Site-selective imination of an anthracenone sensor: Selective fluorescence detection of Barium(II)

Article abstract of DOI:10.1021/jo2013143

Site-selective imination of anthraquinone-based macrocyclic crown ethers using titanium tetrachloride as the catalyst yields imines where only the external carbonyl group of the anthraquinone forms Schiffbases. The following aromatic amines yield monomeri

Full text of DOI:10.1021/jo2013143

ARTICLE
pubs.acs.org/joc
Site-Selective Imination of an Anthracenone Sensor: Selective
Fluorescence Detection of Barium(II)
Prem N. Basa, Arundhati Bhowmick, Mariah M. Schulz, and Andrew G. Sykes*
Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069, United States
S Supporting Information
b
ABSTRACT: Site-selective imination of anthraquinone-based macrocyclic
crown ethers using titanium tetrachloride as the catalyst yields imines
where only the external carbonyl group of the anthraquinone forms Schiff-
bases. The following aromatic amines yield monomeric compounds
(aniline, 4-nitroaniline, 4-pyrrolaniline, and 1,3-phenylenediamine). Reac-
tion of 2 equiv of the macrocyclic anthraquinone host with 1,2- and 1,4-
phenylenediamine yields dimeric imine compounds. The 1,2-diimino host
acts as a luminescence sensor, exhibiting enhanced selectivity for Ba(II) ion. Spectroscopic data indicate that two barium ions
coordinate to the sensor. Due to E/Z isomerization of the imine, the monomeric complexes are nonluminescent. Restricted rotation
about the 1,2 oriented CdN groups or other noncovalent/coordinate-covalent interactions acting between neighboring crown
ether rings may inhibit E/Z isomerization in this example, which is different from current examples that employ coordination of a
metal cation with a chelating imine nitrogen atom to suppress E/Z isomerization and activate luminescence. The 1,4-diimino
adduct, where the crown rings remain widely separated, remains nonluminescent.
’ INTRODUCTION
photophysical properties result.19 For this class of sensor, isomer-
ization of the covalently bridged CdN provides an efficient
pathway for the nonradiative relaxation of photoexcited states.
Coordination of a metal ion to the CdN nitrogen lone pair
suppresses CdN isomerization, leading to production of a photo-
dynamic ‘ON’ state, resulting in enhanced emission.20
Here we demonstrate the selective sensing of Ba(II) ion using
imine-containing macrocycles, whereas imine-based fluores-
cence sensors have predominantly been selective for zinc(II)
ion in the past. The selective and sensitive detection of barium is
important because soluble barium compounds are known to be
poisonous and severity is dependent upon the dose of barium. At
low doses, barium acts as a muscle stimulant, whereas higher doses
affect the nervous system, causing cardiac irregularities, tremors,
weakness, anxiety, dyspnea, and paralysis.21 Studies involving the
selective detection of Ba2+ have been rarely addressed, however,22
although the problems related to its toxicity should encourage the
development of new detection systems for this analyte. Many
of the reported sensors are nonselective in nature,23 generally
containing strong interference from Mg2+, Ca2+, and Sr2+ cations
within the same alkaline earth family.
Since the original discovery of imines by Schiff in the year
1864,1 imines have been used in a variety of applications such as
dyes,2 catalysts,2 and macrocycles,3 and they have served as
model compounds that mimic several enzymes and proteins.4
Imines, also called azomethines or Schiff bases, are typically
synthesized starting from primary amines and corresponding
aldehydes. Because of their limited reactivity, imines derived
from ketones are less common; however, ketones do undergo
nucleophilic substitution in the presence of an appropriate
catalyst. One system that has recently gained prominence
uses Ti(IV) as the catalyst.5 Hall et al. has successfully condensed
anthraquinones with aromatic amines to make imines and
polyimines, using titanium tetrachloride as the catalyst together
with DABCO.5b,6 Recently we have employed this catalyst to
react anilines with compound 1 (Scheme 1), which condenses
exclusively with the external carbonyl group, even in the pre-
sence of excess aromatic amine. The directed synthesis of imines
has been limited to simple asymmetric quinones,7 and has the
potential for the selective derivitization of new antistaphylococ-
cal drugs.8
Imine-based fluorescence sensors, although only recently
employed as a new class of luminescence sensors,9 are attractive
because generally (i) straightforward preparative methods are
available, (ii) starting materials are readily available, (iii) selectivity
toward metal ions is good,10À18 and (iv) attractive electronic and
In this study, we report the site-selective synthesis of a series of
anthraquinone-imine derivatives along with fluorescence screen-
ing with common metal cations and associated stoichiometric
studies. Compound 1, without imine formation, and its deriva-
tives have previously demonstrated selective fluorescence with a
variety of biological and environmentally important cations.24
Received: June 22, 2011
Published: August 23, 2011
r
2011 American Chemical Society
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Scheme 1a
a 2a, R = H; 2b, R = 4-NO2; 2c, R = 4-(1H-pyrrol-1yl); 2d, R = 3-NH2; 3a = 1,2-diimino; 3b, 1,4-diimino.
by Hall et al. for the diimination/polymerization of both car-
bonyl groups in anthraquinones that do not contain a cyclic
polyether.5,6
The synthesis of diimines was also achieved by the addition of
1,2- and 1,4-phenylenediamines with 1 (Scheme 1, compounds
3a and 3b, respectively). The asymmetry of the anthraquinone
aryl protons, integration of aryl vs alkyl proton resonances,
elemental analyses data, and the ESI-MS results confirm the
formation of dimeric compounds, which also are a darker red-
orange in color than the imine monomers (orange) due to
increased conjugation. When using only 1 equiv of 1, reaction
with 1,3-phenylenediamine yielded only the resulting monomer,
compound 2d. Reaction of 1,2- and 1,4-phenylenediamine with
1 equiv of 1 only yielded dimeric compounds upon isolation and
purification.
Selectivity of these new imines as potential metal ion sensors
was examined by adding 10 equiv of metal perchlorate salts to
compounds 2aÀd and 3aÀb in acetonitrile and collecting the
resulting emission spectra (Figures S2ÀS7, Supporting In-
formation). With the exception of 3a, none of the imines shows
selectivity for any of the different metal ions tested. Compound
3a does show a marked selectivity for the addition of Ba(II) ion,
however. Figure S7 also shows that 3a is mildly selective for other
alkaline earth metals, albeit with reduced sensitivity, increasing in
emission intensity as descending down the periodic table. A
competition study for Ba(II) with the same cations (Figure 2)
shows that Ba(II) selectivity has increased 7-fold over the
ligand, while Sr(II) increases ∼2.5 fold and Ca(II) increases
1.5 fold in comparison. Luminescence for the most part is
maintained for Ba(II) even in the presence of most other metal
ions, except for a few paramagnetic transitional metals and Hg
and Pb heavy atoms.
The Job’s plot using fluorescence data to determine the
number of barium(II) ions that associate with 3a is provided
in Figure 3. The apex point of the plot approximates 0.667,
which is the ideal mole ratio for a 1:2 ligand:metal complex,
i.e., [Ba]/([L] + [Ba]) = 0.667. This supports a 1:2 ligand:
metal association reaction, L + 2 M T LM2 where two Ba(II)
ions are involved in production of luminescence.
Figure 1. Thermal ellipsoid diagram (30%) for compound 2b. C(13)À
O(1) = 1.215(2) Å; N(2)ÀO(6) = 1.227(2) Å ; N(2)ÀO(8) =
1.231(2) Å.
’ RESULTS
The reaction of 1 equiv of the anthraquinone macrocycle 1
with 1 equiv of TiCl4 and a slight excess of an aromatic aniline
(Scheme 1) yields the site-selective synthesis of compounds
2aÀd where only the extraannular carbonyl group has been
converted to the corresponding imine. The identities of these
new imines have all been confirmed by elemental analyses
and NMR results. In addition, the X-ray crystal structures
of compound 2b (Figure 1) and 2c (Figure S1, Supporting
Information), the p-nitro and pyrrole adducts, confirm that
imine formation occurs only at the external carbonyl group.
The imine bond for 2b equals 1.288(2) Å in length, typical of a
CdN double bond, and the imine bond angle equals C(6)ÀN-
(1)ÀC(23) 125.99(15)° where the p-nitrophenyl ring is also
rotated away from the anthraquinone plane to avoid steric
interference from the aromatic hydrogen atom of the anthra-
quinone. Since two carbonyl groups are available, the produc-
tion of only one possible imine isomer indicates that the cyclic
polyether ring may trap the Lewis acid catalyst adjacent to the
intraannular carbonyl, preventing loss of H2O after nucleo-
philic attack by the amine nitrogen at the carbonyl carbon.
Only two bands, the starting material 1 and the product band,
were observed upon purification by column chromatography
after each synthesis. The directed synthesis of amines in
this system is different than what has previously been observed
The titration of compound 3a with increasing amounts of
Ba(II) ion is shown in Figure 4. Ba(II) has been added until the
emission intensity saturates, and the insert plots fluorescence
intensity versus Ba(II) concentration. The data points (squares)
follow a sigmoidal curve rather than the standard saturation
behavior that would be expected for a simple 1:1 ligand: metal
association reaction (dotted line in the insert). Higher order
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Figure 2. SelectivityÀcompetition study of compound 3a with added M(II) perchlorate salts. 4.0 Â 10À5 M = [3a]; [M(II)] = 4.0 Â 10À4 M.
λex = 356 nm. Green = 3a; blue = 3a + M(II); red = 3a + M(II) + Ba(II).
Figure 3. Job’s plot showing 1:2 stoichiometry for compound 3a
(L + Ba(II) = constant 2.0 Â 10À4 M): The ideal mole ratio for a 1:2
Figure 4. Titration of 3a (2.0 Â 10À4 M) with Ba(ClO4)2 in acetoni-
complex [Ba]/([L] + [Ba]) = 0.667, λex = 356 nm.
trile, λex = 356 nm. Insert: 9 fluorescence data; --- best fit for 1:1 ligand:
Ba(II) association; À À À best fit for 1:1.5; — best fit for 1:2.
association reactions have been modeled to confirm that a
Association constant for 1:2 best fit equals 1.1 Â 107 MÀ2.
.
second-order association where two metals ions associate with
compound 3a provides the best fit. Also shown in the inset is the
fit (dashed line) for a 1:1.5 association, which would represent a
2 L + 3 M T L2M3 type model. The 1:2 fit is clearly the appro-
priate fit and confirms the stoichiometry previously obtained
from the Job’s plot. The 1:2 association constant equals 1.1 Â
107 MÀ2 ((15%) from the best fit and represents the product
of the individual association constants of the successive steps:
L + M T LM + M T LM2 = K1K2 = 1.1 Â 107. K1 and K2 (esti-
mated at ∼102 to 104 individually) are not particularly large
association constants in either case and account for the shallow
saturation in intensity at approximately 2 equiv (0.004 M) of
added Ba(II).
Crystal structures for compound 2b in the presence of Ca(II)
and Zn(II) perchlorate show two very different coordination
environments for the metal cations. For the calcium adduct
(Figure 5), the calcium is bound completely within the macro-
cycle and exhibits a coordination sphere where the calcium is
complexed to nine oxygen atoms: the intraannular carbonyl
oxygen, three polyether oxygens of the polyether ring, and four
oxygen atoms from two chelating perchlorate anions, one above
and one below the ring. The CaÀO bond distances range from
2.303(3) to 2.643(3) Å, with the carbonyl oxygen to calcium
bond distance being the shortest.
The Zn complex is quite different in the solid state as
compared to the Ca complex (Figure 6). The Zn(II) ion is not
complexed within the macrocyclic ring and has instead a
complete octahedral coordination sphere occupied by six
water molecules. Two of the water molecules coordinated
to the zinc form hydrogen bonds to the carbonyl oxygen,
O(1)ÀO(11) = 2.794(4) Å and O(2)ÀO(11) = 2.849(3) Å,
and a third coordinated water molecule bridges two polyether
oxygens, O(3)ÀO(13) = 2.733(4) Å and O(3)ÀO(15) =
2.743(4) Å. Two additional hydrogen bonds exist between
the coordinated water molecules, a perchlorate anion, and the
methanol solvate (not shown). The [Zn(H2O)6]2+ cation is
also centrosymmetric and is sandwiched between two iden-
tical imine macrocycles. Within the structure, no hydrogen
bonds are found involving the imine nitrogen or the nitro
group, however.
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ARTICLE
transfer (ICT) of the anthraquinone lumophore, and, at the same
time, complexation of the metal ions produce rigidification of
the molecule, increasing radiative relaxation/emission. Because the
macrocyclic hosts occupy the 1,2 positions of the bridging
phenyl group, E/Z isomerization of the R-CdN group may
be restricted. Notably, the 1,4-isomer (3b) is nonluminescent,
lending credibility to this option, because the binding affinity of
the macrocyclic hosts should remain relatively constant; however
E/Z isomerization is still possible, as the two hosts are diametri-
cally opposed to each other across the phenyl spacer. Restricted
rotation about the 1,2 oriented CdN groups or other additional
noncovalent or coordinate-covalent interactions between metal-
occupied neighboring crown rings represents a new photody-
namic mechanism for activation of luminescence in E/Z imine
sensors. This is opposed to the common suppression of E/Z
isomerization by coordination of metal ions to the chelating
imine nitrogen atom. The possibility still remains, however, that
the imine nitrogens in the ortho positions of the phenyl spacer in 3a
act as a potential chelating group, binding to a second metal center
and preventing E/Z isomerization from occurring. Unfortunately, to
resolve this issue, we have not been able to isolate single-crystals of
3a in the presence of Ba(II) or Sr(II). We have made numerous
attempts to grow crystals in the presence of different alkaline earth
cations, different solvents, different amounts of metal ion, different
counteranions, and using different crystal growing methods. Only
small clusters with thin, ill-defined shapes have been obtained.
Figure 5. Thermal ellipsoid diagram (30%) of [2b Ca](ClO4)2 CH2Cl2.
The chloroform has been omitted for clarity.
3
3
’ CONCLUSIONS
The organic site-selective synthesis of new imine macrocycles
is described, which has not been previously reported. Imine
monomers are nonluminescent in the presence of metal cations,
as opposed to the title compound from which they are derived.
E/Z isomerization, typical of imines, allows nonradiative deacti-
vation of the excited state. A diimine dimer based on 1,2-phenyl-
enediamine shows luminescence specificity toward Ba(II) in solu-
tion. E/Z Isomerization of the imine groups may be restricted by
noncovalent/coordinate-covalent interactions between adjacent
crown ether rings and/or steric influences, representing a new mecha-
nism for luminescence enhancement in this class of compounds.
Figure 6. Thermal ellipsoid diagram (30%) of [2b Zn(H2O)6](ClO4)2
MeOH. The methanol solvate and the perchlorate anions have been
omitted for clarity. Hydrogen bonds are drawn as dashed lines.
3
3
’ EXPERIMENTAL SECTION
’ DISCUSSION
Elemental analyses results are the average of two or more determina-
tions. 1H (200 MHz) and 13C (50 MHz) spectra were obtained at room
temperature in CDCl3 and referenced to the residual solvent peak. 1,8-
Oxybis(ethylenethoxyethylenethoxy)-9,10-anthraquinone (1) was syn-
thesized by a previously reported method.25 Melting points were deter-
mined in an open capillary and are uncorrected. ESI-MS spectra were
obtained using an ion trap mass spectrometer.
General Synthesis. In a round-bottom flask, 2.0 mmol of the
aromatic amine (2aÀd) and 7.5 mmol of DABCO were dissolved in
10 mL of toluene and heated at 90 °C for 10 min. Titanium tetrachloride
(1.87 mmol) in 5 mL of toluene was added dropwise. Compound 1
(1.87 mmol) was then added to the above mixture and refluxed for 15 h.
The solvent was removed at reduced pressure, and 50 mL of CH2Cl2 was
added and washed several times with brine solution. The organic layer
was collected, dried, and purified by column chromatography (neutral
alumina) using CH2Cl2 and CH2Cl2/CH3OH (9/1) as eluents, to
obtain pure solids.
Neither Ca(II) or Zn(II) turn on luminescence with any of the
monomeric imine compounds, and these structures indicate that
complexation of a cation with the intraannular carbonyl is not
sufficient to turn on emission within these macrocyclic hosts.
The uncomplexed imine is ostensibly still free to undergo E/Z
isomerization and nonradiatively deactivate the excited state.
This is different behavior than what was previously observed with
compound 1 alone (no imine present), which exhibits signifi-
cantly enhanced luminescence in the presence of Ca(II) and
Pb(II).24b X-ray crystal structures clearly show complexation of
cations within the polyether ring in 1, similar to the calcium
structure found here, including coordination of the intraannular
carbonyl oxygen, which is sufficient to enhance luminescence in
these other nonimine type systems.
The stoichiometric data above indicates that the addition of 2
equiv of metal cations is sufficient to induce luminescence in the
1,2-dimeric imine compound 3a. Two possibilities exist for
activation of luminescence. Both of the polyether macrocycles
could coordinate a metal cation, which promotes internal charge
1,8-Oxybis(ethylenoxyethylenoxy)-10-(phenylimino)anth-
racen-9(10H)-one (2a). Yield, 80%, melting point = 208À210 °C,
elemental analyses calculated for C28H29N1O6: C, 70.72; H, 6.15; N,
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2.95%. Found: C, 70.86; H, 5.83; N, 3.19%. 1H NMR (200 MHz, CDCl3,
25 °C): δ 3.70À4.30 (m, 16H, CH2ÀO, polyether); 6.60À6.75 (t, 1H);
6.80À7.00 (m, 3H); 7.00À7.20 (t, 3H); 7.25À7.40 (t, 2H); 7.45À7.65
(t, 1H); 7.75À7.90 (d, 1H). 13C NMR (CDCl3, 25 °C): δ 68.7; 69.3;
70.1; 70.3; 71.00; 109.7, 114.5, 117.9; 119.3; 120.9; 123.7; 129.2; 131.2;
132.8; 150.8; 156.4; 157.1; 183.1.
1,8-Oxybis(ethylenoxyethylenoxy)-10-[(4-nitrophenyl)-
imino]anthracen-9(10H)-one (2b). Yield, 83%, melting point =
220À222 °C, elemental analyses calculated for C28H28N2O8.H2O: C,
62.45; H, 5.57; N, 5.21%. Found: C, 62.74; H, 5.40; N, 5.22%. 1H NMR
(200 MHz, CDCl3, 25 °C): δ 3.7À4.3 (m, 16H, CH2ÀO, polyether);
6.75À7.20 (dd, 5H); 7.3À7.5 (br, 2H); 8.10À8.25 (d, 2H). 13C NMR
(CDCl3, 25 °C): δ 68.7; 69.4; 70.2; 71.0; 115.5; 119.2; 125.5; 132.4;
143.6; 156.8; 157.5, 158.0, 183.2
close donorÀacceptor distance of 2.76 Å exists between the OH of the
methanol and a perchlorate anion, representative of a traditional
hydrogen bond. A single DFIX instruction restrained the CÀO distance
in the methanol solvate as well. For [2b Ca](ClO4)2 CH2Cl2, the nitro
group is disordered and has been modeled over two positions in a 64:36
ratio and the smaller occupancy kept isotropic. Table S1 in Supporting
Information lists additional crystallographic and refinement information.
3
3
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures,
b
crystallographic data, NMR data and full spectroscopic data
for all new compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
1, 8-Oxybis(ethylenoxyethylenoxy)-10-{[4-(1H-pyrrol-1yl)-
phenyl]imino}anthracen-9(10H)-one (2c). Yield, 74%, melting
point = 222À224 °C, elemental analyses calculated for C32H30N2O6.
H2O: C, 69.05; H, 5.76; N, 5.03%. Found: C, 68.42; H, 5.67; N, 4.93%. 1H
NMR (200 MHz, CDCl3, 25 °C): δ 3.7À4.3 (m, 16H, CH2ÀO, poly-
ether); 6.2À6.4 (s, 2H); 6.6À6.8 (d, 1H); 6.8À7.0 (t, 3H); 7.0À7.2 (m,
4H); 7.3À7.4 (d, 2H); 7.4À7.5 (t, 2H); 7.7À8.0 (d, 2H). 13C NMR
(CDCl3, 25 °C): δ 69.1, 69.3, 70.6, 71.2, 110.6, 114.8, 118.1, 119.4, 121.1,
121.4, 131.7, 133.3, 137.3, 139.8, 148.6, 157.5, 157.6, 183.7.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: asykes@usd.edu.
’ ACKNOWLEDGMENT
The authors thank NSF-EPSCOR (EPS-0554609) and the
South Dakota Governor’s 2010 Initiative for the purchase of a
Bruker SMART APEX II CCD diffractometer. The elemental
analyzer was provided by funding from NSF-URC (CHE-
0532242). P.N.B. thanks NSF-EPSCoR (EPS-0903804) for
financial support. M.S. was supported by the Univ. of South
Dakota Beverley and Truman Schwartz summer research
fellowship.
1,8-Oxybis(ethylenoxyethylenoxy)-10-[(3-aminophenyl)-
imino]anthracen-9(10H)-one (2d). Yield, 73%, melting point =
178À180 °C, elemental analyses calculated for C28H28N2O6: C, 68.85;
1
H, 5.74; N, 5.74%. Found: C, 69.08; H, 5.88; N, 5.50%. H NMR
(200 MHz, CDCl3, 25 °C): δ 3.7À4.3 (m, 16H, CH2ÀO, polyether);
6.0À6.2 (m, 1H); 6.3À6.5 (m, 1H); 6.7À7.2 (m, 5H); 7.4À7.6 (t, 2H);
7.7À7.8 (d, 1H). 13C NMR (CDCl3, 25 °C): δ 68.7; 69.0; 70.0; 70.2;
70.9; 105.5; 109.2; 110.7; 114.3; 117.9; 121.0; 130.0; 147.6; 151.9;
157.0; 183.5.
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3
1
H, 5.68; N, 3.16%. Found: C, 67.42; H, 5.56; N, 3.38%. H NMR
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2H), 7.51 (t, 2H), 7.81 (d, 2H). 13C NMR (CDCl3, 25 °C): δ 68.8, 70.1,
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