Spectroscopic and structural investigations reveal the signaling mechanism of a luminescent molybdate sensor

Article abstract of DOI:10.1021/ic1019422

A heteroditopic ligand H₂-L consisting of a dihydroxybenzene (catechol)-unit linked via an amide bond to a pyridyl-unit and its methyl-protected precursor Me₂-L were synthesized, characterized, and their photophysical properties investigated. The three accessible protonation states of the ligand, H₂-L+, H₂-L, and H-L-, showed distinct 1 H NMR, absorption and emission spectroscopic characteristics that allow pH-sensing. The spectroscopic signatures obtained act as a guide to understand the signaling mechanism of the luminescent pH and molybdate sensor [Re-(bpy)(CO)₃(H₂-L)]+. It was found that upon deprotonation of the 2-hydroxy group of H₂-L, a ligand-based absorption band emerges that overlaps with the Re(dπ)-bpy metal-to-ligand charge transfer (MLCT) band of the sensor, reducing the quantum yield for emission on excitation in the 370 nm region. In addition, deprotonation of the catechol-unit leads to quenching of the emission from the Re(dn)→ bpy 3MLCT state, consistent with photoinduced electron transfer from the electron-rich, deprotonated catecholate to the Re-based luminophore. Finally, reaction of 2 equiv of [Re(bpy)(CO)₃(H₂-L)]+ with molybdate was shown to give the zwitterionic Mo(VI) complex [MoO₂{Re(CO) ₃-(bpy)(L)}₂], as confirmed by electrospray ionization (ESI) mass spectrometry and X-ray crystallography. The crystal structure determination revealed that two fully deprotonated sensor molecules are bound via their oxygen-donors to a cis-dioxo-MoO₂ center.

Full text of DOI:10.1021/ic1019422

Inorg. Chem. 2011, 50, 1105–1115 1105
DOI: 10.1021/ic1019422
Spectroscopic and Structural Investigations Reveal the Signaling Mechanism of a
Luminescent Molybdate Sensor
Vincent A. Corden, Anne-K. Duhme-Klair,* Sarah Hostachy, Robin N. Perutz, and Nicole Reddig
Department of Chemistry, University of York, York YO10 5DD, U.K.
†
€
Hans-Christian Becker and Leif Hammarstrom
Department of Photochemistry and Molecular Science, Uppsala University, P.O. Box 523, S-75120 Uppsala,
Sweden. † Present address: Physical Chemistry, Department of Chemical and Biological Engineering, Chalmers
University of Technology, SE-41296 Gothenburg, Sweden
Received September 22, 2010
A heteroditopic ligand H2-L consisting of a dihydroxybenzene (catechol)-unit linked via an amide bond to a pyridyl-unit
and its methyl-protected precursor Me2-L were synthesized, characterized, and their photophysical properties
investigated. The three accessible protonation states of the ligand, H3-Lþ, H2-L, and H-L-, showed distinct 1H NMR,
absorption and emission spectroscopic characteristics that allow pH-sensing. The spectroscopic signatures obtained
act as a guide to understand the signaling mechanism of the luminescent pH and molybdate sensor [Re-
(bpy)(CO)3(H2-L)]þ. It was found that upon deprotonation of the 2-hydroxy group of H2-L, a ligand-based absorption
band emerges that overlaps with the Re(dπ)fbpy metal-to-ligand charge transfer (MLCT) band of the sensor,
reducing the quantum yield for emission on excitation in the 370 nm region. In addition, deprotonation of the catechol-
unit leads to quenching of the emission from the Re(dπ)fbpy 3MLCT state, consistent with photoinduced electron
transfer from the electron-rich, deprotonated catecholate to the Re-based luminophore. Finally, reaction of 2 equiv of
[Re(bpy)(CO)3(H2-L)]þ with molybdate was shown to give the zwitterionic Mo(VI) complex [MoO2{Re(CO)3-
(bpy)(L)}2], as confirmed by electrospray ionization (ESI) mass spectrometry and X-ray crystallography. The crystal
structure determination revealed that two fully deprotonated sensor molecules are bound via their oxygen-donors to a
cis-dioxo-MoO2 center.
Introduction
axial ligand, such as a halide or pyridine-derivative) are
now proving almost as versatile.8-14
Metal-based luminophores with appended dihydroxyben-
zene (catechol)-units are known to function as sensors,1-3
redox switches,4 photo-5,6 and electrocatalysts.7 Functiona-
lized [Ru(bpy)3]2þ-type luminophores are still the most
commonly studied in the field, but [Re(bpy)(CO)3L]-type
luminophores (with bpy=2, 20-bipyridine and L=monodentate
Several catechol- or phenol-linked pyridine derivatives
that are structurally related to the pyridine-derivative used
as the axial ligand in our study have recently been developed,
and their pH-dependent photophysical properties have been
studied.15-17 A phenol-pyridinium biaryl chromophore, for
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(14) Lo, K. K.-W.; Louie, M.-W.; Zhang, K. Y. Coord. Chem. Rev. 2010,
254, 2603.
*To whom correspondence should be addressed. E-mail: akd1@
york.ac.uk.
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2011 American Chemical Society
Published on Web 01/10/2011
pubs.acs.org/IC

1106 Inorganic Chemistry, Vol. 50, No. 3, 2011
Corden et al.
Scheme 1. Structures of the Accessible Protonation States of [Re(bpy)(CO)3(H2-L)]þ, [Re(bpy)(CO)3(Bn2-L)]þ, H2-L, and Me2-L, with Most Likely
Intramolecular Hydrogen-Bond Arrangements Indicated in Each Case
example, has been investigated as potential photoacid.18 In
addition, pyridine-linked catechol19 or phenol20 derivatives
with more than one protonation state have shown promise as
wide-range or ratiometric pH sensors.
We are interested in catechol-appended luminophores
since they can be applied in the sensing of biologically
relevant oxometalates, such as molybdate, tungstate, or
vanadate.21 In our previous work, we have developed a series
of luminescent chemosensors for such species,22-24 with po-
tential analytical applications in environmental monitoring,25
biochemical research,26,27 and medical analysis.28,29 For
these applications, selectivity for oxometalates over oxo-
2-
2-
anions, such as SO4 and HPO42-, which resemble MoO4
2-
and HVO4 in size, shape, and charge, is a key require-
ment. High selectivity can be achieved with catecholate-based
binding units since the transition metal centers are able to
coordinate to catecholates by enhancing their coordination
number to six,30-32 while sulfur and phosphorus cannot react
in this way. In addition, selectivity for oxometalates over
cations, such as Fe(III) or Cu(II) can be achieved by pH-
control.22
The molybdate sensor[Re(bpy)(CO)3(H2-L)]þ (Scheme 1),
in which the catechol-unit is connected to the monodentate
pyridyl ligand of the luminophore via an amide linker, signals
the presence and concentration of molybdate (MoO42-) in
solution through a decrease in luminescence intensity.23 Here
we report the results of a combined spectroscopic and
structural study to further characterize the signaling mecha-
nism that gives rise to the decrease in the emission intensity of
the sensor upon deprotonation or analyte binding.
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Article
The Re(I)-center in [Re(bpy)(CO)3(H2-L)]þ is coordinated
Inorganic Chemistry, Vol. 50, No. 3, 2011 1107
tube, in four-windowed 10 mm quartz cuvettes. IR spectra were
recorded on a Unicam-Mattson RS-FTIR spectrometer.
Synthesis of Me2-L. 2, 3-Dimethoxybenzoic acid (3.62 g, 20
mmol) was dissolved in dry dimethylformamide (DMF, 150
mL) and combined with triethylamine (5.575 mL, 40 mmol),
DMAP (0.489 g, 4 mmol) and p-nitrobenzenesulfonyl chloride
(4.390 g, 20 mmol). The reaction mixture was stirred for 20 min
before being cooled to 0 °C in an ice bath. A solution of
4-aminopyridine (2.061 g, 22 mmol) in dry DMF (50 mL) was
then added slowly to the reaction mixture. The resulting orange
solution was stirred overnight at room temperature to ensure
completion of the reaction. The solution was then filtered to
remove triethyl ammonium chloride. After removal of the
DMF, the remaining residue was extracted 3 times with CH2Cl2
(50 mL). The combined organic fractions were washed, alter-
nately with saturated sodium hydrogen carbonate solution and
distilled water, a total of 3 times. Finally, the resulting organic
phase was dried over magnesium sulfate. Removal of the solvent
yields the crude product, which was recrystallized from CH2Cl2-
hexane. Yield: 4.75 g (18.4 mmol, 92%).
to two electron-poor acceptor ligands, the bpy ligand and the
pyridine-unit of H2-L. In contrast, the 2, 3-dihydroxybenz-
amide-unit of H2-L is electron rich and becomes reducing
upon deprotonation. Upon UV/vis excitation, several charge
transfer transitions are thus conceivable for the deprotonated
form of sensor [Re(bpy)(CO)3(H-L)], including Re(dπ)fbpy
metal-to-ligand charge transfer (MLCT), intraligand charge
transfer from the deprotonated catecholate to the pyridine-
unit (ILCT), and ligand-to-ligand charge transfer (LLCT)
from the deprotonated catecholate unit to the bpy ligand.
To facilitate the interpretation of the electronic absorption
and emission spectra of [Re(bpy)(CO)3(H-L)], we have
synthesized the benzyl-protected derivative [Re(bpy)(CO)3-
(Bn2-L)]þ, the ligand H2-L, and its methyl-protected deri-
vative Me2-L to serve as control compounds. The benzyl-
protected sensor cannot deprotonate and shows the spec-
troscopic characteristics of Re(dπ)fbpy charge transfer
(MCLT). In addition, pH-dependent studies of the ligand
H2-L provide the spectroscopic signatures of its three
accessible protonation states, H3-Lþ, H2-L, and H-L-.
The comparison of the spectroscopic characteristics of
[Re(bpy)(CO)3(Bn2-L)]þ, H3-Lþ, H2-L, H-L-, HMe2-Lþ,
and Me2-L with those of the molybdate sensor in its pro-
tonated and deprotonated states, [Re(bpy)(CO)3(H2-L)]þ
and [Re(bpy)(CO)3(H-L)], respectively, allows identifica-
tion of important aspects of the photophysical processes
involved in the signaling mechanism.
IR (KBr), ν (cm-1): 3305 (s, br, νNH); 3013 (m); 2936 (w); 1693
(s, νCO); 1581 (s). 1H NMR, (500 MHz, CDCl ) δ [ppm]: 10.29
03
(1H, s, N-H); 8.57 (2H, d, J = 5.5 Hz, Ha, Ha ); 7.79 (1H, dd, J
0
= 8.0, 1.5 Hz, Hf); 7.67 (2H, d, J = 6.0 Hz, Hb, Hb ); 7.26 (1H, t,
J = 8.0 Hz, Hg); 7.17 (1H, dd, J = 8.0, 1.5 Hz, Hh); 4.05 (3H, s,
OMe); 3.97 (3H, s, OMe). 13C NMR, (500 MHz, CDCl3) δ
a0
[ppm]: 163.8 (CdO); 152.6 (Cd); 150.1 (Ca, C ); 147.4 (Cj); 145.7
0
(Ci); 125.8; 125.0; 123.0; 116.5; 114.1 (Cb, Cb ); 61.8 (OMe); 56.2
(OMe). EI-MS: m/z = 259 (100%, [MþH]þ). HRMS Calc. for
C14H15N2O3, 259.2852. Found [MþH]þ, 259.2854 (difference
0.2 mDa).
The time-resolved infrared spectra of [Re(bpy)(CO)3L]
complexes show strong carbonyl bands, the position of which
reflects the electron density of the Re-center. MLCT excited
states give rise to high frequency shifts of the carbonyl bands,
whereas photoinduced electron transfer from ligands ap-
pended to the Re-center causes low frequency shifts.33-37 A
related paper will be published in due course, describing
insights obtained by time-resolved IR spectroscopy. These
complement and extend the findings reported here.
Synthesis of [H3-L]Br. Under an inert atmosphere, a solution
of Me2-L (0.5 g, 1.94 mmol) in dry CH2Cl2 (25 mL) was cooled
to 0 °C. A solution of BBr3 in CH2Cl2 (1 M, 10.75 mL) was
added dropwise. The resulting yellow suspension was stirred at
room temperature overnight. Water (8.60 mL) was added care-
fully at 0 °C (HBr evolution), and the mixture was stirred for 2 h
to ensure complete hydrolysis. The precipitate was then filtered
off and washed with water. The resulting solid was dissolved in
methanol and evaporated three times to remove boron com-
pounds. Finally, the residue was dissolved in methanol, the
resulting solution was acidified with HBr, and the product
precipitated by addition of diethyl ether. Yield: 0.39 g (1.25
mmol, 64%).
Experimental Section
General Methods. Bn2-L, [Re(bpy)(CO)3(Bn2-L)][PF6], and
[Re(bpy)(CO)3(H2-L)][PF6] were synthesized as described
previously,23 with minor modifications, most notably the in-
crease of the hydrogen pressure to 3.5 bar to ensure complete
deprotection of [Re(bpy)(CO)3(Bn2-L)](PF6) to [Re(bpy)(CO)3-
(H2-L)](PF6).
Commercially available reagents and solvents were obtained
from Aldrich and Fluka and used as supplied. Solvents were
dried over molecular sieves where required. NMR spectra were
recorded on a Bruker AMX500 instrument (1H at 500.13 MHz,
13C at 125.76 MHz). Electrospray ionization (ESI) mass spectra
were recorded on a Bruker microTOF electrospray mass spec-
trometer or a ThermoFinnigan LCQ-Classic instrument. UV/
vis spectra were measured on an Agilent 8453 spectropho-
tometer in 10 mm quartz cuvettes. Uncorrected emission and
excitation spectra were recorded on a Hitachi F-4500 fluori-
meter, equipped with a red-sensitive R928F photomultiplier
IR (KBr), ν (cm-1): 3463 (m, νO-H/N-H); 3372 (m, νO-H/N-H);
3253 (s, br, νO-H/N-H); 1665 (s, νCdO); 1506 (m); 1331; 1223. 1H
0
NMR, (500 MHz, CD3OD) δ: 8.66 (2H, d, J = 6.0 Hz, Ha, Ha );
0
8.33 (2H, d, J = 6.0 Hz, Hb, Hb ); 7.50 (1H, d, J = 8.0 Hz, Hf);
7.09 (1H, d, J = 7.5 Hz Hh); 6.89 (1H, t, J = 8.0 Hz, Hg). 13
C
NMR, (500 MHz, CD3OD) δ: 168.0 (CdO); 153.1 (Cd); 147.3
0
(Cj0); 146.1 (Ci); 142.5 (Ca, Ca ); 119.7, 119.6; 117.0; 115.3 (Cb,
Cb ). ESI-MS: m/z = 231 (100%, [MþH]þ). HRMS Calc. for
C12H11N2O3, 231.0764. Found [M]þ, 231.0766 (difference 0.2
mDa). Elemental Analysis: found C, 43.85; H, 3.99; N, 8.48%.
Required for C12H11N2O3Br ꢀ 1 H2O: C, 43.80; H, 3.98; N,
8.51%.
(33) Kuimova, M. K.; Alsindi, W. Z.; Dyer, J.; Grills, D. C.; Jina, O. S.;
ꢀ
Matousek, P.; Parker, A. W.; Portius, P.; Sun, X. Z.; Towrie, M.; Wilson, C.;
Yang, J. X.; George, M. W. Dalton Trans. 2003, 3996.
ꢀ
ꢀ
(34) Busby, M.; Matousek, P.; Towrie, M.; Vlcek, A. J. Phys. Chem. A
2005, 109, 3000.
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(36) Gabrielsson, A.; Hartl, F.; Zhang, H.; Lindsay Smith, J. R.; Towrie,
Synthesis of [MoO2{Re(bpy)(CO)3(L)}2]. [Re(bpy)(CO)3(H2-
L)]PF6 (10 mg, 0.01 mmol) was dissolved in the minimum
ꢀ
M.; Vlcek, A.; Perutz, R. N. J. Am. Chem. Soc. 2006, 128, 4253.
ꢀ
(37) Vlcek, A. Top. Organomet. Chem. 2010, 29, 73.

1108 Inorganic Chemistry, Vol. 50, No. 3, 2011
Corden et al.
amount of a solvent system consisting of a 2:1 acetonitrile/water
mixture, and the pH of the solution was adjusted to 3 with dilute
hydrochloric acid. Five microliters of an aqueous solution of
sodium molybdate dihydrate (242 mg, 1 mmol dissolved in 1 mL
water) were added to obtain a 2:1 ratio of sensor to molybdate.
Equilibration of the mixture resulted in an increase of the pH to
6. Crystals suitable for single X-ray diffraction were obtained
after 6 h. Yield: 3.4 mg (0.002 mmol, 47%).
Table 1. Crystal Data and Summary of Data Collection and Refinement Details
for [MoO2{Re(bpy)(CO)3(L)}2] CH3CN 7(O)
3
3
empirical formula
formula weight
temperature
C52H35MoN9O21Re2
1590.23
100(2) K
wavelength
0.71073 A
triclinic
P1
a = 13.3454(14) A
b = 15.1335(16) A
c = 16.1209(17) A
R = 104.903(2)°
β = 113.199(2)°
γ = 92.983(3)°
2848.5(5) A3
crystal system
space group
unit cell dimensions
1
IR (KBr), ν (cm-1) (CO): 2029(s), 1925 (s). H NMR: (500
0
MHz, DMF-d7): δ: 9.48 (dd, 4 H, Ha, Ha ); 8.85 (dd, 4 H,
0
0
0
Hd, Hd ); 8.52 (dt, 4 H, Hc, Hc ); 6.99 (d, 4 H, Hf, Hf ); 6.76
(d, 2 H, Hg); 6.57 (d, 2 H, Hi); 6.11 (t, 2 H, Hh). T = -40 °C: 8.19
0
0
(d, 4 H, He, He ), 8.14 (t, 4 H, Hb, Hb ). ESI-MS: m/z = 1461
Mo). Elemental Analysis:
(100%, [MþNa]þ for 187Re and
96
V
Z
2
found C, 37.82; H, 2.99; N, 7.08%. Required for C50H32N8O14-
Mo1Re2 ꢀ 9 H2O: C, 37.55; H, 3.15; N, 7.01%.
density (calculated)
absorption coefficient
F(000)
1.854 Mg/m3
4.541 mm-1
1540
crystal size
θ range for data collection
index ranges
0.11 ꢀ 0.04 ꢀ 0.04 mm3
1.41 to 23.54°.
-14 e h e 14
-16 e k e 15
-17 e l e 18
reflections collected
14029
independent reflections
completeness to θ = 23.54°
absorption correction
max. and min transmission
refinement method
8379 [R(int) = 0.0598]
98.9%
semiempirical from equivalents
0.830 and 0.631
full-matrix least-squares on F2
8379/12/785
data/restraints/parameters
goodness-of-fit on F2
0.978
final R indices [I > 2σ(I )]
R indices (all data)
largest diff. peak and hole
R1 = 0.0548, wR2 = 0.1189
R1 = 0.1001, wR2 = 0.1319
2.387 and -1.512 e A-3
X-ray Crystallography. Diffraction data were collected using
a Bruker Smart Apex X-ray diffractometer with an Oxford
cryostream cooling system and Mo-KR radiation source (λ =
0.71073 A) using a SMART CCD camera. The structure was
solved by direct methods using SHELXS-97 and refined by full-
matrix least-squares using SHELXL-97 (G. M. Sheldrick, Uni-
by addition of HCl and [Bu4N]OH. The samples were measured
first in unbuffered solutions in the presence and absence of
oxygen. However, the pH was seen to increase upon laser
exposure. The samples were subsequently buffered with 2,
4-lutidine and the measurements were repeated. A comparison
of the pH values and absorption spectra recorded before and
after the measurements confirmed that the buffer prevented the
pH from drifting and that no significant photodecomposition
had occurred.
€
versity of Gottingen, Germany, 1997). Crystal data and refine-
ment details are summarized in Table 1. The crystal contained
seven water molecules two of which were disordered and
modeled with 50% occupancy over two sites. The hydrogen
atoms of the water molecules could not be located by difference
map and so were omitted. CCDC 799865 contains the crystal-
lographic data, which can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk.
Femtosecond transient absorption spectra were recorded
using an amplified 1 kHz system. Briefly, 800 nm pulses (80 fs,
1 kHz) were generated in a Coherent Vitesse/Legend-HE sys-
tem. The 800 nm light was split into pump and probe pulses.
Pump light (≈100 fs, 355 nm, ≈300 nJ per pulse) was obtained
using a Light Conversion TOPAS set to 710 nm and an external
BBO doubling crystal. Broadband probe light (≈100 fs) was
generated in a 3 mm thick CaF2 plate, and residual 800 nm light
was attenuated using a KG3 filter. The polarization of the pump
light was set to 55° relative to that of the probe light using a
waveplate. Full spectra were detected using a diode array
(Pascher Instruments, Lund, Sweden) for each laser pulse. The
reported spectra and kinetic traces are averages of 5000 to 10000
shots. All measurements were done in 1 mm path length cu-
vettes, which were oscillated to avoid photobleaching and
accumulation of degradation products. The femtosecond tran-
sient absorption data were fitted using iterative reconvolution of
exponential functions with a Gaussian response (200-300 fs)
using the Igor Pro (Wavemetrics, Lake Oswego, OR, U.S.A.)
software package. Spectra are not chirp-corrected, as the chirp
was found to be insignificant at the time scales reported. Non-
linear effects within the pulse width (<300 fs) were either fitted
using a dummy lifetime of 5 fs or removed using visual inspection.
General pH Titration Procedures. (a). Absorption, Excitation,
and Emission Studies. Unless otherwise stated, the following general
conditions apply. All titrations were carried out in air at room
temperature using either aqueous acetonitrile (5% v/v water)
or aqueous DMF (5% v/v water) as solvent, depending on the
solubility of the compound under investigation. Adjustments to the
pH were made with 0.6 mM [Me4N]OH in water and 0.6 mM HCl
or HBr in the respective solvent system, as indicated. The absorp-
tion, excitation, and emission spectra were recorded in 10 mm
quartz cuvettes after equilibration. Excitation and emission spectra
are uncorrected. pH values were determined using a WTW Profilab
pH 597 pH meter with a Mettler Toledo Inlab 422 electrode and are
given as measured in the respective solvent system.
(b). Mass Spectrometry. A solution containing 1 equiv of
potassium molybdate (0.79 mM) and 2 equiv of [Re(bpy)-
(CO)3(H2-L)]þ (1.58 mM) in aqueous DMF (5% water) was
prepared. After equilibration, the pH value was measured and
the ESI-MS was recorded on a ThermoFinnigan LCQ-Classic
instrument.
Transient Absorption Spectroscopy. The measurements were
carried out in the Department of Photochemistry and Molecular
Science at Uppsala University. The concentration of the samples
was 0.017 mM in DMF/water 20:1 (v/v). The pH was adjusted
Results and Discussion
Synthesis and Characterization. Re(bpy)(CO)3(Bn2-L)]-
(PF6) was synthesized as described previously23 and

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 1109
Figure 1. Comparison of the absorption spectra of 0.1 mM solutions of
2, 3-dimethoxybenzoic acid (black), 4-aminopyridine (red) and the result-
ing summation spectrum (green) with the spectrum of the methyl-
protected ligand (blue) in aqueous acetonitrile (5% water) under (a)
acidic and (b) basic conditions.
Figure 2. (a) Absorption spectra recorded between pH 1.8 and 8.4
during the titration of an acidic solution (0.1 mM) of Me2-L in aqueous
acetonitrile (5% water) with [Me4N]OH. (b) Plot of the absorbance at 290
nm as a function of pH.
2, 3-dimethoxybenzoic acid and 4-aminopyridine were
recorded for comparison. All pH values are given as
measured by standard glass electrode in the solvent
system used.
complete debenzylation was achieved by catalytic hydro-
genolysis (3.5 bar H2, 5% Pd on charcoal) to yield [Re-
(bpy)(CO)3(H2-L)](PF6).
Figure 1a shows the absorption spectra of HMe2-Lþ
and Me2-L in comparison with the absorption spectra of
equimolar acidic solutions of the two starting materials 2,
3-dimethoxybenzoic acid and 4-aminopyridine and the
sum of their spectra. The corresponding spectra for basic
solutions are shown in Figure 1b. It is evident that the
absorbance in the region between 225 and 350 nm is
mainly dominated by transitions associated with the
pyridine-moiety of the ligand, with some contributions
from catecholamide-based transitions. Under both con-
ditions, the most intense band observed in the spectrum of
the methyl-protected ligand is broader and clearly red-
shifted, the latter indicating a decrease in the HOMO
-LUMO gap. In addition, the absorbance of the main
band increases significantly in acidic solution if compared
to the summation spectrum, providing additional evi-
dence for an increase in the degree of conjugation.
The absorption spectral changes associated with the
deprotonation of the pyridinium-unit in [HMe2-L]Br
were investigated in more detail by titration of an acidic
solution with small aliquots of base (Figure 2, top). The
pH-dependence of the absorbance at 290 nm indicates
only one step, the deprotonation of the pyridinium unit
and formation of Me2-L. Upon deprotonation, the ab-
sorption maximum shifts to higher energy, from 280 to
263 nm, as observed in similar pyridine-derivatives.38 The
To obtain the Mo(VI) complex, [Re(bpy)(CO)3(H2-
L)]PF6 (0.01 mmol) was dissolved in the minimum
amount of a solvent system consisting of a 2:1 acetoni-
trile/water mixture, and the pH of the solution was
adjusted to 3 with dilute hydrochloric acid. Five micro-
liters of an aqueous solution of sodium molybdate dihy-
drate (1 mmol dissolved in 1 mL water) were added to
obtain a 2:1 ratio of sensor to molybdate. Equilibration of
the mixture resulted in an increase of the pH to 6 and the
crystallization of the dioxo-Mo(VI) complex, which was
isolated and characterized by IR and proton NMR
spectroscopy, ESI mass spectrometry, and X-ray crystal-
lography.
Me2-L was synthesized by reacting commercially avail-
able 2, 3-dimethoxybenzoic acid with 4-aminopyridine.
The downfield shift of the NH proton (δ 10.29) indicates
hydrogen bonding to the adjacent methoxy-oxygen. Me2-
L was demethylated with BBr3 to give the free ligand [H3-
L]Br, which was characterized by mass spectrometry,
elemental analysis, IR spectroscopy, and NMR spectros-
copy. The values of the ν(OH) bands in the IR spectra are
suggestive of intramolecular hydrogen bonding interac-
tions, as indicated in Scheme 1.
Spectroscopic Properties. 1. UV/vis Absorption Spec-
tra of Me2-L and [H3-L]Br. The pH-dependence of the
electronic absorption spectra of Me2-L and [H3-L]Br was
investigated in a mixed solvent system consisting of 5%
water in acetonitrile. The spectra of the starting materials
(38) Cattaneo, M.; Fagalde, F.; Katz, N. E. Inorg. Chem. 2006, 45, 6884.

1110 Inorganic Chemistry, Vol. 50, No. 3, 2011
Corden et al.
Figure 4. (a) Absorption spectra of [Re(bpy)(CO)3(H2-L)]þ recorded
between pH2.5and 10.3 duringthe titrationofacidicsolutionsin aqueous
DMF with [Me4N]OH. (b) Plot of the molar absorption coefficient at 375
nm of [Re(bpy)(CO)3(H2-L)]þ (squares, 0.3 mM) and [Re(bpy)(CO)3-
(Bn2-L)]þ, (circles, 0.26 mM) as a function of pH.
increase in acidity, however, is consistent with the struc-
tural differences, in particular the electron withdrawing
effect of the pyridine-unit in H-L- as opposed to the
electron-donating properties of the two N-methyl-sub-
stituents in N, N0-dimethyl-2, 3-dihydroxybenzamide.
The intense absorption bands with maxima at 277 nm
(pH 9.1) and 290 nm (pH 2.3) are due to overlapping
pyridine- and dihydroxybenzamide-based transitions.
Accordingly, the plot of the absorbance at 290 nm versus
pH reveals both the deprotonation of the pyridinium-unit
to give H2-L (with midpoint at pH 3.9) and the deproton-
ation of the dihydroxybenzamide-unit to give H-L-
(with midpoint at pH 7.6, Figure 3c). The former agrees
well the midpoint obtained for the deprotonation of
[HMe2-L]þ, indicating that the methylation of the cate-
chol-unit has no significant impact on the pKa value of
the pyridine-unit.
Figure 3. (a) Absorption spectra recorded between pH 2.3 and 9.1
during the titration of an acidic solution (0.065 mM) of H3-Lþ in aqueous
acetonitrile (5% water) with [Me4N]OH. (b) Plot of the absorbance at
360 nm as a function of pH. (c) Plot of the absorbance at 290 nm as a
function of pH.
The above assignment of the two protonation sites was
confirmed by H NMR spectroscopy (Figure S1, Sup-
1
1
porting Information). The H NMR spectrum of H2-L
sigmoidal decrease of the absorbance at 290 nm is cen-
tered at around pH 3.9 (Figure 2b).
shows two doublets at 8.48 and 7.85 ppm for the protons
of the pyridine-unit and three resonances at 7.49, 7.04,
and 6.85 ppm for the protons of the catechol unit.
Addition of base and formation of H-L- results in a
significant upfield shift of two of the resonances due to the
catechol-protons to 6.40 and 6.82 ppm, which is consis-
tent with the deprotonation of the 2-hydroxy-position. In
contrast, the pyridine resonances barely shift upon addi-
tion of base. On the other hand, the addition of HBr and
formation of [H3-L]Br does not affect the catechol-domi-
nated area of the spectrum, but results in a marked
downfield shift of the two doublets due to the pyridine
protons to 8.69 and 8.39 ppm. This is consistent with the
protonation of the pyridine-unit.
Subsequently, the absorption spectra of the ligand H2-
L were recorded at a range of pH values (Figure 3a). The
titration of an acidic solution of [H3-L]þ with base
revealed two deprotonation steps, the formation of H2-
L followed by the formation of H-L-. The increase of
the lower energy absorption at around 360 nm above
pH 6 reflects the second step, the deprotonation of the
dihydroxybenzamide-unit of the ligand to give H-L-
(Figure 3b). The midpoint of the sigmoidal increase is
located at pH 7.6, which is slightly lower than corre-
sponding pKa value for N, N0-dimethyl-2, 3-dihydroxy-
benzamide, which was reported as 8.42 in water.39 An
UV/vis Absorption Spectra of [Re(bpy)(CO)3(Bn2-L)]þ
and [Re(bpy)(CO)3(H2-L)]þ. The pH-dependence of the
electronic absorption spectra of [Re(bpy)(CO)3(Bn2-L)]þ
(39) Harris, W. R.; Carrano, C. J.; Cooper, S. R.; Sofen, S. R.; Avdeef,
A. E.; McArdle, J. V.; Raymond, K. N. J. Am. Chem. Soc. 1979, 101, 6097.

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 1111
Figure 6. Excitation spectrum of H-L- monitored at 470 nm (dashed
line, 0.02 mM, pH 11) in comparison with the absorption spectrum (solid
line, 0.065 mM, pH 9).
[Re(bpy)(CO)3(H2-L)]þ is due to the deprotonation of
the dihydroxybenzamide-unit.
For both [Re(bpy)(CO)3(H-L)] and H-L- the back-
titration with acid restored the original absorption spec-
tra, confirming that the catecholate unit is not oxidized
during the titration.
2. Emission Behavior of [H3-L]Br. Acidic solutions of
H3-Lþ in aqueous acetonitrile are not emissive below pH
3. At pH 4, weak emission with maximum at 405 nm is
observed upon excitation into the catecholate-dominated
absorption band at around 360 nm. Further increase of
the pH leads to an increase of the emission intensity up to
a pH value of 9, with a shift of the maximum to 470 nm
(Figure 5a). A sigmoidal fit of the corresponding pH
profile gives a midpoint of 7.5 (Figure 5b). The midpoint
of 7.5 is very similar to the second pKa value of 7.6
obtained from the electronic absorption spectra, suggest-
ing that the increase in emission intensity is due to the
deprotonation of the OH-group in the 2-position of the
catecholamide-unit and formation of H-L-. This conclu-
sion was further confirmed by the excitation spectrum
recorded at pH 11, monitoring the emission intensity at
470 nm (Figure 6). The low energy region in the excitation
spectrum of H-L- shows a band that matches the absorp-
tion band of the deprotonated catecholamide-unit, con-
firming that the emission observed at 470 nm is due to the
excitation of catecholamide-based transitions. Further
addition of base leads to a decrease in emission intensity,
which may be due to the deprotonation of the OH-group
in the 3-position of the dihydroxybenzamide-unit.
The absorption band at 360 nm and the emission band
at 470 nm of H-L- are likely to be associated with the
same electronic excited state. The observation that these
low energy transitions are unique to the deprotonated
form together with the solvent dependence40 of the emis-
sion leads us to assign the excited state as intramolecular
charge transfer state, in which the catecholate acts as the
donor and the pyridylamide as the acceptor.41
Figure 5. (a) Emission spectra (λexc = 360 nm) recorded during the
titration of an acidic 0.1 mM solution of ligand H2-L in aqueous
acetonitrile with [Me4N]OH. (b) pH dependence of the emission intensity
at 470 nm, λexc = 360 nm.
and [Re(bpy)(CO)3(H2-L)]þ was investigated in aqueous
DMF (5% water). In the absorption spectrum of the
benzyl-protected complex [Re(bpy)(CO)3(Bn2-L)]þ, the
lowest energy band (shoulder at ca. 375 nm) can be
assigned to a Re(dπ)fbpy charge transfer (MLCT)
transition by reference to the spectra of similar
complexes.8 In agreement with this assignment, the ab-
sorption spectra of [Re(bpy)(CO)3(Bn2-L)]þ change only
very little with pH (Figure S2, Supporting Information).
In contrast, the titration of an acidic solution of the
deprotected sensor [Re(bpy)(CO)3(H2-L)]þ with base
leads to an increase in the absorbance at 375 nm with
pH (Figure 4a), indicating an overlap of the pH-indepen-
dent MLCT band with pH-dependent ligand-based tran-
sitions. At high pH, a band emerges at 380 nm, which may
be attributed to the deprotonation of the corresponding
OH-group of the dihydroxybenzamide-unit of the sensor.
From the inflection point, a pKa-value of 6.7 can be
estimated (Figure 4b).
The pKa value of 6.7 obtained for [Re(bpy)(CO)3(H2-
L)]þ is lower than the corresponding pKa value of 7.6
obtained for H2-L, but the trend is consistent with the
electron withdrawing effect of the electron poor, rhe-
nium-coordinated pyridine unit in [Re(bpy)(CO)3(H2-
L)]þ. It should also be considered that solubility limita-
tions required the titrations of [Re(bpy)(CO)3(Bn2-L)]þ
and [Re(bpy)(CO)3(H2-L)]þ to be performed in aqueous
DMF, while the ligand was investigated in aqueous
acetonitrile. The pKa values obtained are therefore not
directly comparable. Nevertheless, the ligand spectra
obtained are consistent with the conclusion that the pH-
dependence of the low-energy band in the spectra of
Since the optimal excitation wavelength for the molyb-
date sensor (370 nm) falls into the wavelength range
affected by the intramolecular charge transfer absorption
of H-L-, ligand-based electronic transitions might play a
(40) In chloroform, the emission maximum is located at 445 nm, whilst in
water, the fluorescence is quenched.
(41) Aronica, C.; Venancio-Marques, A.; Chauvin, J.; Robert, V.;
Lemercier, G. Chem.;Eur. J. 2009, 15, 5047.

1112 Inorganic Chemistry, Vol. 50, No. 3, 2011
Corden et al.
Figure 9. pH dependence of the relative emission intensity of a 0.1 mM
solution of [Re(bpy)(CO)3(H2-L)]þ (triangles, λexc = 370 nm, λem = 565
nm) and a 0.1 mM solution of H2-L (squares: λexc = 360 nm, λem = 470
nm) in aqueous acetonitrile (5% water).
Figure 7. Kinetic traces obtained from the transient absorption spectra
of H3-Lþ at pH 3.7 at a selection of wavelengths (dark blue 420 nm, light
blue 470 nm, green 550 nm, red 630 nm. Solid lines are best multi-
component fits to three lifetimes. Nonlinear effects within the pulse width
have been accounted for by the addition of a 5 fs lifetime. The x-axis
contains two scales, as indicated by the axis break).
the excitation pulse. The transient absorption then decays
without significant shift in the band position (Figure 8a).
Since there is virtually no ground state absorption above
410 nm at this pH, the negative signal observed between
425 and 560 nm is caused by stimulated emission of the
sample rather than bleaching of the ground state. Initi-
ally, the minimum of the negative signal is located at
440 nm, but the position of the band then red-shifts to
about 490 nm. By comparison with the fluorescence spectra
(Figure 5a) it is evident that fluorescence from the relaxed
excited state is significant up to 650 nm. Therefore it is most
probable that the decrease in net absorption to the red
(Figure 8) is due to a dynamic Stokes shift of the stimulated
emission and not due to a decrease in the excited state
absorption. Excited state absorption above 500 nm is
present both at high pH and at low pH also at low pH
(Figure 7 and Supporting Information, Figure S3). At low
pH all traces have positive amplitudes. What is absent at
pH = 3.7 is clear stimulated emission, which is in agree-
ment with the weak fluorescence at this pH (Figure 5b).
A three-exponential fit of the decay profile gives life-
times of 1.4, 60, and approximately 2200 ps (Figure 8b).
The spectral envelope does not appear to change much
with time except for the apparent red-shift of the emis-
sion. The wavelength of the emission maximum at t > 100
ps agrees well with that observed by steady state fluores-
cence measurements. Although the exact origin of the life-
times cannot be determined, it is conceivable that the two
short lifetimes correspond to internal molecular and exter-
nal solvent relaxation, especially when considering the
relatively high viscosity of the solvent system used.
Figure 8. (a) Transient absorption/emission spectra of HL- obtained at
pH 7.9 after the indicated time intervals. (b) Kinetic traces at a selection of
wavelengths (violet 400 nm, dark blue 430 nm, light blue 450 nm, green
495 nm, orange 550 nm, red 630 nm. Solid lines are best fits to two
lifetimes. Nonlineareffectswithinthepulse widthhavebeenremoved. The
X-axis contains two scales, as indicated by the axis break).
3. Comparison with the Emission Properties of the
Molybdate Sensor [Re(bpy)(CO)3(H2-L)]þ. In contrast to
the emission properties of H2-L, the pH profile of the emis-
sion of the molybdate sensor [Re(bpy)(CO)3(H2-L)](PF6)
shows a decrease in emission intensity upon deprotonation.
In acidic solution, the excitation at 370 nm gives rise to emis-
sion from the 3MLCT state at around 565 nm, as previously
reported.23 The intensity of the emission decreases sigmoid-
ally with increasing pH. In aqueous acetonitrile, the inflection
point is located at pH 5.9, if the pH scale is uncorrected for the
composition of the solvent system used42 (Figure 9).
role in the signaling mechanism. The photophysical prop-
erties of ligand H2-L were therefore studied further by
ultrafast transient absorption spectroscopy on a 0.1 ps to
2.1 ns time scale with laser excitation at 355 nm.
The transient spectra recorded in aqueous DMF (5%
water) at pH 3.7 are characterized through absorption
bands with maxima at 398 and 420 nm (Figure S3,
Supporting Information). A multicomponent fit of the
decay profiles at a selection of wavelengths gives lifetimes
of 5 and 40 ps (Figure 7).
(42) The inflection point of 4.9 reported in our previous paper23 was
obtained by applying a correction of -1 to the measured value of 5.9 to
account for the effect of the aqueous acetonitrile solvent mixture used.
Upon increase of the pH to 7.9 an absorption band with
maximum at 398 nm is observed almost immediately after

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 1113
Table 2. Selected Spectroscopic Data and Protonation Constants for the Protected Ligand, Deprotected Ligand and the Sensor Moleculea
Me2-LHþ
Me2-L
H3-Lþ
286
H2-L
HL-
[Re(bpy)(CO)3(H2-L)]þ
[Re(bpy)(CO)3(H-L)]
absorption λmax/nm
emission
280
263
∼274
278, 360
shoulder
375c
λ
em/nm
405 weak
(360)
∼460
470
(360)
7.6
565
(370)
565 weak
(370)
(λex/nm)
pKa
(360)
3.9
3.9
6.7c
5.9
pHib
7.5
a Aerated solutions at room temperature in aqueous acetonitrile (5% water). b pHi is the pH at the inflection point of the fluorescence titration curve.
c Determined in aqueous DMF (5% water).
Figure 10. Top: Positive mode ESI mass spectrum of a reaction mixture containing 2 equiv of [Re(bpy)(CO)3(H2-L)]þ and 1 equiv of molybdate in
aqueous DMF (5% water) at pH 10. Bottom: Isotopic distribution pattern calculated for the sodium (left) and potassium (right) adducts of
[MoO2{Re(bpy)(CO)3(L)}2].
Table 2 compares the spectroscopic characteristics
previously obtained23 for [Re(bpy)(CO)3(H2-L)]þ with
those of Me2-L and H2-L. Taken together, the data
confirm that the decrease in emission intensity observed
for [Re(bpy)(CO)3(H2-L)]þ with increasing pH is caused
by the deprotonation of the OH group in the ortho-
position of the catecholamide unit. The ground state
protonation constants and excited state inflection points
obtained for [Re(bpy)(CO)3(H2-L)]þ/[Re(bpy)(CO)3(H-L)]
from the pH-profiles of the absorption and emission data,
respectively, are lower than those obtained for H2-L/
HL-, which is consistent with the positive charge of the
protonated sensor molecule and the electron withdrawing
effect of the electron poor, rhenium-coordinated pyridine
unit in [Re(bpy)(CO)3(H2-L)]þ.
As evident from Figures 5 and 9, the ligand is not
emissive if fully protonated, but emits after deprotona-
tion at 470 nm if excited into the catecholate-dominated
absorption band at 360 nm. Although the excitation
wavelength of the latter is very similar to the optimum
excitation wavelength of the molybdate sensor (370 nm),
no interference from ligand-based fluorescence is ob-
served in the emission spectra of [Re(bpy)(CO)3(H-L)].
The absence of the ligand fluorescence in the sensor may
be due to the intramolecular heavy-atom effect of the Re
leading to a shortening of the lifetime of the ligand-based
excited states, but it could also be the result of photo-
induced electron transfer (PeT) from the electron-rich
catecholate-donor to the Re(bpy)-based acceptor (see
below).

1114 Inorganic Chemistry, Vol. 50, No. 3, 2011
Corden et al.
Table 3. Selected Bond Distances (A) and Angles (deg) for [MoO2{Re(bpy)-
(CO)3(L)}2] CH3CN 7(O)
3
3
Bond Distances
1.685(9)
Mo(1)-O(2)
Mo(1)-O(1)
Mo(1)-O(10)
Mo(1)-O(3)
Mo(1)-O(4)
Mo(1)-O(9)
C(13)-O(6)
C(13)-Re(1)
C(14)-O(7)
C(14)-Re(1)
C(15)-O(8)
C(15)-Re(1)
N(1)-Re(1)
N(2)-Re(1)
N(3)-Re(1)
N(6)-Re(2)
N(7)-Re(2)
N(8)-Re(2)
C(38)-O(12)
C(38)-Re(2)
C(39)-O(13)
C(39)-Re(2)
C(40)-O(14)
C(40)-Re(2)
2.162(11)
2.163(11)
2.204(10)
2.203(10)
2.197(11)
2.151(10)
1.194(17)
1.873(18)
1.111(15)
1.939(15)
1.162(15)
1.918(15)
1.720(9)
1.978(8)
1.980(8)
2.170(8)
2.172(7)
1.154(16)
1.901(16)
1.160(14)
1.894(13)
1.139(16)
1.925(16)
Bond Angles
Figure 11. ORTEP plot of the molecular structure of [MoO2{Re-
(bpy)(CO)3(L)}2] (hydrogen atoms and lattice solvent molecules not
shown).
C(13)-Re(1)-C(15)
N(1)-Re(1)-N(2)
C(14)-Re(1)-N(3)
(1)-Mo(1)-O(2)
89.8(6)
74.4(4)
175.2(5)
104.0(4)
75.4(3)
C(40)-Re(2)-C(39)
N(8)-Re(2)-N(7)
C(39)-Re(2)-N(6)
O(3)-Mo(1)-O(4)
O(3)-Mo(1)-O(10)
89.3(6)
74.6(4)
176.3(5)
75.5(3)
159.3(3)
The long-wavelength emission observed at 565 nm for
[Re(bpy)(CO)3(H2-L)]þ upon excitation at 370 nm in
acidic solutions involves the metal center and is due to
O(9)-Mo(1)-O(10)
3
emission from the Re(dπ)fbpy MLCT excited state.
crystallography could be obtained at pH 6. The molecular
structure of the complex is shown in Figure 11; selected
bond angles and distances are given in Table 3.
The absorption spectroscopic study showed that under
alkaline conditions the MLCT absorption in the spectra
of [Re(bpy)(CO)3(H-L)] overlaps with the catecholate-
dominated intraligand-based transitions, which leads to
an increase in absorbance at the excitation wavelength of
370 nm. The resulting competition contributes to the
observed decrease of the emission intensity at 565 nm
upon addition of base. In addition, intramolecular PeT
from the deprotonated catecholate-donor to the Re(bpy)-
acceptor may lead to the quenching of the emission from
The structural analysis revealed that two fully depro-
tonated sensor molecules are bound to a cis-dioxo-MoO2
center via their catecholate-units in accord with the
evidence from the ESI mass spectra and the emission
titration. The resulting complex has approximate C2
symmetry. Formally, [MoO2{Re(bpy)(CO)3(L)}2] is zwit-
terionic with the cis-dioxo-MoO2(cat)2-center bearing a
2- charge while each Re(bpy)(CO)3(py)-center carries a
positive charge. The crystal contained seven water mole-
cules, two of which were modeled with 50% occupancy
over two sites. The distorted octahedral geometry of the
3
the ReII-bpy* MLCT state. Further evidence for PeT
obtained from time-resolved IR and emission spectros-
copy will be reported in a follow-up paper.
4. Reaction of [Re(bpy)(CO)3(H2-L)]þ with Molybdate
and Molecular Structure of [MoO2{Re(bpy)(CO)3(L)}2].
The intensity of the emission from the ReII-bpy* 3MLCT
excited state of [Re(bpy)(CO)3(H2-L)]þ also decreases in
the presence of molybdate, showing that the complex acts
as a molybdate sensor. As previously reported,23 the
decrease in emission intensity upon titration of [Re-
(bpy)(CO)3(H2-L)]þ with a solution of sodium molybdate
is linear up to a break at a Mo/sensor ratio of 0.5, which is
consistent with the formation of a 1:2 complex that
contains a molybdenum center with two coordinated
sensor molecules. The composition of the molybdenum-
complex formed in solution was now confirmed by ESI-
MS, and the structural features of the 1:2 complex in the
solid state were identified by X-ray crystallography.
The ESI-MS investigation was carried out by reacting 2
equiv of [Re(bpy)(CO)3(H2-L)]þ and 1 equiv of potas-
sium molybdate in aqueous DMF (5% water). After
equilibration, the only Re- and Mo-containing peaks
observed correspond to the sodium or potassium adducts
of the 1:2 complex [MoO2{Re(bpy)(CO)3(L)}2]. The iso-
topic distribution patterns obtained for the two adducts
match those calculated for the respective elemental com-
positions (Figure 10).
Mo-center is typical for MoO2 core structures.43 The
2þ
strong π-donor character of the two terminal oxo ligands
(O(1) and O(2)) leads not only to their cis-orientation but
also to the binding of the weaker 2-benzoxy donors of the
ligands (O(4) and O(9)) trans to them. This results in a
lengthening of the Mo(1)-O(4) and Mo(1)-O(9) bonds
in trans-position (2.170(8) and 2.172(7) A, respectively),
compared to the bonds cis to the oxo groups, Mo(1)-O-
(3) and Mo(1)-O(10) (1.980(8) and 1.978(8) A, re-
spectively). These bond lengths are comparable to those
reported for other dioxomolybdenum(VI) complexes
with catecholamide ligands.31,44-47 The molybdenum-
oxygen bond angles are also similar.
The two rhenium centers in [MoO2{Re(bpy)(CO)3-
(L)}2] show the characteristically distorted octa-
hedral coordination geometry of bipyridine coordi-
nated fac-ReI(CO)3 units, as previously observed for
(43) Stiefel, E. I. In Comprehensive Coordination Chemistry; Wilkinson, G.,
Ed.; Pergamon Press: Oxford, U.K., 1987; p 1380.
(44) Albrecht, M.; Franklin, S. J.; Raymond, K. N. Inorg. Chem. 1994, 33,
5785.
(45) Duhme, A.-K.; Dauter, Z.; Hider, R. C.; Pohl, S. Inorg. Chem. 1996,
35, 3059.
(46) Duhme, A.-K. J. Chem. Soc., Dalton Trans. 1997, 773.
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´
Reaction of 2 equiv of [Re(bpy)(CO)3(H2-L)]þ with 1
equiv of sodium molybdate in aqueous acetonitrile (33%
water) yielded a red solution from which deep red single
crystals of [MoO2{Re(bpy)(CO)3(L)}2] suitable for X-ray
ꢁ
Colon, J. L. Eur. J. Inorg. Chem. 2005, 118.
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Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 1115
typical MLCT absorption and emission bands. After
deprotonation of the catecholate-unit, ligand-based tran-
sitions overlap with Re(dπ)fbpy MLCT transitions. The
quenching of both ligand-based fluorescence and emis-
sion from the Re(dπ)fbpy 3MLCT state under alkaline
conditions is consistent with a heavy atom effect and
photoinduced electron transfer from the electron-rich
catecholate to the Re(bpy)-based acceptor. In addition,
H2-L was found to be an interesting donor-acceptor
system in its own right with three states of protonation
and photophysical behavior suitable for pH sensing by
absorption or emission. Finally, the formation of the
dioxo-Mo(VI)Re2 complex [MoO2{Re(bpy)(CO)3(L)}2]
upon reaction of the sensor [Re(bpy)(CO)3(H2-L)]þ with
molybdate was ascertained in solution and in the solid
state. The crystal structure determination revealed that
two fully deprotonated sensor molecules are bound via
their catecholate units to a cis-dioxo-MoO2 center. Hy-
drogen bonds are formed between the amide proton and
the 2-benzoxy donor of each ligand.
Figure 12. Ball and stick representation of a fragment of [MoO2-
{Re(bpy)(CO)3(L)}2] that illustrates the bending of one of the catechol
ligands out of the pyridine plane (Re: magenta, Mo: orange, C: gray, N:
blue, O: red).
[Re(bpy)(CO)3(Bn2-L)]þ.23 The Re-C and Re-N dis-
tances are within the expected range.48-51 The two ligands
L in [MoO2{Re(bpy)(CO)3(L)}2] each contain an intramo-
lecular hydrogen bond between the amide proton and the
adjacent 2-benzoxy donor (distances N(4) O(4) =
3 3 3
2.683(13) A and N(5) O(9) = 2.650(12) A). The
3 3 3
catechol and pyridine rings in the two ligands deviate
significantly from an ideal coplanar orientation although
the deviation is due to bending of the ligands rather than
twisting (Figure 12). The structural parameters exclude a
radical complex formulation as [Mo(IV)O2{Re(bpy)-
(CO)3(L 3 -)}2].
Acknowledgment. We thank the EU (Aquachem Marie
Curie Research Training Network and COST D35- STSM-
03115) and the K&A Wallenberg Foundation for financial
support, Drs. J. Gun and O. Lev, The HebrewUniversity of
Jerusalem, for preliminary mass spectrometry data and
Ms. Juliet Morgan for experimental assistance.
5. Conclusions. The pyridine-appended catechola-
mides Me2-L and H2-L were synthesized and character-
ized. The investigation of the photophysical properties of
their accessible protonation states revealed important
spectroscopic characteristics relevant to the signaling
mechanism of the molybdate sensor [Re(bpy)(CO)3(H2-
L)]þ. In acidic solution the molybdate sensor exhibits
Supporting Information Available: Figures S1 (1H NMR
spectra of [H3-L]þ, H2-L and H-L-), S2 (Absorption spectra
of [Re(bpy)(CO)3(Bn2-L)]þ), S3 (Transient absorption spectra
obtained for H3-Lþ at pH 3.7), and X-ray crystallographic data
for [MoO2{Re(bpy)(CO)3(L)}2] CH3CN 7(O) in CIF format.
(50) Yam, V. W.-W.; Chong, S. H.-F; Cheung, K.-K. Organometallics
2000, 19, 5092.
(51) Busby, M.; Gabrielsson, A.; Matousek, P.; Towrie, M.; Di Bilio,
3
3
This material is available free of charge via the Internet at http://
pubs.acs.org.
ꢀ
A. J.; Gray, H. B.; Vlcek, A., Jr. Inorg. Chem. 2004, 43, 4994.