Efficient synthesis of ruthenium complexes of the type (R-bpy) 2RuCl2 and [(R-bpy)2Ru(L-L)]Cl2 by microwave-activated reactions (R: H, Me, tert-But) (L-L: Substituted bibenzimidazoles, bipyrimidine, and phenanth

Article abstract of DOI:10.1016/j.ica.2004.07.007

Complexes of the type (R-bpy)₂RuCl₂ (R: H, Me, tert-but) were synthesised by microwave-activated reactions of [Ru(cod)Cl ₂]_n with substituted 2,2′-bipyridines in dimethylformamide in high yields and high purity.

Full text of DOI:10.1016/j.ica.2004.07.007

Inorganica Chimica Acta 357 (2004) 4496–4503
www.elsevier.com/locate/ica
Efficient synthesis of ruthenium complexes of the type
(R-bpy)2RuCl2 and [(R-bpy)2Ru(L–L)]Cl2 by
microwave-activated reactions (R: H, Me, tert-But) (L–L: substituted
bibenzimidazoles, bipyrimidine, and phenanthroline)
a,
Sven Rau , Bernhard Scha¨fer , Andre Grußing , Sebastian Schebesta , Katja Lamm ,
a
a
a
a
*
´
¨
a
a
a,
a
Jana Vieth , Helmar Gorls , Dirk Walther *, Manfred Rudolph ,
¨
Ulrich W. Grummt , Eckard Birkner
b
b
a
Institut fu¨r Anorganische und Analytische Chemie, Friedrich-Schiller-Universita¨t Jena, A.-Bebel Str. 2, D-07743 Jena, Germany
b
Institut fu¨r Physikalische Chemie, Friedrich-Schiller-Universita¨t Jena, D-07743 Jena, Germany
Received 4 May 2004; accepted 10 July 2004
Available online 10 August 2004
Abstract
Complexes of the type (R-bpy)2RuCl2 (R: H, Me, tert-but) were synthesised by microwave-activated reactions of [Ru(cod)Cl2]n
with substituted 2,20-bipyridines in dimethylformamide as the solvent. The complexes were isolated in high yields and high purity
from the reaction mixture. Microwave-assisted or thermal reaction of the (R-bpy)2RuCl2 solutions with substituted bibenzimidaz-
oles, 1,10 phenanthroline or bipyrimidine in dmf/water mixtures resulted in the formation of mixed ligand complexes of the type
[(R-bpy)2Ru(L–L)]Cl2. The complexes were characterised by NMR spectroscopy and MS. Furthermore, their photochemical and
electrochemical properties were investigated and the solid state structure of (4-tert-butyl-bpy)2RuCl2 (3), [(4-tert-butyl-bpy)2Ru(te-
tramethylbibenzimidazole)](PF6)2 (4), and [(4-tert-butyl-bpy)2Ru(bipyrimidine] (PF6)2 (5) was determined by X-ray diffraction anal-
ysis of single crystals.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Microwave-assisted reactions; 2,20Bipyridines; Ruthenium complexes; Bibenzimidazoles; Bipyrimidine
1. Introduction
functional groups to construct photochemically active
supramolecular or nano-sized systems [1]. Such devices
are very important for the construction of sensors
[2,3], photocatalysts [4–8], artificial molecular machines
[9], molecular switches [10], light-collecting molecular
antenna [11], and for conversion of light energy into
electric energy [12].
2,20-bipyridine ruthenium(II) complexes of the type
(R-bpy)2RuCl2 are known to be valuable starting prod-
ucts for the synthesis of photophysically active mixed
ligand complexes of the type [(R-bpy)2Ru(L–L)]Cl2, in
which the chelating ligands L–L can bear additional
Since the known thermal methods for synthesising
complexes of the composition (R-bpy)2RuCl2 (R: H,
Me, tert-but) are not only unselective but also time-con-
suming (typical are reaction times up to 72 h and only
80% moderate yields for these complexes [13,14]), it
would be highly desirable to develop alternatives which
*
Corresponding authors. Tel.: +49 3641 948112/948113; fax: +49
3641 948102 (S. Rau); Tel.: +49 3641 948110 (D. Walther).
E-mail addresses: sven.rau@uni-jena.de (S. Rau), dirk.Walther@
uni-jena.de (D. Walther).
0020-1693/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.07.007

S. Rau et al. / Inorganica Chimica Acta 357 (2004) 4496–4503
4497
allow the selective preparation of these complexes in
short reaction times.
ments were performed in acetonitrile containing 0.25
M tetra-n-butylammonium-hexafluorophosphate under
a blanket of solvent-saturated argon. The ohmic resist-
ance of the solvent, which had to be compensated for,
was determined by measuring the impedance of the sys-
tem at potentials where the faradaic current was negligi-
bly small. The reference electrode was an AgjAgCl
electrode in acetonitrile containing 0.25 M tetra-n-butyl-
ammonium chloride. The potential of this reference sys-
tem was calibrated by measuring the potential of the
ferrocenium/ferrocene couple at the end of each experi-
ment. The working electrode was a 1.5 mm Pt disk elec-
trode (Bioanalytical Systems Inc., West Lafayette,
USA).
It is well-known that microwaves can be used for en-
hancing the reaction rates of coordination compounds
(for reviews see [15]). Typically, in the reaction of RuCl3
with substituted bipyridines complexes of the type (R-
bpy)3RuX2 are formed [16] which, however, cannot be
used for the formation of mixed complexes of the type
[(R-bpy)2Ru(L–L)2]Cl2. To the best of our knowledge,
there is only one report, which describes the formation
of caboxy- and phosphonate-substituted bis(bipyri-
dine)Ru(II)X2 complexes [17].
In this paper, we describe the highly efficient micro-
wave-assisted synthesis of complexes of the type
(R-bpy)2RuCl2 (R: H, Me, t-but) in dmf. These com-
plexes can either be isolated as pure compounds in
high yields or they can be used in solution for further
thermal or microwave-supported reactions with chelat-
ing ligands L–L resulting in mixed ligand complexes
of the type [(R-bpy)2Ru(L–L)2 ]Cl2. (R: tert-but; L–
L: substituted bibenzimidazoles, 1,10-phenanthroline,
2,20-bipyrimidine).
2.3. Luminescence properties
Emission spectra were recorded using a Perkin–Elmer
LS50B spectrometer equipped with a Hamamatsu R928
red-sensitive detector. Luminescence kinetics were inves-
tigated with a CD900LP spectrometer (Edinburgh In-
struments). A N2-laser (k = 337 nm, 800 kW, pulse
width (FWHM) = 0.5 ns) was used for excitation with
direct pulse to pulse observation of the luminescence.
The instrument response pulse was 20 ns FWHM. The
signals were averaged over at least 100 pulses. The detec-
tor was a Photomultiplier M11FVC520 (RFT, Berlin)
connected with a 50 MHz Digital Oscilloscope (TDS
310, Tektronix). Fluorescence kinetics was recorded at
the emission maxima. Decay analysis was performed us-
ing exponential least square fitting software
implemented in the CD900LP software.
2. Experimental
2.1. General procedures
Infrared spectra were recorded using a Perkin–Elmer
1
2000 FT-IR; H NMR spectra were recorded on a Bru-
ker 400/200 MHz spectrophotometer, UV/Visible spec-
tra were obtained using a Varian Cary 1 UV–Vis or
Shimadzu UV 3100 UV–VIS–NIR. The mass spectra
were recorded using a SSQ 170, Finigan Mat at the
Friedrich Schiller University Jena. Electrospray-Mass
spectra were recorded on a Finnnigan MAT, MAT 95
XL. The positive ES mass spectra were obtained with
voltages of 3–4 kV applied to the electrospray needle.
The microwave assisted reactions were carried out us-
ing a Microwave Laboratory Systems MLS EM-2
microwave system. HPLC-chromatographic investiga-
tions were performed using a Jasco 851 AS autosampler,
M480 pump and a diode array UVD 340 U, Gynkotek,
as detector, a Hypersil SAX as column at 35 ꢁC, a flow
rate of 0.5 ml/min and as mobile phase 70/30 AcN: H20
(0.0125 mol K3PO4, pH 8.2). Detection wavelengths
were set at 240, 284, 450, and 540 nm.
2.4. Crystal structure determination
The intensity data for the compounds were collected
on a Nonius KappaCCD diffractometer, using graphite-
monochromated Mo Ka radiation. Data were corrected
for Lorentz and polarisation effects, but not for
absorption [18,19].
The structures were solved by direct methods (SHEL-
XS[20]) and refined by full-matrix least squares
techniques against F o2
(SHELXL-97 [21]). For the am-
ine-groups (N2, N4) and for the water molecule O3 of
7, the hydrogen atoms were located by difference Fou-
rier synthesis and refined isotropically. All other hydro-
gen atoms were included at calculated positions with
fixed thermal parameters. The quality of the data of
compound 4 is too bad. We will only publishing the con-
formation of the molecule and the crystallographic data.
We will not deposit the data in the Cambridge Crystal-
lographic Data Centre. All non-hydrogen atoms were
refined anisotropically [21]. XP (SIEMENS Analytical
X-ray Instruments, Inc.) was used for structure
representations.
2.2. Electrochemical investigation
The electrochemical oxidation of the compounds was
investigated in 3-electrode technique by means of cyclic
square-wave voltammetry using a home-built computer
controlled instrument based on the PCI-6110E data ac-
quisition board (National Instruments). The experi-

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S. Rau et al. / Inorganica Chimica Acta 357 (2004) 4496–4503
2.5. Syntheses
R1all = 0.059, wR2all ¼ 0:102, GOF = 1.012, largest differ-
ꢀ3
˚
ence peak and hole: 0.717/ꢀ0.721 e A
.
RuCl3*xH2O, 2,20-bipyridine, 4,40-dimethyl-2,2-bipy-
ridine, 1,10-phenanthroline, 2,20-bipyrimidine and
DMF (dimethylformamide, water <50 ppm) were of
commercial grade and used without further purification.
[Ru(cod)Cl2]n, [22] 4,40-di-tert-butyl-2,20-bipyridine, [23]
and 5,50,6,60-tetramethyl-2,20-bibenzimidazole [24] were
prepared as described elsewhere. 1H and 13C NMR spec-
tra were recorded on a Bruker AC 200 F spectrometer.
Mass spectra were recorded on a Finnigan MAT SSQ
710.
2.6.4. Ru(tbbpy)2(tmbiH2)](PF6)2 (4)
[Ru(cod)Cl2]n (0.1 g, 0.356 mmol) and 4,40-di-tert-bu-
tyl-2,20-bipyridine (tbbpy) (0.192 g, 0.716 mmol) were
suspended in 60 ml DMF and irradiated for 45 min to
form a deep purple solution of Ru(tbbpy)2Cl2 (micro-
wave set-up: 30 s, 450 W; 45 min, 150 W; 10 min,
ventilation). Water (5 ml) and 5,50,6,60-tetramethyl-
2,20-bibenzimidazole (tmbiH2) (0.104 g, 0.358 mmol)
were added and the reaction mixture was again irradiat-
ed (microwave set-up: 30s, 450 W; 90 min, 150 W; 10
min, ventilation). After evaporation of the solvent, the
crude product was dissolved in 50 ml EtOH and kept
overnight at ꢀ18 ꢁC. Excess ligand was filtered off; the
product was precipitated by addition of an aqueous so-
lution of NH4PF6, removed by filtration, washed with
20 ml water and 50 ml Et2O and dried on air. Yield:
63% (0.273 g). 1H NMR (200 MHz, CDCl3): d
[ppm] = 8.20 (d, J = 1.7 Hz, 2H); 8.13 (s, 2H); 7.88–
7.85 (d, J = 6.0 Hz, 2H); 7.62–7.59 (d, J = 6.0 Hz, 2H);
7.57–7.53 (d/d, J1 = 6.0 Hz, J2 = 1.7 Hz, 2H); 7.19 (s,
2H); 7.09–7.07 (d, J = 6.0 Hz, 2H); 5.36 (s, 2H); 2.15
(s, 6H); 1.94 (s, 6H); 1.45 (s, 18H); 1.31 (s, 18H). MS
(ESI in EtOH) m/z = 927 (100%) [Mꢀ2PF6]+; 464
2.6. General procedure for the preparation of 1–3
A suspension of [Ru(cod)Cl2]n (3.2 g, 0.0114 mmol)
and the corresponding ligand R-bpy (0.0228 mmol) in
315 ml DMF was reacted for 45 min under microwave
irradiation (microwave set-up: 30 s, 600 W; 45 min,
200 W; 10 min, ventilation). The solvent was immediately
removed on the rotary evaporator; the obtained solid
was washed with Et2O and dried on air.
2.6.1. Ru(bpy)2Cl2 (1)
Yield: 98% (6.04 g). 1H NMR (200 MHz, DMSO-d6):
d [ppm] = 9.96 (1 H, d), 8.62 (1 H, d), 8.47 (1 H, d), 8.06
(1 H, t), 7.75 (1 H, t), 7.66 (1 H, t), 7.50 (1 H, d), 7.08 (1
H, t). UV (Ethanol) kmax = 556 nm. MS (FAB in NBA):
m/z = 484 (100%) [M]+.
(31%) [(Mꢀ2PF6)/2]2+
H2O Æ CH4O, Mr = 1515.96 g molꢀ1, red–brown prism,
size 0.03 · 0.02 · 0.01 mm , triclinic, space group P1,
a = 13.1086(6), b = 14.5995(9), c = 18.6155(9) A, a =
.
Crystal Data for 4: C54H66F12N8P2Ru Æ 2CHCl3 Æ 1,5
3
ꢀ
˚
2.6.2. Ru(dmbpy)2Cl2 (2)
1
Yield: 98%, (6.04 g). H NMR (200 MHz, DMSO-
99.976(2), b = 96.148(2), c = 99.686(3) ꢁ, V = 3424.5(3)
3
A , T = ꢀ90 ꢁC, Z = 2, qcalcd. = 1.470 g cmꢀ3, l (Mo
˚
Ka) = 5.89 cmꢀ1, F(000) = 1554, 9282 reflections in
h(0/16), k(ꢀ17/15), l(ꢀ24/23), measured in the range
3.58ꢁ 6 H 6 27.84ꢁ, completeness Hmax = 57%.
d6): d [ppm] = 9.73 (1 H, d), 8.44 (1 H, s), 8.30 (1 H,
s), 7.53 (1 H, d), 7.25 (1 H, d), 6.92 (1 H, d), 2.63 (3
H, s, methyl), 2.53 (3 H, s, methyl). UV (THF)
kmax = 570 nm. MS (FAB in NBA): m/z = 540 (100%)
[M]+.
2.6.5. Ru(tbbpy)2(bpym)](PF6)2 (5)
[Ru(cod)Cl2]n (0.1 g, 0.356 mmol) and 4,40-di-tert-
butyl-2,20-bipyridine (tbbpy) (0.192 g, 0.716 mmol) were
suspended in 60 ml DMF and irradiated for 45 min to
form a deep purple solution of Ru(tbbpy)2Cl2 (micro-
wave set-up: 30 s, 450 W; 45 min, 150 W; 10 min, ven-
tilation). To this solution, 2,20-bipyrimidine (bipym)
(0.058 g, 0.366 mmol) and water (60 ml) were added
and reacted under microwave irradiation (microwave
set-up: 30 s, 600 W; 45 min, 200 W, 10 min, ventilation).
After evaporation of the solvent, the remaining solid
was dissolved in 30 ml EtOH. The product was precip-
itated by addition of an aqueous solution of NH4PF6,
removed by filtration, washed with Et2O and dried on
air. Yield: 82% (0.319 g). 1H NMR (200 MHz, CD2Cl2):
d [ppm] = 9.08–9.06 (d/d, J1 = 4.7 Hz, J2 = 2.0 Hz, 2H);
8.27 (s, 4H), 8.10–8.08 (d/d, J1 = 5.7 Hz, J2 = 2.0 Hz,
2H); 7.69–7.67 (d, J = 6.0 Hz, 2H); 7.65–7.62 (m, 2H);
7.61–7.59 (d, J = 6.0 Hz, 2H); 7.49–7.46 (m, 4H); 1.54
2.6.3. Ru(tbbpy)2Cl2 (3)
Yield: 97% (7.9 g). 1H NMR (200 MHz, acetone-d6) d
[ppm] = 10.02 (1 H, d), 8.54 (1 H, s), 8.42 (1 H, s), 7.68
(1 H, d), 7.48 (1 H, d), 7.08 (1 H, d), 1.51 (9 H, s, tert-
butyl), 1.31 (9 H, s, tert-butyl). UV (THF) kmax = 583
nm. MS (ESI in CHCl3): m/z = 708 (100%) [M]+. Crystal
Data for 3 [25]: C36H48Cl2N4Ru Æ C3H6O, Mr = 766.83
g molꢀ1, red–brown prism, size 0.08 · 0.07 · 0.05 mm3,
monoclinic, space group P21/n, a = 16.2351(2),
˚
b = 11.3965(2), c = 21.9759(3) A, b = 103.220(1)ꢁ,
3
˚
V = 3958.30(10) A , T = ꢀ90 ꢁC, Z = 4, qcalcd. = 1.287
g cmꢀ3, l (Mo Ka) = 5.65 cmꢀ1, F(000) = 1608, 15,077
reflections in h(ꢀ21/21), k(ꢀ14/13), l(ꢀ28/28), measured
in the range 1.77ꢁ 6 H 6 27.47ꢁ, completeness
H
max = 99.8%,
9055
independent
reflections,
Rint = 0.031, 7078 reflections with Fo >4r (Fo), 424 pa-
rameters, 0 restraints, R1obs = 0.039, wR2obs ¼ 0:093,

S. Rau et al. / Inorganica Chimica Acta 357 (2004) 4496–4503
4499
(s, 18 H); 1.42 (s, 18H). MS (ESI in MeOH): m/z = 941
(100%) [MꢀPF6]+; 398 (68%) [(Mꢀ2PF6)/2]2+
the corresponding isotopic pattern. ESI-MS m/z+: 667
(M-H2O-PF6), 639 (M-H2O-PF6-CO), 632 (M-H2O-
PF6-Cl).
Crystal data for 7 [25]: C33H34ClF6N6OPRu Æ
C3H6O Æ H2O, Mr = 888.25 g molꢀ1, brown prism, size
.
Crystal Data for 5 [25]: C44H54F12N8P2Ru Æ C2H3-
N Æ 0.5C7H8, Mr = 1173.08 g molꢀ1, red–brown prism,
3
ꢀ
size 0.03 · 0.03 · 0.02 mm , triclinic, space group P1,
3
˚
ꢀ
a = 11.3347(3), b = 15.1446(5), c = 17.7099(5) A, a =
0.12 · 0.12 · 0.10 mm , triclinic, space group P1, a =
˚
69.575(2), b = 85.251(2), c = 73.201(2) ꢁ, V = 2726.8(1)
9.6974(5), b = 13.8567(6), c = 15.2344(6) A, a = 72.463
3
3
A , T = ꢀ90 ꢁC, Z = 2, qcalcd. = 1.429 g cmꢀ3, l (Mo
(3), b = 84.856(3), c = 83.198(2) ꢁ, V = 1935.07(15) A ,
˚
˚
Ka) = 4.3 cmꢀ1, F(000) = 1206, 20,177 reflections in
h(ꢀ13/14), k(ꢀ18/19), l(ꢀ22/22), measured in the range
T = ꢀ90 ꢁC, Z = 2, qcalcd. = 1.524 g cmꢀ3, l (Mo
Ka) = 5.88 cmꢀ1, F(000) = 908, 11,615 reflections in
h(ꢀ12/11), k(ꢀ17/17), l(ꢀ19/17), measured in the range
2.50ꢁ 6 H 6 27.47ꢁ, completeness Hmax = 94.1%, 8355
independent reflections, Rint = 0.057, 6536 reflections
2.13ꢁ 6 H 6 27.46ꢁ,
completeness
Hmax = 99.4%,
12,399 independent reflections, Rint = 0.032, 9062 reflec-
tions with Fo > 4r (Fo), 620 parameters, 0 restraints,
R1obs = 0.055, wR2obs ¼ 0:137, R1all = 0.087, wR2all
¼ 0:156, GOF = 1.010, largest difference peak and hole:
with Fo > 4r (Fo), 489 parameters,
0 restraints,
R1obs = 0.055, wR2obs ¼ 0:158, R1all = 0.069, wR2all
ꢀ3
˚
1.177/ꢀ1.237 e A
.
¼ 0:165, GOF = 1.075, largest difference peak and hole:
ꢀ3
˚
1.075/ꢀ1.088 e A
.
2.6.6. Ru(tbbpy)2(phen)](PF6)2 (6)
[Ru(cod)Cl2]n (0.1 g, 0.356 mmol) and 4,40-di-tert-
butyl-2,20-bipyridine (tbbpy) (0.192 g, 0.716 mmol)
were suspended in 60 ml DMF and irradiated for 45
min to form a deep purple solution of Ru(tbbpy)2Cl2
(microwave set-up: 30 s, 450 W; 45 min, 150 W; 10
min, ventilation). After addition of 1,10-phenanthro-
line (phen) (0.072 g, 0.363 mmol) and water (60 ml),
the reaction mixture was again irradiated (microwave
set-up: 30 s, 450 W; 45 min, 200 W; 10 min, ventila-
tion). The solvent was completely removed on the ro-
tary evaporator and the resulting solid was dissolved
in 100 ml EtOH. Addition of an aqueous solution
of NH4PF6 afforded the desired product, which was
removed by filtration, washed with Et2O and dried
on air.
3. Results and discussion
Scheme 1 shows the synthetic routes, which have been
developed for the stepwise formation of a variety of bpy
ruthenium complexes by microwave-assisted reactions.
In the first step, the ruthenium complexes of the type
(R-bpy)2RuCl2 (R: H, Me, tert-but) can easily be pre-
pared by microwave-promoted reaction of easily accessi-
ble [Ru(cod)Cl2]n [22] with substituted 2,20-bipyridines
in dimethylformamide. Provided water is excluded dur-
ing this reaction, the formation of the complexes 1–3
proceeds very selectively. 1–3 can be isolated as pure
compounds in excellent yields ( P 90%). The result of
the X-ray structural investigation of 3 is depicted in
Fig. 1.
Yield: 68% (0.272 g). 1H NMR (200 MHz, CD2Cl2): d
[ppm] = 8.56–8.51 (d/d, J1 = 8.2 Hz, J2 = 1.2 Hz, 2H);
8.30–8.29 (d, J = 1.8 Hz, 2H); 8.25–8.24 (d, J = 1.8 Hz,
2H); 8.17 (s, 2H); 8.09–8.06 (d/d, J1 = 5.2 Hz, J2 = 1.2
Hz, 2H); 7.86–7.79 (m, 2H); 7.73–7.70 (d, J = 6.0 Hz);
7.53–7.49 (d/d, J1 = 6.0 Hz, J2 = 1.8 Hz, 2H); 7.41–
7.38 (d, J = 6.0 Hz, 2H); 7.27–7.23 (d/d, J1 = 6.0 Hz,
J2 = 1.8 Hz, 2H); 1.45 (s, 18H); 1.36 (s, 18H). MS (ESI
in EtOH): m/z = 963 (56%) [MꢀPF6]+ ; 409 (100%)
In comparison with the thermal reaction to form 1–3,
which usually requires long reaction times in refluxing
DMF (10–72 h [13,14]), the microwave-activated reac-
tion is much faster (ca 1–2 h) resulting in excellent yields
and products of high purity.
The formation of complexes of the type [(R-bpy)2R-
u(L–L)2]Cl2 from 1 to 3 does not require the isolation
of 1–3, since their solutions react upon addition of water
with chelating ligands L–L to form the bisheteroleptic
complexes either under the influence of microwaves or
by thermal reaction. The potential of this in situ micro-
wave assisted derivatisation was elucidated using 1,10-
phenanthroline and two types of bridging ligands
4,40,5,50-tetramethyl-2,20-bibenzimidazole (tmbiH2) and
2,20-bipyrimidine (bpym) employing tert-butyl-substi-
tuted 2,20-bipyridene/Ru(COD)Cl2n as starting material
for the microwave-assisted synthesis. All reactions pro-
ceeded smoothly with moderate to high yields resulting
in the compounds [(tbbpy)2Ru(tmbiH2)]Cl2 (4),
[(tbbpy)2Ru(bpym)]Cl2 (5), and [(tbbpy)2Ru(phen)]Cl2
(6). No formation of any side products was observed un-
der these conditions.
[(Mꢀ2PF6)/2]2+
.
2.6.7. Isolation of {[(tbbpy)Ru(Cl)(CO)(biH2)]-
(H2O)(PF6)}2 (7)
7 was isolated as a crystalline side product from a syn-
thesis for the preparation of (2,20-bibenzimid-
azole)bis(4,40-di-tert-butyl-2,20-bipyridine)ruthenium(II)
bis(hexafluorophosphate) [Ru(tbbpy)2(biH2)](PF6)2. The
reaction was carried out as described in reference [13]. Af-
ter the recrystallisation of the crude product, small crys-
tals could be isolated. X-ray structural investigation
revealed the structure of 7. ESI-MS investigation of the
crystals resulted in the identification of 7 at 667 m/z with

4500
S. Rau et al. / Inorganica Chimica Acta 357 (2004) 4496–4503
Scheme 1. Microwave assisted synthesis of polypyridyl ruthenium complexes.
Fig. 1. Solid state structure of 3, hydrogen atoms are omitted for
˚
clarity bond length Ru–N 2.015(2)–2.049(2) A, Ru–Cl1 2.4278(7) A,
˚
˚
Ru–Cl2 2.4105(7) A, bond angles in ꢁ N1–Ru–N2 79.08(8), N3–Ru–
N4 78.68(8), Cl1–Ru–Cl2 92.48(2).
Fig. 2. Supramolecular structure of compound 7 (graphical represen-
tation of mononuclear complex 7 in inset) bond length Ru–N 2.056(3)–
˚
˚
˚
2.173(3) A, Ru–C 1.854(4) A, Ru–Cl 2.4175(10) A, bond angles in ꢁ
N5–Ru–N6 78.93(11), N1–Ru–N3 77.33(11), Cl–Ru–C33 90.03(12).
To illustrate the significant improvement of this mi-
crowave assisted synthetic methodology we have isolat-
ed one side product from a previously employed thermal
reaction. The employed reaction conditions always re-
sult in the formation CO and dimethylamine as products
of the thermal decomposition of DMF. Since CO is an
excellent ligand for ruthenium(II)-centres, carbonyl
complexes may be formed as side products under these
reaction conditions [26]. We could isolate a crystalline
sample of such a carbonyl complex when we reacted
tert-but-bpy and Ru(cod)Cl2 in refluxing DMF in an at-
tempt to form 3 followed by addition of 2,20-bibenzim-
idazole and NH4PF6. Under these conditions, not only
the expected complex [(tbbpy)2Ru(biH2)](PF6)2 was iso-
lated as main product but also the side product
[(tbbpy)2Ru(Cl)(CO)(biH2)](H2O)(PF6)2 (7) which was
isolated in less than 1% yield. The X-ray structural in-
vestigation of single crystals of 7 (Fig. 2)reveals some in-
teresting features with respect to the supramolecular
arrangement of the molecules.
While the Ru–N, Ru–C and Ru–Cl bond lengths and
angles are quite comparable with literature values and
therefore need no further discussion, the solid state of
7 contains an extended hydrogen-bonded network (Fig.
3). In this network, the chloro ligand acts as a
Fig. 3. Graphical representation of hydrogen bonding pattern in 7, all
components not involved in the hydrogen bond pattern are omitted for
clarity.

S. Rau et al. / Inorganica Chimica Acta 357 (2004) 4496–4503
4501
hydrogen bond acceptor for both a NH group of a neigh-
al-driven molecular switch [29] and coordinates anions
via hydrogen bonds [30] or water molecules resulting
in an interesting one dimensional water chain [13]. The
new complex 4 is much better soluble in organic solvents
than the related compounds with unsubstituted ligands.
Complexes 4–6 were characterised by one- and two-
dimensional NMR spectroscopy and ESI-MS. All com-
pounds exhibited the expected pattern in the NMR and
corresponding signals in the MS were detected. X-ray
structural investigation of crystalline samples of 4 (crys-
tallising with methanol and water in the unit cell) and 5
(crystallising with acetonitrile) provided unambiguous
proof of their respective molecular structures which
are depicted in Fig. 4.
˚
bouring bibenzimidazole complex (N4–Cl 3.160(7) A)
and the OH group of the water molecule (O3–Cl
˚
3.132(8) A). Furthermore, the oxygen atom of this water
molecule serves as a hydrogen bond acceptor for the oth-
er N–H hydrogen of the bibenzimidazole (N2–O3
˚
2.724(6) A). In addition, the remaining OH bond of the
water molecule forms a hydrogen bond to a peripheral
˚
PF6 anion (O4–F14 2.727(10) A). This supramolecular
pattern results in the formation of dinuclear assemblies,
which are combined by six hydrogen bonds.
The formation of such complex side products like 7
from the thermal reaction illustrates clearly the impor-
tance of more selective and efficient synthetic methodol-
ogies for the preparation of pure ruthenium polypyridyl
complexes.
The high-yield synthesis of pure mixed ligand com-
plexes 4–6 using the new microwave assisted reaction
underlines the potential of this reaction protocol. Espe-
cially, the formation of 4 and 5 is of interest, since both
the bibenzimidazole and the bipyrimidine represent
bridging ligands which potentially may form dinuclear
complexes. Both complexes represent two different types
of ‘‘metalloligands’’, 4 containing tmbiH2 is a strong r-
donor ligand with two deprotonatable NH functions. In
contrast, 5 serves as a very weak r-donor ligand with
relatively low-lying p* orbitals of the bpym bridge. Re-
lated ruthenium complexes with unsubstituted biimidaz-
ole or bibenzimidazole, first prepared and investigated
by Haga, have interesting photophysical properties [2].
Recently, we have shown that the compound
(bpy)2Ru(bibenzimidazole) forms a supramolecular sys-
tem with (bpy)2Os(4-carboxy-40-methyl-2,20-bipyridine)
capable of transferring energy from Ru* to Os [28]. Fur-
thermore, (tbbpy)2Ru(bibenzimidazole) can act as met-
Interestingly, no intermolecular interactions between
these complex cations and surrounding solvent mole-
cules like methanol or water are apparent from the X-
ray structural investigation of 4, which is in contrast
to all our previous findings.
Photo- and electro-chemical properties of 4–6 reveal
the nature of the coordinated ligands. Not surprisingly,
the oxidation potentials of the ruthenium centres in 4
and 5 are very different with 4 being more easily oxidised
by 355 mV than 5, revealing the different electronic in-
fluence of the bridging ligand on the metal centre
(Table 1). While the reduction of 4 proceeds in a series
of redox steps, which are not yet well understood, 5 dis-
plays three fully reversible one electron reductions in the
range between ꢀ1.4 and ꢀ2.2 V vs. F c=F þc . A fourth re-
duction at ꢀ2.5 V is judged from the respective current
densities of forward and backward reaction only revers-
ible at high scan rates, as depicted in Fig. 5. Related
bpym complexes exhibit under similar conditions only
3 reversible reduction steps [Ru(bpy)2bpym](PF6)2
[31,32].
Fig. 4. Structural motif of 4 and solid state structure of 5; anions and hydrogen atoms omitted for clarity, bond length for 5: Ru–N 2.050(3)–2.073 (3)
˚
A, bond angles in ꢁ N–Ru–N 77.92(11)–78.27(11).

4502
S. Rau et al. / Inorganica Chimica Acta 357 (2004) 4496–4503
Table 1
Oxidation potentials and photophysical data for complexes 4–6
Compound
E1/2 ox in V (vs F c=F cþ
)
E1/2 red in V (vs F c=F cþ
)
kmax in nm
kem in nm
Lifetime of excited state in ns
(deaerated) at room temperature
4
0.54
0.90
–
480
650
645
260( 20)
40( 5)/550( 50)
5
ꢀ1.43; ꢀ1.94; ꢀ2.16; ꢀ2.50
ꢀ1.31; ꢀ1.75; ꢀ1.98a
ꢀ1.80; ꢀ1.99; ꢀ 2.23
420
Ref
6
1.09a
0.783b
422a
444b
610b
1420
a
Taken from [31].
Taken from [33].
b
Fig. 5. Square wave potentiometric reduction of 5 vs. F c=F þc in acetonitrile at 1500 Hz scanning frequency.
Reduction of 6 leads to 3 fully reversible peaks at pot-
4. Conclusions
entials very similar to the parent complex (REF.) [31].
Complexes 4–6 are luminescent at room temperature
and display lifetimes of the excited states, which in gen-
eral agree well with literature values of their related un-
substituted bipyridine derivatives, (see Table 1). 4
displays the longest lifetime of the excited state detected
for bibenzimidazole complexes. Whereas excited 4 de-
cays single exponential, two exponentials are necessary
to describe the luminescence decay of 5. The latter is
due to a second emitting species as revealed from sta-
tionary emission and excitation spectra (although HPLC
measurements indicate the presence of only one com-
pound). In order to circumvent this problem, we used
single crystals of 5 for a detailed analysis but still detect-
ed the same decay profiles. Details of the kinetics and
the identity of the emitting species are subject to a forth-
coming study.
The presented microwave-assisted synthetic method-
ology for the formation of complexes of the type
(R-bpy)2RuCl2 and [(R-bpy)2Ru(L–L)]Cl2 represents a
significant improvement over previously employed
methods. The high yield of above 90% and dramatically
decreased reaction time, i.e., 1.5 h for 3 instead of 72 h
using the thermal method, are significant steps en route
to the broader application of molecular building blocks
based on ruthenium polypyridyl complexes. The in situ
derivatisation of the formed starting complexes 1–3
decreases further the required reaction times for the
formation of luminescent heteroleptic ruthenium poly-
pyridyl complexes. Of special interest are the easily
accessible complexes 4 and 5 containing bridging
ligands useful for the construction of oligometallic
complexes.

S. Rau et al. / Inorganica Chimica Acta 357 (2004) 4496–4503
4503
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Acknowledgements
[17] M. Alebbi, E. Costa, C.A. Bignozzi, EP1178084 Production and
use of ruthenium bi- and terpyridinecarboxylate and -phospho-
nate complex dyes.
Financial support from the Deutsche Forschungsg-
emeinschaft (SFB 436) and a grant to S.R. is gratefully
acknowledged.
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Sweet (Eds.), Methods in Enzymology, Macromolecular Crys-
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