New insights into dihydrogenphosphate recognition with dirhenium(i) tricarbonyl complexes bridged by a thiourea moiety

Article abstract of DOI:10.1039/c4ra00912f

Three thiourea bridged 2,2′-bipyridine ligands bearing either a single thiourea group (L1), or two units separated by either a para (L2) or meta-substituted (L3) aromatic spacer, along with the corresponding bis(fac-tricarbonylrhenium(i)) complexes are reported. The three ligands all show the anticipated binding to acetate. However ¹H NMR titrations reveal an unusual cooperative binding to, and selectivity for, two dihydrogenphosphate ions. The rhenium(i) complexes similarly demonstrate unusual sigmoidal titration curves, and in the case of {Re(CO)₃Br} ₂(μ-L1) a surprisingly strong interaction to two anions. These were further exemplified in the emissive behaviour leading to the conclusion that there is an unusual interaction with dihydrogenphosphate, giving an initial increase in the emission, followed by a decrease and a blue shift in wavelength possibly as a result of partial deprotonation. It appears that dihydrogenphosphate binds cooperatively, with the addition of a second anion enhancing the interaction of the first, probably by proton transfer; this could explain the remarkable selectivity for phosphate seen with many reported anion receptors.

Full text of DOI:10.1039/c4ra00912f

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New insights into dihydrogenphosphate
recognition with dirhenium(I) tricarbonyl
Cite this: RSC Adv., 2014, 4, 18442
complexes bridged by a thiourea moiety†
*
Anna L. Blackburn, Naomi C. A. Baker and Nicholas C. Fletcher
Three thiourea bridged 2,20-bipyridine ligands bearing either a single thiourea group (L1), or two units
separated by either a para (L2) or meta-substituted (L3) aromatic spacer, along with the corresponding
bis(fac-tricarbonylrhenium(I)) complexes are reported. The three ligands all show the anticipated binding
1
to acetate. However H NMR titrations reveal an unusual cooperative binding to, and selectivity for, two
dihydrogenphosphate ions. The rhenium(I) complexes similarly demonstrate unusual sigmoidal titration
curves, and in the case of {Re(CO)3Br}2(m-L1) a surprisingly strong interaction to two anions. These were
further exemplified in the emissive behaviour leading to the conclusion that there is an unusual
interaction with dihydrogenphosphate, giving an initial increase in the emission, followed by a decrease
Received 31st January 2014
Accepted 7th April 2014
and
a blue shift in wavelength possibly as a result of partial deprotonation. It appears that
dihydrogenphosphate binds cooperatively, with the addition of a second anion enhancing the interaction
of the first, probably by proton transfer; this could explain the remarkable selectivity for phosphate seen
with many reported anion receptors.
DOI: 10.1039/c4ra00912f
www.rsc.org/advances
by Kitchen et al. highlights a tris-bipyridine complex of ruth-
enium(II) bearing an appended aryl urea where the emission can
Introduction
be “turned on” by the addition of orthophosphate, but
decreased with pyrophosphate.27 Custelcean and co-workers
have also highlighted that de novo computer-aided design can
be used to design a sulfate receptor via the formation of a M4L6
tetrahedral cage using a simple linear ligand with two bipyr-
idine ligands bridged by a methylene urea group (Fig. 1a) which
in the presence of nickel(II) was able to extract tetrahedral
oxoanions from aqueous solution.28,29
The selective recognition of anions has developed over the last
two decades into a vibrant area of research1 with considerable
emphasis placed on the selective recognition of phosphate salts
due to their prevalence in biology.2,3 One successful approach is
the use of metal-based receptors, where positive electrostatic
interactions can be combined with an acidic proton to
encourage hydrogen bonding,4 and then exploit a change in the
redox, photophysical or magnetic properties to create a
sensor.5–7 Anion selective systems able to operate in aqueous
media remains a considerable challenge however.8,9 The inclu-
sion of urea groups has proved to be particularly successful in
the
recognition
of
uoride,
acetate
and
dihy-
drogenphosphate10–12 as shown by the development of systems
involving two13–15 or three urea or thiourea moieties,16–18
including tripodal architectures.19–25 These demonstrate
considerable selectivity for specic anions, even in protic
media.
In the creation of metal-complex anion receptors, 2,20-
bipyridine has been widely exploited,26 however the number of
situations where it has been combined with a urea function
remains surprisingly small. A notable recent example reported
The School of Chemistry and Chemical Engineering, Queen's University Belfast,
Belfast, BT9 5AG, UK. E-mail: n.etcher@qub.ac.uk; Fax: +44 (0)28 9097 6524; Tel:
+44 (0)28 9097 5479
† Electronic supplementary information (ESI) available: Spectroscopic
characterization and titration data. See DOI: 10.1039/c4ra00912f
Fig. 1 (a) Ligand system previously reported by Custelcean and co-
workers28,29 and (b) the dirhenium complex reported by Hanan.30
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Organometallic complexes bearing appropriate hydrogen 5-isothiocyanatomethyl-2,20-bipyridine and 5-aminomethyl-
bond donor groups have been the feature of a number of anion 2,20-bipyridine, exploiting pyridine as both solvent and base,
bonding studies.31–33 One family of metal complexes that has resulted in the ditopic ligand L1 (Scheme 1) in 73% yield, the
1
shown considerable promise in the design of emissive sensors identity of which was conrmed by H NMR spectroscopy and
has been that of the fac-rhenium(I) tricarbonyl diimine frag- electrospray mass spectrometry. Subsequent complexation of
ment, where the sixth coordination site can accommodate ligand L1 with Re(CO)5Br was initially attempted in toluene, but
either a neutral ligand to give a monovalent cation, or a halide the poor solubility of the ligand resulted in the necessity to use
to provide a neutral complex. Anion recognition has been dry DMSO, which was subsequently removed by vacuum
observed in a variety of complexes,34–40 and previously we have distillation and recrystallization from hot toluene giving the
shown that large perturbations in the metal-to-ligand excited product as a bright yellow solid in 62% yield. This was charac-
state luminescence can be achieved on a successful anion terised by 1H NMR spectroscopy, highlighted by a considerable
recognition event.41,42 In our studies we noted that dihy- downeld shi for the bipyridine proton peaks in comparison
drogenphosphate can disrupt an unconventional hydrogen to the spectrum of L1 (Fig. S1 and S2†), and by electrospray
bond from an amide proton to the metal carbonyl. The group of mass spectrometry with a peak at 1110.8530 corresponding to
Lees has also highlighted that dirhenium(I) carbonyl complexes the protonated complex.
with polarized N–H groups, including amidic, thioamidic and
To investigate whether the system could be extended, and to
thiourea bridges, display outstanding sensitivity and selectivity introduce two anion binding centres, isothiocyanatomethyl-
toward a variety of anionic species including acetate and 2,20-bipyridine was also reacted with a range of aromatic
cyanide.43 In another recent example, Hanan30 and co-workers diamines but without success, with only single thiourea adducts
have described a dirhenium species bridged by two thiourea being formed. It was decided therefore to react 5-aminomethyl-
groups (Fig. 1b) which demonstrates a strong affinity for uo- 2,20-bipyridine with the corresponding diisothiocyanates. 1,4-
ride, acetate and dihydrogenphosphate. Interestingly, this latter Phenylenediisothiocyanate was readily prepared following
anion showed an unusual situation where the binding of the Wong's procedure48 however the 1,3-functionalised analogue
second anion appeared to be counter intuitively greater than the could not be isolated, but was obtained using thiophosgene via
binding of the rst anion.
a published method.49 Ligands L2 and L3 (Scheme 2) were
Thiourea offers even greater opportunities for anion recog- subsequently prepared in the presence of triethylamine in 70
nition than urea itself due to the enhanced acidity of the N–H
groups,44 and the observation that it readily permits deproto-
nation with a number of anionic species. Consequently it offers
opportunities to demonstrate remarkably strong anion recog-
nition and possibilities to design sensors capable of operating
in aqueous media. To further explore these phenomena, we
report here the synthesis of the thiourea analogue of the ligand
recently reported by Custelcean et al. and its dirhenium(I)
complex.28,29 We then extend this to two systems which include
two thiourea groups, with the intention of understanding the
remarkable preference for dihydrogenphosphate over other
tetrahedral oxo-anions with what supercially look like simple
linear luminescent complexes.
Results and discussion
Synthesis
The preparation of thiourea and urea adducts derived from 2,20-
bipyridine complexes are not common in the literature,28,45 and
a new synthetic strategy has been developed via an unreported
isothiocyanate derivative from 5-aminomethyl-2,20-bipyridine.46
This was achieved in reasonable yield via the amination of
5-chloromethyl-2,20-bipyridine46 rather than the more tradi-
tional route through the analogous bromo adduct47 which has
proved to be a severe irritant, and in our experience is not a
procedure that can be easily scaled-up to give multi-gram
quantities. Isolation of 5-isothiocyanatomethyl-2,20-bipyridine
Scheme 1 Synthetic pathway to {Re(CO)3Br}2(m-L1); (i) hexamethyle-
netetraamine, DCM, reflux 16 h, then 12 M HCl, reflux 16 h, (ii) CS2,
NEt3, dry THF stirred for 16 h followed by tosyl chloride and stirred for
16 h, (iii) 5-aminomethyl-2,20-bipyridine, pyridine, reflux 48 h, and (iv)
was readily achieved following the procedure reported by Wong
et al.48 via the tosyl chloride mediated decomposition of a
dithiocarbamic acid salt in 61% yield, avoiding the more
traditional route via thiophosgene. Subsequent combination of [Re(CO)5Br], DMSO, 75 ꢀC for 16 h under N2.
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{Re(CO)3Br}2(m-L3) revealed the anticipated ligand centred (LC)
p–p* transition at approximately 290 nm (Table 1), a shoulder
at 320 nm, and a broad metal-to-ligand-charge-transfer (MLCT)
absorption in the range of 370 to 390 nm (Fig. S7†). The cor-
responding emission spectra of all of the complexes show a
weak luminescence around 610 nm (excited at 380 nm) and a
quantum yield of approximately half that of [Re(CO)3(bpy)Br]
(Fig. S8†).50
1H NMR anion binding studies – ligands
Studies were undertaken to screen compound L1's behaviour in
1
the presence of a variety of common anions by H NMR spec-
troscopy in a 50% mixture of CD3CN and DMSO-D6. This
revealed that there is no change in the relative peak position of
any of the observed signals, including that of the thiourea NH
resonance, with the addition of up to ten equivalents of the
tetrabutylammonium (TBA) salts of NO3ꢁ, Brꢁ and HSO4
,
ꢁ
while the addition of the corresponding chloride salt leads to a
small downeld shi of the thiourea proton signal (Dd ¼ 0.25
ppm with ten equivalents, Fig. 2a and S9†). However a
sequential titration with uoride causes the complete loss of the
NH signal, consistent with proton exchange, or even full
deprotonation (Fig. S9†).44,51,52 Acetate gives a shi in peak
position permitting the determination of a binding constant
(pb1 ¼ 2.8) using a simple one to one host to anion model
(WINEQNMR253 Fig. S10a†), which is typical for a simple
hydrogen bond pairing in this competitive solvent mixture
(Table 2).54 TBA H2PO4 also results in a denite change in the 1H
NMR spectrum (Fig. S10b†), however the nature of the observed
shi of the thiourea protons is notably different from a typical
anion titration, as seen here with acetate (Fig. 2a). At low
concentration there is little movement in the NH peak position,
but upon the addition of one equivalent there is a much larger
shi giving an unusual sigmoidal curve suggestive of a coop-
erative effect. A model combining two dihydrogenphosphate
ions to the one ligand gives a good reproducible t (pb1 ¼ 2.6
Scheme 2 Synthetic pathway to L2 and L3.
and 86% yield respectively using a similar procedure as
described for L1. These two ligands were also successfully
bonded to two Re(CO)3Br moieties in DMSO, and characterised
by 1H NMR spectroscopy (Fig. S3–S6†) and high resolution
electrospray mass spectrometry with the molecular ion less one
bromide being observed at 1182.9620 and 1182.9681 respec-
tively (theoretical 1182.9689) with an appropriate isotopic
distribution.
The rhenium complexes all proved to be bright yellow in
colour and the UV/vis spectrum of {Re(CO)3Br}2(m-L2) and
Table 1 Photophysical properties of the isolated rhenium(I) complexes
Absorptiona
Emissiona
Complex
lmax ꢂ 2 nm
3 ꢃ 103 dmꢁ3 molꢁ1 cm
lmax ꢂ 2 nm
3 ꢃ 103 dmꢁ3 molꢁ1 cm
lmax ꢂ 2 nm
Fem
Re(CO)3Br(Bpy)
294
304
317
298
308
321
296
308
321
292
308
321
8.55
8.20
7.33
26.4
(sh)
(sh)
42.2
(sh)
(sh)
28.1
(sh)
(sh)
377
3.02
579
7.8 ꢃ 10ꢁ3
{Re(CO)3Br}2(L1)
{Re(CO)3Br}2(L2)
{Re(CO)3Br}2(L3)
375 (sh)
375 (sh)
380 (sh)
3.38
4.85
5.26
612
606
608
4.8 ꢃ 10ꢁ3
4.0 ꢃ 10ꢁ3
6.4 ꢃ 10ꢁ3
a
Recorded in aerated CH3CN at 298 K, excited at 380 nm, emission quantum yields (Fem) were calculated relative to [Re(CO)3Br(bpy)] (7.8 ꢃ 10ꢁ3) in
acetonitrile.50
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Table 2 Anion-binding stability constants for L and [{Re(CO)3Br}L] in
50% acetonitrile/DMSOa
OAcꢁ
H2PO4
ꢁ
Host
pb1
pb2
pb1
pb2
6.1
8.3
8.0
L1
L2
L3
2.8
3.8
3.5
3.2
4.4
5.0
—
2.6
5.1
4.7
6.0
4.4
—
6.9
7.8
[{Re(CO)3Br}2L1]
[{Re(CO)3Br}2L2]
[{Re(CO)3Br}2L3]
6.9
>10
b
b
3.8
7.4
a
Errors estimated to be #5%; T ¼ 298 K. Data obtained from the
b
movement of the CH2NH signal. Data unavailable due to loss of the
NH signal.
uoride again causes the loss of the NH resonance consistent
with a high degree of deprotonation. The addition of the cor-
responding chloride salt leads to a small downeld shi of the
thiourea protons signal (Dd ¼ 0.25 ppm with ten equivalents)
(Fig. 2b, 2c, S12a and S13a†). A signicant and predictable
interaction is observed with acetate, which ts well to a simple
two to one stoichiometry, and it is assumed that one acetate
binds to each of the two thiourea groups with reasonable pb2
values consistent with the literature54 (pb2 ¼ 6.0 and 4.4 for L2
and L3 respectively; Fig. S12b and 13b†). The observed differ-
ence in the behaviour of the two compounds probably arises
from the differences in the steric constraint resulting from the
para and meta-functionalised spacers (L2 and L3 respectively).
The addition of TBA dihydrogenphosphate to L2 and L3 both
result in a very similar sigmoidal titration curves for both of the
two thiourea NH resonances, similar in shape to that exhibited
by L1 (Fig. S12c and 13c†). Using a two to one model (similar to
that used for acetate), binding constants of approximately pb1
z 5 and pb2 z 8 were obtained for both complexes
(WINEQNMR253). While this does not indicate cooperativity in
itself, there is denitely something unexpected occurring giving
Fig. 2 1H NMR spectroscopic change in NH peak position on the
introduction of TBA OAc (:), TBA Cl (-) and TBA H2PO4 (C); (a) L1, (b)
L2, and (c) L3 (50% DMSO-D6–CD3CN 400 MHz, 298 K, at approx. 1 ꢃ
10ꢁ3 mol dmꢁ3).
and pb2 ¼ 6.1) conrming a surprisingly high degree of coop-
erativity on the introduction of a second dihydrogenphosphate
ion. To further demonstrate this surprising stoichiometry, a 1H
NMR Job plot analysis55 was undertaken (Fig. 3, S11 and Table
S1†) which is highly suggestive of the proposed two anions to
one ligand stoichiometry.
1
For L2 and L3, the H NMR spectroscopic titrations behave
in a similar manner to that observed with L1, with no interac-
tion observed with the introduction of up to ten equivalents of
TBA NO3, Br and HSO4, while with L2 a sequential titration with 1.26 mmol dmꢁ3, 50% DMSO-D6–CD3CN 400 MHz, 298 K).
Fig. 3 Job plot analysis in the 1H NMR spectroscopic change in NH
peak position on the introduction of TBA H2PO4 (total concentration
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rise to the sigmoidal shape to the titration curve. There is a very
small difference in the NH resonances associated with the rst
anion binding, typically Dd ¼ 0.3 ppm on both the NH signals,
while the second anion causes a more dramatic change of the
order of Dd z 1.5 ppm (depending on the sample and the NH
resonance considered). Further, aer one equivalent of the
dihydrogenphosphate with ligand L3, there was considerable
perturbation on the bridging aromatic group with a dramatic
down-eld shi for the signal attributed to H4 and H6, while H2
and H3 demonstrated movement in an up-eld direction
(Fig. S13c†). For all these samples, dilution of the compound
did not result in a change in the peak position of any of the 1H
resonances indicating that this surprising phenomenon is not
as a result of an aggregation. However considering the behav-
iour observed with L1, and the fact that this sigmoidal shape in
the titration curve is not seen with the addition of acetate, along
with the rigid separation between the two thiourea groups in L2
and L3, the situation is probably a little more complicated and
will be discussed subsequently.
1H NMR anion binding studies – complexes
With the inclusion of the metal fragment, the rhenium
1
complexes were similarly investigated using H NMR spectro-
scopic anion titrations under the same conditions used with Lꢁ1
to L3. {Re(CO)3Br}2(m-L1) with NO3ꢁ, HSO4ꢁ, Brꢁ and ClO4
salts resulted in no perturbation to the spectra (in 50% CD3CN–
DMSO-D6; Fig. S14†), as seen with the free ligand L1, while
chloride exhibits a slight deviation in the thiourea peak position
(Dd ¼ 0.2 ppm with ten equivalents; Fig. 4 and S14†). Surpris-
ingly, uoride shows no immediate change in the spectrum, but
once one equivalent had been added, a small downeld shi is
observed before the peaks broaden and become uninterpretable
consistent with proton exchange.44,51,52 With acetate, a similar
behaviour is found to that of the free ligand (Fig. 4a and S15a†),
with a binding constant, pb1 ¼ 3.2, assuming a one to one
stoichiometry, suggesting that the presence of the two Re(CO)3
groups has a marginal affect on the affinity for acetate.
With the introduction of TBA H2PO4 to {Re(CO)3Br}2(m-L1) in
a 50% DMSO-D6–CD3CN mixture, there is a signicant down-
eld shi of the NH proton signal (Fig. S15b†), and a change in
the methylene peak position, with the singlet broadening and
eventually becoming a multiplet implying a conformational
change and a more rigid arrangement around the exible
methylene groups – i.e. they become conformationally frozen
making the two protons diastereotopic. A slight downeld shi
Fig. 4 1H NMR spectroscopic change in CH2NH peak position on the
introduction of TBA OAc (:), TBA Cl (-) and TBA H2PO4 (C); (a)
{Re(CO)3Br}2(m-L1), (b) {Re(CO)3Br}2(m-L2), and (c) {Re(CO)3Br}2(m-L3);
(50% DMSO-D6–CD3CN) 400 MHz, 298 K, at approx. 1 ꢃ 10ꢁ3 mol
dmꢁ3
.
For the complexes {Re(CO)3Br}2(m-L2) and {Re(CO)3Br}2(m-
in the peaks attributed to the 3 and 4 positions on the bipyr- L3), they again show no interaction with NO3ꢁ, HSO4ꢁ, Brꢁ and
idine is also noted. A value of pb2 of over 10 is obtained by ClO4 salts in 50% CD3CN–DMSO-D6, while similarly chloride
ꢁ
modelling the system to a two to one stoichiometry conrming exhibits only a slight deviation in the thiourea peak position
a surprisingly strong interaction with the dihydrogenphosphate (Fig. 4b, 4c, S16a and S17a†). Tentative calculations of pb2 for
(Table 2). Given that these values are too high to be accurate, the chloride are of the order of 2, but given the size of the change in
titration was repeated in DMSO-D6 with 0.5% H2O, however in peak position in both damp DMSO-D6, and 50% CD3CN–DMSO-
the more protic environment there is a rapid loss of the N–H D6, precise values cannot be accurately determined (Dd ¼ 0.2
proton signal consistent with proton exchange. Under the same ppm with ten equivalents). With acetate the complexes show
conditions, the presence of the fac-tricarbonyl rhenium(I) centre similar behaviour in binding to the rst anion to that displayed
results in an enhanced interaction to this anion when compared by the free ligands L2 and L3 respectively. However the associ-
to the free ligand (L1).
ation to the second anion is considerably greater with the
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complex, presumably due to the inuence of the metal centre
and associated carbonyl groups (Fig. S16b and S17b†).
The 1H NMR spectroscopic titrations with {Re(CO)3Br}2(m-L2)
and TBA dihydrogenphosphate were not successful. While it
can be seen from the change in position of the visible NH
protons signals that an interaction is occurring (Fig. 4b and
S16c†), with the addition of over one equivalent of the anion,
the peak broadens to such an extent that it effectively disap-
pears, although the start of a sigmoidal curve similar to that
seen with the free ligand, and in keeping with the other dihy-
drogenphosphate titrations, is in evidence. Similar behaviour is
also observed in DMSO-D6 with 0.5% H2O. With {Re(CO)3Br}2(m-
L3) on the other hand, while the signal broadens indicative of a
degree of proton exchange in the solvent system used, it can be
followed sufficiently to again highlight a sigmoidal curve
(Fig. 4c, S17c†) and gives similar stability constants (pb1 ¼ 3.8
and pb2 ¼ 7.4) to that of the free ligand L3 suggestive of a degree
of cooperativity in the binding of the second anion. And again,
there are similar perturbations to the aromatic linkage as seen
with ligand L3 itself.
UV/vis and emission anion binding studies
The complexes {Re(CO)3Br}2(m-L1), {Re(CO)3Br}2(m-L2) and
{Re(CO)3Br}2(m-L3) UV/vis absorption behaviour with a range of
TBA salts in acetonitrile prove again that NO3ꢁ, HSO4ꢁ, Brꢁ, Clꢁ
and ClO4ꢁ salts result in little or no perturbation to the spectra,
while the addition of TBA uoride, shows a decrease in the LC
transition (Fig. S18†). Similar behaviour is seen by UV spec-
troscopy with acetate (Fig. S19†) and to a lesser extent with
dihydrogenphosphate indicative of a degree of selectivity.
Fig. 5 The change in emissive behaviour of (a) {Re(CO)3Br}2(m-L1) on
the introduction of TBA H2PO4 and (b) the change in integrated area
(CD3CN, 298 K, at approx. 1 ꢃ 10ꢁ5 mol dmꢁ3).
However the observed change in the spectra is very marginal complex, {Re(CO)3Br}2(m-L1), initially gives a slight increase in
and is probably best related to the capacity of the anion to affect the luminescence, before a gradual decrease. This is under-
deprotonation of the thiourea, rather than a measꢁure of a stood to be binding, followed by a degree of deprotonation
ꢁ
“recognition” event with Fꢁ > OAcꢁ > H2PO4 > Cl .44,51 An (Fig. S23a†). However for {Re(CO)3Br}2(m-L2) a signicant
attempt was made to complete
a Job plot analysis of quenching in the emission is observed in a similar fashion to
{Re(CO)3Br}2(m-L1) in the presence of TBA dihy- that seen with uoride. It is therefore assumed that the pres-
drogenphosphate, while examining the concentration corrected ence of the aryl group makes the NH group more acidic in
change in the absorption at both 235 nm and 300 nm. Given the agreement with previous studies.30,44
very marginal perturbations in the UV absorption, the data is
With the addition of TBA dihydrogenphosphate, the emis-
rather tentative but would appear to illustrate the proposed two sion titrations result in interesting behaviour suggestive of two
to one stoichiometry observed with the ligand, and proposed for competing processes. With {Re(CO)3Br}2(m-L1), an increase in
the complexes (Fig. S20†).
luminescence (Fig. 5a) is observed alongside a slight blue shi
There are no observed perturbations in the emission spectra of the order of 10 nm. There is a subsequent gradual decrease; a
of {Re(CO)3Br}2(m-L1), {Re(CO)3Br}2(m-L2) and {Re(CO)3Br}2(ꢁm- plot of the integrated spectra (Fig. 5b) reveals an unusual
L3) (excited at 380 nm) on the addition of NO3ꢁ, Brꢁ, and ClO4
.
sigmoidal titration curve, with a two to one ratio conrming the
Chloride salts result in a very slight drop in the emissive 1H NMR data. With the more extended system {Re(CO)3Br}2(m-
behaviour in the order of 2% quenching for example with L2), the titration results in a reproducibly small initial increase
ꢁ
{Re(CO)3Br}2(m-L1) (Fig. S21†). HSO4 gives a slight increase in luminescence (up to 0.7 equivalents) followed by a signicant
followed by a consistent decrease in the order of 8% quenching decrease along with a 20 nm blue shi in the maximum peak
for example with {Re(CO)3Br}2(m-L3) (Fig. S22†). The introduc- position (Fig. 6a). Complex {Re(CO)3Br}2(m-L3) shows an inter-
tion of uoride anions however results in a gradual decrease mediate situation with initially an increase along with a
(20% and 50% drop with ten equivalents in {Re(CO)3Br}2(m-L1) subsequent and considerable hypsochromic shi before a
and
{Re(CO)3Br}2(m-L2)
respectively)
consistent
with quenching mechanism is invoked (Fig. 6b).
deprotonation.
Although attempts to get stability constants from the emis-
The addition of acetate results in a different behaviour sion data were considered, given the complexity in the system, it
between the various complexes under investigation; the simpler proved to be difficult to obtain meaningful data. The
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blocking a solvent, or oxygen induced radiationless decay
processes. However the act of deprotonation provides an alter-
native non-emissive decay pathway as exemplied by the more
basic anions uoride and acetate, and the more acidic aryl
thioureas in L2 and L3.
Discussion
The strong interactions observed with uoride and acetate are
consistent with similar thiourea complexes. Fluoride enables
deprotonation and the formation of H2F+, while acetate forms a
strong double hydrogen bond with the thiourea (Fig. 7a) with
binding constants consistent with literature values for similar
systems.22–24 However it is evident that there is also a degree of
deprotonation occurring with there being a considerable loss in
the 1H NMR signal for the NH signal on the introduction of this
anion and the UV/vis titration not presenting a clear isobestic
point (Fig. S19†). In fact moving to a solution of DMSO-D6
contain 0.5% water, the 1H NMR signal, for all of the complexes
disappeared on the addition of less than one equivalent of
acetate suggestive of a high degree of proton exchange partic-
ularly with the para-substituted aryl bridged complex
{Re(CO)3Br}2(m-L2).51 However where the NH peak could be
Fig. 6 The change in emissive behaviour of (a) {Re(CO)3Br}2(m-L2) and
(b) {Re(CO)3Br}2(m-L3) on the introduction of TBA H2PO4 (CD3CN, 298
K, at approx. 1 ꢃ 10ꢁ5 mol dmꢁ3).
luminescence in such systems is attributed to a long lived meta
stable excited triplet state predominantly located on the bipyr-
idine ligand, which decays to the HOMO possessing mainly
metal-carbonyl character.56 In the systems investigated here
there appear to be two emissive states at approximately 580 nm
and 610 nm. Initially the transition from the lower energy state
is dominant, but this state appears to be more sensitive to the
anions interacting with the thiourea groups, leading to the
observed blue shi. However this is complicated by there being
two competing processes occurring in the three systems
considered. While further spectroscopic investigations are
required to verify this, it would appear that binding the anion
Fig. 7 Proposed modes of binding of (a) OAcꢁ and H2PO4ꢁ to (b) L and
initially enhances the uorescence, presumably by either (c) {Re(CO)3Br}2(m-L1).
18448 | RSC Adv., 2014, 4, 18442–18452
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followed, the peak position gave a good indication of an asso-
With two rhenium centres in the complex {Re(CO)3Br}2(m-
ciation process, rather than the competing deprotonation, L1), there is a signicant enhancement in the binding of the two
which reveals an insight into the nature of how dihy- dihydrogenphosphate groups over that seen with L1 alone,
drogenphosphate interacts with thiourea groups.
which is not seen with acetate. In light of our previous ndings,
The binding of monovalent dihydrogenphosphate is far it is proposed that this corresponds to an involvement of the
stronger than we would have anticipated in these systems, and carbonyl groups, which are available to form additional
it is anomalously high in comparison to other tetrahedral oxo- chelating hydrogen bonds with the phosphates (Fig. 7c).41,42
anions. Evidently it behaves more like acetate than the majority Unfortunately, the poor solubility of the materials involved in
of the other anions considered, including chloride implying this study prevented further elucidation by a solution IR
that the presence of the two OH groups is important. However it spectroscopy.
is the surprising observation of sigmoidal titration curves and
In adding a second urea to the ligand system (L2 and L3),
the implication of cooperative binding that needs to be both the free ligand and the complexes see a marginal
considered in more depth. Initially we considered a syn/anti enhancement in the binding consistent with two sites being
conformational change in the thiourea group,57,58 however a available to bind anions. In a CD3CN–DMSO-D6 mixture, these
review of the literature reveals that similar anomalies have been t well to
a two to one stoichiometry. However dihy-
reported with dihydrogenphosphate, and not just with thio- drogenphosphate again shows remarkably strong binding to
ureas and urea based sysꢁtems.25,30,59,60 It appears that the the second anion that cannot be adequately explained using
binding of a second H2PO4 strengthens the interaction with this simple model. Given the sigmoidal titration curve, the data
the rst unit, and that acetate can show similar behaviour in did t surprisingly well to the two to one model with goodness-
certain instances. It has been hypothesised to be as a result of of-t of less than 2% in each titration curve examined. But the
the formation of a hydrogen bond with the conjugate base.51
situation is evidently far more complex than this simple model
Lazarides et al. highlighted an unusual situation with a justies and is consistent with the anomalous results observed
widely spaced diamide with a Re(I) tricarbonyl complex,59 where by Hanan and co-workers in the related system.30 In DMSO
similar behaviour is attributed to the formation of a phosphate there was some evidence of a degree of proton exchange with
dimer and they propose two possible congurational explana- the loss of the N–H proton signals on the addition of both
tions. Recently both Blaˇzek et al.14 and Amendola et al.61 have acetate and dihydrogenphosphate, but discounting this
also shown that the binding of phosphate to urea provides dynamic effect, the observed cooperativity is not adequately
additional stability though the formation of secondary explained given the rigid separation of the two binding sites by
hydrogen bonds between the two phosphates groups. In addi- an aromatic spacer. In light of the result proposed for the
tion a study by Gale and co-workers with a diindolylurea simpler system {Re(CO)3Br}2(m-L1), it is conceivable that once
complex has also shown an anomalous interaction with dihy- the two anions are bound to the ligands, a secondary process
drogen phosphate, where neither a simple 1 to 1, nor a 2 to 1, occurs such as dimerisation, or the formation of larger aggre-
binding model adequately accounted for the observed behav- gations. This is also consistent with the observed broadening of
1
iour in the H NMR spectroscopic titrations.62 They concluded the observed aromatic signals in the 1H NMR titrations.
ꢁ
that following binding that the pKa for the H2PO4 dropped to However larger assemblies have not yet been evident by elec-
permit proton transfer to a second dihydrogen phosphate, trospray mass spectrometry and remain a hypothesis.
which was a slow process on the 1H NMR timescale. It is
conceivable that a similar process is occurring here. A review of
the Cambridge Crystallographic Database highlighted several
Conclusions
hundred examples where two or more phosphates are associ- Our results suggest that the binding of acetate and dihy-
ated together through hydrogen bonding, including several drogenphosphate to thioureas is not a simple binding event,
“anion receptors” bearing amidic NH groups, which also show with several competing processes in evidence. As has been
remarkable selectivity for dihydrogenphosphate.63–65
previously described, proton exchange, or even deprotonation,
In the case of ligand L1, where only solution phase behaviour of the NH group readily occurs with uoride and acetate, while
is available to us, and basing this on the previous observations, there is some evidence that this can also occur with dihy-
it is proposed that the second phosphate “piggy backs” on the drogenphosphate. However it is also apparent that the latter
rst formally bonded anion (Fig. 7b). This interaction, with a anion also binds cooperatively, with two units potentially
partial transfer of a proton from the thiourea to the anions then interacting with one thiourea group, and the second anion
encourages one of the protons from the dihydrogenphosphate enhancing the interaction of the rst possibly by proton
from the rst anion onto the second, enhancing the anionic exchange. This accounts for the remarkable selectivity for
character of the formally bound anion thereby strengthening its dihydrogenphosphate in many of the reported protic anion
interaction explaining the observed cooperativity, and the high receptors. While this knowledge will enable understanding to
binding constants. In an attempt to conrm this, a titration was assist in designing systems to both encourage selectivity for this
attempted to add both ligand L1 and the complex intriguing anion, the ramications of these results extend into
{Re(CO)3Br}2(m-L1) into dihydrogenphosphate in DMSO-D6, and understanding many of the commonly observed supramolec-
monitor the change in the 31P resonance, but unfortunately no ular processes in biology involving this ubiquitous anion. For
change is observed.
example, the facile addition of a dihydrogenphosphate to ADP
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to form ATP in ATP synthase could be explained by the coop- recrystallization from CHCl3 (2 cm3) and hexane (70 cm3)
erativity observed in this study, where the initial stage of simply subsequent removal of the solvent gave a crumbly brown solid
bringing two negatively charged groups together cooperatively (yield ¼ 0.147 g, 61%). Found: C: 63.1, H: 4.2, N: 18.3%.
is justied, permitting the subsequent enzyme catalysed
dehydration.
C
11H9N3S requires: C: 63.4, H: 4.0, N: 18.5%. 1H NMR (300 MHz,
0
CDCl3): d 8.68 (1H, d, J ¼ 4.8 Hz, bpyH6 ), 8.62 (1H, s, bpyH6),
8.44 (1H, d, J ¼ 8.1 Hz, bpyH3), 8.39 (1H, d, J ¼ 8.1 Hz, bpyH4),
0
0
7.85–7.79 (2H, m, bpyH3 ,4 ), 7.33 (1H, dd, J ¼ 7.8 and 4.8 Hz,
Experimental
Physical measurements
0
bpyH5 ), 4.80 (2H, s, CH2). 13C NMR (75.4 MHz, CDCl3): d 156.4,
155.3, 149.2, 147.6, 136.9, 135.5, 130.0, 123.1, 121.1, 121.1, 46.3.
EI MS (m/z): 227.1 [M+] (100%). IR (KBr disc) cmꢁ1: 2178, 2093
(s, NCS).
NMR spectra were recorded using Bruker AV300 (ligands),
AV400 (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 spectrouorimeter,
having adjusted the adsorption at the excitation wavelength to
be 0.1. Microanalysis, and E.S. and E.I. mass spectrometry were
performed by A.S.E.P., The School of Chemistry and Chemical
Engineering, Queen's University Belfast.
Ligand L1: N,N0-bis([2,20]-bipyridin-5-yl-methyl)-thiourea. 5-
Aminomethyl-2,20-bipyridine (0.300 g, 1.62 mmol) and 5-iso-
thiocyanatomethyl-2,20-bipyridine (0.368 g, 1.62 mmol) were
dissolved in pyridine (20 cm3) and heated to reux for 48 hours
and cooled to room temperature. Diethyl ether (150 cm3) was
added and the reaction stored at 2 ꢀC for 16 hours. The resulting
precipitate was collected by ltration and washed with diethyl
ether (3 ꢃ 10 cm3) to give the product as a pale brown solid
1
(yield ¼ 0.487 g, 73%). H NMR (400 MHz, DMSO-D6): d 8.68
Materials
0
(2H, d, J ¼ 4.5 Hz, bpyH6 ), 8.61 (2H, s, bpyH6), 8.38 (2H, d, J ¼
0
0
All reagents were purchased from Sigma Aldrich and used as
supplied unless otherwise stated. 5-Chloromethyl-2,20-bipyr-
7.5 Hz, bpyH3/3 ), 8.35 (2H, d, J ¼ 7.5 HZ, bpyH3/3 ), 8.28 (2H, s,
0
NH), 7.96 (2H, dd, J ¼ 6.9 Hz, 7.2 Hz, bpyH4 ), 7.86 (2H, d, J ¼ 7.6
0
idine46 was prepared from 2-acetylpyridine via a Krohnke
¨
Hz, bpyH4), 7.44 (2H, dd, J ¼ 4.8 Hz, 7.2 Hz, bpyH5 ), 4.78 (4H, s,
synthesis to 5-methyl-2,20-bipyridine,66 lithiation and subse-
quent reaction to 5-(trimethylsilyl)methyl-2,20-bipyridine fol-
lowed by conversion to the chlorinated derivative by reaction
with CsF and Cl3C2Cl3.46 1,3-Phenylene diisothiocyanate was
prepared using thiophosgene following a literature procedure.49
CH2). 13C NMR (100.6 MHz, DMSO-D6): d 158.98, 153.81, 149.11,
148.26, 137.23, 136.05, 127.98, 124.16, 123.97, 120.74, 119.97,
16.21. HRMS calculated for [M + H+] C23H21N6S+ (ES+) 413.1548,
found 413.1563. IR (KBr disc) cmꢁ1: 3232 (br, NH stretch), 1551
(s, thiourea bend).
1,4-Phenylene diisothiocyanate. 1,4-Diphenylamine (3.00 g,
27.7 mmol), triethylamine (19.33 cm3, 137.8 mmol) and THF (50
Synthesis
5-Aminomethyl-2,20-bipyridine.67 Hexamethylenetetraamine cm3) were cooled to 0 ꢀC, freshly distilled carbon disulphide
(0.294 g, 2.10 mmol) in DCM (30 cm3) was heated to reux and (10.01 cm3, 166.4 mmol) was added slowly and the reaction
5-chloromethyl-2,20-bipyridine (0.300 g, 1.47 mmol) in DCM (10 stirred for 16 hours at room temperature. The reaction was
cm3) added and reuxed for 16 hours. The solution was cooled cooled to 0 C, tosyl chloride (10.507 g, 55.5 mmol) added and
ꢀ
to room temperature, the resulting precipitate was collected by then stirred for a further 16 hours at room temperature. The
ltration then suspended in ethanol (15 cm3) and 12 M HCl (2 solution was washed with 1 M aqueous NaOH (20 cm3), the
cm3) before being heated to reux for a further 16 hours. Upon product extracted into diethyl ether (3 ꢃ 30 cm3), the organic
removal of the ethanol, CHCl3 (15 cm3) and H2O (15 cm3) were fractions were dried over MgSO4 and solvent removed under
added and the pH adjusted to 13 with 1 M aqueous NaOH. The reduced pressure. The product was puried by recrystallization
product was extracted into DCM (3 ꢃ 30 cm3), dried with MgSO4 from CHCl3 (2 cm3) and hexane (70 cm3) subsequent removal of
and the solvent removed under reduced pressure. The resulting the solvent gave a waxy off white solid (yield ¼ 3.849 g, 72%). 1H
oil, was repeatedly triturated with DCM (2 cm3) and hexane (50 NMR (300 MHz, CDCl3): d 7.20 (4H, s, Ph). 13C NMR (75.4 MHz,
cm3) until removal of the solvent gave a pale yellow powder CDCl3): d 139.9, 128.2, 125.3. HRMS calculated for [M+]
(yield ¼ 0.182 g, 67%). Characterisation in accordance with C8H4N2S2 (ES+) 191.9816, found 191.9818.
+
literature.67
Ligand L2: 1,4-bis(N0-{[2,20]-bipyridin-5-yl-methyl]-thiour-
5-Isothiocyanatomethyl-2,20-bipyridine. 5-Aminomethyl-2,20- eido)-benzene. 5-Aminomethyl-2,20-bipyridine (0.350 g, 1.89
bipyridine (0.196 g, 1.06 mmol), triethylamine (0.59 cm3, 4.22 mmol) and triethylamine (6 cm3) were dissolved in dichloro-
mmol) and THF (40 cm ) were cooled to 0 C, freshly distilled methane (20 cm3). 1,4-Phenylene diisothiocyanate (0.165 g,
3
ꢀ
carbon disulphide (0.32 cm3, 5.28 mmol) was added slowly and 0.587 mmol) in dichloromethane (10 cm3) was added and the
the reaction stirred for 16 hours at room temperature. The solution stirred for 16 hours under N2. The resulting cream
ꢀ
reaction was cooled to 0 ꢀC, tosyl chloride (0.220 g, 1.15 mmol) precipitate was collected by ltration and dried at 60 C for 16
1
added and then stirred for a further 16 hours at room temper- hours (yield ¼ 0.230 g, 70%). H NMR (300 MHz, DMSO-D6): d
0
ature. The solution was washed with 1 M aqueous NaOH (20 9.74, (2H, br, Ph-NH), 8.68 (2H, d, J ¼ 4.8 Hz, bpyH6 ), 8.61 (2H,
0
cm3), the product extracted into diethyl ether (3 ꢃ 30 cm3), the s, bpyH6), 8.33 (2H, d, J ¼ 7.5 Hz, bpyH3/3 ), 8.32 (2H, d, J ¼ 7.8
0
organic fractions were dried over MgSO4 and solvent removed Hz, bpyH3/3 ), 8.27 (2H, br, bpy–CH2–NH), 7.93 (2H, dd, J ¼ 7.5
0
under reduced pressure. The product was puried by Hz, 7.8 bpyH4 ), 7.90 (2H, d, J ¼ 7.8 Hz, bpyH4), 7.43 (2H, dd, J ¼
18450 | RSC Adv., 2014, 4, 18442–18452
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0
4.8 Hz, 7.5 Hz, bpyH5 ), 7.38 (4H, s, phenyl H), 4.82 (4H, br, mmol, 45%). Found C: 33.78, H: 2.43, N: 6.98%;
CH2). 13C NMR (100.6 MHz, DMSO-D6): d 180.94, 155.09, 153.90, 36H26N8Br2O6Re2S2 requires C: 34.24, H: 2.07, N: 8.87. 1H NMR
C
149.22, 148.49, 137.30, 136.29, 135.43, 135.23, 124.05, 123.91, (300 MHz, DMSO-D6): d 10.01 (2H, s, phenyl-NH), 9.01 (2H, d, J
0
120.33, 12ꢁ0.04, 44.65. HRMS calculated for [M
cmꢁ1: 3195 (br, NH stretch), 1519 (s, thiourea bend).
ꢁ
H+] ¼ 4.5 Hz, bpyH6 ), 8.97 (2H, s, bpyH6), 8.71 (4H, d, J ¼ 8.1 Hz,
0
C
30H25N8S2 (ESꢁ) 561.1644, found 561.1660. IR (KBr disc) bpyH3,3 ), 8.41 (2H, br, CH2NH), 8.31 (2H, dd, J ¼ 8.1 Hz, 7.2 Hz,
0
bpyH4 ), 8.24 (2H, d, J ¼ 8.4 Hz, bpyH4), 7.76–7.71 (2H, m,
0
Ligand L3: 1,3-bis(N0-{[2,20]-bipyridin-5-yl-methyl]-thiour- bpyH5 ), 7.51 (1H, s, Ph-H), 7.34 (2H, dd, J ¼ 7.8 Hz, 5.1 Hz, Ph-
eido)-benzene. Prepared by the same procedure employed for H), 7.22 (2H, d, J ¼ 7.8 Hz, PhH), 4.94 (4H, d, J ¼ 5.4 Hz, CH2);
+
L2 using 5-aminomethyl-2,20-bipyridine (0.320 g, 1.73 mmol), HRMS calculated for [M ꢁ Brꢁ] C36H26N8BrO6Re2S2 (ES+)
triethylamine (8 ml) and 1,3-diisothiocyanato-benzene (0.15 g, 1182.9689, found 1182.9681 (corresponds to isotopic pattern);
0.78 mmol) (yield ¼ 0.38 g, 86%). Found: C: 60.30, H: 4.80, N: IR (KBr disc) cmꢁ1: 2021, 1893 (s, C]O stretch), 1540 (br,
18.63%, C30H26N8S2$H2O requires: C: 60.20, H: 5.05, N: 18.73%; thiourea).
1H NMR (500 MHz, DMSO-D6): d 9.81 (2H, br, Ph-NH), 8.67 (2H,
0
d, J ¼ 5.0 Hz, bpyH6 ), 8.63 (2H, s, bpyH6), 8.35 (4H, m, bpyH3 and
Acknowledgements
0
3 ), 8.31 (2H, br, bpy–CH2–NH), 7.93 (2H, dd, J ¼ 7.5 Hz, 8.0 Hz,
0
bpyH4 ), 7.88 (2H, d, J ¼ 8.0 Hz, bpyH4), 7.54 (1H, s, ArH), 7.43 This work was supported by a DELNI studentship (ALB) and
0
(2H, dd, J ¼ 7.5 Hz, 5.0 Hz, bpyH5 ), 7.31 (1H, dd, J ¼ 7.5 Hz, 8.5 QUESTOR and ChemVite Ltd. for the studentship (NCAB). Prof.
Hz, ArH), 7.18 (2H, d, J ¼ 8.0 Hz, ArH), 4.80 (4H, s, CH2). 13C M. D. Ward is thanked for his additional insight.
NMR (125.7 MHz, DMSO-D6): d 180.7, 155.0, 153.9, 149.1, 148.4,
139.0, 137.1, 136.1, 135.0, 129.0, 123.9, 120.2,ꢁ119.9, 119.4, 44.6;
HRMS calculated for [M ꢁ H+] C30H25N8S2 (ESꢁ) 561.1697,
found 561.1660. IR (KBr disc) cmꢁ1: 3220 (br, NH stretch), 1539
(s, thiourea, bend).
Notes and references
1 M. Wenzel, J. R. Hiscock and P. A. Gale, Chem. Soc. Rev.,
2012, 41, 480–520.
{Re(CO)3Br}2(m-L1). Ligand L1 (60.0 mg, 0.146 mmol) dis-
solved in DMSO (5 cm3) was added to [Re(CO)5Br] (31.0 mg,
0.076 mmol) again dissolved in DMSO (10 cm3) and the solution
heated at 75 ꢀC for 16 hours under N2. The solvent was removed
by vacuum distillation and the product recrystallised from hot
toluene (100 cm3) and the precipitate was collected as a yellow
2 A. E. Hargrove, S. Nieto, T. Z. Zhang, J. L. Sessler and
E. V. Anslyn, Chem. Rev., 2011, 111, 6603–6782.
3 A. K. H. Hirsch, F. R. Fischer and F. Diederich, Angew. Chem.,
Int. Ed., 2007, 46, 338–352.
4 V. Amendola, D. Esteban-Gomez, L. Fabbrizzi and
M. Licchelli, Acc. Chem. Res., 2006, 39, 343–353.
5 V. Amendola and L. Fabbrizzi, Chem. Commun., 2009, 513–
531.
6 D. J. Mercer and S. J. Loeb, Chem. Soc. Rev., 2010, 39, 3612–
3620.
7 J. W. Steed, Chem. Soc. Rev., 2009, 38, 506–519.
8 D. Jana, G. Mani and C. Schulzke, Inorg. Chem., 2013, 52,
6427–6439.
powder (yield ¼ 0.051 g, 31%). 1H NMR (500 MHz, DMSO-D6): d
0
9.01 (2H, d, J ¼ 5.1 Hz, bpyH6 ), 8.96 (2H, s, bpyH6), 8.72–8.67
0
(4H, m, bpyH3,3 ), 8.54 (2H, s, NH), 8.31 (2H, dd, J ¼ 7.5 Hz, 8.0
0
Hz, bpyH4 ), 8.19 (2H, d, J ¼ 8.1 Hz, bpyH4), 7.74 (1H, dd, J ¼ 5.5
0
Hz, 7.8 Hz, bpyH5 ), 4.91 (2H, s, CH2). 13C NMR (125.7 MHz,
DMSO-D6): d 197.2, 197.1, 189.4, 155.0, 153.7, 153.0, 151.6,
140.2, 138.7, 127.6, 124.1, 123.8, 59.9, 30.3, 27.8. HRMS calcu-
lated for [M + H+] C29H21N6Br2O6Re2S+ (ES+) 1110.8530, found
9 S. Kubik, Chem. Soc. Rev., 2010, 39, 3648–3663.
10 R. M. Duke, E. B. Veale, F. M. Pfeffer, P. E. Kruger and
T. Gunnlaugsson, Chem. Soc. Rev., 2010, 39, 3936–3953.
11 J. W. Steed, Chem. Soc. Rev., 2010, 39, 3686–3699.
1110.8510 (corresponds to isotopic pattern); IR (KBr disc) cmꢁ1
:
3412 (br, NH stretch), 2018, 1905 (s, C]O stretch), 1604 (br,
thiourea bend).
{Re(CO)Br}2(m-L2). Prepared by the same procedure 12 V. Amendola, L. Fabbrizzi and L. Mosca, Chem. Soc. Rev.,
employed for [{Re(CO)3Br}2(m-L1)] using L2 (65 mg, 0.116 mmol)
2010, 39, 3889–3915.
and [Re(CO)5Br] (0.126 g, 0.310 mmol) (yield ¼ 0.059 g, 0.046 13 C. Caltagirone, C. Bazzicalupi, F. Isaia, M. E. Light,
1
mmol, 40%). H NMR (500 MHz, DMSO-D6): d 9.91 (2H, s, Ph-
V. Lippolis, R. Montis, S. Murgia, M. Olivari and G. Picci,
Org. Biomol. Chem., 2013, 11, 2445–2451.
0
NH), 9.01 (2H, d J ¼ 4.5 Hz, bpyH6 ), 8.95 (2H, s, bpyH6), 8.71
0
(4H, d, J ¼ 7.8 Hz, bpyH3,3 ), 8.31 (2H, dd, J ¼ 7.7 Hz, 8.2 Hz, 14 V. Blaˇzek, K. Molcanov, K. Mlinaric-Majerski, B. Kojic-Prodic
0
bpyH4 ) 8.24 (2H, d, J ¼ 7.8 Hz, bpyH4), 7.74 (2H, dd, J ¼ 4.6 Hz,
and N. Basaric, Tetrahedron, 2013, 69, 517–526.
0
8.1 Hz, bpyH5 ), 7.88 (4H, s, Ph-H), 4.95 (4H, s, bpy–CH2). 13C 15 R. M. Duke and T. Gunnlaugsson, Tetrahedron Lett., 2010, 51,
NMR (125.7 MHz, DMSO-D6): d 197.2, 189.5, 181.2, 155.1, 153.6,
5402–5405.
153.0, 151.6, 140.3, 139.7, 138.9, 135.5, 127.8, 124.2, 123.9, 62.0, 16 Y. L. Zhang, R. Zhang, Y. X. Zhao, L. G. Ji, C. D. Jia and B. Wu,
+
44.3, 27.5; HRMS calculated for [M ꢁ Brꢁ] C36H26N8BrO6Re2S2
New J. Chem., 2013, 37, 2266–2270.
(ES+) 1182.9689, found 1182.9620 (corresponds to isotopic 17 C. Jia, B. Wu, S. Li, X. Huang, Q. Zhao, Q.-S. Li and X.-J. Yang,
pattern); IR (KBr disc) cmꢁ1: 3209 (br, NH stretch), 2021, 1893
(s, C]O stretch), 1551 (br, thiourea bend).
Angew. Chem., Int. Ed., 2011, 50, 486–490.
18 C. Jia, B. Wu, S. Li, Z. Yang, Q. Zhao, J. Liang, Q.-S. Li and
X.-J. Yang, Chem. Commun., 2010, 46, 5376–5378.
{Re(CO)5Br}2(m-L3). Prepared by the same procedure
employed for [{Re(CO)3Br}2(m-L1)] using L3 (70 mg, 0.125 mmol) 19 R. Custelcean, P. Remy, P. V. Bonnesen, D.-E. Jiang and
and [Re(CO)5Br] (0.130 g, 0.320 mmol) (yield ¼ 71.5 mg, 0.005
B. A. Moyer, Angew. Chem., Int. Ed., 2008, 47, 1866–1870.
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Paper
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