Novel [Ru(polypyridine)(CO)2Cl2] and [Ru(polypyridine)2(CO)Cl]+-type complexes: Characterizing the effects of introducing azopyridyl ligands by electrochemical, spectroscopic and crystallographic measurements

Article abstract of DOI:10.1016/j.ica.2008.11.023

The reaction of ruthenium carbonyl polymer ([Ru(CO)₂Cl₂]_n) with azopyridyl compounds (2,2′-azobispyridine; apy or 2-phenylazopyridine; pap) generated new complexes, [Ru(azo)(CO)₂Cl₂] (azo = apy, pap).

Full text of DOI:10.1016/j.ica.2008.11.023

Inorganica Chimica Acta 362 (2009) 2581–2588
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Novel [Ru(polypyridine)(CO)2Cl2] and [Ru(polypyridine)2(CO)Cl]+-type complexes:
Characterizing the effects of introducing azopyridyl ligands by electrochemical,
spectroscopic and crystallographic measurements
b
c
a
Dai Oyama a, , Akio Asuma , Takashi Hamada , Tsugiko Takase
*
a Cluster of Science and Technology, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan
b Faculty of Education, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan
c Graduate School of Science and Technology, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan
a r t i c l e i n f o
a b s t r a c t
The reaction of ruthenium carbonyl polymer ([Ru(CO)2Cl2]n) with azopyridyl compounds (2,20-azobis-
pyridine; apy or 2-phenylazopyridine; pap) generated new complexes, [Ru(azo)(CO)2Cl2] (azo = apy,
pap). [Ru(apy)(CO)2Cl2] underwent photodecarbonylation to give a chloro-bridged dimer complex,
whereas the corresponding pap complex ([Ru(pap)(CO)2Cl2]) was not converted to a dimer. The reactions
of the chloro-bridged dimer containing the bpy ligand (bpy = 2,20-bipyridine) with either apy or pap
resulted in the formation of mixed polypyridyl complexes, [Ru(azo)(bpy)(CO)Cl]+. The novel complexes
containing azo ligands were characterized by various spectroscopic measurements including the deter-
mination of X-ray crystallographic structures. Both [Ru(azo)(CO)2Cl2] complexes have two CO groups
in a cis position to each other and two chlorides in a trans position. The azo groups are situated cis to
the CO ligand in [Ru(azo)(bpy)(CO)Cl]+. All complexes have azo N–N bond lengths of 1.26–1.29 Å. The
complexes exhibited azo-based two-electron reduction processes in electrochemical measurements.
The effects of introducing azopyridyl ligands to the ruthenium carbonyl complexes were examined by
ligand-based redox potentials, stretching frequencies and force constants of CO groups and bond param-
eters around Ru–CO moieties.
Article history:
Received 19 September 2008
Received in revised form 15 November 2008
Accepted 27 November 2008
Available online 7 December 2008
Keywords:
Ruthenium
Azopyridyl ligand
Carbonyl complex
Molecular structure
Redox property
Ó 2008 Elsevier B.V. All rights reserved.
*
1. Introduction
a low-lying azo-centered
p
MO [6]. Therefore, the construction of
complex systems containing azopyridyl derivatives as ancillary li-
gands should be very interesting from the viewpoint of redox and
structural chemistries. A variety of metal complexes including
azopyridyl ligands have been reported [6,7]. We have recently re-
ported specific properties of azopyridyl moieties in ruthenium
carbonyl complexes [8,9]. In these reports, we have demonstrated
that azopyridyl ligands can control the redox behavior or steric
configurations of the complexes. Although metal complexes con-
taining azopyridyl ligands demonstrate a variety of interesting
properties compared with typical polypyridines such as bpy or
phen, there have been no systematic reports on the effects of
introduction of azopyridyl ligands to the ruthenium carbonyl
complexes. In this paper, we report the preparation and charac-
terization of mono-bidentate pyridyl complexes ([Ru(apy)
(CO)2Cl2] and [Ru(pap)(CO)2Cl2]; apy = 2,20-azobispyridine, pap =
2-phenylazopyridine) and bis-bidentate pyridyl complexes
([Ru(apy)(bpy)(CO)Cl]+ and [Ru(pap)(bpy)(CO)Cl]+), both contain-
ing one azopyridyl ligand. Comparisons with other analogous
polypyridyl-containing ruthenium complexes based on electro-
chemical, spectroscopic and crystallographic measurements are
also described.
Ruthenium(II) carbonyl complexes with bidentate pyridyl li-
gands are attractive catalysts for the water–gas shift reaction
and for carbon dioxide reduction [1]. Additionally, ruthenium car-
bonyl complexes incorporating bidentate pyridyl ligands have
been investigated as precursors to heteroleptic tris polypyridyl
ruthenium complexes of the type [Ru(L1)(L2)(L3)]2+ [2]. Currently,
2,20-bipyridine (bpy) and 1,10-phenanthroline (phen) and their
derivatives have been predominantly used as polypyridyl ligands
in this research field. While polypyridyl-based redox reactions
play a significant role as electron reservoirs in photo- and electro-
chemical reductions of carbon dioxide [3,4], it should be possible
to use other polypyridyl ligands which possess non-innocent
properties. The characteristics of these new potential complexes
should be explored. A candidate for the new complex is the azo-
pyridyl derivatives. Azopyridyl compounds are photochemically
active [5]. In addition, azopyridyl compounds can undergo one
or two-electron transfer reactions because these compounds have
* Corresponding author. Tel./fax: +81 24 548 8199.
E-mail address: daio@sss.fukushima-u.ac.jp (D. Oyama).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.11.023

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D. Oyama et al. / Inorganica Chimica Acta 362 (2009) 2581–2588
2.2.4. [Ru(apy)(bpy)(CO)Cl]PF6
2. Experimental
2.1. Physical measurements
The complex was prepared by modification of a previously re-
ported method [13]. A mixture of [Ru(bpy)(CO)Cl2]2 (140 mg,
0.39 mmol) and apy (108 mg, 0.58 mmol) was heated under reflux
in 2-methoxyethanol (10 mL) for 2 h. The mixture was evaporated
to dryness under reduced pressure and the remaining solid was
dissolved in water (24 mL). The solution was filtered and added
to an excess of KPF6. The precipitate was collected by filtration
and washed with water and diethyl ether. The product was then
dried in vacuo. The crude product was purified by column chroma-
tography using Al2O3 (eluent: CH3CN). The volume of the solution
was reduced to ca. 5 mL using a rotary evaporator. The black crys-
tals were precipitated by the addition of diethyl ether, collected,
washed with diethyl ether and finally dried in vacuo. The yield
was 80 mg (31%). Anal. Calc. for C21H16N6OClPF6Ru: C, 38.86; H,
2.68; N, 12.78. Found: C, 38.81; H, 2.48; N, 12.93%. ESI-MS
(CH3CN): m/z = 505 (M+), 477 (MꢀCO+). IR (KBr): 2002 cmꢀ1
Elemental analyses were carried out at the Research Center for
Molecular-Scale Nanoscience, Institute for Molecular Science. IR
spectra were obtained on KBr pellets with a JASCO FT-IR 4100 spec-
trometer. Mass spectra were obtained with a Bruker Daltonics
microTOF mass spectrometer equipped with an electrospray ioni-
zation (ESI) interface. UV–Vis spectra were obtained with a JASCO
V-570 UV/Vis/NIR spectrophotometer. NMR spectra were recorded
on a JEOL JMN-AL300 spectrometer operating at 1H and 13C fre-
quencies of 300 and 75.5 MHz, respectively. Chemical shifts were
calibrated against tetramethylsilane (TMS). Cyclic voltammograms
were obtained in a one-compartment cell consisting of a platinum
working electrode, a platinum counterelectrode and an Ag/AgNO3
(0.01 M) reference electrode in CH3CN containing tetra-n-butylam-
monium perchlorate (0.1 M) as a supporting electrolyte and the
complex (1 mM). The data were collected with an ALS/Chi model
620A electrochemical analyzer. All potentials are reported in volts
versus ferrocene/ferrocenium couple (Fc+/Fc) at 25 °C under N2.
(m
C„O). 1H NMR (CD3CN): d 9.50 (d, 1H), 8.83 (bs, 1H), 8.57 (d,
1H), 8.42–8.20 (m, 6H), 8.07–7.89 (m, 5H), 7.52 (bs, 1H) and 7.29
(t, 1H) ppm. 13C NMR (CD3CN): d 196.48 (CO), 156.81–156.43 (mul-
tiple signals), 155.10, 152.78, 141.51, 140.70, 129.61, 128.91,
128.62 and 125.43–125.28 (multiple signals) ppm.
2.2. Preparation of compounds
2.2.5. [Ru(pap)(bpy)(CO)Cl]PF6
The complex was prepared in a similar manner as outlined in
Section 2.2.4 [9]. Recrystallization of the crude product from aceto-
nitrile/diethyl ether gave single crystals of [Ru(pap)(bpy)(CO)Cl]
PF6. The yield was 54 mg (21%). Anal. Calc. for C22H17N5OClPF6Ru:
C, 40.72; H, 2.64; N, 10.79. Found: C, 40.71; H, 2.83; N, 10.89%.
ESI-MS (CH3CN): m/z = 504 (M+), 476 (MꢀCO+). IR (KBr):
All solvents were dehydrated solvents for organic synthesis and
used without further purification. CH3CN for electrochemical
experiments was distilled over CaH2 under N2 just prior to use.
2-phenylazopyridine (pap), 2,20-azobispyridine (apy), [Ru(CO)2
Cl2]n, and [Ru(bpy)(CO)2Cl2] were prepared according to proce-
dures outlined elsewhere [10,11].
2001 cmꢀ1 C„O). 1H NMR (CD3CN): d 9.53 (d, 1H), 8.54 (d,
(m
1H), 8.41–8.29 (m, 5H), 8.09–7.60 (m, 8H), 7.50 (bs, 1H) and 7.40
(t, 1H) ppm. 13C NMR (CD3CN): d 194.96 (CO), 156.62–156.25 (mul-
tiple signals), 154.00, 152.64, 141.50, 140.80, 129.01, 128.85 and
125.33–125.26 (multiple signals) ppm.
2.2.1. [Ru(apy)(CO)2Cl2]
A methanolic solution (10 mL) containing [Ru(CO)2Cl2]n (200
mg, 0.88 mmol) and apy (200 mg, 0.92 mmol) was refluxed for
1 h. The black precipitate was collected by filtration and washed
with methanol and diethyl ether, and then dried in vacuo. The yield
was 200 mg (55%). Anal. Calc. for C12H8N4O2Cl2Ru: C, 34.96; H, 1.96;
N, 13.60. Found: C, 34.83; H, 2.00; N, 13.58%. IR (KBr): 2074,
2.3. X-ray crystallography
2.3.1. [Ru(apy)(CO)2Cl2] ꢁ CH2Cl2 and [Ru(pap)(CO)2Cl2]
2004 cmꢀ1 C„O). 1H NMR (CD2Cl2): d 9.23 (d, 1H), 8.92 (d, 1H),
(m
The single crystals of the complexes were obtained from dichlo-
8.72 (t, 1H), 8.38–8.28 (m, 2H) and 8.05–7.71 (m, 3H) ppm. 13C
NMR (CD2Cl2): d 196.53, 194.53 (each CO), 163.43, 152.09, 149.27,
141.96, 139.51, 131.27, 130.29, 129.56, 116.89 and 114.90 ppm.
romethane/methanol mixtures.
A black crystal of [Ru(apy)
(CO)2Cl2] ꢁ CH2Cl2 with the dimensions 0.2 ꢂ 0.2 ꢂ 0.2 mm was
mounted on a glass fiber. A dark red crystal of [Ru(pap)(CO)2Cl2]
with the dimensions 0.2 ꢂ 0.2 ꢂ 0.2 mm was mounted on a glass
fiber. All data were collected on a Rigaku/MSC Mercury CCD dif-
2.2.2. [Ru(pap)(CO)2Cl2]
A similar reaction between [Ru(CO)2Cl2]n and pap under the
same conditions described above gave rise to [Ru(pap)(CO)2Cl2]
with a 39% (140 mg) yield. Anal. Calc. for C13H9N3O2Cl2Ru: C,
37.97; H, 2.21; N, 10.22. Found: C, 37.82; H, 2.25; N, 10.23%. IR
fractometer with graphite monochromated Mo
Ka radiation
(k = 0.71070 Å) at ꢀ100(1) °C. Data were collected to a maximum
2h value of 55.0°. All calculations were carried out using the TEXSAN
crystallographic software package [14]. The structures were solved
either by the heavy-atom Patterson method [15] for [Ru(apy)
(CO)2Cl2] ꢁ CH2Cl2 or by a direct method [16] for [Ru(pap)(CO)2Cl2],
and were expanded using Fourier techniques. Empirical absorption
corrections were applied using Lorentz polarization (Lp) and
absorption. Structures were refined using full-matrix least-square
techniques. The non-hydrogen atoms were refined anisotropically,
and hydrogen atoms were built in using standard geometry. The
final cycle of full-matrix least-squares refinements was based on
3860 observations (all data) and 217 variable parameters for [Ru
(apy)(CO)2Cl2] ꢁ CH2Cl2, whereas 3109 observations (all data) and
190 variable parameters were used for [Ru(pap)(CO)2Cl2]. Crystal-
lographic parameters are summarized in Table 1 and selected bond
lengths and angles are listed in Table 3.
(KBr): 2074, 2010 cmꢀ1 C„O). 1H NMR (CD2Cl2): d 9.13 (d, 1H),
(m
8.65 (d, 1H), 8.35 (t, 1H), 8.11–8.08 (m, 2H) and 7.83–7.60 (m,
4H) ppm. 13C NMR (CD2Cl2): d 195.54, 194.57 (each CO), 163.32,
155.47, 151.50, 142.32, 134.14, 130.33, 130.01, 129.88, 123.97
and 123.66 ppm.
2.2.3. [Ru(apy)(CO)Cl2]2
The dimer complex was prepared according to the method in
the literature [12]. [Ru(apy)(CO)2Cl2] (60 mg, 0.14 mmol) was dis-
solved in dichloromethane (60 mL) and filtered. The filtrate was
placed in a quartz flask with a stopper and irradiated with a
100 W mercury lamp for 15 h, after which time a dark green pre-
cipitate had formed. The precipitate was collected by filtration,
washed with diethyl ether and dried in vacuo. The yield was
50 mg (45%). Anal. Calc. for C22H16N8O2Cl4Ru2 ꢁ 3CH2Cl2: C, 29.35;
H, 2.17; N, 10.95. Found: C, 29.42; H, 2.15; N, 10.75%. IR (KBr):
2.3.2. [Ru(apy)(bpy)(CO)Cl]PF6
Crystals suitable for X-ray crystallography were prepared by a
week-long diffusion of diethyl ether into an acetonitrile solution
1991 cmꢀ1
(mC„O).

D. Oyama et al. / Inorganica Chimica Acta 362 (2009) 2581–2588
2583
Table 1
Crystallographic data for [Ru(apy)(CO)2Cl2] ꢁ CH2Cl2, [Ru(pap)(CO)2Cl2] and [Ru(apy)(bpy)(CO)Cl]PF6.
Chemical formula
C13H10N4O2Cl4Ru
C13H9N3O2Cl2Ru
C21H16N6OF6ClPRu
Formula weight
Temperature (K)
Crystal system
Space group
Unit cell parameters
a (Å)
497.13
173(1)
triclinic
411.21
173(1)
triclinic
649.88
173(1)
monoclinic
P21/c (No. 14)
ꢀ
ꢀ
P1 (No. 2)
P1 (No. 2)
7.905(3)
9.038(3)
13.604(6)
7.1251(1)
8.9479(2)
12.3328(5)
9.1342(2)
12.9869(3)
20.5545(6)
b (Å)
c (Å)
a
(°)
83.05(1)
86.41(1)
b (°)
87.81(1)
80.39(1)
100.3311(9)
c
(°)
66.382(8)
884.0(6)
70.441(9)
730.48(6)
V (Å3)
2398.75(10)
Z
l
2
2
4
(Mo K
a
) (cmꢀ1
)
15.04
3860
14.44
3109
9.069
5493
Number of reflections measured
Number of observations
Refinement method
3860 (all)
3109 (all)
4589 (I > 2r(I))
full-matrix least-squares on F
full-matrix least-squares on F2
full-matrix least-squares on F2
Parameters
217
0.0296
190
0.0282
350
Ra
Rw
S
0.0415
0.0701b (all data)
1.523
0.0906b (all data)
1.529
0.0652c (I > 2
r(I))
1.003
P
a
R ¼ jjFoj ꢀ jFcjj=
R
jFoj.
P
P
2
2
1=2
Rw ¼ f wðF2o ꢀ Fc2Þ = wðFo2Þ g
.
b
c
P
P
2
1=2
Rw ¼ f wðjFoj ꢀ jFcjÞ = wF2og
.
Table 2
Electrochemical and UV–Vis spectral data for the prepared complexes.
Complexes
[Ru(apy)(CO)2Cl2]
[Ru(pap)(CO)2Cl2]
[Ru(apy)(bpy)(CO)Cl]+
[Ru(pap)(bpy)(CO)Cl]+
E1/2/Va (azo/azoꢀ)
ꢀ0.56
ꢀ1.33
ꢀ0.68
ꢀ1.57
ꢀ0.70
ꢀ1.46
ꢀ1.93
ꢀ0.81
ꢀ1.52
ꢀ1.94
b
(azoꢀ/azo2ꢀ
(bpy/bpyꢀ)
)
kmax/nmc
(e
/Mꢀ1 cmꢀ1
)
513 (1590)
358 (14700)
484 (1670)
354 (13200)
525 (3910)
502 (4820)
348 (14000)
315 (21200)
305 (19400)
262 (sh)
343 (12000)
314 (17900)
303 (16900)
268 (18100)
237 (19300)
236 (sh)
a
In CH3CN, vs. Fc+/Fc.
Epc values.
In CH3CN.
b
c
Table 3
Selected bond lengths (Å) and angles (°) for [Ru(apy)(CO)2Cl2] ꢁ CH2Cl2, [Ru(pap)(CO)2Cl2] and [Ru(apy)(bpy)(CO)Cl]PF6.
[Ru(apy)(CO)2Cl2] ꢁ CH2Cl2
[Ru(pap)(CO)2Cl2]
[Ru(apy)(bpy)(CO)Cl]+
Bond lengths
Ru1–Cl1
Ru1–Cl2
Ru1–C1
Ru1–C2
Ru1–N1
Ru1–N3
C1–O1
2.389(1)
2.394(1)
1.895(2)
1.905(3)
2.108(2)
2.107(2)
1.136(3)
1.097(3)
1.265(3)
Ru1–Cl1
Ru1–Cl2
Ru1–C1
Ru1–C2
Ru1–N1
Ru1–N3
C1–O1
2.3828(6)
2.4008(5)
1.896(3)
1.909(2)
2.099(2)
2.119(2)
1.134(3)
1.098(3)
1.264(3)
Ru1–Cl1
Ru1–C1
Ru1–N1
Ru1–N3
Ru1–N5
Ru1–N6
C1–O1
2.3859(8)
1.889(3)
2.114(2)
1.998(2)
2.096(3)
2.069(2)
1.116(4)
1.291(4)
C2–O2
N2–N3
C2–O2
N2–N3
N2–N3
Bond angles
Ru1–C1–O1
Ru1–C2–O2
N1–Ru1–N3
174.7(2)
177.7(2)
74.79(7)
Ru1–C1–O1
Ru1–C2–O2
N1–Ru1–N3
176.2(2)
175.5(2)
75.01(7)
Ru1–C1–O1
N1–Ru1–N3
N5–Ru1–N6
173.0(3)
75.89(11)
78.53(10)
of the complex. A black crystal of [Ru(apy)(bpy)(CO)Cl]PF6 with the
dimensions 0.4 ꢂ 0.3 ꢂ 0.1 mm was mounted on a glass fiber. All
data were collected on a Rigaku RAXIS-RAPID diffractometer with
All calculations were carried out using the CrystalStructure crystal-
lographic software package [17]. The structure was solved by a di-
rect method [16], and was expanded using Fourier techniques. A
multi-scan absorption correction was applied [18]. The structure
was refined using full-matrix least-square techniques. The non-
graphite monochromated Mo
Ka radiation (k = 0.71075 Å) at
ꢀ100(1) °C. Data were collected to a maximum 2h value of 55.0°.

2584
D. Oyama et al. / Inorganica Chimica Acta 362 (2009) 2581–2588
hydrogen atoms were refined anisotropically, and hydrogen atoms
were built in using standard geometry. The final cycle of full-
matrix least-squares refinements was based on 4589 observations
(I > 2r(I)) and 350 variable parameters. Crystallographic parame-
ters are summarized in Table 1 and selected bond lengths and an-
[Ru(apy)(CO) Cl ]
2
2
[Ru(pap)(CO) Cl ]
2
2
+
+
[Ru(apy)(bpy)(CO)Cl]
[Ru(pap)(bpy)(CO)Cl]
gles are listed in Table 3.
3. Results and discussion
3.1. Synthesis and characterization of carbonyl complexes
with azopyridyl ligands
200
300
400
500
600
700
800
Wavelength / nm
Synthetic routes and some characterization data for the present
complexes are summarized in Scheme 1 and Table 2, respectively.
When a methanolic solution containing [Ru(CO)2Cl2]n and one
equivalent of azopyridyl ligands was refluxed for 1 h, the reaction
proceeded to form [Ru(azo)(CO)2Cl2] (azo = apy, pap). The com-
plexes imparted an intense purple color (Fig. 1), whereas the com-
Fig. 1. UV–Vis spectra of the prepared complexes (c = 1.0 ꢂ 10ꢀ4 M in CH3CN).
bpy
N
Cl
plexes ([Ru(polypyridine)(CO)2Cl2]) exhibited lighter yellow to
O
N
Cl
Cl
C
*
orange colors. The very low energy of the
p azopyridyl MO is
responsible for the observed long-wavelength metal-to-ligand
C
N
O
apy
charge transfer (MLCT) transitions in [Ru(azo)(CO)2Cl2] [19]. In
Cl
N
bpy
*
addition, the intensities of these MLCT (dp ? p ) transitions are
bpy
(phen-type)
N
typically high because of the large LUMO coefficients at the coordi-
nating nitrogen centers [19]. In the IR measurements, two stretch-
ing bands of the terminal CO ligands were observed. This implied
that two CO groups existed at cis positions relative to each other.
Both complexes displayed two carbonyl carbon signals in 13C{1H}
NMR spectra, indicating that the two CO groups exist in magneti-
cally inequivalent environments.
N
N
N
apy
C
O
O
apy
N
C
N
Cl
N
Cl
Cl
Cl
N
bpy
Cl
Photoirradiation of ruthenium(II) carbonyl complexes with UV
or visible light has been known to result in decarbonylation [20].
For example, irradiation of [Ru(polypyridine)(CO)2Cl2] complexes
(polypyridine = bpy, phen, etc.) in poorly coordinating solvents in
a pyrex flask resulted in mono-decarbonylation and the subse-
quent formation of a chloro-bridged dimer within a few hours
[12]. Photodecarbonylation of [Ru(apy)(CO)2Cl2], however, did
not proceed under the same condition. Irradiation of [Ru(apy)
(CO)2Cl2] using a quartz flask led to the solution gradually chang-
ing from a purple to a greenish color. After several hours, a small
amount of dark green powder had precipitated from the solution.
The [Ru(polypyridine)(CO)Cl2]2 dimers have two different struc-
tural isomers [12]: in [Ru(phen)(CO)Cl2]2, the carbonyls are trans
to the bridging Cl and the terminal Cl trans to the pyridyl nitrogens
(‘‘phen-type” in Scheme 2), whereas in [Ru(dpk)(CO)Cl2]2
(dpk = di(2-pyridyl) ketone), the terminal and bridging Cl are
mutually trans, and the carbonyls are trans to the pyridyl nitrogens
(‘‘dpk-type” in Scheme 2). As a result, structural differences in
C
O
apy
(dpk-type)
Scheme 2. Formation of the [Ru(apy)(bpy)(CO)Cl]+ complex via different precursors.
Based on the IR data, the present dimer ([Ru(apy)(CO)Cl2]2), there-
fore, is expected to have the latter structure. By contrast, the pho-
tochemical reaction of [Ru(pap)(CO)2Cl2] did not produce the
corresponding dimer. We may need to reinvestigate some reaction
conditions such as solvents, temperature or the wavelength of irra-
diation to ascertain whether conditions need to be changed so as to
obtain the dimer.
The [Ru(azo)(bpy)(CO)Cl]+ complexes (azo = apy, pap) were
prepared by reacting [Ru(bpy)(CO)Cl2]2 with the corresponding
azopyridyl ligand in 2-methoxyethanol (Scheme 1). The asymmet-
rical nature of the azopyridyl units led to the possible formation of
several isomers of [Ru(azo)(bpy)(CO)Cl]+. IR spectra of the [Ru
[Ru(polypyridine)(CO)Cl2]2 led to the observed variance in
mCO fre-
(azo)(bpy)(CO)Cl]+ complexes showed single strong
mCO bands,
quencies. The lower
m
CO frequencies (around 1945 cmꢀ1) indicated
indicating that only one isomer formed in both complexes.
Although partial decarbonylation occurs during the reaction of
[Ru(phen)(CO)Cl2]2-type configurations, whereas the higher mCO
(around 1985 cmꢀ1) frequencies indicated [Ru(dpk)(CO)Cl2]2-types.
([Ru(pap)(bpy)(CH3CN)Cl]+)
[Ru(pap)(bpy)(CO)Cl]+
hv
[Ru(pap)(CO)2Cl2]
no reaction
pap
pap
bpy
hv
RuCl3
[Ru(CO)2Cl2]n
apy
[Ru(bpy)(CO)2Cl2]
[Ru(bpy)(CO)Cl2]2
apy
bpy
hv
[Ru(apy)(CO)2Cl2]
[Ru(apy)(CO)Cl2]2
[Ru(apy)(bpy)(CO)Cl]+
Scheme 1. Synthetic routes for the prepared complexes.

D. Oyama et al. / Inorganica Chimica Acta 362 (2009) 2581–2588
2585
the [Ru(bpy)(CO)Cl2]2 dimer with pap ([Ru(pap)(bpy)(CH3CN)Cl]+
in Scheme 1) [9], such decarbonylation behavior was not observed
for the apy ligand introduced in the reaction. The distinctions be-
tween the two reactions may result in the pendant moiety of the
azo ligands. Here the non-coordinating pyridyl nitrogen atom in
the apy ligand prevents or reduces decarbonylation of the adjacent
CO ligand by strong donor–acceptor interaction after coordination
to the ruthenium center (see the next section).
[Ru(azo)(CO)2Cl2] complexes are shown in Figs. 3 and 4. Both com-
plexes have two CO groups in a cis position to each other and two
chlorides in a trans position. The azopyridyl ligands are bound to
the ruthenium ion with the pyridine nitrogen and the azo nitrogen
possessing a bite angle of 74.79(7)° in [Ru(apy)(CO)2Cl2] and
75.01(7)° in [Ru(pap)(CO)2Cl2]. As shown in Table 3, the shorter
C–O lengths at trans position of the azo nitrogen (1.097(3) Å in
[Ru(apy)(CO)2Cl2] and 1.098(3) Å in [Ru(pap)(CO)2Cl2]) compared
Reaction of the apy-dimer ([Ru(apy)(CO)Cl2]2) with bpy should
give a different isomer to [Ru(apy)(bpy)(CO)Cl]+ because the order
of the addition of the ligands confers geometric specificity [13].
Nevertheless, the molecular structure obtained by the reaction of
the apy-dimer with bpy clearly indicates that the reaction of [Ru
(apy)(CO)Cl2]2 with bpy and the inverse reaction yield identical
structures (Scheme 2). This result supports the concept that the
structures of dimers ([Ru(bpy)(CO)Cl2]2 and [Ru(apy)(CO)Cl2]2)
are different as mentioned above. UV–Vis spectra of [Ru(azo)
(bpy)(CO)Cl]+ indicated strong MLCT transitions in the visible re-
gion which were similar to that observed for [Ru(azo)(CO)2Cl2]
(Fig. 1 and Table 2). Since general carbonyl ruthenium complexes
with polypyridyl ligands are poor absorbers of visible light, the
with C–O lengths at trans of the pyridyl nitrogen (1.136(3) and
)RuII ?
p
(azo) backbonding
*
1.134(3) Å) are due to strong (d
p
[26]. Noteworthy is that the dihedral angle between the two aro-
matic rings in [Ru(apy)(CO)2Cl2] (7.99°) is approximately parallel
and this contrasts that observed for [Ru(pap)(CO)2Cl2] (35.29°).
As a result, the distance between the non-coordinating nitrogen
atom (N4) and the CO carbon (C1) in [Ru(apy)(CO)2Cl2] is very
short (2.682(3) Å) and this suggests that a significant interaction
may exist between the non-coordinating pyridine and the adjacent
carbonyl group.
The molecular structure of [Ru(apy)(bpy)(CO)Cl]+ is shown in
Fig. 5. The azo group of the apy ligand is situated cis to the CO li-
gand. The Ru1–C1, C1–O1 and Ru1–Cl1 bond lengths, and the
Ru1–C1–O1 bond angle are 1.889(3), 1.116(4), 2.3859(8) Å and
173.0(3)°, respectively (Table 3). These parameters are consistent
presence of azopyridyl ligands in [Ru(azo)(bpy)(CO)Cl]+ caused a
*
p azopyridyl MO which contributed to a shift in
lowering of the
the MLCT transition into the visible region.
Cyclic voltammograms (CV) of the present complexes exhibited
the successive two-electron reductions based on the low lying
LUMO of the azo moieties in Eq. (1) and Fig. 2 [6]. As shown in
Table 2, the apy-complexes ([Ru(apy)(CO)2Cl2] and [Ru(apy)(bpy)
(CO)Cl]+) undergo reductions at a more positive region (ca. 0.1 V)
than the corresponding pap-copmlexes ([Ru(pap)(CO)2Cl2] and
[Ru(pap)(bpy)(CO)Cl]+). These observations suggest that the apy
Cl1
C4
C5
O2
C3
N1
C2
C1
C6
Ru1
C7
N2
ligand is better
p-acceptor than the pap ligand. The irreversible
N3
C8
behavior of the second step indicated the dissociation of a chloro
or a carbonyl ligand accompanied by two-electron reductions
[21,22]. In addition to these reductions, [Ru(azo)(bpy)(CO)Cl]+
showed another quasi-reversible reduction wave at a more nega-
tive region (Table 2). This redox wave is assigned to the bpy-based
reduction [23]. In comparison to [Ru(bpy)2(CO)L]+ (L = anionic li-
gands) [3c,24,25], the reduction potentials show that the replace-
ment of the bpy ligand with an azopyridyl ligand caused a shift
to more positive potentials by 0.8–1.2 V. No oxidation wave caused
by Ru3+/2+ was observed within the potential window of the pres-
ent complexes.
N4
C9
O1
Cl2
C10
C12
C11
Fig. 3. ORTEP drawing of [Ru(apy)(CO)2Cl2] with the atom numbering scheme.
Hydrogen atoms are omitted for clarity. The thermal ellipsoids are shown at the 50%
probability level.
e-
e-
ð1Þ
N
N
N
N
-N N-
Cl1
3.2. Single crystal X-ray analysis
C4
O2
We carried out X-ray analyses to obtain detailed structural
information of the complexes. The molecular structures of the
C3
C5
C2
N1
C6
Ru1
C7
5 x 10-6
A
N3
N2
C13
i
C8
C1
O1
C9
C12
Cl2
C10
C11
-2.0
-1.5
-1.0
+
-0.5
E / V vs. Fc /Fc
Fig. 4. ORTEP drawing of [Ru(pap)(CO)2Cl2] with the atom numbering scheme.
Hydrogen atoms are omitted for clarity. The thermal ellipsoids are shown at the 50%
probability level.
Fig. 2. Cyclic voltammogram of [Ru(pap)(CO)2Cl2] (scan rate = 0.1 V sꢀ1).

2586
D. Oyama et al. / Inorganica Chimica Acta 362 (2009) 2581–2588
gands). The one-electron reduced (anion radical) ligands have a
O1
C1
C11
bond lengths of about 1.35 Å, whereas the two-electron reduced
(hydrazido) forms have single bonds with >1.40 Å lengths. All mol-
ecules in this study have N–N bond distances of 1.26–1.29 Å. This
clearly suggests the presence of the unreduced forms of the azo-
pyridyl ligands. The same results were obtained for other azopyr-
idyl ruthenium(II) complexes [28].
C10
C13
C12
N5
N6
N4
Cl1
C14
C15
Ru1
N1
N3
C9
C17
C16
C7
N2
C21
C8
C18
C19
3.3. Comparison with analogous complexes using spectroscopic
and crystallographic data
C6
C5
C20 C2
C3
Some physical data of terminal carbonyl groups are generally
useful indicators for electronic states around metal centers in me-
tal complexes. In particular, bond lengths and vibrational spectra
are considered important [29]. In addition to these data, we calcu-
lated the stretching force constants of the CO group using IR spec-
tral data. This was used to distinguish between the electronic
states of the azo and other polypyridyl ligands in the complex sys-
tems [30,31]. The stretching force constants of the CO group in the
present and analogous complexes are summarized in Tables 4 and
C4
Fig. 5. ORTEP drawing of [Ru(apy)(bpy)(CO)Cl]+ with the atom numbering scheme.
Hydrogen atoms are omitted for clarity. The thermal ellipsoids are shown at the 50%
probability level.
with those observed in [Ru(pap)(bpy)(CO)Cl]+ [9]. The ruthenium–
azo nitrogen bond length (Ru1–N3) is shorter than the ruthenium–
pyridine nitrogen in the apy ligand of the complex. This agrees
with the corresponding pap complex [9], and can be explained
5 together with the values of
mCO, d(Ru–C) and d(C–O). As shown in
Table 4, Fig. 6a and b, the electron density in the ruthenium center
did not significantly change when the bidentate pyridyl ligand was
exchanged in the mono-pyridyl systems. At least two points should
be considered: (1) the average values of the parameters in the two
CO groups were used, and (2) an equatorial plane of octahedral
configuration has been constituted by the two nitrogen atoms of
the bidentate pyridyl ligands and the two carbonyl carbons includ-
ing the ruthenium atom. Synergistic effects of both (1) and (2)
might reduce the differences of the parameters in the mono-pyri-
by the stronger
p-backbonding effect [27]. The short interatomic
distance between the non-coordinating nitrogen atom (N4) and
the CO carbon (C1) of 2.711(5) Å implies a donor–acceptor interac-
tion between these atoms as mentioned above.
The N–N distance in an azo moiety is an excellent indicator of
the charge on an azo group [6]. The values for unreduced azo N–
N bonds are 1.25–1.30 Å in metal complexes (ca. 1.24 Å for free li-
Table 4
Stretching frequencies, force constants, and bond lengths of the CO groups in cis(CO),trans(Cl)-[Ru(polypiridine)(CO)2Cl2].
Polypyridinea
m
CO (cmꢀ1
)
k (N cmꢀ1
)
d(Ru–C) (Å)
d(C–O) (Å)
Ref.
moppt
4,40-dnbpy
mppt
pap
4,40-dpbpy
tpyr
apy
apt
pynp
bpy
4,40-dbbpy
4,40-dmbpy
mopip
6,60-dmbpy
2077, 2020
2074, 2018
2074, 2012
2074, 2010
2073, 2011
2072, 2011
2074, 2004
2073, 2005
2064, 2012
2067, 2003
2066, 2000
2063, 2001
2066, 1999
2063, 1999
16.96
16.92
16.87
16.85
16.85
16.84
16.80
16.80
16.78
16.74
16.70
16.69
16.69
16.67
1.876(3), 1.888(3)
1.877(4), 1.890(4)
1.862(3), 1.871(3)
1.896(3), 1.909(2)
1.893(9), 1.864(1)
1.875(4), 1.882(4)
1.895(2), 1.905(3)
1.851(6), 1.881(6)
1.901(7), 1.880(6)
1.835(17), 1.817(8)
1.878(4), 1.878(4)
1.879(10), 1.866(12)
1.886(3), 1.873(3)
1.873(6), 1.872(5)
1.127(4), 1.125(3)
1.118(4), 1.122(4)
1.126(4), 1.135(4)
1.134(3), 1.098(3)
1.132(1), 1.148(1)
1.140(5), 1.130(5)
1.136(3), 1.097(3)
1.135(8), 1.131(8)
1.126(9), 1.129(7)
1.133(22), 1.159(11)
1.137(5), 1.137(5)
1.117(11), 1.129(11)
1.132(3), 1.122(3)
1.131(7), 1.138(6)
[33]
[34]
[33]
this work
[34]
[33]
this work
[35]
[36]
[1 h]
[34]
[37]
[33]
[38]
a
moppt = 3-(4-methoxyphenyl)-pyridin-2-yl-1,2,4-triazole; 4,40-dnbpy = 4,40-dinitro-2,20-bipyridine; mppt = 3-(4-methylphenyl)-pyridin-2-yl-1,2,4-triazole; 4,40-dpbpy=
4,40-diphosphonic acid-2,20-bipyridine; tpyr = 2-(4H-[1,2,4]triazol-30-yl)-pyrazine; apt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole; pynp = 2-(2-pyridyl)-1,8-naphthyri-
dine; 4,40-dbbpy = 4,40-di-tert-butyl-2,20-bipyridine; 4,40-dmbpy = 4,40-dimethyl-2,20-bipyridine; mopip = 3,6-bis[(4-methoxyphenyl)iminomethyl]pyridazine.
Table 5
Stretching frequencies, force constants, and bond lengths of the CO groups in cis-[Ru(polypiridine)2(CO)Cl]+.
Polypyridinesa
m
CO (cmꢀ1
)
k (N cmꢀ1
)
d(Ru–C) (Å)
d(C–O) (Å)
Ref.
apy/bpy
pap/bpy
4,40-Me2bpy/5,60-Me2phen
4,70-Me2phen/dpk
5,50-Me2bpy/dpa
bpy/dpk
4,40-Me2bpy/4,70-Me2phen
bpy/dpa
bpy/phen
bpy/bpy
biq/biq
2002
2001
1978
1976
1972
1970
1967
1967
1966
1966
1964
16.20
16.18
15.81
15.80
15.72
15.68
15.64
15.64
15.62
15.62
15.59
1.889(3)
1.891(2)
1.867(5)
1.820(11)
1.917(4)
1.880(5)
1.95(2)
1.883(16)
1.857(4)
1.861(29)
1.91(2)
1.116(4)
1.122(3)
1.112(7)
1.146(13)
1.026(6)
1.119(7)
1.05(2)
this work
[9]
[13]
[13]
[13]
[13]
[13]
[13]
[13]
[39]
[40]
1.08(2)
1.126(6)
1.122(32)
1.01(2)
a
4,40-Me2bpy = 4.40-dimethyl-2,20-bipyridine; 5,60-Me2phen = 5,60-dimethyl-1,10-phenanthroline; dpa = di(2-pyridyl)amine; biq = 2,20-biquinoline.

D. Oyama et al. / Inorganica Chimica Acta 362 (2009) 2581–2588
2587
a
b
c
d
Fig. 6. Relationship between the force constants (k) and bond lengths (d) in [Ru(polypyridine)(CO)2Cl2] (a and b) and [Ru(polypyridine)2(CO)Cl]+ (c and d). The values of the
bond lengths in a and b are derived from the mean values of two CO groups.
dyl systems. Conversely, while obvious characteristics about the
bond lengths (Ru–C and C–O) are not shown in the bis-pyridyl
complexes, the force constant (k) and the stretching frequencies
of CO groups can be classified into two groups (Table 5, Fig. 6c
and d): the k values of the azo-containing complexes were higher
ca. 0.4–0.6 N cmꢀ1 than of the values for the bis-pyridyl complexes
without azopyridines. As the decrease of the k value of [Ru(bpy)2
(CO)(quinoline)]2+ caused by quinoline-based one-electron reduc-
tion is 0.56 N cmꢀ1 [32], it is presumed that the electron density
of the ruthenium center considerably decreases when an azo group
bpy and phen. Since azopyridine-coordinated complexes show
some interesting properties, we are currently extending the inves-
tigation to other systems containing Ru–CO moieties.
Acknowledgments
Professor Koji Tanaka of the Institute for Molecular Science is
acknowledged for helpful discussions and insight. Dr. Kazushi Shi-
ren provided crystallographic assistance. The authors thank the
Joint Studies Program (2006–2007) of the Institute for Molecular
Science for generous financial support of this research.
(which has greater p-acidity) coordinates to the ruthenium atom.
Accordingly, it can be concluded that complex formation of the
bis-pyridyl compound with azopyridyl ligands induced relatively
high k values. This observation can be ascribed to the decrease of
Appendix A. Supplementary material
the
p back-donation from the ruthenium center to the CO group.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2008.11.023.
4. Conclusion
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present complexes have the unreduced azo moieties. All the com-
plexes showed the azo-localized reduction at more positive
regions. This suggests that the azo ligands serve as prominent elec-
tron reservoirs compared with other polypyridyl ligands such as
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