Unexpected lability of the [RuIII(phtpy)Cl3] complex

Article abstract of DOI:10.1039/c7dt03658b

Ruthenium(iii) complexes are known for their high stability and inertness. To the best of our knowledge, the only well-characterized example of a labile Ru(iii) complex is [Ru^III(edta)(H₂O)] as a consequence of an intramolecular hydrogen bonding leading to the formation of a large opening in the molecule front, thus changing the mechanism from dissociative to associative. Compelling experimental evidence is presented demonstrating that the [Ru^III(phtpy)Cl₃] complex is labile, also indicating that the Ru(iii)-phtpy bond is much weaker than expected, in contrast to the strongly π-back-bonding stabilized Ru(ii)-phtpy bond.

Full text of DOI:10.1039/c7dt03658b

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Unexpected lability of the [RuIII(phtpy)Cl3] complex
Paola A. Benavides,a Tiago A. Matiasa and Koiti Araki*a
Received 00th January 20xx,
Accepted 00th January 20xx
Ruthenium(III) complexes are known for their high stability and inertness. To the best of out knowledge, the only well
characterized example of labile Ru(III) complex is [RuIII(edta)(H2O)] as consequence of an intramolecular hydrogen bonding
leading to the formation of a large opening in the molecule front, thus changing the mechanism from dissociative to
associative. Compeling experimental evidences are presented demonstrating that the [RuIII(phtpy)Cl3] complex is labile
also indicating that the Ru(III)-phtpy bond is much weaker than expected, in contrast with the strongly π-back-bonding
DOI: 10.1039/x0xx00000x
www.rsc.org/
stabilized
Ru(II)-phtpy
bond.
.
binuclear complex [{RuII(bpy)(H2O)}2(tpy2ph)](PF6)4 (where bpy
is 2,2’-bipyridine and tpy2ph is 1,3-bis(4′-2,2′:6′,2′′-terpyridin-
4-yl)benzene), when compared with the mononuclear
counterpart [RuII(phtpy)(H2O)(bpy)](PF6)2 (where phtpy is 4’-
phenyl-2,2’:6’,2’’-terpyridine) evidencing significant synergic
effects involving the metal sites.20 This fact has encouraged us
to investigate binuclear complexes for catalytic water
oxidation such as the [{RuII(phtpy)(H2O)}2(dpimbH2)]4+
complex, where dpimbH2 is the bis(2-pyridyl)benzodiimidazole
ligand. However, unexpected difficulties were faced in the
preparation of the respective chloro complex precursor
[{RuII(phtpy)Cl}2(dpimbH2)]2+ based on the well-established
Introduction
The concept of lability/inertness and thermodynamic stability
of transition metal complexes are fundamental in coordination
chemistry,
particularly
when
considering
the
catalytic/electrocatalytic activity of such compounds, since the
binding of a molecular substrate to the active site followed by
the reaction and release of the product are the key steps
controlling their efficiency.1,2 Accordingly, the mechanism and
rate of substitution reactions in transition metal complexes
were extensively studied in the last century and are well
established.3,4 For example, it is known that representative
elements cations and first row transition metal elements in 2+
oxidation state tend to generate labile coordination
compounds with high ionic character metal-ligand bonds. The
covalent character and stability of those complexes are
enhanced in the 3+ oxidation state species, but the ligand
exchange continue to be fast except for the Cr(III) and Co(III)
complexes in which the ligand field stabilization energy is
maximized.5–9 Nevertheless, the covalent character and
stability of second row transition metal complexes are much
larger and are all low spin and quite inert,10–13 particularly in
the case of Ru(II) d6 complexes with π-acceptor polypyridyl
ligands and respective Ru(III) complexes.14–16 Hence, except for
the [RuIII(edta)(H2O)]- complex in which the aqua ligand can be
easily substituted by associative or interchange mechanism,17–
reaction of [RuIII(phtpy)Cl3]
ligands in the presence of
(1
)
complex with bidentated
weak reducing agent.20
a
Surprisingly, the main reaction product was always the
[RuII(phtpy)2]2+ complex suggesting that the terdentated N-
heterocyclic ligand phtpy is actually dissociating from that
Ru(III) complex and reacting with reduced [RuII(phtpy)Cl3]
species.
Interestingly enough, such mind bogging result was
reproducible even when changing reaction parameters such as
solvent, reaction time, and temperature, as well as in the
presence of weak reducing species, precluding the preparation
of the desired [RuII(phtpy)(LL)(H2O)]2+ complexes, where LL=
dpimbH2 and bpy, with suitable yields. Accordingly, we
decided to carefully investigate that reaction, in order to shed
light on the possible reasons for the formation of the
[RuII(phtpy)2]2+ complex as main product. Therefore, herein
described are the experimental evidences demonstrating the
lability of the [RuIII(phtpy)Cl3] complex while reporting the
attempts to solve such an improbable synthetic problem.
19
the complexes are inert and the ligand exchange by
dissociation mechanism tend to be slow.
Recently, we showed the enhancement by 20 times of the
catalytic activity for water oxidation of the weakly coupled
Results and discussion
In a typical reaction, dpimbH2 ligand, [RuIII(phtpy)Cl3] (2
equiv.) and LiCl (10 equiv.) were added to a DMF/H2O 3:1 (v/v)
mixture in a two-necked round bottom flask (Scheme 1). Then
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4-ethylmorpholine (NEM) was added and the reaction mixture very similar, exhibiting the phtpy and dpimH2 intraligand bands
refluxed for 3 h while monitoring by UV-vis spectroscopy and as well as the metal-to-ligand charge-transfer (MLCT) bands in
thin layer chromatography (TLC). NEM is added to facilitate the the
visible
region,
as
expected
for
the
reduction of the final substitution product, or the reduction of [{RuII(phtpy)Cl}2(dpimH2)]2+ complex. However, the main
the [RuIII(phtpy)Cl3] complex to the respective Ru(II) complex, fraction labelled as A, corresponding to 54% yield, displayed a
thus facilitating the dissociation of chloro ligands and the contrasting spectral profile characterized by the absence of the
binding of dpimbH2, whereas LiCl assures the formation of the band belonging to the bridging ligand and the presence of a
desired monochloro complexes 3 and 4 (Scheme 1).
strong MLCT band centred at 490 nm. Analysis of this fraction
The UV-Vis spectrum of the reaction mixture after 3 h of by UV-Vis and 1H-NMR spectroscopy (see SI Fig. S2) showed
reflux in DMF (Fig. 1a), showed that the phtpy ligand that, under these experimental conditions, the major reaction
absorption bands were bathochromically shifted respectively product is the [RuII(phtpy)2]2+ complex. This outcome was
from 253 and 290 nm (free ligand) to 290 and 320 nm, completely unexpected since dpimbH2 is a strong σ-donor and
whereas the bands of the free dpimbH2 bridging ligand were π-acceptor bis-bidentated bridging ligand, known to stabilize
shifted from 351 and 368 nm to 382 and 406 nm. In addition, binuclear complexes generated upon reaction with two
the ruthenium complex generated in the reaction exhibited equivalents of [Co(bpy)2Cl2], [Rh(bpy)2Cl2], and [Ru(bpy)2Cl2]
broad overlapping bands spanning the whole visible range, complexes.21,22
that can be attributed to metal-to-ligand charge-transfer
Similar results were obtained in reactions carried out in
transitions characteristic of ruthenium(II) polypyridine ethanol and methanol even in the absence of NEM. Thus, the
species.20
possible reduction of Ru(III) to Ru(II) in methanol solution was
confirmed by dispersing solid [RuIII(phtpy)Cl3] in MeOH at 50 °C
under stirring, monitoring the spectral evolution as a function
of time (Fig. 2a), and then reacting with hydrogen peroxide
(Fig. 2b). The absorption at 440 nm decayed with the
concomitant rise of the bands at 379, 490 and 573 nm, while
the colour changed from pale brown to purple and the
scattering contribution decreased, as confirmed visually by the
decrease of turbidity. Such spectroscopic behaviour was
attributed to the formation of soluble reduced
[RuII(phtpy)L1L2L3] derivatives (where Ln = solvent or Cl-) and
the [RuII(phtpy)2]2+ complex. In order to confirm this
hypothesis, diluted hydrogen peroxide was added into the
Scheme 1. Reaction scheme for the synthesis of the binuclear
[{RuII(phtpy)(Cl)}2(dpimbH2)]2+ species (two of
6 possible
isomers are shown) from the [RuIII(phtpy)Cl3] complex and
dpimbH2 ligand.
purple
solution
and
the
mixture
monitored
spectrophotometrically. The bands at 379 and 573 nm
disappeared rapidly after the addition of H2O2 concomitantly
with the rise of the band at 440 nm, suggesting the
regeneration of the initial Ru(III) species (Fig. 2b). Interestingly,
the final spectrum of the solution, except for the absorption at
440 nm, matched that of the bis-terpyridine complex
indicating that the redox potential of [RuII(phtpy)2]2+ is more
positive than that of H2O2, thus remaining unchanged in
solution.
In a parallel experiment, [RuIII(phtpy)Cl3] was reduced with
zinc almagam, Zn(Hg), (see SI Fig. S3a) a strong reducing agent,
in MeOH at 50 °C under N2 atmosphere, in order to evaluate
the stability of the Ru(II)-phtpy complex and its eventual
contribution to the formation of the bis-phtpy complex. After
addition of Zn(Hg) into the solution, strong absorption bands,
characteristic of the MLCT bands of reduced [RuII(phtpy)L1L2L3]
species, started to appear in the 450 to 750 nm range as the
band of [RuIII(phtpy)Cl3] at 440 nm fade. After 30 min, the
reduction reaction and consequent solubilisation process was
complete. The Zn(Hg) was removed and the solution
monitored spectrophotometrically for up to 2.5 h with no
significant spectral change. Then H2O2 was added leading to
the immediate disappearance of the MLCT bands and
regeneration of the spectral pattern of the Ru(III) complex,
with no evidence of formation of the [Ru(phtpy)2]2+ species
Figure 1. a) UV-Vis spectra of the crude reaction mixture of
[RuIII(phtpy)Cl3] with dpimbH2 ligand after 3 h reflux in DMF,
compared with the authentic free dpimbH2 and phtpy ligands
in MeOH and b) UV-Vis spectrum of the five representative
fractions isolated by silica gel column chromatography.
As shown in Figure 1b, the spectroscopic profiles of
fractions C, D and E obtained by column chromatography are
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exhibiting a sharp MLCT band at 490 nm (Fig. S3b). This result transmission UV-Vis and by TLC as a function of time. In this
clearly indicates that phtpy ligand does not dissociate from the case, the evolution of the reaction was monitored by UV-Vis
reduced ruthenium complex, in agreement with the behaviour reflectance spectroscopy directly at the spots in the
reported for cis/trans-[RuII(DMSO)4Cl2] that promptly react chromatoplates (Fig. 3). The spectral profile of the supernatant
with
tpy
generating
the
respective
cis/trans- solution changed continuously, as seen by the rise of the
[RuII(tpy)(DMSO)Cl2] (tpy= 2,2’:6’,2’’-terpyridine) complexes in absorption bands at 390 and 490 nm, suggesting the build-up
high yields (>95%) without formation of [RuII(tpy)2]2+ species, of [Ru(phtpy)2]2+ complex in solution. However, although the
as expected for the high stability of RuII-tpy bond.23
absorption band at 490 nm increased in the first 15 min, the
Considering that there is no significant amount of free formation of that homoleptic species was confirmed only after
1
phtpy ligand as impurity of complex 1 (as shown below by H- 2 h at room temperature (Fig. 3a). In fact, the spot in the
NMR) to explain the formation of [RuII(phtpy)2]2+ complex as chromatoplate with spectrum matching the authentic
major product, the reaction conditions (solvents, temperature compound increased in size for longer reaction times,
and stoichiometry of reagents) were varied in an attempt to indicating that phtpy ligand is dissociating from
1 at room
avoid the formation of that complex (see SI Table 1). However, temperature and possibly reacting with [RuII(phtpy)Cl3]
since all reactions gave similar results, we accepted the complex or solvated derivatives.
possibility that the [RuIII(phtpy)Cl3] precursor complex is in fact
releasing the coordinated phtpy ligand to generate the
homoleptic complex, as well as solvated RuCl3.
Figure 3. a) Series of UV-Vis spectra collected while monitoring
a suspension of [RuIII(phtpy)Cl3] complex in MeOH as a
function of time for up to 2 h. Note that there is almost no
[RuII(phtpy)2]2+ complex MLCT band at 490 nm at t = 0 min. b)
UV-Vis reflectance spectrum of solid [RuIII(phtpy)Cl3] precursor
(brown line), of authentic [RuII(phtpy)2]2+ complex (red line),
and the spots in the chromatoplate after TLC analysis of the
supernatant solution of a [RuIII(phtpy)Cl3] complex suspension
in MeOH after 2 h (orange line).
F
igure 2. a) Evolution of the UV-Vis spectrum of a
[RuIII(phtpy)Cl3] complex suspension in methanol at 50 °C,
under stirring, as a function of time up to 129 min; and photo
of the initial solution and final solution; b) spectroscopic
behaviour of the final purple solution as a function of time
after addition of diluted H2O2 solution, at room temperature.
Considering the well stablished inertness and stability of
ruthenium(III) terpyridine derivatives, the only other possible
explanation for the results described above was the presence
of significant amounts of free phtpy ligand as impurity of
Similar experimental fact was previously reported by
Ziegler et. al.24 for a RuIII complex containing a more sterically
hindered ‘pineno’-fused terpyridyl (L1) ligand which
dissociates releasing RuIII and free L1 in dilute solutions of
polar solvents, that in the presence of terpyridine ligand
yielded statistical mixtures of hetero and homoleptic
ruthenium complexes.25 Although the authors did not give
additional evidences to confirm that behaviour, it is supportive
of the fact that the terdentated terpyridine ligand may be
dissociating from our precursor leading to the formation of
[RuII(phtpy)2]2+, a quite unusual hypothesis considering the
well-known high stability of chelate species and the supposed
inertness of ruthenium(III) complexes.
complex 1
, thus leading to the formation of [RuII(phtpy)2]2+ as
major product. To verify this hypothesis, we performed a
careful characterization of the [RuIII(phtpy)Cl3] complex used as
precursor by 1H-NMR spectroscopy in search of organic
impurities, particularly free phtpy ligand.
The 1H-RMN spectrum of complex
1 in DMSO-d6 is
characterized by the presence of signals spanning from -37
ppm to 10 ppm (see SI Fig. S4). All signals in the high field
region below 1 ppm, and at 7.31 ppm and 9.70 ppm were
assigned to the [RuIII(phtpy)Cl3] complex, whereas the five
peaks indicated with stars can be attributed to the respective
reduced Ru(II) species. As expected, the paramagnetic effect
In an effort to get stronger evidences, the supernatant
solution of a [RuIII(phtpy)Cl3] complex suspension in methanol,
under stirring at room temperature, was monitored by
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due to the presence of S=1/2 low spin Ru(III) ion is changing species decreased, while the weak signals of the free phtpy
the local magnetic field and strongly shifting most of the phtpy ligand (indicated in light purple) increased concomitantly.
ligand signals to higher fields. It is also evident the broadening
This result clearly demonstrates that the oxidized species
of the peaks corresponding to the oxidized species such as the dissociates releasing free phtpy ligand, which in turn is
peaks at 7.31 ppm and -35.81 ppm, respectively attributed to reacting with reduced [RuII(phtpy)L1L2L3] species generating
the H3’ and H6 (see Figure S4), as expected for the shortening the homoleptic complex, while more compound
1 reduced
of the nuclear spin relaxation time of the coordinated ligand species is slowly generated in solution. Notice that there is no
protons26. Notice also the presence of four signals in the 7.5- significant amount of reduced ruthenium precursor left, yet
9.0 ppm range (labelled with dots) assigned to the phtpy ligand the peaks of initially absent free phtpy ligand can now be
coordinated to a diamagnetic center.23
observed in the spectrum of the solution after 8 h (Fig. 4),
strongly indicating that chelating ligand is in fact dissociating
from the [RuIII(phtpy)Cl3] complex. This seems to be the driving
force pushing the reaction towards the formation of
[RuII(phtpy)2]2+ as the major reaction product. In fact, reactions
of the bis-bidentated ligand dpimbH2 with the [RuII(phtpy)Cl3]
complex, prepared by previous reduction of
1 with zinc
amalgam, successfully lead to the formation of the desired
binuclear complexes in high yield, indicating that the back-
bonding interactions in the reduced species strengthen the
RuII-phtpy bonds and weakens the RuII-Cl bonds facilitating the
chloro ligand substitution reactions. The expected binuclear
complex was also obtained upon reaction of dpimbH2 with
[Ru(bpy)2Cl2] in suitable conditions. This behaviour seems to
parallel that of Fe(III/II) complexes with bidentated polypyridyl
ligands such as bpy, phen and tpy,29,30 whose Fe(II) complex
formation constants are significantly much larger than that of
the respective Fe(III) complex because of the major
contribution of π-backbonding. Thus, Ru(III)-phtpy bond
should have much higher ionic character than expected and
suggest to be weaker than Ru(III)-Cl bonds because of the
favourable electrostatic contribution absent in the first one,
despite of its low spin nature, in contrast with the highly
covalent Ru(II)-phtpy bond. In fact, the bidentated polypyridyl
ligands of the [RuII(bpy)2Cl2] and [RuII(phen)2Cl2] complexes
have no tendency to dissociate, in contrast with the chloro
ligands that tend to dissociate more promptly.
1
Figure 4. H-RMN in DMSO-d6 of the [RuIII(phtpy)Cl3] complex
contaminated with the respective reduced species (t=0h,
brown line), the [RuII(phtpy)2]2+ complex (orange line), free
phtpy ligand (purple line) and the solution obtained after
heating a suspension of 1 in EtOH for 8 h (red line).
Accordingly, a typical sample of the [Ru(phtpy)Cl3] complex
used as precursor in the preparation of ruthenium terpyridine
derivatives is a mixture of the Ru(III) and Ru(II) species formed
during its preparation, where the solvent methanol acts as a
weak reducing agent.25,27 The weak signals labelled with dots
perfectly matched the spectrum of [RuII(phtpy)2]2+, a complex
formed as byproduct in the synthesis of [RuIII(phtpy)Cl3].26,28 It
is important to remember that the amount of this impurity
(about 4%) is much smaller than that found after reaction of
[Ru(phtpy)Cl3] complex with the dpimbH2 ligand (can reach up
to 54% under conditions shown in scheme 1), thus not being
enough to explain its presence as major reaction product.
Finally, a dispersion of 1 in methanol under stirring for
eight hours at 60 °C was dried in a flash evaporator, and its 1H-
NMR spectrum in DMSO-d6 (Figure 4) compared with the
spectrum of phtpy ligand, the [RuII(phtpy)2]2+ and freshly
prepared solution of the [RuIII(phtpy)Cl3] complex. This was
Conclusions
Summarizing, the improbable dissociation of the phtpy ligand
from the [RuIII(phtpy)Cl3] complex was demonstrated by the
espontaneous formation of the [Ru(phtpy)2]2+ complex in
methanol, ethanol and DMF solution, as confirmed by UV-vis
and 1H-NMR spectroscopy, as well as by TLC and column
chromatography. In fact, its reduction with a strong reducing
agent (zinc amalgam) completely shut down that process
indicating that the Ru(II)-tpy bond is highly stabilized by π-
back-bonding, whilst the Ru(III)-tpy bond has much higher than
expected ionic character and lower bond strength as
confirmed by the unexpected lability of that terdentated
polypyridyl ligand from a Ru(III) complex.
found to contain a mixture of
1 and its respective reduced
species, as well as a small amount of [RuII(phtpy)2]2+ complex
as impurity, as discussed above. After 8 h at 60°C in methanol,
however, the amount of this homoleptic species increased to
14.5% and the signals assigned to the reduced [RuIII(phtpy)Cl3]
Experimental
Instrumentation and methods.
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1
The H-NMR spectra were acquired in a Bruker AIII 300 MHz cooled to room temperature and the brown precipitate
spectrometer using samples dissolved in pure deuterated filtered out and washed with water and ethanol. The solid was
solvents or mixtures of deuterated solvents, in order to dispersed in hot ethanol, filtered and washed with hot ethanol
generate 2-3 mg/500 μL solutions, using TMS as internal to get the final product. Yield 60%. CHN Analysis %found
reference. ESI-MS spectra were recorded in a Bruker Daltonics (%calc) for C21H15Cl3N3·4H2O: C 45.00 (45.62); H 3.25 (3.46); N
Esquire 3000 plus mass spectrometer using samples dissolved 7.28 (7.60).
in N,N-dimethylformamide diluted in methanol just before
injection, setting the capillary voltage to 4 kV. Elemental Reduction of the [RuIII(phtpy) Cl3] complex with Zn amalgam.
analyses of the samples were recorded in a Perkin Elmer 2400 Zinc amalgam was prepared by transferring granulated Zn
series II elemental analyzer equipped with
a thermal pellets into a 250 mL erlenmeyer, activating them by washing
conductivity detector. The electronic spectra of the tree times with a 50% v/v aqueous HCl solution, and reacting
compounds in the UV-Vis region (190 nm to 1100 nm) were with HgCl2 powder under manual shaking for five minutes. The
recorded in a Hewlett Packard 8453A spectrophotometer. The bright amalgamated Zn pellets were carefully washed with
UV-vis absorption and reflectance spectra were registered distilled water, then with methanol, and immediately added
respectively in a HP8453A diode-array spectrophotometer into a suspension of the [RuIII(phtpy)Cl3] complex in methanol,
(190-1100 nm range), employing 1.00 cm optical path length previously purged with nitrogen gas. The solution color
quartz cuvettes, and in
spectroradiometer from Analytical Spectral Devices (350-2500 remained after 30 min, indicating the complete reduction of
nm range). Thin layer chromatography was carried out using the ruthenium(III) complex, as confirmed
a
FieldSpect
3 fiber optic changed from brown to bluish-violet and no precipitate
silica gel 60 F254 TLC chromatoplates employing spectrophotometrically (no further spectral change). The
acetone:methanol:KNO3(saturated solution) 3:2:1 as eluent pellets were removed while N2 gas was continuously bubbled
mixture.
in the solution to keep an inert atmosphere, thus avoiding re-
The percentage yields calculations by 1H-NMR were oxidation by O2 in air.
realized by comparing the peak areas normalized with respect
to the H3’ proton peak, which has a integration value
corresponding to 2 protons in the phtpy complexes, to
determine their relative amounts in the mixtures. The
experiment shown in figure 2 were carried out using a solution
of 1 mg of [Ru(phtpy)Cl3] dispersed in 25 mL of MeOH,
whereas the experiments shown in figure 3 used a suspension
of 2 mg of [Ru(phtpy)Cl3] in 4 mL of MeOH.
Conflicts of interest
There are no conflicts to declare
Acknowledgements
Authors gratefully acknowledge the financial support by the
Brazilian Agencies Conselho Nacional de Desenvolvimento
Synthetic Details
Científico
e
Tecnológico (CNPq 302368/2010-8 and
All solvents and reactants were of analytical grade and
employed as received, without further purification.
133876/2015-2), and Fundação de Amparo à Pesquisa do
Estado de São Paulo (FAPESP 2013/24725-4). PAB thanks
Coordenação de Aperfeiçoamento de Pessoal de Nível
Superior (CAPES) for the fellowship.
4-phenyl-2,2’:6’,2’’-terpyridine (phtpy) ligand. This polypyridyl
ligand was prepared by following a modified Kröhnke reaction
for the synthesis of substituted terpyridines.20, 31 1g (9.4 mmol)
of benzaldehyde was transferred to a 250 mL round-bottom
flask containing 40 mL of a 0.3 M KOH solution in ethanol.
After 10 minutes of stirring at room temperature, 2.2 g (19
mmol) of 4-acetylpyridine and 27 mL of NH4OH 28% were
added into. Four hours later the precipitate dispersed in a
yellow-green solution was collected, washed with ethanol and
water, and dried under vacuum to obtain a greenish solid,
which yielded a white solid after two times recrystallization
from ethanol. Yield: 60%. CHN Analysis %found (%calc) for
C21H15N3·4H2O: C 81.76 (81.53); H 4.99 (4.89); N 13.53 (13.58).
Notes and references
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2
3
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4
5
20
.
[RuIII(phtpy)Cl3] precursor complex
0.27 g (1 mmol) of
RuCl3·3H2O was transferred into a two-necked round-bottom
flask of 100 mL and dissolved with 70 mL of ethanol. Then, a
saturated solution of the phtpy ligand in chloroform was
slowly added into the ruthenium complex solution employing
an addition funnel, while keeping the temperature constant at
70 °C. After three hours of reflux, the reaction mixture was
6
7
8
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