Supramolecular structures and columnar mesophase induction in nondiscoid pyrazoles by complexation to rhodium(I)

Article abstract of DOI:10.1021/ic0614216

Several new cis-[RhCl(CO)₂(Ln)] complexes have been prepared using different polycatenar pyrazole ligands (Ln) in order to obtain columnar liquid crystalline arrangements. The topology of the ligand plays an essential role, and a mesophase is induced at room temperature from a nonmesogenic pyrazole only when it is symmetrically substituted with six decyloxy chains. The single-crystal structure of a methoxy-substituted analogue, 3,5-bis(3,4,5-trimethoxyphenyl)pyrazole, is formed by globular tetrameric structures held together by H-bonding. However, parallel dimers are present in the corresponding c/s-chlorodicarbonylrhodium(I) complex, a situation that explains the induction of a columnar mesophase in the decyloxy-substituted complex. The XRD pattern of the mesophase is consistent with a hexagonal symmetry in which the columns are formed by molecules assembled in an antiparallel mode. The crystal-to-mesophase transition was also detected by spectroscopic techniques as a shift in the IR carbonyl stretching bands and the appearance of a charge-transfer band in the absorption spectrum with thermochromic behavior.

Full text of DOI:10.1021/ic0614216

Inorg. Chem. 2006, 45, 10363−10370
Supramolecular Structures and Columnar Mesophase Induction in
Nondiscoid Pyrazoles by Complexation to Rhodium(I)
Raquel Gime´nez,*,† Anabel Elduque,*,‡ Jose´ Antonio Lo´pez,‡ Joaqu´ın Barbera´,† Emma Cavero,†
Ignacio Lantero,‡ Luis A. Oro,‡ and Jose´ Luis Serrano†
Departamento de Qu´ımica Orga´nica and Departamento de Qu´ımica Inorga´nica, Facultad de
CienciassInstituto de Ciencia de Materiales de Arago´n, UniVersidad de Zaragoza-CSIC, 50009
Zaragoza, Spain
Received July 28, 2006
Several new cis-[RhCl(CO)2(Ln)] complexes have been prepared using different polycatenar pyrazole ligands (Ln)
in order to obtain columnar liquid crystalline arrangements. The topology of the ligand plays an essential role, and
a mesophase is induced at room temperature from a nonmesogenic pyrazole only when it is symmetrically substituted
with six decyloxy chains. The single-crystal structure of
a methoxy-substituted analogue, 3,5-bis(3,4,5-
trimethoxyphenyl)pyrazole, is formed by globular tetrameric structures held together by H-bonding. However, parallel
dimers are present in the corresponding cis-chlorodicarbonylrhodium(I) complex, a situation that explains the induction
of a columnar mesophase in the decyloxy-substituted complex. The XRD pattern of the mesophase is consistent
with a hexagonal symmetry in which the columns are formed by molecules assembled in an antiparallel mode. The
crystal-to-mesophase transition was also detected by spectroscopic techniques as a shift in the IR carbonyl stretching
bands and the appearance of a charge-transfer band in the absorption spectrum with thermochromic behavior.
Introduction
crystal phases from these types of metal complexes7 (named
metallomesogens). This approach is a way to combine the
self-organizational ability of the mesogens and the different
properties introduced by the metal center.
There are several drawbacks associated with metallomeso-
gens,8-10 and one is that the geometry imposed by the metal
is not always compatible with a columnar organization of
molecules. A second drawback is the difficulty in preserving
the liquid crystalline order at room temperature as metal
complexes usually have high melting points. For these
reasons, careful design of the ligand is needed in order to
overcome these situations.11,12 Although the most favorable
shape for columnar liquid crystals is the discoid shape, in
Square-planar d8 rhodium(I) or iridium(I) complexes are
interesting for molecular electronics as their disposition along
a columnar axis may give rise to extended metallic interac-
tions1 andresultinmaterialswithanisotropicsemiconductive2-4
or pyroelectric5,6 properties. Materials with metal arrange-
ments having different bond strengths, usually weaker than
a covalent intermetallic bond, have been prepared by growing
good quality single crystals or by LB film deposition. A third
strategy that could lead to analogous arrangements, and
related to this work, is the generation of columnar liquid
* To whom correspondence should be addressed. E-mail: rgimenez@
unizar.es (R.G.), anaelduq@unizar.es (A.E.).
† Departamento de Qu´ımica Orga´nica.
‡ Departamento de Qu´ımica Inorga´nica.
(7) Trzaska, S. T.; Swager, T. M. Chem. Mater. 1998, 10, 438-443.
(8) Serrano, J. L. Metallomesogens: Synthesis, Properties and Applica-
tions; VCH: Weinheim, 1996.
(1) Bera, J. K.; Dunbar, K. R. Angew. Chem., Int. Ed. 2002, 41, 4453-
4457.
(2) Pitt, C. G.; Monteith, L. K.; Ballard, L. F.; Collman, J. P.; Morrow,
J. C.; Roper, W. R.; Ulkii, D. J. Am. Chem. Soc. 1966, 88, 4286-
4287.
(9) Gime´nez, R.; Lydon, D. P.; Serrano, J. L. Curr. Opin. Solid State
Mater. Sci. 2002, 6, 527-535.
(10) Donnio, B.; Guillon, D.; Deschenaux, R.; Bruce, D. W. In Compre-
hensiVe Coordination Chemistry II; McCleverty, J. A., Meyer, J. J.,
Eds.; Elsevier: Oxford, 2003; Vol. 7, p 357-627.
(3) Aullo´n, G.; Alvarez, S. Chem.sEur. J. 1997, 3, 655-664.
(4) Miller, J. S.; Epstein, A. J. In Progress in Inorganic Chemistry;
Lippard, S. J., Ed.; John Wiley and Sons: New York, 1976; Vol. 20.
(5) Richardson, T.; Topac¸li, a.; Majid, W. H. A.; Greenwood, M. B.;
Bruce, D. W.; Thornton, A.; Marsden, J. R. AdV. Mater. Opt. Electron.
1994, 4, 243-251.
(11) Gime´nez, R.; Manrique, A. B.; Uriel, S.; Barbera´, J.; Serrano, J. L.
Chem. Commun. 2004, 2064-2065.
(12) Terazzi, E.; Suarez, S.; Torelli, S.; Nozary, H.; Imbert, D.; Mamula,
O.; Rivera, J.-P.; Guillet, E.; Benech, J.-M.; Bernardinelli, G.;
Scopelliti, R.; Donnio, B.; Guillon, D.; Bu¨nzli, J.-C. G.; Piguet, C.
AdV. Funct. Mater. 2006, 16, 157-168.
(6) Wong, J. E.; Bruce, D. W.; Richardson, T. H. Synth. Met. 2005, 148,
11-14.
10.1021/ic0614216 CCC: $33.50
Published on Web 11/15/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 25, 2006 10363

Gime´nez et al.
some cases a subtle combination of noncovalent dipole-
dipole,7,13-16 ligand-ligand and/or ligand-metal17-23 or weak
metal-metal7,13,15 interactions can induce or stabilize a
columnar mesophase with nondiscoid molecules. These
interactions promote time-averaged disklike structures that
self-organize to form columns. The only report of columnar
mesomorphism in rhodium and iridium compounds described
in the literature is an example of this behavior and concerns
dicarbonyl-1,3-diketonate complexes [M(CO)2(dk)].7
In previous publications, we have reported that pyrazoles
are versatile heterocycles for the generation of coordination
compounds with columnar phases at or near room tempera-
ture.24-27 Modulation of the intermolecular forces can be
achieved in several ways using pyrazoles, either by incor-
porating appropriate substituents on the carbon atoms,
through hydrogen bonding (as the pyrazole is a good donor
and acceptor of such bonds) and/or by coordination to metals.
Mononuclear rhodium complexes of the type cis-[RhCl(CO)2-
(HPz)] could be good candidates to show liquid crystallinity
due to the properties observed for the related 1,3-diketonate
complexes [Rh(CO)2(dk)]. The presence of both the chloro
ligand and the NH-pyrazole introduces additional dipolar and
hydrogen bond interactions that may give rise to different
molecular packing from diketonates and have a decisive
influence on the properties.
Figure 1. Structures of the rhodium complexes 1-6.
crystals, but columnar mesomorphism has never been found.
In the case where L ) pyrazole, it was observed that the
mesomorphic properties of the ligand are worse33 and only
one compound (with nematic mesomorphism) has been
reported.34
In the work described here, several cis-[RhCl(CO)2(Hpz)]
compounds (1-6, Figure 1) containing HPz ligands derived
from different 3,5-diarylpyrazoles (L1-L6) were prepared.
To promote columnar mesomorphism, the pyrazole ligands
are substituted at the phenyl rings with four, five, or six long
alkoxy chains (complexes 1-5). These different substitution
patterns allowed us to assess the influence of the structure
of the pyrazole ligand on the formation of columnar phases.
Finally, model compound 6 and its respective pyrazole L6
were structurally characterized in the solid state in order to
gain an insight into the tendency of these compounds to form
columnar assemblies.
Complexes with a similar structure {cis-[MCl(CO)2(L)],
where M ) Rh, Ir and L ) nitrile,28 pyridine,29,30 or
stilbazole31,32} have been studied in the field of liquid
(13) Coco, S.; D´ıez-Expo´sito, F.; Espinet, P.; Ferna´ndez-Mayordomo, C.;
Mart´ın-AÄ lvarez, J. M.; Levelut, A. M. Chem. Mater. 1996, 8, 3666-
3671.
Experimental Section
(14) Morale, F.; Date, R. W.; Guillon, D.; Bruce, D. W.; Finn, R. L.;
Wilson, C.; Blake, A. J.; Schro¨der, M.; Donnio, B. Chem.sEur. J.
2003, 9, 2484-2501.
Materials. The preparation of the metal complexes was carried
out at room temperature under argon using standard Schlenk
techniques. Pyrazole ligands L1-L627 and [Rh(µ-Cl)(CO)2]235 were
prepared according to literature methods. Dichloromethane was
dried over calcium hydride and distilled under argon prior to use.
Deuterated chloroform for NMR experiments was stirred with
sodium carbonate and filtered prior to use.
Instrumentation. Elemental analysis was performed with a
Perkin-Elmer 240C microanalyzer. IR spectra were measured on a
Nicolet Avatar FTIR instrument using KBr pellets. 1H NMR spectra
were recorded on Bruker ARX 300 or Avance 400 spectrometers.
Mass spectra were obtained with a VG Autospec spectrometer with
positive ion FAB-LSIMS using nitrobenzyl alcohol as matrix.
Optical absorption spectra were recorded with a UV4-200 UV-
vis spectrophotometer from ATI-Unicam. Mesomorphic behavior
and transition temperatures were determined using an Olympus
BH-2 polarizing microscope equipped with a Linkam THMS 600
hot-stage central processor and a CS196 cooling system. Differential
scanning calorimetry (DSC) was carried out using a DSC 2910
from TA Instruments with samples sealed in aluminum pans and a
scanning rate of 10 °C/min under a nitrogen atmosphere. The
apparatus was calibrated with indium (156.6 °C, 28.4 J g-1) as the
(15) Coco, S.; Espinet, P.; Mart´ın-AÄ lvarez, J. M.; Levelut, A. M. J. Mater.
Chem. 1997, 7, 19-23.
(16) Barberio, G.; Bellusci, A.; Crispini, A.; Ghedini, M.; Golemme, A.;
Prus, P.; Pucci, D. Eur. J. Inorg. Chem. 2005, 181-188.
(17) Serrette, A. G.; Swager, T. M. J. Am. Chem. Soc. 1993, 115, 8879-
8880.
(18) Zheng, H.; Lai, C. K.; Swager, T. M. Chem. Mater. 1994, 6, 101-
103.
(19) Atencio, R.; Barbera´, J.; Cativiela, C.; Lahoz, F. J.; Serrano, J. L.;
Zurbano, M. M. J. Am. Chem. Soc. 1994, 116, 11558-11559.
(20) Serrette, A. G.; Swager, T. M. Angew. Chem., Int. Ed. Engl. 1994,
33, 2342-2345.
(21) Cocker, T. M.; Bachman, R. E. Mol. Cryst. Liq. Cryst. 2004, 408,
1-19.
(22) Lai, C. K.; Pang, Y. S.; Tsai, C. H. J. Mater. Chem. 1998, 8, 2605-
2610.
(23) Terazzi, E.; Benech, J. M.; Rivera, J. P.; Bernardinelli, G.; Donnio,
B.; Guillon, D.; Piguet, C. Dalton Trans. 2003, 769-772.
(24) Barbera´, J.; Gime´nez, R.; Serrano, J. L. AdV. Mater. 1994, 6, 470-
472.
(25) Barbera´, J.; Elduque, A.; Gime´nez, R.; Oro, L. A.; Serrano, J. L.
Angew. Chem., Int. Ed. Engl. 1996, 35, 2832-2835.
(26) Barbera, J.; Elduque, A.; Gimenez, R.; Lahoz, F. J.; Lopez, J. A.;
Oro, L. A.; Serrano, J. L. Inorg. Chem. 1998, 37, 2960-2967.
(27) Barbera´, J.; Gime´nez, R.; Serrano, J. L. Chem. Mater. 2000, 12, 481-
489.
(28) Bruce, D. W.; Lalinde, E.; Styring, P.; Dunmur, D. A.; Maitlis, P. M.
J. Chem. Soc., Chem. Commun. 1986, 581-582.
(29) Esteruelas, M. A.; Sola, E.; Oro, L. A.; Ros, M. B.; Serrano, J. L.
Chem. Commun. 1989, 55-56.
(30) Esteruelas, M. A.; Sola, E.; Oro, L. A.; Ros, M. B.; Marcos, M.;
Serrano, J. L. J. Organomet. Chem. 1990, 387, 103-111.
(31) Bruce, D. W.; Dunmur, D. A.; Esteruelas, M. A.; Hunt, S. E.;
Lelagadec, R.; Maitlis, P. M.; Marsden, J. R.; Sola, E.; Stacey, J. M.
J. Mater. Chem. 1991, 1, 251-254.
(32) Adams, H.; Bailey, N. A.; Bruce, D. W.; Hudson, S. A.; Marsden, J.
R. Liq. Cryst. 1994, 16, 643-653.
(33) Torralba, M. C.; Cano, M.; Campo, J. A.; Heras, J. V.; Pinilla, E.;
Torres, M. R. J. Organomet. Chem. 2002, 654, 150-161.
(34) Barbera, J.; Elduque, A.; Gimenez, R.; Lahoz, F. J.; Lopez, J. A.;
Oro, L. A.; Serrano, J. L.; Villacampa, B.; Villalba, J. Inorg. Chem.
1999, 38, 3085-3092.
(35) Hieber, W.; Lagally, H. Z. Anorg. Allg. Chem. 1943, 251, 96.
10364 Inorganic Chemistry, Vol. 45, No. 25, 2006

Structures and Mesomorphism of Rhodium(I) Complexes
Table 1. Crystallographic Data for L6 and 6‚CH2Cl2
standard. Powder X-ray diffraction experiments were carried out
using a Pinhole camera (Anton Paar) operating with a Ni-filtered
Cu KR beam. The sample was held in a Lindemann glass capillary
(1 mm diameter) and heated with a variable-temperature attachment.
The X-ray patterns were collected on flat photographic films.
General Procedure for the Synthesis of the Mononuclear
Rhodium Complexes cis-[RhCl(CO)2(Ln)], 1-6. To a solution
of [Rh(µ-Cl)(CO)2]2 (15.6 mg, 0.04 mmol) in dry dichloromethane
(10 mL) was added a solution of the appropriate pyrazole ligand
(0.08 mmol in 15 mL of dry dichloromethane). The reaction mixture
was stirred for 30 min, and the volume reduced to 0.5 mL. The
addition of methanol led to the precipitation of a yellow solid or a
yellow oil. The solids were purified by recrystallization from
methanol, and the oils were washed several times with methanol
and dried under vacuum. Oily samples crystallized as yellow solids
upon storage in the refrigerator for several days. Yields: 80-90%.
Compound 1. Found: C, 65.9; H, 8.9; N: 2.7. Calcd for C57H92-
ClN2O6Rh: C, 65.8; H, 8.9; N, 2.7%. νmax(KBr)/cm-1 3198, 3179,
3150, 3129 (N-H), 2083, 2069, 2016, 2005 (CO), 1608, 1516
(arC-C), 1266, 1244 (C-O). δΗ (300 MHz; CDCl3; Me4Si) 0.84-
0.88 (m, 12H, CH3-), 1.25-1.50 (m, 56H, -CH2-), 1.80-1.84
(m, 8H, -CH2-CH2-O-), 4.01-4.07 (m, 8H, -CH2-O-), 6.56
(s, 1H, He), 6.91 (d, J ) 8.5 Hz, 1H, Hh), 6.99 (d, J ) 8.5 Hz, 1H,
Hb), 7.09 (d, J ) 2.0 Hz, 1H, Hg), 7.11 (dd, J ) 8.5 Hz, J ) 2.0
Hz, 1H, Hd), 7.24 (m, 2H, Ha, Hc), 11.63 (s, 1H, Hf). δC (75 MHz;
CDCl3; Me4Si) 14.1 (CH3-), 22.6, 26.0, 29.1, 29.2, 29.3, 29.4, 29.5,
31.9 (-CH2-), 69.2, 69.5 (-CH2-O-), 102.8 (dCH- pyrazole),
111.3, 113.2, 113.5, 114.9, 118.9, 119.8 (Car-H), 122.4, 124.5 (Car-
pz), 145.5, 149.0, 149.6, 150.8, 157.1 (Car-O, Cpz). m/z (FAB+)
1004 [M - Cl]+.
L6
6‚CH2Cl2
crystal color, habit
crystal size, mm3
chem formula
fw
colorless, irregular block yellow, prismatic block
0.26 × 0.32 × 0.34
0.09 × 0.13 × 0.34
C23H24ClN2O8Rh‚CH2Cl2
679.73
C
21H24N2O6
400.42
cryst syst
space group
a (Å)
b (Å)
c (Å)
tetragonal
I41/a
21.2193(16)
21.2193(16)
18.4841(14)
90
8322.6(11)
16
1.278
monoclinic
P21/n
14.6971(14)
14.2494(14)
27.234(2)
94.254(3)
5687.8(9)
8
â (deg)
V (Å3)
Z
D
calcd (g cm-3
)
1.588
0.930
µ (mm-1
)
0.094
θ range data collec (deg) 1.90-25.25
1.50-25.25
33971
collected reflns
21937
unique reflns
3768 (Rint ) 0.0511)
3768/0/298
0.0396
0.0954
1.019
10235 (Rint ) 0.0756)
10235/0/697
0.0410
0.0824
0.957
data/restraints/params
R(F) [F2 > 2σ(F2)]a
wR(F2) [all data]b
Sc
a R(F) ) ∑||Fo| - |Fc||/∑|Fo|, for 2899 and 7464 observed reflections,
2 2
2
respectively. b wR(F2) ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo ) ]}1/2
.
c S ) [∑[w(Fo
2
- Fc )2]/(n - p)]1/2; n ) number of reflections, p ) number of parameters.
-CH2-O-), 4.07 (t, J ) 6.4 Hz, 4H, -CH2-O-), 4.24 (t, J )
7.2 Hz, 2H, -CH2-O-), 6.65 (d, J ) 2.4 Hz, 1H, He), 6.70 (d, J
) 8.8 Hz, 1H, Hb), 6.95 (s, 2H, Hd), 7.28 (d, J ) 8.8 Hz, 1H, Ha),
12.34 (d, J ) 2.4 Hz, 1H, Hf). δC (75 MHz; CDCl3; Me4Si) 14.1
(CH3-), 22.7, 26.0, 26.1, 29.3, 29.4, 29.4, 29.5, 29.6, 29.7, 29.8,
30.1, 30.3, 31.9 (-CH2-), 68.9, 69.4, 73.6, 73.9, 75.0 (-CH2-
O-), 102.6 (dCH- pyrazole), 108.4, 108.5, 113.6 (Car-H), 122.5,
127.2 (Car-pz), 139.8, 141.8, 143.2, 150.9, 153.3, 155.0, 155.9 (Car-
O, Cpz). m/z (FAB+) 1317 [M - Cl]+.
Compound 5. Found: C, 68.4; H, 9.8; N, 2.05. Calcd for
C77H132ClN2O8Rh: C, 68.4; H, 9.8; N, 2.1%. νmax(KBr)/cm-1 3187,
3162, 3129 (N-H), 2084, 2078, 1997 (CO), 1592, 1504 (arC-C),
1245 (C-O). δΗ (300 MHz; CDCl3; Me4Si) 0.83-0.97 (m, 18H,
CH3-), 1.26-1.48 (m, 84H, -CH2-), 1.71-1.87 (m, 12H, -CH2-
CH2-O-), 3.99-4.08 (m, 12H, -CH2-O-), 6.59 (s, 1H, He),
6.70 (s, 2H, Hd), 6.94 (s, 2H, Ha), 11.77 (s, 1H, Hf). δC (75 MHz;
CDCl3; Me4Si) 14.1 (CH3-), 22.7, 26.1, 29.4, 29.4, 29.6, 29.6,
29.7, 29.7, 30.3, 31.9, 31.9 (-CH2-), 69.4, 73.6 (-CH2-O-),
103.3 (dCH- pyrazole), 104.6, 108.2 (Car-H), 122.1, 126.7 (Car-
pz), 139.9, 153.3, 153.8 (Car-O, Cpz). m/z (FAB+) 1317 [M -
Cl]+.
Compound 2. Found: C, 67.3; H, 9.4; N, 2.3. Calcd for C67H112
-
ClN2O7Rh: C, 67.3; H, 9.4; N, 2.3%. νmax(KBr)/cm-1 3197, 3172,
3128 (N-H), 2087, 2074, 2003 (CO), 1591, 1506 (arC-C), 1262
(C-O). δΗ (300 MHz; CDCl3; Me4Si) (see also Figure 2) 0.88 (m,
15H, CH3-), 1.27-1.45 (m, 60H, -CH2-), 1.45-1.55 (m, 10H,
-CH2-), 1.73-1.84 (m, 10H, -CH2-CH2-O-), 3.98-4.09 (m,
10H, -CH2-O-), 6.59 (s, 1H, He), 6.71 (s, 0.8H, Hd), 6.9 (s,
1.2H, Hd′), 6.94 (d, J ) 8.4 Hz, 0.6H, Hb′), 6.97 (d, J ) 8.4 Hz,
0.4H, Hb), 7.03 (d, J ) 2.0 Hz, 0.6H, Hc′), 7.13 (dd, J ) 8.4 Hz,
J ) 2 Hz, 0.6H, Ha′), 7.26 (m, 0.8H, Ha, Hc), 11.62 (s, 0.4H, Hf),
11.70 (s, 0.6H, Hf′). δC (75 MHz; CDCl3; Me4Si, partial) 14.1
(CH3-), 22.7, 25.9, 26.1, 28.8, 28.9, 29.0, 29.1, 29.2, 29.3, 29.4,
29.6, 29.6, 29.7, 29.9, 29.9, 31.9 (-CH2-), 69.2, 69.2, 69.4, 69.6,
73.6 (-CH2-O-), 104.6 (dCH- pyrazole), 108.3, 111.3, 113.3,
113.6, 119.0 (Car-H), 141.5, 148.3, 151.0, 153.3, 153.3, 153.8,
153.8 (Car-O, Cpz). m/z (FAB+) 1161 [M - Cl]+.
Compound 6. Found: C, 46.5; H, 4.1; N, 4.7. Calcd for C23H24-
ClN2O8Rh: C, 46.4; H, 4.1; N, 4.7%. νmax(KBr)/cm-1 3186, 3153,
3129 (NH), 2093, 2084, 2011, 2001 (CO), 1591, 1572, 1503 (arC-
C), 1244 (C-O). δΗ (300 MHz; CDCl3; Me4Si) 3.89 (m, 18H,
CH3O-), 6.61 (s, 1H, He), 6.77 (br s, 2H, Hd), 7.11 (br s, 2H,
Ha), 12.18 (s, 1H, Hf). δC (75 MHz; CDCl3; Me4Si) 56.4 (CH3O-
), 61.0 (CH3O-), 102.8 (dCH- pyrazole), 103.2 (br, Car-H), 106.2
(br, Car-H), 122.3 (br, Car-pz), 126.4 (br, Car-pz), 139.5 (Car-O),
153.4 (br, Car-O, Cpz). m/z (FAB+) 559 [M - Cl]+, 531 [M - Cl
- CO]+, 501 [M - Cl - 2CO]+.
Crystal Structure Determination for L6 and 6‚CH2Cl2. A
summary of crystal data and refinement parameters is given in Table
1. Single-crystal X-ray diffraction data were collected on a Bruker
SMART diffractometer equipped with an APEX CCD detector, with
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å), using
the ω scan method. The crystals were covered with inert oil,
mounted on glass fibers and cooled to the data collection temper-
ature (100 K).
Compound 3. Found: C, 70.9; H, 10.3; N: 1.9. Calcd for
C87H152ClN2O7Rh: C, 70.8; H, 10.4; N, 1.9%. νmax(KBr)/cm-1
3153, 3120 (N-H), 2090, 2078, 2016 (CO), 1607, 1506 (arC-C),
1262 (C-O). δΗ (300 MHz; CDCl3; Me4Si) (see also Figure 2)
0.88 (m, 15H, CH3-), 1.27-1.45 (m, 60H, -CH2-), 1.45-1.55
(m, 10H, -CH2-), 1.73-1.84 (m, 10H, -CH2-CH2-O-), 3.98-
4.09 (m, 10H, -CH2-O-), 6.59 (s, 1H, He), 6.71 (s, 0.8H, Hd),
6.9 (s, 1.2H, Hd′), 6.94-6.98 (m, J ) 8.4 Hz, 1H, Hb, Hb′), 7.03
(d, J ) 2.0 Hz, 0.6H, Hc′), 7.13 (dd, J ) 8.4 Hz, J ) 2 Hz, 0.6H,
Ha′), 7.30 (m, 0.8H, Ha, Hc), 11.68 (s, 0.4H, Hf), 11.73 (s, 0.6H,
Hf′). m/z (FAB+) 1440 [M - Cl]+.
Compound 4. Found: C, 68.4; H, 9.8; N, 2.1. Calcd for C77H132
-
ClN2O8Rh: C, 68.4; H, 9.8; N, 2.1%. νmax(KBr)/cm-1 3405, 3300
(NH), 2078, 2009 (CO), 1601, 1584, 1488 (arC-C), 1240, 1115
(C-O). δΗ (300 MHz; CDCl3; Me4Si) 0.86-0.90 (m, 18H, CH3-
), 1.28-1.40 (m, 72H, -CH2-), 1.34-1.51 (m, 12H, -CH2-),
1.74-1.89 (m, 12H, -CH2-CH2-O-), 3.98-4.02 (m, 6H,
Inorganic Chemistry, Vol. 45, No. 25, 2006 10365

Gime´nez et al.
Data were corrected for Lorentz and polarization effects and also
for absorption using SADABS.36 The organic structure (L6) was
solved by direct methods using SIR9737 and the rhodium complex
by the Patterson method with SHELXS-97.38 Both of these
structures were completed with Fourier techniques and refined by
full-matrix least squares on F2 with SHELXL-97.38 Atomic scat-
tering factors, corrected for anomalous dispersion, were used as
implemented in the refinement program. Anisotropic displacement
parameters were used in the last cycles of refinement for all non-
hydrogen atoms. Hydrogen atoms were placed in calculated
positions, except those bonded to aromatic carbons and the nitrogen
atom in compound L6, which were located in a difference Fourier
map; all of them were refined riding on bonded atoms. Largest
peak and hole in the final difference maps were 0.198 and -0.163
e Å-3 for L6, 0.944 and -0.620 e Å-3 for 6‚CH2Cl2. The program
ORTEP-3,39 in the WINGX package,40 was used for diagrams.
Results and Discussion
Synthesis and Characterization of the Complexes in
Solution. The rhodium complexes were readily obtained by
reaction of the corresponding pyrazole ligands with [Rh(µ-
Cl)(CO)2]2 in a 2:1 molar ratio. All complexes are air-stable
materials and were isolated from the reaction media as yellow
oils or pale-yellow waxy solids. Oils solidified upon storage
in the refrigerator for several days. In solution these
complexes are slightly unstable and decompose after several
hours to give dark brown solutions. The analytical data and
MS, IR, and NMR spectra are consistent with the proposed
formulas.
Figure 2. (a) Isomers found for 2. The same proportion was found for 3
(with the same substitution in the phenyl rings but with C14 alkoxy chains).
1
The H NMR signals for the aromatic protons have been assigned in the
Experimental Section according to these numbering schemes. (b) Structure
of 4.
Compounds 2 and 3 were obtained as mixtures of two
isomers in a 3:2 ratio, as evidenced by the 1H NMR spectra.
This is a consequence of the nonsymmetrical substitution
pattern of the ligand. Comparison of the spectra of the
complexes, and also results of COSY and NOESY experi-
ments, shows that in the major isomer the metal atom is
situated next to the less-substituted aromatic ring (see Figure
2a). Two broad singlets for the NH proton can also be
observed, and these are in the same ratio as found for the
aromatic protons.
1
The aromatic proton signals in the H NMR spectra of
samples in CDCl3 are generally shifted upfield on complex-
ation. Significant changes and the opposite trend are found
for the NH signal. In the pyrazole ligands, the NH signal
appears as an extremely broad signal at around 11 ppm due
to fast interchange, and this signal is barely visible. A broad
singlet in the region 11.5-12.3 ppm is clearly detected for
the complexes. In all cases, resonances for the protons of
the phenyl ring near the metal (Figure 2, Hd, Hg) are highly
shielded and appear at lower chemical shifts than the
corresponding protons near to the NH group (Figure 2, Ha,
Hc).
Compound 6 displays two broad singlets for the resonances
of the aromatic protons at room temperature, suggesting the
existence of a metallotropic equilibrium (dynamic character
resulting from site exchange of the metal between adjacent
nitrogen atoms). However, the other complexes display sharp
signals at room temperature, indicating that dynamic pro-
cesses do not take place.
Compound 4, although containing a nonsymmetrically
substituted pyrazole, is formed as only one isomer. Therefore,
metal complexation is influenced by steric effects imposed
by the ligand. It appears that the rhodium atom only reacts
with the less congested nitrogen atom, and in the case of
compound 4, this is next to the 2,3,4-tridecyloxyphenyl ring,
assuming a conformation in which the decyloxy chain in the
ortho position is far from the coordination site (Figure 2b).
1
It can also be seen from the H NMR spectrum an unusual
4J coupling between the N-H and the C-H of the pyrazole,
with both signals appearing as doublets with a coupling
constant of 2.4 Hz. This is indicative of a slow proton
interchange involving the NH in chloroform solution.
The IR spectra (KBr pellets) display sets of two, three, or
four sharp bands in the 2090-1995 cm-1 region correspond-
ing to the carbonyl stretching of mononuclear cis-dicarbo-
nylrhodium(I) complexes. The pattern depends on the nature
of the compound and also on the phase or state (see
Experimental Section). Compound 6 was studied at different
temperatures, and it was found that in the crystalline state
at room temperature there are four bands for the carbonyl
stretching vibration. These bands appear at 2093, 2084, 2011,
and 2001 cm-1 and appear as two split absorptions of similar
intensity. The splitting is attributed to solid-state effects as
In particular, the long-chained compound 5 displays
aromatic signals at slightly higher fields than the methoxy
compound 6, probably due to the shielding caused by folding
of the decyloxy chains around the core.
(36) Sheldrick, G. M. SADABS, 2.03 ed.; University of Go¨ttingen:
Go¨ttingen, Germany, 2002.
(37) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
(38) Sheldrick, G. M. SHELXS-97, release 97-2; Institu¨t fu¨r Anorganische
Chemie der Universita¨t: Go¨ttingen, Germany, 1998.
(39) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
(40) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
10366 Inorganic Chemistry, Vol. 45, No. 25, 2006

Structures and Mesomorphism of Rhodium(I) Complexes
Table 2. Selected Bond Distances (Å) and Angles (deg) for 6a
Rh(1)-Cl(1)
2.3687(10);
2.3621(10)
Rh(1)-N(1)
2.106(3); 2.099(3)
Rh(1)-C(22)
C(22)-O(7)
N(1)-N(2)
N(2)-C(3)
1.849(5); 1.853(5)
1.133(5); 1.134(5)
1.360(4); 1.347(4)
1.353(4); 1.356(4)
Rh(1)-C(23)
C(23)-O(8)
N(1)-C(1)
1.843(4); 1.845(4)
1.144(4); 1.143(5)
1.349(5); 1.356(4)
Cl(1)-Rh(1)- 92.20(9); 88.72(8)
N(1)
Cl(1)-Rh(1)- 87.24(13); 92.06(13) N(1)-Rh(1)-
Cl(1)-Rh(1)- 175.47(13);
C(22) 178.97(13)
89.32(14);
90.42(15)
C(22)-Rh(1)- 91.25(18);
C(23)
N(1)-Rh(1)-
C(23)
C(22)
179.42(16);
179.10(15)
C(23)
88.81(17)
Figure 3. Molecular structure drawing for L6. ORTEP representation (50%
probability ellipsoids) with atom numbering scheme. Selected bond distances
(Å): N(1)-N(2) 1.354(2), N(1)-C(1) 1.342(2), N(2)-C(3) 1.354(2), C(1)-
C(2) 1.397(2), C(2)-C(3) 1.382(2).
Rh(1)-C(22)- 175.7(4); 178.5(4)
O(7)
Rh(1)-C(23)- 179.7(4);
O(8)
177.4(4)
a The two values correspond to the two independent molecules.
3-methyl-5-phenylpyrazole.43,44 Each molecule interacts with
both perpendicular molecules through two hydrogen bonds
involving the acceptor and donor nitrogens of the pyrazole
unit. In this way, two parallel molecules are situated
perpendicular to the other two parallel molecules (Figure 5).
In this compound, the steric hindrance introduced by the
methoxy groups in positions 3 and 5 of the phenyl groups
precludes the formation of planar dimeric structures as in,
for instance, 3,5-diphenyl-4-bromopyrazole41 or 3,5-bis(4-
butoxyphenyl)pyrazole,33 and the molecule adopts a globular
organization that maintains the intermolecular H-bonding
interactions. The supramolecular (-N-N-H-)4 ring has a
solvent accessible void or internal free volume of 53(3) Å3
(calculated with the VOID45 procedure as implemented in
the PLATON46 program). Stacking along the c axis gives
rise to a channel-like supramolecular structure.
Figure 4. Molecular structure drawing for one of the two independent
molecules of 6. ORTEP representation (50% probability ellipsoids) with
atom numbering scheme.
On the other hand, rhodium complex 6 crystallizes with
two crystallographically independent molecules that have
similar conformations. In this case, a parallel molecular
dimeric structure is obtained, and in each molecule the
coordination plane of the metal is rotated with respect to
the pyrazole ring [66.50(11)° and 60.58(11)° for the two
independent molecules]. The dimeric structure is formed
through two weak H-bonds between the chloro atom of one
molecule and the NH group of another one situated below
(Figure 6). In the dimer, the Rh-Rh distance is 3.4456(5)
Å. These parallel dimeric structures are arranged alternately
in an antiparallel disposition and stacked along the b axis
(Figure 7). The structure described here is rather unusual
since in the previously studied [RhCl(CO)2(Hpz)] complexes
the metal plane is coplanar to the pyrazole ring, showing
NH‚‚‚Cl intramolecular interactions, or almost coplanar (ca.
16°) to the pyrazole ring without H-bonding.47-49 In com-
pound 6, the congested pyrazole structure and its tendency
this phenomenon disappears at temperatures above the
melting point. Indeed, in the isotropic liquid state (at 167
°C) there are two broad bands at 2080 and 2011 cm-1. Weak
N-H stretching bands are also found in the region 3200-
3120 cm-1, and these correspond to hydrogen bonded
nitrogens. These bands are slightly shifted to higher wave-
numbers on increasing the temperature, indicating a weakn-
ing of the hydrogen bonds in the isotropic liquid.
Single-Crystal Structure of Compound 6 and Its Pyra-
zole Ligand L6. Yellow prismatic single crystals of com-
pound 6 were obtained by vapor diffusion of hexane into a
solution of the compound in dichloromethane. L6 was
obtained as colorless plates from slow evaporation of a
dichloromethane solution. Molecular drawings of these
structures are shown in Figures 3 and 4. Selected bond
distances for L6 are included in Figure 3, and selected bond
distances and angles for 6 are listed in Table 2.
In the crystal state, the pyrazole L6 has a tetrameric
structure supported by H-bonds, which form a 12-membered
ring (-N-N-H-)4. The internal symmetry of this tetramer
is S4 due to a crystallographic 4-fold inversion axis. Some
other tetrameric structures, albeit with lower internal sym-
metry, have been described for 3,5-diphenylpyrazole41,42 and
(42) Raptis, R. G.; Staples, R. J.; King, C.; Fackler, J. P., Jr. Acta
Crystallogr. 1993, C49, 1716.
(43) Maslen, E. N.; Cannon, J. R.; White, A. H.; Willis, A. C. J. Chem.
Soc., Perkin 2 1974, 1298.
(44) Moore, F. H.; White, A. H.; Willis, A. C. J. Chem. Soc., Perkin 2
1975, 1068.
(45) van der Sluis, P.; Spek, A. L. Acta Crystallogr. 1990, A46, 194-201.
(46) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7-13.
(47) Decker, M. J.; Fjeldsted, D. O. K.; Stobart, S. R.; Zaworotko, M. J.
Chem. Commun. 1983, 1525-1527.
(48) Torralba, M. C.; Cano, M.; Campo, J. A.; Heras, J. V.; Pinilla, E.;
Torres, M. R. J. Organomet. Chem. 2001, 633, 91-104.
(41) Aguilar-Parrilla, F.; Scherer, G.; Limbach, H.-H.; Foces-Foces, M.
C.; Herna´ndez Cano, F.; Smith, J. A. S.; Toiron, C.; Elguero, J. J.
Am. Chem. Soc. 1992, 114, 9657-9659.
Inorganic Chemistry, Vol. 45, No. 25, 2006 10367

Gime´nez et al.
Figure 6. Dimeric structure of 6 held by double intermolecular H-bonds.
Figure 7. Packing of 6 along the b axis.
Table 3. Transition Temperatures and Thermodynamic Data for the
DSC First Heating Cycle at 10 °C/min
Figure 5. Tetrameric structure of ligand L6. (a) View along the c axis.
(b) Detail of the intermolecular H-bonding pattern (G ) 3,4,5-trimethox-
yphenyl).
compd
phase transitiona
T, °C
∆H, kJ mol-1
1
2
Cr-I
91.3
40.5
50.0
40.4
27.7
31.4
46.9
45.7
5.4
72.23
22.14
42.89
96.60
50.38
45.96
2.08
-2.42
-48.75
22.42
Cr-Cr′
Cr′-I
to stack may be responsible for the twisting of the metal
plane in a staggered orientation with respect to the closest
molecule to establish a double intermolecular NH‚‚‚Cl bond.
This tendency for stacking in 6 is more favorable than the
structure exhibited by ligand L6 in terms of forming a
columnar mesophase.
Thermal and Mesomorphic Properties. Pyrazole ligands
L1-L6 are not liquid crystalline27 although they could give
rise to disk-shaped dimeric entities through hydrogen bond-
ing, in a similar way to 3,5-bis(4-alkoxyphenyl)pyrazoles.33
DSC data corresponding to the first heating cycle for
compounds 1-6 are gathered in Table 3.
3
4
5
Cr-I
Cr-I
Cr-Colh
Colh-I
I-Colh
Colh-Cr
Cr-I
6
147.9
a Cr, Cr′, crystal phase; I, isotropic liquid phase; Colh, columnar
mesophase.
melting points decrease on increasing the number of aliphatic
chains in the structure. Compound 1, with four decyloxy
chains, is a crystalline solid that melts directly to the isotropic
liquid. An increase in the number of chains to five (com-
pound 2), elongation of the chain (compound 3), or introduc-
tion of a six-chained ligand with reduced symmetry and steric
hindrance (compound 4) all gave rise to lower melting points
and the absence of liquid crystal behavior.
It can be observed that the peripheral chain structure plays
a crucial role in inducing mesomorphism. Compounds 1-4
are not liquid crystalline but are solids at room temperature,
melting to the isotropic liquid with high enthalpies. The
It is interesting to note that compounds 2 and 4, although
they are not liquid crystalline, do not crystallize on cooling
(49) Cano, M.; Campo, J. A.; Heras, J. V.; Lafuente, J.; Rivas, C.; Pinilla,
E. Polyhedron 1995, 14, 1139-1147.
10368 Inorganic Chemistry, Vol. 45, No. 25, 2006

Structures and Mesomorphism of Rhodium(I) Complexes
hexagonal columnar assembly of molecules stacked 3.6 Å
apart. The presence of scattering at 7.2 Å in the meridian
region reveals the existence of an additional modulation of
the electronic density along the column axis. A distance of
7.2 Å corresponds to twice 3.6 Å, and it can therefore be
deduced that the molecular orientation alternates along the
column axis in an antiparallel way. The value of 7.2 Å
represents the distance between two equally oriented mol-
ecules.
High-temperature X-ray diffractograms recorded at 40 °C
are very similar to those recorded at room temperature.
Although oriented samples were not investigated, the patterns
contained the same maxima as those described above. The
difference is that the scattering maxima at 7.2 and 3.6 Å are
weaker and more diffuse, which suggests a shorter correlation
length of the stacking.
Figure 8. (a) Microphotograph of the texture of the columnar mesophase
exhibited by compound 5 at 41 °C (crossed polarizers). (b) Proposed model
for the columnar arrangement of 5.
and remain as isotropic liquids at room temperature before
becoming glasses at 14 and 0 °C, respectively. These glasses
only crystallize over time on storing the samples in the
refrigerator.
The structure proposed for 5 is promoted by the molecular
shape, which resembles a half disk. Such molecules stacked
in an alternating antiparallel way are able to generate columns
with an almost circular cross section, consisting of an inner
part containing the rhodium atoms and the aromatic rings
and a peripheral part consisting of the disordered hydrocarbon
chains uniformly spread around the inner core (Figure 8b).
Indeed, in the antiparallel mode dipolar forces are satisfied.
A similar type of organization with analogous lattice
constants (a ) 27 Å) has been found in the hexagonal
columnar mesophases of some nondiscoid metal-containing
and metal-free molecules with six decyloxy chains, which
are joined together through interactions that involve the metal
atom7 or dipole-dipole forces.50 The column diameter is
smaller than twice the molecular radius, indicating that slight
interdigitation exists between antiparallel neighbors.
In the columnar mesophase, it is very probable that NH‚
‚‚Cl interactions exist, as the IR N-H stretching frequencies
of compound 5 in the mesophase are very similar to those
found in the crystal of 6 (several bands between 3187 and
3129 cm-1). However, it is not possible to clarify whether
the H-bonding inside the columns is intermolecular or
intramolecular in origin, as both would be consistent with
the presence of stacked half-disks in an antiparallel arrange-
ment in the mesophase. Therefore, the induction of meso-
morphism in nondiscoid cis-[RhCl(CO)2(Ln)] complexes is
the result of a subtle combination of an appropriately
substituted pyrazole, the planar arrangement of the pyrazole
ligands in the complex, and the dipolar and/or hydrogen
bonding interactions.
As one would expect, compound 6, which does not contain
long alkoxy chains, is not mesomorphic and has the highest
melting point. On cooling, this material does not crystallize
but becomes a glass at 80 °C and remains in this state over
time.
Only complex 5 (with six decyloxy chains) is a liquid
crystal. This compound shows an enantiotropic columnar
mesophase in the range 31-47 °C before clearing to the
isotropic liquid. On cooling, the mesophase reforms and
remains down to 5 °C, when crystallization occurs. This
mesophase shows a characteristic pseudo-focal-conic texture
with large homeotropic areas (Figure 8a). The application
of mechanical shear to the sample gives rise to a uniformly
oriented birefringent texture.
In summary, induction of mesomorphism exclusively
occurs in the case of compound 5. The symmetrical pyrazole
ligand with six decyloxy groups does not show mesogenic
behavior, but the corresponding rhodium complex shows a
columnar hexagonal mesophase that is stable at room
temperature on cooling.
In an effort to determine the structure for the mesophase
of 5, this compound was studied by X-ray diffraction on an
aligned sample obtained by mechanical stress at the me-
sophase temperature followed by freezing to room temper-
ature.
The diffraction pattern is characteristic of a hexagonal
columnar (Colh) mesophase with a lattice constant a ) 27
Å, as deduced form the d10 reflection (23.5 Å). As this
reflection is located in the equator (plane perpendicular to
the stress direction), the column axes are oriented parallel
to the stress direction and the two-dimensional hexagonal
lattice lies in the perpendicular plane.
In the large-angle region, an almost isotropic diffuse halo,
characteristic of the disordered hydrocarbon chains, is
observed at 4.6 Å. In addition, the diffraction pattern contains
scattering bands perpendicular to the meridian corresponding
to distances of 7.2 and 3.6 Å. The maximum at 3.6 Å
corresponds to the stacking distance, and a combination of
this value, the hexagonal lattice constant a, and the molecular
mass (1352.24) allowed a density of 0.99 g cm-3 to be
calculated, assuming that the mesophase of 5 consists of a
Spectroscopic Study of the Phase Transitions of Com-
pound 5. Compound 5 exhibits marked changes in the phase
transition from the crystalline state to the liquid crystal phase.
This transition displays thermochromism that can be easily
followed by the naked eye and is associated with changes
in both the UV-vis (Figure 9) and IR (Figure 10) spectra.
Compound 5 in the crystalline state is a pale yellow solid
and, at room temperature, shows a broad absorption band
with a maximum at 277 nm and two shoulders at 305 and
(50) Omenat, A.; Barbera´, J.; Serrano, J. L.; Houbrechts, S.; Persoons, A.
AdV. Mater. 1999, 11, 1292-1295.
Inorganic Chemistry, Vol. 45, No. 25, 2006 10369

Gime´nez et al.
the metal coordination plane and the intermolecular interac-
tions in the crystalline state. Indeed, several different
structures have been obtained for such complexes, and these
include planar molecules with intramolecular NH-Cl bonds
packed without metallic interactions, or unidimensional
stacks of molecules with an out-of-plane angle for the
pyrazole ligand of 16° and some degree of intermetallic
interaction.48 The low-frequency carbonyl stretching band
for the former example appears at higher wavenumbers than
for the latter. Therefore, the position of the carbonyl bands
can be related not only to the degree of metal-carbonyl back-
bonding and the metal valence but also to the conformation
of the pyrazole ligand with respect to the metal coordination
plane.
On the basis of a combination of the data found for these
previously studied structures, the presence or absence of the
intramolecular charge-transfer band, and the shifting of the
carbonyl stretching band, we propose a more planar confor-
mation in the mesophase than in the crystal state for
compound 5.
Figure 9. Variable temperature UV-vis spectra of 5. Crystal phase at 25
°C (s). Columnar hexagonal mesophase at 42 °C (---). Isotropic liquid at
63 °C (- ‚ -).
Conclusions
The induction of mesomorphism in nondiscoid cis-[RhCl-
(CO)2(Hpz)] complexes 1-6 is the result of a combination
of several factors. The most important is the ligand-ligand
interaction, as only an appropriately substituted pyrazole with
six decyloxy chains arranged symmetrically leads to liquid
crystalline properties. Second, dipolar and hydrogen bonding
interactions involving the chloro and the NH groups of a
single molecule or neighboring stacked molecules are also
important.
Figure 10. Variable temperature FTIR carbonyl stretching bands of 5.
Crystal phase at 25 °C (s). Columnar hexagonal mesophase at 42 °C (---).
Isotropic liquid at 50 °C (- ‚ -).
The columnar liquid crystal phase and the isotropic liquid
are more similar to the isolated molecule, behaving as
monomeric entities or systems with only weak intermolecular
interactions. The spectroscopic data found in the mesophase
are consistent with planar molecules exhibiting intramolecu-
lar NH-Cl bonds but packed without intermetallic interac-
tions. This situation is also in agreement with the proposed
structure from the X-ray experiments in the mesophase.
However, aggregates exist in the crystalline state, and the
low-energy intramolecular charge-transfer band disappears
completely on cooling from the mesophase to the crystal,
probably due to the pyrazole ligand being out of plane with
respect to the metal coordination plane.
330 nm. When the compound enters the liquid crystalline
phase, it changes to an intense yellow color, and in the UV-
vis spectrum the highest energy band shifts slightly to 280
nm, the shoulders disappear, and a new band at 400 nm
appears. The isotropic liquid shows the same spectrum as
the liquid crystalline phase. Moreover, these latter two spectra
are also similar to those observed in dilute solutions, although
in solution the low energy band is much weaker compared
with the rest of the bands.
On the other hand, the transition from crystal to liquid
crystal is associated with a shift of the carbonyl stretching
toward higher wavenumbers, indicating a decrease in the
π-back-bonding in the metal-carbonyl bonds.
We can therefore assume that H-aggregates exist in the
crystal, and on melting, they break and a new bathochro-
mically shifted band appears at 400 nm that is related to the
isolated molecule. This band at 400 nm can be assigned to
a metal-to-ligand charge transfer (MLCT) band (d-π*
character), which is also associated with stronger π-back-
bonding of the metal-pyrazole bond and weaker π-back-
bonding of the carbonyl-metal bond, as observed by IR
spectroscopy.
It is worth mentioning that, in previously reported cis-
[RhCl(CO)2(HPz)] complexes, different sets and wavenum-
bers for the carbonyl stretching bands are found depending
on the conformation of the pyrazole ligand with respect to
Acknowledgment. We thank the CICYT-FEDER
MAT2003-07806-C02-01, BQU2000/1170, and the Pro-
grama Ramo´n y Cajal from the Ministerio de Educacio´n y
Ciencia (MEC), Spain, and the Gobierno de Arago´n for
financial support.
Supporting Information Available: Crystallographic data for
L6 and 6 as a CIF file. Figure S1 showing the molecular structure
drawing for the two independent molecules of 6. Figure S2 showing
the XRD diffraction pattern of the mesophase of an aligned sample
of compound 5. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC0614216
10370 Inorganic Chemistry, Vol. 45, No. 25, 2006