A prospective study of the use of the [Os(tpy)2]2+ (tpy = 2,2′;6′:2″-terpyridine) core as signalling scaffolding for the development of chemical sensors

Article abstract of DOI:10.1002/ejic.200501122

The heteroleptic osmium(II) complex [Os(L¹)(Me-phtpy)]-[PF ₆]₂ (L¹ = 1-[4′-p-tolyl-(2,2′: 6′,2″-terpyridyl)]-1,4,8,11-tetraazacyclotetradecane, Me-phtpy = 4′-p-tolyl-2,2′:6′,2″-terpyridine) has been prepared, characterized and its potential use for chemical sensing purposes studied. The complex contains a polyazacycloalkane as binding site and a Os(tpy)₂²⁺ core that acts as a signalling subunit . The emission intensity of the osmium complex was quenched in acetonitrile/water (1:1 v/v) by Cu²⁺ and Ni²⁺. Potentiometric titrations of [Os(L¹)(Me-phtpy)]²⁺ in the presence of Cu²⁺ were carried out in order to determine thermodynamic binding constants in acetonitrile/water (70:30 v/v, 0.1 mol·dm ^-3 tetrabutylammonium perchlorate). With Cu²⁺ the receptor [Os(L¹)-(Me-phtpy)]²⁺ forms the complex {Cu[Os(L ¹)(Me-phtpy)]}⁴⁺ at neutral pH. Moreover it undergoes two stepwise protonation processes at acid pH related to the partial protonation of the cyclam core and forms two hydroxo complexes at basic pH. The Ni²⁺ complexes of the osmium receptor [Os(L¹)(Me-phtpy)]²⁺ are able to act as an anion fluorogenic chemosensor in acetonitrile/water mixtures. A displacement of the quenching effect to more acidic pH in the Ni ²⁺-[Os(L¹)(Me-phtpy)]²⁺ system was observed for anions in the order ATP > AMP > chloride. A prospective study of the use of the [Os(L¹)-(Me-phtpy)]²⁺ complex as a fluorogenic sensor for oxygen was also carried out by incorporation of the complex in solgel silica films made with TEOS and MTEOS. Significant quenching of the osmium fluorescence by oxygen was observed. The oxygen response of the films showed good stability and repeatability. The study suggested that the Os(tpy) ₂²⁺ core might act as suitable signalling scaffolding in chemical sensing systems of cations, anions and gases. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200501122

FULL PAPER
DOI: 10.1002/ejic.200501122
2
+
A Prospective Study of the Use of the [Os(tpy) ] (tpy = 2,2Ј;6Ј:2ЈЈ-
2
Terpyridine) Core as Signalling Scaffolding for the Development of Chemical
Sensors
[a]
[a]
[a]
[b]
Jose V. Ros-Lis, Ramón Martínez-Máñez,* Juan Soto, Colette McDonagh,* and
Adrian Guckian[
b]
Keywords: Chemosensors / Osmium terpyridines / Macrocyclic ligands / Fluorescent probes / Thin films
1
2+
1
The heteroleptic osmium(II) complex [Os(L )(Me-phtpy)]-
pH. The Ni complexes of the osmium receptor [Os(L )(Me-
1
2+
[PF
6
]
2
(L
=
1-[4Ј-p-tolyl-(2,2Ј:6Ј,2ЈЈ-terpyridyl)]-1,4,8,11-
phtpy)] are able to act as an anion fluorogenic chemosensor
tetraazacyclotetradecane, Me-phtpy = 4Ј-p-tolyl-2,2Ј:6Ј,2ЈЈ-
terpyridine) has been prepared, characterized and its poten-
tial use for chemical sensing purposes studied. The complex
contains a polyazacycloalkane as “binding site” and a
in acetonitrile/water mixtures. A displacement of the
2
+
1
quenching effect to more acidic pH in the Ni –[Os(L )(Me-
phtpy)]2 system was observed for anions in the order ATP Ͼ
+
1
AMP Ͼ chloride. A prospective study of the use of the [Os(L )-
2
+
2+
Os(tpy)
2
core that acts as a “signalling subunit”. The emis-
(Me-phtpy)] complex as a fluorogenic sensor for oxygen
sion intensity of the osmium complex was quenched in aceto-
nitrile/water (1:1 v/v) by Cu and Ni . Potentiometric ti-
was also carried out by incorporation of the complex in sol-
gel silica films made with TEOS and MTEOS. Significant
quenching of the osmium fluorescence by oxygen was ob-
served. The oxygen response of the films showed good sta-
2
+
2+
1
2+
2+
trations of [Os(L )(Me-phtpy)] in the presence of Cu were
carried out in order to determine thermodynamic binding
–
3
constants in acetonitrile/water (70:30 v/v, 0.1 mol·dm tetra-
butylammonium perchlorate). With Cu the receptor [Os(L )- Os(tpy)
Me-phtpy)] forms the complex {Cu[Os(L )(Me-phtpy)]}
bility and repeatability. The study suggested that the
2+
1
2+
2
core might act as suitable signalling scaffolding in
2+
1
4+
(
chemical sensing systems of cations, anions and gases.
at neutral pH. Moreover it undergoes two stepwise proton-
ation processes at acid pH related to the partial protonation
of the cyclam core and forms two hydroxo complexes at basic
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
6
Introduction
the polypyridyl complexes of d metal cations, with their
strong metal-to-ligand charge transfer absorptions and
emitting excited states, are good candidates as signalling re-
porters. In this sense, the most widely used metal-based sig-
The development of new chemosensors for the selective
sensing of cations and anions based on the covalent or non-
covalent coupling of binding sites and signalling subunits is
an area of interest. These programmed molecular systems,
when appropriately designed, are capable of transducing
guest binding processes into suitable signalling events such
as changes in colour, fluorescence or redox properties of the
formed molecular ensemble. Among different output sig-
nals, those based on changes in emission properties are es-
pecially attractive and a number of different fluorogenic
2+
[4]
nalling framework has been the Ru(bpy)
core. In con-
3
trast, the analogous Ru(tpy)22 scaffolding has been used
+
[
5–10]
2+
as a signalling unit in very few cases.
Ru(tpy)2 might
show certain advantages versus Ru(bpy)32 , that is, it is rel-
atively easy to functionalise the terpyridine core. The most
serious drawback in using the Ru(tpy)22 scaffolding is its
usually very low quantum yield when compared with that
found for the Ru(bpy)32 systems. In order to maintain the
use of the terpyridine framework but in an attempt to en-
hance the emission properties of the signalling unit at room
temperature, we have moved in this paper from ruthenium
to osmium as an alternative metal centre. Thus, for instance,
the quantum yield at room temperature of the Ru(tolyl-
tpy)22 complex (tolyl-tpy = bis[4Ј-(p-tolyl)-2,2Ј:6Ј,2ЈЈ-ter-
+
+
+
chemosensors have been described for anion[ and cation
1]
[2]
sensing. Both all-organic and metal-based fluorogenic sub-
units have been widely used. Among metal-based lumines-
cent systems, the complexes of ruthenium and rhenium are
well known and have been used as signalling subunits in a
considerable number of reported chemosensors.[ In fact,
3]
+
–5
pyridine]) is 3.2×10 , whereas that observed with osmium
[
a] Instituto de Química Molecular Aplicada, Departamento de
Química, Universidad Politécnica de Valencia,
Camino de Vera s/n, 46022 Valencia, Spain
E-mail: rmaez@qim.upv.es
2+
as the metal centre, Os(tolyl-tpy)
, is more than 500 times
2
[11]
larger (φ = 0.021).
Although the osmium–terpyridine
complexes have received less consideration than other
metal–terpyridine systems, their photophysical properties
[b] National Centre for Sensor Research, Dublin City University,
Dublin 9, Ireland
Eur. J. Inorg. Chem. 2006, 2647–2655
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2647

J. V. Ros-Lis, R. Martínez-Máñez, J. Soto, C. McDonagh, A. Guckian
FULL PAPER
are well known because of comparative studies with the ru- reacted in a THF/water (1:1 v/v) mixture in the presence of
[
12]
thenium analogues.
hydrazine hydrate as reductant to yield a complex mixture
Some of us have previously reported ruthenium bis-ter- containing several Os–tpy derivatives. The desired hetero-
1
pyridine functionalised systems as chemosensors for metal leptic derivative [Os(L )(Me-phtpy)][PF ] was isolated
6
2
cations and anions.[6–10] However, and as far as we know, from the reaction mixture by column chromatography in
there are very few examples of the use of the Os(tpy)22
+
silica using a mixture of acetonitrile/water/saturated aque-
core as a signalling subunit for chemosensing purposes. As ous solution of KNO (85:10:5 v/v/v) as eluent and further
3
part of our interest in the development of chemical sensors precipitation with ammonium hexafluorophosphate (see
[
13]
[14]
[15]
for cations,
anions
and neutral guests,
we report Scheme 1). The high-resolution FAB mass spectrum of the
1
here a prospective use of the osmium–terpyridine scaffold- [Os(L )(Me-phtpy)][PF ] complex showed peaks at m/z
ing as signalling reporter for the fluorogenic sensing of 1325, 1180 and 1033 corresponding to [Os(L )(Me-
metal cations, anions and gases.
6
2
1
1
1
phtpy)][PF ] , [Os(L )(Me-phtpy)][PF ] and [Os(L )(Me-
6 2 6
phtpy)] fragments, respectively. The high-resolution FAB
mass spectra also showed the expected isotopic distribution
of peaks corresponding to the Os(192), Os(190) and
Os(189) isotopes. The 1H NMR spectra of the osmium
complex were also in agreement with the proposed formula-
tion.
Results and Discussion
Synthesis and Characterisation
1
The ligand L was synthesised following previously re-
[
7]
ported procedures by reaction of 4Ј-[(4-bromomethyl)-
phenyl]-2,2Ј:6Ј,2ЈЈ-terpyridine with an excess of cyclam in
Fluorogenic Sensing of Metal Cations
dichloromethane in the presence of NEt at 30 °C for 24 h.
Equimolar amounts of L and [Os(Me-phtpy)O OH·-
3
1
2
1
The synthesised osmium() complex [Os(L )(Me-
(
NO ) ] (Me-phtpy = 4Ј-p-tolyl-2,2Ј:6Ј,2ЈЈ-terpyridine) were
3 2
2+
phtpy)] contains a polyazacycloalkane as “binding site”
and an Os(tpy)22 core able to act as a “signalling subunit”.
In the first step, the studies shown below were carried out
to identify the effect that the coordination of metal cations
on the cyclam fragment has on the emission properties of
the signalling osmium–terpyridine reporter. The electronic
+
1
2+
spectrum of the [Os(L )(Me-phtpy)] complex is as ex-
pected for an Os(tpy)22 chromophore with two bands in
+
[11]
the visible region because of the spin-allowed dǞπ metal-
to-ligand charge transfer (MLCT) transition (λmax
=
=
470 nm) and the spin-forbidden MLCT band (λmax
670 nm).
The studies below were performed in acetonitrile–water
mixtures because of the insolubility in pure water of the
1
[
Os(L )(Me-phtpy)][PF ] complex in a wide pH range. Ex-
6 2
citation at 670 nm resulted in the emission at 740 nm. This
1
2+
emission of the [Os(L )(Me-phtpy)] complex remains un-
altered within 5% in the 2–12 pH interval (see Figure 1).
This behaviour is similar to that found for some ruthenium–
[
8]
terpyridine–polyamine analogues. This indicates that in
these systems there are no pH-dependent PET processes
from the lone pair of the cyclam nitrogens to the photoex-
cited fluorophore as is usually found in polyazaalkanes
bearing polycyclic aromatic hydrocarbons as signalling
[
2]
units.
Steady-state fluorescence experiments as a function of
the pH in the presence of equimolar amounts of metal cat-
ions M2 (M = Cu , Zn , Cd , Hg , Pb and Ni )
+
2+
2+
2+
2+
2+
2+
2+
2+
resulted in noticeable changes only in the presence of Cu
2
+
and Ni , whereas the emission intensity versus pH profile
stayed unaltered in the presence of the remaining cations.
2+
The metal cation Cu induced an important quenching of
the Os(tpy)22 emission in the 3–12 pH range, whereas Ni
+
2+
gave a similar quenching at basic pH (see Figure 1). This
behaviour is significantly different to that found for the
Scheme 1.·L and synthesis of [Os(L )(Me-phtpy)]2+.
1
1
2648
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 2647–2655

[
Os(tpy)
2
]2+ Core as Signalling Scaffolding for Development of Chemical Sensors
FULL PAPER
Figure 1. Emission intensity of the system [Os(L )(Me-phtpy)]2+–
1
2
+
+
M
–H as a function of the pH in acetonitrile/water (70:30 v/v)
1
2+
–4
–3
mixture, [{Os(L )(Me-phtpy)} ] = 10 mol·dm , in the presence
of 1 equiv. of metal cation (λex = 670 nm, λem = 740 nm).
Figure 2. Steady-state fluorescence intensity (᭡) and contribution
1
2+
of the lifetime of the free [Os(L )(Me-phtpy)] to the global decay
1
2+
2+
analogous [Ru(L )(Me-tpy)] compound (Me-tpy = 4Ј- (᭿) as a function of the Cu equivalents in acetonitrile/water
methyl-2,2Ј:6Ј,2ЈЈ-terpyridine),[ for which quenching was (70:30 v/v) mixture.
9]
observed in a narrower pH window (5–9 pH range). In re-
cent years a certain number of fluorogenic molecular
[
16–18]
probes for copper and nickel have been reported.
1
In order to obtain more information about the nature of
When compared with them, the complex [Os(L )(Me-
2
+
the cation–ligand interaction, we have performed additional
phtpy)] shows some advantages; it displays fluorogenic
sensing properties in mixed aqueous solutions and shows
emission properties in the far end of the visible window
where usually few fluorescence interferences are found.
Additional time-resolved emission studies of the ligand
2+
1
2+
coordination studies for the Cu –[Os(L )(Me-phtpy)]
system by the use of potentiometric titrations.
1
2+
2+
[
Os(L )(Me-phtpy)] in the presence of Cu were carried
Potentiometric Studies
out in acetonitrile and in acetonitrile/water (70:30 v/v) solu-
tions at neutral pH. In both cases the concentration of the
The protonation and coordination behaviour of the
–
5
1
1
2+
osmium complex was 3×10 . The ligand [Os(L )(Me- [Os(L )(Me-phtpy)] complex was studied by potentio-
2
+
phtpy)] shows a monoexponential decay in both acetoni- metric titrations with KOH of previously acidified acetoni-
trile and acetonitrile/water mixtures with lifetime values of trile/aqueous solutions (70:30 v/v, 0.1 mol·dm tetrabu-
07 ns and 112 ns, respectively. In the presence of certain tylammonium perchlorate) of [Os(L )(Me-phtpy)][PF ] .
amounts of Cu , a biexponential decay was observed. One The resulting protonation constants are shown in Table 1.
–3
1
1
6 2
2
+
1
2+
of the species has the same lifetime as [Os(L )(Me-phtpy)] , For comparison purposes, the protonation constants of the
whereas the other showed significantly shorter lifetime ligand L1 are also included in Table 1. Os –terpyridine
values of 22 and 28 ns respectively for acetonitrile and ace- complexes are very stable and kinetically inert, therefore ti-
tonitrile/aqueous solutions. This second lifetime was as- tration experiments could be carried out without observing
[9]
II
2
+
1
2+
2+
1
signed to the formation of Cu –[Os(L )(Me-phtpy)]
demetallation on the Os(tpy)2 framework. L gives rise to
complexes. In fact, when we compare the intensity of five protonation constants in acetonitrile/aqueous solutions.
steady-state fluorescence and the contribution of the 107 ns The first two correspond to protonation of the cyclam frag-
component to the global decay, we observe that they follow ment whereas the remaining three are more difficult to as-
the same tendency (see Figure 2), which is in agreement sign and might correspond to either protonation of the ter-
with the assignment of the 25 ns species to the copper com- pyridine or the cyclam units. The existence of a large
1
plex that induced the quenching of the fluorescence of the number of protonation sites in L makes the last two pro-
2
+
2+
Os(tpy)2 core. Upon increasing amounts of Cu , in tonations too acidic and those constant values could not
1
2+
water, the lifetime at 112 ns does not undergo significant be determined. For the [Os(L )(Me-phtpy)] complex only
changes. On the contrary, when only acetonitrile is em- three protonations were observed. This is in agreement with
ployed as solvent, the lifetime at 108 ns decreases to a value the results found for cyclam in acetonitrile/water (70:30 v/v)
lower than 95 ns in the presence of an excess of Cu . This mixtures, in which only three protonation processes (logK =
suggests that in acetonitrile the free Cu displays some 11.6, 9.49 and 2.01, respectively) were found. The proton-
kind of additional interaction with the fluorogenic group ation constants for the [Os(L )(Me-phtpy)] complex are
that is avoided in the presence of water, probably because very close to those reported for the analogous Ru-contain-
of a stronger solvation of the metal. The curves in Figure 2 ing complex [Ru(L )(Me-tpy)] . This was an expected be-
also reach an asymptotic behaviour between 1 and 2 equiv., haviour that indicated that the simple modification of the
2
+
2
+
[9]
1
2+
1
2+ [9]
1
2+
2+
suggesting the formation of [Os(L )(Me-phtpy)] :Cu 1:1 metal centre in the M–terpyridine core does not signifi-
complexes.
cantly affect the protonation of the anchored cyclam unit.
Eur. J. Inorg. Chem. 2006, 2647–2655
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2649

J. V. Ros-Lis, R. Martínez-Máñez, J. Soto, C. McDonagh, A. Guckian
FULL PAPER
1
Table 1. Logarithms of the protonation constants of L and
From the stability constants obtained from potentio-
metric methods and emission studies shown in Figure 1 it
is possible to determine which species are responsible for
1
2+
[
0
Os(L )(Me-phtpy)]
in acetonitrile/water (70:30 v/v) (298 K,
.1 mol·dm–3 tetrabutylammonium perchlorate).
2+
Reaction
logβ
the quenching observed in the interaction of Cu with the
Os(tpy)22 signalling unit. This can be seen in Figure 3,
which shows both the distribution diagram and the emis-
+
1
2+
+
1
3+
[
[
[
[
[
[
[
Os(L )(Me-phtpy)] + H p [Os(L )(Me-phtpy)H]
10.42(3)
Os(L )(Me-phtpy)H] + H+ p [Os(L )(Me-phtpy)H
1
3+
1
]4+ 18.90(4)
2
1
3+
+
1
5+
Os(L )(Me-phtpy)H
2
]
+ H p [Os(L )(Me-phtpy)H
3
]
23.81(5)
19.34(4)
23.95(3)
26.89(3)
29.51(2)
1
sion intensity versus pH profile for the [Os(L )(Me-
1
+
+
1
]2+
L H] + H p [L H
2
2+
+
2+
phtpy)] –H –Cu system. It can be observed that the
1
2+
3+
4+
+
1
3+
4+
5+
L H
2
]
3
]
4
]
+ H p [L H
3
]
4
]
5
]
2+
1
+
1
quenching of the emission of the Os(tpy)
2
core is related
L H
+ H p [L H
+ 1
2+
1
2+
1
L H
+ H p [L H
to the formation of Cu –[Os(L )(Me-phtpy)] complexes
independently of the nature of the species formed. Neither
the partial coordination of the cyclam macrocycle by
1
5+
the protons to form {Cu[Os(HL )(Me-phtpy)]}
and
Table 2 shows the stability constants obtained for the for-
1
6+
–
2
+
1
2+
{Cu[Os(H L )(Me-phtpy)]} nor the coordination of OH
mation of Cu complexes with [Os(L )(Me-phtpy)] in
acetonitrile/water (70:30 v/v) mixtures from potentiometric
titrations with KOH of previously acidified solutions of 1:1
2
+
to the Cu2 cation to form {Cu(OH)[Os(L )(Me-phtpy)]}
1
3+
1
2+
and {Cu(OH) [Os(L )(Me-phtpy)]} produce significant
changes in the emission intensity of the Os(tpy)2
2
2
+
2
+
1
core.
mixtures of Cu and [Os(L )(Me-phtpy)][PF ] . The distri-
6
2
1
2+
+
2+
The enhancement at acid pH is due to demetallation of the
bution diagram of [Os(L )(Me-phtpy)] –H –Cu
is
2
+
1
2+
1
2+
Cu to recover the emissive [Os(L )(Me-phtpy)] species.
This behaviour contrasts significantly with that observed
shown in Figure 3. The receptor [Os(L )(Me-phtpy)]
1
4+
forms the complex {Cu[Os(L )(Me-phtpy)]} that exists at
neutral pH (see Figure 3). This complex undergoes two
stepwise protonation processes at acid pH and forms two
hydroxo complexes at basic pH. The hydroxo complexes are
formed by OH coordination to the Cu centre to give
most likely an octahedral configuration, whereas proton-
ation processes at acidic pH involve the partial protonation
of the cyclam subunit. The logarithm of the stability con-
stant for the formation of the {Cu[Os(L )(Me-phtpy)]}
complex is 14.2. This value is very close to that found for
the formation of the analogous ruthenium complex
Cu[Ru(L )(Me-tpy)]} (logK = 13.30), in agreement with
the almost identical nature of the receptor. The constant
for the formation of the {Cu[Os(L )(Me-phtpy)]} com-
plex can be compared with that found, under similar condi-
tions (acetonitrile/water, 70:30), for the formation of the
complex [Cu(tmfcyclam)] (logK = 19.06), where tmfcy-
clam is a cyclam-substituted derivative [tmfcyclam = 1,4,8,11-
tetrakis(ferrocenylmethyl)-tetraazacyclotetradecane].
Despite the somewhat different nature of both coordination
sites in these complexes, the lower stability constants found
2+
1
2+
for the Cu –[Ru(L )(Me-tpy)]
quenching of the emission intensity of the Ru(tpy)2 core
was only found at neutral pH and attributed to the exis-
tence of the {Cu[Ru(L )(Me-tpy)]} complex in the solu-
tion. In that case, the partial protonation of the macro-
cycle or Cu coordination with OH induced the recupera-
tion of the emission intensity. In our system the different
complexes formed have no effect on the emission of the
system, for which the
2+
1
4+
–
2+
[
9]
II
–
1
4+
2+
1
2+
+
2+
Os(tpy)2 core in the [Os(L )(Me-phtpy)] –H –Cu sys-
tem, suggesting that the deactivation path should be dif-
2
+
1
1
4+
ferent to that of Ru(tpy)
in the analogous [Ru(L )(Me-
{
2
2+
+
2+
[9]
tpy)] –H –Cu ensemble. For the latter, a deactivation by
energy transfer (ET) mechanisms was suggested, bearing in
mind that photoelectron transfer (PET) processes were un-
likely because of the large difference between the reduction
1
4+
of the Cu(cyclam)2 fragment and the oxidation of the Ru
+
II
2
+
core.
[
9]
From a simple thermodynamic cycle the free energy asso-
ciated to a photoelectron transfer from the excited state of
the Os or Ru complex to the Cu–cyclam core can be calcu-
lated from the redox potentials of the metal–terpyridine and
copper–cyclam cores and the λem from the excited state of
the metal–terpyridine group by using the equation: ∆G =
2
+
for the former with Cu are in agreement with the electro-
2
+
static repulsions imposed by the Os(tpy)
Cu –cyclam coordination process.
core on the
2
2
+
–
F[E II
Cu /CuI
– EM
III/MII
] – hcN /λ where M = Os or Ru.
A em
II
2+
The oxidation of the Ru core in [Ru(Me-tpy) ] is +1.3 V
2
II
Table 2. Logarithms of the stability constants of the system versus SCE, whereas the oxidation of the Os metal centre
Os(L )(Me-phtpy)] –H –Cu in acetonitrile/water (70:30 v/v)
1
2+
+
2+
1
2+
[
(
in [Os(L )(Me-phtpy)] is found at +0.84 V versus SCE.
Additionally, the λem for the Os and Ru complexes is 740
and 670 nm, respectively. Unfortunately, the Os system did
298 K, 0.1 mol·dm–3 tetrabutylammonium perchlorate).
Reaction
logβ
II
Os(L )(Me-phtpy)] + Cu + 2H+
1
2+
2+
not show any defined reduction peak for the Cu –cyclam
[
[
[
[
[
[
[
[
[
[
p
1
6+
Os(L )(Me-phtpy)CuH
2
]
24.38(9)
20.64(8)
14.20(8)
5.01(5)
core. Nevertheless, it might be possible to get some conclu-
sion from the comparison between the oxidation potentials
and λem from the Ru and Os complexes. Thus, from the
above equation it can be observed that a reduction in the
oxidation potential of the M(tpy)22 core would favour the
occurrence of photoelectron transfer processes, whereas a
Os(L )(Me-phtpy)] + Cu + H+
1
2+
2+
p
1
5+
Os(L )(Me-phtpy)CuH]
1
2+
2+
Os(L )(Me-phtpy)] + Cu
p
1
4+
Os(L )(Me-phtpy)Cu]
+
Os(L )(Me-phtpy)] + Cu2+ + H
1
2+
2
+O p
1
3+
2+
Os(L )(Me-phtpy)Cu(OH)] + H
1
2+
Os(L )(Me-phtpy)] + Cu + 2H
2
O+ p
reduction of the energy of the excited state (longer λem
)
1
]2+ + 2H
Os(L )(Me-phtpy)Cu(OH)
2
–5.45(7)
would disfavour this. When we move from Ru to Os there
2650
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Eur. J. Inorg. Chem. 2006, 2647–2655

[
Os(tpy)
2
]2+ Core as Signalling Scaffolding for Development of Chemical Sensors
FULL PAPER
1
2+
+
2+
Figure 3. Species distribution diagram of the system [Os(L )(Me-phtpy)] –H –Cu in acetonitrile/water (70:30 v/v) from the data of
1
2+
1
2+
2+
Table 2. The experimental values of intensity of fluorescence of [Os(L )(Me-phtpy)] (᭿) and [Os(L )(Me-phtpy)] –Cu (o) [Cx =
1
Os(L )(Me-phtpy)] are also included.
1
2+
is a fall in the oxidation potential of almost 500 mV that is profile of the [Os(L )(Me-phtpy)] complex remained un-
–
–
not compensated by the difference in λem from the excited altered in the presence of the anions studied (Br , Cl , phos-
states of Os and Ru (λem at 740 and 670 nm), which corre- phate, sulfate, ATP, ADP and GMP). As an alternative, we
2+
1
spond to an energy of only 180 mV. This indicates that a also carried out similar studies with the Cu –[Os(L )(Me-
2
+
2+
PET process from the excited state of the M(tpy)
core to phtpy)] systems. However, unfortunately no change in the
2
II
2+
the Cu –cyclam moiety is thermodynamically more favour- emission intensity of the Os(tpy)2 core was observed at
2
+
1
2+
able to occur for Cu –[Os(L )(Me-phtpy)] than for the any pH in the presence of the above-mentioned anions. This
2
+
1
2+
analogous ruthenium Cu –[Ru(L )(Me-phtpy)] complex. might be because, as we have found above, to observe a
2+
1
Attempts to determine stability constants from potentio- recovery of emission intensity in the Cu –[Os(L )(Me-
2
+
1
2+
2+
metric titrations for the Ni –[Os(L )(Me-phtpy)] system phtpy)] system it appears necessary to remove the copper
were unsuccessful, most likely because of the presence of from the macrocycle, whereas none of the anions is able to
kinetically slow coordination processes. Nevertheless, the produce an interaction strong enough to take out the Cu2
+
2
+
Ni cation would be expected to form similar complexes from the cyclam.
to those found for Cu , that is, the formation of the
Ni[Os(L )(Me-phtpy)]} complex at near neutral pH and ing the effect that the presence of anions has on the emis-
2
+
The research in this section has been completed by study-
1
4+
{
2+
1
2+
the formation of hydroxo complexes and protonation of the sion behaviour of the Ni –[Os(L )(Me-phtpy)] system
cyclam macrocycle at basic and acidic pH, respectively. As- (see Figure 4). As can be seen, the anions ATP, ADP and
suming this general behaviour it might tentatively be sug- chloride show a remarkable behaviour and are able to sig-
2
+
gested that the quenching in the case of the Ni cations nificantly change the emission intensity versus pH profile
2+
1
2+
occurs in the pH range where the square-planar 1:1 of the Ni –[Os(L )(Me-phtpy)] ensemble. Other anions
Ni[Os(L )(Me-phtpy)]} complex and the corresponding (Br , phosphate, sulfate and GMP) are not shown but they
1
4+
–
{
hydroxo complexes exist in solution.
did not induce significant emission changes. In all cases the
2+
Ni quenching effect is shifted to the acidic range, suggest-
ing these anions stabilise the formation of the nickel com-
2+
1
plex. In fact, the interaction of the Ni –[Os(L )(Me-
phtpy)]2 complex with anions would reduce the neat
charge of complex. This would decrease the repulsion with
the protonated cyclam core at acidic pH, resulting in a sta-
Anion Sensing
+
1
As we have found above, the complex [Os(L )(Me-
2
+
phtpy)] can protonate in acetonitrile/water media to give
highly positively charged species. We believed that some of
these formed species in mixed aqueous environments might
be used as chemosensors for the fluorogenic sensing of
anions. In order to check this possibility, certain anions
2+
1
2+
bilization of the Ni –[Os(L )(Me-phtpy)] system in a
wider pH range with the concomitant fluorescence quench-
ing due to the presence of the Ni2 cation in the cyclam
core. On the contrary, it seems that the anions are not able
+
2+
1
2+
to produce an extra stabilisation of Cu in the macrocycle
because the quenching effect of Cu2 is not shifted in the
presence of anions (see above). The pH value at which the
were added to acidified solutions of [Os(L )(Me-phtpy)]
+
2
+
and the emission of the Os(tpy)2 core monitored at dif-
ferent pH values. Unfortunately, the emission versus pH
Eur. J. Inorg. Chem. 2006, 2647–2655
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2651

J. V. Ros-Lis, R. Martínez-Máñez, J. Soto, C. McDonagh, A. Guckian
FULL PAPER
Figure 4. Emission intensity of the system [Os(L )(Me-phtpy)]2+–Ni2+–H –A as a function of the pH in acetonitrile/water (70:30 v/v)
1
+
n–
1
2+
–4
–3
mixture, [{Os(L )(Me-phtpy)} ] = 10 mol·dm , in the presence of 1 equiv. of anion unless otherwise noted (λex = 670 nm, λem
=
1
2+
2+
7
40 nm). [Os(L )(Me-phtpy)] (᭡), Ni (᭿), chloride (᭺), ADP, (ᮀ) and ATP (᭝).
demetallation is produced (about 4 units lower than in the Oxygen Sensing
2
+
case of Ni ) is probably too low and the compensation of
charge with anions is too weak to produce a significant ef-
fect in the fluorescence emission.
Finally, we have also tested the oxygen-sensing properties
of the osmium complex [Os(L )(Me-phtpy)] . There are a
1
2+
Selectivity can, for example, be achieved by changing the certain number of examples in the literature of the use of
2+
pH. For instance, addition of anions to the Ni – ruthenium–polypyridyl complexes for the detection of oxy-
1
2+
[19]
[20]
[
Os(L )(Me-phtpy)] system at pH 6.5 would result in a gen both in air
and as dissolved oxygen in water.
significant quenching only in the presence of ATP and ADP These studies are usually carried out either by immobilising
anions. A displacement of the quenching effect to more the complex in polymer films or in sol-gel-derived films. In
acidic pH is observed for anions in the order ATP Ͼ AMP these systems the oxygen concentration is proportional to
Ͼ chloride. A similar sequence would also be expected the degree of quenching of the ruthenium complex emission
(
ATP Ͼ ADP Ͼ chloride) in relation with the strength of in the presence of oxygen. In most of the cases the com-
1
the interaction of these anions with the {Ni[Os(L )(Me- plexes used for the preparation of oxygen sensors are Ru-
phtpy)]} complex. This is a somewhat attractive behav- (bipy)3 . On the contrary, there are very few examples of
iour that indicated that the Ni
4
+
2+
2
+
[21]
complex of the oxygen sensors using osmium complexes.
1
2+
[
Os(L )(Me-phtpy)] system acts as an anion fluorogenic
Thus, we have performed studies directed to test the use
of the [Os(L )(Me-phtpy)] complex as a gas-phase oxygen
1
2+
chemosensor in mixed aqueous environments.
2 2
Figure 5. Emission intensity in different N :O mixtures. MTEOS (᭿), MTEOS/TEOS, 1:1 (᭡), TEOS (᭹).
2652
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[
Os(tpy)
2
]2+ Core as Signalling Scaffolding for Development of Chemical Sensors
FULL PAPER
sensor. The studies have been carried out using sol-gel silica tivity of sol-gel-based thin films in which the complex has
films in which the osmium complex has been immobilised. been immobilised. All these studies indicate that osmium–
Additionally, in order to study the influence of the sol-gel terpyridine systems might behave as suitable and versatile
matrix we have used two different silica precursors, tetra- signalling subunits for different sensing purposes.
ethoxysilane (TEOS) and methyl tryethoxysilane
(
MTEOS). These precursors have a different number of hy-
drolysable positions and different speed of hydrolysis. Experimental Section
Furthermore, the MTEOS precursor has a methyl group
that increases the hydrophobicity of the final matrix. The
emission intensity of the [Os(L )(Me-phtpy)] complex in
the presence of different O /N mixtures (λ = 470 nm, λem ing to published methods. All other reagents were obtained from
General Remarks: [Os(Me-phterpy)O
ylphenyl]-2,2Ј:6Ј,2ЈЈ-terpyridine and 1-[4Ј-p-tolyl-(2,2Ј:6Ј,2ЈЈ-terpyr-
idyl)]-1,4,8,11-tetraazacyclotetradecane (L ) were prepared accord-
2 3 2
OH·(NO ) ], 4Ј-[4-meth-
1
2+
1
2
2
ex
=
740 nm) was measured and it was observed that oxygen commercial sources and were used as received.
produced a significant quenching of the osmium fluores-
cence. Figure 5 plots the emission intensity of the osmium
1
Synthesis of [Os(L )(Me-phtpy)][PF
6
]
2
: The synthesis of the osmium
1
2+
complex [Os(L )(Me-phtpy)] was carried out following a general
core as a function of the concentration of oxygen for three procedure described previously.[ A mixture of L (195.06 mg,
22]
1
2 3 2
different films made with TEOS, MTEOS and TEOS/ 0.35 mmol), [Os(Me-phterpy)O OH·(NO ) ] (238.6 mg, 0.35 mmol)
MTEOS in 1:1 molar ratio. Table 3 shows the characteris- and hydrazine hydrate (0.65 mL) as a reducing agent in water/THF
(
1:1 v/v) were heated to 50 °C for 10 min. Ammonium hexafluoro-
tics of the films prepared. As can be seen in Figure 5 and
Table 3, the choice of precursor has a significant effect on
the sensor response. Thus, films made with TEOS display a
larger quenching (63% of the original emission intensity) in
the presence of oxygen than that found for films with
MTEOS (13% of the original osmium emission). As can
also be seen in Table 3, the response time (time for the in-
phosphate was added to the resulting deep red solution and the
THF was eliminated under reduced pressure until the appearance
of a precipitate that was filtered. This precipitate was purified by
column chromatography over silica using acetonitrile/aqueous/sat-
urated KNO
finally isolated as its PF
complexes are toxic and they should be handled carefully. H NMR
3
aqueous solution 85:10:5 as eluent. The product was
–
6
salt (19 mg, 4.1% yield). The osmium
1
tensity to drop to 90% of the original value for a certain (300 MHz, CD CN, 25 °C) δ = 2.5–3.5 (m), 4.1 (s, 2 H), 7.13–7.20
3
concentration of O ) for films fabricated with TEOS was (m, 4 H), 7.368 (t, 4 H), 7.61–7.70 (m, 4 H), 7.864 (t, 4 H), 8.137
2
approximately 10 times larger than for the films made with (d, 2 H), 8.282 (d, 2 H), 8.699 (t, 4 H), 9.082 (s, 2 H), 9.109 (s,
+
1
2+
–
1
2
H). FAB : 1325 [Os(L )(Me-phtpy)] ·2PF
6
, 1180 [Os(L )(Me-
MTEOS. This is most likely due to the presence of silanol
groups in the former films that are able to retain some water
in the structure hence slowing the sensor response time. The
presence of the hydrophobic CH3 groups in the matrix
made with MTEOS avoids this effect and enhances the ac-
cessibility of the gas in the films, resulting in shorter re-
sponse times. The films prepared showed a good stability
and repeatability, indicating a very reversible process (see
inset of Figure 5).
2
+
–
1
2+
+
6
phtpy)] ·PF , 1033 [Os(L )(Me-phtpy)] . High resolution FAB
C
C
C
54
54
54
H
57
H
57
H
57
F
12
F
12
F
12
N
N
N
10OsP
10OsP
10OsP
2
2
2
(192) calcd. 1327.36662, found 1327.36961.
(190) calcd. 1325.36358, found 1325.35995.
(189) calcd. 1324.36328, found 1324.38587.
Preparation of the Films for Oxygen Sensing: Aqueous HCl (0.1 )
and the corresponding Si precursor were added to [Os(L )(Me-
phtpy)][PF ] (20 mg, 15 µ) solved in acetonitrile (see Table 4).
6 2
The solution was stirred for 30 min at room temperature and aged
in the oven at 70 °C for several hours. Glass slides were coated by
dip coating (3 mm·s ), and the films aged in the oven at 70 °C for
4 h.
1
–
1
Table 3. Response parameters of the films.
2
Film
Quenching O
2
[%] Response time 90% [s]
Table 4. Details for the preparation of the oxygen-sensing films.
MTEOS
MTEOS/TEOS, 1:1
TEOS
13
52
63
6.7
4.4
44.1
TEOS
[%]
ACN
[mL]
HCl
[mL]
MTEOS
[mL]
TEOS
[mL]
Oven
[h]
0
3.280
2.995
2.840
0.630
0.536
0.563
1.800
0.847
–
–
1
4
5
50
0.946
1.800
100
Conclusions
Physical Measurements and Instrumentation: Photochemical data
A new osmium bis-terpyridine derivative attached to a were obtained with a FS900CDT steady-state T-Geometry Fluo-
1
cyclam receptor unit has been prepared and its ability to rometer from Edinburgh Analytical Instruments. H NMR spectra
were recorded with a Varian Gemini spectrometer. Potentiometric
act as a fluorogenic chemosensor has been studied. The os-
1
2+
titrations were carried out in acetonitrile/water (70:30, v/v,
mium complex [Os(L )(Me-phtpy)] is able to act as a
fluorogenic sensor of copper and nickel in mixed aqueous
environments, whereas the emission of the osmium–terpyri-
dine core remains unaltered in the presence of other metal
cations. Additionally, changes in the emission of the nickel
complex have shown to be capable of sensing anions such
–
3
0
.1 mol·dm tetrabutylammonium perchlorate) under nitrogen
using a vessel water-thermostatted at 25.0± 0.1 °C. The tritant was
added by means of a Crison microburette 2031. Further details of
[23]
the potentiometric experiments have been published previously.
The concentrations of the metal ions were determined using stan-
[24]
dard methods. The computer program SUPERQUAD was used
as ATP, ADP and chloride in mixed aqueous solutions. Fi- to calculate the protonation and stability constants. The titration
nally we have demonstrated good gas-phase oxygen sensi- curves for each system (about 250 experimental points, correspond-
Eur. J. Inorg. Chem. 2006, 2647–2655
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2653

J. V. Ros-Lis, R. Martínez-Máñez, J. Soto, C. McDonagh, A. Guckian
FULL PAPER
ing to at least three titration curves; pH range investigated
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2
1
.5–10.2; ligand and metal ions concentration about
.0×10–3 mol·dm ) could be treated either as a single set or as
–3
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Acknowledgments
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We thank the Ministerio de Ciencia y Tecnología (projects
REN2002-04237-C02-01 and MAT2003-08568-C03-02) and UPV
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