Preparation of an ester-substituted triscatechol ligand and characterization of the corresponding M4L4 titanium(IV) complex

Article abstract of DOI:10.1055/s-0030-1259086

The ester-substituted triscatechol ligand L-H₆ can be easily prepared in a triple imine condensation reaction. The required trisamine is prepared starting from diphenylamine and 2-bromobenzoic acid methylester. Upon coordination of titanium(IV)

Full text of DOI:10.1055/s-0030-1259086

LETTER
121
Preparation of an Ester-Substituted Triscatechol Ligand and
Characterization of the Corresponding M4L4 Titanium(IV) Complex
Preparation of
an
E
ster
a
-Substitute
r
d
T
riscat
k
e
chol
L
igandus Albrecht,* Bianca A. Stöckel
Institut für Organische Chemie, RWTH Aachen University, Landoltweg 1, 52070 Aachen, Germany
E-mail: markus.albrecht@oc.rwth-aachen.de
Received 12 October 2010
of chiral or reactive groups, catalytically active sites, or
simply to interlink containers to obtain novel porous soft
materials.
Abstract: The ester-substituted triscatechol ligand L-H6 can be
easily prepared in a triple imine condensation reaction. The required
trisamine is prepared starting from diphenylamine and 2-bromoben-
zoic acid methylester. Upon coordination of titanium(IV) ions the
tetrahedral M4L4 coordination compound is obtained, which due to
the symmetry breaking substituent exists as a mixture of isomers.
However, the complex can be unambiguously characterized by ESI-
MS.
In here, the preparation of the triscatechol imine ligand L-
H6 is described, which forms the tetranuclear titanium(IV)
complex Na8[L4Ti4]. Due to the formation of a mixture of
isomers, the complex cannot be well characterized by
NMR spectroscopy but ESI-MS proves to be the ideal tool
for the characterization of the container.
Key words: ligand, self assembly, coordination compound, imine,
container molecule
Synthesis of the ligand L-H6 starts with the copper-medi-
ated coupling of diphenylamine (1) with 2-bromo methyl-
benzoate (2) in the presence of potassium carbonate and
sodium sulfate in nitrobenzene as solvent at 220 °C fol-
lowing the protocol of Ahn.5 After recrystallization from
n-hexane–ethyl acetate (9:1) and column chromatography
using the same solvent mixture, triaryl amine 3 is obtained
in 53% yield (Scheme 1).
The formation of molecules with well-defined internal
space for host–guest chemistry is a challenge. Such com-
pounds are of interest due to the features of the interior, al-
lowing the stabilization of unusual species, or the
promotion or catalysis of chemical reactions. Molecular
containers can be either formed in a step-by-step covalent
synthesis or more easily in self-assembly processes utiliz-
ing noncovalent interactions like hydrogen bonding, elec-
trostatics, or metal coordination.1
The triple nitration of compound 3 to obtain 4 proved to
be tricky. Under mild conditions the ester-substituted unit
does not react, while harsher nitrating conditions afforded
overnitrated product. Finally, nitration of one equivalent
of 3 in glacial acetic acid–acetic anhydride (4:1) with ad-
dition of one drop of concentrated nitric acid at 10 °C,
heating to 75 °C and addition of further nitric acid (total
of 3.2 equiv) afforded after chromatographic workup (sil-
ica, n-hexane–EtOAc = 3:1) the desired product 4 in 43%.
The nitro groups of 4 are reduced using palladium on car-
bon in methanol under 30 bar of hydrogen atmosphere.
The trisamine 5 is isolated in close to quantitative yield
(98%) after simple filtration over Celite and removal of
the solvent.
Since several years we investigate the preparation and co-
ordination behavior of triangular ligands, which bear
chelating-metal-coordination sites at the corners
(Figure 1).2,3 With appropriate metal ions they form in a
self-assembly process M4L4 metallosupramolecular tetra-
hedra exhibiting a huge internal cavity. The complexes
are able to encapsulate guest molecules in their interior.4
Container molecules are of interest due to their ability for
guest uptake. However, in order to modify the container
itself, it is of interest to attach some anchor to the ligand
which in subsequent studies would allow the attachment
In our hands, imine formation of 5 with 2,3-dihydroxy-
benzaldehyde (6) in methanol did not work well on a
small scale. However, condensation of 511 mg of 5 (1.5
mmol) with 628 mg of 2,3-dihydroxybenzaldehyde (6, 4.6
mmol) in a concentrated methanol solution at room tem-
perature resulted in the precipitation of the product L-H6
as a dark red solid in 71% yield (744 mg).6 1H NMR
shows as characteristic signals the resonance of the ester
methyl group at d = 3.48 ppm and two singlets of imine
protons at d = 9.02 and 8.93 ppm in the ratio 1:2. Mass
spectrometry reveals the molar peak at m/z = 709 [M +
H]+.
Figure 1 Schematic representation of the self-assembly of M4L4
container molecules from tritopic triangular ligands and appropriate
metal ions
The ligand L-H6 was introduced into coordination studies
with titanium(IV) ions. Therefore, l-H6, titanoyl acetylac-
etonate [TiO(acac)2] and sodium carbonate are dissolved
SYNLETT 2011, No. 1, pp 0121–0123
Advanced online publication: 07.12.2010
DOI: 10.1055/s-0030-1259086; Art ID: G28610ST
x
x.
x
x.
2
0
1
1
© Georg Thieme Verlag Stuttgart · New York

122
M. Albrecht, B. A. Stöckel
LETTER
AcOH,
Ac2O,
HNO3
K2CO3,
Ph2NH
Na2SO4, Cu,
1
+
PhNO2
N
CO2Me
43%
53%
Br
CO2Me
3
2
H2N
NH2
O2N
NO2
H2, Pd/C,
N
N
MeOH
CO2Me
CO2Me
98%
NH2
NO2
5
4
Figure 2 Orientational isomers of an M4L4 tetrahedron with a sub-
stituent (œ) located at one of the ligand arms
OH
O
HO
HO
OH
OH
OH
N
N
N
N
6
CO2Me
MeOH
71%
OH
OH
L-H6
Scheme 1 Synthetic scheme for the preparation of ligand L-H6
in a 1:1:1 ratio in a small amount of DMF. After 12 hours
at room temperature DMF is removed, and the residue is
washed with methanol. The complex Na8[L4Ti4] is ob-
tained in 30% yield (197 mg). Introduction of the ester re-
duces the symmetry of the triangular ligand leading to a
series of different isomers due to different relative posi-
tions of the four esters in the complex. The four orienta-
tional isomers are indicated in Figure 2. Furthermore, the
substituents can be directed towards the inside or to the
outside of the tetrahedron. Consequently NMR spectros-
copy is not informative in case of Na8[L4Ti4].
Figure 3 Part of the ESI-MS spectrum of Na8[L4Ti4]
pendant ester groups will be a functional unit for future
work in order to add recognition or reaction sites or to link
several containers.
Acknowledgment
This work was supported by the Fonds der Chemischen Industrie.
Figure 3 shows a part of the ESI-MS spectrum. This ana-
lytical technique proved to be ideal for the characteriza-
tion of the Na8[L4Ti4] tetrahedron.7 The spectrum shows
solely peaks of the M4L4 container with varying amounts
of countercations (Na+ or H+). The part of the ESI-MS
spectrum shown in Figure 3 reveals the peaks of double,
triple, or quadruple negatively charged species. The sig-
nals show the correct isotopic distribution.
We thank Dr. W. Bettray for the measurement of the ESI-MS spec-
tra.
References and Notes
(1) (a) Cram, D. J.; Cram, J. M. Container molecules and their
guests; RSC: Cambridge, 1994. (b) Warmuth, R. Chem.
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Chem. 2006, 265, 1. (d) Saalfrank, R. W.; Maid, H.;
Scheurer, A. Angew. Chem. Int. Ed. 2008, 47, 8794; Angew.
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2009, 48, 3418; Angew. Chem. 2009, 121, 3470.
In this paper we described the preparation of an unsym-
metric tris(4-aminoaryl)amine 5 with an ester group at one
of the aryls. Condensation with 2,3-dihydroxybenzalde-
hyde (6) affords a triangular triscatechol ligand L-H6,
which forms a M4L4 tetrahedron upon complexation of ti-
tanium(IV) ions in the presence of sodium carbonate. The
(f) Mugridge, J. S.; Bergman, R. G.; Raymond, K. N. J. Am.
Chem. Soc. 2010, 132, 1182.
Synlett 2011, No. 1, 121–123 © Thieme Stuttgart · New York

LETTER
Preparation of an Ester-Substituted Triscatechol Ligand
123
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7807.
(6) Characterization of L-H6: mp 179 °C; 1H NMR (300 MHz,
DMSO-d6): d = 13.36, 12.89, 9.18 (three broad signals, 6 H
total, OH), 9.02 (s, 1 H, HC=N), 8.93 (s, 2 H, HC=N), 7.78–
7.65 (m, 2 H, CHar), 7.40–7.32 (m, 5 H, CHar), 7.17–6.96 (m,
9 H, CHar), 6.85–6.74 (m, 4 H, CHar), 3.48 (s, 3 H, OCH3).
MS (EI, 70 eV): m/z = 709 [M + H] +. Anal. Calcd for
C41H32N4O8 (708.71): C, 69.48; H, 4.55; N, 7.91. Found: C,
69.39; H, 4.08; N, 8.00.
(3) Related work by others: (a) Amoroso, A. J.; Jefferey, J. C.;
Jones, P. L.; McCleverty, J. A.; Thornton, P.; Ward, M. D.
Angew. Chem., Int. Ed. Engl. 1995, 34, 1443; Angew. Chem.
1995, 107, 1577. (b) Paul, R. L.; Amoroso, A. J.; Jones,
P. L.; Couchman, S. M.; Reeves, Z. R.; Rees, L. H.; Jefferey,
J. C.; McCleverty, J. A.; Ward, M. D. J. Chem. Soc., Dalton
Trans. 1999, 1563. (c) Brückner, C.; Powers, R. E.;
Raymond, K. N. Angew. Chem. Int. Ed. 1998, 37, 1837;
Angew. Chem. 1998, 110, 1937. (d) Caulder, D.; Brückner,
(7) For ESI-MS as characterization tool in supramolecular
chemistry, see: (a) Schalley, C. A. Int. J. Mass Spectrom.
2000, 194, 11. (b) Schalley, C. A. Mass Spectrom. Rev.
2001, 20, 253.
Synlett 2011, No. 1, 121–123 © Thieme Stuttgart · New York

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