Preparation, spectroscopic, and electrochemical properties of two 4,5-diazafluorene-containing ruthenium(II) complexes: Bridging ligands composed of two kinds of nonequivalent coordinating sites

Article abstract of DOI:10.1080/15533174.2013.809750

Two tetrapodal ligands L¹ and L² containing 4,5-diazafluorene fragments have been prepared and characterized. Both ligands are composed of two kinds of nonequivalent coordinating sites. Ligand L ¹ consists of the 4-(4,5-di

Full text of DOI:10.1080/15533174.2013.809750

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Preparation, Spectroscopic, and Electrochemical
Properties of Two 4,5-Diazafluorene-Containing
Ruthenium(II) Complexes: Bridging Ligands Composed
of Two Kinds of Nonequivalent Coordinating Sites
Feixiang Chenga, Chixian Hea, Hongju Yina & Ning Tangb
a College of Chemistry and Chemical Engineering, Qujing Normal University, Qujing, P. R.
China
b College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, P. R. China
Accepted author version posted online: 21 Jan 2014.Published online: 02 Jun 2014.
To cite this article: Feixiang Cheng, Chixian He, Hongju Yin & Ning Tang (2014) Preparation, Spectroscopic, and
Electrochemical Properties of Two 4,5-Diazafluorene-Containing Ruthenium(II) Complexes: Bridging Ligands Composed of Two
Kinds of Nonequivalent Coordinating Sites, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry,
44:10, 1443-1449, DOI: 10.1080/15533174.2013.809750
To link to this article: http://dx.doi.org/10.1080/15533174.2013.809750
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:1443–1449, 2014
Copyright ꢀ Taylor & Francis Group, LLC
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ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2013.809750
Preparation, Spectroscopic, and Electrochemical Properties
of Two 4,5-Diazafluorene-Containing Ruthenium(II)
Complexes: Bridging Ligands Composed of Two Kinds
of Nonequivalent Coordinating Sites
Feixiang Cheng,1 Chixian He,1 Hongju Yin,1 and Ning Tang2
1College of Chemistry and Chemical Engineering, Qujing Normal University, Qujing, P. R. China
2College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, P. R. China
of oligonuclear Ru(II) polypyridyl complexes are the bridg-
Two tetrapodal ligands L1 and L2 containing 4,5-diazafluorene
fragments have been prepared and characterized. Both ligands
are composed of two kinds of nonequivalent coordinating sites.
Ligand L1 consists of the 4-(4,5-diazafluoren-9-ylimino)phenoxy
and 4,5-diazafluoren-9-yliminoxy moieties, ligand L2 involves
the 2-(4,5-diazafluoren-9-ylimino)phenoxy and 4,5-diazafluoren-9-
yliminoxy moieties. The Ru(II) complexes [(bpy)8Ru4(L1)](PF6)8
and [(bpy)8Ru4(L2)](PF6)8 (bpy = 2,2ꢁ-bipyridine) have been syn-
thesized by refluxing Ru(bpy)2Cl2·2H2O and each ligand in 2-
methoxyethanol. Both complexes exhibit metal-to-ligand charge
transfer absorptions at around 443 nm, and emission at around
577 nm. Electrochemical studies of both complexes display one
Ru(II)-centered oxidation at around 1.32 V and four ligand-
centered reductions.
ing ligands, since the interactions between the bridged units,
and thereby the ground and excited state properties of polynu-
clear complexes, are strongly dependent on the size, shape,
and electronic nature of the bridging ligands. Tuning the size,
shape, and electronic properties of the bridging ligands can
induce desirable changes in the electrochemical and spectro-
scopic properties of Ru(II) polypyridyl complexes.[9–13] Thus,
synthesizing appropriate bridging ligands is the most important
step in realizing molecular devices based on oligonuclear Ru(II)
complexes. A large number of bridging ligands have been pre-
pared in order to assemble Ru(II) polypyridine building blocks
over the past decade. However, the vast majority of such stud-
ies have focused on systems containing symmetric bridging
ligands. The study of polynuclear Ru(II) complexes, bridged
with ligands containing two kinds of nonequivalent coordinat-
ing sites, has attracted less attention.[14–16] Borgstrom et al.[17]
reported a novel bichromophoric system ([(bpy)2Ru(bpy-Ph-
tpy)Ru(Metpy-PI)][PF6]3, where ligand bpy-Ph-tpy consists of
a 2,2ꢀ-bipyridine unit and a 2,2ꢀ:6’,2”-terpyridine unit. Excita-
tion of the (bpy)2Ru(bpy-Ph-tpy) unit results in an initial energy
transfer to the (bpy-Ph-tpy)Ru(Metpy-PI) unit. Subsequent elec-
tron transfer to the PI acceptor results in the formation of the
charge separation state. Chao et al.[18] reported a tetranuclear
Ru(II) polypyridine complex based on the asymmetric bridg-
ing ligand pdtp. The bridging ligand directs center-to-periphery
energy transfer occurring in the dendritic tetranuclear complex.
Toward the aim of preparing novel oligonuclear Ru(II) com-
plexes with interesting spectroscopic and electrochemical prop-
erties, herein, we describe the synthesis and characterization of
two tetrapodal ligands incorporating two kinds of nonequivalent
chelating sites. Ligand L1 consists of the 4-(4,5-diazafluoren-
9-ylimino)phenoxy and 4,5-diazafluoren-9-yliminoxy moieties,
ligand L2 involves the 2-(4,5-diazafluoren-9-ylimino)phenoxy
and 4,5-diazafluoren-9-yliminoxy moieties. The electronic ab-
sorption, emission, and electrochemical properties of both com-
plexes are also presented and discussed.
Keywords asymmetric tetrapodal ligand, electrochemistry, photo-
physics, Ru(II) complex
INTRODUCTION
Ru(II) polypyridyl complexes have attracted much interest
as artificial photosynthesis, molecular recognition, DNA in-
tercalation, pH switching and so on because of their unique
combination of chemical stability, redox properties, reactivity,
and emission.[1–5] Oligometallic complexes incorporating Ru(II)
polypyridyl fragment have received special attention in recent
years in connection with the development of artificial multi-
component systems for photoinduced electron or energy trans-
fer and other related photonic devices.[6–8] Key components
Received 17 April 2013; accepted 27 May 2013.
Address correspondence to Feixiang Cheng, College of Chemistry
and Chemical Engineering, Qujing Normal University, Qujing 655011,
P. R. China.. E-mail: chengfx2010@163.com
Color versions of one or more of the figures in the article can be
found online at www.tandfonline.com/lsrt.
1443

1444
F. CHENG ET AL.
(d, J = 4.8 Hz, 2H). ESI-MS: m/z 955.4 (M + H)+. Found: C,
EXPERIMENTAL
66.5; H, 4.3; N, 8.6. Anal. Calcd. for C53H42N6O8S2: C, 66.7;
H, 4.4; N, 8.8.
Materials and Physical Measurements
2,2ꢀ-Bipyridine, 1,10-phenanthroline, 4-aminophenol, 2-
aminophenol, p-toluenesulfonyl chloride, pentaerythritol, tetra-
butylammonium perchlorate (TBAP), ethyl acetate, 2-
methoxyethanol, NH2OH·HCl, RuCl3·3H2O, NH4PF6, K2CO3,
CH3CN, CH2Cl2, EtOH, MeOH, and DMF were purchased from
the Tianjin Chemical Reagent Factory. Solvents and raw mate-
rials were of analytical grade and used as received, apart from
CH3CN, which was filtered over activated alumina and dis-
tilled from P2O5 immediately prior to use. 4,5-Diazafluoren-9-
one,[19] 9-(4-hydroxy)phenylimino-4,5-diazafluorene,[20] 9-(2-
hydroxy)phenylimino-4,5-diazafluorene,[20] 4,5-diazaflu-oren-
9-oxime,[20] pentaerythrityl tetratosylate,[21] and Ru(bpy)2
Cl2·2H2O[22] were prepared according to literature procedures.
1H NMR spectra were obtained on a Mercury Plus 300
spectrometer and a Mercury Plus 400 spectrometer using
TMS as internal standard. ESI-MS spectra were obtained
on a Bruker Daltonics Esquire 6000 mass spectrometer. El-
emental analyses were obtained using a Perkin-Elmer 240C
analytical instrument. Absorption spectra were obtained on
a Varian Cary-100 UV-Visible spectrophotometer and emis-
sion spectra with a Hitachi F-4600 spectrophotometer. Emis-
1,1ꢀ-Di[2-(4,5-diazafluoren-9-ylimino)phenoxymethyl]-1ꢀꢀ,1ꢀꢀꢀ
-di(p-tosyloxymethyl)-methane (compound 2)
Compound 2 was prepared by the same procedure as that
described for compound 1, except 9-(2-hydroxy)phenylimino-
4,5-diazafluorene (533 mg, 1.95 mmol) was used instead of
9-(4-hydroxy)phenylimino-4,5-diazafluorene to react with pen-
taerythrityl tetratosylate (712 mg, 0.95 mmol). Yield: 192 mg
(21.3%) of a red solid. 1H NMR (400 MHz, CDCl3): δ = 2.35
(s, 6H), 3.63 (s, 4H), 3.73 (s, 4H), 6.50 (d, J = 7.6 Hz, 2H),
6.85 (dd, J = 7.6, 1.6 Hz, 2H), 6.89–6.94 (m, 4H), 7.02 (td,
J = 7.6, 1.2 Hz, 2H), 7.09 (td, J = 8.0, 1.2 Hz, 2H), 7.14 (d,
J = 8.4 Hz, 4H), 7.36–7.40 (m, 6H), 8.16 (dd, J = 7.6, 1.2 Hz,
2H), 8.59 (dd, J = 4.4, 2.0 Hz, 2H), 8.81 (dd, J = 4.4, 2.0 Hz,
2H). ESI-MS: m/z 955.5 (M + H)+. Found: C, 66.9; H, 4.5; N,
8.6. Anal. Calcd. for C53H42N6O8S2: C, 66.7; H, 4.4; N, 8.8.
1,1ꢀ-Di[4-(4,5-diazafluoren-9-ylimino)phenoxymethyl]-1ꢀꢀ,1ꢀꢀꢀ
-di[(4,5-diazafluoren-9-yliminoxy)methyl]-methane (L1)
A mixture of compound 1 (507 mg, 0.53 mmol), 4,5-
diazafluoren-9-oxime (429 mg, 2.18 mmol), and K2CO3
(416 mg, 3.01 mmol) in DMF (50 mL) was heated to 90◦C
for 72 h under nitrogen atmosphere. The solution was poured
into 500 mL of water after cooling down to room temperature,
and a red precipitate that formed was collected by filtration. The
crude product was purified twice by column chromatography on
silica, being eluted with CH2Cl2-EtOH (10:1, v/v) to afford the
desired product as a red solid. Yield: 117 mg (21.9%). 1H NMR
(400 MHz, CDCl3): δ = 4.48 (s, 4H), 5.05 (s, 4H), 6.93 (dd, J =
7.6, 4.8 Hz, 2H), 7.02 (d, J = 8.8 Hz, 4H), 7.07–7.13 (m, 6H),
7.20 (dd, J = 7.6, 4.8 Hz, 2H), 7.25–7.27 (m, 2H), 7.41 (dd, J =
7.6, 4.8 Hz, 2H), 7.91 (dd, J = 7.6, 1.6 Hz, 2H), 8.26 (dd, J =
7.6, 2.0 Hz, 2H), 8.46 (dd, J = 8.0, 1.6 Hz, 2H), 8.65 (dd, J =
4.8, 1.2 Hz, 2H), 8.72 (d, J = 4.4 Hz, 4H), 8.82 (dd, J = 4.8,
1.2 Hz, 2H). ESI-MS: m/z 1005.3 (M + H)+. Found: C, 72.6;
H, 3.9; N, 16.5. Anal. Calcd. for C61H40N12O4: C, 72.9; H, 4.0;
N, 16.7.
2+
sion quantum yields were calculated relative to Ru(bpy)3
(ꢀstd = 0.376) in EtOH-MeOH (4:1, v/v) glassy matrix.[23]
Electrochemical measurements were carried out at room tem-
perature using a CHI 660D electrochemical workstation. Cyclic
voltammetry and differential pulse voltammetry were per-
formed in CH3CN and DMF solutions using a micro cell
equipped with a platinum disk working electrode, a plat-
inum auxiliary electrode, and a saturated potassium chloride
calomel reference electrode with 0.1 mol/L TBAP as support-
ing electrolyte. All samples were purged with nitrogen prior to
measurement.
Synthesis
1,1ꢀ-Di[4-(4,5-diazafluoren-9-ylimino)phenoxymethyl]-1ꢀꢀ,1ꢀꢀꢀ
-di(p-tosyloxymethyl)-methane (compound 1)
A mixture of pentaerythrityl tetratosylate (652 mg,
0.87 mmol), 9-(4-hydroxy)phenylimino-4,5-diazafluorene
(486 mg, 1.78 mmol), and K2CO3 (273 mg, 1.98 mmol) in
DMF (20 mL) was heated to 80◦C for 24 h under nitrogen
atmosphere. The solution was poured into 200 mL of water
1,1ꢀ-Di[2-(4,5-diazafluoren-9-ylimino)phenoxymethyl]-1ꢀꢀ,1ꢀꢀꢀ
-di[(4,5-diazafluoren-9-yliminoxy)methyl]-methane (L2)
L2 was prepared by the same procedure as that described for
after cooling down to room temperature, and a red precipitate L1, except compound 2 (602 mg, 0.63 mmol) was used instead of
that formed was collected by filtration. The crude product was compound 1 to react with 4,5-diazafluoren-9-oxime (510 mg,
chromatographed on silica, being eluted first with CH2Cl2-ethyl 2.59 mmol). Yield: 122 mg (19.3%) of a red solid. H NMR
1
acetate (2:1, v/v) to remove impurities, then with CH2Cl2-EtOH (400 MHz, CDCl3): δ = 3.91 (s, 4H), 4.18 (s, 4H), 6.68 (dd,
(25:1, v/v) to afford the desired product as a red solid. Yield: J = 8.0, 1.2 Hz, 2H), 6.72 (dd, J = 8.0, 4.8 Hz, 2H), 6.94 (dd,
1
233 mg (28.2%). H NMR (300 MHz, CDCl3): δ = 2.44 (s, J = 7.6, 1.6 Hz, 2H), 6.98 (dd, J = 7.6, 1.6 Hz, 2H), 7.08–7.14
6H), 4.09 (s, 4H), 4.31 (s, 4H), 6.84 (d, J = 8.7 Hz, 4H), 6.97 (m, 8H), 7.25–7.31 (m, 2H), 7.74 (dd, J = 7.6, 1.6 Hz, 2H),
(d, J = 9.0 Hz, 4H), 7.06 (d, J = 3.0 Hz, 4H), 7.31 (d, J = 8.08 (dd, J = 7.6, 1.6 Hz, 2H), 8.14 (dd, J = 7.6, 1.6 Hz,
8.1 Hz, 4H), 7.41 (dd, J = 7.5, 4.8 Hz, 2H), 7.76 (d, J = 8.4 Hz, 2H), 8.42 (dd, J = 4.8, 1.2 Hz, 2H), 8.66 (dd, J = 5.2, 1.6 Hz,
4H), 8.25 (d, J = 7.5 Hz, 2H), 8.67 (d, J = 3.6 Hz, 2H), 8.81 2H), 8.69 (dd, J = 4.8, 1.6 Hz, 2H), 8.75 (dd, J = 5.2, 1.6 Hz,

Ru(II) COMPLEX CONTAINING 4,5-DIAZAFLUORENE
1445
2H). ESI-MS: m/z 1005.5 (M + H)+ Found: C, 72.7; H, 3.9; N,
react with pentaerythrityl tetratosylate in DMF for 24 h afford-
ing compounds 1 and 2, respectively. Compounds 1 and 2 react
with 4,5-diazafluoren-9-oxime in DMF for 72 h yielding ligands
L1 and L2, respectively. The Ru(II) complexes were prepared by
refluxing Ru(bpy)2Cl2·2H2O with the appropriate ligand in 2-
methoxyethanol solution, and isolated as their PF6− salts. These
compounds were characterized by elemental analyses, ESI-MS,
and 1H NMR spectroscopy.
◦
16.5. Anal. Calcd. for C61H40N12O4: C, 72.9; H, 4.0; N, 16.7.
[(bpy)8Ru4(L1)](PF6)8
A mixture of ligand L1 (67 mg, 0.07 mmol) and
Ru(bpy)2Cl2·2H2O (179 mg, 0.34 mmol) in 2-methoxyethanol
(50 mL) was heated to 120◦C for 12 h under nitrogen to give a
clear deep red solution, then the solvent was evaporated under
reduced pressure. The residue was purified twice by column
chromatography on alumina, being eluted first with CH3CN-
EtOH (8:1, v/v) to remove impurities, then with MeOH to afford
the complex [(bpy)8Ru4(L1)]Cl8. This complex was dissolved
in the minimum amount of water followed by dropwise addition
of saturated aqueous NH4PF6 until no more precipitate formed.
The precipitate was recrystallized from CH3CN-Et2O mixture
(vapor diffusion method) to afford a red solid. Yield: 105 mg
Absorption Spectra
The UV-Vis absorption spectra of both complexes have been
studied in CH3CN solution, at a working concentration of 5 ×
10−6 mol/L. The energy maxima and absorption coefficients are
summarized in Table 1, and the spectra are shown in Figure 1.
Assignments of the absorption bands of the complexes have been
made on the basis of the well-documented optical transitions of
analogous Ru(II) polypyridyl complexes.[24,25] The absorption
spectra of the complexes both show three well-resolved bands.
Those at ca. 286 and 238 nm can be assigned to intraligand π
→ π∗ transitions centered on the 2,2ꢀ-bipyridine. The lowest
energy band at around 443 nm is attributed to MLCT, dπ → π∗
transition, which consists of overlapping dπ(Ru) → π∗(bpy)
and dπ(Ru) → π∗(L) components. The two complexes contain
two different kinds of ligands with different accepting proper-
ties (2,2ꢀ-bipyridine and 4,5-diazafluorene), which results in the
appearance of a non-symmetrical MLCT band. The MLCT ab-
sorption maxima of both complexes are blue-shifted by about
1
(41.2%). H NMR (400 MHz, DMSO-d6): δ = 4.51 (s, 4H),
5.05 (s, 4H), 7.18–7.21 (m, 6H), 7.31 (d, J = 5.2 Hz, 4H),
7.52–7.57 (m, 16H), 7.56 (d, J = 5.6 Hz, 4H), 7.63–7.67 (m,
6H), 7.76 (d, J = 5.6 Hz, 2H), 7.83–7.87 (m, 8H), 8.05–8.08
(m, 6H), 8.15–8.24 (m, 22H), 8.36 (d, J = 7.6 Hz, 4H), 8.49
(d, J = 7.6 Hz, 2H), 8.80–8.86 (m, 16H). ESI-MS: m/z 1763.7
(M – 2PF6)2+, 1127.4 (M – 3PF6)3+, 810.0 (M – 4PF6)4+, 618.5
(M – 5PF6)5+. Found: C, 44.7; H, 2.9; N, 10.5. Anal. Calcd. for
C141H104F48N28O4P8Ru4: C, 44.4; H, 2.8; N, 10.3.
[(bpy)8Ru4(L2)](PF6)8
7 nm compared with that of Ru(bpy)3 ,
2+ [26] suggesting that the
[(bpy)8Ru4(L2)](PF6)8 was prepared by the same proce-
dure as that described for [(bpy)8Ru4(L1)](PF6)8, except L2
(73 mg, 0.07 mmol) was used instead of L1 to react with
Ru(bpy)2Cl2·2H2O (193 mg, 0.37 mmol). Yield: 107 mg
donor properties of ligands L1 and L2 are weaker than that of
2,2ꢀ-bipyridine.
Emission Spectra
1
(38.5%) of a red solid. H NMR (400 MHz, DMSO-d6): δ =
Both Ru(II) complexes are non-emissive in CH3CN solution
at room temperature upon excitation into the MLCT band. The
emission properties of Ru(II) polypyridyl complexes generally
follow the energy gap law.[27,28] The 3MLCT state is reasonably
long-lived and is thought to be deactivated by three processes:
radiative decay, kr, radiationless decay, knr, and thermal pop-
ulation of a higher lying excited state, koexp(-δE/RT). For the
last process, the thermally accessible excited state has been
designated as a ligand field excited state. The energy of the
ligand field state should depend on the ligand field strength.
The emission intensities follow the model shown in Figure 2
originally proposed by Crosby, Meyer, and others.[29–33] The
values of δE for the Ru(II) diimine complexes containing 4,5-
3.96 (s, 4H), 4.14 (s, 4H), 7.42 (d, J = 5.6 Hz, 4H), 7.50–7.56
(m, 16H), 7.59–7.64 (m, 8H), 7.69 (d, J = 4.8 Hz, 2H), 7.76
(d, J = 5.6 Hz, 4H), 7.82–7.89 (m, 10H), 8.08–8.13 (m, 8H),
8.16–8.23 (m, 16H), 8.50(td, J = 7.6, 1.2 Hz, 4H), 8.72–8.76
(m, 6H), 8.81 (dd, J = 5.2, 1.2 Hz, 2H), 8.83–8.88 (m, 16H).
ESI-MS: m/z 1127.6 (M – 3PF6)3+, 810.7 (M – 4PF6)4+, 619.8
(M – 5PF6)5+. Found: C, 44.6; H, 2.9; N, 10.5. Anal. Calcd. for
C141H104F48N28O4P8Ru4: C, 44.4; H, 2.8; N, 10.3.
RESULTS AND DISCUSSION
Synthesis
The outline of the preparation of ligands L1 and L2 and diazafluorene are substantially lower than the corresponding
the corresponding Ru(II) complexes [(bpy)8Ru4L1](PF6)8 and value for Ru(bpy)32+. These results are consistent with ligand
[(bpy)8Ru4L2](PF6)8 is presented in Scheme 1. The starting field theory, because 4,5-diazafluorene derivatives are known to
compounds 9-(4-hydroxy)phenylimino-4,5-diazafluorene, 9-(2- be lower than 2,2ꢀ-bipyridine in the spectrochemical series,[34–36]
hydroxy)phenylimino-4,5-diazafluorene, and 4,5-diazafluoren- hence the ligand field excited state energy will be lowered if 2,2ꢀ-
9-oxime were prepared from 4,5-diazafluoren-9-one according bipyridine ligands are replaced by 4,5-diazafluorene derivatives.
to the literature procedure.[20] Tetrapodal ligands L1 and L2 have Consequently, population of the ligand field state is very effi-
been prepared by two steps, 9-(4-hydroxy)phenylimino-4,5- cient for these complexes and they are essentially nonemissive
diazafluorene and 9-(2-hydroxy)phenylimino-4,5-diazafluorene at room temperature. However, the energy transfer is inhibited

1446
F. CHENG ET AL.
SCH. 1. Synthesis of tetrapodal ligands L1–2 and their Ru(II) complexes.

Ru(II) COMPLEX CONTAINING 4,5-DIAZAFLUORENE
1447
TABLE 1
Photophysical and electrochemical data of Ru(II) polypyridyl complexes
Absorption
Emissiona
E1/2, V (δEp, mV)b
Complex
λmax, nm (104ε, M−1cm−1
)
λmax, nm
ꢀ
Oxidation
Reduction
[(bpy)8Ru4(L1)]8+
[(bpy)8Ru4(L2)]8+
443 (7.57) 286 (32.92) 238 (19.98)
444 (7.21) 286 (33.71) 237 (17.50)
577
576
0.273
0.258
1.32 (63)
1.33 (82)
−0.82 (55) −0.98 irr −1.39 (91) −1.64 (106)
−0.81 (52) −1.01irr −1.40 (86) −1.65 (112)
2+
aThe emission quantum yields are calculated relative to Ru(bpy)3 (ꢀstd = 0.376) in EtOH-MeOH (4:1, v/v) glassy matrix at 77 K, the
b
uncertainty in quantum yields is 15%. Oxidation potentials are recorded in 0.1 mol/L TBAP/CH3CN, reduction potentials are recorded in
0.1 mol/L TBAP/DMF, and potentials are given vs SCE, where scan rate = 200 mV/s and δEp is the difference between the anodic and cathodic
waves.
at 77 K, so both complexes show similar emission spectra to
2+
that of Ru(bpy)3 in EtOH-MeOH (4:1, v/v) glassy matrix at
77 K (Figure 3).[34–36] The complexes (10−5 mol/L) show char-
acteristic emission at around 577 nm and a shoulder at around
620 nm in EtOH-MeOH (4:1, v/v) glassy matrix at 77 K when
excited at 436 nm (Table 1).
Electrochemistry
The electrochemical behaviors of both complexes have been
studied in DMF and CH3CN solutions with 0.1 mol/L TBAP as
supporting electrolyte. The reduction waves of the complexes
are not well behaved in CH3CN solution due to adsorption of the
reduced species onto the surface of the platinum electrode. In
DMF solution, the complexes display clear reduction processes,
but do not exhibit the oxidative waves due to the insufficient an-
odic window of the solvent. Therefore, the oxidation potentials
were recorded in CH3CN solution, and the reduction potentials
were recorded in DMF solution (Table 1).
The complex [(bpy)8Ru4(L1)]8+ exhibits a Ru(II)-centered
reversible oxidation wave at 1.32 V (Figure 4). This potential
is slightly more negative (by about 70 mV) than that of the
parent complex [(bpy)2Ru(dafo)]2+ (dafo = 4,5-diazafluoren-
FIG. 2. Energy state diagram based on the Crosby-Meyer model.
1.8
1.5
1.2
0.9
0.6
0.3
0.0
6000
5000
4000
3000
2000
1000
0
500
550
600
650
700
200
300
400
500
600
Wavelength (nm)
Wavelength (nm)
FIG. 3. Emission spectra of complexes [(bpy)8Ru4(L1)](PF6)8 (black) and
[(bpy)8Ru4(L2)](PF6)8 (red) in EtOH-MeOH (4:1, v/v) glassy matrix at 77 K.
FIG. 1. Absorption spectra of complexes [(bpy)8Ru4(L1)](PF6)8 (black) and
[(bpy)8Ru4(L2)](PF6)8 (red) in CH3CN solution at room temperature.

1448
F. CHENG ET AL.
0.000010
0.000005
0.000000
-0.000005
-0.000010
-0.000015
a
Potential / V
1.6
1.5
1.4
1.3
1.2
0.000012
1.1
1.0
0.9
0.000010
0.000008
0.000006
0.000004
0.000002
0.000000
1.1
1.2
1.3
1.4
1.5
1.6
1.7
Potential / V
0.000014
b
0.00005
0.00004
0.00003
0.00002
0.00001
0.00000
-0.00001
0.000012
0.000010
0.000008
0.000006
0.000004
0.000002
0.000000
d'
c'
b'
a'
d
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
c -1.8
Potential / V
b
a
-0.3
-0.6
-0.9
-1.2
-1.5
-1.8
-2.1
Potential / V
FIG. 4. Cyclic voltammetry and differential pulse voltammetry of complex [(bpy)8Ru4(L1)](PF6)8 (5 × 10−4 mol/L, scan rate = 200 mV/s): (a) oxidation
potential is recorded in 0.1 mol/L TBAP CH3CN solution, (b) reduction potentials are recorded in 0.1 mol/L TBAP DMF solution.
9-one),[34] but slightly more positive (by about 40 mV) than that
broadening in differential pulse voltammetry (Figure 4), which
indicates that the small redox potential difference caused by
these different coordination environments is not resolved by the
electrochemical means.
Electrochemical studies of both complexes display four
ligand-centered reductions. The first reversible reduction
wave of complex [(bpy)8Ru4(L1)]8+ is consistent with the
of Ru(bpy)3 (+1.28 V vs. SCE),[37] which indicates that the
2+
ligand L1 is a stronger π-acceptor than 2,2ꢀ-bipyridine but a
weaker π-acceptor than dafo. Complex [(bpy)8Ru4(L1)]8+ has
four Ru(II) centers; two of one type of coordination environ-
ment, while the other two are different. The complex shows a
single wave in cyclic voltammetry and a single peak without

Ru(II) COMPLEX CONTAINING 4,5-DIAZAFLUORENE
1449
addition of two electrons to the LUMO localized on 4,5-
diazafluoren-9-oxime fragment of ligand L1, giving the
5. Tan, L.F.; Liu, J.; Shen, J.L.; Liu, X.H.; Zeng, L.L.; Jin, L.H. Inorg. Chem.
2012, 51, 4417–4419.
6. Lin, J.L.; Tsai, C.N.; Huang, S.Y.; Endicott, J.F.; Chen, Y.J.; Chen, H.Y.
Inorg. Chem. 2011, 50, 8274–8280.
7. Saita, T.; Nitadori, H.; Inagaki, A.; Akita, M. J. Organomet. Chem. 2009,
694, 3125–3133.
8. Ventura, B.; Barbieri, A.; Barigelletti, F.; Diring, S.; Ziessel, R. Inorg.
Chem. 2010, 49, 8333–8346.
9. Yamamoto, Y.; Tamaki, Y.; Yui, T.; Koike, K.; Ishitani, O. J. Am. Chem.
Soc. 2010, 132, 11743–11752.
10. Fan, S.H.; Wang, K.Z.; Yang, W.C. Eur. J. Inorg. Chem. 2009, 4, 508–518.
11. Wang, C.F.; Zuo, J.L.; Ying, J.W.; Ren, T.; You, X.Z. Inorg. Chem. 2008,
47, 9716–9722.
12. Jurss, J.W.; Concepcion, J.J.; Butler, J.M.; Omberg, K.M.; Baraldo, L.M.;
Thompson, D.G.; Lebeau, E.L.; Hornstein, B.; Schoonover, J.R.; Jude, H.;
Thompson, J.D.; Dattelbaum, D.M.; Rocha, R.C.; Templeton, J.L.; Meyer,
T.J. Inorg. Chem. 2012, 51, 1345–1358.
13. Kim, M.; Kang, C.H.; Hong, S.; Lee, W.Y.; Kim, B.H. Inorg. Chim. Acta
2013, 395, 145–150.
species [(bpy)2Ru(bpy)2RuL2−Ru(bpy)2Ru(bpy)2]6+ [38,39]
The
.
second irreversible reduction at around −0.98 V adds
two electrons to the 9-(4-hydroxy)phenylimino-4,5-
diazafluorene fragment of ligand L1, giving complex [(bpy)2
Ru(bpy)2RuL4−Ru(bpy)2Ru(bpy)2]4+
. The third reduction
with a peak at −1.43 V and a return wave at −1.34 V is
quasireversible. This reduction is located on one of the two
2,2ꢀ-bipyridine ligands on each metallic terminal, adding
electrons to the 2,2ꢀ-bipyridine to give the species [(bpy·−)(bpy)
Ru(bpy)(bpy·−)RuL4−Ru(bpy·−)(bpy)Ru(bpy·−)(bpy)]. Sim-
ilar to the oxidation process, the reductions of the remote
2,2ꢀ-bipyridine appear at the same potential, indicating no inter-
action between the four sites. The fourth reduction at −1.64 V
is quasi-reversible and affords the species [(bpy·−)(bpy·−)
Ru(bpy·−)(bpy·−)RuL4−Ru(bpy·−)(bpy·−)Ru(bpy·−)(bpy·−)]4−
.
14. Shi, S.; Liu, J.; Yao, T.; Geng, X.; Jiang, L.; Yang, Q.; Cheng, L.; Ji, L.
Inorg. Chem. 2008, 47, 2910–2912.
15. Elias, B.; Herman, L.; Moucheron, C.; Mesmaeker, A.K. Inorg. Chem.
2007, 46, 4979–4988.
The electrochemical behavior of complex [(bpy)8Ru4(L2)]8+ is
similar to that of [(bpy)8Ru4(L1)]8+
.
16. Guelfi, M.; Puntoriero, F.; Arrigo, A.; Serroni, S.; Cifelli, M.; Denti, G.
Inorg. Chim. Acta 2013, 398, 19–27.
17. Borgstrom, M.; Ott, S.; Lomoth, R.; Bergquist, J.; Hammarstrom, L.; Jo-
hansson, O. Inorg. Chem. 2006, 45, 4820–4829.
18. Chao, H.; Qiu, Z.R.; Cai, L.R.; Zhang, H.; Li, X.Y.; Wong, K.S.; Ji, L.N.
Inorg. Chem. 2003, 42, 8823–8830.
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3341.
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Inorg. Chem. 2008, 47, 7286–7294.
CONCLUSION
In summary, two 4,5-diazafluorene-containing tetranuclear
Ru(II) polypyridyl complexes incorporating two kinds of
nonequivalent chelating sites have been prepared. The UV-Vis
absorption and emission properties of both complexes are dom-
inated by MLCT transitions and excited states. The emission
properties of both Ru(II) complexes follow the ligand field the-
ory, they exhibit intense emission at around 577 nm originating
from the lowest energy MLCT excited state in EtOH-MeOH
(4:1, v/v) glassy matrix at 77 K. Cyclic voltammetry and dif-
ferential pulse voltammetry of both complexes show one single
Ru(II)-centered oxidation wave without broadening. The photo-
physical and electrochemical properties of both complexes are
25. Bilakhiya, A.K.; Tyagi, B.; Paul, P. Inorg. Chem. 2002, 41, 3830–3842.
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2008, 47, 2–4.
2+
somewhat different to those of Ru(bpy)3 due to the differ-
ent electronic nature of the tetrapodal ligands L1 and L2. Both
complexes have potential applications in the research area of
electron or energy transfer.
¨
28. Abrahamsson, M.; Ja¨ger, M.; Kumar, R.J.; Osterman, T.; Persson, P.;
Becker, H.C.; Johansson, O.; Hammarstro¨m, L. J. Am. Chem. Soc. 2008,
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7037–7042.
FUNDING
The authors are grateful to the National Natural Science
Foundation of China (21261019) and the Yunnan provincial 31. Hipps, K.W.; Crosby, G.A. J. Am. Chem. Soc. 1975, 97, 7042–7048.
32. Meyer, T.J. Pure. Appl. Chem. 1990, 62, 1003–1009.
33. Barigelletti, F.; Belser, P.; A. von Zelewsky, Juris, A.; Balzani, V. J. Phys.
science and technology department (2010ZC148) for financial
support.
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