Syntheses, spectroelectrochemistry and photoinduced electron-transfer processes of novel Ru and Os dyad and triad complexes with functionalized diimide ligands

Article abstract of DOI:10.1135/cccc20010307

A series of mono-and dinuclear Ru complexes containing a bridging ligand, L-diimide-L (L = 2-(2-pyridyl)benzimidazole; diimide = benzene-1,2:4,5-bis(dicarboximide) (bdi; pyromellitimide) or naphthalene-1,8:4,5-bis(dicarboximide) (ndi), with either propane

Full text of DOI:10.1135/cccc20010307

Ru and Os Dyad and Triad Complexes
307
SYNTHESES, SPECTROELECTROCHEMISTRY AND PHOTOINDUCED
ELECTRON-TRANSFER PROCESSES OF NOVEL Ru AND Os DYAD
AND TRIAD COMPLEXES WITH FUNCTIONALIZED DIIMIDE LIGANDS
a
b
M. Delower HOSSAIN , Masa-aki HAGAa1,*, Bobak GHOLAMKHASS ,
Koichi NOZAKIb1, Minoru TSUSHIMA , Noriaki IKEDA and Takeshi OHNO
b
b2
b3
a
Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University,
1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan; e-mail: 1 mhaga@apchem.chem.chuo-u.ac.jp
Department of Chemistry, Faculty of Science, Osaka University, Machikaneyama, Toyonaka,
b
1
2
Osaka 560, Japan; e-mail: nozaki@ch.wani.osaka-u.ac.jp, ikeda@ch.wani.osaka-u.ac.jp,
ohno@ch.wani.osaka-u.ac.jp
3
Received Novem ber 8, 2000
Accepted Jan uary 20, 2001
This paper is dedicated to the memory of the late Professor Antonín A. Vlček. We will always remem-
ber his pioneering work, particularly in electrochemistry and spectroelectrochemistry of a wide range of
transition metal complexes.
A series of m on o- an d din uclear Ru com plexes con tain in g a bridgin g ligan d, L-diim ide-L (L =
2-(2-pyridyl)ben zim idazole; diim ide = ben zen e-1,2:4,5-bis(dicarboxim ide) (bdi; pyrom ellit-
im ide) or n aph th alen e-1,8:4,5-bis(dicarboxim ide) (n di), with eith er propan e or xylen e group
as lin kers, h ave been prepared. Th e m on o- an d din uclear Ru com plexes con tain in g th e bdi
or n di ligan d, exh ibit ch aracteristic m etal-to-ligan d ch arge tran sfer (MLCT) tran sition at 458
n m . Th e m on o- an d din uclear Ru/Os com plexes exh ibit a rich redox ch em istry arisin g from
both M(II) to M(III) oxidation an d ligan d-based con secutive reduction of diim ide an d
2,2′-bipyridin e (bpy) ligan ds. Th e em ission decays fit well with double- or triple-expon en tial
decay m odels. Th e n on -expon en tial decay curve reveals th e existen ce of several con form ers
in solution due to th e flexible propan e or p-xylen e lin ker. Th e m uch sh orter em ission life-
tim es of th e [M(bpy)2(L-diim ide-L)] com plexes com pared with th e paren t [M(bpy)2L] in di-
cates th e in tram olecular electron -tran sfer from th e excited M(bpy)2 m oiety to th e diim ide.
Th e fastest rates of electron tran sfer (3 · 1010 s–1) are attributable to th e folded con form ers
suitable for th e electron don or/acceptor th rough -space in teraction . A tim e-resolved absorp-
tion spectroscopic study of th e din uclear bdi an d n di com plexes revealed appearan ce of th e
electron -tran sfer products, M(III) an d th e diim ide radical an ion , an d th eir rapid disappear-
an ce. Th e effect of th e lin kers of th e ligan d L-diim ide-L on th e rates of electron tran sfer an d
th e back electron tran sfer is also discussed.
Keyw ord s: Electroch em istry; Ph otoch em istry; Ruth en ium com plexes; Osm ium com plexes;
Ph otoin duced electron tran sfer; Diim ides; Bipyridin es.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)
doi: 10.1135/cccc20010307

308
Hossain et al.:
Electron tran sfer reaction s play a key role in biological processes such as
ph otosyn th esis, an d are also at h eart of m an y im portan t ch em ical pro-
cesses1. Th e ph otosyn th etic cen tre uses a m ultistep electron tran sfer
th rough a set of don or an d acceptor m oieties in order to ach ieve lon g-ran ge
ch arge separation an d to reduce th e rapid ch arge recom bin ation by spa-
tially separatin g th e electron an d h ole. Durin g th e past two decades
ph otoin duced in tram olecular electron tran sfer reaction s in dyad, triad an d
tetrad assem blies h ave been exten sively exam in ed towards m im ickin g th e
ph otosyn th etic cen tre2–8. In a triad system , several differen t m olecular se-
quen ces for a com bin ation of electron don ors (D) an d acceptors (A) can be
con sidered. Most studied sequen ces are D–D*–A an d D*–A–A triad system s,
in wh ich porph yrin an d [Ru(bpy)3]2+ (bpy = 2,2′-bipyridin e) are used as
ch rom oph ores (D) an d p-quin on e an d m eth ylviologen as electron accep-
tors2–5,9–11. Th ese two types of m olecules can in prin ciple give rise to
lon g-lived, ch arge-separated states, D+–D–A–, as a result of sequen tial elec-
tron tran sfer reaction s provided th e redox poten tials of th e don ors an d ac-
ceptors are judiciously selected. Recen tly, th e diim ide un its h ave been used
as an electron acceptor in porph yrin triad system s12,13. Sin ce th e diim ide
radical an ion possesses ch aracteristic in ten se absorption ban ds between
600–700 n m , th e tran sien t species resultin g from th e ph otoin duced elec-
tron tran sfer can be easily detected by tran sien t absorption spectra14–18. Fur-
th erm ore, th e n aph th alen ebis(dicarboxim ide) is used as DNA-bin din g
m olecules or don or–acceptor π-stacked supram olecular h ost system s19–23. So
far, several reports on a com bin ation of [Ru(bpy)3]2+ as an electron don or
an d diim ide un its as electron acceptors, h ave been publish ed20,24. A n ovel
bridgin g ligan d, L-diim ide-L was prepared, in wh ich th e diim ide un it is
lin ked by two biden tate ligan d un its (L) such as bpy or 2-(2-pyridyl)-
ben zim idazole (pbim H). Th e bpy-diim ide-bpy ligan d was also used as a
buildin g block lin ker for self-assem blin g to m etallom acrocycles23. In th e
presen t paper, we describe syn th eses of a series of Ru/Os com plexes with
th is n ovel bridgin g ligan d con tain in g th e carboxim ide group (Sch em e 1).
Their photoinduced electron transfer reactions were exam ined and the results
are reported h erein after. A prelim in ary result for a RuII-(L-diim ide-L)-OsIII
com plex h as been reported elsewh ere24
.
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Ru and Os Dyad and Triad Complexes
309
EXPERIMENTAL
Materials
Acrylon itrile (Nacalai) an d ruth en ium trich loride trih ydrate (En gelh ardt) were used with out
furth er purification . Aceton itrile was purified twice by distillation over P2O5. Tetra-
butylam m on ium tetrafluoroborate (TBABF4, Nacalai) an d tetrabutylam m on ium perch lorate
(TBAClO4, Nacalai) were recrystallized from eth an ol–water (4 : 1 v/v) an d dried in vacuo. All
oth er supplied ch em icals were of stan dard reagen t grade quality.
Th e com poun ds 2-(2-pyridyl)ben zim idazole25
, 4,4′-bis(trifluorom eth yl)-2,2′-bipyridin e
(btfm b)26, [Ru(bpy)2Cl2]·2H2O (ref.27), [Ru(btfm b)2Cl2]·2H2O (ref.26) an d [Os(bpy)2Cl2]·2H2O
(ref.27) were syn th esized accordin g to literature procedures.
O
N
O
O
N
O
N
N
M(L')2
H3C
N
mndi
N
N
(L')2M
M(L')2
Q
N
N
N
N
propane linker 1
N
N
N
N
(L')2M
M(L')2
N
N
Q
p-xylene linker 2
M = Ru or Os; L' = bpy, dmbpy, btmfb
O
N
O
O
N
O
O
Q =
N
or
N
O
O
O
pbdi: linker 1
xbdi:
pndi: linker 1
xndi:
2
2
SCHEME 1
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Hossain et al.:
Syntheses of Ligands
1-(Cyan oeth yl)-2-(2-pyridyl)ben zim idazole (2)
2-(2-Pyridyl)ben zim idazole (1) (8.6 g, 44.1 m m ol) was dissolved in warm 1,4-dioxan e
(120 m l). Next, tetram eth ylam m on ium h ydroxide in 25% m eth an ol solution (0.5 m l) was
added. Th e ligh t pin k colour of th e reaction solution disappeared. To th e resultin g solution
acrylon itrile (5.84 g, 0.11 m ol) was added dropwise. Durin g th e addition th e colour turn ed
red an d a sm all am oun t of precipitate was form ed. Th e solution m ixture was h eated at
120 °C overn igh t. After coolin g to room tem perature, th e solution was filtered to rem ove a
red brown precipitate. Th e filtrate was evaporated in vacuo. Th e resultin g oily residue was
redissolved in a m in im um volum e of aceton e, followed by th e addition of a large volum e of
eth er to precipitate th e product. Th e precipitate was collected an d crystallized from m eth a-
n ol–water as wh ite n eedles. Yield 6.5 g (60%). Th e product is soluble in com m on organ ic
solven ts. M.p. 105–110 °C. IR (KBr disk): ν(CN) at 2 280 cm –1 1H NMR (CDCl3): 3.16 (t, 2 H,
.
J = 3.7 ); 5.07 (t, 2 H, J = 3.1); 7.34 (t, 1 H, J = 6.1); 7.38 (t, 1 H, J = 3.7 ); 7.40 (t, 1 H, J =
7.0); 7.49 (d, 1 H, J = 7.6); 7.86 (d, 1 H, J = 3.7); 7.88 (t, 1 H, J = 1.8); 8.48 (d, 1 H, J = 2.1);
8.70 (d, 1 H, J = 3.1). MS (EI), m/z: 248 (M+). For C15H12N4 calculated: 72.56% C, 4.87% H,
22.57% N; foun d: 72.54% C, 5.09% H, 22.54% N.
1-(3-Am in opropyl)-2-(2-pyridyl)ben zim idazole (3)
1-(2-Cyan oeth yl)-2-(2-pyridyl)ben zim idazole (2) (6.0 g, 24.2 m m ol) was dissolved in m eth a-
n ol (200 m l), followed by addition of solid cobalt(II) ch loride h exah ydrate (11.1 g,
48.4 m m ol) un der stirrin g. To th is solution , 1 g of solid sodium boroh ydride was added each
tim e un til th e total am oun t reach ed 15 g. Ca u tion ! A vigorous reaction with violent gas evolu-
tion occurred upon the addition of NaBH4. The addition of NaBH4 should be carried out with cau-
tion. As a result, th e pin k solution colour turn ed black. After com plete addition of sodium
boroh ydride th e reaction m ixture was stirred at room tem perature for an oth er 3 h ; durin g
th is tim e th e solution colour gradually turn ed ligh t brown . Th e solution was th en acidified
by dropwise addition of con cen trated HCl an d a wh ite precipitate form ed. After filtration
an d dryin g in vacuo, a blue solid product was left. It was dissolved in water an d th e solution
was m ade alkalin e dropwise addition of 25% NH3 solution . Th e resultin g solution was ex-
tracted with ch loroform (5 × 80 m l), an d th e com bin ed ch loroform layer was dried over an -
h ydrous MgSO4. A red-brown oily product was obtain ed after evaporation of ch loroform .
Th is product was dissolved in a m in im um volum e of pure ben zen e an d a large volum e of di-
eth yl eth er was added to precipitate im purities. After filtration , th e pure product was ob-
tain ed as red oil by evaporation of th e solven t. Yield 4.91 g (80.5%). IR: ν(–NH2) at 3 350
an d 3 280 cm –1 1H NMR (CDCl3): 1.62 (s, 2 H); 2.08 (t, 2 H); 2.76 (t, 2 H); 4.94 (t, 2 H);
.
7.32 (t, 1 H); 7.33 (t, 1 H); 7.33 (t, 1 H); 7.52 (d, 1 H); 7.85 (d, 1 H); 7.88 (t, 1 H); 8.43 (d,
1 H); 8.69 (d, 1 H). MS (EI), m/z: 253 (M+). M = C15H16N4.
1-(4-Cyan oben zyl)-2-(2-pyridyl)ben zim idazole (4)
2-(2-Pyridyl)ben zim idazole (1) (8.75 g, 44.9 m m ol) in dim eth ylform am ide (DMF; 50 m l) was
reacted with sodium h ydride (1.97 g, 49.3 m m ol, 60% dispersion in m in eral oil) wh ich was
wash ed un der n itrogen with h eptan e as to rem ove th e m in eral oil. A vigorous reaction oc-
curred, accom pan ied by evolution of H2 gas. Th e pale yellow solution colour turn ed dark
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Ru and Os Dyad and Triad Complexes
311
brown on h eatin g at 80 °C for
2
h . After th e gas evolution ceased, 4-(brom o-
m eth yl)ben zon itrile (8.79 g, 44.9 m m ol) in 30 m l of DMF was added dropwise an d th e solu-
tion was refluxed for 14 h . After coolin g, th e solven t was evaporated in vacuo. Th e residue
was redissolved in CH2Cl2. Th e solution was filtered to rem ove in soluble im purities. Th e fil-
trate was evaporated in vacuo to obtain a brown product th at was recrystallized from a m eth -
an ol–water m ixture. Yield 12.7 g (91%). Th e product is soluble in com m on organ ic solven ts.
M.p. 154–158 °C. IR (KBr disk): ν(CN) at 2 200 cm –1 1H NMR (CDCl3): 6.23 (t, 2 H); 7.26 (d,
.
2 H, J = 3.5); 7.29 (d, 2 H, J = 2.0); 7.32 (t, 1 H, J = 4.0); 7.36 (t, 1 H, J = 4.0); 7.54 (d, 1 H); 7.56
(d, 1 H); 7.82 (t, 1 H, J =1.5); 7.85 (t, 1 H, J = 4.5); 7.87 (d, 1 H, J = 2.0); 7.90 (d, 1 H, J =
5.4). MS (EI), m/z: 309 (M+). For C20H14N4 calculated: 77.40% C, 4.55% H, 18.05% N; foun d:
77.55% C, 4.76% H, 7.83% N.
[4-(Am in om eth yl)ben zyl]-2-(2-pyridyl)ben zim idazole (5)
1-(4-Cyan oben zyl)-2-(2-pyridyl)ben zim idazole (4) (6.0 g,19.3 m m ol) an d CoCl2·6H2O (9.2 g,
38.6 m m ol) were dissolved in m eth an ol with stirrin g at room tem perature. To th is solution
solid NaBH4 (8 g, 0.2 m ol) was added in 0.5 g portion s. Ca u tion ! A vigorous reaction with vio-
lent gas evolution occurred upon the addition of NaBH4. The addition of NaBH4 should be carried
out with caution. After com plete addition of NaBH4 th e reaction m ixture was stirred at room
tem perature for 2 h . At th is stage th e black solution colour turn ed ligh t brown . Th e solution
was th en acidified with th e dropwise addition of con cen trated HCl. A wh ite precipitate was
rem oved by filtration . Th e filtrate was evaporated in vacuo to obtain a blue solid. Th e solid
was redissolved in 25% NH3 (50 m l). Th e solution was extracted with five 50 m l portion s of
ch loroform . Th e com bin ed extracts were dried over an h ydrous MgSO4 an d evaporated to
dryn ess. A red-brown oily residue was again dissolved in ben zen e an d treated with
dieth yleth er to precipitate in soluble im purities th at were rem oved by filtration . Th e filtrate
was again evaporated to dryn ess. A pure product was obtain ed as a red oil. Yield 4.7 g
(77.3%). IR (KBr disk): ν(NH2) at 3 220 an d 3 280 cm –1 1H NMR (CDCl3): 1.66 (s, 2 H); 3.79
.
(t, 2 H); 6.18 (t, 2 H); 7.15 (d, 2 H, J = 8.2); 7.17 (d, 2 H, J = 4.9); 7.25 (t, 1 H, J = 5.9); 7.30
(t, 1 H, J = 2.0); 7.32 (d, 1 H, J = 3.0); 7.33 (d, 1 H, J = 2.0); 7.81 (t, 1 H, J = 7.9); 7.86 (t,
1 H, J = 1.3); 8.43 (d, 1 H, J = 7.9); 8.63 (d, 1 H, J = 4.9). MS (EI), m/z: 313 (M+). M =
C
20H18N4.
N-Meth yl-N′-{3-[2-(2-pyridyl)ben zim idazol-1-yl]propyl}n aph th alen e-1,8:4,5-bis-
(dicarboxim ide) (m n di)
N-{3-[2-(2-Pyridyl)ben zim idazol-1-yl]propyl}n aph th alen e-1,8:4,5-bis(dicarboxim ide) (0.5 g,
1 m m ol) was dissolved in DMF (50 m l) at 120 °C un der an n itrogen atm osph ere with in 1 h .
To th e resultin g solution m eth ylam in e (0.03 g, 1 m m ol) was added an d h eatin g was con tin -
ued for an oth er 8 h . Th e solven t was th en evaporated in vacuo. A brown product was
recrystallized from a m ixture of ch loroform –m eth an ol. Yield 0.41 g (83%). Th e product was
soluble in CHCl3, DMF, DMSO, CH2Cl2. M.p. 230–234 °C. 1H NMR (CD3CN): 2.43 (t, 2 H,
J = 7.3); 3.47 (s, 3 H, J = 2.0); 4.22 (t, 2 H, J = 7.3); 4.96 (t, 2 H, J = 7.6); 7.21 (t, 1 H, J = 4.6);
7.26 (t, 1 H, J = 6.3); 7.30 (t, 1 H, J = 7.6); 7.61 (d, 1 H, J = 8.9); 7.66 (d, 1 H, J = 7.9); 7.83
(t, 1 H, J = 5.6); 8.27 (d, 1 H, J = 1.0); 8.30 (d, 1 H, J = 7.9); 8.64 (s, 4 H, J = 4.6). MS (EI),
m/z: 515 (M+). For C30H21N5O4·0.5H2O calculated: 68.70% C, 4.23% H,13.35% N; foun d:
69.09% C; 4.33% H,13.40% N.
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Hossain et al.:
Syntheses of Bridging Ligands
N,N′-Bis{3-[2-(2-pyridyl)ben zim idazol-1-yl]propyl}ben zen e-1,2:4,5-bis-
(dicarboxim ide) (pbdi)
Pyrom ellitic dian h ydride (6) (1.8 g, 8.26 m m ol) was dissolved in toluen e (150 m l), followed
by addition of trieth ylam in e (2 m l). Wh en th e solution becam e clear, 4.16 g (16.5 m m ol) of
1-(3-am in opropyl)-2-(2-pyridyl)ben zim idazole (3) was added to th e reaction m ixture. Th e so-
lution colour turn ed yellow. Th e reaction m ixture was th en refluxed at 200 °C for 5 h , wh ile
con tin uously rem ovin g water with a water separator. Wh en th e volum e of water reach ed to
th eoretical value, th e reaction was stopped an d th e solven t was evaporated to dryn ess. Th e
product was obtain ed as a yellow solid th at was dried in vacuo. Th e crude product was th en
recrystallized from DMF–water (2 : 1). Yield 4.05 g (71.5%). Th e product is soluble in DMF,
.
DMSO an d eth ylen e glycol. M.p. 268–272 °C. IR: ν(C=O) at 1 765 an d 1 708 cm –1 1H NMR
(DMSO-d6): 2.21 (t, 4 H); 3.74 (t, 4 H); 4.90 (t, 4 H); 7.26 (t, 2 H, J = 7.6); 7.32 (t, 2 H, J =
7.3); 7.39 (t, 2 H); 7.71 (d, 2 H, J = 7.9); 7.75 (d, 2 H, J = 7.6); 7.95 (t, 2 H); 8.17 (s, 2 H);
8.31 (d, 2 H, J = 7.9); 8.39 (d, 2 H). MS (EI), m/z: 686 (M+). M = C40H30N8O4. For
C
40H30N8O4·0.5 H2O calculated: 69.05% C, 4.49% H, 16.11% N; foun d: 69.04% C, 4.67% H,
15.59% N.
N,N′-Bis{3-[2-(2-pyridyl)ben zim idazol-1-yl)]propyl}n aph th alen e-1,4:5,8-bis-
(dicarboxim ide) (pn di)
A
m ixture of n aph th alen e-1,4:5,8-bis(dicarboxan h ydride) (7) (2.0 g, 7.46 m m ol),
1-(3-am in opropyl)-2-(2-pyridyl)ben zim idazole (3.76 g, 14.9 m m ol) an d trieth ylam in e (2 m l)
in toluen e (150 m l) was h eated at 180 °C for 7 h , wh ile rem ovin g th e water form ed. Th e
grey solution colour solution turn ed ligh t brown . Th e solven t was rem oved in vacuo. Th e
crude brown product was recrystallized from DMF. Yield 3.70 g (67.3%). Th e product is solu-
ble in DMF, DMSO an d eth ylen e glycol. M.p. 226–230 °C. IR (KBr disk): ν(C=O) at 1 700 an d
1 660 cm –1 1H NMR (DSMO-d6): 2.20 (t, 4 H); 4.15 (t, 4 H); 4.95(t, 4 H); 7.15 (t, 2 H, J =
.
7.3); 7.24 (t, 2 H, J = 7.3); 7.31 (t, 2 H, J = 4.9); 7.68 (d, 2 H, J = 7.9); 7.72 (d, 2 H, J = 8.2);
7.91 (t, 2 H, J = 7.6); 8.28 (d, 2 H, J = 7.9); 8.31 (d, 2 H, J = 0.9); 8.60 (s, 4 H). MS (EI), m/z:
736 (M+). M = C44H32N8O4. For C44H32N8O4·0.5H2O calculated: 70.86% C, 4.46% H,
15.02% N; foun d: 70.72% C, 4.51% H, 14.76% N.
N.N′-Bis(4-{[2-(2-pyridyl)ben zim idazol-1-yl]m eth yl}ben zyl)n aph th alen e1,4:5,8-bis-
(dicarboxim ide) (xn di)
Naph th alen e-1,4:5,8-bis(dicarboxan h ydride) (7) (0.94 g, 3.49 m m ol) an d trieth ylam in e
(1.5 m l) were dissolved in toluen e (50 m l) at 180 °C un der stirrin g. Next, 1-[4-(am in o-
m eth yl)ben zyl]-2-(2-pyridyl)ben zim idazole (5) (2.2 g, 6.99 m m ol) was added. Th e reflux was
con tin ued for an oth er 12 h , durin g wh ich tim e th e grey solution colour turn ed brown . Th e
solven t was evaporated in vacuo. Th e resultin g brown solid was recrystallized from
DMF–m eth an ol. Yield 2.2 g (73%). Th e product is soluble in DMF, DMSO an d eth ylen e gly-
.
col. M.p. > 280 °C. IR: ν(C=O) at 1 660 an d 1 692 cm –1 1H NMR (DSMO-d6): 5.16 (t, 4 H);
6.17 (t, 4 H); 7.07 (d, 4 H, J = 7.9); 7.22 (t, 2 H, J = 8.9); 7.25 (t, 2 H, J = 6.9); 7.47 (d, 4H; J =
7.4); 7.51 (t, 2 H, J = 3.5); 7.71 (d, 2 H, J = 5.9); 7.74 (d, 2 H, J = 3.0); 7.98 (t, 2 H, J = 7.9);
8.34 (d, 2 H, J = 7.4); 8.61 (s, 4 H); 8.65 (d, 2 H, J = 4.9). MS (EI), m/z: 860 (M+). M =
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Ru and Os Dyad and Triad Complexes
313
C54H36N8O4. For C54H36N8O4·H2O calculated: 73.79% C, 4.36% H, 12.75% N; foun d:
73.77% C, 4.49% H, 12.04% N.
Ca u tion ! Perchlorate salts are potentially explosive. Although no detonation tendencies have
been observed, caution is advised and handling of only small quantities is recommended.
Syntheses of Mononuclear Complexes
[Ru(bpy)2(pbdi)](ClO4)2·3H2O
Th e ligan d, pbdi (0.30 g, 0.44 m m ol), was dissolved in eth ylen e glycol (30 m l) by h eatin g.
Next, solid [Ru(bpy)2Cl2] (0.21 g, 0.44 m m ol) was added to th e solution . Th e tem perature of
180 °C was m ain tain ed for an oth er 5 h . Durin g th is tim e th e violet colour of th e reaction
m ixture turn ed red. After coolin g, water (30 m l) was added to th e reaction m ixture, followed
by filtration . To th e filtrate, a saturated solution of NaClO4 was added dropwise to com plete
precipitation . Th e precipitate was collected an d dried in vacuo. Its purification was per-
form ed by SP-Seph adex C-25 colum n ch rom atograph y, usin g aceton itrile–buffer (1 : 1) of
differen t pH as an eluen t. Th e desired ban d was eluted at pH 5.0. Evaporation of aceton itrile
from th e eluate an d addition of a saturated NaClO4 solution afforded a red precipitate th at
was purifed by recrystallization from m eth an ol. Yield 0.22
g
(37%). For
C
60H46Cl2N12O12Ru·3H2O calculated: 53.26% C, 3.87% H, 12.42% N; foun d: 53.53% C,
4.12% H, 12.0% N.
[M(bpy)2(pn di)](ClO4)2·xH2O (M = Ru, x = 2; M = Os, x = 4)
Th e syn th etic procedure is th e sam e as th at used for th e correspon din g m on on uclear pbdi
com plex. For th e Os com plex, a lon ger reflux tim e was used.
M = Ru, x = 2. Yield 45%. For C64H48Cl2N12O12Ru·2H2O calculated: 55.50% C, 3.78% H,
12.13% N; foun d: 55.90% C, 4.05% H, 12.11% N.
M = Os, x = 4. Yield 41%. For C64H48Cl2N12O12Os·4H2O calculated: 47.50% C, 4.23% H,
10.39% N; foun d: 47.61% C, 3.66% H, 10.34% N.
[Ru(bpy)2(xn di)](ClO4)2·7H2O
Th e syn th etic procedure is th e sam e as th at used for th e correspon din g m on on uclear pn di
com plex. Yield 0.18 g (35%). For C74H52Cl2N12O12Ru·7H2O calculated: 55.57% C, 4.16% H,
10.51% N; foun d: 55.65% C, 3.75% H, 10.39% N.
[Ru(bpy)2(m n di)](ClO4)2.2H2O
Th e m n di ligan d (0.20 g, 0.39 m m ol) was stirred in 30 m l of eth ylen e glycol at 180 °C un der
a n itrogen atm osph ere for 1 h . Solid [Ru(bpy)2Cl2] (0.19 g, 0.39 m m ol) was added to th e re-
action m ixture an d h eated for an oth er 5 h . Durin g th is tim e th e brown solution colour
turn ed red. After coolin g to room tem perature, water (30 m l) was added an d th e reaction
m ixture was filtered. To th e filtrate, a saturated NaClO4 solution was added. A red precipi-
tate was collected an d dried. Th e sam ple was th en loaded on a SP-Seph adex C-25 colum n
an d eluted with aceton itrile–buffer (1 : 1 v/v), adjustin g pH. Th e desired ban d was eluted at
pH 4.5. Th e solution was con cen trated in vacuo. Next, a saturated NaClO4 solution was
added to precipitate th e desired com plex. Th e precipitate was collected by filtration an d
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

314
Hossain et al.:
recrystallized from m eth an ol. Yield 0.24 g (55%). For C50H37Cl2N9O12Ru·2H2O calculated:
51.55% C, 3.52% H, 10.83% N; foun d: 51.24% C, 2.84% H, 10.73% N.
Syntheses of Dinuclear Complexes
[{Ru(bpy)2} (µ-pbdi)](ClO4)4·3H2O
2
A solution of [Ru(bpy)2Cl2] (0.17 g, 0.35 m m ol) in eth ylen e glycol (30 m l) was h eated at
about 180 °C for 30 m in . Solid pbdi (0.12 g, 0.17 m m ol) was added to th e h ot solution an d
th e h eatin g was con tin ued for an oth er 8 h . Th e colour of th e solution turn ed red. After
coolin g th e solution to room tem perature, water (30 m l) was added to precipitate un reacted
pbdi ligan d. After filtration , a saturated aqueous NaClO4 solution was added to com plete th e
precipitation . Th e red precipitate was collected an d dried. Th e crude product was purified by
SP-Seph adex C-25 colum n ch rom atograph y, usin g aceton itrile–buffer (1 : 1 v/v) eluen t. Th e
desired ban d was eluted at pH 7.0. Furth er purification was ach ieved by recrystallization
from m eth an ol. Yield 0.30 g (54%). MS (ESI) (CH3CN), m/z: 856 ([M – 2 X]2+), 537 ([M –
3 X]3+), 378 ([M – 4 X]4+). For C80H62Cl4N16O20Ru2·3H2O calculated: 48.89% C, 3.49% H,
11.40% N; foun d: 48.67% C, 3.81% H, 11.20% N.
[{M(bpy)2} (µ-pn di)](ClO4)4·xH2O (M = Ru, x = 3; M = Os, x = 4)
2
Th ese com plexes were syn th esized in a sim ilar m an n er as [{Ru(bpy)2}(µ-pbdi)](ClO4)4·3H2O,
except th at pn di was used in stead of pbdi. Yield 40%.
M = Ru. MS (ESI) (CH3CN), m/z: 881 ([M – 2 X]2+), 554 ([M – 3 X]3+), 391 ([M – 4 X]4+).
For C84H64Cl4N16O20Ru2·3H2O calculated: 50.06% C, 3.50% H, 11.12% N; foun d: 49.90% C,
3.73% H, 11.07% N.
M = Os. For C84H64Cl4N16O20Os2·4H2O calculated: 45.62% C, 3.28% H, 10.13% N; foun d:
45.43% C, 3.40% H, 10.06% N.
[{RuL } (µ-pn di)](ClO ) ·xH O (L′ = dm bpy, x = 7; L′ = btm fb, x = 2)
′
2
2
4
4
2
Th ese com plexes were prepared in a sim ilar m an n er as th e correspon din g h om odin uclear
pbdi com plex above, except th at [RuL Cl ] (L′ = dm bpy or btm fb) were used as startin g com -
′
2
2
plexes. Purification was accom plish ed by colum n ch rom atograph y usin g th e SP-Seph adex
C-25 resin , followed by recrystallization from m eth an ol.
L′ = dmbpy. Yield 0.24 g (50%). For C92H80Cl4N16O20Ru2·7H2O calculated: 50.23% C,
4.31% H, 10.19% N; foun d: 50.36% C, 4.21% H, 10.21% N.
L′ = btmfb. Yield 0.23 g (55.8%). For C92H56Cl4F24N16O20Ru2·2H2O calculated: 43.48% C,
2.38% H, 8.82% N; foun d: 43.55% C, 2.68% H, 8.45% N.
[Ru(bpy)2(µ-pbdi)Os(bpy)2](ClO4)4·4H2O
[Ru(bpy)2Cl2] (0.052 g, 0.11 m m ol) in eth ylen e glycol (30 m l) was h eated at 180 °C un der a
n itrogen atm osph ere for 1 h . Next, [Os(bpy)2(pbdi)](ClO4)2 (0.15 g, 0.11 m m ol) was added
to th e solution . Th e h eatin g was con tin ued for an oth er 10 h . Th e red solution colour turn ed
ligh t green . Water (30 m l) was added to rem ove th e un reacted pbdi ligan d. A saturated solu-
tion of NaClO4 was added to th e filtrate to com plete th e precipitation . Th e resultin g green
precipitate was filtered an d dried in vacuo. Th e crude product was purified by ch rom atogra-
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Ru and Os Dyad and Triad Complexes
315
ph y on SP-Seph adex C-25 resin , usin g aceton itrile–buffer (1 : 1 v/v) as eluen t. Th e desired
ban d was eluted at pH 7.0. Th e ligh t green powder product was recrystallized from m eth a-
n ol. Yield 0.13 g (61%). For C80H62Cl4N16O20OsRu·4H2O calculated: 46.36% C, 3.40% H,
10.81% N; foun d: 46.89% C, 4.03% H, 10.65% N.
[Ru(bpy)2(µ-pn di)Os(bpy)2](ClO4)4·4H2O
[Ru(bpy)2Cl2] (0.099 g, 0.21 m m ol) was dissolved in 30 m l of eth ylen e glycol at 180 °C un -
der
a n itrogen atm osph ere wh ile stirrin g for 1
h . Next, [Os(bpy)2(pn di)]2+ (0.30 g,
0.21 m m ol) was added an d h eatin g was con tin ued for an oth er 12 h , durin g wh ich tim e th e
red solution colour turn ed ligh t green . After bein g cooled to room tem perature, 30 m l of wa-
ter was added an d th e solution was filtered. A saturated aqueous solution of NaClO4 was
added dropwise to th e filtrate to com plete th e precipitation . A ligh t green precipitate was
collected an d purified by colum n ch rom atograph y on SP-Seph adex C-25 resin , usin g
aceton itrile–buffer (1 : 1 v/v) as eluen t. Th e desired ban d was eluted at pH 7.0. Th e volum e
of th e collected eluate was reduced to th e h alf an d NaClO4 was added to com plete th e pre-
cipitation . Th e precipitate was collected an d recrystallized from m eth an ol. Yield 0.26 g
(62%). MS (ESI) (CH3CN), m/z: 926 ([M – 2 X]2+), 584 ([M – 3 X]3+), 413 ([M – 4 X]4+). For
C
84H64Cl4N16O20OsRu·4H2O calculated: 47.53% C, 3.42% H, 10.56% N; foun d: 47.50% C,
3.84% H, 10.42% N.
[{Ru(bpy)2} (µ-xn di)](ClO4)4·5H2O
2
Th e syn th etic procedure was th e sam e as th at used for th e din uclear pn di an alogue, substi-
tutin g pn di for xn di. Yield 0.34
g (61%). For C94H680Cl4N16O2Ru2·5H2O calculated:
51.89% C, 3.6% H, 10.30% N; foun d: 51.90% C, 3.48% H, 10.31% N.
[Ru(bpy)2(µ-xn di)Os(bpy)2](ClO4)4·10H2O
[Os(bpy)2Cl2] (0.06 g, 0.1 m m ol) was dissolved in eth ylen e glycol (30 m l) at 180 °C un der a
n itrogen atm osph ere wh ile stirrin g for 1 h . Next, th e solid, [Ru(bpy)2(xn di)]2+ (0.15 g,
0.1 m m ol), was added an d h eatin g was con tin ued for an oth er 10 h , durin g wh ich tim e th e
black solution colour turn ed ligh t green . Upon coolin g, 30 m l of water was added to rem ove
excess un coordin ated ligan d. To th e filtrate a saturated NaClO4 solution was added. Th e re-
sultin g precipitate was collected an d dried in vacuo. Purification was don e by SP-Seph adex
C-25 colum n ch rom atograph y, usin g aceton itrile–buffer (1 : 1 v/v) as an eluen t. Th e eluted
ban d at pH 7.0 was collected an d th e eluate volum e reduced to th e h alf by rem oval of
aceton itrile. To th e con cen trated solution , a saturated solution of NaClO4 was added. Th e re-
sultin g red-green precipitate was collected an d recrystallized from m eth an ol. Yield 0.09 g
(41%). For C94H68Cl4N16O20OsRu·10H2O calculated: 47.94% C, 3.77% H, 9.52% N; foun d:
47.54% C, 3.35% H, 9.24% N.
Physical Measurements
Electron ic absorption spectra were recorded on
a Hitach i U-3210 spectroph otom eter
(200–850 n m ) an d a Hitach i U-3400 spectroph otom eter (800–2 500 n m ). NMR spectra were
m easured with a 270 MHz JEOL JNM-EX spectrom eter. Ch em ical sh ifts are given in ppm
(δ-scale), couplin g con stan ts, J, in Hz. IR spectra were recorded on a Nicolet FT-IR Magn a
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

316
Hossain et al.:
560 spectrom eter. Th e m ass spectra were m easured on a Sh im adzu QP1000EX spectrom eter
for organ ic ligan ds, an d on Fin n igan –Matt TSQ-700 triple quadrupole m ass spectrom eter,
equipped with ESI in terface system , for th e com plexes.
Electroch em ical m easurem en ts were m ade at 20 °C with a BAS 100 B/W electroch em ical
workstation . Th e workin g electrode was a BAS glassy carbon (3-m m diam eter) or BAS plati-
n um (1.6-m m diam eter) disk. Th e auxiliary electrode was a platin um wire. Th e referen ce
electrode was Ag/AgNO3 (10–2
M
solution in 10–1
M
TBABF4/CH3CN). Th e
ferrocen e/ferrocen ium (Fc/Fc+) couple was used as an in tern al referen ce stan dard; all poten -
tials in th is work are reported vs Fc/Fc+. Th e value E1/2(Fc/Fc+) was +0.09 V vs Ag/Ag+ an d
+0.43 V vs SCE. In situ UV-VIS spectroelectroch em istry was perform ed by usin g a platin um
m in igrid (80 m esh ) workin g electrode in a th in -layer cell (optical path of 0.05 cm ) as de-
scribed elsewh ere28. For th e gen eration of RuII-n di-OsIII m ixed-valen t com plexes in th e
course of th e tran sien t absorption spectral m easurem en t, a flow-th rough electrolytic m eth od
was used as previously described29. Th e quan titative oxidative electrolysis of RuII-n di-OsII
com plex was perform ed at +0.5 V in aceton itrile solution con tain in g 10–1 M TBAClO4.
A Hitach i spectrofluorim eter, Model MPF-2A, was used to m easure em ission spectra at
room tem perature. Em ission lifetim es were determ in ed by m ean s of th e tim e-correlated
sin gle-ph oton -coun tin g system 30. A deaerated sam ple solution in 2-m m quartz cell was ex-
cited with th e secon d h arm on ics of a cavity-dum ped Ti3+: Sapph ire laser (400 n m ,
1 n J/pulse, 2 kHz–76 MHz) or a LD-excited solid-state laser (532 n m , 1 µJ/pulse, 10 kHz,
Nan olase). Th e typical full width at h alf m axim um (fwh m ) of in strum en tal respon se fun c-
tion usin g th e form er laser is 40 ps an d 1 n s for th e latter. Tran sien t absorption spectra were
obtain ed after exposure of a sam ple solution in quartz cell of 2 m m optical len gth to th e
secon d h arm on ic pulse of th e m ode-locked Nd3+-YAG laser (2–7 m J/pulse, fwh m = 17 ps,
10 Hz, PY61C-10 Con tin uum ). Th e details of th e m easurem en t system were reported else-
wh ere31. Tim e-resolved differen ce absorption spectra in a delay tim e ran ge of 0–6 000 ps
were m easured usin g th e system . Excitation pulses of adequate en ergy were selected by usin g
a PIN ph otodiode en ergy m on itor. A wh ite-ligh t pulse was produced by focusin g th e
fun dam en tal-oscillation ligh t in to flowin g H2O–D2O (1 : 1 v/v). Th e pulse delay was tun ed
usin g an optical delay with a m ovable m irror. Th e tim e origin of th e wh ite-ligh t pulse was
assign ed to a tim e wh en th e absorban ce ch an ge due to a prim ary ph otoproduct was on e-h alf
of its m axim um value. Two sets con sistin g of an im agin g spectrograph with
150 grooves/m m (Ch rom ex 250IS) an d an air-cooled MOS-type lin ear im age sen sor with 512
elem en ts (Ham am atsu S4805-512), were used for th e detection of m on itorin g ligh t in fron t
of an d beh in d a sam ple cell of 1–10 m m optical len gth . Sign als of th e im age sen sor were
digitized an d fed to a com puter, th ereby h avin g selected m on itorin g pulses of adequate in -
ten sity. Th e dark sign als with out m on itorin g th e ligh t pulse were averaged over 200 m ea-
surem en ts, with an d with out laser excitation .
RESULTS AND DISCUSSION
Preparation of Ligands and Complexes
We succeeded to fun ction alize 2-(2-pyridyl)ben zim idazole (pbim H) to ob-
tain a bridgin g ligan d32–35 by substitutin g th e im in o NH group in pbim H
with 2-cyan oeth yl an d 3-am in opropyl groups (Sch em e 2). Th is allowed us
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Ru and Os Dyad and Triad Complexes
317
to syn th esize a wide variety of ligan ds with electron acceptor/don or or ad-
dition al coordin ation sites. 1-(3-Am in opropyl)- (3) an d [4-(am in o-
m eth yl)ben zyl]-2-(2-pyridyl)ben zim idazole (5) are good precursors of
exten ded m ultifun ction al ligan ds or labellin g reagen ts in biom olecules.
Novel bis(biden tate) ligan ds con tain in g diim ide as an electron acceptor
were obtain ed by th e con den sation reaction of an am in o derivative of
ben zim idazole an d a tetracarboxylic dian h ydride in toluen e. Th e ligan ds
are well soluble in polar solven ts such as DMF an d DMSO an d sparin gly sol-
uble in eth ylen e glycol an d alcoh ol. Mon on uclear Ru or Os com plexes were
syn th esized from th e reaction of th e ligan d with on e m olar equivalen t of
[Ru(bpy)2Cl2]·2H2O or [Os(bpy)2Cl2]·2H2O in eth ylen e glycol. However, th e
product was always con tam in ated with a sm all am oun t of din uclear com -
plex th at was carefully separated by SP-Seph adex cation -exch an ge ch rom a-
tograph y. Hom odin uclear Ru com plexes were obtain ed by th e reaction of
th e ligan d with two m olar equivalen ts of [RuL′ Cl2]·2H2O (L′
=
2
2,2′-bipyridin e (bpy), 4,4′-dim eth yl-bpy (dm bpy), 4,4′-bis(trifluorom eth yl)-
bpy (btm fb)). Furth erm ore, th e reaction of [Ru(bpy)2Cl2] with on e m olar
equivalen t of a m on on uclear Os com plex, [Os(bpy)2L]2+ produced th e corre-
spon din g h eterodin uclear Ru-L-Os com plex. Th e com position of th ese com -
plexes was establish ed with electrospray m ass spectroscopy, NMR an d
elem en tal an alyses.
N
N
N
N
N
N
H
N
N
CN
N
2
4
1
CN
pbdi
or
pndi
xbdi
or
xndi
N
N
N
N
6 or 7
6 or 7
N
N
3
5
NH2
NH2
O
O
O
O
O
O
O
O
O
O
O
O
6
7
SCHEME 2
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

318
Absorption Spectra
Hossain et al.:
Th e spectral data are sum m arized in Table I. Th e n ovel ligan ds sh ow π–π*
absorption at 312 n m for pbdi, an d at 381, 360 an d 312 n m for pn di36. Th e
m on on uclear an d din uclear Ru(II) an d Os(II) com plexes con tain in g th e
diim ide ligan d possess m etal-to-ligan d (bpy) ch arge tran sfer (MLCT) ban d
at 458 n m ( for Ru) an d 620 n m (for Os), in addition to ligan d π–π* ban ds.
Th e en ergy of th e MLCT tran sition rem ain s un altered wh en th e bridgin g
ligan d is ch an ged from pbdi to pn di. Th e UV-VIS spectrum of th e
m ixed-m etal RuOs din uclear com plex, [RuII(bpy)2(µ-pn di)OsII(bpy)2]4+, is
essen tially th e sum of th e spectra of th e com pon en ts. Th is com plex sh ows
in traligan d π–π* tran sition s of pn di at 322 an d 378 n m .
Redox Properties of the Complexes
Redox poten tials of th e in vestigated com plexes are collected in Table II. All
th e studied Ru(bpy) com plexes sh ow on ly on e reversible Ru(II)/Ru(III) an -
odic wave at +0.78 ± 0.02 V vs Fc/Fc+, regardless th e diim ide group an d th e
n uclearity of th e com plex. Coulom etry of th e din uclear com plexes revealed
th at th e oxidation in volves two electron s. Th us, th e poten tial differen ce be-
tween th e two un resolved on e-electron steps is th e statistical m in im um of
40 m V (Kc = 4), judgin g from th e peak-to-peak separation in th e cyclic
voltam m ogram by usin g th e sim ulated curve reported by Rich ardson an d
Taube37. Th ese results suggest n egligible in teraction between th e two Ru
cen tres. Th e reduction processes are stron gly depen den t on th e diim ide
group in th e com plexes. Mon on uclear [Ru(bpy)2(pbdi)]2+ sh ows th ree re-
versible on e-electron cath odic waves in CH3CN; th e pyrom ellitim ide group
is reduced first, followed by reduction of each bpy ligan d. On th e oth er
h an d, din uclear [{Ru(bpy)2}2(µ-pbdi)]4+ exh ibits on ly two two-electron cath -
odic waves th at can be assign ed to bpy-localized reduction by com parison
of th e reduction poten tials with th ose for m on on uclear pbdi derivatives.
Hen ce, th e two bpy pairs are reduced in depen den tly in two 1e + 1e steps
due to th e lack of electron ic com m un ication between th e Ru cen tres. Th e
pyrom ellitim ide reduction is n ot observed for th e din uclear com plex sin ce
th e pyrom ellitim ide group is covered by th e m olecular surface of th e
Ru(bpy)2 m oieties, th ereby preven tin g a direct electron tran sfer from th e
cath ode to th e pyrom ellitim ide m oiety. Figure 1 sh ows a cyclic voltam -
m ogram of [{Ru(bpy)2}2(µ-pn di)]4+ in CH3CN at room tem perature. Th e
pn di com plex exh ibits five reduction processes, th e first two bein g
diim ide-localized, an d th e rest bpy-localized. Th e large cath odic peak for
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Ru and Os Dyad and Triad Complexes
319
TABLE I
Absorption an d em ission data for Ru/Os com plexes in CH3CN at room tem perature
Absorption m axim a (ε), n m (M cm –1
)
–1
Em ission
Com plex
m axim um ,
n m
bpy(π–π*)
BL(π–π*)
MLCT(dπ–π*)
Mon on uclear
[Ru(bpy)2(pbdi)]2+
[Ru(bpy)2(m n di)]2+
289 (74 000)
289 (76 900)
–
458 (14 000)
640
a
376 (31 700) 458 (17 100)
355 (32 000)
337 (34 300)
320 (36 100)
a
[Ru(bpy)2(pn di)]2+
[Ru(bpy)2(xn di)]2+
[Os(bpy)2(pbdi)]2+
[Os(bpy)2(pn di)]2+
289 (99 000)
287 (82 000)
292 (88 200)
292 (90 000)
378 (38 500) 457 (18 000)
357 (36 500)
379 (32 900) 459 (15 800)
358 (30 500)
650
a
453 (14 200) 602 (4 000)
487 (15 600)
a
378 (38 000) 626 (4 100)
357 (33 000) 484 (15 000)
Din uclear
[{Ru(bpy)2} (µ-pbdi)]4+
289 (130 000)
289 (159 000)
–
458 (28 200)
642
2
a
[{Ru(bpy)2} (µ-pn di)]4+
378 (45 400) 459 (35 600)
320 (68 500)
2
[{Ru(bpy)2} (µ-xn di)]4+
289 (144 900)
292 (133 000)
288 (138 200)
379 (40 900) 459 (32 800)
320 (60 600)
660
2
[{Os(bpy)2} (µ-pn di)]4+
378 (42 000) 617 (7 000)
321 (51 000) 486 (26 000)
378 (39 900) 464 (31 300)
356 (40 400)
a
2
[{Ru(dm bpy)2} (µ-pn di)]4+
a
2
321 (61 600)
[{Ru(btfm b)2} (µ-pn di)]4+
290 (153 500)
378 (44 200) 478 (25 100)
340 (55 700)
688
2
a
[Ru(bpy)2(µ-pbdi)Os(bpy)2]4+ 291 (128 600)
[Ru(bpy)2(µ-pn di)Os(bpy)2]4+ 290 (147 000)
[Ru(bpy)2(µ-xn di)Os(bpy)2]4+ 290 (152 100)
–
610 (3 458)
459 (25 760)
a
378 (45 000) 620 (3 900)
322 (60 000) 457 (29 500)
379 (47 100) 605 (4 300)
357 (44 000) 458 (31 200)
322 (62 000)
654
a
Not detected at room tem perature.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

320
Hossain et al.:
th e 4th reduction process in Fig. 1, labelled with an asterisk, arises from ad-
sorption of reduction products on to th e electrode surface.
Varyin g th e substituen ts R on th e bpy ligan d in [{RuL′ } (µ-pn di)]4+ (R =
2
2
H, L′ = bpy; R = CH3, L′ = dm bpy; R = CF3, L′ = btm fb), th e oxidation poten -
tial is sh ifted in th e positive direction in th e order of R = H < CH3 < CF3; al-
th ough , th e first reduction poten tial rem ain s alm ost un altered, in dicatin g
th e pn di reduction .
TABLE II
Electroch em ical data for Ru/Os com plexes in CH3CN con tain in g 10–1 M TBAB as a support-
in g electrolyte
Oxidation a
Reduction a
Diim ide0/n– bpy0/–
Com plex
Os(II/III)
Ru(II/III)
Mon on uclear
[Ru(bpy)2(pbdi)]2+
[Ru(bpy)2(m n di)]2+
[Ru(bpy)2(pn di)]2+
[Ru(bpy)2(xn di)]2+
[Os(bpy)2(pbdi)]2+
[Os(bpy)2(pn di)]2+
+0.79
+0.77
+0.78
+0.78
–1.18
–1.79, –1.98
–1.78, –1.98
–1.79, –1.97
–1.74, –1.98
–1.67, –1.95
–1.73, –1.92
–0.95, –1.34
–0.93, –1.34
–0.93, –1.31
–1.18
+0.36
+0.36
–0.94, –1.35
Din uclear
[{Ru(bpy)2} (µ-pbdi)]4+
+0.76
+0.78
+0.78
–1.74, –2.08
–1.76, –1.98
–1.72, –1.99
–1.67, –1.93
–1.84, –2.03
–1.52, –2.04
–1.72, –1.99
–1.73, –1.95
–1.67, –1.97
2
[{Ru(bpy)2} (µ-pn di)]4+
–0.91, –1.30
–0.92, –1.32
–0.91, –1.29
–0.91, –1.30
–0.90, –1.31
2
[{Ru(bpy)2} (µ-xn di)]4+
2
[{Os(bpy)2} (µ-pn di)]4+
+0.36
2
[{Ru(dm bpy)2} (µ-pn di)]4+
+0.67
+1.04
+0.75
+0.79
+0.78
2
[{Ru(btfm b)2} (µ-pn di)]4+
2
[Ru(bpy)2(µ-pbdi)Os(bpy)2]4+
[Ru(bpy)2(µ-pn di)Os(bpy)2]4+
[Ru(bpy)2(µ-xn di)Os(bpy)2]4+
+0.33
+0.37
+0.36
–0.91, –1.28
–0.91, –1.31
a
E1/2 , in V vs Fc/Fc+.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Ru and Os Dyad and Triad Complexes
321
Oxidative spectroelectroch em istry of [{Ru(bpy)2}2(µ-pn di)]4+ at +0.95 V vs
Fc/Fc+ sh ows disappearan ce of th e structured MLCT ban d at 478 n m an d
appearan ce of th e bpy π–π* in traligan d ban d at 317 n m , wh ich in dicates
th e form ation of th e Ru(III) state (Fig. 2a). Sim ilar spectral ch an ge h as also
been observed durin g th e oxidation of [{Ru(bpy)2}2(µ-pbdi)]4+. On th e oth er
h an d, th e reductive spectroelectroch em istry at –1.1 V vs Fc/Fc+ results in
appearan ce of n ew ban ds at 608, 682 an d 756 n m th at agree with th e re-
ported UV-VIS absorption data of th e pn di radical an ion 15 (Fig. 2b). Th is re-
sult stron gly in dicates th at th e first reduction is localized on th e bridgin g
pn di m oiety. Sin ce n o reduction wave for th e bdi m oiety was observed, th e
reductive spectroelectroch em istry could n ot be perform ed in th e latter case.
Cyclic voltam m ogram of [RuII(bpy)2(µ-pn di)OsII(bpy)2]4+ in CH3CN
sh ows two an odic processes at +0.37 an d +0.79 V an d four cath odic pro-
cesses at –0.91, –1.28, –1.73 an d –1.95 V vs Fc/Fc+ (Fig. 3). Com parin g th e
oxidation poten tials with th ose for th e correspon din g Ru an d Os com -
plexes, th e oxidation occurs first at th e Os site, an d th en at th e Ru site. Th e
poten tial differen ce between Ru(II)/Ru(III) an d Os(II)/Os(III) oxidation s is
0.41 V for th e studied din uclear Ru-Os com plex. Th e first two reduction s
occur at th e n aph th alen e diim ide m oiety as sequen tial on e-electron pro-
cesses, followed by th e con secutive bpy-localized on e-electron reduction
steps.
*
5 µA
1.0
0.0
–1.0
E, V vs Fc/Fc
–2.0
+
FIG. 1
Cyclic voltam m ogram of [{Ru(bpy)2} (µ-pn di)]4+ in CH3CN at glassy carbon workin g elec-
2
trode at room tem perature. Th e asterisk in dicates adsorption at th e electrode surface
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

322
Hossain et al.:
Figure 4 sh ows th e oxidative spectroelectroch em istry of [RuII(bpy)2-
(µ-pn di)OsII(bpy)2]4+ in CH3CN. For th e con secutive on e-electron oxida-
tion s at +0.50 an d +0.95 V, th e in ten sities of th e MLCT ban ds at 457 an d
620 n m gradually decrease; th en , at +0.95 V, com plete disappearean ce of
Ru(II)-to-bpy MLCT ban d at 459 n m an d appearan ce of n ew ban d n ear 700
n m origin atin g from bpy-to-Ru(III) LMCT tran sition , were observed. For th e
first reduction at –1.1 V vs Fc/Fc+, n ew ban ds appear at 608, 682 an d 756
n m , in addition to in creased absorban ce at 471 n m , wh ich stron gly in di-
cates th e form ation of n di radical an ion 15 (Fig. 5).
0.9
a
0.0
400
500
600
700
800
λ, nm
0.9
b
0.0
400
500
600
700
800
λ, nm
FIG. 2
UV-VIS spectroelectroch em ical respon se of 1 · 10–4 M [{Ru(bpy)2} (µ-pn di)]4+: a oxidation at
2
+0.95 V, b reduction at –1.1 V vs Fc/Fc+
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Ru and Os Dyad and Triad Complexes
323
General Aspects of Emission Decays and Transient Absorption Spectra
Th e paren t com plexes, [M(bpy)2L] (L = 2-(2-pyridyl)ben zim idazole)) an d
[M(bpy)2(L-L)] (L-L = bis[2-(2-pyridyl)ben zim idazole]), sh ow em ission at
640–750 n m arisin g from M-to-bpy CT state29,33,38 (M = Ru or Os). However,
h alf-lives of em ission of th e studied [M(bpy)2(L-diim ide-L)] com plexes are
in gen eral trem en dously sh ort com pared with th ose for th e paren t
[M(bpy)2(L-L)]2+ (660 n s for M = Ru an d 34 n s for M = Os)39 in gen eral. Th e
sh ort em ission h alf-lifes of th e studied pbdi, xn di or pn di com plexes in di-
cate th at electron tran sfer occurs between th e excited M(bpy)2 m oiety an d
diim ide. As sh own for [{Ru(bpy)2}2(µ-pn di)]4+ an d [{Ru(btfm b)2}2(µ-pn di)]4+
in Fig. 6, th e em ission decays, i(t), fit well with a m ulti-expon en tial decay
m odel below (Eq. (1)),
i(t) = A exp (−t / τ1 ) + A2 exp (t / τ2 ) + A3 exp (−t / τ3 ) , (τ1 < τ2 < τ3)
(1)
1
wh ere Ai an d τi are th e fraction an d th e lifetim e of th e decay com pon en t
(i = 1, 2, 3). Th e fraction s of th e th ird com pon en ts are on ly 3–8%, except
for [Ru(bpy)2(pn di)]2+, if an y. Table III sum m arizes th e determ in ed em ission
lifetim es for th e Ru/Os com plexes un der study. Sin ce several con form -
ation al isom ers are presen t in solution for a propan e-lin ked com poun d, th e
*
5 µA
1.0
0.0
–1.0
–2.0
+
E, V vs Fc/Fc
FIG. 3
Cyclic voltam m ogram of [RuII(bpy)2(µ-pn di)OsII(bpy)2]4+ in CH3CN at glassy carbon workin g
electrode. Th e asterisk in dicates adsorption at th e electrode surface
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

324
Hossain et al.:
m ulti-com pon en t decay of em ission is in terpreted in term s of th e con for-
m ation isom erism in th e propan e ch ain lin kin g. Th e first com pon en ts can
be assign ed to th e con form ers in wh ich th e distan ce between th e electron -
don atin g Ru(bpy)2 an d th e electron -acceptin g diim ide is sufficien tly sh ort
to un dergo th rough -space in teraction . Th e slower-decay com pon en ts are at-
tributable to a com m on an d weak th rough -bon d in teraction between th e
electron -don atin g Ru(bpy)2 an d th e electron -acceptin g diim ide for all con -
form ers. Tran sien t absorption spectra recorded durin g th e picosecon d laser
ph otolysis h ave ascertain ed th at quen ch in g th e em ission bleach ed th e
MLCT ban d. At th e sam e tim e a n ew absorption ban d rised at 715 n m due
1.3
a
0.0
300
400
500
600
700
800
λ, nm
1.3
b
0.0
300
400
500
600
700
800
λ, nm
FIG. 4
Oxidative UV-VIS spectroelectroch em istry of 2 · 10–4 M [RuII(bpy)2(µ-pn di)OsII(bpy)2]4+ at +0.5
an d +0.95 V vs Fc/Fc+ in CH3CN: a first oxidation , b secon d oxidation
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Ru and Os Dyad and Triad Complexes
325
1.3
0.0
400
500
600
700
800
λ, nm
FIG. 5
Reductive UV-VIS spectroelectroch em istry of 2 · 10–4 M [RuII(bpy)2(µ-pn di)OsII(bpy)2]4+ at
–1.1 V vs Fc/Fc+ in CH3CN
a
b
30
0
10
0
–30
–10
3
τ
τ
τ
= 0.064 ns
= 0.35 ns
= 2.5 ns
10
1
2
3
τ
τ
= 26 ns
= 68 ns
1
2
3
10
2
10
1
10
2
10
0
10
0
2
4
0
100
200
Time, ns
Time, ns
FIG. 6
Typical decay curves for [{Ru(bpy)2} (µ-pn di)]4+ (a) an d [{Ru(btfm b)2} (µ-pn di)]4+ (b), both re-
2
2
corded at 630 n m in CH3CN at room tem perature. Excitation wavelen gth 400 n m (a) an d
532 n m (b)
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

326
Hossain et al.:
TABLE III
Em ission lifetim es (CH3CN, room tem perature) calculated by m ulti-expon en tial decay m odel
Em ission lifetim eb (fraction of am plitude)
λm on i-
a
λexcit
Com plex
tored
τ1, n s
τ2, n s
τ3, n s
Mon on uclear
[Ru(bpy)2(pn di)]2+
[Ru(bpy)2(m n di)]2+
[Ru(bpy)2(pn di)]2+
[Os(bpy)2(pn di)]2+
A
A
A
A
630
630
630
730
0.064 (0.55)
0.055 (0.47)
0.082 (0.49)
0.070 (0.62)
0.46 (0.23)
0.48 (0.45)
0.48 (0.37)
0.40 (0.35)
>2.8 (0.2)
4.4 (0.08)
3.0 (0.14)
Din uclear
[{Ru(bpy)2} (µ-pbdi)]4+
A
A
B
A
A
B
630
630
630
750
630
630
0.17 (0.21)
0.064 (0.58)
6.4 (0.92)
3.4 (0.67)
0.35 (0.38)
27 (0.08)
325 (0.12)
2.5 (0.04)
2
[{Ru(bpy)2} (µ-pn di)]4+
2
[{Ru(bpy)2} (µ-xn di)]4+
2
[{Os(bpy)2} (µ-pn di)]4+
0.12 (0.70)
0.060 (0.69)
26 (0.60)
0.47 (0.30)
0.32 (0.28)
68 (0.40)
2.8 (0.03)
4.0 (0.03)
2
[{Ru(dm bpy)2} (µ-pn di)]4+
2
[{Ru(btfm b)2} (µ-pn di)]4+
2
[Ru(bpy)2(µ-pbdi)Os(bpy)2]4+
A
A
630
730
26 (0.94)
32 (1.00)
175 (0.06)
2.4 (0.03)
4.9 (0.04)
[Ru(bpy)2(µ-pn di)Os(bpy)2]4+
A
A
650
750
0.060 (0.49)
0.12 (0.57)
0.33 (0.47)
0.42 (0.43)
[Ru(bpy)2(µ-pn di)Os(bpy)2]5+
[Ru(bpy)2(µ-xn di)Os(bpy)2]5+
[Ru(bpy)2(µ-xn di)Os(bpy)2]4+
A
A
640
630
0.043 (0.67)
64 (0.33)
0.35 (0.30)
197 (0.67)
B
B
620
750
1.8 (0.44)
2.2 (0.95)
6.3 (0.56)
8.3 (0.05)
[Ru(bpy)2(µ-xn di)Os(bpy)2]5+
B
630
1.4 (0.58)
5.3 (0.42)
a
b
Excitation wavelen gth ; A = 400 n m , B = 532 n m . Th e values in paren th eses sh ow th e
fraction of em ission in ten sity.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Ru and Os Dyad and Triad Complexes
327
to th e radical an ion of bdi15 (Fig. 7), or n ew ban ds at 470, 610, 682 an d 756
n m due to th e radical an ion of n di15 (Fig. 8).
A sim ilar triple-expon en tial decay was observed for m on on uclear
[Ru(bpy)2L]2+ (L = N-[3-(2,2′-bipyridin e-5-carboxam ide)propyl]-N′-[3-(di-
m eth ylam in o)propyl]n aph th alen e-1,4:5,8-bis(dicarboxim ide)), th e first
com pon en t of wh ich was proposed to arise from som e in teraction between
th e h ydroph obic groups of an electron -don atin g ligan d, bpy-5-dicarbox-
am ide, an d favourably orien ted an d located electron -acceptor n di20.
Th e studied propan e-lin ked Ru-diim ide com plexes also possess
con form ation al isom ers, as sh own in Sch em e 3. Sin ce th e th rough -bon d in -
teraction is sim ilar for all th e con form ers, th e rate of electron tran sfer m ay
n ot be discrim in ated. A th rough -space exch an ge in teraction , such as
ch arge-tran sfer in teraction between th e diim ide an d Ru(bpy) m oieties,
would in crease th e fraction of th e folded con form ers (e an d f). Mean wh ile,
a th rough -space in teraction could n ot en h an ce th e electron tran sfer rate up
to 1010 s–1 for th e exten ded con form ers (a, b, c an d d). Recen tly, both
in tram olecular an d in term olecular stackin g between an n di an d Ru(bpy)
m oieties was reported for th e bin uclear Ru com poun d with aliph atic side
0.1
–20 ps
0.0
20 ps
–0.1
50 ps
100 ps
1000 ps
3000 ps
6000 ps
400
500
600
700
800
λ, nm
FIG. 7
Picosecon d tran sien t differen ce absorption spectra of [{Ru(bpy)2} (µ-pbdi)]4+ in BuCN, fol-
2
lowin g laser pulse excitation (532 n m ) of 20 ps duration
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

328
Hossain et al.:
Extended form
(a)
Possible conformers on
one of propane linkers
= Ru(bpy)
= diimide
2
(b)
(c)
Folded form
(d)
(e)
(f)
SCHEME 3
0.1
0.0
–20 ps
15 ps
–0.1
50 ps
200 ps
500 ps
1000 ps
2000 ps
400
500
600
700
800
λ, nm
FIG. 8
Picosecon d tran sien t differen ce absorption spectra of [{Ru(bpy)2} (µ-pn di)]4+ in BuCN
2
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Ru and Os Dyad and Triad Complexes
329
1
ch ain -lin ker, revealed by H NMR spectroscopy19. In order to exam in e th e
possibility of in tram olecular in teraction between arom atic rin gs, th e tem -
1
perature-depen den t H NMR spectra of [RuII(bpy)2(µ-pn di)OsII(bpy)2]4+ were
m easured in DMF-d7. Th e 1H NMR sign als of m eth ylen e proton s of th e
im in o group of n di an d ben zim idazole m oiety were observed at δ 4.25 an d
5.23, respectively. Th e form er sign als were broad an d rem ain ed un ch aged
on tem perature variation . On th e oth er h an d, th e latter proton sign als of
th e m eth ylen e groups boun d to 2-(2-pyridyl)ben zim idazole m oiety, exh ibit
tem perature depen den ce. Th e two com pon en ts of th e broad sign al at room
tem perature coalesced in to broad un resolved sign al at –50 °C. Th is result
sh ows th e presen ce of con form ers in solution an d th e dyn am ic m ovem en t
of th e propan e side arm in addition to th e restricted rotation about th e N–C
bon d of n aph th alen ediim ide.
Photoinduced Electron-Transfer and Back Electron-Transfer Processes
in Dyad Complexes
[{Ru(bpy)2} (µ-pbdi)]4+
2
From th e viewpoin t of drivin g force, th e ph otoin duced electron tran sfer
from th e excited Ru(bpy)2 m oiety to pbdi is feasible (Sch em e 4). Figure 7
sh ows th e picosecon d tran sien t spectra of [{Ru(bpy)2}2(µ-pbdi)]4+ in BuCN,
recorded after laser pulse excitation (532 n m ). Th is absorption ban d is ch ar-
acteristic of th e an ion radical of bdi15, th ough it is som ewh at broader. Th e
appearan ce of th is ban d in dicates th at th e an ion radical of bdi is form ed as
a result of fast ph otoin duced electron tran sfer processes from th e excited
state of th e RuII(bpy) m oiety to bdi. However, th e em ission data reveal th e
3
III
II
( CT) Ru -pbdi-Ru
k
FET
III
–
II
Ru -[pbdi] -Ru
2.05 V
1.97 V
hν
k
BET
0 V
II
II
Ru -pbdi-Ru
SCHEME 4
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

330
Hossain et al.:
presen ce of addition al slower-decay com pon en ts (1.9 an d 79 n s), suggestin g
th e m ultiple con form ation s of th is com plex. Broadn ess of th e absorption
ban d of th e an ion radical of bdi an d m uch sh orter lifetim e (50 ps) com -
pared to th e oth er com pon en ts, suggest th at th e fastest electron tran sfer oc-
curs with in th e con form er in wh ich th e bdi an d RuII(bpy) m oieties are in
close con tact. Th us, th e bridgin g bdi an ion radical was form ed after
ph otoirradiation as a result of ph otoin duced electron tran sfer form RuII to
bdi (Fig. 7).
Th e propan e-lin ker well establish ed th at several con form ation al isom ers
are presen t in solution 40. Som e of th e possible con form ers are sh own in
Sch em e 3. In th e studied [{Ru(bpy)2}2(µ-pbdi)]4+ com plex, th e two Ru(II)
cen tres repel each oth er an d th e m olar excess of th e expan ded con form ers
(a), (b), or (c) in Sch em e 3 m ay be large. Furth er, th e distan ce between th e
Ru-bpy cen tre an d th e diim ide m oiety in th e con form er (b) is alm ost th e
sam e as th at in th e con form er (c). As a result, th e ph otoin duced electron
tran sfer is govern ed by th ese two m ajor con form ers, as revealed by two
kin etically differen t decay curves.
[{Ru(bpy)2} (µ-pn di)]4+
2
Th e oxidative quen ch in g of th e excited Ru(bpy) m oiety in [Ru(bpy)2L] is
feasible for th is com plex, with th e Gibbs en ergy differen ce of –0.37 eV. Th e
picosecon d tim e-resolved absorption spectrum of [{Ru(bpy)2}2(µ-pn di)]4+ in
BuCN, recorded 15 ps after excitation , exh ibits th e bleach in g of th e MLCT
ban d at 460 n m an d th e rise in absorption aroun d 400 an d 550 n m due to
th e radical an ion of th e bpy ligan d, poin tin g to th e form ation of an 3MLCT
excited state of th e Ru(bpy)22 + m oiety. In 200 ps, several absorption peaks
appeared at 470, 610, 682 an d 756 n m , wh ich are ch aracteristics of th e an -
ion radical of n di15 (Fig. 8). Th ese n ew tran sien t absorption peaks th en dis-
appeared in a sin gle expon en tial decay to reproduce th e groun d state. Th e
tim e profile reveals th at th e rise an d decay tim es of th e absorban ce at 610
n m are 91 an d 480 ps, respectively. Th e electron tran sfer rate con stan t (kET)
is assum ably calculated accordin g to Eq. (2),
1
1
kET
=
−
,
(2)
τ
τm odel
wh ere τ an d τm odel are lifetim es of th e com plex of in terest an d a m odel com -
plex, respectively. Th us, th e rates of th e ph otoin duced an d th e back elec-
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Ru and Os Dyad and Triad Complexes
331
tron tran sfers between th e Ru(bpy)2 m oiety an d bridgin g n di for th is
com plex are determ in ed as 1.6 · 1010 an d 2.9 · 109 s–1, respectively, as deter-
m in ed from th e excited state decay an d th e ch arge separation yield.
Ch an gin g th e substituen t on th e bpy ligan d from R = H to CH3, both for-
ward an d backward electron tran sfers becom e sligh tly faster. However,
[{Ru(btfm b)2}2(µ-pn di)]4+ in BuCN does em it an d th e ph otoin duced electron
tran sfer processes are very slow (kET = 3.8 · 107 s–1 from th e em ission life-
tim e), due to th e localization of th e excited electron alm ost exclusively on
th e btfm b ligan d, com pared with th e existen ce of th e excited electron both
on th e bridgin g ligan d, th e 2-(2-pyridyl)ben zim idazole m oiety, an d th e L′
ligan d in th e [{RuL′ } (µ-pn di)]4+ (L′ = bpy, dm bpy)33 (Sch em e 5).
2
2
3
III
II
( CT) Ru -pbdi-Ru
2.05 V
k
FET
III
–
II
Ru -[pbdi] -Ru
1.68 V
hν
k
BET
0 V
II
II
Ru -pbdi-Ru
SCHEME 5
[{Os(bpy)2} (µ-pn di)]4+
2
In th e [{Os(bpy)2} (µ-pn di)]4+ com plex, th e 3MLCT excited state of th e
2
Os(bpy)22 + m oiety decayed faster th an th at of th e correspon din g
diruth en ium com plex (rate is 2.7 · 1010 s–1), yieldin g th e ch arge separated
state (OsIII-[pn di]–-OsII) th at was proved by th e tran sien t absorption spectra.
Th e triple-expon en tial decay was also observed for th is diosm ium com plex.
Oth er Dyads
Th e m on on uclear com plexes, [M(bpy)2L]2+ (M = Ru or Os; L = pbdi or
pn di), beh ave n early in th e sam e way as th e correspon din g h om odin uclear
com plexes [{M(bpy)2} (µ-L)]4+. Th e lifetim e values an d electron -tran sfer
2
rates are collected in Table IV.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

332
Hossain et al.:
Photoinduced Electron-Transfer and Back Electron-Transfer Processes
in Triad Complexes
[RuII(bpy)2(µ-pn di)OsII(bpy)2]4+
Wh en th e RuII-pn di-OsII com plex is excited with 400-n m ligh t, two em is-
sion m axim a are clearly observed aroun d 650 an d 750 n m . Th e em ission is
assign ed to 3MLCT excited states of Ru(bpy)22 + an d Os(bpy)22 + m oieties, re-
spectively. Th e in ten sity of th e em ission from Ru(bpy)22 + decreases faster
TABLE IV
Rate con stan ts of ph otoin duced electron tran sfer reaction s in m on on uclear an d din uclear
com plexes, determ in ed from picosecon d tran sien t absorption spectra in CH3CN
FET rate
d
f
Con di-
tion sb
–∆GFET
eV
Back ET ratee
( 109 s–1
–∆GBET
eV
Com plex
from *Mc
)
( 109 s–1
)
Mon on uclear
[Ru(bpy)2(pbdi)]2+
[Ru(bpy)2(pn di)]2+
[Ru(bpy)2(m n di)]2+
[Os(bpy)2(pn di)]2+
A
A
A
A
≈30e, 16
0.08
0.34
0.33
0.39
≈5
3.0
1.97
1.71
1.72
1.30
12
18c
28e, 14
–
8, 2.8
Din uclear
[{Ru(bpy)2} (µ-pbdi)]4+
[Ru(bpy)2(µ-pbdi)Os(bpy)2]5+
A
B
≈18e, 5.9
0.08
0.08
≈2
1.97
0.42
2
0.83
–
A
B
16
0.37
0.37
2.9
3.2
1.69
1.69
[{Ru(bpy)2} (µ-pn di)]4+
2
12e
[{Ru(bpy)2} (µ-xn di)]4+
[Ru(bpy)2(µ-pn di)Os(bpy)2]5+
[Ru(bpy)2(µ-xn di)Os(bpy)2]4+
A
B
A
A
0.16
23 (Ru)
0.55c
23
0.37
0.37
0.37
0.42
(n ot detected)
0.0091g
–
1.70
0.42
1.69
1.27
2
[{Os(bpy)2} (µ-pn di)]4+
10, 2.8
2
a
b
n di = n aph th alen e-1,4:5,8-bis(dicarboxim ide) an d bdi = pyrom ellitdiim ide.
A = aceto-
n itrile on ly, B = aceton itrile with 10–1 M TBAClO4 as supportin g electrolyte. Gibbs en ergy
ch an ge for th e forward electron tran sfer.
c
d
Gibbs en ergy ch an ge for th e back electron tran s-
e
f
fer. Calculated from th e first com pon en t of em ission lifetim es, τ1 in Table III . From th e
tran sien t absorption decay curve. Dyn am ic m ovem en t in th e com plexes reveals two differen t
decays for [Os(bpy)2(pn di)]2+ an d [{Os(bpy)2} (µ-pn di)]4+ due to th e presen ce of differen t
2
g
con fom ers. Back ET rate for Ru(III)-Os(II)→Ru(II)-Os(III).
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Ru and Os Dyad and Triad Complexes
333
th an th at from th e excited Os(bpy)22 + m oiety. Two quen ch in g m ech an ism s
m ay operate, i.e., en ergy tran sfer an d electron tran sfer. In th e studied
Ru-pn di-Os com plex, th e em ission lifetim e of th e Ru(bpy)2 m oiety is th e
sam e as th at of [{Ru(bpy)2}2(µ-pn di)]4+. In addition , n ew absorption peaks at
470, 610, 682 an d 756 n m due to th e pn di radical an ion arise in th e tran -
sien t absorption spectra after excitation , in dicatin g th at th e en ergy tran sfer
from excited *Ru(bpy)2 to Os(bpy)2 is n ot th e m ajor path way for th e
quen ch in g. Th erefore, th e electron -tran sfer quen ch in g m ech an ism is dom i-
n an t for th is h eterodin uclear com plex, th ough m an y exam ples h ave been
reported for th e en ergy tran sfer m ech an ism in th e Ru(bpy)2-lin ker-Os(bpy)2
type com plexes8,41–43. However, th e quan titative an alysis of th e electron
tran sfer even t is too com plex to determ in e th e correspon din g kin etic data
in th e system un der study.
[RuII(bpy)2(µ-pn di)OsIII(bpy)2]5+
Th e m ixed-valen t RuII-pn di-OsIII com plex was selectively produced by
quan titative oxidative electrolysis of [RuII(bpy)2(µ-pn di)OsII(bpy)2]4+ at
+0.5 V in aceton itrile solution con tain in g 1 · 10–1 M TBAClO4 (ref.24). Th e
tran sien t absorption spectrum of th e m ixed-valen t com plex, [RuII(bpy)2-
(µ-pn di)OsIII(bpy)2]5+, reveals th e form ation of th e RuII-to-bpy CT excited
state directly after th e excitation , as sh own in Fig. 9. A tin y peak arose at
608 n m in 50 ps, followed by th e appearan ce of absorption ban ds at 500
an d 650 n m due to th e OsII-to-bpy CT excited state. Th e weak absorption at
608 n m , appearin g on ly at th e earliest stage, in dicates form ation of th e an -
ion radical of n di. Th e fast electron tran sfer from th is radical an ion to th e
Os(III) site produces a ch arge-separated state, RuIII-pn di-OsII, wh ich is th e
redox isom er state of th e startin g m ixed-valen t com plex. Th e back electron
tran sfer from th e [n di]– an ion to th e Ru(III) site would reduce th e popula-
tion of th e ch arge-separated state, estim ated to reach 75% on groun ds of
th e tran sien t absorption at 650 n m , usin g m olar absorption coefficien ts of
–1
th e m odel com plexes [*Ru(bpy)2L]2+ (∆ε = –7 800 M cm –1 at 460 n m ),
–1
–1
[Ru(bpy)2(L)]3+ (2 000 M cm –1 at 650 n m ), [Os(bpy)2(L)]2+ (3 500 M cm –1
at 650 n m ) an d [Os(bpy)2(L)]3+ (780 M cm –1 at 650 n m ), wh ere L =
–1
1-m eth yl-2-(2-pyridyl)ben zim idazole)44. Con siderin g th e sim ilarity between
th e decay rate of th e (3CT)Ru excited state of RuII-pn di-OsIII for th e com plex
[RuII(bpy)2(µ-pn di)OsIII(bpy)2]5+ an d th at for th e h om odin uclear Ru com -
plex, [{Ru(bpy)2}2(µ-pn di)]4+, th e electron tran sfer occurs first from Ru(II) to
n di, followed by rapid electron tran sfer from th e radical an ion of n di to
Os(III) (Sch em e 6). Th e direct ph otoin duced electron -tran sfer process from
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

334
Hossain et al.:
(3CT)Ru to Os(III) is m uch slower th an th is stepwise electron tran sfer be-
cause th e ch arge-separated state was n ot appreciably observed in
RuII-pbdi-OsIII com plex with in 100 ps. Th e rate of back electron tran sfer
from Os(II) to Ru(III) is calculated as 9.1 · 106 s–1 (τ = 110 n s) (Table IV).
Th is slow ch arge recom bin ation rate im plies th at th e propan e lin ker keeps
3
III
III
( CT) Ru -pbdi-Os
2.05 V
k
FET
III
–
III
Ru -[pbdi] -Os
1.68 V
hν
III
–
II
Ru -[pbdi] -Os
6
–1
k
= 9.1·10 s
BET
0 V
II
III
Ru -pbdi-Os
SCHEME 6
0.1
0.0
–20 ps
15 ps
–0.1
100 ps
500 ps
1000 ps
6000 ps
400
500
600
700
800
λ, nm
FIG. 9
Tran sien t differen ce absorption spectra of th e m ixed-valen ce com plex [RuII(bpy)2(µ-pn di)-
OsIII(bpy)2]5+ , gen erated in an electroch em ical flow cell in CH3CN (3 · 10–4 M with 1 · 10–1
TBABF4 as supportin g electrolyte) at room tem perature
M
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Ru and Os Dyad and Triad Complexes
335
th e Ru(III) an d Os(II) m oieties apart from each oth er. Th e Ru–Os distan ce is
estim ated as ≈21 Å from th e optim ized m olecular geom etry.
Dependence of Photoinduced Electron-Transfer Processes on Linker Groups
[RuII(bpy)2(µ-xn di)OsIII(bpy)2]5+
In order to elucidate th e distan ce an d con form ation al depen den ce in
Ru/Os-diim ide com plexes, th e lin ker group between M(bpy)2L an d diim ide
was ch an ged from propan e to p-xylen e (see Sch em e 1; th e n di an d bdi
bridges are n ow abbreviated as xn di an d xbdi, respectively). Due to th is
ch an ge, th e rate of electron tran sfer from Ru(bpy)2 to xn di becom es slow.
From th e viewpoin t of th e Gibbs en ergy differen ces, th e values are th e
sam e. From th e m olecular m odelin g, th e cen tre-to-cen tre distan ce between
th e Ru m oiety an d n di in th e exten ded con form er is ≈10 Å for th e propan e
lin ker (pn di) an d ≈17 Å for th e p-xylen e on e (xn di). Th e distan ce between
Ru an d n di is th e m ost im portan t factor for th e ph otoin duced electron
tran sfer in th e presen t Ru-diim ide system . Furth erm ore, th e con form ation al
rigidity is h igh er for th e p-xylen e lin ker, even th ough several con form ers
can be con sidered.
Conclusion
In con clusion , we h ave prepared an d ch aracterized a series of m on o- an d
din uclear Ru/Os com plexes con tain in g a bridgin g ligan d, L-diim ide-L (L =
2-(2-pyridyl)ben zim idazole, diim ide = pyrom ellitim ide (bdi) or n aph th a-
len e-1,4:5,8-bis(dicarboxim ide) (n di)) with differen t lin kers. Th e
ph otoin duced electron tran sfer from th e M(bpy)2 m oiety (M = Ru or Os) to
th e diim ide site takes place, in volvin g several path ways with in th e
con form ation al isom ers in solution . Th e electron -tran sfer rates decrease in
th e order of pn di > pbdi ≈ xn di > xbdi wh en ch an gin g th e lin ker an d
diim ide. Th e din uclear com plexes prefer an exten ded con form ation because
of coulom bic repulsion between th e cation ic M(bpy)2 groups. On th e oth er
h an d, th e ch arge tran sfer in teraction between M(bpy)2 an d diim ide is possi-
ble, leadin g to a folded con form ation 19,45. Th ese two con form ers are in -
volved in th e electron tran sfer even t in th e din uclear dyad an d triad
com plexes un der study, as th e rotation about th e C–C bon d is slower com -
pared with th e lifetim e of th e ch arge-separated state in th e studied system .
However, con trollin g dyn am ic m ovem en t between th e folded an d ex-
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

336
Hossain et al.:
ten ded con form ation s th rough th e lin ker part will becom e a target towards
th e ligh t-driven m olecular actuator in th e future.
M. H. gratefully acknowledges financial support from the Ministry of Education, Science, Sports and
Culture by a “Grant-in-Aid for Scientific Research” (No. 12440188), a “Grant-in-Aid for Scientific Re-
search on Priority Areas of Metal-assembled Complexes and Creation of Delocalized Electronic Sys-
tems” (No. 10146103), and also support from the Promotion and Mutual Aid Corporation for Private
Schools of Japan. M. D. H. also acknowledges the Ministry of Education for a Japanese Government
(Monbusho) Scholarship.
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