Supramolecular structure of N,N-Bis(2-hydroxybenzyl)alkylamine: Flexible molecular assembly framework for host without guest and host with guestz

Article abstract of DOI:10.1021/jp061778v

N,N-Bis(2-hydroxybenzyl)alkylamine derivatives form a cage-like assembly consisting of two molecules via inter- and intramolecular hydrogen bonds. The derivatives exhibit themselves as host to accept copper-ion guests under the double-oxygen-bridged dimeric system. Quantum chemical calculation suggested that the host-guest interaction is based on a charge-transfer coordination. Comparison of the crystal structures before and after complexation clarifies a rare example of a host-guest compound where the hosts maintain their cage framework through the change of hydrogen bonds to coordination bonds.

Full text of DOI:10.1021/jp061778v

J. Phys. Chem. B 2006, 110, 21365-21370
21365
Supramolecular Structure of N,N-Bis(2-hydroxybenzyl)alkylamine: Flexible Molecular
Assembly Framework for Host without Guest and Host with Guest
Suttinun Phongtamrug,† Kohji Tashiro,‡ Mikiji Miyata,§ and Suwabun Chirachanchai*,†
The Petroleum and Petrochemical College, Chulalongkorn UniVersity, Phyathai Road,
Pathumwan, Bangkok 10330, Thailand, Department of Future Industry-Oriented Basic Science and Materials,
Graduate School of Engineering, Toyota Technological Institute, Tempaku, Nagoya 468-8511, Japan, and
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity,
Yamadaoka, Suita, Osaka 565-0871, Japan
ReceiVed: March 22, 2006; In Final Form: July 3, 2006
N,N-Bis(2-hydroxybenzyl)alkylamine derivatives form a cage-like assembly consisting of two molecules via
inter- and intramolecular hydrogen bonds. The derivatives exhibit themselves as host to accept copper-ion
guests under the double-oxygen-bridged dimeric system. Quantum chemical calculation suggested that the
host-guest interaction is based on a charge-transfer coordination. Comparison of the crystal structures before
and after complexation clarifies a rare example of a host-guest compound where the hosts maintain their
cage framework through the change of hydrogen bonds to coordination bonds.
Introduction
complex structure under variations of host molecules to establish
the supramolecular chemistry of the HBA.
Benzoxazine is a unique heterocyclic compound obtained
from cyclization between phenol, formaldehyde, and amine.1
As shown in Scheme 1, theoretically the ring-opening reaction
of p-substituted phenol-based benzoxazine with phenol deriva-
tives gives a linear aza-methylene-linked phenol-based polymer.
We found, however, that the actual reaction tends to terminate
at the dimer formation stage, which might be due to the fact
that reactive hydroxyl groups are stabilized by inter- and
intramolecular hydrogen bonds as reported previously.2 The
thus-created dimer, N,N-bis(2-hydroxybenzyl)alkylamine, here-
inafter referred to as HBA, takes a cage structure to induce a
supramolecular assembly by accepting other molecules or ionic
species (Figure 1). As host-guest complexes are known as
organometallic catalysts,3 biomimetic metalloproteins,4 and
enzymatic complex systems,5 establishment of HBA for su-
pramolecular chemistry might bring useful applications.
In previous papers,6,7 we clarified the inclusion phenomena
of HBA with various guests in either solution or the solid state.
It is important to note that parallel to our work there are some
reports on HBA derivatives with metal ions. For example,
Tshuva et al. reported on the zirconium complex of amine-bis-
(phenolate) as a candidate catalyst for 1-hexene polymerization.8
Malathy Sony et al. showed a biomimetic model complex of
N-[(2-hydroxylato-5-methyl)benzyl-(2′-hydroxylato-3′,5′-dim-
ethylbenzyl)]ethylamine dicopper(II).9 Although those com-
plexes demonstrated the precise crystal structures of the HBA
derivative and metal ions, the role of HBA self-assembly and
its development to form a complex were not clarified. As we
succeeded in preparing a series of HBA derivatives via a simple,
effective, and selective ring-opening reaction of benzoxazines2,
it is possible to carry out systematic work on the supramolecular
In the present article, we focus on two types of benzoxazine
dimer derivatives (N,N-bis(2-hydroxy-5-methylbenzyl)cyclo-
hexylamine, HBA1, and N,N-bis(2-hydroxy-3,5-dimethylbenzyl)
methylamine, HBA2) and on their copper-ion complexation
(Scheme 1). On the basis of the information obtained from
crystal structure analysis, thermal analysis, vibrational spec-
troscopy, and quantum chemical calculation, the derivatives have
been found for the first time to show their uniqueness in
accepting guests without destroying the host framework. This
is a rare example among many already revealed hosts. In the
present paper, we describe various unique behaviors concerning
the specific supramolecular structure of HBA on the basis of
the following significant experimental findings: (i) the double-
oxygen-bridged charge-transfer host-metal complex, (ii) the
well-superimposition structure of the hydrogen-bond network
of HBA and coordination systems of HBA with metal guest,
and (iii) the existence of multiguest species, i.e., ion and neutral
molecules, in a single host-guest framework.
Experimental Section
Preparation of HBA Derivatives. p-Cresol, 2,4-dimeth-
ylphenol, formaldehyde, methylamine, and cyclohexylamine
were obtained from Merck, Germany. Copper(II) acetate mono-
hydrate and sodium hydroxide were purchased from Fluka,
Switzerland. 1,4-Dioxane, diethyl ether, 2-propanol, and metha-
nol were from Labscan, Ireland. All chemicals and solvents used
for synthesis were of reagent grade and used without purifica-
tion.
N,N-Bis(2-hydroxy-5-methylbenzyl)cyclohexylamine
(HBA1) and N,N-bis(2-hydroxy-3,5-dimethylbenzyl)methy-
lamine (HBA2) were prepared from a ring-opening reaction of
the relevant benzoxazine and phenol derivatives as reported
previously.7 Mixtures of 3,4-dihydro-6-methyl-3-cyclohexyl-2H-
1,3-benzoxazine and p-cresol (1:1) were prepared and stirred
at 60 °C. The mixtures were allowed to react until the solution
became viscous, and they were left for precipitation. The
precipitates obtained were collected and washed with diethyl
* To whom correspondence should be addressed. E-mail: csuwabun@
chula.ac.th.
† Chulalongkorn University.
‡ Toyota Technological Institute.
§ Osaka University.
10.1021/jp061778v CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/29/2006

21366 J. Phys. Chem. B, Vol. 110, No. 42, 2006
Phongtamrug et al.
SCHEME 1
ether before vacuum drying. HBA1 was recrystallized in
chloroform to prepare single crystals. Similarly, 3,4-dihydro-
3,6,8-trimethyl-2H-1,3-benzoxazine was reacted with 2,4-dim-
ethylphenol to obtain HBA2.
solution into methanol solutions of HBA1 and HBA2 and
leaving them for several days. The single crystals of HBA
derivatives are colorless, whereas those of the copper inclusions
are dark green. X-ray diffraction measurement of HBA1 and
HBA2 was carried out using an X-ray imaging plate system
DIP3000 (MAC Science Co., Ltd., Japan). The graphite mono-
chromatized Mo KR line (λ ) 0.71073 Å), which was generated
from the SRA-M18XHF rotating anode X-ray generator (50 kV
and 200 mA), was used as an incident X-ray source. Data
correction was performed with XDIP software (MAC Science).
The sample was oscillated in a range of 3° over a total rotation
angle of 0-180° around the ω axis. The exposure time was 30
min for one image. It took about 12 h to collect the 24 images
in total. Data were analyzed using DENZO and SCALEPACK
software.10,11 The crystal structure was solved using maXus
(NoniusBV, Delft) software. The direct method was used to
find out the initial models, where the SIR92 software developed
by Altomare et al. was used.12 Least-squares refinement was
made on the basis of the full-matrix method using the quantity
N,N-Bis(2-hydroxy-5-methylbenzyl)cyclohexylamine
(HBA1): 80% yield; Rf ) 0.30 (5% MeOH in CHCl3); clear
and colorless solid; mp ) 181 °C; FTIR (KBr, cm-1) 3226 (br,
OH), 1500 (vs, C-C), 1449 (m, N-CH), 1249 (s, C-N), 1210
(m, C-N-C), 819 (s, C-N-C); 1H NMR (200 MHz, CDCl3,
ppm) δH 1.1 (2H, m, CH2), 1.45 (4H, m, CH2), 1.82 (4H, m,
CH2), 2.22 (6H, s, CH3-Ar), 2.70 (1H, m, CH), 3.72 (4H, s,
Ar-CH2-N), 6.68 (2H, d, Ar-H), 6.85 (2H, s, Ar-H), 6.90
(2H, d, Ar-H). Anal. Calcd for C22H29NO2: C, 77.88; H, 8.55;
N, 4.13. Found: C, 77.90; H, 8.56; N, 4.16.
N,N-Bis(2-hydroxy-3,5-dimethylbenzyl)methylamine
(HBA2): 80% yield; Rf ) 0.39 (5% MeOH in CHCl3); clear
and colorless solid; mp ) 123 °C; FTIR (KBr, cm-1) 3399 (br,
OH), 1484 (vs, C-C), 1427 (m, N-CH3), 1243 (m, C-N), 1214
1
and 1201 (m, C-N-C), 847 (m, C-N-C); H NMR (200
2
2 2
MHz, CDCl3, ppm) δH 2.22 (12H, s, Ar-CH3), 2.25 (3H, s,
N-CH3), 3.68 (4H, s, Ar-CH2-N), 6.72 (2H, s, Ar-H), 6.81
(2H, s, Ar-H). Anal. Calcd for C19H25NO2: C, 76.26; H, 8.36;
N, 4.68. Found: C, 76.27; H, 8.34; N, 4.69.
∑ω(|Fo| - |Fc| ) as a minimized function with weight ω )
exp[FA sin2 θ/λ2]/[σ2(Fo) + FBFo ], where σ2(Fo) is the squared
2
standard deviation of the observed structure factor Fo and
coefficients FA and FB were set to 0.0 and 0.03, respectively.
The reflections satisfying the cutoff condition of |Fo| > 3σ(|Fo|)
were used in the least-squares refinement. Because no detectable
Instrumentation. Fourier transform infrared spectra (FTIR)
were recorded by the Nujol mull method in the range 4000-
400 cm-1 at a resolution of 2 cm-1 using a HORIBA FT-720
infrared spectrometer. Thermogravimetric-differential thermal
analysis (TG-DTA) was performed with a Rigaku Thermoplus
TG8120 from 50 to 300 °C at a heating rate of 5 °C/min under
a nitrogen atmosphere.
Structural Analysis. Single crystals of HBA with copper
ions were prepared by dropping the methanolic copper acetate
Figure 1. Cage structure of HBA1 consisting of two HBA1 molecules
(the broken line represents the hydrogen bonds).
Figure 2. FTIR spectra of (a) HBA1, (b) HBA1-Cu, (c) HBA2, and
(d) HBA2-Cu after Nujol peaks were subtracted.

N,N-Bis(2-hydroxybenzyl)alkylamine-Cu Supramolecule
J. Phys. Chem. B, Vol. 110, No. 42, 2006 21367
Figure 3. TG and DTA thermograms of (a) HBA1, (b) HBA1-Cu, (c) HBA2, and (d) HBA2-Cu.
Results and Discussion
Infrared Spectral Change by Cu Complexation. Green
crystals of HBA1 and HBA2 including copper ions, abbreviated
as HBA1-Cu and HBA2-Cu, were characterized by FTIR to
observe the vibrational spectral changes caused by copper
inclusion. Since metal-ion exchange might occur relatively easily
when KBr powder is mixed with HBA1-Cu and HBA2-Cu
complexes, samples for IR measurements were prepared by the
Nujol mull method. Figure 2 shows the FTIR spectra of the
single crystals of HBA1, HBA2, and their copper inclusions
where the contribution of Nujol bands was already subtracted.
Although HBA1 and HBA2 give the peak positions and width
of the bands differently, due to the strength and environment
of the hydrogen bonds, the OH peaks are clearly observed before
complex formation for both compounds (Figure 2a and 2c).
After entrapping copper ions, the significant decrease of the
OH band intensity was observed, as shown in Figure 2b and
2d, corresponding to loss of inter- and intramolecular hydrogen
bonds. This implies that HBA1 and HBA2 form a complex with
copper ions. The newly observed peaks for HBA1-Cu (Figure
2b) are at 1305 and 1289 cm-1, whereas for HBA2-Cu (Figure
Figure 4. Coordination compound of HBA1-Cu.
effect was found, the absorption correction for the observed
intensity was not included in the structural refinement. As a
measure of the reasonableness of the structural analysis, the
reliability factors, R and Rw, were defined by the following
equations: R ) ∑||Fo| - |Fc||/∑|Fo| and Rw ) [∑ω(|Fo| -
|Fc|)2/∑ω|Fo|2]1/2. X-ray diffraction measurement of HBA1-
Cu and HBA2-Cu was performed using a Rigaku R-AXIS
RAPID/FS diffractometer with graphite-monochromated Mo KR
radiation at 296 K. The structure was solved by direct methods
(SIR92)12 and refined by the full-matrix least-squares procedure
2d) they are at 1267 and 1255 cm-1 14
These bands may be
.
assigned to the C-N vibrational modes. However, these are
different from the usual ones at 1249 and 1210 cm-1 for HBA1
and at 1243, 1214, and 1201 for HBA2. This result indicates
the change of the vibrational mode of the aza group after copper-
ion inclusion. In this way, the OH and C-N groups of HBA
were affected remarkably by introducing copper ions.
Thermal Stability of Complex. Thermal stabilities of the
compounds before and after inclusion of the copper ion were
investigated. As shown in Figure 3a, HBA1 gives a sharp
melting at 177 °C followed by an exothermic peak at 185 °C
and an endothermic peak at 225-240 °C with a remarkable
weight loss. As the measurement was done in a nitrogen
atmosphere, the endothermic peak reflects the continuous
thermal degradation in nonoxidative conditions. However, in
the case of HBA1-Cu, the melting peak was difficult to detect
and only an exothermic peak due to thermal degradation was
observed (Figure 3b). For HBA2, the melting peak was located
at 125 °C and it disappeared after complexation (Figure 3c and
3d). Thermal degradation of HBA2 was observed at 225 °C,
whereas that of HBA2-Cu was identified at 250 °C. The
2
on |F| . All non-hydrogen atoms were refined for anisotropic
thermal parameters as well as the coordinates. Hydrogen atoms
were detected from the different Fourier map, and positions were
refined assuming isotropic thermal parameters. All calculations
were performed using the TEXSAN crystallographic software
package.13
Molecular Modeling. The electron density distribution was
calculated for the original cage structure and the complex with
Cu ions utilizing DMol3 software (Material Studio, version 3.0,
Accelrys) on the basis of density function theory. The atomic
orbital basis set was DND (double numerical plus d-functions),
and the type of exchange-correlation potential was a local LDA.
The molecular structures were transferred directly from the
X-ray analysis results, and electron densities were calculated
for the isolated molecules without energy optimization.

21368 J. Phys. Chem. B, Vol. 110, No. 42, 2006
Phongtamrug et al.
Figure 5. Comparison of cage structure between HBA1 (red color) and HBA1-Cu complex (blue color) viewed from different directions.
SCHEME 2
increase in thermal degradation indicates a remarkable thermal
stability of the HBA1 and HBA2 complexes.
O-H stretching at 3200-3400 cm-1 is hardly observed for the
HBA1-Cu complex.
Crystal Structures. To clarify the crystal structures of the
HBA-copper complexes, single-crystal X-ray analysis was
performed. Figure 4 shows the inclusion structure of HBA1-
Cu with doubly bridged Cu-oxygen linkages. Each copper is
bridged by three phenoxy oxygens and an amine donor. The
bond lengths of Cu-O(1), Cu-O(2), Cu-O(2)*, and Cu-N
are almost identical (1.85-2.05 Å). These distances are in the
range suitable for stable Cu-O and Cu-N coordination bonds.15
The bond angles of O(1)-Cu-N, O(2)-Cu-N, O(1)-Cu-
O(2)*, and O(2)-Cu-O(2)* are nearly 90°, as illustrated in
Scheme 2, and therefore, the inclusion structure appears in a
distorted square-planar geometry. Similar coordination is also
identified for other types of asymmetric hydroxybenzylalky-
lamine with different substituted groups (R, R′, and R′′ in
Scheme 1) including the derivatives reported previously.9 This
type of molecular geometry was reported also by Malathy Sony
et al.9 It is important to note that the square-planar geometry is
a unique characteristic of the HBA derivatives, which form
coordination bonds with metals.
Although, in general, it is difficult to determine the proton
positions by single-crystal X-ray analysis, we succeeded in
extracting the protons of OH groups for HBA1 and HBA2 with
reasonable bond distances. In the case of HBA1-Cu, however,
the hydrogen atoms could not be detected but the oxygen atoms
were found to be directly linked to Cu atoms, as shown in Figure
4. Here, we could not identify the acetate counteranions,
although there were reports that inclusion complexes occasion-
ally show their counterions.16 This causes us to speculate that
the hydrogen atoms of the hydroxyl groups might perform as
proton donors for acetate anions, resulting in formation of acetic
acid molecules after complexation (Scheme 2). This speculation
is consistent with the infrared spectrum (Figure 2) in which
As shown in Figure 1, the HBA derivatives form the inter-
and intramolecular hydrogen bonds between N atoms and OH
groups (O(1)-H‚‚‚O(2)* and N‚‚‚H-O(2)), resulting in an
assembly of two HBA molecules.2 As for the HBA with and
without guest ions, the point to be raised here is that the cage
is formed between the two HBA molecules either before or after
complex formation (Scheme 2). As described later, the electron
density distribution was calculated for the original cage structure
and the complex with Cu ions on the basis of density function
theory utilizing DMol3 software. As demonstrated in Figure 5,
the size and shape of the cage structure composed of two HBA
molecules are maintained even after complexation with Cu ions,
although the O-H‚‚‚O and O-H‚‚‚N inter- and intramolecular
hydrogen bonds, respectively, are replaced with coordination
linkages of Cu-O and Cu-N types. This geometrical relation
between the HBA and HBA-Cu complex may allow us to
speculate on the mechanism of benzoxazine-copper complex
formation (as discussed later).
Stabilization of Supramolecular Structure by Solvent
Molecules. Such a reservation of cage structure can also be seen
for HBA2-Cu complexation (Figure 6). However, it is impor-
tant to note that complexation of HBA2 is a little different from
that of HBA1.17 As shown in Figure 7, three water molecules
are coexistent with the dimeric HBA2-Cu molecules in the
crystal lattice. The distances between the phenoxy oxygens
(O(3)-H‚‚‚O(1)) and water oxygens (O(4)-H‚‚‚O(3)) are 2.70
and 2.82 Å, respectively. The positions of the hydrogen atom
belonging to the water molecules were extracted successfully
from the difference Fourier synthesis map. The O(1)‚‚‚H
distance is 1.78 Å, which is acceptable for the interatomic
distance of the hydrogen bond, as found for cases of phenol
and water (1.80 Å).18 The structure reveals that HBA2-Cu

N,N-Bis(2-hydroxybenzyl)alkylamine-Cu Supramolecule
J. Phys. Chem. B, Vol. 110, No. 42, 2006 21369
Figure 6. Comparison of cage structure between HBA2 and HBA2-Cu viewed from different directions.
Figure 7. Structure of HBA2-Cu complex coordinated by water molecules. (Hydrogen atoms are omitted for clarity.)
complex takes a unique bowl-shaped network structure and
accepts those small solvent molecules to stabilize the whole
structure.
essence of the charge distribution change in the formation
process of the complex crystal, and it is also expected that the
infrared frequencies and intensities obtained from the calculation
will support the results in Figure 2.
Comparison of crystal volumes also supports this concept
about the role of the water molecules in formation of HBA2-
Cu complex. For HBA2-Cu, one complex framework com-
posed of the two HBA2 molecules has a volume of 963.6 Å3
whereas a single HBA2 framework has a volume of 859.1 Å3.
This volume increment after Cu-complex formation is different
from that of HBA1, showing a decrease in volume after
complexation from 974.1 to 959.9 Å3.
Essential Features of Benzoxazine-Copper Complexes.
The characteristic features of these copper complexes of HBA
are seen in the formation of the double square planes connected
by common oxygen atoms. To reveal the characteristic features,
the atomic charge distribution was calculated on the basis of
density function theory with DMol3 (Material Studio Version
3.0, Accelrys). The atomic orbital basis set was DND (double
numerical plus d-functions), and the type of exchange-correlation
potential was a local LDA. In the calculation the X-ray-analyzed
structure was used without any further optimization. Of course,
it is more ideal to take the intermolecular interactions into
account in the geometrical calculation, but the DFT calculation
of the whole crystal lattice is quite a hard task for such a
complicated structure, as in the present case. The calculation
was made for a rough estimation of the atomic charge distribu-
tion in this characteristic complex structure. More detailed and
accurate calculation will be made in the future to clarify the
Figure 8 shows the calculated electrostatic potential (ESP)
charges. In the case of HBA1, the typical charge distribution is
observed for the O-H‚‚‚O hydrogen bond. After complexation
(Figure 8b), the charges of oxygen atoms connected directly to
the Cu atom change from -0.47 and -0.58 to -0.53 and -0.78
eV. At the same time the charges of nitrogen atoms decrease
from -0.34 to -0.55 eV. The phenol carbon atoms connected
to oxygen also change their charges. As for the Cu atom, the
charge decreases from +2 (Cu2+ of Cu(CH3COO)2) to +1.02
is shown in Figure 8b. The changes in the atomic charge
distribution suggest that the electrons flow from carbon (or
aromatic rings) to oxygen and nitrogen and to copper atoms.
Referring to the case of π-conjugated oligo(phenylene ethy-
nylene),19 we speculate that our complex might be formed by
the charge-transfer mechanism. Considering the assembly
networks of HBA1 and HBA2, it is natural to emphasize that
the charge-transfer coordination networks of HBA1-Cu and
HBA2-Cu are formed with only a minor destruction of the
original packing structure. In other words, the charge-transfer
complex is initiated smoothly by substituting the hydrogen atoms
with Cu atoms. This type of dimeric charge-transfer complex-
ation is, to our knowledge, quite rare to find in general
complexation.

21370 J. Phys. Chem. B, Vol. 110, No. 42, 2006
Phongtamrug et al.
Supporting Information Available: Crystallographic in-
formation files (CIF); HBA1, HBA2, HBA1-Cu; CCDC
258998, and HBA2-Cu; CCDC 258999. This material is
available free of charge via the Internet at http://pubs.acs.org.
References and Notes
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In general, a host molecule accepts a guest species under the
interaction induced by favorable conditions, e.g., solvent dis-
solution, melting, and irradiation. To date, it is known that the
host likely dissolves its original framework and establishes its
new channel to incorporate the guest. The present work clarified
a unique type of complex formation in that the host retains its
framework even after accepting guests: HBA1 and HBA2 form
complexes with copper ions without losing the cage-like
structure through modification from a hydrogen-bonded self-
assembly to a coordination-bonded host-guest system, as
illustrated in Scheme 2. The energy calculation suggested
formation of an HBA-copper complex through the atomic
charge transfer among the phenoxy oxygen atom, aromatic ring,
and aza methylene group, leading to a double-oxygen-bridged
network.
Acknowledgment. One of the authors, S.C., acknowledges
financial support from The Thailand Research Fund (grant no.
RSA4680025) and extends his appreciation for the support from
the Hitachi Scholarship Foundation. The authors gratefully thank
Mr. Katsunari Inoue (Graduate School of Engineering, Osaka
University, Japan) for help in single-crystal analysis.