Polymorphism and metal-metal interactions on [Rh(Cl)(CO)2(HpzR)] complexes

Article abstract of DOI:10.1016/S0022-328X(01)01058-0

The novel pyrazoles containing 3-[4-n-hexyloxyphenyl] (hp), 3-[4-n-octyloxyphenyl] (op) and 3-[4-n-decyloxyphenyl] (dp) substituents, Hpz^R (R=hp, op, dp; 1-3), and their corresponding Rh(I) compounds [Rh(Cl)(LL)(Hpz^R)] (LL=2,5-norbor

Full text of DOI:10.1016/S0022-328X(01)01058-0

Journal of Organometallic Chemistry 633 (2001) 91–104
www.elsevier.com/locate/jorganchem
Polymorphism and metal–metal interactions on
[Rh(Cl)(CO)2(HpzR)] complexes
M.C. Torralba a, M. Cano a,*, J.A. Campo a, J.V. Heras a, E. Pinilla a,b, M.R. Torres b
a Departamento de Qu´ımica Inorga´nica I, Facultad de Ciencias Qu´ımicas, Uni6ersidad Complutense, E-28040 Madrid, Spain
b Laboratorio de Difraccio´n de Rayos-X, Facultad de Ciencias Qu´ımicas, Uni6ersidad Complutense, E-28040 Madrid, Spain
Received 17 April 2001; accepted 1 June 2001
Abstract
The novel pyrazoles containing 3-[4-n-hexyloxyphenyl] (hp), 3-[4-n-octyloxyphenyl] (op) and 3-[4-n-decyloxyphenyl] (dp)
substituents, HpzR (R=hp, op, dp; 1–3), and their corresponding Rh(I) compounds [Rh(Cl)(LL)(HpzR)] (LL=2,5-norbornadi-
ene NBD, 1,5-cyclooctadiene COD, 2CO; R=hp, op, dp; 4–12) have been prepared and characterised. The influence of the
pyrazol substituent on the properties of the Rh(I) complexes has been analysed. Two crystalline polymorphs (yellow and red) have
been isolated for [Rh(Cl)(CO)2(Hpzdp)] (12) in contrast to the single red crystalline form isolated for the related complexes
[Rh(Cl)(CO)2(HpzR)] (R=hp, op; 10 and 11). X-ray crystal structures of the red forms of 10–12 as well as the yellow one of 12
have been solved. The red compounds display one-dimensional stacking of square-planar molecules with metal–metal interactions
along the c-axis. The yellow form consists of dimeric unities held together by Cl bridges, and without the one-dimensional
metal–metal contacts. In both cases an intramolecular hydrogen bond was present, being stronger in the yellow form than in the
red one. Thermochromic behaviour has also been observed for these dicarbonyl complexes. No liquid crystals properties were
found for the new compounds, however, the double melting behaviour observed for 10–12 could be closely related to
mesomorphism. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Long chain-substituted pyrazoles; Rhodium–pyrazol complexes; Crystalline polymorphism; One-dimensional stacking; Ther-
mochromism
1. Introduction
pyrazol derivative having liquid crystal properties [14].
This behaviour opens new spectatives for the use of
extended rod-like pyrazol ligands for generating related
metallomesogenic compounds.
The study of liquid crystal materials containing tran-
sition metals has increased in recent years [1–6]. In this
regard, the chemistry of complexes of the nickel triad
have been extensively developed and, comparatively in
less extension, also iridium and rhodium derivatives.
Specifically, the calamitic liquid crystal behaviour of
some rod-like complexes of type cis-[M(Cl)(CO)2(L)]
(M=Rh, Ir; L=N-donor ligand with extended long-
chain substituents) has been well established [3,5,7–13].
It is interesting to note that the complex [Rh(Cl)-
(CO)2(HpzMe2,4R)] (HpzMe2,4R=4-[(4%-((4¦-n-decyloxy-
benzoyl)oxy)phenoxy)carbonyl] - 3,5 - dimethylpyrazole)
has recently been described as the first organometallic
Previous work from this laboratory has dealt with
Rh(I) complexes of the type cis-[Rh(Cl)(CO)2(HpzR%,R¦)]
(R%=t-Bu, Ph, An, R¦=H; R%=t-Bu, Ph, R¦=Me)
containing bulky pyrazol ligands substituted in the 3(5)
or 3 and 5 positions and some interesting features have
been found depending on the substituents on the pyra-
zol ring [15]. Although in all cases the colour of com-
plexes in solution was orange–yellow, in the solid state
they were found to be blue–black, green–violet or
violet, this dark colour being associated with the pres-
ence of one-dimensional metal–metal interactions as a
consequence of the crystal packing of the square-planar
molecules. This arrangement was confirmed through
the crystalline structure of the green–violet compound
* Corresponding author. Fax: +34-91-3944352.
E-mail address: mmcano@eucmax.sim.ucm.es (M. Cano).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01058-0

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M.C. Torralba et al. / Journal of Organometallic Chemistry 633 (2001) 91–104
[Rh(Cl)(CO)2(HpzAn)] which revealed to an intermetal-
loxyphenyl)pyrazole (Hpzop) (2) and 3-(4-n-decyloxy-
phenyl)pyrazole (Hpzdp) (3), have been synthesised as
well as their corresponding complexes [Rh(Cl)(CO)2-
(HpzR)] (Scheme 1).
The aim of this work is to explore the structural
consequences that the changes of the length of the
chain of the n-alkoxyphenyl group, CnH2n+1O (n=6,
8, 10), on the pyrazole, would have on the complexes as
well as on their potential liquid crystal properties. On
this basis, the presence of different crystalline forms
and their X-ray structures are analysed and related with
the thermal behaviour of the complexes.
,
lic RhꢀRh distance (3.398(3) A) slightly shorter than
that previously found for [Rh(Cl)(CO)2(Hpz)]
,
(3.4522(4) A) which contains a non-substituted pyrazol
,
ligand [16], but 0.13 A longer than in [Rh(acac)(CO)2]
[17] which has been described as an intrinsic semicon-
ductor [18]. Additionally, the complex [Rh(Cl)(CO)2-
(Hpzbp)] (Hpzbp=3-[4-n-butoxyphenyl]pyrazole), which
was also described for us [19], presented the blue colour
characteristic of the previously mentioned metal–metal
interactions although no crystals of sufficient quality
for X-ray diffraction could be isolated in this case.
The molecular packing in the above complexes con-
sisted of columns defined by stacking of parallel square-
planar molecules which were alternatively staggered.
The molecules of contiguous columns were oriented in
opposite directions, and no short intermolecular con-
tacts between the columns were found. Considering this
arrangement it is possible that the introduction of long
substituents on the pyrazol groups could give rise to
new types of interactions between molecules of contigu-
ous columns as depicted in Fig. 1.
2. Experimental
2.1. Materials and physical measurements
All commercial reagents were used as supplied. Syn-
theses of the starting Rh-complexes [Rh(m-Cl)(LL)]2
(LL=2,5-norbornadiene NBD, 1,5-cyclooctadiene
COD) have been described previously [20,21]. The
other starting Rh-complex [Rh(m-Cl)(CO)2]2 was pur-
chased from Sigma–Aldrich and used as supplied.
The 4-n-hexyloxyacetophenone, 4-n-octyloxyace-
tophenone and 4-n-decyloxyacetophenone derivatives
were synthesised by alkylation of 4-hydroxyacetophe-
none with the corresponding n-alkyliodide in the ace-
By the extension of these previous results, towards
the design of Rh(I) complexes with liquid crystal prop-
erties, we have now prepared and studied complexes of
the type [Rh(Cl)(CO)2(HpzR)] having pyrazol ligands
with long-chain substituents at the 3(5) position. In
particular, three new substituted pyrazol ligands 3-(4-n-
hexyloxyphenyl)pyrazole (Hpzhp) (1), 3-(4-n-octy-
Fig. 1. Intermolecular Cl-bridge interactions (B) suggested on the basis of a molecular gliding (indicated by the arrows) from molecules of adjacent
columns (A).

M.C. Torralba et al. / Journal of Organometallic Chemistry 633 (2001) 91–104
93
samples as KBr pellets or in CH2Cl2 solution in the
4000–350 cm−1 region.
1H- and 13C{1H}-NMR spectra were performed on a
Varian VXR-300 or on a Bruker AM-300 spectropho-
tometers of the NMR Service of the Complutense
University from solutions in CDCl3. Chemical shifts l
are listed in ppm relative to Me4Si using the signal of
the deuterated solvent as reference, and coupling con-
stants J are in Hz. Multiplicities are indicated as s
(singlet), d (doublet), t (triplet), q (quintuplet), m (mul-
tiplet) and br s (broad signal). The atomic numbering
used in the assignment of the NMR signals is shown in
1
Scheme 1. The H- and 13C-NMR chemical shifts are
accurate to 0.01 and 0.1 ppm, respectively, and cou-
1
pling constants are accurate to 0.3 Hz for H-NMR
spectra.
The electronic spectra were registered on a CARY
5G spectrophotometer with solutions ca. 10−4 M of the
complexes in CH2Cl2.
Phase studies were carried out by optical microscopy
using an Olympus BX50 microscope equipped with a
Linkam THMS 600 heating stage. The temperatures
were assigned on the basis of optic observations with
polarised light.
Measurements of the transition temperatures were
made using a Perkin–Elmer Pyris 1 differential scan-
ning calorimeter with the sample (2–6 mg) hermetically
sealed in aluminium pans and with a heating or cooling
rate of 10 °C min−1
.
2.2. Synthetic methods
2.2.1. Preparation of HpzR (R=hp, op, dp) (1–3)
To a solution of the corresponding substituted ace-
tophenone (9.42 mmol) in ethyl formate (18.84 mmol)
and toluene (20 ml) was added a slurry of anhydrous
sodium methoxide (9.42 mmol) in toluene (30 ml). The
reaction mixture was stirred for 24 h at room tempera-
ture (r.t.). The solid formed was isolated by filtration
and washed with CH2Cl2 and hexane.
The solid was dissolved in MeOH (30 ml) and hydra-
zine chlorhydrate (9.42 mmol) in water (10 ml) was
added very slowly. The solution was stirred for 12 h at
r.t., then the solvent was removed and CH2Cl2 was
added as the extracting solvent. The extracts were dried
Scheme 1.
tone solution of K2CO3 as previously described for
related compounds [22].
Elemental analyses for carbon, hydrogen and nitro-
gen were carried out by the Microanalytical Service of
the Complutense University. IR spectra were recorded
on a FTIR Nicolet Magna-550 spectrophotometer with
Table 1
IR data, isolated yield and elemental analyses of HpzR (R=hp, op, dp) (1–3)
a
IR (cm−1
)
Yield (%)
Molecular formula
Calculated (%)
Found (%)
C
w(NH)
w(CN)
C
H
N
H
N
1
2
3
3142
3142
3143
1615
1615
1614
53
30
50
C
15H20N2O
73.8
75.0
76.0
8.2
8.8
9.3
11.5
10.3
9.3
73.5
74.6
75.5
8.2
8.8
9.5
11.5
10.1
9.2
C17H24N2O
C19H28N2O
a In KBr discs.

94
M.C. Torralba et al. / Journal of Organometallic Chemistry 633 (2001) 91–104
Table 2
IR data, colour, isolated yield and elemental analyses of [Rh(Cl)(LL)(HpzR)] (LL=NBD, COD, 2CO; R=hp, op, dp) (4–12)
a
IR (cm−1
)
Colour
Yield (%)
Molecular formula
Calculated (%)
Found (%)
w(NH)
w(CN)
w(CO)
C
H
N
C
H
N
4
5
6
7
8
9
10
3291
3293
3297
3221
3227
3227
3326
1613
1613
1613
1614
1614
1615
1615
Yellow
Yellow
Yellow
Yellow
Yellow
Yellow
Red
60
82
75
63
55
67
60
C
22H28N2OClRh
55.6
57.3
58.8
56.3
57.9
59.3
46.5
5.9
6.4
6.8
6.5
6.9
7.3
4.6
5.9
5.6
5.3
5.7
5.4
5.1
6.4
55.3
57.3
58.6
56.5
57.5
59.5
46.9
5.9
6.5
6.7
6.5
6.8
7.5
4.6
5.9
5.7
5.3
5.8
5.5
5.2
6.5
C24H32N2OClRh
C26H36N2OClRh
C23H32N2OClRh
C25H36N2OClRh
C27H40N2OClRh
C17H20N2O3ClRh
2086
2061
2015
1995
2087
2060
2014
1995
2086
2060
2014
1995
2069
2007
11
3327
3329
3285
1615
1615
1611
Red
73
19
68
C19H24N2O3ClRh
C21H28N2O3ClRh
48.9
51.0
51.0
5.2
5.7
5.7
6.0
5.7
5.7
48.9
51.6
50.1
5.2
5.8
5.5
6.0
5.7
5.6
12-red
12-yellow
Red
Yellow
C21H28N2O3ClRh
a In KBr discs.
on Na2SO4 and filtered. The solution was concentrated
and a solid was obtained by precipitation with hexane.
Yields are given in Table 1.
(12-red) and yellow crystals of [Rh(Cl)(CO)2(Hpzdp)]
(12-yellow) were grown by layering CH2Cl2 solutions
with hexane. The crystals were mounted on a SMART
CCD-Bruker diffractometer with graphite monochro-
2.2.2. Preparation of [Rh(Cl)(LL)(HpzR)] (LL=NBD,
COD; R=hp, op, dp) (4–9)
,
mated Mo–Ka radiation (u=0.71073 A) operating at
50 kV and 20–25 mA. A summary of the fundamental
crystal and refinement data for the four structures is
given in Table 3.
To a solution of [Rh(m-Cl)(LL)]2 (0.10 mmol) in
CH2Cl2 (5 ml) was added the corresponding pyrazole
HpzR (0.20 mmol). The reaction mixture was stirred for
1–2 h, and then the volume was reduced in vacuo to
approximately 2 ml. Addition of hexane led to the
precipitation of a yellow solid, which was filtered off,
washed with hexane and dried in vacuo. Yields are
given in Table 2.
For 12-red a total of 5476 reflections were collected
over a quadrant of the reciprocal space by combination
of the two frame sets, and for 12-yellow a total of 7316
reflections were collected over an hemisphere of the
reciprocal space by combination of the three frame sets.
Each exposure of 30 s covered 0.3° in ꢀ. The first 50
frames were recollected at the end of the data collection
to monitor crystal decay, and no appreciable decay was
observed. The structure was solved by Patterson and
Fourier techniques and refined by full-matrix least-
squares on F2 [23]. Anisotropic parameters were used in
the last cycles of refinement for all non-hydrogen
atoms, except the terminal carbon atoms C20 and C21
in 12-red for which only two cycles of refinement were
used and the thermal anisotropical factors fixed. The
hydrogen atoms were included in calculated positions
and refined as riding on the respective carbon atom
with a common anisotropic displacement, except the
hydrogen atom H2 bonded to N2 which was located in
a Fourier synthesis, included and refined as riding on
the N2 atom or fixed for 12-red and 12-yellow, respec-
tively. Similar refinement to that of 12-red has been
carried out for 10 and 11.
2.2.3. Preparation of [Rh(Cl)(CO)2(HpzR)] (R=hp, op,
dp) (10–12)
To a solution of [Rh(m-Cl)(CO2)]2 (0.15 mmol) in
CH2Cl2 (2 ml) was added another of the corresponding
pyrazole HpzR (0.30 mmol) in the same solvent (6 ml).
The yellow solution was stirred for 2 h, and then the
volume was reduced in vacuo to approximately 2 ml.
Addition of hexane led to the precipitation of a yellow
or a red solid depending on the pyrazole. The solid was
filtered off, washed with hexane and dried in vacuo.
Yields are given in Table 2.
2.3. X-ray structure determination
Red prismatic crystals of [Rh(Cl)(CO)2(Hpzhp)] (10),
[Rh(Cl)(CO)2(Hpzop)] (11) and [Rh(Cl)(CO)2(Hpzdp)]

M.C. Torralba et al. / Journal of Organometallic Chemistry 633 (2001) 91–104
95
In agreement with the related aryl-substituted pyra-
zoles, two tautomeric forms, Hpz3R or Hpz5R, or a
tautomeric equilibrium could be expected for these
heterocycles [27]. However, the NMR spectra of 1–3
show two well-defined signals as doublets for the H4
and H5 protons, indicating the presence of only one
tautomer or a very fast equilibrium between the two
tautomeric forms on the NMR time-scale at room
temperature. On the other hand, the X-ray structure of
the related pyrazole bearing the 4-phenoxyphenyl sub-
stituent demonstrated that this heterocycle was isolated
as the 3-tautomer [19]. On this basis, we can suggest for
the new pyrazoles 1–3 that the 3-tautomer must be
present in both the solution and solid states.
The largest residual peaks in the final difference map
−3
,
were 0.33, 0.24, 0.40 and 0.34 e A
for 10, 11, 12-red
and 12-yellow, respectively, in the vicinity of the Rh
atom. The refinement converged to R values of 0.031,
0.029, 0.042 and 0.035 for 10, 11, 12-red and 12-yellow,
respectively.
3. Results and discussion
3.1. Synthetic and characterisation studies
3.1.1. Ligands HpzR (R=hp, op, dp) (1–3)
Compounds 1–3 were prepared by analogous proce-
dures to those described for other pyrazol ligands
[19,24–26]. The physical properties, IR data and ele-
mental analyses are given in Table 1. The 1H- and
13C-NMR data are summarised in Tables 4 and 5,
respectively. Assignment of the signals was made fol-
lowing the literature precedents [19,27–29].
3.1.2. Compounds [Rh(Cl)(LL)(HpzR)] (LL=NBD,
COD; R=hp, op, dp) (4–9)
Reactions of the dimers [Rh(m-Cl)(LL)]2 (LL=NBD,
COD) with HpzR (R=hp, op, dp) were performed in
dichloromethane in 1:2 molar ratios (Scheme 2). All the
Table 3
Crystal and refinement data for [Rh(Cl)(CO)2(HpzR)] (R=hp, op, dp) (10, 11, 12-red, 12-yellow)
10
17H20ClN2O3Rh
11
12-red
12-yellow
Empirical
formula
C
C19H24ClN2O3Rh
C21H28ClN2O3Rh
C21H28ClN2O3Rh
Formula weight 438.71
Temperature (K) 296(2)
466.76
296(2)
494.81
296(2)
494.81
296(2)
Crystal system
Space group
Unit cell dimensions
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/c
Triclinic
(
P1
,
a (A)
19.357(2)
13.930(2)
7.092(1)
21.315(2)
13.982(1)
7.070(1)
23.099(3)
14.021(2)
7.059(1)
7.6304(8)
10.019(1)
15.738(2)
82.273(2)
84.567(2)
71.782(2)
1130.7(2)
2
,
b (A)
,
c (A)
h (°)
i (°)
97.826(3)
97.721(2)
93.168(3)
k (°)
3
,
V (A )
1894.6(4)
4
1.538
1.058
2088.0(3)
4
1.485
0.965
2282.7(6)
4
1.440
0.887
Z
zcalc. (g cm−3
)
1.453
0.896
508
v (mm−1
F(000)
)
888
952
1016
Crystal size
(mm)
0.28×0.12×0.06
0.26×0.08×0.08
0.50×0.08×0.03
0.15×0.10×0.08
Scan technique
Data collected
ƒ and ꢀ
ƒ and ꢀ
ƒ and ꢀ
ƒ and ꢀ
(−8, −13, −6) to
(19, 13, 6)
1.81–20.81
4008
(−23, −15, −7) to
(23, 10, 7)
1.75–23.25
9179
(−23, −14, −7) to
(15, 14, 3)
1.70–21.96
5476
(−9, −12, −22) to
(10, 14, 19)
3.41–30.56
7316
q range (°)
Reflections
collected
Independent
reflections
Reflections
observed
1761 (Rint=0.07)
2990 (Rint=0.05)
2396 (Rint=0.08)
5395 (Rint=0.05)
1101
1778
1166
2566
[I]2|(I)]
a
R
0.031
0.063
0.029
0.066
0.042
0.080
0.035
0.070
b
RwF
a
ꢀ[ꢁFoꢁ−ꢁFcꢁ]/SꢁFoꢁ.
b (ꢀ[w(Fo2−Fc2)2]/ꢀ[w(Fo2)2])1/2
.

96
M.C. Torralba et al. / Journal of Organometallic Chemistry 633 (2001) 91–104
Table 4
1H-NMR data of the pyrazoles HpzR (R=hp, op. dp) (1–3) in CDCl3 at 298 K
Pyrazole
NH
ꢀC6H4ꢀ
Ho
ꢀOR
H4
H5
Hm
1
2
3
n.o.
6.51d
7.58d
7.63d
6.92d
CH2-1%: 3.97t
CH2-2%: 1.78q
CH2-3%,4%,5%: 1.5–1.1 m
CH3-6%: 0.89t
CH2-1%: 3.96t
CH2-2%: 1.78q
CH2-3%,4%,5%,6%,7%: 1.5–1.1 m
CH3-8%: 0.87t
CH2-1%: 3.96t
CH2-2%: 1.78q
J1%2%=6.5
J2%3%=7.8
J5%6%=6.8
J45=2.1
J
om=8.8
n.o.
n.o.
6.52d
45=2.1
7.58d
7.58d
7.63d
om=8.6
6.91d
6.92d
J1%2%=6.5
J2%3%=7.2
J7%8%=6.8
J
J
6.51d
45=2.1
7.63d
om=8.7
J1%2%=6.6
J2%3%=7.4
J9%10%=6.7
J
J
CH2-3%,4%,5%,6%,7%,8%9%: 1.5–1.1 m
CH3-10%: 0.86t
n.o.: not observed.
Table 5
13C{1H}-NMR data of the pyrazoles HpzR (R=hp, op. dp) (1–3) in CDCl3 at 298 K
Pyrazole carbons
ꢀC6H4ꢀ
Ch
ꢀOR
C3
C4
C5
Co
Cm
Cp
1
2
3
148.4
102.0
133.7
124.0
127.0
114.8
159.2
C1%: 68.0
C2%,3%,4%,5%: 31.5, 29.1, 25.6, 22.5
C6%: 13.9
C1%: 68.1
C2%,3%,4%,5%,6%,7%: 31.8, 29.3, 29.2, 29.1, 26.0, 22.6
C8%: 14.0
C1%: 68.1
148.4
148.4
102.1
102.1
133.7
133.7
124.0
124.0
127.1
127.1
114.8
114.8
159.3
159.3
C2%,3%,4%,5%,6%,7%,8%,9%: 31.9, 29.6, 29.4, 29.3, 29.2, 26.0, 22.7
C10%: 14.1
Scheme 2.
3.1.3. Compounds [Rh(Cl)(CO)2(HpzR)] (R=hp, op,
dp) (10–12)
complexes isolated are yellow air-stable solids charac-
terised as [Rh(Cl)(LL)(HpzR)] (LL=NBD, COD; R=
hp, op, dp) (4–9) by analytical and spectroscopic (IR
The complexes [Rh(Cl)(CO)2(HpzR)] (R=hp, op, dp)
(10–12) were formed in a single step by reaction of the
dimer [Rh(m-Cl)(CO)2]2 with the corresponding pyra-
zole HpzR in dichloromethane in 1:2 molar ratios
(Scheme 2). An alternative synthesis previously used in
related compounds [15,19] consisting of bubbling CO
1
1
and H-NMR) techniques (Tables 2 and 6). The H-
NMR spectra at room temperature consist of well-re-
solved signals corresponding to the coordinated pyrazol
ligand with the expected intensity ratios and multiplic-
ities. However, the olefinic protons appear as broad
signals, which is attributed to the conformational flux-
ionality of the coordinated diene, as has been previ-
ously observed in related Rh(I) complexes [15,19,30].
through
a dichloromethane solution of [Rh(Cl)-
(LL)(HpzR)] (LL=NBD, COD) also gave rise to the
dicarbonyl complexes 10–12, but only compound 12,

M.C. Torralba et al. / Journal of Organometallic Chemistry 633 (2001) 91–104
97
pound 12, having the 10-carbon chain substituent, was
isolated pure in a good yield.
form (12-red). The analytical and spectroscopic (IR and
1H-NMR) data of the dicarbonyl complexes 10–12 are
collected in Tables 2 and 6.
In all cases, the reaction solution was yellow but
some differences were found for the isolated solid prod-
ucts. Compounds 10 and 11 were obtained by partial
evaporation of the dichloromethane solution and pre-
cipitation with hexane as red microcrystalline solids,
which after recrystallisation in dichloromethane–hex-
ane gave red needle crystals. On the other hand, com-
pound 12 precipitated as a yellow solid from the
reaction solution, which was filtered off and character-
ised by analytical and spectroscopic data as the dicar-
bonyl derivative [Rh(Cl)(CO)2(Hpzdp)] (12-yellow).
From the filtrate a second product was formed as red
microcrystals by slow crystallisation at room tempera-
ture. The analytical and spectroscopic data showed that
this red product corresponded to the same dicarbonyl
complex [Rh(Cl)(CO)2(Hpzdp)] but in a different solid
1
The H-NMR spectra of 10–12 in CDCl3 solution at
room temperature show all the expected signals. No
differences between the spectra of the red and yellow
forms for 12 were found, indicating that the difference
between the forms applies only to the solid state. In all
cases, the presence of a single tautomer with the corre-
sponding pyrazole bearing the substituents at the 3
position, was deduced from the values of the coupling
constant between the H4 and H5 protons of ca. 2 Hz
(lower values of ca. 1.5 Hz are indicative of a substitu-
tion at the 5 position) [15,19,29,31–33]. Consequently
no evidences of a metallotropic equilibrium were found.
In all spectra a signal at ca. 12 ppm was observed
which corresponds to the NH proton. Also remarkable
is the presence of triplets for the H4 and H5 protons of
Table 6
1H-NMR data of compounds [Rh(Cl)(LL)(HpzR)] (LL=NBD, COD, 2CO; R=hp, op, dp) (4–12) in CDCl3 at 298 K
Pyrazole
NH
ꢀC6H4ꢀ
Ho
ꢀOR
LL
H4
H5
Hm
4
5
6
7
8
9
12.07br 6.39d
6.74d
7.41d
6.91d CH2-1%: 3.96t
CH2-2%: 1.77q
J1%2%=6.5
J1%2%=6.6
CH2-7: 1.5–1.2
CH-2,3,5,6: 4.00br s
CH-1,4: 3.85 br s
J45=2.1
J
om=8.7
CH2-3%,4%,5%: 1.5–1.2m
CH3-6%: 0.89t
6.90d CH2-1%: 3.95t
CH2-2%: 1.75q
12.07br 6.37d
6.73d
7.40d
om=8.7
CH2-7: 1.6–1.2
CH-2,3,5,6: 4.04br s
CH-1,4: 3.84br s
J45=2.1
J
CH2-3%,4%,5%,6%,7%: 1.6–1.2m
CH3-8%: 0.86t
6.90d CH2-1%: 3.95t
CH2-2%: 1.76q
J7%8%=6.9
J1%2%=6.6
12.08br 6.34 br
6.72br 7.40d
CH2-7: 1.5–1.2
CH-2,3,5,6: 4.04br s
CH-1,4: 3.84br s
Jom=8.7
CH2-3%,4%,5%,6%,7%,8%,9%: 1.5–1.2m
CH3-10%: 0.86t
6.91d CH2-1%: 3.95t
CH2-2%: 1.84m
J9%10%=6.9
J1%2%=6.6
12.50br 6.42d
7.04d
7.43d
CH-1,2,5,6: 4.40br s
CH2-3,4,7,8: 2.45m, 1.84m
J
45=2.1
Jom=8.7
CH2-3%,4%,5%: 1.5–1.2m
CH3-6%: 0.86t
6.93d CH2-1%: 3.97t
CH2-2%: 1.76q
J5%6%=6.8
J1%2%=6.6
12.50br 6.44br
7.06br 7.45d
CH-1,2,5,6: 4.22br s
CH2-3,4,7,8: 2.49m, 1.85m
Jom=8.7
CH2-3%,4%,5%,6%,7%: 1.5–1.2m
CH3-8%: 0.88t
6.91d CH2-1%: 3.95t
CH2-2%: 1.77q
J7%8%=6.6
J1%2%=6.6
12.49br 6.42d
7.04d
7.59t
7.59t
7.59t
7.43d
CH-1,2,5,6: 4.37br s
CH2-3,4,7,8: 2.46m, 1.85m
J45=2.1
J
om=8.7
CH2-3%,4%,5%,6%,7%,8%,9%: 1.5–1.1m
CH3-10%: 0.86t
6.95d CH2-1%: 3.98t
CH2-2%: 1.78q
J9%10%=6.9
J1%2%=6.6
10 12.18br 6.55t
7.46d
Jom=8.7
J
J
45=2.1
NH4=2.1
CH2-3%,4%,5%: 1.5–1.3m
CH3-6%: 0.90t
6.95d CH2-1%: 3.97t
CH2-2%: 1.78q
J5%6%=6.9
J1%2%=6.6
11 12.18br 6.55t
7.46d
om=9.0
J
J
45=2.1
NH4=2.1
J
CH2-3%,4%,5%,6%,7%: 1.5–1.2m
CH3-8%: 0.90t
6.95d CH2-1%: 3.97t
CH2-2%: 1.78q
J7%8%=6.9
J1%2%=6.6
12 12.18br 6.55t
7.46d
om=8.7
J
J
45=2.1
NH4=2.1
J
CH2-3%,4%,5%,6%,7%,8%,9%: 1.5–1.2m
CH3-10%: 0.86t
J9%10%=6.9

98
M.C. Torralba et al. / Journal of Organometallic Chemistry 633 (2001) 91–104
Fig. 2. 1H-NMR spectra in the aromatic region for 12 (a) and after irradiation on the NH signal (b). The asterisks show the H4 and H5 protons
of the pyrazole.
shoulder at ca. 340 nm (m=4600 dm3 mol−1 cm−1
could be associated with a transition involving metal
orbitals.
The above results indicate that two polymorphs (red
and yellow) were isolated for 12, whilst only one (red
form) for the related 10 and 11. Then the length of the
alkyloxy substituent on the pyrazol ligands appears to
be responsible for the differences observed in polymor-
phic behaviour.
)
the pyrazole, which is attributed to an unusual coupling
between these protons with the NH proton, as confi-
rmed from the spectra obtained by irradiation on the
NH signal (Fig. 2).
The IR spectra of 10–12 have been studied both in
the solid state and in solution. In dichloromethane all
three compounds showed the two expected w(CO)
bands at ca. 2088 and 2016 cm−1, consistent with a
mononuclear cis-dicarbonyl complex. However, in the
solid state four bands at ca. 2086, 2060, 2015 and 1995
cm−1 were observed for 10, 11 and 12-red. By contrast
only two bands at 2069 and 2007 cm−1 were found for
12-yellow. The red colour and the extra bands are not
typical of mononuclear square-planar Rh(I) complexes
and therefore these results are probably a consequence
of their particular stacking, possibly analogous to that
observed in related complexes which display one-di-
mensional metal–metal interactions [15,16,34]. The yel-
low form probably corresponds to a different packing
arrangement in which metal–metal interactions for
chains of complexes are not present.
The electronic spectra in the dichloromethane solu-
tion of both the yellow and red forms of 12 were
identical, consistent with the presence of only one form
in solution. The pattern of bands, which was also found
for the red forms of 10 and 11, consists of a main band
centred at ca. 310 nm similar to that observed in their
corresponding ligands (ꢀ320 nm) assigned to a p“p*
intraligand transition [14]. A second absorption as a
The existence of polymorphic forms for related meso-
morphic compounds has previously been reported. In a
recent work, Bruce et al. described two modifications
(one burgundy and other orange) of the complex cis-
[Ir(Cl)(CO)2(9-OPhVPy)] (9-OPhVPy=trans-4-nony-
loxy-4%stilbazole), although no X-ray crystalline
structures of any of those forms were reported [10].
However, their IR patterns in the carbonyl region were
analogous to those described above for our red and
yellow materials. Also two polymorphic forms (yellow
and red or yellow and blue) were found, respectively,
for [Rh(CO)2(b-diketonate)] (b-diketonate=3-[(4%-((4¦-
n-decyloxybenzoyl)oxy)phenoxy)carbonyl]-2,4-pentane-
dionate) and [Ir(CO)2(b-diketonate)] (b-diketonate=3-
[(4% - ((4¦ - n - decyloxybenzoyl)oxy)phenoxy)carbonyl] - 2,
4-pentanedionate,
3-[(4%-(4¦-n-decyloxybenzoyl)oxy)-
benzyl]-2,4-pentanedionate) [14]. In this case, the X-ray
crystal structure of the yellow form of [Ir(CO)2(b-diket-
onate)] (b-diketonate=3-[(4%-((4¦-n-decyloxybenzoyl)-
oxy)phenoxy)carbonyl]-2,4-pentanedionate) indicated

M.C. Torralba et al. / Journal of Organometallic Chemistry 633 (2001) 91–104
99
molecular stacking along the b-axis with the metal
3.2. X-ray crystal structures of 10–12
coordination planes being parallel but in a disposition
which appeared to exclude direct metal–metal interac-
tions [14].
The molecular geometry of the red form of
[Rh(Cl)(CO)2(Hpzdp)] (12-red) is shown in Fig. 3, and
Table 7 lists the selected bond distances and angles. A
square-planar environment around the Rh atom is
defined by the Cl ligand, two CO groups and the
pyrazolic N atom, the Rh atom being displaced by
In contrast to the above examples, no polymorphic
forms have previously been described for complexes of
the type [Rh(Cl)(CO)2(HpzR)]. Indeed a red or blue
compound has exclusively been found for these com-
plexes containing unsubstituted or some 3(5) or 3 and 5
substituted pyrazol ligands [15,16,19], and in addition
the complex [Rh(Cl)(CO)2(HpzMe2,4R)] (HpzMe2,4R=4-
[(4%-((4¦-n-decyloxybenzoyl)oxy)phenoxy)carbonyl]-3,5-
dimethylpyrazole) containing 4-substituted pyrazole has
recently been isolated as a yellow solid [14].
With the above precedents in mind, we were inter-
ested in proving why increase of the length of the
substituents at the 3 position on the pyrazole is respon-
sible for the polymorphism for 12. Crystals of sufficient
quality for X-ray diffraction of the two polymorphs
were isolated after repeated recrystallisations and their
X-ray structures solved. We have similarly solved the
X-ray crystal structures of the red compounds 10 and
11 in order to deduce the influence of the length of the
chains.
,
0.0011 A from the best least-squares plane N1C1C2Cl1.
The pyrazol ring is rotated 16.1(3)° from the coordina-
tion plane and the phenyl ring deviates by 12.5(3) and
12.4(2)° from the pyrazol plane and the coordination
plane, respectively. However, the best least-squares
plane defined for the carbon atoms C12ꢀC21 of the
aliphatic chain presents a higher deviation from the
above-mentioned planes, the angle with the phenyl ring
being 62.6(3)° (Table 8). An interesting feature is the
hydrogen bonding interaction between the NH group
,
and the Cl ligand with a N2···Cl1 distance of 3.105(7) A
(Table 9).
However, the most interesting aspect of the structure
is the molecular packing which consists of stacking of
the square-planar unities along the crystallographic c-
axis to form one-dimensional chains which are almost
Fig. 3. Stacking along the c-axis of 12-red showing the atomic numbering scheme. Hydrogen atoms have been omitted for clarity. Compounds
10 and 11 present the same disposition.

100
M.C. Torralba et al. / Journal of Organometallic Chemistry 633 (2001) 91–104
Table 7
rings and consequently the alkyl chains, in order to
minimise steric congestion. As a consequence of the
parallel disposition of the molecular cores, defined by
the coordination, pyrazol and phenyl planes, in each
column a close approach between the Rh centres along
the stacking axis should be favoured. However, this
effect could partially be inhibited by the molecular
bending produced by the long-chain substituent.
,
Selected bond distances (A) and bond angles (°) for
[Rh(Cl)(CO)2(Hpzdp)] (12-red and 12-yellow)
12-red
12-yellow
RhꢀN1
RhꢀC1
RhꢀC2
RhꢀCl1
C1ꢀO1
C2ꢀO2
N1ꢀN2
N1ꢀC5
N2ꢀC3
N2ꢀH2
2.061(7)
1.829(8)
1.803(9)
2.361(2)
1.135(8)
1.165(9)
1.321(7)
1.365(9)
1.341(8)
0.821
2.081(3)
1.845(4)
1.835(4)
2.338(1)
1.126(4)
1.137(4)
1.355(3)
1.327(4)
1.334(4)
0.971
Adjacent molecules in the ab plane are arranged
head-to-tail along the b-axis and in the same sense
through the molecular axis (Fig. 4). Individual stacks
,
are spaced at distances greater than 5 A from one
another giving rise to Rh···Rh distances longer than 10
,
A.
Overall the structure can therefore be described as
N1ꢀRhꢀC1
N1ꢀRhꢀC2
N1ꢀRhꢀCl1
C1ꢀRhꢀC2
C1ꢀRhꢀCl1
C2ꢀRhꢀCl1
H2ꢀN2ꢀN1
H2ꢀN2ꢀC3
90.3(3)
178.1(4)
89.4(2)
90.8(4)
178.4(3)
89.5(3)
121.1
90.7(1)
176.9(1)
88.7(1)
89.6(2)
177.7(1)
90.9(1)
117.7
formed by layers in the ac plane with some interdigita-
tion between the consecutive layers.
The structure of 12-red shows that one-dimensional
metal–metal interactions exist in the solid state despite
the steric consequences of the alkyl long-chains give rise
122.7
128.4
,
to an intermolecular Rh···Rh distance 0.13 A longer
than that observed in the related complex [Rh(Cl)-
(CO)2(HpzAn)] for which the same type of molecular
stacking had been found [15]. However, the columnar
chain structure, which is very scarce amongst rhodium
derivatives, is maintained.
Table 8
Selected angles (°) between the least-squares sets defined by the
specified atoms for [Rh(Cl)(CO)2(Hpzdp)] (12-red and 12-yellow)
12-red
12-yellow
Complexes 10 and 11 are almost isostructural with
the above-described 12-red. The slight differences ob-
served in the Rh···Rh distances and Rh···Rh···Rh angles
(Table 10) suggest that the increase in the length of the
alkyl chain favours the intermetallic interaction.
The crystal structure of the yellow form of
[Rh(Cl)(CO)2(Hpzdp)] (12-yellow) was solved and com-
pared with that of 12-red. Selected bond distances and
angles are recovered in Table 7. Comparing both struc-
tures, firstly there are some differences between the
1 — C1N1C2Cl1
2 — N1N2C3C4C5
3 — C6C7C8C9C10C11
1–2 16.1(3)
1–3 12.4(3)
1–4 64.3(2)
12.0(1)
34.1(1)
37.5(2)
23.3(1)
4 — C12C13C14C15C16C17C18- 2–3 12.5(3)
C19C20C21
2–4 75.1(3)
3–4 62.6(3)
26.9(2)
3.6(2)
Table 9
Interaction distances (A) and angles (°) for [Rh(Cl)(CO)2(Hpzdp)]
(12-red and 12-yellow)
,
12-red
12-yellow
N2ꢀH2
0.821
3.105(7)
2.582
0.971
3.059(3)
2.384
N2···Cl1
Cl1···H2
Rh···Cl1%
3.921(1)
N2ꢀH2···Cl1%
123.0
126.2
(%) Symmetry operation: 1−x, 2−y, 1−z.
linear (178.86(5)°). The coordination planes are almost
orthogonal to the b-axis, and the RhꢀRh distance of
,
3.5296(5) A is short enough to be considered a metal–
metal interaction along the stacking axis.
The columnar arrangement (Fig. 3) implicates an
alternatively staggered disposition of coordinated
groups around the metal, so that all the pyrazol planes
are alternatively staggered as well as those of the phenyl
Fig. 4. Drawing of the packing in 12-red in the ab plane.

M.C. Torralba et al. / Journal of Organometallic Chemistry 633 (2001) 91–104
101
Table 10
,
Rh···Rh% distances (A) and Rh%···Rh···Rh¦ angles (°) along the colum-
nar stacking for complexes [Rh(Cl)(CO)2(HpzR)] (R=hp, op, dp)
(10–12)
Rh···Rh%
Rh%···Rh···Rh¦
a
10
3.5464(5)
3.5354(4)
3.5296(5)
3.3631(1)
178.88(4)
178.82(2)
178.86(5)
11 a
12-red a
12-yellow b
Fig. 6. Drawing of the packing through the b-axis in 12-yellow
showing the rod-like distribution.
a Symmetry operation: (%) x, 1/2−y, 1/2+z; (¦) x, 1/2−y, −1/2+z.
b Symmetry operation: (%) 1−x, 2−y, 1−z.
,
this axis is too long (7.6304(8) A) for metal–metal
interactions.
geometrical parameters of the molecules of each form.
In particular, the molecular geometry of 12-yellow, as
shown in Fig. 5, is apparently closer to planarity as
defined by angles between the coordination plane and
those of the pyrazol ring, the aromatic group and the
best least-squares plane of the chain atoms (Table 8).
In striking contrast, the molecular packing in crystals
of 12-yellow is dominated by the stacking of dimeric
units along the a-axis. Each almost planar molecule in
columnar stacking of square metal-centred units inter-
acts with a second molecule of the neighbour column
via their corresponding Cl ligands. Specifically, Cl of
one coordination plane lies above that of the related
molecule and at the same time Cl of the latter is
situated below the rhodium-centred coordination plane
of the first one (Figs. 5 and 6). Both RhꢀCl distances of
It is reasonable to ask why the structure of the yellow
form consists of dimers stacked rather than the uniform
stacking arrangement of monomers observed in the red
form. By the inspection of the nearest contacts inside
the structure we observed that a hydrogen bonding
interaction exists between the NꢀH hydrogens and the
Cl atoms which was stronger than that found in the red
form (12-red) described above (Table 9). This enhance-
ment could be a consequence of the orientation of the
Cl ligand to act as a bridge in the dimer. In other words
the Cl ligand of each molecule engages in both intra-
and inter-molecular interactions with an intramolecular
NH group and the Rh atom of the neighbour,
H2···Cl1···Rh1% (Fig. 5). This feature is also supported
by the IR results of the NꢀH frequency values: the
yellow form presents a lowering of ca. 60 cm−1 com-
pared to the red one (Table 2) as a consequence of the
weakness of NꢀH bond produced by the hydrogen
bonding interaction.
,
3.921(1) A suggest the presence of weak intermolecular
interactions through Cl bridges and as a consequence
the Rh···Rh distance between the metal centres of inter-
,
acting molecules is 3.3631(1) A. Therefore, the structure
could be described as formed of weakly bonded dimers.
These dimers are stacked along the a-axis in a linear
fashion but the Rh···Rh distance between dimers along
The above structural results together with the fact
that the yellow form is favoured with long-chain sub-
stituents need further comments. An explanation
Fig. 5. Dimeric unity in 12-yellow showing the atomic numbering scheme. Hydrogen atoms, except H2, have been omitted for clarity.

102
M.C. Torralba et al. / Journal of Organometallic Chemistry 633 (2001) 91–104
3.3. Thermal studies
emerges by considering the interdigitated layer structure
of the red forms in which some gliding between layers
in order to obtain a higher proximity between metal
centres of molecules of adjacent columns would be
easier in 12 than in 10 and 11 (Fig. 1). In this way, the
dimeric unities of the yellow form are raised only for 12
at room temperature. Consistent with this proposal the
red form is converted to the yellow one by heating as
observed by polarised optical microscopy.
The thermal behaviour of the pyrazol ligands 1–3
and their corresponding Rh–dicarbonyl complexes 10–
12 were studied by polarised optical microscopy and
differential scanning calorimetry. The results are recov-
ered in Table 11. The thermal data from the olefinic
derivatives 4–9 are also included.
Unfortunately no mesomorphism was observed for
any compound. However, thermochromic behaviour
was found for the dicarbonyl complexes 10–12. Specifi-
cally, on heating the corresponding microcrystalline red
forms a transformation to a yellow solid phase oc-
curred in all cases. This transition from the red to
yellow solid is a slow process which occurs over a wide
temperature range. In all cases, before the yellow solid
begins to form, a darkening is observed.
The evolution from the red to the yellow phase is
slower and has a higher onset temperature for 11 than
for 12-red, these observations being consistent with the
greater ease of gliding between layers in 12-red in
relation to another which has shorter chains. Com-
pound 10 follows the same sequence of temperature for
that transformation. However, in this case it is difficult
to observe the evolution to the yellow phase because
this process is overlapped with the melting of the red
solid. Melting of some of the red solid was also ob-
served for 11 and 12-red at temperatures of ca. 85 and
87 °C. Finally, for the three compounds melting of the
yellow solid phase is clearly observed at similar temper-
atures (ca. 95 °C).
Table 11
Phase properties of compounds 1–12 determined by DSC
Transition
T (°C)
DH (kJ mol−1
)
1
2
3
4
5
6
7
8
9
K“IL
K“IL
K“IL
K“IL
K“IL
K“IL
K“IL
K“IL
K“IL
K1“K1%
K1“IL
K2“IL
K1“K1%
K1“IL
K2“IL
K1“K1%
K1“IL
K2“IL
K2“IL
59
67
77
105
119
115
107
98
13.5
21.8
32.8
19.2
26.2
17.3
22.9
22.1
23.3
9.5
101
10
88 a
103
73
19.7
11
11.9
91 b
99
b
21.6
11.2
31.5
12-red
68
95 c
The sequence of phase changes of these compounds
is shown in Scheme 3, and that for 12-red is taken as a
representative example:
12-yellow
98
46.0
a The transformations K1“K1% and K1“IL appear overlapped.
b This transition is observed as a weak and broad signal (DH value
has not been possible to be determined).
“ The sample exists as red needle-like crystals (K1) at
room temperature.
“ When the sample is heated to 67 °C a darkening of
the solid occurs (K%1).
c The two melting processes K1“IL and K2“IL appear over-
lapped.
“ By heating to 69 °C the yellow solid phase (K2)
begins to formed.
“ By holding the temperature of the sample at 87 °C
for 20 min, the red crystalline form (K1) melts.
“ Finally, by heating to 95 °C the yellow form (K2)
melts.
This double melting behaviour observed by polaris-
ing microscopy is also confirmed by DSC measure-
ments. The precise temperature of the red–yellow phase
transition could not be determined by DSC observa-
tions because this phase transition is very slow and
because the superheating of K1 crystals occurs easily.
Although the superheating of K1 crystals originates
such double melting behaviour, at the same time this
makes it difficult to detect the precise temperature of
the solid–solid phase transition.
For compound 12-yellow only the expected transition
to isotropic liquid is observed at 97 °C by both DSC
and polarising microscope.
Scheme 3. Phase transitions observed by polarised microscopy for the
red compounds [Rh(Cl)(CO)2(HpzR)]: (a) R=hp 10; (b) R=op 11;
(c) R=dp 12-red.

M.C. Torralba et al. / Journal of Organometallic Chemistry 633 (2001) 91–104
103
The double melting behaviour and the textures ob-
Acknowledgements
served by polarising microscopy could suggest for these
compounds [Rh(Cl)(CO)2(HpzR)] (R=hp, op, dp) a
thermal behaviour close to mesomorphism. Such multi-
ple melting behaviour has been related to phenomena
for states between crystals and liquid crystals [35].
On the other hand, in all cases, on cooling the
isotropic liquid at 10 °C per minute, the thermograms
show two broad peaks corresponding to transforma-
tions to crystalline phases at ca. 65–90 °C, in agree-
ment with that observed by polarising light microscope.
By comparing the thermal behaviour of the com-
plexes 10–12 with that of their respective ligands 1–3,
an increase in the melting points is observed. All the
types of interactions found in the described complexes
appear to be responsible for this increase.
Financial support from the DGES of Spain is grate-
fully acknowledged (Project no. PB98-0766).
References
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[2] S.R. Collinson, D.W. Bruce, in: J.P. Sauvage (Ed.), Transition
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[3] J.L. Serrano (Ed.), Metallomesogens. Synthesis, Properties and
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Director, CCDC, 12 Union Road, Cambridge CB2
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