Synthesis, characterization and solid-state photoluminescence studies of six alkoxy phenylene ethynylene dinuclear palladium(ii) rods

Article abstract of DOI:10.1039/c4dt00493k

A rare family of six discrete binuclear [PdCl(PEt₃)₂] phenylene ethynylene rods with alkoxy side chains (methoxy, ethoxy and heptoxy) have been developed, and their solid-state photoluminescence results have been presented and discus

Full text of DOI:10.1039/c4dt00493k

Dalton
Transactions
PAPER
Synthesis, characterization and solid-state
photoluminescence studies of six alkoxy
phenylene ethynylene dinuclear palladium(II) rods†
Cite this: Dalton Trans., 2015, 44,
4003
João Figueira,‡a Wojciech Czardybon,§a José Carlos Mesquita,a João Rodrigues,*a
c
Fernando Lahoz,*b Luca Russo,¶c Arto Valkonenk and Kari Rissanen*c
A rare family of six discrete binuclear [PdCl(PEt3)2] phenylene ethynylene rods with alkoxy side chains
(methoxy, ethoxy and heptoxy) have been developed, and their solid-state photoluminescence results
have been presented and discussed. The shorter bridging ligands are of the general formula H–CuC–
C6H2(R)2–CuC–H, where R = H, OCH3, OC2H5, and OC7H15, whereas the longer ones are based on H–
CuC–C6H4–CuC–C6H2(R)2–CuC–C6H4–CuC–H, where R = OCH3, OC2H5. These ligands display
increasing length in both the main dimension (backbone length) as well as the number of carbons in the
side chains (R, alkoxide side chain) that stem from the central phenylene moiety. The X-ray crystal struc-
tures of two of the prepared complexes are reported: one corresponds to a shorter rod, 1,4-bis[trans-
(PEt3)2ClPd-CuC]-2,5-diethoxybenzene (6c), while the second one is associated with a longer rod, the
binuclear complex 1,4-bis[trans-(PEt3)2ClPd-4-(–CuC–C6H4–CuC)]-2,5-diethoxybenzene (7c). All new
compounds were characterized by NMR spectroscopy (1H, 13C{1H} and 31P{1H}) as well as ESI-MS(TOF),
EA, FTIR, UV-Vis, cyclic voltammetry and solid-state photoluminescence. Our work shows the influence
of the alkoxy side chains on the electronic structure of the family of binuclear Pd rods by lowering its oxi-
dation potential. In addition to this, the increase of the length of the bridge results in a higher oxidation
potential. Solid state photoluminescence results indicate that Pd complexes are characterized by a
marked decrease in both the emission intensity and the fluorescence lifetime values as compared to their
ligands. This behaviour could be due to some degree of ligand-to-metal charge transfer.
Received 16th February 2014,
Accepted 7th January 2015
DOI: 10.1039/c4dt00493k
www.rsc.org/dalton
insulators.1–7 It is well known that the incorporation of tran-
sition metal centers enables even finer tuning of the properties
Introduction
Conjugated one-dimensional rods have been the centre of of these rods.8–11
attention of several recent studies related to the development
Transition metal alkynyl complexes, with their ability to
of molecular electronic devices such as wires, switches or perturb electronic properties through metal alkynyl dπ–pπ
interactions,12,13 are primary candidates for the preparation of
metallomolecular devices.14 The added rigid framework pro-
vided by the phenylene ethynylene ligand, as well as the easy
aCQM – Centro de Química da Madeira, MMRG, Universidade da Madeira, Campus
Universitário da Penteada, 9000-390 Funchal, Portugal. E-mail: joaor@uma.pt;
processability, improves its potential for molecular device
http://www.uma.pt/jrmmrg; Fax: +351 291705149/249; Tel: +351 291705108
preparation using the building block approach, which allows
for the fine tuning of magnetic, electronic and photo-
luminescence properties.9–11,14,15 The introduction of side
chains into the aromatic moieties has the major advantage of
not only increasing solubility,16,17 but also it facilitates the
preparation of thin films, improves interface with other co-
polymers,18 contributes to electroluminescence enhance-
ment,19 inhibits chain-to-chain interactions and also gives
more regular polymerizations products,20 refines emission
color shifting in electroluminescent materials,21 helps improve
and tune the formation of thin films for photovoltaic cells22 as
well as increase delocalization and consequently inter-metal
communication in binuclear complexes.23–26
bDepartamento de Física Fundamental y Experimental, Electrónica y Sistemas,
Faculdad de Física, Universidad de La Laguna, 38206 La Laguna, Spain.
E-mail: flahoz@ull.es
cDepartment of Chemistry, NanoScience Center, University of Jyväskylä, PO. Box 35,
40014 JYU, Finland. E-mail: kari.t.rissanen@jyu.fi
†Electronic supplementary information (ESI) available: 1H, 13C, 31P NMR, ESI-
MS, FTIR and UV-Vis spectra as well as the remainder of the cyclic voltammo-
grams can be found in the ESI. CCDC 882324 and 882325. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c4dt00493k
‡Present address: Clinical Neuroscience Unit, Department of Pharmacology and
Clinical Neuroscience, Umeå University, 90187 Umeå, Sweden.
§Present address: Selvita S.A. ul. Bobrzyńskiego 1430-348 Kraków, Poland.
¶Present address: Rigaku Europe SE, Am Hardtwald 11, 76275 Ettlingen, Germany.
kPresent address: Tampere University of Technology, Department of Chemistry
and Bioengineering, P.O. Box 541, 33101 Tampere, Finland.
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Moreover, the decrease of the oxidation potential as a result (77 K) with a decay time of about 2.9 μs. In the case of the
of methyl (donor) β-substitution in thiophene,27 as well as the platinum analogues, the luminescence was strongly quenched
decrease in the Eg (energy gap, EHOMO–ELUMO) yielded by in CH2Cl2 solutions. This was evidenced by the short lifetime
methoxy (also donor) substitution in phenylene vinylene oligo- observed in the solution (<0.1 μs) as compared to that
mers, was previously reported.28
measured in the solid state, which was similar to the palla-
If Pd-σ-alkynyls are often used for the preparation of dium counterparts. Moreover, when testing glass samples of
homometallic3,4,29–33 or heterometallic34–36 (with the incorpor- this series (77 K, EtOH–MeOH 4 : 1, v/v), the luminescence
ation of Ru, Fe or Ni metal centers) organometallic polymers, intensity increased drastically, and the lifetime was augmented
the synthesis of discrete binuclear Pd rods is comparatively to 50 μs.
much more scarce and is not so often found in the
Previous results from our group and others showed the
importance of solid-state photoluminescence studies in the
literature.5,37–39
Previous studies showed that [PdCl2(PEt3)2] compounds characterization of new photonic hybrid waveguides,60–62 and
form square planar systems with the bridging oligophenylene prompted us to systematically study the effect of substituents
ligands.15 Consequently, an increase of the π delocalization attached to bridging ligands on the electronic communication
from the ring to the metal center would be expected, which between the two metal centers and on the solid-state photo-
could improve the conducting properties of the rod. This co- luminescence efficiency of a new family of binuclear [PdCl-
planarity between the metal center and the π system of the (PEt3)2] phenylene ethynylene rods with alkoxy side chains
ligand is dictated by the ligand as Onitsuka and colleagues (methoxy, ethoxy and heptoxy). As such, a family of palladium
reported for 1,4-diethynylbenzene palladium complexes.40 terminated dinuclear rods (Scheme 1, 6a–d, 7b–c), based on
This report shows that the metal centers are slightly twisted 1,4-diethynylbenzene derivatives (Scheme 2, 1a–d, 4b–c), was
out of the plane because of steric hindrance caused by the prepared and characterized.
phosphane groups of the metal center. As such, choosing a
bridging ligand that, in addition to a solid conjugate back-
bone, also promotes co-planarity and processability is of great
importance. As a matter of fact, the structure and geometry of
Results and discussion
Synthesis
the target systems greatly influence their properties, resulting
in an insulator or a conductor molecule.41–48
We have synthesized phenylene ethynylene based bridging
ligands of one or three benzene aromatic rings (compounds
The luminescence properties of organometallic compounds
in solution are well-documented.9–11,49–59 The inclusion of
transition metal moieties gives access to efficient spin–orbit
coupling, which, in turn, enables the population of excited
states of triplet character and phosphorescence characteristics
of organometallic compounds. As such the coordination of the
organic compounds to metal centers can enrich the emission
properties by enabling access to new excited-state species.57
Yam and co-workers52 reported the carbazole bridged dinuc-
lear Pd and Pt rods which were non-emissive in solution
(CH2Cl2), whereas it showed luminescence in the solid state
Scheme 2 The 1,4-diethynylbenzene derivatives used as bridging
ligands.
Scheme 1 The series of trans-[PdCl(PEt3)2]+ rods prepared in this work.
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aromatic protons show very similar chemical shifts in compari-
son with the free ligands (e.g. 6b, OCH3 6.69 vs. 6.98 ppm in
the free ligand). The same observation can be made for the
first protons of the side chains (e.g. 6b, OCH3 3.74 vs.
3.87 ppm in the free ligand). Furthermore, the values for the
equivalent protons across Pd rods are relatively closer. This
can be observed for the side chains as well as for the aromatic
protons. The palladium rods (6a–d, 7b–c) all show 31P NMR
singlets at around 18 ppm which is only about 1 ppm more
than the free palladium starting material trans-[Pd(PEt3)2Cl2]
(5). There is no significant change in this value throughout the
series. The main difference in the NMR data of the palladium
rods comes from the virtual coupling observed in the 13C
NMR. This C–P coupling70 was found at around 15 ppm in the
13C spectra for all the rods.
The Pd complexes were also characterized by ESI-TOF.
An [M − Cl]+ peak was found for most complexes, except for
complex 6d, for which a peak at 2240.51 m/z [2M + Na + H]+
was observed. Furthermore, [M + Na]+ ion peaks were also
observed for all the studied compounds. For 6a, an ion peak of
727.0 m/z was found, which corresponds to [M − Cl − PEt3 +
H]+. Several other ions were visible in the case of the longer
complexes. For example, for 7b, 881.3 m/z presents a peak with
the formal composition [M − (Pd(PEt3)2Cl) + PEt3]+. Similar
fragmentation and cleavage have been observed in the
literature.35
Scheme 3 Preparation of the longer bridging ligands (4b–c);
i. Sonogashira–Hagihara coupling, 1-bromo-4-iodobenzene, 45 °C; ii.
Sonogashira–Hagihara coupling, trimethylsilylacetylene, 55 °C; iii.
Degassed CH2Cl2–CH3OH (1 : 1 mixture), KF, room temp.
1a–d and 4b–c, respectively, Scheme 3). In the case of the
longer bridging ligands (4b–c), the Sonogashira–Hagihara
coupling conditions were used (Scheme 3).19,41,48,62–69 Inter-
mediary products as well as the final products previously
reported (1a–d)34,62,63 were confirmed at least by 1H and 13C
(see ESI†). The yields for these reactions are in the range of
those reported in the literature (i.e. 50–76%). The tris ringed
analogues of 1a and 1d (R = H and OC7H15 respectively 4a and
4d) were not prepared by us due to isolation and purification
problems encountered in the synthesis of these ligands. Never-
theless, the numbering was done taking into account these
compounds to help the direct comparison of compounds 1b/
6b/7b and 1c/6c/7c.
The preparation of the palladium rods (6a–d, 7b–c,
Scheme 1) was performed with good yields (60–80%) by follow-
ing the methodology reported in the literature40 and using
trans-[PdCl2(PEt3)2] (5). These rods are sufficiently stable to be
washed with water, and two new X-ray structures were obtained
for a shorter rod (6c) and a longer rod (7c) both with the same
1,4-diethoxybenzene central moiety.
UV-Vis studies
Significant information about the influence of the chemical
structure on the optoelectronic properties of the compounds
can be obtained from the analysis of the UV-Vis absorption
spectra.71
The absorption is dominated by spin allowed π→π* tran-
sitions of the delocalized electronic charge of the ethynylben-
zene moieties, which are rich in conjugated bonds. Two main
bands are observed for the tris ringed ligands 4b and 4c at
about 316 and 376 nm. However, the absorption bands of the
single ring ligand 1b are notably blue shifted, showing a
maximum at 346 nm (Fig. 1). It should be mentioned that the
Characterization
The new compounds were characterized by NMR spectroscopy
(1H, 13C{1H}, and 31P{1H}) as well as ESI-MS(TOF), EA, FTIR,
UV-Vis, cyclic voltammetry and solid-state photoluminescence
(see the Experimental section and ESI†). Briefly, the FTIR
studies for the new palladium rods (6a–d, 7b–c) show the
characteristic shift of the νCuC band with the coordination to
the metal. This was also observed by the shift, albeit small, of
the singlet observed in the 31P NMR. Furthermore the Pd rod
complexes display (in comparison with the free ligands), in the
1H spectrum, mostly a shielding of the aromatic (no more
Fig. 1 Absorption spectra (ε vs. λ) for the shorter (1b) and longer free
than 0.5 ppm) and side chain protons (ca. 0.1 ppm only). The ligands (4b–c) in CH2Cl2.
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absorption bands of the tris ringed ligands show a red shift
when compared with a similar 1,4-diphenylethynylbenzene
reported by Nakatsuji,72 with a peak at 322 nm. Nevertheless,
Nakatsuji and co-workers performed this study in a CHCl3
solution which might account for the high ε that was observed
(62.0 × 103 M−1 cm−1). No significant change was observed in
the position of the absorption bands of the 4b and 4c com-
pounds. Probably this observation is due to the small differ-
ence in the length of side chains (OCH3 to OC2H5) of the
prepared compounds.
Concerning the palladium complexes, the spectra of the
palladium rod (6a) based on the single ringed free ligand
without any side chains (1a) are presented and compared to
the starting materials in Fig. 2. The bands of the absorption
spectrum of 6a can be clearly assigned to those of the starting
materials as they show similar positions with a slight blue
shift. It seems that the coordination of the bridge to the palla-
dium termini has little effect on the π→π* absorption bands.
In the case of the other palladium rods, the shift is in the
expected low energy direction (red shift) as further conjugation
is brought about by the coordination. Moreover, the high simi-
larity between the absorption of the starting ligand and the
resulting complex is an indication of the predominantly ligand
character of the transitions (π→π*) responsible for the absorp-
tion spectrum of the complex. This observation was also
reported by several other authors32,73–79 who additionally
admitted some degree of MLCT (metal-to-ligand charge trans-
fer) for the low energy bands (dπ→π*(CuC–R)) in similar com-
pounds (ClL2Pd–CuC–C6H4–R, where R = (CuC–C6H4–CuC–
PdLCl)2 or analogous Pt complexes).
We have analyzed the effect of the length of the ring back-
bone on the absorption properties. A table which summarizes
the position of the relevant absorption bands for both the
ligands and the Pd complexes is available in the ESI
(Table S1†). In the case of 1b to 6b (shorter bridging ligands),
the variation is from 346 to 362 nm, and for 4b to 7b (longer
bridging ligands), it changes from 375 to 385 nm which is
even lower that the single ringed case. The increase of ε is also
Fig. 3 Electronic spectra (ε vs. λ) for the palladium rods (6b–d) in
CH2Cl2.
lower in the case of the tris ringed rods (7b–c, Fig. S2,† about
twofold relative to free ligands) when compared to the single
ringed counterparts (6b–d, about 3 fold in relation to the free
ligands). This can point to lesser conjugation, which might be
due to trapped electron density on the ligand. Furthermore,
the increase in side chain length is mostly inconsequential
after a length of 6c (OC2H5), in the particular case of the
studied alkoxy chains, as only slight variations in the range of
1–5 nm are observed when changing only the alkyl length.
Nevertheless, with respect to ε, a marked decrease is observed
from 6c to 6d (31–25 × 103 M−1 cm−1, Fig. 3). Finally, the sig-
nificant differences found in both the positions as well as the
intensities of the absorption bands of the Pd-complexes com-
pared to their associated free ligands are directly related to
π→π* electronic transition properties. It might be an indi-
cation of different electronic conduction properties, which are
important for optoelectronic applications. Moreover, this point
will be further analyzed in the solid-state fluorescence decay
studies section.
Electrochemistry studies
The free ligands only present an irreversible oxidation wave in
the working potential intervals when the side chain is intro-
duced, i.e., the 1a ligand does not show any oxidation wave,
but 1b–d and 4b–c, all present high potential oxidations (ca.
1.6 V for the shorter – 1a–d – and 1.4 V for the longer bridging
ligands – 4b–c). This wave is most likely the one-electron oxi-
dation of the phenyl ring which shifts to lower energies by the
electron donating effect of the alkoxy side chains. Other
authors,80 working with similar compounds, have also per-
formed the same attribution.
As for the shorter rods (Fig. 4, 6a–d), it is possible to
observe the existence of two electrode processes. The first one
corresponds to a reversible process while the second one is
probably irreversible. Reduction for the first process is only
observable at 100 mV s−1 in the compound with the longer
side chain (6d) but is relatively weak. However, when the scan
rate is increased, the reduction is observed more intensely,
Fig. 2 Absorption spectra (ε vs. λ) for the palladium rod 6a as well as
the respective free ligand (1a) and starting complex 5 in CH2Cl2.
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Fig. 4 Cyclic voltammograms for the shorter palladium rods without
(6a) and with alkoxy side chains (6b–d) at 100 mV s−1 vs. Ag/AgCl (KCl
saturated) in CH2Cl2.
Fig. 5 Cyclic voltammograms for the longer palladium rods (7b–c)
with alkoxy side chains at 100 mV s−1 vs. Ag/AgCl (KCl saturated) in
CH2Cl2.
indicating a following chemical reaction step. The first oxi-
dation wave width of the compounds with alkoxy side chains
(6b–d) suggests that this conversion is 1e−, which can indicate
the formation of the PdII/PdIII pair.81 A following conversion to
PdIII/PdIV could characterize the process at higher potentials,
but once again, the similarity to the free ligands (as observed
in the UV studies), as well as the region where it is found, is
close to the electrochemical process of the solvent making it
difficult to characterize. The first process observed for the rods
with smaller side chains (6a–b) tends to irreversibility.
The most striking aspect of the aggregated voltammograms
in Fig. 4 is that there is a shift to higher anodic potentials
when the side chain is introduced (890 (6a) to ca. 996 mV for
6b). This value then shifts back down with the increase of the
side chain’s length (6c and 6d). This potential appears to
stabilize after a side chain of OC2H5 (6c) since for 6d (side
chain of OC7H15), the values are very close (945 and 927 mV
respectively). Nevertheless, this decrease from OCH3 to OC2H5
side chains is not very pronounced when observing the longer
palladium rods, since the anodic potentials for these oxidation
processes are already very close (1.31 V for both compounds
7a–b, Fig. 5) and actually higher than that of the 6a (with no
side chain). Moreover, they are very close to the ones observed
for the free ligands (4b–c). This observation further cements
the expected low delocalization not only through the palla-
dium centre but also through these more extended ligands.82
Fig. 6 ORTEP-383 plot of (6c). H atoms are excluded for clarity.
this work (6b–d). Apart from a published example bearing gold
complex termini,86 no X-ray structure of a dinuclear rod
bridged by a tris ringed ligand was found. Table 1 summarizes
the crystallographic data for the two reported structures
(selected bond angle and distance values are available in the
ESI†).
The structure of 6c (Fig. 6) contains half a molecule in the
asymmetric unit and, in a first look, there appears to be a co-
planarity of the phenylene ring with the palladium center,
which has the expected square planar geometry. Nevertheless
and although the ring is not twisted in relation to the Pd
centre, the coordination plane around the Pd intersects the
plane formed by the phenyl ring with an angle of 9.8(3)°. The
ethoxy side chain does not run across the same plane formed
by the phenylene ring, bending slightly to one side due to,
apparently, packing short contacts. In contrast, the free
ligand62 shows co-planar side chains.
X-ray crystal structure analysis
Two X-ray structures were solved for the prepared Pd rods. One
is related with the shorter rod (6c) and the other to the analo-
gous longer palladium rod (7c). Their ORTEP-383 plots are pres-
ented in Fig. 6 and 7, respectively. A search of the Cambridge
Structural Database (CSD)84 reported only three similar struc-
tures.37,85 These are related to PdCl(PR3)2 (where R = butyl,
ethyl) rods bridged by 1,4-diethynylbenzene,37,85 as well as
another one37 with the central decorated phenyl ring with
OC8H19 side chains similar to the shorter Pd rods reported in
As for bond distances for this structure, CuC, is 1.176(9) Å
and Pd–C is 1.964(7) Å which are the typical values for metal-σ-
acetylide moieties.36,37,40,85,87 Furthermore Pd–P is 2.320(2) Å
and Pd–Cl is 2.362(2) Å. Regarding the bridging ligand, CvC
and C–O are 1.385(8)–1.411(9) and 1.385(8) Å long, respect-
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Fig. 7 ORTEP-383 plot of the major component in 7c. H atoms and disorder are excluded for clarity.
short-contacts between the terminal chlorides and the
H-atoms from the phosphanes and the ethoxy side chains. The
observed packing was very similar to that observed by Lo
Sterzo and co-workers for [Cl(PBu3)2Pd–CuC]2-1,4-benzene.37
The same authors reported a sheet-like packing when the
alkoxy side chain was the longer octyloxy which clearly drives
the formation of the lattice.37 It is noteworthy that this highly
oriented lattice comes at the cost of a loss of co-planarity
between the Pd centre and the phenylene ring. This could be
regarded as a disadvantage for some applications,89–92 As
described by Mayor and co-workers,93 who found, in a family
of biphenyl compounds, conductance values thirty times lower
when a full torsion (90°) is seen. Further explanation for this
observation could also be based on the lack of co-planarity
between the central phenyl ring and the two Pd coordination
planes.
Table 1 Crystallographic data for the structures of 6c and 7c
6c
7c
CCDC Depository
Crystallization solvents
882324
CH2Cl2–diethyl
ether
882325
CH2Cl2–diethyl
ether
Empirical formula
T (K)
C38H72Cl2O2P4Pd2
173(2)
C54H80Cl2O2P4Pd2
123(2)
λ (Å)
0.71073
Monoclinic
P21/n
12.093(2)
15.652(3)
13.128(3)
90
0.71073
Monoclinic
P21/n
13.4614(3)
11.7059(3)
18.7397(4)
90
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
107.02(3)
90
2376.0(8)
2
1.348
1.032
103.7050(10)
90
2868.89(12)
2
1.353
0.868
γ (°)
V (Å3)
Z
ρcalc (g cm−3
)
µ (mm−1
)
The crystal structure of 7c shows disorder in all groups co-
ordinated to the Pd atom, except for the acetylide group (see
ESI†). The major component (60% population) is presented in
Fig. 7. The central and terminal phenyl rings in 7c, which also
contains half a molecule in the asymmetric unit, do not show
too much twisting between each other, with the angle formed
between the corresponding planes being only 23.3(2)°. Never-
theless, there is an 86.7(2)° angle (major, 84.2(2)° for the
minor component) formed between the central phenyl moiety
and the coordination plane around Pd. The bond distances
Pd–P of 2.340(4) and 2.283(7) Å as well as Pd–Cl of 2.346(6) Å
in 7c (values for the minor component in ESI†) are in agree-
ment with the literature reported distances of 2.306 Å for Pd–P
and 2.334 Å for Pd–Cl.37,85 The acetylide to palladium distance,
Pd–CuC, is of 1.947(6) Å, which is also close to reported
values of 1.939 Å. The acetylenic bond lengths are 1.184(8) Å
for the Pd coordinated and 1.200(8) Å for the isolated CuC
bonds. These are in accordance with the reported values
(1.198 Å)37,85 and only slightly shorter when coordinated to the
Pd. Finally, the CAr–O of 1.360(8) Å with the CAr–O–CH2 angle
Total reflections
Unique reflections
Rint
Crystal size (mm)
Colour
Habit
F(000)
thetamin–thetamax (°)
Data
Restraints
Parameters
19 588
5367
0.1385
0.4 × 0.02 × 0.01
Orange
Flat needle
996
3.46–27.52
5367
9254
5186
0.0292
0.13 × 0.6 × 0.06
Yellow
Plate
1212
2.34–25.25
5186
421
0
217
1.077
424
1.055
Goodness-of-fit (GOF) on
F2
R indices (all data)
R1
wR2
Final R indices (I > 2σ(I))
R1
wR2
0.1425
0.1003
0.0765
0.1461
0.0709
0.0814
0.658 and −0.711
0.0591
0.1343
1.304 and −0.957
Largest difference in peak
and hole (e Å−3
)
ively, which are also similar to other binuclear Pd rods.37,85
The Pd–CuC angle is 175.9(7)° which is in agreement with the of 118.6(5)° are similar to the literature.37,85 The coordination
expected linear geometry. The P–Pd–Cl angles are 88.17(7) and around the Pd atoms, viz., the angle values for P–Pd–Cl, is of
96.5(2)° and 89.1(2)° with P–Pd–P 171.1(2) and C–Pd–Cl 177.8(2)°,
96.13(7)° and the P–Pd–C is 177.2(2)° which consequently
means a slight deviation from fully square planar geometry. as expected, close to the previously reported values for
The values for these angles and the corresponding distortion related systems.37,85 Fig. 8 shows the packing diagram of
complex 7c (along the a-axis) where it is possible to observe
are in good agreement with those previously reported by other
authors.32,37,88 Intermolecular interactions commence through the layer orientation of the molecules.
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Paper
Fig. 8 Packing diagram of 7c (along the a-axis), which shows the different orientation of the molecules between the layers. The molecule in the
asymmetric unit is emphasized (ball-and-stick). The disorder and hydrogen atoms are removed for clarity.
Solid-state fluorescence decay studies
Since a potential application of these rods would be as solid-
state electronic devices, solid-state fluorescence studies would
help to determine the conjugation properties of these
materials.94 We have studied the decay of the fluorescence
when the powder samples were excited with a pulsed laser
(80 ps pulse width) at 405 nm. The results obtained for the
free ligands and their respective Pd rods are shown in Fig. 9
and 10, respectively. The fluorescence decay was detected at
about the maximum of the emission bands. A non-exponential
decay was observed in all the cases, which is very common for
non-diluted solid state samples. The non-exponential behavior
of the decay curves can be due to ligand-to-ligand or ligand-to-
metal interactions, as well as to the contribution of a distri-
bution of environments, which is typical in powder samples.
The photoluminescence decays are consistent with a multi-
exponential equation, which can be useful to estimate an
average lifetime that represents the decay rate of the
complex.37 Good agreement was found for a three exponential
decay curve (eqn (1)).
Fig. 10 Decay of the fluorescence for the Pd rods (6a–d, 7b–c).
burgh Instruments. The average lifetime is then calculated
using the following equation (eqn (2)):95
2
2
2
B1τ1 þ B2τ2 þ B3τ3
B1τ1 þ B2τ2 þ B3τ3
τav
¼
ð2Þ
IðtÞ ¼ B1eðꢀt=τ Þ þ B2eðꢀt=τ Þ þ B3eðꢀt=τ Þ
ð1Þ
1
2
3
The results for the free ligands and for the Pd complexes
are given in Table 2.
The fitting was done using an IRF (instrumental response
function) reconvolution analysis with F900 software by Edin-
The free ligands (1a–d, 4b–c) show average lifetime values
comparable to those found in the literature for the new hybrid
composites of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-pheny-
lenevinylene) (MEH-PPV) and clay (montmorillonite), purposely
prepared for use in optoelectronic devices (like OLEDs).96
These nanocomposites show increased lifetimes when the con-
centration is higher, but values are still on the picosecond
Table 2 Solid-state fluorescence lifetime values for the free ligands
(1a–d, 4b–c) and the respective Pd rods (6a–d, 7b–c)
Bridge
λ/nm
τ/ns
Rod
λ/nm
τ/ns
1a
1b
1c
1d
4b
4c
510
510
510
510
510
490
0.80
1.00
0.70
0.90
1.20
0.95
6a
6b
6c
6d
7b
7c
454
510
510
510
450
510
0.24
0.60
0.30
0.30
0.05
0.40
Fig. 9 Decay of the fluorescence of the free ligands (1a–d, 4b–c) IRF.
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Dalton Trans., 2015, 44, 4003–4015 | 4009

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Dalton Transactions
scale. Moreover, lasing capabilities were reported97 for p-(phe- distilled over potassium hydroxide; n-hexane and dichloro-
nylene ethynylene) polymers, but in solution phase. Reports methane over calcium hydride; tetrahydrofuran, diethyl ether
show98 that fluorescence lifetime values of 2 ns were obtained and toluene over sodium/benzophenone ketyl. Dimethylsulfox-
for phenylene ethynylenes compounds similar to the studied ide and glacial acetic acid were used as received and degassed
here. These reported values are nevertheless for chloroform under nitrogen (with stirring or sonication). Methanol was
solutions. Furthermore, this report98 also shows similar life- degassed under argon (with sonication). Solvents used in
time values for the fluorescence when comparing a tris ringed column chromatography were used as received. CuI and CuCl
with its twisted (by a secondary chain) counterpart, even were pre-dried at 120 °C for at least 2 h (usually overnight)
though a significantly lower (21 fold) quantum yield was found before usage. Compound 5, trans-[PdCl2(PEt3)2], was used as
for the twisted compound. The authors98 do not account for received from Acros Organics taking care to store it at 4 °C.
this discrepancy and they attribute it to the equipment’s fast
Physical measurements
decay.
When comparing the values presented in Table 2, higher The infra-red spectra were obtained using a Bruker Vertex 70
lifetime values are found for the longer ligands (4b–c). Further- FTIR spectrophotometer equipped with an ATR (attenuated
more, a clear trend is observed for all the complexes that were total reflectance) diamond add-on. Samples were collected at a
analyzed (6a–d, 7b–c), the average lifetime decreases in the Pd resolution of 2 cm−1 using a spectral range of 4000 to
compounds as compared to their respective free ligands. More- 400 cm−1. NMR spectra were recorded with a Bruker 400 MHz
over, a decrease in the emission intensity of the Pd complexes NMR (Bruker Avance II+). 1H, 13C{1H} and 31P{1H} NMR
of one or two orders of magnitude was detected when com- spectra were collected at 400 MHz, 101 MHz and 161 MHz
pared to their associated free ligands. This could be accounted respectively. The chemical shifts (δ) are reported in ppm and
for by the charge distribution (from ligand to metal centre) referenced to residual solvent peaks99 for 1H and 13C. 31P NMR
reducing available electronic density that would induce a non- is referenced to external 85% H3PO4. Melting points were
radiative relaxation path, which would decrease the lifetime determined with an Electrothermal 9200 melting point appar-
and radiative emission quantum yield.
atus, using sealed capillaries without calibration. Mass spectra
In particular, for the two compounds for which X-ray data were obtained using a Micromass LCT spectrometer ESI-TOF
are available, 6c and 7c, their average lifetimes can be com- spectrometer. Elemental analyses were carried out using
pared with those of their respective free-ligand molecules, 1c
a VariolEL instrument from Elementar Analysensysteme.
and 4c, respectively. In the case of the single ring sample, it Cyclic voltammetry was performed using a PARSAT advanced
reduces from 0.7 ns, in the free ligand 1c, to 0.3 ns for the Pd electrochemical equipment with tetra-N-butylammonium hexa-
coordinated complex, 6c. The lifetime diminishes from the fluorophosphate 0.1 M as the supporting electrolyte in CH2Cl2
0.95 ns found in the tris ringed free ligand 4c to 0.4 ns in the with an analyte concentration of 1 × 10−3 M. The reference
corresponding metal complex, 7c. This represents a similar electrode used was the Ag/AgCl (saturated KCl). Platinum was
reduction of lifetime of 43% and 42% for the single and tris used as the working and secondary electrodes. The potentials
ringed systems. If we consider an important structural para- were calibrated against the ferrocene/ferrocenium couple.
meter obtained from the X-ray analysis, such as the co-planar- Absorption spectra were recorded using a GBC-Cintra 40 in dry
ity angle between the central phenyl and the Pd coordination and degassed (argon) CH2Cl2 at 1 × 10−5 M for the free ligands
plane, it changes from 10.96° in the single ring complex, 6c, to (1a–d, 4b–c) and trans-[PdCl2(PEt3)2] (5) and at 1 × 10−6 M for
86.56° for the tris ringed metal compound, 7c. One could the rods (6a–d, 7b–c). All solution measurements were per-
expect a higher overlapping of the π delocalized electronic formed using dry and degassed solvent and at room tempera-
density to the metal center for a small co-planarity angle. ture unless otherwise noted. Lifetime fluorescence studies
However, the similar lifetime reduction observed in both were carried out by measuring the samples in the form of
compounds seems to indicate that a significant ligand to powder between two microscope slides. The measurements
metal charge transfer exists in both cases, despite the low co- were then performed using a LifeSpec II Spectrometer from
planarity exhibited in the 7c complex. This can be due to the Edinburg Instruments, with a pulsed 405 nm diode laser with
partial tilting between the central and terminal phenyl rings 80 ps pulse duration as the excitation source. The Instrumental
(23.29°), and also to the strong electronegativity of the Pd Response Function (IRF) of the spectrometer has been
center.
measured in order to correct the lifetime data. As the powder
samples did not present all the same particle size and the
sample size was different, integrated fluorescence spectra can
only be compared by orders of magnitude of difference.
Accordingly, samples are compared to their average lifetimes,
which is independent of the factors referred above.
Experimental section
Materials
Chemicals were used as received unless otherwise noted. All
reactions were carried out under a dry nitrogen atmosphere
Synthesis of the bridging ligands
using standard Schlenk techniques. Solvents were freshly dis- The new 1,4-diethynylbenzene single ringed derivatives
tilled under nitrogen. Triethylamine and diethylamine were (Scheme 1) were prepared according to methods previously
4010 | Dalton Trans., 2015, 44, 4003–4015
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Dalton Transactions
Paper
described in the literature (1a,34 1b63 and 1c–d62). The longer Calcd for C30H22O2·0.45 H2O (422.2): C, 85.26; H, 5.46; found
bridging ligands (4b–c) were prepared by following the C, 85.2; H, 5.56.
Sonogashira–Hagihara procedures19,41,62,65–69 by coupling the
shorter analogues (1b–c, respectively) with 1-iodo-4-bromoben-
Synthesis of the palladium complexes
zene, followed by another coupling to insert the terminal acety- The bridged palladium complexes (Scheme 1) were obtained
lenic moiety (Scheme 3). Cleavage of the trimethylsilyl modifying the method reported by Onitsuka,40 coordinating
protecting group was achieved with KF or K2CO3.100 The prepa- the described ligands (1a–d, 4b–c) to the trans-[PdCl2(PEt3)2]
ration of the intermediaries (2b–c, 3b–c) is detailed in (5) moiety in diethylamine (solvent and base) using copper (I)
the ESI.†
1,4-Dimethoxy-2,5-bis((4-ethynylphenyl)ethynyl)benzene, 4b.
(CuCl 5% molar equiv.) as a catalyst.
1,4-Bis[trans-(PEt3)2ClPd–CuC]benzene,
6a. Compound
1,4-Dimethoxy-2,5-bis((4-trimethylsilylethynylphenyl)ethynyl) 1,4-bis[trans-(PEt3)2ClPd–CuC]benzene (6a) was first prepared
benzene (3b, 350 mg, 0.65 mmol) was dissolved in CH2Cl2– by us, but Mukherjee and co-workers85 were able to publish
MeOH (60 mL, 1 : 1), and K2CO3 (211 mg, 1.52 mmol) was previously its preparation, with only minor differences. For
added into one portion. The reaction mixture was stirred for that reason and because its preparation was confirmed by
16 h at room temperature and then filtered. The resulting solu- FTIR, NMR and MS spectroscopy, no EA analysis was per-
tion was evaporated under low pressure and purified by formed in that case. Compound 6a is prepared in the same
column chromatography (neutral alumina, petroleum ether way as the other short Pd rods (6b–d). Briefly, the starting
(40–60 °C) and 5% diethyl ether). Compound 4b was obtained materials trans-[PdCl2(PEt3)2] (5, 200 mg, 0.48 mmol) and 1a
as a yellow powder (200 mg, 58.8%). Mp = 170–172 °C. 1H (41 mg, 0.32 mmol) were dissolved in NHEt2 (20 mL). CuCl
NMR (400 MHz, CDCl3): δ 3.18 (s, 2H, CuC–H), 3.91 (s, 6H, (1 mg, 0.01 mmol) was added into one portion and the mix
OCH3), 7.03 (s, Ar–H, central ring), 7.47 (d, 4H, Ar–H outer was stirred at room temperature for 14 h. It was then evapor-
ring, JH,H = 8.20 Hz), 7.52 (d, 4H, Ar–H outer ring, JH,H = 8.28 ated and redissolved in CH2Cl2. The resulting solution was
Hz). 13C{1H} NMR (101 MHz, CDCl3): δ 56.9 (s, OCH3), 79.3, washed with water, dried over Na2SO4 and evaporated under
83.7, 88.0, 94.9 (s, CuC, internal and external), 113.8, 116.0, reduced pressure. A yellow powder was obtained (142 mg,
1
122.4, 124.1, 131.9, 132.4 (s, Ar. central and outer rings), 154.4 49.6%). Mp = 119 °C (decomp.). H NMR (400 MHz, CDCl3): δ
(s, Ar. C–O–CH3). FTIR (cm−1): ˜ν = 3271 (m, νCuC−H), 2102 (vw, 1.51 (t, J = 21 Hz, 36H, PCH2–CH3), 1.92 (q, J = 8 Hz, 24H,
νCuC), 655 (s, δCuC–H), 1510 (w), 1500 (w), 1491.79 (w), 1461 PCH2), 7.07 (s, 4H, Ar–H). 13C{1H} NMR (101 MHz, CDCl3): δ
(w), 1936 (m) (ar. νCvC). TOF-MS (ESI+) of C28H18O2: m/z = calc. 8.3 (PCH2CH3), 15.4 (vt, JC–P = 13.80 Hz, PCH2CH3), 125.1,
409.12, found 409.12 [M + Na]+; calc. 441.15, found 441.13 [M 130.4; 31P{1H} NMR (161 MHz, CDCl3): δ 18.1. FTIR (cm−1): ˜ν =
+ Na + CH3OH]+; calc. 493.10, found 493.13 [M + K + Cl + 2118 (w, νCuC); 1498 (m), 1452 (m), 1409 (m), 1378 (m) (ar.
CH3OH + H]+; calc. 795.25, found 795.23 [2M + Na]+. Anal. νCvC). TOF-MS (ESI+) of C34H64Cl2P4Pd2: m/z = calc. 727.1,
Calcd for C28H18O2·2H2O (422.48): C, 79.60; H, 5.25 found C, found 727.0 [M − Cl − PEt3 + H]+, calc. 845.2, found 845.1
66.5; H, 5.17.
[M − Cl]+, 903.1 [M + Na]+.
1,4-Diethoxy-2,5-bis((4-ethynylphenyl)ethynyl)benzene, 4c.
1,4-Bis[trans-(PEt3)2ClPd–CuC]-2,5-dimethoxybenzene, 6b.
Following the same conditions as those described for 4b, 1,4- The starting materials trans-[PdCl2(PEt3)2] (5, 200 mg,
diethoxy-2,5-bis((4-trimethylsilylethynylphenyl)ethynyl)benzene 0.48 mmol) and 1b (40 mg, 0.12 mmol) were dissolved in
(3c, 25 mg, 0.44 mmol) was dissolved in CH2Cl2–MeOH NHEt2 (20 mL). CuCl (1 mg, 0.01 mmol) was added into one
(50 mL, 1 : 1) and K2CO3 (15 mg, 0.10 mmol) was added at portion and the mixture was stirred at room temperature for
once. The reaction mixture was stirred for 16 h at room tem- 14 h. It was then evaporated and redissolved in CH2Cl2. The
perature and then filtered. The final solution was evaporated resulting solution was washed with water, dried over Na2SO4
under low pressure and purified by column chromatography and evaporated under reduced pressure. Complex 6b was
(neutral alumina, petroleum ether (40–60 °C) and 5% diethyl obtained as a yellow powder (128 mg, 62.9%). Mp = 153 °C
ether). The target compound 4c was obtained as a yellow (decomp.) 1H NMR (400 MHz, CDCl3): δ 1.21 (t, J = 21 Hz, 36H,
1
powder (14 mg, 75.5%). Mp = 212–215 °C. H NMR (400 MHz, PCH2–CH3), 2.02 (q, J = 8 Hz, 24H, PCH2), 3.75 (s, OCH3), 6.69
CDCl3): δ 1.48 (t, 6H, JH,H = 6.97 Hz), 3.17 (s, 2H, CuC–H), (s, 2H, Ar–H). 13C{1H} NMR (101 MHz, CDCl3):
δ 8.0
4.11 (s, 6H, OCH3), 7.01 (s, Ar–H, central ring), 7.47 (d, 4H, Ar– (PCH2CH3), 15.3 (t, JC–P = 13.80 Hz, PCH2CH3), 30.9 (Pd–CuC),
H outer ring, JH,H = 8.56 Hz), 7.49 (d, 4H, Ar–H outer ring, JH,H 42.2 (Pd–CuC), 56.2 (C1), 115.7 (C3), 154.3 (C2). 31P{1H} NMR
= 8.60 Hz). 13C{1H} NMR (101 MHz, CDCl3): δ 15.1 (s, (161 MHz, CDCl3): δ 18.1. FTIR (cm−1): ˜ν = 2115 (s, νCuC), 1493
OCH2CH3), 65.5 (s, OCH2CH3), 79.1, 83.5, 88.1, 94.6 (s, CuC, (m), 1454 (m), 1409 (m), 1385 (m) (ar. νCvC). TOF-MS (ESI+) of
internal and external) 117.4, 122.1, 124.1, 131.6, 132.2 (s, Ar. C36H68O2Cl2P4Pd2: m/z = 907.21, found 907.70 [M − Cl + 2H]+;
central and outer rings), 153.7 (s, Ar. C–O–CH2CH3). FTIR calc. 963.15, found 963.09 [M
+
Na]+. Anal. Calcd for
(cm−1): ˜ν = 3275 (m, νCuC–H), 2105 (vw, νCuC), 661 (s, δCuC–H), C36H68Cl2O2P4Pd2·1.05H2O (969.46) C, 45.07; H, 7.36; found C,
1516 (m), 1499 (w), 1489 (w), 1474 (w), 1417 (m), 1403 (m), 44.74; H, 7.03.
1392 (m) (ar. νCvC). TOF-MS (ESI+) of C30H22O2: m/z = calc.
1,4-Bis[trans-(PEt3)2ClPd–CuC]-2,5-diethoxybenzene,
6c.
437.15, found 437.20 [M + Na]+; calc. 469.18, found 469.22 The starting materials trans-[PdCl2(PEt3)2] (5, 250 mg,
[M + Na + CH3OH]+; calc. 851.31, found 851.37 [2M + Na]+. Anal. 0.60 mmol) and 1c (58 mg, 0.27 mmol) were dissolved in
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Dalton Transactions
NHEt2 (20 mL). CuCl (1 mg, 0.01 mmol) was added into one was then evaporated and redissolved in CH2Cl2. The resulting
portion, and the mixture was stirred at room temperature for solution was washed with water, dried over Na2SO4 and evapor-
14 h. It was then evaporated and redissolved in CH2Cl2. The ated under reduced pressure, giving 7b as a yellow powder
1
resulting solution was washed with water, dried over Na2SO4 (273 mg, 74.4%). Mp = 155 °C (decomp.). H NMR (400 MHz,
and evaporated under reduced pressure. The final product was CDCl3): δ 1.22 (t, J = 8.18 Hz, 36H, PCH2–CH3), 1.98 (q, J = 3.61
obtained as a yellow powder (214 mg, 76.3%). Orange crystals Hz, 24H, PCH2), 3.90 (s, 6H, OCH3), 7.01 (s, 2H, Ar–H), 7.21 (d,
3
3
were obtained by diffusion of diethyl ether in a saturated 4H, JH,H = 8.08 Hz), 7.42 (d, 4H, JH,H = 8.12 Hz). 13C{1H}
dichloromethane solution at −20 °C. Mp = 145 °C (decomp.). NMR (101 MHz, CDCl3): δ 8.7 (s, PCH2CH3), 15.8 (t, JC–P
=
1H NMR (400 MHz, CDCl3): δ 1.19 (t, J = 20 Hz, 36H, PCH2– 13.97 Hz, PCH2CH3), 56.7 (s, OCH3), 86.8, 95.8 (s, CuC,
CH3), 1.46 (t, J = 18 Hz, 6H, OCH2–CH3), 2.01 (q, J = 18 Hz, internal), 99.5 (t, Pd–CuC, JC–P = 16.13 Hz), 107.1 (t, Pd–CuC,
24H, PCH2), 3.97 (q, J = 18 Hz, 4H, OCH2), 6.69 (s, 2H, Ar–H). JC–P = 5.87 Hz), 113.9, 116.0, 120.5, 128.3, 130.9 (s, Ar. central
13C{1H} NMR (101 MHz, CDCl3): δ 8.7 (PCH2CH3), 15.5 (s, and outer rings), 154.3 (s, Ar. C–O–CH3). 31P{1H} NMR
OCH2CH3), 15.7 (t, JC–P = 13.97 Hz, PCH2CH3), 64.8 (s, (161 MHz, CDCl3): δ 18.4. FTIR (cm−1): ˜ν = 2110 (w, νCuC),
OCH2CH3), 100.1 (t, JC–P = 16.06 Hz, Pd–CuC), 102.9 (t, JC–P
=
1501 (m), 1488 (m), 1457 (m), 1397 (m) (ar. νCvC). TOF-MS
5.83 Hz, Pd–CuC), 116.0 (s, C1), 117.4 (s, Ar.), 153.2 (s, Ar. (ESI+) of C54H80Cl2O2P4Pd2: m/z = calc. 763.19, found 763.19
C–O–CH3). 31P{1H} NMR (161 MHz, CDCl3): δ 18.0. FTIR (cm−1): [M − (PdCl(PEt3)2) + H]+; calc. 881.22, found 881.26 [M − (PdCl-
˜ν = 2113 (w, νCuC), 1498 (m), 1478 (m), 1409 (m), 1389 (m, ar. (PEt3)2) + H + PEt3]+; calc. 987.18, found 987.15 [M − Cl − PEt3
νCvC). TOF-MS (ESI+) of C38H72Cl2O2P4Pd2: m/z = calc. 933.24, + H]+; calc. 1105.26, found 1105.26 [M − Cl]+. Anal. Calcd for
found 933.14 [M − Cl + H]+; calc. 991.18, found 991.10 [M + C52H76Cl2O2P4Pd2·0.35H2O (1147.09) C, 54.45; H, 6.74; found:
Na]+. Anal. Calcd for C38H72Cl2O2P4Pd2·0.75CH2Cl2·0.35H2O C, 54.33; H, 6.59.
(1038.59): C, 44.81; H, 7.20; found: C, 44.85; H, 7.24.
1,4-Bis[trans-(PEt3)2ClPd-4-(CuC–C6H4–CuC)]-2,5-diethoxy-
1,4-Bis[trans-(PEt3)2ClPd–CuC]-2,5-diheptoxybenzene, 6d. benzene, 7c.
The starting materials trans-[PdCl2(PEt3)2] (5, 250 mg,
0.60 mmol) and 1d (96 mg, 0.27 mmol) were dissolved in
NHEt2 (20 mL). CuCl (1 mg, 0.01 mmol) was added into one
portion and the mixture was stirred at room temperature for
14 h. It was then evaporated and redissolved in CH2Cl2. The
The starting materials trans-[PdCl2(PEt3)2] (5, 250 mg,
resulting solution was washed with water, dried over Na2SO4
and evaporated under reduced pressure. Compound 6d was
obtained as a yellow powder (193 mg, 62.5%). Mp = 98.9 °C
0.60 mmol) and 4c (100 mg, 0.24 mmol) were dissolved in
NHEt2 (30 mL). CuCl (1 mg, 0.01 mmol) was added into one
portion and the mixture was stirred at room temperature for
14 h. It was then evaporated and redissolved in CH2Cl2. The
resulting solution was washed with water, dried over Na2SO4
and evaporated under reduced pressure. The target compound
7c was obtained as a yellow powder (241 mg, 85.9%). Yellow
crystals were obtained by diffusion of diethyl ether in a satu-
rated dichloromethane solution at −20 °C. Mp = 150 °C
1
(decomp.). H NMR (400 MHz, CDCl3): δ 0.89 (t, J = 6.86 Hz,
6H, O(CH2)6CH3), 1.19–1.31 (m, 52H, PCH2–CH3,
O
(CH2)2(CH2)4CH3), 1.48–1.93 (m, OCH2CH2(CH2)4CH3), 3.89 (t,
J = 6.82 Hz, 4H, OCH2(CH2)5CH3), 2.03 (q, J = 7.4 Hz, 24H,
PCH2), 6.69 (s, 2H, Ar–H). 13C{1H} NMR (101 MHz, CDCl3): δ
8.0 (PCH2CH3), 15.3 (t, JC–P = 13.80 Hz, PCH2CH3), 5.7, 13.8,
22.6, 26.1, 29.2, 29.7, 31.8 (s, (OCH2)6CH3), 99.6 (s, Pd–CuC,
JC–P = 16.13 Hz), 102.7 (Pd–CuC, JC–P = 5.50 Hz), 115.6, 117.1 (s,
Ar. C), 152.8 (s, Ar. C–O–CH2). 31P{1H} NMR (161 MHz, CDCl3):
δ 18.5. FTIR (cm−1): ˜ν = 2113 (w, νCuC), 1498 (m), 1478 (m),
1404 (m), 1389 (m) (ar. νCvC). TOF-MS (ESI+) of
C48H92Cl2O2P4Pd2: m/z = calc. 1131.34, found 1131.34 [M +
Na]+; calc. 2240.69, found 2240.51 [2M + Na + H]+; Anal. Calcd
for C48H92Cl2O2P4Pd2·2H2O·0.6CH2Cl2 (1192.74): C, 48.81; H,
8.19; found: C, 48.83; H, 8.21.
1
(decomp.). H NMR (400 MHz, CDCl3): δ 1.22 (t, J = 8.02 Hz,
36H, PCH2–CH3), 1.48 (t, 6H, OCH2CH3) 1.98 (q, J = 3.61 Hz,
24H, PCH2), 4.10 (s, 4H, OCH2CH3), 6.95 (s, 2H, Ar–H), 7.21 (d,
3
3
4H, JH,H = 8.44 Hz), 7.40 (d, 4H, JH,H = 8.44 Hz). 13C{1H}
NMR (101 MHz, CDCl3):
δ 8.7 (s, PCH2CH3), 15.3 (s,
OCH2CH3), 15.8 (t, JC–P = 1393 Hz, PCH2CH3), 5.7 (s, OCH3),
87.1, 95.6 (s, CuC, internal), 99.4 (t, Pd–CuC, JC–P = 15.80
Hz), 107.1 (t, Pd–CuC, JC–P = 5.53 Hz), 114.6, 117.6, 120.5,
128.2, 130.9 (s, Ar. central and outer rings), 153.8 (s, Ar. C–O–
CH3). 31P{1H} NMR (161 MHz, CDCl3): δ 18.1 ppm. FTIR
(cm−1): ˜ν = 2112 (w, νCuC), 1510 (m), 1487 (m), 1454 (m), 1413
(m), 1377 (m) (ar. νCvC). TOF-MS (ESI+) of C54H80Cl2O2P4Pd2:
m/z = calc. 1133.29 found 1133.33 [M − Cl]+. Anal. Calcd for
C54H80Cl2O2P4Pd2 (1168.83): C, 55.49; H, 6.90; found: C, 56.10;
H, 6.74.
1,4-Bis[trans-(PEt3)2ClPd-4-(CuC–C6H4–CuC)]-2,5-dimethoxy-
benzene, 7b.
The starting materials trans-[PdCl2(PEt3)2] (5, 300 mg,
0.72 mmol) and 4b (126 mg, 0.32 mmol) were dissolved in
Crystallographic data collection and structure determination
NHEt2 (40 mL). CuCl (1 mg, 0.01 mmol) was added at once X-ray data for compounds 6c and 7c were collected using a
and the mixture was stirred at room temperature for 14 h. It Bruker-Nonius KappaCCD diffractometer with an APEX-II
4012 | Dalton Trans., 2015, 44, 4003–4015
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
detector at 173(2) and 123(2) K, respectively, using MoKα radi- fer. Based on the current results, the development of new
ation (λ = 0.71073 Å). The structures were solved by direct hybrid molecular/semiconductor systems formed by non-
methods with SIR-2004101 and refined by least squares using metallated or metallated π-conjugated oligo(phenylene ethyny-
SHELXL-97.102 Hydrogen atoms were included at calculated lene)s molecular wires, covalently grafted onto not previously
positions and refined as riding atoms with isotropic displace- functionalized porous silicon substrates is currently underway
ment parameters coupled to those of the parent atoms. The in our laboratory, in view of their possible application as
non-hydrogen atoms were refined using anisotropic para- waveguides.
meters. ORTEP-383 plots have been drawn with 30% prob-
ability ellipsoids. For disordered 7c, a large number of
restraints were applied to stabilize the structure of major and Conflict of Interest
minor components. The Pd–Cl, Pd–P and P–C bond distances
The authors declare no competing financial interest.
were restrained to be equal (s = 0.02) in both components.
Also, all C–C distances in ethyl groups bonded to P atoms as
well as all P–methyl distances over two bonds were restrained
to be equal (s = 0.02). Anisotropic displacement parameters for
Acknowledgements
Cl, P and disordered C atoms were restrained to be equal (s =
0.01 for P and methylene C, and s = 0.02 for Cl and methyl C).
Finally, two atoms of the minor component of disordered 7c,
were restrained to improve the isotropicity of this structure’s
We thank the Fundação para a Ciência e a Tecnologia (FCT-IP)
for the generous support of our work through the network con-
tracts PTNMR-2013, RNEM-2013, as well as the research
project PTDC/CTM/098451/2008, the PhD grant SFRH/BD/
shape.
29325/2006 (JF), the post-doc grant SFRH/BPD/20695/2004/
7Y55 (WC) and CQM pluri-annual funding (PEst-OE/QUI/
UI0674/2011-2013). The Academy of Finland is gratefully
Conclusions
acknowledged for funding (KR grant no. 122350, 140718,
265328 and 263256). Support from Agencia Canaria de Investi-
gación, Innovación y Sociedad de la Información, Gobierno de
Canarias, through project SolSubC200801000088 is also
acknowledged. We gratefully acknowledge the continued
support of our work by Vidamar Madeira Resort. We are grate-
ful to Dr César Fernandes and Laboratório Regional de Engen-
haria Civil for the usage of the ATR-FTIR equipment.
The design and synthesis of novel molecular non-metallated
or metallated systems is currently a hot topic of research, since
the potential integration of these functional molecular struc-
tures within electronic devices could extend the life of silicon
technology. In this work, we have reported the preparation,
structural characterization and electrochemical properties of a
rare family of [PdCl(PEt3)2]+ rods based on derivatives of 1,4-
diethynylbenzene (phenylene ethynylene rods with alkoxy side
chains: methoxy, ethoxy and heptoxy). We also systematically
studied the effect of substituents attached to bridging ligands
on the electronic communication between the two metal
centers and on the solid-state photoluminescence efficiency of
this family of binuclear [PdCl(PEt3)2] phenylene ethynylene
rods. Preparation of bridging ligands was done using Sonoga-
shira–Hagihara coupling, and two new ligands (4b–c) were
obtained in good yields as well as the resulting bimetallic
rods. The structures of 1,4-bis[trans-(PEt3)2ClPd–CuC]-2,5-
diethoxybenzene (6c) and the longer rod 1,4-bis[trans-
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