Rhenium bipyridine complexes for the recognition of glucose

Article abstract of DOI:10.1021/ic010202b

Bipyridine ligands containing pendant methyl, amino, and amino-boronic acid groups were synthesized. Coordination complexes of these ligands with rhenium were prepared straightforwardly and in good yield. The fluorescence behavior of the Re complexes was studied as a function of pH and exposure to various concentrations of glucose. The methyl bipyridine complex showed no change in fluorescence with pH, the amino derivative showed a rapid decrease from low pH to neutral, and the amino-boronate derivative showed little change from pH 4 to 10. Fluorescence quenching was observed at high pH as expected on the basis of a photoinduced electron transfer (PET) signaling mechanism. This behavior can be explained on the basis of the first oxidation and reduction potentials of these complexes. Glucose testing showed a significant dependence on the solvent system used. In pure methanol, the rhenium boronate complex exhibited a 55% fluorescence intensity increase upon increasing glucose concentration from 0 to 400 mg/dL However, in 50 vol % methanol/phosphate buffered saline, none of the complexes showed significant response in the glucose range of physiological interest.

Full text of DOI:10.1021/ic010202b

Inorg. Chem. 2002, 41, 1662−1669
Rhenium Bipyridine Complexes for the Recognition of Glucose
,†
Douglas R. Cary,* Natasha P. Zaitseva,† Kelsey Gray,† Kira E. O’Day,† Christopher B. Darrow,†
,†
Stephen M. Lane,† Thomas A. Peyser,† Joe H. Satcher, Jr.,* William P. Van Antwerp,‡
A. J. Nelson,† and John G. Reynolds†
UniVersity of California, Lawrence LiVermore National Laboratory, P.O. Box 808,
L-092, LiVermore, California 94551, and MiniMed Inc., 12744 San Fernando Rd.,
Sylmar, California 91342
Received February 20, 2001
Bipyridine ligands containing pendant methyl, amino, and amino-boronic acid groups were synthesized. Coordination
complexes of these ligands with rhenium were prepared straightforwardly and in good yield. The fluorescence
behavior of the Re complexes was studied as a function of pH and exposure to various concentrations of glucose.
The methyl bipyridine complex showed no change in fluorescence with pH, the amino derivative showed a rapid
decrease from low pH to neutral, and the amino-boronate derivative showed little change from pH 4 to 10.
Fluorescence quenching was observed at high pH as expected on the basis of a photoinduced electron transfer
(PET) signaling mechanism. This behavior can be explained on the basis of the first oxidation and reduction
potentials of these complexes. Glucose testing showed a significant dependence on the solvent system used. In
pure methanol, the rhenium boronate complex exhibited a 55% fluorescence intensity increase upon increasing
glucose concentration from 0 to 400 mg/dL. However, in 50 vol % methanol/phosphate buffered saline, none of the
complexes showed significant response in the glucose range of physiological interest.
Introduction
Because of the importance of developing a continuous in
vivo glucose sensor, a number of research groups are
attempting to address this problem using different ap-
proaches.2-5 The overwhelming majority of sensors reported
in the literature and available commercially are based on the
electrochemical detection of hydrogen peroxide produced by
the reaction of glucose oxidase with glucose. By comparison,
there are relatively few studies of systems using optical
detection, some of which rely on proteins for sensing.4,6-8
Designs based on large proteins, such as glucose oxidase or
Concanavalin A show promise9,10 but have the limitation of
The development of sensors for the continuous determi-
nation of blood glucose levels in vivo remains an important
goal in the treatment of diabetes.1,2 Current therapy for
insulin-dependent diabetics requires the sampling of blood
at discrete intervals throughout the day. This is both
unpleasant for the patient and fails to give an accurate signal
of the continually changing glucose levels in the blood stream
with time. The continuous measurement of glucose concen-
trations is crucial in providing feedback to insulin delivery
devices. Small insulin pumps are now commercially available
for the precisely controlled injection of insulin. When
combined with a continuous sensor, these systems could then
be used to mimic the glucose detection and insulin secretion
functions of the pancreas.
(3) Burmeister, J. J.; Arnold, M. A. Clin. Chem. 1999, 45, 1621-1627.
(4) Heinemann, L.; Schmelzeisen-Redeker, G. Diabetologia 1998, 41,
848-854.
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Soc. 1998, 145, 4119-4125.
* To whom correspondence should be addressed. Phone (J.H.S.): (925)
422-7794.E-mail(J.H.S.): satcher1@llnl.gov.E-mail(D.R.C.): dcary@pcpy.com.
† University of California, Lawrence Livermore National Laboratory.
‡ MiniMed Inc.
(1) For recent information on glucose sensors, insulin pumps, and diabetes
research in general, see the following web sites for example: American
Diabetes Association Home Page, http://www.diabetes.org; Children
with Diabetes Home Page, http://www.childrenwithdiabetes.com; The
Diabetes Mall, http://www.diabetesnet.com.
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1662 Inorganic Chemistry, Vol. 41, No. 6, 2002
10.1021/ic010202b CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/21/2002

Re bpy Complexes for Glucose Recognition
Chart 1
Figure 1. Schematic drawing of PET sensors: (a) in the absence of analyte,
the electron donor (D) quenches the fluorescence of the electron acceptor/
fluorophore (A). (b) In the presence of analyte, electron transfer does not
occur, and A fluoresces.
using molecules that can potentially denature. Denaturing
of the protein typically occurs because of hydrogen peroxide
produced as a byproduct of the enzymatic reaction.
chemical transduction.11,18-23 A schematic of a PET sensor
is shown in Figure 1. An electron donor (D), such as an
amine, quenches the fluorescence of an electron acceptor (A)
in the absence of an analyte. When the analyte is present,
the oxidation potential of D is raised, and electron transfer
does not occur, giving an increase in fluorescence. Other
detection systems are known to operate by PET, such as the
ruthenium and rhenium bipyridine complexes reported by
Meyer et al.,24,25 Murtaza et al.,26 and Ziessel et al.,27 the
naphthalimide and perylene systems reported by de Silva et
al.,28,29 a variety of molecular sensors reported by Beer,30
and visible wavelength lanthanide calix[4]arene complexes.31
In these systems, the most common recognition elements are
pendant amines, calixarenes, or crown ethers used for
measuring pH or alkali metal concentration. Such systems
are limited to the relatively few types of recognition elements
available. Yam and Kai32 have reported a rhenium bipyridyl
complex for the transduction of saccharides by absorbance,
and Mizuno et al. have previously synthesized other types
of organic and inorganic bipyridyl boronic acids.33,34 In this
paper, we report the synthesis and fluorescence studies of
new boronic acid derivatives of rhenium bipyridine com-
plexes for potential use as glucose sensing molecules.
Thus, we are designing new glucose detection systems
based on more robust, small molecule transducers that
respond by fluorescence. The goal is to develop such
complexes for attachment to membranes and implant these
materials subcutaneously. Once implanted, the membranes
will be expected to remain in place for long periods in time,
with glucose measured through the skin by optical excitation
and detection. In terms of chemistry, a key to this design is
the synthesis of small molecule transducers for glucose. A
number of systems have been published previously which
primarily involve detection by colorimetry and circular
dichroism spectroscopy.11-13 A smaller set of compounds
makes use of fluorescence detection,14-16 the most promising
of these being boronic acid derivatives of anthracene,
compounds 1 and 2.17 Compound 1 shows an impressive
120% increase in fluorescence upon increasing glucose, with
the glucose concentration of a solution increasing from 0 to
300 mg/dL in 33% MeOH/phosphate buffered saline (PBS,
pH 7.4). The main factors limiting the utility of this
compound for use in a commercial sensor are low water
solubility and short wavelength excitation and emission.
Water solubility is clearly needed for operation in vivo;
fluorescence at long wavelengths is important because of
greater transmission of visible light through the skin toward
the red end of the spectrum.4 New molecules need to be
developed that overcome these limiting properties. A recent
study by Kukrer and Akkaya has used this approach to create
a squaraine dye with an appended boronic acid group that
shows modest increases in fluorescence with increasing
glucose concentration.15
(18) Balzani, V.; Scandola, F. In Supramolecular photochemistry; Ellis
Horwood: New York, 1991; Chapter 5.
(19) Grigg, R.; Holmes, J. M.; Jones, S. K.; Norbert, W. Chem. Commun.
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VCH Publishers: New York, 1993; Chapter 1.
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G. J. E.; Lehn, J.-M. Chem.sEur. J. 1999, 5, 3523-3527.
(23) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461.
(24) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163-170.
(25) Shen, Y. Ph.D. Thesis, University of Wyoming, Laramie, WY, 1996.
(26) Murtaza, Z.; Chang, Q.; Rao, G. V.; Lin, H.; Lakowicz, J. R. Anal.
Biochem. 1997, 247, 216-222.
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(28) Daffy, L. M.; de Silva, A. P.; Gunaratne, H. Q. N.; Huber, C.; Lynch,
P. L. M.; Werner, T.; Wolfbeis, O. S. Chem.sEur. J. 1998, 4, 1810-
1815.
To design new molecular transducers, it is important to
understand the mechanism by which signaling occurs. In the
case of 1, fluorescence switching occurs by photoinduced
electron transfer (PET), a common mechanism for such
(11) James, T. D.; Sandanayake, K. R. A. S.; Shinkai, S. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 1911-1922.
(12) Lewis, P. T.; Davis, C. J.; Cabell, L. A.; He, M.; Read, M. W.;
McCarroll, M. E.; Strongin, R. M. Org. Lett. 2000, 2, 589-592.
(13) Ward, C. J.; Patel, P.; Ashton, P. R.; James, T. D. Chem. Commun.
2000, 229-230.
(14) Kijima, H.; Takeuchi, M.; Robertson, A.; Shinkai, S.; Cooper, C.;
James, T. D. Chem. Commun. 1999, 2011-2012.
(15) Kukrer, B.; Akkaya, E. U. Tetrahedron Lett. 1999, 40, 9125-9128.
(16) Yoon, J.; Czarnik, A. W. J. Am. Chem. Soc. 1992, 114, 5874-5875.
(17) James, T. D.; Sandanayake, K. R. A. S.; Iguchi, R.; Shinkai, S. J.
Am. Chem. Soc. 1995, 117, 8982-8987.
(29) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A.
J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997,
97, 1515-1566.
(30) Beer, P. D. Acc. Chem. Res. 1998, 31, 71-80.
(31) Masumoto, H.; Ori, A.; Inokuchi, F.; Shinkai, S. Chem. Lett. 1996, 4,
301-302.
(32) Yam, V. W.-W.; Kai, A. S.-F. Chem. Commun. 1998, 109-110.
(33) Mizuno, T.; Fukumatsu, T.; Takeuchi, M.; Shinkai, S. J. Chem. Soc.,
Perkin Trans. 1 2000, 407-413.
(34) Mizuno, T.; Takeuchi, M.; Hamachi, I.; Nakashima, K.; Shinkai, S.
J. Chem. Soc., Perkin Trans. 2 1998, 2281-2288.
Inorganic Chemistry, Vol. 41, No. 6, 2002 1663

Cary et al.
protected boronate ester [(bpyNB)Re(CO)3(py)](OTf) for our
spectroscopic studies, rather than isolating and using the unprotected
compound, because of ease of synthesis. Control experiments were
carried out by NMR and fluorescence spectroscopy to demonstrate
that the presence of even small amounts of water in methanolic
solution resulted in immediate and quantitative removal of the
neopentyl glycol protecting group. This is in accordance with results
by James et al. indicating extremely weak binding constants for
simple diols compared to saccharides for other boronate systems17
and NMR studies by Bielecki et al. in which trace amounts of water
affect the binding mode of saccharides and boronates.39
Experimental Section
General. All reactions were performed under an atmosphere of
N2, followed by workup in air. Protected boronate esters were stored
under vacuum to prevent hydrolysis during storage. Toluene and
THF were distilled from sodium/benzophenone under N2; dichlo-
romethane and acetonitrile were distilled from calcium hydride
under N2. 4,4′-Dimethyl-2,2′-bipyridine (bpyMe) was purchased
from Aldrich or GFS Chemicals. The compounds 4-(bromomethyl)-
4′-methyl-2,2′-bipyridine (bpyCH2Br)35-37 and 2,2-dimethylpro-
pane-1,3-diyl[o-(bromomethyl)phenyl]boronate (3)5 were prepared
by literature methods. Samples for FTIR spectroscopy were
prepared as solutions in CHCl3, and only the CdO stretches are
reported. Unless otherwise stated, all NMR spectra were recorded
at 500 MHz for 1H and 125 MHz for 13C at 20-25 °C using CDCl3
as the solvent. Unless stated otherwise, mass spectra were obtained
using electrospray ionization (50 V) with a 50% methanol/water
solvent mixture with 1% acetic acid added. Electrochemical
experiments were carried out in argon-purged acetonitrile solutions
with a BAS 100 multipurpose instrument interfaced to a personal
computer, using cyclic voltammetry (CV) techniques. The working
electrode was a glassy carbon electrode; its surface was polished
with a 0.05 mm alumina-water slurry on a felt surface immediately
prior to use. The counter electrode was a Pt wire, and the reference
electrode was Ag/AgCl. The concentration of the examined
compounds was typically 5.0 × 10-4 M; 0.1 M tetrabutylammonium
perchlorate was added as supporting electrolyte. XPS samples were
prepared as solids on cellulose tape using C 1s at 284.6 eV as
reference and were analyzed as described elsewhere.38
Compound Synthesis. 4-(Methylaminomethyl)-4′-methylbi-
pyridine (bpyCH2NHMe). A stream of MeNH2 gas was bubbled
through a solution of bpyCH2Br (1.52 g, 5.76 mmol) in 75 mL of
THF at 0 °C for 15 min to give a cloudy colorless solution. The
solution was allowed to warm to ambient temperature, and stirring
continued for 16 h. The solvent was removed to give an oily white
solid. A pale yellow solution was extracted from a white powder
with diethyl ether and the solvent removed under vacuum to give
bpyCH2NMeH (1.20 g, 98%) as a pale yellow liquid. 1H NMR: δ
8.52 (d, 1H, J ) 5.0 Hz), 8.44 (d, 1H, J ) 5.0 Hz), 8.24 (s, 1H),
8.14 (s, 1H), 7.20 (d, 1H, J ) 4.9 Hz), 7.02 (d, 1H, J ) 4.9 Hz),
3.74 (s, 2H), 2.36 (s, 3H), 2.32 (s, 3H), 1.65 (br s, 1H).13C{1H}
NMR: δ 156.3, 155.9, 150.3, 149.2, 148.9, 148.1, 124.7, 122.9,
122.0, 120.4, 54.9, 36.1, 21.2. GC/EIMS: m/z 212 (M - H)+, 184
(bpyMe)+.
4-(N-Benzylmethylaminomethyl)-4′-methylbipyridine (bpyN).
A solution of HNMe(CH2Ph) (0.680 mL, 5.27 mmol) in 5 mL of
CH2Cl2 was added dropwise to a solution of bpyCH2Br (0.693 g,
2.63 mmol) in 30 mL of CH2Cl2 over a few minutes to gradually
give a golden solution. Stirring was continued for 2 h and the
solvent removed under vacuum. The crude product was dissolved
in 40 mL of CHCl3 and then 30 mL of H2O were added. A saturated
solution of sodium carbonate was added to adjust the pH to 8. The
layers were separated, followed by washing of the organic layer
with 2 × 30 mL H2O. The solvent was removed to give an oily
white solid. A pale yellow solution was extracted from a white
powder with diethyl ether and the solvent removed under vacuum
Fluorescence Measurements. To avoid the dilution corrections
necessary for pH and glucose titrations, stock solutions of known
pH, solvent composition, and glucose concentration for each data
point to be collected were prepared. A 30.0 µL aliquot of a 1.00
mM solution of the fluorophore in the chosen solvent was added
to 3.00 mL of an appropriate stock solution using the same solvent.
For pH studies, the solvent system consisted of 0.05 M aqueous
NaCl adjusted to the appropriate pH with 0.5 M HCl or 0.5 M
NaOH and then mixed in a 1:1 ratio with methanol (50 vol % saline/
methanol solvent). For glucose studies, the solvent consisted of
glucose dissolved in either methanol or 50 vol % methanol/PBS
(phosphate buffered saline, pH 7.4). Fluorescence experiments were
performed in dilute (∼10-5 M) solutions at room temperature by
using an ISA SPEX FluoroMax-2 spectrofluorimeter equipped with
a Hamamatsu 1527 photomultiplier. Luminescence maxima reported
are uncorrected for photomultiplier response. Excitation spectra
were performed on all luminescent compounds, obtaining a good
match with the respective absorption spectra. Absorption spectra
were recorded with an HP 8453 UV-vis spectrophotometer.
Fluorescence measurements were made as a function of pH and
glucose concentration by exciting the fluorophore at its peak
excitation wavelength and measuring fluorescence intensity at the
peak emission wavelength. Excitation and emission peak maxima
are reported for each of these compounds in 50% methanol/PBS
solution. The samples were run in the signal to reference mode,
with slit widths between 1 and 5 mm, in polystyrene cells. The
fluorimeter was calibrated on quartz-distilled water. We used the
1
to give bpyN (1.20 g, 98%) as a pale yellow liquid. H NMR: δ
8.63 (d, 1H, J ) 5.0 Hz), 8.55 (d, 1H, J ) 5.0 Hz), 8.35 (s, 1H),
8.23 (s, 1H), 7.41 (d, 1H, J ) 4.9 Hz), 7.38 (d, 2H, J ) 7.3 Hz),
7.32 (t, 2H, J ) 7.5 Hz), 7.24 (m, 1H), 7.12 (d, 1H, J ) 4.9 Hz),
3.60 (s, 2H), 3.57 (s, 2H), 2.42 (s, 3H), 2.21 (s, 3H).13C{1H}
NMR: δ 156.9, 156.6, 150.4, 149.8, 149.6, 148.7, 139.4, 129.5,
128.9, 127.7, 125.3, 124.4, 122.7, 121.9, 62.8, 61.4, 43.0, 21.8.
GC/EIMS: m/z 302 (M - H)+, 184 (M - NMeCH2Ph)+.
2,2-Dimethylpropane-1,3-diyl[o-(methylaminomethyl)phenyl]-
boronate (4). A stream of MeNH2 gas was bubbled through a
solution of 3 (7.02 g, 24.8 mmol) in 150 mL of CH3CN at 0 °C for
20 min to give a yellow solution. Stirring was continued for 3 h
and the reaction allowed to warm to room temperature. The solvent
was removed under vacuum to give a yellow powder. A yellow
solution was extracted from a white powder with chloroform and
the solvent removed under vacuum to give 4 (5.48 g, 95%) as a
1
pale yellow powder. H NMR: δ 7.54 (d, 2H, J ) 6.8 Hz), 7.19
(m, 2H), 6.98 (d, 2H, J ) 7.2 Hz), 3.82 (s, 2H), 3.53 (s, 4H), 1.01
(s, 6H).13C{1H} NMR: δ 140.8, 130.0, 127.7, 127.3, 123.0, 72.5,
55.8, 35.2, 32.4, 22.5. ESIMS: m/z 234.1 (M + H)+.
4-{N-[o-(5,5-Dimethylborinan-2-yl)benzyl]-N-[methylamino]-
methyl}-4′-methylbipyridine (bpyNB). Method A. A solution of
(35) Hamachi, I.; Tanaka, S.; Tsukiji, S.; Shinkai, S.; Oishi, S. Inorg. Chem.
1998, 37, 4380-4388.
(36) Imperiali, B.; Prins, T. J.; Fisher, S. L. J. Org. Chem. 1993, 58, 1613-
1616.
(37) Strouse, G. F.; Schoonover, J. R.; Duesing, R.; Boyde, S.; Jones, W.
E.; Meyer, T. J. Inorg. Chem. 1995, 34, 473-487.
(38) Nelson, A. J.; Reynolds, J. G.; Baumann, T. F.; Fox, G. A. Appl. Surf.
Sci. 2000, 167, 205-215.
(39) Bielecki, M.; Eggert, H.; Norrild, J. C. J. Chem. Soc., Perkin Trans.
2 1999, 449-455.
1664 Inorganic Chemistry, Vol. 41, No. 6, 2002

Re bpy Complexes for Glucose Recognition
3 (1.60 g, 5.64 mmol) in 60 mL of THF was added dropwise over
1 h to a solution of bpyCH2NMeH (1.204 g, 5.65 mmol) and NEt3
(0.787 mL, 5.65 mmol) in 60 mL of THF to give a cloudy white
mixture by the end of addition. Stirring was continued for 1 h and
then the solvent removed under vacuum. The crude solid was
extracted with diethyl ether and the solvent removed to give pure
bpyNB (1.91 g, 82%) as a cream colored to white powder. Method
B. A mixture of bpyCH2Br (0.525 g, 2.00 mmol) and 4 (0.927 g,
3.99 mmol) was dissolved in 40 mL of THF to immediately give
a cloudy white mixture. The reaction was stirred for 1 h and then
the solvent removed under vacuum. The crude solid was extracted
with diethyl ether and the solvent removed under vacuum to give
heated at 50 °C for 17 h and exhibited no visual change. The solvent
was removed under vacuum to give a bright yellow powder.
Chromatography on silica with acetonitrile gave pure [(bpyMe)-
Re(CO)3(py)](OTf) as a yellow powder (555 mg, 80%). 1H NMR:
δ 8.91 (d, 2H, J ) 5.7 Hz), 8.59 (s, 2H), 8.18 (d, 2H, J ) 5.7 Hz),
7.83 (t, 1H, 7.7 Hz), 7.54 (d, 2H, 5.5 Hz), 7.37 (t, 2H, 7.0 Hz),
2.62 (s, 6H). 13C{1H} NMR: δ 196.0, 191.5, 155.6, 154.9, 152.1,
151.8, 140.0, 129.7, 127.3, 126.7, 21.8. ESIMS: m/z 533.9 (M)+.
IR: νCO ) 2034, 1932 cm-1. Anal. (wt %) Calcd for ReC21H17N3O6-
SF3: C, 36.95; H, 2.51; N, 6.16. Found: C, 36.62; H, 2.79; N,
5.75.
[(bpyN)Re(CO)3(py)](OTf). The procedure used was analogous
to that for [(bpyMe)Re(CO)3(py)](OTf). The reaction of AgOTf
(154 mg, 0.599 mmol) and (bpyN)Re(CO)3Cl (363 mg, 0.596
mmol), followed by chromatography on silica with 1:2 toluene/
acetonitrile gave pure [(bpyN)Re(CO)3(py)](OTf) as a yellow
1
pure bpyNB (0.424 g, 51%) as a pale yellow solid. H NMR: δ
8.64 (d, 1H, J ) 5.0 Hz), 8.54 (d, 1H, J ) 5.0 Hz), 8.32 (s, 1H),
8.24 (s, 1H), 7.64 (d, 1H, J ) 7.0 Hz), 7.34-7.23 (m, 4 H), 7.14
(d, 1H, J ) 4.9 Hz), 3.79 (br, 2H), 3.77 (s, 4H), 3.73 (br s, 2H),
2.44 (s, 3H), 2.29 (s, 3H), 1.05 (s, 6H).13C{1H} NMR: d 156.5,
156.0, 149. 5, 149.3, 149.2, 148.3, 133.2, 129.0, 127.7, 125.0, 124.9,
123.9, 122.4, 122.2, 121.5, 72.5, 61.3, 59.9, 42.1, 32.1, 22.2, 21.4.
ESIMS: m/z 348.0 (unprotected M + H)+.
1
powder (260 mg, 54%). H NMR: δ 8.96 (d, 1H, J ) 5.7 Hz),
8.90 (d, 1H, J ) 5.7 Hz), 8.61 (s, 1H), 8.52 (s, 1H), 8.18 (d, 2H,
J ) 5.2 Hz), 7.84 (d, 1H, J ) 5.6 Hz), 7.81 (t, 1H, J ) 7.5 Hz),
7.54 (d, 1H, J ) 5.6 Hz), 7.36 (t, 4H, J ) 7.2 Hz), 7.32 (t, 2H, J
) 7.5 Hz), 7.24 (t, 1H, J ) 7.2 Hz), 3.85 (s, 2H), 3.66 (s, 2H),
2.65 (s, 3H), 2.31 (s, 3H). 13C{1H} NMR: δ 196.0, 195.9, 191.3,
156.7, 155.8, 155.5, 154.9, 152.4, 152.2, 151.8, 140.0, 138.5, 129.8,
129.2, 128.6, 128.3, 127.6, 127.3, 126.8, 125.4, 62.5, 59.9, 43.0,
21.9. ESIMS: m/z 652.9 (M)+. IR: νCO ) 2034, 1931 cm-1. Anal.
(wt %) Calcd for ReC29H26N4O6SF3: C, 43.44; H, 3.27; N, 6.99.
Found: C, 40.50; H, 3.52; N, 6.56.
(bpyMe)Re(CO)3Cl. A mixture of Re(CO)5Cl (774 mg, 2.14
mmol) and bpyMe (394 mg, 2.14 mmol) in 40 mL of CH2Cl2 and
120 mL of toluene was heated at reflux for 2 h to give a bright
yellow-orange solution. The solvent was removed under vacuum
and the crude material recrystallized from CH2Cl2/hexanes to give
1
(bpyMe)Re(CO)3Cl as a pure yellow powder (1.00 g, 95%). H
NMR: δ 8.89 (d, 2H, J ) 5.6 Hz), 7.98 (s, 2H), 7.34 (d, 2H, J )
5.6 Hz), 2.58 (s, 6H).13C{1H} NMR: δ 197.5, 155.6, 152.8, 151.4,
128.1, 123.9, 21.9. ESIMS: m/z 454.8 (M - Cl)+. IR: νCO ) 2022,
1917, 1895 cm-1. Anal. (wt %) Calcd for ReC15O3H12N2Cl: C,
36.77; H, 2.47; N, 5.74. Found: C, 36.44; H, 2.59; N, 5.65.
(bpyN)Re(CO)3Cl. The procedure used was analogous to that
for (bpyMe)Re(CO)3Cl. The reaction of Re(CO)5Cl (299 mg, 0.827
mmol) and bpyN (251 mg, 0.827 mmol) gave (bpyN)Re(CO)3Cl
as a yellow powder (486 mg, 96%). 1H NMR: δ 8.91 (d, 1H, J )
5.6 Hz), 8.86 (d, 1H, J ) 5.6 Hz), 8.19 (s, 1H), 7.98 (s, 1H), 7.50
(d, 1H, J ) 5.6 Hz), 7.38 (m, 4H), 7.31 (m, 2H), 3.66 (br s, 4H),
2.57 (s, 3H), 2.32 (s, 3H).13C{1H} NMR: δ 197.5, 190.0, 155.8,
155.6, 153.7, 152.9, 152.7, 151.5, 138.2, 129.1, 128.8, 128.1, 127.8,
126.8, 124.1, 122.6, 62.5, 60.0, 43.1, 21.9. ESIMS: m/z 574.0 (M)+.
IR: νCO ) 2022, 1917, 1895 cm-1. Anal. (wt %) Calcd for
ReC23H21O3N3Cl: C, 45.35; H, 3.48; N, 6.90. Found: C, 43.58;
H, 3.58; N, 6.55.
(bpyNB)Re(CO)3Cl. The procedure used was analogous to that
for (bpyMe)Re(CO)3Cl. The reaction of Re(CO)5Cl (310 mg, 0.857
mmol) and 6 (357 mg, 0.860 mmol) gave 7 as a yellow powder
(560 mg, quantitative yield). 1H NMR: δ 8.84 (d, 1H, J ) 5.4 Hz),
8.83 (d, 1H, J ) 5.4 Hz), 8.10 (s, 1H), 8.02 (s, 1H), 7.70 (d, 1H,
J ) 7.0 Hz), 7.35-7.22 (m, 5H), 3.87 (d, 1H, J ) 12.2 Hz), 3.77
(d, 1H, 12.2 Hz), 3.72 (s, 4H), 3.46 (br s, 2H), 2.58 (s, 3H), 2.35
(s, 3H), 1.00 (s, 6H).13C{1H} NMR: δ 197.5, 190.1, 155.7, 155.6,
153.4, 152.6, 152.5, 151.3, 144.1, 134.2, 129.7, 129.2, 127.9, 127.2,
127.0, 124.4, 124.0, 72.5, 62.5, 59.2, 44.0, 32.0, 22.0, 21.7.
ESIMS: m/z 654.0 (unprotected M + H)+, 618.0 (unprotected M
- Cl)+. IR: νCO ) 2021, 1917, 1895 cm-1. Anal. (wt %) Calcd
for ReC27H30O5N3BCl: C, 45.74; H, 4.27; N, 5.93. Found: C,
45.35; H, 4.40; N, 5.59.
[(bpyNB)Re(CO)3(py)](OTf). The procedure used was analo-
gous to that for [(bpyMe)Re(CO)3(py)](OTf). The reaction of
AgOTf (115 mg, 0.448 mmol) and (bpyNB)Re(CO)3Cl (292 mg,
0.447 mmol), followed by chromatography on basic alumina with
9:1 acetonitrile/methanol gave pure [(bpyNB)Re(CO)3(py)](OTf)
as a yellow powder (182 mg, 45%). 1H NMR: δ 8.94 (d, 1H, J )
5.7 Hz), 8.90 (d, 1H, J ) 5.7 Hz), 8.36 (s, 1H), 8.24 (s, 1H), 8.18
(d, 2H, J ) 5.1 Hz), 7.79 (t, 1H, J ) 7.7 Hz), 7.65 (d, 1H, J ) 5.6
Hz), 7.62 (d, 1H, J ) 5.6 Hz), 7.53 (d, 1H, J ) 7.3 Hz), 7.35 (t,
2H, J ) 7.0 Hz), 7.14 (m, 2H), 7.06 (m, 1H), 3.86 (d, 2H, J )
12.8 Hz), 3.73 (s, 4H), 3.63 (q, 2H, J ) 15.0 Hz), 2.64 (s, 3H),
2.38 (s, 3H), 0.98 (s, 6H). 13C{1H} NMR: δ 195.7, 195.6, 191.4,
155.9, 155.3, 155.2, 154.3, 152.4, 152.3, 151.8, 144.1, 139.9, 135.9,
133.9, 130.0, 129.4, 129.0, 128.7, 127.2, 126.8, 126.1, 125.4, 72.4,
62.4, 59.6, 44.5, 31.9, 21.9, 21.7. ESIMS: m/z 697.2 (unprotected
M)+. IR: νCO ) 2034, 1931 cm-1. Anal. (wt %) Calcd for
ReC33H35O8BSF3N4: C, 43.95; H, 3.91; N, 6.21. Found: C, 43.69;
H, 3.95; N, 6.12.
Results
Bipyridine Ligand Synthesis. We have synthesized new
boronate and benzyl bipyridine ligands by the routes shown
in Scheme 1. Previous work has shown that compound
bpyCH2Br provides the simplest entry into a variety of
functionalized bipyridine compounds.35-37 While the prepa-
ration of bpyCH2Br can only be carried out in moderate
yields, the final two alkylation steps generally occur in 70-
80% yield, allowing multigram batches of bpyN or bpyNB
to be conveniently prepared. Compound 3 was introduced
by James et al. as a useful reagent for appending the
o-tolylboronic acid group to fluorophores such as ArCH2-
NHMe (Ar ) phenyl, naphthyl, anthryl).17 We have prepared
the amino boronate reagent 4, which can also be used in the
synthesis of bpyNB. In this case, either the linear or
convergent route gives acceptable results, although in other
[(bpyMe)Re(CO)3(py)](OTf). A solution of AgOTf (263 mg,
1.02 mmol) in 5 mL of THF was added to a solution of (bpyMe)-
Re(CO)3Cl (499 mg, 1.02 mmol) in 50 mL of CH2Cl2 to im-
mediately give a cloudy yellow mixture. After stirring for 2 h, the
precipitate was removed by filtration, and 100 mL of ethanol and
10.0 mL of pyridine were added to the yellow solution. This was
Inorganic Chemistry, Vol. 41, No. 6, 2002 1665

Cary et al.
Scheme 1. Synthesis of Bipyridine Ligands
Scheme 2. Preparation of Rhenium Complexes
Table 1. XPS Binding Energies for Selected Re Compounds
compound
Re4f7/2, eV
Re4f5/2, eV
(bpyMe)Re(CO)3Cl
(bpyN)Re(CO)3Cl
(bpyNB)Re(CO)3Cl
[(bpyMe)Re(CO)3(py)](OTf)
[(bpyN)Re(CO)3(py)](OTf)
[(bpyNB)Re(CO)3(py)](OTf)
40.6
40.2
40.5
40.8
40.9
40.8
42.8
42.6
42.8
42.9
43.1
42.9
(2034 and 1931 cm-1).25 It is worth noting that the carbonyl
stretching frequencies are invariant among the set of chloro
compounds and among the set of pyridinium complexes. This
suggests that the substituent changes on the periphery of the
bipyridyl ligands do not substantially alter the electron
density at the metal center (see electrochemistry and XPS
below). This is further substantiated by the fact that the
absorbance spectra between 250 and 500 nm for both
[(bpyN)Re(CO)3(py)](OTf) and [(bpyNB)Re(CO)3(py)](OTf)
are virtually identical, having maxima at 255, 310, 320, and
345 nm. One compound, [(bpyN)Re(CO)3(py)](OTf), ex-
hibits a C elemental analysis value that is 8% lower than
that calculated for the formula ReC29H26N4O6SF3. The cause
of this difference is probably due, at least in part, to a ligand
impurity, but this is not clear and is addressed later.
The binding energies for Re derived from XPS are shown
in Table 1. The positions for all the compounds fall within
40-41 eV for the Re4f7/2 and 42.4-43.5 eV for Re4f5/2.
These data indicate the binding around the metal center is
similar in all the complexes, and as seen in the IR and
electronic spectral data, the substitutions on the periphery
of the bipyridine ligand do not substantially alter the electron
density on the metal. Similar behavior is seen in the XPS
binding energies for the bipyridyl N atoms. The full XPS
analyses of these compounds will be reported elsewhere.
Figure 2 shows the excitation and emission spectra in 50%
MeOH/PBS for [(bpyX)Re(CO)3(py)][OTf] where bpyX )
bpyMe, bpyN, and bpyNB compounds. The excitation
spectra were taken between 280 and 450 nm. Two excitation
systems 3 or 4 were found to be more convenient depending
on the transformation required.
Rhenium Complex Synthesis. Rhenium complexes
[(bpyX)Re(CO)3Cl] and [(bpyX)Re(CO)3(py)](OTf) (bpyX
) bpyMe, bpyN, and bpyNB; py ) pyridine; OTf-
)
-
CF3SO3 ) were prepared as shown in Scheme 2 using the
bipyridyl ligands bpyMe, bpyN, and bpyNB. These reactions
are analogous to previously reported reactions and can be
carried out in high yield.25,30,40 The three ligand derivatives
were prepared for rhenium in order to aid in the interpretation
of the fluorescence and electrochemical data discussed later.
1
The H and 13C{1H} NMR spectra and MS data clearly
confirm the identity of the compounds. In the IR spectra of
the three chloro complexes, [(bpyX)Re(CO)3Cl] (bpyX )
bpyMe, bpyN, and bpyNB), each exhibit carbonyl stretches
at 2022, 1917, at 1895 cm-1; CO resonances are observed
at 2034 and 1931 cm-1 for each of the pyridinium complexes
[(bpyX)Re(CO)3(py)](OTf). These data are in accord with
the reported values for [(bpyCH2NEt2)Re(CO)3Cl] (2021,
1915, at 1895 cm-1) and [(bpyCH2NEt2)Re(CO)3(py)](OTf)
(40) Li, L.; Castellano, F. N.; Gryczynski, I.; Lakowicz, J. R. Chem. Phys.
Lipids 1999, 99, 1-9.
1666 Inorganic Chemistry, Vol. 41, No. 6, 2002

Re bpy Complexes for Glucose Recognition
Table 2. Electrochemical Data for [(bpyX)Re(CO)3(py)][OTf] where
bpyX ) bpyMe, bpyN, bpyNBa
compound
amine oxidation (V)
fluorophore reduction (V)
bpyMe
bpyN
bpyNB
nd
1.3
1.25
-1.3
-1.2
-1.25
a All measurements done using glassy carbon working electrode, Pt
counter electrode, and Ag/AgCl reference electrode in 0.1 M TBAP/ACN;
nd ) none detected.
Figure 3. Relative fluorescence intensity at the excitation and emission
peak maxima of 10.0 µM [(bpyX)Re(CO)3(py)](OTf) (bpyX ) bpyMe,
bpyN, bpyNB) in 50% methanol/saline solution vs pH.
Figure 2. Excitation and emission spectra (intensity vs wavelength) for
[(bpyX)Re(CO)3(py)](OTf) (bpyX ) bpyMe (top), bpyN (middle), bpyNB
(bottom)). Excitation spectra recorded from 280 to 450 nm and emission
spectra recorded from 375 to 600 or 650 nm. For emission spectra, λex
values are the following: bpyMe, 316 and 350 nm (see text); bpyN, 360
nm; bpyNB, 358 nm.
fluorophore reduction potentials. These values are consistent
with the amine oxidation values reported by Ziessel et al.
where the rhenium metal oxidation potentials for a series of
related compounds are shown to be considerably higher.27
pH Dependence of Fluorescence. Figure 3 shows the
relative fluorescence intensity of [(bpyX)Re(CO)3(py)](OTf)
(bpyX ) bpyMe, bpyN, bpyNB) as a function of pH in 50%
methanol/saline solution. As expected for compounds without
a pendant amine functionality, the relative fluorescence of
[(bpyMe)Re(CO)3(py)](OTf) remains constant over the pH
range studied; [(bpyN)Re(CO)3(py)](OTf) exhibits a decrease
in fluorescence intensity of 71% upon decreasing pH from
2 to 7, with an inflection point at about pH 4.3 (pKa ≈ 9 for
amines related to bpyN). This discrepancy between the
apparent pKa from fluorescence data and the reported pKa
based on acid-base titrations is well-known.41 In contrast,
[(bpyNB)Re(CO)3(py)]OTf shows a fairly level fluorescence
intensity over the pH range 4-10. At pH 11.4, [(bpyNB)-
Re(CO)3(py)](OTf) undergoes a single step decrease in
intensity of 86%.
maxima are seen for all three [(bpyX)Re(CO)3(py)][OTf]
compounds at the following positions: bpyMe, 316 and 350
nm; bpyN, 316 and 360 nm; bpyNB, 318 and 358 nm.
The emission spectra were taken from 375 to 650 nm. For
the emission spectra of [(bpyMe)Re(CO)3(py)][OTf], λex
)
316 and 350 nm, with the higher intensity spectrum corre-
sponding to the shorter wavelength excitation. The spectra
are identical except for relative intensities, having a λem
maximum at 540 nm. For the emission spectrum of [(bpyN)-
Re(CO)3(py)][OTf], λ
) 360 nm, and two emission
ex
maxima are seen, at 406 and 540 nm. The emission spectrum
from λ ex ) 316 nm (not shown) exhibited the same emission
spectrum, except for higher intensity. For the emission
spectrum of [(bpyNB)Re(CO)3(py)][OTf], λ ) 358 nm,
ex
and only one emission maximum is seen, as in the [(bpyMe)-
Re(CO)3(py)][OTf] case. The emission spectrum from λ
ex
) 318 nm (not shown) exhibited the same emission
spectrum, except for higher intensity.
Glucose Testing. Figure 4 shows the testing of [(bpyX)-
Re(CO)3(py)](OTf) (bpyX ) bpyMe, bpyN, and bpyNB)
with glucose at various concentrations in 50% methanol/
PBS at pH 7.4. All show similar gradual fluorescence
responses upon increasing glucose levels in the 10,000-
30,000 mg/dL range. At concentrations below 5000 mg/dL,
the response of all is complicated and not useful for analytical
Absorption spectra for [(bpyN)Re(CO)3(py)][OTf] and
[(bpyNB)Re(CO)3(py)][OTf] were essentially identical ex-
cept for relative intensities. The spectra were nearly identical
to the excitation spectra having maxima at 253, 306 (sh),
318, and 351 nm.
Table 2 shows the electrochemical potentials of the OTf
complexes. The values for the amine oxidation potentials
are similar (nonexistent for the Me derivative), as are
(41) Skoog, D. A. Principles of instrumental analysis, 3rd ed.; Saunders:
Philadelphia, 1985.
Inorganic Chemistry, Vol. 41, No. 6, 2002 1667

Cary et al.
at 406 nm, the origin of which is not clear. The bpyN ligand
does have fluorescence properties λex ) 325 nm and λem
)
432 nm. In the middle trace of Figure 2, a shoulder at 432
nm on the maximum at 406 nm can be seen, which is
probably due to a ligand impurity (contributing to the
discrepancy in the C analysis for [(bpyN)Re(CO)3(py)]-
(OTf)). However, the 406 nm maximum in [(bpyN)Re(CO)3-
(py)][OTf] may not be due to a ligand impurity. It may be
due to a ligand centered excited state involving the lone pair
of the N. The same intense emission is seen in [(bpyN(CH2-
CH3)2)Re(CO)3(py)][OTf] at λex ) 365 nm and λem ) 414
and 544 nm. However, the higher energy emission is not
seen in either [(bpyNB)(Re(CO)3(py)][OTf] or the bpyNB
ligand perhaps because of delocalization of the lone pair with
the boron empty orbitals. This is further substantiated by
the pH dependence of [(bpyN(CH2CH3)2)Re(CO)3(py)][OTf]
which does not exhibit this emission maximum at low pH,
probably because of protonation of the amino nitrogen. In
addition, the nitrogen XPS data (not shown) indicate only
one type of N at N bonded to Re energies.
Figure 4. Relative fluorescence intensity at the excitation and emission
peak maxima of 10.0 µM [(bpyX)Re(CO)3(py)](OTf) (bpyX ) bpyMe,
bpyN, bpyNB) in 50% methanol/saline solution vs [glucose].
Electrochemistry. The search for new fluorescent glucose
transducers based on PET requires, among other things, an
understanding of the electrochemistry of any potential
donor-acceptor pair. The free energy of PET (∆Gel) can be
calculated using the Rehm-Weller equation shown in eq 1,
∆Gel(kcal mol-1) ) 23.06[E°(D+/D) - E°(A/A-)] - wp -
wr - ∆G00 (1)
where E°(D+/D) is the oxidation potential of the donor, E°
(A/A-) is the reduction potential of the acceptor, and ∆G00
is the free energy corresponding to the zero point energy,
E00.21 The quantities wp and wr are Coulombic terms for the
products and reactants and are found to be small in polar
solvents. In this present case, wr ) 0 because the reactants
are neutral, and to simplify predictions, wp is assumed to be
zero. E00 was determined from the overlap of the lowest
energy absorbance maximum and the emission maximum
spectra (λ00).21,45
The measurement of the first reduction potential for the
transition metal complexes [(bpyN)Re(CO)3(py)](OTf) and
[(bpyNB)Re(CO)3(py)](OTf) and corresponding ligand oxi-
dation potentials, given in Table 2, and the estimated λ00
values (458 and 425 nm, respectively) yield calculated ∆Gel
values of -4.75 and -9.65 kcal/mol, respectively, showing
the bpyNB derivative to be more favorable for PET, as
observed. In considering the significance of these values, it
is worth comparing them to ∆Gel for 1. For 1, the first
oxidation potential and reduction potential are 1.10 and
-2.11 V vs Ag/AgCl, respectively, and this compound has
a λ00 value of 405 nm (using criteria of Kavarnos and
Turro45). This yields a ∆Gel value of 3.47 kcal/mol for 1,
Figure 5. Relative fluorescence intensity at the excitation and emission
peak maxima of 10.0 µM [(bpyX)Re(CO)3(py)](OTf) (bpyX ) bpyMe,
bpyN, bpyNB) in methanol vs [glucose].
purposes. In addition, the bpyNB complex fails to show a
large fluorescence increase in the region of physiological
interest, 90-300 mg/dL.
Figure 5 shows the effects of changing the solvent system
to methanol for glucose testing in the 0-1600 mg/dL range.
[(bpyMe)Re(CO)3(py)](OTf) and [(bpyN)Re(CO)3(py)](OTf)
show little change in fluorescence intensity with addition of
glucose, similar to the behavior seen in Figure 4. [(bpyNB)-
Re(CO)3(py)](OTf), however, shows a 55% intensity increase
upon increasing glucose concentration from 0 to 400 mg/
dL, and ultimately, a 113% increase at 1600 mg/dL.
Discussion
Fluorescence Behavior. All three Re complexes show
fluorescence emission maxima at 540-550 nm, consistent
with Re diimine complexes in general and bipyridyl com-
plexes specifically,27,32,42-44 which have been assigned to
metal to ligand charge transfer (MLCT). The [(bpyN)Re-
(CO)3(py)](OTf) complex also shows a strong emission band
(42) Caspar, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984, 23,
2104-2109.
(43) Kotch, T. G.; Lees, A. J.; Fuerniss, S. J.; Papthomas, K. I. Chem.
Mater. 1991, 3, 25-26.
(44) Kotch, T. G.; Lees, A. J.; Fuerniss, S. J.; Papthomas, K. I. Chem.
Mater. 1992, 4, 675-683.
(45) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401-449.
1668 Inorganic Chemistry, Vol. 41, No. 6, 2002

Re bpy Complexes for Glucose Recognition
suggesting that relative driving force for PET for [(bpyNB)-
Re(CO)3(py)](OTf) is approximately 13 kcal mol-1 more
favorable than that for 1.46
mg/dL) by Yoon and Czarnik.16 By this metric, our com-
pounds exhibit comparable fractional transduction levels and
longer wavelengths than many of the currently available
systems. The major drawback is that these systems only
perform well in nonaqueous solvents, which makes them less
useful for operation in the body. Thus, one of our continuing
focuses is to develop improved transducers for use in purely
aqueous solvents. Most systems reported to date have been
studied in organic/aqueous solvent mixtures because of
limitations in solubility and performance.
Fluorescence Spectroscopy: pH Dependence. The most
direct test of the validity of these calculations is to examine
the response of these compounds to pH changes. At high
pH, the pendant amino group is expected to quench
fluorescence; by protonation of the amine at low pH,
quenching should not occur, giving maximum fluorescence.
This has been the basis for a number of previous molecular
transducers for pH detection, including bipyridine complexes
of ruthenium and rhenium.24,26,28,30 The magnitude of frac-
tional intensity change observed with respect to pH puts an
approximate upper bound on the level of response that can
be expected with respect to glucose concentration. In
[(bpyNB)Re(CO)3(py)](OTf), the modulation of amine quench-
ing by an attached glucose-boronate complex is more
complex and more difficult to predict than modulation of
this effect by a proton. Understanding the nature of the
boron-nitrogen interaction is crucial in the development of
a more effective glucose transducer by this approach.
Glucose Response. This level of performance is promising
in comparison to known small-molecule glucose transducers.
James et al. report a 180% intensity increase from 0 to 1600
mg/dL for 1 in 33% methanol/water,17 while more moderate
increases are observed for the squaraine system (8% increase
from 0 to 3000 mg/dL) by Kukrer and Akkaya15 and the
anthracene boronate system (8% decrease from 0 to 3000
Summary
The results obtained from this work are being applied to
the development of improved sensors for glucose. For that
purpose, it has been worthwhile to demonstrate that a new
small molecule transducer for glucose can be designed by a
rational approach based on initial studies of electrochemistry
and fluorescence properties, thus saving on the time spent
synthesizing new saccharide transducers.
Acknowledgment. This work was performed under the
auspices of the U.S. Department of Energy by University of
California Lawrence Livermore National Laboratory under
Contract No. W-7405-Eng-48. Funding for this work was
obtained through the U.S. Department of Energy Laboratory
Directed Research and Development program, DOC NIST
ATP Award 70NANB8H4065, NIH Grant DK-98-008, and
MiniMed Inc. We would like to thank Prof. B. Imperiali of
Massachusetts Institute of Technology for providing helpful
data on the preparation of bipyridine ligands and Robert A.
Reibold of LLNL for experimental assistance.
(46) Note: The ∆Gel values are meant to be for relative comparison only.
The positive ∆Gel value in this calculation is probably due to
anthracene boronate not having a MLCT and the λ00 needing to be
estimated by a different method.45
IC010202B
Inorganic Chemistry, Vol. 41, No. 6, 2002 1669