Luminescent liquid crystalline materials based on palladium(II) imine derivatives containing the 2-phenylpyridine core

Article abstract of DOI:10.1039/c3dt52137k

In this work we report our studies concerning the synthesis and characterisation of a series of imine derivatives that incorporate the 2-phenylpyridine (2-ppy) core. These derivatives were used in the cyclometalating reactions of platinum(II) or palladium(II) in order to prepare several complexes with liquid crystalline properties. Depending on the starting materials used as well as the solvents employed, different metal complexes were obtained, some of them showing both liquid crystalline behaviour and luminescence properties at room temperature. It was found that, even if there are two competing coordination sites, the cyclometalation process takes place always at the 2-ppy core with (for Pt) or without (for Pd) the imine bond cleavage. We successfully showed that it is possible to prepare emissive room temperature liquid crystalline materials based on double cyclopalladated heteroleptic complexes by varying the volume fraction of the long flexible alkyl tails on the ancillary benzoylthiourea (BTU) ligands. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3dt52137k

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Luminescent liquid crystalline materials based
on palladium(II) imine derivatives containing the
2-phenylpyridine core†
Cite this: Dalton Trans., 2014, 43,
1151
Marin Micutz,a Monica Iliş,a Teodora Staicu,a Florea Dumitraşcu,b Iuliana Pasuk,c
Yann Molard,d Thierry Roisneld and Viorel Cîrcu*a
In this work we report our studies concerning the synthesis and characterisation of a series of imine
derivatives that incorporate the 2-phenylpyridine (2-ppy) core. These derivatives were used in the cyclo-
metalating reactions of platinum(II) or palladium(II) in order to prepare several complexes with liquid crys-
talline properties. Depending on the starting materials used as well as the solvents employed, different
metal complexes were obtained, some of them showing both liquid crystalline behaviour and lumine-
scence properties at room temperature. It was found that, even if there are two competing coordination
sites, the cyclometalation process takes place always at the 2-ppy core with (for Pt) or without (for Pd)
the imine bond cleavage. We successfully showed that it is possible to prepare emissive room temperature
liquid crystalline materials based on double cyclopalladated heteroleptic complexes by varying the
volume fraction of the long flexible alkyl tails on the ancillary benzoylthiourea (BTU) ligands.
Received 5th August 2013,
Accepted 16th October 2013
DOI: 10.1039/c3dt52137k
www.rsc.org/dalton
order to raise the metal-centred (MC) state, as it is the case for
cyclometalating ligands, mostly with 2-arylpyridine or 2-thienyl-
Introduction
Liquid crystals incorporating metal ions (metallomesogens) pyridine derivatives.9–11 Bruce et al. reported phosphorescent
with luminescent properties are of great interest for their liquid crystalline complexes of platinum(II) showing a stimu-
promising application in electrooptical devices, in particular lus-dependant emission12 as well as highly luminescent
for display applications based on liquid crystals or organic (yields higher than 0.5) Pt(II) containing metallomesogens.13
light-emitting diode (OLED) technology. Emissive properties The recently reported Pt(II) metallomesogens bearing 3-
were reported for a series of purely organic liquid crystals1 or (2-pyridyl)pyrazole chelates show quantum yields near 1, when
metallomesogens and the latter ones were recently reviewed.2 recorded in degassed dichloromethane.4a All these platinum(II)
There are several studies dealing with light-emitting metallo- luminescent metallomesogens reported so far show relatively
mesogens based on palladium(II)3 or platinum(II)4 complexes, high transition temperatures with mainly columnar and, in
most of them showing the metal in a cyclometalating few cases smectic phases stable at high temperatures. These
environment.5–8 The explanation might be that one of the best photophysical studies were done either in the solid state,
strategies used to promote luminescence in d8 metal com- liquid state or in films obtained by solidification of the meso-
plexes is to employ ligands with a very strong ligand field in phase in order to preserve the specific ordering of the liquid
crystalline state. The luminescence properties of d8 systems
depend on the presence of a sufficiently high energy gap
aDepartment of Inorganic Chemistry, University of Bucharest, 23 Dumbrava Rosie st,
sector 2, Bucharest 020464, Romania. E-mail: viorel_carcu@yahoo.com,
viorel.circu@g.unibuc.ro
between the lowest emitting excited state and the upper-lying
metal-centered excited states when high-field ligands are
employed.
bCentre for Organic Chemistry “C. D. Nenitzescu”, Romanian Academy, Spl.
Independentei 202B, Bucharest 060023, Romania
By comparison with Pt(II) complexes, it has been proved in
several cases that the use of cyclometalated ligands is not
enough to promote room temperature luminescence properties
in Pd(II) complexes. For this reason, luminescent Pd(II) com-
plexes at room temperature are very rare when compared to
their Pt(II) analogues, in particular because of the presence
of low-lying metal-centred excited states which deactivate
the potentially luminescent metal-to-ligand charge transfer
cNational Institute of Materials Physics, P.O. Box MG-7, Magurele, 077125, Romania
dSciences Chimiques de Rennes UMR 6226 CNRS Université de Rennes 1, Avenue du
Général Leclerc, 35042 Rennes Cedex, France
†Electronic supplementary information (ESI) available: Crystallographic data
including the cif files, emission and absorption spectra, powder X-ray diffraction
data, DSC traces and polarising optical microscopy pictures. CCDC
948666–948668. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt52137k
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1151–1161 | 1151

View Article Online
Paper
Dalton Transactions
(MLCT) and ligand-centered (LC) levels through thermally acti-
vated processes.14 In most cases, luminescent Pd(II) and Pt(II)
complexes contain heterocyclic ligands, usually with one or
two pyridine rings, while imine ligands were completely
ignored from this point of view in spite of their wide spread in
the design of Pd(II) or Pt(II)15 metallomesogens. Several
examples of luminescent, either cycloplatinated16 or platinum(II)
complexes with N,N-donor diimine ligands are known.17 The
double cyclopalladation reaction with various substrates has
been intensively studied18 while its application in the design
of liquid crystalline materials was less explored.19
In this paper we describe the first example of a doubly
cyclopalladated columnar Pd(II) metallomesogen based on an
unsymmetrical imine ligand containing the 2-ppy unit that
shows luminescence in the solid state and in solution at room
temperature.
Fig. 1 Molecular structure of 3.
This system can offer the possibility of developing a whole
new range of such complexes due to the different possibilities
for tuning the mesogenic properties of such compounds, as
well as other related physico-chemical properties.
Schiff base ligand, were obtained in refluxing toluene and
their structure was confirmed by single-crystal X-ray diffraction
studies.
Such a chemical behaviour was reported in few other cases
when reaction between [PtCl2(DMSO)2] with various Schiff
bases having a pyridyl ring or other various pyridine-contain-
ing ligands25 led to monocoordination of the ligand through
the pyridinic N atom by replacing only one molecule of DMSO
solvent of the initial starting molecule.26
The crystal structure shows that the platinum(II) atom has a
square-planar geometry as expected and it is surrounded by
two chlorine atoms in trans positions, one sulfur atom of the
coordinated solvent and the nitrogen atom from the pyridine
ring of the Schiff base (Fig. 2).
All the bond lengths and angles at the central Pt atom are
in the normal range for such complexes (Pt–Cl 2.3037(13) Å
and 2.3016(13) Å respectively, Pt–N 2.078(4) Å and Pt–S 2.2170(13)
Å)27 (see ESI, Table 2†).
Results and discussion
We have started from a series of imine ligands that contains
the 2-phenylpyridine core, 1a–f (see Scheme 1), a widely used
mesogenic unit,20,21 in order to take advantage of its ability to
promote emission properties when linked to either Pt(II) or
Pd(II) ions.
First we were interested to prepare the analogous complexes
of Pt(II) by using various starting materials, such as K2PtCl4,
[Pt(μ-Cl)(η3-C4H7)]2, or [PtCl2(DMSO)2] that are extensively
used in the cycloplatination reaction.22 The product of the first
cyclometallation step was not purified further (2), but used
subsequently in the following step involving the bridge-split-
ting reaction with a simple BTU derivative.
Surprisingly, in all cases, except when using [PtCl2(DMSO)2]
as a starting material, the final product shows that the split-
ting of the imine bond occurred in the first step when the
1H-NMR of the dinuclear product 2 shows a signal at ∼10 ppm
assigned to CHO proton and, only the simple heteroleptic
complex (3) with 4-(2-pyridyl)benzaldehyde and BTU ancillary
ligands was obtained. Such a cleavage of an imine ligand was
previously shown to occur with various metals and starting
materials.23 Yellow-orange crystals were isolated from the reac-
tion mixture and studied by single-crystal X-ray diffraction.
The crystal structure confirmed firmly that the Pt(II) ion shows
a very strong preference for the 2-phenylpyridine coordination
site and that the BTU ligand is coordinated in a normal
fashion via the O and S atoms to complete the square-planar
geometry around the Pt(II) ion24 (Fig. 1). Moreover, these
complexes show interesting luminescence properties with
quantum yields around 8% in non-degassed dichloromethane
solution. If the starting material used in attempted cyclometa-
lation reaction was [PtCl2(DMSO)2], then light-yellow com-
Following these experimental results, we turned our atten-
tion to the cyclopalladation reaction by employing simply the
palladium acetate as a starting material and, in this way, we
were able to isolate in toluene red products of doubly cyclo-
palladated imine ligands as confirmed by 1H and 13C NMR
1
spectroscopy, as well as by MS. Thus, in the H NMR spectra
two different methyl signals assigned to two different acetato
groups could be seen in the 2.5–1.5 ppm range. As previously
described in the literature,19b such a product could have a
polymeric structure. Additional support for the nuclearity of
the new complexes arises from mass spectrometry. The MS
plexes of the general formula trans-[PtL(DMSO)Cl2], where L = Fig. 2 Molecular structure of 4b.
1152 | Dalton Trans., 2014, 43, 1151–1161
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
pattern for complex 5b shows m/z values for [M + Na]+ (739.3) deprotonated form was confirmed by the disappearance of νNH
and [M + AcO + 2Na]+ (823.2), together with some other higher (∼3300 cm−1) and νCvO (∼1670 cm−1) frequencies (compared
m/z values fragments and some other recombination to the IR spectra of free ligands) together with a shift of νC–N
fragments.
frequency towards lower wavenumbers. All these information
Our aim was to prepare new emissive liquid crystalline suggest the absence of NH hydrogen located between carbonyl
materials and, for this reason, the products of the first cyclo- and thiocarbonyl groups of the benzoyl thioureic moiety, infor-
metalation step were not deeply investigated. According to mation that is further supported by 1H-NMR spectroscopy.
these observations we proceeded further with these μ-acetato-
Despite the BTU derivative being unsymmetric, the final
bridged Pd(II) complexes and the bridge cleavage reaction was products (9–12) were found to contain only one of the two
performed in the presence of either a simple BTU derivative or possible syn and anti isomers with respect to the positions of
by employing BTU derivatives containing a long alkoxy meso- sulfur atom of the BTU ligand and nitrogen atom of the metal-
genic group in order to establish a correlation structure – lated imine ligand. For the purpose of elucidating the products
mesomorphic behaviour for such doubly cyclopalladated of this reaction, N,N-dipropyl-N′-benzoylthiourea ligand was
complexes.
employed in the first stage. Successful isolation of yellow crys-
The synthesis pathway used to prepare the palladium(II) tals by slow evaporation from a mixture of dichloromethane
complexes is presented in Scheme 1. The double cyclopalla- and ethanol afforded a single crystal X-ray study to be per-
dated complexes were prepared by reacting the acetato-bridged formed and the molecular structure to be unequivocally con-
complexes with BTU derivatives in dichloromethane solvent in firmed. The molecular structure of doubly cyclopalladated
the presence of potassium carbonate. All of the complexes complex having eight carbon atoms in the terminal alkyl chain
9–12 were obtained in good yield as yellow to orange products, is shown in Fig. 3. The complex has two different Pd(II) ions
which are stable under ambient conditions. The formation of with a square-planar arrangement, one coordinated in a cyclo-
the mixed-ligands double cyclopalladated complexes can be metalated fashion to the 2-phenylpyridine core and the
confirmed readily by IR and 1H-NMR spectroscopy when the second, in a similar cyclometalated fashion, coordinated to
coordination of the N-benzoylthiourea derivatives in the the imine core, both having completed the coordination
Scheme 1 Synthetic routes and chemical structures of the complexes.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1151–1161 | 1153

View Article Online
Paper
Dalton Transactions
Table 1 Thermal and liquid crystalline properties
Compound
Transition
T/°C
ΔH/kJ mol−1
1a
1b
1c
1d
1e
1f
7
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–Cr′
Cr′–I
I–Cr
118
142
116
140
114
137
115
134
118
132
118
128
87
96
84
171
87a
156a
152b
24
32.3
1.0
36.2
0.9
45.8
1.2
52.2
1.2
57.4
1.7
64
1.8
10.5
49.3
50.0
49.1
61.2
0.2
—
—
—
—
10
11
Cr–I
Cr–Colh
Colh–I
I–Colh
Colh-g
g-Colh
Colh–I
12
21
Fig. 3 Molecular structure of 9.
165b
a Data taken from the first heating-cooling cycle. b Temperatures
measured by POM.
environment by the deprotonated mononegative BTU ancillary
ligand. All the bond distances as well as the angles at the two
Pd atoms are similar to those found in either cyclometalated
2-phenylpyridine Pd(II) complexes with different ancillary
ligands or cyclometalated imine Pd(II) complexes with various
BTU derivatives. Such complexes have an overall rigid planar
conformation, making them very interesting candidates for
discotic materials by subsequent appropriate substitution with
promesogenic groups.
Now, if the simple BTU derivative is replaced with alkoxy-
substituted BTU derivatives, yellow-orange solids can be
obtained and, according to the number of mesogenic groups
per BTU ligand, a different mesomorphic behaviour could be
seen. Room-temperature liquid crystals are materials with
melting points below 25 °C. In order to decrease the melting
point of the mesogenic materials, our strategy was to increase
the volume fraction of the long flexible carbon chains around
the doubly cyclopalladated rigid core.28 In this respect, we pro-
gressively increased the number of alkyl chains onto the BTU
ancillary ligands, taking advantage of our previous strategy to
attach long alkyl chains either in 4; 3,4 or 3,4,5 positions of
the benzoyl fragment of these ligands.29
atoms in the alkyl chain. The investigation of complexes 10–12
using POM and DSC showed that only complexes 11 and 12
form liquid crystalline phases. On heating, the complex 10
melts straight to the isotropic phase around 170 °C while on
cooling the isotropic phase the complex simply starts to soli-
dify around 120 °C without crystallisation. This behaviour
shows that the five alkyl chains around the discotic molecular
shape of compound 10 are not sufficient enough to reduce the
melting point and stabilise a liquid crystalline phase. A
reduced stability of liquid crystalline phases for Pd(II) com-
plexes having alkylated 2-phenylpyridine cyclometalated
ligands and acetyl-acetonates as ancillary ligands was noticed
and this was explained by a slight deviation from a planar
geometry of this unit with consequences on the packing of
aromatic organometallic cores.20 In contrast, by successive
addition of alkyl chains onto the BTU ligands, complexes 11
and 12 display large range liquid crystalline phases. The DSC
trace for complex 11 exhibits two transitions on the first heating
cycle. The first endothermic transition at 80 °C was assigned as
a transition from a crystalline state to a columnar phase (Colh)
as identified by XRD studies. This transition was followed by a
much weaker one around 160 °C assigned to isotropisation with
an enthalpy of 0.2 kJ mol−1. The cooling trace showed no tran-
sition corresponding to an isotropic liquid to liquid crystalline
phase, this broad transition that starts at around 152 °C being
detected only by polarising optical microscopy. Further cooling
resulted in no crystallisation process and only a glass transition
could be detected at 24 °C (temperature recorded at half inflex-
ion point). The following heating–cooling cycles exhibited no
peaks, but the glass transition in the same interval meaning
that the complex 11 remains in the mesophase from 24 °C up
to 155 °C. The mesophase formed on cooling from the isotropic
Liquid crystal properties
All the six new Schiff bases derived from the 2-phenylpyridine
unit exhibited an enantiotropic nematic phase over a relatively
short range, between 10 and 30 °C. This phase was evidenced
by polarising optical microscopy observations when a fluid and
homogeneous texture, either Schlieren- or marbled-textures, was
developed on cooling from the isotropic liquid (Table 1).
Additionally, the enthalpy values associated with these tran-
sitions fall well within the typical values for a nematic to iso-
tropic transition. The melting and clearing points follow a
normal trend with a little variation on passing to the superior
analogue, with respect to increasing the number of carbon
1154 | Dalton Trans., 2014, 43, 1151–1161
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Fig. 5 X-ray diffraction pattern of 11 at 85 °C after cooling from isotro-
pic phase.
Fig. 4 Columnar texture of compound 11 at 90 °C on cooling from the
isotropic phase (a); mosaic texture of 12 seen between crossed polari-
sers at 80 °C on cooling from the isotropic phase (b) (magnification
×200).
liquid of compound 11 displayed a typical texture for columnar
mesophases (Fig. 4).
The texture of 11 showed large homeotropic areas with
columnar axes orientated perpendicular to the glass sub-
strates, meaning that the material has a strong tendency to
orient with its optic axis perpendicular to the surface of the
untreated glass microscope slides. This texture remains vir-
tually the same after cooling to room temperature when the
compound solidifies as a glass. The complex 12 that possesses
one additional alkyl chain onto the BTU ligand and nine alkyl
chains all together in the molecule was isolated as a meso-
Fig. 6 X-ray diffraction pattern for complex 12 recorded at 100 °C on
cooling from isotropic liquid.
The X-ray patterns for both Pd(II) compounds show the
morphic paste from the reaction mixture. Its DSC trace shows intensity of the d11 signal extremely weak, but detectable, com-
no transition on the first heating cycle, except a glass tran- pared to the d10 and d30 signals (for 11) or d10 and d20 signals
sition with the inflexion point at 21 °C. The clearing tempera- (for 12). This rather unusual diffraction pattern observed for
ture, 165 °C, was detected by POM. Powder X-ray diffraction both Pd(II) complexes was reported in several other cases
studies were performed on cooling the samples from the iso- showing a columnar hexagonal mesophases and it was attribu-
tropic liquid to the mesophase. All compounds show diffrac- ted to an incomplete powder averaging resulting in unreliable
tion patterns typically observed for mesophases in the relative intensities of the reflections in the X-ray diffracto-
temperature range of liquid crystal phase and frozen, glassy gram.30 Along with these reflections, a broad peak around
mesophases. Additionally, virgin samples of 11 and 12 were 4.6 Å corresponding to the molten state of the chains and a
subjected to X-ray diffraction studies at room temperature and less intense diffraction around 3.6 Å corresponding to short-
the results confirmed the crystalline nature of complex 11 range π–π stacking interactions (Fig. 5) were also observed in
(Fig. S3, ESI†) before melting to the liquid crystalline state. In the X-ray pattern of complex 11. The two-dimensional hexago-
contrast, complex 12 shows a slightly similar pattern with the nal parameter (d10) was evaluated as 36.2 Å corresponding to a
ones recorded at higher temperatures, thus confirming its distance of 41.8 Å between neighbouring columns. Addition-
liquid crystalline state at room temperature. The 1D-WAXS pat- ally, a relative sharp signal around 7.7 Å was found for 11 and
terns recorded for complexes 11 and 12 (Fig. 5 and 6) are this was assigned to Pd–Pd interactions as the closest Pd–Pd
typical of a disordered hexagonal columnar mesophase and distance deduced from the solid-state structure of 9 was 7.5 Å.
contains a series of several Bragg diffraction peaks in the low
In the X-ray pattern of 12, in the wide angle region, besides
angle region with their ratio 1 : 31/2 : 2 : 71/2 : 3… that resembles the diffuse scattering at 4.7 Å, due to the liquid-like state of
a hexagonal lattice. The results are collected in ESI (Table 3†).
the terminal aliphatic chains, an additional broad scattering
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1151–1161 | 1155

View Article Online
Paper
Dalton Transactions
Fig. 7 Proposed model for the molecular organisation of the building
blocks in the hexagonal lattice of complex 11 (left) and 12 (right).
of weak intensity at ∼6.5 Å was found (Fig. 6). This corres-
ponds to about twice the stacking distance of the aromatic
cores in the columnar phase and points to the formation of an
alternating molecular arrangement along the columns (Fig. 7),
in order to allow a better space filling of the discs, similar to
the observations made for other palladium(II) compounds with
2-phenylpyridine or imine cores.20 Interestingly, the hexagonal
lattice parameters for the two complexes are significantly
different (41.8 Å for 11 and 38 Å for 12, respectively), with a
smaller value for the complexes having a greater number of
alkoxy chains at the periphery implying a different organis-
ation within the mesophase. In order to better understand this
difference in the mesophase structure, we estimated the
number of molecules Z forming a columnar hexagonal unit,
using the following equation:31
Fig. 8 Top-view of the columnar hexagonal packing of complex 11 (a)
and complex 12 (b).
Z ¼ 31=2NAa2hρM ꢀ1 2ꢀ1
chains, two different supramolecular organisations within the
columns are expected for the two complexes 11 and 12.
where ρ = density of the liquid crystalline phase (estimated to
be ∼1 g cm−3), NA = Avogadro’s constant, a = columnar lattice
parameter, h = height of a single columnar unit and M =
molecular mass.
Photophysical properties
The photophysical properties of Pd(II) complexes have been
investigated and the results are summarised in Table 2.
In the case of complex 11 this value was found to be
between one and two molecules in the slice of the column,
while in the case of complex 12 this value amounts around 1.
To propose a packing model for these two complexes, extensive
molecular modelling (Hyperchem) were employed based on
the X-ray molecular structure of complex 9 (Fig. 8). In this way,
for the Colh phase of compound 11, two molecules instead of
only one as for complex 12, could be put together in the cross-
section of the columns in order to have a lattice parameter of
41.8 Å. Complex 11 with seven decyloxy tails showed a regular
intracolumnar distance along the columns, as revealed by the
reflection corresponding to h0 ≈ 3.6 Å. To accommodate two
molecules per disc unit it is necessary for complex 11 to adopt
approximately a half disc shape resulting from the folding of
aniline unit around the NH group of the BTU ligand along the
long axis of the molecule defined by the 2-phenylpyridine
imine ligand in order to facilitate the efficient space filling
and to adopt approximately a hemidisc shape. In fact, this
bent orientation of the flexible aniline fragment of the BTU
ligands is often seen in the X-ray solid state structure of several
Ph-NH(CO)-(CS)NH-R BTU molecules and even for palladium
complexes with such derivatives and is due to the formation of
hydrogen bonding between NH proton and CvS groups.32
Their UV-VIS absorption spectra in CH2Cl2 (Fig. 9) feature
two highly intense absorption bands at around λmax 290 (ε >
5.2 × 104 mol−1 dm3 cm−1) and 330 nm (ε > 6.6 × 104 mol−1
dm3 cm−1) that are assigned to 1LC(ππ*) ligand centred tran-
sitions of the cyclometalated and BTU ligands, based on simi-
larities with the absorptions of the free ligands. The first band
Table 2 Absorption and emission data for the Pd(II) complexes in
dichloromethane solution (concentration 5 × 10−4 M) and in solid state
Emission, λem/nm
(λexc/nm; Φ/%)
Absorption, λmax/nm
Compound
(ε × 10−3/M−1 cm−1
)
Solutiona
Solid state
10
254(sh, 107.2), 290(115.6),
324(127.2), 381(sh, 58.9),
405(sh, 42.2), 450(15.9),
247(sh, 80.7), 284(52.5),
338(66.9), 407(sh, 22.1),
457(8.2)
251(sh, 73.7), 285(sh, 54.8),
331(66.9), 403(sh, 27.2),
452(10.2)
410(285/
0.28)
518, 667,
727, 817(sh)
11
12
365(283/
0.55)
546, 676,
736, 819(sh)
360(283/
0.39)
556, 683,
744, 823(sh)
a Quantum yields were determined with respect to [Ru(bpy)3]2+ in
This means that, depending on the number of terminal alkoxy water.
1156 | Dalton Trans., 2014, 43, 1151–1161
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
was found that the emission bands slightly red-shift with
increasing number of alkoxy chains in the molecules. A
similar trend was seen for other luminescent metallomeso-
gens, including Pd(II) complexes,3c with different number of
alkoxy tails.34 Taking into account the previous reports on
luminescent Pd(II) complexes, this emission can be attributed
3
to originate from excimeric ππ* intraligand excited states as a
dσ*–π* MMLCT assignment is possibly ruled out due to the
lack of evidence for appreciable Pd–Pd contacts (the shortest
Pd–Pd distance in the crystal lattice of complex 9 is 7.5 Å).
On the other hand, if the samples are irradiated with λexc
=
380 nm, another less intense structured emission band was
observed, with λmax around 450 nm and two shoulders on each
side located approximately at 410 nm and 470 nm, respectively
(ESI, Fig. S10†). These latter high-energy emission bands could
be tentatively assigned to intraligand emission (3IL) by the
cyclometalated ligand in analogy with several other lumine-
scent 2-phenylpyridine containing Pd(II) complexes.14b,35 The
luminescent properties of the Pd(II) complexes were also
studied in solution when only one emission band was
observed, with its maximum located in the 360–410 nm range
assigned to an intraligand transition which are blue-shifted
when compared to the solid-state emission spectra, as it is nor-
mally expected (ESI, Fig. S9†). However, the quantum yields
of all compounds recorded in non-degassed CH2Cl2 estimated
with [Ru(bpy)3]Cl2 as a standard (Φ = 2.8% in air-equilibrated
water)36 were relatively low, and were found in the range
0.28%–0.55%, very similar to the values reported for other
Pd(II) luminescent complexes.14b–d
Fig. 9 The UV-VIS spectra of palladium(II) complexes recorded in
dichloromethane solution.
around 290 nm appeared like a shoulder in the case of
complex 12. Sometimes, these are seen like shoulders probably
due to broadening resulted from the superposition of π–π*
transitions of the two different coordinated ligands. In
addition, several shoulders are also present in their UV-VIS
spectra (around 250 nm for all complexes), and a moderately
intense band that is absent for the ligands is observed at λmax
450 nm (ε > 0.8 × 104 mol−1 dm3 cm−1), with a shoulder at
around 400 nm (ε > 2.2 × 104 mol−1 dm3 cm−1). These less
intense low-energy absorptions located at λ > 350 nm could be
assigned to a mixture of spin-allowed metal-to-ligand charge
transfer (1MLCT) and ligand-centred (1LC) transitions, and
this assignment is consistent with previous results based on
related cyclometalated Pd(II) compounds.33
Now, if the emission of the two mesomorphic Pd(II) com-
plexes was recorded in the mesophase, at room temperature,
after previous heating to 100 °C followed by rapid cooling to
The solid-state emission spectra recorded at room tempera-
ture for complexes 10–12 are shown in Fig. 10. The emission
spectra of all three Pd(II) complexes show two maxima at λmax
around 660–685 and 725–745 nm, respectively, with a shoulder
ambient temperature,
a slight red-shifted emission was
observed for complex 11 while the emission features of 12
were virtually the same (ESI, Fig. S11 and S12†). These latter
findings confirmed again the existence of 12 in the liquid crys-
talline state at room temperature. For 11, a slight red shift of
the red emission was observed due to the columnar organis-
ation in the liquid crystalline phase (ESI, Fig. S11†).
around 820 nm when the samples are irradiated with λexc
=
480 nm. This orange-red emission has also been visually
detected at the optical microscope when the samples were ir-
radiated in the 380–420 nm region (ESI, Fig. S1 and S2†). It
Conclusions
It has been shown that by incorporating the 2-phenylpyridine
unit into the imine ligands and varying the number of peri-
pheral alkoxy chains on the ancillary ligands we can prepare
red-emissive Pd(II)-containing liquid crystalline materials with
columnar mesophases stable at room temperature. An impli-
cation of different number of alkoxy chains on the organis-
ation within the mesophase was seen and this was assigned to
a different way of packing inside the columns. We successfully
proved that it is possible to prepare red-emissive room temp-
erature liquid crystalline materials based on double cyclopalla-
dated heteroleptic complexes by varying the volume fraction of
the long flexible alkyl tails on the ancillary benzoylthiourea
(BTU) ligands.
Fig. 10 The solid-state emission spectra of complexes 10–12 (λexc
480 nm).
=
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1151–1161 | 1157

View Article Online
Paper
Dalton Transactions
1
mmol) in ethanol (10 cm3), 4-(2-pyridyl)benzaldehyde
Experimental section
(0.183 g, 1 mmol) followed by a few drops of glacial acetic acid
Dichloromethane was distilled over phosphorus pentoxide; were added. The mixture was heated under reflux for 2 h and
other chemicals were used as supplied. C, H, N and S analyses then cooled to −25 °C to give the crude product. Crystallisation
were carried out with a Perkin Elmer instrument. IR spectra from hot ethanol gave the analytically pure product as an off-
were recorded on a Bruker spectrophotometer using KBr white crystalline solid.
pellets. UV-VIS absorption spectra were recorded using a Jasco
Compound 1b. Yield 76%. Calc. for C26H30N2O: %C: 80.79;
V-660 spectrophotometer. 1H and 13C NMR spectra were %H 7.82; %N 7.25. Found: %C 80.65; %H 7.78; %N 7.18. H
recorded on a Varian Gemini 300 BB spectrometer operating at NMR (CDCl3, 300 MHz): 8.77 (d, br, 1H); 8.58 (s, 1H); 8.15 (d,
300 MHz, using CDCl3 as a solvent. 1H chemical shifts were J = 8.5 Hz, 2H); 8.03 (d, J = 8.2 Hz, 2H); 7.84–7.81 (m, 2H);
referenced to the solvent peak position, δ 7.26 ppm. ESI-MS 7.33–7.27 (m, 3H); 6.97 (d, J = 8.8 Hz, 2H); 4.02 (t, J = 6.6 Hz,
1
analysis was performed on an Agilent VL mass spectrometer.
2H); 1.87–1.78 (m, 2H); 1.55–1.30 (m, 10H); 0.85 (m, 3H).
The phase assignments and corresponding transition temp- 13C NMR (CDCl3, 75 MHz); 158.2, 157.7, 156.8, 150.0, 144.8,
eratures for the palladium(II) complexes were determined by 141.8, 137.1, 137.0, 129.1, 127.3, 122.7, 122.4, 120.9, 115.2,
polarising optical light microscopy (POM) using a Nikon 68.5, 32.0, 29.5, 26.2, 22.8, 14.2. νmax(KBr)/cm−1: 3430br,
50iPol microscope equipped with a Linkam THMS600 hot 2954m, 2922s, 2855s, 1621s, 1581m, 1502s, 1468s, 1435w,
stage and TMS94 control processor. Temperatures and enthal- 1391w, 1290m, 1251vs, 1191w, 1111w, 1004m, 837s, 775m,
pies of transitions were investigated using differential scan- 729w, 538w. Compounds 1b–f show the same chemical shifts
ning calorimetry (DSC) with a Diamond DSC Perkin Elmer. with corresponding integration for methylene protons.
The materials were studied at scanning rates of 5 and 10 °C
Synthesis of Pt(II) complex 3
min−1 after being encapsulated in aluminium pans. Two or
more heating/cooling cycles were performed on each sample. To a methanolic solution of [Pt(μ-Cl)(η3-C4H7)]2 (0.057 g,
Mesophases were assigned by their optical texture and powder 0.1 mmol in 20 ml) solid 1 (0.25 mmol) was added and the
X-ray diffraction studies.
suspension was stirred at room temperature for 48 h. The
The powder X-ray diffraction measurements were made on resulting orange solid was filtered off, washed several times
1
a D8 Advance diffractometer (Bruker AXS GmbH, Germany), in with cold methanol and dried. The H-NMR spectroscopy evi-
parallel beam setting, with monochromatized Cu-Kα1 radiation denced the CHO signal around 10 ppm. This product was
(λ = 1.5406 Å), scintillation detector, and horizontal sample reacted further with an excess amount of sodium salt of N,N-
stage. The measurements were performed in symmetric (θ–θ) dipropyl-N′-benzoylthiourea (BTUPr2Na) (0.072 g, 0.25 mmol)
geometry in the 2θ range from 0.5° to 10° or 30° in steps of in dichloromethane to give an orange crystalline solid of 3 in
0.02°, with measuring times per step in the 5–40 s range. The 69% yield. Single-crystals were obtained by cooling a mixture
temperature control of the samples during measurements was of acetone–methanol at −25 °C.
achieved by adapting a home-made heating stage to the
Compound 3. Yield 78%. Calc. for C26H27N3O2PtS: %C:
sample stage of the diffractometer. X-ray single-crystal data for 48.74; %H 4.25; %N 6.56; %S 5.01. Found: %C 48.56; %H 4.37;
1
complexes 3, 4a and 9 were collected with a Bruker-AXS %N 6.48; %S 4.77. H NMR (CDCl3, 300 MHz): 10.03 (s, 1H);
APEXII diffractometer. The structure was solved by direct 9.37 (d, J = 6.4 Hz, JPt–H = 31 Hz, 1H); 8.20 (d, J = 8.5 Hz, 2H);
methods using the SIR97 program,37 and then refined with 8.10–7.92 (m, 2H); 7.87 (m, 1H); 7.63 (s, 2H); 7.56–7.38 (m,
full-matrix least-square methods based on F2 (SHELXL-97)38 4H); 3.91–3.79 (m, 4H); 1.95 (m, 2H); 1.77 (m, 2H); 1.09 (t, J =
with the aid of the WINGX39 program. All non-hydrogen atoms 7.4 Hz, 3H); 0.98 (t, J = 7.4 Hz, 3H) 13C NMR (CDCl3, 75 MHz);
were refined with anisotropic atomic displacement para- 193.3, 169.0, 168.4, 164.0, 151.0, 145.3, 138.8, 138.5, 137.6,
meters. H atoms were finally included in their calculated posi- 136.2, 135.2, 131.3, 129.2, 128.3, 123.9, 123.6, 120.0, 54.6, 53.6,
tions. Luminescence spectra were recorded on a Fluorolog-3™ 21.4, 20.9, 11.7, 11.6. νmax(KBr)/cm−1: 3444br, 2963w, 2927w,
fluorescence spectrometer (FL3-22, Horiba Jobin Yvon) in the 2871w, 1735s, 1519s, 1483s, 1420vs, 1361m, 1308w, 1228w,
solid state and employing a Jasco FP-6300 spectrofluorometer 1199m, 1101w, 1067w, 870w, 829w, 775w, 745w, 711m.
(operating parameters: band width – 5 nm; data pitch –
Synthesis of Pt(II) complex 4
0.5 nm; scanning speed – 100 nm min−1; spectrum accumu-
lation – 3; path length – 10 mm using Quartz SUPRASIL cells) [PtCl2(DMSO)2] (0.084 g, 0.2 mmol) was added to a solution of
in dichloromethane solution.
1 (0.2 mmol) in toluene (50 ml) and the resulting solution was
heated under reflux for 24 hours. On cooling the solution to
−25 °C a yellow crystalline product was formed which was fil-
Synthesis of imine ligands
The imine ligands were prepared by the condensation reaction tered off, washed with cold toluene and dried. Single crystals
between the 4-(2-pyridyl)benzaldehyde and corresponding suitable for X-ray studies were grown by slow evaporation from
4-alkoxyanilines, catalyzed by glacial acetic acid, in absolute a dichloromethane solution.
ethanol, using the same procedure described elsewhere for the
Compound 4b. Yield 47%. Calc. for C28H36Cl2N2O2PtS: %C:
preparation of imine ligands.40 Below is presented the prepa- 46.03; %H 4.97; %N 3.83; %S 4.39. Found: %C 45.85; %H 5.08;
ration of 1a. To a solution of p-hexyloxyaniline (0.193 g, %N 3.65; %S 4.18. 1H NMR (CDCl3, 300 MHz): 8.90 (d, br, 1H);
1158 | Dalton Trans., 2014, 43, 1151–1161
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
8.59 (s, 1H); 8.17 (d, J = 8.5 Hz, 2H); 8.08 (d, J = 8.2 Hz, 2H); study were obtained by slow evaporation of a solution of 9 in
7.95 (td, J = 7.7 Hz, J = 1.6 Hz, 1H); 7.62 (d, J = 6.7 Hz, 1H); dichloromethane–ethanol 1/1.
7.49 (ddd, J = 6.0 Hz, J = 1.6 Hz, 1H); 7.29 (d, J = 8.8 Hz, 2H);
Compound 10. Yield 65%. Calc. for C86H114N6O7Pd2S2: %C:
6.96 (d, J = 8.8 Hz, 2H); 4.00 (t, J = 6.6 Hz, 2H); 3.30 (s, 6H); 63.73; %H 7.09; %N 5.18; %S 3.96. Found: %C 64.02; %H 7.01;
1
1.86–1.77 (m, 2H); 1.64–1.25 (m, 10H); 0.91 (m, 3H). 13C NMR N 5.48; %S 3.71. H NMR (CDCl3, 300 MHz): 8.99 (d, br, 1H);
(CDCl3, 75 MHz): 161.2, 158.3, 156.9, 152.3, 139.4, 130.4, 8.17–8.07 (m, 3H); 7.97–7.84 (m, 4H); 7.71 (d, br, 2H);
129.3, 128.3, 127.2, 124.2, 122.4, 118.6, 115.1, 68.4, 44.0, 31.8, 7.48–7.42 (m, 6H); 7.35–7.27 (m, 2H); 6.99–6.90 (m, 7H); 6.72
29.4, 29.3, 29.2, 26.1, 22.7, 14.1. νmax(KBr)/cm−1: 3425br, (d, J = 9 Hz, 2H); 4.05–3.95 (m, 6H); 3.90 (d, J = 5.8 Hz, 1H);
2923s, 2859s, 1615s, 1583m, 1505s, 1289m, 1248vs, 1125s.
3.85 (d, J = 5.8 Hz, 1H); 1.89–1.67 (m, 6H); 1.65–1.25 (m, 52H);
1.96 (m, 21H). 13C NMR (CDCl3, 75 MHz): 162.6, 158.8, 147.7,
124.6, 114.7, 114.5, 114.1, 113.6, 110.2, 68.8, 68.5, 39.5, 32.0,
30.7, 29.6, 29.5, 29.4, 29.2, 26.2, 24.0, 23.2, 22.8, 14.3, 11.3.
νmax(KBr)/cm−1: 2923m, 2854m, 1604m, 1525s, 1507vs, 1473s,
1435s, 1404s, 1362m, 1297m, 1248s, 1220m, 1115m, 1097w,
1027w, 909w, 828m, 764w, 670w, 517w.
Compound 11. Yield 56%. Calc. for C118H178N6O9Pd2S2:
%C: 67.43; %H 8.54; %N 4.00; %S 3.05. Found: %C 67.25; %H
8.67; %N 3.85; %S 2.80. 1H NMR (CDCl3, 300 MHz): 9.00 (d,
br, 1H); 8.16 (s, 1H); 7.96–7.67 (m, 6H); 7.50–7.27 (m, 10H);
6.96–6.86 (m, 6H); 6.67 (d, J = 8.6 Hz, 1H); 4.05–3.94 (m, 12H);
3.68 (t, br, 2H); 1.92–1.20 (m, 118H); 0.89 (m, 21H). 13C NMR
(CDCl3, 75 MHz): 174.2, 164.3, 158.8, 152.6, 152.5, 149.0,
148.3, 148.2, 146.7, 141.5, 138.6, 134.5, 132.1, 130.1, 124.8,
124.5, 122.6, 115.1, 114.7, 114.5, 112.3, 111.8, 110.2, 69.2, 69.1,
68.6, 68.5, 32.1, 32.0, 29.9, 29.8, 29.7, 29.6, 29.5, 29.4, 29.3,
26.3, 26.2, 22.9, 14.3. νmax(KBr)/cm−1: 2921vs, 2851s, 1601w,
1540s, 1500vs, 1473s, 1437s, 1405s, 1302s, 1269s, 1244s,
1160m, 1131m, 826w, 757w, 720w, 652w, 517w.
Compound 12. Yield 63%. Calc. for C142H226N6O11Pd2S2:
%C: 69.04; %H 9.22; %N 3.40; %S 2.60. Found: %C 68.75; %H
9.47; %N 3.18; %S 2.29. 1H NMR (CDCl3, 300 MHz): 8.99 (d,
br, 1H); 8.13 (s, 1H); 7.96–7.83 (m, 3H); 7.47–7.29 (m, 10H);
7.05 (s, 2H); 6.93–6.82 (m, 6H); 4.04–3.85 (m, 14H); 3.65 (t, br,
4H); 1.9–1.62 (m, 18H); 1.55–1.20 (m, 140H); 0.95 (m, 27H).
13C NMR (CDCl3, 75 MHz): 164.0, 158.8, 155.6, 152.6, 152.5,
149.3, 142.5, 125.1, 124.4, 114.5, 114.4, 110.2, 108.5, 73.6, 73.5,
69.1, 68.9, 68.4, 61.9, 32.1, 32.0, 29.9, 29.8, 29.7, 29.6, 29.5,
26.4, 26.3, 22.8, 13.4, 13.2. νmax(KBr)/cm−1: 2922vs, 2852s,
1603m, 1525s, 1509vs, 1467m, 1437s, 1405m, 1360s, 1307m,
1243s, 1154m, 1113s, 1037m, 829w, 760w, 721w, 582w, 518w.
Synthesis of μ-acetato-bridged Pd(II) complexes
Palladium(II) acetate (0.5 mmol) was added to a solution of 1
(0.5 mmol) in toluene (25 ml). The resulting mixture was
heated under reflux for 24 h when a red-orange solid was
formed. The solid was filtered off, washed with cold toluene
and dried.
1
Compound 5b. Yield 57%. H NMR (CDCl3, 300 MHz): 7.96
(td, J = 7.8 Hz, J = 1.6 Hz, 1H); 7.66 (d, J = 7.9 Hz, 1H); 7.50 (s,
1H); 6.97 (m, 1H); 6.80 (s, 1H); 6.70 (m, 4H); 6.58 (s, 1H); 3.95
(t, J = 6.7 Hz, 2H); 2.34 (s, 3H); 1.83 (m, 5H); 1.61–1.26 (m,
10H); 0.90 (m, 3H).
Synthesis of N-benzoylthiourea derivatives (BTU)
The N-benzoylthiourea derivatives (6 and 8) as well as their
sodium salts used in this work were prepared according to the
methods published in the literature29,32 (Scheme 1, ESI†).
Compound 7. Yield 64%. Calc. for C46H76N2O4S: %C: 73.36;
%H 10.17; %N 3.72; %S 4.26. Found: %C 73.15; %H 10.36;
1
%N 3.55; %S 4.01. H NMR (CDCl3, 300 MHz): 12.47 (s, 1H);
9.01 (s, 1H); 7.55 (d, J = 8.8 Hz, 2H); 7.42–7.40 (m, 2H); 6.92 (d,
J = 8.7 Hz, 3H); 4.08–4.03 (m, 4H); 3.96 (t, J = 6.6 Hz, 2H);
1.90–1.20 (m, 52H); 0.85 (m, 9H). 13C NMR (75 MHz): 178.8,
166.7, 157.9, 154.7, 149.5, 130.6, 126.9, 123.6, 121.6, 114.7,
112.6, 112.2, 69.8, 69.4, 68.3, 32.1, 31.9, 29.9, 29.7, 29.4, 29.3,
29.2, 26.2, 26.1, 22.8, 14.5. νmax(KBr)/cm−1: 2921vs, 2851s,
1733m, 1664s, 1597s, 1561s, 1537s, 1510vs, 1467m, 1353m,
1277s, 1245m, 1209m, 1072s, 828w, 751w, 657w, 518w.
Synthesis of Pd(II) complexes 9–12
Corresponding solid N-benzoylthiourea compound, 6–8,
(0.30 mmol) or sodium salt of N,N-dipropyl-N′-benzoylthiourea
(BTUPr2Na) for complex 9, was added to a suspension of
μ-acetato-bridged palladium complex 5 (0.10 mmol) and
Acknowledgements
K2CO3 in dichloromethane (15 cm3) and the mixture was This work was supported by a grant of the Romanian Authority
stirred at room temperature for 24 hours. Evaporation of the for Scientific Research, CNCS-UEFISCDI, project number
solvent gave yellow to orange solids, which were purified by PN-II-ID-PCE-2011-3-0384.
chromatography on silica using dichloromethane as an eluant
to yield the final products. They were further crystallized from
a mixture of dichloromethane–ethanol (1/1) at −25 °C.
Notes and references
1
Compound 9. Yield 72%. H NMR (CDCl3, 300 MHz): 9.10
(d, br, 1H); 8.28 (s, 1H); 8.18 (d, J = 6.6 Hz, 2H); 7.97–7.85 (m,
2H); 7.75 (d, J = 7.4 Hz, 2H); 7.63 (s, 1H); 7.48–7.35 (m, 7H);
7.25–7.17 (m, 3H); 6.99 (d, J = 8.8 Hz, 2H); 4.03 (t, J = 7.1 Hz,
2H); 3.92 (m, 4H); 3.80 (m, 4H); 1.97–1.69 (m, 10H); 1.58–1.27
(m, 10H); 1.13–0.85 (m, 15H). Single-crystals suitable for X-ray
1 (a) T. Kato and K. Tanabe, Chem. Lett., 2009, 634;
(b) S. Yamane, Y. Sagara and T. Kato, Chem. Commun.,
2009, 3597; (c) J. H. Olivier, F. Camerel, G. Ulrich,
J. Barbera and R. Ziessel, Chem.–Eur. J., 2010, 16, 7134.
2 K. Binnemans, J. Mater. Chem., 2009, 19, 448.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1151–1161 | 1159

View Article Online
Paper
Dalton Transactions
3 (a) M. J. Mayoral, P. Ovejero, J. A. Campo, J. V. Heras,
E. Oliveira, B. Pedras, C. Lodeiro and M. Cano, J. Mater.
Chem., 2011, 21, 1255; (b) M. Ghedini, D. Pucci, A. Crispini,
A. Bellusci, M. La Deda, I. Aiello and T. Pugliese, Inorg.
Chem. Commun., 2007, 10, 243; (c) C.-R. Wen, Y.-J. Wang,
H.-C. Wang, H.-S. Shen, G.-H. Lee and C. K. Lai, Chem.
Mater., 2005, 17, 1646.
(b) A. S. Mocanu, M. Ilis, F. Dumitrascu, M. Ilie and
V. Cîrcu, Inorg. Chim. Acta, 2010, 363, 729; (c) V. Cîrcu,
P. N. Horton, M. B. Hursthouse and D. W. Bruce, Liq.
Cryst., 2007, 34, 1463; (d) J. Buey, L. Diez, P. Espinet,
H.-S. Kitzerow and J. A. Miguel, Chem. Mater., 1996, 8,
2375; (e) L. Diez, P. Espinet, J. A. Miguel and M. B. Ros,
J. Mater. Chem., 2002, 12, 3694.
4 (a) C.-T. Liao, H.-H. Chen, H.-F. Hsu, A. Poloek, H.-H. Yeh, 16 (a) Y. Yu Scaffidi-Domianello, A. A. Nazarov, M. Haukka,
Y. Chi, K.-W. Wang, C.-H. Lai, G.-H. Lee, C.-W. Shih and
P.-T. Chou, Chem.–Eur. J., 2011, 17, 546; (b) K. Venkatesan,
P. H. J. Kouwer, S. Yagi, P. Muller and T. M. Swager,
J. Mater. Chem., 2008, 18, 400; (c) Y. Wang, Q. Chen, Y. Li,
Y. Liu, H. Tan, J. Yu, M. Zhu, H. Wu, W. Zhu and Y. Cao,
M. Galanski, B. K. Keppler, J. Schneider, P. Du,
R. Eisenberg and V. Yu Kukushkin, Inorg. Chem., 2007, 46,
4469; (b) S. U. Pandya, K. C. Moss, M. R. Bryce,
A. S. Botsanov, M. A. Fox, V. Jankus, H. A. Al Attar and
A. P. Monkman, Eur. J. Inorg. Chem., 2010, 1963.
J. Phys. Chem. C, 2012, 116, 5908; (d) Y. F. Wang, Y. Liu, 17 (a) C.-M. Che, C.-C. Kwok, S.-W. Lai, A. F. Rausch,
J. Luo, H. R. Qi, X. S. Li, M. J. Ni, M. Liu, M. X. Zhu,
W. G. Zhu and Y. Cao, Dalton Trans., 2011, 40, 5046;
(e) M. Spencer, A. Santoro, R. G. Freeman, A. Diez,
P. R. Murray, J. Torroba, A. C. Whitwood, L. J. Yellowless,
W. J. Finkenzeller, N. Zhu and H. Yersin, Chem.–Eur. J.,
2010, 16, 233; (b) C.-M. Che, S.-C. Chan, H.-F. Xiang,
M. C. W. Chan, Y. Liu and Y. Wang, Chem. Commun., 2004,
1484.
J. A. G. Williams and D. W. Bruce, Dalton Trans., 2012, 41, 18 (a) K. Selvakumar and S. Vancheesan, Polyhedron, 1995, 14,
14244; (f) A. Diez, S. J. Cowling and D. W. Bruce, Chem.
Commun., 2012, 48, 10298; (g) T. Sato, H. Awano, O. Haba,
H. Katagiri, Y.-J. Pu, T. Takahashi and K. Yonetake, Dalton
Trans., 2012, 41, 8379.
5 C. Damm, G. Israel, T. Hegmann and C. Tschierske,
J. Mater. Chem., 2006, 16, 1808.
6 F. Neve, M. Ghedini and A. Crispini, Chem. Commun., 1996,
2463.
7 Y. Wang, Y. Liu, J. Luo, H. Qi, X. Li, M. Nin, M. Liu, D. Shi,
W. Zhu and Y. Cao, Dalton Trans., 2011, 40, 5046.
8 S. P. Jiang, K. J. Luo, J. H. Wang, X. Wang, Y. Jiang and
Y. Y. Wei, Chin. Chem. Lett., 2011, 22, 1005.
9 J. A. Gareth Williams, S. Develay, D. L. Rochester and
L. Murphy, Coord. Chem. Rev., 2008, 252, 2596.
10 K. P. Balashev, M. V. Puzyk, V. S. Kotlyar and
M. V. Kulikova, Coord. Chem. Rev., 1997, 159, 109.
11 L. Chassot and A. von Zelewsky, Inorg. Chem., 1987, 26,
2814.
2091; (b) J. J. Wu, J. H. Liu, L. Yang and Q. Zhu, Polyhedron,
1997, 16, 335; (c) C. M. Harsthorn and P. J. Steel, Organo-
metallics, 1998, 17, 3487; (d) P. Steenwinkel, S. L. James,
D. M. Grove, H. Kooijman, A. L. Spek and G. van Koten,
Organometallics, 1997, 16, 513; (e) D. J. de Geest,
B. J. O’Keefe and P. J. Steel, J. Organomet. Chem., 1999, 579,
97; (f) D. J. de Geest and P. J. Steel, Inorg. Chem. Commun.,
1998, 1, 358; (g) A. G. J. Ligtenbarg, E. K. van den Beuken,
A. Meetsma, N. Veldman, W. J. J. Smeets, A. L. Spek and
B. L. Feringa, J. Chem. Soc., Dalton Trans., 1998, 263;
(h) J. W. Slater and J. P. Rourke, J. Organomet. Chem., 2003,
688, 112; (i) B. J. O’Keefe and P. J. Steel, Organometallics,
2003, 22, 1281; ( j) M. Curic, D. Babic, A. Visnjevac and
K. Molcanov, Inorg. Chem., 2005, 44, 5975; (k) D. Babic,
M. Curic, K. Molcanov, G. Ilc and J. Plavec, Inorg. Chem.,
2008, 47, 10446; (l) M. Marino, J. Martinez, M. Caamano,
M. T. Pereira, J. M. Ortigueira, E. Gayoso, A. Fernandez and
J. M. Vila, Organometallics, 2012, 31, 890.
12 V. N. Kozhevnikov, B. Donnio and D. W. Bruce, Angew. 19 (a) B. Heinrich, K. Praefcke and D. Guillon, J. Mater. Chem.,
Chem., Int. Ed., 2008, 47, 6286.
1997, 7, 1363; (b) B. Gündogan and K. Praefcke, Chem. Ber.,
1993, 126, 1253; (c) J. W. Slater, D. P. Lydon and
J. P. Rourke, J. Organomet. Chem., 2002, 645, 246;
(d) D. P. Lydon and J. P. Rourke, Chem. Commun., 1997,
1741; (e) J. W. Slater, D. P. Lydon, N. W. Alcock and
J. P. Rourke, Organometallics, 2001, 20, 4418.
13 A. Santoro, A. C. Whitwood, J. A. Gareth Williams,
V. N. Kozhevnikov and D. W. Bruce, Chem. Mater., 2009, 21,
3871.
14 (a) D. Patra, P. Pattanayak, J. L. Pratihar and
S. Chattopadhyay, Polyhedron, 2013, 51, 46; (b) M. Dolores
Santana, R. Garcia-Bueno, G. Garcia, G. Sanchez, J. Garcia, 20 T. Hegmann, J. Kain, S. Diele, B. Schubert, H. Bogel and
J. Perez, L. Garcia and J. L. Serrano, Inorg. Chim. Acta, 2011, C. Tschierske, J. Mater. Chem., 2003, 13, 991.
378, 49; (c) M. Dolores Santana, R. Garcia-Bueno, G. Garcia, 21 (a) Y. Wang, Y. Liu, J. Luo, H. Qi, X. Li, M. Nin, M. Liu,
G. Sanchez, J. Garcia, A. R. Kapdi, M. Naik, S. Pednekar,
J. Perez, L. Garcia, E. Perez and J. L. Serrano, Dalton Trans.,
2012, 41, 3832; (d) M. Dolores Santana, R. Garcia Bueno,
G. Garcia, G. Sanchez, J. Garcia, J. Perez, L. Garcia and
J. L. Serrano, Dalton Trans., 2011, 40, 3537; (e) D.-Y. Ma,
L.-X. Zhang, X.-Y. Rao, T.-L. Wu, D.-H. Li and X.-Q. Xie,
J. Coord. Chem., 2013, 66, 1486.
D. Shi, W. Zhu and Y. Cao, Dalton Trans., 2011, 40, 5046;
(b) W.-L. Chia, S.-W. Shen and H.-C. Liu, Tetrahedron Lett.,
2001, 42, 2177; (c) W.-L. Chia, C.-L. Li and C.-H. Lin, Liq.
Cryst., 2010, 37, 23; (d) A. Santoro, A. M. Prokhorov,
V. N. Kozhevnikov, A. C. Whitwood, B. Donnio, J. A. Gareth
Williams and D. W. Bruce, J. Am. Chem. Soc., 2011, 133,
5248.
15 (a) V. Cîrcu, D. Manaila-Maximean, C. Rosu, M. Ilis, 22 See for example: (a) A. Capape, M. Crespo, J. Granell,
Y. Molard and F. Dumitrascu, Liq. Cryst., 2009, 36, 123;
M. Font-Bardia and X. Solana, Dalton Trans., 2007, 2030;
1160 | Dalton Trans., 2014, 43, 1151–1161
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
(b) A. Capape, M. Crespo, J. Granell, M. Font-Bardia and 27 (a) G. M. Arvanitis, C. E. Holmes, D. G. Johnson and
X. Solans, J. Organomet. Chem., 2005, 690, 4309;
(c) B. Bilgin-Eran, H. Ocak, C. Tschierske and
U. Baumeister, Liq. Cryst., 2012, 39, 467; (d) K. Praefcke,
M. Berardini, Acta Crystallogr., Sect. C: Cryst. Struct.
Commun., 2000, 56, 1332; (b) W. I. Sundquist, K. J. Ahmed,
L. S. Hollis and S. J. Lippard, Inorg. Chem., 1987, 26, 1524.
B. Bilgin, J. Pickardt and M. Borowski, Chem. Ber., 1994, 28 S. Debnath, H. F. Srour, B. Donnio, M. Fourmigue and
127, 1543. F. Camerel, RSC Adv., 2012, 2, 4453.
23 (a) M. Crespo, R. Martin, T. Calvet, M. Font-Bardia and 29 M. Ilis, M. Bucos, F. Dumitrascu and V. Cîrcu, J. Mol.
X. Solans, Polyhedron, 2008, 27, 2603; (b) D. Sukanya, Struct., 2011, 987, 1.
M. R. Evans, M. Zeller and K. Natarajan, Polyhedron, 2007, 30 M. J. Sienkowska, H. Monobe, P. Kasynski and Y. Shimizu,
26, 4314; (c) J. Bravo, C. Cativiela, R. Navarro and J. Mater. Chem., 2007, 17, 1392.
E. P. Urriolabeitia, J. Organomet. Chem., 2002, 650, 157; 31 (a) M. Lehmann, C. Köhn, H. Meier, S. Renker and
(d) H. Adams, S. Clunas and D. E. Fenton, Inorg. Chem.
Commun., 2001, 4, 667; (e) X. Chen, F. J. Femia,
J. W. Babich and J. Zubieta, Inorg. Chim. Acta, 2000, 308,
80; (f) S. Gourtatsis, S. P. Perlepes, I. S. Butler and
N. Hadjiliadis, Polyhedron, 1999, 18, 2369; (g) J. M. Vila,
A. Oehlhof, J. Mater. Chem., 2006, 16, 441; (b) S. Sauer,
N. Steinke, A. Baro, S. Laschat, F. Giesselmann and
W. Kantlehner, Chem. Mater., 2008, 20, 1909;
(c) M. Butschies, J. C. Haenle, S. Tussetschläger and
S. Laschat, Liq. Cryst., 2013, 40, 52.
M. Gayoso, T. Pereira, M. L. Torres, J. J. Fernandez, 32 A. C. Tenchiu, M. Ilis, F. Dumitrascu, A. C. Whitwood and
A. Fernandez and J. M. Ortigueira, J. Organomet. Chem.,
1996, 506, 165.
24 V. Cîrcu, M. Ilie, M. Ilis, F. Dumitrascu, I. Neagoe and
S. Pasculescu, Polyhedron, 2009, 28, 3739.
V. Cîrcu, Polyhedron, 2008, 27, 3537.
33 S.-W. Lai, T.-C. Cheung, M. C. W. Chan, K.-K. Cheung,
S.-M. Peng and C.-M. Che, Inorg. Chem., 2000, 39, 255.
34 E. Cavero, S. Uriel, P. Romero, J. L. Serrano and
R. Gimenez, J. Am. Chem. Soc., 2007, 129, 11608.
25 (a) A. Klein, S. Elmas and K. Butsch, Eur. J. Inorg. Chem.,
2009, 2271; (b) L. Maguire, C. M. Seward, S. Baljak, 35 C. A. Craig and R. J. Watts, Inorg. Chem., 1989, 28, 309.
T. Reumann, Y. Ortin, W. Banide, K. Nikitin, H. Müller- 36 K. Nakamaru, Bull. Chem. Soc. Jpn., 1982, 55, 2697.
Bunz and M. J. McGlinchey, Eur. J. Inorg. Chem., 2009, 37 A. Altomare, M. C. Burla, M. Camalli, G. Cascarano,
3250; (c) P. Marqués-Gallego, H. den Dulk, J. Brower,
C. Giacovazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori
H. Kooijman, A. L. Spek, O. Roubeau, S. J. Teat and
and R. Spagna, J. Appl. Crystallogr., 1999, 32, 115.
J. Reedijk, Inorg. Chem., 2008, 47, 11171; (d) A. D. Ryabov, 38 G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystal-
I. M. Panyashkina, V. A. Polyakov and A. Fischer, Organo-
metallics, 2002, 21, 1633.
logr., 2008, 64, 112.
39 L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837.
26 T. A. K. Al-Allaf and A. Z. M. Sheet, Polyhedron, 1995, 14, 40 V. Cîrcu, M. Ilis and F. Dumitrascu, J. Optoelectron. Adv.
239.
Mater., 2008, 10, 3454.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1151–1161 | 1161