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Chapter 5
Crystal engineering: molecular
architectures in the solid state.
5.1 Concepts, Examples and Perspectives.
The assembly of individual molecular units driven by intermolecular
interactions represents the central scope of supramolecular chemistry.1 Its principles
can be applied in the solid state to the formation of supramolecular networks. These
networks can be mono dimensional 1D, 2D or 3D and this depends on the number of
translations operating at the molecular assembly core.2 The solid state is the preferred
phase to develop such aim due to the fact that analysis by X-ray techniques allows
accurate structural studies, but in principle molecular networks could be obtained in
any type of condensed phase. This field of research which recalls scientists from many
different areas is frequently called crystal engineering,3a or molecular tectonics.3b
A tecton is the basic molecular building block. It is rationally designed in such
a way to bear proper recognition sites able to lead to the desired organization upon
association with itself or with another tecton. Again molecular recognition is the
fundamental step towards complexity. A crystal solid can be viewed as the
supermolecule par excellence.4 This vision replaced the old-fashioned concept for
which a crystal architecture was the result of the orror vacui, i.e. the need to avoid
emptiness in the crystal. The geometrical motifs and the architecture of a crystalline
solid are the result of many weak interactions whose complete understanding is, still
nowadays, far from completion. Indeed, any crystal structure of organic molecules
cannot be predicted a priori.5 The present knowledge of covalent valence can be
considered satisfactory and by the covalent approach it can reasonably be stated that
every target molecule can be synthesized. In variance, the non-covalent valence still
need many efforts towards understanding and rationalization. This problem is common
to several other related fields. The structure-reactivity relationship in the folding of
proteins is just one example. However, despite many evident difficulties, the following
question made by Feyman in 1960 “What would the properties of materials be if we
could really arrange the atoms the way we want them?” opened and fuelled the
continuously growing area of research on crystal engineering.6 Indeed, obtaining a
supramolecular assembly in the solid state means the creation of a new material which
can posses new features and properties. Applications in several field of research is
Chapter 5
therefore possible and highly appealing. Many examples can be found in literature
showing that even if crystal prediction is very elusive and much remains in the realm
of pure speculation, short term applications are not precluded and some of them are
even of consummate structural beauty.
Figure 1: Enatiopure bulding block 1 forms highly organized crystal structures (a) which are
provided with large chiral channels (b), they can be exploited in enantioselective catalysis.
The enantiopure chiral organic building block 1 is easily synthesized from D-tartaric
acid.7 It coordinates to Zn2+ ions to produce a homochiral open-framework solid
(referred as D-POST-1). The enantiomorphic L-POST-1 is obtained from the
enantiomer of 1 and the Zn2+ ions under the same conditions. In D-POST-1, three zinc
ions, held together with six carboxylate groups of the chiral ligands and a bridging oxo
oxygen, form a trinuclear subunit. These trinuclear units are interconnected through
coordinative bonds between the zinc ions and pyridyl groups of 1, thereby generating
two-dimensional (2D) infinite layers. (Figure 1a and 1b). The average interlayer
separation is 15.47 Å. Most notably, large chiral 1D channels are formed and their
cross-section is best described as an equilateral triangle with a side length of 13.4 Å
(Fig. 1b). Almost 50% of the total volume is void filled with water molecules. These
chiral channels are responsible for the fact that this solid compound is a catalysis of
transesterification reactions. In Figure 2 different rates in the ethanolysis of
2,4-dinitro-phenoxyacetate in the presence of POST-1, the pyridine N-methylated
derivative and in the absence of catalyst are shown. Moderate enhancements are
observed. Moreover POST-1 revealed enantioselective catalytic activity in this
transesterfication reaction. Studies showed that the reaction of the latter with a large
excess of racemic 1-phenyl-2-propanol in the presence of D-POST-1 or the
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Crystal engineering.
enantiomorphic L-POST-1 produces the corresponding esters with 8% enantiomeric
excess in favour of S or R enantiomer, respectively.
Figure 2: Ethanolysis reaction catalysed by POST-1.
This enantioselectivity is modest but still noteworthy because asymmetric induction
has never been observed in reactions mediated by modular porous materials.
Many extended metal-organic frameworks (MOF) with rich structural, thermal,
magnetic, and sorptive properties by the use of simple diazaaromatic anions have been
reported. For instance neutral and flexible 3D sodalite-type MOFs of formula
[Cu(2-pymo)2]n show heterogeneous solid-liquid sorption responsible for guest
induced crystal-to-crystal phase transitions (pymo=pyrimidinolate).8
Figure 3: a-d) Green balls: Cu; purple balls: MNO3; green sticks: pymo N,N' bridges; e)
LiNO3 tetraaquo complex.
An ab initio X-ray powder diffraction (XRPD) study on the hydrated [Cu(2-pymo)2]n
rhombohedral material (1R, Figure 3a) reveals its distorted 3D sodalite-type
framework. This 3D framework is not rigid but, upon exposure to an aqueous methanol
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Chapter 5
solution of MNO3 (M= NH4+, Li+, Na+, K+, Rb+ and Tl+), a transition to a cubic phase
(MNO3@1C, Figure 3b), is observed. The kinetically controlled crystal-to-crystal
inclusion process of the 3D framework 1R to MNO3@1C is, indeed, followed by
further incorporation of MNO3 ion pairs, leading to isomorphous orthorhombic
layered materials of type [Cu(2-pymo)2]n‚(MNO3)n/2 (MNO3@1O) (Figure 3c).
Despite these large guest-induced structural changes, the original 1R phase can
be restored. For instance, refluxing KNO3@1O or RbNO3@1O in MeOH for 6 days
with 18-crown-6-ether removes the MNO3 guests, giving an empty layered
[Cu(2-pymo)2]n species (1O, Figure 3d) which can be readily converted to the original
1R phase by exposing it to water. A particular of the LiNO3@1C structure reveals the
Li+ ion as a tetraaquo complex (Figure 3e).
Figure 4: Legenda: blue polyhedron= zinc; yellow balls= accessible space for storage.
Metal-organic frameworks of composition Zn4O(BDC)3 where BDC stays for
1,4-benzenedicarboxylate (Figure 4, A) and related structures (Figure 4, B and C),
form a cubic three-dimensional extended porous structure which is able to adsorb
hydrogen up to 4.5 weight percent (17.2 hydrogen molecules per formula unit) at 78 K
and 1.0 weight percent at room temperature and pressure of 20 bar. These materials
seem to be competitive with metal hydrides which are expensive and with carbon
nanotubes and similar compounds which have been beset by mixed results.9
Supramolecular architectures in the solid state can also be used as a template to
induce chemical reaction as shown in the next example. The [n]ladderanes are
molecule that consist of n edge-sharing (n>2) cyclobutane rings and are viewed as a
molecular equivalent of a macroscopic ladder.10a Ladderanes are promising in
optoelectronics devices and have been found to posses an important role in biological
systems. Despite the apparent simplicity of the intermolecular photochemical
dimerization of two all-trans-poly-m-enes (m=2, 3, 4…) which should occur to
generate ladderanes, generally such transformations fail. A crystal engineering
approach can be very useful. If a properly designed crystal architectures is created in
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Crystal engineering.
such a way to put the two polyenes in an organized solvent free environment (as in the
case shown in Figure 5), the desired reaction takes place upon UV irradiation. Two
crystal structures of the starting and ending material are reported in Figure 5. The
conversion of the polyene into the ladderane is quantitative.10b
a)
b)
Figure 5: Solid state conversion by UV irradiation of a polyene a) into ladderane, b).
Designing molecular architectures requires great effort as already shown.
However a strategy for a rational design must consider three different, consecutive
steps of increasing complexity which refer to three grade of sub-structures. The first
step must account for the tecton structure and its features, being it a metal center or a
molecule. A metal should feature a coordination geometry directing the interacting
ligands towards association in the desired geometry; a molecule has to bear binding
sites properly disposed to interact, with an high grade of directionality, with the
molecular partner. The geometrical features of the tecton or tectons is the first substructure to be considered. The secondary structure corresponds to the object formed
by the association of tectons with each others. This assembly process can lead to rows,
layers or 3D objects depending on the primary structure. The way in which secondary
subunits interact with each others determines the tertiary structure. This final
arrangement highly influences the properties of the crystal and it has to be taken in
great consideration in the a priori design process even if it is the most elusive to
prediction. Polymorphism is the risk and it should be avoided in order to obtain always
the same solid with the same properties upon crystallization. In the case of a 3D
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Chapter 5
secondary sub-unit the tertiary structure coincides with the secondary for obvious
reason.
Building the desired object in the solid state is very complex and subtle factors
can determine big failure or success. To try to minimize the sources of errors, strong
and highly directional interactions are required to produce high affinities between
tectons and to avoid polymorphism. Not surprisingly H-bonds and coordinative bonds
are the most used tools in crystal engineering.11 Some example are described.
Figure 6: The organization in the solid
state of the porphyrin in figure create
cavities of 22 Å (edge to edge).
In Figure 6 a porphiryn is functionalized with H-bond donor and acceptor groups and
the crystal structure obtained shows interesting structural features which suggest the
use of such compound as molecular sieves.12
Figure 7: Polymetal oxalate complexes
that form multilayers with magnetic
properties.
128
Crystal engineering.
Obtaining materials with unusual magnetic properties can be achieved by the
formation of successive layers of different composition. Several metal elements can be
used. Their oxalate complexes can be arranged in infinite layers between which
cromocenium or ferrocenium complexes intercalate. The result, shown in Figure 7,
reveals a complex structure whose bulk magnetic properties can be tuned depending on
the metal.13
Figure 8: High symmetry in molecular networks based on the [Re 6-(3-Q)8]2+ core (molecular
formula, left and X-ray structure of two superimposing trigonal units, right).
The [Re6-(3-Q)8]2+, where Q is S or Se, revealed to be a very versatile building block
to build complex molecular networks.14 In Figure 8 only one over the manifold
possible geometries is reported. The versatility of the building blocks along with the
high synthetic accessibility of the cluster core itself provide access to many possible
geometries and applications. The cluster core is inert and thus it prohibits
stereochemical scrambling, ensuring a fixed geometry which limits the structural
possibilities of a multicluster array upon assembly.
Clearly all kinds of intermolecular forces have a role in crystal packing and
can be exploited to obtain the desired geometry. Therefore, - stacking and charge
transfer interactions have been proposed to be good candidates in molecular tectonics
as well.15 Strong ion-ion electrostatic forces,16 along with weaker CH··· and
CH···O17 and CH···X18 interactions can also give remarkable contributions to this
field.
Even if a rational design is the first compulsory step towards engineered
crystals, serendipitous, yet extremely fascinating organizations are reported in
literature. With surprise, Rissanen and co-workers found that mixing basic building
blocks consisting in tetramethylresorcinarene 2 and the diquat 3 resulted in an
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Chapter 5
intriguing nano-tubular structure held together by intra-tubolar  interactions, as
shown in Figure 9.19
a)
b)
Figure 9: a) molecular building blocks 2 and 3 and b) their supramolecular assembly (some
representations).
5.2 UO2- and Al-Salophen and Salen Complexes as Building Block for
Molecular Tectonics.
The careful and rational design of interacting tectons, have led to very complex
molecular architectures with interesting properties which have been reported so far.
Sometimes even very simple tectons can display interesting geometry in the solid state,
provided that they posses suitable binding sites. An interesting example is given by
compound 4. It is a very simple molecule which can be dissected into two domains.
The aromatic moiety is hydrophobic and its optimal large surface gives - staking.
O1
H
O2H
O
4
C1
10C
C2
C9
C3
C4
C8
C7
C5
C6
Figure 10: Tecton 4 and the molecular organization in staked layers which shows - and
H-bonding interactions. Colour scheme: black, grey=C, red=O; cyan dotted lines= H-bond, blue
lines=- stacking.
130
Crystal engineering.
The aldehydic moieties are polar and provide donor and acceptor sites for H-bonding.
The crystal structure is reported in Figure 10. The organization at the supramolecular
level shows rows of molecules connected by a four center hydrogen bond between the
hydroxyaldehyde moieties and several - interactions between the aromatic surfaces.
Distances of the aromatic carbon atoms C3 and C5 to the closest centroid on the
naphthyl moieties are 3.504 and 3.7489 Å (blue lines). The four centered hydrogen
bond displays interacting oxygens (carbonyl and hydroxyl ones) 3.022 Å apart.
Naphthyl-CH2 between different rows also show close contacts, distances ranging from
3.442 to 3.493 Å.
Figure 11: Tecton 5 and the molecular organization in staked layers. Colour scheme: black,
grey=C, red=O; blue dotted line= H-bond.
When a more complex molecule is taken into account, things can change dramatically.
With tecton 5 the recognition pattern displayed by 4 is absent. The four center H-bond
is replaced with a four center CH···O, OH···O bond with CH···O=C, CH···OH,
C=O···HOinter and C=O···HOintra distances of 3.736, 3.338, 3.093 and 2,675 Å,
respectively.
Salophen-Uranyl complexes have been reported to be effective receptors for
hard anions in polar solvents.20 The uranyl cation complexed in a salophen unit prefers
a pentagonal bipyramidal coordination, with the two oxygens at the apical positions
and with both the four-coordinating sites of the salophene moiety and a guest molecule
in the equatorial positions. In 1994 a crystal structure of Salophen-UO2 complex 6 with
H2PO4- have been documented.21 As shown in Figure 12, the phosphate ion is
coordinated to the UO2, not surprisingly (the association constant of 6 with H2PO4 was
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Chapter 5
measured: K= 1.5x104 M-1 in MeCN:DMSO 99:1). Moreover the anion forms a doubly
H-bonded bridge which connect the two salophen molecules. Thus the two phosphate
molecules, connected by H-bonding, act as a spacer between the two receptor units.
N
Figure 12: Receptor
6 and its H2PO4- 2:2
complex in the solid
state (guest P=green,
O=red).
N
UO2
O
O
O
O
6
In order to build infinite chains of this kind in the solid state the receptor has to be
ditopic with the two binding sites pointing in opposite directions. For this purpose we
synthesised salophen-UO2 complex 7 (Figure 13). This compound is formed as a
complex with two pyridine molecules (present in the reaction mixture). Unfortunately
the compound is soluble only in DMSO and this fact renders the crystallization process
very slow. The phosphate ion can therefore act as a linear spacer upon dimerization in
the solid state via H-bond. If a different spacer is considered diverse geometries, apart
from the linear, could be expected as shown in Figure 14.
Figure 13: Ditopic receptor 7.
1
H- and 13C-NMR spectra.
Pyridine signals are
denoted with an x
7
132
Crystal engineering.
O
N
O
O
UO2
N
N
N
N
N
UO2
O N O
N
N
O
N
N
UO 2
O N O
N
N
O
N
N UO
2 O
N
UO 2
N
O
O
UO2 N
O
N
N
N
N
N
N
O UO
2 N
N
O
N
O
UO 2
UO2
O
N
O
N
N
N
UO 2
O N O
Figure 14: Some of the possible spatial organization in the association of bi- and tri-dentate
nitrogen ligands with 7
As a preliminary study the complexation between bipyridyl and the simple
salophene-UO2 complexes in solution showed moderate binding in 95:5 chloroform:
methanol solution. In Figure 15 is shown a 1H NMR titration experiment between the
Host 0 bipyridyl
two species.
9.2
9.1
N
N
UO2
9.0
O

O
C
N
8.9
N
4,4'-bipyridyl
8.8
8.7
0.000
0.002
0.004
0.006
[host]
Figure 15: 1H NMR titration between salophen-uranyl host C and the bipyridyl ligand.
133
Chapter 5
The data can be easily fitted with a 1:1 binding isotherm, suggesting that the second
equilibrium in Scheme 1 is not favourable in these conditions (K1=400±30 M-1,
=-0.56; K2 is negligible). Anyway, the binding occurs and it supports the possibility
of the desired complexation in the solid state.
N
N
N
UO 2
O
O
N
N
UO 2
O
N
O
N
N
UO 2
O
O
N
UO 2
O
O
N
N
N
K1
K2
N
O
O
N
UO 2
N
N
Scheme 1: Multiple equilibria in the association between salophen-UO2 complex C and the
bipyridyl ligand.
The creation of proper architecture in the solid state could lead also to the exploitation
of the concave nature of the salophen-UO2 compounds. The salophen derivative 8 that
we prepared does not posses suitable secondary binding sites for the recognition of
organic cations due to poor electron density on the aromatic rings. Still, in the solid
state it displays evident interactions with a N-Methylquinuclidinium bromide
exploiting the concave structure derived by the distortion of the ligand caused by the
large ionic radius of the UO22+. The uranyl oxygens may display weak Lewis basicity
and interact with cationic partners. Moreover phenoxy rings on the host can be
involved in cation- interactions. The X-ray structure obtained is shown in Figure 16,
along with the tripeptide-NMQI complex reported by Kubik.22 A clear resemblance can
be noted. However, as reported in Table 1, cation- interactions play an ancillary role
in the complex formation.
Figure 16: Comparison between two NMQ complexes
134
Crystal engineering.
O
N
N
U
O
8
O
O
S
C9
C13
C11 C12
C9 4.407
C10 4.007
51-O=U 3.293
52 O=U 3.407
S
C11 3.726
54 O=S
3.366
C12 3.873
53 O=S
3.257
C13 4.337
57 O=S
3.865
C14 4.585
U-Br
2.784
O
O
Br
51C
N
C14
C10 A
O
O
O
O
C52
C53
C54
C57
Table 1: Relevant interatomic distances
in 8-NMQBr (Å).
CH-O and coord.
Cation···
NMQBr
Figure 17: Numbering scheme for the 8-NMQBr complex
Aluminium complexes are finding an increasing importance in many field of
organic synthesis, especially as catalysts for polymerisation and Lewis acid promoted
reduction of aldehydes and ketones.23 Moreover they posses some interesting features
from a spectroscopic and structural point of view. Salen-Al complexes are the most
widely used for their synthetic accessibility and inertness even in the presence of Al-C
bonds. Aluminium cations are naturally electron deficient and they act as hard Lewis
acids. The X-ray structure in Figure 18 shows a square-pyramidal geometry, not very
common for Al3+.24 In this case the complex is a salen-Al-ethyl crystallized by slow
cooling of a dry acetonitrile solution. The four donors of the tetradentate ligand are on
the same plane, while the alkyl chain is on the apical position.
Figure 18: Square pyramidal geometry in the X-ray structure of the salen-Al -ethyl complex.
The coordination geometry can change in the presence of coordinative solvent
molecule such as water or methanol. The geometry of the Al center switches from
square-pyramidal to octahedral with the two solvent molecules in the apical positions
while the salen ligand donors occupy the equatorial plane. Many example of salen
derivatives are reported in literature. Some of them are shown in Figure 19.25
135
Chapter 5
Figure 19: Planar octahedral complexes of salen-Al derivatives a-f.
All these case are summarized in the scheme 2, (right hand side). The situation is
different when an alkyl group is coordinated to the metal center. In these case the
protonation of alkyl group, which forms the corresponding alkane and deprotonated
solvent, leads to a dimerization with two bridging solvent molecules. The process is
described by the left-hand portion of Scheme 2. A crystal structure of a salen derivative
complexed with methanol is reported in Figure 20.25b In this case the geometry is
different compared to previous examples and the four donor atoms from the salen
ligand are not on the equatorial plane anymore.
R1
N
R
O
O
O
O
O
O
Al
N
N
N
R
R
Al
R1
R1OH
N
X = Alkyl
R= CH2-CH2, C6H4
R1= Me, H
X
Al
N
2 R1OH
O
O
N
R
X = Chloride
R= CH2-CH2, C6H4
R1= Me, H
O
X
Al
N
O
1/2
Scheme 2: Behavior of salen- salophen- Al complexes depending on the X ligand nature.
Figure 20: doubly
MeOH
bridged
salen
derivative
compound X-ray
structure
(left),
and the chemical
structure, (right)
136
OR1
OR1
Crystal engineering.
Salen-Aluminium complexes spectroscopic properties have been only recently
documented and they possess interesting luminescence.26
To our knowledge Salophen-Al complexes have never been synthesized. The
aluminium center is a strong Lewis acid and therefore can interact with suitable Lewis
bases to form organized structures in the solid state. 4,4'-bipyridyl has been chosen as a
good starting point, being a ditopic ligand with the two nitrogen donor atoms pointing
outwards. It should in principle be able to form infinite linear structures when
associated with salophen- or salen-aluminium complexes (Fig. 21a). With this idea in
mind Al-complexes 9-12 has been synthesized (Fig. 21b).
Many crystallization attempts in obtaining the desired geometry failed and the
only single crystal suitable to X-rays analysis resulted to be analogous to the “dimeric”
complex shown in Figure 20. The structure is reported in Figure 21c.
a)
9:
N
R
N
b)
O
X
Al
O
X= Cl
R= CH2-CH2
10: X= Me
R= CH2-CH2
11: X= Cl
R= C6H4
12: X= Me
R= C6H4
c)
Figure 21: a) Possible linear organization in the solid state of bipyridyl ligand with Al-complex
(represented as a blue ellipsoid); b) salen- and salophen-Al complexes synthesized c) crystal
structure of the methanol solvated dimeric species: H=white, C=dark grey, N= blue, O=red, Al=
purple; ORTEP view with 50% ellipsoid probability (H in their calculated positions).
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Chapter 5
5.3 Experimental part.
Synthesis of compound 7 (Scheme 3): 0.344 g of salycilaldehyde are added to 200mg solution
of 1,2,4,5-tetraaminobenzene tetrachlorohydrate and 0.23 ml of pyridine in 40ml of MeOH
under argon atmosphere. The solution is refluxed for 30 minutes, whereupon solid uranyl
diacetate is added (0.59 g). The solution turns deep red with the formation of abundant
precipitate. The solid corresponding to 7·2C5H5N is filtered. Yield: 45%. 1H NMR (DMSO): 
9.88 (s, 4H),  8.5786-8.5703 (d, 2H, pyr),  8.299 (s, 2H),  7.814-7.7693 (m, 6H),  7.6508 (t,
4H),  7.3816 (t, 4H, pyr),  7.0489-7.0323 (d, 6H),  6.745 (t, 4H). 13C NMR:  170.095, 
166.981,  149.524, d 146.576, d 136.284, d 136. 155, d 136.028, d 124.184, d 123.884, d
120.720, d 116.717. Mass spectrum (ESI), m/z 2181.50, 1091.3, 1125.28 ([2M+H] +, [M+H]+,
[M+Cl]- calcd for C68H45N8O16U4, C34H23N4O8U2 and C34H22N4O8U2Cl 2181.50,
1091.25 and 1125.21, respectively).
H2N
NH2
.
H2N
Scheme 3
O
4 HCl
+
OH
4
2 UO2(OAc)2
+
NH2
N
CH3OH, reflux
O
N
N
O
O
UO2
UO2
N
N
.
2
N
O
7
Synthesis of 2-Hydroxy-3-(phenylsolphoxy)benzaldehyde (Scheme 4): To a suspension of
NaH (0.565 g, 80% in oil), prewashed with n-pentane, in DMSO (10 mL) a solution of
2,3-dihydroxybenzaldehyde (1 g, 7.764mmol) in DMSO (5 mL) was added at 20-25°C. After 1
h of stirring, neat benzenesulphonylchloride (1.37 g, 7.764 mmol) was added, and stirring was
continued for 24 h, whereupon the mixture was poured into water (50 mL) and extracted with
CHC13 (3x20ml). The aqueous layer was acidified with 6 M HCl to adjust the pH to 3 and was
again extracted with CHC13 (3x50 mL). The latter combined CHC13 layers were washed with 1
M HCl (2x20 mL). Column (SiO2/CHCl3) gives pure compound as a yellow solid: yield 53%;
138
Crystal engineering.
H NMR (CDCl3):  10.90 (s, 1H, CHO),  9.86 (s, 1H, -OH),  7.96-7.484 (m, 7H),  7.0039
(t, 1H). mass spectrum (EI), m/z 278 (M+, calcd for C13H10O5S 278.28).
1
Synthesis of compound 8 (Scheme 5): To a refluxing solution of 2-hydroxy-3(phenylsolphoxy)benzaldehyde (0.9 g, 3.23 mmol) in methanol (50 mL) was added dropwise a
solution of 1,2-benzenediamine (0.174 g, 1.615 mmol) in MeOH (15mL). After 1.5 h
UO2(OAc)2 2H20 (0.693 g, 1.615 mmol) was added and reflux was maintained for 15 min
whereupon the mixture was allowed to cool to room temperature overnight. The red solid
formed is filtered and dried in vacuo. Yield 63%; mass spectrum (ESI), m/z 919.4, 935.24
([M+Na]+, calcd for C32H22O10N2S2UNa 919.1; [M+K]+, calcd for 323H22O10N2UK
935.1). 1H NMR (CDCl3):  9.3534 (s, 2H, CH=N),  8.14-7.200 (m, 18H),  6.6161 (t, 2H).
O
O
H
H
1. NaH 2 eq.
OH
OH
2. R
OH
O
Br
SO2
O
H
N
N
UO2
O
O
methanol
2
OH
O
+
H2N
+
NH2
R
UO 2(AcO)2
O
O
SO2
O2S
8
Scheme 4
General synthesis of compound 9-12: To a refluxing solution of salycilaldehyde (1ml, 1.146g,
9.38 mmol) in toluene (30 ml) ethylendiammine (or orthophenylendiammine, 0.5 eq.) is added
under stirring. After 1h Al(CH3)3 (or AlCl3 0.5 eq) is added with caution whereupon the solution
is cooled to room temperature overnight. The solid formed is filtered and crystallized from a hot
mixture MeCN:CHCl3: MeOH 10:10:1. Yields 50-70 %.
139
Chapter 5
5.4 Conclusions.
The formation of highly organized structures in the solid state is the aim of the
emerging field of organic-inorganic chemistry, called Molecular Tectonics. Our efforts
were aimed to the exploitation of UO2- and Al-based salen and salophen complexes in
the formation of crystal architectures endowed with appealing features. Up to this point
no conclusion can be given, since the desired single crystals have not been obtained
yet. However, new compounds that could provide interesting properties have been
synthesized and the crystal structure of the first salophen-Al complex has been
obtained.
References:
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