Photoisomerizable metallomesogens and soft crystals based on orthopalladated complexes

Article abstract of DOI:10.1021/ic1010665

With the aim of obtaining light-responsive liquid-crystalline palladium complexes, six palladium complexes derived from an orthometalated imine, bearing one or two azocarboxylato bridges, have been synthesized: [Pd ₂(μ-SC₁₀H₂₁)(μ-O₂CAzo)(L ^1,2)₂] (7 and 8), [Pd₂(μ-SC ₁₀H₂₁)(μ-O₂CAzo3C₁₀)(L ^1,2)₂] (9 and 10), [Pd(μ-O₂CAzo)L ¹]₂ (11), and [Pd(μ-O₂CAzo3C ₁₀)L¹]₂ (12), in which L¹ = p-H ₂₁C₁₀OC₆H₃CHNC₆H ₄OC₁₀H₂₁-p, L² = p-H ₂₁C₁₀OC₆H₃CHNC₁₄H ₂₉, AzoCO₂^- = p-(phenylazo)benzoate, and Azo3C₁₀CO₂^- = p-(2′,3′,4′- tris-n-decyloxyphenylazo)benzoate. Three of them (7-9), as well as the precursor Azo3C₁₀CO₂H (3), are thermotropic liquid crystals displaying nematic and smectic A mesophases, while 10-12 have been identified by X-ray diffraction to give rise to soft crystal phases. Electronic spectroscopy and ¹H NMR show that all of them undergo a transcis isomerization of the azobenzene moiety at λ = 365 nm. The molecular structure determines the photoresponse in solution, which is faster and more stable when the trisubstituted azocarboxylate is present and the motion of the azo group is not hindered by the orthometalated imine. The photoresponse has also been observed in the condensed phases, which change from the ordered phase to the isotropic liquid upon irradiation, except for compound 10, a soft crystal in which a permanent photoalignment highly sensitive to light polarization is produced. The latter is a behavior with potential applications, rather unusual in low-molecular-weight compounds.

Full text of DOI:10.1021/ic1010665

8904 Inorg. Chem. 2010, 49, 8904–8913
DOI: 10.1021/ic1010665
Photoisomerizable Metallomesogens and Soft Crystals Based on Orthopalladated
Complexes
†
,†
‡
§
,‡
´
M. J. Baena, P. Espinet,* C. L. Folcia, J. Ortega, and J. Etxebarrıa*
†
´
ꢀ
IU CINQUIMA/Quımica Inorganica, Facultad de Ciencias, Universidad de Valladolid, 47071 Valladolid,
‡
´
´
Spain, Departamento de Fısica de la Materia Condensada, Facultad de Ciencia y Tecnologıa, Universidad del
Paıs Vasco, Apartado 644, 48080 Bilbao, Spain, and Departamento de Fısica Aplicada II, Facultad de Ciencia y
§
´
´
´
´
Tecnologıa, Universidad del Paıs Vasco, Apartado 644, 48080 Bilbao, Spain
Received May 26, 2010
With the aim of obtaining light-responsive liquid-crystalline palladium complexes, six palladium complexes derived from an
orthometalated imine, bearing one or two azocarboxylato bridges, have been synthesized: [Pd2(μ-SC10H21)(μ-O2CAzo)(L1,2)2]
(7and 8),[Pd2(μ-SC10H21)(μ-O2CAzo3C10)(L1,2)2] (9and 10),[Pd(μ-O2CAzo)L1]2 (11), and [Pd(μ-O2CAzo3C10)L1]2 (12),
in which L1 = p-H21C10OC6H3CHdNC6H4OC10H21-p, L2 = p-H21C10OC6H3CHdNC14H29, AzoCO2- = p-(phenylazo)ben-
zoate, and Azo3C10CO2- = p-(20,30,40-tris-n-decyloxyphenylazo)benzoate. Three of them (7-9), as well as the precursor
Azo3C10CO2H (3), are thermotropic liquid crystals displaying nematic and smectic A mesophases, while 10-12 have been
identified by X-ray diffraction to give rise to “soft” crystal phases. Electronic spectroscopy and 1H NMR show that all of them
undergo a trans-cis isomerization of the azobenzene moiety at λ = 365 nm. The molecular structure determines the
photoresponse in solution, which is faster and more stable when the trisubstituted azocarboxylate is present and the motion of
the azo group is not hindered by the orthometalated imine. The photoresponse has also been observed in the condensed
phases, which change from the ordered phase to the isotropic liquid upon irradiation, except for compound 10, a soft crystal in
which a permanent photoalignment highly sensitive to light polarization is produced. The latter is a behavior with potential
applications, rather unusual in low-molecular-weight compounds.
Introduction
different photoinduced effects can be produced, such as
changes in color,3 changes in the molecular orientation,2a,b,h
isothermal phase transitions,4 induction of chirality,5 birefrin-
gence,6 changes in polarization7 and in magnetism,8 photome-
chanical effects,9 or light-driven plasmonic switching.2i These
photoresponsive moieties are included in devices as dopants or
as part of polymers and also as part of supramolecular self-
assemblies.2j
Liquid crystals are versatile materials for applications.1 The
incorporation of liquid crystals in photoresponsive devices (for
optical data storage, photomechanics, or nonlinear optics) is one
of the main applications, and several recent reviews on this topic
can be found.2 In order to display functions, the liquid crystal
has to contain entities able to react to luminous stimuli. For
example, azobenzenes, stilbenes, spirocompounds, etc., undergo
either an isomerization or a photochemical reaction when irra-
diated with the adequate wavelength. Upon irradiation, the
shape of the photoreactive entity changes and, as a consequence,
Azobenzene and its derivatives are well-known to under-
go reversible trans-cis isomerization,10 which changes the
(4) (a) Ikeda, T.; Horiuchi, S.; Karanjit, D. B.; Kurihara, S.; Tazuke, S.
Macromolecules 1990, 23, 36. (b) Ikeda, T.; Horiuchi, S.; Karanjit, D. B.;
Kurihara, S.; Tazuke, S. Macromolecules 1990, 23, 42. (c) Kurihara, S.; Ikeda, T.;
Sasaki, T.; Kim, H.-B.; Tazuke, S. J. Chem. Soc., Chem. Commun. 1990, 1751.
(5) (a) Mitsuoka, T.; Sato, H.; Yoshida, J.; Yamagishi, A.; Einaga, Y.
Chem. Mater. 2006, 18, 3442. (b) Yoshida, J.; Sato, H.; Hoshino, N.; Yamagishi,
A. J. Phys. Chem. B 2008, 112, 9677. (c) Tejedor, R. M.; Oriol, L.; Serrano, J. L.;
Sierra., T. J. Mater. Chem. 2008, 18, 2899.
*To whom correspondence should be addressed. E-mail: espinet@qi.uva.es
(P.E.), j.etxeba@ehu.es (J.E).
(1) Tschierske, C. J. Mater. Chem. 2008, 18, 2869.
(2) (a) Ichimura, K. Chem. Rev. 2000, 100, 1847. (b) Natansohn, A.; Rochon, P.
Chem. Rev. 2002, 102, 4139. (c) Shibaev, V.; Bobrovsky, A.; Boiko, N. Prog. Polym.
Sci. 2003, 28, 729. (d)Lemieux, R. P. Soft Matter 2005, 1, 348. (e) Mizoshita, N.; Seki,
T. Soft Matter 2006, 2, 157. (f) Sato, H.; Yamagishi, A. J. Photochem. Photobiol. C
2007, 8, 67. (g) Ikeda, T.; Mamiya, J.; Yu, Y. Angew. Chem., Int. Ed. 2007, 46, 506.
(h) Mataharu, A. S.; Jeeva, S.; Ramanujam, P. S. Chem. Soc. Rev. 2007, 36, 1868.
(i) Hsiao, V. K. S.; Zheng, Y. B.; Juluri, B. K.; Huang, T. J. Adv. Mater. 2008, 20, 3528.
( j) Yagai, S.; Kitamura, A. Chem. Soc. Rev. 2008, 37, 1520. (k) Tejedor, R. M.; Vera, F.;
Oriol, L.; Sierra, T.; Serrano, J. L. Mol. Cryst. Liq. Cryst. 2008, 489, 105.
(3) (a) Sackmann, E. J. Am. Chem. Soc. 1971, 93, 7088. (b) Irie, M. Chem.
(6) Okano, K.; Shishido, A.; Ikeda., T. Adv. Mater. 2006, 18, 523.
(7) Walton, H.; Coles, H. Ferroelectrics 1993, 147, 223.
(8) Nakatsuji, S. Chem. Soc. Rev. 2004, 33, 348.
(9) (a) Kondo, M.; Yu, Y.; Ikeda, T. Angew. Chem., Int. Ed. 2006, 45,
1378. (b) Yagai, S.; Iwashima, T.; Kishikawa, K.; Nakahara, S.; Karatsu, T.;
Kitamura, A. Chem.;Eur. J. 2006, 12, 3984.
(10) (a) Rau, H. In Photochromism. Molecules and Systems; D€urr, H. B.,
Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, The Netherlands, 1990; p 165. (b)
Rau, H. Photochemistry of Azobenzenes. In Photochemistry and Photophysics;
Rabek, J. K., Ed.; CRC Press: Boca Raton, FL, 1990; Vol. II.
ꢀ
Rev. 2000, 100, 1683. (c) Lee, H.-Y.; Doi, K.; Harada, H.; Tsutsumi, O.;
Kanazawa, A.; Shiono, T.; Ikeda, T. J. Phys. Chem. B 2000, 104, 7023. (d)
Bossi, M.; Murgida, D.; Aramendía, P. F. J. Phys. Chem. B 2006, 110, 13804.
r
pubs.acs.org/IC
Published on Web 09/10/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8905
molecular geometry (planar rodlike for the trans isomer to
nonplanar bent-shaped for the cis isomer) and the dipolar
moment of the molecule.11 Azobenzene-containing com-
pounds have been incorporated in very different systems to
obtain photoresponsive materials. There are still very few
examples2f,5a,5b,12-15 where the material involved in the
photoreactivity of azo-containing compounds is a metallo-
mesogen.16 The photoresponsivity is usually studied on
isotropic solutions of these compounds, so the property of
being a liquid crystal in condensed phases is irrelevant for the
photoresponsivity in these cases.12,13 In most cases, the azo
compound is used as a chiral dopant of a nematic solvent, and
the isomerization of the azo group induces a change in the
pitch value of the cholesteric phase, not a change of the
phase.2f,5a,5b Only in two cases has the photoresponsivity
been proven and studied in the mesophase of the pure
compound;14,15 it is worth noting that the discotic assemblies
formed by the macrocyclic azo complex reported in ref 15
show high light sensitivity only in the lyotropic state.
Scheme 1. Synthesis of 3a
With these few precedents, we aimed at exploring the
photoisomerization of the azobenzene function in metal-con-
taining liquid crystals. Some structural types of orthometalated
iminepalladium complexes previously reported by our group17
were conveniently modified to introduce the azobenzene moiety
by replacing one or two acetate bridges for 4-(phenylazo)ben-
zoate by 4-[20,30,40-tris(decyloxyphenylazo)]benzoate.18 The
first kind of carboxylate was chosen to check whether the
complex with an azobenzene moiety at the bridging position of
the dimer was or was not photoresponsive. The second intro-
duces a bulkier moiety, which might affect more effectively the
molecular order when the azo function isomerizes.
aAryl labels are for the 1H NMR signals.
Results and Discussion
Synthesis and Characterization. The synthesis of 4-[20,30,
40-tris(decyloxyphenylazo)]benzoic acid (3) was achieved
in three steps (Scheme 1). First, an azo coupling between
1,2,3-tris(hydroxybenzene) and the diazonium salt of the
corresponding benzoate ester afforded 1; 1H NMR revealed
that the preferred position of coupling was position 4 of
the trihydroxybenzene. Then, alkoxylation of the OH
groups afforded ester 2. Finally, treatment of the ester in
a refluxing basic medium, followed by acidification,
yielded acid 3.
The dinuclear complexes with one azocarboxylate bridge,
7-10, were obtained from the analogous complexes with
orthopalladated imine and chlorothiolato mixed bridges,17a
by reaction with the silver salt of the azocarboxylic acid
(Scheme 2). 1H NMR shows that the relative position of the
orthometalated imines is syn, as in the chlorothiolato pre-
cursors.
The complexes with two carboxylate bridges, 11 and
12, were obtained by reaction of the azocarboxylic acid
with the corresponding dinuclear hydroxy-bridged com-
plex (Scheme 3).17b In this case, 1H NMR shows that the
anti isomer is exclusively obtained.
Mesogenic Behavior and Structural Properties. The
mesomorphism of the different compounds is shown
in Table 1. The transition temperatures and enthalpies
correspond to the second cycle for both heating and cooling
runs.
(11) Kumar, G. S.; Neckers, D. C. Chem. Re.v 1989, 89, 1915.
(12) (a) Aiello, I.; Ghedhini, M.; La Deda, M.; Pucci, D.; Francescangeli,
O. Eur. J. Inorg. Chem. 1999, 1367. (b) Ghedini, M.; Pucci, D.; Crispini, A.;
Aiello, I.; Barigelletti, F.; Gessi, A.; Francescangeli, O. Appl. Organomet. Chem.
1999, 13, 565.
(13) Antharjanam, P. K. S.; Mallia, V. A.; Das, S. Chem. Mater. 2002, 14,
2687.
(14) Arias, J.; Bardajı
Inorg. Chem. 2009, 48, 6205.
(15) Pecinovsky, C. S.; Hatakeyama, E. S.; Gin, D. L. Adv. Mater. 2008,
20, 174.
, M.; Espinet, P.; Folcia, C. L.; Ortega, J.; Etxebarrıa, J.
´ ´
(16) A metallomesogen is a metal complex displaying liquid-crystal
properties. Reviews on this subject: (a) Giroud-Godquin, A. M.; Maitlis,
P. M. Angew. Chem., Int. Ed. Engl. 1991, 30, 375. (b) Espinet, P.; Esteruelas,
M. A.; Oro, L. A.; Serrano, J. L.; Sola, E. Coord. Chem. Rev. 1992, 117, 215. (c)
Hudson, S. A.; Maitlis, P. M. Chem. Rev. 1993, 93, 681. (d) Serrano, J. L., Ed.
Metallomesogens; VCH: Weinheim, Germany, 1996. (e) Bruce, D. W. Metal-
Containing Liquid Crystals. In Inorganic Materials; Bruce, D. W., O'Hare, D.,
Eds.; Wiley: Chichester, U.K., 1996. (f) Neve, F. Adv. Mater. 1996, 8, 277. (g)
Collison, S. R.; Bruce, D. W. Metallomesogens: Supramolecular Organization of
Metal Complexes in Fluid Phases. In Transition Metal in Supramolecular
Chemistry; Sauvage, J. P., Ed.; Wiley: Chichester, U.K., 1999. (h) Donnio, B.;
Bruce, D. W. Struct. Bonding (Berlin) 1999, 95, 193. (i) Donnio, B. Curr. Opin.
Colloid Interface Sci. 2002, 7, 371. (j) Gharbia, M.; Gharbi, A.; Nguyen, H. T.;
^
Malthete, J. Curr. Opin. Colloid Interface Sci. 2002, 7, 312. (k) Binnemans, K.;
€
Gorller-Walrand, C. Chem. Rev. 2002, 102, 2303. (l) Serrano, J. L.; Sierra, T.
Coord. Chem. Rev. 2003, 242, 73. (m) Lin, I. J. B.; Vasam, C. S. J. Organomet.
Chem. 2005, 690, 3498. (n) Porta, B.; Khamsi, J.; Noveron, J. C. Curr. Org.
Chem. 2008, 12, 1298.
Compound 3 displays a monotropic nematic (N) phase
upon cooling from the isotropic liquid (I), which is thermally
metastable in a range of about 20 °C; upon further cooling,
three subsequent crystalline phases are detected. It must be
pointed out that in this compound hydrogen bonding
between the acidic groups makes the mesogenic entity a
dimer bearing six long-chain substituents. This polycatenar
dimer might be expected to present a discotic shape,16h but,
ꢀ
(17) (a) Buey, J.; Diez, G. A.; Espinet, P.; Garcıa-Granda, S.; Perez
´
~
Carreno, E. Eur. J. Inorg. Chem. 1998, 9, 1235. (b) Diez, L.; Espinet, P.; Miguel,
J. A.; Rodríguez Medina, M. P. J. Organomet. Chem. 2005, 690, 261.
(18) Complexes bearing palladacycles have been extensively studied as
metallomesogens. It is usually easy to modulate their steric and electronic
properties. For a general revision, see: Donnio, B.; Bruce, D. W. Liquid
crystalline Ortho-Palladated Complexes. In Palladacycles: Synthesis, Char-
acterization and Applications; Dupont, J., Pfeffer, M., Eds.; Wiley-VCH:
Weinheim, Germany. 2008.

8906 Inorganic Chemistry, Vol. 49, No. 19, 2010
Scheme 2. Synthesis of 7-10a
Baena et al.
Table 1. Thermal and Thermodynamic Parameters of Compounds 3 and 7-12
(Second Heating)a
ΔH
compound
transition
T (°C)
(kJ mol-1
)
3
3
C f C0
46.4
68.2
83.4
98.1
(100.0)
94.6
127.6
144.2
155.7
101.2
(75.8)
87.9
56.9
64.0
68.5
14.3
3.3
27.1
-3.5
(-0.4)
-12.3
13.2
0.2
1.3
C0 f C00
C00 f C000
C000 f I
(I f N)
C f C0
7
8
C0 f SmA
SmA f N
N f I
C f I
(I f SmA)
N f I 0
56.1
(-1.7)
0.6
12.3
-8.3
41.3
9
10
C0f C
C f C00
C00 f I
(I f X)
(X f C)
C f I
(41.6)
(25.5)
159.2
(147.1)
(135.6)
94.1
(-0.7)
(-33.5)
39.5
(-17.1)
(-9.8)
2.1
11
12
(I f C0)
(C0 f C)
C f C0
C0 f I
103.2
63.0
aC, C0, C00, C000 = crystalline phases, N = nematic, SmA = smectic
A, I = isotropic liquid, and X = unidentified metastable mesophase.
Monotropic transitions are in parentheses.
aAryl labels are for the 1H NMR signals.
Scheme 3. Synthesis of 11 and 12a
supported by the good alignment (see the textures below)
obtained in all of the mesophases, which is a typical behavior
of calamitic liquid crystals and is hardly observed in the
discotic case.
Compound 7 shows N-SmA mesomorphism, which
was determined by texture observations. In compound 8,
only a monotropic SmA phase is observed upon cool-
ing. Compound 9 displays an N phase that gets frozen
upon cooling. In the latter compound, the thermogram of
the first heating run reveals that two phase transitions are
produced (solid-solid and solid-mesophase) before the
clearing point, but the solid phase is not recovered upon
cooling or in the subsequent heating cycle.
Compound 10 presents a phase sequence with a com-
plicated dynamics, showing kinetic effects involving sev-
eral metastable phases. Upon heating, DSC data reveal
three transitions, including a cold crystallization (see the SI).
Upon cooling from the isotropic phase, there is an unidenti-
fied phase X, which probably is a mesophase because the
transition has a small enthalpy. However, this phase is meta-
stable and could not be studied. X-ray diffraction experi-
ments carried out below the isotropic phase upon cooling ac-
tually indicated a crystalline phase. At small angles, a sharp
peak and several harmonics were observed (Figure 1a), which
might suggest a lamellar structure, and at wide angles, a set of
well-defined peaks superimposed to a diffuse halo appeared,
indicating that the phase (named C00 in Table 1) is, in fact, a
“soft” crystal. The whole X-ray diagram could be indexed
(Figure 1a-c). Under the proposed indexation scheme, the
corresponding unit cell is monoclinic, with lattice parameters
a=32.0 A, b=5.3 A, c=30.3 A, and β=87.6°, with b being
the monoclinic axis. Essentially the same indexation results at
two different temperatures (30 and 40 °C). The identification
of this phase as C00 was made by comparing the diffraction
patterns obtained upon heating the material from room
temperature. In this regard, it must be pointed out that two
aAryl labels are for 1H NMR signals.
in fact, the nature of the N phase of compound 3 is calamitic
(i.e., it is an N phase with positive birefringence). This point
was confirmed experimentally: by using a Berek compensa-
tor (see the Supporting Information, SI), it was checked that
the azobenzene group lies along the rubbing direction of the
cell in an oriented sample because this direction determines
the slow axis of the optical indicatrix; moreover, this axis
coincides with the optic axis of the material, which could be
established by measuring the birefringence of the sample as a
function of the incidence angle as the sample was rotated
along the rubbing direction. The same conclusion was drawn
for all of the mesophases studied in the present work and,
therefore, the existence of discotic-shaped molecules in the
mesophases could be discarded. This conclusion is also

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8907
Figure 2. ORTEP diagram for the molecular structure of compound 11.
Figure 1. X-ray diffraction pattern of compound 10 as a function of 2θ
and the Miller indices: (a) whole diffraction pattern at 30 °C with the (00l)
reflections indexed; (b and c) detailed wide-angle regions.
Figure 3. X-ray diffraction pattern vs 2θ of compound 12 at 70 °C.
additional different patterns, which should correspond to C
and C0, were observed at lower temperatures.
the palladium coordination planes are almost parallel,
making the molecule markedly rodlike. The Pd-Pd dis-
tance of 2.92 A, shorter than twice the van der Waals
radius of palladium (3.26 A) but larger than twice its co-
valent radius (2.62 A),21 is usually considered nonbond-
ing.20 Upon cooling from the isotropic liquid, compound
11 shows a very viscous monotropic phase, named C0 in
Table 1, which could not be aligned. It presents a mosaic
texture typical of very ordered smectic phases (B, E, G, H,
J, or K). The high enthalpy of the isotropic liquid-C0
phase transition reveals a high molecular order as well.
Finally, compound 12 presents a “soft” crystal phase
below the clearing point (named C0 in Table 1). This point
was confirmed by X-ray diffraction measurements in the
phase (Figure 3). Similar to compound 10, the material
shows a strong peak at small angles that points to a lamellar
structure, but at wide angles, several peaks appear over a
weak diffuse halo, suggesting a crystalline structure, which is
supported by the high melting enthalpy.
Optimization of the molecular structure, using an
algorithm based on the CHARMM parametrization,19
indicates that the molecule has roughly the shape of a rec-
tangular parallelepiped with well-defined sides of about
32 ꢀ 15 ꢀ 5 A. The c parameter is compatible with
the longest molecular distance, which could explain the
lamellar-like diffraction pattern observed at small angles
because, along the c direction, cores would be segregated
from tails, giving rise to highly defined X-ray (00l) diffrac-
tion peaks. On the other hand, the intermediate and small
molecular sizes are compatible with two molecules of 10
per unit cell, which would imply a density of 1.26 g cm-3
.
This result is in excellent agreement with the measured
3
density of the material (1.27 g cm-3) and supports the
validity of the proposed indexation scheme.
3
Single crystals were obtained for compound 11 from
CH2Cl2/petroleum ether. Figure 2 shows the molecular
structure determined by X-ray diffraction (see the SI for
complete information). The molecular structure is book-
shaped, with the two Pd coordination planes making an
angle of 26° and the carboxylates in the hinge of the book,
as found in related complexes.17b,20 It is remarkable that
Photoisomerization in Solution. UV-Vis Spectra.
The new compounds were first tested for their behavior in
solution under irradiation by simply recording their electro-
nic spectra before and after their solutions had been irra-
diated with an intensity of 7 mW cm-2 at a wavelength of
3
(19) Brooks, B. R.; Bruccoleri, R. E.; Olafson, B. D.; States, D. J.;
Swaminathan, S.; Karplus, M. Comput. J. Chem. 1983, 4, 187.
(20) Angles found in the literature for acetate-bridged orthopalladated
imine complexes range from 25° to 51°. For example, see:(a) Pereira, M. T.;
Vila, J. M.; Gayoso, E.; Gayoso, M.; Hiller, W.; Strahle, J. J. Coord. Chem.
365 nm, near the π f π* transition band of the trans isomer
of azobenzene.10b Upon irradiation, the trans starting com-
plexes isomerize to the cis form, giving rise to a severe
decrease in the absorbance of the π f π* band and a slight
increase in that of the n f π* band of the cis isomer at
lower frequencies (Figure 4). The values for the maximum
ꢀ
ꢀ
1988, 18, 245. (b) Teijido, B.; Fernandez, A.; Lopez-Torres, M.; Castro-Juiz, S.;
ꢀ
ꢀ
Suarez, A.; Ortigueira, J. M.; Vila, J. M.; Fernandez, J. J. J. Organomet. Chem.
ꢀ
ꢀ
ꢀ
2000, 71, 598. (c) Castro-Juiz, S.; Fernandez, A.; Lopez-Torres, M.; Vazquez-
ꢀ
ꢀ
García, D.; Suarez, A. J.; Vila, J. M.; Fernandez, J. J. Organometallics 2009, 28,
6657.
(21) Emsley, J. The Elements; Oxford University Press: New York, 1998.

8908 Inorganic Chemistry, Vol. 49, No. 19, 2010
Baena et al.
Table 3. Maximum Percentage of the Cis Isomer Obtained under Irradiation with
λ = 365 nm and I = 7 mW cm-2 (Measured by 1H NMR Integration)
3
compound
max cis isomer %
time (min)
3
7
8
9
10
11
12
39
23
49
31
67
32
47
45
50
120
95
110
10
80
the more efficient and faster the trans-cis isomerization
process (compare 360 s to reach saturation in 4-(phenyl-
azo)benzoic acid versus 180 s for compound 3).
On the other hand, the extent of conjugation on the
orthometalated imine ligand plays some role (electronic
effect and/or steric hindrance) in the trans-cis isomeriza-
tion because we observe that, independent of the sub-
stitution of the azocarboxylato bridge, the complexes
bearing the imine with only one phenyl ring (8 and 10)
respond better to irradiation than those with two phenyl
rings in the imine (7 and 9). This can be explained by
taking into account the two pathways proposed for
isomerization of the azo group: an inversion reaction
and the N-N rotation.3c The presence of the aryl ring
in the azobenzene neighborhood is expected to hinder the
rotation motion.
The behavior of complexes 11 and 12 is consistent with
the proposal that the environment of the azobenzene
moiety in the molecule is very important for the photo-
response. In fact, the molecular shapes of 11 and 12, with
the coordination planes of the palladium nearly parallel,
make the azocarboxylato groups dangle away from the
rigid core and move freely. Thus, their photoresponses
are similar and higher than those for the related com-
pounds 7 and 9.
Figure 4. UV-vis spectra in a THF solution before (black line) and
after (colored line) irradiation with λ = 365 nm. The time of exposure is
indicated for each compound.
1H NMR Spectra. The photoisomerization of the azo
group in solution can be sharply observed by 1H NMR.
The experiments were carried out by irradiation of the
NMR tubes containing the solutions at 295 K (con-
centrations ca. 2 ꢀ 10-2 M) with the same light as that
used in the UV-vis experiment (λ = 365 nm; I = 7
Table 2. Wavelength of Maximum Absorbance and Molar Absorptivity for the
Trans Isomer
compound
λ
max (nm)
amax (M-1 cm-1
)
3
AzoCO2Ha
3
7
8
326.5
374.0
326.5
325.0
376.0
371.4
327.5
374.0
18 500
20 034
59 722
41 086
38 203
35 384
61 098
54 875
mW cm-2). Before illumination, only the trans isomer
3
was observed. After light exposure, the signals of the two
isomers were present, with no signs of decomposition
products. All of the NMR peaks were affected by the
trans-to-cis isomerization. The most shifted protons were
those of the azobenzene moiety because the paramagnetic
anisotropic shielding undergone in the cis conformation
is stronger. Typically, the shift of the aromatic protons
closest to the azo group is about 1 ppm. The chemical
shifts of the trans isomers of all of the complexes are given
in the Experimental Section. Those of the cis isomers are
also given when unambiguous assignment could be made.
In order to check the maximum percentage of the cis
isomer that can be obtained under these conditions, the
illumination was maintained as far as the resonances of
the cis isomer were observed to grow up. The spectra at
different irradiation times for 10 are displayed in the SI.
A total trans-to-cis-photoinduced transformation was
never reached because of competition with the cis-to-
trans thermal conversion. Table 3 collects these results.
The trend observed is in good accordance with that
obtained from the UV-vis measurements.
9
10
11
12
a4-(Phenylazo)benzoic acid.
absorbance and the correspondent molar absorptivity are
listed in Table 2.
The spectra show that all of the complexes are photo-
responsive, but to different extent, mainly depending on
the electronic distribution over the (phenylazo)benzoato
bridge and, apparently, also on how sterically hindered
the isomerization is.
The trisubstitution on the phenylazo group causes a
bathochromic shift of the π f π* band, which explains
why the complexes with a 3C10AzoCO2- group are more
sensitive to irradiation than those having an azobenzene
without chains: It must be noted that the closer the
excitation frequency is to the maximum of absorption,

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8909
Figure 5. Textures of compound 3 in the N phase at T = 70 °C in a
planar cell. The good quality of the alignment is clearly observed.
Polarizers are at 0° and 90°. The molecular director makes angles of 45°
(a) and 0° (b) with respect to the polarizer.
Figure 7. Textures of compound 8 in the SmA phase at T = 70 °C in a
planar cell. The good quality of the alignment is clearly observed.
Polarizers are at 0° and 90°. The molecular director makes angles of 45°
(a) and 0° (b) with respect to the polarizer.
Figure 8. Compound 9 in the N phase. The directions of the polarizer P,
analyzer A, and molecular director n are indicated. (a) Illumination of the
alignedmesophase at 83°C. The black centralarea is the I phase. The laser
polarizationisalong the analyzerdirection. (b) Illuminationofthe aligned
mesophase at 80 °C. Most of the sample is dark because of the extinction
condition. The yellow central area is an N phase that shows no extinction
for any rotation angle of the sample. This means that the n direction is not
well-defined in that region.
Figure 6. Textures of compound 7 in the N phase at T = 153 °C in a
planar cell. Polarizers are at 0° and 90°, and the rubbing direction is at 45°.
(a) Texture of the virgin sample. The same area is shown after 10 s (b) and
40 s (c) of illumination with the He-Cd laser. The dark areas in part c
correspond to the I phase.
The effect is also appreciated in the SmA mesophase,
although to a smaller degree. The relatively weak photo-
response can be attributed to the higher difficulty to
modify the molecular shape in such a bulky molecule
(at least in comparison to compound 3), leading to just a
small percentage of the cis isomer produced (consistent
with the solution behavior).
In order to know whether the photoresponse is more
effective when illuminated along the director or in a
perpendicular direction, the experiment was carried out
with crossed polarizers at þ45° and -45°, with the optic
axis of the material (i.e., the director n) being at 0° or 90°
and the laser light polarized at 0°. For both phases (SmA
and N), the conclusion was the same: the material re-
sponded to both polarizations, but the response was more
efficient when the polarization was parallel to the direc-
tor. The result makes sense because the azobenzene group
in the trans conformation is dichroic, with maximum
absorption for the parallel light.
Compound 8. This material could be aligned in the SmA
phase (see Figure 7). The measured birefringence was Δn =
0.20 at 70 °C. The response to illumination was qualitatively
similar to that of both preceding compounds, with an inter-
mediate sensitivity looking at the illumination time required
to achieve the isotropic phase. This is also consistent with the
behavior observed in solution.
Compound 9. The material has an N phase that could be
well aligned, with birefringence Δn = 0.19 at T = 83 °C
and Δn = 0.22 at T = 65 °C. In general, the compound
responded well to the He-Cd laser light, although the
kind of response was temperature-dependent and the
effect disappeared soon after the illumination ceased. At
83 °C, the material went rapidly to the I phase (Figure 8a).
Photoisomerization in the Condensed Phases. It is known
that the molecular order in the mesophase of azobenzene-
containing liquid crystals is disturbed when the calamitic-
like trans isomer isomerizes to the bent conformation of
the cis isomer.2a,b,g Consequently, isotropization of the
mesophase can be reached, promoting the trans-cis iso-
merization by irradiation. In some cases, the trans-cis
isomerization only induces a reduction of the birefringence
with no change of the mesophase. Below we describe
the response of the mesophases of compounds 3 and
7-12 to irradiation with a He-Cd laser of wavelength
442 nm and intensity 1 W cm-2, as well as the effects on
3
the birefringence.
Compound 3. The material could be easily aligned in
planar cells in the N mesophase (see Figure 5). A birefrin-
gence Δn = 0.22 at 70 °C was obtained. Upon illumination
with the laser light, the material underwent an isothermal
transition to the isotropic phase. The change was very fast,
and the aligned N phase was restored immediately when the
illumination was switched off.
Compound 7. The sample showed good planar align-
ment in both the N and SmA mesophases. A birefringence
Δn = 0.23 was measured in the SmA phase (140 °C), and
a slightly lower value was obtained in the N phase. When
the material was illuminated for a few seconds in the N
phase, a decrease in the birefringence was observed from
the original pink color to orange and yellow tones. In
contrast to compound 3, a longer irradiation period was
necessary to get black isotropic areas (Figure 6). The
change was reversible when the illumination ceased.

8910 Inorganic Chemistry, Vol. 49, No. 19, 2010
Baena et al.
Figure 10. (a) Effect of laser illumination on the optically isotropic
texture of compound 10. A slightly birefringent texture (bright area) is
induced, with the director orientation perpendicular to the laser polariza-
tion. (b) Mark created on a sample of compound 10 cooled from the I
phase under laser illumination. The illuminated area is birefringent and
appears dark because the director is parallel to the polarizer P direction.
Figure 9. Textures of compound 10 upon cooling from the I phase with
the lamp of the microscope switched off (left) or on (right). The illumi-
nated area is darker because of the very small size of the domains, which
gives rise to an almost optically isotropic texture.
At 80 °C, the response was different because the laser only
modified the alignment of the mesophase, which went to a
state where n is not very well-defined (bright area in
Figure 8b). We think that this can be due to competition
between the directions of alignment imposed by the sur-
face treatment and the trend of the illuminated molecules
to orient the director perpendicularly to the laser polar-
ization direction. In this material, the response was very
sensitive to light polarization. In fact, when the polariza-
tion was perpendicular to the director, no effect could be
observed. Finally, at 65 °C, the material hardly responded
to the light, irrespective of its polarization.
Compound 10. As reported in the Mesogenic Behavior
and Structural Properties section, this compound pre-
sents an unidentified mesophase X that could not be
studied because of its metastability. However, the stable
phase C, identified as a “soft” crystal, has a very special
behavior with the light. As in the cases above, the material
turns out to be marked by the light, but interestingly here
the mark remains in the absence of the beam, being
permanent at least for 15 h. This persistence has never
been reported before for a low-molecular-weight soft
crystal (though it is typical for liquid-crystal polymers).
In the case of liquid crystals, the relatively low viscosity of
the mesophases restores the original alignment in a few
seconds after the light is switched off. In the present case,
however, the phase is viscous enough to retain the light
alignment but soft enough to permit the light to mark the
sample. The mark can only be erased by heating to the
I phase and cooling down again.
Figure 11. Textures of compound 11 at T = 146 °C, in the very vicinity
of the clearing point, before (a) and after (b) illumination with the He-Cd
laser light. The black area on the left is the I phase.
the induced structure (slow axis of the optical indicatrix)
is perpendicular to the laser polarization. The birefrin-
gence induced after a few seconds of laser illumination is
typically 0.02. In addition, we have checked that, once the
mark has been created, a change of the light polarization
direction also produces a counterpart in the director
orientation, with both being perpendicular to each other
in the steady state (the entire reorientation process can
last a few minutes).
Upon cooling from the I phase with the laser on, the
birefringent mark is directly created, without mediating
the optically isotropic texture (Figure 10b). In all cases,
the laser produces a permanent photoalignment of the
material, where the director direction is controlled by the
laser polarization.
Compound 11. The phase below the I phase is meta-
stable and could not be aligned. It is very viscous and
probably shows crystalline order. The birefringence has a
value of 0.28 at 145 °C. Upon illumination, there was no
change to the isotropic liquid (except at very high tem-
perature; see Figure 11), but there is always a reduction in
the birefringence. In this case, the high viscosity is another
factor that reduces the effect of the light on the phase.
Compound 12. The material showed a soft crystalline
phase that could not be aligned. The small domain size
prevented the performance of reliable measurements of
the birefringence. There was hardly any effect of the laser
light on the material texture even close to the clearing
point.
The way the light affects the sample is peculiar and
deserves a comment. If the material is cooled down in the
dark from the isotropic phase, a small-size domain tex-
ture is obtained (Figure 9, left). However, if a moderate
light source (e.g., the microscope lamp itself) is on during
the cooling process, the domain size is much smaller
(Figure 9, right). In fact, in the later conditions, an almost
isotropic texture is created (the transmitted intensity is
small and independent of the angular position of the
sample, i.e., Δn ∼ 0). This suggests that the domain
dimensions are on the order of magnitude of the visible
wavelength.
Conclusions
Complexes derived from orthopalladated imines bearing
one or two azocarboxylato bridges were synthesized, with
three of them (7-9) behaving as liquid crystals, while 10-12
are “soft” crystals. Because of the presence of the azobenzene
It is in this situation where the sample is most sensitive
to laser illumination. Figure 10a shows a laser mark
(bright area) on this dark texture, where the director of

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8911
moiety, these compounds photoisomerize in solution with
light of λ = 365 nm. As deduced from their electronic and 1H
NMR spectra, the best responses are achieved when three
alkoxy chains are linked to the azo bridge and even better
when the orthometalated part of the molecule does not
hinder the trans-to-cis motion.
The photoresponse of all compounds has also been studied
in their condensed phases. Some parallelism has been found
with the behavior in solution, although the particular kind of
phase and, especially, its viscosity have turned out to be
crucial for the performance of the materials. All of them
respond to the light, although to very different degrees,
ranging from compound 3 (the free acid, the most sensitive
and fast) to compound 12, which is almost inactive. The case
of compound 10, a soft crystal, is especially interesting, with a
moderate but permanent photoresponse and high sensitivity
to the light polarization. These properties are unusual and
could be used or further elaborated to produce materials for
permanent optical memories.
positions were located in difference Fourier maps and refined
anisotropically. Powder X-ray diffraction measurements were
carried out with a powder diffractometer equipped with a high-
temperature attachment. Data were collected in Debye-Scherrer
operation mode using Lindemann capillaries of diameter 0.5 mm.
A linear position-sensitive detector, with an angular resolution
better than 0.01°, was employed to detect the diffracted intensity
in the 2θ interval 0.5-35° (θ is the Bragg angle). Monochromatic
Cu KR1 radiation (1.5406 A) was used.
Commercial reagents and solvents were used as provided.
Literature procedures were used to synthesize the hydroxo-
bridged17b and thiolatochloro-bridged17a precursors. Synthesis
of the trisubstituted azocarboxylate was achieved in three steps,
as described below, based on the reported synthesis of 4-(40-n-
tetradecyloxyphenylazo)benzoic acid,27 with adequate modifi-
cations.
Preparation of 4-[20,30,40-Tris(n-decyloxyphenylazo)]benzoic
Acid (3). Step 1: Ethyl 4-[(20,30,40-Tris(hydroxyphenylazo)]ben-
zoate (1). A solution of 2.27 g of sodium nitrite (32.04 mmol) in
water (15.72 mL) was added dropwise over a suspension of ethyl
p-aminobenzoate (4.5 g, 26.70 mmol) in a mixture of hydro-
chloric acid and water (10 and 30 mL, respectively) below 5 °C. The
reaction mixture was stirred for 30 min, and then 4.04 g of pyro-
gallol (32.04 mmol) was added. After stirring for 1 h at e5 °C, the
cold reaction mixture was poured into a NaHCO3 saturated
solution to reach a pH of 6-7. A brown solid appeared that was
filtered off and washed with water. This solid was treated with
diethyl ether to remove an insoluble fraction. The solution was
evaporated, the residue was dissolved in acetone, and the final
brown product was precipitated with petroleum ether. Yield: 30%.
1H NMR (300 MHz, acetone-d6): δ 13.30 (br signal, -OH), 8.15
(d, J = 8.9 Hz, 2H), 7.93 (d, J = 8.9 Hz, 2H), 7.36 (d, J = 9.0 Hz,
1H), 6.67 (d, J = 9.0 Hz, 1H), 4.35 (q, J = 7.3 Hz, 2H), 3.00 (br
signal including -OH), 1.37 (t, 7.3 Hz, 3H). IR (KBr): 3409, 1692,
Experimental Section
General Procedures. 1H NMR spectra were recorded in a
Bruker AC-300 or ARX-300 MHz spectrometer, in CDCl3.
Elemental analyses were done in a Perkin-Elmer 2400 micro-
analyzer. IR spectra were recorded with a Perkin-Elmer FT
1720X spectrophotometer (4000-400 cm-1) using KBr pellets.
The textures of the mesophases were studied with a Leica model
DMRB polarizing microscope, equipped with a Mettler FP-
82HT hot stage and a Mettler FP-90 temperature controller.
Transition temperatures and enthalpies were measured by dif-
ferential scanning calorimetry (DSC), with a Perkin-Elmer
DSC-7, using aluminum crucibles. The apparatus was cali-
1604, 1284 cm-1
.
brated with indium (156.6 °C; 28.45 J g-1) as the standard.
Step 2: Ethyl 4-[20,30,40-Tris(n-decyloxyphenylazo)]benzoate
(2). Over a mixture of 1 (1.18 g, 3.9 mmol), K2CO3 (2.7 g, 20
mmol), and KI (0.3 g) in dry acetone (150 mL) under a nitrogen
atmosphere was added 1-bromodecane (5.2 g, 23.5 mmol). The
reaction was stirred under reflux for 48 h and checked by thin-
layer chromatography (silica gel, CH2Cl2). The product ob-
tained is a mixture of, at least, the mono-, di-, and trialkoxylated
derivatives. The acetone was removed, and the bulk of the
reaction was treated with toluene, filtered off, and chromato-
graphed in a silica column, eluting with toluene to collect
exclusively the trisubstituted derivative. The product is a red
solid after solvent removal. Yield: 29%. Mp: 41.4 °C (ΔH =
3
The electronic spectra were recorded with a Shimadzu UV 1603
UV-vis spectrophotometer from THF solutions. These solu-
tions and those for NMR were irradiated with a UV lamp able to
provide light of 254 and 365 nm (I = 7 mW cm-2). Planar
3
samples were prepared in commercially available cells (Linkam)
of a nominal thickness of 5 μm. The inner glass surfaces were
coatedwith polyimide andrubbed unidirectionally. Samples were
illuminated with polarized laser light, and the textures were
simultaneously observed in the polarizing microscope. We em-
ployed a He-Cd laser emitting at 442 nm, and the average
intensity of irradiation on the samples was about 1 W cm-2
.
3
The birefringence was measured using a Berek compensator. The
opticalpathdifference was determined through the rotationangle
of a calcite plate cut perpendicular to the polarizing microscope
axis.22 Single-crystal X-ray diffraction was carried out in a Bruker
SMART CCD diffractometer. Cell parameters were retrieved
using SMART23 software and refined with SAINT24 on all
observed reflections. Data reduction was performed with the
SAINT software and corrected for Lorentz and polarization
effects. Absorption corrections were based on multiple scans
(program SADABS).25 The structure was solved by direct meth-
ods and refined anisotropically on F2.26 All non-hydrogen atomic
55.8 kJ mol-1). 1H NMR (300 MHz, CDCl3): δ 8.18 (d, J = 8.7
3
Hz, 2H), 7.93 (d, J = 8.9 Hz, 2H), 7.52 (d, J = 9.3 Hz, 1H), 6.72
(d, J = 9.3 Hz, 1H), 4.42 (q, J = 7.0 Hz, 2H), 4.25 (t, J = 6.7 Hz,
2H), 4.06 (t, J = 6.7 Hz, 4H), 1.91-1.79 (m, 6H), 1.57-1.28 (m,
45H), 0.91-0.86 (m, 9H). IR (KBr): 2916, 2851, 1718, 1587,
1472, 1296, 1273 cm-1
.
Step 3: Compound 3. Over an ethanolic solution (200 mL) of 2
(0.88 g, 1.2 mmol) was added 0.4 g (8.4 mmol) of solid KOH, and
the mixture was refluxed for 5 h. After cooling to room
temperature, the reaction was poured into 500 mL of water.
Concentrated hydrochloric acid was added to reach an acidic
pH. The orange precipitate formed was collected and washed
1
(22) Wahlstrom, E. E. Optical crystallography with particular reference
to the use and theory of the polarizing microscope. John Wiley: New York,
1960. See also the comment about the Berek compensator given in the SI.
(23) SMART software for the CCD Detector System, version 5.051; Bruker
Analytical X-ray Instruments Inc.: Madison, WI, 1998.
(24) SAINT integration software, version 6.02; Bruker Analytical X-ray
Instruments Inc.: Madison, WI, 1999.
(25) Sheldrick, G. M. SADABS: A program for absorption correction with
with water, EtOH, and acetone successively. Yield: 85%. H
NMR (300 MHz, CDCl3): azo group in the trans conformation
(see Scheme 1 for labels) δ 8.26 (d, J = 8.5 Hz, 2H, Hb), 7.96 (d,
J = 8.5 Hz, 2H, Ha), 7.54 (d, J = 9.1 Hz, 1H, Hc), 6.72 (d, J =
9.3 Hz, 1H, Hd), 4.27 (t, J = 6.4 Hz, 2H), 4.07 (m, 4H),
1.89-1.80 (m, 6H), 1.51-1.27 (m, 42H), 0.91-0.86 (m, 9H);
azo group in the cis conformation δ 7.98 (d, J = 8.8 Hz, 2H,
€
€
the Siemens SMART system; University of Gottingen; Gottingen, Germany,
1996.
(26) SHELXTL program system, version 5.1; Bruker Analytical X-ray
Instruments Inc.: Madison, WI, 1998.
´
(27) Folcia, C. L.; Alonso, I.; Ortega, J.; Etxebarrıa, J.; Pintre, I.; Ros,
M. B. Chem. Mater. 2006, 18, 4617.

8912 Inorganic Chemistry, Vol. 49, No. 19, 2010
Baena et al.
Hb), 6.94 (d, J = 8.7 Hz, 2H, Ha), 6.45 (d, J = 8.8 Hz, 1H, Hc),
6.31 (d, J = 8.8 Hz, 1H, Hd), 4.03 (t, J = 6.4 Hz, 2H), 3.89 (t,
J = 6.4 Hz, 2H), 3.78 (t, J = 6.4 Hz, 2H), 1.89-1.80 (m, 6H),
1.51-1.27 (m, 42H), 0.91-0.86 (m, 9H). IR (KBr): 2921, 2851,
2664, 2548, 1692, 1460, 1585, 1286 cm-1. Elem anal. Found
(exptl): C, 74.00 (74.31); H, 10.05 (10.15); N, 4.11 (4.03).
Preparation of [Pd(μ-SC10H21)(μ-Cl)(L2)2]. This compound
is the precursor of compounds 8 and 10. Nevertheless, its
synthesis is very similar to that reported for other orthopalla-
dated complexes with mixed bridges;17a this is the first that is
derived from an imine with only one aromatic ring, and thus its
characterization as well as that of its precursors is reported here.
1. HL2, H21C10OC6H4CHdNC14H29 (4). The synthesis was
carried out as previously reported.28 A yellow solid was ob-
7.67 (d, J = 8.4 Hz, 2H, Hg), 7.55 (d, J = 2.2 Hz, 2H, Ha),
7.53-7.45 (m, 5H, Hh, Hj, Hk), 7.17 (d, J = 8.8 Hz, 4H, He),
7.16 (d, J = 8.8 Hz, 2H, Hc), 6.67 (d, J = 8.8 Hz, 4H, Hf), 6.45
(dd, J = 2.2 and 8.3 Hz, 2H, Hb), 4.1-3.95 (m, 4H), 3.55-3.65
(m, 4H), 2.51 (t, J = 7.4 Hz, 2H), 1.89-1.77 (m, 10H),
1.55-1.10 (m, 70 H), 0.92-0.81 (m, 15H). IR (KBr): 2923,
2853, 1609, 1583, 1546, 1505, 1466, 1390, 1249, 1200 cm-1. Elem
anal. Found (exptl): C, 66.87 (66.94); H, 7.94 (8.21); N, 3.45
(3.51).
Compound 8. 1H NMR (300 MHz, CDCl3): azo group in the
trans conformation (see Scheme 2 for labels) δ 8.20 (d, J = 8.8
Hz, 2H, Hg), 7.97-7.91 (m, 4H, Hi, Hh), 7.85 (s, 2H, Hd),
7.57-7.49 (m, 3H, Hj, Hk), 7.45 (d, J = 2.6 Hz, 2H, Ha), 7.13 (d,
J = 8.4 Hz, 2H, Hc), 6.48 (dd, J = 2.8 and 8.4 Hz, 2H, Hb),
4.08-3.97 (m, 4H), 3.79-3.70 (m, 2H), 3.55-3.47 (m, 2H), 2.46
(t, J = 7.5 Hz, 2H), 1.89-1.77 (m, 10H), 1.55-1.10 (m, 86 H),
0.92-0.81 (m, 15H); azo group in the cis conformation δ
7.97-7.81 (overlapped signal, 2H, Hg), 7.81 (s, 2H, Hd), 7.42
(d, J = 2.4 Hz, 2H, Ha), 7.27-7.14 (m, 3H, Hj, Hk), 7.11 (d, J =
8.3 Hz, 2H, Hc), 6.90-6.85 (m, 2H, Hh), 6.83 (d, J = 8.7 Hz, 2H,
Hi), 6.46 (dd, J = 2.6 and 8.2 Hz, 2H, Hb), 4.08-3.97 (m, 4H),
3.69-355 (m, 2H), 3.55-3.38 (m, 2H), 2.39 (t, J = 7.6 Hz, 2H),
1.89-1.77 (m, 10H), 1.55-1.10 (m, 86 H), 0.92-0.81 (m, 15H).
1
tained in 75% yield. Mp: 33.2 °C (ΔH = 62.4 kJ mol-1). H
3
NMR (300 MHz, CDCl3): δ 8.19 (s, 1H), 7.66 (d, J = 8.7 Hz,
2H), 6.91 (d, J = 8.7 Hz, 2H), 3.99 (t, J = 6.6 Hz, 2H), 3.57 (t,
J = 6.8 Hz, 2H), 1.83-1.26 (m, 40H), 0.91-0.87 (m, 6H). IR
(KBr): 2938, 1634, 1607, 1509, 1470, 1244 cm-1. Elem anal.
Found (exptl): C, 81.24 (81.34); H, 11.97 (12.11); N, 3.18 (3.06).
2. [Pd(μ-Cl)L2]2 (5). Over a hot solution (80 °C) of palladium
acetate (0.15 mg, 0.22 mmol) in 30 mL of toluene was added the
imine ligand HL2 (0.3 g, 0.66 mmol). After 2 h of heating, the
reaction was slowly cooled down to room temperature. The
dinuclear acetate-bridged complex was not isolated. A metha-
nolic solution of hydrochloric acid (0.5 mmol) was added to the
bulk of the reaction and stirred for 15 min. The mixture of
solvents was removed, and the residue was chromatographed in
a silica gel column with CH2Cl2 as the eluent. A yellowish solid
was obtained after concentration and precipitation with petro-
leum ether. Yield: 57%. 1H NMR (300 MHz, CDCl3): δ 7.66 (s,
2H), 7.10 (d, J = 8.4 Hz, 2H), 6.90 (d, J = 2.4 Hz, 2H), 6.54 (dd,
J = 2.6 and 8.5 Hz, 2H), 3.97 (t, J = 6.4 Hz, 4H), 3.56-3.52 (m,
4H), 1.83-1.76 (m, 8H), 1.56-1.25 (m, 72H), 0.90-0.86 (m,
IR (KBr): 2922, 2852, 1618, 1586, 1551, 1466, 1390, 1228 cm-1
.
Elem anal. Found (exptl): C, 66.88 (66.95); H, 9.09 (9.12); N,
3.89 (3.67).
Compound 9. 1H NMR (300 MHz, CDCl3; see Scheme 2 for
labels): δ 7.94 (s, 2H, Hd), 7.63 (d, J = 8.6 Hz, 2H, Hg), 7.58 (d,
J = 2.3 Hz, 2H, Ha), 7.47 (d, J = 8.6 Hz, 2H, Hc), 7.46 (d, J =
9.1 Hz, 1H, Hi), 7.25-7.21 (m, 6H, He, Hh), 6.71 (d, J = 8.8 Hz,
4H, Hf), 6.69 (d, J = 9.2 Hz, 1H, Hj), 6.51 (dd, J = 2.1 and 8.3
Hz, 2H, Hb), 4.25 (t, J = 6.6 Hz, 2H), 4.10-4.04 (m, 8H),
3.76-3.70 (br signal, 4H), 2.51 (t, J = 7.4 Hz, 2H), 1.89-1.20
(m, 128H), 0.94-0.83 (m, 24H). IR (KBr): 2924, 2853, 1582,
1548, 1507, 1467, 1388, 1288, 1252, 1198 cm-1. Elem anal.
Found (exptl): C, 68.97 (69.19); H, 9.15 (9.27); N, 2.79 (2.71).
Compound 10. 1H NMR (300 MHz, CDCl3): azo group in the
trans conformation (see Scheme 2 for labels) δ 8.17 (d, J = 8.8
Hz, 2H, Hg), 7.89 (d, J = 8.5 Hz, 2H, Hh), 7.85 (s, 2H, Hd), 7.51
(d, J = 9.1 Hz, 1H, Hi), 7.45 (d, J = 2.7 Hz, 2H, Ha), 7.13 (d,
J = 8.4 Hz, 2H, Hc), 6.71 (d, J = 9.2 Hz, 1H, Hj), 6.48 (dd, J =
2.2 and 8.4 Hz, 2H, Hb), 4.25 (t, J = 6.6 Hz, 2H), 4.09-4.00 (m,
8H), 3.77-3.73 (m, 2H), 3.51-3.47 (m, 2H), 2.45 (t, J = 7.4 Hz,
2H), 1.86-1.17 (m, 144H), 0.92-0.81 (m, 24H); azo group in the
cis conformation δ 7.89 (d, J = 8.5 Hz, 2H, Hg), 7.81 (s, 2H,
Hd), 7.42 (d, J = 2.3 Hz, 2H, Ha), 7.10 (d, J = 8.4 Hz, 2H, Hc),
6.88 (d, J = 8.6 Hz, 2H, Hh), 6.46 (dd, J = 2.2 and 8.3 Hz, 2H,
Hb), 6.37 (d, J = 8.9 Hz, 1H, Hi), 6.17, (d, J = 8.8 Hz, 1H, Hj),
4.25 (t, J = 6.6 Hz, 2H), 4.09-4.00 (m, 8H), 3.75-3.55 (m, 4H),
2.40 (t, J = 7.4 Hz, 2H), 1.86-1.17 (m, 144H), 0.92-0.81 (m,
12H). IR (KBr): 2920, 2849, 1618, 1580, 1467, 1265, 1228 cm-1
.
Elem anal. Found (exptl): C, 61.81 (62.20); H, 8.95 (9.09); N,
2.36 (2.34). Thermal behavior: C, 103.1 °C (23.7 kJ mol-1); SA,
3
121.9 °C (6.6 kJ mol-1) I.
3
3. [Pd(μ-SC10H21)(μ-Cl)(L2)2] (6). This compound was ob-
tained by the method reported in the literature.17a A yellowish
solid was obtained in 90% yield. 1H NMR (300 MHz, CDCl3): δ
7.79 (s, 2H), 7.31 (d, J = 2.4 Hz, 2H), 7.16 (d, J = 8.3 Hz, 2H),
6.53 (dd, J = 2.4 and 8.3 Hz, 2H), 4.01 (t, J = 6.4 Hz, 4H), 3.56
(br signal, 4H), 2.92 (t, J = 7.2 Hz, 2H), 2.05-1.22 (m, 96H),
0.91-0.87 (m, 15H). IR (KBr): 2921, 2851, 1617, 1580, 1551,
1467, 1265, 1230, 1210 cm-1. Elem anal. Found (exptl): C, 64.69
(64.77); H, 9.53 (9.74); N, 2.15 (2.10). Thermal behavior: C,
26.8 °C (11.8 kJ mol-1); SA, 68.1 °C (6.9 kJ mol-1) I.
3
3
Preparation of [Pd2(μ-SC10H21)(μ-O2CAzo)(L1,2)2] (7 and 8)
and [Pd2(μ-SC10H21)(μ-O2CAzo3C10)(L1,2)2] (9 and 10). Over a
solution of the corresponding acid [4-(phenylazo)benzoic acid
(50 mg, 0.15 mmol) or 3 (104 mg, 0.15 mmol)] in CH2Cl2 (25 mL)
was added 0.3 mL of a 0.5 M solution of NaOH, and the
resulting solution was stirred for a few minutes. Then, AgNO3
was added (25 mg, 0.15 mmol) and vigorously stirred for 0.5 h.
The palladium complex [Pd(μ-SC10H21)(μ-Cl)(L1,2)2] (0.15
mmol) was dissolved in 25 mL of CH2Cl2 and added to the
silver salt solution. The reaction was protected from the light
and stirred for 4 h. Anhydrous MgSO4 was added to dry the
mixture, which was filtered off through a kiesel gel pad. After
evaporation of the solvent, the residue was washed with water
and then triturated with EtOH. An orange solid was obtained,
filtered off, and washed with EtOH and acetone successively.
Yield: 85-80%.
24H). IR (KBr): 2922, 2852, 1586, 1553, 1466, 1392, 1289 cm-1
.
Elem anal. Found (exptl): C, 68.61 (68.69); H, 9.92 (9.98); N,
2.85 (2.79).
Preparation of [Pd(μ-O2CAzo)L1]2(11) and [Pd(μ-O2CAzo-
3C10)L1]2 (12). Over a suspension of [Pd(μ-OH)L1]2 (0.1 mmol)
and MgSO4 in dry CH2Cl2 was added 0.2 mmol of the corre-
sponding acid [4-(phenylazo)benzoic acid or compound 3]. The
mixture was stirred for 6 h and filtered off through a pad of
kiesel gel. After solvent removal, the solid residue was recrys-
tallized from CH2Cl2/EtOH to obtain an orange solid. Yield:
78-80%.
Compound 11. 1H NMR (300 MHz, CDCl3; see Scheme 3 for
labels): δ 8.25 (d, J = 8.7 Hz, 4H, Hh), 7.94 (dd, J = 2.2 and 8.1
Hz, 4H, Hi), 7.88 (d, J = 8.5 Hz, 4H, Hg), 7.58 (s, 2H, Hd),
7.53-7.50 (m, 6H, Hj, Hk), 7.19 (d, J = 8.3 Hz, 2H, Hc), 6.73 (d,
J = 8.9 Hz, 4H, He), 6.55 (dd, J = 2.3 and 8.0 Hz, 2H, Hb), 6.47
(d, J = 8.8 Hz, 4H, Hf), 6.01 (d, J = 2.2 Hz, 2H, Ha), 3.77 (m,
2H), 3.65 (m, 2H), 3.40 (m, 2H), 3.00 (m, 2H), 1.88-1.07 (m,
64H), 0.91-0.84 (m, 12H). IR (KBr): 2922, 2852, 1606, 1578,
Compound 7. 1H NMR (300 MHz, CDCl3; see Scheme 2 for
labels): δ 7.92 (dd, J = 2 and 8.2 Hz, 2H, Hi), 7.83 (s, 2H, Hd),
(28) Baena, M. J.; Espinet, P.; Ros, M. B.; Serrano, J. L. J. Mater. Chem.
1996, 6, 1291.

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8913
1561, 1507, 1466, 1384, 1256, 1230, 1199 cm-1. Found (exptl): C,
66.79 (67.02); H, 6.95 (7.21); N 4.88 (5.10).
Acknowledgment. We gratefully acknowledge financial sup-
port by the DGI (Project CTQ2008-03954/BQU and INTECAT
Consolider Ingenio-2010 CSD2006-0003), the Junta de Castilla y
ꢀ
Leon (Projects VA012A08 and GR169), the Basque Government
(GIC10/45), and the MICIIN (Project MAT2009-14636-C03-03).
Compound 12. 1H NMR (300 MHz, CDCl3; see Scheme 3 for
labels): δ 8.21 (d, J = 8.4 Hz, 4H, Hh), 7.84 (d, J = 8.4 Hz, 4H,
Hg), 7.58 (s, 2H, Hd), 7.51 (d, J = 9.2 Hz, 2H, Hi), 7.18 (d, J =
8.4 Hz, 2H, Hc), 6.74 (d, J = 8.9 Hz, 4H, He), 6.69 (d, J = 9.7
Hz, 2H, Hj), 6.54 (dd, J = 2.1 and 8.4 Hz, 2H, Hb), 6.47 (d, J =
8.9 Hz, 4H, Hf), 6.01 (d, J = 2.2 Hz, 2H, Ha), 4.24 (t, J = 6.6 Hz,
4H), 4.08-4.02 (m, 8H), 3.77 (m, 2H), 3.65 (m, 2H), 3.40 (m,
2H), 3.00 (m, 2H), 1.88-1.07 (m, 160H), 0.91-0.84 (m, 30H). IR
(KBr): 2922, 2854, 1604, 1579, 1560, 1466, 1385, 1289, 1253,
1228, 1198 cm-1. Found (exptl): C, 70.51 (70.59); H, 8.92 (9.26);
N, 3.12 (3.25).
Supporting Information Available: DSC thermograms for
compounds 2-12, microphotographs for Figures 5-11 de-
picted at a higher size, 1H NMR spectra for compound 10 after
different irradiation times, comment on the Berek compensator,
X-ray diffraction data for compound 11, and X-ray crystal-
lographic data for compound 11 in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.