Cyclometalated palladium(II) metallomesogens with Schiff bases and N-benzoyl thiourea derivatives as co-ligands

Article abstract of DOI:10.1016/j.molliq.2017.02.114

Two series of cyclometalated palladium(II) complexes based on Schiff bases and N-benzoylthiourea (BTU) ligands having four alkoxy groups at their periphery, with different number of carbon atoms, have been designed and prepared. The liquid crystalline behavior of these palladium(II) complexes was investigated by a combination of polarized optical microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray diffraction while their thermal stability was studied by thermogravimetric analysis (TGA). The palladium(II) complexes are thermally stable on a broad temperature range up to 230?°C showing, depending on the alkoxy chain length, either monotropic or, for longest alkyl chain, enantiotropic smectic A and C phases. The luminescent properties of palladium(II) complexes were reported. These complexes show a yellow-orange solid-state emission at room temperature with two emission maxima at λ_max around 580 and 650?nm, respectively, with a shoulder around 710?nm, when the samples are irradiated in the 330–380?nm range.

Full text of DOI:10.1016/j.molliq.2017.02.114

Accepted Manuscript
Cyclometalated palladium(II) metallomesogens with Schiff bases
and N-benzoyl thiourea derivatives as co-ligands
Monica Ilis, Dan Batalu, Iuliana Pasuk, Viorel Cîrcu
PII:
DOI:
Reference:
S0167-7322(17)30099-5
doi: 10.1016/j.molliq.2017.02.114
MOLLIQ 7027
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24 February 2017
27 February 2017
Please cite this article as: Monica Ilis, Dan Batalu, Iuliana Pasuk, Viorel Cîrcu ,
Cyclometalated palladium(II) metallomesogens with Schiff bases and N-benzoyl thiourea
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ACCEPTED MANUSCRIPT
Cyclometalated palladium(II) metallomesogens with Schiff bases
and N-benzoyl thiourea derivatives as co-ligands
Monica Ilisa, Dan Batalub, Iuliana Pasukc, Viorel Cîrcu*a
a
Department of Inorganic Chemistry, University of Bucharest, 23 Dumbrava Rosie st, sector 2,
Bucharest 020464, Romania, e-mail: viorel.circu@chimie.unibuc.ro
bUniversity 'Politehnica' of Bucharest, Splaiul Independentei 313, 060042, Romania
c National Institute of Materials Physics, P.O. Box MG-7, Magurele, 077125, Romania
Abstract
Two series of cyclometalated palladium(II) complexes based on Schiff bases and N-
benzoylthiourea (BTU) ligands having four alkoxy groups at their periphery, with different
number of carbon atoms, have been designed and prepared. The liquid crystalline behavior of
these palladium(II) complexes was investigated by a combination of polarized optical
microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray diffraction while
their thermal stability was studied by thermogravimetric analysis (TGA). The palladium(II)
complexes are thermally stable on a broad temperature range up to 230oC showing, depending on
the alkoxy chain length, either monotropic or, for longest alkyl chain, enantiotropic smectic A
and C phases. The luminescent properties of palladium(II) complexes were reported. These
complexes show a yellow-orange solid-state emission at room temperature with two emission
maxima at λmax around 580 and 650 nm, respectively, with a shoulder around 710 nm, when the
samples are irradiated in the 330 - 380 nm range.
Introduction
The research area dealing with the design and investigation of metal-containing liquid crystals
(metallomesogens) expanded considerably in the past few decades. [1,2] There is a growing
interest to study the unique combination of the properties of anisotropic fluids (anisotropy of
physical properties and fast orientational response to external fields) specific to liquid crystals
(LC) with the particular properties of metals (geometry of coordination, electronic, magnetic or
the purely structural role, depending on the metal ion). [3] The different coordination geometries
adopted by metal centers enabled the rich structures and topologies of these metal complexes
1

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and, consequently, more diversified LC properties to those found for pure organic derivatives.
[4] Liquid crystalline materials have found various applications such as the manufacturing of
LCD, molecular sensors and detectors, optical switches, spatial light modulator, etc.[5-7] In
particular, the liquid crystalline materials based on palladium(II) complexes were given a
tremendous attention. [8-18] Amongst various advantages, their square planar geometry that
favors the stabilization of LC phases and their relative ease of preparation are worth to mention
here. Moreover, the LC and emission properties could be controlled easier by a variety of
available options to change the chemical structure around the palladium metal center. [19-23] A
judicious design of the ligands coordinated to palladium(II) metal could bring improved LC
properties, reflected in lower transition temperatures preventing chemical decomposition and
stability of the mesophases near room temperature. [24, 25] In the past years, we have started the
investigation of a new class of palladium(II) complexes with LC properties based on
cyclometalated imine ligands and N-benzoylthiourea (BTU) derivatives as auxiliary ligands. The
BTU compounds represent an interesting class of chelating ligands bearing two very strong
donor groups (carbonyl and thioamide). In most cases they produce neutral metal complexes
with S,O-coordination. [26-28] Moreover, the BTU compounds decorated with alkoxy groups
show interesting mesogenic behavior. [29-32] The specific influence of the alkoxy-substituted
BTU derivatives on the thermal behavior of square-planar Pd(II) and Pt(II) cyclometalated
complexes together with their emissive properties were reported. [33, 34] When simple N-
benzoyl thiourea derivatives were used as co-ligands for palladium(II) metal accompanied by
cyclometalated imine ligands, only monotropic nematic or smectic A phases were observed. [35-
39] On the other hand, the use of BTU ligands having various number of alkoxy terminal groups,
can lead to mono- or dinuclear palladium(II) complexes with lower transition temperatures and
rich mesomorphic behavior ranging from calamitic to discotic materials. [34] In this work, we
report the design and a systematic investigation of the LC properties of a series of palladium(II)
complexes based on cyclometalated Schiff bases and BTU ligands with alkoxy groups of various
chain length grafted in terminal positions. The emission properties of the palladium(II)
complexes in solid state were also investigated.
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Experimental
Characterization methods. All the chemicals were used as supplied. C, H, N analyses were
carried out with an EuroEA 3300 instrument. IR spectra were recorded on a Bruker
1
13
spectrophotometer using ATR technique. H and C NMR spectra were recorded on a Bruker
spectrometer operating at 300 MHz, using CDCl3 as solvent. 1H chemical shifts were referenced
to the solvent peak position, 7.26 ppm. The phase assignments and corresponding transition
temperatures for all palladium complexes 1, 2 a-f, were evaluated by polarizing optical light
microscopy (POM), placed on untreated glass slides, using a Nikon 50iPol microscope equipped
with a Linkam THMS600 hot stage and TMS94 control processor. Temperatures and enthalpies
of transitions were collected from their corresponding thermograms recorded by using
differential scanning calorimetry (DSC) technique employing a Diamond DSC Perkin Elmer
instrument. The materials were studied at 10o/min scanning rate after being encapsulated in
aluminium pans. Three heating/cooling cycles were performed on each sample. The nature of LC
phases was established by their optical texture and powder X-ray diffraction data. The powder
X-ray diffraction measurements were made on a D8 Advance diffractometer (Bruker AXS
GmbH, Germany), in parallel beam setting, with monochromatized Cu–Kα1 radiation (λ=1.5406
Å), scintillation detector, and horizontal sample stage. The measurements were performed in
symmetric (θ-θ) geometry in the 2θ range from 1.5o to 30o in steps of 0.02o, with measuring
times per step in the 5-40 s range. The temperature control of the samples during measurements
was achieved by adapting a home-made heating stage to the sample stage of the diffractometer.
Thermogravimetric analysis was performed on a TA Q50 TGA instrument using alumina
crucibles and nitrogen as purging gas. The heating rate employed was 10°C min-1 from room
temperature (approx. 25°C) to 1000oC. The variable-temperature emission spectra in solid state
were recorded with an OceanOptics QE65PRO spectrometer attached to the microscope and
using a Nikon Intensilight excitation source.
Synthesis of Pd(II) complexes 1,2a-f
Corresponding solid N-benzoylthiourea compound (0.30 mmol), prepared as described elsewhere
[32, 54], was added to a suspension of dinuclear -acetato-bridged palladium complexes (0.10
mmol) and K2CO3 in dichloromethane (15 cm3) and the mixture stirred at room temperature for
24 hours. Evaporation of the solvent gave yellow solids, which were purified by chromatography
3

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on silica using dichloromethane as eluant to yield the final products. They were further
crystallized from a mixture of dichloromethane/ethanol (1/1) at –25ºC.
Only the first compound of each series is described below as their 1H NMR spectra are identical
except for corresponding decrease in integration of the hydrogens of the alkoxy chains at 1.55–
1.25ppm.
1a Yellow solid. Yield 76%. Calc. for C51H69N3O5PdS: %C: 64.98; %H 7.38; %N 4.46. Found:
%C 64.65; %H 7.66; %N 4.32.
1H-NMR (CDCl3, 300 MHz): 8.13 (s, 1H), 7.90 (s, br, 1H), 7.65 (d, br, 2H), 7.49-7.33 (m, 6H),
3
6.98 (d, J=8.5Hz, 2H), 6.90 (d, J=8.3Hz, 2H), 6.69 (d, J=8.8Hz, 2H), 6.60 (dd, J=8.0Hz,
4J=2.1Hz, 1H), 4.06-3.92 (m, 8H), 1.90-1.82 (m, 8H), 1.55-1.25 (m, 24H), 0.95-0.90 (m, 12H).
13C NMR (CDCl3, 75 MHz): 171.1, 162.3, 160.3, 158.4, 157.9, 141.6, 140.1, 132.3, 130.1,
129.8, 126.3, 124.5, 118.9, 114.7, 114.5, 113.5, 110.9, 68.6, 68.4, 68.1, 32.2, 32.1, 29.9, 29.7,
29.6, 29.5, 29.4, 29.3, 26.4, 22.9, 14.3.
IR (ATR, cm-1): 3227, 2955, 2923, 2856, 1606, 1583, 1541, 1506, 1473, 1422, 1312, 1297, 1247,
1167, 1111, 1034, 915, 834, 792, 768, 669, 576, 519.
1c. Yield 84%. Yellow solid. Analytical: Calc. for C59H85N3O5PdS: %C: 67.18; %H 8.12; %N
3.98. Found: %C 66.89; %H 8.45; %N 3.81.
1H-NMR (CDCl3, 300 MHz): 8.13 (s, 1H), 7.91 (s, br, 1H), 7.65 (d, br, 2H), 7.50-7.32 (m, 6H),
3
6.98 (d, J=8.5Hz, 2H), 6.90 (d, J=8.3Hz, 2H), 6.69 (d, J=8.8Hz, 2H), 6.60 (dd, J=8.0Hz,
4J=2.1Hz, 1H), 4.06-3.92 (m, 8H), 1.90-1.82 (m, 8H), 1.55-1.25 (m, 40H), 0.95-0.90 (m, 12H).
13C NMR (CDCl3, 75 MHz): 171.1, 162.3, 160.3, 158.4, 157.9, 141.5, 140.2, 132.3, 130.1,
129.8, 126.3, 124.5, 118.9, 114.7, 114.4, 113.5, 110.9, 68.7, 68.4, 68.1, 32.2, 32.1, 29.9, 29.7,
29.6, 29.5, 29.4, 29.3, 29.2, 26.4, 22.9, 14.3.
IR (ATR, cm-1): 3227, 2955, 2923, 2856, 1606, 1583, 1541, 1506, 1473, 1422, 1312, 1297, 1247,
1167, 1111, 1034, 915, 834, 792, 768, 669, 576, 519.
1d. Yield 88%. Yellow solid. Analytical: Calc. for C63H93N3O5PdS: %C: 68.11; %H 8.44; %N
3.78. Found: %C 67.87; %H 8.67; %N 3.55.
1H-NMR (CDCl3, 300 MHz): 8.14 (s, 1H), 7.90 (s, br, 1H), 7.65 (d, br, 2H), 7.49-7.33 (m, 6H),
3
6.98 (d, J=8.5Hz, 2H), 6.89 (d, J=8.3Hz, 2H), 6.69 (d, J=8.8Hz, 2H), 6.60 (dd, J=8.0Hz,
4J=2.1Hz, 1H), 4.04-3.90 (m, 8H), 1.91-1.83 (m, 8H), 1.55-1.25 (m, 48H), 0.95-0.90 (m, 12H).
13C NMR (CDCl3, 75 MHz): 171.2, 162.3, 160.3, 158.5, 157.9, 141.5, 140.1, 132.3, 130.1,
4

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129.8, 126.3, 124.5, 118.9, 114.6, 114.5, 113.5, 110.9, 68.6, 68.4, 68.1, 32.2, 32.1, 29.9, 29.7,
29.6, 29.5, 29.4, 29.3, 29.2, 29.1, 26.4, 22.9, 14.3.
IR (ATR, cm-1): 3230, 2955, 2923, 2857, 1606, 1582, 1541, 1507, 1473, 1425, 1312, 1297, 1247,
1167, 1111, 1034, 916, 834, 792, 769, 670, 576, 519.
1e. Yield 65%. Yellow solid. Analytical: Calc. for C67H101N3O5PdS: %C: 68.95; %H 8.72; %N
3.60. Found: %C 68.71; %H 8.97; %N 3.48.
1H-NMR (CDCl3, 300 MHz): 8.15 (s, 1H), 7.90 (s, br, 1H), 7.65 (d, br, 2H), 7.49-7.33 (m, 6H),
3
6.98 (d, J=8.5Hz, 2H), 6.90 (d, J=8.3Hz, 2H), 6.69 (d, J=8.8Hz, 2H), 6.60 (dd, J=8.0Hz,
4J=2.1Hz, 1H), 4.06-3.92 (m, 8H), 1.90-1.82 (m, 8H), 1.55-1.25 (m, 56H), 0.95-0.90 (m, 12H).
13C NMR (CDCl3, 75 MHz): 171.1, 162.3, 160.3, 158.4, 157.9, 141.6, 140.1, 132.3, 130.1,
129.8, 126.3, 124.5, 118.9, 114.7, 114.5, 113.5, 110.9, 68.6, 68.4, 68.1, 32.2, 32.1, 29.9, 29.7,
29.6, 29.5, 29.4, 29.3, 26.4, 22.9, 14.3.
IR (ATR, cm-1): 3227, 2952, 2924, 2856, 1608, 1583, 1544, 1506, 1473, 1421, 1312, 1297, 1247,
1167, 1113, 1035, 915, 834, 792, 768, 669, 576, 521.
1f. Yield 83%. Yellow solid. Analytical: Calc. for C71H109N3O5PdS: %C: 69.72; %H 8.98; %N
3.44. Found: %C 69.56; %H 9.16; %N 3.23.
1H-NMR (CDCl3, 300 MHz): 8.14 (s, 1H), 7.88 (s, br, 1H), 7.65 (d, br, 2H), 7.49-7.33 (m, 6H),
3
6.98 (d, J=8.5Hz, 2H), 6.90 (d, J=8.3Hz, 2H), 6.67 (d, J=8.8Hz, 2H), 6.62 (dd, J=8.0Hz,
4J=2.1Hz, 1H), 4.07-3.93 (m, 8H), 1.90-1.82 (m, 8H), 1.55-1.25 (m, 64H), 0.95-0.90 (m, 12H).
13C NMR (CDCl3, 75 MHz): 171.3, 162.3, 160.3, 158.4, 157.9, 141.5, 140.1, 132.3, 130.1,
129.9, 126.3, 124.5, 118.9, 114.9, 114.6, 113.5, 111, 68.6, 68.4, 68.2, 32.2, 32.1, 29.9, 29.7, 29.6,
29.5, 29.4, 29.3, 29.2, 29.1, 26.4, 22.9, 14.2.
IR (ATR, cm-1): 3231, 2955, 2925, 2857, 1606, 1583, 1544, 1506, 1473, 1425, 1313, 1297, 1243,
1167, 1112, 1034, 915, 834, 792, 768, 669, 576, 519.
2a Yield 90%. Yellow solid. Analytical: Calc. for C55H77N3O5PdS: %C: 66.14; %H 7.77; %N
4.21. Found: %C 65.78; %H 7.97; %N 3.98.
1H-NMR (CDCl3, 300 MHz): 8.12 (s, 1H), 7.92 (s, br, 1H), 7.66 (d, br, 2H), 7.51-7.33 (m, 6H),
3
6.98 (d, J=8.5Hz, 2H), 6.90 (d, J=8.3Hz, 2H), 6.69 (d, J=8.8Hz, 2H), 6.61 (dd, J=8.0Hz,
4J=2.4Hz, 1H), 4.06-3.92 (m, 8H), 1.92-1.82 (m, 8H), 1.55-1.25 (m, 32H), 0.95-0.90 (m, 12H).
5

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13C NMR (CDCl3, 75 MHz): 171.1, 162.5, 160.3, 158.5, 157.9, 141.7, 140.1, 132.4, 130.1,
129.8, 126.3, 124.5, 118.9, 114.8, 114.6, 113.5, 110.9, 68.7, 68.5, 68.1, 32.2, 32.1, 29.9, 29.7,
29.6, 29.5, 29.4, 29.3, 26.4, 22.9, 14.3.
IR (ATR, cm-1): 3221, 2957, 2922, 2853, 1603, 1586, 1540, 1504, 1472, 1416, 1314, 1236, 1209,
1167, 1109, 1031, 964, 910, 836, 789, 765, 724, 660, 623, 567, 519.
2c. Yield 78%. Yellow solid. Analytical: Calc. for C63H93N3O5PdS: %C: 68.11; %H 8.44; %N
3.78. Found: %C 67.88; %H 8.65; %N 3.57.
1H-NMR (CDCl3, 300 MHz): 8.13 (s, 1H), 7.91 (s, br, 1H), 7.67 (d, br, 2H), 7.51-7.33 (m, 6H),
3
6.98 (d, J=8.5Hz, 2H), 6.90 (d, J=8.3Hz, 2H), 6.70 (d, J=8.8Hz, 2H), 6.61 (dd, J=8.0Hz,
4J=2.3Hz, 1H), 4.06-3.92 (m, 8H), 1.92-1.82 (m, 8H), 1.55-1.25 (m, 48H), 0.95-0.90 (m, 12H).
13C NMR (CDCl3, 75 MHz): 171.1, 162.5, 160.4, 158.5, 157.8, 141.7, 140.1, 132.4, 130.1,
129.8, 126.5, 124.6, 118.9, 114.8, 114.6, 113.5, 110.9, 68.7, 68.5, 68.2, 32.2, 32.1, 29.9, 29.7,
29.6, 29.5, 29.4, 29.3, 29.2, 26.4, 22.9, 14.3.
IR (ATR, cm-1): 3225, 2957, 2922, 2853, 1603, 1586, 1539, 1504, 1472, 1416, 1314, 1236, 1209,
1168, 1109, 1033, 964, 910, 836, 789, 765, 724, 662, 623, 567, 519.
2d. Yield 79%. Yellow solid. Analytical: Calc. for C67H101N3O5PdS: %C: 68.95; %H 8.72; %N
3.60. Found: %C 68.73; %H 9.03; %N 3.48.
1H-NMR (CDCl3, 300 MHz): 8.14 (s, 1H), 7.92 (s, br, 1H), 7.66 (d, br, 2H), 7.51-7.33 (m, 6H),
3
6.98 (d, J=8.5Hz, 2H), 6.90 (d, J=8.3Hz, 2H), 6.69 (d, J=8.8Hz, 2H), 6.60 (dd, J=7.9Hz,
4J=2.4Hz, 1H), 4.06-3.92 (m, 8H), 1.92-1.82 (m, 8H), 1.55-1.25 (m, 56H), 0.95-0.90 (m, 12H).
13C NMR (CDCl3, 75 MHz): 171.2, 162.5, 160.3, 158.5, 157.9, 141.7, 140.1, 132.4, 130.1,
129.8, 126.3, 124.3, 118.9, 114.8, 114.5, 113.5, 110.9, 68.7, 68.5, 68.1, 32.2, 32.1, 29.9, 29.7,
29.6, 29.5, 29.4, 29.3, 29.2, 29.1, 26.4, 22.9, 14.3.
IR (ATR, cm-1): 3224, 2955, 2924, 2853, 1603, 1586, 1540, 1504, 1472, 1416, 1314, 1237, 1209,
1167, 1110, 1031, 965, 910, 836, 789, 765, 724, 660, 623, 568, 520.
2e. Yield 86%. Yellow solid. Analytical: Calc. for C71H109N3O5PdS: %C: 69.72; %H 8.98; %N
3.44. Found: %C 69.53; %H 9.15; %N 3.27.
1H-NMR (CDCl3, 300 MHz): 8.13 (s, 1H), 7.92 (s, br, 1H), 7.67 (d, br, 2H), 7.52-7.34 (m, 6H),
3
6.98 (d, J=8.5Hz, 2H), 6.91 (d, J=8.3Hz, 2H), 6.69 (d, J=8.8Hz, 2H), 6.61 (dd, J=8.0Hz,
4J=2.4Hz, 1H), 4.06-3.92 (m, 8H), 1.92-1.82 (m, 8H), 1.55-1.25 (m, 64H), 0.95-0.90 (m, 12H).
6

ACCEPTED MANUSCRIPT
13C NMR (CDCl3, 75 MHz): 171.1, 162.5, 160.3, 158.5, 157.9, 141.7, 140.1, 132.4, 130.1,
129.8, 126.3, 124.5, 118.9, 114.8, 114.6, 113.5, 110.9, 68.7, 68.5, 68.1, 32.2, 32.1, 29.9, 29.7,
29.6, 29.5, 29.4, 29.3, 29.2, 29.1, 26.4, 22.9, 14.3.
IR (ATR, cm-1): 3225, 2956, 2921, 2853, 1603, 1589, 1540, 1504, 1473, 1416, 1314, 1236, 1209,
1165, 1112, 1031, 964, 912, 838, 789, 765, 724, 660, 623, 567, 519.
2f. Yield 77%. Yellow solid. Analytical: Calc. for C75H117N3O5PdS: %C: 70.42; %H 9.22; %N
3.28. Found: %C 70.19; %H 9.51; %N 3.07.
1H-NMR (CDCl3, 300 MHz): 8.11 (s, 1H), 7.92 (s, br, 1H), 7.65 (d, br, 2H), 7.51-7.33 (m, 6H),
3
6.98 (d, J=8.5Hz, 2H), 6.90 (d, J=8.3Hz, 2H), 6.70 (d, J=8.7Hz, 2H), 6.60 (dd, J=8.0Hz,
4J=2.4Hz, 1H), 4.05-3.91 (m, 8H), 1.92-1.82 (m, 8H), 1.55-1.25 (m, 72H), 0.95-0.90 (m, 12H).
13C NMR (CDCl3, 75 MHz): 171.3, 162.5, 160.2, 158.5, 157.9, 141.7, 140.1, 132.4, 130.1,
129.7, 126.3, 124.6, 118.9, 114.8, 114.6, 113.5, 111, 68.7, 68.5, 68.1, 32.2, 32.1, 29.9, 29.7, 29.6,
29.5, 29.4, 29.3, 29.2, 29.1, 26.4, 22.9, 14.3.
IR (ATR, cm-1): 3225, 2957, 2926, 2853, 1605, 1586, 1540, 1504, 1472, 1416, 1314, 1236, 1209,
1167, 1112, 1034, 964, 910, 836, 789, 765, 725, 660, 624, 567, 517.
Results and Discussion
The preparation of cyclometalated palladium(II) complexes along with their numbering scheme
is presented in Scheme 1. Their synthesis has been achieved by treatment of the corresponding
dinuclear -acetato-bridged species with BTU derivatives in dichloromethane, in the presence of
potassium carbonate at room temperature. The final mononuclear complexes were purified on
silica by using dichloromethane as eluant, followed by repeated recrystallization from a mixture
of dichloromethane and ethanol, to give the desired compounds in good yields as yellow to
yellow-brown solid products. The new mononuclear palladium(II) complexes were characterized
1
by using several techniques: elemental analysis, IR, H and 13C NMR spectroscopy and, to
evaluate the potential LC properties, a combination of differential scanning calorimetry (DSC),
polarizing optical microscopy (POM) and powder X-ray diffraction were employed.
7

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OCnH2n+1
OCmH2m+1
OCmH2m+1
OCmH2m+1
Pd(OAc)2
BTU/K2CO3
CH2Cl2/R.T.
OAc
O
Pd
Pd
N
CH2Cl2/R.T.
S
N
N
N
2
HN
OCnH2n+1
OCmH2m+1
OCmH2m+1
OCmH2m+1
1a-f: m=6
2a-f: m=8
n=6,8,10,12,14,16
Scheme 1. Preparation of mononuclear palladium(II) complexes
1
Several features in the H NMR and IR spectra of palladium(II) complexes confirm the
coordination of the N-benzoylthiourea derivatives in the chelating deprotonated form to the metal
center. While the 1H NMR spectra of the BTU derivatives contains two singlets, one in the range
12.48–12.37 ppm and a second one in the range 9.02– 8.98 ppm, which were assigned to the two
existing NH groups, the 1H-NMR spectra of the corresponding palladium(II) complexes show no
presence of these two signals as a consequence of ligand coordination.[32, 33] The other signals,
sometimes overlapped as multiplets, assigned to aromatic protons show the normal splitting
pattern of a 1,4-or 1,3,4-substitution of the aromatic rings. These data suggest the absence of the
NH hydrogen atom located between the thiocarbonyl and carbonyl groups belonging to the N-
benzoylthiourea unit. Moreover, there are no absorption bands in the range between 1610 and
1690 cm-1 in the infrared spectra of all palladium(II) complexes, which is the normal range for
νC=O stretches in the spectra of the free BTU ligands. [40] Such a large shift of more than 200
cm-1 to lower wavenumbers indicates chelate formation with a high degree of electron
delocalization within the chelate rings; this behavior has been observed previously for other
chelate complexes of BTU derivatives with various metals. [41, 42] Still, the IR spectrum of the
palladium(II) compounds exhibit a sharp, medium absorption around 3225 cm-1 assigned to the
NH vibration of the remaining NH-R group. These BTU derivatives are asymmetric and they can
lead to mixture of cis and trans isomers for mononuclear palladium(II) complexes. The signal
1
assigned to H-C=N- iminic hydrogen atom appears as a singlet around 8.10 ppm in H NMR
spectra of palladium(II) complexes, well apart from the other aromatic signals, and, for this
reason it could be useful to evaluate the presence of cis and trans isomers mixture in final
products.[25] Thus, the presence of one signal in the region, together with the presence of only
8

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one set of other signals, led to the conclusion that the products contain mainly one of the two
possible isomers with respect to the positions of sulfur atom of the BTU ligand and nitrogen
atom of the cyclometalated imine ligand.
Liquid crystals properties
The thermal data for all palladium(II) complexes are presented in Table 1. The LC phase
assignment was done based on their optical texture and X-ray measurements. [43, 44] Several
typical features for SmA and SmC features were observed by POM. For instance, the SmA phase
was assigned based on its characteristic focal fan shape texture with several homeotropic regions
when developed from the isotropic state. The SmC phase was assigned based on its broken fan
shape texture which was developed from the previous SmA phase (Figure 2). We were interested
to employ BTU derivatives containing different numbers of carbon atoms in the alkoxy
mesogenic group in order to establish a correlation structure – mesomorphic behavior for such
mononuclear cyclopalladated complexes. Two different approaches were undertaken: varying the
number of carbon atoms of the alkoxy groups of Schiff base (6 or 8 carbon atoms), on one hand,
and, on the other hand, change of chain length of the terminal alkoxy groups (6, 8, 10, 12, 14 and
16 carbon atoms) attached to the BTU derivatives while keeping unchanged the Schiff base
ligand. In this respect, the length of alkyl chains onto the BTU auxiliary ligands was
progressively increased by taking advantage of our previous synthetic strategy to attach long
alkyl chains in para position of the benzoyl moiety of these ligands.[32] Indeed, these
palladium(II) complexes display, depending on the alkoxy chain length, either monotropic or, for
longest alkyl chain, enantiotropic smectic A and C phases. For example, the first series of
palladium(II) complexes, having two hexyloxy in terminal position of the Schiff bases and
different chain length on the BTU ligand, 1a-f, can be divided in two groups. The palladium(II)
complexes with shorter chain length (6 to 12, 1a-d) display monotropic SmA and SmC phases on
cooling from the liquid state. Complexes with longer alkyl chain (14 or 16 carbon atoms) 1e and
1f, exhibit an enatiotropic SmA phase and an additional monotropic SmC on the cooling run. For
both groups, the SmA phase was stable on a short thermal range, up to 6oC, and the two thermal
events could not be well separated on the DSC trace.
By changing the chain length of alkoxy group on the Schiff base from 6 to 8, complexes 2a-f,
now both SmA and SmC phases were enantiotropic for the longest chain (14 and 16, complexes
9

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2e and 2f, respectively), while the SmA phase shows an enantiotropic character starting with
shorter alkoxy groups on the BTU derivative, only ten carbon atoms, complex 2c. Obviously,
there is a very fine balance between these two competing structural features (the number of
carbon atoms of either alkoxy groups of the Schiff base or BTU derivative) that lead to the SmA
and SmC phase stabilization for these palladium(II) complexes. Their melting points slightly
depend on the chain length of the BTU ligand, with lower transition temperatures recorded for
higher number of carbon atoms on the BTU ligand. The SmC phase of complexes 2d-f was
stable down to room temperature on the cooling run and the subsequent heating run exhibit a
cold crystallization step preceding the normal crystal-mesophase and isotropisation transitions.
For each of the above-mentioned complexes, three consecutive heating–cooling cycles were
recorded by DSC. It is important to mention here that, with several exceptions, complexes with
the longest chain length, the transition temperatures recorded both on heating and cooling the
samples are slightly shifted (one or two degrees between consecutive cycles) towards lower
values on subsequent heating-cooling cycles. Such behavior could indicate a possible partial
decomposition of the complexes. Their thermal behavior was investigated by TG analysis, in the
25 – 1000oC temperature range, and the decomposition curves of selected samples are presented
in Figure 3. It was found that these palladium(II) complexes show a small weight loss, less than
3%, around 150oC, slightly above the clearing points. The poorer thermal stability, also found for
other palladium(II) complexes with mixed ligands, Schiff bases and BTU derivatives, could
account for the observed shifting of the transition temperatures recorded by DSC.
Table 1. Thermal parameters for palladium(II) complexes
_____________________________________________________________________________________________
Compound
Transitions, T/oC (H/kJ.mol-1)
_____________________________________________________________________________________________
1a
1bc
1c
1d
1e
1f
2a
2bc
2c
2d
2e
2f
Cr 147 (44.8) Iso 138 (9.9)b SmA 134 SmC 106 (11.5) Cr
Cr 143 (46.1) Iso 136 (10.8)b SmA 132 SmC 114 (17.9) Cr
Cr 141(51.7) Iso 137 (11.8)b SmA 134 SmC 86 (10.2) Cr
Cr 135 (48.3) Iso 137 (10.9)b SmA 132 SmC 81 (3.6) Cr
Cr 63(19.2) Cr’ 129 (48.0) SmA 135 Iso 133(12.3)b SmA 127 SmC 81(9.7) Cr
Cr 82 (28.5) Cr’ 126 (32.6) SmA 132 (10.2) Iso 132(12.4)b SmA 129 SmC 77 (8.9) Cr
Cr 138 Iso 137 (10.7)b SmA 124 SmC
Cr 137 (62.3) Iso 136 (14.0)b SmA 133 SmC 63 (2.7) Cr
Cr 132 (59.2) SmA 136 Iso 135(12.4)b SmA 132 SmC 110 (18.3) Cr
Cr 125 (54.6) SmA 132 Iso 131(9.1)b SmA 126 SmC
Cr 118 (25.7) SmC 128(0.4) SmA 132 (4.4) Iso 132(7.4)b SmA 128 SmC
Cr 54 (13.5) Cr’ 113 (27.6) SmC 124 (0.3) SmA 127 (3.4) Iso 127(5.7)b SmA 123 SmC
_____________________________________________________________________________________________
b
aCr, Cr’ = crystalline phases, Iso=isotropic phase, SmA = smectic A phase, SmC = smectic C phase; Combined
enthalpy of the two transitions Iso-SmA and SmA-SmC; cdata reported in ref [33].
10

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30
40
50
60
70
80
90 100 110 120 130 140 150
T/oC
30
40
50
60
70
80
90 100 110 120 130 140
T/oC
(a)
(b)
Figure 1. The DSC traces of compounds 1c (a) and 1f (b), corresponding to the first heating-
cooling cycle.
(a)
(b)
(c)
11

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Figure 2. POM pictures (x200) of complex 2c on cooling at 135oC (a), 132oC (b) and at 125oC
(c).
100
80
60
40
20
0
100
80
60
40
20
0
0
200
400
600
800
1000
0
200
400
600
800
1000
T/oC
T/oC
(a)
(b)
100
80
60
40
20
0
100
80
60
40
20
0
0
200
400
600
800
1000
0
200
400
600
800
1000
T/oC
T/oC
(c)
(d)
Figure 3. The TG curves for complexes 1d(a), 1f (b), 2d(c) and 2f(d).
In order to confirm the smectic nature of the LC phases displayed by these palladium(II)
complexes, compound 1c was selected for X-ray powder diffraction studies. Thus, the X-ray
pattern of 1c, recorded on cooling from the isotropic state, after a previous heating run above
clearing temperature, contains a series of four Bragg diffraction peaks in the low angle region
with their ratio 1:2:3:4, which is typical to a layered phase, with layer periodicity in the SmC
phase of 59.5Å (Figure 4). Such an observation indicates a lamellar structure, whereby the
12

ACCEPTED MANUSCRIPT
calculated interlayer spacing is comparable to the molecular length derived from molecular
modelling in an all-trans conformation, by considering the X-ray single-crystal structures for
similar palladium(II) complexes previously reported by us.[33, 38] Along with these reflections,
a broad peak around 4.6 Å, corresponding to the molten state of the chains, was also observed in
the X-ray pattern of complex 1c, supporting the presence of an LC phase.
5000
d001
4000
3000
2000
d002
d003
1000
0
hch
d004
5
10
15
20
25
30
2/deg
Figure 4. XRD powder pattern for compound 1c recorded at 95oC on cooling from the isotropic
state.
Emission properties
Luminescent metallomesogens are of great interest for their application in electrooptical devices,
especially, for display applications based on liquid crystals or organic light-emitting diode
(OLED) technology. [45-48] The emissive Pd(II) complexes at room temperature are very rare
when compared to their Pt(II) analogues, in particular because of the presence of low-lying
metal-centered excited states which deactivate the potentially luminescent metal-to-ligand charge
transfer (MLCT) and ligand-centered (LC) levels through thermally activated processes.[49-53]
Therefore, the prepared palladium(II) complexes were investigated for their emission properties
in solid state at room temperature and on heating to isotropic phase. These palladium(II)
complexes were almost non-emissive in dichloromethane solution at room temperature. The
emission spectrum of a selected complex 1c, recorded in solid-state at room temperature, is
13

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shown in Figure 5. The emission spectra of all reported palladium(II) complexes show two
maxima at λmax around 580 and 650 nm, respectively, with a shoulder around 710 nm when the
samples are irradiated in the 330 - 380 nm range. No significant change of position of the
maxima emission was observed on changing the carbon chain length of alkoxy groups. The
intensity of this yellow-orange emission varies amongst the different palladium(II) complexes,
but no correlation with their mesogenic groups could be made.
Recording the emission signal on heating the sample to reach the liquid crystalline state, revealed
that the emission is rapidly quenched, which could be explained both by the influence of
temperature and self-assembly in the mesophase. By cooling back to room temperature, the
emission signal is restored slowly, once the crystallization process developed.
Figure 5. The variable-temperature solid-state emission spectrum of complex 1c when irradiated
in the 330-380 nm region. Inset: microscope pictures taken at room temperature in normal light
(top) and under UV light irradiation (bottom).
14

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Conclusions
Yellow-orange-emissive palladium(II)-containing liquid crystalline materials with SmA and
SmC mesophases were prepared by using the alkoxy-substituted N-benzoyl thiourea derivatives
as auxilliary ligands into the cyclopalladated Schiff base system. The number of carbon atoms of
long flexible alkyl tails on the ancillary BTU derivatives as well as on the Schiff base ligands has
significant effects on the LC phase character (either monotropic or enantiotropic), thermal ranges
of SmA and SmC phases and the corresponding transition temperatures, assigned to different
ways of packing inside the LC phases. For the first series 1a-f, palladium(II) complexes with
Schiff base having two hexyloxy terminal groups, there is a minimum chain length of 14 carbon
atoms on the BTU ligand for complexes to display enantiotropic SmA phase. In contrast, for the
second series 2a-f, complexes with Schiff base having octyloxy terminal groups, the minimum
chain length required for changing the monotropic to enatiotropic character of SmA phase
decreased significantly to only ten carbon atoms. The thermal stability of these palladium(II)
metallomesogens is limited by a slight decomposition on heating above the clearing points. The
complexes show interesting luminescent properties with two emission maxima at λmax around
580 and 650 nm, respectively, with a shoulder around 710 nm, when the samples are irradiated in
the 330 - 380 nm range.
Acknowledgements
This work was supported by a grant of the Romanian Authority for Scientific Research, CNCS-
UEFISCDI, project number PN-II-ID-PCE-2011-3-0384 and Core Programme PN 16 48-3/2016.
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Cyclometalated palladium(II) metallomesogens with Schiff bases
and N-benzoyl thiourea derivatives as co-ligands
Graphical abstract
O
C
H
n
2
n
+
1
O
C
H
m
2
m
+
1
O
S
P
d
N
N
N
H
O
C
H
n
2
n
+
1
O
C
H
m
2
m
+
1
,
m
n
=
6
,
8
=
6
,
8
1
0
,
1
2
,
1
4
,
1
6
21

ACCEPTED MANUSCRIPT
Highlights
Cyclometalated palladium(II) metallomesogens with N-benzoyl thiourea ancillary ligands
were prepared and studied.
•
•
Palladium(II) metallomesogens display either monotropic or enantiotropic SmA and SmC
phases.
•
•
Longer alkyl chains leads to SmA and SmC phase stabilization.
Palladium(II) metallomesogens show yellow-orange emission in solid-state.
22