Complexation and extraction of PAHs to the aqueous phase with a dinuclear PtII diazapyrenium-based metallacycle

Article abstract of DOI:10.1002/chem.201002051

New palladium and platinum metallacycles have been synthesized by reaction between a 2,7-diazapyrenium-based ligand and Pd^II and Pt^II complexes. The inclusion complexes between the metallacycles and polycyclic aromatic hydrocarbons (PAHs) in CD₃NO₂ and D₂O were studied by NMR spectroscopy. The structures of the inclusion complexes of the Pt metallacycle as host with pyrene, phenanthrene, and triphenylene were confirmed by single crystal X-ray crystallography. The association constants between the Pt metallacycle and the selected PAHs were determined in CH ₃CN following the characteristic charge-transfer band displayed in their UV/Vis absorption spectrum. Although in aqueous solution all the complexes showed a 1:1 stoichiometry, in CH₃CN the Job plot indicated a 2:1 stoichiometry for complexes with triphenylene and benzo[a]pyrene. The estimated association constants in water correlate with the hydrophobicity of the PAH, indicating that hydrophobic forces play an important role in the complexation process. SuPAH extraction! New metallacycles have been synthesized by self-assembly between a 2,7′-diazapyrenium-based ligand and Pd ^II and Pt^II complexes. The inclusion complexes between the metallacycles and polycyclic aromatic hydrocarbons (PAHs) in CD₃CN and D₂O were studied. The structures of complexes with pyrene, phenanthrene, and triphenylene were confirmed by single-crystal X-ray crystallography (see graphic). The estimated association constants in water correlate with the hydrophobicity of the PAH.

Full text of DOI:10.1002/chem.201002051

FULL PAPER
DOI: 10.1002/chem.201002051
Complexation and Extraction of PAHs to the Aqueous Phase with a
Dinuclear PtII Diazapyrenium-Based Metallacycle**
Vꢀctor Blanco,[a] Marcos D. Garcꢀa,[a] Alessio Terenzi,[b] Elena Pꢀa,[a]
Antonio Fernꢁndez-Mato,[a] Carlos Peinador,*[a] and Josꢂ M. Quintela*[a]
Dedicated to Professor Josꢀ Barluenga on the occasion of his 70th birthday
Abstract: New palladium and platinum
metallacycles have been synthesized by
reaction between a 2,7-diazapyrenium-
based ligand and Pdii and Ptii com-
plexes. The inclusion complexes be-
tween the metallacycles and polycyclic
aromatic hydrocarbons (PAHs) in
CD3NO2 and D2O were studied by
NMR spectroscopy. The structures of
the inclusion complexes of the Pt met-
allacycle as host with pyrene, phenan-
threne, and triphenylene were con-
firmed by single crystal X-ray crystal-
lography. The association constants be-
tween the Pt metallacycle and the se-
lected PAHs were determined in
CH3CN following the characteristic
charge-transfer band displayed in their
UV/Vis absorption spectrum. Although
in aqueous solution all the complexes
showed a 1:1 stoichiometry, in CH3CN
the Job plot indicated a 2:1 stoichiome-
try for complexes with triphenylene
and benzo[a]pyrene. The estimated as-
sociation constants in water correlate
with the hydrophobicity of the PAH,
indicating that hydrophobic forces play
an important role in the complexation
process.
Keywords: inclusion compounds ·
metallacycles · polycyclic aromatic
hydrocarbons
·
self-assembly
·
supramolecular chemistry
Introduction
tal persistence, and potentially deleterious effect on human
health have resulted in an increasing interest by the research
community.[3] PAHs are characterized by their hydrophobici-
ty and tendency to accumulate in the organic matter. Ap-
propriate extraction techniques should enhance the solubili-
ty of the compounds, to extract their entire labile fraction.
Cyclodextrin and resin extractions have shown their poten-
tial as suitable approaches for addressing the problem.[4]
Metal-directed self-assembly has been one of the most ex-
ploited areas in supramolecular chemistry in recent years.[5]
The development of this supramolecular strategy has al-
lowed the preparation of topologically unusual supramole-
cules, such as catenanes, rotaxanes, and knots.[6] Likewise,
metal-coordination-directed self-assembly has also led to the
preparation of molecular triangles, squares, pentagons, and
hexagons, which can be regarded as potential hosts because
of their cavities.[7] Although, the number of those molecular
receptors capable of complexing medium-size substrates re-
mains low.[8] The combination of pyridyl ligands with palla-
dium or platinum centers has become a powerful and versa-
tile tool to synthesize 2D and 3D suprastructures.[9] The suc-
cess of this approach is based on the structural diversity of
pyridyl ligands, the geometry of the metal center, and the
modulation of the lability of the coordination bond.
Polycyclic aromatic hydrocarbons (PAHs) represent an im-
portant group of environmental pollutants characterized by
two or more fused aromatic rings.[1] PAHs are introduced
into the environment from various sources, such as natural
oil seeps, refinery and oil storage wastes, accidental spills
from oil tankers, petrochemical industrial effluents, coal tar
processing wastes, combustion processes, and wood preser-
vative wastes.[2] Their omnipresent distribution, environmen-
[a] Dr. V. Blanco, Dr. M. D. Garcꢀa, E. Pꢀa, Dr. A. Fernꢁndez-Mato,
Prof. C. Peinador, Prof. J. M. Quintela
Departamento de Quꢀmica Fundamental, Universidade da CoruÇa
Facultad de Ciencias, A Zapateira, s/n, 15008 La CoruÇa (Spain)
Fax : (+34)981-167065
E-mail: carlos.peinador@udc.es
jqqoqf@udc.es
[b] A. Terenzi
Dipartimento di Chimica Inorganica e Analitica “S. Cannizzaro”
Universitꢂ di Palermo, Viale delle Scienze
Parco dꢃOrleans II, Edificio 17, 90128 Palermo (Italy)
[**] PAH=polycyclic aromatic hydrocarbon.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002051.
Chem. Eur. J. 2010, 16, 12373 – 12380
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12373

In a recent communication, we reported the complexation
of pyrene in aqueous solution with a self-assembled rectan-
gular palladium metallacycle.[10] The molecular recognition
process is based on hydrophobic forces and p-donor/p-ac-
ceptor interactions between pyrene and the electronic p-de-
ficient 4,4’-bipyridinium moieties of the metallacycle. It is
well known that 2,7-diazapyrene is a better electron accept-
or than 4,4’-bipyridine increasing the hydrophobic nature of
the resulting metallacycle due to the extended aromatic sur-
face and increasing in that manner the potential of the re-
ceptor to complex p-extended aromatic species on an aque-
ous environment. With the aim of completing and expanding
the former study, in this paper we report a new diazapyreni-
um-based metallacyclic receptor and its inclusion complexes
with pyrene and other PAHs. This water-soluble host has a
suitable size to form inclusion complexes with a wide range
of PAHs. Moreover, the potential of this host to selectively
complex targeted aromatic substrates is also outlined by the
use of competitive extraction experiments of several PAHs
to an aqueous phase.
2,7-diazapyrene derivative. The proposed structure of ligand
3·PF6 was confirmed by means of X-ray crystallography of
suitable single crystals obtained by vapor diffusion of ethyl
ether into a CH3CN solution of a monoprotonated form of
the ligand (see the Supporting Information). Thus, the struc-
ture showed the pyridine ring protonated at the nitrogen
atom and, consequently, the presence of two hexafluoro-
phosphate counteranions. The dihedral angle between the
planes defined by the phenylene and diazapyrene systems is
528.
Synthesis of PdII and PtII metallacycles 5a and 5b: Accord-
ing to the usual methodology employed in our investiga-
tions,[12] we checked if metallacycle 5a could be self-assem-
bled from ligand 3·NO3 and the palladium complex 4a. The
1H and 13C NMR spectra of an equimolar solution of 3·NO3
(5.0 mm) and 4a in D2O showed chemical shifts compatible
with the metallacycle formation. The downfield shift of pro-
tons Ha and Hh and the upfield shift of Hb–e clearly suggest-
ed the complexation of ligand 3 to the metal center and the
formation of the rectangle (Figure 1). The presence of other
cyclic structures was discarded by dilution experiments
1
Results and Discussion
monitored by H NMR spectroscopy, since the spectra ex-
hibited no changes over the 2.5–0.5 mm range. The diffusion
coefficients obtained from DOSY (diffusion-ordered NMR
spectroscopy) experiments of metallacycle 5a·6NO3 showed
that the metallacycle is larger than its components as the
diffusion coefficient is lower than the values obtained for
the free components in separate experiments. Moreover, the
signals from the ligand and palladium complex displayed the
same diffusion coefficients, implying both components dif-
fusing as a whole.
The aromatic nucleophilic substitution of 4-chloro-1,3-dini-
trobenzene with 2,7-diazapyrene produced the activated in-
termediate N-(2,4-dinitrophenyl)-2,7-diazapyrenium salt (1;
Scheme 1). For the preparation of ligand 3·NO3 we used the
Zincke reaction.[11] The exchange of the 2,4-dinitroaniline
moiety by 4-(pyridin-4-ylmethyl)aniline (2) gave, presuma-
bly by an addition of the nucleophile, ring opening and ring
closing (ANRORC) mechanism, the ligand 3·PF6. The ni-
trate salt was prepared by metathesis of 3·PF6 with tetrabu-
tylammonium nitrate (TBAN). To the best of our knowl-
edge, this is the first example of the Zincke reaction with a
The self-assembly also proceeded in CH3NO2 solution
with similar results. However, the 1H NMR spectrum of
5a·6PF6 in [D3]nitromethane revealed the existence of other
Scheme 1. Synthesis of metallacycles 5a and 5b and selected PAHs used in this work. en=ethylenediamine.
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Chem. Eur. J. 2010, 16, 12373 – 12380

Complexation and Extraction of PAHs
FULL PAPER
Complexation of metallacycles 5a and 5b with PAHs: With
metallacycles 5a and 5b in our hands, we approached a sys-
tematic study of their complexes with PAHs, either in solu-
tion or solid state. To achieve a representative group, six
PAHs with different sizes and shapes were chosen: naphtha-
lene (NAP), anthracene (ANT), phenanthrene (PHE),
pyrene (PYR), triphenylene (TRI), and benzo[a]pyrene
(BPY) (Scheme 1). The host–guest interaction in solution
was examined by NMR and UV/Vis spectroscopy. The addi-
tion of 1 equiv of PYR to a solution of metallacycle
5a·6NO3 in water (2.5 mm, sonication for 2 h was necessary
to dissolve the PYR) resulted in the appearance of a red-
colored solution, originating from a charge-transfer band be-
tween the host and guest centered at l=437 nm. The contin-
uous variation method (Job plot) confirmed the stoichiome-
try of complex formation between PYR and 5b·6PF6 in
CH3CN solution to be 1:1 (Figure 2). However, the Job plot
for the complexes of BPY and TRI with 5b·6PF6 showed a
2:1 (metallacycle/PAH) stoichiometry. In addition, the titra-
tion curves were poorly fitted to a 1:1 binding isotherm, but
nicely fitted to a 2:1 binding isotherm (see the Supporting
Information).
Figure 1. Partial 1H NMR (D2O, 500 MHz) spectra of a) ligand 3·NO3,
b) metallacycle 5a·6NO3, and c) inclusion complex PYRꢁ5a·6NO3.
Pyrene signals are marked as black dots.
minor species when the concentration is 1 mm or higher,
while at concentrations below 0.5 mm only the dinuclear
metallacycle was detected.
As described above, the self-assembly of palladium metal-
lacycles 5a proceeded instantaneously at room temperature.
ꢀ
Nevertheless, the known lability of the Pd N(Py) bond
makes their isolation and an accurate structural characteri-
ꢀ
zation difficult. The use of the more inert Pt N bond should
ꢀ
allow us to surpass those inconveniences. The Pt N coordi-
nation bond has double characteristics owing to its inertness
at low temperature, which is lost when the temperature is
increased. This dual feature has been exploited by Fujita to
introduce the “molecular lock” concept.[13] Employing this
strategy we could prepare the platinum analogues of metal-
lacycles 5b. Obviously, the reaction time (8 d) and tempera-
ture (1008C) were higher to assure the thermodynamic con-
trol over the self-assembly. The hexafluorophosphate salt of
platinum metallacycle 5b was isolated in 96% yield and
characterized by NMR spectroscopy and ESI-HRMS. Mass
spectrometry showed peaks resulting from the loss of be-
tween two and five hexafluorophosphate anions. A compari-
son between the calculated and experimental isotopic mass
distribution data for the triply charged species supported
the structure of [5b·3PF6]3+
.
To obtain additional information on the structure and size
of compound 5b, this molecule was characterized by means
of DFT calculations (B3LYP model). Geometry optimiza-
tion of 5b was carried out in water without constrains (see
the Supporting Information). The calculated structure evi-
denced that the rectangular metallacycle is 14.4 ꢅ in length
and 6.6 ꢅ width (distances measured between the corre-
sponding centroids of each side). Taking into account the
van der Waals radius of an sp2 carbon (1.7 ꢅ), the size of
the resulting cavity is suitable to accommodate a polycyclic
aromatic guest. Likewise, the calculated dimensions are very
similar to those of the bipyridium analogue 6.[10]
Figure 2. Job plots showing the 1:1 stoichiometry of complex
PYRꢁ5b·6PF6 and 2:1 stoichiometry of TRIꢁ
ACHTUNGTRENN(NUG 5b·6PF6)2 in CH3CN.
Chem. Eur. J. 2010, 16, 12373 – 12380
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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12375

C. Peinador, J. M. Quintela et al.
Table 1. 1H and 13C NMR chemical shift data (Dd) for metallacycle 5a·6NO3 (2.5 mm) and complexes PAHꢁ5a·6NO3 (2.5 mm).
Compound[a]
H/C
a
b
c
d
e/e’
f/f’
g/g’
h/h’
q[c]
r[c]
s[c]
t[c]
v[c]
1H
13C
1H
13C
1H
13C
1H
13C
1H
13C
1H
13C
0.05 ꢀ0.5
ꢀ0.4
ꢀ1
ꢀ0.5
ꢀ0.7
ꢀ0.26
0.02
2
0.02
0.35
1.3
0.12
ꢀ0.2
0.09
ꢀ0.1
0.15
[b]
5a·6NO3
9.7
0
0.8
ꢀ1.2
ꢀ0.24
ꢀ0.5
ꢀ0.17
ꢀ0.2
ꢀ0.3
ꢀ0.3
ꢀ0.19
ꢀ0.3
0.42
0.4
0.39
0.2
0.32
0.2
0.4
0.1
0.36
0.2
0.52
0.5
0.11 ꢀ0.42 ꢀ1.11
ꢀ0.65
ꢀ0.4
ꢀ0.42
ꢀ0.2
ꢀ0.63
ꢀ0.6
ꢀ0.5
ꢀ0.2
ꢀ3.03 ꢀ2.15
ꢀ1.9 ꢀ1.3
ꢀ1.13
ꢀ1.1
PYRꢁ5a·6NO3
PHEꢁ5a·6NO3
TRIꢁ5a·6NO3
ANTꢁ5a·6NO3
0
ꢀ1.3
0.11 ꢀ0.26 ꢀ0.83
0.1 ꢀ2.7
ꢀ0.8
0.07 ꢀ0.58 ꢀ1.05
ꢀ1 ꢀ3
0.17 ꢀ0.11 ꢀ0.99
0.2 ꢀ2.7
ꢀ0.6
ꢀ3.7
0.34
0.3
ꢀ3.31 ꢀ2.99 ꢀ2.14 ꢀ1.12
ꢀ2.5
ꢀ1.5
ꢀ3
ꢀ2.8
0.44
0.4
ꢀ1.42 ꢀ2.05
0
ꢀ0.1
ꢀ3
ꢀ1.8
0.34
0.4
0
ꢀ2.23 ꢀ2.15
ꢀ1.4 ꢀ3.1
ꢀ1.81
ꢀ2.6
ꢀ0.2
0.58
0.7
0.04 ꢀ0.64 ꢀ1.14/ꢀ1.35 ꢀ0.83/ꢀ0.88 ꢀ0.36/ꢀ0.53
0.1 ꢀ3.9/ꢀ4.1 ꢀ0.9 ꢀ0.7
ꢀ1.8
0.08/0.01
[d]
BPYꢁ5a·6NO3
ꢀ0.4/0.1
[a] Hydrogen and carbon labels are defined in Scheme 1. The Dd values are compared with those of the free metallacycle 5a·6NO3. [b] The Dd values
for 5a·6NO3 are compared with those of free ligand 3·NO3. [c] The Dd values are compared with those of free PAH in CDCl3 solution. [d] Twelve reso-
1
nances from BPY were identified in the H NMR spectrum, but they could not be assigned.
1
The H NMR spectra of equimolar solutions of 5a·6NO3
(2.5 mm) and PAHs in D2O recorded upon sonication for 2 h
at room temperature (except for TRI and BPY, which re-
quired 20 h of sonication to complete the inclusion process-
es) showed signals compatible with the formation of the
complexes. The protons of the corresponding PAH were
dramatically shielded (for example in the PHE complex:
Dd=ꢀ3.31 and ꢀ2.99 ppm for Hq and Hr, respectively) as a
consequence of the insertion of the guest. As usual, the pro-
tons at the central positions of the long side of the rectangle
(Hd–h) are specially shifted upfield, whereas the probable
CH–p interactions induce the downfield shift of the protons
at the short side (pyridine ring, protons Ha and Hb; Fig-
ure 1c and Table 1). At room temperature, the rate of the
equilibrium complexation process for all the PAHs (except
NAP) is slow on the NMR spectroscopy time scale. Thus,
the addition of an excess of receptor 5a·6NO3 to a solution
of PYRꢁ5a·6NO3 allowed us to observe two sets of signals,
one for PYRꢁ5a·6NO3 and one attributable to the void
metallacycle 5a·6NO3 (Figure 3). The same pattern of chem-
ical shifts, but to a lower extent because of fast exchange,
can be observed for the complex with NAP. The formation
Figure 3. Partial 1H NMR (D2O, 500 MHz) spectra of a) metallacycle
5a·6NO3 (2.5 mm), b) a solution of metallacycle 5a·6NO3 (2.5 mm) and
PYR (0.75 mm), and c) a solution of metallacycle 5a·6NO3 (2.5 mm) and
&
*
PYR (2.5 mm).
PYRꢁ5a·6NO3.
=metallocycle 5a·6NO3;
=inclusion complex
of
the
inclusion
complexes
TRIꢁ5a·6NO3
and
BPYꢁ5a·6NO3 was monitored by 1H NMR spectroscopy.
The spectra showed separated signals for the inclusion com-
plexes and the void metallacycle suggesting that the ex-
change rates are again slow on the NMR spectroscopy time
scale. Surprisingly, the complexes of 5a·6NO3 with TRI and
BPY presented a 1:1 stoichiometry in D2O, in contrast to
the 2:1 stoichiometry observed in CH3CN solution with
these guests. The 1:1 stoichiometry was determined on the
is well solvated, whereas in CH3CN solution a partial inser-
tion could induce the formation of a ternary complex. Inter-
estingly, the signals assigned to Hr and Hq of the guest in
TRIꢁ5a·6NO3 appeared as two double doublets; moreover,
only eight resonances for the aromatic protons of the metal-
lacycle were detected. This observation in conjunction with
the slow rate exchange indicated that the rotation of TRI
around its C3 axis inside the metallacycle is fast on the
NMR spectroscopy time scale. Although, the dynamic be-
havior of BPYꢁ5a·6NO3 is slightly different, its 1H NMR
spectrum revealed two doublets with a geminal coupling
constant (J=13.0 and 13.4 Hz) for the methylene group in-
dicating that the equatorial plane of the metallacycle is not
a symmetry plane for BPYꢁ5a·6NO3. Consequently, all the
protons of each diazapyrenium moiety are non-equivalent
and they appear as eight resonances, suggesting that the ro-
1
basis of H NMR spectra integration. Even if an excess of
metallacycle is present, the 1:1 stoichiometry of the complex
is preserved. In other words, the H NMR spectra recorded
1
while the formation of the inclusion complexes was moni-
tored showed the signals assigned to the inclusion complex
integrating according to a 1:1 stoichiometry. Solvent polarity
and hydrophobic forces could play a key role in determining
the stoichiometry. Probably, in water the guests are com-
pletely inserted into the metallacycle and the whole complex
ꢀ
tation of these aromatic systems around the N N axis is
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Complexation and Extraction of PAHs
FULL PAPER
1
slow on the H NMR spectroscopy time scale (protons e, e’–
fined by the guest and the diazapirenium moieties is very
close to the optimal separation maximizing p–p interactions.
The metallacycle increases slightly its length to about
14.90 ꢅ and reduces the width to about 7.0 ꢅ. The planes
defined by the diazapyrene systems are parallel to each
other and nearly perpendicular to the equatorial plane of
the metallacycle defined by their four corners. Possibly, this
disposition of the diazapyrene rings also forces the PAH
plane to be orthogonal (about 868) to the same plane, in
contrast to the angle measured in PYRꢁ6a·6NO3 (708)
where the diazapyrenium system is replaced by a 4,4’bi-
pyridium one.[10]
h, hꢃ in Table 1). On the contrary, the rotation of the pyri-
dine and phenylene rings is fast and such splitting of signals
was not detected.
The DOSY experiments also supported the complexation,
all the signals attributable to the complex, including those of
the PAHs resonances, showed the same diffusion coefficient
(Figure 4).
The position of PYR and PHE inside the cavity deviates
slightly from the longitudinal insertion mode defined by the
coincidence of the axis H2–H7 and the equatorial plane of
5a·6PF6. The angles formed by this axis and the equatorial
plane of the metallacycle are 198 (PYR) and 158 (PHE). In
the case of TRIꢁ5a·6PF6 the mode of binding in the solid
state is close to the longitudinal insertion, being the angle
between one of the C2 axes of TRI and the equatorial plane
of the metallacycle corresponding to 88.
Figure 4. DOSY (D2O, 500 MHz, 298 K) experiment of TRIꢁ5a·6NO3.
Determination of binding constants: To measure the associa-
tion between metallacycles 5a, 5b, and 6a and the selected
PAHs quantitatively, the association constants were deter-
mined in acetonitrile solution. The results found are sum-
marized in Table 2. As could be expected, the diazapyrene
The affinity of metallacycle 5a·6NO3 for PAHs in aqueous
media was monitored by 1H NMR spectroscopy. Competi-
tion assays revealed that NAP and ANT present the lowest
affinity because the addition of 1 equivalent of PYR can dis-
place them completely from the cavity of metallacycle
5a·6NO3 (after sonication). The PHE inclusion complex is
more stable and PYR can displace PHE only partially (see
the Supporting Information). These results are in agreement
with the values of the association constants determined in
aqueous media (see below).
[a]
Table 2. Association constants Ka (Lmolꢀ1 or Lmolꢀ2
)
in CH3CN for
the complexes formed between metallacycles 5b or 6a and PAHs.
Entry Metallacycle PAH
Ka [mꢀ1] or [mꢀ2
]
lmax
[nm]
1
2
3
4
5
6
7
8
9
6a·4OTf·2PF6 pyrene
6a·4OTf·2PF6 phenanthrene
6a·4OTf·2PF6 triphenylene
145ꢂ7
1:1 452
156ꢂ6
1:1 425
nd 410
1:1 435
1:1 436
1:1 437
1:1 437
376ꢂ10
5b·6PF6
5b·6PF6
5b·6PF6
5b·6PF6
5b·6PF6
5b·6PF
naphthalene
anthracene
pyrene
phenanthrene
triphenylene
<10
Crystal structures of PYRꢁ5a·6PF6, PHEꢁ5a·6PF6, and
TRIꢁ5a·6PF6: The formation of the 1:1 inclusion complexes
with PYR, PHE, and TRI in the solid state was confirmed
by X-ray crystallography analysis of single crystals obtained
by vapor diffusion of isopropyl ether into CH3CN solutions
of the corresponding PAH and 5a·6PF6 (Figure 5). Crystal
structures show the inclusion of the PAH inside of the hexa-
cationic metallacycle. The distance between the planes de-
3135ꢂ82
10704ꢂ766
3426ꢂ121
1.46ꢆ107 ꢂ3ꢆ105 2:1 438
benzo[a]pyrene 4.19ꢆ107 ꢂ8ꢆ105 2:1 438
6
[a] The association constants reported for TRI and BPY complexes (en-
tries 8 and 9) are the product of Ka1 ꢆKa2.
based metallacycle 5b displays higher association constants
than the bipyridine metallacycle 6a for all the PAH mea-
sured. Furthermore, the magnitude of the association con-
stants for the 1:1 complexes of 5b (Table 2, entries 4–7) is
well correlated with the size and extended p character of
the PAHs used in the study.
An important feature of these metallacycles is their ability
for the extraction of PAHs to an aqueous phase. The associ-
ation constants in water were estimated by liquid–liquid ex-
traction experiments and are summarized in Table 3.[14]
A
Figure 5. Crystal structures of the inclusion complexes TRIꢁ5a·6PF6,
PYRꢁ5a·6PF6, and PHEꢁ5a·6PF6 (from left to right). Hydrogen atoms,
solvent molecules, and counterions have been omitted for clarity. The
color-labeling scheme is as follows: carbon (gray), palladium (light gray),
and nitrogen (dark gray).
10 mm solution of PAH in heptane was extracted for 24 h at
room temperature with a 2 mm solution of 5b·6NO3. The
concentration of PAH in the heptane solution upon extrac-
Chem. Eur. J. 2010, 16, 12373 – 12380
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12377

C. Peinador, J. M. Quintela et al.
Table 3. Association constants Ka (Lmolꢀ1) in H2O for 1:1 complexes
formed between metallacycle 5b·6NO3 and PAHs.
inclusion complexes (with PYR, PHE, and TRI) have been
presented and discussed. The association constants of plati-
num receptor 5b with PAHs in organic and aqueous media
were measured. The potential of the designed metallacycle
5b, for the selective extraction of PAHs from an organic
media to an aqueous phase, was explored by means of com-
petitive extraction experiments. The obtained results
showed a good correlation between the association con-
stants in water for the inclusion complexes and the resulting
percentages of PAHs extracted.
Entry
PAH
Ka [mꢀ1
]
G
2
1
2
3
4
5
naphthalene
anthracene
pyrene
phenanthrene
benzo[a]pyrene
2.59ꢆ103
nd
2.47ꢆ10ꢀ4
4.10ꢆ10ꢀ7
6.63ꢆ10ꢀ7
7.24ꢆ10ꢀ6
1.51ꢆ10ꢀ8
5.59ꢆ105
6.47ꢆ104
7.69ꢆ106
[a] Taken from reference [16].
tion ([PAH]hep) was determined by GC–MS. The total con-
centration of pyrene present in the aqueous solution
[PAH]H O(tot) can be calculate by subtracting [PAH]hep from
2
Experimental Section
the initial concentration of pyrene (10 mm). It can be as-
sumed that the amount of uncomplexed PAH in the aque-
ous solution can be considered constant and equal to the
Compounds 1,[17] 2,[18] 6,[9] and 2,7-diazapyrene[19] were prepared accord-
ing to published procedures. All other reagents used were commercial
grade chemicals from freshly opened containers. Milli-Q water was puri-
fied with a Millipore Gradient A10 apparatus. Merck 60 F254 foils were
used for thin layer chromatography, and Merck 60 (230–400 mesh) silica
gel was used for flash chromatography. Proton and carbon nuclear mag-
netic resonance spectra were recorded on a Bruker Avance 300 or
Bruker Avance 500 spectrometer equipped with a dual cryoprobe for 1H
and 13C, using the deuterated solvent as lock and the residual protiated
solvent as internal standard. DOSY experiments were referenced using
the value 1.92ꢆ10ꢀ9 m2 sꢀ1 for the DHO signal in D2O at 298 K[20] and
the value 1.97ꢆ10ꢀ9 m2 sꢀ1 for the CHD2NO2 signal in CD3NO2 at
298 K.[21] Mass spectrometry experiments were carried out in an LC-Q-q-
TOF Applied Biosystems QSTAR Elite spectrometer for low- and high-
resolution ESI. Determinations of PAH by GCMS were carried out in a
Thermo Finnigan Polaris Q system equipped with a J&W, DB-XLB
column.
maximum concentration of PAH in water ([PAH]H O).[15]
2
The concentration of PAH complexed is [PAH]H O(tot)
ꢀ
2
[PAH]H O, which in a 1:1 complexation is equal to the con-
2
centration of [PAHꢁ5b·6NO3]H O. The concentration of free
2
metallacycle is obtained by subtracting [PAHꢁ5b·6NO3]H O
2
from the initial concentration of 5b·6NO3 (2 mm). There-
fore, all concentrations of equation (1) are known and Ka is
determined from equation (2).
ð5 b ꢃ 6NO3ÞH O þ PAHH O Ð ðPAH ꢁ 5 a ꢃ 6NO3ÞH O
ð1Þ
ð2Þ
2
2
2
½PAH ꢁ 5 b ꢃ 6NO3ꢄH O
2
Ka ¼
½5 b ꢃ 6NO3ꢄH O½PAHꢄH O
2
2
Ligand 3·PF6: A solution of 2-(2,4-dinitrobenzyl)-2,7-diazapiren-2-ium
chloride (1·Cl) (1.03 g, 2.77 mmol) and 4-(pyridin-4-ylmethyl)aniline (2)
(2.04 g, 11.07 mmol) in EtOH (75 mL) was refluxed for 3 d, after cooling,
the solvent was evaporated in vacuo. The resulting residue was dissolved
in H2O (200 mL) and extracted with EtOAc (200 mL). The organic layer
was further extracted with H2O (2ꢆ150 mL) and the combined aqueous
extracts were washed with EtOAc (4ꢆ75 mL). The solvent was removed
under reduced pressure to give a crude product, which was purified by
column chromatography (SiO2, acetone/NH4Cl 1.5m/MeOH 5:4:1). The
product-containing fractions were combined and the solvents were re-
moved in vacuo. The residue was dissolved in H2O/CH3OH (95:5,
250 mL) and an excess of KPF6 was added until no further precipitation
was observed. The solid was filtered and washed with water to give 3·PF6
(0.97 g, 70%) as a dark yellow solid. 1H NMR (500 MHz, CD3NO2): d=
4.68 (s, 2H), 7.84 (d, J=8.6 Hz, 2H), 8.08 (d, J=8.6 Hz, 2H), 8.11 (d, J=
6.7 Hz, 2H), 8.69 (d, J=9.1 Hz, 2H), 8.82 (d, J=6.8 Hz, 2H), 8.84 (d, J=
9.1 Hz, 2H), 9.91 (s, 2H), 9.92 ppm (s, 2H); 13C NMR (125 MHz,
CD3NO2): d=42.1 (CH2), 125.5 (C), 127.2 (CH), 127.3 (CH), 127.6 (C),
129.3 (CH), 130.4 (C), 130.6 (C), 133.1 (CH), 133.2 (CH), 139.3 (CH),
142.5 (CH), 142.7 (C), 144.8 (C), 149.9 (CH); MS (ESI): m/z: 372.2
[MꢀPF6ꢀ]+; elemental analysis calcd (%) for C26H18F6N3P: C 60.35, H
3.51, N 8.12; found: C 60.08, H 3.86, N 8.33.
The association constants in aqueous media show a clear
correlation with the hydrophobicity of the PAH. The larger
the aromatic surface, the greater the association constant for
its formation. Finally, a series of competitive extraction ex-
periments was carried out to illustrate the selectivity of this
molecular recognition process. On one hand, PYR was ex-
clusively extracted from a 1:1 molar mixture of PYR/NAP.
On the other hand, a slight excess of PYR (54% molar) was
extracted to the aqueous phase from an equimolar solution
of PYR and PHE, according to the respective binding con-
stants and structural similarity between PYR and PHE. Sim-
ilarly, a correspondence was found between association con-
stants and the molar extraction of an equimolar solution of
five PAHs (NAP, 9%; ANT, 19%; PHE, 20%; PYR, 21%;
BPY, 31%).
Conclusion
Ligand 3·NO3: Ligand 3·PF6 (500.0 mg, 0.97 mmol) was dissolved in the
minimum amount of CH3CN and an excess of Bu4NNO3 was added until
no further precipitation was observed. The white precipitate was filtered
and washed with CH3CN to yield 3·NO3 (356.0 mg, 85%) as a brown
solid. 1H NMR (500 MHz, D2O): d=4.56 (s, 2H), 7.77 (d, J=8.5 Hz,
2H,), 7.98 (m, 4H), 8.46 (d, J=9.1 Hz, 2H), 8.56 (d J=9.1 Hz, 2H), 8.70
(d, J=6.8 Hz, 2H), 9.57 (s, 2H), 9.95 ppm (s, 2H); 13C NMR (125 MHz,
D2O): d=40.5 (CH2), 123.9 (C), 125.5 (CH), 126.1 (CH), 126.2 (C), 127.3
(CH), 128.0 (C), 128.7 (C), 131.0 (CH), 131.6 (CH), 138.2 (CH), 141.1
(C), 141.2 (CH), 142.6 (C), 146.8 (CH), 161.5 ppm (C); MS (ESI): m/z:
372.2 [MꢀNO3ꢀ]+; elemental analysis calcd (%) for C26H18N4O3: C 71.88,
H 4.18, N 12.90; found: C 71.60, H 3.88, N 13.20.
New dinuclear metallacycles were self-assembled in aqueous
media from a 2,7-diazapyrenium-based ligand and square-
planar palladium(II) or platinum(II) complexes. These rec-
tangular metallacycles have proven to be receptors for
PAHs, such as pyrene, phenanthrene, triphenylene, and ben-
zo[a]pyrene. Their cavities present a nearly optimal size to
form supramolecular complexes with PAHs through p-stack-
ing and hydrophobic forces. The crystal structures of three
12378
www.chemeurj.org
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 12373 – 12380

Complexation and Extraction of PAHs
FULL PAPER
Table 4. Summary of crystal data, intensity measurements, and structure
refinement.
calcd (%) for C56H52N16O18Pt2: C 41.33, H 3.22, N 13.77; found: C 41.56,
H 3.08, N 13.67.
Computational methods: All calculations were performed using the
Gaussian 09 (Revision A.02)[22] program package with the B3LYP[23,24]
three-parameter hybrid density functional. Geometry optimization of 5b
was carried out in water without constrains. Initial geometries were con-
structed using the GaussView program[25] and standard bond distances
and angles. In these calculations we used the standard 6-31G(d) basis set
for C, H, and N atoms, whereas for Pt atoms the effective core potential
of Wadt and Hay (Los Alamos ECP) included in the LanL2DZ basis set
was applied.[26] The stationary point found on the potential energy sur-
face as a result of the geometry optimizations of 5b has been tested to
represent energy minima rather than saddle points by frequency analysis.
PYRꢁ5a·6PF6
C91H86F36N20P6Pt2 C39H36F18N7P3Pt
PHEꢁ5a·6PF6
TRIꢁ5a·6PF6
C46H45.5F18N9.5P3Pt
1361.42
0.71073
formula
Mr
2719.80
1232.75
0.71073
wavelength 0.71073
[ꢅ]
crystal size
0.50ꢆ0.32ꢆ0.27
0.25ꢆ0.06ꢆ0.03
triclinic
0.22ꢆ0.15ꢆ0.10
monoclinic
[mm3]
crystal
system
monoclinic
¯
P1
space group C2/c
Cc
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
b [8]
41.102(5)
12.359(5)
13.052(5)
15.650(5)
96.747(5)
105.968(5)
110.945(5)
2200.3(14)
2
41.335(3)
11.5259(8)
28.6254(19)
90
129.298(3)
90
10553.8(12)
8
1.714
Crystal structure analysis: CCDC-783769, 783767, 783766, and 783768
contain the supplementary crystallographic data for 3·PF6,
PYRꢁ5a·6PF6, PHEꢁ5a·6PF6, and TRIꢁ5a·6PF6, respectively. These
data can be obtained free of charge from The Cambridge Crystallograph-
ic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Crystal data are
reported in Table 4. The structures were solved by direct methods and re-
fined with the full-matrix least-squares procedure (SHELX-97)[27] against
F2. The X-ray diffraction data were collected on a Bruker X8 ApexII dif-
fractometer. Non-solvent hydrogen atoms were placed in idealized posi-
tions with Ueg(H)=1.2Ueg(C) and were allowed to ride on their parent
atoms. Solvent hydrogen atoms were placed in idealized positions with
Ueg(H)=1.5Ueg(C) and were allowed to ride on their parent atoms.
11.488(5)
28.310(5)
90.000(5)
128.761(5)
90.000(5)
10423(5)
4
g [8]
V [ꢅ3]
Z
1calcd
1.733
1.861
[Mgmꢀ3
]
]
m [mmꢀ1
2.894
2.27 to 28.35
3.415
2.858
q range [8]
2.27 to 28.47
ꢀ16ꢅhꢅ16,
ꢀ17ꢅkꢅ17,
ꢀ20ꢅlꢅ20
100(2)
1.43 to 25.15
ꢀ49ꢅhꢅ49,
ꢀ13ꢅkꢅ13,
ꢀ34ꢅlꢅ34
100(2)
index range ꢀ54ꢅh ꢅ46,
ꢀ15ꢅkꢅ15,
ꢀ37ꢅlꢅ37
T [K]
100(2)
independent 12898
11046
18669
Acknowledgements
reflections
refinement
method
[Rint =0.0308]
[Rint =0.0310]
[Rint =0.0483]
based on F2
based on F2
based on F2
This research was supported by Ministerio Ciencia e Innovaciꢇn and
FEDER (CTQ2007-63839/BQU, and CTQ2010-16484/BQU). V.B. and
E.P. thank Ministerio de Educaciꢇn (FPU Program) and Universidade da
CoruÇa for predoctoral fellowships. M.D.G. thanks the Xunta de Galicia
for financial support under “Programa Isidro Parga Pondal”.
final R
indices
R1 =0.0496,
wR2 =0.1334
R1 =0.0334,
wR2 =0.0856
R1 =0.0432,
wR2 =0.1106
[I>2s(I)]
R indices
(all data)
R1 =0.0560,
wR2 =0.1386
R1 =0.0348,
wR2 =0.0864
R1 =0.0539,
wR2 =0.1212
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Metallacycle 5b·6PF6: A solution of ligand 3·NO3 (120.0 mg, 0.276 mmol)
and (en)Pt(NO3)2 (4b) (104.7 mg, 0.276 mmol) in H2O (60 mL) was
ACHTUNGTRENNUNG
heated at 1008C for 8 d. Upon cooling to room temperature, an excess of
KPF6 was added until no further precipitation was observed. The solid
was filtered to yield 5b·6PF6 (281.7 mg, 96%) as a brown solid. 1H NMR
(500 MHz, CD3NO2): d=3.19 (m, 8H), 4.26 (s, 4H), 7.42 (d, J=8.6 Hz,
4H), 7.51 (d, J=6.8 Hz, 4H), 7.63 (d, J=8.7 Hz, 4H), 8.63 (d, J=9.2 Hz,
4H), 8.67 (d, J=9.2 Hz, 4H), 8.82 (d, J=6.8 Hz, 4H), 9.69 (s, 4H),
10.00 ppm (s, 4H); 13C NMR (125 MHz, CD3NO2): d=41.1 (CH2), 49.8
(CH2), 49.9 (CH2), 126.2 (C), 126.3 (CH), 128.9 (C), 129.5 (CH), 130.0
(CH), 130.4 (C), 130.7 (C), 131.6 (CH), 132.3 (CH), 140.5 (CH), 143.6
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]
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3+
ꢀ 4+
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563.0860; found 563.0853; calcd for [Mꢀ4PF6
]
386.0733;
ꢀ
5+
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(CH), 128.8 (C), 129.0 (C), 129.7 (CH), 130.6 (CH), 139.0 (CH), 141.8
(C), 142.7 (C), 149.0 (CH), 151.6 (CH), 155.1 ppm (C); elemental analysis
Chem. Eur. J. 2010, 16, 12373 – 12380
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: July 19, 2010
Published online: October 4, 2010
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