Paramagnetic Ru(III) complexes of tridentate ligands: Characterization of useful intermediates for heteroleptic Ru(II) complexes

Article abstract of DOI:10.1016/j.inoche.2010.12.011

Paramagnetic Cl₃Ru(L) complexes of tridentate ligands (2a: L = 1a = 4′-(p-bromophenyl)-2,2′:6′,2″-terpyridine; 2b: L = 1b = 6-(p-bromophenyl)-2,4-dipyrid-2-yl-1,3,5-triazine) were synthesized in a high-yield method with facile isolation of these useful synthons. The complexes were isolated in high purity and were characterized by several methods, including standard techniques such as ¹H NMR and electrospray ionization mass spectrometry. The ¹H NMR of the complexes displayed peaks from + 10 to - 37 ppm, with the protons ortho to the nitrogen atoms coordinated to the paramagnetic centre being shifted the most (2a: H _6,6″ = - 35.3 ppm; 2b: H_6,6″ = - 26.1 ppm), while the protons on the non-bonding phenyl rings were relatively unchanged with respect to their uncomplexed ligands. The electronic absorption spectra of the complexes displayed both ¹LMCT bands (Cl-to-Ru, 2a: λmax = 405 nm; 2b: λmax = 420 nm) and ¹MLCT (Ru-to-L, 2a: λmax = 486 nm; 2b: λmax = 567 nm) bands. Due to the ease of purification and high yields, the use of complex 3, first introduced by Chatt, is the method of choice to form Cl₃Ru(L) complexes of tridentate ligands.

Full text of DOI:10.1016/j.inoche.2010.12.011

Inorganic Chemistry Communications 14 (2011) 399–402
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Paramagnetic Ru(III) complexes of tridentate ligands: Characterization of useful
intermediates for heteroleptic Ru(II) complexes
b,
b,
Marie-Pierre Santoni a,b,1, Amlan K. Pal a,1, Garry S. Hanan a, , Anna Proust , Bernold Hasenknopf
⁎
⁎
⁎
a
Département de Chimie, Université de Montréal, 2900 Edouard-Montpetit, Montréal, Québec, Canada, H3T-1J4
b
Institut Parisien de Chimie Moléculaire, UMR7201, Université Pierre et Marie Curie, Case Courrier 42, 4 place Jussieu, 75252 Paris cedex 05, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Paramagnetic Cl3Ru(L) complexes of tridentate ligands (2a: L=1a=4′-(p-bromophenyl)-2,2′:6′,2″-
terpyridine; 2b: L=1b=6-(p-bromophenyl)-2,4-dipyrid-2-yl-1,3,5-triazine) were synthesized in a high-
yield method with facile isolation of these useful synthons. The complexes were isolated in high purity and
were characterized by several methods, including standard techniques such as 1H NMR and electrospray
ionization mass spectrometry. The 1H NMR of the complexes displayed peaks from +10 to −37 ppm, with
the protons ortho to the nitrogen atoms coordinated to the paramagnetic centre being shifted the most
(2a: H6,6″ =−35.3 ppm; 2b: H6,6″ =−26.1 ppm), while the protons on the non-bonding phenyl rings were
relatively unchanged with respect to their uncomplexed ligands. The electronic absorption spectra of
the complexes displayed both 1LMCT bands (Cl-to-Ru, 2a: λmax=405 nm; 2b: λmax=420 nm) and 1MLCT
(Ru-to-L, 2a: λmax=486 nm; 2b: λmax=567 nm) bands. Due to the ease of purification and high yields, the
use of complex 3, first introduced by Chatt, is the method of choice to form Cl3Ru(L) complexes of tridentate
ligands.
Received 9 August 2010
Accepted 9 December 2010
Available online 16 December 2010
© 2010 Elsevier B.V. All rights reserved.
Ruthenium polypyridyl complexes are attractive species as
chromophores in light-harvesting asssemblies which aim to convert
solar energy into chemical energy [1–5]. They display desirable
electrochemical and photophysical properties such as : (i) a strong
absorption in the visible region of the spectrum; (ii) a relatively long
lifetime of the emissive 3MLCT (Metal-to-Ligand Charge-Transfer)
excited-state at room temperature (r.t.); (iii) a capacity for energy
and electron transfer; (iv) and in some cases, a capacity for reversible
multi-electron storage [6]. The prototypical Ru(bpy)23+ unit (bpy=
2,2′-bipyridine) has a long-lived r.t. excited state [7], however, the
stereogenic Ru(bpy)23+ motif leads to complicated mixtures of
isomers in larger supramolecular assemblies [8–10]. In contrast, the
achiral Ru(tpy)22+ (tpy=2,2′:6′,2″-terpyridine) motif gives unique
products in polymetallic complexes when substituted symmetrically
on the tpy rings [10,11], thus simplifying the synthesis of oligomeric
species. The synthetic pathway commonly followed to obtain
heteroleptic Ru(II)-polypyridine complexes is via intermediate deri-
vatives of Ru(III), LRuCl3 (with L=neutral tridentate polypyridine
ligand) [11–15]. In a subsequent step, this intermediate is tradition-
ally refluxed in a polar solvent with silver salts and 1 equiv. of L′
(where L′=different neutral tridentate polypyridine ligand), to give a
heteroleptic complex [LRuL′]2+. Although, this protocol is widely
used, characterization of the Ru(III) intermediates remains limited
due to the paramagnetic nature of Ru(III)-polypyridine complexes
produced in this way, even though it has been demonstrated that
coordination complexes of Ru(III) can be studied by 1H NMR [16], an
essential tool in synthetic chemistry. Another important technique
is ESI-MS (Electro-Spray Ionization Mass Spectrometry), commonly
used in identification of coordination complexes, and even, in the
elucidation of reaction mechanisms [17]. The main difficulty with
poorly soluble neutral species lies in the successful ionization of the
compound and its identification, which is sometimes obscured by
fragmentation processes arising from the ionization process. We
report herein the characterization by 1H NMR and ESI-MS of neutral
paramagnetic Ru(III) intermediates, 2a–b, using a Ru(III) precursor
initially described by Chatt et al. over thirty years ago [18].
Complex 3 presents significant advantages when the syntheses of
tridentate ligands are multi-step or are only available in low yields
[19]. In particular, 3 is: (i) more soluble than RuCl3·xH2O, allowing a
variety of solvents to be used for synthesis; and (ii) successfully
complexed by tridentate ligands in refluxing CH3CN, due to the easy
displacement of thioether ligands avoiding the need for polar and
reducing solvents (such as alcohols and DMF). Meyer et al. [20].
previously reported the synthesis of (tpy)RuCl3 as being more
convenient and high-yielding starting from RuCl3·xH2O than from
3. However, even in the case of ligands 1a–b, we found out that 3 leads
⁎
Corresponding authors. Hanan is to be contacted at Tel.: +1 514 343 7056; fax: +1
514 343 2468. Proust and Hasenknopf, Fax: + 33 1/44273841.
E-mail addresses: garry.hanan@umontreal.ca (G.S. Hanan), anna.proust@upmc.fr
(A. Proust), bernold.hasenknopf@upmc.fr (B. Hasenknopf).
1
Tel.: +1 514 343 7056; fax: +1 514 343 2468.
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.12.011

400
M.-P. Santoni et al. / Inorganic Chemistry Communications 14 (2011) 399–402
Br
Br
Br
Br
N
N
N
N
H
S
N
N
O
N
N
N
N
S
Ru
Ru
Ru
N
N
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
N
N
Cl
N
N
1a
1b
2a
2b
3
Chart 1. Chart. Structures for ligands 1a–b, complexes 2a–b and precursor 3.
Scheme 1. Synthesis of LRuCl3 (2a–b) from RuCl3·xH2O (classical way) vs. from RuCl3
(iPrSPh)2(CH3OH) (3).
to easier purification of the desired complexes 2a–b. Indeed, removal
of the cationic homoleptic Ru(II) complex which forms in the syn-
thesis of is of crucial importance when one aims at further complexa-
tion, in a ‘step-by-step’ approach toward supramolecular asssemblies
[21] (Chart 1, Scheme 1 and Table 1).
Ligands 1a [22] and 1b [23] were synthesized following published
procedures. Precursor 3 was synthetized following the initial report
from Chatt et al. [18],. and was fully characterized by 1H NMR, ESI-MS,
elemental analysis, UV–visible absorption and X-ray diffraction (see
Supplementary Information). The high-resolution ESI-MS analysis
allowed identification of 3 and some fragmentation pathways : [3-
2Cl–CH3OH+e-]+; [3-2Ph]+ and [3-Ph-iPr]+. The X-ray structure con-
firmed the formation of the mer isomer (Fig. 1), predicted by Chatt, and
required for complexation to a tridentate polypyridine-type ligand.
The presence of the paramagnetic centre Ru(III) significative-
ly influences the chemical shifts of the protons from the organic moiety
(Fig. 2). Signals observed between 9.8 and 7.3 ppm correspond to a
diamagnetic decomposition by-product, forming in a few hours in
DMSO by slow reduction of Ru(III)16c (over the duration of data
acquisition). Signals corresponding to the paramagnetic species (major
product) were observed between 10 and −37 ppm (Fig. 2).
Fig. 1. ORTEP diagram of 3 (thermal ellipsoides drawn at 50% probability), confirming
the mer geometry, needed to accommodate a coordinating tridentate ligand.2
Table 1
Absorption data, measured in acetonitrile, for complexes 2a–b and 3.
The unpaired electron of Ru(III) accelerates the relaxation processes
of neighboring nuclei, masking the coupling interaction 1H–1H with a
distance-dependancy of 1/r3. Thus, all signals appear as broad singlets,
except for the most distant protons H2″′ and H3″′, belonging to the
phenyl ring (vide infra). Assignment of the different signals remains an
important step in the understanding of these species : (a) giving access
to information on electronic effects, in the molecule itself and by
comparision within a family of derivatives; (b) allowing NMR 1H
monitoring of complexation reactions (particularly important whenev-
er Thin-Layer Chromatography is not a viable option). At first, the
observed loss of signals multiplicity prevented a meaningful assignment
Compound
λ/nm (ε/M−1 cm−1
)
2a
227
285
306
405
486
(2950)
MLCT
Ru→1a
567
(2700)
MLCT
Ru→1b
479
(1110)
MLCT
(21145)
(19950)
(14900)
(6010)
LMCT
Cl→Ru
420
LC π–π* 1a
294
LC π–π* 1a
320 (sh)
2b
3
–
(24600)
LC π–π*
1b
(7480)
LMCT
Cl→Ru
404
LC π–π*
1b
253
–
–
(15440)
LC π–π*
Sulfide
(4570)
LMCT
Cl→Ru
Ru→sulfide
2
CIF information is available as CCDC 774603.

M.-P. Santoni et al. / Inorganic Chemistry Communications 14 (2011) 399–402
401
Fig. 2. 1H NMR spectra for 2a and 2b, recorded at r.t. (400 MHz, DMSO-d6): the upfield shift of the protons from the ligand, due to the Ru(III) centre, is not the same in 2b compared to
2a, the protons being further influenced by the electron-deficient effect of the triazine.
of the peaks, except for the peaks at 9.86 ppm and −1.02 ppm. These
two peaks were the only ones to display multiplicity (doublets), due to
their remoteness with respect with the paramagnetic centre, and they
were assigned to the protons of the phenyl ring (this assignment was
further confirmed by COSY 1H–1H, showing coupling between these two
protons). HMQC 1H–13C and TOCSY (Total Correlation Spectroscopy)
experiments on model complex 2a led to the majority of peaks being
assigned. However, the signals at −2.67, −8.76 and −9.17 ppm were
not assigned as the proton at −35.3 ppm did not couple with its
neighboring proton(s) even with longer acquisition times. Finally, 1H
NMR (d6-DMSO, 700 MHz, 298 K) gave the following assignment: δ 9.85
(d, J=16 Hz, 2H, H3″′), 7.28 (br, 2H, H3′), −1.04 (d, J=17 Hz, 2H, H2″′),
−2.67 (br, 2H, H3/4/5), −8.76 (br, 2H, H3/4/5), −9.17 (br, 2H, H3/4/5), and
−35.3 (br, 2H, H6) ppm. In 2b, the electron deficiency of the triazine
motif has a contrary effect on the chemical shifts of the protons from the
pyridyl rings. Unexpectedly, in 2a, the shielding effect of the para-
magnetic centre is more important on the H2″′ protons than on the H3′
protons, suggesting that the angular dependancy of the paramagnetic
effect has to be taken into account.
Compounds 2a–b have an absorption profile common to rutheni-
um polypyridyl complexes, albeit with bands at higher energy than
usually observed, due to the effect of the chloro ligands (Fig. 3). The
intense absorption bands in the UV region of the spectrum are due
to π–π* transitions Ligand-Centered (LC), whereas the bands in the
visible region are due to 1MLCT transitions. Two main low-lying
transitions are expected for each compound: a 1MLCT Ru→1a–b and
a
1LMCT Cl→Ru, with the latter at higher energy than the former.
In conclusion, the characteristic 1H NMR signals for intermediate
Ru(III) complexes make them useful tools to verify the purity of these
important precursors for heteroleptic Ru(II) complexes. Precursor 3
turned out to be generally more useful for complexation of tridentate
ligands than RuCl3·xH2O, leading to easier purification of reaction
mixtures, and of particular use when the tridentate ligands are only
afforded in multiple steps and in low yields.
Acknowledgments
We thank Pr. André L. Beauchamp (Université de Montréal) for
advice on Ru(III) paramagnetism and Mrs. Sylvie Bilodeau, Dr. Phan
Viet Minh Tan (Université de Montréal) for NMR 1H decoupling
studies. This work was supported by the Université de Montréal.
MPS thanks FQNRT — Consulat Français à Montréal for a Frontenac
scholarship. GSH thanks Johnson Matthey PLC for a loan of precious
metals, NSERC of Canada, Fond France Canada de Recherche (FFCR)
and Direction des Relations Internationales of Université de Montréal.
Appendix A. Supplementary data3
Supplementary data to this article can be found online at
doi:10.1016/j.inoche.2010.12.011.
Fig. 3. Electronic absorption spectra of 2a (solid line), 2b (dotted line) and 3 (dashed line)
in acetonitrile.
3
See Supplementary Information for synthetic descriptions and characterization.

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