Synthesis, structural characterisation and luminescent anion sensing studies of a Ru(II)polypyridyl complex featuring an aryl urea derivatised 2,2′-bpy auxiliary ligand

Article abstract of DOI:10.1016/j.ica.2011.10.026

The inclusion of a urea functionality into the coordination sphere of a Ru(II)-polypyridyl complex (Ru·L₁) resulted in a system that can function as an effective long wavelength emissive fluorescent anion sensor. The MLCT emission of Ru·L₁

Full text of DOI:10.1016/j.ica.2011.10.026

Inorganica Chimica Acta 381 (2012) 236–242
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structural characterisation and luminescent anion sensing studies
of a Ru(II)polypyridyl complex featuring an aryl urea derivatised 2,20-bpy
auxiliary ligand
⇑
⇑
Jonathan A. Kitchen , Elaine M. Boyle, Thorfinnur Gunnlaugsson
School of Chemistry, Centre for Synthesis and Chemical Biology, Trinity College Dublin, Dublin 2, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 18 October 2011
The inclusion of a urea functionality into the coordination sphere of a Ru(II)-polypyridyl complex (RuꢀL1)
resulted in a system that can function as an effective long wavelength emissive fluorescent anion sensor.
The MLCT emission of RuꢀL1 is sensitive to the binding of acetate, phosphate and pyrophosphate but not
fluoride in organic solvent. In addition, RuꢀL1 can distinguish between phosphate and pyrophosphate
Fluorescence Spectroscopy: from Single
Chemosensors to Nanoparticles Science –
Special Issue
with an emission increase upon binding of H2POꢁ (‘‘turn on’’ sensor) and an emission decrease upon
4
binding of HP2O3ꢁ (‘‘turn off’’ sensor), which occurs via hydrogen bonding to the urea receptor moiety
7
as demonstrated by carrying out NMR titrations as well as by employing [Ru(II)bpy3]ðPFꢁÞ as a model
6
2
Keywords:
Ru(II)
compound that lacks the anion receptor moiety.
Ó 2011 Elsevier B.V. All rights reserved.
Polypyridyl
Anion sensing
Luminescence
Urea receptors
Hydrogen bonding
1. Introduction
is a more versatile means of achieving both better selectivity and
sensitivity for the anion recognition [16–19]. Herein, we explore
the use of combined electrostatic binding and hydrogen bonding
for the recognition and sensing of anions; an area of research we
have recently started investigating through the use of transition
metal ion complexes in competitive solvents [14,20,21]. We show
that by appending a urea moiety to part of a bi-pyridine ligand,
via a phenyl spacer, and using such ligands in the formation
of Ru(II) polypyridyl complexes, anion sensing can be achieved
through both hydrogen bonding as well as through electrostatic
interactions [22]. Moreover, we demonstrate that depending on
the nature of the anion, i.e. charge, structure and size, the metal
to ligand charge transfer (MLCT) emission from such complexes,
can be either ‘switched on’ or ‘switched off’, upon binding to anions.
We also demonstrate that phosphate anions can be selectively de-
tected over other anions, such as halides, using such Ru(II) polypyr-
idyl complexes through monitoring of changes in both the ground
and the excited states as a consequence of anion binding and/or ex-
change. The example featured in this communication, the Ru(II)
complex RuꢀL1, is a prime example of our design, where the emis-
sion, which occurs at a long wavelength, is highly sensitive to the
nature of the anions, and we demonstrate, that in the case of RuꢀL1,
the primary recognition mode affecting the MLCT emission is
hydrogen bonding and not deprotonation [23], as the emission aris-
ing from RuꢀL1 is not sensitive to anions such as Fꢁ, while being
highly sensitive to both phosphate, and pyrophosphate.
The luminescent sensing of anions has become an active area of
research in the last decade [1–5]. Such sensing usually requires the
use of a receptor motif that can bind anions; often through hydro-
gen bonding, or through electrostatic binding to cationic or Lewis
Acid centres, such as ammonium moieties and metal ions. For the
latter, the ion-pairing can give rise to strong binding, which can
be employed in competitive media [6,7]. In particular, the use of
luminescent metal ion complexes, where the anion binding/recog-
nition occurs directly at the meal ion; and hence, is a consequence
of competitive binding or exchange of anions for solvent molecules
within the first coordination sphere of the metal ion, has been
shown to be a powerful means of forming luminescent sensors
[8,9]. Such binding usually results in significant perturbation of
the various photophysical properties of the metal ion complexes;
giving rise to changes in wavelengths, lifetimes or quantum yields,
all of which can be monitored to quantify the anion binding affinity
[10,11]. Alternatively, the anion recognition/sensing can take place
at a designed receptor moiety that is part of the auxiliary ligands
used in the formation of such metal ion complexes [12–15]. This
⇑
Corresponding authors. Tel.: + 353 1 896 3459; fax: +353 1 671 2826.
E-mail
addresses:
jkitchen@tcd.ie
(J.A.
Kitchen),
gunnlaut@tcd.ie
(T. Gunnlaugsson).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.10.026

J.A. Kitchen et al. / Inorganica Chimica Acta 381 (2012) 236–242
237
using published procedures [25]. Solvents were HPLC grade and
were used without further purification.
2. Experimental
2.1. General information
2.3. Synthesis of 1-(4-[2,20]Bipyridyl-4-phenyl)-3-(3-nitrophenyl)-
All reactions were carried out in 2–5 mL or 10–20 mL Biotage
Microwave Vials in a Biotage Initiator Eight EXP microwave reactor.
Elemental analyses were carried out at the Microanalytical Labora-
tory, School of Chemistry and Chemical Biology, University College
urea (L1)
3-Nitro-phenyl-isocyanate (66 mg, 0.40 mmol) was added to a
solution of 4-[2,20]bipyridyl-4-phenylamine (100 mg, 0.404 mmol)
in CH3CN (15 mL) and heated at 100 °C under microwave irradia-
tion for 40 min. The resulting pale precipitate was filtered and
dried in vacuo to yield an off white powder (98 mg, 59%). Anal. Calc.
for C23H17N5O3 (411.13 g molꢁ1): C 67.15, H 4.16, N 17.07. Found:
C 67.05, H 4.22, N 16.85%. NMR dH (600 MHz, DMSO-d6): 9.32 (1H,
s, N4H), 9.16 (1H, s, N3H), 8.75–8.72 (2H, m, C1H & C10H), 8.67 (1H, s,
C7H), 8.60 (1H, s, C19H), 8.44 (1H, d, C4H), 7.98 (1H, t, C3H), 7.86
(3H, d, C12H & C16H & C23H), 7.77 (1H, d, C9H), 7.74 (1H, d, C21H),
7.69 (2H, d, C13H & C15H), 7.59 (1H, t, C22H), 7.50 (1H, dd, C2H).NMR
dC (150 MHz, DMSO-d6): 155.9 (C6), 155.2 (C5), 152.3 (C17), 149.9
(C10), 149.3 (C1), 148.1 (C20), 147.7 (C8), 140.9 (C18), 140.7 (C14),
137.4 (C3), 130.7 (C11), 130.1 (C22), 127.4 (C12 & C16), 124.5, 124.3
(C2), 120.9 (C9), 120.6 (C4), 118.9 (C13 & C15), 116.9 (C7), 116.5
(C21), 112.2 (C19). IR (neat) 3060, 1723, 1590, 1550, 1521, 1461,
1417, 1390, 1342, 1317, 1240, 1184, 1167, 1097, 1074, 998, 820,
Dublin. Infrared spectra were recorded on
a Perkin–Elmer-
Spectrum-One FT-IR spectrometer equipped with a Universal-ATR
sampling accessory; solid samples were recorded directly as neat
samples; in cmꢁ1. NMR data were recorded on a Bruker-DPX-400-
Avance spectrometer (400.13 (1H) and 100.6 MHz (13C)) or a
Bruker-AV-600 spectrometer (600.13 (1H) and 150.2 MHz (13C)),
in commercially available deuterated solvents; d in ppm relative
to SiMe4 (=0 ppm) referenced relative to the internal solvent signals,
J in Hz; data were processed with Bruker Win-NMR 5.0 and Topspin
2.1 softwares. Absorption spectra were measured in 1-cm quartz
cuvettes with a Varian-Cary-50 spectrophotometer. Baseline cor-
rection was applied for all spectra. Emission spectra were measured
with a Varian-Cary-Eclipse luminescence spectrometer. X-ray data
were collected on a Rigaku Saturn 724 CCD Diffractometer using
graphite-monochromated Mo K
a radiation (k = 0.71073 Å). The
789, 739, 726, 678 cmꢁ1
.
HRMS (ESI) 412.1411 ([M+H]+,
datasets were collected using Crystal clear-SM 1.4.0 software. Data
integration, reduction and correction for absorption and polariza-
tion effects were all performed using Crystalclear-SM 1.4.0 soft-
ware. Space group determinations were obtained using Crystal
structure ver. 3.8. The structures were solved by direct methods
C23H18N5O3 requires 412.1410).
2.4. Synthesis of [RuII(L1)(bpy)2](PF6)2 (RuꢀL1)
(
SHELXS-97) and refined against all F2 data (SHELXL-97) [24]. All H-
Ligand L1 (85 mg, 0.2 mmol) was suspended in 4 mL of ethanol/
water (1:1) in a 2–5 mL Biotage Microwave Vial. To this suspension
was added solid cis-bis-(2,20-bipyridine) ruthenium chloride
(98 mg, 0.2 mmol) and the resulting dark mixture heated at 120 °C
under microwave irradiation for 45 min. The resulting clear deep-
red solution was added to an aqueous solution of excess ammonium
hexafluorophosphate resulting in a bright orange precipitate. This
was purified by column chromatography on silica eluting with
40:4:1 CH3CN:H2O:NaNO3(sat. aq.) to yield a red solid (45 mg, 20%).
Anal. Calc. for C43H33N9O3F12P2Ru (1115.103 g molꢁ1): C 46.17, H
atoms, except for O–H and N–H protons, were positioned geometri-
cally and refined using riding model with d(CH) = 0.95 Å,
Uiso = 1.2 Ueq (C). OH and NH protons were found from the differ-
ence map and fixed to the attached atoms with UH = 1.2UX.
a
2.2. Reagents
All chemicals were purchased from commercial sources and
used as received. 4-[2]Bipyridyl-4-phenylamine was prepared
Fig. 1. (a) Schematic representation of synthesis of L1 and RuꢀL1. (b) Observed (red) and calculated (blue) mass spectra of [RuII(bpy)2(L1)(PF6)]+. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)

238
J.A. Kitchen et al. / Inorganica Chimica Acta 381 (2012) 236–242
2.98, N 11.30. Found: C 45.81, H 2.98, N 11.40%. NMR dH (400 MHz,
CD3CN) 8.75 (1H, d), 8.71 (1H, d), 8.60 (1H, t), 8.54 (4H, dd), 8.11–
8.07 (4H, m), 8.01 (1H, s, NH), 7.94 (1H, s, NH), 7.89 (3H, m), 7.85
(1H, d), 7.79–7.71 (8H, m), 7.65 (1H, dd), 7.57 (1H, t), 7.45–7.42
(6H, m). IR (neat) 3094, 1711, 1594, 1520, 1466, 1426, 1347, 1320,
1239, 1190, 824, 761, 730, 658 cmꢁ1 HRMS (MALDI) 970.1414
([M-PF6]+, C43H33N9O3RuPF6 requires 970.1404).
characteristic shifts associated with complexation. Moreover, the
1H NMR (400 MHz, CD3CN) data for RuꢀL1 indicated successful for-
mation of the desired complex; the formation of which was further
confirmed by ESI MS, which showed the expected peak and isoto-
pic distribution pattern for the formation of RuꢀL1 (found,
970.1414; calc. for [RuꢀL1-PF6]+, 970.1404 m/z), see Fig. 1b.
The molecular structure of RuꢀL1 was also established by X-ray
diffraction experiments. Small, poor quality, red rod shaped crys-
tals were grown from the vapour diffusion of diethyl ether into
an acetonitrile solution of RuꢀL1 and the low temperature structure
(108 K) determined. Unfortunately the data set was not of suffi-
cient quality to allow for a full refinement. Nevertheless, it did al-
low for the connectivity of RuꢀL1 to be established (See TOC).
3. Results and discussion
3.1. Synthesis and characterisation of L1 and Ru(II) complex RuꢀL1
Ligand L1 was prepared by the reaction of 4-[2,20]bipyridyl-4-
phenylamine with 3-nitro-phenyl-isocyanate in CH3CN at 100 °C
under microwave irradiation to give an off-white precipitate in
59% yield (Fig. 1a). Both 1H NMR (600 MHz, DMSO-d6) spectra
and microanalytical data were consistent with the expected struc-
ture. The expected m/z peaks and isotopic distribution patterns
were also observed in the mass spectra of L1 (found, 412.1411;
calc. for [L1+H]+, 412.1410 m/z).
3.2. Photophysical properties of RuꢀL1 and anion complexation studies
The photophysical properties of both the ligand, L1, and the Ru(II)
polypyridyl complex, RuꢀL1, were investigated. The UV/vis spectrum
of L1 was similar to those obtained for other urea based compounds,
with a broad peak at kmax = 288 nm in CH3CN. The UV–Vis absorp-
tion spectra of RuꢀL1 in CH3CN gave rise to a broad MLCT band cen-
tred at 450 nm and three bands at shorter wavelengths of 334 nm,
286 nm and 245 nm, respectively (Fig. 3a). The fluorescence emis-
sion spectra of RuꢀL1 showed a broad emission band at 615 nm upon
excitation into the MLCT absorption band at 450 nm, and the fluo-
rescence excitation spectra of RuꢀL1 (kem = 615 nm) structurally
matched that of the absorption spectra (Fig. 3).
The molecular structure of L1 was furthermore, established by
X-ray crystallography. Poor quality pale yellow irregular plates of
L1ꢀH2O were grown from the slow evaporation of ethanol and the
low temperature (108 K) X-ray structure determined. The ligand L1
crystallises as a mono-hydrate (L1ꢀH2O) in the monoclinic space
group C2/c with one molecule in the asymmetric unit (Fig. 2a). The
molecule is relatively flat with mean plane angles <29° and the
2,20-bpy nitrogen atoms (N(1) and N(2)) are orientated in a trans like
configuration. Packing interactions in L1ꢀH2O, shown in Fig. 2b, in-
volve the urea nitrogen atoms acting as H-bond donors to the inter-
stitial water molecule [N(3)ꢀ ꢀ ꢀO(100) = 2.896(5) Å, <(N(3)-H(3X)ꢀ ꢀ ꢀ
O(2)) = 160° and N(4)ꢀ ꢀ ꢀO(100) = 2.943(5) Å, <(N(4)-H(4X)ꢀ ꢀ ꢀO(2))
= 156°]. In addition, the water molecule is hydrogen bonded to pyr-
idyl nitrogen atoms on two symmetry generated adjacent mole-
cules[O(100)ꢀ ꢀ ꢀN(1)0 = 2.866(5) Å, <(O(100)-H(10X)ꢀ ꢀ ꢀN(1)0) = 168°
and O(100)ꢀ ꢀ ꢀN(2)0 = 2.843(5) Å, <(O(100)-H(10Y)ꢀ ꢀ ꢀN(2)0) = 176°].
The complexation of L1 and cis-bis-(2,20-bipyridine) ruthenium
chloride was carried out in a solvent mixture of ethanol and water
(1:1) under microwave irradiation at 120 °C for 45 min. The result-
ing clear red/orange solution was added to an aqueous solution of
excess ammonium hexafluorophosphate yielding a bright orange
precipitate. This was purified further by flash column chromatog-
raphy on silica (eluting with 40:4:1 CH3CN:H2O:NaNO3(sat.aq.)) to
give RuꢀL1 in 20% yield. Microanalytical data was consistent with
that expected for RuꢀL1, while the IR spectra showed the
The effect of the anion interacting with RuꢀL1 was next investi-
gated in CH3CN. These preliminary results are summarised in
Fig. 4, and showed that in general, the addition of an excess of
some anions (including H2POꢁ, HP2O3ꢁ, OAcꢁ, SO24ꢁ and Fꢁ) as their
4
7
tetrabutyl ammonium (TBA) salts to RuꢀL1 resulted in dramatic
changes to the absorption and emission spectra; in particular the
addition of HP2O3ꢁ was quite significant, giving rise to a large de-
7
gree of quenching of the emission spectrum. Having established
that anions give rise to modulation of the photophysical properties
of RuꢀL1 we carried out detailed anion titrations, observing the
changes to both the absorption spectra and the fluorescence emis-
sion spectra of RuꢀL1 upon the addition of H2POꢁ, HP2O3ꢁ, OAcꢁ,
4
7
SO24ꢁ and Fꢁ as their TBA salts in this solvent.
The changes observed in the absorption spectra upon titration
with OAcꢁ are shown in Fig. 5a and show a slight decrease in all
absorption bands and a significant red-shift for the 334 nm transi-
tion. Only a slight decrease in the MLCT band was observed with no
noticeable shift in kmax. We were able to analyse these results by
Fig. 2. (a) Perspective view of the crystal structure ofL1ꢀH2O. (b) View of packing in L1ꢀH2O viewed down the crystallographic c-axis.

J.A. Kitchen et al. / Inorganica Chimica Acta 381 (2012) 236–242
239
Fig. 3. (a) The absorption spectra of L1
kem = 615 nm) of RuꢀL1.
(
) and RuꢀL1
(
) in CH3CN (ca. 1 ꢂ 10ꢁ5 M). (b) The emission spectrum ( , kex = 450 nm) and resulting excitation spectrum (
,
Fig. 4. (a) The changes in the absorption spectra of RuꢀL1 in CH3CN in the absence and presence of Fꢁ, H2PO4ꢁ, HP2O73ꢁ and OAcꢁ. (b) The corresponding changes in the
emission spectra upon excitation at 450 nm.
Fig. 5. The changes observed in the absorption (a) and emission (b) spectra of RuꢀL1 in CH3CN upon titration with TBA OAcꢁ. Inset (a): experimental binding isotherms for the
absorption titration and their corresponding fit by means of SPECFIT ( ); (b) the speciation distribution derived from the fitting of the changes in the emission spectra.
fitting the changes in the absorption spectra using the non-linear
regression analysis program SPECFIT, to 1:1, 1:2 and 1:3 (host:-
guest) stoichiometries, which demonstrated high affinity of RuꢀL1
for OAcꢁ, with a binding constant of logK1:1 = 7.72 ( 0.26) for the
1:1 binding; while we also observed some higher order stoichiom-
etries, with logK1:2 = 6.06 ( 0.63) and logK1:3 = 2.16 ( 0.78), being
determined for the 1:2 and 1:3 (sensor:anion) binding, respec-
tively .The high 1:1 binding clearly demonstrates the role of the
Lewis Acid centre in RuꢀL1, which makes the urea moiety a much
better hydrogen bonding donor. The binding of OAcꢁ gave
moderate changes to the emission spectra of RuꢀL1, as shown in
Fig. 5b and fitting the observed changes gave similar binding con-
stants to that of the absorbance data with logK1:1 = 6.86 ( 0.19),
logK1:2 = 4.41 ( 0.51) and logK1:3 = 2.65 ( 0.99). The origin of this
binding we attribute to the interaction of the anion with the urea
moiety, which takes place through hydrogen bonding. Indeed,
analysis of the binding event, by constructing a speciation distribu-
tion diagram demonstrated that within the addition of one equiv-
alent of the anion, ca. 90% of the sensor is complexed to the anion,
Fig. 5b, inset. However, in competition to this binding, is the 1:2

240
J.A. Kitchen et al. / Inorganica Chimica Acta 381 (2012) 236–242
stochiometry, which can either be due to a weaker interaction with
the urea moiety, or more likely, through electrostatic interactions
with the metal complex itself. This latter phenomenon, is more
likely, as at higher anion concentrations, the 3:1 stochiometry be-
comes the dominant one; a species most likely to be that of elec-
trostatic interactions and charge balancing.
data gave the same speciation as observed for the absorption data
fit with 1:1 and 1:2 binding stoichiometries. The binding constants
for HP2O3ꢁ were determined as logK1:1 = 6.52 ( 0.19) and
7
logK1:2 = 3.48 ( 0.39), where the binding constant for the 1:1 stoi-
chiometry was slightly higher than that observed for the absorp-
tion data. To further explore the nature of the anion binding
interactions seen above for RuꢀL1, we also investigated the changes
in the luminescence of [Ru(II)bpy3]ðPFꢁÞ upon addition of these
Larger changes in the absorption spectra of RuꢀL1 were observed
upon titration with H2POꢁ; where all the transitions were red-
4
6
2
shifted to a greater extent than seen for OAcꢁ (Fig. 6a). Once more,
fitting of the changes observed was carried out using non-linear
regression analysis, and in this case, they were again best fit to 1:1,
1:2 and 1:3 binding stoichiometries, with logK1:1 = 6.55 ( 0.20),
logK1:2 = 5.48 ( 0.37) and logK1:3 = 4.18 ( 0.41), being determined.
Oncemore, theinitial1:1bindinggaverise totheformationof higher
anion:complex species (See Fig. 6b, inset). In this case the 1:1 bind-
ing constant was slightly lower than that of OAcꢁ, and the hydrogen
bonding complex was formed in a slightly lower yield. Interestingly,
the emission spectra were quite different to the titration of OAcꢁ.
Rather than a slight quenching on addition up to ꢃ1 equivalent of
anion, as was observed in the case of OAcꢁ, there was an increase
in the emission intensity, accompanied by a red-shift. However,
same anions in CH3CN, as this system would function as a model
compound, lacking the urea anion receptor. These titrations
showed that in the case of OAcꢁ and Fꢁ no significant changes oc-
curred in either the absorption or the emission spectra upon addi-
tion of 400 equivalents of these anions. In the case of H2POꢁ,
4
HP2O3ꢁ, and SO24ꢁ no spectral changes occurred either; but after
7
the addition of 28 equivalents of H2POꢁ, 5 equivalents of HP2O3ꢁ
,
4
7
and 3 equivalents of SO24ꢁ a precipitation occurred, which would
suggest some anion exchange interactions with [Ru(II)bpy3]ðPFꢁÞ .
6
2
These results clearly support our findings above that the modula-
tion in the Ru(II) centred MLCT emission are primarily due to the
binding of these anions to the urea moiety in RuꢀL1.
We also carried out titrations of RuꢀL1 using SO2ꢁ and Fꢁ, unfor-
4
upon addition of an excess of H2POꢁ the emission intensity began
tunately SO24ꢁ caused precipitation (in a similar manner to that
4
to decrease. However, it never fell below the initial intensity of RuꢀL1
(as shown in Fig. 6b). Fitting of the changes observed in the
fluorescence titration using non-linear regression analysis gave a
similar binding profile of 1:1, 1:2 and 1:3 binding stoichiometries,
with comparable binding constants where logK1:1 = 6.85 ( 0.12),
logK1:2 = 5.85 ( 0.24) and logK1:3 = 4.14 ( 0.35). These results again
showed that both the absorption and the emission spectra could be
monitored to observe the anion sensing of RuꢀL1.
seen for [Ru(II)bpy3] ðPFꢁÞ of the RuꢀL1, in the early stages of the
6
2
titration and we were unable to obtain any binding constants.
Titration of RuꢀL1 with Fꢁ gave very surprising results; no change
in either the absorption nor the emission spectra even upon addi-
tion of up to 400 equivalents of anion (Fig. 8), mirroring that seen
for [Ru(II)bpy3]ðPFꢁÞ , despite the fact that this anion can function
6
2
as a base and deprotonate the urea receptor in CH3CN. This result is
quite strange, and in contrast to many other anion sensors
developed in which Fꢁ often results in dramatic changes to the
absorbance or emission spectra through deprotonation of N–H
receptors and subsequent modulation in the various photophysical
properties of the sensors.
However, the most significant changes in the absorption spectra
of RuꢀL1 were observed upon titration with HP2O3ꢁ (Fig. 7a). Whilst
7
the same red-shift was observed as seen above; there was a much
more dramatic decrease in the intensity of all transitions upon
addition of excess HP2O3ꢁ. In this instance, the fitting of the
7
3.3. 1H NMR titrations of RuꢀL1
changes in absorption were best represented by 1:1, which was
formed in high yield (as seen as an inset in Fig. 7b, inset) and 1:2
binding stoichiometries alone; with no 1:3 stoichiometry being
Further studies are currently underway in our laboratory on
RuꢀL1 and related structures, including in-depth 1H NMR anion
titrations in order to help deduce the nature of the anion recogni-
tion events occurring. In particular, we are focusing on elucidating
the nature of the 1:2 and 1:3 stoichiometries that were required to
obtain good fits for the absorption/fluorescence titrations. Preli-
minary 1H NMR titrations (400 MHz) carried out on RuꢀL1 using
H2POꢁ, HP2O3ꢁ, and Fꢁ in DMSO-d6 have shown that in the case
determined, and
a logK1:1 = 5.63 ( 0.09) and logK1:2 = 3.12
( 0.18). Large changes were also observed in the emission spec-
trum upon addition of HP2O3ꢁ. However, significant quenching of
7
the emission band was also observed along with a red-shifting of
the emission maximum. After addition of a large excess (>100
equivalents) of HP2O3ꢁ there was almost complete quenching
7
(>90%) of the emission band (Fig. 7b). Fitting of the fluorescence
4
7
Fig. 6. The changes observed in the absorption (a) and emission (b) spectra of RuꢀL1 in CH3CN upon titration with TBAH2POꢁ. Inset (a): experimental binding isotherms for
4
the absorption titration and their corresponding fit by means of SPECFIT ( ); (b) the speciation distribution derived from the fitting of the changes in the emission spectra.

J.A. Kitchen et al. / Inorganica Chimica Acta 381 (2012) 236–242
241
Fig. 7. The changes observed in the absorption (a) and emission (b) spectra of RuꢀL1 in CH3CN upon titration with TBAHP2O3ꢁ. Inset (a): experimental binding isotherms for
7
the absorption titration and their corresponding fit by means of SPECFIT ( ); (b) the speciation distribution derived from the fitting of the changes in the emission spectra.
Fig. 8. The changes observed in the absorption (a) and emission (b) spectra of RuꢀL1 in CH3CN upon titration with Fꢁ.
of H2POꢁ and HP2O3ꢁ, the N–H resonances assigned to the urea
system that can function as an effective long wavelength emissive
fluorescent anion sensor for several anions in organic solutions,
where the binding sensitivity is very high. Preliminary 1H NMR
titrations have also demonstrated that these interactions are pre-
dominantly due to hydrogen bonding between the urea receptor
and the anions. We also demonstrate that by simple design, the
use of hydrogen bonding motifs and electrostatic binding can give
rise to good selectivity for such structures. In particular, we have
demonstrated that RuꢀL1 is able to differentiate between phos-
phate and pyrophosphate in CH3CN as it acts as an emission ‘‘turn
4
7
protons were all in fast exchange, and became broadened and
shifted down-field upon binding to H2POꢁ and HP2O3ꢁ. For the for-
4
7
mer, such changes occurred between 0.1 ? 1 equivalents, after
which a precipitation occurred. These results again, show the di-
rect interaction of the urea protons with the anion and confirm
that the changes seen above in the absorption and the emission
spectra are indeed due to anion binding event through hydrogen
bonding. In contrast, in the case of HP2O3ꢁ, such precipitation oc-
7
curred earlier in the titration, after the addition of ca. 0.5 equiva-
lents, while in the case of Fꢁ the resonances became broadened,
and impossible to follow after ca. 0.3 equivalents. These changes
were also accompanied by the formation of a triplet at ca.
16 ppm which is indicative of the formation of bifluoride (HFꢁ2 )
[26,27]. While such deprotonation was not evident in the absorp-
tion or the luminescence studies above, however; this could be
due to lower concentrations of both the sensor and the anions in
CH3CN. We are currently investigating this in greater detail using
various NMR analyses, as well as solid state crystallographic anal-
ysis, which will shed some light on this phenomenon, and be cru-
cial to both designing anion selective sensors and elucidating
complex binding phenomenon – where both hydrogen bonding,
deprotonation and electrostatic binding interactions dictate the
overall anion sensing action.
on’’ sensor for H2POꢁ and an emission ‘‘turn-off’’ sensor for
4
HP2O3ꢁ. In contrast, using [Ru(II)bpy3]ðPFꢁÞ did not in general
7
6 2
lead to such spectroscopic changes. Interestingly there was no
change on addition of Fꢁ to RuꢀL1, and only subtle changes on
the addition of OAcꢁ and SO42ꢁ. We are currently investigating
the use of other, similar, Ru(II)-polypyridyl complexes for use as
luminescent anion sensors and DNA binding agents by altering
the number of receptors, and the nature of the anion binding
groups.
Acknowledgements
This work was supported by The Irish Research Council for
Science, Engineering & Technology (IRCSET) (Postdoctoral Fellow-
ship to JAK), the Irish Higher Education Authority, through the
PRTLI Cycle 4 (CSCB funding; postgraduate award to EMB), Trinity
College Dublin and Science Foundation Ireland (SFI RFP Award
2009 and SFI PI Award 2010).
4. Conclusions
In summary, we have shown that the incorporation of a urea
moiety into a Ru(II)-polypyridyl complex (RuꢀL1) results in a

242
J.A. Kitchen et al. / Inorganica Chimica Acta 381 (2012) 236–242
[12] (a) D. Saha, S. Das, C. Bhaumik, S. Dutta, S. Baitalik, Inorg. Chem. 49 (2010)
References
2334;
(b) E. Berni, I. Gosse, D. Badocco, P. Pastore, N. Sojic, S. Pinet, Chem. Eur. J. 15
(2009) 5145.
[13] D.R. Turner, E.C. Spencer, J.A.K. Howard, D.A. Tocher, J.W. Steed, Chem.
Commun. (2004) 1352.
[1] R.M. Duke, E.B. Veale, F.M. Pfeffer, P.E. Kruger, T. Gunnlaugsson, Chem. Soc.
Rev. 39 (2010) 3936.
[2] (a) T. Gunnlaugsson, M. Glynn, G.M. Tocci, P.E. Kruger, F.M. Pfeffer, Coord.
Chem. Rev. 250 (2006) 3094;
[14] R.B.P. Elmes, T. Gunnlaugsson, Tetrahedron Lett. 51 (2010) 4082.
[15] (a) M.S. Goodman, J. Weiss, A.D. Hamilton, Tetrahedron Lett. 35 (1994) 8943;
(b) T.B. Norsten, K. Chichak, N.R. Branda, Chem. Commun. (2001) 1794.
[16] (a) S.-H. Fan, Q. Li, K.-Z. Wang, Inorg. Chem. Acta 362 (2009) 5155;
(b) C.M.G. dos Santos, T. Gunnlaugsson, Supramol. Chem. 21 (2009) 173.
[17] (a) S. Goswami, R. Chakrabarty, Tetrahedron Lett. 44 (2009) 5994;
(b) F. Zapata, A. Caballero, A. Espinosa, A. Tárraga, P. Molina, Org. Chem. 73
(2008) 4034;
(c) M.G. Fisher, P.A. Gale, M.E. Light, S.J. Loeb, Chem. Commun. 20 (2008) 5695.
[18] (a) J. Perez, L. Riera, Chem. Soc. Rev. 37 (2008) 2658;
(b) A. Ghosh, B. Ganguly, A. Das, Inorg. Chem. 46 (2007) 9912;
(c) D. Pelleteret, N.C. Fletcher, A.P. Doherty, Inorg. Chem. 46 (2007) 4386;
(d) J.V. Ros-Lis, R. Martinez-Manez, J. Soto, C. McDonagh, A. Guckian, Eur. J.
Inorg. Chem. (2006) 2647.
[19] (a) S.R. Bayly, P.D. Beer, Struct. Bond. 129 (2008) 45;
(b) M.D. Pratt, P.D. Beer, Tetrahedron 49 (2004) 11227.
[20] G.J. Ryan, S. Quinn, T. Gunnlaugsson, Inorg. Chem. 47 (2008) 401.
[21] (a) A.M. Nonat, S.J. Quinn, T. Gunnlaugsson, T. Inorg. Chem. 48 (2009) 4646;
(b) K. Senechal-David, S.J.A. Pope, S. Quinn, S. Faulkner, T. Gunnlaugsson, Inorg.
Chem. 45 (2006) 10040.
[22] (a) C.M.G. dos Santos, T. Gunnlaugsson, Dalton Trans. 20 (2009) 4712;
(b) C.M.G. dos Santos, P.B. Fernandez, S.E. Plush, J. Leonard, T. Gunnlaugsson,
Chem. Commun.20 (2007) 3389.
(b) C.M.G. dos Santos, A.J. Harte, S. Quinn, T. Gunnlaugsson, Coord. Chem. Rev.
252 (2008) 2512;
(c) J.P. Leonard, C.B. Nolan, F. Stomeo, T. Gunnlaugsson, Top. Curr. Chem. 281
(2007) 1.
[3] (a) P.A. Gale, Chem Soc. Rev. 39 (2010) 3746;
(b) A.-F. Li, J.-H. Wang, F. Wang, Y.-B. Jiang, Chem. Soc. Rev. 39 (2010) 3729;
(c) V. Amendola, L. Fabbrizzi, L. Mosca, Chem Soc. Rev. 39 (2010) 3889.
[4] (a) P. Padros, R. Quesada, Supramol. Chem. 20 (2008) 201;
(b) P.A. Gale, S.E. Garcia-Garrido, S.E. Garric, J. Chem. Soc. Rev. 37 (2008) 151;
(c) P.A. Gale, R. Quesada, Coord. Chem. Rev. 250 (2006) 3219;
(d) T. Gunnlaugsson, H.D.P. Ali, M. Glynn, P.E. Kruger, G.M. Hussey, F.M. Pfeffer,
C.M.G. dos Santos, J.J. Tierney, J. Fluoresc. 15 (2005) 287.
[5] (a) R. Martínez-Máñez, F. Sancenón, Chem. Rev. 103 (2003) 4419;
(b) C. Suksai, T. Tuntulani, Chem. Soc. Rev. 32 (2003) 192;
(c) J.F. Callan, A.P. de Silva, D.C. Magri, Tetrahedron 36 (2005) 8551.
[6] L. Fabbrizzi, M. Licchelli, G. Rabaioli, A.F. Taglietti, Coord. Chem. Rev. 205
(2000) 85.
[7] (a) T. Gunnlaugsson, F. Stomeo, Org. Biomol. Chem. 5 (2007) 1999;
(b) S.J.A. Pope, Polyhedron 26 (2007) 4818;
(c) S. Pandya, J. Yu, D. Parker, Dalton Trans. (2006) 2757.
[8] (a) J. Massue, S.J. Quinn, T.J. Gunnlaugsson, Am. Chem. Soc. 130 (2008) 6900;
(b) C.S. Bonnet, J. Massue, S.J. Quinn, T. Gunnlaugsson, Org. Biomol. Chem. 7
(2009) 3074;
[23] Cametti, M., Rissanen, K., 2009. Chem. Commun. 2809.
[24] G.M. Sheldrick, Acta Crystallogr., Sect. A A64 (2008) 112.
[25] O. Johansson, M. Borgstrom, R. Lomoth, M. Palmblad, J. Bergquist, L.
Hammarstrom, L. Sun, B. Akermark, Inorg. Chem. 42 (2003) 908.
[26] (a) R.M. Duke, J.E. O’Brien, T. McCabe, T. Gunnlaugsson, Org. Biomol. Chem. 6
(2008) 4089;
(c) S.E. Plush, T. Gunnlaugsson, Dalton Trans. 20 (2008) 3801;
(d) S.E. Plush, T. Gunnlaugsson, Org. Lett. 9 (2007) 1919;
(e) H. Tsukube, S. Shinoda, Chem. Rev. 102 (2002) 2389;
(f) D. Parker, Chem. Soc. Rev. 33 (2004) 156;
(g) T. Gunnlaugsson, A.J. Harte, J.P. Leonard, M. Nieuwenhuyzen, Chem.
Commun. 20 (2002) 2134.
(b) C.M.G. dos Santos, T. McCabe, G.W. Watson, P.E. Kruger, T.J. Gunnlaugsson,
J. Org. Chem. 73 (2008) 9235;
(c) R.M. Duke, T. Gunnlaugsson, Tetrahedron Lett. 48 (2007) 8043;
(d) T. Gunnlaugsson, P.E. Kruger, P. Jensen, F.M. Pfeffer, G.M. Hussey,
Tetrahedron Lett. 44 (2003) 8909.
[9] (a) P.D. Beer, V. Timoshenko, M. Maestri, P. Passaniti, V. Balzani, Chem.
Commun. (1999) 1755;
(b) P.D. Beer, Chem. Commun. (1996) 689.
[10] (a) J.P. Leonard, C.M.G. dos Santos, S.E. Plush, T. McCabe, T. Gunnlaugsson,
Chem. Commun. (2007) 129;
(b) T. Gunnlaugsson, A.J. Harte, J.P. Leonard, M. Nieuwenhuyzen, Supramol.
Chem. 15 (2003) 505.
[11] V. Amendola, M. Boiocchi, B. Colasson, L. Fabbrizzi, Inorg. Chem. 45 (2006)
6138.
[27] (a) P.A. Gale, J.R. Hiscock, S.J. Moore, C. Caltagirone, M.B. Hursthouse, M.E.
Leight, Chem. Asian J. 5 (2010) 555;
(b) M. Cametti, K. Rissanen, Chem. Commun. 5 (2009) 2809;
(c) M. Boiocchi, L. Del Boca, D.E. Gomez, L. Fabbrizzi, M. Licchelli, E. Monzani, J.
Am. Chem. Soc. 50 (2004) 16507.