Intramolecularly sensitized precipitons: A model system for application to metal sequestration

Article abstract of DOI:10.1021/ja0561340

We have investigated light-triggered or catalytically activated precipitation agents and have proposed the name "precipiton" for such molecules or molecular fragments. A phase separation Is induced when the precipiton isomerizes to a low-solubility form.

Full text of DOI:10.1021/ja0561340

Published on Web 12/13/2005
Intramolecularly Sensitized Precipitons: A Model System for
Application to Metal Sequestration
Mark R. Ams and Craig S. Wilcox*
Contribution from the Department of Chemistry and The Combinatorial Chemistry Center,
UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260
Received September 6, 2005; E-mail: daylite@pitt.edu
Abstract: We have investigated light-triggered or catalytically activated precipitation agents and have
proposed the name “precipiton” for such molecules or molecular fragments. A phase separation is induced
when the precipiton isomerizes to a low-solubility form. In this paper we describe the first intramolecularly
activated precipitons. The isomerization process is induced by intramolecular triplet energy transfer from
a covalently attached metal complex. As expected, intramolecular sensitization leads to a more rapid
isomerization than can be achieved by intermolecular sensitization at accessible concentrations. Two
isomeric bichromophoric precipiton species, each containing [Ru(bpy)3]2+ and 1,2-bis(biphenyl)ethene units
covalently linked together by an ether tether, have been synthesized and characterized, and their
photochemical properties have been investigated. The rates of photoisomerization of these complexes,
[((Z)-1,2-bis(biphenyl)ethene-bpy)Ru(bpy)2](PF6)2 (2Z) and [((E)-1,2-bis(biphenyl)ethene-bpy)Ru(bpy)2](PF6)2
(2E), were compared to those of their untethered analogues, (Z)-1,2-bis(biphenyl)ethene-OTBS (1Z) and
(E)-1,2-bis(biphenyl)ethene-OTBS (1E), where ruthenium sensitization occurred through an intermolecular
pathway. Upon irradiation with visible light (λ g 400 nm) in degassed solution, 2Z/E and 1Z/E obeyed
reversible first-order rate kinetics. The intramolecularly sensitized precipiton 2Z isomerized 250 times faster
(k2Zf2E ) 1.0 × 10-3 s-1 with a 51% neutral density filter) than the intermolecular case 1Z (k1Zf1E ) 0.80
× 10-5 s-1). For 1E and 2E, the isomerization rates were k1Ef1Z ) 11.0 × 10-5 s-1 and k2Ef2Z ) 1.6 × 10-3
s-1, respectively. The average Z/E mole ratio at the photostationary state was 62/38 for 2Z/E and 93/7 for
1Z/E. The impetus for this study was our desire to evaluate the possibility of using metal-binding precipitons
that would precipitate only upon metal-to-precipiton binding and would be inert to visible light in the absence
of metals.
Introduction
metal impurities have been reported, and all these methods rely
on passive equilibrium binding events. For example, silica and
The effective removal of metal contaminants from aqueous
and organic media continues to be an important challenge with
implications for public health and economic competitiveness.1
Reducing toxic metal impurities from organic media to the parts
per million (ppm) level is required for active pharmaceutical
ingredients (APIs), and meeting this need can increase costs
for the pharmaceutical industry.2
synthetic-polymer-derived supports have been used to carry thiol
and amine ligands so that expended catalysts, if sufficiently
sequestered by these ligating groups, can be removed (scav-
enged) from a reaction mixture by filtration.2c,3
Many pharmaceutical intermediates and final drugs, because
of their heterocyclic structures, have good affinities for these
metals, a fact that exacerbates the challenge. When a binding
agent is used in stoichiometric amounts, in order for such a
binding agent or scavenger to be effective at removing a metal
impurity, the scavenger ligands must have a much stronger
affinity for the metal than the drugs and APIs that are also
present. For example, to remove 99% of a contaminant metal
from 20 kg of an API, one would need 20 g of a ligand that has
a 100 000 times greater affinity for the metal than for the API.
Although such ligands can certainly be prepared, their use may
be prohibitively expensive.
The most common sources of metal contaminants in APIs
are the catalysts used in their synthesis. These catalysts are of
great importance in reducing the costs of production, and there
are no good alternatives that would allow them to be replaced.
Commonly observed contaminants are palladium, platinum,
ruthenium, and rhodium. Several methods to remove these trace
(1) (a) Kim, S.; Kim, K.; Stuben, D. J. EnViron. Eng. 2002, 128, 705-715.
(b) Agrawal, A.; Sahu, K. K.; Pandey, B. D. Colloids Surf., A 2004, 237,
133-140. (c) Wingenfelder, U.; Hansen, C.; Furrer, G.; Schulin, R. EnViron.
Sci. Technol. 2005, 39, 4606-4613. (d) Rahman, M. A.; Ahsan, S.; Kaneco,
S.; Katsumata, H.; Suzuki, T.; Ohta, K. J. EnViron. Manage. 2005, 107-
110.
(2) (a) Rosso, V. W.; Lust, D. A.; Bernot, P. J.; Grosso, J. A.; Modi, S. P.;
Rusowicz, A.; Sedergran, T. C.; Simpson, J. H.; Srivastava, S. K.; Humora,
M. J.; Anderson, N. G. Org. Process Res. DeV. 1997, 1, 311-314. (b)
Manley, P.; Acemoglu, M.; Marterer, W.; Pachinger, W. Org. Process Res.
DeV. 2003, 7, 436-445. (c) Urawa, Y.; Miyazawa, M.; Ozeki, N.; Ogura,
K. Org. Process Res. DeV. 2003, 7, 191-195.
The challenge can be stated in this way: Given 20 kg of an
API or final pharmaceutical containing 1 g of a metal contami-
(3) (a) Brown, J.; Mercier, L.; Pinnavaia, T. J. Chem. Commun. 1999, 69-70.
(b) Liu, A. M.; Hidajat, K.; Kawi, S.; Zhao, D. Y. Chem. Commun. 2000,
1145-1146. (c) Nooney, R. I.; Kalyanaraman, M.; Kennedy, G.; Maginn,
E. J. Langmuir 2001, 17, 528-533. (d) Vieira, E. F. S.; Simoni, J. de A.;
Airoldi, C. J. Mater. Chem. 1997, 7, 2249-2252.
9
250
J. AM. CHEM. SOC. 2006, 128, 250-256
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Intramolecularly Sensitized Precipitons
A R T I C L E S
Scheme 1. An Energy-Activated Precipitation Process To Remove
Chart 1. Formulas of the Investigated Compounds
Metal Impurities
nant (50 ppm), what is the most economical way to reduce this
contaminant to less than 5 ppm (a level currently considered
acceptable to the industry for the common catalyst-derived
contaminants)? Ideally, one would need not more than one molar
equivalent of the scavenging ligand, no matter the scale, and
this ligand would be recoverable.
We expect that one way in which the problem of ligand
competitors in metal scavenging may be overcome is through
the use of an energy-driven separation process, wherein the
ligated metal would activate isomerization and cause precipita-
tion. In every event that the metal does bind to the precipiton
in the Z form, if the resulting complex can absorb light, an
isomerization process is “triggered” via intramolecular sensitiza-
tion to produce the E isomer. At any time after the point at
which the E precipiton has reached saturation, the E precipiton
would precipitate together with the metal, thus decreasing the
amount of metal in solution. In short, only a “loaded” ligand
will precipitate.4 The amount of metal bound to scavenger,
originally limited by the equilibrium constants for binding to
the precipiton ligand (K1) and competitors (K2), is increased
due to precipitation (Scheme 1). The energy to drive the process
is stored in the structure of the scavenging ligand.
In this paper we report on our initial studies of metal complex
activated isomerization. A large number of transition metal
complexes containing polyimine ligands are capable of visible
light absorption and formation of metal-to-ligand charge-transfer
(MLCT) triplet states.5 Within this group, polypyridyl Ru(II)
complexes have been widely used as photosensitizers for a
variety of reactions, including photoisomerization. In the 1970s,
researchers from the Wrighton laboratory demonstrated inter-
molecular energy transfer from the triplet excited states of Ru-
(bpy)32+ complexes to trans-stilbene.6 Since then, intramolecular
energy-transfer processes from the 3MLCT state of Ru(II)
complexes to stilbene-containing ligands have also been inves-
tigated.7 Studies of related bichromophoric receiver-antenna
systems have demonstrated that Ru(bpy)3 complexes are
capable of intramolecular energy-transfer processes with co-
valently linked organic chromophores.8
Herein we describe the preparation, photophysical properties,
and photoisomerization kinetics of precipitons9 that are co-
valently linked and unlinked to a Ru(bpy)32+ unit. The structures
of these compounds are shown in Chart 1. The photoisomer-
ization kinetics at low concentration (10 µM) and photostation-
ary-state ratios of the Ru(II)-linked precipitons 2Z/E were
compared to those of the unlinked examples 1Z/E.
2+
Results and Discussion
Synthesis. The preparation of 1,2-bis(biphenyl)ethene TBS
ether 1Z9g and 1,2-bis(biphenyl)ethene alcohol 3Z9d has been
reported previously from our laboratory, and 5-(bromomethyl)-
2,2′-bipyridine was synthesized according to a published
procedure.10 The syntheses of 1E, 2Z, and 2E are summarized
in Schemes 2, 3, and 4, respectively.
(7) (a) Zarnegar, P.; Bock, C. R.; Whitten, D. G. J. Am. Chem. Soc. 1973, 95,
4367-4372. (b) Shaw, J. R.; Webb, R. T.; Schmehl, R. H. J. Am. Chem.
Soc. 1990, 112, 1117-1123.
(8) (a) Morales, A.; Accorsi, G.; Armaroli, N.; Barigelletti, F.; Pope, S.; Ward,
M. D. Inorg. Chem. 2002, 41, 6711-6719. (b) Wilson, G. J.; Launikonis,
A.; Sasse, W. H. F.; Mau, A. W. H. J. Phys. Chem. A 1997, 101, 4860-
4866. (c) Juris, A.; Prodi, L. New J. Chem. 2001, 25, 1132-1135. (d)
Wilson, G. J.; Sasse, W. H. F.; Mau, A. W. H. Chem. Phys. Lett. 1996,
250, 583-588. (e) Tyson, D.; Bignozzi, C.; Castellano, F. J. Am. Chem.
Soc. 2002, 124, 4562-4563. (f) Tyson, D. S.; Castellano, F. N. J. Phys.
Chem. A 1999, 103, 10955-10960. (g) Wilson, G. J.; Launikonis, A.; Sasse,
W. H. F.; Mau, A. W. H. J. Phys. Chem. A 1998, 102, 5150-5156. (h)
Tyson, D. S.; Henbest, K. B.; Bialecki, J.; Castellano, F. N. J. Phys. Chem.
A 2001, 105, 8154-8161. (i) Harriman, A.; Hissler, M.; Khatyr, A.; Ziessel,
R. Chem. Commun. 1999, 735-736.
(4) This requires the right relationship between rates of exchange and
precipitation.
(5) (a) Wrighton, M.; Giordano, P. J. Am. Chem. Soc. 1979, 101, 2888-2897.
(b) Sakaki, S.; Okitaka, I.; Ohkubo, K. Inorg. Chem. 1984, 23, 198-203.
(c) Balzani, V.; Juris, A.; Venturi, M. Chem. ReV. 1996, 96, 759-833. (d)
Yam, V.; Chor-Yue, V.; Wu, L. J. Chem. Soc., Dalton Trans. 1998, 1461-
1468. (e) Schanze, K. S.; Lucia, L. A.; Cooper, M.; Walters, K. A.; Ji, H.;
Sabina, O. J. Phys. Chem. A 1998, 102, 5577-5584. (f) Vlcek, A. Coord.
Chem. ReV. 2000, 200-202, 933-977. (g) Sun, S.; Robson, E.; Dunwoody,
N.; Silva, A.; Brinn, I.; Lees, A. J. Chem. Commun. 2000, 201-202. (h)
Yam, V. W.; Yang, Y.; Zhang, J.; Chu, W.; Zhu, N. Organometallics 2001,
20, 4911-4918. (i) Sun, S.; Lees, A. J. Organometallics 2002, 21, 39-
49.
(9) (a) Bosanac, T.; Yang, J.; Wilcox, C. S. Angew. Chem., Int. Ed. 2001, 40,
1875-1879. (b) Bosanac, T.; Wilcox, C. S. Chem. Commun. 2001, 1618-
1619. (c) Bosanac, T.; Wilcox, C. S. Tetrahedron Lett. 2001, 42, 4309-
4212. (d) Bosanac, T.; Wilcox, C. S. J. Am. Chem. Soc. 2002, 124, 4194.
(e) Honigfort, M.; Brittain, W. J.; Bosanac, T.; Wilcox, C. S. Macromol-
ecules 2002, 35, 4849-4851. (f) Honigfort, M.; Shingtza, L.; Rademacher,
J.; Malaba, D.; Bosanac, T.; Wilcox, C. S.; Brittain, W. J. ACS Symp. Ser.
2003, 854, 250. (g) Bosanac, T.; Wilcox, C. S. Org. Lett. 2004, 6, 2321-
2324.
(10) Ballardini, R.; Balzani, V.; Clemente-Leon, M.; Credi, A.; Gandolfi, M.;
Ishow, E.; Perkins, J.; Stoddart, F.; Tseng, H.; Wenger, S. J. Am. Chem.
Soc. 2002, 124, 12786-12796.
(6) Wrighton, M.; Markham, J. J. Phys. Chem. 1973, 77, 3042.
9
J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006 251

A R T I C L E S
Ams and Wilcox
Scheme 2. Synthesis of Compound 1E
Table 1. Spectroscopic and Photophysical Data (CH3CN, 298 K)
absorption
λ
max/nm
emission
max/nm
1
compound
(ꢀ
/M-1 cm-
)
λ
1Z
265 (61 500), 309 (44 300)
335 (59 300)
247 (30 122), 286 (198 400), 341 (33 100),
1Ec
2Z
612a
612a
450 (18 260)
245 (66 757), 286 (178 080), 341 (59 300),
450 (10 755)
288 (57 400), 450 (10 500)
268 (31 000), 331 (31 600)
274 (34 200), 290 (33 200), 330 (24 800)
Scheme 3. Synthesis of Ruthenium Complex 2Z
2E
2+
Ru(bpy)3
3Z
4Z
613a
315b
b
a λexc ) 450 nm. λexc ) 285 nm. c Measured in CH2Cl2 at 298 K.
Scheme 4. Synthesis of Ruthenium Complex 2E
Figure 1. Room-temperature absorption spectra recorded for degassed 10
µM acetonitrile solutions of 1Z, 1E (performed in CH2Cl2), 2Z, 2E, and
2+
Ru(bpy)3
.
Absorption and Emission Spectroscopy. The photophysical
data for these new compounds are summarized in Table 1.
Figure 1 shows the absorption spectra for 1Z, 1E, 2Z, 2E, and
Ru(bpy)3Cl2 at room temperature. Compounds 2Z, 2E, and Ru-
(bpy)3Cl2 were dissolved in neat acetonitrile, while 1E had to
be dissolved in neat CH2Cl2. Comparison of the absorption
profiles displayed in this figure allows the following assignments
to be made. Complexes 2E and 2Z are essentially composed of
1,2-bis(biphenyl)ethene TBS ether 1Z/E appended to a Ru-
Synthesis of Compound 1E. The synthesis of complex 1E
is outlined in Scheme 2. Chemical isomerization of TBS ether
1Z with 0.1 equiv of diphenyl diselenide in THF afforded the
E isomer in 59% yield.
Synthesis of Complex 2Z. To begin the preparation of
complex 2Z (Scheme 3), alkylation of 1,2-bis(biphenyl)ethene
alcohol (3Z) with 5-(bromomethyl)-2,2′-bipyridine afforded a
73% yield of the bipyridine-tagged 1,2-bis(biphenyl)ethene
precipiton 4Z. The ruthenium complex 2Z was obtained as its
bis(hexafluorophosphate) salt in 49% overall yield, after re-
fluxing of cis-dichlorobis(2,2′-bipyridine)ruthenium(II) with
ligand 4Z in acetone for 48 h and counterion exchange (NH4-
PF6/H2O/CH3CN).
2+
(bpy)3 moiety. With reference to previous work on similar
systems, the band maximum at 450 nm is tentatively assigned
as the MLCT (Ru f bpy) transition, as shown in the same
2+ 8a
region for Ru(bpy)3
.
The features in the 270-300 nm
spectroscopic region are those of bpy-centered π f π*
transitions. The absorption characteristics in the region 260-
370 nm are identified as being of phenylene origin and are
ascribed to population of the 1phenylene excited state.11 Absorp-
tion in the region 321-377 nm, which is seen to be most intense
for the trans isomer 2E, is attributed to the π f π* transition
localized on the ethylene functionality for each precipiton. The
absorption spectra of Figure 1 indicate that absorption results
from additive properties of scarcely interacting Ru-based and
1,2-bis(biphenyl)ethene chromophores. This is consistent with
the expectation that the ether linkage provides for electronic
separation between the Ru and 1,2-bis(biphenyl)ethene photo-
Synthesis of Complex 2E. The synthesis of complex 2E is
outlined in Scheme 4. Sensitized isomerization of bipyridine-
tagged (Z)-1,2-bis(biphenyl)ethene precipiton 4Z with erythrosin
B in THF afforded (73%) the E isomer 4E. The ruthenium
complex 2E was obtained as its bis(hexafluorophosphate) salt
in 49% overall yield, after refluxing of cis-dichlorobis(2,2′-
bipyridine)ruthenium(II) with ligand 4E in 1:1 acetone:THF
followed by counterion exchange (NH4PF6/H2O/CH3CN). It was
observed that 2E and 2Z quickly interconvert under normal room
illumination. Therefore, operations with pure isomers were
carried out under red light.
(11) Crews, P.; Rodriguez, J.; Jaspars, M. Optical and Chiroptical Techniques:
Ultraviolet Spectroscopy. In Organic Structure Analysis; Houk, K. N.,
Loudon, G. M., Eds.; Topics in Organic Chemistry Series; Oxford
University Press: New York, 1998; pp 349-373.
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252 J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006

Intramolecularly Sensitized Precipitons
A R T I C L E S
Figure 3. UV-vis spectral changes observed with complex 1E upon
irradiation at λ g 400 nm at 0, 50, 100, 210, 300, 410, and 565 min (a-g,
respectively) in 10 µM 70:30 THF:CH3CN.
Figure 2. Room-temperature emission spectra recorded for degassed 10
µM acetonitrile solutions of complexes 2Z, 2E, and Ru(bpy)3Cl2.
active units. From the absorption properties of 2Z, 2E, and their
components, it seems reasonable that the use of λexc g 400 nm
2+
irradiation would result in selective excitation of the Ru(bpy)3
chromophore. Conversely, use of λexc e 370 nm irradiation
would produce excited states of both Ru and 1,2-bis(biphenyl)-
ethene components.
Excitation of complexes 2Z and 2E at λexc ) 450 nm in
acetonitrile at room temperature results in luminescence centered
at 570-680 nm. The data are summarized in Table 1, and Figure
2 shows the excitation spectra of 2Z, 2E, and Ru(bpy)32+. The
spectra exhibit overlapping profiles, with the band maxima
peaking at 612 nm. The emission intensity for the cis isomer is
shown to be approximately 2.5 orders of magnitude more intense
than that of the trans isomer and slightly red-shifted, by 3 nm.
It is known for stilbene that the triplet-state energy for the cis
isomer (63 kcal/mol) is higher than that for the trans isomer
(49 kcal/mol).12 In the case of 2E, it is likely that the triplet
energy from the MLCT excited state of Ru(bpy)3 (49 kcal/
mol) is favorably intramolecularly transferred to the trans-alkene
moiety as a deactivation pathway. For the cis isomer 2Z, on
the other hand, such an intramolecular process is disfavored
due to the higher triplet-state energy of the cis isomer.5d
Figure 4. UV-vis spectral changes observed with complex 2Z (8.2 µM
in degassed acetonitrile) upon irradiation at λ g 400 nm at 0, 20, 40, 60,
80, and 100 min (a-f, respectively).
changes with a clean and well-defined isosbestic point were
observed. The isomerizations were expectedly slow, requiring
over 9 h to give no further UV-vis spectroscopic changes. For
1E, we observed an isosbestic point at 304 nm (Figure 3).
From initial photoisomerization experiments with 2Z/E, we
learned that the reaction rate was too rapid for direct observation
under the conditions used for 1Z/E. To slow the reaction rate,
the light intensity was reduced by 51% through equipping the
lamp with a neutral density filter. Upon irradiation of a degassed
acetonitrile solution of 2Z (8.2 µM) or 2E (7.4 µM), UV-vis
spectral changes with a clean and well-defined isosbestic point
were observed. Complex 2Z showed an increase in absorption
intensity at 341 nm as well as an isosbestic point at 305 nm
(Figure 4). Complex 2E showed a decrease in absorption
intensity at 341 nm as well as an isosbestic point at 303 nm
(Figure 5). The photostationary state was reached within 45 min.
The isomers 2Z/E are distinguishable by 1H NMR spectroscopy
due to the chemical shift differences of the alkene and methylene
protons. Observation of the changes in NMR chemical shift and
peak intensity of 2Z/E during photoisomerization confirmed that
the UV-vis spectroscopic changes are associated with the
isomerization of the carbon-carbon double bond.
3
2+
Photoisomerization Reactivity in O2-Free Solution
Electronic Absorption Studies. For compounds 1Z/E and
2Z/E, direct visible light excitation at λ g 400 nm gave cis T
trans isomerization as the only observed photoreaction event.
To determine if 2Z/E isomerizes under these conditions
primarily due to an intramolecular interaction with the bound
2+
Ru(bpy)3 sensitizer, the concentration was varied and the
photoreactivity of analogue 1Z/E with Ru(bpy)32+ was studied
for comparison. At dilute concentrations (e10 µM), intermo-
lecular quenching processes between donor and acceptor can
be reduced so that the observed quenching is primarily due to
intercomponent interactions.13
A 10 µM degassed solution of 1Z/E in 70:30 THF:CH3CN
was irradiated in a darkened room using a 25-W incandescent
lamp equipped with a 400-nm cutoff filter. UV-vis spectral
Kinetics. To further investigate the process occurring in
complexes 2Z/E and compounds 1Z/E, quantitative rate studies
were undertaken. Solutions of pure 1Z and 1E (10 µM, 70:30
THF:CH3CN) were irradiated at λ g 400 nm in the presence of
10 µM Ru(bpy)32+ sensitizer and monitored by UV-vis at 350
nm. The concentration of cis and trans precipiton present is
(12) Saltiel, J.; Marchland, G.; Kaminska, E.; Smothers, W.; Meuller, W.;
Charlton, J. J. Am. Chem. Soc. 1984, 106, 3144.
(13) Schlicke, B.; Belser, P.; De Cola, L.; Sabbioni, E.; Balzani, V. J. Am. Chem.
Soc. 1999, 121, 4207.
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J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006 253

A R T I C L E S
Ams and Wilcox
Figure 5. UV-vis spectral changes observed with complex 2E (7.4 µM
in degassed acetonitrile) upon irradiation at λ g 400 nm at 0, 20, 40, 60,
80, and 100 min (a-f, respectively).
Figure 7. Photochemical production of photostationary state concentrations
upon irradiation at λ g 400 nm of 2Z f 2E and 2E f 2Z. (Top) [2E] vs
time. (Bottom) [2Z] vs time. a[Z] vs time starting with 8.2 µM 2Z. b[E] vs
time starting with 7.2 µM 2E.
Figure 6. Photochemical production of photostationary state concentrations
upon irradiation at λ g 400 nm of 1Z/E starting from pure 1Z.
constants were determined to be kZfE ) 1.0 × 10-3 s-1 and
kEfZ ) 1.6 × 10-3 s-1. When factoring in the light reduction
due to the 51% neutral density filter, the rate constant for cis
f trans isomerization for 2Z is 250 times faster than for 1Z,
and the trans f cis isomerization for 2E is 29 times faster than
for 1E.
Table 2. Isomerization Rate Constants and Photostationary State
Ratios for the Photoisomerization Process of 1Z/E and 2Z/E upon
Irradiation at λ g 400 nm
1
reaction
k (s-
)
×
103
Z/E average ratio
1Z f 1E
1E f 1Z
2Z f 2E
2E f 2Z
0.0080
0.11
1.0
93/7
62/38
A control experiment to correct for the presence of any
background cis T trans isomerization occurring from simple
exposure to direct irradiation was performed on the untethered
precipiton 3Z. A 20 mM solution of 3Z in acetonitrile was
prepared and irradiated at λ g 400 nm for 1 h. Only 3% had
1.6
plotted as a function of irradiation time in Figure 6. After 9.4
h of irradiation, solutions containing each pure isomer came to
a final Z/E photostationary state, the average ratio of which was
determined to be 93/7. The plots fit well to a reversible first-
order rate curve (see Figure 6), and the rate constants were
determined to be kZfE ) 0.80 × 10-5 s-1 and kEfZ ) 11.0 ×
10-5 s-1 (Table 2).
1
isomerized, as determined by H NMR spectroscopy analysis,
and produced a small amount of white precipitate. Formation
of the trans isomer, however, was not observable by NMR due
to its high insolubility.
Conclusions
Solutions of 2Z and 2E (CH3CN) were irradiated under the
same conditions14 as 1Z/E and came to a final Z/E photosta-
tionary state, the average ratio of which was determined to be
62/38. Figure 7 plots the relative concentration of 2Z/E formed
or depleted in each reaction as a function of time.15 The plots
fit well to a reversible first-order rate curve, and the rate
We have synthesized and characterized two new stilbene-
based precipitons, 2Z and 2E, both featuring a covalently bound
2+
Ru(bpy)3 sensitizer. Upon irradiation at visible wavelengths
(λ g 400 nm), the complexes undergo photoisomerization. The
1
process was monitored by electronic absorption and H NMR
spectroscopy. The reversible first-order rate constants for 2Z/E
have been determined and compared to those of the intermo-
lecularly sensitized untethered analogues 1Z/E. At low con-
centration (e10 µM), 2Z isomerized 250 times faster than 1Z
+ Ru(bpy)32+. Each isomer came to a final Z/E photostationary
state, the average ratio of which was determined to be 62/38
(14) The irradiation light, however, was reduced 51% using a neutral density
filter.
(15) 1H NMR spectroscopy and elemental analysis of 2E revealed that 12% of
4E was present in the sample prior to irradiation. To account for this, the
absorbance due to 4E was subtracted from the observed absorbance. The
correction simply displaces curve and has no affect on the derived rate
constants.
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254 J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006

Intramolecularly Sensitized Precipitons
A R T I C L E S
J ) 8 Hz, 2H), 7.90 (dd, J ) 8, 2 Hz, 1H), 7.85 (dd, J ) 8, 2 Hz, 1H),
7.66-7.62 (m, 12H), 7.50 (app d, J ) 9 Hz, 3H), 7.43 (app s, 1H),
7.38-7.34 (m, 2H), 7.20 (s, 2H), 4.68 (s, 2H), 4.67 (s, 2H); 13C NMR,
too insoluble to obtain; HRMS(EI) m/z calcd for C38H31N2O [M + H]
531.2436, found 531.2452.
for 2Z f 2E and 93/7 for 1Z f 1E. In this model study we
have been able to demonstrate a significant isomerization rate
enhancement for precipitons containing a bound metal sensitizer
compared to their unbound analogues. This is a significant step
forward in the development of precipitons as energy-driven
metal scavengers.
(Z)-[5-(4′-(2-Biphenyl-4-yl-vinyl)-biphenyl-4-ylmethoxymethyl]-
[2,2′]bipyridine]-bis(2,2′-bipyridine)ruthenium(II) Bis(hexafluoro-
phosphate) (2Z). A solution of cis-dichlorobis(2,2′-bipyridine)ruthenium-
(II) dihydrate (90.8 mg, 0.188 mmol) and silver hexafluoroantimonate(V)
(132 mg, 0.376 mmol) in acetone (12.5 mL) under N2 was heated at
reflux for 48 h, followed by filtration of AgCl. Bipyridine 4Z (99.8
mg, 0.188 mmol) was added to the filtrate, and the mixture was heated
at reflux for 24 h. Volatile components of the reaction mixture were
removed in vacuo to afford a crude red solid that was purified by flash
chromatography (SiO2, 19:1 CH3CN:0.4 M KNO3(aq)). The desired
fractions were combined, and volatile components were reduced in
volume to 25 mL. The solution was combined with 0.25 M NH4PF6-
(aq) (10 mL), stirred at 23 °C for 20 min, and then extracted with
CHCl3 (3 × 100 mL). The organic extract was washed with H2O (2 ×
200 mL). Volatile components of the organic layer were removed in
vacuo to afford complex 2Z as a red solid (114 mg, 49%): Rf 0.16
(19:1 CH3CN:0.4 M KNO3(aq) on pretreated silica); mp 164-166 °C;
IR (KBr) 3027, 2919, 2853, 1603, 1464, 1446, 1093, 838, 761, 730,
699; UV-vis (CH3CN, 10 µM) λmax 247 nm, ꢀ 30 122 M-1 cm-1, 286
nm, ꢀ 198 400 M-1 cm-1, 341 nm, ꢀ 33 100 M-1 cm-1, 450 nm, ꢀ
10 755 M-1 cm-1; 1H NMR (300 MHz, CD3CN) δ 8.49-8.46 (m, 4H),
8.44-8.39 (m, 2H), 8.06-8.01 (app t, J ) 15, 8 Hz, 4H), 7.97 (dd, J
) 5, 2 Hz, 1H), 7.92 (dd, J ) 8, 1 Hz, 1H), 7.73-7.69 (m, 5H), 7.65-
7.60 (m, 6H), 7.58-7.55 (m, 3H), 7.47-7.45 (m, 2H), 7.42-7.34 (m,
9H), 7.33-7.27 (m, 1H), 7.23 (app d, J ) 8, 2 Hz, 2H), 6.74 (s, 2H),
4.51 (ABq, J ) 14 Hz, 2H), 4.47 (s, 2H); 13C NMR (75 MHz, CD3-
CN) δ 158.5, 158.4, 158.3, 157.4, 153.1, 153.0, 150.9,141.7, 141.2,
141.0, 140.8, 140.6, 139.3, 139.2, 138.7, 138.1, 138.0, 137.7, 131.5,
131.4, 130.9, 130.8, 130.3, 129.8, 129.7, 129.6, 129.07, 129.04, 129.01,
128.97, 128.94, 128.90, 128.90, 128.2, 128.1, 125.7, 125.6, 125.5, 125.3,
73.6, 69.6; HRMS(ES) m/z calcd for C58H46F6N6OPRu [M - PF6]+
1089.2418, found 1089.2452, 944.14 [M - 2PF6]+. Anal. Calcd: C,
56.45; H, 3.76; Ru, 8.19. Found: C, 56.55; H, 3.74; Ru, 7.22.
Experimental Section
General Synthetic Methods. The complexes Ru(bpy)2Cl2 and
Ru(bpy)3Cl2 were purchased from Strem Chemicals Inc. and used as
received. All other starting materials were purchased from Aldrich
Chemical Co. and were used as received. Preparation and purification
of the complexes 2Z and 2E were conducted in a darkened laboratory
under red light illumination.
(E)-4′-(2-Biphenyl-4-yl-vinyl)-biphenyl-4-ylmethoxy-tert-butyl-di-
methyl-silane (1E). A solution of TBS ether 1Z9g (175 mg, 368 µmol)
and diphenyl diselenide (12 mg, 37 µmol) in THF (1.5 mL) was heated
at reflux for 4 h. The solution was cooled to 23 °C, and volatile
components of the reaction mixture were removed in vacuo. The crude
solid was triturated with Et2O (4 mL), filtered, and washed with Et2O
(2 × 2 mL). The volatile components of the filtrate were removed in
vacuo, and the crude solid was triturated with hexanes (3 × 3 mL) and
filtered. Solids were collected to afford TBS ether 1E as a white
crystalline solid (93 mg, 53%): Rf 0.44 (9:1 Hex:EtOAC); mp 260
°C; IR (KBr) 2949, 2927, 2854, 1495, 1471, 1462, 1408, 1378, 1252,
1089, 1062, 973, 835, cm-1; UV-vis (CH2Cl2, 10 µM) λmax 335 nm,
1
ꢀ 59 300 M-1 cm-1; H NMR (300 MHz, CDCl3) δ 7.66-7.60 (m,
12H), 7.49-7.33 (m, 5H), 7.20 (s, 2H), 4.81 (s, 2H), 0.98 (s, 9H),
0.14 (s, 6H); 13C NMR, too insoluble to obtain; HRMS m/e calcd for
C33H36OSi 476.2535, found 476.2515.
(Z)-5-[4′-(2-Biphenyl-4-yl-vinyl)-biphenyl-4-ylmethoxymethyl]-
[2,2′]bipyridine (4Z). To a cooled solution (0 °C) of benzyl alcohol
3Z9d (500 mg, 1.38 mmol) in THF (138 mL) was added dry NaH (66.2
mg, 2.76 mmol). The solution was warmed to 23 °C and stirred for 2
h. 5-(Bromomethyl)-2,2′-bipyridine (354 mg, 1.42 mmol) was then
added, and the solution was stirred for 2 h at 23 °C and then heated at
reflux for 17.5 h. The solution was cooled to 23 °C, and volatile
components of the reaction mixture were removed in vacuo. The crude
solid was combined with H2O (300 mL), and the solution was extracted
with CH2Cl2 (3 × 300 mL). The combined organic phases were washed
with brine (900 mL), dried with MgSO4, and filtered. Volatile
components in the filtrate were removed in vacuo, and the residue was
purified by flash chromatography (SiO2, CH2Cl2 then 99:1 CH2Cl2:
MeOH) to afford 4Z as a pale green solid (537 mg, 73%): Rf 0.31
(CH2Cl2); mp 113-115 °C; IR (KBr) 3030, 3003, 2857, 1598, 1588,
1557, 1460,1395, 1353, 1088, 881, 815; UV-vis (CH3CN, 10 µM)
λmax 274 nm, ꢀ 34 200 M-1 cm-1, 290 nm, ꢀ 33 200 M-1 cm-1, 330
(E)-[5-(4′-(2-Biphenyl-4-yl-vinyl)-biphenyl-4-ylmethoxymethyl]-
[2,2′]bipyridine]-bis(2,2′-bipyridine)ruthenium(II) Bis(hexafluoro-
phosphate) (2E). A solution of cis-dichlorobis(2,2′-bipyridine)ruthenium-
(II) dihydrate (183 mg, 0.377 mmol) and silver hexafluoroantimonate(V)
(259 mg, 0.754 mmol) in acetone (8 mL) under N2 was stirred at 23
°C for 72 h, followed by filtration of AgCl. Bipyridine 4E (200 mg,
0.377 mmol) was added to the filtrate and diluted with acetone (4 mL)/
THF (12 mL). The mixture was heated at reflux for 28 h. The solvent
was removed under reduced pressure, and the crude product was
dissolved in CH3CN (10 mL). Next, 0.25 M NH4PF6(aq) (10 mL) was
added, and the mixture was stirred at 23 °C for 30 min. The solution
was then extracted into CH2Cl2 (3 × 200 mL) and washed with H2O
(200 mL). Volatile components of the organic layer were removed in
vacuo to afford a red solid. The solid was dissolved in CH3CN (125
mL) and precipitated with diethyl ether (100 mL). The resulting
precipitate was filtered to afford complex 2E as a red solid (46.5 mg,
80%): Rf 0.84 (19:1 CH3CN:0.4 M KNO3(aq) on pretreated silica);
UV-vis (CH3CN, 10 µM) λmax 245 nm, ꢀ 66 757 M-1 cm-1, 286 nm,
ꢀ 178 080 M-1 cm-1, 341 nm, ꢀ 59 300 M-1 cm-1, 450 nm, ꢀ 10 755
1
nm, ꢀ 24 800 M-1 cm-1; H NMR (300 MHz, CD3CN) δ 8.69-8.66
(m, 2H), 8.43-8.41 (app d, J ) 8 Hz, 2H), 7.95-7.90 (m, 2H), 7.72-
7.63 (m, 5H), 7.56 (app d, J ) 7 Hz, 4H), 7.46 (app d, J ) 8.5 Hz,
4H), 7.42 (app s, 1H), 7.34 (app d, J ) 8 Hz, 4H), 6.72 (s, 2H), 4.67
(s, 2H), 4.65 (s, 2H); 13C NMR (75 MHz, CD3CN) δ 156.1, 156.8,
149.3, 148.8, 140.8, 140.4, 140.0, 139.6, 137.0, 136.6, 136.5, 136.4,
130.2, 130.1, 129.6, 129.5, 129.8, 128.4, 127.4, 127.1, 127.0, 126.9,
124.0, 121.2, 121.0, 72.3, 69.6; HRMS(EI) m/z calcd for C38H30N2O
530.2358, found 530.2383.
1
(E)-5-[4′-(2-Biphenyl-4-yl-vinyl)-biphenyl-4-ylmethoxymethyl]-
[2,2′]bipyridine (4E). A solution of bipyridine 4Z (227 mg, 0.428
mmol) and erythrosine B (17.0 mg, 0.0214 mmol) in THF (4.28 mL)
was stirred and irradiated with a 250-W incandescent lamp for 21 h.
The solution was cooled to 0 °C, paper filtered, and washed with cold
THF (4 × 10 mL) to afford 4E as a yellow solid (173.4 mg, 87%):
mp 300-310 °C; IR (KBr) 3048, 3030, 2855, 1911, 1589, 1557, 1496,
1461, 1436, 1397, 1357, 1081, 970, 832, 807, 795, 763, 752, 742, 731,
M-1 cm-1; H NMR (300 MHz, CD3CN) 8.50-8.42 (m, 6H), 8.07-
7.99 (m, 4H), 7.99-7.93 (m, 2H), 7.80-7.60 (m, 16H), 7.57-7.54
(m, 1H), 7.54-7.44 (m, 2H), 7.44-7.30 (m, 9H), 7.26 (d, J ) 8 Hz,
2H), 4.58-4.50 (m, 2H), 4.50-4.44 (m, 2H); 13C NMR (151 MHz,
CD3CN) δ 157.97, 157.94, 157.90, 156.9, 152.72, 152.66, 152.63,
150.4, 141.26, 141.12, 140.82, 140.49, 140.31, 138.80, 138.79, 138.17,
137.85, 137.59, 137.17, 129.95, 129.28, 129.22, 129.04, 128.60, 128.56,
128.49,128.42, 128.25, 128.22, 128.09, 128.04, 127.70, 125.25, 125.13,
124.85, 73.09, 69.12; HRMS(ES) m/z calcd for C58H46F6N6OPRu [M
1
691; H NMR (300 MHz, CDCl3) δ 8.75-8.69 (m, 2H), 8.46 (app d,
9
J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006 255

A R T I C L E S
Ams and Wilcox
- PF6]+ 1089.2418, found 1089.2368. Anal. Calcd: C, 56.45; H, 3.76;
Ru, 8.19. Found: C, 57.64; H, 3.94; Ru, 7.23.
the sample. The light emitted was filtered by using a cutoff filter (λ e
400 nm).
Photochemical Experiments. All of the measurements were per-
formed at room temperature in degassed CH3CN or THF/CH3CN (70:
30) solutions. UV-vis absorption spectra were recorded with an Agilent
8453 spectrophotometer. Photoisomerization experiments were moni-
tored by recording the UV-vis absorption of 2Z/E and 1Z/E at 341
nm (exclusively) with a PerkinElmer Lambda 9 UV-vis spectrometer.
All experiments were performed in the dark or under red light due to
the high sensitivity of the compounds to normal ambient lighting.
Uncorrected luminescence spectra were recorded with a Cary Eclipse
fluorescence spectrometer equipped with Varian software. The excita-
tion light used for all photoisomerization experiments was produced
by a 25-W Bausch & Lomb microscope lamp positioned 5 cm from
Acknowledgment. Financial support from the National
Institute of General Medical Sciences is gratefully acknowl-
edged. The authors would also like to recognize the help of
Prof. Stephane Petoud, Prof. Dave Waldeck, and Ms. Demetra
Chengelis in acquiring the absorption spectra.
Supporting Information Available: Experimental details, 1H
and 13C NMR spectra, and complete characterization data for
all compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA0561340
9
256 J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006