Ruthenium(II) and osmium(II) polypyridyl complexes of an asymmetric pyrazinyl- and pyridinyl-containing 1,2,4-triazole based ligand. Connectivity and physical properties of mononuclear complexes

Article abstract of DOI:10.1039/b206667j

The synthesis, purification and characterisation of two coordination isomers of ruthenium(II) and osmium (II) complexes containing the ligand 3-(pyrazin-2′-yl)-5-(pyridin-2′-yl)-1,2,4-triazole (Hppt) are described. The X-ray and molecular structure of the complex [Ru(bipy)₂(ppt)]PF₆·CH₃OH (1a) is reported, where the Ru(bipy)₂-centre is bound to the ppt^- ligand via the pyridine nitrogen and the N1 atom of the triazole ring. ¹H NMR spectroscopic measurements confirm that in the second isomer (1b) the Ru(bipy)₂-moiety is bound via the N2 atom of the triazole ring and the pyrazine ring. Partially deuteriated metal complexes are utilised to facilitate interpretation of ¹H NMR spectra. The redox and electronic properties indicate that there are significant differences in the electronic properties of the two coordination isomers obtained. The acid-base properties of the compounds are also reported and show that the pK_a of the 1,2,4-triazole ring varies systematically depending on the nature of the non-coordinating substituent. Analysis of these data indicates a significant electronic interaction between the pyridyl/pyrazyl rings and the 1,2,4-triazole ring in the coordinated ppt^- ligand.

Full text of DOI:10.1039/b206667j

View Online / Journal Homepage / Table of Contents for this issue
Ruthenium(II) and osmium(II) polypyridyl complexes of an
asymmetric pyrazinyl- and pyridinyl-containing 1,2,4-triazole
based ligand. Connectivity and physical properties of
mononuclear complexes†
a
a
a
b
c
Wesley R. Browne, Christine M. O’Connor, Helen P. Hughes, Ronald Hage, Olaf Walter,
c
d
a
Manfred Doering, John F. Gallagher and Johannes G. Vos*
a
National Centre for Sensor Research, School of Chemical Sciences, Dublin City University,
Dublin 9, Ireland. E-mail: johannes.vos@dcu.ie
Unilever Research Laboratory, Olivier van Noortlaan 120, 3133 AT, Vlaardingen,
The Netherlands
ITC-CPV, Forshungszentrum Karlsruhe, PO Box 3640, 76021 Karlsruhe, Germany
National Institute for Cellular Biotechnology, School of Chemical Sciences,
Dublin City University, Dublin 9, Ireland
b
c
d
Received 9th July 2002, Accepted 2nd September 2002
First published as an Advance Article on the web 30th September 2002
The synthesis, purification and characterisation of two coordination isomers of ruthenium() and osmium ()
complexes containing the ligand 3-(pyrazin-2Ј-yl)-5-(pyridin-2Љ-yl)-1,2,4-triazole (Hppt) are described. The X-ray
and molecular structure of the complex [Ru(bipy) (ppt)]PF ؒCH OH (1a) is reported, where the Ru(bipy) -centre
2
6
3
2
Ϫ
1
is bound to the ppt ligand via the pyridine nitrogen and the N1 atom of the triazole ring. H NMR spectroscopic
measurements confirm that in the second isomer (1b) the Ru(bipy) -moiety is bound via the N2 atom of the triazole
2
1
ring and the pyrazine ring. Partially deuteriated metal complexes are utilised to facilitate interpretation of H NMR
spectra. The redox and electronic properties indicate that there are significant differences in the electronic properties
of the two coordination isomers obtained. The acid–base properties of the compounds are also reported and show
that the pK of the 1,2,4-triazole ring varies systematically depending on the nature of the non-coordinating
a
substituent. Analysis of these data indicates a significant electronic interaction between the pyridyl/pyrazyl
Ϫ
rings and the 1,2,4-triazole ring in the coordinated ppt ligand.
Introduction
the nature of the lowest excited states of these multinuclear
compounds and on metal–metal interaction has been examined
in detail. It was found that by using ligands such as 3,5-
bis(pyridin-2Ј-yl)-1,2,4-triazole (Hbpt, Fig. 1) the lowest energy
For many years, ruthenium() and osmium() polypyridyl
complexes have attracted attention due to their well-defined
spectroscopic, photophysical, photochemical, and electro-
1
chemical properties. These properties are of particular use in
2
the construction of supramolecular systems and in the3
development of photochemically driven molecular devices.
Ruthenium() polypyridyl complexes have also received exten-
sive attention as functional models for water-oxidation catalysis
4
in photo-system II and in the catalytic photochemical cleavage
5
of water. Of particular interest is their incorporation into the
design of multinuclear structures capable of directing and
3
modulating electron and energy transfer processes. The ability
to tune the excited state properties of these complexes is central
to their potential for practical applications.
For these reasons there has been a detailed investigation
of the synthesis and characterisation of multinuclear ruthen-
ium() and osmium() polypyridyl complexes using negatively
6
,7
charged triazole based bridging ligands. It was shown that
with these ligands relatively strong interaction can be obtained
between metal centres, so that both electron transfer and energy
Fig. 1 Ligands described in text.
excited state is located on the polypyridyl ligands (i.e. bipy)
whilst with the ligand 3,5-bis(pyrazin-2Ј-yl)-1,2,4-triazole
(Hbpzt, Fig. 1) the emitting state is located on the pyrazine
8
transfer processes can occur efficiently. The immobilisation of
such dinuclear complexes on nanocrystalline TiO has also been
reported. In these studies the importance of the 1,2,4-triazole
bridge in mediating electron transfer was demonstrated. By
2
9
6,7
bridging ligand. This has important consequences for the
photochemical properties of these compounds and indeed
Ϫ
Ϫ
systematically changing the nature of the bridge, the effect on
dinuclear bpt and bpzt complexes show different products
10
when photolysed under the same conditions. In addition,
resonance Raman evidence suggested that in the dinuclear
bpzt complexes the excited state is located on one of the
1
†
Electronic supplementary information (ESI) available: H NMR spec-
tra of 1a and 1b. See http://www.rsc.org/suppdata/dt/b2/b206667j/
Ϫ
4
048
J. Chem. Soc., Dalton Trans., 2002, 4048–4054
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b206667j

View Online
1
1
pyrazine rings of the bridge. These results show clearly that
the photophysical properties of these dinuclear structures can
be manipulated by a careful choice of the bridging ligand.
To further investigate these systems we have extended our
study to include the ligand 3-(pyrazin-2Ј-yl)-5-(pyridin-2Љ-yl)-
X-Ray crystallography
The molecular structure of 1a is shown in Fig. 3. Complex 1a
co-crystallised with a molecule of methanol and a hexafluoro-
phosphate counter anion (not shown). From the crystal
structure it is clear that the ligand is bound through the
pyridine-N and N1 of the triazole ring (via N(1) and N(2) in
Fig. 3). The bite angle of the N(1)–Ru(1)–N(2) is 77.98 (6)Њ,
which corresponds well with the bite angle obtained by Hage et
1
,2,4-triazole (Hppt) (see Fig. 1). The synthesis of these
dinuclear complexes presents a major synthetic challenge. The
asymmetry of the ligand may result in the formation of up to
four different mononuclear coordination isomers as shown in
Fig. 2 and as expected the reaction of one equivalent of the
o
6
al. of 78(1) for [Ru(bipy) (bpt)]PF ؒ½H O, and of 77.9(1)Њ for
2
6
2
[
Ru(bipy) (3-(2-hydroxy-phenyl)-5-(pyridin-2-yl)-1,2,4-triazole)]-
2
14
PF ؒCH COCH . Bite angles of 78.87(7)Њ and 78.72(7)Њ for
6
3
3
bipyridyl ligands are also typical for this class of complex.
Ruthenium–nitrogen distances of 2.0398(16)–2.1033(17) Å are
15
also comparable to those found in other complexes. The
ruthenium–pyridine distance Ru(1)–N(1) at 2.1033(17) Å is the
longest Ru–N bond in the complex and the ruthenium–triazole
distance Ru(1)–N(2) at 2.0398(16) Å is the shortest, in
agreement with previously reported structures (e.g. 2.03(2) Å
6
[
Ru(bipy) (bpt)]PF ؒ½H O). The isomer obtained is therefore
2
6
2
identified as 1a, where the metal centre is bound to N1 of the
triazole ring and the pyridine moiety (Fig. 2).
ϩ
Fig. 2 Possible coordination isomers of [Ru(bipy) (ppt)] .
2
bridging ligand with two equivalents of a [M(bipy) ] precursor
2
leads to the formation of a mixture of different coordination
isomers. Therefore in order to obtain well-defined dinuclear
complexes an indirect route via the formation of mononuclear
precursors has to be developed.
In this contribution the synthesis, purification and character-
isation of two mononuclear coordination isomers of Ru(bipy)2
and Os(bipy) complexes with Hppt are reported. The com-
2
pounds obtained have been separated using chromatographic
1
techniques and were characterised using H NMR, electronic
spectroscopy and electrochemistry. The X-ray crystal structure
of one of the coordination isomers is reported. The acid-base
properties of the coordination isomers are also investigated and
are discussed, together with data obtained for related com-
plexes. In particular the effect of substitution in the C5 position
of the 1,2,4-triazole on the acid base properties is rationalised
on the basis of the electron withdrawing/donating properties of
the substituent.
Fig. 3 Molecular structure of the cation in 1a.
1
H NMR spectroscopy
1
It has been shown previously that H NMR spectroscopy
Results and discussion
can be used effectively to determine the coordination mode of
16
1
1
triazole based ligands. To this end, H and H COSY NMR
Synthesis and purification
spectroscopy together with specific deuteriation have been
26
Ϫ
The synthesis of the ligand Hppt and of the mononuclear
employed. In Table 1, the chemical shifts (δ) of the ppt proton
6,7
complexes
were carried out by previously reported pro-
resonances are presented together with those of the free ligand
Ϫ
Ϫ
cedures. Hppt is inherently asymmetric and as a result the
synthesis of its mononuclear complexes is complicated by the
possibility of the formation of four coordination isomers (Fig.
and of the analogous bpt and bpzt Ru(bipy) complexes. The
2
bipy resonances are as expected and are not listed (see
16
supplementary material for spectra of 1a/1b).
Ϫ
Ϫ
2
). The metal centre may be bound via N1 of the triazole ring
Results obtained for the analogous bpt and bpzt mono-
nuclear compounds show that coordination of a pyridyl/
pyrazyl moiety results in a significant change in the chemical
shift of its H6 proton while only minor changes to the H6
and the pyridine ring (1a), via N2 of the triazole ring and
the pyrazine ring (1b), via N4 of the triazole ring and the pyr-
idine ring (1c) and finally via N4 of the triazole ring and the
pyrazine ring (1d). It has been shown for several 1,2,4-triazoles
that the presence of bulky substituents in the 5- position of the
6,12
proton of the free ring are observed. This is illustrated in the
1
H NMR spectra of the partially deuteriated analogues 2a and
1
,2,4-triazole ring results in preferential formation of the N2
2b (Fig. 4). Partial deuteriation simplifies the analysis of the
spectra, since ppt based signals can be identified with ease. For
Ϫ
isomer. The amount of N4 bound isomers varies from 50%
12
(
5 position occupied by H) to less than 5% (5 position
2a, the H6 resonance of the coordinated pyridyl ring at 7.56
ppm is significantly different from the free ligand value of
8.72 ppm, in agreement with coordination being via the N1 of
the 1,2,4-triazole ring and the pyridine moiety. Similarly for 2b
the change in the H6 resonance of the pyrazyl ring from
8.73 ppm in the free ligand to 7.63 ppm shows that coordin-
ation in this case, and for 1b, is via the triazole N2 atom and the
pyrazine ring.
13
occupied by Br). HPLC analysis of reaction products before
purification shows that, in agreement with this observation, the
amount of N4 bound isomers present is estimated at less than
5
% and these are removed by recrystallisation. The remaining
two coordination isomers 1a and 1b were subsequently separ-
ated by column chromatography as detailed in the experimental
section.
J. Chem. Soc., Dalton Trans., 2002, 4048–4054
4049

View Online
1
Ϫ
Ϫ
Ϫ
Table 1 H NMR resonances observed for the mononuclear ppt complexes in CD CN, together with bpt and bpzt analogues
3
3
4
5
6
Compound
Hppt
H
H
H
H
pz
py
pz
py
pz
py
pz
py
pz
py
9.42 s
8.26 d
–
8.68 d
7.52 m
8.73 d
8.72 d
8.00 m
ϩ
ϩ
2
2
3
3
a [Ru(d -bipy) (ppt)]
9.20 (Ϫ0.22) s
8.22 (0.04) d
9.33 (Ϫ0.09) s
8.02 (Ϫ0.14) d
9.19 (Ϫ0.16) s
8.13 (Ϫ0.07) d
9.25 (Ϫ0.10) s
8.46 (ϩ0.26) d
8.23 (ϩ0.08) d
8.06 (Ϫ0.09) d
9.30 (Ϫ0.04) s
9.23 (Ϫ0.09) s
8.32 (ϩ0.17) d
8.23 (ϩ0.08) d
9.23 (Ϫ0.11) s
9.20 (Ϫ0.14) s
–
8.52 (Ϫ0.16) d
7.20 (Ϫ0.32) m
8.28 (Ϫ0.40) d
7.27 (Ϫ0.25) m
8.50 (Ϫ0.22) d
7.07 (Ϫ0.49) m
7.64 (Ϫ1.08) d
7.75 (ϩ0.19) m
7.26 (Ϫ0.26) m
7.20 (Ϫ0.32) m
8.36 (Ϫ0.42) d
8.46 (Ϫ0.32) d
7.30 (Ϫ0.26) m
7.59 (Ϫ0.07) m
8.06 (Ϫ0.72) d
8.46 (Ϫ0.32) d
8.46 (Ϫ0.27) d
7.56 (Ϫ1.16) d
7.63 (Ϫ1.10) d
8.54 (Ϫ0.18) d
8.43 (Ϫ0.29) d
7.42 (Ϫ1.37) d
8.13 (Ϫ0.59) d
8.53 (Ϫ0.26) d
7.74 (Ϫ0.98) d
8.45 (Ϫ0.22) d
7.61 (Ϫ1.17) d
8.46 (Ϫ0.32) d
7.74 (Ϫ0.97) d
8.65 (Ϫ0.02) d
7.57 (Ϫ1.21) d
8.46 (Ϫ0.32) d
8
2
7.94 (Ϫ0.06) m
b [Ru(d -bipy) (ppt)]
–
8
2
7.78 (Ϫ0.22) m
ϩ
a [Os(bipy) (ppt)]
–
2
7.75 (Ϫ0.28) m
ϩ
b [Os(bipy) (ppt)]
–
2
7.31 (Ϫ0.72) m
8.01 (ϩ0.01) m
7.74 (Ϫ0.26) m
ϩ
a
[
[
[
[
Ru(bipy) (bpt)]
Ring Ab
2
Ring B
ϩ
a
Ru(bipy) (bpzt)]
Ring Ab
Ring B
–
–
2
ϩ c
a
Os(bipy) (bpt)]
Ring Ab
7.94 (Ϫ0.06) m
8.14 (ϩ0.07) m
2
Ring B
ϩ
a
Os(bipy) (bpzt)]
Ring Ab
Ring B
–
–
2
Values in parantheses are change with respect to free ligand {i.e. Hppt (in CD CN), Hbpt (in (CD ) SO), Hbpzt (in (CD ) SO)}. pz = pyrazine ring,
3
3
2
3 2
a
b
c
py is pyridine ring {s = singlet, d = doublet, dd = doublet of doublets and m = multiplet}. Coordinated ring. Free ring. From ref. 28 measured in
CD ) CO.
(
3
2
6,7
pyridyl and pyrazine triazole complexes, however, there are
some significant differences in the electronic properties of the
two isomers as the comparison below shows.
Ϫ
For the pyridine bound complexes 1a and bpt , the absorp-
tion spectra of the protonated and deprotonated species are
very different, with a substantial blue shift in the λmax occurring
upon protonation (∼ 40 nm) (see Fig. 5). However, for the pyra-
1
Fig. 4 H NMR (400 MHz) spectra of 2a (upper spectrum) and 2b
(
lower spectrum) in CD CN.
3
For the osmium() complexes 3a/3b the upfield shift of the
H6 proton of the coordinated ring is greater than for the
ruthenium complexes. This has been observed previously, and
has been attributed to increased π-backbonding to the ligands
by osmium() compared with ruthenium(), which increases
the ring current of the aromatic rings and hence the shielding
Fig. 5 UV/vis absorption and emission spectra of 1a, H1a in
acetonitrile.
17
effect felt by the H6 proton.
In conclusion, with the X-ray structure of 1a as reference, the
Ϫ
zine bound complexes (1b and bpzt ) only a small blue shift in
Ϫ
coordination mode of mononuclear ppt complexes in general
1
the λmax of the MLCT absorption bands occurs upon proton-
1
can be determined conveniently by H NMR spectroscopy.
ation (∼ 10 nm) (Fig. 6). The effect of protonation on the
pyridine and pyrazine bound isomers are similar to those of
related mononuclear complexes (see Table 2). For the osmium
analogues a similar behaviour is observed, however, the pres-
Electronic properties
The visible absorption and emission data for the compounds
obtained are listed in Table 2. The lowest energy absorption
feature for the ruthenium complexes is assigned to several
3
ence of formally spin forbidden transitions ( MLCT) results in
more complex spectra.
1
overlapping singlet metal-to-ligand-charge-transfer ( MLCT)
All complexes examined are luminescent in acetonitrile at
room temperature and at 77 K. The ruthenium complexes
examined all emit in the 650 to 700 nm region and a large blue
shift is observed going from 300 to 77 K, typical for MLCT
emission. As for the absorption spectra, the emission max-
transitions (log ε ∼ 4.2), involving both bpy and Hppt ligands,
2
by comparison with other ruthenium polypyridyl complexes.
3
All compounds show strong absorptions (log ε ∼ 5) at about 280
2
nm which are π–π* in nature. For the osmium() complexes 3a/
3
Ϫ
3
b formally forbidden MLCT transitions are present at longer
imum of 1a (676 nm) is close to that of the analogous bpt
wavelengths (550 nm to 750 nm) similar to those observed for
complex (678 nm). Upon protonation of 1a, a blue shift in the
spectrum and a reduction in the emission lifetime is observed
2ϩ 18
[
Os(bipy) ] . Overall the electronic properties are typical for
3
4
050
J. Chem. Soc., Dalton Trans., 2002, 4048–4054

View Online
Table 2 Redox and electronic data for non-deuteriated complexes
a
Oxidation
Reduction
Emission/
Compound
potential/V
potential/V
Absorption/nm (log ε)
nm (τ/ns)
ϩ
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
Ru(bipy) (ppt)] (1a)
0.95
1.25
1.05
1.20
0.85
1.06
0.99
1.24
0.52
0.80
0.63
0.77
0.49
0.89
0.64
1.08
Ϫ1.45, Ϫ1.70
–
Ϫ1.50, Ϫ1.75
–
Ϫ1.47, Ϫ1.72
–
Ϫ1.42, Ϫ1.62
–
Ϫ1.42, Ϫ1.69
–
Ϫ1.44, Ϫ1.72
–
Ϫ1.41, Ϫ1.69
–
Ϫ1.41, Ϫ1.69
–
475 (4.04)
438
457 (4.35)
432
475 (4.05)
429 (4.19)
453 (4.15)
446 (4.07)
676 (72)
640 (5)
2
2
ϩ
Ru(bipy) (Hppt)] (H1a)
2
ϩ
Ru(bipy) (ppt)] (1b)
657 (82)
647 (29)
678 (72)
645 (45)
654 (88)
675 (28)
750 (45)
730 (39)
760 (16)
743 (5)
762 (55)
n/a (n/a)
783 (14)
791 (n/a)
2
2
ϩ
Ru(bipy) (Hppt)] (H1b)
2
ϩ b
Ru(bipy) (bpt)]
2
2
ϩ b
Ru(bipy) (Hbpt)]
2
ϩ c
Ru(bipy) (bpzt)]
2
2
ϩ c
Ru(bipy) (Hbpzt)]
2
ϩ
Os(bipy) (ppt)] (3a)
385 (3.95) 446 (3.74) 498 (3.82) 620
392 432 470 600
408 (4.04) 432 (4.07) 474 (4.11) 610
408 450 600
392 (1.34) 438 (1.06) 486 (1.08) 610 (0.28)
394 (1.45) 438 (1.38) 476 (1.42) 570 (sh)
404, 436 (3.66), 483 (3.70), 600
416, 452
2
2
ϩ
Os(bipy) (Hppt)] (H3a)
2
ϩ
Os(bipy) (ppt)] (3b)
2
2
ϩ
Os(bipy) (Hppt)] (H3b)
2
ϩ b
Os(bipy) (bpt)]
2
2
ϩ b
Os(bipy) (Hbpt)]
2
ϩ
Os(bipy) (bpzt)]
2
2
ϩ
Os(bipy) (Hbpzt)]
2
Ϫ1
Redox potentials (vs. SCE) were obtained using CV and DPP in 0.1 M TEAP–acetonitrile (scan rate:10 mV s ). Protonation by addition of
a
b
c
concentrated HClO . Measured in aerated acetonitrile at 298 K. Values obtained from ref. 6. Values obtained from ref. 7.
4
Ϫ
likewise the pyrazine bound isomer 1b and the bpzt based
complex) it would be expected that the pK values for the
a
related complexes would be quite similar. Unusually the
acid–base properties of 1a/1b are found to display behaviour
quite different to this expectation.
Table 3 shows that the acidity of the coordinated triazole ring
is strongly dependent on the nature of the non-coordinated
substituent in the C5 position. This dependence is reflected
in the ∆pK values, with respect to the unsubstituted C–H
a
analogues, given in Table 3. The effect of the introduction of a
pyridine or pyrazine ring is particularly relevant. A comparison
Ϫ
Ϫ
of the pK values of the pztr and bpzt complexes shows that
a
the triazole ring becomes more acidic by 1.7 pH units in the
presence of an additional, free, pyrazine ring. A comparison of
Ϫ
Ϫ
the values observed for the pytr and bpt complexes shows
that the effect of the introduction of a pyridine ring is far less
dramatic and does not result in a significant change in the pKa
(
ϩ 0.1 pH unit). This indicates that the free pyrazine group acts
Fig. 6 UV/vis absorption and emission spectra of 1b, H1b in
acetonitrile.
as a strong electron-withdrawing group. Within this framework
the pKa values are as expected and indicate that there is
substantial interaction between the different components of the
Hppt ligand. Similar trends are observed for the analogous
osmium() complexes. It is interesting to note the comparison
of the substituent effect on the acid–base properties of the
complexes and “Hammett” effects observed for organic systems
such as substituted benzoic acids.
for the pyridine bound isomer. This reduction in emission
lifetime, in contravention of the energy gap law, is due to the
reduction in the σ-donor strength of the 1,2,4-triazole ligand
3
and hence a lowering of the MC excited state, which is
thermally accessible and provides a route for rapid non-
6
radiative excited state deactivation. For 1b a 10 nm blue shift in
the λmax occurs upon protonation, however, the reduction in
emission lifetime is somewhat less dramatic than for 1a. For the
osmium complexes emission is observed at much lower energy
Electrochemical properties
18
(
> 700 nm) as expected. Both 3a and 3b undergo a blue shift in
Electrochemical potentials for pyridine bound and pyrazine
bound complexes are presented together with those of their
both absorption and emission spectra upon protonation.
Ϫ
Ϫ
analogous bpt and bpzt complexes in Table 2. As expected
the pyrazine bound isomer (1b) has a more positive metal-based
oxidation potential than the pyridine bound isomer (1a). This is
due mainly to the weaker σ-donor/stronger π-acceptor prop-
Acid–base properties
The acid–base properties of all compounds have been investi-
gated by studying the pH dependence of their absorption
19
erties of the pyrazine over the pyridine ligand. The first two
spectra. The pK values obtained from these studies and of
a
reduction potentials of the deprotonated complexes are revers-
some related complexes are presented in Table 3. The behaviour
observed can be explained by protonation/deprotonation of the
triazole moiety as indicated in eqn. (1).
2ϩ
ible and are similar to those of [Ru(bipy) ] , suggesting
3
20
they are bipy based reductions. Due to hydrogen formation
reduction potentials could not be obtained for the protonated
complexes. The oxidation potentials of the osmium complexes,
3a and 3b, are approximately 400 mV lower than those of the
corresponding ruthenium complexes. This is normal behaviour
for these types of systems and is attributed to the higher energy
ϩ
ϩ
2ϩ
[
Ru(bipy) (ppt)] ϩ H ↔ [Ru(bipy) (Hppt)]
(1)
2
2
Although protonation of the coordinated pyrazine ring is
possible, this occurs at only at negative pH (pK ∼ Ϫ1.5).
8
17
of the 5d orbitals compared to the 4d orbitals.
a
Given the structural similarities between the pyridine bound
complex 1a and the bpt based mononuclear complex (and
Overall the redox properties of all complexes are as
expected. However the influence of the free pyrazine ring
Ϫ
J. Chem. Soc., Dalton Trans., 2002, 4048–4054
4051

View Online
Table 3 Ground state pK values of 1,2,4-triazole based mononuclear complexes
a
Pyrazine bound complexes
pKa
∆pKa
Pyridine bound complexes
pKa
∆pKa
ϩ a
ϩ a
[
[
1
[
[
[
Ru(bipy) (pztr)]
3.7
2.0
3.8
4.2
1.4
1.2
3.5
0
[Ru(bipy) (pytr)]
4.1
2.7
4.2
4.9
1.3
2.1
4.0
0
2
2
ϩ a
Ru(bipy) (bpzt)]
Ϫ1.7
ϩ0.1
ϩ0.5
Ϫ2.3
Ϫ2.5
Ϫ0.2
1a
Ϫ1.4
ϩ0.1
ϩ0.8
Ϫ2.8
Ϫ2.0
Ϫ0.1
2
ϩ a
b
[Ru(bipy) (bpt)]
2
ϩ a
ϩ a
Ru(bipy) (Mepztr)]
[Ru(bipy) (Mepytr)]
[Ru(bipy) (Brpytr)]
2
2
ϩ b
ϩ b
Ru(bipy) (Brpztr)]
2
2
ϩ
Os(bipy) (bpzt)]
3a
2
ϩ a
3
b
[Os(bipy) (bpt)]
2
a
b
All measurements are carried out in Britton–Robinson buffer, values ± 0.1. From ref. 28. From ref. 29.
Ϫ
of 1a and the bpzt based complexes is of note with the
redox potentials of these complexes being in every case signif-
icantly higher than for their –H substituted analogues. This
is in agreement with the acid–base properties of these com-
plexes, with the electron withdrawing influence of the pyrazine
ring resulting in a reduction in the σ-donor strength of
Syntheses
3
-(Pyrazin-2Ј-yl)-5-(pyridin-2Љ-yl)-1H-1,2,4-triazole (Hppt) was
26
prepared by standard procedures.
2
-Pyrazylhydrazide. 15 g of 2-pyrazylcarboxylic acid (0.12
3 3
mol) were heated at reflux for 3 h in 90 cm ethanol and 15 cm
concentrated H SO . The solution was neutralised with satur-
the 1,2,4-triazole ring (resulting in a lowering of the pK ) and
consequently a stabilisation of the metal centre towards
oxidation.
a
2
4
ated Na CO solution and filtered. The volume was reduced
2
3
3
in vacuo and extracted with four 30 cm aliquots of dichloro-
methane. The combined fractions were washed with 10 cm of
3
water and dried over MgSO . The dichloromethane was
4
Conclusions
removed in vacuo to yield the ethyl 2-pyrazylcarboxylate, which
3
In this contribution the synthesis, structural, electrochemical
and electronic characterisation of a series of ruthenium() and
osmium() (bipy) complexes based on the ligand Hppt are
reported. The acid–base properties of the two major coordin-
ation isomers, taken together with a series of related complexes,
provide for a much improved understanding of the effect of
crystallised on cooling. The ester was dissolved in 20 cm of
ethanol and an equimolar amount of hydrazine monohydrate
was added drop-wise. The solution was then kept a Ϫ4 ЊC for
12 h and crystalline 2-pyrazylhydrazide formed. The solid
2-pyrazylhydrazide was filtered off and air-dried. Yield 12 g, 74
2
1
%. H NMR in (CD ) SO: δ 10.15 (1H, s, NH), 9.11 (1H, s),
3
2
“
spectator” or non-coordinated substituents on the 1,2,4-
8.81 (1H, d), 8.67 (1H, d), 4.65 (2H, s).
triazole ring. It is shown that these substituents are of major
importance in determining the ground state properties of both
the ruthenium() and osmium() based complexes. Overall the
effect of protonation on the electronic and electrochemical
properties of the complexes examined are as expected and con-
1-(Pyrazin-2Ј-yl)-4-(pyridin-2Љ-yl)acylamidrazone. Equimolar
amounts of 2-cyanopyridine (as to the 2-pyrazylhydrazide, 12g)
3
were dissolved in 30 cm of methanol with 0.7 g of sodium and
heated under reflux for 3 h. The 2-pyrazylhydrazide was added
to the solution and heated for 15 min. The solution was cooled
to room temperature and the precipitate formed (yellow) was
Ϫ
firm the assignments of coordination mode of the ppt ligand
1
made on the basis of H NMR spectroscopy and X-ray
1
crystallography.
filtered off and air-dried for 2 h. Yield 12.4 g, 67%. H NMR in
The isolation and identification of the major coordination
isomers formed has been achieved and allows for the prep-
aration of both homo- (RuRu, OsOs) and hetero- (RuOs)
binuclear complexes of the ligand Hppt in a systematic and
controlled manner. The inherent asymmetry of the Hppt ligand
results in important differences in the ground state properties of
the mononuclear coordination isomers. In further studies the
effects of these differences on the excited state properties of the
(CD ) SO: δ 9.19 (1H, d, H3Љ), 8.73 (1H, d, H6Љ), 8.60 (1H, dd,
H5Љ), 8.19 (1H, d, H3Ј), 7.90 (1H, t, H4Ј), 7.51 (1H, t, H5Ј), 8.84
(1H, d, H6Ј).
3
2
3-(Pyrazin-2Ј-yl)-5-(pyridin-2Љ-yl)-1H-1,2,4-triazole (Hppt).
12.4
g (0.05 mol) of 1-(pyrazin-2Ј-yl)-4-(pyridin-2Љ-yl)-
3
acylamidrazone was heated under reflux for 1 h in 10 cm of
ethylene glycol to yield the Hppt ligand. The ligand (white) was
Ϫ
1
two parts of the ppt ligand will be investigated in greater
recrystalised from hot ethanol. Yield: 8 g (52%). H NMR in
detail. Of particular interest is the location of the excited state
and, for binuclear complexes, the degree of interaction between
the metal centres mediated by the bridging ppt ligand. In these
(CD ) SO δ 9.33 (1H, d, H3Љ), 8.72 (2H, m, H5Љ, H6Љ), 8.17 (1H,
d, H3Ј), 8.01 (1H, t, H4Ј), 7.54 (1H, t, H5Ј), 8.77 (1H, d, H6Ј).
3
2
Ϫ
studies selective deuteriation will be invaluable in understand-
ing the excited state properties of mono- and bi-nuclear com-
plexes, in particular in the interpretation of their resonance
[Ru(bipy) (ppt)]PF ؒ3H O 1a/[Ru(bipy) (ppt)]PF ؒH O.CH -
2
6
2
2
6
2
3
OH 1b. 520 mg (1 mmol) of cis-[Ru(bipy) Cl ]ؒ2H O was added
2
2
2
3
to 0.45 g (2.1 mmol) Hppt dissolved in 50 cm of hot ethanol–
water (1:1).The solution was heated at reflux for 4 h and
21
Raman spectra.
3
evaporated to dryness. The residue was dissolved in 10 cm of
3
water and three drops of conc. aqueous ammonia and 5 cm
Experimental
of saturated aqueous ammonium hexafluorophosphate were
added to precipitate 1a/1b. The isomers were separated by
column chromatography using neutral alumina and acetonitrile
as eluent. The pyridine bound isomer (1a) eluted first followed
by the pyrazine bound isomer (1b). Only very small amounts of
the N4 bound isomers were obtained subsequently by elution
with methanol and were not further investigated. Further
purification took place by recrystallisation from acetone–water
(1:1 v/v). Yield of combined fractions: 420 mg (51 %). Mass
Materials
All solvents employed were of HPLC grade or better and used
as received unless otherwise stated. For all spectroscopic
measurements Uvasol (Merck) grade solvents were employed.
All reagents employed in synthetic procedures were of reagent
2
2
grade 3or better. [D ]-2,2Ј-bipyridine,
cis-[Ru(bipy) Cl ]ؒ
cis-[Os(bipy) Cl ]ؒ
2 2
25
8
2 2
2
2
23
2
2
H O,
2
cis-[Ru([D ]-bipy) Cl ]ؒ2H O,
8
2
2
2
4
ϩ
ϩ
H O, and tetraethylammonium perchlorate (TEAP) were
spec: m/z 1a 637 (M ), 1b 637 (M ). Elemental analysis: Found
2
prepared by previously reported methods.
052 J. Chem. Soc., Dalton Trans., 2002, 4048–4054
(calc. for C H F N PRuؒ3H O (1a)): C 44.6 (44.55); H 3.2
31
23
6
10
2
4

View Online
1
1
Table 4 X-Ray parameters for [Ru(bipy) (ppt)]PF ؒCH OH
an Exador analytical CE440. H and H COSY spectra were
recorded on a Bruker Avance (400 MHz) NMR spectrometer.
Peak positions are relative to residual solvent peaks. Electro-
chemical measurements were carried out on a Model 660 elec-
trochemical workstation (CH Instruments). Typical complex
concentrations were 0.5 to 1 mM in anhydrous acetonitrile
(Aldrich 99.8%) containing 0.1 M TEAP. A Teflon shrouded
glassy carbon or platinum working electrode, a Pt wire auxiliary
electrode and SCE reference electrode were employed. Solu-
tions for reduction measurements were deoxygenated by pur-
2
6
3
Empirical formula
Formula weight/g mol
Temperature/K
Wavelength/Å
Space group
Crystal system
a/Å, α/Њ
C H F N OPRu
32 27 6 10
Ϫ1
813.67
200(2)
0.71073
¯
P1 (No. 2)
Triclinic
9.9097(7), 93.8570(10)
12.5731(9), 93.4100(10)
14.1156(10), 111.4390(10)
1626.6(2), 2
1.661
b/Å, β/Њ
c/Å, γ/Њ
Volume/Å , Z
3
Ϫ3
ging with N or Ar gas for 15 min prior to the measurement.
Measurements were made in the range of Ϫ2.0 to 2.0 V (w.r.t
SCE electrode). Protonation of complexes was achieved by
addition of 0.1 M HClO to the electrolyte solution. The scan
Density (calc.)/Mg m
2
Crystal dimensions/mm
Absorption coeff./mm
0.6 × 0.5 × 0.4
0.612
Ϫ1
F(000)
820
1.45–28.30
Ϫ12 ≤ h ≤ 12;
Ϫ15 ≤ k ≤ 16;
Ϫ18 ≤ l ≤ 18
17142
4
θ range for data collection/Њ
Limiting indices
Ϫ1
rates used were typically 100 or 200 mV s . UV/Vis absorption
spectra were recorded on a Shimadzu UV/Vis-NIR 3100 spec-
trophotometer interfaced with an Elonex PC466 using UV/Vis
data manager. Absorption maxima, ±2 nm. Molar absorption
coefficients are ± 10%. Emission spectra (accuracy ±5 nm) were
recorded at 298 K using a LS50B luminescence spectro-
photometer, equipped with a red sensitive Hamamatsu R928
PMT detector, interfaced with an Elonex PC466 employing
Perkin-Elmer FL WinLab custom-built software. Emission and
excitation slit widths were 10 nm at 298 K and spectra are
uncorrected for photomultiplier response; 10 mm pathlength
quartz cells were used for recording spectra. pH titrations were
carried out in Britton-Robinson buffer (0.04 M H BO , 0.04 M
No. eflections collected
Independent reflec. (Rint
)
734 (0.0185)
7634/0/469
1.048
Data/restraints/parameters
2
GOF F
Final R indices [I > 2σ(I)] R1 (wR2)
R indices all data R1 (wR2)
Largest difference peak and hole/e Å
0.0293 (0.0726)
0.0342 (0.0749)
0.702 and Ϫ0.524
Ϫ3
(
3.11); N 16.2 (16.77) %. Found (calc. for C H F N -
31 23 6 10
PRu.H OؒCH OH (1b)): C 45.0 (46.10); H 3.3 (3.36); N 17.2
2
3
3
3
(
16.81) %. Crystals of 1a suitable for X-ray analysis were grown
H PO , 0.04 M CH CO H). The pH was adjusted using concen-
3 4 3 2
from a methanol solution of 1a.
trated sulfuric acid or sodium hydroxide solution.
[
Ru([D ]-bipy) (ppt)]PF ؒCH OHؒ2H O 2a/[Ru([D ]-bipy) -
8 2 6 3 2 8 2
Acknowledgements
(
7
ppt)]PF ؒ2H O 2b. As for 1a/1b. Yield of combined fractions:
12 mg (93%). Mass spec: m/z 2a 653 (M ), 2b 653 (M ).
Elemental analysis: Found (calc. for C H D F N -
6
2
ϩ
ϩ
The authors thank Enterprise Ireland for financial assistance.
31
7
16
6
10
PRu.CH OH.2H O (2a)): C 44.60 (44.29); H 2.85 (3.34); N
3
2
1
5.84 (16.15) %. Found (calc. for C H D F N PRu.2H O
References
31
7
16
6
10
2
(
2b)): C 44.46 (44.66); H 2.77 (3.00); N 16.32 (16.81) %.
1
(a) V. Balzani, A. Juris, M. Venturi, S. Campagna and S. Serroni,
Acc. Chem. Res., 1998, 31, 26; (b) C. A. Slate, D. R. Striplin,
J. A. Moss, P. Chen, B. W. Erickson and T. J. Meyer, J. Am. Chem.
Soc., 1998, 120, 4885; (c) Y.-Z. Hu, S. Tsukiji, S. Shinkai, S. Oishi
and I. Hamachi, J. Am. Chem. Soc., 2000, 122, 241; (d ) V. Balzani,
A. Juris, M. Venturi, S. Campagna and S. Serroni, Chem. Rev., 1996,
96, 759.
(a) A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and
A. von Zelewsky, Coord. Chem. Rev., 1988, 84, 85; (b) T. J. Meyer,
Acc. Chem. Res., 1989, 22, 163; (c) B. O’Regan and M. Graetzel,
Nature (London), 1991, 335, 737; (d ) L. De Cola and P. Belser,
Coord. Chem. Rev., 1998, 177, 301; (e) C. A. Bignozzi, J. R.
Schoonover and F. Scandola, Prog. Inorg. Chem., 1997, 44, 1; ( f )
M.-J. Blanco, M. C. Jiménez, J.-C. Chambron, V. Heitz, M. Linke
and J.-P. Sauvage, Chem. Soc. Rev., 1999, 28, 293.
(a) V. Balzani and F. Scandola, Supramolecular Photochemistry, Ellis
Horwood, Chichester, 1991; V. Balzani (Editor), Supramolecular
Photochemistry, Reidel, Dordrecht, 1997.
H. Dürr and S. Bossmann, Acc. Chem. Res., 2001, 34, 905.
P. R. Rich, Faraday Discuss. Chem. Soc., 1982, 75, 349.
(a) R. Hage, R. Prins, J. G. Haasnoot, J. Reedijk and J. G. Vos,
J. Chem. Soc., Dalton Trans., 1987, 1389; (b) H. A. Nieuwenhuis,
J. G. Haasnoot, R. Hage, J. Reedijk, T. L. Snoeck, D. J. Stufkens and
J. G. Vos, Inorg. Chem., 1990, 30, 48; (c) B. E. Buchanan, R. Wang,
J. G. Vos, R. Hage, J. G. Haasnoot and J. Reedijk, Inorg. Chem.,
[
Os(bipy) (ppt)]PF ؒ2H O 3a/[Os(bipy) (Hppt)](PF ) ؒ2H O
2
6
2
2
6
2
2
H3b. As for 1a/1b except 575 mg (1 mmol) of cis-[Os(bipy) Cl ]
was heated at reflux with 223 mg (1 mmol) of Hppt for 4 d with
0 mg of zinc powder. Yield of combined fractions: 0.82 g (78
). Mass spec: m/z 3a 725 (M ), Elemental analysis: Found
calc. for C H F N POsؒ2H O (3a)): C 41.1 (41.05); H 2.80
2
2
5
%
(
(
ϩ
2
31
23
6
10
2
2.76); N 15.28 (15.45) %. Found (calc. for C H F N P Osؒ
31 24 12 10 2
2
H O (3b)): C 35.6 (35.35); H 2.6 (2.47); N 13.5 (13.31) %.
2
[
Os(bipy) (Hbpzt)](PF ) ؒH O. As for 3a/3b except 440 mg
2 6 2 2
(
2 mmol) of Hbpzt was heated at reflux with 575 mg (1 mmol)
of cis-[Os(bipy) Cl ]ؒ2H O. Yield: 470 mg (0.42 mmol, 42%).
Elemental analysis: Found (calc. for C H F N P OsؒH O): C
5.6 (34.78); H 2.34 (2.32); N 14.93 (14.83) %.
2
2
2
3
30
23 12 11
2
2
3
4
5
6
X-Ray crystallography
Data for 1a were collected on a Siemens SMART 1000 CCD-
diffractometer fitted with a molybdenum tube (K , λ = 0.71073
Å) and a graphite monochromator. Relevant experimental data
are presented in Table 4. A full sphere of data was collected
with the irradiation time of 4 s per frame. The structures were
solved with direct methods and all non hydrogen atoms refined
anisotropically with the SHELX program (refinement by
least-squares against F ).
CCDC reference number 187996.
See http://www.rsc.org/suppdata/dt/b2/b206667j/ for crystal-
lographic data in CIF or other electronic format.
α
1
990, 29, 3263; (d ) W. R. Browne, C. M. O’Connor, C. Villani and
J. G. Vos, Inorg. Chem., 2001, 40, 5461.
7 (a) R. Hage, A. H. J. Dijkhuis, J. G. Haasnoot, R. Prins,
J. Reedijk, B. E. Buchanan and J. G. Vos, Inorg. Chem., 1988, 27,
2185; (b) F. Barigelletti, L. De. Cola, V. Balzani, R. Hage,
J. G. Haasnoot and J. G. Vos, Inorg. Chem., 1989, 28, 4344.
R. Hage, J. G. Haasnoot, H. A. Nieuwenhuis, J. Reedijk, D. J. A.
De Ridder and J. G. Vos, J. Am. Chem. Soc., 1990, 112, 9249.
C. Lees, C. J. Kleverlaan, C. A. Bignozzi and J. G. Vos, Inorg. Chem.,
27
2
8
9
2
001, 40, 5343.
Instrumental methods
10 H. P. Hughes and J. G. Vos, Inorg. Chem., 1995, 34, 4001.
1
1 G. Coates, T. E. Keyes, H. P. Hughes, P. M. Jayaweera,
J. J. McGarvey and J. G. Vos., J. Phys. Chem. A, 1998, 102,
5013.
Elemental analysis on C, H and N was carried out at the
Microanalytical Laboratory at University College Dublin using
J. Chem. Soc., Dalton Trans., 2002, 4048–4054
4053

View Online
20 (a) C. M. Elliot and E. J. Hershenhart, J. Am. Chem. Soc.,
1
2 (a) H. A. Nieuwenhuis, J. G. Haasnoot, R. Hage, J. Reedijk,
T. L. Snoeck, D. J. Stufkens and J. G. Vos, Inorg. Chem., 1991, 30, 48;
1982, 104, 7519; (b) R. Hage, J. G. Haasnoot, D. J. Stufkens,
T. L. Snoeck, J. G. Vos and J. Reedijk, Inorg. Chem., 1989, 28,
1413.
(
b) S. Fanni, T. E. Keyes, C. M. O’Connor, H. Hughes, R. Wang and
J. G. Vos, Coord. Chem. Rev., 2000, 208, 77.
3 S. Fanni, C. Di Pietro, S. Serroni, S. Campagna and J. G. Vos, Inorg.
Chem. Commun., 2000, 3, 42.
1
1
21 W. R. Browne and J. G. Vos, Coord. Chem. Rev., 2001, 761,
787.
4 R. Hage, J. G. Haasnoot, J. Reedijk, R. Wang, E. M. Ryan, J. G. Vos,
A. L. Spek and A. J. M. Duisenberg, Inorg. Chim. Acta, 1990, 174,
22 W. R. Browne, C. M. O’Connor, J. S. Killeen, A. L. Guckian,
M. Burke, P. James, M. Burke and J. G. Vos, Inorg. Chem., 2002, 41,
4245.
7
7.
5 P. Rillema, D. S. Jones, C. Woods and H. A. Levy, Inorg. Chem.,
992, 31, 2935.
1
1
1
23 B. P. Sullivan, D. J. Salmon and T. J. Meyer, Inorg. Chem., 1978, 17,
1
3334.
6 R. Hage, R. Prins, J. G. Haasnoot, J. Reedijk and J. G. Vos, J. Chem.
Soc., Dalton Trans., 1987, 1389.
7 (a) G. M. Bryant and J. E. Fergusson, Aust. J. Chem., 1971, 24, 441;
24 M. Kober, K. A. Goldsby, D. N. S. Narayana and T. J. Meyer, J. Am.
Chem. Soc., 1983, 105, 4303.
25 R. Wang, J. G. Vos, R. H. Schmehl and R. Hage, J. Am. Chem. Soc.,
1992, 114, 1964.
26 R. Prins, R. A. G. de Graaff, J. G. Haasnoot, C. Vader and
J. Reedijk, J. Chem. Soc., Chem. Commun., 1986, 1430.
27 G. M. Sheldrick, SHELX-97, University of Göttingen, Göttingen,
1997.
28 R. Hage, Thesis Ph.D., Leiden University, Leiden, 1991.
29 C. Di Pietro, S. Serroni, S. Campagna, M. T. Gandolfi, R. Ballardini,
S. Fanni, W. Browne and J. G. Vos, Inorg. Chem., 2002, 41,
2871.
(
b) Y. Ohsawa, M. K. DeArmond, K. W. Hanck and C. G.
Moreland, J. Am. Chem. Soc., 1985, 107, 5383; (c) J. M. de Wolf,
R. Hage, J. G. Haasnoot, J. Reedijk and J. G. Vos, New J. Chem.,
1
991, 15, 501.
8 G. M. Bryant, J. E. Fergusson and H. J. K. Dowell, Aust. J. Chem.,
971, 24, 257.
9 (a) E. S. Dodsworth and A. B. P. Lever, Chem. Phys. Letts., 1986,
24, 152; (b) D. P. Rillema, G. Allen, T. J. Meyer and D. Conrad,
Inorg. Chem., 1983, 22, 1617.
1
1
1
1
4
054
J. Chem. Soc., Dalton Trans., 2002, 4048–4054