Selective Sensing of Phosphates by a New Bis-heteroleptic RuIIComplex through Halogen Bonding: A Superior Sensor over Its Hydrogen-Bonding Analogue

Article abstract of DOI:10.1002/chem.201604049

The selective phosphate-sensing property of a bis-heteroleptic Ru^IIcomplex, 1[PF₆]₂, which has a halogen-bonding iodotriazole unit, is demonstrated and is shown to be superior to its hydrogen-bonding analogue, 2[PF₆]₂. Complex 1[PF₆]₂, exploiting halogen-bonding interactions, shows enhanced phosphate recognition in both acetonitrile and aqueous acetonitrile compared with its hydrogen-bonding analogue, owing to considerable amplification of the Ru^II-center-based metal-to-ligand charge transfer (MLCT) emission response and luminescence lifetime. Detailed solution-state studies reveal a higher association constant, lower limit of detection, and greater change in lifetime for complex 1 in the presence of phosphates compared with its hydrogen-bonding analogue, complex 2. The¹H NMR titration study with H₂PO₄^?ascertains that the binding of H₂PO₄^?occurs exclusively through halogen-bonding or hydrogen-bonding interactions in complexes 1[PF₆]₂and 2[PF₆]₂, respectively. Importantly, the single-crystal X-ray structure confirms the first ever report on metal-assisted second-sphere recognition of H₂PO₄^?and H₂P₂O₇^2?with 1 through a solitary C?I???anion halogen-bonding interaction.

Full text of DOI:10.1002/chem.201604049

DOI: 10.1002/chem.201604049
Full Paper
&
Ruthenium Complexes
Selective Sensing of Phosphates by a New Bis-heteroleptic RuII
Complex through Halogen Bonding: A Superior Sensor over Its
Hydrogen-Bonding Analogue
Bijit Chowdhury, Sanghamitra Sinha, and Pradyut Ghosh*[a]
Dedicated to Professor Parimal K. Bharadwaj on the occasion of his 65th birthday
Abstract: The selective phosphate-sensing property of a bis-
heteroleptic RuII complex, 1[PF6]2, which has a halogen-
bonding iodotriazole unit, is demonstrated and is shown to
be superior to its hydrogen-bonding analogue, 2[PF6]2. Com-
plex 1[PF6]2, exploiting halogen-bonding interactions, shows
enhanced phosphate recognition in both acetonitrile and
aqueous acetonitrile compared with its hydrogen-bonding
analogue, owing to considerable amplification of the RuII-
center-based metal-to-ligand charge transfer (MLCT) emis-
sion response and luminescence lifetime. Detailed solution-
state studies reveal a higher association constant, lower limit
of detection, and greater change in lifetime for complex 1 in
the presence of phosphates compared with its hydrogen-
bonding analogue, complex 2. The 1H NMR titration study
ꢀ
ꢀ
with H2PO4 ascertains that the binding of H2PO4 occurs ex-
clusively through halogen-bonding or hydrogen-bonding in-
teractions in complexes 1[PF6]2 and 2[PF6]2, respectively. Im-
portantly, the single-crystal X-ray structure confirms the first
ever report on metal-assisted second-sphere recognition of
ꢀ
2ꢀ
H2PO4 and H2P2O7 with 1 through a solitary CꢀI···anion
halogen-bonding interaction.
bonding.[1g,2d,7] In general, the XB receptors developed to date
display excellent selectivity for halides over oxo anions.[3b,g,8]
However, there have been very few reports showing the recog-
nition of phosphates through charge-assisted XB interac-
tions.[3g,9]
Introduction
Substantial research efforts have been devoted to the recogni-
tion of anions, particularly phosphate, because of the signifi-
cant roles played by the latter in chemical, biological, medical,
and environmental processes.[1] Although anion coordination
chemistry has been enriched greatly by hydrogen-bonding
(HB) interactions over recent years,[1e,f,2] the potential of
charge-assisted halogen bonding (XB) interactions for selective
anion recognition is still a flourishing topic in the area of
supramolecular chemistry.[3] Halogen bonding is a directional,
noncovalent attractive interaction, which generally arises be-
tween the positive region of the electrostatic potential surface
of an electrophilic halogen atom and the negative sites of
a Lewis base such as N, O, S, P, or halogen functionalities and
p-electron donors.[3,4] Disruption of electron density through
the formation of a carbonꢀhalogen (CꢀX) bond leads to the
development of a positive electrostatic potential,[5] whereas
the strength of halogen bonding depends on the polarizability
of the halogen attached to the receptor.[4,5c,6] Again, the major-
ity of receptors designed for phosphates are mainly concerned
with coulombic interactions, metal coordination, and hydrogen
On the other hand, among the traditional luminescent metal
complexes for the recognition of anions, RuII complexes de-
rived from bidentate chelating ligands[10,11b,16c] are particularly
attractive because of their interesting photophysical, photo-
chemical, and electrochemical properties associated with the
metal-to-ligand charge-transfer (MLCT) excited states.[10c,11b,c,15b]
In our recently published articles we have shown the selective
recognition of phosphates by bis-heteroleptic RuII complexes
of click-derived triazole-pyridine and phenanthroline hybrid li-
gands through CꢀH···anion interactions.[12] Complexation with
the RuII center enhances the acidity of the triazole CꢀH proton
and makes it an effective anion recognition unit, and the rec-
ognition event is understood easily through the change in
photophysical properties of the RuII complexes.[10a–c] However,
metal-complex-based second-sphere anion recognition
through XB interactions is yet to be explored. Beer et al. have
reported recognition of halides through solitary charge-assist-
ed XB interactions using a RuII or ReI-bipyridyl complex-based
rotaxane host system.[3h,8d] However, direct attachment of the
XB subunit to the metal center has not yet been explored.
Here, we report two new bis-heteroleptic RuII complexes
with iodotriazole and triazole CꢀH groups as XB and HB donor
units to compare their potentiality toward the binding of
phosphate through XB and HB interactions, respectively. We
[a] B. Chowdhury, S. Sinha, Prof. P. Ghosh
Department of Inorganic Chemistry
Indian Association for the Cultivation of Science
2A&2B Raja S. C. Mullick Road, Kolkata 700032 (India)
E-mail: icpg@iacs.res.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201604049.
Chem. Eur. J. 2016, 22, 1 – 10
1
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
show that the complex exploiting XB provides stronger bind-
S14, Supporting Information), and elemental analysis tech-
niques. Finally, single-crystal X-ray diffraction analysis con-
firmed the structures of both complexes.
ꢀ
ing toward H2PO4 than its HB analogue in both acetonitrile
and the aqueous acetonitrile mixed solvent system. Moreover,
XB-assisted phosphate binding has been studied extensively in
solution, which is further supported by single-crystal X-ray dif-
fraction studies. To the best of our knowledge, this is the first
ever report on metal-assisted second-sphere phosphate bind-
ing through solitary XB interactions.
Photophysical studies
The presence of the anion-coordinating iodotriazole (CꢀI)
group and photoresponsive RuII center in 1[PF6]2 allows us to
explore its anion-binding affinities by observing the extent of
perturbation of its photophysical properties in the presence of
ꢀ
various anions such as Fꢀ, Clꢀ, AcOꢀ, BzOꢀ, NO3ꢀ, HCO3
,
Results and Discussion
HSO4ꢀ, HOꢀ, HP2O73ꢀ, H2PO4ꢀ, and so on. The absorption spec-
trum of complex 1[PF6]2 shows three characteristic bands at
262, 403, and 445 nm (Figure S15, Supporting Information).
The MLCT bands at 403 and 445 nm can be attributed to
Ru(dp)!phenanthroline and Ru(dp)!pyridine-triazole charge-
transfer transitions.[10b,c,11b,15] Practically no change in the typical
MLCT band at 445 nm is observed upon the addition of ten
equivalents of the anions into 40 mm acetonitrile solutions of
Designing aspect of complex
In our recently published articles we have shown selective
sensing of phosphates by rigidification of the flexible frame-
work of sensors in which the binding event was mainly attrib-
utable to the CꢀH···anion interactions. The triazole CꢀH unit
was the main phosphate-coordinating unit in these complexes.
These studies show that coordination of the pyridine-triazole
unit with the RuII center serves the dual purpose of enhancing
the anion-coordinating ability of the triazole CꢀH group and
helping in establishing the anion recognition phenomenon
through the change in photophysical properties of the RuII
complexes.[12] At this juncture, it would be appropriate to in-
corporate an XB subunit in the complex structure to explore
the effect of an XB donor group on the sensing of phosphates.
Accordingly, we have designed a new bis-heteroleptic RuII
complex, 1[PF6]2, comprising the iodotriazole group, and its HB
analogue, 2[PF6]2, for anion-sensing studies (Scheme 1).
ꢀ
1[PF6]2 (Figure S16, Supporting Information), except for H2PO4
and HP2O73ꢀ. The addition of H2PO4 and HP2O7 causes
a slight redshift in the MLCT band of 1[PF6]2, indicating an in-
teraction between 1 and the corresponding anions. The slight
redshift of the MLCT bands can be visualized from the point of
lowering of the MLCT band energies owing to a slight increase
in the electron density at the metal center through CꢀI···anion
interactions.[11b,16c]
ꢀ
3ꢀ
Complex 1[PF6]2 is very weakly emissive (quantum yield:
0.0027) and is found to exhibit a broad luminescence band at
585 nm upon excitation at its MLCT absorption maximum of
403 nm (Figure S15, Supporting Information). However, excita-
tion at either 403 or 445 nm results in similar emission behav-
ior, indicating that the emission occurs from the same triplet
MLCT excited state.[10a] The intensity of the broad centered
ꢀ
weak emission band increases only in the presence of H2PO4
3ꢀ
(17-fold) and HP2O7 (5-fold), whereas the addition of other
competitive anions promotes no change in the PL spectrum
even at high concentrations (10 equiv; Figure 1).
Quantum yields calculated for 1[PF6]2 as well as in the pres-
ence of various anions in acetonitrile are listed in Table S1
(Supporting Information). The presence of flexible side arms in
1[PF6]2 results in a lowering of its quantum yield (f ; Fig-
f
ure S17, Supporting Information) by a factor of 23 compared
Scheme 1. Chemical structures of the two RuII complexes.
with the standard ([Ru(bpy)3](PF6)2; f =0.062). The f value of
f
f
1[PF6]2 remains almost unperturbed in the presence of various
anions, but increases 17-fold and 5-fold in the presence of
Synthesis and characterization
ꢀ
H2PO4 and HP2O73ꢀ, respectively. This may be caused by the
Compounds L1, L2, and cis-[Ru(phen)2Cl2] were synthesized ac-
cording to the reported literature procedures.[13,14] The ruthe-
nium(II) complexes, 1[PF6]2 and 2[PF6]2, were synthesized by
heating ligands L1 or L2 at reflux with cis-[Ru(phen)2Cl2] in an
ethanol/water (2:1, v/v) mixture under an argon atmosphere
lowering of the non-radiative decays through rotational and vi-
brational relaxation pathways by rigidification of the frame-
ꢀ
3ꢀ [2d,11b,16]
work upon selective binding of H2PO4 and HP2O7
.
To gain a quantitative insight into the receptor–anion inter-
actions, we performed UV/Vis and PL titrations of 1[PF6]2 with
ꢀ
ꢀ
for 24 h, followed by exchange of Clꢀ with PF6 (Scheme 2).
H2PO4 and HP2O73ꢀ. Upon gradual addition of increasing
1
ꢀ
3ꢀ
Both the complexes were characterized fully by H, DEPT-135,
amounts of H2PO4 and HP2O7 to 1[PF6]2 (40 mm in acetoni-
trile), the absorption bands at 403 and 445 nm decreased, and
a well-defined single isosbestic point appeared at 455 nm (Fig-
ure 2a and Figure S18, Supporting Information). This indicates
13C, 1H-1H COSY, 1H-DEPT-135 HSQC, and 1H-13C HMBC NMR
spectroscopy (Figures S1–S12, Supporting Information), elec-
trospray ionization mass spectrometry (ESI-MS; Figures S13 and
&
&
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
2
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
Scheme 2. Synthetic procedure for the two RuII complexes.
ꢀ
tained as 1.94ꢁ105 and 5.6ꢁ104 mꢀ1 for 1[PF6]2 with H2PO4
and HP2O73ꢀ, respectively, by fitting the respective titration
data to a nonlinear 1:1 curve-fitting model.[7c,l]
ꢀ
The selectivity of 1[PF6]2 toward H2PO4 over other anions
was verified by adding one equivalent of H2PO4 to the aceto-
ꢀ
nitrile solution of 1[PF6]2 in the presence of ten equivalents of
other competitive anions. As expected, the selectivity and
emission enhancement remained unperturbed in the presence
of all the anions (Figure 3a). The PL intensity change (IꢀI0) of
1[PF6]2 shows a good linear response against increasing con-
centration of H2PO4ꢀ, and the detection limit is found to be as
low as approximately 0.018 mm on the basis of specific quanti-
tative analyses (Figure 3b and Figure S24, Supporting Informa-
tion).
The luminescence response of the metalloreceptor (1[PF6]2)
upon binding with various anions is further confirmed through
time-resolved emission studies (Figure 4a and Table 1). Com-
plex 1[PF6]2 displays biexponential decay, with an initial lumi-
nescence lifetime of approximately 5 ns followed by a relatively
longer-lived component with an excited-state lifetime of
around 9 ns. Comparing the lifetime data with those of previ-
ously reported complexes, one can say that the first compo-
Figure 1. Changes in the PL (10 mm) spectrum of 1[PF6]2 in the presence of
various anions (10 equiv) in acetonitrile at room temperature.
the presence of a single equilibrium between the non-com-
plexed (free complex 1) and complexed species (phosphate
adduct of complex 1). The UV/Vis titration data show a satura-
3
nent may be attributed to the MLCT state based on the phe-
ꢀ
tion point at the addition of one equivalent of H2PO4 and
nanthroline unit, whereas the second component originates
from the equilibrium with the triplet state of the pyridine-tria-
3ꢀ
HP2O7 (Figure S19, Supporting Information). During PL titra-
3ꢀ
3
tion, upon incremental addition of H2PO4ꢀ/HP2O7 into 1[PF6]2
zole moiety, which repopulates the MLCT state after the initial
(10 mm in acetonitrile), the intensity of the broad centered
emission band at 585 nm is continually enhanced up to the
addition of one equivalent of the guest anion with a slight red-
shift (5 nm) of the emission maxima (Figure 2b and Figure S20,
Supporting Information). No further change in the emission in-
tensity is observed beyond this, indicating a saturation point
(Figure S21, Supporting Information). Job’s plot analyses sug-
gest 1:1 host–guest stoichiometric binding between 1[PF6]2
emission.[10b,11e]
ꢀ
The addition of various anionic guests, except H2PO4 and
HP2O73ꢀ, does not change the decay pattern or the lifetime of
ꢀ
1[PF6]2 (Figure 4a and Table 1). In the presence of H2PO4 and
HP2O73ꢀ, the lifetimes of both the components increase in gen-
eral. On further addition of these anions, the lifetime of the
first component decreases, whereas that of the second compo-
nent increases. The bi-exponential decay pattern in the pres-
ꢀ
3ꢀ
ꢀ
3ꢀ
and H2PO4 /HP2O7 (Figure S22, Supporting Information). The
ence of H2PO4 /HP2O7 indicates the presence of two distinct
luminescent species; the one with the shorter lifetime can be
assigned as the free metalloreceptor, and the other is the
ꢀ
3ꢀ
PL titration data of 1[PF6]2 with H2PO4 and HP2O7 fit well
with the 1:1 binding model (Figure S23, Supporting Informa-
tion), and apparent 1:1 association constant values are ob-
3ꢀ
H2PO4ꢀ/HP2O7 adduct of the receptor, with a longer lifeti-
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
3
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
ꢀ
Figure 2. a) UV/Vis (40 mm) and b) PL (10 mm) titration of 1[PF6]2 with H2PO4
(0–1.5 equiv) in acetonitrile at room temperature.
Figure 3. a) Selectivity graph of 1[PF6]2 (10 mm) with H2PO4ꢀ in the presence
of other anions in acetonitrile. Color code: Light gray bars correspond to
1[PF6]2 in the presence of all anions; dark gray bars correspond to 1[PF6]2 in
Table 1. Lifetimes of 1[PF6]2 and 1[PF6]2 in the presence of various anions
(3 equiv) in acetonitrile at 258C.
the presence of all anions and H2PO4ꢀ. Codes used: A) Only 1[PF6]2; B) Fꢀ;
ꢀ
C) Clꢀ; D) Brꢀ; E) Iꢀ; F) AcOꢀ; G) BzOꢀ; H) NO3ꢀ; I) HCO3ꢀ; J) HOꢀ; K) ClO4
;
Entry
Lifetime [ns]
Entry
Lifetime [ns]
L) HSO4ꢀ; M) HP2O73ꢀ; N) H2PO4ꢀ. b) Calibration curve for H2PO4ꢀ over the
concentration range 0 to 5.0 mm derived from the PL titration with 1[PF6]2
(10 mm).
1[PF6]2
5.78
5.99
5.92
5.88
5.96
5.79
1[PF6]2 +BzOꢀ
1[PF6]2 +HCO3
5.85
5.66
5.67
6.65
34.20
108.55
ꢀ
1[PF6]2 +Fꢀ
1[PF6]2 +Clꢀ
1[PF6]2 +Brꢀ
1[PF6]2 +Iꢀ
1[PF6]2 +AcOꢀ
ꢀ
1[PF6]2 +NO3
1[PF6]2 +HSO4
ꢀ
3ꢀ
1[PF6]2 +HP2O7ꢀ
1[PF6]2 +H2PO4
tered excited state (3MC).[10b,11b,16c] This change in the lifetime
and decay pattern of complex 1[PF6]2 continues up to the ad-
ꢀ
3ꢀ
dition of one equivalent of H2PO4 /HP2O7 and stops beyond
that, supporting the 1:1 host–guest stoichiometry obtained
from UV/Vis and PL titration data (Figure 4b, and Figure S25
me.[11b] Combination of the above two components leads to
the overall enhancement in lifetime. The increase in the life-
ꢀ
3ꢀ
ꢀ
time of 1[PF6]2 with H2PO4 /HP2O7 can be explained by the
lowering of the chances of rotational/vibrational relaxation
modes of non-radiative decay through rigidification in the re-
ceptor framework upon complexation with the aforemen-
and Table S2, Supporting Information). Again, between H2PO4
and HP2O73ꢀ, the former is proved to cause a better change in
the decay pattern and lifetime of 1[PF6]2, which makes com-
ꢀ
plex 1[PF6]2 a suitable lifetime-based sensor for H2PO4 over
3
3ꢀ
tioned anions, which leads to stabilization of the MLCT state
HP2O7 and other competitive anions in acetonitrile.
of the complexes, and hence, increases the energy gap of the
3MLCT level with the corresponding non-emitting metal-cen-
For a comparison between the sensing abilities of 1[PF6]2
(with the XB iodotriazole unit) and its HB analogue 2[PF6]2, de-
&
&
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
4
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
ꢀ
ized by comparing the mode of H2PO4 binding through two
distinct noncovalent interactions, that is, CꢀI···anion XB interac-
tions in 1[PF6]2 and CꢀH···anion HB interactions in 2[PF6]2.
Moreover, to examine the effect of water on the lumines-
cence sensing behaviors of the XB and HB receptors toward
ꢀ
H2PO4 and HP2O73ꢀ, we performed analogous PL experiments
in 5%, 10%, 15%, and 20% water/acetonitrile mixed solvents
(Figure 5 and Figures S39–S41, Supporting Information). It was
observed that the XB receptor 1[PF6]2 is capable of sensing
ꢀ
3ꢀ
H2PO4 and HP2O7 in up to 20% water/acetonitrile mixtures
through moderate changes in PL intensities. The degree of
emission enhancement is reduced upon switching from anhy-
drous acetonitrile to aqueous acetonitrile owing to the hydra-
tion of the anion in the aqueous medium.
3ꢀ
Further, trianionic HP2O7 shows weak binding with XB and
ꢀ
HB receptors in aqueous acetonitrile compared with H2PO4
,
probably owing to its higher hydration energy than that of
H2PO4ꢀ. The apparent 1:1 association constant and lower limit
ꢀ
of detection values for 1[PF6]2 with H2PO4 are calculated as
1.46ꢁ104 mꢀ1 and 0.28 mm, respectively, in 20% water/acetoni-
Figure 4. Time-resolved luminescence decays of 1[PF6]2 (10 mm) in acetoni-
trile solution at 258C: a) upon addition of various anions as their corre-
sponding tetrabutylammonium salts, and b) upon addition of increasing
amounts of TBAH2PO4.
tailed photophysical studies were performed for 2[PF6]2 (Figur-
es S26–S38 and Tables S3–S5, Supporting Information). The
combined solution-state output for complexes 1[PF6]2 and
2[PF6]2 is summarized in Table S6 (Supporting Information).
ꢀ
This clearly shows that 2[PF6]2 selectively senses H2PO4 similar-
ly to 1[PF6]2; however, the degree of emission enhancement
(sixfold), apparent 1:1 association constant (5.59ꢁ104 mꢀ1), sen-
sitivity (0.05 mm), and lifetime enhancement (sixfold) are much
lower in the case of 2[PF6]2. Here, it is important to mention
that our previously reported methyl-substituted pyridine-tria-
zole-based RuII complex, 2’[PF6]2, which is structurally quite
similar to complex 2[PF6]2, the benzyl-substituted one, also
showed similar outputs in the presence of H2PO4ꢀ. Thus,
changing the appending methyl group in 2’[PF6]2 to a benzyl
group in 2[PF6]2 does not perturb the basic photophysical
properties (UV/Vis, PL) or the anion-sensing behavior.
The selectivities of complexes 1[PF6]2 and 2[PF6]2 toward
ꢀ
3ꢀ
H2PO4 over other competitive anions and HP2O7 can be at-
tributed to the high basicity and high charge density of
H2PO4ꢀ, which assist the formation of strong adducts between
host and guest. However, the higher degree of enhancement
Figure 5. PL titration of 1[PF6]2 (10 mm) with H2PO4ꢀ (0–1.5 equiv) in a) 15%
water/acetonitrile, and b) 20% water/acetonitrile at room temperature.
ꢀ
in case of 1[PF6]2 than in 2[PF6]2 with H2PO4 can be rational-
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
trile (Figure S42, Supporting Information). Interestingly, al-
though the HB analogue (2[PF6]2) shows a change in emission
ꢀ
3ꢀ
behavior with H2PO4 and HP2O7 in 5% water/acetonitrile, it
fails to give signaling output in the presence of the aforemen-
tioned anions in water/acetonitrile mixtures of 10% or above.
Thus, complex 1[PF6]2 with the XB donor group is proved to
ꢀ
3ꢀ
be a better sensor for the detection of H2PO4 over HP2O7
and other competitive anions than its HB analogue 2[PF6]2 in
acetonitrile and aqueous acetonitrile mixed solvent.
1
Mechanistic investigation by H NMR experiments
To obtain mechanistic information on the binding mode of the
two receptors (1[PF6]2 and 2[PF6]2) with various anions, we per-
1
formed H NMR titration experiments involving the gradual ad-
dition of aliquots of the anions to a solution of the receptors
in [D6]DMSO. There was no shift in any proton signal in the
ꢀ
1H NMR spectrum of 1[PF6]2 in the presence of H2PO4 (Fig-
ꢀ
ure 6a), which implies that the binding of H2PO4 with
1 occurs through the CꢀI···anion XB interaction only, without
the assistance of any proton. On the other hand, the addition
ꢀ
3ꢀ
of H2PO4 and HP2O7 promotes a significant downfield shift
of the triazole CꢀH proton of 2[PF6]2, from 9.29 to 9.77 ppm
(Dd=0.48 ppm) and 9.57 ppm (Dd=0.28 ppm), respectively,
whereas the other protons are not affected. This indicates that
complex 2[PF6]2 provides CꢀH···anion HB interactions toward
ꢀ
3ꢀ
the binding of H2PO4 /HP2O7 (Figure 6b and Figure S43, Sup-
porting Information). However, the addition of other competi-
tive anions does not induce any noticeable change in the
1H NMR spectrum of 2[PF6]2, except for very high amounts of
Fꢀ and AcOꢀ (Figure S44, Supporting Information) owing to
the acid–base interaction between the acidic triazole CꢀH
proton and these highly basic anions.
Figure 6. 1H NMR titration profile of a) 1[PF6]2 (7 mm) and b) 2[PF6]2 (6.7 mm)
upon addition of increasing amounts of H2PO4 (0–2.0 equiv) in [D6]DMSO.
ꢀ
For confirmation of whether the anion-recognition processes
in solution could involve the formation of polymeric supra-
1
ꢀ
molecular structures, H DOSY NMR spectroscopic experiments
H2PO4 at 1.85 ppm is shifted downfield to 3.72 ppm and
were performed in a 9:1 CD3CN/CD3OD solution of 1[PF6]2
3.23 ppm in the presence of 1[PF6]2 and 2[PF6]2, respectively
ꢀ
before and after the addition of one equivalent of H2PO4 and
(Figure 7a). Similarly, it is observed that the initial 31P signals of
HP2O73ꢀ. Interestingly, the diffusion coefficient of the free com-
free HP2O7 at ꢀ3.31 and ꢀ7.44 ppm are shifted downfield
3ꢀ
plex (1[PF6]2; 1.52ꢁ10ꢀ8 m2 sꢀ1) decreases significantly upon
significantly to 1.85 and ꢀ3.95 ppm in the presence of 1[PF6]2
and 1.45 and ꢀ3.98 ppm upon the addition of 2[PF6]2 (Fig-
ure 7b). This downfield shifting of the 31P signals of free
ꢀ
the addition of one equivalent of H2PO4 (6.37ꢁ10ꢀ9 m2sꢀ1
)
3ꢀ
and HP2O7 (6.72ꢁ10ꢀ9 m2sꢀ1), indicating the formation of
polymeric supramolecular structures in solution. It is evident
from the diffusion spectroscopic data that the sizes of the
phosphate adducts of complex 1 in solution are significantly
larger than that of the free complex. The similar diffusion coef-
ficients for all the peaks in the 1H DOSY NMR spectra (Fig-
3ꢀ
HP2O7 (Dd=5.16 and 3.49 ppm with 1[PF6]2 and Dd=4.76
ꢀ
and 3.46 ppm with 2[PF6]2) and H2PO4 (Dd=1.87 and
1.38 ppm with 1[PF6]2 and 2[PF6]2, respectively) in the presence
ꢀ
of 1[PF6]2 and 2[PF6]2 indicates that the binding of H2PO4 and
3ꢀ
HP2O7
occurs through the participation of neighboring
ꢀ
3ꢀ
ꢀ
3ꢀ
ure S45–S47, Supporting Information) of the H2PO4 /HP2O7
oxygen centers of P in H2PO4 and HP2O7 .
adducts of complex 1 suggest the formation of a single poly-
meric species for these two phosphate adducts in solution.
Single-crystal X-ray structural studies
Single crystals suitable for X-ray diffraction of complexes 1 and
2 with PF6 counter anions were grown by diffusing diethyl-
ether into acetone/THF (9:1, v/v) solutions of the complexes at
31P NMR experiments
ꢀ
ꢀ
3ꢀ
31P NMR spectra of free H2PO4 and HP2O7 as well as those in
the presence of one equivalent of 1[PF6]2 and 2[PF6]2 were re-
corded in [D6]DMSO using triphenylphosphine (PPh3) as exter-
nal standard (ꢀ4.8 ppm in [D6]DMSO). The 31P signal of free
approximately 108C. Both 1[PF6]2 and 2[PF6]2 are found to crys-
¯
tallize in the triclinic crystal system with P1 space group. The
RuII centers adopt a distorted octahedral geometry through co-
&
&
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
Figure 7. Partial 31P NMR spectra of a) TBAH2PO4 and b) TBA3HP2O7 in the
presence of 1[PF6]2 and 2[PF6]2; PPh3 as external standard (ꢀ4.8 ppm in
[D6]DMSO).
ordination with two ancillary phenanthrolines and one triazole-
pyridine ligand (Figure 8). The RuꢀN bond lengths and NꢀRuꢀ
N angles (Tables S7 and S8, Supporting Information) are in line
with the previously reported RuII polypyridyl complex-
es.[10b,c,11b,12,16c] Analysis of the crystal packing reveals that both
the complexes form noncovalent 3D networks through CꢀI···F
and CꢀH···F interactions (Figure S48, Supporting Information).
To obtain solid-state support of phosphate binding with com-
Figure 8. Single-crystal X-ray structures: a) complex 1 and b) complex 2 with
PF6 counter anions (thermal ellipsoids drawn at the 30% probability level).
ꢀ
to be 2.833 and 2.801 ꢂ in 1[H2PO4]2 and 12[H2P2O7]2, respec-
tively, and the XB angles (CꢀI···O) are observed as 178.588 and
177.728 for 1[H2PO4]2 and 12[H2P2O7]2, respectively. Importantly,
these XB (O···I) distances are 81% and 80% of the sum of their
van der Waals radii in 1[H2PO4]2 and 12[H2P2O7]2, respectively
(Table S12, Supporting Information). This clearly designates
a noticeably strong and directional XB interaction, as observed
in previously reported systems.[3g,9a] Interestingly, apart from
halogen bonding, the oxygen atoms of the phosphate anion
are also involved in the formation of a hydrogen-bonded poly-
meric assembly of phosphates, which results in the develop-
ment of infinite anionic chains. On the other hand, the pyro-
phosphate adduct shows 2:2 host–guest stoichiometry be-
tween complex 1 and H2P2O72ꢀ, in which apart from the CꢀI···O
XB interaction, the pyrophosphate anion is involved in a hydro-
gen-bonded dimeric assembly. Analysis of the crystal packing
in 1[H2PO4]2 and 12[H2P2O7]2 reveals the formation of a 3D net-
work through CꢀI···O XB interactions between the O atoms of
ꢀ
3ꢀ
plex 1 we attempted to crystallize H2PO4 and HP2O7 adducts
of 1. Single crystals of X-ray diffraction quality of dihydrogen
phosphate and pyrophosphate adducts of complex 1, namely
1[H2PO4]2 and 12[H2P2O7]2, were grown by slow evaporation of
complex 1 with the corresponding phosphates as their tetra-
butylammonium salts from an acetonitrile/methanol (9:1, v/v)
mixture. Crystallographic parameter details of complex 3
(1[H2PO4]2) and complex 4 (12[H2P2O7]2) are listed in Table S9
(Supporting Information).
1[H2PO4]2 and 12[H2P2O7]2 crystallize in triclinic crystal sys-
¯
tems with the P1 space group (Figure 9 and Figure S49, Sup-
porting Information), in which the geometry around the RuII
centers remains similar to that in the free complexes (Ta-
bles S10 and S11, Supporting Information). Interestingly, in
these two structures, phosphate or pyrophosphate is bound
by strong and directional XB interactions between the iodine
atom of the iodotriazole group and the oxygen atom of the
corresponding phosphates. The XB distances (O···I) are found
2ꢀ
H2PO4ꢀ/H2P2O7 and the iodine center of complex 1 (Fig-
ure S50, Supporting Information).
In this context, note that Resnati et al. first reported structur-
al evidence of phosphate recognition by charge-assisted XB in-
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
lar sensing behavior. However, the detailed solution-state stud-
ies performed in this work strongly establish the superiority of
1[PF6]2 over 2[PF6]2 as a phosphate sensor. Mechanistic investi-
gation using NMR titration experiments has confirmed the key
functioning modes in the complexes: CꢀI···anion XB interac-
tions in 1[PF6]2 and CꢀH···anion HB interactions in 2[PF6]2. Final-
ly, single-crystal X-ray structural analyses corroborated the
ꢀ
2ꢀ
binding of H2PO4 /H2P2O7 with complex 1 through solitary
CꢀI···anion XB interactions. This is the first ever report of metal-
assisted second-sphere phosphate binding through solitary Cꢀ
I···anion XB interactions.
Experimental Section
The synthetic procedures and characterization of the complexes
and other experimental details are provided in the Supporting In-
formation.
Acknowledgements
P.G. gratefully acknowledges the Science and Engineering Re-
search Board (SERB; EMR/2016/000900), India, for financial sup-
port and the Alexander von Humboldt Foundation for donat-
ing a Fluorimeter. B.C. and S.S. would like to thank CSIR, India,
for research fellowships. The authors acknowledge Koushik
Sarkar and Saptarshi Mondal IACS, Kolkata, India, for their help
in solving the crystal structure of complex 4 and in the fitting
of PL titration data, respectively.
Figure 9. Single-crystal X-ray structures of a) infinite propagated chain of
halogen-bonded dihydrogen phosphate adduct of complex 1, and b) pyro-
phosphate adduct of complex 1. Short contacts involving anions are shown
as dashed black lines.
teractions, but that is the only solid-state structural evidence
of phosphate binding through XB interactions to date.[9a]
Again, to the best of our knowledge, the crystal structures of
1[H2PO4]2 and 12[H2P2O7]2 constitute the first ever report of
metal-complex-based second-sphere phosphate recognition
through solitary XB interactions. Both 1[H2PO4]2 and 12[H2P2O7]2
were further characterized by ESI-MS techniques, and good fits
of the experimental isotope patterns with their respective si-
mulated patterns are observed (Figure S51, Supporting Infor-
mation).
Keywords: CꢀH···anion interaction
· halogen bonding ·
phosphates · ruthenium · X-ray diffraction
[1] a) B. Alberts, D. Bray, J. Lewis, M. Raff, K. Roberts, J. D. Watson, Molecular
Biology of the Cell, 3rd ed., Garland, New York, 1994; b) P. D. Beer, P. A.
Gale, Angew. Chem. Int. Ed. 2001, 40, 486–516; Angew. Chem. 2001, 113,
502–532; c) P. A. Gale, S. E. Garcia-Garrido, J. Garric, Chem. Soc. Rev.
2008, 37, 151–190; d) K. Bowman-James, Acc. Chem. Res. 2005, 38,
671–678; e) S. Kubik, Chem. Soc. Rev. 2009, 38, 585–605; f) Y. Hua, A. H.
Flood, Chem. Soc. Rev. 2010, 39, 1262–1271; g) S. K. Kim, D. H. Lee, J.-I.
Hong, J. Yoon, Acc. Chem. Res. 2009, 42, 23–31; h) R. T. Sturbelle, L. Far-
ias da Costa de Avila, T. B. Roos, J. L. Borchardt, R. de Cassia dos Santos
da Conceicao, O. A. Dellagostin, F. P. L. Leite, Vet. Microbiol. 2015, 180,
245–252.
[2] a) T. D. Ashton, K. A. Jolliffe, F. M. Pfeffer, Chem. Soc. Rev. 2015, 44,
4547–4595; b) N. Busschaert, C. Caltagirone, W. Van Rossom, P. A. Gale,
Chem. Rev. 2015, 115, 8038–8155; c) S. Lee, K. K. Y. Yuen, K. A. Jolliffe, J.
Yoon, Chem. Soc. Rev. 2015, 44, 1749–1762; d) A. E. Hargrove, S. Nieto,
T. Zhang, J. L. Sessler, E. V. Anslyn, Chem. Rev. 2011, 111, 6603–6782.
[3] a) T. Shirman, M. Boterashvili, M. Orbach, D. Freeman, L. J. W. Shimon,
M. Lahav, M. E. van der Boom, Cryst. Growth Des. 2015, 15, 4756–4759;
b) S. M. Walter, F. Kniep, L. Rout, F. P. Schmidtchen, E. Herdtweck, S. M.
Huber, J. Am. Chem. Soc. 2012, 134, 8507–8512; c) T. M. Beale, M. G.
Chudzinski, M. G. Sarwar, M. S. Taylor, Chem. Soc. Rev. 2013, 42, 1667–
1680; d) C. C. Robertson, R. N. Perutz, L. Brammer, C. A. Hunter, Chem.
Sci. 2014, 5, 4179–4183; e) N. K. Beyeh, F. Pan, K. Rissanen, Angew.
Chem. Int. Ed. 2015, 54, 7303–7307; Angew. Chem. 2015, 127, 7411–
7415; f) C. J. Massena, A. M. S. Riel, G. F. Neuhaus, D. A. Decato, O. B. Ber-
ryman, Chem. Commun. 2015, 51, 1417–1420; g) R. Tepper, B. Schulze,
M. Jaeger, C. Friebe, D. H. Scharf, H. Goerls, U. S. Schubert, J. Org. Chem.
2015, 80, 3139–3150; h) M. J. Langton, I. Marques, S. W. Robinson, V.
Felix, P. D. Beer, Chem. Eur. J. 2016, 22, 185–192; i) B. R. Mullaney, B. E.
Partridge, P. D. Beer, Chem. Eur. J. 2015, 21, 1660–1665.
Assuming a spherical shape of the crystal structure, we cal-
culated the molecular diameter of the 2:2 dimeric supramolec-
2ꢀ
ular assembly between complex 1 and H2P2O7 by averaging
the dimensions of the major and minor axes (Figure S52 and
Table S13, Supporting Information). Interestingly, this value
(16.40 ꢂ) matches quite well with the diameter obtained in so-
1
lution from the H DOSY NMR experiment (16.66 ꢂ). This result
suggests that the 2:2 dimeric adduct, that is, (1:1 receptor/
2ꢀ
anion)2, between complex 1 and H2P2O7 observed in the
solid-state investigation also exists as a stable species in solu-
tion.
Conclusions
In summary, an XB-based highly sensitive luminescence “OFF-
ON” chemosensor, 1[PF6]2, has been developed, which can
ꢀ
3ꢀ
detect H2PO4 /HP2O7 selectively at very low concentrations,
even in the presence of excess amounts of other competitive
anions. The HB analogue of 1[PF6]2, that is, 2[PF6]2, shows simi-
&
&
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
8
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
[4] G. Cavallo, P. Metrangolo, R. Milani, T. Pilati, A. Priimagi, G. Resnati, G.
Terraneo, Chem. Rev. 2016, 116, 2478–2601.
2013, 13, 871–877; i) M. G. Chudzinski, C. A. McClary, M. S. Taylor, J. Am.
Chem. Soc. 2011, 133, 10559–10567; j) J. Y. C. Lim, M. J. Cunningham,
J. J. Davis, P. D. Beer, Chem. Commun. 2015, 51, 14640–14643.
[9] a) M. Cametti, K. Raatikainen, P. Metrangolo, T. Pilati, G. Terraneo, G. Res-
nati, Org. Biomol. Chem. 2012, 10, 1329–1333; b) F. Zapata, A. Caballero,
P. Molina, I. Alkorta, J. Elguero, J. Org. Chem. 2014, 79, 6959–6969.
[10] a) S. Khatua, D. Samanta, J. W. Bats, M. Schmittel, Inorg. Chem. 2012, 51,
7075–7086; b) S. Karmakar, D. Maity, S. Mardanya, S. Baitalik, Inorg.
Chem. 2014, 53, 12036–12049; c) W. K. C. Lo, G. S. Huff, J. R. Cubanski,
A. D. W. Kennedy, C. J. McAdam, D. A. McMorran, K. C. Gordon, J. D.
Crowley, Inorg. Chem. 2015, 54, 1572–1587.
[11] a) V. Balzani, A. Juris, M. Venturi, S. Campagna, S. Serroni, Chem. Rev.
1996, 96, 759–833; b) D. Maity, S. Das, S. Mardanya, S. Baitalik, Inorg.
Chem. 2013, 52, 6820–6838; c) B. Laramꢃe-Milette, F. Lussier, I. Ciofini,
G. S. Hanan, Dalton Trans. 2015, 44, 11551–11561; d) T. D. Nguyen, H.-R.
Tseng, P. C. Celestre, A. H. Flood, Y. Liu, J. F. Stoddart, J. I. Zink, Proc. Natl.
Acad. Sci. USA 2005, 102, 10029–10034; e) A. Ghosh, B. Ganguly, A. Das,
Inorg. Chem. 2007, 46, 9912–9918.
[12] a) B. Chowdhury, S. Khatua, R. Dutta, S. Chakraborty, P. Ghosh, Inorg.
Chem. 2014, 53, 8061–8070; b) B. Chowdhury, R. Dutta, S. Khatua, P.
Ghosh, Inorg. Chem. 2016, 55, 259–271.
[13] W. S. Brotherton, R. J. Clark, L. Zhu, J. Org. Chem. 2012, 77, 6443–6455.
[14] J. E. Collins, J. J. S. Lamba, J. C. Love, J. E. McAlvin, C. Ng, B. P. Peters, X.
Wu, C. L. Fraser, Inorg. Chem. 1999, 38, 2020–2024.
[15] a) M. Felici, P. Contreras-Carballada, Y. Vida, J. M. M. Smits, R. J. M. Nolte,
L. De Cola, R. M. Williams, M. C. Feiters, Chem. Eur. J. 2009, 15, 13124–
13134; b) A. K. Pal, S. Nag, J. G. Ferreira, V. Brochery, G. La Ganga, A. San-
toro, S. Serroni, S. Campagna, G. S. Hanan, Inorg. Chem. 2014, 53, 1679–
1689.
[16] a) P. D. Beer, Acc. Chem. Res. 1998, 31, 71–80; b) J. A. Kitchen, E. M.
Boyle, T. Gunnlaugsson, Inorg. Chim. Acta 2012, 381, 236–242; c) D.
Maity, C. Bhaumik, D. Mondal, S. Baitalik, Inorg. Chem. 2013, 52, 13941–
13955; d) R. Martꢄnez-MꢅÇez, F. Sancenon, Chem. Rev. 2003, 103, 4419–
4476.
[5] a) A. Priimagi, G. Cavallo, P. Metrangolo, G. Resnati, Acc. Chem. Res.
2013, 46, 2686–2695; b) P. Politzer, J. S. Murray, T. Clark, Phys. Chem.
Chem. Phys. 2010, 12, 7748–7757; c) L. C. Gilday, S. W. Robinson, T. A.
Barendt, M. J. Langton, B. R. Mullaney, P. D. Beer, Chem. Rev. 2015, 115,
7118–7195; d) B. Schulze, U. S. Schubert, Chem. Soc. Rev. 2014, 43,
2522–2571.
[6] a) K. E. Riley, J. S. Murray, J. Fanfrlik, J. Rezac, R. J. Sola, M. C. Concha,
F. M. Ramos, P. Politzer, J. Mol. Model. 2011, 17, 3309–3318; b) G. Caval-
lo, P. Metrangolo, T. Pilati, G. Resnati, M. Sansotera, G. Terraneo, Chem.
Soc. Rev. 2010, 39, 3772–3783.
[7] a) J. L. Sessler, J. Cai, H.-Y. Gong, X. Yang, J. F. Arambula, B. P. Hay, J. Am.
Chem. Soc. 2010, 132, 14058–14060; b) D.-J. Oh, K.-M. Kim, K.-H. Ahn,
Chem. Asian J. 2011, 6, 2034–2039; c) P. Sokkalingam, D. S. Kim, H.
Hwang, J. L. Sessler, C.-H. Lee, Chem. Sci. 2012, 3, 1819–1824; d) G. San-
chez, A. Espinosa, D. Curiel, A. Tarraga, P. Molina, J. Org. Chem. 2013, 78,
9725–9737; e) O. G. Tsay, S. T. Manjare, H. Kim, K. M. Lee, Y. S. Lee, D. G.
Churchill, Inorg. Chem. 2013, 52, 10052–10061; f) I. Ravikumar, P. Ghosh,
Inorg. Chem. 2011, 50, 4229–4231; g) L. J. Liang, X. J. Zhao, C. Z. Huang,
Analyst 2012, 137, 953–958; h) J. Massue, S. J. Quinn, T. Gunnlaugsson,
J. Am. Chem. Soc. 2008, 130, 6900–6901; i) P. Mateus, R. Delgado, P.
Brandao, V. Felix, J. Org. Chem. 2009, 74, 8638–8646; j) H. Gan, A. G.
Oliver, B. D. Smith, Chem. Commun. 2013, 49, 5070–5072; k) X. Liu, H. T.
Ngo, Z. Ge, S. J. Butler, K. A. Jolliffe, Chem. Sci. 2013, 4, 1680–1686; l) J.
Cai, B. P. Hay, N. J. Young, X. Yang, J. L. Sessler, Chem. Sci. 2013, 4, 1560–
1567.
[8] a) S. H. Jungbauer, S. M. Huber, J. Am. Chem. Soc. 2015, 137, 12110–
12120; b) F. Zapata, A. Caballero, N. G. White, T. D. W. Claridge, P. J.
Costa, V. Felix, P. D. Beer, J. Am. Chem. Soc. 2012, 134, 11533–11541;
c) M. G. Sarwar, B. Dragisic, S. Sagoo, M. S. Taylor, Angew. Chem. Int. Ed.
2010, 49, 1674–1677; Angew. Chem. 2010, 122, 1718–1721; d) B. R.
Mullaney, A. L. Thompson, P. D. Beer, Angew. Chem. Int. Ed. 2014, 53,
11458–11462; Angew. Chem. 2014, 126, 11642–11646; e) S. Chakrabor-
ty, R. Dutta, P. Ghosh, Chem. Commun. 2015, 51, 14793–14796; f) C. J.
Kassl, D. C. Swenson, F. C. Pigge, Cryst. Growth Des. 2015, 15, 4571–
4580; g) R. Tepper, B. Schulze, H. Goerls, P. Bellstedt, M. Jaeger, U. S.
Schubert, Org. Lett. 2015, 17, 5740–5743; h) K. Raatikainen, G. Cavallo,
P. Metrangolo, G. Resnati, K. Rissanen, G. Terraneo, Cryst. Growth Des.
Received: August 25, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
9
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
Tipping the balance: Selective “OFF-
ON” luminescent sensing, based on hal-
ogen bonding, of dihydrogen phos-
phate, and its superiority over the Cꢀ
H···anion interaction are demonstrated
(see figure). Solid-state second-sphere
recognition of dihydrogen phosphate
through solitary halogen-bonding inter-
actions is also shown.
&
Ruthenium Complexes
B. Chowdhury, S. Sinha, P. Ghosh*
&& – &&
Selective Sensing of Phosphates by
a New Bis-heteroleptic RuII Complex
through Halogen Bonding: A Superior
Sensor over Its Hydrogen-Bonding
Analogue
&
&
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
10
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!