Binuclear, ruthenium - manganese complexes as simple artificial models, for photosystem II in green plants

Article abstract of DOI:10.1021/ja962511k

As part of a project aimed at developing models for photosystem II (PSII) in green plants, we have prepared a series of model compounds (7, 8, and 13). In these compounds, a photosensitizer, ruthenium(II) tris(bipyridyl) complex (to mimic the function of P₆₈₀ in PSII), was covalently linked to a manganese(II) ion through different bridging ligands. The structures of the compounds were characterized by electron paramagnetic resonance measurements and electrospray ionization mass spectrometry. The interaction between the ruthenium and manganese moleties within the complex was probed by steady- state and time-resolved emission measurements. When the binuclear complexes are exposed to flash photolysis in the presence of an electron acceptor such as methylviologen (MV²⁺), it could be shown that after the initial electron transfer from the excited state of Ru(II) in compound 7, forming Ru(III) and MV^+. , an intramolecular electron transfer from coordinated Mn(II) to the photogenerated Ru(III) occurred with a first-order rate constant of 1.8 x 10⁵ s^-1, regenerating Ru(II). This is believed to be the first supramolecular system where a manganese complex has been used as an electron donor to a photo-oxidized photosensitizer. Possible extensions to develop the manganese donor, and thus to approach the function of reaction center in PSII, are indicated.

Full text of DOI:10.1021/ja962511k

6996
J. Am. Chem. Soc. 1997, 119, 6996-7004
Binuclear Ruthenium-Manganese Complexes as Simple
Artificial Models for Photosystem II in Green Plants
Licheng Sun,† Helena Berglund,‡ Roman Davydov,§ Thomas Norrby,†
Leif Hammarstro1m,‡ Peter Korall,† Anna Bo1rje,† Christian Philouze,† Katja Berg,†
Anh Tran,† Michael Andersson,‡ Gunnar Stenhagen,| Jerker Ma˚rtensson,|
Mats Almgren,*,‡ Stenbjo1rn Styring,*,§ and Bjo1rn A° kermark*,†,
Contribution from the Department of Chemistry, Organic Chemistry, Royal Institute of
Technology, S-100 44 Stockholm, Sweden, Department of Physical Chemistry,
Uppsala UniVersity, P.O. Box 532, S-751 21 Uppsala, Sweden, Department of Biochemistry,
Center for Chemistry and Chemical Engineering, UniVersity of Lund, P.O. Box 124,
S-221 00 Lund, Sweden, and Department of Organic Chemistry, Chalmers UniVersity of
Technology, S-412 96 Go¨teborg, Sweden
ReceiVed July 22, 1996X
Abstract: As part of a project aimed at developing models for photosystem II (PSII) in green plants, we have prepared
a series of model compounds (7, 8, and 13). In these compounds, a photosensitizer, ruthenium(II) tris(bipyridyl)
complex (to mimic the function of P680 in PSII), was covalently linked to a manganese(II) ion through different
bridging ligands. The structures of the compounds were characterized by electron paramagnetic resonance
measurements and electrospray ionization mass spectrometry. The interaction between the ruthenium and manganese
moieties within the complex was probed by steady-state and time-resolved emission measurements. When the binuclear
complexes are exposed to flash photolysis in the presence of an electron acceptor such as methylviologen (MV2+),
it could be shown that after the initial electron transfer from the excited state of Ru(II) in compound 7, forming
Ru(III) and MV+•, an intramolecular electron transfer from coordinated Mn(II) to the photogenerated Ru(III) occurred
with a first-order rate constant of 1.8 × 105 s-1, regenerating Ru(II). This is believed to be the first supramolecular
system where a manganese complex has been used as an electron donor to a photo-oxidized photosensitizer. Possible
extensions to develop the manganese donor, and thus to approach the function of reaction center in PSII, are indicated.
Introduction
electrons to the photo-oxidized form, P680+. The ET is mediated
by an tyrosyl residue, tyrosinez,4 located ∼5 Å away from the
The conversion of solar energy into fuel and electricity
emerges as an important part of future sustainable energy
systems. An attractive way to develop efficient systems for
this conversion is to mimic natural photosynthesis in green
plants. In this process, light energy is transformed into “fuel”
by reduction of carbon dioxide. Simultaneously, water is
oxidized to molecular oxygen.1 This complex process starts at
the reaction center of photosystem II (PSII) in green plants. The
central part of this reaction center consists of a heterodimer of
two proteins denoted D1 and D2 which bind most of the redox
components including the primary photoelectron donor, a
chlorophyll dimer P680. This dimer is surrounded by a large
number (ca. 30) of protein subunits, including some chlorophyll
binding proteins that absorb light.2 After absorption of one
quantum of light, P680 is excited, and a very rapid “down hill”
electron transfer (ET) chain starts, involving a primary electron
acceptor pheophytin and two quinones, QA and QB. This is
the first step of the conversion of light into chemical energy.3
In green plants, a tetramanganese cluster serves as an electron
donor to regenerate the photosensitizer P680 by transfer of
5
manganese cluster and 10-15 Å from P680
.
In the unit of the
oxygen evolving center (OEC), four electrons are sequentially
transferred to P680+ and then all four electrons are recovered in
a one-step, four-electron oxidation of two water molecules to
molecular oxygen.
Since the crystal structure of the PSII reaction center has not
been determined and the knowledge of the structure and function
of the OEC is still fairly limited,6 model systems 7-9 have been
extensively studied. Such systems have in general been mainly
structural models based on X-ray and EPR. Fewer examples
(3) Nobel reviews: (a) Huber, A. Angew. Chem. 1989, 101, 849; Angew.
Chem., Int. Ed. Engl. 1989, 28, 848. (b) Deisenhofer, J.; Michel, H. Angew.
Chem. 1989, 101, 872; Angew. Chem., Int. Ed. Engl. 1989, 28, 829; EMBO
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Am. Chem. Soc. 1996, 118, 4672-4679. (b) Tommos, C.; Tang, X.-S.;
Warncke, K.; Hoganson, C. W.; Styring, S.; McCracken, J.; Diner, B. A.;
Babcock, G. T. J. Am. Chem. Soc. 1995, 117, 10325-10335. (c) Gilchrist,
M. L. J.; et al. Proc. Natl. Acad. Sci. U.S.A. 1995, 12, 9545-9547.
(5) Kodera, Y.; Hara, H.; Astashkin, A. V.; Kawamori, A.; Ono, T.
Biochim. Biophys. Acta 1995, 1232, 43-51.
(6) (a) George, G. N.; Prince, R. C.; Cramer, S. P. Science 1989, 243,
789-791. (b) Dau, H.; Andrews, J. C.; Roelofs, T. A.; Latimer, M. J.; Liang,
W.; Yachandra, V. K.; Sauer, K.; Klein, M. P. Biochemistry 1995, 34, 5274-
5287. (c) Riggs-Gelasco, P. J.; Mei, R.; Ghanotakis, D. F.; Yocum, C. F.;
Penner-Hahn, J. E. J. Am. Chem. Soc. 1996, 118, 2387-2399. (d) Riggs-
Gelasco, P. J.; Mei, R.; Ghanotakis, D. F.; Yocum, C. F.; Penner-Hahn, J.
E. J. Am. Chem. Soc. 1996, 118, 2400-2410.
† Royal Institute of Technology.
‡ Uppsala University.
§ Lund University.
| Chalmers University of Technology.
E-mail: bear@orgchem.kth.se.
X Abstract published in AdVance ACS Abstracts, July 1, 1997.
(1) (a) Barber, J.; Andersson, B. Nature 1994, 370, 31-34. (b)
Andersson, B.; Styring, S. In Current Topics in Bioenergetics; Lee, C. P.,
Ed; Academic Press: San Diego, 1991; Vol. 16, pp 1-81
(2) (a) Svensson, B.; Etchebest, C.; Tuffery, P.; van Kan, P.; Smith, J.;
Styring, S. Biochemistry 1996, 35, 14486-14502. (b) Irrgang, K.-D.; Shi,
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(b) Naruta, Y.; Sasayama, M.-A.; Sasaki, T. Angew. Chem., Int. Ed. Engl.
1994, 33, 1839-1841.
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S0002-7863(96)02511-5 CCC: $14.00 © 1997 American Chemical Society

Binuclear Ruthenium-Manganese Complexes
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 6997
Brucker-400 MHz or on Bruker-500 MHz spectrometers. X-Band EPR
spectra were recorded on a Brucker ESP380 spectrometer equipped
with an Oxford Instruments temperature controller. Infrared spectra
were obtained on a Perkin Elmer FT-IR spectrometer 1725X or 1760X.
Time-resolved emission measurements were conducted on single-
photon-counting equipment employing a mode-locked Nd:YAG laser
to pump a DCM dye laser. The output from the dye laser was frequency
doubled to 327 nm and used to excite the samples and the instrumental
response function (fwhm) was 200 ps. The emission was observed
around 610 nm using an interference filter with 10 nm band width. All
solutions used for photophysical measurements were deoxygenated by
purging with nitrogen for 15 min before the measurements were taken
and then kept under an atmosphere of nitrogen. The solvents used,
acetonitrile (AN) and dimethyl sulfoxide (DMSO), were of spectro-
scopic grade. In the flash photolysis experiments, an excimer laser
pumped a dye laser to produce excitation pulses of 15 ns width at λex
) 458 nm. Transient absorption measurements were made at 452 nm
(Ru(II)) and 600 nm (methylviologen radical (MV+•)), respectively.
All solutions were deoxygenated. The electrospray ionization mass
spectrometry (ESI-MS) experiments were performed on a ZacSpec mass
spectrometer (VG Analytical, Fisons Instrument). Electrospray condi-
tions were the following: Needle potential, 3 kV; acceleration voltage,
4 kV; bath and nebulizing gas, nitrogen. Liquid flow was 50 µL/min
using a syringe pump (Phoenix 20, Carlo Erba, Fisons instrument).
Solvent compositions were the following: CH3CN/H2O 50:50 or
CH3CN/MeOH 90:10. Accurate mass measurements were obtained by
the use of polyethylene glycol (PEG) as an internal standard.
exist where the OEC models have undergone some electro-
chemical or light-induced process.10 For the primary charge
separation, on the other hand, several functional models exist,
with electron donor (D) and acceptor (A) assembled with a
photosensitizer (S).11 In some cases, detailed features of the
bacterial reaction center have been mimicked in artificial
D-S-A systems,12 but most studies have only shown the
general principle of photoinduced charge separation. Both
porphyrins and ruthenium polypyridyl complexes have been
widely used as photosensitizers, and their photophysical and
photochemical properties have been extensively studied.13 As
electron acceptors, quinones and viologens have often been
used,14,15 and various donors, including sacrificial donors such
as EDTA, have been applied to study the photoinduced ET
reaction.16 However, we are not aware of any studies of using
manganese ions as electron donors to mimic the photoinduced
+
ET process from OEC to P680 in the reaction center of PSII.
Since this appears to be a crucial step in attempts to construct
a good model for PSII, we have initiated a project to mimic
+
photoinduced ETs both from P680 to QA and from OEC to P680
.
We have synthesized some model systems, which contain a
ruthenium tris(bipyridyl) type complex as photosensitizer, linked
to a coordinated manganese ion. Here we report on the
preparation and the photophysical studies of three such systems
(7, 8, and 13). It was found that in the presence of an external
electron acceptor such as methylviologen (MV2+) excitation of
system 7 with visible light led to an intermolecular ET from
the excited state of the Ru(II) moiety to MV2+, with the
formation of MV+• and Ru(III). The latter is then reduced back
to Ru(II) by an intramolecular ET from the manganese part of
the complex. This result shows that an electron transfer from
coordinated Mn(II) to a photo-oxidized photosensitizer is
possible, opening the way to mimic an individual step of ET
Materials. 4,4′-Dimethyl-2,2′-bipyridine, 2,2′-bipyridine (bpy), 4,4′-
bipyridine, ruthenium trichloride, n-butyllithium in n-hexane, N-
bromosuccinimide (NBS), silver triflate, triethylamine, ammonium
hexaflorophosphate, potassium fluoride, and 1,2-dibromoethane were
purchased from Aldrich and used as received. Silica gel 60 (230-400
mesh, Merck, Darmstadt, Germany) and aluminum oxide (Aldrich) were
used for column chromatography. cis-Dichloro-bis(bipyridine)ruthe-
nium (cis-Ru(bpy)
2Cl2‚2H2O) was prepared by the method reported by
Meyer et al.17 Diisopropylamine was distilled from CaH2, and
tetrahydrofuran was distilled from sodium benzophenone ketyl radical
under nitrogen immediately prior to use. All other solvents were dried
by standard methods. The precursor complex Mn(II)(bpy)Cl2‚2H2O
(1) was prepared by refluxing manganese(II) chloride tetrahydrate
solution in methanol together with a molar equivalent of bpy for 3 h.
After the solution was cooled to room temperature, a light yellow
crystalline solid was obtained by filtration, washed with cold methanol,
and dried for 4 h in vacuum at 40 °C. Similarly, the complex
Mn(II)(bpy)2Cl2 (2) was obtained as yellow crystalline solid using 2
equiv of bpy. Methylviologen dihexafluorophosphate was prepared
by refluxing 4,4′-bipyridine and methyl iodide in acetonitrile solution
for 10 h; the orange solid obtained was filtered, redissolved in water,
and then precipitated by addition of a concentrated NH4PF6 solution.
White needle crystals were obtained by recrystallization twice from
ethanol/water mixture (90:10, v/v).
+
from OEC to P680 in PSII.
Experimental Section
General Methods. The electronic absorption spectra and steady-
state emission spectra were recorded on a Varian Cary 5E UV-vis-
near IR spectrophotometer and Perkin Elmer LS-5 luminescence
spectrometer, respectively. 1H NMR spectra were measured either on
(9) (a) Frapart, Y.-M.; Boussac, A.; Albach, R.; Anxolabehere-Mallart,
E.; Delroisse, M.; Verlhac, J.-B.; Blondin, G.; Girerd, J.-J.; Guilhem, J.;
Cesario, M.; Rutherford, A. W.; Lexa, D. J. Am. Chem. Soc. 1996, 118,
2669-2678. (b) Berry, K. J.; Moubaraki, B.; Murray, K. S.; Nicols, P. J.;
Schulz, L. D.; West, B. O. Inorg. Chem. 1995, 34, 4123-4133. (c) Kalsbeck,
W. A.; Thorp, H. H. J. Electroanal. Chem. 1991, 314, 335-343. (d)
Gamelin, D. R.; Kirk, M. L.; Stemmler, T. L.; Pal, S.; Armstrong, W. H.;
Penner-Hahn, J. E.; Solomon, E. I. J. Am. Chem. Soc. 1994, 116, 2392-
2399.
(10) Ruettinger, W.; Dismukes, G. C. Chem. ReV. 1997, 97, 1-24.
(11) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461.
(12) (a) Hasharoni, K.; Levanon, H.; Greenfield, S. R.; Gosztola, D. J.;
Svec, W. A.; Wasielewski, M. R. J. Am. Chem. Soc. 1995, 117, 8055. (b)
Osuka, A.; Nakajima, S.; Okada, T.; Taniguchi, S.; Nozaki, K.; Ohno, T.;
Yamazaki, I.; Nishimura, Y.; Mataga, N. Angew. Chem., Int. Ed. Engl. 1996,
35, 92.
1,2-Bis[4-(4′-methyl-2,2′-bipyridyl)]ethane (Mebpy-Mebpy, 5).
This compound was prepared by a modified literature procedure.18
Lithium diisopropyl amide (LDA) was prepared by dropwise addition
of n-butyllithium solution (7.00 mL, 1.6 M solution in n-hexane, 11
mmol) to freshly distilled diisopropylamine (1.42 mL, 10.86 mmol)
solution in freshly distilled tetrahydrofuran (THF) (25 mL) by a syringe
at -20 °C under magnetic stirring and Ar atmosphere. After the
solution was cooled to -78 °C by dry ice/acetone, a 4,4′-dimethylbi-
pyridine (2.00 g, 10.86 mmol) solution in freshly distilled THF (35
mL) (the solubility of 4,4′-dimethylbipyridine in THF was relatively
low) was added dropwise to the solution which became grey im-
mediately. After another 1.5 h of stirring, 1,2-dibromoethane (2.00
mL, 22 mmol) was added rapidly to the solution; the temperature was
then allowed to rise to room temperature, and water (5 mL) was added
(13) (a) Kurreck, H.; Hubert, M. Angew. Chem., Int. Ed. Engl. 1995,
34, 849-866. (b) Balzani, V., et al. Chem. ReV. 1994, 94, 993-1019. (c)
Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin
Complexes; Academic: London, 1992.
(14) (a) Watkinson, M.; Whiting, A.; McAuliffe, C. A. J. Chem. Soc.,
Chem. Commun. 1994, 2141-2142. (b) Ashmawy, F. M.; McAuliffe, C.
A.; Parish, R. V.; Tames, J. J. Chem. Soc., Dalton Trans. 1985, 1391-
1397. (c) Aurangzeb, N.; Hulme, C. E.; McAuliffe, C. A.; Pritchard, R. G.;
Watkinson, M.; Bermejo, M.; Garcia-Deibe, A.; Rey, M.; Sanmartin, J.;
Sousa, A. J. Chem. Soc., Chem. Commun. 1994, 1153-1155.
(15) (a) Xu, X.; Shreder, K.; Iverson, B. L.; Bard, A. J. J. Am. Chem.
Soc. 1996, 118, 3656-3660. (b) Kelly, L. A.; Rodgers, M. A. J. J. Phys.
Chem. 1995, 99, 13132-13140.
(17) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17,
3334.
(18) (a) Elliott, C. M.; Freitag, R. A.; Blaney, D. D. J. Am. Chem. Soc.
1985, 107, 4647-4655. (b) Wallendael, S. V.; Rillema, D. P. Coord. Chem.
ReV. 1991, 111, 297-318. (c) Policar, C.; Artaud, I.; Mansuy, D. Inorg.
Chem. 1996, 35, 210-216. (d) Elliott, C. M.; Derr, D. L.; Ferrere, S.;
Newton, M. D.; Liu, Y.-P. J. Am. Chem. Soc. 1996, 118, 5221-5228.
(16) (a) Amouyal, E. Sol. Energy Mater. Sol. Cells 1995, 38, 249-276.
(b)Willner, I.; Willner, B. Photoinduced Electron Transfer III. In Topics in
Current Chemistry; Springer-Verlag: Berlin, 1991; Vol. 159, p 153.

6998 J. Am. Chem. Soc., Vol. 119, No. 30, 1997
Sun et al.
to quench the reaction. The resulting yellow cloudy solution was
extracted with ether (3 × 50 mL), and the organic phases were
combined and dried over anhydrous sodium sulfate. The crude product
was obtained by evaporating the solvent in vacuum. After purification
by medium pressure liquid chromatography (MPLC)19 on silica gel
(230-400 mesh), (eluents first CH2Cl2/acetone 90:10 to recover the
starting material 4,4′-dimethylbipyridine, then a gradient of CH2Cl2 and
CH2Cl2/MeOH 97:3), the desired fractions (checked by TLC on silica
8.48 (d, J ) 4.8 Hz, 1H, Mebpy-Mebpy-H), 8.54 (d, J ) 4.9 Hz, 1H,
Mebpy-Mebpy-H), 8.79 (s, 1H, Mebpy-Mebpy-H), 8.85 (m, 4H, bpy-
H), 8.88 (s, 1H, Mebpy-Mebpy-H). ESI-MS: m/z 925.205 (M - PF6-),
C44H38N8RuPF6 requires 925.192. Anal. Calcd for C44H38N8RuP2F12‚
H2O‚0.25EtOAc: C, 48.70; H, 3.81; N, 10.10. Found: C, 49.00; H,
3.90; N, 9.97.
[Ru(bpy)2Mebpy-MebpyMnCl2‚H2O]Cl2 (7). To a solution of 6
(50 mg, 0.058 mmol) in methanol (10 mL) was added manganese(II)
chloride tetrahydrate (12 mg, 0.060 mmol) under stirring at room
temperature. The resulting red solution was heated at reflux for 2 h
and then evaporated to dryness under vacuum. A red solid was
obtained, which was washed with a cold mixture of EtOAc/methanol
80:20 and dried under vacuum for 5 h at 40 °C to give 7 in 90% yield.
1H NMR (400 MHz, DMSO-d6) signals broadened due to paramagnetic
Mn(II). ESI-MS: m/z 940.085 (M - Cl-); C44H38N8Cl3RuMn requires
940.071. For the X-band EPR spectrum of 7 in powder, see: Figure
2a. Anal. Calcd for C44H42N8O2Cl4RuMn‚2H2O: C, 50.39; H, 4.42;
N, 10.68; Found: C, 49.63; H, 4.34; N, 10.44.
[Ru(bpy)2Mebpy-MebpyMn(bpy)Cl2]Cl2 (8). To a solution of 6
(85 mg, 0.10 mmol) in methanol (10 mL) was added 1 (31 mg, 0.11
mmol). After the solution was heated at reflux for 2 h, it was
concentrated to about 0.5 mL by evaporating the solvent at reduced
pressure on a rotary evaporator. Addition of ethyl acetate (15 mL)
gave a red precipitate which was washed with ethyl acetate and dried
in vacuum for 4 h at 40 °C to give 8 in 92% yield. 1H NMR (400
MHz, DMSO-d6) signals broadened due to paramagnetic Mn(II). ESI-
MS: m/z 940.046 (M - bpy-Cl-), C44H38N8Cl3RuMn requires 940.071.
X-Band EPR spectrum of 8 in powder (see Figure 2b). Anal. Calcd
for C54H46N10Cl4RuMn‚4H2O: C, 53.83; H, 4.52; N, 11.62; Cl, 11.77;
Mn, 4.56. Found: C, 53.32; H, 4.67; N, 11.34; Cl, 11.49; Mn, 4.69.
4-Bromomethyl-4′-methylbipyridine (10). This compound was
made according to a literature procedure.21 A pale yellow solid was
obtained as raw product, and it was washed with ethyl acetate and
n-hexane and then purified by column chromatography (MPLC) on
silica gel (eluents in gradient, CH2Cl2 and CH2Cl2/acetone 98:2) to give
essentially pure 10 (450 mg, 18%). 1H NMR (400 MHz, CDCl3) δ:
2.43 (s, 3H, CH3), 4.49 (s, 2H, CH2Br), 7.15 (d, J ) 4.0 Hz, 1H, Py-
H), 7.33 (dd, J ) 3.3 Hz, J′ ) 1.3 Hz, 1H, Py′-H), 8.24 (s, 1H, Py-H),
8.42 (s, 1H, Py′-H), 8.54 (d, J ) 5.1 Hz, 1H, Py-H), 8.65 (d, J ) 5.1
Hz, 1H, Py′-H).
Ru(bpy)2(4-BrCH2-4′-CH3-bpy)(PF6)2 (11). A mixture of cis-
Ru(bpy)2Cl2‚2H2O (890 mg, 1.73 mmol) and silver triflate (889 mg,
3.46 mmol) in anhydrous acetone (65 mL) was stirred for 8 h at room
temperature under Ar atmosphere. The solution turned gradually red,
and a white precipitate of AgCl was formed. This was removed by
filtration, and 10 (447 mg, 1.71 mmol) was added to the filtrate. The
solution was stirred for another 2 h at room temperature and then
evaporated to dryness at 40 °C in vacuum. The red viscous residue
obtained in this way was redissolved in methanol (10 mL) and mixed
with water (20 mL), and concentrated aqueous NH4PF6 solution was
added, to give a red precipitate which was isolated and washed with
water and diethyl ether and then purified by column chromatography
(MPLC) on neutral Al2O3 (eluent toluene/CH3CN 1:1). The second
orange band was collected to give the desired product 11 in 52% yield.
1H NMR (400 MHz, DMSO-d6) δ: 2.49 (s, 3H, CH3), 4.76 (d, J )
15.0 Hz, 1H, CHaBr), 4.80 (d, J ) 15.0 Hz, 1H, CHbBr), 7.38 (d, J )
6.4 Hz, 1H, 4-Me-4′-BrMe-bpy(dmb)-H), 7.50-7.57 (m, 6H, bpy-4H
+ dmb-2H), 7.69-7.74 (m, 5H, bpy-4H + dmb-1H), 8.16 (d, J ) 7.9
Hz, 4H, bpy-4H), 8.73 (unresolved d, 1H, dmb-H), 8.82 (d, J ) 8.6
Hz, 4H, bpy-4H), 8.91 (d, J ) 1.8 Hz, 1H, dmb-1H). ESI-MS: m/z
821.025 (M - PF6-), C32H27N6BrRuPF6 requires 821.017.
1
gel, eluent CH2Cl2/MeOH 97:3, Rf ≈ 0.40 and H NMR in CDCl3)
were collected and further purified by recrystallization from ethyl acetate
to give 5 (755 mg, 38%) as white crystals. 1H NMR (CDCl3) δ: 2.43
(s, 6H, -CH3), 3.08 (s, 4H, -CH2CH2-), 7.08-7.15 (m, 4H, 5,5′-bpy-
H), 8.23 (s, 2H, 3-bpy-H), 8.31 (s, 2H, 3′-bpy-H), 8.50-8.58 (m, 4H,
6,6′-bpy-H).
N-Methyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine (Mebi-
spicen, 9). A solution of N-methylethane-1,2-diamine (0.88 mL, 10
mmol) and 2 equiv of triethylamine in acetonitrile (50 mL, 99.9%)
was mixed with KF/Celite (8 g, prepared by adding Celite (4 g) to a
solution of KF (4 g) in 8 mL of water and then removing the water by
heating at 70 °C in an rotary evaporator, followed by drying in an
oven overnight at 120 °C immediately before use). A solution of
2-(chloromethyl)pyridine (3.28 g, 20 mmol) in acetonitrile (20 mL)
(prepared from the hydrochloride by treatment with an equivalent
amount of triethylamine and removal of triethylamine hydrochloride
by filtration) was added at room temperature over a period of 30 min.
The reaction mixture was then heated at reflux with magnetic stirring
for 2 h under Ar atmosphere. After the solution was cooled to room
temperature, additional triethylamine hydrochloride crystallized, which
was filtered. The filtrate was concentrated by evaporating the solvent
in vacuum and then purified by chromatography on neutral aluminum
oxide (eluent in gradient, CH2Cl2 and CH2Cl2/MeOH 98:2), to yield
compound 9 (768 mg, 30%) as a slightly yellow oil. 1H NMR (400
MHz, CDCl3) δ: 2.28 (s, 3H, N-CH3), 2.67 (t, J ) 5.4 Hz, 2H,
CH3NCH2CH2N-), 2.80 (t, J ) 5.4 Hz, 2H, CH3NCH2CH2N-), 3.68 (s,
2H, PyCH2N), 3.91 (s, 2H, Py′CH2N), 7.10-7.19 (m, 2H, Py-H), 7.32
(d, J ) 6.4 Hz, 1H, Py-H), 7.45 (d, J ) 6.4 Hz, 1H, Py′-H), 9.60-
7.69 (m, 2H, Py-H), 8.48-8.57 (m, 2H, Py-H).
N,N′-Dimethyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine
(Me2bispicen). This compound and its Mn(II) complex, Mn(II)Me2-
bispicenCl2 (3), were prepared according to the literature procedure.20
Ru(bpy)2(Mebpy-Mebpy)Cl2 (6). To a solution of Mebpy-Mebpy
(300 mg, 0.82 mmol) in methanol (40 mL) was added cis-Ru(bpy)2-
Cl2‚2H2O (141 mg, 0.27 mmol) under magnetic stirring and Ar
atmosphere. The resulting brown solution was heated, and the solution
became clear orange after refluxing for 2 h. The solvent was evaporated
under a vacuum, and a red solid residue was obtained. The residue
was redissolved in water (20 mL), and the unreacted Mebpy-Mebpy
was removed by filtration. After the residue was washed with water
(5 mL), the filtrates were combined and evaporated to dryness in
vacuum at 70 °C to give a red viscous oil. Purification by MPLC on
neutral aluminium oxide (eluents in gradient, CH2Cl2 and CH2Cl2/
MeOH 94:6) gave a nice separation of three bands, the first violet one
was the starting material cis-Ru(bpy)2Cl2‚2H2O and the third deep
orange one was the byproduct binuclear ruthenium complex. Com-
pound 6 (188 mg, 82%) was obtained from the second orange band as
a red solid after evaporation of the solvent. Dissolution of 6 in a
minimum amount of water followed by addition of a concentrated
NH4PF6 solution gave a red precipitate, which was recrystallized from
acetone/EtOAc 1:1 to give Ru(bpy)2(Mebpy-Mebpy)(PF6)2 (6a) in 95%
yield. 1H NMR (400 MHz, DMSO-d6) δ: 2.41 (s, 3H, -CH3(on free
ligand)), 2.52 (s, 3H, -CH3(on Ru-bound ligand)), 3.14 (unresolved t,
2H, -CH2-(near free ligand)), 3.15 (unresolved t, 2H, -CH2-(near Ru)),
7.29 (m, 2H, Mebpy-Mebpy-H), 7.37 (d, J ) 5.7 Hz, 1H, Mebpy-
Mebpy-H), 7.44 (dd, J ) 5.8 Hz, J′ ) 0.7 Hz, 1H, Mebpy-Mebpy-H),
7.54-7.46 (m, 5H, Mebpy-Mebpy-H + bpy-H), 7.57 (d, J ) 5.8 Hz,
1H, Mebpy-Mebpy-H), 7.73-7.65 (m, 4H, bpy-H), 8.15 (m, 4H, bpy-
H), 8.22 (s, 1H, Mebpy-Mebpy-H), 8.32 (s, 1H, Mebpy-Mebpy-H),
Ru(bpy)2(4-CH3-4′-(N′-CH3-N,N′-bis(2-pyridylmethyl)-1,2-ethanedi-
amine)-CH2-bpy)(PF6)2 (12). In a dry flask, 9 (80 mg, 0.31 mmol)
and triethylamine (0.1 mL, 0.75 mmol) were dissolved in acetonitrile
(2.0 mL, 99.9%), KF/Celite (200 mg) was added to the solution. Under
stirring, a solution of 11 (200 mg, 0.20 mmol) in acetonitrile (2.0 mL)
was added over a period of 5 min, and then the reaction mixture was
heated at reflux for 2 h, cooled, and filtered. The filtrate was evaporated
to dryness under vacuum. Repetitive column chromatography (MPLC)
(19) Ba¨ckstro¨m, P.; Stridh, K.; Li, L.; Norin, T. Acta. Chem. Scand. 1987,
B41, 442-447.
(20) Glerup, J.; Goodson, P. A.; Hazell, A.; Hazell, R.; Hodgson, D. J.;
McKenzie, C. J.; Michelson, K.; Rychlewska, U.; Toftlund, H. Inorg. Chem.
1994, 33, 4105-4111.
(21) Gould, S.; Strouse, G. F.; Meyer, T. J.; Sullivan, B. P. Inorg. Chem.
1991, 30, 2942-2949.

Binuclear Ruthenium-Manganese Complexes
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 6999
on Al2O3 (eluents in gradient, CH2Cl2 and CH2Cl2/MeOH 98:2) gave
12 in 70% yield. 1H NMR (400 MHz, DMSO-d6) δ: 2.03 (s, 3H,
-NCH3), 2.42 (s, 3H, dmb-CH3), 2.62-2.70 (m, 4H, -CH2CH2-), 3.52
(s, 2H, NCH2-dmb), 3.79 (s, 2H, -NCH2-py), 3.91 (s, 2H, NCH2-py),
7.16-7.22 (m, 2H, py-H), 7.31 (d, J ) 7.2 Hz, 1H, py-H), 7.36 (d, J
) 6.2 Hz, 1H, dmb-H), 7.42 (d, J ) 7.6 Hz, 1H, py-H), 7.48-7.57
(m, 8H, bpy-4H, dmb-2H, py-2H), 7.65-7.74 (m, 5H, bpy-4H, dmb-
H), 8.15 (d, J ) 6.2 Hz, 4H, bpy-4H), 8.38 (d, J ) 3.6 Hz, 1H, py-H),
8.44 (d, J ) 4.4 Hz, 1H, py-H), 8.64 (s, 1H, dmb-H), 8.81 (d, J ) 7.9
Hz, 4H, bpy-4H). ESI-MS: m/z 997.266 (M - PF6-), C45H46N10RuPF6
requires 997.261.
bpyRu(bpy)2. The binuclear complex can be removed from the
product mixture by column chromatography. Complex 6 is
soluble in water, and the counterion chloride can be changed
to hexafluorophosphate by adding ammonium hexafluorophos-
phate to an aqueous solution of 6, leading to precipitation of
6a. To prepare ruthenium-manganese binuclear complexes,
manganese dichloride MnCl2‚4H2O or the manganese dichloride
bpy complex 1 were reacted with 6 in methanol (Scheme 1).
Ru(bpy)2(4-CH3-4′-(N′-CH3-N,N′-bis(2-pyridylmethyl)-1,2-ethanedi-
amine)-CH2-bpy)Mn(PF6)2Cl2 (13). To a stirred solution of 12 (40
mg, 0.035 mmol) in methanol (2 mL) MnCl2‚4H2O (9.7 mg, 0.048
mmol) was added at room temperature, giving immediately a red
precipitate. The reaction mixture was stirred for another 20 min, and
the red precipitate was filtered and washed with a small amount of
cold methanol, followed by diethyl ether (20 mL), and dried in air to
give 13 as a red solid in 86% yield. 1H NMR (400 MHz, DMSO-d6)
δ: similar to 12, but all peaks were broadened because of the
paramagnetic Mn(II). Anal. Calcd for C47H46N10Cl2P2F12RuMn‚
2H2O: C, 43.30; H, 3.87; N, 10.74; Found: C, 43.46; H, 3.88; N,
10.43. ESI-MS: m/z 1124.136 (M - PF6-), C47H46N10Cl2PF6RuMn
requires 1124.135. For X-band EPR spectra of 13 in powder and in
acetonitrile, see: Figures 3a and 4a, respectively.
Results and Discussion
Synthesis. In order to link the ruthenium tris(bipyridyl)
complex covalently to manganese complexes (Scheme 1), we
have used two different bridging ligands, 1,2-bis[4-(4′-methyl-
2,2′-bipyridyl)]ethane (Mebpy-Mebpy) and 4-methyl-4′-(N′-
methyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine)methylbi-
pyridine (bpy-bispicen). Reaction of excess Mebpy-Mebpy with
cis-Ru(bpy)2Cl2 (molar ratio 4:1) in aqueous solution at 60-
90 °C gave a mononuclear ruthenium complex 6 in more than
90% yield.
The binuclear complexes 7 and 8 were formed and characterized
by elemental analysis and electrospray ionization mass spec-
trometry (ESI-MS) and confirmed by emission-quenching
1
measurements. In the H NMR spectra of both compounds 7
and 8 in DMSO-d6, all peaks are broadened because of the
presence of paramagnetic Mn(II). ESI-MS results show that
in acetonitrile/methanol (90:10) solution there is an equilibrium
between 7 and 6, which can also be monitored by emission
quenching (see text below) of these complexes in solution.
To make the binuclear complex 13, one route is to synthesize
Addition of excess Mebpy-Mebpy is very important to reduce
formation of the binuclear byproduct, Ru(bpy)2Mebpy-Me-
Scheme 1

7000 J. Am. Chem. Soc., Vol. 119, No. 30, 1997
Sun et al.
Scheme 2
first the bridging ligand bpy-bispicen and then introduce Ru(II)
and Mn(II). However, the reaction of bpy-bispicen with cis-
Ru(bpy)2Cl2 in aqueous solution afforded a complicated mixture,
due to the poor ligand selectivity for cis-Ru(bpy)2Cl2. It was
very difficult to separate the desired 12 from other byproducts.
Therefore, another synthetic route was designed (Scheme 2).
tions gave geometries with distances between the Ru and Mn
centers of 13 Å for 7 and 9 Å for 9 (Figure 1). These distances
are reasonable for intramolecular electron transfer between the
two metal centers.
EPR Characterization of Mn(II) complexes. Ruthenium(II)
tris(bipyridine) complexes are diamagnetic, while manganese(II)
complexes are EPR active, with S ) 5/2. The spectra have line
shapes and g values that depend strongly on the ligands. Free
Mn(II) in DMSO and MeCN gives a well-resolved six-line
narrow EPR signal around g ) 2.0. The EPR spectrum of 7 in
powder at low temperature (77 K) is characterized as a N2X2
ligand donor set (Figure 2a) with a broad g ≈ 2 centered
resonance without manganese hyperfine structure. A powder
spectrum of manganese 4,4′-dimethylbipyridine (dmb) dichloride
(Mn(II)(dmb)Cl2 (not shown)) is similar to that of 7. Dowsing
et al. observed a similar g ≈ 2 centered resonance for the
complex Mn(II)(picoline)2Cl2.24 They explained these observa-
tions as resulting from a polymeric structure giving extensive
intermolecular magnetic interactions. Related EPR studies in
solution were difficult due to the low solubility of the compound
7 in acetonitrile, and also because it is not stable in DMSO or
DMF or in MeOH at low concentration. The powder EPR
spectrum of complex 8 can be characterized as an N4X2 ligand
donor set and displays a broad fine structure resonance without
any resolved hyperfine patterns (Figure 2b). A similar X-band
EPR spectrum is afforded by 1 mM Mn(bpy)2Cl2 (2) in MeOH
and in DMF (not shown) at 4 K.25 Powdered Mn(o-phen)2Cl2
also displays a similar spectrum which may be simulated using
zero-field splitting parameter D ) 0.12 cm-1 and E/D ) 0.04.24
The complex 13 in powder state and in DMSO at 77 K gives
rise to the rhombic EPR spectra shown in Figure 3a,c, which
are similar to that of 8. At 77 K in acetonitrile solution, the
EPR spectrum of 13 (Figure 4a) is similar to that in solid 13
(Figure3a). It is interesting to note that in acetonitrile solution
In this route, silver triflate in dry acetone solution was used
to remove two chlorides from cis-Ru(bpy)2Cl2, forming inter-
mediate cis-Ru(bpy)2(acetone)2. Reaction of this with ligand
4-bromomethyl-4′-methylbipyridine (10) at room temperature
followed by addition of ammonium hexafluorophosphate gave
Ru(bpy)2(4-BrCH2-4′-Me-2,2′-bpy)(PF6)2 (11). Direct com-
plexation of cis-Ru(bpy)2Cl2 with the ligand 10 in aqueous
solution at 60-90 °C gave lower yield of 11, the reason
probably being self quaternization of ligand 10 with the active
bromomethyl group. The nucleophilic reaction of 11 with ligand
N-methyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine (9), in
refluxing dry acetonitrile solution in the presence of triethy-
lamine and potassium floride on Celite as bases, gave the
ruthenium complex 12. Column chromatography on aluminum
oxide provided a practical way to obtain pure 12 in about 70%
yield. The ruthenium-manganese binuclear complex Ru(bpy)2(4-
methyl-4′-(N′-methyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanedi-
amine)-bpy)MnCl2 (PF6)2 (13) was produced quantitatively as
a red precipitate by adding manganese dichloride (MnCl2‚4H2O)
to a solution of 12 in methanol at room temperature. The
structures of compounds 11 and 12 were fully characterized by
1H NMR and ESI-MS and 13 by ESI-MS and EPR spectros-
copy.
Molecular Modeling. The distance and orientation between
the electron donor and acceptor in a covalently linked system
are very important for ET.11,22 In order to estimate the
maximum Ru-Mn distance in molecules 7 and 13, we
performed molecular mechanics calculations.23 The optimiza-
(24) (a) Dowsing, R. D.; Gibson, J. F.; Hayward, P. J. J. Chem. Soc. A
1969, 187-193. (b) Dowsing, R. D.; Gibson, J. F.; Goodgame, D. M. L.;
Goodgame, M.; Hayward, P. J. Nature 1968, 219, 1037-1038.
(25) (a) Policar, C.; Artaud, I.; Mansuy, D. Inorg. Chem. 1996, 35, 210-
216. (b) Lynch, W. B.; Boorse, R. S.; Freed, J. H. J. Am. Chem. Soc. 1993,
115, 10909-10915. (c) Mabad, B.; Cassoux, P.; Tuchagues, J.-P.; Hen-
drickson, D. N. Inorg. Chem. 1986, 25, 1420-1431.
(22) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993,
26, 198. (b) Ranasinghe, M. G.; Oliver, A. M.; Rothenfluh, D. F.; Salek,
A.; Paddon-Row, M. N. Tetrahedron Lett. 1996, 37, 4797-4800.
(23) The optimizations were performed starting from the most extended
conformations using constrained metal-ligand distances. The Sybyl force
field was used in Spartan SGI Version 4.1.1 OpenGL.

Binuclear Ruthenium-Manganese Complexes
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 7001
Figure 3. X-Band EPR spectra of microcrystalline 13 (a) and 3 (b) as
well as 3 mM of 13 in DMSO (c) at 77 K. Instrumental parameters are
the same as in Figure 2.
Figure 1. Computational representations of the binuclear compound
7 (a) and 13 (b) obtained by force field calculation. The parameters
for ruthenium are based on the crystal structure geometry of complex
4.
Figure 4. X-Band EPR spectra of 13 in acetonitrile at 77 K (a) and in
a mixture of 13 with Ru(III)(bpy)3 (molar ratio 1:7) in acetonitrile at
77 K (b). Instrumental parameters are the same as in Figure 2.
control experiments were performed where chemically produced
Ru(III) in the form of Ru(III)(bpy)3 was added to the complexes
containing Mn(II). Since both Ru(III)(bpy)3 and the manga-
nese(II) in complexes 7, 8, and 13 have characteristic EPR
spectra while Ru(II) and Mn(III) species are EPR-silent,26
oxidation of Mn(II) by Ru(III) should be readily monitored by
EPR. This is observed in the case when complex 7 was mixed
with a molar equivalent amount of Ru(III)(bpy)3 in acetonitrile.
The signals from Ru(III) and Mn(II) rapidly disappeared,
showing that Ru(III) can indeed oxidize Mn(II) in complex 7
(not shown). From a measurement using the stopped-flow
technique, monitoring the disappearance of Ru(III) optically, a
second-order rate constant for the reaction of ca. 1 × 107 s-1
M-1 was obtained. In a similar experiment with compound 13,
the EPR signal from Ru(III) rapidly disappeared as expected,
but an EPR signal from a Mn(II) species with a modified EPR
pattern was still observable at room temperature. We suspect
that the result is due to oxidation of the bispicen ligand.
Addition of Ru(bpy)33+ to this solution at a molar ratio 7:1 did
not result in oxidation of Mn(II) into Mn(III), but resulted in a
dramatic modification of the EPR pattern (see Figure 4b). These
Figure 2. X-Band EPR spectra of microcrystalline 7 (a) and 8 (b) at
77 K. Instrumental parameters: modulation frequency, 100 kHz;
modulation amplitude, 10 G; microwave power, 10 mW; microwave
frequency, 9.47 GHz.
13 appears to be EPR invisible at room temperature, while under
the same condition in DMSO, 13 displays an EPR spectrum
5
which has typical Mn(II) (S ) /2) line shape.
A comparison of the powder spectra of 13 (Figure 3a) and
the related manganese complex 3 (Figure 3b) indicates that
ruthenium(II) tris(bipyridyl) complex in 13 has some effect on
the structure of Mn(II) center within the molecule. The
spectrum in Figure 3b is characteristic of a mononuclear
Mn(II)N4X2 complex and may be simulated using the parameters
0.1 cm-1 for D and e0.1 for E/D.24
3+
experiments indicate that in acetonitrile solution Ru(bpy)3
modifies the ligand of the manganese in complex 13 rather than
oxidizing Mn(II).
Photophysical and Photochemical Studies. The absorption
spectra of the compounds 6, 6a, 7, 8, 12, and 13 in acetonitrile
Chemical Oxidation of Manganese(II) by Ruthenium(III).
In the anticipated photochemical reaction of the binuclear
complexes 7, 8, and 13, the excited state of Ru(II) is expected
to deliver an electron to the added electron acceptor to yield
Ru(III). Ru(II) would then be regenerated by intramolecular
ET from the coordinated Mn(II) to the photogenerated Ru(III).
In order to determine if such a process is reasonable, some
(26) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of
Transition Ions; Dover: New York, 1986.

7002 J. Am. Chem. Soc., Vol. 119, No. 30, 1997
Sun et al.
Figure 5. Time-resolved emission spectra of compound 6a (i),
compound 7 (ii) and compound 7 + excess MnCl2‚4H2O (iii) in N2-
purged acetonitrile at room temperature: λex ) 327 nm, λem ) 610
nm, and the concentration of the complex, 5.0 × 10-5 M, MnCl2‚4H2O,
1.0 × 10-3 M.
Figure 6. Time-resolved emission spectra of compound 12 and 13 in
acetonitrile solution (a) and in DMSO solution (b), at room temperature,
λex ) 327 nm, λem ) 610 nm, and the concentration of the complex,
5.0 × 10-5 M.
Table 1. Emission Life-Time Data at Room Temperature
cmpd
solventa
τ/ns
fraction
concn (µM)
6a
7
AN
AN
τ
950
906
255
956
258
1015
259
982
240
1009
240
1037
1046
7
41
39
curve, it was found to be 6 × 105 M-1 for complex 7 in
τ1
τ2
τ1
τ2
τ1
τ2
τ1
τ2
τ1
τ2
τ
0.20
0.80
0.16
0.84
0.25
0.75
0.31
0.69
0.49
0.51
acetonitrile solution.
7
7
8
8
AN
AN
AN
AN
54
11
66
13
The behavior of 8 is similar with a double exponential
emission decay with lifetime of τ1 ) 980 ns and τ2 ) 240 ns,
showing that also 8 dissociates to form 6. As judged from the
relative intensities of these two emission components, compound
8 is more dissociated than 7 at equal concentrations. As for 7,
addition of Mn(II) increased the relative contribution of the
species with τ ) 240 ns. The differences between compounds
7 and 8 may be fortuitous, however, since there is strong
evidence from ESI-MS that the bipyridyl ligand of Mn(II) in 8
is dissociated in fluid solution. The fraction of nondissociated
8 was probably small, and the behavior of compound 8 was
therefore not further investigated. The compound 12 appears
to have a lifetime slightly longer (τ ) 1040 ns in acetonitrile)
than that of compound 6a. When Mn(II) was added, the
concentration of this species was decreased and a new species
with lifetime of 7 ns appeared, which was identified as
compound 13 (Figure 6a). In comparison with 7 and 8, 13 was
less dissociated in acetonitrile, proving that bispicen is a stronger
ligand for Mn(II) than bpy. In DMSO, the lifetime of 13 was
increased to 870 ns, the same as the mononuclear complex 12
(Figure 6b), suggesting that complete dissociation takes place
in this solvent at the low concentrations used in the photo-
physical measurements.
Addition of small amounts of DMSO to the acetonitrile
solution of 13 also leads to a relative decrease of the fast decay
component, showing that the presence of DMSO induces
dissociation of manganese from the complex 13 in acetonitrile
solution.
Electron Transfer from Manganese(II) to Photogenerated
Ruthenium(III). In the attempts to observe photoinduced
intramolecular ET from manganese(II) to photogenerated ru-
thenium(III), the compounds 6a, 7, and 13 were dissolved in
acetonitrile and methylviologen (MV2+) was added as external
electron acceptor. When the deoxygenated solution was excited
with a laser flash (15 ns) at 458 nm, an electron was transfered
from the excited state of Ru(II) to MV2+, to give Ru(III) and
MV+• at a concentration of about 2 µM each. The recovery of
Ru(II) and the disappearance of MV+• were then monitored at
12
13
AN
AN
64
67
τ1
τ2
τ
0.07
0.93
12
13
DMSO
DMSO
874
866
69
69
τ
a AN ) acetonitrile, DMSO ) dimethylsulfoxide, and τ and fraction
are given by the curve fit program.
are quite similar, all with a ruthenium metal to ligand charge
transfer (MLCT) band at ca. 452 nm and with similar extinction
coefficients. Also, the shapes and positions of the emission
spectra are similar, but the coordination of manganese(II) has a
profound influence on the emission intensity and decay rate.
The emission decay of 6a and 7 in acetonitrile are shown in
Figure 5. In this solution, compound 6a has a single exponential
decay with a lifetime (τ) of 950 ns, whereas a double exponential
decay was observed for compound 7, having one component
with τ1 ) 980 ns (20%) and another with τ2 ) 260 ns (80%).
Addition of Mn(II) lead to a relative increase of the component
with short lifetime and a decrease in the one with long lifetime
(Figure 5, iii). This suggests that the long-lived species is the
mononuclear compound 6 and the shorter-lived species the
binuclear compound 7. In the very dilute solutions used, partial
dissociation occurs and an equilibrium between 6 and 7 is
established. The fractions of the rapid and slow components
were found to vary with the concentration of complex according
to a simple dissociation equilibrium (Table 1). The equilibrium
was also studied by observing the decrease in steady-state
emission intensity of an acetonitrile solution of 6 which was
titrated with a solution of manganese(II) chloride. After 1.2
equiv of Mn(II) was added, the emission of 6 became constant.
By a determination of the stability constant from the titration

Binuclear Ruthenium-Manganese Complexes
J. Am. Chem. Soc., Vol. 119, No. 30, 1997 7003
Figure 7. Transient absorption curves of deoxygenated acetonitrile
solutions of 6a and 7 (7.0 × 10-5 M) in the presence of MV2+ (1.0 ×
10-2 M) at room temperature, laser pulse 15 ns, λex ) 458 nm. The
Ru(II) complex absorption recovery is monitored at 452 nm for 6a (a)
and 7 (b); (c) shows the decay of the MV+• to MV2+ as measured by
the decrease in absorption at 600 nm.
Figure 8. Photoinduced electron transfer pathway of compound 7 in
the presence of MV2+ in acetonitrile: (a) transfer of an electron from
excited state of Ru(II) complex to MV2+; (b) intramolecular electron
transfer from coordinated Mn(II) to photogenerated Ru(III).
452 and 600 nm,27 respectively. With the compound 6a, this
process occurred by a second-order, diffusion-controlled (rate
constant 5 × 109 s-1 M-1) recombination of Ru(III) and MV+•
(i.e., with a first half-life of about 0.1 ms at the concentrations
produced). This is essentially the same rate constant as observed
for the electron transfer between Ru(III)(bpy)3 and MV+•.28
Interestingly, with 7, a different behavior was observed. The
production of MV+• and its decay were similar to those observed
for 6a. In contrast, the recovery of Ru(II) was 1 order of
magnitude faster (t1/2 ) ca. 10 µs) (Figure 7).
This means that the photogenerated Ru(III) in the bimetallic
complex 7 must have received an electron from a source not
present in complex 6a and which is not MV+•. We propose
that this is the Mn(II) which is coordinatively bound ca. 13 Å
from the Ru center in complex 7. In this case, the rate of
recovery of Ru(II) would be independent of the concentration
of 7. This hypothesis was tested in an experiment where the
concentration of 7 was varied from 1.5 × 10-5 M to 1.0 ×
10-4 M. The results of this experiment (not shown) revealed
that more than one process occurred in parallel. The recovery
of Ru(II) was found to be concentration dependent and nonex-
ponential. However, the results could be satisfactorily described
by a mechanism (Scheme 3) involving one concentration
independent reaction path, involving the major part of the
Ru(III), and one minor path that was concentration dependent.
The concentration independent reduction of Ru(III) in the
photo-oxidized compound 7 occurs with a rate constant of ca.
1.8 × 105s-1, in competition with back electron transfer from
MV+•. We assign this process to intramolecular electron
transfer from coordinated Mn(II) to the photogenerated Ru(III).29
The concentration dependence is more difficult to explain, but
careful analysis of the data (a detailed description of these
experiments and our kinetic analysis will be published else-
where30) reveals that it stems from those complex molecules in
which the Mn(II) is dissociated (see above), and this fraction
was determined independently from the emission decay curves
(compare Figure 5). The recovery of Ru(II) in these complexes
only occurs after a rate-determining reassociation of Mn(II)
(Scheme 3) and has a bimolecular rate constant of 2.9 × 109
M-1 s-1. Since the concentration of nonbound Mn(II) was
below 10 µM, the pseudo-first-order rate constant for this path
was always lower than 3 × 104 s-1
.
The first-order process, with rate constant of about 1.8 ×
105 s-1, is thus assigned to intramolecular electron transfer from
coordinated Mn(II) to photogenerated Ru(III). The demonstra-
tion of this reaction is important as it shows that the concept of
intramolecular electron transfer from coordinated Mn to the
photo-oxidized Ru center is viable and this process is illustrated
in Figure 8.
The photoinduced ET of compound 13 could not be studied
conclusively. This was mainly due to the rapid intramolecular
quenching of the excited Ru(II) state (7 ns, Table 1), and because
of this, very high concentrations of MV2+ were needed to
intercept an electron from the excited species before deactiva-
tion. It was also confirmed by EPR experiments (see above)
that chemical oxidation of 13 by Ru(III)(bpy)3 is rapid but does
not produce Mn(III); instead it appears that the ligand is
oxidized.
Conclusions
Several conclusions pertaining to the development of an
artificial model for mimicking an individual step of electron
transfer from the oxygen-evolving complex to P680+ in PSII can
be drawn from the present study. The most important is that a
coordinated manganese ion can restore the activity of a photo-
oxidized photosensitizer by intramolecular electron transfer. This
is a crucial event in PSII and clearly not a trivial act. A second
conclusion is that the distance between the photosensitizer and
the manganese is important. If the distance is short, which
would promote efficiency of electron transfer, efficient quench-
ing of the excited state of the photosensitizer may inhibit the
primary electron transfer to an acceptor system. This is clearly
a problem with the system 13, which has a very short-lived
(27) (a) Watanabe, T.; Honda, K. J. Phys. Chem. 1982, 86, 2617. (b)
Yoshimura, A.; Hoffman, M. Z.; Sun, H. J. Photochem. Photobiol. A 1993,
70, 29.
(28) (a) Kalyanasundaram, K. Photochemistry of Polypyridine and
Porphyrin Complexes; Academic: London, 1992. (b) Kelly, L. A.; Rodgers,
M. A. J. J. Phys. Chem. 1995, 99, 13132-13140.
(29) In the experiments with chemical oxidation of 7 by Ru(III)(bpy)3,
a second-order rate constant of ca. 1 × 107 s-1 M-1 was determined for
intermolecular ET from Mn(II) in 7. Such an intermolecular ET would not
show up in the flash photolysis, where the concentration of complex 7 was
(30) Berglund, H.; Sun, L.; Hammarstro¨m, L.; Styring, S.; A° kermark,
B.; Almgren, M. Manuscript in preparation.
less than 1 × 10-4 M, and the rate would be about 1 × 103 s-1
.

7004 J. Am. Chem. Soc., Vol. 119, No. 30, 1997
Sun et al.
Scheme 3
excited state compared to 7 (ca. 2 orders of magnitude shorter).
The quenching effect is observed even for 7 which has a lifetime
(τ) ca. 250 ns as compared to 950 ns for 6a, which lacks the
manganese.
its function is probably more sophisticated than just working
+
as an electron donor to P680
. Nevertheless, an obvious
extension of the present work is to continue to seek inspiration
from PSII and thus insert a tyrosine or an analogous phenol
group between the ruthenium and the manganese ion(s) in
systems related to 7 and 13. Also, multielectron transfer from
manganese(s) is necessary for oxidation of water to oxygen at
a reasonable potential. Work is therefore in progress to
synthesize supramolecular complexes, where more than one
manganese ion is coordinated together with ruthenium.
The slow electron transfer from coordinated manganese in 7
suggests that a distance of ca. 13 Å may be somewhat too long
for efficient electron transfer. However, already at 9 Å, the
undesirable quenching of the excited photosensitizer by the
manganese complex is very fast. Although the rate of this
quenching may be adjusted somewhat by changing the ligand
environment of Mn(II), it will probably be favorable to insert
an intermediate, an electron donor, between the ruthenium and
manganese parts of the molecule. This donor should be chosen
so that its quenching of the excited state of the photosensitizer
is small. Also, the reorganization energy for electron transfer
to Ru(III) should preferably be lower than for the Mn complex,
which would allow a faster donor reaction. In PSII, a tyrosine
moiety is present between P680 and the manganese cluster, and
Acknowledgment. This is publication no. 5 from the
Swedish Consortium for Studies of Artificial Photosynthesis,
financially supported by the Knut and Alice Wallenberg
Foundation, Sweden. This work was also supported by the
Swedish Natural Science Research Council.
JA962511K