Ruthenium cryptates with an unusual selectivity for nitrate

Article abstract of DOI:10.1039/c2dt12446g

The synthesis of two new tripodal complexes [Ru(L3)](PF₆) ₂ and [Ru(L4)](PF₆)₂, encapsulating a ruthenium(ii) cation, has been successfully achieved and the products fully characterized, including by X-ray structural determination. The smaller cavity, built around a tris(2-aminoethyl)amido scaffold demonstrated only moderate and predictable interactions with a range of anions and no significant spectroscopic change with nitrate, chloride and bromide, although dihydrogen phosphate did result in an almost stoichiometric precipitation. The expansion of the cavity to include the more rigid 1,3,5-benzenetricarbonylamide group creates a larger cavity, which shows a decrease in the emission on the introduction of chloride, bromide, hydrogen sulfate and nitrate salts, with the ¹H NMR titrations giving a surprisingly high binding affinity for nitrate over the smaller and simpler halides.

Full text of DOI:10.1039/c2dt12446g

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PAPER
Ruthenium cryptates with an unusual selectivity for nitrate†
Naomi C. A. Baker,a Nicholas C. Fletcher,*a Peter N. Hortonb and Michael B. Hursthouseb
Received 19th December 2011, Accepted 4th April 2012
DOI: 10.1039/c2dt12446g
The synthesis of two new tripodal complexes [Ru(L3)](PF6)2 and [Ru(L4)](PF6)2, encapsulating a
ruthenium(II) cation, has been successfully achieved and the products fully characterized, including by
X-ray structural determination. The smaller cavity, built around a tris(2-aminoethyl)amido scaffold
demonstrated only moderate and predictable interactions with a range of anions and no significant
spectroscopic change with nitrate, chloride and bromide, although dihydrogen phosphate did result in an
almost stoichiometric precipitation. The expansion of the cavity to include the more rigid
1,3,5-benzenetricarbonylamide group creates a larger cavity, which shows a decrease in the emission on
the introduction of chloride, bromide, hydrogen sulfate and nitrate salts, with the 1H NMR titrations
giving a surprisingly high binding affinity for nitrate over the smaller and simpler halides.
in two comparable mer and fac isomers of the complex
Introduction
[Ru(L2)3](PF6)2, and we also compared these to the ditopic
receptor [Ru(L1)3](PF6)2 (Fig. 1).26 From these studies the con-
clusion was drawn that the C3-symmetric “pockets” provided by
[Ru(L1)3]2+ is conducive to the recognition of a range of anions
Arising from the unique combination of structural stability, well
behaved redox activity, and photophysical properties, the poly-
pyridine complexes of ruthenium(II) have attracted considerable
attention with potential applications in solar energy conver-
sion,1,2 photocatalysis,3,4 and biological imaging5 as well as pro-
viding suitable candidates for the structural recognition of
DNA,6,7 protein,8–11 and small molecules.12 Their consideration
in anion specific recognition dates back over 15 years,13 with
numerous complexes having been reported with varying degrees
of selectivity.12,14–17 Beer and co-workers importantly illustrated
that the use of the C3-symmetric cavity formed by arranging
three 5,5′-functionalized-2,2′-bipyridine ligands around a ruthe-
nium(II) metal centre could be used to selectively recognize a
range of small inorganic anions through hydrogen bonding with
the three amides.18 A similar approach has subsequently been
adopted by a number of research groups, where anion inter-
actions have been used to template the C3-symmetric structures
around a numberof labile metal centres including iron(II), cobalt(II)
and zinc(II).19–25 Recently, Wu and Janiak have illustrated
that a ruthenium(II) complex, with bipyridine ligands bearing
two carbamate groups, has a good selectivity for sulfate with a
stoichiometric quenching of the fluorescence.23
In a recent paper we explored how the relative orientation of
three functional groups would affect the anion binding behaviour
aThe School of Chemistry and Chemical Engineering, Queen’s
University Belfast, Belfast, UK BT9 5AG. E-mail: n.fletcher@qub.ac.uk;
Fax: +44 28 9097 6524; Tel: +44 28 9097 5479
bUniversity of Southampton, Chemistry Department, EPSRC National
Crystallography Service, Southampton, UK
†Electronic supplementary information (ESI) available: Tabulated data
for the significant ruthenium–nitrogen bond lengths and angles, and the
significant hydrogen bonding interactions. CCDC reference numbers
859683 and 859684. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2dt12446g
Fig. 1 (a) [Ru(L1)3](PF6)2 and (b) [Ru(L2)3](PF6)2.
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while the fac-isomer of [Ru(L2)3]2+ has an enhancement in the
binding interactions over the mer form with dihydrogen phos-
phate in dimethyl sulfoxide (DMSO), although the difference is
much less marked with the spherical chloride ions. It was con-
cluded that the fac isomer may be of interest in the design of
new sensors in future studies, although it was conceded that the
synthesis of fac isomers remains a considerable challenge around
a ruthenium metal centre.
As has been previously reported, the fac isomer can be selec-
tively formed by “tethering” the three bipyridine ligands
together.27–30 In our attempts to achieve this, it was recognized
that there is the synthetic potential to include a tripodal cavity
adjacent to the metal centre, which could potentially allow a
degree of selectivity into the observed anion recognition. If the
cavity was sufficiently small, and tailored appropriately around
the C3-axis, then there is the potential to be able to selectively
discriminate the environmentally important anion; nitrate over
phosphate. It is recognized that tripodal cavities have been
widely explored in the selective detection of anions14,15,31–37
although the number of tripodal cavities formed around a tran-
sition metal template using diimine tris-chelates are relatively
scarce,35,38–44 while those with ruthenium are extremely rare pre-
sumably due to synthetic difficulties.27–30,42 However, the work
of Amendola et al.35,39 and Sato et al.42 have clearly demon-
strated that positively charged trisimidozolium cages built
around iron(II) and cobalt(III) centres can be used to develop
selectivity for small spherical and linear anions such as chloride
and azide.
Fig. 2 ORTEP plot of [Ru(L3)](PF6)2 with ellipsoids at 50% prob-
ability. Hydrogen atoms, solvent and anion disorder have been deleted
for clarity.
observed to retain both salt and solvent in the lattice despite
several attempts to recrystallize the product from acetone/methanol
and water.
Crystals of [Ru(L3)](PF6)2 suitable for X-ray analysis were
grown by diffusion of pentane into a saturated acetone solution.
The structure demonstrates similar Ru–N bond lengths (in the
region of 2.021(8) to 2.079(9) Å) and internal N–Ru–N bipyri-
dine bond angles (78.1(4) to 78.7(3)°) to related published struc-
tures (Fig. 2).26,29 All hydrogen atoms were fixed using a riding
model and some highly disordered solvent molecules were
removed using the SQUEEZE programme.45 The two PF6
anions are observed to be external to the cavity in the complex
and one of the PF6 anions is disordered over two sites (which
has been appropriately modelled). There appears to be no other
solvent hydrogen bonding to the complex. We have previously
observed a high variation in the deviation from planarity
between the pyridyl and amide in the complex [Ru(L1)3]2+,26
yet for [Ru(L3)](PF6)2, the torsion angles between the ring and
the carbonyl function (12.4, 101.9 and 145.6°) indicate the struc-
ture is significantly strained, with one carbonyl group pointing
into the cavity, probably governed by the occurrence of a hydro-
gen bond to an amidic NH at a distance of 2.96 Å.
The cavity observed in the X-ray structural analysis of [Ru-
(L3)](PF6)2 would not appear to be able to accommodate any-
thing other than a small spherical anion such as chloride. In
order to increase the cavity size, 5-carboxy-2,2′-bipyridine was
reacted with a large excess of ethylenediamine, giving disap-
pointingly high yields of the biproduct N,N′-1,2-ethanediylbis-
([2,2′-bipyridine]-5-carboxamide). To overcome this, one of the
nitrogens was BOC protected using a literature procedure
(Scheme 1)46 and subsequently liberated in 3 M HCl in ethyl
acetate. The formation of a tripodal system was achieved by then
reacting this amine with a solution of 1,3,5-benzenetricarbonyl
chloride giving ligand L4 as an extremely insoluble solid that
In this paper we report the synthesis of two new tripodally
capped ruthenium(II) tris-chelate complexes and explore if
closing the “face” of the anion recognition “pocket” can be used
to encourage selectivity for nitrate over chloride or even dihydro-
gen phosphate.
Results and discussion
Synthesis and characterization
The first tripodally capped ligand was formed by reaction of
5-carboxy-2,2′-bipyridineandtris(2-aminoethyl)amineviatheacyl
chloride using an adapted literature procedure to give ligand L3
in 62% yield (Scheme 1).29 The resulting ligand was sparingly
1
soluble in DMSO permitting characterisation by H NMR spec-
troscopy, and was observed to retain water, despite prolonged
drying in a cool oven (50 °C). L3 was subsequently bound to
ruthenium(II) by reaction with ruthenium trichloride via parallel
slow addition of the reagents, dissolved in DMSO and ethanol,
respectively, to a large volume of refluxing ethanol containing
silver nitrate. These high dilution conditions were employed to
minimize the formation of unwanted by-products of higher
nuclearity. The resulting red-brown solution was reduced in
volume, diluted with water and loaded directly onto a SP Sephadex
C-25 cation-exchange column, washing through with water
to remove the DMSO. The desired mono-nuclear complex was
then eluted from the column using aqueous 0.275 M sodium
chloride and isolated as the hexafluorophosphate salt in a
respectable yield of 57% with the structure being confirmed
using 1H NMR spectroscopy, CHN analysis and mass spec-
trometry. Unsurprisingly, the resulting solid product was
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Scheme 1 (i) SOCl2, (ii) tris(2-aminoethyl)amine, NEt3, dry tetrahydrofuran (THF), (iii) (2-amino-ethyl)-carbamic acid tert-butyl ester, NEt3, dry
THF, (iv) 3M aqueous HCl/ethyl acetate for 16 hours then 0.2 M aqueous NaOH, (v) 1,3,5-benzenetricarbonyl chloride, NEt3, dry THF, (vi)
RuCl3·xH2O in DMSO/EtOH/H2O in the presence of AgNO3 followed by KPF6.
retained considerable solvent and salt, despite repeated washing
with, and drying from, a variety of solvents. Complexation of L4
to ruthenium(II) was completed following the same procedure
used in the isolation of [Ru(L3)](PF6)2, and subsequently
purified by passage down a Sephadex LH-20 column and iso-
lated as a red solid with a disappointing yield of 17%. The low
yield is attributed to the extremely poor solubility of the tripodal
ligand, and the higher degree of flexibility exhibited by L4 over
L3 resulting in a greater chance of species with a higher nuclear-
ity being produced.
Crystals of [Ru(L4)]Br2, suitable for X-ray structural analysis
were grown by slow evaporation of an acetonitrile solution con-
taining tetrabutylammonium bromide. A riding model was again
used to fix the C–H hydrogens in the structure. The N–H and
water atoms were restrained using known lengths and angles and
disordered acetonitrile was removed using the SQUEEZE pro-
gramme45 (Fig. 3). As with [Ru(L3)](PF6)2, the Ru–N bond
lengths were between 2.028(6) to 2.080(6) Å and the internal
bipyridine N–Ru–N bond angles range from 78.1(2) to 78.5(2)
Å in concurrence with similar published values.26,29 Signifi-
cantly, the amide groups all point out of the central cavity with
torsion angles between the ring and the carbonyl function almost
ideally located at 9.5, 10. 9 and 15.7°. Both of the bromide ions
are located at surprisingly short distances from the amidic hydro-
Fig. 3 ORTEP plot of [Ru(L4)](Br)2 with ellipsoids at 50% prob-
ability. Hydrogen atoms and solvent have been deleted for clarity.
gens (N–Br separation of just 3.32 and 3.41 Å) and the adjacent
pyridine CH4s (3.63 and 3.71 Å), but outside of the cavity. Each
of the “faces” of the cavitand is further secured with water, held
in place by a network of hydrogen bonds (Fig. S1†). While the
cavity does not contain the bromide presumably because the
cavity is too big to accommodate the ions comfortably, it is
anticipated that in this case perhaps the cavity is more suited to
nitrate.
A range of other tripodal systems were also considered using
other tris-acid compounds, in a similar process used in the iso-
lation of our previously reported fac-orientated ester tripod but
unfortunately without success.27 Similarly, attempts were also
made to extend the cavity by the use of aromatic diamines
including 1,2-, 1,3- and 1,4-phenylene diamine, and whilst the
ligands could be isolated, the corresponding ruthenium com-
plexes were not formed.
The electronic absorption and emission spectra of [Ru(L3)]-
(PF6)2 and [Ru(L4)](PF6)2 (Table 1 and ESI Fig. 2†) are compar-
able with the [Ru(bipy)3](PF6)2.47 There are however a number
of interesting observations. The π–π* transition for the parent
complex [Ru(bipy)3]2+ is typically reported at 288 nm, with an
extinction coefficient of approximately 77 000 dm−3 mol−1
cm−1 48
With the addition of an amidic function at the 5 position
.
of the ring this is then bathochromically shifted to 293 nm for
the first amide [Ru(L2)3](PF6)2, and to 301 nm for the second
[Ru(L1)3] (PF6)2.26 This is not observed with [Ru(L3)](PF6)2
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Table 1 Photophysical properties in aerated CH3CN at 298 K
LC
MLTC
Emissiona
λmax 1/nm
ε/dm−3 mol−1 cm−1
λmax 1/nm
ε/dm−3 mol−1 cm−1
λmax 2/nm
Φem 5%
[Ru(L1)3](PF6)2
Mer-[Ru(L2)3](PF6)2
Fac-[Ru(L2)3](PF6)2
[Ru(L3)](PF6)2
301
293
293
281
286
81 800
66 410
66 270
50 720
55 600
482
468
470
470
458
7978
8351
8422
6570
7070
650
635
632
634
640
0.027
0.058
0.056
0.029
0.038
[Ru(L4)](PF6)2
a Excited at 450 nm, absorption 0.1, concentration typically in the range 1–2 × 10−5 mol dm−3, emission quantum yields (Φem) were calculated
relative to [Ru(bpy)3](PF6)2 as a standard in acetonitrile (0.062).31,32
and [Ru(L4)](PF6)2, where a notable drop in the extinction
coefficient and a broadening of the absorption is observed. This
would be in keeping with a distortion from the ideal planar
configuration of the carbonyl and pyridine and subsequent loss
of conjugation particularly in the case of the more strained
complex [Ru(L3)](PF6)2, and is consistent with previous obser-
vations in other reported cage structures.49 The MLCT absorp-
tions around 400 to 500 nm effectively follows the same pattern
for both the complexes, with an observed red-shift with respect
to the parent complex [Ru(bipy)3]2+ (452 nm)48 to 458 nm for
[Ru(L4)]2+ and then to 470 nm for [Ru(L3)]2+. For [Ru(L4)]2+
this is consistent with the cage like systems reported by de Cola
et al.,48–50 where there are competing influences causing strain
within the structure, both in terms of deviation from the assumed
pseudo-octahedral coordination geometry, and the degree of con-
jugation with the electron-withdrawing carbonyl groups. As has
been commented on previously, the very broad MLCT absorp-
tion arises from a number of overlapping bands, and subtle
changes in the importance of these bands can cause a consider-
able change in peak shape, and position of the maximum value.
It is also complicated by the energy that can accompany
vibrational relaxation, and interference from solvent,51 influences
that could be very important in a constrained system such as this.
The observed luminescence again is effectively red shifted with
respect to the parent species [Ru(bipy)3](PF6)2 (615 nm)49 and
there is a significant drop in the observed quantum yield consist-
ent with the inclusion of carbonyl groups at the 5 position, along
with the structural restrictions presented by a strained cage like
architecture.
Fig. 4 Percentage change in the emission quantum yield of [Ru(L4)]-
(PF6)2 (approx. 5 × 10−6 M, CH3CN, 25 °C) on the addition of [N(n-
C4H9)4] nitrate, hydrogen sulfate, bromide and chloride in aerated aceto-
nitrile at room temperature.
hydrogen sulfate with [Ru(L4)](PF6)2 accompanied by a signifi-
cant drop in the intensity of the main absorption bands.
Emission spectroscopic anion titrations again were carried out
in acetonitrile at 298 K using the TBA anions (nitrate, dihydro-
gen phosphate, hydrogen sulfate, chloride and bromide) using a
previously reported protocol (ESI Fig. S3 and S4†).52–55 The
observed changes in the luminescence during the titrations of
TBA nitrate, chloride and bromide with [Ru(L3)](PF6)2 could be
attributed to solvent dilution as the patterns observed resembled
the titration carried out with only acetonitrile, while the addition
of dihydrogen phosphate resulted in a slight decrease in lumines-
cence up to 1.5 equivalents, but of insufficient significance to
warrant detailed consideration. However, after the initial
additions the luminescence increased substantially probably
associated with the precipitation observed in the UV/vis absorp-
tion study. A similar pattern was observed with hydrogen sulfate
although a blue shift was observed after 3 equivalents. The titra-
tions of [Ru(L4)](PF6)2 with nitrate, hydrogen sulfate chloride
and bromide showed a significant decrease in the luminescence
(Fig. 4). The dihydrogen phosphate titration initially showed
a marginal decrease in luminescence of approximately 5%
Anion binding studies
UV/Vis spectroscopic anion titrations were attempted in aceto-
nitrile by the sequential addition of tetrabutylammonium nitrate,
dihydrogen phosphate, chloride, bromide and hydrogen sulfate
to both complexes [Ru(L3)](PF6)2 and [Ru(L4)](PF6)2. In both
cases the addition of anions gave no significant changes with
nitrate, chloride, and bromide, while there was an interesting
observation with dihydrogen phosphate in that there was little
change until approximately 5 equivalents were added, after
which a bathochromic shift of 5–10 nm was observed in both the
LC and the MLCT transitions, accompanied by an increased
background scattering, so is likely to result from precipitation as
observed previously noted with complex [Ru(L2)3](PF6)2.26
Similar behaviour was also demonstrated on the addition of
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1
Table 2 Observed change in chemical shift in the H NMR spectroscopic peak position on the addition of one equivalent of tetrabutylammonium
salts to[Ru(L4)](PF6)2 in CD3CN at 298 Ka
Proton/δ (ppm)
Anion
NH1
NH2
BipyH6
CH2(a)
CH2(b)
CH2(c)
CH2(d)
Chloride
Bromide
Nitrate
0.39
0.37
0.28
n/a
n/a
0.10
0.13
0.16
0.10
−0.19
−0.19
−0.24
0.06
0.07
0.07
−0.13
−0.13
−0.11
0.14
0.16
0.14
a Titration completed at a concentration of 2.5 × 10−6 mol dm−3
.
Table 3 Anion-binding stability constants for [Ru(L1)3](PF6)2, [Ru-
(L3)](PF6)2 and [Ru(L4)](PF6)2 in CD3CN at 298 K calculated using
WINEQNMR56 a
followed by an increase in luminescence along with a bathochro-
mic shift, apportioned to precipitation.
The observed changes in the emission are in stark contrast to
those observed by the complexes [Ru(L1)3](PF6)2 and mer/fac-
[Ru(L2)3](PF6)2 where a significant drop in the fluorescence is
observed on the addition of dihydrogen phosphate, while it has
been noted that the behaviour of similar species can vary
significantly,53–55 and appears to be very dependent on the pres-
ence of competing hydrogen bonding groups (such as water), as
well as precipitation. As the X-ray crystallographic study has
indicated, the observed “binding pocket” in complex [Ru(L3)]-
(PF6)2 appears to be too small, with the amides being consider-
ably strained from the ideal planarity by the tris(2-aminoethyl)
amine group. As a result, it does not bind to the anions and so
no quenching in the emission is observed. [Ru(L4)](PF6)2 on the
other hand does show an interaction with nitrate, chloride and
bromide which was not observed with the emission from [Ru-
(L1)3](PF6)2, [Ru(L2)3](PF6)2 or [Ru(L3)](PF6)2; this must arise
from the ability to hold anions in close proximity to the location
of the excited state. This could occur either in, or adjacent to,
the cryptand like binding cavity, with the indication that this
space appears to be too small to accommodate phosphate, which
then forms ion pairs in solution and results in significant
precipitation.
−
Cl−
Br− c
NO3
Host
pβ1
pβ2
pβ1
pβ2
pβ1
pβ2
b
[Ru(L1)3](PF6)2
[Ru(L3)](PF6)2
[Ru(L4)](PF6)2
5.5
4.0c
2.1
8.7
7.0c
4.5
4.2
4.0c
2.9
7.0
6.3c
4.5
3.7
3.6
6.4
6.1
5.4
9.2
a Errors estimated to be ≤5%; T = 298 K and consistent with the
analysis of three different protons within the complex to within 5%
variation. b Values taken from ref. 26. c Calculated only using the shifts
in the NH proton alone.
than observed in the latter complex where there are twice as
many opportunities for binding to the amidic protons. It is noted
that the data for the halide binding constants to [Ru(L3)]2+ could
only be calculated using just the chemical shift on the amide
proton as there were remarkably small observed changes in
chemical shifts elsewhere within the structure. From the data, it
is concluded that the cavity in [Ru(L3)]2+ appears to be too
small to permit anions to readily enter; the anions interact exter-
nal to the cavity giving a similar binding interaction to that
observed for mer and fac-[Ru(L2)3](PF6)2.26
1H NMR spectroscopic titrations in deuterated acetonitrile
were used to examine the interactions with a variety of tetrabuty-
lammonium salts including nitrate, dihydrogen phosphate,
bromide, chloride and hydrogen sulfate. The sequential addition
of dihydrogen phosphate and hydrogen sulfate to both [Ru(L3)]-
(PF6)2 and [Ru(L4)](PF6)2 unsurprisingly gave similar results;
both complexes began to precipitate on the addition of the first
aliquot which prevented detailed analysis (ESI Fig. S5 and S6†).
While other solvents were explored, including d6-DMSO and d4-
MeOD, in every case an almost stoichiometric precipitation was
observed, and in the later case the position of the amidic proton
could not be monitored due to deuterium exchange.
The addition of TBA nitrate, chloride and bromide to both
[Ru(L3)](PF6)2 and [Ru(L4)](PF6)2 resulted in a significant
downfield shift of the amide NH signal providing firm evidence
of an interaction occurring with these simple anions (ESI
Fig. S7–S9†, Table 2). Stability constants were calculated using
WINEQNMR,56 using an appropriate model to fit the data to
two anions with one cation in keeping with the previous studies
(Table 3).18,26,57 For complex [Ru(L3)](PF6)2, the anticipated
change in pβ2 is observed, with a decrease in the binding ability
of the complexes as the size of the anion increases, consistent
with the data observed for [Ru(L1)3]2+ although notably smaller
The titration with [Ru(L4)](PF6)2 gave a surprising result.
Firstly, the halide binding constants (Table 2) proved to be sig-
nificantly smaller than observed for either [Ru(L3)](PF6)2 or [Ru-
(L1)3](PF6)2, which would indicate that the amidic protons in
this complex are much less available for bonding of the halide
ions, despite reasonable changes observed in the chemical shifts
on the addition of these anions (Table 3) and the significant
quenching observed in the emission titrations. This could be as a
result of the observed rigidity in the structure, with water
retained in the cavity competing for the amidic protons, as
observed in the crystal structure of [Ru(L4)](Br)2, or alterna-
−
tively the structure has an unusual interaction with the PF6
which could potentially be included in the cavity. Certainly, the
X-ray structural analysis indicates that the cavity is ill-suited to
bromide, which binds externally to the faces of the cavity, rather
than within it. However the complex illustrates a very high selec-
tivity for nitrate, with a pβ1 of over 6. There are clear indications
in the 1H NMR spectrum that the bipyridine H6 proton is
strongly influenced by the addition of nitrate, while the four ali-
phatic signals from the diastereotopic alkyl CH2 protons show an
interesting behaviour; two experiencing a downfield shift and the
other two shifting up-field indicative of a change in the cavity
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conformation. From these changes we conclude that nitrate
enters the cavity giving a remarkably enhanced selectivity, pre-
sumably due to the complementary C3-symmetry and hydrogen
bonds between the two amidic protons on each of the tripodal
linkages.
2.50 mmol) was refluxed in SOCl2 (50 mL) under nitrogen until
all the solid had dissolved. The excess SOCl2 was removed by
distillation and the resulting solid dried in vacuo for 2 hours.
The solid was dissolved in dry THF (70 mL) and to this was
added a solution of tris(2-aminoethyl)amine (0.10 g, 0.69 mmol)
and Et3N (1 ml) in dry THF (30 mL) dropwise over 30 min. The
reaction was stirred under nitrogen for a further 16 hours at room
temperature. The solution was concentrated to approximately
10 mL and the resulting solid was collected by filtration and sub-
sequently steeped in a solution of saturated NaHCO3. The solid
was filtered and washed with distilled water (30 mL), EtOH
(30 mL) and diethyl ether (30 mL), giving a light brown solid
(yield 296 mg, 62%). Found: C: 62.13, H: 5.03, N: 18.23%,
Conclusions
The interactions of [Ru(L3)](PF6)2 and [Ru(L4)](PF6)2 with
phosphate salts are ambiguous, with an almost stoichiometric
precipitation observed in almost all cases. Whether this is a con-
sequence of a selective interaction, or a simple ion-pairing inter-
action is debatable, as [Ru(bipy)3]2+ also readily precipitates in
the presence of phosphate salts. The primary objectives within
this study were to determine if a tripodal cavity could be used to
gain selectivity for anions with a C3-symmetry. That certainly
appears not to be the case with the [Ru(L3)]2+, where the amidic
protons, typically associated with binding anions, are held
together by internal hydrogen bonding. This results in a small
strained unit that does not readily permit the inclusion of chlor-
ide as had initially been predicted. In fact this complex shows
very similar binding to mer-[Ru(L2)3]2+ where there is less
directionality and selectivity than the open C3 binding “pockets”
in fac-[Ru(L2)3]2+. However expansion in the size and rigidity
of the cavity, as demonstrated by complex [Ru(L4)]2+ leads not
only to an enhanced selectivity for nitrate over the smaller
halides chloride and bromide, but a surprising decrease in the
association with these anions, despite the inclusion in the struc-
ture of a larger number of amide protons. It is disappointing,
however, that despite the selectivity for nitrate, the emission is
still quenched to a greater extent by the halides.
1
C39H36N10O3·3H2O requires C: 62.71, H: 5.66, N: 18.75%; H
NMR (500 MHz, d6-DMSO): δ 8.99 (3H, s, H6), 8.63 (3H, d,
J = 4.0 Hz, H6′), 8.57 (3H, t, J = 5.0 Hz, NH), 8.31–8.29 (6H, m,
H3,3′), 8.21 (3H, d, J = 8.5 Hz, H4), 7.87 (3H, dd, J = 8.0, 7.5
Hz, H4′), 7.44 (3H, dd, J = 8.0, 5.0 Hz, H5′), 3.46 (6H, dd, J =
a
b
11.5, 5.0 Hz, CH2 ), 2.78 (6H, t, J = 11.5 Hz, CH2 ); 13C NMR
(125.7 MHz, d6-DMSO): δ 164.6, 156.8, 154.2, 149.1, 148.0,
137.1, 135.7, 129.5, 124.4, 120.8, 119.6, 52.8; EI-MS (m/z) 183
[BpyCO]+; IR (KBr disc) cm−1: 3390, 3286 (br, NH stretch),
1636 (CvO stretch), 766, 754 (s, unattributed).
1,3,5-Benzene tricarboxylic acid tris-({2-[([2,2′]bipyridinyl-5-
carbonyl)-amino]-ethyl}-amide) (L4). A solution of 1,3,5-benze-
netricarbonyl chloride (0.24 g, 0.88 mmol) in dry THF (40 mL)
was added to a stirring solution of 2,2′-bipyridyl-5-carboxylic
acid (2-amino-ethyl) amide (0.65 g 2.67 mmol) and triethyl-
amine (5 mL) in dry THF (40 mL) under nitrogen. A white sus-
pension formed. The reaction was stirred for 16 hours at room
temperature. The resulting solid was collected by filtration. The
product was suspended in saturated aqueous NaHCO3 solution
(100 mL) and gently shaken for 10 min and the remaining solid
collected by filtration, and washed with ethanol repeatedly until
a “dry” appearance was obtained (yield 0.43 g, 55%.). Found: C:
58.95, H: 5.56, N: 15.45%. C48H42N12O6·3(MeOH)·(NaHCO3)
Experimental
Physical measurements
NMR spectra were recorded using Bruker DPX300 (ligands and
titrations) and DRX500 (complexes) spectrometers. Electronic
absorption studies were performed with a Perkin-Elmer Lambda
800 spectrophotometer and emission spectra were recorded on a
Perkin-Elmer LS55 spectrofluorimeter, having adjusted the
adsorption at the excitation wavelength to be 0.1. Microanalysis,
and E.S. and E.I. mass spectroscopy were performed by
A.S.E.P., the School of Chemistry and Chemical Engineering,
Queen’s University Belfast.
1
requires C: 58.75, H: 5.21, N: 15.81%; H NMR (500 MHz, d6-
DMSO): δ 9.91 (3H, br, NHa), 9.10 (3H, s, H6), 8.89 (3H, br,
NHb), 8.71 (3H, d, J = 4.7 Hz, H6′), 8.48–8.41 (9H, m, H3,3′ and
ArH), 8.34 (3H, d, J = 8.0 Hz, H4), 7.96 (3H, dd, J = 8.0 Hz,
7.5 Hz, H4′), 7.48 (3H, dd, J = 4.7 Hz, 7.5 Hz, H5′), 3.51 (12H,
s, CH2); 13C NMR (125.7 MHz, d6-DMSO): δ 165.6, 164.7,
157.0, 154.3, 149.3, 148.2, 137.3, 136.1, 134.8 (q), 129.8, 128.5
(q), 124.5, 120.9, 119.7; EI-MS (m/z): 181 [BpyCOH]+. IR (KBr
disc) cm−1: 3281 (Br, NH stretch), 1646, 1593, 1540 (CvO
stretch).
Materials
Acetonitrile, potassium hexafluorophosphate and ethylene glycol
were obtained from Aldrich and used as supplied. Silica gel 60
PF254 was supplied by Merck and RuCl3·3H2O by Strem. 2,2′-
Bipyridine-5-carboxylic acid,58 BOC protected ethylene
diamine59 and 2,2′-bipyridyl-5-carboxylic acid (2-amino-ethyl)
amide were prepared according to literature procedures.46
[Ru(L3)3](PF6)2. A solution of RuCl3·3H2O (45 mg,
0.22 mmol) and L3 (0.15 g, 0.22 mmol) in DMSO (5 mL) and
ethanol (15 mL) was added to refluxing ethanol (500 mL) and
AgNO3 (37 mg) using a peristaltic pump over 2 hours. The reac-
tion was refluxed for a further 20 hours giving a red solution.
The reaction was cooled and NaCl (0.50 g) was added. The
AgCl was removed by filtration and the filtrate was concentrated
to 50 mL. The residue was diluted with distilled water (250 mL)
and the product isolated by cation exchange chromatography on
SP Sephadex C-25. The crude product was eluted from the
column with 0.25 aqueous NaCl. KPF6 (200 mg, 10.8 mmol)
Preparations
N,N,N-Tris([2,2′]bipyridinyl-5-carboxylic acid(2-amidoethy-
lene)) amine (L3). 2,2′-Bipyridine-5-carboxylic acid (0.50 g,
7010 | Dalton Trans., 2012, 41, 7005–7012
This journal is © The Royal Society of Chemistry 2012

View Article Online
was added to the first major red fraction and the product was
extracted into DCM (3 × 50 mL). The solvent was concentrated
giving a red solid. The product was recrystallized from acetone
and methanol (yield 133 mg, 0.12 mmol, 57%). Found: C:
33.00, H: 3.07, N: 8.80%, C39H36N10O3RuP2F12. 2(KPF6)2·2
(MeOH) requires C: 33.54, H: 2.88, N: 9.10%; 1H NMR
(300 MHz, CD3CN): δ 8.50 (6H, d, J = 8.1 Hz, H3,3′), 8.27 (3H,
d, J = 8.4 Hz, H4), 8.12 (3H, dd, J = 8.4, 7.8 Hz, H4′), 7.88 (3H,
d, J = 5.7 Hz, H6′), 7.70 (3H, s, H6), 7.57 (3H, dd, J = 7.8,
b = 14.048(2) Å, c = 16.370(3) Å, α = 65.572(9)°, β = 89.168(7)°,
γ = 62.936(8)°, U = 2458.2(7) Å3, Z = 2, μ = 0.475 mm−1
,
T = 120 K, a total of 30 256 reflections were measured for the
angle range 2.96–27.48°, 15 445 independent reflections were
used in the refinement (Rint = 0.1039). The final parameters
were wR2 = 0.2851 and R1 = 0.1316 [I > 2σI], 1.042.
Crystal data for [Ru(L4)]Br. Crystals suitable for X-ray analy-
sis were grown by evaporation of approximately a 0.5 mM sol-
ution CD3CN solution of [Ru(L4)](PF6)2 in the presence of 10
a
5.7Hz, H5′), 6.98 (3H, br, NH), 3.55–3.36 (6H, m, CH2 ), 2.94
b
(6H, m, CH2 ); 13C NMR (125.7 MHz, CD3CN): δ 164.9,
equivalents
of
tetrabutyl
ammonium
bromide;
ˉ
C48H48Br2N12O9Ru: M = 1197.87, triclinic, space group P1, a =
12.9778(8) Å, b = 15.1648(11) Å, c = 18.1787(14) Å, α =
101.915(3)°, β = 98.313(4)°, γ = 92.738(4)°, U = 3452.7(4) Å3,
Z = 2, μ = 1.434 mm−1, T = 120 K, a total of 47 358 reflections
were measured for the angle range 2.94–27.48°, 15 327 indepen-
dent reflections were used in the refinement (Rint = 0.0623). The
final parameters were wR2 = 0.2304 and R1 = 0.0958 [I > 2σI],
GOF 1.065.
160.6, 157.4, 148.8, 139.1, 136.9, 135.7, 129.3, 125.8, 53.8,
37.7; LSIMS-MS (m/z) : 939 [M − PF6]+, 793 [MH − 2PF6]+
(corresponds to the modelled isotopic pattern for
C39H36N10O3Ru).
[Ru(L4)](PF6)2. Was prepared by a similar procedure to that
used for [Ru(L3)3](PF6) in an ethanol (300 mL)/distilled water
(100 mL) mixture using L4 (0.15 g, 0.17 mmol) and RuCl3·H2O
(35 mg, 0.17 mmol). The product was eluted from a SP Sepha-
dex C-25 cation-exchange column with 0.275 M aqueous NaCl.
A saturated aqueous KPF6 solution was added to the resulting
first major fraction and the solvent removed under reduced
pressure. Inorganic salts were removed by dissolving the product
in methanol and removing the insoluble white solid by filtration
from the solution. The solvent was removed and the process
repeated before the red compound was then passed through a
Sephadex LH-20 column eluted with a 1 to 1 acetone–methanol
mixture, followed by precipitation from an acetone–water
mixture by evaporation in the presence of a small quantity of
aqueous KPF6 solution, to obtain a red solid (yield = 36 mg,
Stability constant determination
1
The determination of stability constants from the H NMR data
was achieved using WINEQNMR,56 and an appropriate model
to fit the data to two anions with one cation. Typically, stability
constants would be calculated from the average obtained follow-
ing the analysis of four different protons (unless otherwise
stated) and the standard deviation of the experimental points to
the calculated curve in all cases is less than 5%.
17%).
Found:
C:
32.96, H:
3.03,
N:
8.05%,
Acknowledgements
C48H42N12O6RuP2F12·3(KPF6)·2(Acetone)·2(H2O) requires: C:
33.79, H: 2.96, N: 8.50; H NMR (300 MHz, CD3CN): δ 8.53
1
We thank QUESTOR and ChemVite Ltd for the studentship to
NCAB.
(3H, d, J = 8.4 Hz, H3), 8.48 (3H, d, J = 8.1 Hz, H3′), 8.31 (3H,
d, J = 8.4 Hz, H4), 8.18 (3H, s, H6), 8.04 (3H, dd, J = 8.1,
7.8Hz, H4′), 7.88 (3H, s, ArH), 7.65 (3H, d, J = 5.1 Hz, H6′),
7.36 (3H, br, NHa), 7.42–7.34 (6H, m, NHb, pyH5′), 4.74 (3H,
m, CHb), 4.00 (3H, m, CHa), 3.35 (3H, d, J = 13.8, CHc), 3.11
(3H, d, J = 13.5, CHd); LSI-MS (m/z): 1129 [M − PF6]+, 985
[MH − 2 PF6]+ (corresponds to the modelled isotopic pattern for
C48H43N12O6Ru).
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