Diastereoisomerically pure pyrazole-3,5-dicarboxylate-bridged dinuclear ruthenium(II) and osmium(II) complexes of 2, 2′-bipyridine: Structural, electrochemical and spectral studies

Article abstract of DOI:10.1016/j.ica.2003.07.003

Pyrazole-3,5-dicarboxylate-bridged dinuclear ruthenium(II) and osmium(II) complexes of 2,2′-bipyridine of composition [(bpy)₂Ru(pzdc) Ru(bpy)₂](ClO₄)·H₂O (1) and [(bpy) ₂Os(pzdc)Os(bpy)₂](C

Full text of DOI:10.1016/j.ica.2003.07.003

Inorganica Chimica Acta 357 (2004) 699–706
www.elsevier.com/locate/ica
Diastereoisomerically pure pyrazole-3,5-dicarboxylate-bridged
dinuclear ruthenium(II) and osmium(II) complexes of 2,20-bipyridine:
structural, electrochemical and spectral studies
a
Sujoy Baitalik , Pradip Bag , Ulrich Florke , Kamalaksha Nag
a
b
a,
*
€
a
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
b
Anorganische und Analytische Chemie, Universit€at Paderborn, D-33098 Paderborn, Germany
Received 23 April 2003; accepted 29 July 2003
Abstract
Pyrazole-3,5-dicarboxylate-bridged dinuclear ruthenium(II) and osmium(II) complexes of 2,20-bipyridine of composition
[(bpy)2Ru(pzdc)Ru(bpy)2](ClO4) ꢀ H2O (1) and [(bpy)2Os(pzdc)Os(bpy)2](ClO4) ꢀ H2O (2) have been obtained in high yield and have
been separated to their homochiral ðKK=DDÞ rac (1a, 2a) and heterochiral ðKD=DKÞ meso (1b, 2b) diastereoisomers. The distinctive
1
structural features of these diastereoisomers have been characterized by 1-D and 2-D H NMR spectroscopy. The X-ray crystal
structure of rac-[(bpy)2Os(pzdc)Os(bpy)2](ClO4) ꢀ H2O (2a) has been determined. The electrochemical and electronic spectral studies
have established that there remain difference in properties and hence difference in intermetallic communication between the dia-
stereoisomeric forms in each case.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Ruthenium(II) complexes; Osmium(II) complexes; Dinuclear complexes; Diastereoisomers; Crystal structure; 1H NMR; Electrochemistry
1. Introduction
We reported [13–15] spectroscopic properties and
proton-coupled redox activities of mononuclear and
diastereoisomerically pure diruthenium(II) polypyridine
complexes with pyrazole-3,5-bis(benzimidazole) [H3p-
zbzim] and pyrazole-3,5-bis(benzothiazole)-[Hpzbzth] as
the bridging ligands. To examine the effect of increasing
anionic charge of the bridging ligand on the properties
of the complexes, we have recently reported [16] homo-
and heterodinuclear ruthenium(II) and osmium(II)
complexes derived from 2,20(-bipyridine (bpy) and pyr-
azole-3,5-dicarboxylic acid [H3pzdc]. Although the di-
nuclear complexes were expected to be isolated as
homochiral rac ðKK=DDÞ and heterochiral meso
ðKD=DKÞ diastereoisomers, we could isolate the
RuIIRuII, OsIIOsII and RuIIOsII complexes exclusively
in the rac form. To resolve this unexpected observation,
we have reexamined the synthetic methods. Using
modified procedures, we have now been able to obtain
the homodinuclear complexes as the mixture of two
diastereoisomers in high yield. The separation of these
diastereoisomers into rac and meso forms has enabled us
The electrochemical, photochemical and photophys-
ical properties of di- and oligonuclear ruthenium(II) and
osmium(II) complexes in which the mononuclear poly-
pyridine units are anchored by bridging ligands have
received much attention [1–5]. The length, charge, to-
pology and conjugation of the bridging ligands in these
assemblies play important roles to regulate photoin-
duced intramolecular electron and energy transfer pro-
cesses [6,7]. The physicochemical properties of these
systems are also influenced by the stereoisomeric rela-
tionship between component species. Since most of the
earlier reported studies have been carried out with
mixture of diastereoisomers, lately, separation of the
diastereoisomers or stereospecific synthesis of optical
isomers has been the focus of interest [8–12] to address
the role of chirality on the physicochemical properties.
* Corresponding author. Fax: +91-33-2473-2805.
E-mail address: ickn@mahendra.iacs.res.in (K. Nag).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.07.003

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S. Baitalik et al. / Inorganica Chimica Acta 357 (2004) 699–706
to compare their physicochemical properties. The
structure of rac-[(bpy)2Os(pzdc)Os(bpy)2](ClO4) ꢀ H2O
is also reported here.
and the dark product that deposited was filtered after
2 h. The product was dissolved in requisite amount of
water, filtered and to the filtrate 1 g of NaClO4 dissolved
in 5 ml of water was slowly added. The mixture was kept
at 10 °C for 6 h, after which the microcrystalline product
that deposited was filtered. The product was recrystal-
lized from methanol; yield 0.48 g (76%). Anal. Found: C,
42.2; H,2.85; N, 10.85%. Calc. for C45H35N10ClO4Os2:
C, 42.35; H, 2.75; N, 10.95.
Similar to 1, compound 2 was also found to be a
mixture of rac (2a) and meso (2b) diastereoisomers in ca.
1:1 ratio. The two isomers were separated by fractional
recrystallization (four times) from 1:1 methanol–ethanol
in which the meso form again is relatively more soluble.
2. Experiment
2.1. Preparation of complexes
All the complexes were prepared under a nitrogen
atmosphere. Caution: The perchlorate salts reported
here are potentially explosive and, therefore, should be
handled with care.
2.2. Physical measurements
1H NMR (300 MHz) spectra were obtained on a
Bruker Avance DPX spectrometer using DMSO-d6 so-
lutions. The spectral assignments were made with the
assistance of COSY and taking into consideration the
following typical 3J coupling patterns for bipyridine
protons: H(3)[d, J ꢁ 8 Hz], H(4)[t, J ꢁ 8 Hz], H(5)[dd/t,
J ꢁ 8:5 Hz], H(6)[d, J ꢁ 5 Hz]. UV–Vis absorption
spectra were recorded for methanol, ethanol, acetoni-
trile, N,N-dimethylformamide (DMF) and dimethyl-
sulfoxide (DMSO) solutions of the complexes. Emission
spectra were recorded at 77 K for 1:4 methanol–ethanol
solutions ð1 ꢂ 10ꢃ5 M) of the complexes.
2.1.1. [(bpy)2Ru(pzdc)Ru(bpy)2](ClO4) ꢀ H2O (1) and
separation of rac (1a) and meso (1b) diastereoisomers
A mixture of cis-[Ru(bpy)2Cl2] ꢀ 2H2O (1.04 g, 2
mmol), H3pzdc ꢀ H2O (0.17 g, 1 mmol) and triethylamine
(0.3 g, 3 mmol) in 100 ml of 1:1 ethanol–water was he-
ated under reflux with constant stirring for 12 h. The
solution was filtered and concentrated to about 10 ml on
a rotary evaporator. An aqueous solution (5 ml) of
NaClO4 (1.5 g) was slowly added to this solution when a
red product deposited, which was collected by filtration.
The product was dissolved in minimum volume of
boiling methanol, filtered and the filtrate was kept in a
refrigerator for overnight period. The microcrystalline
orange red compound that deposited was filtered on a
glass frit; yield 0.90 g (82%). Anal. Found: C, 49.0; H,
3.25; N, 12.85%. Calc. for C45H35N10ClO9Ru2: C, 49.25;
H, 3.2; N, 12.75.
The compound thus obtained was found to be an
almost 1:1 mixture of homochiral ðKK=DDÞ rac (1a) and
heterochiral ðKD=DKÞ meso (1b) diastereoisomers. The
previously characterized rac form [16] was found to be
relatively less soluble in methanol or ethanol as com-
pared to the meso form. Their separation was achieved
by fractional crystallization from 1:1 methanol–ethanol.
Four cycles of fractionation afforded the pure diastere-
oisomers.
The electrochemical measurements were performed
under nitrogen using a BAS-100B electrochemistry sys-
tem. Cyclic, square wave and differential pulse voltam-
mograms were recorded in acetonitrile solution of the
complexes ðꢄ1 ꢂ 10ꢃ3 M) containing 0.1 M tetraethy-
lammonium perchlorate as the supporting electrolyte
using a platinum or glassy carbon working electrode and
AgjAgCl reference electrode. The reference electrode
was separated from the bulk electrolyte by a salt bridge
(acetonitrile/0.1 M[Et4N](ClO4)). Under the experi-
mental condition used, the ferrocene/ferrocenium couple
was observed at 365 mV.
2.3. Crystal structure determination of rac ðKK=
DDÞ[(bpy)2Os(pzdc)Os(bpy)2](ClO4) ꢀ H2O (2a)
Single crystals used for structure determination were
obtained by slow evaporation of a methanol solution of
2a. X-ray diffraction data were collected using Siemens
SMART CCD diffractometer using graphite mono-
chromated Mo Ka radiation. Crystal data and details of
data collection are listed in Table 1. The intensity data
were corrected for Lorentz and polarization effects and
semi-empirical absorption correction was made from w-
scans. A total of 12 039 reflections were collected in the
range h ¼ 2:17–27.53° with h ¼ ꢅ36, k ¼ ꢃ1 to 15 and
2.1.2. [(bpy)2Os(pzdc)Os(bpy)2](ClO4) ꢀ H2O (2) and
separation of rac (2a) and meso (2b) diastrereoisomers
A mixture of cis-[Os(bpy)2Cl 2] (0.57 g, 1 mmol),
H3pzdc ꢀ H2O (0.09 g, 0.5 mmol) and triethylamine (0.15
g, 1.5 mmol) in 100 ml of 1:1 ethanol–water was heated
under reflux with constant stirring for 72 h. The solution
was filtered and the filtrate was reduced to about 10 ml
on a rotary evaporator. This was cooled in an icebath

S. Baitalik et al. / Inorganica Chimica Acta 357 (2004) 699–706
701
Table 1
Crystallographic data for rac-[(bpy)2Os(pzdc)Os(bpy)2] (ClO4) ꢀ H2O
(2a)
salt of the complexes to a small volume, followed by
precipitation of their perchlorate salt, has led to the iso-
lation of compounds 1 and 2 in about 80% yield. The 1H
NMR spectra of 1 and 2 (Figs. 1 and 2) show that both
are mixture of rac and meso diastereoisomers in about 1:1
ratio. The separation of the diastereoisomers has been
accomplished by fractional recrystallization from 1:1
Empirical formula
Formula weight
Crystal size (mm)
Crystal system
Space group
C45H35ClN10O9Os2
1275.68
0.38 ꢂ 0.35 ꢂ 0.26
orthorhombic
Pna21
27.972(8)
12.239(6)
12.640(3)
4327(4)
4
ꢀ
a (A)
ꢀ
b (A)
ꢀ
c (A)
3
ꢀ
V (A )
Z
Dcalc (g cmꢃ3
k (Mo KaÞ (A)
)
ꢀ
1.958
0.71073
293
5.992
T (K)
l (mmꢃ1
F ð000Þ
)
2464
12 039
Reflections measured
Unique reflections
Final R indices
5643
a
R1 ¼ 0.0451
b
½I > 2rðIÞꢆ
wR2 ¼ 0.0832
R indices (all data)
R1 ¼ 0:0862
WR2 ¼ 0:1033
P
P
a R1 ¼ jjF0j ꢃ jFcjj= jF0j.
P
P
2
2 1=2
b wR2ðF 2Þ½ wðF02 ꢃ Fc2Þ = wðF02Þ ꢆ
.
l ¼ ꢃ16 to 1, of which 5643 independent reflections
ðRint ¼ 0:0689Þ were used for structure determination.
The structure was solved by direct and Fourier methods
and refined by full-matrix least-squares based on F 2
using the programs SHELXTL-PLUS [17] and SHELXL-97
[18]. All non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were placed in calculated posi-
tions (riding model) and were not refined. The
refinement ½I > 2:00rðIÞꢆ converged to R1 ¼ 0:0451 and
wR2 (all data) ¼ 0.1033. The goodness-of-fit on F2 was
1.072 and the maximum and minimum peak on the final
difference Fourier map corresponded to 1.038 and
Fig. 1. The 1H NMR spectra (DMSO-d6, 300 MHz) of unseparated
and separated diastereoisomers of [(bpy)2Ru(pzdc)Ru(bpy)2] þ: (a) rac
ðKK=DDÞ form; (b) unseparated mixture of rac and meso forms;
(c) meso ðKD=DKÞ form.
3
ꢀ
)1.021 eA , respectively.
3. Results and discussion
3.1. Separation of diastereoisomers
In our earlier study [16] the complexes
[(bpy)2Ru(pzdc)Ru(bpy)2](ClO4) ꢀ H2O and [(bpy)2Os
(pzdc)Os(bpy)2](ClO4) ꢀ H2O were obtained in 35–40%
yield solely as the homochiral rac ðKK=DDÞ form. Since
these compounds were isolated as the perchlorate salts
from fairly dilute ethanol–water solution, in retrospect, it
appeares that the heterochiral meso ðKD=DKÞ diastereo-
isomer remained unrecovered from the solution. The fact
that the chloride salts of these dinuclear complexes are
more soluble in methanol, ethanol or water as compared
to the perchlorate salts has been used with advantage in
the present modified syntheses. Indeed, reduction of the
ethanol–water reaction solution containing the chloride
Fig. 2. The 1H NMR spectra (DMSO-d6, 300 MHz) of unseparated
and separated diastereoisomers of [(bpy)2Os(pzdc)Os(bpy)2]þ: (a) rac
ðKK=DDÞ form; (b) unseparated mixture of rac and meso forms;
(c) meso ðKD=DKÞ form.

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S. Baitalik et al. / Inorganica Chimica Acta 357 (2004) 699–706
methanol–ethanol in which the meso ðKD=DKÞ form is
relatively more soluble. As shown in Figs. 1 and 2, clean
separation of the two isomers in both cases has been
achieved after four recrystallization steps.
1
3.2. H NMR spectra
1
The H NMR of the separated pairs of the diastere-
oisomers la ðKK=DDÞ and 1b ðKD=DKÞ and 2a ðKK=DDÞ
and 2b ðKD=DKÞ have been assigned using a combina-
tion of one- and two-dimensional techniques as reported
earlier [16]. The results are summarized in Table 2. As
may be seen in Figs. 1 and 2, there are significant dif-
ferences in the spectral patterns displayed by the ho-
mochiral ðKK=DDÞ and heterochiral ðKD=DKÞ
diastereoisomers of the diruthenium(II) and dios-
mium(II) complexes. Interestingly, the spectra exhibited
by the rac pair la and 2a or the meso pair 1b and 2b show
generally similar features but they differ in detail, sig-
nifying subtle influence of the metal centres on the
chemical shifts of bpy ligand in a given stereochemical
arrangement.
Fig. 3. ORTEP representation of the rac-[(bpy)2Os(pzdc)Os(bpy)2]þ
cation (2aþ) showing the 50% probability of thermal ellipsoids.
the chelating ligands around the two osmium centres
(Fig. 3) depicts homochirality, indicating it to be the rac
ðKK=DDÞ diastereoisomer. The rac form of diruthenium
(1a), diosmium (2a) and ruthenium–osmium (3a) com-
plexes all crystallize in the orthorhombic space group
Pna21 with four dinuclear units in the unit cell. The meso
diastereoisomer for none these compounds could be
obtained as suitable single crystals for structure deter-
mination despite many attempts.
3.3. Crystal structure of rac-[(bpy)2Os(pzdc)Os(bpy)2]
(ClO4) ꢀ H2O (2a)
The crystal structures of the homodinuclear rac-
[(bpy2Ru(pzdc)Ru(bpy)2](ClO4) ꢀ H2O (1a) and the
heterdinuclear rac-[(bpy2Ru(pzdc)Os(bpy)2](ClO4) ꢀ (3a)
complexes have already been reported [16]. An ORTEP
representation of 2aþ cation along with the atom labels
is shown in Fig. 3. Selected bond distances and angles
are given in Table 3. The stereochemical disposition of
The coordination environment around each os-
mium(II) in 2a is slightly distorted octahedral with the
average Os–N(bpy), Os–N(pyrazolate) and Os–O(car-
ꢀ
boxylate) distances being 2.03(2), 2.08(1) and 2.09(2) A,
respectively. The corresponding distances observed for
Table 2
1H NMR dataa for homochiral rac ðKK=DDÞ and heterochiral meso ðKD=DKÞ diastereoisomers of [(bpy)2Ru(pzdc)Ru(bpy)2](ClO4) ꢀ H2O (1) and
[(bpy)2Os(pzdc)Os(bpy)2](ClO4) ꢀ H2O (2)
Proton
pz-CH
rac la
meso 1b
rac 2a
meso 2b
6.85, 1H
6.82, 1H
6.84, 1H
6.82, 1H
H(3)
H(4)
H(5)
H(6)
7.89, 2H, 8.1
8.34, 2H, 8.1
8.40, 2H, 8.1
8.53, 2H, 8.2
7.98, 2H, 8.0
8.10, 2H, 8.1
8.49, 2H, 8.0
8.69, 2H, 8.0
7.88, 2H, 8.0
8.35, 4H, 8.0
7.94, 2H, 8.2
8.12, 2H, 8.1
8.43, 2H, 8.1
8.64, 2H, 8.1
8.49, 2H, 8.2
7.34, 2H, 7.7
7.77, 2H, 7.7
7.95, 2H, 7.8
8.11, 2H, 7.5
7.75, 2H, 7.3
7.79, 2H, 7.3
8.01, 2H, 7.7
8.14, 2H, 7.6
6.97, 2H, 7.6
7.51, 4H, 7.7
7.07, 2H, 7.7
7.52, 2H, 7.8
7.68, 2H, 7.4
7.73, 2H, 7.3
7.80, 2H, 7.7
6.68, 2H, 6.7
7.08, 2H, 6.5
7.24, 2H, 6.4
7.84, 2H, 6.4
6.61, 2H, 6.4
7.07, 2H, 6.5
7.43, 2H, 6.3
7.47, 2H, 6.8
6.45, 2H, 6.6
6.90, 2H, 6.6
7.02, 2H, 6.5
7.72, 2H, 6.5
6.12, 2H, 6.7
6.92, 2H, 7.0
7.32, 2H, 6.6
7.38, 2H, 6.8
6.36, 2H, 5.6
6.63, 2H, 5.6
7.00, 2H, 5.3
8.77, 2H, 5.4
6.81, 2H, 5.1
6.98, 4H, 5.4
6.34, 2H, 5.6
6.40, 2H, 5.5
6.62, 2H, 5.5
8.53, 2H, 5.6
6.46, 2H, 5.8
6.94, 2H, 5.9
7.92, 2H, 5.9
7.98, 2H, 5.6
8.18, 2H, 5.2
a For 1H NMR data, respectively: chemical shift (ppm), number of protons, J (Hz).

S. Baitalik et al. / Inorganica Chimica Acta 357 (2004) 699–706
703
Table 3
þ
ꢀ
Selected bond distances (A) and angles (°) for rac-[(bpy)2Os(pzdc)Os(bpy)2] (C1O4) ꢀ H2O (2a)
Bond distances
Os(1)–N(11)
Os(1)–N(21)
Os(1)–N(22)
Os(1)–N(12)
Os(1)–N(51)
Os(1)ꢀ ꢀ ꢀOs(2)
2.026(12)
2.034(11)
2.043(10)
2.048(11)
2.094(11)
4.662(2)
Os(2)–N(31)
Os(2)–N(41)
Os(2)–N(42)
Os(2)–N(32)
Os(2)–N(52)
2.012(13)
2.014(9)
2.048(11)
2.055(12)
2.073(12)
Bond angles
N(11)–Os(1)–N(21)
N(11)–Os(1)–N(22)
N(21)–Os(1)–N(22)
N(11)–Os(1)–N(12)
N(21)–Os(1)–N(12)
N(22)–Os(1)–N(12)
N(11)–Os(1)–O(51)
N(21)–Os(1)–O(51)
N(22)–Os(1)–O(51)
N(12)–Os(1)–O(51)
N(11)–Os(1)–N(51)
N(21)–Os(1)–N(51)
N(22)–Os(1)–N(51)
N(12)–Os(1)–N(51)
O(51)–Os(1)–N(51)
88.4(5)
100.7(5)
77.7(5)
79.9(5)
98.7(5)
176.2(5)
82.9(5)
168.5(5)
96.5(5)
87.2(5)
159.6(5)
111.8(5)
87.3(4)
93.4(5)
77.5(4)
N(31)–Os(2)–N(41)
N(31)–Os(2)–N(42)
N(41)–Os(2)–N(42)
N(31)–Os(2)–N(41)
N(31)–Os(2)–N(42)
N(42)–Os(2)–N(32)
N(31)–Os(2)–O(53)
N(41)–Os(2)–0(53)
N(42)–Os(2)–O(53)
N(32)–Os(2)–O(53)
N(31)–Os(2)–N(52)
N(41)–Os(2)–N(52)
N(42)–Os(2)–N(52)
N(32)–Os(2)–N(52)
O(53)–Os(2)–N(52)
82.3(5)
100.5(5)
78.5(5)
82.3(5)
100.5(5)
177.9(5)
89.1(4)
167.8(5)
94.6(4)
86.9(4)
166.3(4)
110.4(5)
87.5(5)
94.2(5)
79.1(4)
the diruthenium(II) analogue in la are 2.05(1), 2.10(1)
ꢀ
and 2.10(1) A [16]. It is also of interest to note that the
diastereoisomers, two fully reversible metal-centred re-
dox couples MIIMII/MIIMIII and MIIMIII/MIIIMIII are
observed. The separations between these two redox
couples ðDE1=2 ¼ E12=2 ꢃ E11=2Þ are 170 mV (for 1a) and
200 mV (for 1b) with the diruthenium diastereoisomers,
while in the cases with the diosmium analogues these
values are 165 mV (for 2a) and 195 mV (for 2b). Almost
identical differences in redox separation for the two di-
astereoisomers of the two metals indicate that the me-
tal–metal interactions in these compounds are chirality
dependent. The differences in redox potentials of the
rac and the meso forms of the same complex may be
ꢀ
Osꢀ ꢀ ꢀOs separation of 4.662 A in 2a is slightly shorter
ꢀ
compared to the Ruꢀ ꢀ ꢀRu distance of 4.685 A in 1a,
indicating tighter bonding in the osmium compound.
The bite angles of bpy and pzdc ligands are nearly equal,
79(1)°. On the other hand, the shorter C–O distance of
ꢀ
the free carbonyl moiety of pzdc (1.22(3) A) relative to
ꢀ
that of the metal-bound C–O (1.32(2) A) is consistent
with pronounced double bond character of the former.
3.4. Electrochemical studies
used to determine their comproportionation constant
1=2Þ
The electrochemical characteristics of the homo- and
heterochiral diruthenium (1a and 1b) and diosmium (2a
and 2b) complexes have been examined by cyclic vol-
tammetry, square wave voltammetry and differential
pulse voltammetry and the observed redox potentials are
given in Table 4. As shown in Fig. 4, for each of the
ðKc ¼ 10ð16:92ꢂDE
at 298 K). The Kc values given in
Table 4 indicate that the mixed-valence (MIIMIII) spe-
cies derived from the meso form has at least threefold
greater stability compared to that of the rac form.
Differences in the electrochemical properties of the dia-
stereoisomers of some other diruthenium complexes
Table 4
Redox potentialsa for the diastereoisomers of [{M(bpy)2}2(pzdc)]þ in acetonitrile
E11=2 (mV)
E12=2 (mV)
b
Complex
DE1=2 (V)
Kc (298 K)
MIIMII/MIIMIII
MIIMIII/MIIIMIII
[{Ru(bpy)2}2(pzdc)]þ
rac (1a)
meso (1b)
750
710
920
910
0.170
0.200
750
2400
[{Os(bpy)2}2(pzdc)]þ
rac (2a)
meso (2b)
305
300
470
495
0.165
0.195
620
1990
a All the potentials are referenced against AgjAgCl electrode with E1=2 ¼ 365 for ferrocene/ferrocenium couple. Uncertainty in E1=2 values ꢅ5 mV.
b DE1=2 ¼ E12=2 ꢃ E11=2
.

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S. Baitalik et al. / Inorganica Chimica Acta 357 (2004) 699–706
meso form (1b) occurs by 5–7 nm relative to the rac form
(1a). The variation of the solvents has the effect of sol-
vation of both the diastereoisomers almost to the same
extent, as a result the relative difference in peak positions
between the two stereoisomers remains more or less
same in all the solvents (Table 5).
The absorption spectra of the two diastereoisomers of
the diosmium(II) complex again follow the same trend.
For example, the spectra of 2a (rac) and 2b (meso) re-
corded in dimethyl sulfoxide (Fig. 6) show that relative to
the rac form, the metal-to-ligand charge transfer band of
the meso form is shifted from 550 to 542 nm. Clearly, there
is a real difference, albeit small, in the electronic transition
energies between the homochiral and heterochiral dia-
stereoisomers. In other words, the intermetallic electronic
communication in polymetallic systems depends, among
other things, on chiral relationships.
The luminiscence spectral behaviour of the diastere-
oisomers 1a and 1b of the diruthenium(II) complex has
been examined in methanol–ethanol (1:4) glass at 77 K,
The excitation of the MLCT band at 516 nm (for 1a)
and 510 nm (for 1b) provided an emission peak at 615
nm in both the cases, indicating the same excited state
behaviour of the two diastereoisomers.
Fig. 4. Cyclic voltammograms for the oxidation of the diastereoiso-
mers of [(bpy)2Ru(pzdc)Ru(bpy)2]þ (a) and [(bpy)2Os(pzdc)Os
(bpy)2]þ (b) in acetonitrile: —, rac form; –––, meso form.
have been reported [11,19–21]. However, it is not pos-
sible to predict for which of the diastereoisomers the
comproportionation constant will be greater.
4. Conclusion
The homodinuclear complexes [(bpy)2Ru(pzdc)
Ru(bpy)2](ClO4) ꢀ H2O and [(bpy)2Os(pzdc)Os(bpy)2]
(ClO4) ꢀ H2O have been synthesized in high yield as ap-
proximately 1:1 mixture of their homochiral ðKK=DDÞ
rac and heterochiral ðKD=DKÞ diastereoisomers. The
separation of the diastereoisomers has been accom-
plished by fractional recrystallization, taking into ad-
vantage of the relatively greater solubility of the meso
form. The difference in spatial orientations of the bi-
pyridine ligands in each pair of the diastereoisomers is
reflected in their 1H NMR spectra. The crystal structure
of rac-[(bpy)2Os(pzdc)Os(bpy)2]þ has been determined.
The diastereoisomers of both the metals show a signifi-
cant difference in their metal-centred redox potentials.
The separation between the two redox couples MIIMII/
MIIMIII and MIIMIII/MIIIMIII are about 170 mV for the
rac form and 200 mV for the meso form in both the
complexes. In terms of their comproportionation con-
stants, these values indicate relatively stronger metal–
metal interaction in the meso form compared to the rac
form. The difference in physico-chemical properties of
the diastereoisomers also become evident when their
absorption spectra measured in different solvents are
considered. For both the metal complexes, the MLCT
band of the meso form is shifted to a lower wavelength by
5–7 nm relative to the rac form, again indicating small but
definite dependence of metal–metal coupling on the chi-
ral combination of the metal centres. In contrast, the
In the reductive mode, all the diastereoisomers (1a,
1b, 2a, 2b) exhibited two redox couples with the E1=2
values ꢃ1:60 ꢅ 0:05 and ꢃ2:05 ꢅ 0:05 V. Each of these
redox couples corresponds to two-electron process, in-
dicating simultaneous transfer of one electron to two
complimentary bipyridine moieties. Differential pulse
voltammetry or square wave voltammetry failed to re-
solve the two redox couples further.
3.5. Electronic spectra
The absorption spectra of the homochiral rac and the
heterochiral meso diastereoisomers of the diruthe-
nium(II) and diosmium(II) complexes have been re-
corded in several solvents to find out whether any
spectral difference observed between the two diastereo-
isomers is due to their difference in stereochemistry or
due to their variation in solvation. The spectral data (in
the range 300–900 nm) for the stereoisomers 1a/1b and
2a/2b in different solvents are given in Table 5.
The spectra obtained for 1a and 1b in methanol are
shown in Fig. 5. It may be noted that the metal-to-ligand
RuðdpÞ ! bpyðpꢇÞ charge transfer transition observed
at 512 nm for the homochiral rac form (1a) is blue-
shifted to 507 nm for the heterochiral meso form (1b).
The spectral data given in Table 5 indicate that in other
solvents also similar blue-shift of the MLCT band of the

S. Baitalik et al. / Inorganica Chimica Acta 357 (2004) 699–706
705
Table 5
Electronic spectral data for diastereoisomers of diruthenium(II) and diosmium(II) complexes in different solvents
Complex
Solvent
k )
max, nm (e, Mꢃ1 cmꢃ1
rac
meso
[(bpy)2Ru(pzdc)Ru(bpy)2]þ
MeOH
346(16 870)
460(9450)
512(13 480)
348(17 780)
458(9840)
507(13 900)
EtOH
MeCN
DMF
346(17 540)
459(9740)
517(14 380)
350(19 000)
462(10 300)
511(14 810)
357(17 000)
474(9380)
530(4300)
358(19 200)
470(10 220)
525(15 430)
358(17 420)
475(9500)
533(14 300)
358(18 100)
472(10 140)
529(15 260)
DMSO
356(18 200)
474(10 110)
534(15 250)
356(18 420)
470(10 830)
527(15 400)
[(bpy)2Os(pzdc)Os(bpy)2]þ
MeOH
MeCN
DMSO
356(17 300)
430(13 540)
530(12 700)
720b(3100)
360(18 100)
424(13 960)
526(13 220)
720b(3100)
362(17 360)
436(12 680)
542(12 470)
725b(3200)
358(17 800)
432(12 850)
537(12 120)
725b(3200)
362(17 600)
438(13 230)
550(13 020)
740b(3280)
360(16 950)
432(13 360)
542(13 230)
740b(3200)
Fig. 6. Absorption spectra of the diastereoisomers of
[(bpy)2Os(pzdc)Os(bpy)2]þ in dimethyl sulfoxide: —, meso form; –-–
rac form.
Fig. 5. Absorption spectra of the diastereoisomers of
[(bpy)2Ru(pzdc)Ru(bpy)2]þ in methanol: —, meso form; –-–, rac form.

706
S. Baitalik et al. / Inorganica Chimica Acta 357 (2004) 699–706
[9] (a) U. Knof, A. von Zelewsky, Angew. Chem., Int. Ed. Engl. 38
(1999) 302;
luminiscent spectral behaviour of the two diastereoiso-
mers of the diruthenium(II) complex is identical.
(b) H. Muerner, P. Belser, A. von Zelewsky, J. Am. Chem. Soc.
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Acknowledgements
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Chem. 38 (1999) 692.
We thank the Council of Scientific and Industrial
Research, India for financial support.
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