pH-induced luminescence changes of chromophore-quencher tricarbonylpolypyridylrhenium(I) complexes with 4-pyridinealdazine

Article abstract of DOI:10.1002/ejic.200700860

The new chromophore-quencher tricarbonylrhenium(I) complexes [Re(4,4′-X₂-bpy)(CO)₃(PCA)]⁺, [(4,4′-X₂-bpy)(CO)₃Re(μ-PCA)Re(CO) ₃(4,4′-X₂-bpy)]²⁺, and [(4,4′-X₂

Full text of DOI:10.1002/ejic.200700860

FULL PAPER
DOI: 10.1002/ejic.200700860
pH-Induced Luminescence Changes of Chromophore-Quencher
Tricarbonylpolypyridylrhenium(I) Complexes with 4-Pyridinealdazine
Mauricio Cattaneo,[a] Florencia Fagalde,[a] Néstor E. Katz,*[a] Claudio D. Borsarelli,[b] and
Teodor Parella[c]
Keywords: Donor-acceptor systems / Rhenium / Photochemistry / Molecular devices / Mixed-valent compounds
The new chromophore-quencher tricarbonylrhenium(I) com-
plexes [Re(4,4Ј-X2-bpy)(CO)3(PCA)]+, [(4,4Ј-X2-bpy)(CO)3-
Re(µ-PCA)Re(CO)3(4,4Ј-X2-bpy)]2+, and [(4,4Ј-X2-bpy)(CO)3-
ReI(µ-PCA)RuII(NH3)5]3+ (X = Ph or CO2Me and PCA = 4-
protonation of both the pyridinyl and imine N atoms of PCA
in aqueous solution leads to unusual bell-shaped curves for
both the absorption and the emission intensities vs. pH be-
cause of opposite effects on the electronic delocalization of
PCA. This latter property can be employed to devise novel
luminescent pH sensors of the on-off-on type. The corre-
sponding unsymmetrical dinuclear complexes [(4,4Ј-X2-bpy)-
(CO)3ReI(µ-PCA)RuIII(NH3)5]4+ have been obtained in situ by
oxidizing the [ReI,RuII] precursors in CH3CN solution. Spec-
tral and electrochemical measurements with these com-
plexes lead to the values of the metal-to-metal electronic
–
pyridinealdazine) have been synthesized as their PF6 salts
and characterized by spectroscopic, electrochemical, and
photophysical techniques. In contrast to previously reported
species with X = Me or H, these complexes emit at room
temperature in CH3CN. The recovery of luminescence can
thus be ascribed to the change of energy levels induced by
adding electron-withdrawing substituents to the 2,2Ј-bipyr-
idine ring, since the emissive ReII(X2bpy–) excited state be- coupling elements.
comes much lower in energy than the ReII(PCA–) non-emis-
sive excited state. For the mononuclear and symmetric dinu-
clear rhenium(I) complexes with X = CO2Me, consecutive
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
bpy)(CO)3(PCA)]+ and [(4,4Ј-X2-bpy)(CO)3Re(µ-PCA)-
Re(CO)3(4,4Ј-X2-bpy)]2+ (bpy = 2,2Ј-bipyridine, X = CH3
or H, and PCA = 4-pyridinealdazine).[6,7] Emission from
the lowest-energy metal-to-ligand charge-transfer excited
Introduction
Tricarbonylpolypyridylrhenium(I) complexes of the type
fac-[Re(α-diimine)(CO)3L]+ (L = auxiliary monodentate li-
gand) have very interesting properties, such as relatively
high thermal and photochemical stabilities, high redox po-
tentials, and luminescence in the visible region that have
led to their applications as light switches,[1] ion sensors,[2]
intercalating agents for nucleic acids,[3] probes for biological
molecules,[4] and models in fundamental studies of energy-
and electron-transfer reactions relevant to solar energy con-
version.[5] Tuning the physicochemical properties of these
complexes by changing the nature of the diimine and/or
L offers a high degree of versatility for the design of new
supramolecular systems that may perform useful specific
functions.[5r]
3
state MLCT [dπ(Re)Ǟπ*(X2bpy)] is completely quenched
in these complexes by the presence of PCA in the mononu-
clear species at room temperature in CH3CN. Quenching of
luminescence has been observed before in tricarbonylpoly-
pyridylrhenium(I) complexes with electron-acceptor ligands
(L) such as N-methyl-4,4Ј-bipyridinium (MQ+), N-benzyl-
4,4Ј-bipyridinium (BzQ+), or N-(4-pyridyl)-β-(N-methylpyr-
idinium-3-yl)acrylamide (pyAm-Mepy+).[8] In the case of L
= PCA, the emission was also found to be quenched in
aqueous solution at high pH values, and when the pH was
lowered until one of the imine N atoms of PCA became
protonated, alteration of the ligand conjugation brought
about a recovery of luminescence. We thus concluded that
these complexes can be employed as luminescent pH sen-
sors.[7]
In this work, we have prepared and studied the new
complexes [Re(4,4Ј-X2-bpy)(CO)3(PCA)]+, [(4,4Ј-X2-bpy)-
(CO)3Re(µ-PCA)Re(CO)3(4,4Ј-X2-bpy)]2+, and [(4,4Ј-X2-
bpy)(CO)3ReI(µ-PCA)RuII/III(NH3)5]3+/4+ (X = Ph and
CO2Me), thereby completing the previously studied series.
We expected that introducing substituted bipyridines with
strongly electron-withdrawing groups would have different
We have recently described the novel chromophore-
quencher tricarbonylrhenium(I) complexes [Re(4,4Ј-X2-
[a] Instituto de Química Física, Facultad de Bioquímica, Química
y Farmacia, Universidad Nacional de Tucumán,
Ayacucho 491 (T4000INI), San Miguel de Tucumán, Argentina
Fax: +54-381-424-8169
E-mail: nkatz@fbqf.unt.edu.ar
[b] Instituto de Ciencias Químicas, Facultad de Agronomía y
Agroindustrias, Universidad Nacional de Santiago del Estero,
Av. Belgrano (S) 1912 (G4200ABT), Santiago del Estero,
Argentina
[c] Servei de RMN, Universitat Autònoma de Barcelona,
Bellaterra, 08193 Barcelona, Spain
Eur. J. Inorg. Chem. 2007, 5323–5332
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5323

M. Cattaneo, F. Fagalde, N. E. Katz, C. D. Borsarelli, T. Parella
FULL PAPER
effects on the emission properties. Indeed, we found that
the emission is recovered at high pH values, and proton-
ation changes in the PCA ligand can now be employed to
devise novel luminescent switches of the on-off-on type. The
effect of pH on the emission properties of the previously
reported complex [Re(bpy)(CO)3(4,4Ј-bpy)]+ (4,4Ј-bpy =
4,4Ј-bipyridine)[9] was also studied in order to compare the
behavior of complexes containing an auxiliary ligand with
one protonation site (4,4Ј-bpy) with that of those contain-
ing an auxiliary ligand with two protonation sites (PCA).
Finally, dinuclear unsymmetrical complexes with [Ru-
(NH3)5]n+ as electron donor (n = 2) or acceptor (n = 3)
groups have been prepared and characterized in relation to
intramolecular electron transfer in the Marcus inverted re-
gion.[10]
Results and Discussion
Syntheses, Solubilities, and IR and NMR Spectral
Characterizations
The synthetic methods used to obtain the new mono-
and dinuclear tricarbonylpolypyridylrhenium complexes de-
scribed in this work are slightly different from those pre-
viously developed in our laboratory.[6,7,11,12] All the com-
plexes are soluble in acetonitrile, dichloromethane, and ace-
tone and their purity was confirmed by chemical analysis,
CV, and IR, NMR, and UV/Vis spectroscopy. The ester
complexes are soluble in water and are stable for several
days in neutral solutions at low concentrations (Յ10–4 ),
whereas the phenyl complexes are not soluble in water, even
with Cl– as the counterion. The solubility of rhenium(I)
complexes in water has a direct impact on their applications
in radiopharmaceuticals.[7] The values of the carbonyl
stretching frequencies (νCO) in the IR spectra of complexes
1 and 2 (2032 and 1918 cm–1, respectively) are almost the
same as those reported for related complexes of the type
fac-[Re(4,4Ј-X2-bpy)(CO)3(L)]PF6,[6,7,11,12] whereas the val-
ues of the carbonyl stretching frequencies in the IR spectra
of complexes 5 and 6 (2037 and 1927 cm–1, respectively) are
slightly higher than those of 1 and 2, thereby indicating a
stronger π-backbonding effect from Re to the bipyridine
rings substituted with moderately strong electron-with-
drawing groups, as reported previously for complexes such
The structures of the ligands, along with the atom num-
bering for NMR assignments, are shown in Scheme 1; the
structures of the complexes are shown in Scheme 2.
as fac-[Re(4,4Ј-X2-bpy)(CO)3(L)]PF6 {X
= C(O)NEt2,
CO2Et2 and L = py-PTZ [10-(4-picolyl)phenothiazine]}.[13]
All these values of νCO are consistent with a facial configu-
ration of the carbonyl groups of local C3v symmetry. Both
the mono- and dinuclear species were completely charac-
terized by NMR spectroscopy (see Experimental Section).
Scheme 1. Ligands.
Scheme 2. A: Complexes 1 (X = Ph) and 5 (X = CO2Me); B: Complexes 2 (X = Ph) and 6 (X = CO2Me); C: Complexes 3 (X = Ph) and
7 (X = CO2Me).
5324
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5323–5332

pH-Induced Luminescence Changes
Six signals are observed for the protons of coordinated of the two pyridinyl nitrogen atoms of PCA in the mononu-
PCA in the mononuclear complexes due to coordination of clear species appear at δ ≈ +323 (free N) and +238 ppm (N
only one of the pyridinyl N atoms to one rhenium center. bonded to Re), whereas a single resonance appears at δ ≈
For the dinuclear complexes, only three signals are observed +238 ppm in the dinuclear complexes. Figure 1 shows the
for the protons of PCA due to the symmetry imposed by 1H NMR spectrum of complex 2 as a representative exam-
coordination of both pyridinyl nitrogen atoms of PCA to ple.
two rhenium centers. As a general trend, metal coordina-
tion induces slight displacements of the chemical shifts of
UV/Vis Spectra
the PCA moiety in all complexes to higher fields with re-
spect to the values in the free ligands. Diffusion coefficients
(D) were also determined to gain more insight into their
overall molecular size.[14] The D values (in CD3CN at
25 °C) of the mononuclear species 1 and 5 are 10.00ϫ10–10
and 10.23ϫ10–10 m2 s–1, respectively, and for the dinuclear
species 2 and 6 7.94ϫ10–10 and 8.31ϫ10–10 m2 s–1, respec-
tively. These changes of 25% in the D value between the
mono- and dinuclear species strongly agree with the corre-
sponding increase in molecular weights (about 75%) and
hydrodynamic radius (7.2 and 7.0 Å for the mononuclear
species 1 and 5 and 9.0 and 8.7 Å for the dinuclear com-
plexes 2 and 6, respectively). Furthermore, the characteris-
tic 15N chemical shifts of the nitrogen atoms from the dif-
ferent ligands provide direct experimental evidence for the
existence of an Re–N bond. Thus, the 15N chemical shifts
Table 1 shows the spectroscopic data for complexes 1–8
in CH3CN. Characteristic spin-allowed intraligand πǞπ*
transitions of the polypyridyl ligands give rise to intense
UV absorptions between 200 and 330 nm.[6,7,9] The mono-
nuclear complexes 1 and 5 present minimum-energy absorp-
tions at λmax = 346 and 352 nm, respectively, which can be
assigned, as observed previously in the analogous com-
plexes [Re(X2bpy)(CO)3(L)]+ (λmax = 330–380 nm),[6,7] to
overlapping metal-to-ligand charge transfers (MLCT) of
the type dπ(Re)Ǟπ*(X2bpy) and dπ(Re)Ǟπ*(PCA). These
same bands are redshifted to λmax = 350 and 355 nm,
respectively, in the homodinuclear ReI–ReI complexes 2 and
6, as reported previously for symmetric dinuclear complexes
of the type [(X2bpy)(CO)3Re(L)Re(CO)3(X2bpy)]2+ [6,7,9]
.
Figure 2 shows the UV/Vis spectrum of complex 6 as a rep-
resentative example. It is noteworthy that both the πǞπ*
transitions of X2bpy as well as the MLCT
dπ(Re)Ǟπ*(X2bpy) transitions are shifted to longer wave-
lengths when increasing the electron-accepting strength of
the substituent X, as already reported for similar spe-
cies.[9,13,15] On the other hand, when comparing the effect
of ligands L in complexes of the type [Re(bpy)(CO)3L]+, a
Figure 1. 1H NMR spectrum of complex
500.13 MHz.
2
in CD3CN at
Figure 2. UV/Vis spectrum of [{4,4Ј-(MeO2C)2-bpy}(CO)3Re(µ-
PCA)Re(CO)3{4,4Ј-(MeO2C)2-bpy}]2+ (6) in CH3CN.
Table 1. UV/Vis spectroscopic data (in CH3CN) at 22 °C.
–3
Complex
λmax [nm] (10
ε
[–1 cm–1])
max
[Re(Ph2bpy)(CO)3(PCA)]+ (1)
346 (18.6), 330 (29.1), 284 (57.6)
350 (26.8), 330 (40.4), 284 (76.5)
560 (14.6), 350 (20.2), 330 (30.8), 283 (60.4)
496 (4.7), 350 (24.4), 330 (36.4), 281 (66.6)
352 (9.2), 335 (18.8), 320 (21.8), 290 (27.0)
[(Ph2bpy)(CO)3Re(µ-PCA)Re(CO)3(Ph2bpy)]2+ (2)
[(Ph2bpy)(CO)3ReI(µ-PCA)RuII(NH3)5]3+ (3)
[(Ph2bpy)(CO)3ReI(µ-PCA)RuIII(NH3)5]4+ (4)
[Re{4,4Ј-(MeO2C)2-bpy}(CO)3(PCA)]+ (5)
[{4,4Ј-(MeO2C)2-bpy}(CO)3Re(µ-PCA)Re(CO)3{4,4Ј-(MeO2C)2-bpy}]2+ (6) 355 (17.5), 335 (31.0), 321 (31.8), 291 (34.9)
[{4,4Ј-(MeO2C)2-bpy}(CO)3ReI(µ-PCA)RuII(NH3)5]3+ (7)
[{4,4Ј-(MeO2C)2-bpy}(CO)3ReI(µ-PCA)RuIII(NH3)5]4+ (8)
560 (13.6), 355 (10.9), 334 (20.9), 320 (23.3), 284 (28.8)
501 (2.0), 355 (13.6), 335 (24.5), 321 (26.6), 280 (30.6)
Eur. J. Inorg. Chem. 2007, 5323–5332
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5325

M. Cattaneo, F. Fagalde, N. E. Katz, C. D. Borsarelli, T. Parella
FULL PAPER
blueshift of the (ReǞbpy) MLCT band can be observed oxidation. In effect, the dπ(Ru)Ǟπ*(PCA) MLCT band at
with increasing electron-withdrawing power of L; in effect, λmax = 560 nm disappears completely, and a new MMCT
the values of λmax in CH3CN decrease in the order 4-Etpy band (combined with an LMCT band) appears near
(λmax = 352 nm)[16] Ͼ 4,4Ј-bpy (λmax = 340 nm)[9] Ͼ PCA 500 nm. When reverting the oxidation process in the spec-
(λmax = 332 nm),[7] thus pointing to an increasing stabiliza- troelectrochemical experiment, the intensity of the
tion of the dπ(Re) energy levels.
dπ(Ru)Ǟπ*(PCA) MLCT band at λmax = 560 nm is not re-
With respect to the UV/Vis spectra of the unsymmetrical covered completely, which is consistent with the results ob-
dinuclear complexes 3, 4, 7, and 8, we can assign the bands tained upon Sn2+ reduction. We consider that, besides mere
in the ReI–RuII complexes 3 and 7 at λmax = 350 and electron transfer, the formation of a cis isomer is possibly
560 nm to dπ(Re)Ǟπ*(X2bpy, PCA) and dπ(Ru)Ǟπ*(PCA) favored on oxidation.
MLCT transitions, respectively. For the ReI–RuIII com-
plexes 4 and 8, which were prepared in situ by oxidation of
Photophysical Properties
3 and 7, respectively, with bromine, the bands at λmax
=
560 nm disappear completely, and the new bands that ap-
pear at λmax = 497 and 501 nm, respectively, whose values
were obtained by Gaussian deconvolution, as shown in Fig-
ure 3 for complex 4, can be assigned to ReIǞRuIII MMCT
transitions, with some contribution from PCAǞRuIII
LMCT bands, as discussed previouly for the analogous spe-
cies with Me2bpy and bpy.[6,7]
In contrast to the previously reported complexes with
Me2bpy and bpy,[6,7] the complexes studied in this work
emit in CH3CN solution at room temperature. The shape
and position of the emission maxima are typical of ReII-
(diimine–) MLCT excited states.[13] Table 2 shows the data
obtained at 22 °C for the emission maxima, lifetimes, and
emission quantum yields for complexes 1, 2, 5, and 6. The
same lifetimes were obtained both by laser flash photolysis
and by pulsed luminescence decay in argon-saturated solu-
tions, thereby indicating that the luminescence properties
are not due to impurities. Furthermore, the reported chemi-
cal analyses and the spectral and electrochemical data point
to a high degree of purity of the complexes under study.
The observed quantum yields, which were calculated and
corrected by taking into account the refractive indices of
the solvents and absorbance differences, as described in the
literature,[17] are much lower (by a factor of about 102) than
those reported for related complexes with the same chromo-
phore,[13] which can be attributed to intramolecular lumi-
nescence quenching induced by the presence of the PCA
ligand.[6,7] This quenching was attributed in the Me2bpy de-
rivative to crossing to a neighboring [(X2bpy)(CO)3ReII-
(PCA–)]+ excited state, as confirmed by laser flash photoly-
sis and low-temperature emission measurements.[6] The ra-
diative rate constants kr (approx. 103–104 s–1), on the other
hand, are lower than those expected (approx. 105 s–1) for
ReI complexes of the type [Re(bpy)(CO)3X] (X = substi-
tuted pyridine).[13] Energy transfer from 3MLCT (ReǞbpy)
to PCA to give an excited intraligand triplet state (3PCA*)
that decays non-radiatively to the ground state may be the
cause of these low values of kr. In support of this argument,
it should be noted that changes in photophysical properties
brought about by MLCT or intraligand excited states
placed very close to the luminescent excited state have al-
Figure 3. Gaussian deconvolution of the visible bands of the
mixed-valent complex [(Ph2bpy)(CO)3ReI(µ-PCA)RuIII(NH3)5]4+
(4; approx. 3ϫ10–5 ) obtained by Br2 oxidation (in CH3CN) of
the dinuclear complex 3.
Figure 4 shows the results of a controlled-potential elec-
trolysis of complex 3 in CH3CN at 800 mV. The results ob-
tained are consistent with those obtained upon bromine
Table 2. Photophysical data (in CH3CN) of complexes of the
type [Re(4,4Ј-X2-bpy)(CO)3(PCA)]+ and [(4,4Ј-X2-bpy)(CO)3Re-
(µ-PCA)Re(CO)3(4,4Ј-X2-bpy)]2+ at 22 °C.
[a]
X
Complex
λem [nm]
Φem
τ [ns]
Ph
1
2
5
6
552
552
570
565
0.0010
0.0045
0.0004
0.0017
690
865
132
187
Figure 4. Controlled potential electrolysis of a CH3CN solution of
[(Ph2bpy)(CO)3ReI(µ-PCA)RuII(NH3)5]3+ (3; approx. 2ϫ10–5 ) at
800 mV in an OTTLE cell. The arrow indicates increasing times: t
= 0, 1, 2, 3, 4, 8, and 10 min.
CO2Me
[a] Calculated as described previously.[7,17]
5326
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5323–5332

pH-Induced Luminescence Changes
ready been observed in related complexes.[18] Moreover, a Protonation Equilibria
recent review by Schmehl et al. gives a detailed discussion
These studies were carried out with complexes 1, 2, 5,
of similar systems in which reversible energy transfer be-
3
3
and 6 since the heterodinuclear complexes 3, 4, 7, and 8
have very low stability in aqueous solutions, as already re-
ported for analogous complexes.[7] Table 3 shows the acidity
constants obtained from the spectrophotometric and spec-
trofluorometric titrations of all the measured complexes.
Complexes 1 and 2 were measured in a 1:4 mixture of
CH3CN/H2O because of their poor solubility in water. The
effect of the organic solvent could be to lower all pKa values
with respect to those in aqueous solution. In water, com-
plexes 5 and 6 emit very weakly between λmax = 565 nm at
pH 8.5 and λmax = 575 nm at pH 1.5 (λex = 380 nm). When
performing the experiment at constant ionic strength, a de-
crease in pH brings about not only an increased emission
from the ReǞX2bpy MLCT excited state but also a notice-
able isomerization/hydrolysis process of the PCA and
(CO2Me)2bpy ligands, which becomes enhanced at extreme
pH values. Evidence for ligand isomerization has been pre-
sented before,[6,7] and evidence of ester hydrolysis emerges
when comparing the final spectra, obtained after several
hours, with the spectra of the corresponding complexes of
carboxylic acid substituted bipyridines. On the other hand,
more accurate values of the protonation constants could be
obtained when the protonation studies were done with
larger volumes of solution since the decomposition pro-
cesses were negligible during the time taken for the mea-
surements. This latter method has therefore been used for
reporting the values in Table 3, except for the reference
complex [Re(bpy)(CO)3(4,4Ј-bpy)]+, which proved to be
very stable in aqueous solutions.
tween a long-lived IL state and a MLCT state leads to
3
longer lived MLCT emission.[19] In complex 2, laser flash
photolysis showed the emergence of a transient with λmax
=
380 nm, which is typical of the absorption of the radical
anion of bpy ligands, as shown in Figure 5, together with
emission bleaching at λmax = 550 nm. The lifetime (τ) is
865 ns, which is about two orders of magnitude higher than
the lifetime of the excited state of [Re(Me2bpy)(CO)3-
(PCA)]+. These findings are consistent with formation of
an ReII(Ph2bpy–) excited state. The mononuclear complex
[Re(Ph2bpy)(CO)3(PCA)]+ has a lower lifetime (τ = 690 ns)
but with a similar transient spectrum. We conclude that,
since Ph2bpy is a much better π-acceptor than Me2bpy and
bpy, the energy of the emissive ReII(Ph2bpy–) excited state
becomes much lower than that of the non-emissive ReII-
(PCA–) excited state, with consequent recovery of lumines-
cence at room temperature. As shown in Table 2, the quan-
tum yields and lifetimes for the complexes containing
(CO2Me)2bpy are lower than those of the Ph2bpy com-
plexes. Luminescence in the series of complexes [Re(4,4Ј-
X2-bpy)(CO)3(PCA)]+ and [(4,4Ј-X2-bpy)(CO)3Re(µ-PCA)-
Re(CO)3(4,4Ј-X2-bpy)]2+ can thus be finely tuned by chang-
ing the nature of X: when X = Me or H, the emission is
almost completely quenched,[6,7] whereas when X = Ph or
CO2Me, emission is partially recovered because of the less
effective intramolecular quenching by PCA.
Consistent and reproducible values of pKa for both
ground and excited states could only be obtained for the
mononuclear complex 5 starting from basic aqueous solu-
tions. Both the absorbance and luminescence data can be
fitted with a three-species diagram involving protonation
equilibria of the ground and excited states of PCA com-
plexes, as shown in Scheme 3. Two ground-state pKa values
were detected by fitting the UV/Vis absorptivity changes
with pH at λ = 330 nm, as shown in Figure 6(a). The first
of these (pKa1 = 2.15) corresponds to one of the imine N
atoms of coordinated PCA and the second one (pKa2
=
5.59) corresponds to the free pyridine N atom of coordi-
nated PCA. This latter value is 0.4 units larger than the
Figure 5. Transient spectra of [(Ph2bpy)(CO)3Re(µ-PCA)Re-
(CO)3(Ph2bpy)]2+ (2) in deaerated CH3CN solution at 22 °C.
Table 3. Values of pKa for the ground and excited states of complexes 1, 2, 5, and 6 at 22 °C in aerated aqueous solution.
Complex
pKa
pKa*
[Re(Ph2bpy)(CO)3(PCA)]+ (1)[a]
pKa1 Ͻ 1
pKa2 = 5.08Ϯ0.23
pKa1 Ͻ 1
pKa1 = 2.15Ϯ0.03
pKa2 = 5.59Ϯ0.06
pKa Ͻ 1
pKa1* Ͻ 1
[(Ph2bpy)(CO)3Re(µ-PCA)Re(CO)3(Ph2bpy)]2+ (2)[a]
[Re{4,4Ј-(MeO2C)2-bpy}(CO)3(PCA)]+ (5)[b]
pKa1* Ͻ 1
pKa1* = 2.34Ϯ0.17
pKa2* = 3.33Ϯ0.05
not detected
[{4,4Ј-(MeO2C)2-bpy}(CO)3Re(µ-PCA)Re(CO)3{4,4Ј-(MeO2C)2-bpy}]2+ (6)[c]
[Re(bpy)(CO)3(4,4Ј-bpy)]+[c]
pKa = 3.70Ϯ0.07
pKa1* = 4.03Ϯ0.05
[a] In CH3CN/H2O (1:4). [b] Data obtained from bulk experiments. [c] Data obtained from experiments at constant ionic strength (I =
0.1 ).
Eur. J. Inorg. Chem. 2007, 5323–5332
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5327

M. Cattaneo, F. Fagalde, N. E. Katz, C. D. Borsarelli, T. Parella
FULL PAPER
value for the free ligand, probably because of the ReIǞPCA The result is a bell-shaped curve which can be reproduced
π-backbonding effect. Both pKa values are very similar to in an inverted way when fitting the emission intensities at
those corresponding to the previously studied complex different pH values. This is in contrast to similar carbon-
[Re(bpy)(CO)3(PCA)]+ (pKa1 = 1.98 and pKa2 = 5.41),[7] ylrhenium complexes, where only sigmoid curves have been
thereby indicating little influence of the nature of the sub- observed.[2,7] As shown in Figure 6(b), complex 5 presents
stituent X on the basicity of coordinated PCA.
two adjustable excited-state pKa* values: the first one,
pKa1* = 2.34, corresponds to the imine N atom of coordi-
nated PCA and the second one, pKa2* = 3.33, corresponds
to the free pyridine N atom of coordinated PCA. No better
fitting could be obtained between pH = 5 and 9, and in any
case the observed deviations from the fits are within the
experimental uncertainties of the intensity measurements.
The first basicity constant of the excited state is higher than
that of the ground state (by around 0.2 pK units), as ex-
pected for tricarbonylrhenium(I) complexes,[2] but lower
than that of the complex [Re(bpy)(CO)3(PCA)]+ (by around
0.4 units),[7] thereby evidencing the effect of decreasing the
electron density on PCA as a result of including a more
electron-accepting bipyridine ligand. Strikingly, the value of
pKa2* is 2.26 units lower than that of the ground state,
which points to the fact that since more electron density has
been delocalized in the excited state, [(CO2Me)2bpy–(CO)3-
ReII(PCA)]+, to the substituted bpy ligand, less electron
density is available at the remote N atom of the coordinated
PCA ligand, and possibly evidencing some contribution of
the pKa of the carboxylate-substituted bpy that results from
hydrolysis of the ester-substituted bpy.
Scheme 3. Three-species diagram for the protonated equilibria of
the ground and excited states of PCA complexes. B is the non-
protonated species.
Figure 6. Variation of (a) absorbance A (at λ = 330 nm) and (b)
emission intensity (at λex = 380 nm and λem = 570 nm) for [Re{4,4Ј-
(MeO2C)2-bpy}(CO)3(PCA)]+ (5) in aqueous buffer, with pH. The
lines indicate the best fits using a three-species model in each case.
As found before in related unsubstituted bpy com-
plexes,[7] protonation of the pyridine N atom of PCA leads
to an absorbance increase at λ = 330 nm due to an in-
creased dπ(Re)-π*(PCA) coupling. The MLCT band corre-
sponding to this transition is expected to fall in this re-
gion.[6] Protonation of the imine N atom of PCA leads in-
stead to an absorbance decrease at λ = 330 nm because the
energy of the lowest-lying π* orbital of PCA is increased
Figure 7. Variation of (a) absorbance A (at λ = 350 nm) and (b)
emission quantum yield Φem (at λex = 350 nm and λem = 552 nm)
for [Re(bpy)(CO)3(4,4Ј-bpy)]+ in aqueous buffer, with pH. The lines
and dπ(Re)-π*(PCA) coupling is consequently diminished. indicate the best fits using a two-species model in each case.
5328 Eur. J. Inorg. Chem. 2007, 5323–5332
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

pH-Induced Luminescence Changes
There is only one protonation site possible in the model cies 2, and the ReI–RuII dinuclear species 3 are very similar,
complex [Re(bpy)(CO)3(4,4Ј-bpy)]+, namely the free pyridyl with E1/2 values between 1.8 and 1.9 V, with some degree of
N atom. As shown in Figure 7, a value of the ground-state irreversibility, as expected for carbonyl(diimine)rhenium(I)
pKa of 3.70, which agrees with those reported for polypyrid- complexes.[6,7,11,12] The redox potentials for the ReII/ReI
ylruthenium complexes, is obtained by fitting absorbance couple in complexes 5, 6, and 7 are higher than 2.0 V and
changes with pH.[20] However, we obtain a pKa* value of could not be detected. The PCA ligand is reduced irrevers-
4.03 for the excited state when fitting the emission quantum ibly at Epeak ≈ –0.8/–1.0 V in all complexes except 7. The
yields with pH. This value is higher than that of the ground first reduction of X2bpy appears at Epeak ≈ –1.2 V, while a
state by 0.33 units and points to much less electron density second irreversible reduction at Epeak ≈ –1.4 V can be attrib-
being delocalized onto the unsubstituted bpy ligand in the uted to an ReI/0 couple, as reported previously for the re-
excited state.
lated Me2bpy complex.[6]
We conclude that when the free pyridine N atom of coor-
The voltammetric wave that appears in the asymmetric
dinated PCA is protonated, the π* excited state of PCA ReI–RuII dinuclear complexes 3 and 7 at E1/2 = 0.46 and
decreases in energy, and the emission is partially quenched, 0.48 V, respectively, which is absent in the mononuclear rhe-
as observed before for MQ+,[8] but when the imine N atom nium complexes, can be readily assigned to the RuIII/RuII
of coordinated PCA is protonated, the π* excited state of couple by comparison with similar complexes.[6,7] The high
PCA increases in energy, due to alteration of the ligand ∆Ep values (92–134 mV) suggest redox-induced ligand
conjugation, and emission is recovered again. In effect, isomerization processes, and experimental evidence of re-
when decreasing the pH from 4 to 1, the band assigned to dox-induced ligand isomerization has been described pre-
the π-π* transition of PCA at λmax = 290 nm is shifted to viously.[6] Varying the sweep rate gave no better ∆E values
lower wavelengths, thereby indicating a proton-induced en- at higher sweep rates. The differences between the redox
ergy increase of the π* levels of PCA. The mononuclear potentials of both metallic couples in 3 and 7 are: ∆E1/2
=
species 5 can therefore be used as a novel luminescent pH E1/2(ReII/ReI) – E1/2(RuIII/RuII) = 1.52 and 1.54 V, respec-
sensor of the on-off-on type.
tively.
Electrochemistry
Intramolecular Electron Transfer
Table 4 shows the electrochemical data for the studied
The reorganization energy, λ, for the intramolecular elec-
complexes. The redox potentials for the ReII/ReI couple of tron transfer through the PCA bridge can be calculated
the mononuclear ReI species 1, the ReI–ReI dinuclear spe- from the Marcus–Hush equations and the experimental
Table 4. Electrochemical data (in CH3CN) at 22 °C.[a]
Complex
Process
E1/2 [V] (∆Ep [mV])
Epeak [V]
[{4,4Ј-(Ph)2-bpy}(CO)3Re(PCA)]+ (1)
Re2+/+
1.85
–0.98
PCA0/–
Ph2bpy0/–
Re+/0
–1.14 (51)
–1.28 (94)
[{{4,4Ј-(Ph)2-bpy}(CO)3Re}2PCA]2+ (2)
Re2+/+
1.86
–0.78
–1.18
PCA0/–
Ph2bpy0/–
Re+/0
–1.27 (97)
1.77 (166)
0.48 (134)
[{4,4Ј-(Ph)2-bpy}(CO)3ReI(µ-PCA)RuII(NH3)5]3+ (3)
Re2+/+
Ru3+/2+
PCA0/–
–0.95
–1.16
–1.31
Ͼ2
Ph2bpy0/–
Re+/0
[{4,4Ј-(MeO2C)2-bpy}(CO)3Re(PCA)]+ (5)
Re2+/+
PCA0/–
–0.81
(MeO2C)2bpy0/–
Re+/0
–0.96 (172)
–1.32
Ͼ2
–0.81
[{{4,4Ј-(MeO2C)2-bpy}(CO)3Re}2PCA]2+ (6)
[{4,4Ј-(MeO2C)2-bpy}(CO)3ReI(µ-PCA)RuII(NH3)5]3+ (7)
Re2+/+
PCA0/–
(MeO2C)2bpy0/–
Re+/0
–0.96 (172)
–1.32
Ͼ2
Re2+/+
Ru3+/2+
PCA0/–
0.46 (92)
–0.64
–1.01
–1.22
(MeO2C)2bpy0/–
Re+/0
[a] All CV data were obtained at a scan rate of 200 mVs–1.
Eur. J. Inorg. Chem. 2007, 5323–5332
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5329

M. Cattaneo, F. Fagalde, N. E. Katz, C. D. Borsarelli, T. Parella
FULL PAPER
on-line PC for data transfer and analysis. Typically, about 100–200
laser cycles with the excitation laser operating at 15 Hz were
averaged in order to obtain the decay times with a suitable signal-
to-noise ratio. NMR spectra were recorded in CD3CN with an Av-
ance Bruker 500.13 MHz spectrometer. Full 1H, 13C, and 15N
chemical shift assignments were performed by collecting 2D COSY,
2D 1H-13C HSQC, 2D 1H-13C HMBC, 2D 1H-15N HMBC, and
2D NOESY (mixing time: 500 ms) experiments. Diffusion NMR
coefficients were measured at 25 °C using the LEDBP pulse se-
quence (diffusion time: 150 ms) with sample spinning at 20 Hz to
data of the MMCT transition in the heterodinuclear com-
plexes 4 and 8[21] to be approximately 0.9 eV, a value similar
to that obtained previously for the analogous complexes
with Me2bpy and bpy.[6,7] The values determined for the
metal–metal electronic coupling elements (HAB = 8.8ϫ102
and 6.2ϫ102 cm–1 for the Ph and CO2Me derivatives,
respectively) are consistent with those obtained previously
for mixed-valent symmetric and unsymmetrical dinuclear
complexes of PCA.[6,7] These complexes might be consid- minimize convection effects.[14] Argon was bubbled through the
solutions for 15 min prior to electrochemical and photophysical
measurements. Chemical analyses for C, H, and N were done at
INQUIMAE, University of Buenos Aires, Argentina, with an esti-
mated error of Ϯ0.5%.
ered as prototypes for simulating primary charge separa-
tions in the inverted region.
Conclusions
Preparation of PCA and 4,4Ј-(MeO2C)2-bpy: The ligands 4-pyr-
idinealdazine and 4,4Ј-bis(methoxycarbonyl)-2,2Ј-bipyridine were
prepared as described previously.[23,24]
Continuing our previous studies on proton-induced lu-
minescence of chromophore-quencher tricarbonylpolypyr-
idylrhenium(I) complexes, we have shown in this work that
quenching of luminescence in these complexes can not only
be turned on and off by changing the substituents on the
2,2Ј-bipyridine acceptor group but also by changing the
acidity of their aqueous solutions. In particular, the mono-
nuclear complex [Re{4,4Ј-(MeO2C)2-bpy}(CO)3(PCA)]+
Preparation of [Re(Ph2bpy)(CO)3(PCA)]PF6·H2O (1) and [(Ph2bpy)-
(CO)3Re(µ-PCA)Re(CO)3(Ph2bpy)](PF6)2 (2): These salts were pre-
pared by a procedure similar to that reported in the literature.[6,7]
In a typical experiment, [Re(CO)5Cl] (181 mg, 0.50 mmol) and 4,4Ј-
diphenyl-2,2Ј-bipyridine (154 mg, 0.50 mmol) were heated at reflux
in toluene (20 mL) for 1 h. It was cooled and then precipitated with
hexane (40 mL). The solid obtained after additional cooling was
can be employed as a novel luminescent pH sensor of the collected, washed with hexane and diethyl ether, and dried in vacuo.
The resulting complex [Re(Ph2bpy)(CO)3Cl] (272 mg, 0.443 mmol)
and Ag(CF3SO3) (114 mg, 0.443 mmol) were heated at reflux in thf
(40 mL) for 30 min. PCA (139 mg, 0.664 mmol) was then added to
the reaction mixture and heating at reflux was continued for 2 h.
After removal of AgCl by filtration, the solvent was removed in a
rotary evaporator to give a yellow oil. This oil was dissolved in
80 mL of 3:1 (v:v) MeOH/H2O, 2 g of NH4PF6 dissolved in 20 mL
of water was added, and the mixture was put in the freezer. The
on-off-on type rather than the recently reported lumines-
cent off-on-off switches based on polypyridylruthenium
complexes.[22]
Experimental Section
Materials and Techniques: All chemicals used were p.a. grade.
CH3CN was distilled from P4O10. Tetrakis(n-butyl)ammonium yellow precipitate that formed was filtered and washed with copi-
hexafluorophosphate (TBAH) was recrystallized from ethanol
three times and dried at 150 °C for 72 h. IR spectra were recorded
(as KBr pellets) with an FTIR Perkin–Elmer Spectrum RX-I spec-
trophotometer. UV/Vis spectra were recorded with a Varian Cary
50 spectrophotometer for solutions in 1-cm cells. Redox titrations
were performed stoichiometrically by adding aliquots of a stock
solution of Br2 in CH3CN which had previously been standardized
by known procedures.[20] Reductions were carried out by adding
solid SnCl2 to CH3CN solutions of the oxidized complexes. Elec-
trochemical measurements were performed in CH3CN (0.1 
TBAH) with a BAS Epsilon electrochemical equipment. A stan-
dard three-electrode compartment cell was used, with Ag/AgCl
(3  KCl) as a reference electrode, vitreous C as a working elec-
trode, and Pt wire as an auxiliary electrode. All redox potentials
(E1/2) are referenced to Ag/AgCl. Spectroelectrochemical measure-
ments were performed in an OTTLE (optically transparent thin
layer electrolysis) type cell from BAS. Emission studies were per-
formed with a Shimadzu RF-5301 PC spectrofluorometer in 1-cm
fluorescence cells. Laser-flash photolysis (LFP) experiments were
carried out with a Q-switched Nd:YAG laser (Continuum Minilite
II) generating 355-nm pulses (fwhm: 10 ns; 5 mJ per pulse). The
signals were recorded with a Luzchem m-LFP 112 system and fed
into a transient Tektronik TDS 3032B recorder. Time-resolved lu-
minescence detection was carried out with a home-made system
composed of an f/4 monochromator (PTI-101, 1200 blazes) cou-
pled with a red-extended PMT (Hamamatsu R928) placed at right
angles to the excitation laser pulse. The output of the detector was
fed to the Tektronix TDS3032B digital oscilloscope linked to an
ous amounts of H2O and three times with portions of diethyl ether.
This solid was dissolved in a minimum amount of 1:4 (v/v) acetoni-
trile/dichloromethane, adsorbed onto a silica gel (Kieselgel 60) col-
umn and eluted with the same solvent. The yellow fractions (the
first is complex 2 and the second is complex 1) were concentrated
to dryness, redissolved in acetone, and precipitated with hexane. 1:
Yield: 66 mg (14%). C37H28F6N6O4PRe (951.84): calcd. C 46.7, H
2.97, N 8.83; found C 46.5, H 2.74, N 8.10. IR (KBr): ν = 2032
˜
(s), 1918 (s), 1615 (m), 1542 (w), 1474 (m), 1413 (m), 1235 (w), 840
(s), 766 (m), 739 (w), 695 (w), 627 (w), 558 (m) cm–1. 1H NMR
(500.13 MHz, CD3CN, 25 °C): δ = 9.24 (d, J = 5.8 Hz, 2 H, HA),
8.78 (d, J = 1.8 Hz, 2 H, HC), 8.68 (dd, J = 1.6, 4.5 Hz, 2 H, H2,
8.44 (s, 1 H, H4), 8.42 (s, 1 H, H4Ј), 8.41 (dd, J = 5.4, 1.7 Hz, 2 H,
H2Ј), 8.06 (dd, J = 1.8, 5.8 Hz, 2 H, HB), 7.97 (m, 4 H, HD), 7.68
(m, 4 H, H3 and H3Ј), 7.62 (m, 6 H, HE and HF) ppm. 13C NMR
(125.6 MHz CD3CN, 25 °C): δ = 123.0 (C3), 123.5 (CC), 125.7
(C3Ј), 127.0 (CB), 128.7 (CD), 130.5 (CE), 132.1 (CF), 136.2 (C),
141.4 (C), 145.0 (C), 151.6 (C2), 153.7 (CA), 153.8 (C2Ј), 154.9 (C),
157.3 (C), 158.4 (C4Ј), 161.4 (C4), 190.0 (CCOtrans), 197.1 (CCOcis
)
ppm. 15N NMR (50.69 MHz CD3CN, 25 °C; chemical shifts ex-
1
tracted from the H-15N HMBC spectrum): δ = 234.8 (NAЈ), 239.3
(N1Ј), 322.1 (N1), 374.3 (N5Ј), 379.7 (N5) ppm. 2: Yield: 48 mg
(12%). C62H42F12N8O6P2Re2 (1657.4): calcd. C 44.9, H 2.55, N
6.76; found C 44.8, H 2.51, N 6.37. IR (KBr): ν = 2032 (s), 1918
˜
(s), 1615 (m), 1542 (w), 1474 (m), 1413 (m), 1236 (w), 840 (s), 766
(m), 739 (w), 696 (w), 627 (w), 558 (m) cm–1. 1H NMR
(500.13 MHz, CD3CN, 25 °C): δ = 9.25 (d, J = 6.0 Hz, 4 H, HA),
8.79 (d, J = 1.7 Hz, 4 H, HC), 8.42 (dd, J = 1.7, 5.2 Hz, 4 H, H2),
5330
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5323–5332

pH-Induced Luminescence Changes
8.32 (s, 2 H, H4), 8.07 (dd, J = 5.6, 1.6 Hz, 4 H, HB), 7.98 (dd, J
(s, 6 H, Me) ppm. 13C NMR (125.6 MHz CD3CN, 25 °C): δ = 54.3
= 5.6, 1.6 Hz, 8 H, HD), 7.68–7.64 (m, J = 1.7, 5.2 Hz, 16 H, H3, (C of Me), 123.0 (C3), 125.3 (C), 125.7 (C3Ј), 129.0 (CB), 141.4 (C),
HF and HE) ppm. 13C NMR (125.6 MHz CD3CN, 25 °C): δ =
142.7 (C), 145.0 (C), 151.6 (C), 153.8 (C), 153.8 (C2Ј), 156.1 (C2),
157.6 (C4Ј), 158.3 (C), 161.4 (C4), 164.4 (C) ppm. 15N NMR
122.6 (CC), 124.8 (C3), 126.0 (CB), 128.2 (CD), 129.8 (CF), 131.3
(CE), 135.6 (C), 143.3 (C) 152.8 (C), 153.2 (C2), 154.4 (CA), 156.1 (50.69 MHz CD3CN, 25 °C; chemical shifts extracted from the
(C), 157.1 (C4), 191.6 (CCOtrans), 196.3 (CCOcis) ppm. The 1H NMR 1H-15N HMBC spectrum): δ = 248.0 (NAЈ), 237.2 (N1Ј),322.9
spectrum is shown in Figure 1.
(N1), 373.0 (N5Ј), 375.5 (N5) ppm. 6: Yield: 48 mg (12%).
C46H34F12N8O14P2Re2 (1585.2): calcd. C 34.9, H 2.2, N 7.1; found
Preparation of
[(Ph2bpy)(CO)3ReI(µ-PCA)RuII(NH3)5](PF6)3·
C 34.7, H 2.1, N 6.8. IR (KBr): ν = 2037 (s), 1927 (s), 1735 (s),
˜
6CH2Cl2 (3): Complex 1 (30 mg, 0.032 mmol) was stirred in ace-
tone (10 mL) under argon for 30 min and [Ru(NH3)5(H2O)](PF6)2
(16 mg, 0.032 mmol), prepared as described in the literature,[25] was
then added. The mixture was stirred under argon in the dark for
2 h. Diethyl ether (100 mL) was added to precipitate the complex,
which was re-dissolved in a minimum amount of acetone. Dichloro-
methane (40 mL) and diethyl ether (40 mL) were added to re-pre-
cipitate the complex, which was filtered and washed with dichloro-
methane, diethyl ether, and water. It was finally dissolved in aceto-
nitrile and purified by chromatography on Sephadex LH-20, using
acetonitrile as the eluting solvent. The first, blue fraction was col-
lected, concentrated to dryness, re-dissolved in acetone, precipi-
tated with dichloromethane, filtered, washed with dichloromethane
and diethyl ether, and dried in vacuo over P4O10. Yield: 30 mg
(66%). C43H53Cl12F18N11O3P3ReRu (1919.6): calcd. C 26.9, H 2.8,
N 8.0; found C 26.6, H 2.8, N 8.5.
1618 (m), 1560 (w), 1440 (m), 1408 (m), 1327 (m), 1307 (m), 1266
(m), 1232 (m), 1134 (w), 981 (w), 843 (s), 785 (w), 766 (m), 722 (w),
1
644 (w), 558 (m) cm–1. H NMR (500.13 MHz, CD3CN, 25 °C): δ
= 9.37 (dd, J = 5.6, 0.6 Hz, 4 H, HA), 8.88 (dd, J = 1.7, 0.6 Hz, 4
H, HC), 8.27 (dd, J = 1.7, 5.2 Hz, 4 H, H2), 8.27 (s, 2 H, H4), 8.20
(dd, J = 5.6, 1.6 Hz, 4 H, HB), 7.57 (dd, J = 1.7, 5.2 Hz, 4 H, H3),
4.00 (s, 12 H, Me) ppm. 13C NMR (125.6 MHz CD3CN, 25 °C): δ
= 54.2 (CMe), 125.3 (CC), 125.7 (C), 129.0 (C), (C), 142.7 (C), 144.6
(C3), 153.9 (C2), 156.0 (CA), 157.5 (C4), 158.5 (C), 164.4 (C), 191.6
(CCOtrans), 196.3 (CCOcis) ppm. 15N NMR (50.69 MHz CD3CN,
25 °C; chemical shifts extracted from the 1H-15N HMBC spec-
trum): δ = 250.5 (NAЈ), 238.1 (N1), 377.6 (N5) ppm.
Preparation of [{4,4Ј-(MeO2C)2-bpy}(CO)3ReI(µ-PCA)RuII(NH3)5]-
(PF6)3·CH2Cl2 (7): Complex 5 (50 mg, 0.056 mmol) was stirred in
acetone (10 mL) under argon for 30 min, and [Ru(NH3)5-
(H2O)](PF6)2 (28 mg, 0.056 mmol), prepared as described in the lit-
erature,[25] was added. The mixture was stirred continuously under
argon in the dark for 2 h. Diethyl ether (100 mL) was then added
to precipitate the complex, which was redissolved in a minimum
amount of acetone. Dichloromethane (60 mL) and diethyl ether
(60 mL) were added to re-precipitate the complex, which was fil-
tered and washed with dichloromethane, diethyl ether, and water.
It was finally dissolved in acetonitrile and purified by chromatog-
raphy on Sephadex LH-20 using acetonitrile as the eluting solvent.
The first, blue fraction was collected, concentrated to dryness, re-
dissolved in acetone, precipitated with dichloromethane, filtered,
washed with dichloromethane and diethyl ether, and dried in vacuo
over P4O10. Yield: 55 mg (71%). C31H39Cl2F18N11O7P3ReRu
(1470.8): calcd. C 25.3, H 2.7, N 10.5; found C 25.1, H 2.7, N 10.8.
Preparation of [(Ph2bpy)(CO)3ReI(µ-PCA)RuIII(NH3)5]4+ (4): This
heterodinuclear ion was generated in situ by adding bromine to
an acetonitrile solution of 3 or by electrochemical oxidation. The
oxidation progress was monitored by measuring the absorbance
changes in the 200–1100 nm range.
Preparation of [Re{4,4Ј-(MeO2C)2-bpy}(CO)3(PCA)]PF6·H2O (5)
and [(4,4Ј-(MeO2C)2-bpy)(CO)3Re(µ-PCA)Re(CO)3{4,4Ј-(MeO2C)2-
bpy}](PF6)2 (6): [Re(CO)5Cl] (181 mg, 0.50 mmol) and 4,4Ј-bis-
(methoxycarbonyl)-2,2Ј-bipyridine (136 mg, 0.50 mmol) were
heated at reflux in toluene (20 mL) for 1 h. Precipitation was
achieved by adding hexane (20 mL) to the cooled solution. The red
solid obtained after additional cooling was collected, washed with
hexane and diethyl ether, and dried in vacuo. The resulting complex
[Re{4,4Ј-(MeO2C)2-bpy}(CO)3Cl] (280 mg, 0.485 mmol) and
Ag(CF3SO3) (114 mg, 0.443 mmol) were heated at reflux in thf
(40 mL) for 30 min. PCA (153 mg, 0.728 mmol) was then added to
the reaction mixture and heating at reflux was continued for 2 h.
After removal of AgCl by filtration, the solvent was removed in a
rotary evaporator to give an orange oil. This oil was dissolved in
40 mL of 3:1 (v/v) MeOH/H2O, 2 g of NH4PF6 dissolved in 10 mL
of water was added, and the mixture was put in the freezer. The
yellow precipitate that formed was filtered and washed with copi-
ous amounts of H2O and three times with portions of diethyl ether.
It was then dissolved in a minimum amount of 1:4 (v/v) acetoni-
trile/dichloromethane, adsorbed onto a silica gel (Kieselgel 60) col-
umn and eluted with the same solvent. The yellow fractions (the
first is complex 6 and the second is complex 5) were concentrated
to dryness, redissolved in acetone, and precipitated with hexane. 5:
Yield: 107 mg (24%). C29H24F6N6O8PRe (915.72): calcd. C 38.0,
Preparation
of
[{4,4Ј-(MeO2C)2-bpy}(CO)3ReI(µ-PCA)RuIII
-
(NH3)5]4+ (8): This heterodinuclear ion was generated in situ by
adding bromine to an acetonitrile solution of 7 or by electrochemi-
cal oxidation. The oxidation progress was monitored by measuring
the absorbance changes in the 200–1100 nm range.
Preparation of [Re(bpy)(CO)3(4,4Ј-bpy)]PF6 (9): This complex was
available from previous studies.[11]
Protonation Studies: For pKa determinations, pH titrations by spec-
trophotometric and spectrofluorometric techniques were per-
formed with two kinds of experiments. In the first one, Britton and
Robinson’s buffer, which consists of a mixture of 0.04  acetic acid,
0.04  phosphoric acid, and 0.04  boric acid with variable
amounts of a solution of 0.2  NaOH, was used, and the ionic
strength was fixed at 0.1  with a solution of NaCl. The complexes
were dissolved in aqueous solutions of 0.1  NaCl and stirred for
about 5 h until the filtered solution showed a final average ab-
H 2.6, N 9.2; found C 37.9, H 2.1, N 9.0. IR (KBr): ν = 2037 (s),
˜
1927 (s), 1735 (s), 1616 (m), 1560 (w), 1440 (m), 1408 (m), 1327 sorbance of about 0.4. Each sample was prepared just before mea-
(m), 1308 (m), 1266 (m), 1232 (m), 1134 (w), 981 (w), 843 (s), 785
(w), 766 (m), 722 (w), 644 (w), 558 (m) cm–1. 1H NMR
(500.13 MHz, CD3CN, 25 °C): δ = 9.40 (dd, J = 5.7, 0.7 Hz, 2 H,
surement with 2 mL of buffer and 2 mL of a stock solution of the
complex. The pH value of each fraction was then determined with
a Metrohm pH-meter. Reversal of the pH values of the extreme
HA), 8.91 (dd, J = 1.6, 0.7 Hz, 2 H, HC), 8.70 (dd, J = 1.7, 4.5 Hz, fractions was done by adding 3  HCl or 3  NaOH. In each ti-
2 H, H2), 8.45 (s, 1 H, H4), 8.42 (s, 1 H, H4Ј), 8.30 (dd, J = 5.2,
1.6 Hz, 2 H, H2Ј), 8.22 (dd, J = 1.6, 5.6 Hz, 2 H, HB), 7.69 (dd, J
= 1.7, 4.5 Hz, 2 H, H3), 7.64 (dd, J = 1.6, 5.2 Hz, 2 H, H3Ј), 4.02
tration, 12–15 points were recorded. In the second experiment, the
ionic strength was not regulated. Only one solution (approx.
30 mL) was used, with Britton and Robinson’s buffer, which con-
Eur. J. Inorg. Chem. 2007, 5323–5332
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5331

M. Cattaneo, F. Fagalde, N. E. Katz, C. D. Borsarelli, T. Parella
FULL PAPER
Feliz, G. Ferraudi, Inorg. Chem. 1998, 37, 2806–2810; m) D. R.
Striplin, G. A. Crosby, Coord. Chem. Rev. 2001, 211, 163–175;
n) D. M. Dattelbaum, M. K. Itokazu, N. Y. Murakami Iha,
T. J. Meyer, J. Phys. Chem. A 2003, 107, 4092–4095; o) O. S.
Wenger, L. M. Henling, M. W. Day, J. R. Winkler, H. B. Gray,
Inorg. Chem. 2004, 43, 2043–2048; p) I. E. Pomestchenko, D. E.
Polyansky, F. N. Castellano, Inorg. Chem. 2005, 44, 3412–3421;
q) K. K. W. Lo, K. H. K. Tsang, W. K. Hui, N. Zhu, Inorg.
Chem. 2005, 44, 6100–6110; r) M. Ghirotti, C. Chiorboli, M. T.
Indelli, F. Scandola, M. Casanova, E. Iengo, E. Alessio, Inorg.
Chim. Acta 2007, 360, 1121–1130.
sists of 0.02  acetic acid, 0.02  phosphoric acid and 0.02  boric
acid with a small amount of 3  NaOH solution up to pH = 10.
The complex was dissolved by adding a drop of acetonitrile and
rapidly mixing with the buffer solution. 3  HCl (40–60 µL) was
added to lower the pH to 1.5. In each titration, 40–50 points were
recorded. Both methods gave consistent results. Luminescence and
absorption measurements were performed with air-saturated solu-
tions. We used a previously reported technique to fit the obtained
data for all complexes.[7] The experimental data were fitted with
the PSI Plot software. The fits were all satisfactory, giving consis-
tent and reasonable values of pKa and pKa*.
[6] M. Cattaneo, F. Fagalde, N. E. Katz, A. M. Leiva, R. Schmehl,
Inorg. Chem. 2006, 45, 127–136.
[7] M. Cattaneo, F. Fagalde, N. E. Katz, Inorg. Chem. 2006, 45,
6884–6891.
Acknowledgments
[8] a) T. D. Westmoreland, H. LeBozec, R. W. Murray, T. J. Meyer,
J. Am. Chem. Soc. 1983, 105, 5952–5954; b) P. Chen, E. Dan-
ielson, T. J. Meyer, J. Phys. Chem. 1988, 92, 3708–3711; c) P.
Chen, M. Curry, T. J. Meyer, Inorg. Chem. 1989, 28, 2271–2280;
d) R. Duesing, G. Tapolsky, T. J. Meyer, J. Am. Chem. Soc.
1990, 112, 5378–5379; e) N. E. Katz, S. L. Mecklenburg, T. J.
Meyer, Inorg. Chem. 1995, 34, 1282–1284.
We thank the Universidad Nacional de Tucumán (UNT), the
Consejo Nacional de Investigaciones Científicas
y Técnicas
(CONICET), and the Agencia Nacional de Promoción Científica
y Tecnológica (ANPCyT), all from Argentina, for financial sup-
port. The Deutscher Akademischer Austauschdienst (DAAD) is
gratefully acknowledged for the donation of the Bioanalytical Sys-
tems equipment. F. F., N. E. K., and C. D. B are members of the
Research Career from CONICET, Argentina. M. C. thanks
CONICET for a graduate fellowship and specially his father for
being an exemplary person.
[9] G. Tapolsky, R. Duesing, T. J. Meyer, Inorg. Chem. 1990, 29,
2285–2297.
[10] R. A. Marcus, N. Sutin, Biochem. Biophys. Acta 1985, 811,
265–322.
[11] F. Fagalde, N. E. Katz, J. Coord. Chem. 2001, 54, 367–377.
[12] M. G. Mellace, F. Fagalde, N. E. Katz, Polyhedron 2003, 22,
369–374.
[13] L. Sacksteder, A. P. Zipp, E. A. Brown, J. Streich, J. N. Demas,
B. A. DeGraff, Inorg. Chem. 1990, 29, 4335–4340.
[14] N. Esturau, F. Sánchez-Ferrando, C. Roumestand, M. A.
Delsuc, J. A. Gavin, T. Parella, J. Magn. Reson. 2001, 153, 48–
55.
[1] A. S. Polo, M. K. Itokazu, K. M. Frin, A. O. de Toledo Patroc-
ínio, N. Y. Murakami Iha, Coord. Chem. Rev. 2006, 250, 1669–
1680.
[2] B. Higgins, B. A. DeGraff, J. N. Demas, Inorg. Chem. 2005, 44,
6662–6669, and references cited therein.
[15] L. Worl, R. Duesing, P. Chen, L. Della Ciana, T. J. Meyer, J.
Chem. Soc. Dalton Trans. 1991, 849–858.
[16] J. K. Hino, L. Della Ciana, W. J. Dressick, B. P. Sullivan, Inorg.
Chem. 1992, 31, 1072–1080.
[3] H. D. Stoeffler, N. B. Thornton, S. L. Temkin, K. S. Schanze,
J. Am. Chem. Soc. 1995, 117, 7119–7128.
[4] K. K.-W. Lo, K. H.-K. Tsang, K.-S. Sze, Inorg. Chem. 2006,
45, 1714–1722.
[5] a) M. Wrighton, D. L. Morse, J. Am. Chem. Soc. 1974, 96, 998–
1003; b) T. D. Westmoreland, K. S. Schanze, P. E. Neveux Jr, E.
Danielson, B. P. Sullivan, P. Chen, T. J. Meyer, Inorg. Chem.
1985, 24, 2596–2597; c) K. Kalyanasundaram, J. Chem. Soc.
Faraday Trans. 1986, 2, 2401–2415; d) P. Chen, T. D. West-
moreland, E. Danielson, K. S. Schanze, D. Anthon, P. E. Ne-
veux Jr, T. J. Meyer, Inorg. Chem. 1987, 26, 1116–1126; e) P.
Chen, E. Danielson, T. J. Meyer, J. Phys. Chem. 1988, 92, 3708–
3711; f) A. Juris, S. Campagna, I. Bidd, J.-M. Lehn, R. Ziessel,
Inorg. Chem. 1988, 27, 4007–4011; g) B. P. Sullivan, J. Phys.
Chem. 1989, 93, 24–26; h) W. Kaim, H. E. A. Kramer, C.
Vogler, J. Reiker, J. Organomet. Chem. 1989, 367, 107–115; i)
L. Wallace, D. C. Jackman, D. P. Rillema, J. W. Merkert, Inorg.
Chem. 1995, 34, 5210–5214; j) S. Berger, A. Klein, W. Kaim, J.
Fiedler, Inorg. Chem. 1998, 37, 5664–5671; k) L. C. Abbott,
C. J. Arnold, T.-Q. Ye, K. C. Gordon, R. N. Perutz, R. E. Hes-
ter, J. N. Moore, J. Phys. Chem. A 1998, 102, 1252–1260; l) M.
[17] J. V. Caspar, T. J. Meyer, J. Am. Chem. Soc. 1983, 105, 5583–
5590.
[18] D. J. Stufkens, A. Vlcˇek Jr, Coord. Chem. Rev. 1998, 177, 127–
179, and references cited therein.
[19] X. Wang, A. Del Guerzo, R. H. Schmehl, J. Photochem. Pho-
tobiol. C 2004, 5, 55–77.
[20] F. Fagalde, N. E. Katz, J. Chem. Soc. Dalton Trans. 1993, 571–
575.
[21] C. Creutz, Prog. Inorg. Chem. 1983, 30, 1–73.
[22] F. Gao, H. Chao, F. Zhon, B. Peng, L.-N. Ji, Inorg. Chem.
Commun. 2007, 10, 170–173.
[23] D. M. Ciurtin, Y.-B. Dong, M. D. Smith, T. Barclay, H.-C.
zur Loye, Inorg. Chem. 2001, 40, 2825–2834.
[24] F. H. Case, J. Am. Chem. Soc. 1946, 68, 2574–2577.
[25] J. E. Sutton, H. Taube, Inorg. Chem. 1981, 20, 3125–3134.
Received: August 17, 2007
Published Online: October 31, 2007
5332
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5323–5332