquid crystals; metallomesogens; orthometallation; palladium; smectic
Introduction
Liquid-crystalline materials that contain metal ions (metallomesogens) are typical
examples where the unique properties of anisotropic fluids are combined with the
specific properties of metals such as color, polarizability, and electrical and magnetic
properties. The organometallic compounds of palladium(II) based on ortho-
metallated complexes with imine ligands have been extensively studied, in particular
due to the design flexibility and possibility of varying their chemical and physical
properties by systematic change of the ligands around the palladium ion. These type
of complexes are thermally stable and consist of both dinuclear and mononuclear
organometallic species [1,2]. The dinuclear ortho-metallated palladium(II)
complexes show various liquid-crystalline phases depending on the number of
peripheral chains and the nature of the bridging group. Previous studies on the
dinuclear acetato-bridged Pd(II) complexes with four peripheral chains revealed
ˆ
Address correspondence to Dr. Viorel Cırcu, Inorganic Chemistry Department,
University of Bucharest, 23 Dumbrava Rosie St., 020464, Bucharest, sector 2, Romania.
E-mail: viorel_carcu@yahoo.com
41

ˆ
V. Cırcu and F. Dumitras¸cu
42
the absence of mesomorphism, because the acetate bridge was found to be ineffective
in promoting mesomorphism in such systems, in particular due to the formation of
nonplanar complexes [3–5]. On the other hand, it is well known that the analogous
dimeric chloro compounds with ortho-palladated imine with two long alkyl chains
form smectic A and C phases at high temperatures [6].
Additional chains in the molecule produce more disc-like shape. Thus, whereas
the dinuclear m-chloro-bridged palladium ortho-metallated complexes with six per-
ipherial chains show smectic A and C phases [7], the related compounds with eight
alkyl chains, due to their sheet-like shape, show a nematic discotic mesophase [8].
We were interested to study the influence of the nature of the bridging group as
well as the alkyl chain length on the mesomorphic behavior of dinuclear palladium(II)
complexes with six peripherial alkoxy chains with the aim of extending our previous
studies on organometallic palladium(II) complexes with imine ligands, including a
thermally stimulated depolarization currents (TSDC) method [9–11]. In this article
we present the synthesis, characterization, and the liquid-crystalline phase behavior
of six novel palladium mesogens that contain six alkoxy chains in the molecule and
acetato or chloro bridges.
Results and Discussion
The synthetic pathway used to prepare the imine ligands and the palladium(II)
complexes is depicted in Schemes 1 and 2. The dinuclear acetato-bridged palladium
complex 2 was obtained by the ortho-palladation reaction of the imine 1, by using pal-
ladium(II) acetate, with good yields. The crude product obtained by the ortho-metal-
lation of imine 1 with palladium acetate was used in the next step without further
purification, by reacting it with HCl in methanolic solution in order to prepare the
chloro-bridged analogue 3. The new complexes were characterized by elemental
1
analysis, Infrared (IR), and H and 13C nuclear magnetic resonance (NMR) spec-
troscopy, whereas their liquid-crystal properties were investigated by differential
scanning calorimetry (DSC) and polarizing optical microscopy (POM).
These dinuclear complexes can show both cis and trans isomers depending on
how the nitrogen atoms are positioned in respect to the bridge, although, in general,
the acetato- and chloro-bridged palladium(II) systems were found to exist as single
1
trans compounds. The H NMR spectra indicate the presence of only one isomer
in solution by the presence of only one set of signals. This can be observed, in
Scheme 1. Preparation of the imine ligands.

Dinuclear ortho-Metallated palladium(II) Complexes
43
Scheme 2. Preparation of dinuclear Pd(II) ortho-metallated complexes.
particular, for the iminic proton and for the two singlets assigned to the aromatic
protons of the Pd-substituted phenyl ring.
Thermal Behavior
The DSC data and the type of mesophases observed by microscopy are presented in
Table 1.
Table 1. Thermal data for dinuclear palladium(II) complexes
Compound
Transition
Cr-Cr0
Cr-I
T (ꢀC)
DH (kJ molꢁ1
)
1a
35
68
57
74
64
74
79
(76)
50
88
(70)
76
96
109
(74)
110
(75)
111
175
152
108
177
22.3
48.1
36.1
57.6
46.7
1b
Cr-Cr0
Cr-I
Cr-I
Cr-I
Cr-I
(I-SmA)
Cr-Cr0
Cr0-I
(I-SmA)
Cr-Cr0
Cr0-Cr0
Cr0-I
(I-SmA)
Cr-I
(I-SmA)
Cr-Cr0
1c
1d
2a
56.3
(4.8)
20.9
18.0
(5.6)
50.3
32.4
26.1
(5.9)
64.8
(4.3)
40.7
33.7
42.7
45.8
36.2
2b
2c
2d
3a
Cr0-SmA(dec.)
Cr-SmA(dec.)
Cr-Cr0
3b
3c
Cr0-SmA(dec.)

ˆ
44
V. Cırcu and F. Dumitras¸cu
The imine ligands with three alkoxy chains used as ligands do not show any
liquid crystal properties and they melt from the crystalline phase straight to the
isotropic liquid (1a: Cr-I, 74ꢀC, 1b: Cr-I, 73ꢀC, 1c: Cr-I, 71ꢀC).
Both acetate- and chloro-bridged dinuclear palladium(II) complexes with
six alkoxy chains show mesomorphic behavior. The acetato complexes exhibit a
monotropic smectic A phase that could be detected easily by the typical separation
as battonets from the isotropic liquid and by the fan-like texture, which can be
aligned homeotropically (Figs. 1 and 2) [12], whereas the chloro complexes show
extensive decomposition on melting from the crystalline state to the SmA phase.
An interesting thermal behavior is seen for complex 2c, which exhibits three succe-
sive crystalline phases on the first heating run (Fig. 3a). On cooling, the transition
to the smectic phase at 74ꢀC is accompanied by a glass transition followed on the
next heating run by crystallization and only one crystalline phase (Fig. 3b).
The transition temperatures are significantly lower for the acetato-bridged
complexes compared to their chloro-bridged analogues, and this mesophase destabi-
lization can be attributed to the nonplanarity of the molecules induced by the acetate
ion. On the other hand, the introduction of an additional alkoxy group on the imine
ligand has a great effect on promoting the mesomorphic behavior. There is a clear
tendency of increasing the melting points of the acetato complexes with the increase
in chain length either on the Pd-substituted ring or on the aniline ring of the imine
ligand. However, the temperature of the I-SmA transition does not follow the
same trend. The shortest chain homologue, 2a, shows the highest temperature of
Figure 1. Optical microphotographs of compound 2a (200ꢂ): transition from isotropic liquid
ꢀ
ˆ
to the SmA phase, separation of battonets from isotropic liquid (a) SmA phase at 76 C (b)
fan-shaped texture of the SmA phase at 67ꢀC (c) and crystallisation occurs below 40ꢀC (d).

Dinuclear ortho-Metallated palladium(II) Complexes
45
Figure 2. The fan-shaped texture of the SmA phase of complex 2b at 67ꢀC.
the I-SmA transition; increasing the chain lengths of the alkoxy groups on the
Pd-substituted ring leads to a decrease of 6ꢀC of this transition, whereas the increase
of the alkoxy chain length of the aniline ring produces only a 2ꢀC decrease; when all
alkoxy groups, both on the Pd-substituted ring and on the aniline ring, are
substituted with decyloxy groups, the I-SmA is almost unchanged compared to
the shortest analogue. The temperatures corresponding to the Cr-SmA transition
of the chloro-bridged palladium complexes bearing six alkoxy chains are higher than
the temperatures reported for the chloro-bridged palladium(II) complex with four
decyloxy chains and two hexyl chains, previously reported in the literature, but this
fact can be explained by the presence of two alkyl chains on the aniline rings instead
of alkoxy chains in the latter case [7].
Figure 3. DSC thermogram of compound 2c showing the first (a) and second (b) heating and
cooling cycles.

ˆ
V. Cırcu and F. Dumitras¸cu
46
Experimental
Dichloromethane was distilled from phosporus pentoxide; other chemicals were
used as supplied (Merck). The IR spectra were recorded using a BioRad FTS-135
spectrometer. Proton and carbon NMR spectra were recorded on a Bruker DFX
1
spectrometer (Sigma-Aldrich) operating at 270 MHz, using CDCl3 as solvent. H
chemical shifts were referenced to the solvent peak position, d 7.26.
Transition temperatures were measured using A Linkam THMS600 hot stage
and a TMS94 temperature controller attached to a Nikon 50i Pol polarizing micro-
scope. These temperatures were confirmed by differential scanning calorimetry
(Perkin-Elmer Diamond instrument using 10ꢀC=min or 5ꢀC=min heating rate).
Two or more heating=cooling cycles were performed on each sample. The
mesophases were assigned by their optical texture.
Synthesis of Imine Ligands 1
The imine ligands were prepared by the condensation reaction between the corre-
sponding aldehydes and anilines, catalyzed by glacial acetic acid, in absolute ethanol.
The preparation of 1b is presented below. To a solution of p-hexyloxyaniline
(0.193 g, 1 mmol) in ethanol (10 ml), 3,4-didecyloxybenzaldehyde (0.387 g, 1 mmol)
followed by five drops of glacial acetic acid was added. The mixture was heated
under reflux for 2 h and then cooled to ꢁ25ꢀC to give the crude product. Recrystal-
lization from hot ethanol gave the analytically pure product as off-white crystals.
The preparation and characterization of 1a and 1d were reported elsewhere [11,13].
Compound 1b. Yield 47%, off-white crystals, m.p. 73ꢀC. Anal. Calc. for C39H63NO3:
1
C, 78.9; H, 10.7; N, 2.4; Found: C, 78.7; H, 11.0; N, 2.1. H NMR (300 MHz,
=
CDCl3): 8.35 (s, 1H, CH N), 7.60 (d, broad, 1H), 7.27 (dd, J ¼ 1.8 Hz and
J ¼ 8.3 Hz, 1H), 7.19 (AA0XX0 system, J ¼ 8.9 Hz, 2H aniline ring), 6.91 (d,
J ¼ 8.9 Hz, 2H, aniline ring), 4.12–4.03 (m, 4H, 2OCH2 groups), 3.97 (t,
J ¼ 6.5 Hz, 2H, OCH2), 1.90–1.25 (m, 40H, CH2 groups), 0.95–0.85 (m, 9H, CH3
groups). 13C NMR: 158.0, 157.4, 151.8, 149.3, 129.5, 123.7, 121.9, 114.8, 112.4,
111.1, 69.0, 68.2, 31.7, 29.1, 29.0, 25.9, 22.5, 14.0. IR (cmꢁ1): 2926 vs, 2856 s,
1621 m, 1599 m, 1574 m, 1513 vs, 1468 m, 1432 m, 1391 m, 1293 m, 1266 vs, 1243
vs, 1168 w, 1135 m, 1024 m, 870 w, 831 s, 723 w, 617 w.
Compound 1c. Yield 55%, off-white crystals, m.p. 64ꢀC. Anal. Calc. for C37H59NO3:
1
C, 78.5; H, 10.5; N, 2.5; Found: C, 78.1; H, 10.9; N, 2.3. H NMR (300 MHz,
=
CDCl3): 8.36 (s, 1H, CH N), 7.57 (d, broad, 1H), 7.26 (dd, J ¼ 1.8 Hz and
J ¼ 8.3 Hz, 1H), 7.19 (AA0XX0 system, J ¼ 8.8 Hz, 2H aniline ring), 6.91 (AA0XX0
system, J ¼ 8.8 Hz, 2H, aniline ring), 4.12–4.03 (m, 4H, 2OCH2 groups), 3.97 (t,
J ¼ 6.6 Hz, 2H, OCH2 group), 1.90–1.25 (m, 36H, CH2 groups), 0.95–0.85 (m, 9H,
CH3 groups). 13C NMR: 158.0, 157.4, 151.8, 149.3, 129.5, 123.7, 121.9, 114.8,
112.4, 111.1, 69.0, 68.2, 31.7, 29.1, 25.9, 22.5, 14.0. IR (cmꢁ1): 2956 s, 2922 vs,
2851 s, 1623 m, 1601 m, 1577 m, 1511 vs, 1467 m, 1433 m, 1390 m, 1267 vs, 1237 vs,
1170 w, 1136 m, 1017 m, 866 w, 830 s, 723 w, 614 w.
Synthesis of Acetato-Dinuclear Palladium(II) Complexes 2a-c
The following is the typical procedure for acetato-bridged palladium dinuclear
complexes. The imine ligand (2.5 mmol) was dissolved in dichloromethane (30 ml)

Dinuclear ortho-Metallated palladium(II) Complexes
47
and palladium acetate (0.83 mmol) was added. The resulting mixture was stirred at
room temperature for 18 h, after which the solvent was removed in vacuo. The resi-
due was crystallized from a mixture of dichloromethane–ethanol several times to
give the yellow crystalline products, which were washed with cold ethanol and dried
under vacuum. The preparation and characterization of 2d was reported elsewhere
1
[11]. The yields and elemental analysis results are presented below. The H and
13C NMR spectra are identical for the three Pd(II) complexes, except the different
integration values corresponding to the signals in the 1.52–1.24 ppm region of the
1H NMR spectra, assigned to hydrogens form the alkoxy chains of imine ligands.
Compound 2a. Yield 63%, yellow crystals. Anal. Calc. for C74H114N2O10Pd2: C,
1
63.3; H, 8.2; N, 2.0; Found: C, 63.0; H, 8.5; N, 1.8. H NMR (300 MHz, CDCl3):
=
7.43 (s, 1H, HC N), 6.82 (s, 1H, Pd-substituted phenyl ring), 6.65 (q, J ¼ 7.0 Hz,
4H, aniline ring), 5.99 (s, 1H, Pd-substituted phenyl ring), 3.99–3.45 (m, 6H), 1.89
(s, 3H, CH3 acetate group), 1.78 (m, 6H), 1.52–1.24 (m, 26H), 0.95–0.85 (m, 9H).
13C NMR: 180.2, 170.1, 158.1, 151.1, 150.7, 145.7, 141.3, 136.8, 124.0, 115.6,
114.3, 113.5, 70.8, 68.3, 32.0, 29.5, 26.3, 24.3, 22.8, 14.3. IR (cmꢁ1): 2927 vs,
2856 s, 1654 m, 1597 s, 1517 vs, 1453 m, 1291 vs, 1259 s, 1186 m, 1149 m, 1058 w,
1019 w, 832 w, 723 w.
Compound 2b. Yield 71%, yellow crystals. Anal. Calc. for C82H130N2O10Pd2: C,
64.9; H, 8.6; N, 1.8; Found: C, 64.5; H, 8.9; N, 1.6.
Compound 2c. Yield 68%, yellow crystals. Anal. Calc. for C78H122N2O10Pd2: C,
64.1; H, 8.4; N, 1.9; Found: C, 63.8; H, 8.8; N, 1.6.
Synthesis of Chloro-Dinuclear Palladium(II) Complexes 3a-c
In the case of chloro-bridged dinuclear palladium(II) complex, the corresponding
acetato-bridged complex was used in the next step without further purification.
Thus, the crude product obtained as previously described was treated with 1 mL
of HCl in 10 mL of methanol. The mixture was further stirred for 8 h, after which
the solvent was removed in vacuo. The residue was crystallized several times from
a mixture of dichloromethane–ethanol to give the yellowish-brown crystalline
chloro-bridged dinuclear palladium(II) complexes. The yields, elemental analysis
1
results, as well as H and 13C NMR data are presented below.
Compound 3a. Yield 47%, yellow-brown crystals. Anal. Calc. for
C70H108Cl2N2O6Pd2: C, 63.7; H, 8.5; N, 1.9; Found: C, 63.5; H, 9.0; N, 1.6.1H
=
NMR (300 MHz, CDCl3): 8.15 (s, 1H, HC N), 8.00 (s, 1H, Pd-substituted phenyl
ring), 7.97 (s, 1H, Pd-substituted phenyl ring), 6.97 (AA0XX0 system, J ¼ 8.8 Hz,
2H aniline ring), 6.87 (AA0XX0 system, J ¼ 8.8 Hz, 2H aniline ring), 4.42 (m, 2H,
OCH2), 4.07 (t, J ¼ 6.6 Hz, 2H, OCH2), 3.87 (t, J ¼ 6.3 Hz, 2H, OCH2), 1.95–1.20
(m, 32H, CH2 groups), 0.94–0.86 (m, 9H, CH3 groups).13C NMR: 160.4, 158.3,
150.1, 145.2, 129.6, 122.5, 119.4, 115.7, 113.8, 112.3, 70.4, 69.7, 68.6, 32.0,
29.5, 29.0, 26.4, 26.1, 22.8, 14.2. IR (cmꢁ1): 2925 vs, 2857 s, 1653 m, 1599 s, 1515
vs, 1466 m, 1450 m, 1392 w, 1285 vs, 1253 s, 1180 m, 1148 s, 1012 w, 832 w,
810 w, 722 w.
Compound 3b. Yield 40%, yellowish-brown crystals. Anal. Calc. for C78H124Cl2
N2O6Pd2: C, 63.7; H, 8.5; N, 1.9; Found: C, 63.3; H, 9.0; N, 1.7. 1H NMR

ˆ
V. Cırcu and F. Dumitras¸cu
48
=
(300 MHz, CDCl3): 8.16 (s, 1H, HC N), 7.95 (s, 1H, Pd-substituted phenyl ring),
7.92 (s, 1H, Pd-substituted phenyl ring), 6.97 (AA0XX0 system, J ¼ 8.5 Hz, 2H
aniline ring), 6.90 (AA0XX0 system, J ¼ 8.8 Hz, 2H aniline ring), 4.23 (m, 2H,
OCH2), 4.09 (t, J ¼ 6.8 Hz, 2H, OCH2), 3.91 (t, J ¼ 6.6 Hz, 2H, OCH2), 1.97–1.20
(m, 40H, CH2 groups), 0.92–0.87 (m, 9H, CH3 groups). 13C NMR: 160.4, 158.3,
150.1, 145.2, 129.6, 122.5, 119.4, 115.7, 113.8, 112.3, 70.4, 69.7, 68.6, 32.0, 29.5,
29.0, 28.8, 26.4, 26.1, 22.8, 14.2. IR (cmꢁ1): 2926 vs, 2857 s, 1653 m, 1599 s, 1515
vs, 1466 m, 1449 m, 1392 w, 1285 vs, 1253 s, 1179 m, 1148 s, 1015 w, 832 w,
807 w, 722 w.
Compound 3c. Yield 52%, yellow-brown crystals. Anal. Calc. for C74H116Cl2N2
1
O6Pd2: C, 62.9; H, 8.3; N, 2.0; Found: C, 62.4; H, 8.8; N, 1.7. H NMR (300 MHz,
=
CDCl3): 8.19 (s, 1H, HC N), 7.99 (s, 1H, Pd-substituted phenyl ring), 7.96 (s, 1H,
Pd-substituted phenyl ring), 6.97 (AA0XX0 system, J ¼ 8.8 Hz, 2H aniline ring), 6.88
(AA0XX0 system, J ¼ 8.8 Hz, 2H aniline ring), 4.41 (m, 2H, OCH2), 4.08 (t,
J ¼ 6.6 Hz, 2H, OCH2), 3.89 (t, J ¼ 6.3 Hz, 2H, OCH2), 1.95–1.20 (m, 36H, CH2
groups), 0.94–0.86 (m, 9H, CH3 groups). 13C NMR: 160.4, 158.4, 150.1, 145.2,
129.6, 122.5, 119.4, 115.7, 113.8, 112.2, 70.5, 69.7, 68.6, 32.0, 29.5, 29.0, 26.4, 26.1,
22.8, 14.2. IR (cmꢁ1): 2926 vs, 2858 s, 1653 m, 1599 s, 1515 vs, 1470 m, 1449 m,
1392 w, 1285 vs, 1253 s, 1180 m, 1148 s, 1015 w, 830 w, 807 w, 722 w.
Conclusions
The imines 1a-c with three alkoxy chains used as ligands do not show any
liquid-crystalline properties. Increasing the alkoxy chain lengths, either at the aniline
or benzaldehyde unit, leads to a decrease of transition temperatures, higher in the
case of increasing the two alkoxy chains lengths from the benzaldehyde phenyl ring.
The stability of the SmA phase is higher for chloro-bridged Pd(II) complexes com-
pared to their acetate-bridged analogues. It is clear that the mesogenic behavior of
acetato-bridged ortho-metallated Pd(II) complexes is destabilized due to the
promotion of nonplanarity in such complexes leading to monotropic mesogenic
behavior.
Acknowledgments
The authors acknowledge the financial support from CNCSIS-UEFISCSU, project
number PNII-IDEI ID_954/2007 and CNCSIS A1640_GR 96/2007.
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