Pyridyl and pyridiniumyl β-diketones as building blocks for palladium(ii) and allyl-palladium(ii) isomers. Multinuclear NMR structural elucidation and liquid crystal behaviour

Article abstract of DOI:10.1039/c0nj00938e

A series of novel β-diketone ligands bearing alkyloxyphenyl R = C ₆H₄OC_nH_2n+1 and pyridyl substituents at 1 and 3 positions of the β-diketone core [HL^R(n)py] have been prepared and structurally charac

Full text of DOI:10.1039/c0nj00938e

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PAPER
Pyridyl and pyridiniumyl b-diketones as building blocks for palladium(II)
and allyl–palladium(II) isomers. Multinuclear NMR structural elucidation
and liquid crystal behaviourw
a
b
b
a
Marıa Jose Mayoral, Pilar Cornago, Rosa M. Claramunt and Mercedes Cano*
´
´
Received (in Montpellier, France) 28th November 2010, Accepted 8th March 2011
DOI: 10.1039/c0nj00938e
A series of novel b-diketone ligands bearing alkyloxyphenyl R = C6H4OCnH2n+1 and pyridyl
substituents at 1 and 3 positions of the b-diketone core [HLR(n)py] have been prepared and
structurally characterized. The corresponding b-diketonate complexes [Pd(k2-LR(n)py)2] were
isolated as a mixture of cis and trans isomers each of them exhibiting the ligand in a NO- or
OO-bidentate coordinative mode which were established by multinuclear NMR studies
(1H, 13C and 15N). However none of those characteristics were determining to achieve
liquid-crystalline properties on the new complexes. By contrast reactions of the aforementioned
ligands [HLR(n)py] or those containing the pyridiniumyl substituent [HLR(n)pyH +
towards the
]
allyl–palladium fragment were successful to afford ionic metallomesogenic materials isolated as
PF6 salts of [Pd(Z3-C3H5)(k2-HLR(n)py)][PF6] (I) and [Pd(Z3-C3H5)(k2-LR(n)pyH)][PF6] (II) types.
Structural characterization and mesomorphic properties were established by multinuclear NMR
and the use of DSC and POM techniques. In complexes I and II the corresponding b-diketone
ligands were coordinated as neutral bidentate NO-donor or zwitterionic OO-donor systems,
respectively. Complexes of type II exhibit a range of the SmC mesophases significantly greater
than that found in the type I.
the dispersive forces and the intermolecular interactions.
Taking into account the above considerations we thought that
metal complexes containing long lateral chains and pyridine
b-diketone type ligands could provide a new opportunity to
reach metallomesogenic materials. In this point it is also
interesting to note that most of the palladium b-diketonate
complexes with two, three or four lateral chains display a
square planar coordination geometry leading to discotic liquid
crystal phases.1–12
Introduction
Coordination compounds constitute probably the largest
category in the field of metallomesogenic materials. The
interest of the current research on liquid crystal coordination
compounds is directed to achieve a favourable mesomorphism
avoiding the usual decomposition of the metallorganic systems
at the temperatures frequently required to allow the supra-
molecular ordering of the mesophases.
In this context while the effects of the coordination geometry,
the lateral substituents or their polar nature are investigated
on many bis-b-diketonate metal systems,1–12 the study of the
introduction of a functional group on the molecular periphery
of the ligand has not been commonly reported,12 even though the
formation of organic mesogenic materials from functionalized
macrocyclic ligands is a subject of current interest.2,13
On the other hand we have previously established that
the metallic allyl–palladium fragment was able to induce a
mesomorphic behaviour by coordination to mesomorphic or
non-mesomorphic ligands leading to a stabilization of the
mesophases. In those systems the electronic delocalization of
the p–allyl system appears to be a favourable element to
achieve the liquid crystalline properties.14
The b-diketonate core is an electron rich moiety in which the
introduction of a polar group like pyridine could modify
In this work we describe the study of novel palladium
complexes based on b-diketone type ligands containing long
chained alkyloxyphenyl substituents and pyridyl or pyridiniumyl
groups at the 1 and 3 positions of the b-diketonate core, named
a
´
´
Departamento de Quımica Inorganica I, Facultad de Ciencias
Quı´micas, Universidad Complutense de Madrid, E-28040 Madrid,
Spain. E-mail: mmcano@quim.ucm.es; Fax: +34 91394 4352
[HLR(n)py] and [HLR(n)pyH +
(Scheme 1a). Two systems were
]
b
selected to be studied: those of the type [Pd(k2-LR(n)py)2] produced
by coordination of two [HLR(n)py] ligands to a Pd(II) center
(Scheme 1b) and those obtained by coordination of the afore-
mentioned ligands to the [Pd(Z3-C3H5)]+ fragment (Scheme 1c).
´
´
´
Departamento de Quımica Organica y Bio-Organica,
Facultad de Ciencias, UNED, Senda del Rey 9, E-28040 Madrid,
Spain. E-mail: rclaramunt@ccia.uned.es; Fax: +34 91398 8372
w Electronic supplementary information (ESI) available: Tables S1–S8
and Fig. S1–S3. See DOI: 10.1039/c0nj00938e
c
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Scheme 2
molecular structure. The protonated b-diketonate pyridiniumyl
derivatives isolated as chloride salts (5–8) described in a
previous paper15 are also included for comparative purposes.
Scheme 1
Synthesis and characterisation
The ligands [HLR(n)py] (1–4) were readily prepared by reaction
of the corresponding 4-n-alkyloxyacetophenone and 2-ethyl-
picolinate in tetrahydrofuran. The reaction mixture was
refluxed for 24 h (Scheme 2).
The related complexes [Pd(k2-LR(n)py)2] (9–12) were synthe-
sized by reaction of the Pd(Ac)2 with the corresponding
ligands [HLR(n)py] in a NaH/dichloromethane media under
reflux (Scheme 3a). All the complexes were isolated as yellow
solids which were air-stable and soluble in common polar
organic solvents.
The allyl–palladium derivatives [Pd(Z3-C3H5)(HLR(n)py)]-
[PF6] (13–16) and [Pd(Z3-C3H5)(LR(n)pyH)][PF6] (17–20) were
obtained when the dimer [Pd(m-Cl)(Z3-C3H5)]2 was allowed to
react with the neutral ligands [HLR(n)py] or with the related
chloride salts [HLR(n)pyH]Cl in dichloromethane/acetone
(Scheme 3b and c).
Fig.
1
Potential coordinative forms of (a) neutral [HLR(n)py],
(b) anionic [LR(n)py] and (c) zwitterionic [LR(n)pyH +
ligands
]
The work is directed to establish the ability of the selected
b-diketones 1-(2-pyridyl)-3-(4-n-alkyloxyphenyl)propane-1,3-
diones and the related salts 2-[3-(4-n-alkyloxyphenyl)-
propane-1,3-dion-1-yl]pyridiniumyl chlorides to be coordinated
as NO- or OO-chelating ligands (Fig. 1) towards Pd(II) or
allyl–palladium fragment, so allowing us to study the influence
of the metallic coordination environment on the mesomorphic
behaviour of the complexes.
Thermal studies of all compounds are carried out in order to
investigate their potential as liquid crystal materials.
Spectroscopic studies
1
The H-NMR spectra of ligands 1–4 were obtained in CDCl3
solution. All of them exhibited a similar pattern (see ESIw).
The multinuclear NMR study (1H, 13C and 15N) of compound
1, as a representative example, is reported in Table 2.
Results and discussion
All the new compounds (1–4; 9–20) described in this work are
schematically presented in Table 1 including the number
used to identify them. Schemes 2 and 3 depict the synthetic
processes to obtain each type of compounds and the proposed
The 1-(2-pyridyl)-3-(4-n-alkyloxyphenyl)propane-1,3-dione
ligands (1–4) can exist in solution as a pair of enol tautomers
(Fig. 2a and b) and a keto form (Fig. 2c).16 The 1H-NMR
spectra exhibit in all cases a similar pattern which is consistent
with the enol form as the major one (490%) (Fig. 2a and b) as
deduced from the signal intensity ratios of the CH2/CH
Table 1 Schematic formulation of the compounds
1
protons. In addition, the combined use of H and 13C-NMR
Compounds
[HLR(n)py
n
Number
]
12
14
16
18
12
14
16
18
12
14
16
18
12
14
16
18
12
14
16
18
1a
2
3
4
5
6
7
[HLR(n)pyH]Cl
8a
9a
10
11
12
13
14a
15
16
17
18a
19
20
[Pd(k2-LR(n)py)2]
[Pd(Z3-C3H5)(HLR(n)py)][PF6] Type I
[Pd(Z3-C3H5)(LR(n)pyH)][PF6] Type II
a
Selected compounds as representative examples for multinuclear
NMR studies.
Scheme 3 Molecular structure of the palladium complexes.
c
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Table 2 Selected NMR chemical shifts (d in ppm) for 1 at 300 K
Table 3 Selected NMR chemical shifts (d in ppm) for 8 at 323 K
Enol (94%)
1a c 1b
Keto (6%)
1c
Enol (93%)
8a c 8b
Keto (7%)
8c
Atoma
Atoma
C7
C7
CDCl3
CPMAS
H8/C8
CDCl3
CPMAS
C9
CDCl3
CPMAS
OH
181.2
180.7
196.8
CDCl3
CPMAS
H8/C8
CDCl3
CPMAS
C9
CDCl3
CPMAS
OH
174.1
170.5
n.o.
7.52/92.9
91.1
4.80/48.0
193.6
8.11/96.0
94.8
4.82/n.o.
n.o.
187.1
187.0
189.9
188.6
CDCl3
C2
16.64
CDCl3
C2
16.48
CDCl3
CPMAS
H3/C3
CDCl3
CPMAS
H4/C4
CDCl3
CPMAS
H5/C5
CDCl3
CPMAS
H6/C6
CDCl3
CPMAS
Ho/Co
CDCl3
CPMAS
Hm/Cm
CDCl3
CPMAS
Cp
152.6
151.2
152.7
CDCl3
CPMAS
H3/C3
CDCl3
CPMAS
H4/C4
CDCl3
CPMAS
H5/C5
CDCl3
CPMAS
H6/C6
CDCl3
CPMAS
Ho/Co
CDCl3
CPMAS
Hm/Cm
CDCl3
CPMAS
Cp
149.9
142.6, 144.4
n.o.
8.15/122.0
118.5
8.17/122.1
7.89/137.0
7.47/126.1
8.62/149.0
7.96/130.9
6.93/114.3
163.4
8.39/123.8
118.2
n.o.
7.87/137.1
135.4
8.25/141.7
134.1
7.96/n.o.
6.92/n.o.
8.64/n.o.
7.43/126.1
122.7
7.59/126.8
127.1
8.71/149.1
151.2
8.89/145.6
150.1, 153.0
8.06/129.7
129.8
8.26/130.8
130.0
7.91
130.6
6.96/114.4
112.4
7.00/114.8
113.5
6.91/114.3
n.o.
CDCl3
CPMAS
Cipso
163.1
163.6
CDCl3
CPMAS
Cipso
164.0
165.6
CDCl3
CPMAS
N1
127.8
126.2
127.3
CDCl3
CPMAS
N1
127.6
124.2
n.o.
CDCl3
CPMAS
ꢀ73.5
ꢀ68.3
ꢀ66.3
CPMAS
ꢀ181.6
The numbering of the atoms is depicted in Fig. 3; n.o. = not
a
a
The numbering of the atoms is depicted in Fig. 2.
observed.
Fig. 3 Keto–enol equilibrium for [HLR(n)pyH]Cl (5–8).
Fig. 2 Keto–enol equilibrium for [HLR(n)py] (1–4).
As in the related compound 1 the NMR data of the chloride
salt 8 agree with the presence in solution of the enol and keto
forms being the first ones the predominant (93%). In addition
in this equilibrium 8a is the main enol form (Fig. 3, Table 3).
We could establish from the 2D-NMR experiments
[(1H–1H)gs-COSY, (1H–13C)gs-HMQC, (1H–13C)gs-HMBC]
in CDCl3 solution that both forms 1a and 8a are the major ones
(Tables 2 and 3). We have been able to assign the corresponding
data point out to the 1a form as the predominant enol (see
Table 2 and further discussion for 8a).16
The 1H-NMR results of the 2-[3-(4-n-alkyloxyphenyl)-
propane-1,3-dion-1-yl]pyridiniumyl chlorides (5–8) have been
reported in a previous paper.15 However a more complete
study is now carried out by considering new data from 13C and
15N-NMR spectra taken for compound 8 (Table 3).
c
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1693 cmꢀ1 related to the presence in the ligand of an uncoordi-
nated carbonyl group (NO-trans and NO-cis isomers, Fig. 4).
In addition the second n(CO) band at lower frequency,
1600 cmꢀ1, corresponds to a bidentate coordination involving
the two carbonyl groups of the ligands (OO-trans and OO-cis
isomers, Fig. 4).18,19
A final proof allowing us to support our conclusions comes
out from the 15N-NMR data of complex 9. In the solid state
the CPMAS spectra show the presence of an uncoordinated
pyridine nitrogen at ꢀ66.9 ppm, together with a coordinated
one at ꢀ158.7 ppm related to the OO or NO isomers,
respectively. In CDCl3 solution, the results agree with the
presence of two major forms via NO- at ꢀ163.1 ppm and
OO-coordination at ꢀ72.6 ppm, both assigned to the
commonly more stable trans disposition, although a weak
signal from the NO-cis isomer could also be observed
(Table 4).
The coordination chemical shift Dd (Dd = dcomplex ꢀ
dligand) for the NO-coordinated isomers is similar to that
observed in palladium complexes containing the pyrazole
ligand which exhibits a strong covalent Pd–N bond,20 so
suggesting a related bond for these isomers.
Fig. 4 Coordination and geometrical isomers of [Pd(k2-LR(n)py)2]
(9–12).
signals to each 1H and 13C nuclei. On one side, C2 at
152.6/149.9 ppm is bonded to C7 which appears at 181.2/
174.1 ppm; on the other side the Cipso at 127.8/127.6 ppm is
linked to C9 (carbonyl group CQO) at 187.1/189.9 ppm. In
both cases the results agree with those previously found by us
for related b-diketones.16
Reactions of the allyl–palladium fragment with the neutral
ligands [HLR(n)py] were carried out in order to attain
new organometallic compounds as potential liquid
crystal materials. In this way complexes of the type
[Pd(Z3-C3H5)(HLR(n)py)][PF6] (type I; 13–16, Scheme 3b) were
obtained.
All palladium compounds containing 1,3-unsymmetrical
disubstituted-propane-1,3-dionate units of the type [Pd(k2-LR(n)py)2]
(9–12; Scheme 3a) were isolated as a mixture of coordination
and geometrical isomers which exhibit a OO- or NO-
coordination of the donor ligands and a trans or cis distribution
(Fig. 4). The ratio of these isomers was determined by analysis
of the multinuclear NMR results for compound 9 selected as
representative example (Table 4).
In fact the CDCl3 solution 1H-NMR spectrum of 9 indicates
the presence of four signals for each proton of the phenyl and
pyridyl groups as well as for the C(8)H of the b-diketonate
central core, those features suggesting the presence of the four
isomers depicted in Fig. 4. In the mixture, the isomers are
found in the following proportions: OO-trans (40%), OO-cis
(10%), NO-trans (40%) and NO-cis (10%).
Because of the versatile coordination of these ligands
towards the palladium(II) metal centre observed in the above
compounds [Pd(k2-LR(n)py)2] (9–12), we carried out the structural
characterization of the new derivatives by multinuclear NMR
(Table 5).
The 1H-NMR spectra of the complexes I (Scheme 3b) in
CDCl3 solution at 300 K exhibit in all cases the corresponding
signals of the ligands. For those compounds an enol
coordinated form of the b-diketone core could be proposed
on the basis of the signal at ca. 7.2–7.3 ppm corresponding to
the C(8)H. A closer analysis of the NMR data of complex 14
as representative example of the series I shows the OH
1
signal at 14.8 ppm, and the other H, 13C chemical shifts in
agreement with the proposed structure involving a coordination
through the N-pyridyl and O-keto atoms of the enol ligand
(Scheme 3b). Here again the 15N-NMR in the solid state has
proved to be crucial to demonstrate that the pyridine nitrogen
at ꢀ130.0 ppm is involved in the coordination around the
palladium(II) center.21
For each OO- or NO-coordinative form we have considered
the trans geometry as the major one on the basis of the
literature data,17 as such a geometry avoids the steric congestion.
However, weak signals from the cis forms were also observed
in CDCl3 solution, for all carbon and proton nuclei of each
coordinative isomer (Table 4).
If we analyze the allyl group, signals of meso (Hmeso),
syn (Hs) and anti (Ha) protons are clearly established and
observed as three well separated signals at 5.80, 4.38 and
3.39 ppm, respectively, for 14. Because this group exhibits a
non-symmetrical environment the above results suggest syn–syn
and anti–anti interchanges (Fig. 5), which are analogous to
those described in the literature for related compounds.22
The protons of the pyridine group could also be clearly
observed in their characteristic regions. In this context it is
interesting to note that the signal of the H6 proton appears at
8.70 ppm, this value being similar to that found for the same
proton in the related compound [Pd(k2-LR(12)py)2] (9).
The four signals at 7.44 and 7.48/5.45 and 5.44 ppm of the
C(8)H protons exhibit intensity ratios of 40 : 10/40 : 10.
Similar features were found for the signals of the meta protons
at 6.92 (40) and 6.91 (10)/6.74 (40) and 6.77 (10). In the case of
the ortho protons at 7.91, 7.96 and 8.14 ppm the ratio is
40 : 10 : 50 indicating two overlapped signals at 8.14 ppm.
Finally, the pyridyl protons show the same trendline (Table 4).
The IR spectra of the complexes 9–12 in the solid state
(Fig. S1, ESIw) support the coexistence of the aforementioned
four isomers, as they display a n(CO) absorption band at
c
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Table 4 NMR chemical shifts (d in ppm) for 9 at 300 K
Atoma
OO-trans (40%)
OO-cis (10%)
NO-trans (40%)
NO-cis (10%)
C7
CDCl3
CPMAS
H8/C8
CDCl3
CPMAS
C9
CDCl3
CPMAS
C2
182.5
186.6
181.9
199.8
200.1
199.7
7.44/94.1
7.48/93.9
178.0
5.45/57.3
58.0
5.44/57.8
196.6
177.0
177.1
196.7
195.9
CDCl3
CPMAS
H3/C3
CDCl3
CPMAS
H4/C4
CDCl3
CPMAS
H5/C5
CDCl3
CPMAS
H6/C6
CDCl3
CPMAS
Ho/Co
CDCl3
CPMAS
Hm/Cm
CDCl3
CPMAS
Cp
CDCl3
CPMAS
Cipso
154.6
157.3
155.0
158.3
158.3
158.5
7.98/121.7
123.1
8.05/121.9
7.80/136.8
7.37/125.3
8.62/148.8
7.96/129.7
6.91/113.9
162.0
7.87/122.2
124.1
7.87/122.3
8.10/139.9
7.60/126.9
8.82/146.3
8.14/130.7
6.77/113.6
162.37
7.83/136.7
139.4
8.10/139.9
143.6
7.36/125.2
125.4
7.60/127.0
126.5
8.64/148.5
148.6
8.78/146.4
146.4
7.91/129.5
132.3
8.14/130.6
134.9
6.92/114.2
116.8/118.5
6.74/113.5
110.8
162.1
162.9
162.40
162.9
CDCl3
CPMAS
N1
130.7
130.5
130.7
131.4
130.6
131.3
CDCl3
CPMAS
CH3
ꢀ72.6
ꢀ66.9
ꢀ163.1
ꢀ158.7
ꢀ163.4
CDCl3
CPMAS
(CH2)x
CDCl3
CPMAS
OCH2
CDCl3
CPMAS
0.86–0.89/14.1
14.6
0.86–0.89/14.1
1.27–1.86/22.7–31.9
3.99/68.2
0.86–0.89/14.1
15.2
0.86–0.89/14.1
1.27–1.86/22.7–31.9
3.90/68.0
1.27–1.86/22.7–31.9
24.2–34.1
1.27–1.86/22.7–31.9
24.2–34.1
4.03/68.2
67.6
3.90/68.0
67.6
a
The numbering of the atoms is depicted in Fig. 4.
In order to support the NO-coordination of the enol form of
which were again structurally characterized by 1H-NMR in
all cases.
the ligand, the IR spectra of all complexes I were recorded. All
of them exhibit a n(OH) absorption band at 3400–3450 cmꢀ1
,
As they presented a similar pattern of signals with a C(8)H
proton of the b-diketonate core at about 7.53 ppm, consistent
with the enolate coordinative form (Scheme 3c), we decided to
do a further study by 13C- and 15N-NMR which was carried
out for 18 as representative example of this family. All
chemical shifts gathered in Table 6 for this complex agree
with such a coordination mode. In particular, it is interesting
to note that the N1 chemical shift in the solid state is
ꢀ185.6 ppm, a value close to that found in ligand 8
(Table 3) and typical of a protonated pyridine.
which is characteristic of the enol form of the free b-diketones
without intermolecular associations.19
On the other hand the ionic nature of complexes I was
confirmed by the presence of the PF6 counteranion bands at
842 n(P–F) and 558 g(F–P–F) cmꢀ1 18
and therefore with the
,
neutral nature of the enol ligand in the complexes, so supporting
their NO-coordination.
As a strategy to favour a bidentate OO-coordination of the
ligand by blocking the N-pyridine group, the related cationic
species [HLR(n)pyH +
]
were also used towards the mentioned
The allyl group shows three characteristic resonances
corresponding to the meso, syn and anti protons. The two
doublets from the syn and anti protons are at about 4.23 and
allyl–palladium fragment yielding to the new compounds
[Pd(Z3-C3H5)(LR(n)pyH)][PF6] (type II; 17–20, Scheme 3c)
c
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Table 5 Selected NMR chemical shifts (d in ppm) for 14 at 323 K
Atoma
Chemical shifts (d)
C7
CDCl3
CPMAS
H8/C8
CDCl3
CPMAS
C9
185.3
189.8
7.23/94.1
92.1
CDCl3
OH
CDCl3
C2
185.3/180.7
14.80
CDCl3
CPMAS
H3/C3
CDCl3
CPMAS
H4/C4
CDCl3
CPMAS
H5/C5
CDCl3
CPMAS
H6/C6
CDCl3
CPMAS
Ho/Co
152.8
149.6
8.58/126.6
127.1
8.11/141.3
139.0
7.64/129.7
131.7
Fig. 5 Syn–syn and anti–anti interchanges in CDCl3 solution for
allyl–palladium complexes.
8.70/151.9
151.6
Table 6 Selected NMR chemical shifts (d in ppm) for 18 at 323 K
CDCl3
CPMAS
Hm/Cm
CDCl3
CPMAS
Cp
CDCl3
CPMAS
Cipso
CDCl3
CPMAS
CH2-allyl (Hs)
CDCl3
CPMAS
CH2-allyl (Ha)
CDCl3
CPMAS
CH-allyl (Hmeso
CDCl3
CPMAS
N1
8.16/131.6
131.7
Atoma
Chemical shifts (d)
7.01/115.5
119.7
C7
CDCl3
H8/C8
CDCl3
C9
CDCl3
NH
185.4
165.8
163.0
7.53/94.6
185.4
123.7
122.0
CDCl3
C2
14.60
4.38/61.0
66.6/68.3
CDCl3
H3/C3
CDCl3
H4/C4
CDCl3
H5/C5
CDCl3
H6/C6
CDCl3
Ho/Co
CDCl3
Hm/Cm
CDCl3
Cp
CDCl3
Cipso
152.7
8.52/126.0
8.16/140.7
7.67/128.9
8.78/152.2
8.20/131.3
7.02/115.2
165.2
3.39/61.0
66.6/68.3
)
5.80/115.8
113.1/116.3
CPMAS
ꢀ130.0
The numbering of the atoms is depicted in Fig. 5.
a
3.19 ppm, respectively, and the third multiplet signal at 5.68 ppm
is assigned to the meso proton. The presence of a unique
set of signals for each allyl proton suggests again the dynamic
nature of the allyl group in the complex that allows the
equivalence of the syn–syn, anti–anti protons despite the
non-symmetrical environment produced by the b-diketonate
ligand, similarly as it was described in the related complexes of
type I (Fig. 5).
By considering the analogous results of the 1H-NMR
spectra in CDCl3 solution of compounds 14 (family I) and
18 (family II) it becomes apparent that both correspond to the
same species in solution. This feature could be explained on
the basis of a fast exchange between the enol proton in I and
the pyridiniumyl proton in II, through a neutral intermediate
as depicted in Fig. 6.
CDCl3
CH2-allyl (Hs)
CDCl3
CH2-allyl (Ha)
CDCl3
CH-allyl (Hmeso
CDCl3
N1
124.2
4.23b/61.0
3.19c/61.0
5.68/115.2
ꢀ185.6
)
CPMAS
a
The numbering of the atoms is depicted in Fig. 5. b sJ = 6.4 Hz.
c aJ = 11.8 Hz.
However from the multinuclear NMR studies of 14 and 18
in the solid state (Tables 5 and 6) we can conclude that for
c
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complexes I the ligand [HLR(n)py] is in the enol form exhibiting
NO-coordination (Scheme 3b, type I). By contrast
for complexes of type II with the [HLR(n)pyH +
ligand an
Table 7 Phase behaviour of complexes 13–20 determined by DSC
(scan rate: 10 K minꢀ1
)
a
]
T/1C (DH/kJ molꢀ1
)
OO-coordinated enolate form is deduced (Scheme 3c,
type II), giving rise in both cases to cationic species which
were isolated as PF6 salts.
Cr
-
Cr0
-
SmC
-
I
n
12 13
14 14
16 15
18 16
12 17
14 18
16 19
18 20
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
180 (41.1)
173 (32.1)
172 (9.5)
153 (9.2)
174 (40.5)
170 (28.1)a
160b
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
76 (11.6)
90 (7.0)
87 (6.5)
ꢁ
ꢁ
ꢁ
IR spectra of type I complexes (13–16) in the solid state
exhibit a n(CO) absorption band at 1600 cmꢀ1 and a n(OH)
band at ca. 3450 cmꢀ1. Both results agree with the presence of
an enol form of the ligand in which the keto group is involved
in the coordination leaving free the OH group. This feature
155 (19.7)
144 (18.5)
ꢁ
ꢁ
84 (7.5)
78 (7.2)
ꢁ
ꢁ
158 (28.1)a
107 (19.7)
94 (18.9)
ꢁ
ꢁ
ꢁ
144 (5.2)
supports a NO-coordination of neutral ligands [HLR(n)py
]
a
b
Overlapped processes. Temperature observed by POM.
giving rise to cationic species. In agreement with this proposal
the characteristic bands of the PF6 ion observed at 842 n(P–F)
and 558 g(F–P–F) cmꢀ1 indicate its presence as
a
counteranion.18
thermal data of the compounds exhibiting liquid crystal
For complexes of type II (17–20) again one n(CO) band at
1602 cmꢀ1 suggests that at least one carbonyl group is
involved in the coordination of the ligands. In addition in all
cases a strong and splitted band appears in the range
of 3549–3415 cmꢀ1, which is attributed to n(NH) of the
pyridiniumyl group (Fig. S2, ESIw). On these basis a bidentate
OO-coordination of the b-diketonate ligand is suggested.
Therefore, in the cationic complexes [Pd(Z3-C3H5)(LR(n)pyH)]+
(II) the ligand, which acts as bidentate b-diketonate should
be in a zwitterionic form in order to maintain the neutral
character of the complexes.
behaviour (13–20).
b-Diketone ligands bearing a pyridyl substituent [HLR(n)py
]
(1–4) were not liquid crystal materials. In all cases the DSC
thermograms show a single peak at ca. 80 1C corresponding to
the endothermic transition from solid to the isotropic liquid.
In contrast to the mesomorphic behaviour of the related
disubstituted-alkyloxyphenyl b-diketone ligands,7,24 the
absence of mesomorphism in compounds 1–4 requires an
explanation which can be potentially established by considering
that the length to width molecular ratio of these compounds,
which is significantly lower than that of the related disubstituted
ones, results inadequate to achieve the molecular ordering of
the mesophases. On the other hand the expected increase in
the molecular polarization, produced by the inclusion of the
pyridyl group, should not be enough to compensate the
molecular relationship mentioned above.
In summary we can establish the presence of two different
structural environments for complexes I and II in the solid
state, this fact being also reflected in the different liquid crystal
behaviour observed in both cases (see the following discussion).
In addition current photoluminescent studies performed on
the complexes 14 (type I) and 18 (type II) show differences in
the emission features in the solid state while the study carried
out in dichloromethane solution displayed how the emission
bands are similar in both cases.23 These results were expected
on the basis of the equilibrium described in Fig. 6.
On the other hand all of the chloride salts previously
reported that [HLR(n)pyH]Cl (5–8) displayed thermotropic
polymorphism only consistent with crystal–crystal phase
transitions.15
The complexes [Pd(k2-LR(12)py)2] (9–12) behave as non-
mesomorphic materials suggesting that none of the isomers
present in the complexes should exhibit liquid crystal
behaviour. The presence of a mixture of compounds should
not avoid observation of a liquid crystal behaviour if one of
the components acts as mesomorphic.25 We have inferred that,
independently of the NO- or OO-coordination of the
b-diketonate ligands in the trans or cis isomers, the square-
planar bis-b-diketonate palladium(II) complexes yield to
unfavourable molecular requirements to allow the supra-
molecular order of the mesophases.
Thermal studies
The thermal behaviour of all compounds was determined by
using polarized-light optical microscopy (POM) and differential
scanning calorimetry (DSC) techniques. The nature of the
mesophases was confirmed by means of X-ray diffraction
(XRD) at variable temperature. Table 7 summarises the
For the organometallic compounds studied in this work
corresponding to the allyl–palladium systems the results were
much more interesting. So, whereas the [HLR(n)py] (1–4)
ligands were non-mesomorphic, their coordination to
the [Pd(Z3-C3H5)]+ fragment through the reaction with
[Pd(Z3-C3H5)(acetone)2][PF6] (Scheme 3b) gives rise to the
new species (HLR(n)py)][PF6] (type I; 13–16) which behave as
liquid crystal materials in the cases 15 and 16 with extended
alkyl chains (n = 16, 18 respectively).
The optical observations by POM in 15 and 16 confirm the
presence of a single mesophase characterized as SmC on the
basis of their typical Schlieren texture (Fig. 7).
Fig. 6 Equilibrium of the allyl–palladium complexes in CDCl3
solution.
c
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Fig. 7 Microphotograph of the SmC mesophase observed by POM of
[Pd(Z3-C3H5)(HLR(16)py)][PF6] (15) at 160 1C on cooling (the white bar
indicates 100 mm).
Fig. 8 Microphotograph of the SmC mesophase observed by POM of
[Pd(Z3-C3H5)(LR(14)pyH)][PF6] (18) at 165 1C on heating (the white bar
indicates 100 mm).
The related complexes 13 and 14 with shorter alkyl chains
were not mesomorphic as deduced by POM, exhibiting in both
cases an endothermic peak corresponding to the crystal-
isotropic liquid phase transition observed on their DSC
thermograms at 180 and 173 1C, respectively. Additionally,
the thermogram of compound 14 shows a peak at 76 1C
related to a preceding solid–solid phase transition.
temperature (Table 7). The partial decomposition causes a
non-reproducibility of the thermal data in the successive
heating cycles, this fact making difficult the observation of
the mesophase on cooling from the isotropic liquid.
To determine the temperature of solidification, preventing
decomposition, the samples were heated at temperatures
slightly higher than those of the formation of the mesophase
and then cooled. In this way the solid phase is reached and
detected by DSC.
In the DSC thermograms of complexes 15 and 16, after a
solid–solid phase transition at 90 and 87 1C, respectively, new
peaks at 155 and 144 1C are attributed to the transformation
from the solid to the SmC mesophase in agreement with the
POM observations.
Again, the increase in the length of the alkyl chain leads to a
decrease in the melting and clearing temperatures, avoiding
the partial decomposition.
It was remarkable that on cooling the DSC thermograms of
13–16 did not show exothermic peaks with significant enthalpy
values. This fact can be explained on the basis of the partial or
extended decomposition suffered by the complexes on the first
heating. That decomposition was less extended for the liquid
crystals 15 and 16 which displayed the lower clearing
temperatures. The appearance of the liquid-crystalline phase
is favoured with the elongation of the alkyl chain, which
also determines a decrease of both melting and clearing
temperatures.
At this point of the discussion the effect of the coordination
environment and the molecular shape in the liquid crystalline
properties deserves a special comment.
For complexes II with the protonated ligand [LR(n)pyH +
] the
bidentate OO-coordination involves the formation of a
five-membered ring with an electronic delocalization similar
to that observed in the related bis-b-diketonate palladium
derivatives, while the protonated pyridyl group could
probably not to be coplanar with the mentioned ring so
introducing some distortion in the overall molecular geometry.
In order to achieve liquid crystal behaviour the balance
between the cores and the molecular chains interactions is
crucial. Thus, as it has been studied, when the contribution of
the chains prevail, transition temperatures generally decrease
as the chain length increases due to higher flexibility, making
the interactions between the molecular cores more difficult.
However, an opposite result was found if the core contribution
is the determining factor.26
For the complexes of the type II, compound 17 (n = 12) did
not show liquid crystal properties exhibiting only a single
phase transformation from solid to the isotropic liquid at
174 1C. By contrast the other members of the series 18–20
(n = 14–18) are enantiotropic liquid crystals showing SmC
mesophases which were identified by POM through their
typical focal conic broken and Schlieren textures (Fig. 8).
The DSC thermograms recorded on the first heating show
two peaks corresponding to the solid-mesophase and mesophase-
isotropic liquid phase transitions preceded by a solid–solid
transition (Table 7). On cooling, the solidification process
from the mesophase was not seen by DSC, but it was
observable by POM.
The main features observed in the complexes here studied
agree with the first of these behaviours. Then, it is possible to
suggest that the core–core interactions are disadvantageous
compared to those of the alkyl chains, this effect being related
to the absence of planarity of the molecular core.
For palladium-complexes 18–20 it should be noted that at
the temperatures close to the clearing a slight decomposition is
observed which is smaller as the length of the alkyl chain
increases, in agreement with the decrease in the clearing
Moreover, from the comparative study of the two families
of complexes I and II we have been able to establish a selected
design in order to improve the liquid crystal behaviour. In
particular compounds I with a bidentate NO-coordination of
c
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s
(singlet),
d
(doublet),
t
(triplet),
m
(multiplet). The
1H chemical shifts and coupling constants are accurate to
ꢂ0.01 ppm and ꢂ0.3 Hz, respectively. In addition, the solution
spectra of the compounds 1, 8, 9, 14 and 18 were recorded, at
300 K save specified, on a Bruker DRX 400 (9.4 Tesla,
400.13 MHz for 1H, 100.62 MHz for 13C and 40.56 MHz
for 15N) spectrometer with a 5 mm inverse detection H–X
probe equipped with a z-gradient coil for 1H, 13C and 15N.
Chemical shifts (d in ppm) are given from internal solvents,
CDCl3 at 7.26 for 1H and at 77.0 for 13C. An external reference
CH3NO2 (0.00) for 15N-NMR was used.27 2D (1H–1H)
gs-COSY and inverse proton detected heteronuclear shift
correlation spectra, (1H–13C) gs-HMQC, (1H–13C) gs-HMBC,
(1H–15N) gs-HMQC, and (1H–15N) gs-HMBC, were acquired
and processed using standard Bruker NMR software and in a
non-phase-sensitive mode.28 Gradient selection was achieved
through a 5% sine truncated shaped pulse gradient of 1 ms.
Variable temperature experiments were recorded on the same
spectrometer using a Bruker BVT3000 temperature unit to
control the temperature. Solid state 13C (100.73 MHz) and
15N (40.60 MHz) CPMAS NMR spectra of 1, 8, 9, 14 and 18
have been obtained on a Bruker WB 400 spectrometer at
300 K using a 4 mm DVT probe head and a 4 mm diameter
cylindrical zirconia rotor with Kel-F end-caps. The non-
quaternary suppression (NQS) technique to observe only the
quaternary carbon atoms was employed.29 13C-NMR spectra
were originally referenced to a glycine sample and then
the chemical shifts were recalculated to the Me4Si (for the
carbonyl atom d(glycine) = 176.1 ppm) and 15N-NMR
spectra to 15NH4Cl and then converted to nitromethane scale
using the relationship: d15N(nitromethane) = d15N(ammonium
chloride) ꢀ 338.1 ppm.
Fig. 9 Bar diagrams indicating the range of the phases (Cr, Cr0 and
SmC) present in the complexes of the types I and II.
Table 8 X-Ray diffraction data
Complex T/1C Position (1)/2y d-spacing/A Miller indices (h k l)
15
160
2.6
5.2
7.7
10.2
20.0
2.4
4.7
7.0
34.3
17.1
11.4
8.7
(0 0 1)
(0 0 2)
(0 0 3)
(0 0 4)
a
4.5
19
130
37.1
18.7
12.6
4.5
(0 0 1)
(0 0 2)
(0 0 3)
a
20.0
a
Halo of the molten alkyl chains.
the ligand exhibit liquid-crystalline phases when they have
extended alkyl chains, with n = 16 and 18 carbon atoms, while
those of the type II, containing the zwitterionic OO-coordinated
ligand, were liquid crystals in the cases of n = 14–18 carbon
atoms. As it is depicted in Fig. 9 the compounds of type II
have better mesomorphic properties exhibiting a range of the
SmC mesophases significantly higher than that on the related
complexes of type I.
Elemental analyses for carbon, hydrogen and nitrogen were
carried out by the Microanalytical Service of Complutense
University.
IR spectra were recorded on a FTIR Thermo Nicolet
200 spectrophotometer with samples as KBr pellets in the
4000–400 cmꢀ1 region: vs (very strong), s (strong), m
(medium), w (weak).
Finally, selected metallomesogens were subjected to
temperature dependent powder X-ray diffraction (XRD)
measurements. The results are summarized in Table 8. XRD
studies of the compounds confirm the smectic nature of the
fluid phases observed by POM. Two set of reflections were
observed in the diffractograms, one at small angles which
display well-defined peaks in a 1 : 1/2 : 1/3 : 1/4 ratio from
the (0 0 1), (0 0 2), (0 0 3) and (0 0 4) reflections corresponding
to a lamellar structure, the other one consists in a diffuse halo
at wide angles (ca. 4.5 A) owing to the liquid-like order of the
molten alkyl chains (Fig. S3, ESIw).
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 scanning
calorimeter with the sample (1–4 mg) sealed hermetically in
aluminium pans and with a heating or cooling rate of
10 K minꢀ1
.
Experimental section
The X-ray diffractograms at variable temperature were
recorded on a Panalytical X’Pert PRO MPD diffractometer
in a y–y configuration equipped with a Anton Paar HTK1200
heating stage (X-Ray Diffraction Service of Complutense
University).
Materials and physical measurements
All commercial reagents were used as supplied. Commercial
solvents were dried prior to use.
1H-NMR spectra were performed at room temperature on a
Bruker DPX-300 spectrophotometer (NMR Service of
Complutense University) from solutions in CDCl3. Chemical
shifts d are listed relative to Me4Si using the signal of the
deuterated solvent as reference (7.26 ppm), and coupling
constants J are in hertz. Multiplicities are indicated as
Preparation of the [HLR(n)py] compounds (1–4)
To a suspension of NaH (60 wt% in mineral oil, 15.3 mmol) in
tetrahydrofuran was added 9.0 mmol of the corresponding
4-n-alkyloxyacetophenone and it was stirred at room
c
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temperature for 30 min. Then 13.5 mmol of 2-ethylpicolinate
in tetrahydrofuran were slowly added, and the reaction
mixture was refluxed for 24 h. The solution was left to cool
to room temperature and then was poured in ca. 400 mL of
water and HCl solution (spec. grav. 1.19). The mixture was
stirred for 24 h at room temperature. The pale yellow
precipitate was filtered off and washed with n-hexane. Spectro-
scopic data are given for 1. Data for the other homologues are
essentially identical.
Preparation of the complexes [Pd(g3-C3H5)(LR(n)pyH)][PF6]
(17–20)
The synthetic process is analogous to that described above for
13–16 complexes, but in this case the corresponding
[HLR(n)pyH]Cl salts were added. Spectroscopic data are given
for 18. Data for the other homologues are essentially identical.
[Pd(Z3-C3H5)(LR(14)pyH)][PF6] (18). nmax(KBr)/cmꢀ1 3549m,
3487m, 3415s (NH), 1602s (CQC) + (CQC), 783m g(CH)py,
845vs n(P–F), 558w g(F–P–F).
[HLR(12)py
] (1). n
max(KBr)/cmꢀ1 3389w (OH), 1602vs
Yields, elemental analysis and NMR data are collected in
(CQO) + (CQC), 785m g(CH)py.
Yields, elemental analysis and NMR data are given as ESIw
(Tables S1 and S2).
ESIw (Tables S7 and S8).
Conclusions
Preparation of the [HLR(n)pyH]Cl salts (5–8)
The palladium(II) bis-b-diketonate complexes based on the
strategically designed versatile ligands [HLR(n)py] which can
coordinate in a bidentate NO- or OO-fashion were prepared
and their molecular structures investigated. The complexes
were isolated as a mixture of trans and cis isomers each of
them having the ligand in one or other coordinative forms.
The absence of liquid-crystalline properties indicate that
none of the components of the mixture was liquid crystal,
independently of all the variables found. By contrast the ionic
complexes of the types [Pd(Z3-C3H5)(HLR(n)py)][PF6] (I) and
[Pd(Z3-C3H5)(LR(n)pyH)][PF6] (II) containing the neutral
ligand pyridyl b-diketone NO-coordinated or the related
zwitterionic pyridiniumyl b-diketonate OO-coordinated
proved to be metallomesogens exhibiting enantiotropic SmC
mesophases independently of the different coordination
environment around the metal. It appears that the p-delocalization
of the allyl–palladium group was determining to produce
lamellar mesophases, the better mesomorphic results being
achieved for complexes of the type II. Therefore we can
suggest that the allyl–palladium and the zwitterionic
b-diketonate ligand are the best components in designing
mesomorphic ionic compounds.
The synthetic route for [HLR(n)pyH]Cl is shown in Scheme 1a.
The detailed procedure and characterization by 1H-NMR and
IR has been previously reported by us.15 New NMR data are
also reported as ESIw (Table S3).
Preparation of the complexes [Pd(j2-LR(n)py)2] (9–12)
The corresponding [HLR(n)py] (0.30 mmol) and NaH (60 wt%
in mineral oil, 0.60 mmol, 14.40 mg) were added to 75 ml of
dichloromethane and stirred for 30 min. Palladium(II) acetate
(0.15 mmol, 33.67 mg) which was dissolved in a small amount
of dichloromethane was added to the reaction mixture and
refluxed for 24 h. The hot reaction mixture was filtered over
Celite and then was cooled to room temperature; the resulting
orange precipitate was collected by filtration and then
washed with hexane and dried in vacuo. Spectroscopic data
are given for 9. Data for the other homologues are essentially
identical. Yields and elemental analysis are given as ESIw
(Table S4).
[Pd(k2-LR(12)py)2] (9).
n
max(KBr)/cmꢀ1 1693s (CQC),
1601vs (CQO) + (CQC), 782m g(CH)py.
Acknowledgements
Preparation of the complexes [Pd(g3-C3H5)(HLR(n)py)][PF6]
(13–16)
We thank the Spanish Ministry of Science and Innovation
(grants no. CTQ2009-11880, CTQ2010-16122 and CTQ2010-
19470), and UCM-Santander (GR58-08-910300) for the
financial support.
To a solution of [Pd(m-Cl)(Z3-C3H5)]2 (100 mg, 0.273 mmol) in
dry acetone (25 mL) was added AgPF6 (138.1 mg, 0.546 mmol)
under a nitrogen atmosphere. The mixture was stirred
overnight in the absence of light and then filtered over Celite.
The corresponding [HLR(n)py] (0.546 mmol) in dichloro-
methane (20 mL) was added to the resulting solution and let
stirring overnight at room temperature. Then the solvent
was removed in vacuo and the solid recrystallised in
dichloromethane/hexane leading to the precipitation of a
yellow solid, which was filtered off, washed with hexane and
dried in vacuo. Spectroscopic data are given for 14. Data for
the other homologues are essentially identical.
Notes and references
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4 K. Ohta, A. Ishii, H. Muroki, I. Yamamoto and K. Matsuzaki,
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(OH), 1600s (CQC) + (CQC), 780m g(CH)py, 842vs n(P–F),
558w g(F–P–F).
Yields, elemental analysis and NMR data are reported in
ESIw (Tables S5 and S6).
c
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